YEAR IN REVIEW 

2014
YEAR IN REVIEW 

2014

Editor:

Emine Begum Gulsoy, Ph.D.

Associate Director, CHiMaD

ON THE COVER Self-assembly of colloids with anisotropic dielectric properties (red and 

blue surfaces), bound by charged nanoparticles (green). Depending on the solvent selected, the 

nanoparticles preferentially bind (electrostatically) to surfaces with either low or high dielectric constant, thus opening the possibility of generating solvent-dependent structures. From 

molecular dynamics simulations with fully resolved dielectric effects, performed by Huanxin 

Wu and Erik Luijten.

Booklet Design: Kathleen P. Mandell
TABLE OF CONTENTS

YEAR IN REVIEW 2014 | i

FOREWORD: A Message from the Directors ..................................................ii

MISSION AND VISION ................................................................................... 1

RESEARCH ..................................................................................................... 2

Data Mining ............................................................................................. 2

Polymer Matrix Materials......................................................................... 6

Low-Dimensional Nanoelectronic Materials...........................................12

Precipitation-Strengthened Alloys..........................................................18

In-Situ Si-Composite Materials ............................................................. 26

Directed Self-Assembly of Block Copolymers Films for Lithographic 

 Applications....................................................................................... 29

Biomaterials........................................................................................... 38

Organic Bulk Heterojunction Polymer Solar Cells................................. 43

TOOL DEVELOPMENT .................................................................................. 50

OUTREACH .................................................................................................. 52

HONORS AND AWARDS .............................................................................. 54

PUBLICATIONS ............................................................................................ 56

CONTACTS ................................................................................................... 57
FOREWORD

ii | CENTER FOR HIERARCHICAL MATERIALS DESIGN

We are pleased to present the first annual report of the Center for Hierarchical Materials Design 

(CHiMaD), the National Institute for Standards and Technology (NIST) Center of Excellence in Advanced Materials. CHiMaD is focused on achieving the goals of the Materials Genome Initiative 

(MGI), set forth by President Obama and his administration. We are committed to achieving these goals 

through the development of broad research and outreach programs that involve participants from university, national laboratory and industry. Central to these programs are the many collaborative efforts that we 

have developed with NIST. It is these collaborative efforts that make our center unique, and which ensure 

that we shall attain the goals of the MGI on a national level. 

CHiMaD is demonstrating the power and potential of bringing together data science, computational approaches and state-of-the-art experiments to design materials ranging from cobalt alloys for bearing applications to polymers for photovoltaics. The research effort compasses 35 principal investigators, 27 

postdoctoral research fellows, 38 graduate students and four undergraduate students working on CHiMaD 

research projects. This report presents the objectives and accomplishments of our eight use-case groups, 

and multiple tool development efforts. The breadth of our portfolio allows us to interact with a broad spectrum of the industrial community, as well as to involve many NIST scientists. We are also working towards 

being a resource for the Nation through the sponsorship of workshops and summer schools. During this 

inaugural year we have already sponsored four workshops on topics from the development of CALPHAD 

protodata repositories to computational methods for coarse grain simulations of polymers. CHiMaD is also 

developing the computational and experimental tools needed for materials design and the Nation’s materials research community. 

As you will see from the following report, CHiMaD is off to a very fast start. Our first year has set a firm foundation for success in the future. We look forward to making the promise of the Materials Genome Initiative 

a reality. 

Juan De Pablo, Gregory Olson, Peter Voorhees

Co-Directors, CHiMaD
YEAR IN REVIEW 2014 | 1

MISSION AND VISION

The Center for Hierarchical Materials Design (CHiMaD) is 

a NIST-sponsored center of excellence for advanced 

materials research that is focused on developing the 

next generation of computational tools, databases, and experimental techniques in order to enable the accelerated design of novel materials and their integration to industry, one 

of the primary goals of the Obama administration’s Materials 

Genome Initiative (MGI).

This Chicago-based consortium includes Northwestern 

University (NU), University of Chicago (UChicago), 

Northwestern-Argonne Institute for Science and Engineering 

(NAISE) — a partnership between Northwestern University 

and Argonne National Laboratory (ANL) — and the 

Computational Institute (CI) — a partnership between 

University of Chicago and Argonne National Laboratory. The 

consortium is also partnered with Questek Innovations, a pioneering materials design company, ASM International and 

Fayetteville State University.

Designing novel materials of specific properties for a particular application requires simultaneously utilizing physical 

theory, advanced computational methods and models, materials properties databases and complex calculations. This 

approach stands in contrast to the traditional trial-and-error method of materials discovery. CHiMaD aims to focus 

this approach on the creation of novel “hierarchical materials” that exploit distinct structural details at various scales, 

from the atomic on up, to obtain enhanced properties. The 

Center’s research focuses on both organic and inorganic 

advanced materials, in fields as diverse as self-assembled 

biomaterials, smart materials for self-assembled circuits, 

organic photovoltaic materials, advanced ceramics and 

metal alloys. 

MISSION Accelerating materials discovery 

and commercialization by design and development of hierarchical methods and materials 

and enabling the complete integration of theory, computation and experimentation by 

building a strong community of current and 

future researchers.

CHiMaD OBJECTIVES

• To foster hierarchical materials discovery, in 

accordance with the goals of MGI and NIST, 

by developing the next generation of computational tools, databases and experimental 

techniques

• To connect and convene the current experts 

and the next generation of scientists by 

advancing the multi-disciplinary and multisector communication through workshops, 

seminars, training opportunities and meetings

• To provide opportunities to transition new 

breakthroughs in advanced materials to 

industry

• To adopt a seamless integration of prediction, 

measurement and interpretation

• To maintain and advance the collective knowledge on material systems and 

methodologies 

• To serve, together with NIST, as a national 

resource for verified codes and curated 

databases that will enable proliferation of a 

materials-by-design strategy and materials 

discovery through US industry partners

• To build on the established success in metal 

alloy systems to prove generality of the materialsby-design approach through application to both 

inorganic and organic advanced systems for 

both structural and multifunctional applications 

• To expand the 3D and 4D characterization 

methods for hard materials design and to bring 

such methods to the forefront of soft materials 

design
2 | CENTER FOR HIERARCHICAL MATERIALS DESIGN

RESEARCH

DESIGN GOALS

The primary goal for this reporting period was to identify important problems in materials science that can benefit from 

data-driven informatics, and kick-off collaboration(s) with 

domain scientists within the CHiMaD team. To this end, we 

have initiated three specific collaborations in materials informatics building on our prior work in the field. One of them 

aims to design an effective interface between data mining 

techniques and the CALPHAD methodology for materials 

genomic design, with an illustrative application of advanced 

steel design using NIMS steel data. Another collaboration 

aims to use machine learning to discover functional relationships between material properties and quantitative attributes derived from the composition of the material. It is 

built on our previous work on data-driven discovery of stable 

compounds, and we expect our technique to be applicable 

DATA MINING 

Alok Choudhary (co-lead)1, Ankit Agrawal (co-lead)1, Gregory B. Olson2, Christopher Wolverton2, Wei Chen3

1

Electrical Engineering and Computer Science, Northwestern University

2

Materials Science and Engineering, Northwestern University

3

Mechanical Engineering, Northwestern University

The CHiMaD data mining team is working towards making significant advances in data-driven informatics to 

accelerate materials discovery. In this era of big data, we are being bombarded with huge volumes of data 

from a variety of different sources (experiments, simulations) at unprecedented rates in practically all fields of 

science and engineering, and materials science is no exception. Advanced techniques for data-driven analytics are needed to analyze all this data in ways that can help extract meaningful information and knowledge 

from it, and thus contribute to accelerating materials discovery and the vision of Materials Genome Initiative. 

The diversity and expertise within the CHiMaD team provides unique opportunities for collaborations among 

materials scientists and computer scientists to realize this goal. This requires designing and developing 

specific analytics and data mining approaches for problems in materials science, and evaluating the resulting 

insights/discoveries via necessary experiments and simulations. The long-term vision is to work with NIST 

to enable data-to-discovery (or actionable insights) at unprecedented latency through summary statistics, 

query, reporting, analysis, bottom-up as well as hypothesis-driven discoveries and data mining on various 

heterogeneous and big databases that are complex, high dimensional, structured and semi-structured.

to and capable of solving a broad variety of materials problems. Finally, the third collaboration uses machine learning 

to identify key microstructure descriptors from a vast array

of candidates as potential microstructural design variables, 

which is an important research question in microstructure 

representation. 

SIGNIFICANT ACCOMPLISHMENTS 

A team of undergraduates in our special Murphy Scholar section of Freshman Design collaborated with Agrawal, Choudhary, Olson, and research associate Wei Xiong 

to initiate our research on the integration of a processproperty database with a CALPHAD database for fusion of 

prior knowledge with data in the adaptation of data mining 

techniques.
YEAR IN REVIEW 2014 | 3

In collaboration with Agrawal, Choudhary, and Wolverton, 

doctoral student Logan Ward has developed a new hierarchical modeling technique on DFT data that has been demonstrated to be very effective for the problem of band gap 

prediction from material composition.

In collaboration with Chen and Choudhary, doctoral students Hongyi Xu and Ruoqian Liu developed a machine 

learning based design representation method for designing heterogeneous microstructures. This work won the best 

paper award at the 40th ASME Design Automation Conference, 2014. 

RESEARCH ACCOMPLISHMENTS 

Integrating CALPHAD and data mining for advanced 

materials design

The CALPHAD (CALculation of PHAse Diagrams) approach 

is currently considered as one of the most promising approaches for materials genomic design. The great advantage of the CALPHAD approach is model-prediction of a 

multi-component system based on self-consistent optimization. The developed CALPHAD database is easy to couple 

with other models in order to extend its capability of describing multi-scale phase behavior. Thanks to mature computational software, like Thermo-Calc, the development and 

applications of the CALPHAD approach have been greatly 

accelerated over the years. However, so far, there is no comprehensive research available integrating the CALPHAD and 

data mining methodologies for advanced materials design. 

This work involving a collaboration of Agrawal, Choudhary, 

Olson, and research associate Wei Xiong, aims to design an 

effective interface between data mining techniques and the 

CALPHAD methodology for materials genomic design. The 

research studies are performed on the carbon steels with 

raw data sets obtained from experiments provided by NIMS 

(National Institute for Materials Science, Japan).

In Spring 2014, a team of undergraduates in our special Murphy Scholar section of Freshman Design collaborated with our team to initiate our research on the 

integration of a process-property database with a CALPHAD database for fusion of prior knowledge with data in the 

adaptation of data mining techniques for greater accuracy 

Figure 1. Flowchart for implementing theoretical models 

using CALPHAD database in data mining.

Figure 2. Performance of three different strategies to locate 30 compounds 

with a band gap energy between 0.9 and 1.7 eV out of a set of 2500 candidates from the ICSD. The first strategy is to randomly select a nonmetalcontaining compound. The other two strategies are to use a model produced 

using the machine-learning-based method described in this work that was 

trained on the band gap energies of a distinct set of 22667 compounds 

from the ICSD to locate compounds with band gap energy within the desired 

range. The first of those two strategies uses a single model trained on all 

compounds in the training set, and the latter uses the composite predictions 

of a several models each trained on different groups of similar compounds. 

Error bars represent the 95% confidence interval.
4 | CENTER FOR HIERARCHICAL MATERIALS DESIGN

materials science is to use these datasets to create predictive 

models for material properties, which may be approached 

using machine learning. While previous approaches have 

been successful in predicting a single property of a single 

class of materials, no one has yet introduced a single method that is capable of producing predictive models capable 

of solving a broad variety of materials problems. In a collaboration among Agrawal, Choudhary, and Wolverton, 

doctoral student Logan Ward has developed such a method 

and applied it to a variety of material property databases, 

including ones containing crystalline and amorphous materials. The method works by using machine learning to discover functional relationships between material properties 

and quantitative attributes derived from the composition of 

the material. It employs a set of attributes broad enough to 

capture important chemical effects for multiple materials 

problems and introduce a hierarchical modeling strategy to 

enhance the accuracy of our models by partitioning the databases into subsets of similar materials. This new method 

can be used to learn novel design rules, predict the properties of yet-unstudied materials, and dramatically accelerate combinatorial searches. Figure 2 presents an illustrative 

application of the method for a simulated search for compounds suitable for use in a single-layer solar cell. In order to 

optimize efficiency, a candidate compound needs to at least 

have a band gap between 0.9 – 1.7 eV. Many other properties are necessary for a suitable solar cell material, but a 

Figure 3. The framework of machine learning-based microstructure descriptor identification.

of process-structure-property correlation. Using a subset of 

the full process-property database, the team demonstrated 

stronger correlation for a subset of steels compared to previous data mining using the process-property data alone. 

Research associate Wei Xiong, who is a CALPHAD technology expert educated at the Royal Institute of Technology 

(KTH) in Sweden, coached the team. 

After the student pilot project, Xiong continued the research 

developing a TCICP-based link to automate interaction of 

the Thermo-Calc CALPHAD software with the full processproperty database and mining tools in collaboration with 

Agrawal. An interface is being built between the ThermoCalc Gibbs energy module and the input database for 

data mining using TQ toolkit in the Thermo-Calc software package, which can read both experimental data and 

thermodynamic models from Thermo-Calc database, and 

further convert them into important physical properties using 

physical metallurgical models of steels. The flow chart of 

the constructed interface is shown in Figure 1. The resulting 

data would be analyzed by data mining techniques to mine 

process-structure-property (PSP) relationships. 

General-Purpose Machine Learning Strategy for 

Predicting Properties of Inorganic Materials

In recent years, there has been significant focus towards using material property databases and combinatorial searches to accelerate materials discovery. One goal of modern 
YEAR IN REVIEW 2014 | 5

Data Mining

Group Researchers and Their Roles 

Name Position

Institution/

Affiliation Project Title (PI)

Alok Choudhary PI NU/EECS Data Mining

Ankit Agrawal PI NU/EECS Data Mining

Gregory B. Olson PI NU/MSE Materials Design

Christopher Wolverton PI NU/MSE DFT

Wei Chen PI NU/ME Design Methods

Wei Xiong Postdoctoral Researcher NU/MSE Materials Design (Olson)

Logan Ward Graduate student NU/MSE DFT (Wolverton)

Amar Krishna Graduate student NU/EECS Data Mining (Choudhary/Agrawal)

Ruoqian Liu Graduate student NU/EECS Data Mining (Choudhary/Agrawal)

favorable band gap energy is a requirement. The ability to 

locate materials that are likely to have a desirable band gap 

can therefore act as an initial screen.

Machine Learning-based Design Representation 

Method for Designing Heterogeneous Microstructures

In designing microstructural materials systems, one of the 

key research questions is how to represent the microstructural design space quantitatively using a descriptor set that 

is sufficient yet small enough to be tractable. Existing approaches describe complex microstructures either using a 

small set of descriptors that lack sufficient level of details 

and require prior empirical identification and selection, or 

using generic high order microstructure functions of infinite dimensionality without explicit physical meanings. In 

a collaboration between Chen and Choudhary, doctoral students Hongyi Xu and Ruoqian Liu have developed a 

machine learning-based method (Figure 3) that solves this 

problem by effectively reducing the infinite dimension of the 

microstructure design space to a small set of descriptors 

without a significant information loss. With a large number 

of candidate microstructure descriptors collected from literature covering a wide range of microstructural material 

systems, the method is able to perform 4-fold of functionalities: (1) eliminate redundant microstructure descriptors via 

image analyses, (2) identify key microstructure descriptors 

based on microstructure morphology, (3) identify key microstructure descriptors based on structure-property relationship, and (4) determine the final set of microstructure design 

variables. The benefits are demonstrated by an example 

of polymer nanocomposites optimization. The designs are 

compared using key microstructure descriptors versus using empirically chosen microstructure descriptors to validate 

the proposed method. 

PUBLICATIONS

Xu, H., Liu, R., Choudhary, A., and Chen, W., “A Machine 

Learning-Based Design Representation Method for Designing Heterogeneous Microstructures”, IDETC2014-34570, 

August 17-20, Buffalo, New York, 2014. Best Paper Award, 

40th ASME Design Automation Conference. This work was 

not directly supported by CHiMaD, but reported as a research highlight, and laid the foundation for the PIs’ collaboration within CHiMaD. 
6 | CENTER FOR HIERARCHICAL MATERIALS DESIGN

POLYMER MATRIX MATERIALS

L. Catherine Brinson (lead)1,2, Wei Chen2, Sinan Keten3, Erik Luijten1,4,5, Juan de Pablo6

1

Department of Materials Science and Engineering, Northwestern University

2

Department of Mechanical Engineering, Northwestern University

3

Civil and Environmental Engineering, Mechanical Engineering, Northwestern University

4

Engineering Sciences and Applied Mathematics, Northwestern University

5

Physics and Astronomy (by courtesy), Northwestern University

6Institute for Molecular Engineering, University of Chicago 

While polymer composites are lightweight and corrosion resistant with tailorable properties, their capabilities are 

vastly under-utilized due to lack of: 1) understanding of multiphase composite properties under small constituent 

changes, 2) ability to tailor electrical and mechanical properties simultaneously, and 3) durability predictions for 

long-term properties under use conditions. In this Use Case, we promote development of a “materials informatics 

initiative” including integrated databases, curation, visualization, and analysis tools to relate macroscale polymer 

composite behavior to chemical constituent and kinetic behavior, and linking these resources to further development 

of high-performance modeling and predictive tools.

We focus in this project on polymer nanocomposites initially, with long term vision for general composites. The 

key of multiphase polymer response is to understand the interaction of filler particles with the matrix material and 

to explore the physical properties of the interphase zone. This is accomplished both by experimental efforts and 

physics-based modeling using Molecular Dynamics and Monte Carlo simulations to predict interphase polymer 

response as a function of confinement, crosslinking density and monomer chemistry. At the same time, we are assembling the known data into a central resource, which will be used with supervised learning methods to identify 

overarching trends. Combining the physics based methodology with the curated experimental data will provide an 

unprecedented ability to develop new predictive models of composite behavior.

DESIGN GOALS 

The ultimate goal for this project is to develop databases, 

models and tools to enable prediction of hierarchical composite behavior based on constituent components and 

processing. 

The short-term goals of the use-case group are: (1) to design cellulose-polymer nanocomposites with optimized interphase behavior for high modulus/diffusion resistance; (2) to 

develop Databases/Models/tools to predict interphase properties as well as diffusion and elastic modulus of the system.

Specifically, areas of interest that will be investigated are as 

follows: 

Experiments on local interphase properties: Brinson and 

Gilman (NIST) will be conducting mechanophore, aquaphore 

and Atomic Force Microcopy (AFM) experiments on local 

polymer modulus in interphases.

Models for cellulose polymer nanocomposites: De Pablo, 

Keten, Luijten, Phelan (NIST), Douglas (NIST) will be collaborating to model fluorophores near an interface, predict 

partitioning of energies and interpret lifetime measurements. 

They will also be modeling mechanophores and investigating how much energy is required for the bonds to break. In 

addition, they will work on multi-scale modeling of effective 

properties such as modulus and diffusion as well as connecting and synergizing Molecular Dynamics, Monte Carlo 

and Self-Consistent Field models. 

Data Structures and databases for poly-mers and composites:

Brinson and Chen will continue building “NanoMine” data 
YEAR IN REVIEW 2014 | 7

resource for nanocomposites, while 

de Pablo will continue developing

the “Polymer Design Database.” These 

data resources will then be integrated with 

each other as well as the large dataset resource Ian Foster

is developing and larger NIST efforts.

SIGNIFICANT ACCOMPLISHMENTS 

Brinson group, in collaboration with Chen, created the database system “NanoMine” for polymer nanocomposite material systems and developed associated relevant tools, e.g., 

Niblack local thresholding based TEM/SEM image binarization tool, set up an entry portal into NanoMine, and began to 

populate a few sample data sets. Brinson, in collaboration 

with Gilman (NIST), also performed local AFM experiments 

for interphase characterization of epoxy-CNC systems.

Chen group developed a machine learning-based method 

for identifying the key microstructure descriptors and collaborated with Brinson for the database system development.

Keten group studied the elastic properties of interfacial region between graphene and PMMA and predicted the local modulus gradient map, comparing to data from Brinson. 

The confinement of the polymer layer to nanoscale dimensions alters the mechanical response, giving rise to elastic 

properties that are a function of material geometry as well as 

bulk material properties. In collaboration with Phelan (NIST), 

this group is also developing models and algorithms for integration with COMSOFT Workbench.

Luijten group focused on the development of coarsegrained models based upon quantum-mechanical all-atom 

simulations, with an emphasis on the applicability of these 

models in self-assembly studies. In the first stage of this 

project, an all-atom model for linear polyethyleneimine has 

been developed based upon the CHARMM force field.

De Pablo group developed models to address the rheology of block copolymers and nanocomposites to obtain 

structure and dynamic information. These results will be 

connected to the data resources (Polymer Design Database developed and in beta testing) as well as to higher level 

nanocomposite property prediction schemas by Keten and 

Phelan. 

RESEARCH ACCOMPLISHMENTS 

The polymer matrix materials group has made significant 

progress, including experimental measurements of local interphase properties, atomistic, coarse-grained scale simulations, and creating of a platform for data structures and 

databases for polymers and composites. Below, we summarized the major research accomplishments.

Data Structures and Databases for Polymers and 

Composites 

A significant barrier to understanding and predicting polymer nanocomposite properties is the difficulty of gathering and analyzing all the existing published data. Currently, 

each researcher must assemble their own limited set of 

comparison data sets from individual pdf files downloaded from journals, extracting information manually and often with many gaps of details. Huge strides can be made 

by creating a centralized repository of data which can then 

be investigated with simple to sophisticated data analysis 

tools. A significant effort was devoted to developing a novel data resource for polymer nanocomposite, called “NanoMine” (Brinson, Chen, Zhao). It leverages the Materials Data 

Curator (MDC) system developed at NIST with customized 

data structures for experimental and computational polymer 

nanocomposites data from both the literature and PIs’ labs. 

Detailed templates and material data structures (“schemas”) 

have been developed to account for processing, structure 

and property information of material systems. The schemas 

for “NanoMine” material data are set up and follow a systematic approach to group and categorize parameters so 

that the data entered to the database can be properly curated. For polymer nanocomposite data, processing, structure 

and property parameters are first divided into five sections, 

materials, sample processing, measurement methods, data/

measured values, microstructure information. Each section contains some key parameters and designing/analysis tools. Figure 1 shows the main page and one data entry 

snippet. This data system will connect with Polymer Design 

Database (de Pablo) for multiphase polymers which is being developed as a computational tool to predict structural, 

thermodynamic and dynamic properties of functional polymers and block co-polymers, including phase association of 

nanoparticles in BCPs.

As a key component of the “NanoMine” system, the image 

binarization tool is critically needed to characterize the microstructure from TEM/SEM images. A new approach was 

developed by utilizing the Niblack local thresholding in conjunction with the assumption that the background information of each pixel is highly correlated with the gray-scale 

values of its neighbor pixels (Brinson and Cheni). This technique is an important pre-processing step before the statistical characterization of the materials’ microstructure. 

By adjusting the size of the neighborhood, we were able to 

Figure 1. The main page and data entry for the data system “NanoMine” for polymer nanocomposite.
8 | CENTER FOR HIERARCHICAL MATERIALS DESIGN

achieve better binarization results than the most commonly 

used methods.

To build the relationship between microstructure and physical properties, one needs to select and extract the most significant descriptors from a large variable pool. A machine 

learning-based method was developed for identifying the 

key microstructure descriptors from the vast array of candidates as potential microstructural design variables (Chen). 

With a large number of candidate microstructure descriptors collected from literature covering a wide range of microstructural material systems, our method is able to perform 

4-fold of functionalities: (1) eliminate redundant microstructure descriptors via image analyses, (2) identify key microstructure descriptors based on microstructure morphology, 

(3) identify key microstructure descriptors based on structure-property relationship, and (4) determine the final set 

of microstructure design variables. The benefits are demonstrated by an example of polymer nanocomposites optimization. We compare designs using key microstructure 

descriptors versus using empirically-chosen microstructure 

descriptors to validate the proposed method. This work will 

also supplement the “NanoMine” development (Brinson)

and be used in the generation of the models for the 

nanocomposites.

Experimental Measurements of the Local Mechanical 

Properties

The key of multiphase polymer response is to understand 

the interaction of filler particles with the matrix material and 

to explore the physical properties of the interphase zone. To 

characterize the local mechanical 

properties of polymer thin films 

and polymer nanocomposites a 

technique has been developed 

using AFM indentations (Brinson, 

Cheng), wherein a spatial map 

of the gradient of the modulus 

near an interfaces is obtained. 

This has been demonstrated on 

both model nanocomposite systems (flat 1-d controlled systems) 

and in situ nanocomposites. The 

properties of the interphase will 

be used in the models (Keten, 

de Pablo, Luijten) for the nanoreinforcement-matrix properties.

Nanocellulose is chosen as the 

reinforcement for our Composites Use Case due to their inherent high modulus, surface area and tunable surface 

chemistry, plus their renewable nature. The Cellulose Nanocrystal (CNC) fibers can be extracted from multiple sources, 

including wood, bacteria, and tunicate, and exhibit a large 

range of variations in morphology which were examined by 

AFM, TEM and NMR. The CNC fibers were further treated by 

different processing methods, resulting in different surface 

functional groups and different surface energies. To form 

nanocomposite materials, the CNC fibers were blended with 

DGEBA epoxy resin (Diglycidyl Ether of Bisphenol A) using 

a high speed shear mixer, and then casted into a mold with 

Polyetheramine curing agent. Model composites were made 

by bilayer spin-coating of CNC and epoxy onto wafers. The 

spatial arrangement of CNC networks inside a polymer matrix were investigated by using Förster resonance energy 

transfer (FRET) method from NIST (Gilman), in which, the 

interface between CNC and polymer was labeled by fluorescence dye: 5-(4,6-dichlorotriazinyl)-aminofluorescein. Via 

this method, the dispersion, as well as the interfacial damaging of CNC fibers can be clearly revealed. The mechanical properties of interphase formed by CNC and epoxy 

are being determined via AFM nanomechanical mapping 

(Brinson).

Atomistic and Coarse-Grained Simulations

To address the difficulty of accurate prediction of composite properties, a multi-pronged approach has been taken, 

wherein several modeling approaches at different length 

scales have been explored and will be integrated and coupled with the database resources. These models all incorporate coarse graining in order to account for the length and 

Figure 2. CGMD calculated and model predicted elastic modulus of confined polymer phase for PMMA and soft polymer (with reduced interchain 

interaction). Mw is the number of monomers per polymer chain, N is the number of graphene sheets in graphene phase, h is the thickness of 

confined polymer phase, Ep is the elastic modulus of confined polymer phase.
YEAR IN REVIEW 2014 | 9

time-scales relevant for polymers and polymer-nanoparticle systems. At present, some 

efforts explicitly consider nanocellulose, while others address model polymers and block copolymer systems and 

integration of generic nanoparticles. 

Numerous studies have shown that a large differential in 

elastic properties of hard and soft phases in cellulose nanocomposites helps in mitigating crack propagation. However,

to date, measurements on the mechanical response of 

soft interphase layers in such systems and their sizedependent properties have been a source of controversy. 

We carried out predictive coarse-grained molecular dynamics (CGMD) simulations to address this issue by studying the 

mechanical properties of atomically layered nanocomposites (Keten). The research thus far has focused on model systems consisting of graphene and poly(methyl methacrylate) (PMMA) for which we have previously established 

experimentally validated CGMD potentials (Hsu et al. J. 

Chem. Theo. and Comp, 2014, Ruiz et al., Carbon, 2014). We 

show directly through simulations on model layer-by-layer 

graphene-polymer nanocomposites that the confinement of 

the polymer layer to nanoscale dimensions alters the mechanical response, giving rise to elastic properties that are a 

function of material geometry as well as bulk material properties. We propose a simple analytical model that quantitatively captures the effect from confinement on the elastic 

modulus of polymers. (Figure 2) We compare to AFM data 

from Brinson’s group on CNT-PMMA systems. We are exploring methods to integrate the coarse graining method developed here into the COMSOFT workbench. 

The interfacial mechanics of cellulose nanocrystals (CNCs) 

where MD simulations were conducted on (110) and (200) 

CNCs surfaces interacting with PMMA and amorphous cellulose was also investigated by Keten. For hydrophobic 

material PMMA, interfacial adhesion energy does not vary 

significantly for different surfaces; while for the hydrophilic material amorphous cellulose, interfacial adhesion energy shows stark differences depending on surface chemistry. 

The analyses indicate that for the hydrophilic (110) surface, 

electrostatic interactions would contribute more to the interfacial interactions; and for hydrophobic (200) surface, Van 

der Waals interactions are dominant. These results pave the 

way for carrying out systematic MD calculations to tailor interfacial properties in nanocomposites to tune matrix properties and failure mechanisms. We will compare to data from 

the Brinson group. 

Significant efforts have also been focused on the development 

of coarse-grained models based upon quantum-mechanical 

all-atom simulations, with an emphasis on the applicability of these models in self-assembly studies (Luijten). As a 

model test case, we chose linear polyethyleneimine (l-PEI), 

a cationic polymer that has numerous applications and offers significant flexibility, as it can be functionalized with side 

groups and synthesized in other structures as well, notably dendritic (branched) structures (as shown in Figure 3). 

In the first stage of this project, we have generated an allatom model for l-PEI based upon the CHARMM force field. 

This work has enabled us to make important improvements 

compared to existing models for the same polymer, notably 

in the potential-energy surface for the dihedrals. We are currently investigating the consequences of these refinements 

on the predicted conformations. We will compare to the data 

from Brinson and Keten groups.

A particularly important need in the general area of nanocomposites is to develop models capable of describing the 

linear and non-linear rheology on the basis of molecular 

models. A model has been developed by relying on a fieldtheoretic description of the polymeric matrix, a slip-spring 

description of entanglements, and an explicit representation 

of the nanoparticles embedded in the polymeric matrix (de

Pablo). Preliminary results indicate that the model is able 

to describe the phase behavior of homopolymer nanocomposites, and that of diblock copolymer nanocomposites (see 

Figure 4). The predictions of the model for macromolecular 

and nanoparticle diffusion are consistent with experimentally observed trends, and work is now underway to characterize the rheology of the model under shear and elongational 

deformations. These results will be important in developing 

predictions of morphology and thus properties, and will be 

connected to the data resources under development (NanoMine and Polymer Design Database) as well as to higher 

level nanocomposite property prediction schemes (Keten, 

Phelan).

COLLABORATIONS

Collaboration with Materials Data Curator at NIST

Collaborators: Alden Dima, Carelyn Campbell

Materials Science and Engineering Division, NIST

Richard Zhao in Brinson group is working with Alden Dima and 

Carrie Campbell at NIST in using the Materials Data Curator 

(MDC) developed at NIST for polymer nanocomposite database. Detailed interactions on developing schemas for the data 

and needs for the MDC as it moves from alpha to beta testing.

Figure 3. All-atom model for linear polyethyleneimine (PEI, left) mapped onto a coarse-grained model (right). The coarse-grained model will be 

employed in large-scale molecular dynamics simulations for the prediction of complexation phenomena and self-assembly involving PEI.
10 | CENTER FOR HIERARCHICAL MATERIALS DESIGN

Collaboration on Local 

Polymer Response

Collaborators: Jeff Gilman, 

Chelsea Davis

Materials Science and 

Engineering Division, NIST

Xu Cheng (Brinson group) is 

working with Jeff Gilman and 

Chelsea Davis at NIST on characterizing the local response of 

polymer near nanocellulose in epoxy. Samples have been 

exchanged and tested in both labs and Xu Cheng will visit 

NIST in late January with a longer visit planned later in the 

year.

Collaboration on Polymer Nanocomposite Databases

Collaborators: Linda Schadler

Materials Science and Engineering

Rensselaer Polytechnic Institute (RPI)

Richard Zhao (Brinson group) is working together with Wei 

Chen at NU and Linda Schadler at RPI on creating and 

curating data on polymer nanocomposites and developing 

the first comprehensive database for polymer nanocomposites. The database is called “NanoMine”. 

Collaboration on Machine Learning-based Method

Collaborators: Alok Choudhary

Department of Electrical Engineering and Computer Science, Northwestern University

Chen’s group is work together with Ruoqian Liu 

(Choudhary group) and Alok Choudhary on the work of 

“A Machine Learning-based Design Representation Method 

for Designing Heterogeneous Microstructures”

Collaboration on Polymer Nanocomposite Databases

Collaborators: Irene Hassinger, Linda Schadler

Materials Science and Engineering, RPI 

Xiaolin Li (Chen group) is collaborating with He Zhao 

(Brinson group) and Brinson at Northwestern and Irene 

Hassinger (Schadler group) and Linda Schadler at RPI on 

creating and curating data on polymer nanocomposites and 

developing the first comprehensive database for polymer 

nanocomposites.

Collaboration on the Development of Force Fields for 

Polymers in Self-assembly

Collaborator: Hai-Quan Mao

Materials Science and Engineering, Johns Hopkins 

University

Luijten group has a long-standing collaboration with Prof. 

Mao’s group (partly NIH-funded) on the study of self-assembled nanoparticles for gene delivery. We now have leveraged 

this collaboration to develop force fields for atomistic simulations of the copolymers involved in this self-assembly. These 

force fields, which are crucial for quantitative predictive capabilities for polymeric materials, will be tested directly against 

experimental results obtained in the Mao group, permitting iterative improvement of our models.

CHiMaD Student Zonghui Wei (Luijten) visited the Mao group at 

Johns Hopkins University for several days to ensure close collaboration between the two research groups. Prof. Mao visited 

Northwestern University. In addition, we have frequent communications by phone and via Skype.

PUBLICATIONS

Xu, H., Liu, R., Choudhary, A., and Chen, W., “A Machine 

Learning-Based Design Representation Method for Designing 

Heterogeneous Microstructures”, IDETC2014-34570, August 17-

20, Buffalo, New York, 2014. Best Paper Award, 40th ASME Design Automation Conference. 

DATABASES

NanoMine

System of interest: processing, structure and property parameters are first divided into five sections, materials, sample 

processing, measurement methods, data/measured values, 

microstructure information, where the Materials section includes the chemical component and basic properties of the 

matrix polymer and nano-phase filler. Sample Processing accounts for the procedure during sample preparation, especially 

the parameters in composite mixing that are important to morphology and subsequently properties of the composite materials. Detailed processing steps for thermoset and thermoplastic 

are included within respective modules for each type of polymers. Measurement Methods section contains information of 

the measurement equipment, such as SEM/TEM, DMTA and 

IR. Such information is the key to account for the conditions 

Figure 4. Representative configurations of block copolymer nanocomposite for different nanoparticle loadings. Nanoparticles induce a transition 

from a lamellar to a spherical phase, in agreement with experiments.
YEAR IN REVIEW 2014 | 11

where the property data are obtained. Measured Values section contains property data of the sample with each class of 

properties in individual module. Finally, the Microstructure 

section provides the micrograph images and morphology in 

terms of statistical descriptors.

Methodology: Literatures, Expeirments, Finite Element 

Analysis

Distribution: Public

Link to beta version: http://puma.mech.northwestern.

edu:8001

Collaborations: He Zhao, Xiaolin Li, L. Catherine Brinson, 

Wei Chen, Irene Hassinger (RPI), Linda Schadler (RPI)

Polymer Design Database

System of interest: Characteristic properties for design of 

polymer blends and copolymers for engineering applications, including DSA. The database is now being populated by resorting to automated techniques that extract 

target content from the literature. Human intervention is 

then required to verify relevance and accuracy of computergenerated entries. The database will be tested through a 

formal course, to be taught at the University of Chicago in 

Spring 2015, with collaborators from NIST (Debra Audus and 

Jack Douglas). The database also includes a thermodynamic property prediction set of modules based on state-of-theart molecular models. 

Methodology: Theory and experiment 

Distribution: Not released yet 

Collaborations: Juan de Pablo, Ian Foster, Paul Nealey, and 

Heinrich Jaeger

Prediction of Phase Behavior of Polymers and 

Polyelectrolytes 

Database and Simulation Software, this software is being

developed jointly with the “Directed-Self Assembly of 

Block Copolymers Films for Lithographic Applications” use 

case group. Please refer to that Use Case for additional 

information. 

System of interest: Polymer blends, polyelectrolytes, 

copolymers 

Methodology: Computational and experimental 

Distribution: Under development 

Collaborators: Juan de Pablo, Ian Foster, Paul Nealey, Matthew Tirrell, Heinrich Jaeger, Jack Douglas, Carl Freed 

Polymer Matrix Materials

Group Researchers and Their Roles 

Name Position

Institution/

Affiliation Project Title (PI)

L. Catherine Brinson PI NU/MSE Database development for polymer nanocomposites and interphase property characterization

Wei Chen PI NU/ME A Machine Learning-based Design Representation 

Method for Designing Heterogeneous Microstructures

Juan de Pablo PI UC/IME Rheology models of BCPs and copmosites; Polymer 

Design Database development development

Sinan Keten PI NU/CIVIL Modeling of interface and interphase phenomena in 

cellulose nanocomposites

Erik Luijten PI NU/MSE Force-field-based Development of Effective Potentials for 

Polymeric Materials

Zhiwei Cui Research Assistant Professor NU/ME Computational modeling of polymers and nanoparticles. Also coordinator for Use Case Group (Brinson)

Marc Palmeri Postdoctoral Researcher NU/MSE Experimental analysis of polymer composites and 

nanocomposites (Brinson)

Zequn Wang Postdoctoral Researcher NU/ME Hierarchical Materials Design Project (Chen)

Chen Shao Postdoctoral Researcher NU/ME Study on mechanical properties of layer-by-layer 

nanocomposites (Keten)

Richard (He) Zhao Graduate Student NU/ME Databases for Polymer Nanocomposites (Brinson)

Xiaolin Li Graduate Student NU/ME Image binarization for dielectric polymer nanocomposites 

(Brinson)

Xu Cheng Graduate Student NU/ME AFM for local mechanical properties of polymers in 

composites (Brinson)

Xin Qin Graduate Student NU/ME Interfacial Mechanics of Cellulose Nanocrystals and 

Nanocomposites (Keten)

Zonghui Wei Graduate Student NU/AP Development of effective force fields for polymers in 

self-assembly (Luijten)

Yongrui Su Graduate Student UC/IME Rheology models of BCPs and composites (de Pablo)
12 | CENTER FOR HIERARCHICAL MATERIALS DESIGN

LOW-DIMENSIONAL NANOELECTRONIC MATERIALS

Mark C. Hersam (lead), Lincoln J. Lauhon

Department of Materials Science and Engineering, Northwestern University

The Low-Dimensional Nanoelectronic Materials use-case group seeks to understand and realize p-type and n-type 

doping in the low-dimensional limit. Since low-dimensional nanoelectronic materials have exceptionally high surface 

area to volume ratios, doping can be realized via two qualitatively different pathways: conventional substitutional 

doping and surface chemical functionalization. Furthermore, contacts can be designed to selectively inject carriers 

into the conduction or valence band, thus providing effective n-type and p-type behavior, respectively. Experimental 

methods in this use-case group include charge transport measurements, optical spectroscopy (e.g., absorbance, 

photoluminescence, and Raman spectroscopy), and high-resolution microscopy (e.g., scanning probe microscopy, 

electron microscopy, and atom probe tomography). These experimental methods are complemented by an extensive 

suite of computational techniques including multi-scale modeling, molecular dynamics, density functional theory, 

and finite element methods. The interplay between experiment and computation accelerates the understanding and 

design of doped low-dimensional nanoelectronic materials and their heterostructures.

DESIGN GOALS

For the previous funding period, the Low-Dimensional 

Nanoelectronic Materials use-case group made significant 

progress towards two design goals:

(1) Producing Low-Dimensional Semiconductors of Controlled Carrier Concentration. Towards this end, Lauhon is 

advancing the synthesis of transition metal dichalcogenides 

with substitutional dopants and developing characterization methods that can correlate composition with electrical 

properties. For example, the stoichiometry of MoS2

 grown 

by chemical vapor deposition was analyzed by X-ray photoemission spectroscopy and electrical characterization. In 

addition, methods were developed to prepare and analyze 

two-dimensional semiconducting samples by atom probe 

tomography. 

(2) Developing p-n Heterojunctions Consisting of LowDimensional Semiconductors. Of particular interest are 

p-n heterojunctions that allow gate-tunability over the carrier concentrations in both of the constituent materials. 

Towards this end, Hersam has realized wafer-scale p-n 

heterojunction diodes using solution-processed p-type 

semiconducting single-walled carbon nanotubes (s-SWCNTs) and n-type amorphous indium gallium zinc oxide 

(a-IGZO) thin films via photolithographic and etching techniques. These p-n heterojunctions show highly reproducible anti-ambipolar charge transport with high on/off ratios 

and provide insight into the band alignment and charge distribution at the interface between doped low-dimensional 

semiconductors.

SIGNIFICANT ACCOMPLISHMENTS 

(1) In a CHiMaD collaboration involving Lincoln Lauhon and 

Mark Hersam, the stoichiometry of MoS2

 grown by chemical vapor deposition was analyzed by X-ray photoemission spectroscopy and electrical characterization, revealing 

that variations in sulfur loading produced controlled variations in electrical properties. This work resulted in a collaborative publication in ACS Nano involving Lauhon and 

Hersam: I. S. Kim, V. K. Sangwan, D. Jariwala, J. D. Wood, 

S. Park, K.-S. Chen, F. Shi, F. Ruiz-Zepeda, A. Ponce, M. 
YEAR IN REVIEW 2014 | 13

Figure 1. Analysis along (001) direction: (a) before 

APT and (b) after APT; analysis perpendicular to (001) 

direction: (c) before APT and (d) after APT. (e) mass 

spectrum of MoS2 analyzed by APT.

Jose-Yacaman, V. P. Dravid, T. J. Marks, M. C. Hersam, and 

L. J. Lauhon, “Influence of Stoichiometry on the Optical and 

Electrical Properties of Chemical Vapor Deposition Derived 

MoS2,” ACS Nano, 8, 10551 (2014).

(2) In a CHiMaD collaboration involving Mark Hersam and 

Lincoln Lauhon, wafer-scale p-n heterojunctions were realized using solution-processed p-type semiconducting 

single-walled carbon nanotubes (s-SWCNTs) and n-type 

amorphous indium gallium zinc oxide (a-IGZO) thin films via 

photolithographic and etching techniques. These p-n heterojunctions showed highly reproducible anti-ambipolar 

charge transport with high on/off ratios. This work resulted in a collaborative publication in Nano Letters involving 

Hersam and Lauhon: D. Jariwala, V. K. Sangwan, J.-W. T. 

Seo, W. Xu, J. Smith, C. H. Kim, L. J. Lauhon, T. J. Marks, 

and M. C. Hersam, “Large-Area, Low-Voltage, Antiambipolar Heterojunctions from Solution-Processed Semiconductors,” Nano Letters, 15, 416 (2015).

RESEARCH ACCOMPLISHMENTS

One goal of the Low-Dimensional Nanoelectronic Materials use case group is to produce low-dimensional semiconductors of controlled carrier concentration. Towards 

that end, Lauhon is advancing the synthesis of transition 

metal dichalcogenides (TMDCs) with substitutional dopants 

and developing characterization methods that can correlate 

composition with electrical properties. In initial research, 

the stoichiometry of MoS2

 grown by chemical vapor deposition was analyzed by X-ray photoemission spectroscopy 

and electrical characterization. Variations in sulfur loading 

produced controlled variations in electrical properties, and 

the results were reported in collaborative publication in ACS 

Nano involving Lauhon and Hersam: I. S. Kim, V. K. Sangwan, D. Jariwala, J. D. Wood, S. Park, K.-S. Chen, F. Shi, F. 

Ruiz-Zepeda, A. Ponce, M. Jose-Yacaman, V. P. Dravid, T. J. 

Marks, M. C. Hersam, and L. J. Lauhon, “Influence of Stoichiometry on the Optical and Electrical Properties of Chemical Vapor Deposition Derived MoS2

,” ACS Nano, 8, 10551 

(2014).

With the goal of nanoscale characterization of composition 

in TMDC alloys, methods to prepare and analyze samples 

by Atom Probe Tomography (APT) were developed. This 

CHiMaD supported research was conducted by Xiaochen 

Ren (Lauhon group graduate student). Natural MoS2

 flakes 

were purchased and prepared for APT analysis in a dualbeam focused-ion beam (FIB). MoS2

 was analyzed parallel 

to (001) planes 

(Fig.1a,b) and 

perpendicular 

to (001) planes 

(Fig. 1c,d). The 

weak interactions between 

MoS2 layers 

led to tip fracture on (001) 

planes (Fig. 

1b,d). While 

tomographic 

imaging was 

not achieved, 

mass spectra 

were obtained 

from samples 

analyzed perpendicular to 

(001) planes 

(Fig. 1d). A typical mass spectrum obtained under 30 K with a laser pulse energy of 30 

pJ is shown in Fig. 1e. The S/Mo ratio was calculated to be 

1.85, and no impurities were detected. The upper bound 

limit for possible impurities was calculated to be 589 ppm 

for W, 752 ppm for Se, 200 ppm for Cl and 70 ppm for O.

The Low-Dimensional Nanoelectronic Materials use -

case group is also exploring heterojunctions between lowdimensional semiconductors. Of particular interest are p-n 

heterojunctions that allow gate-tunability over the carrier 

concentrations in both of the constituent materials. Over 

the past funding period, Hersam has realized wafer-scale 

heterojunction p-n diodes using solution-processed, ptype semiconducting single-walled carbon nanotubes (sSWCNTs) and n-type amorphous indium gallium zinc oxide 

(a-IGZO) thin films via photolithographic and etching techniques. The resulting device structure has a s-SWCNT 

field-effect transistor (FET), s-SWCNT/a-IGZO p-n heterojunction, and a-IGZO FET in series. This device architecture 

enables electrical characterization of the p-n heterojunction 

in addition to control FETs from the individual semiconductors. Figs. 2 a-b show output curves of a representative p-n 

heterojunction at different gate biases (VG). Rectifying behavior is observed with rectification ratios exceeding 103. 
14 | CENTER FOR HIERARCHICAL MATERIALS DESIGN

The forward current at VG = 4 V is 

low (~20 nA) but abruptly increases for intermediate VG values and 

then falls to the instrumental noise 

floor (~10 pA) at VG = 0 V. This behavior is also evident in the p-n heterojunction transfer plots (I-VG) (Fig. 

2c, green). This anti-ambipolar transfer plot shows one current maximum 

(on-state) in between two off-states 

at either extremes of the gate voltage range. The voltage dependence 

of the anti-ambipolar plot is approximately a superposition of the transfer plots of the p-type and n-type 

unipolar FETs in red and blue, respectively (Fig. 2c). 

Due to screening from the ~20 nm 

thick a-IGZO, the junction itself is less 

modulated by the gate field compared 

to p-n heterojunctions based solely on 

two-dimensional materials, thus simplifying the charge transport mechanism and facilitating the realization of reproducible 

and spatially homogeneous characteristics. For example, Fig. 

2b shows that the reverse saturation current possesses a relatively weak gate-dependence. In Fig. 2d, three-dimensional plots of current (ID) as a function of VG and forward bias 

voltage (VD) illustrate that the charge transport is primarily a 

result of two semiconductors with opposite carrier types in series. Specifically, cross-sections along the VD axis at VG > 2.2 

V (point of maximum current) resemble the output plots of sSWCNT FETs under positive bias, whereas VG < 2.2 V shows 

a saturating behavior that correlates with the output plots of 

a-IGZO FETs. These results the results were reported in collaborative publication in Nano Letters involving Lauhon and 

Hersam: D. Jariwala, V. K. Sangwan, J.-W. T. Seo, W. Xu, 

J. Smith, C. H. Kim, L. J. Lauhon, T. J. Marks, and M. C.

Hersam, “Large-Area, Low-Voltage, Antiambipolar Heterojunctions from Solution-Processed Semiconductors,” Nano 

Letters, 15, 416 (2015).

COLLABORATIONS

Chemical Vapor Transport Growth of Transition Metal 

Dichalcogendies 

Collaborator: Albert Davydov

Materials Science and Engineering Division, NIST

The Davydov group is growing transition metal dichalcogenide (TMDC) crystals by chemical vapor transport (CVT) that 

will be characterized using atom probe tomography (APT). 

In Year 1, NbSe2 bulk single crystals were grown and provided to Northwestern University. Davydov visited Northwestern University on Sept 18, 2014 to meet with Lauhon, 

Hersam, and Voorhees to discuss ongoing efforts and coordinate plans for Years 1 and 2. Davydov, Hersam, and 

Lauhon also conducted multiple teleconferences and email exchanges during the year to define and coordinate the 

Low-Dimensional Nanoelectronic Materials use case group.

Figure 2. (a,b) Output characteristics of a s-SWCNT/a-IGZO p-n heterojunction at different gate voltages on a linear (a) and 

semi-log (b) y-axis. The weak gate modulation of the reverse saturation current magnitude can be seen in b. The plot colors in 

a and b represent the same gate voltage values as indicated in the legend of a. (c) Semi-log transfer characteristics of a p-type 

s-SWCNT FET (blue), n-type a-IGZO FET (red), and s-SWCNT/a-IGZO p-n heterojunction (green). (d) Three-dimensional representation of the anti-ambipolar transfer characteristics at varying drain biases. The grid lines running along the VD

 axis represent 

the forward output characteristics at the indicated gate voltage (VG).
YEAR IN REVIEW 2014 | 15

Chemical Vapor Deposition of MoS2

Thin Films

Collaborators: Albert Davydov (see 

affiliation previous page), 

James Maslar

Chemical Sciences Division, NIST

Davydov and Maslar have designed 

and built two chemical vapor deposition 

(CVD) reactors for large-scale growth of 

TMDC thin films to be shared with the 

NU team for compositional characterization (Lauhon) and device fabrication (Hersam). Davydov is focusing on 

chloride CVD chemistry for large-scale 

MoS2

 and related alloy thin film growth 

with controlled composition, doping, 

thickness, and microstructure. Maslar 

will employ a warm-wall, vertical-flow, 

single-wafer reactor that can be operated in atomic layer deposition (ALD) 

mode (with sequentially-injected precursors) or in CVD mode (with co-injected precursors). Different precursor 

chemistries are being investigated including organometallic compounds, 

metal carbonyls, metal halides, thiols, 

and elemental sources.

Device Fabrication and Transport 

Measurements on TMDC FETs

Collaborators: Albert Davydov (see 

affiliation previous page), Abhishek 

Motayed (Guest Researcher in the 

Davydov group)

Davydov is collaborating with both Lauhon and Hersam

on fabricating field-effect transistor (FET) devices based 

on mono- to few-layer thin MoS2

 and related thin films. In 

year one, the NIST group tested transfer characteristics 

and low-frequency noise in single and multilayer MoS2

FETs as a function of the active channel thickness. The 

NIST and NU groups have positioned themselves to start 

sharing device structures for complimentary electrical and 

optoelectronic testing in year two. NIST’s advanced double-channel system for temperature-dependent 1/f noise 

measurements with low instrumental background noise is 

well-suited to study the effect of non-stoichiometry and 

In Soo Kim1, Vinod K. Sangwan1, Deep 

Jariwala1

, Joshua D. Wood1, Spencer Park1, 

Kan-Sheng Chen1, Fengyuan Shi1, Francisco 

Ruiz-Zepeda5

, Arturo Ponce5, Miguel JoseYacaman5

, Vinayak P. Dravid1,4, Tobin J. 

Marks1,2, Mark C. Hersam1,2,3*, and Lincoln 

J. Lauhon1*

1

Department of Materials Science and 

Engineering, Northwestern University, 

Evanston, IL 60208 USA

2

Department of Chemistry, Northwestern 

University, Evanston, IL 60208 USA

3Department of Medicine, Northwestern 

University, Evanston, IL 60208 USA

4

International Institute of Nanotechnology, 

Northwestern University, Evanston, IL 

60208 USA

5Department of Physics and Astronomy, 

University of Texas at San Antonio, San 

Antonio, TX 78249 USA

SCIENTIFIC ACHIEVEMENT

The stoichiometry of MoS2 grown by 

chemical vapor deposition was analyzed 

by X-ray photoemission spectroscopy, 

and controlled variations in stoichiometry were achieved by varying the sulfur 

loading. Analysis of the electrical properties of field effect transistors demonstrated 

improvements in device characteristics in 

sub-stoichiometric material. This surprising 

finding is consistent with the existence of a 

mobility edge; in materials with significant 

charge carrier scattering due to defects or 

interfaces, a minimum carrier concentration must be achieved to enable delocalized 

carrier transport.

SIGNIFICANCE

Semiconducting transition 

metal dichalcogenides are layered materials 

that can be made as thin as one monolayer, 

opening up new opportunities in electronic 

and photonic devices. Point defects such 

as sulfur vacancies providing additional 

charge carriers but also introduce scattering channels. Understanding and control 

over defects in these emerging materials is 

essential for their insertion into new devices 

and technologies.

CITATION INFORMATION

Kim et al, ACS Nano 8, 10551 (2014)

ACKNOWLEDGEMENTS

This work was supported by the Materials 

Research Science and Engineering Center 

(MRSEC) of Northwestern University 

(National Science Foundation Grant DMR1121262), NSF PREM Grant DMR-0934218, 

the Office of Naval Research (Grant 

N00014-14-1-0669), the Keck Foundation, 

and the Center for Hierarchical Materials 

Design (US Department of Commerce, 

NIST 70NANB14H012). The microscopy 

work was supported by grants from the 

National Center for Research Resources (5 

G12RR013646-12) and the National Institute 

on Minority Health and Health Disparities 

(G12MD007591) from the National Institutes 

of Health.

RESEARCH HIGHLIGHT

CONTROL OF STOICHIOMETRY OF CHEMICAL VAPOR 

DEPOSITION DERIVED MoS2
16 | CENTER FOR HIERARCHICAL MATERIALS DESIGN

Low Dimensional Use-Case Nanoelectronic Materials 

Group Researchers and Their Roles 

Name Position

Institution/

Affiliation Project Title

Lincoln J. Lauhon PI NU/MSE Substitutional doping and alloy formation in transition metal dichalcogenides

Mark C. Hersam PI NU/MSE Low-dimensional semiconducting heterojunctions

Xiaochen Ren Graduate student NU/MSE Substitutional doping and alloy formation in transition metal dichalcogenides

Jung-Woo T. Seo Graduate student NU/MSE Low-dimensional semiconducting heterojunctions

Hadallia Bergeron Graduate student NU/MSE Low-dimensional semiconducting heterojunctions

doping in MoS2 and related TMDCs on the FET performance. Active material and device exchange between the 

NU and NIST groups for electrical testing and correlation to 

doping and other materials properties is planned for Year 

Two.

Low-Dimensional Nanoelectronic Materials Modeling

Collaborators: Francesca Tavazza, Benjamin Burton, 

Project Leaders, and Arunima Singh, Guest Researcher, 

Materials Science and Engineering Division, NIST

Burton, with assistance from Singh, has initiated a firstprinciple modeling of phase diagrams for multicomponent 

TMDC systems, starting from the MoS2

-MoTe2, MoS2-

MoSe2, and MoSe2-MoTe2 pseudo-binary sections, as well 

as building the FPPS (First Principles Phase Stability) repository. To accurately treat a layered structure of TMDC 

compounds, van der Waals interlayer interactions were included into the DFT based ab-initio simulation. Since experimental phase diagrams for the above systems do not 

exist, the phase diagram simulation is the most critical step 

in developing guidance for designing TMDC alloys with the 

composition-defined band gap values. Preliminary simulation for the MoS2

-MoTe2 system indicates formation of one 

ordered phase and two miscibility gaps, while the MoS2-

MoSe2

 system forms a continuous solid solution with no 

phase segregation.

In year two, the WSe2-NbSe2 and MoS2-NbS2 diagrams will 

be considered since niobium has been proven to act as an 

efficient p-dopant in both WSe2 and MoS2. The simulation 

at NIST will complement experimental efforts from Lauhon

for developing substitutional p- and n-doping in TMDC compounds and alloys.

In parallel, Tavazza and Singh have applied DFT theory to 

evaluate the effect of high-k dielectric oxide substrates on 

the electronic properties of ultra-thin TMDC layers (including 

possible doping effects). Initial simulation of MoS2

 mono- to 

three- layer films on sapphire has shown progressively weak 

interaction between MoS2 and Al2O3 with increasing numbers of layers, while the n-doping effect due to charge transfer from the Al2

O3 into the MoS2 layer remains constant. The 

band gap and effective masses of MoS2 are found to be unaffected by the underlying substrate. In year two, Tavazza 

and Singh will continue assessing the influence of sapphire 

on the electronic structure and transport properties of CVDgrown MoS2

 and related TMDC films. The ab-initio modeling 

will be correlated with experimental work of NU (Lauhon, 

Hersam) and NIST (Davydov, Motayed) on the corresponding TMDC/substrate heterostructures.

A long term goal for the above NIST/NU modeling effort 

is to develop an open-source tool for the high-throughput 

screening of suitable substrates for the growth and functionalization of 2D materials. While several 2D materials were 

produced experimentally, many more 2D materials with useful properties have been theoretically predicted. Their synthesis requires knowledge of substrates that can stabilize 

the 2D materials. Specific goals include: (i) perform highthroughput DFT based computational screening of suitable 

substrates for the synthesis of 2D materials, (ii) characterize the mechanical and electronic properties of 2D materials 
YEAR IN REVIEW 2014 | 17

adsorbed on substrates, and (iii) build a database of suitable 

substrates of 2D materials. Initially, a systematic and highthroughput substrate search for the following theoretically 

predicted but non-yet-synthesized experimentally 2D materials will be undertaken by Tavazza and Singh: three groupIII monochalcogenides (AX, with A = In and X = S, Se, Te), 

twelve group IV-monochalcogenides (BX, with B = Ge, Sn, 

Pb and X = O, S, Se, Te), and twenty seven metal dichalcogenides (MX2, M = Ti, V, Cr, Mn, Fe, Co, Ni, Ta, W and X = 

S, Se, Te). This computational screening will help in identifying likely and eliminating unlikely substrates, allowing for 

more efficient use of experimental resources and accelerating possible applications.

PUBLICATIONS

I. S. Kim, V. K. Sangwan, D. Jariwala, J. D. Wood, S. Park, 

K.-S. Chen, F. Shi, F. Ruiz-Zepeda, A. Ponce, M. Jose-Yacaman, V. P. Dravid, T. J. Marks, M. C. Hersam, and L. J. 

Lauhon, “Influence of Stoichiometry on the Optical and 

Electrical Properties of Chemical Vapor Deposition Derived 

MoS2,” ACS Nano, 8, 10551 (2014). Partial CHiMaD support.

D. Jariwala, V. K. Sangwan, J.-W. T. Seo, W. Xu, J. Smith, 

C. H. Kim, L. J. Lauhon, T. J. Marks, and M. C. Hersam, 

“Large-Area, Low-Voltage, Antiambipolar Heterojunctions 

from Solution-Processed Semiconductors,” Nano Letters, 

15, 416 (2015). Partial CHiMaD support.

TECHNOLOGY TRANSFER

Inventions Disclosed:

• System and method for anti-ambipolar heterojunctions 

from solution-processed semiconductors

• US Provisional Patent application filed on 1/9/15.

• Inventors: D. M. Jariwala, V. K. Sangwan, W. Xu, H. Kim, 

T. J. Marks, and M. C. Hersam

• This invention describes a method for forming largearea, anti-ambipolar p-n heterojunctions using solutionprocessed p-type semiconducting single-walled 

carbon nanotubes (s-SWCNTs) and n-type amorphous 

indium gallium zinc oxide (a-IGZO) thin films via 

photolithographic and etching techniques. 
18 | CENTER FOR HIERARCHICAL MATERIALS DESIGN

PRECIPITATION-STRENGTHENED ALLOYS

G. B. Olson (co-lead)1, D. C. Dunand (co-lead)1, D. N. Seidman1, P. W. Voorhees1, C. M. Wolverton1, W.-K. Liu2, 

A. Umantsev5, W. Chen2, J. Sebastian4, D. Snyder4, W. W. Zhang4, J. Saal4, A. Choudhary3, A. Agrawal3, and 

M. Bedzyk1

1

Department of Materials Science and Engineering, Northwestern University

2

Department Mechanical Engineering, Northwestern University

3

Department Electrical Engineering and Computer Science, Northwestern University

4

QuesTek Innovations

5

Department of Chemistry and Physics, Fayetteville State University

New capabilities in predictive design of precipitation-strengthened alloys are being applied to Co-based superalloys 

and high-performance Shape Memory Alloys, both employing low-misfit coherent aluminide precipitates closely 

analogous to the γ- γ’ Ni-based superalloys. The Co-based alloys are of interest for both high-temperature creep 

and fatigue resistance in turbine applications, as well as ambient temperature wear-resistance applications such as 

aerospace bushings to replace Be-Cu alloys. The shape memory alloys are of high interest for device and actuation 

applications in medicine, aerospace and automotive sectors where ultrahigh-cycle fatigue is the primary limitation 

of current alloys; here, efficient precipitation strengthening can eliminate the accommodation slip associated with 

thermoelastic martensitic transformation, standing as the principal cause of cyclic instability.

DESIGN GOALS 

Short-term goals for the Co alloys address ambient temperature applications, where the intrinsic low surface friction 

characteristics of Co-based alloys makes their precipitation strengthened variants of clear near-term interest as a 

replacement for Be-Cu aerospace bushing applications of 

current environmental concern. Long-term goals address 

high-temperature applications where metallic superalloys 

play a vital role in electrical power generating systems (gas 

and steam turbines) and jet engines. While achievement of 

the full set of properties necessary for these applications 

is an ambitious goal, the potential of the new family of Cobased superalloys to achieve the same strength levels of current Ni-based superalloys, but with a temperature capability 

100-150 oC higher would enable a major advance in the fuel 

efficiency of gas turbines and jet engines. 

In support of new capabilities in additive manufacturing by 

3D printing, precipitation-strengthened superalloy research 

will also address process optimization in the 718+ Ni-based 

alloy being developed under the DARPA Open Manufacturing program in collaboration with Honeywell.

The breadth of planned databases for Shape Memory alloy 

design will support both the near-term optimization of high 

Pd alloys for medical applications favoring high radiopacity 

and low Ni content for biocompatibility, and the longer-term 

design of lower-cost high Zr alloys to meet the requirements 
YEAR IN REVIEW 2014 | 19

of aerospace and automotive actuators. This will allow the 

new class of cyclically-stable alloys to have broad technological impact.

SIGNIFICANT ACCOMPLISHMENTS 

• QuesTek (Olson, Sebastian, Snyder, Zhang, Saal) has 

demonstrated a Co-based alloy for aerospace bushing 

applications meeting all primary property requirements 

in testing by Northrup Grumman. Database extensions 

to support further process optimization have been 

conducted, and alloy qualification test plans have been 

established.

• In a database workshop at NIST, plans were made for 

the pre-CALPHAD protodata repository to be hosted at 

NIST, and the metal alloy project was selected as the 

first to begin populating this database using a specified 

format.

• An undergraduate team in Northwestern University’s 

Materials Design class (Olson) placed third in the 

2014 ASM Design Competition. Their design of a 

multicomponent aluminide-strengthened shape 

memory alloy for medical applications employed our 

current CALPHAD databases to optimize mixed Pd-Zr 

aluminides for greater strengthening efficiency.

• A team of undergraduates in our special Murphy 

Scholar section of Freshman Design collaborated 

with Olson, Choudhary and Agrawal to initiate our 

research on the integration of a process-property 

database with a CALPHAD database for fusion of prior 

knowledge with data in the adaptation of datamining 

techniques for greater accuracy of process-structureproperty correlation. The team demonstrated stronger 

correlation compared to previous datamining using the 

process-property data alone.

RESEARCH ACCOMPLISHMENTS 

Discussion at the Inaugural CHiMaD meeting established 

quantitative property objectives and system structures for 

both the Co-based alloys and Shape Memory Alloys. The hierarchical system structure for the near-term case of the Cobased aerospace bushing alloy is represented in Figure 1.

In support of fundamental data development for high-temperature Co alloys (Dunand and Seidman) arc melted alloy 

buttons of compositions (at.%) Co-7Al-8W-4Ti-1Ta-10Ni0.15B and Co-7Al-8W-4Ti-1Ta-15Ni-0.15B, under argon atmosphere. The alloys were flipped and melted over ten times 

to ensure homogenization, then sectioned and vacuum encapsulated. The encapsulated samples were homogenized 

at 1300C for 24 hours in box furnace, water quenched, and 

aged at 1000C for 1000h in vacuum, in preparation for equilibrium studies

Activities at QuesTek (Olson, Sebastian, Snyder, Zhang, 

Saal) have centered on achieving higher yield strength in the 

bushing alloy of Figure 1 through simulation-driven thermal 

process optimization. In particular, aging treatment optimization has addressed more efficient precipitation strengthening and a reduction in the anneal time to peak strength 

Figure 1: Hierarchical system structure of Co-based aerospace bushing alloy.
20 | CENTER FOR HIERARCHICAL MATERIALS DESIGN

(previously ~70 hours). Higher precipitation strengthening is 

achieved through higher precipitate phase fractions and refined precipitate size. Zhang has extended QuesTek’s Co 

thermodynamic and mobility databases with experimental and literature data, applied in the PrecipiCalc® software 

(now commercially available as TC-Prisma through ThermoCalc®) to simulate a range of aging treatments that target 

higher precipitate strengthening by these means. 

As this tool is rooted in the kinetics of multicomponent precipitation theory, the key inputs to PrecipiCalc are thermodynamic and kinetic databases, which are fundamental 

databases that describe phase stability and transformation 

kinetics within multicomponent systems. Such databases 

were developed internally by QuesTek since no commercial 

databases are currently available for cobalt alloy systems. In 

support of the thermodynamic assessment, Sebastian and 

Saal analyzed LEAP atom probe tomography data at three 

distinct aging conditions at 780C: under-aged (2hr), peak 

aged (72hr) and over-aged (260hr). The LEAP reconstructions and corresponding phase compositions are presented 

in Figure 2. 

For mobility database assessment, Zhang has collected 

available literature data for diffusivities in cobalt alloys. Using 

the refined databases, Saal and Snyder applied the PrecipiCalc precipitation simulator to model the effect of varying 

aging treatments on the predicted strength of the alloy. Figure 3 plots the predicted increase in strength during aging 

at various conditions. The condition highlighted by the blue 

star is that was applied to the current material shown to have 

a yield strength of 126 ksi. As represented in Figure 4, a twostep anneal process was then predicted to reduce the time 

to peak strength from ~70 hours to ~24 hours.

Two of the most promising modified heat treatment conditions were selected from this simulation study for tensile 

test validation in comparison with the previous standard 

isothermal treatment condition. The results confirm 

Figure 2: Experimental LEAP analyses of Co bushing alloy used to calibrate QuesTek’s Co thermodynamic databases in preparation for PrecipiCalc simulations.

Figure 3: Calculated isothermal aging curves for the Co bushing alloy showing the predicted 

increment in yield strength due to precipitation of Co3Ti during aging as a function aging 

time. Note: a yield strength increment of “0” is the as-solution treated strength in the absence of precipitation hardening.

Figure 4: Calculated aging curves for Co alloy with a two-step anneal process, predicting a 

much reduced processing time with two-step treatment.
YEAR IN REVIEW 2014 | 21

predicted reduction in required aging time to achieve equivalent strength.

Higher yield strength is now being sought through HallPetch grain refinement strengthening. The current alloy 

prototype shows a relatively coarse grain size of 75 μm diameter, in association with a lower than specified carbon 

content. QuesTek has placed an order for an intermediate 

scale heat of the design composition for desired TiC-based 

grain refinement. Production of a 300-lb heat has been initiated at MetalWerks PMD (Aliquippa, PA) and is anticipated 

to complete in the first quarter of 2015. 

Toward further enhancement of the AIM methodology for accelerated qualification of new materials using the Co bushing alloy as test case, QuesTek staff are collaborating with 

Chen to better codify the current AIM probabilistic minimum 

property forecasting method and quantify its uncertainty. In 

late 2014, a meeting was held at QuesTek to plan the research approach. All property distribution and corresponding simulation data were transferred from QuesTek to NU 

for the three previous demonstrations of the AIM method. 

A meeting with Northrup Grumman staff was also held at 

QuesTek in December to establish the qualification test plan 

for the Co bushing alloy and assess possible sources for its 

funding.

Discussions with Wolverton have established priorities for 

initial applications of high-throughput DFT methods to support CALPHAD database development. A database of solute-vacancy binding energies has been identified as of high 

potential impact in design of high-temperature creep resistance. Co based and Ni based systems will be assessed in 

parallel to take advantage of the greater availability of validation information in Ni based systems.

An identified opportunity to leverage related activities in the 

new technology of additive manufacturing is the DARPA 

Open Manufacturing program where QuesTek collaborates 

with Honeywell in the accelerated process optimization and 

AIM process qualification of the 718+ Ni based superalloy. 

Under a Year 1 collaboration supporting this, QuesTek has 

supplied as-deposited material to Lyle Levine of NIST who 

has collected the raw data for a 4D tomographic study of 

microstress evolution at the current stress relief temperature, using the APS synchrotron at ANL. Preliminary analysis 

shows high promise for the desired quantification of underlying phenomena.

The general system structure for both near-term and 

long-term design of coherent aluminide nanodispersionstrengthened shape memory alloys for enhanced fatigue 

performance is represented in Figure 5. The doctoral research of Dana Frankel, supervised by Olson, has focused 

on the design of biocompatible, PdTi-base SMAs for biomedical applications. Phase relations, precipitation kinetics, 

transformation temperatures, and mechanical properties 

have been characterized in the (Pd,Ni,Fe)50(Ti,Zr,Al)50 system. Controlled precipitation of nanoscale, low-misfit, L21

Heusler aluminides provides excellent strengthening and 

resistance to accommodation slip, maintaining high levels of thermal and mechanical cyclic stability for improved 

Figure 5. System structure of coherent aluminide nanodispersion-strengthened shape memory alloys for high fatigue performance.
22 | CENTER FOR HIERARCHICAL MATERIALS DESIGN

alloy by Frankel confirmed enhanced room 

temperature hardness 

and near-zero values of 

interphase misfit during aging at 600°C. In 

the Fall Quarter, a team 

of graduate students in 

the ICME MS program 

(Olson, Wolverton) was 

organized under Frankel’s coaching to initiate 

follow-on research addressing the design of 

a high strength Ni-free 

(Pd,Fe)(Ti,Zr,Al) superelastic alloy.

While inhibition of accommodation slip through nanoscale 

dispersion strengthening is vital to cyclic stability, of 

equal importance is the control of nonmetallic inclusions 

(the bottom structure subsystem of Figure 5) which act 

as the potent fatigue nucleants determining minimum fatigue strength. Liu group has applied a finite element crystal plasticity modeling approach to quantify the effects of 

such nucleants on the fatigue properties of shape memory alloys, starting with the basic TiNi system. Studies 

have focused on fatigue nucleation at Ti(C,O) carbide and 

(Ti4

Ni2Ox) oxide inclusions within single crystal grains of the 

B2 TiNi parent phase matrix. Analysis of several interacting inclusion clusters has shown that increasing the matrix strength by 50% results in a two order-of-magnitude 

increase in fatigue life, supporting the benefits of nanodispersion strengthening. Further studies have modeled 

a stringer cluster of inclusions based on 3D focused ion 

beam tomographic reconstruction in collaboration with 

Olson. Preliminary results indicate that fatigue life is less 

sensitive to prior interfacial damage (from deformation processing) than to achievable variations in matrix strength. 

Moore has also served as an advisor to the ICME MS graduate student team who are applying his results to assess 

the combined effects of strengthening and inclusion control on SMA fatigue performance.

As an extension of the Phase Field model development 

effort led by Voorhees, Liu met with Umantsev of FSU 

to explore common elements of Umantsev’s Phase Field 

fatigue life. Atom probe tomography (APT) has been employed to measure phase compositions and particle sizes, 

used to calibrate LSW models for coarsening kinetics and 

Gibbs-Thompson models for composition trajectories under 

unstable equilibrium. When combined with models for solid solution strengthening, precipitation strengthening, and 

transformation temperature, the predictive design of an optimally-strengthened superelastic alloy is demonstrated. Using Fe as an austenite stabilizer, a Ni-free (Pd,Fe)(Ti,Al) alloy 

with near-ambient transformation temperatures has been 

designed and characterized. The peak strengthened alloy, 

aged at 600°C for 3.5h to achieve a mean particle radius of 

2.3nm, exhibits compressive transformation strains of 3% 

and extremely stable transformation behavior during thermal cycling. 

While Pd is disadvantageous to strength, both DFT calculations and experimental data from the Ni(Ti,Zr,Al) system 

indicate that Zr is a highly effective strengthener. Frankel 

pursued this concept through coaching of an undergraduate team in Northwestern’s Materials Design class (Olson) in 

the Spring Quarter. Their graphical design, integrating DFT 

calculated shear constants of the Heusler phase, is represented in Figure 6. The team won 3rd prize in the 2014 ASM 

Undergraduate Design Competition. Dr. Shenyen Li of NIST 

also attended the design class as a CHiMaD Visiting Scholar, participating in a related project on a transformation plasticity alloy. Subsequent experimental validation of the SMA 

class project low-Ni, high strength “hybrid” (Pd,Ni)(Ti,Zr,Al) 

Figure 6. Graphical CALPHAD-based design of a multicomponent precipitation-strengthened shape memory alloy for fatigue-resistant medical devices.
YEAR IN REVIEW 2014 | 23

microstructural modeling and Liu’s nonlocal mechanics 

methods to establish a uniform strategy for fatigue modeling. An approach was determined to address the full scope 

of fatigue nucleation and crack growth based on the mesoscopic Fatimi-Socie damage parameter incorporating sizedependent gradient plasticity.

Northwestern University’s SRG materials design consortium 

joined the CHiMaD organization, offering the leveraging opportunity of the CALPHAD-based metals design projects 

with topics and sponsors listed in Figure 7. These projects 

will help demonstrate the generality of the methods, tools, 

and database infrastructure of CHiMaD. The SRG Annual 

Meeting held each March at Northwestern will continue to 

serve as a review of CHiMaD metals activities offering interaction with a broader community.

In a database workshop at NIST April 28-29, organized by 

Carelyn Campbell (NIST), plans were made for the preCALPHAD protodata repository to be hosted at NIST, and 

the metal alloy project was selected as the first to begin 

populating this database using a specified DSpace format. A subsequent teleconference meeting of the Co alloy 

team led to the identification of highest priority alloying elements for thermodynamic and mobility database development. This then formed the basis for thin-film-based initial 

high-throughput experiment design by Chung and Bedzyk

at Northwestern, to employ APS characterization facilities at 

ANL.

A team of undergraduates in our special Murphy Scholar section of Freshman Design collaborated with Olson, 

Choudhary and Agrawal to initiate our research on the integration of a process-property database with a CALPHAD 

database for fusion of prior knowledge with data in the adaptation of datamining techniques for greater accuracy of 

process-structure-property correlation. Using a subset of 

the full process-property database, the team demonstrated 

stronger correlation compared to previous datamining using 

the process-property data alone. The team was coached by 

Research Associate Wei Xiong, a CALPHAD technology expert educated at the Royal Institute of Technology (KTH) in 

Sweden. After the student pilot project, Xiong continued the 

research developing a TCIPC-based link to automate interaction of the ThermoCalc CALPHAD software with the full 

process-property database and mining tools in collaboration with Agrawal. 

Figure 7. SRG materials design projects.

Figure 8. Alloy components for Co database development with priorities.

Figure 9. Performance of three different strategies to locate 30 compounds with a band gap 

energy between 0.9 and 1.7 eV out of a set of 2500 candidates from the ICSD. The first strategy is to randomly select a nonmetal-containing compound. The other two strategies are to 

use a model produced using the machine-learning-based method described in this work that 

was trained on the band gap energies of a distinct set of 22667 compounds from the ICSD 

to locate compounds with a band gap energy within the desired range. The first of those two 

strategies uses a single model trained on all compounds in the training set, and the latter 

uses the composite predictions of a several models each trained on different groups of similar 

compounds. Error bars represent the 95% confidence interval.
24 | CENTER FOR HIERARCHICAL MATERIALS DESIGN

In support of further fundamental database generation and 

datamining capabilities, doctoral student Logan Ward, supervised by Wolverton, has explored accelerated expansion of the Open Quantum Materials Database (OQMD 

– www.oqmd.org, @TheOQMD) which currently consists of 

DFT calculations of ~300,000 compounds taken from the 

Inorganic Crystal Structure Database (ICSD) as well as hypothetical prototype compounds. There are however a very 

large number (>100,000) entries in the Powder Diffraction 

File (PDF) for which compounds have been synthesized, diffraction data obtained, and yet a complete crystal structure 

solution is not available. To aid in the accelerated solution 

of these structures, the First-Principles Assisted Structure 

Solution (FPASS) method was previously developed [B. 

Meredig and C. Wolverton, “A Hybrid Computational-Experimental Approach for Crystal Structure Solution” Nature 

Materials 12, 123 (2013).] The use of FPASS to solve crystal 

structures for a large number of compounds currently “unsolved” in the PDF will enable a much more complete highthroughput database. However, the previous implementation 

of FPASS was not user-friendly, and could only be used in a 

“case-by-case” basis. Logan and Wolverton have now developed software to automate the solution of crystal structures with the First-Principles Assisted Structure Solution 

(FPASS) method, and used these tools to validate the ability FPASS to solve a large number of “test cases” (i.e., cases for which the structural solution is known). A wide array 

of structure types, symmetries, and unit cell sizes was tested, and the automated version of FPASS found the correct 

structure in all 100 test cases. The results from this validation study have also been used to determine how to adjust algorithm parameters (e.g., genetic algorithm mutation 

Precipitation-Strengthened Alloys 

Group Researchers and Their Roles 

Name Position

Institution/

Affiliation Project Title (PI)

Gregory B. Olson PI NU/MSE Materials Design

David C. Dunand PI NU/MSE Co alloys

David N. Seidman PI NU/MSE Co alloys

Peter W. Voorhees PI NU/MSE Phase Field Methods

Chistopher M. Wolverton PI NU/MSE DFT 

Michael Bedzyk PI NU/MSE Rapid Assessment of Phase Relations

Wing Kam Liu PI NU/ME

Alexander Umantsev PI FSU/Chem

Alok Choudhary PI NU/EECS Data Mining

Ankit Agrawal PI NU/EECS Data Mining

Jason Sebastian PI Questek

David Snyder PI QuesTek AIM

Weiwei Zhang PI Questek CALPHAD

James Saal PI Questek AIM

Shengyen Li Postdoctoral Researcher NIST Materials Design (Olson)

Wei Xiong Postdoctoral Researcher NU/MSE Materials Design (Olson)

Seyed Naghavi Postdoctoral Reseracher NU/MSE (Wolverton)

Zequn Wang Postdoctoral Researcher NU/ME (Chen)

Qinyuan Liu Graduate Student NU/MSE (Dunand, Seidman)

Dana Frankel Graduate Student NU/MSE (Olson)

John Moore Graduate Student NU/ME (Liu)

Logan Ward Graduate Student NU/MSE (Wolverton)
YEAR IN REVIEW 2014 | 25

probabilities) to improve performance — a key step in the 

production of a tool intended to be used to solve the structures of large numbers of compounds. The automation toolkit uses the Materials Interface software to perform FPASS, 

which has been made publically available [L. Ward, K. Michel, C. Wolverton, in preparation, 2015]. In recent months, 

use of FPASS has been initiated to determine the structures 

of unsolved entries from the Powder Diffraction File. To date, 

10 solutions have passed all consistency checks. 

In collaboration with Agrawal and Choudhary, Logan and 

Wolverton have explored a general-purpose machine learning strategy for predicting properties of inorganic materials. 

While previous approaches have been successful in predicting a single property of a single class of materials, they have 

introduced a single method that is capable of producing predictive models capable of solving a broad variety of materials problems, and applied it to a variety of material property 

databases, including ones containing crystalline and amorphous materials. The method uses machine learning to discover functional relationships between material properties 

and quantitative attributes derived from material composition. It employs a set of attributes broad enough to capture 

important chemical effects for multiple materials problems 

and introduces a hierarchical modeling strategy to enhance 

the accuracy of our models by partitioning the databases 

into subsets of similar materials. This new method can be 

used to learn novel design rules, predict the properties of 

yet-unstudied materials, and dramatically accelerate combinatorial searches.

COLLABORATIONS

Co-alloy Microanalytical Characterization

Collaborators: Eric Lass, Kil-Won Moon, Maureen Williams

Materials Science and Engineering Division, NIST

Dunand and Seidman research groups are collaborating 

with colleagues at NIST on the microanalysis of prototype 

Co-alloys

Co-alloy Database Development

Collaborators: Carelyn Campbell, Ursula Kattner

Materials Science and Engineering Division, NIST

This collaboration by Gregory Olson involves the sharing of 

pre-CALPHAD data for database development.

Stress relief dynamics in SLM 718+

Collaborator: Lyle Levine

Materials Science and Engineering Division, NIST

Gregory Olson is collaborating with Lyle Levine on APS synchrotron 4D tomographic measurement of a SLM 718+ alloy 

sample from DARPAOpen Manufacturing program on additive manufacturing.

Materials Design Class

Collaborator: Shenyen Li

Materials Science and Engineering Division, NIST

NIST post-doctoral researcher, Li, attended the Materials 

Design class, taught by Olson at Northwestern University 

throughout Spring Quarter 2014. 
26 | CENTER FOR HIERARCHICAL MATERIALS DESIGN

IN-SITU Si-COMPOSITE MATERIALS

Peter Voorhees (lead), Christopher Wolverton

Department of Materials Science and Engineering, Northwestern University

Using computations, verified databases, and experiments in collaboration with Dow Corning we shall design 

Si-based alloys that are both tough and melt castable. This will yield a composite that has many of the desirable 

characteristics of ceramics such as good wear resistance and excellent corrosion resistance, yet can be processed using industry standard casting approaches. A hallmark of this use-case group is the close interaction 

between Dow-Corning scientists in every aspect of this project. Through this close interaction the codes and databases that are developed are being transferred to industry. The databases will be assembled from open literature 

and commercially available sources and then concatenated into a CALPHAD databases. These databases will be 

available to the public. 

DESIGN GOALS 

The over-arching goal of the project is to the design of meltcastable ceramic materials, and to design high performance 

Si-based composite materials for structural applications. 

The focus of the use-case group is on Si-Cr-Ti-Co alloys. A 

major challenge with these materials is that the Si is a strong 

compound former. Since both Si, at room temperature, and 

Si compounds are brittle phases, the challenge is to design 

a polyphase microstructure that increases the toughness of 

the materials beyond that of the constituent phases. Thus, 

CALPHAD modeling is needed to choose the alloy compositions and predict the phases present for a given alloy 

composition. The required toughness of the composite can 

be attained through a fine spatial dispersion of one brittle 

phase in another. This polyphase mixture should have microstructural features sufficiently small, and thus interfacial 

area sufficiently high, to yield significant crack deflection. 

Such crack deflection is well known to yield a tough material using brittle component phases. The fine distribution 

of the phases will be created via solidification of eutectic or 

near-eutectic alloys. Thus, models of the eutectic solidification process are needed to design the processing path and 

molds to yield these fine microstructures. Finally, since Si 

expands on freezing, it is necessary to choose alloy compositions that minimize solidification induced cracking. The 

goal is to produce to produce a castable-ceramic with a 

toughness in the 4-6 MPa m1/2 range.

SIGNIFICANT ACCOMPLISHMENTS 

Central to the design of melt-castable Si-based ceramic materials is the ability to predict the phases that form for a given 

alloy composition, and the processing path needed to insure that these phases are present in the microstructure. To 

this end, Kattner and Voorhees are developing a publically 

available CALPHAD database for the Si-Cr-Ti-Co system using the density functional theory calculations of Skinner and 

Wolverton to fix the energy of the end members of the phase 
YEAR IN REVIEW 2014 | 27

diagram. Previous assessments have not used DFT data in 

their calculations and have relied on optimization of metastable structures with little to no experimental information, 

which may cause problems when extrapolating to multicomponent systems. In addition, the Wolverton Open Quantum 

Mechincal Database predicts a different CrSi2 space group 

and prototype (I4/mmm MoSi2) than the experimental result 

(P6422 CrSi2), implying that the structure is entropically stabilized. Experimental information in the literature has been 

reassessed, and has resulted in the removal of the high temperature β-Cr5

Si3 from the diagram due to the effect of C 

contamination. Using this information, the Si-rich corner of 

the ternary is shown in Figure 1. The new liquidus projection 

predicts a eutectic reaction of L → CrSi2 + CoSi2 + Si that is 

lower in Si content than the Si+CrSi2 eutectic. 

RESEARCH ACCOMPLISHMENTS 

The systems design chart gives the overall design approach 

that we are employing see Figure 1. The objective of this 

chart is to show the interrelationship between processing, 

structure, properties, and performance.

In order to avoid cracking during solidification, the volume 

expansion on freezing should be as small as possible. This 

can be done by choosing an alloy composition that yields a 

volume of the liquid that is close or identical to the volume 

of the solid. However, during solidification the composition 

of the liquid changes due to solute segregation into the liquid. Thus, any calculation of the volume change on solidification must also consider the solidification path of the alloy. 

The key idea is to match the molar volumes of the liquid and 

solid near the end of solidification. So, design of an alloy 

that minimizes volume expansion induced cracking requires 

knowledge of how the molar volumes of the liquid can solid change with composition and temperature, as well as the 

solidification path that is given by the phase diagram. To address this goal we have created a molar volume database. 

The inputs to the database are from experiment and ab inito 

molecular dynamics (AIMD) for the liquid. The ab initio molecular dynamics calculations employ density functional theory calculations at each time step to determine the forces 

between atoms. The AIMD calculations were performed by 

Kwan Skinner (Dow Corning) and Wolverton. The densities 

from the AIMD calculations, and experimentally measured 

densities were interpolated using a classical CALPHAD 

method by Voorhees. 

The binary alloy systems Si-Cr, Si-Co, and Si-Ti have been 

reassessed using up-to-date experimental information 

coupled with DFT calculations for end member phases. In 

addition to the DFT data, the experimental data has been reassessed. This has resulted in the removal of the high temperature β-Cr5

Si3 from the binary phase diagram. A recent 

study did not find the presence of this phase in the binary 

system and a study done on the Si-Cr-C system found this 

phase only in the presence of C2. The original study used 

graphite dyes and therefore, we presume the creation of 

β-Cr5Si3 is due to contamination and has not been included 

in our assessment. The integration of DFT in our assessment 

has also shown discrepancies between the experiments and 

first principle calculations. DFT predicts a different CrSi2

space group and prototype (I4/mmm MoSi2) compared to 

the experimental (P6422 CrSi2). In addition, the low temperature αCr5

Si3 and CrSi are also unstable, implying that these 

structures are all entropically stabilized. 

With the newly optimized Si-Cr and Si-Co binaries, predictions of the ternary Si-Co-Cr system, are possible. In the Sirich corner of the ternary, the liquidus projection predicts a 

eutectic reaction of L → CrSi2

 + CoSi2 + Si that is lower in 

Si content than the Si+CrSi2 eutectic (See Figure 1). This is 

beneficial, as it should reduce the amount of expansion in 

the last to freeze. Using a Scheil model, the solidification 

path could be predicted and an alloy composition could be 

selected to reduce the expansion as well as maintain eutectic structure (shown in gold). The alloys tested had significant reduction in cracking.

The need to understand and to predict the growth of 

eutectics has lead to the development of general theories 

for eutectic growth in binary alloys. Among them, the 

Jackson-Hunt theory establishes relations between the 

growth velocity, the eutectic spacing and the undercooling 

Figure 1. Isotherms on the liquidus surface of the recently reassessed Si-Co-Cr phase diagram 

showing the three-phase eutectic. The gold line is the Scheil solidification path.
28 | CENTER FOR HIERARCHICAL MATERIALS DESIGN

(temperature of the solid/liquid interface compared to the 

eutectic temperature). These relations are the following 

v = A1∆T2,λ2v = A2 λΔT = A3 where A1 and A2 depend on the 

alloy diffusivity in the liquid, thermodynamic properties and 

interfacial energies at the solid/liquid interface, and A3 depends only on the interfacial energies of the three interfaces 

in the eutectic. This model is an approximation, since the SiCrSi2

 eutectic is a faceted eutectic.

Thus, through knowledge of these constants and the evolution of the solidification front during the casting process it 

is possible to predict the eutectic spacing that develops at 

a point in the casting and set the desired microstructure by 

choosing the alloy composition and processing conditions. 

However, it is difficult to measure quantities such as the diffusion coefficient. Skinner thus employed abinitio molecular 

dynamics to determine the intrinsic diffusion coefficients in 

a Si-Cr liquid. In addition to this information, it is also necessary to know the thermodynamic factor to predict the 

interdiffusion coefficient, which depends on the second derivative of the molar free energy of the liquid with respect to 

composition. We obtained this from the CALPHAD fit to the 

free energy of the liquid. By combining the predictions from 

density functional theory and CALPHAD free energies we 

determined a very difficult to measure liquid diffusivity, and 

hence reduce the range of uncertainty in the coefficients A1 

and A2. This leads to a more accurate prediction the development of the eutectic microstructure.

In-Situ Si-Composite Materials

Group Researchers and Their Roles 

Name Position

Institution/

Affiliation Project Title (PI)

Peter Voorhees PI NU/MSE In-Situ Si Composite Materials

Christopher Wolverton PI NU/MSE In-Situ Si Composite Materials

Oriane Senninger Postdoctoral Researcher NU/MSE In-Situ Si Composite Materials (Voorhees)

Shiqiang Hao Postdoctoral Researcher NU/MSE In-Situ Si Composite Materials (Wolverton)

Matthew Peters Graduate Student NU/MSE In-Situ Si Composite Materials (Voorhees)

COLLABORATIONS

Collaboration on In-Situ Si Composite Materials

Collaborators: Vasgen Shamamian, Kwan Skinner, Andres 

Becerra, Lance Wu

Si-Alloys group, Dow Corning Corporation

The scientists from Dow Corning are deeply involved in the 

In-Site Si Composites project. Hence, the collaboration is in 

every aspect of the project and they are collaborating with 

everyone in the use-case group.

Collaboration on CALPHAD Assesment of Si-Alloys

Collaborators: Ursula Kattnera, James Warrena,b

aMaterials Science and Engineering Division, NIST

bMaterial Genomics, Material Measurement Lab, NIST 
YEAR IN REVIEW 2014 | 29

DIRECTED SELF-ASSEMBLY OF BLOCK COPOLYMERS 

FILMS FOR LITHOGRAPHIC APPLICATIONS

Paul Nealey (lead)1, Juan de Pablo1, Stephen Sibener2, Luping Yu2, Heinrich Jaeger2, Ian Foster3,4

1

Institute for Molecular Engineering, University of Chicago

2

Department of Chemistry, University of Chicago

3

Mathematics and Computer Science, Argonne National Laboratory

4

Computation Institute, University of Chicago

Directed Self-Assembly (DSA) has the potential to revolutionize nanomanufacturing. The interest and exponential 

growth in research activity and expenditure is driven in the semiconductor industry by the prospect of manufacturing future generations of computer chips according to Moore’s law, without having to invest billions in new 

fabrication facilities (i.e. based on extreme ultra violet lithography) that may or may not be able to meet the resolution requirements already being demonstrated by DSA. For hard drives, block copolymer lithography is the 

only known technology that is feasible to fabricate nanoimprint masters to manufacture bit patterned media at 

the required storage densities (at least greater than 2 Terabit/inch2

). Finally, commercialization of DSA in nanomanufacturing is currently focused on block copolymers, but the general concepts and measurement tools, once 

established, are anticipated to be broadly applicable to other self-assembling materials and technologies.

DESIGN GOALS 

Here, we seek to lower the resolution limit, such that the DSA 

manufacturing processes can serve three to four future generations of products. We will rely on “Theoretically Informed 

Coarse Grained” (TICG) simulations, informed by atomistic simulations and experiments, coupled to evolutionary 

search strategies of material-pattern combinations, to design so-called high χ block copolymer systems capable of 

self-assembling into the sub 10 nm regime. We aim to identify a “champion” process flow and sub 10 nm DSA materials system employing the same strategy we used to develop 

the materials and processes used for generation 1 DSA, 

now implemented at IMEC, an international industrial consortium, HGST International, a leader in production of hard 

drives, and a host of other companies around the globe. We 

will advance our fundamental understanding of DSA and 

structure in the block copolymer films through a collaborative partnership between center researchers, NIST (Joseph 

Kline, Alex Liddle), and Argonne National Laboratory (ANL). 

TICG simulations on Argonne’s powerful computers, coupled to state-of-the art experimental characterization at Argonne’s Advanced Photon Source, will capture the details of 

the three-dimensional structure of individual block copolymer domains, and the collective averages of three-dimensional structure and pattern perfection over large areas, all 

as a function of materials, processing and boundary conditions used in DSA. Experiments will include transmission 

electron microscopy and tomography, and resonant soft Xray scattering (RSoXS) in transmission-tomography mode.

SIGNIFICANT ACCOMPLISHMENTS 

• Resonant soft X-ray scattering of PS-b-PMMA DSA 

with different boundary conditions (Nealey/de Pablo)

• Resonant soft X-ray scattering of PS-b-P2VP DSA at C 

and N absorption edges (Nealey/de Pablo)

• Resonant soft X-ray scattering of multilayered DSA 

with top-and-bottom patterns (Nealey/de Pablo)
30 | CENTER FOR HIERARCHICAL MATERIALS DESIGN

• Three dimensional DSA using 2D chemical patterns 

(Nealey/de Pablo)

• In-situ membrane sample preparation for block 

copolymer DSA (Nealey)

• Intermediate X-ray Energy beamline commission at 

the Advanced Photon Source (Nealey)

• End-to-End Alignment of Gold Nanorods on Topographically Enhanced Cylinder-Forming Templates 

and their Surface Enhanced Raman Properties 

(Sibener)

• Particle Organization through Two Levels of Selectivity 

— Making the Most of Defects in Block Copolymers 

(Sibener)

• Dynamics of Block Copolymer Swelling by a Selective 

Solvent for the Spatially Resolved Molecular Deposition 

(Sibener)

• Evolutionary design of polymer brushes and triblock 

materials for sub-10nm DSA (de Pablo/Nealey/Jaeger)

• Coarse grained simulation of DSA for contact holeshrinking (de Pablo)

Brandon L. Peters1, Ben Rathsack2, Mark Somervell2, Takeo 

Nakano3, Gerard Schmid4 and Juan J. de Pablo1

1

Institute for Molecular Engineering, University of Chicago, 

Illinois 60637

2

Clean Track Advanced Technology Group, Tokyo Electron 

America, Austin, Texas 78741

3Clean Track Advanced Technology Group, Tokyo Electron 

Limited, Minato-ku, Tokyo, Japan 

4Strategic Lithography Technology, Global Foundries, Malta, 

New York 12020

SCIENTIFIC ACHIEVEMENT

Monte Carlo simulations were performed with block copolymers 

to explore the hole-shrink problem for lithographic applications. The commensurability of the natural periodicity of the 

block copolymer and the diameter of the hole is a key factor 

for creation of polymer cylinders that extend through the hole. 

When a system deviates from the ideal commensurability conditions, rational changes of the wall-polymer interactions, the hole 

geometry, or the composition of a blend with homopolymers 

are shown to provide viable strategies for formation of cylinders 

that extend through the hole and are useful for lithographic 

patterning.

SIGNIFICANCE

This research uses a Monte Carlo simulation to simulate experimentally relevant systems. Importantly, we demonstrate that by 

deforming circular holes into ellipses, it becomes possible to 

form full cylinders over a wider range of conditions than in perfectly cylindrical holes. We also find that homopolymer addition, 

at concentrations in the vicinity of 20%, increases the range of 

conditions over which full cylinders are observed. These results 

allow simulations to identify conditions and material combinations that will alleviate defects in experiments.

CITATION INFORMATION

Peters, B. L., Rathsack, B., Somervell, M., Nakano, T., Schmid, 

G., & de Pablo, J. J. (2014). Graphoepitaxial assembly of 

cylinder forming block copolymers in cylindrical holes. Journal 

of Polymer Science Part B: Polymer Physics. DOI: 10.1002/

polb.23652

ACKNOWLEDGEMENTS

This work is supported by the Center for Hierarchical Materials 

Design (CHIMAD). Additional support from the Semiconductor 

Research Corporation for development of fast simulation codes 

is gratefully acknowledged.

RESEARCH HIGHLIGHT

GRAPHOEPITAXIAL ASSEMBLY OF CYLINDER FORMING BLOCK 

COPOLYMERS IN CYLINDRICAL HOLES
YEAR IN REVIEW 2014 | 31

RESEARCH ACCOMPLISHMENTS

Over the 2014-2015 funding period, Nealey group has developed membrane sample fabrication techniques through 

extensive collaboration with Argonne National Laboratory 

(ANL) and industrial collaborators such as IMEC and HGST 

that enable transmission characterization of the in-film 

structures of blockcopolymer DSA. Collaborating with Joseph Kline from NIST, the Nealey group performed RSoXS 

experiments on these membrane DSA samples at the Advanced Light Source (ALS) to characterize the in-film structure of block copolymer. Four examples are given below:

1) RSoXS of PS-b-PMMA DSA with different boundary 

conditions

DSA of perpendicularly oriented lamella-forming BCP is of 

great interest for semiconductor industry and it’s important 

for the lamella structures to propagate all the way through 

the film. However, molecular simulation suggests the existence of in-film 3D morphologies even when the top surface 

is well-aligned, and the formation of such structures is highly influenced by the boundary condition. DSA samples with 

Figure 1. (a) Calculated in-film lamella structures by fitting scattering data of BCP DSA. (b) Molecular simulations for different guiding stripe widths 

show similar in-film structures.

Figure 2. Qx-Qz scattering data of PS-b-P2VP lamella DSA at the C edge (a) and N edge (b) respectively.

different guiding stripe widths were prepared at IMEC using an 

all-track process previously developed by the Nealey group, a 

first in the industry. Preliminary results show that the lamella 

goes through the film at optimal guiding stripe width but Ushape structures were formed in-film at wider guiding stripes, 

similar to those predicted by molecular simulations from the 

de Pablo group.

2) RSoXS of PS-b-P2VP DSA at C and N absorption edges

PS-b-P2VP is a high χ system that can deliver smaller dimensions beyond the limit of PS-b-PMMA, and it’s an important 

candidate for 2nd generation DSA materials. Through collaborations with ANL and HGST, DSA samples with different guiding 

stripe widths were prepared on SiN membrane by the Nealey

group. The existence of nitrogen in P2VP block also enables us 

to leverage the element-specific contrast that’s unique to soft 

X-ray to identify the two blocks. Scattering experiments were 

performed at the C edge as well as the N edge, which is the 

first of such study on BCP DSA.
32 | CENTER FOR HIERARCHICAL MATERIALS DESIGN

Figure 4. (a) Schematic showing 10 layers of spheres assembled on a 2D pattern that matches the BCC(001) plane. (b)

(c) X-ray diffraction pattern of the BCC(001) assembly with incident angle of 90° (a) and 45° (b) respectively, perfectly 

matching crystallographic predictions.

Figure 5. (a) Back etch process to prepare SiN membrane sample in-situ. (b) 4” wafers and individual membranes after back etch.

3) RSoXS of multilayered DSA with top-and-bottom 

patterns

DSA with guiding patterns on both top and bottom can direct the assembly in two directions; the resulting morphology at the interface is not clear. Techniques developed by 

the Nealey group to fabricate DSA patterns on membranes 

has made possible the transmission characterization of 

the entire film. Through molecular transfer printing, the top 

coat pattern was printed onto a patterned SiN membrane 

with different angles between the two patterns. Preliminary 

scattering results show two sets of peaks, suggesting DSA 

along 2 directions. By fitting the scattering in both directions we will be able to delineate the interfacial morphology.

4) Three dimensional DSA using sphere-forming 

PS-b-PMMA

The Nealey group has developed strategies to extend the 

concept of DSA into 3D lattice systems by using a 2D chemical pattern to match one plane of the 3D lattice. These strategies are supported by simulations by the de Pablo group. 

DSA studies were done on sphere-forming PS-b-PMMA as 

a pilot system, which forms body-centered–cubic (BCC) lattice in the bulk. When the film thickness is commensurate 

Figure 3. (a) Schematic of the lamella DSA with top and bottom patterns. (b)(c) Scattering pattern of the sample with pattern angle 

of 45° (b) and 0° (c) respectively.
YEAR IN REVIEW 2014 | 33

Figure 6. (a) The in-vacuum Kappa-diffractometer of the scattering end-station at 29ID. (b) Detailed view of the sample holder 

and the detectors. (c) Panoramic view of the Intermediate X-ray Energy Beamline at Sector 29 of the APS.

with plane spacing, DSA can be 

achieved through more than 10 

layers of spheres on patterns 

matching BCC (001) geometry. 

The resulting structure is comparable to a single crystal, and soft 

X-ray diffraction-tomography 

was performed to understand 

the in-film lattice packing and the 

wobbling of spheres from their 

lattice points.

Enabling all the aforementioned transmission measurements, strategies have been developed by the Nealey group 

with Center for Nanoscale Materials at ANL to prepare membrane samples in-situ to avoid the unwanted artifacts inherent to conventional membrane preparation techniques. DSA 

was done on SiN coated wafers and in the final step back 

etched the Si to get SiN membranes. The polymer is protected by a specially designed holder and morphological details 

are thus perfectly preserved.

The Nealey group is also collaborating with the ANL to commission a new Intermediate X-ray Energy Beamline at Sector 

29 of the Advanced Photon Source. The first beam was taken in the scattering chamber in late August 2014 and the instruments are going through final stages of characterization 

and calibration. The chamber hosts a seven-axis in-vacuum 

diffractometer with kappa geometry, which allows access to 

a larger area of reciprocal-space than standard two- and 

three-circle diffractometers typically found in RSoXS endstations. With the diffractometer and analyzer fully operational, we are ready for the first RSoXS experiment, which is 

scheduled for February 2015.

Figure 7. (a) Nanorods are drawn into registry with the corrugated polymer template with excellent (>97%) alignment 

(b) as the deposition solvent recedes Figure 8. Hierarchical alignment of gold nanorods on 

pre-aligned DSA polymer scaffolds for a series of nanorod 

densities producing unprecedented end-to-end nanorod 

arrays. (a) 40 AuNRs/μm2, (b) 160 AuNR/μm2, and (c) 

>250 AuNRs/μm2.

Figure 9. Novel SERS substrates show high sensitivity to 

the incident laser polarization. Benzenethiol SERS signals 

across the arrays with (a) parallel and (b) perpendicular 

incident laser polarizations

Over the 2014-2015 funding period, the Sibener group has 

been engaged in three projects concerning the directed selfassembly (DSA) of block copolymer (BCP) templates. These 

projects are unified by the common theme of using BCP 

substrates as a scaffold for the hierarchical organization and 

DSA of functional nanoparticles and molecules. 

In the first of these projects, we aimed to produce end-toend nanorod assemblies, due to the optimal field enhancement of end-to-end junctions. However, deposited nanorods 

naturally form side-by-side aggregates. To overcome these 

challenges, we have employed corrugated block copolymer templates for DSA of end-to-end nanorod arrays. Acetic acid vapor annealing was used to selectively swell and 

collapse the PMMA domains, producing a highly corrugated 

polymer template. Capillary forces draw deposited nanorods into the nanogrooves and enforce conformal alignment 

(>97%) with the groove (Figure 7). Prior alignment of polymer cylinders within lithographic channels enables global 

alignment of end-to-end nanorods (Figure 8). Our success 

in these goals has yielded SERS substrates with high sensitivity to incident laser polarization (Figure 9).
34 | CENTER FOR HIERARCHICAL MATERIALS DESIGN

Figure 10. Conformal and continuous alignment of 

nanoconfined cylinders in kinked channels produces 

high strain energy grain boundaries

Figure 11. Engineered artificial grain boundaries enable 

highly precise localization of nanoparticles

We have also investigated the use of artificial grain boundaries for the DSA of nanoparticle aggregates. Our approach involves two levels of control, (1) chemical selectivity 

and (2) defect selectivity, in order to direct the positions of 

nanoparticle additives. First, PS-capped nanoparticles are 

selectively incorporated into PS domains due to chemical 

compatibility. Next, particles aggregate along defect sites in 

the polymer matrix in order to relieve elastic strain. We can 

engineer defect locations by using the conformal alignment 

of nanoconfined cylinders within kinked (zigzag) channels 

to produce highly strained artificial grain boundaries (Figure 

10). This hierarchical DSA approach has enabled the precise localization of nanoparticle defects at engineered defect sites (Figure 11).

Finally, in situ AFM of imaging block copolymer islands under 

a selective solvent allows direct observation of swelling front 

kinetics. These observations are relevant to understanding 

diffusion rates for emerging polymer-encapsulated drug delivery schemes. Moreover, by using the solvent as a carrier 

for small molecules, they may be incorporated into the polymer matrix. This spatially resolved incorporation has been 

observed for the fluorophore BODIPY (Figure 12). As this ongoing work continues, the use of DSA polymer templates will 

enable positional control over functional molecule gradients.

Using evolutionary strategies, de Pablo, in collaboration with 

Jaeger and Nealey, has demonstrated the use of evolutionary computation strategies for design of chemical patterns 

capable of self-assembling triblock copolymers into features 

with characteristic dimensions below 14 nm. Patterning at 

sub-10 nm is one of the design goals for our team, and the 

evolutionary work of our team, which currently places us 

below 14 nm, put us on a promising pathway to achieving 

some of our long-term goals. In different work, de Pablo

also demonstrated the use of DSA for hole-shrinking applications at length scales below 20 nm. That work was a collaboration with scientists and engineers from TEL and from 

Global Foundries. More specifically, the graphoepitaxial assembly of cylinder-forming block copolymers assembled 

into holes was investigated through theoretically informed 

coarse grained Monte Carlo simulations (TICG MC). The 

aim was to identify conditions leading to assembly of cylinders that span the entire thickness of the holes, thereby 

enabling applications in lithography. Three hole geometries 

were considered, including cylinders, elliptical cylinders, and 

capsule-shaped holes. Four distinct morphologies of cylinder-forming poly(styrene-b-methyl methacrylate) (PS-bPMMA) block copolymers were observed in cylinders and 

elliptical holes, including cylinders, spheres, partial cylinders, 

and wall-bound cylinders. Additional morphologies were observed in capsule-shaped holes. PMMA cylinders that extend through the entire hole were found with PMMA-wetting 

surfaces; a weak wetting condition was needed on the bottom of the hole and a strong wetting condition was necessary on the sides of the hole. Simulations were also used to 

explore the morphologies that arise when holes are overfilled, or when PMMA homopolymers are added in blends 

with copolymers. We found that overfilling can alter considerably the morphological behavior of copolymers in cylinders and, for blends; we found that when the homopolymer 

concentration is >10%, the range of conditions for formation of PMMA cylinders that extend through the entire hole is 

increased. In general, results from simulations (TICG) were 

shown to be comparable to those of self-consistent (SCFT).

Figure 12. Fluorescence map of BODIPY incorporation demonstrates precise and selective 

molecular incorporation along the periphery 

of polymer islands
YEAR IN REVIEW 2014 | 35

Gurdaman S. Khaira1, Jian Qin1, Grant P. Garner1, Shisheng 

Xiong1, Lei Wan2, Ricardo Ruiz2, Heinrich M. Jaeger3, Paul 

F. Nealey1,4, and Juan J. de Pablo1,4

1

Institute for Molecular Engineering, The University of 

Chicago, Chicago, IL 60637 

2

HGST, San Jose, CA 95135,

3James Franck Institute, The University of Chicago, 

Chicago, IL 60637

4Argonne National Laboratory, Argonne, IL 60439

SCIENTIFIC ACHIEVEMENT

We used an evolutionary optimization method, coupled 

with molecular simulations, to predict the optimal chemical 

patterns for directed self-assembly of block polymers 

with density multiplication of lines and spaces. In contrast 

to blind search algorithms, the evolutionary approach is 

able to rapidly identify the optimal combination of pattern 

geometry, chemistry of the guiding stripes and chemistry 

of the background material. This work is the first to use 

computationally intensive, but realistic three-dimensional 

molecular simulations to address the inverse problem of 

design of guiding patterns for defect-free self-assembly of 

block copolymers in lithographic applications.

SIGNIFICANCE

Most practical applications of block copolymers rely on 

complex morphologies that are inherently non-equilibrium 

in nature. Designing a suitable guiding pattern for directed 

assembly into a sought after target represents one of 

the grand challenges for the use of block copolymers 

in lithography. The guiding patterns need not have a 

one-to-one correspondence with a target morphology. 

Moreover, a large number of independent variables makes 

this inverse design problem intractable by trial-and-error. 

The method described in this work represents a ``smart’’ 

strategy to efficiently search for an optimal combination 

of these system variables. From a user’s perspective, 

no-prior information on the sensitivity of a given parameter 

towards the final morphology is required. This approach is 

not limited to pattern or materials variables, but can also 

be extended to searches for optimal processing conditions, including annealing temperature, solvent evaporation 

rate, etc..

CITATION INFORMATION

Khaira, G. S., Qin J., Garner G. P., Xiong S., Wan L., 

Ruiz R., Jaeger H. M., Nealey P. F., and de Pablo J. J. 

“Evolutionary Optimization of Directed Self-Assembly of 

Triblock Copolymers on Chemically Patterned Substrates.” 

ACS Macro Letters 3, no. 8 (2014): 747-752..

ACKNOWLEDGEMENTS

This research work was performed under award 

70NHNB14H012 from the U.S. Department of Commerce, 

National Institute of Standards and Technology as part of 

the CHiMad. Additional support from the Semiconductor 

Research Corporation for the development of new 

processing strategies for sublithographic patterning is 

gratefully acknowledged. H.M.J acknowledges support 

from NSF CBET 1334426.

RESEARCH HIGHLIGHT

EVOLUTIONARY OPTIMIZATION OF DIRECTED SELF-ASSEMBLY OF TRIBLOCK COPOLYMERS OF CHEMICALLY PATTERNED SUBSTRATES
36 | CENTER FOR HIERARCHICAL MATERIALS DESIGN

COLLABORATIONS

Design of materials for graphoepitaxy and holeshrinking processes

Collaborators: Scientists from TEL and Global Foundries

The de Pablo Research group is collaborating with scientists and engineers from TEL and Global Foundries on 

computational designs of materials for graphoepitaxy. The 

collaboration resulted in a publication.

Design of materials for commercial chemical epitaxy 

processes.

Collaborators: Scientists from IMEC (Belgium) and INTEL

The de Pablo and Nealey research groups are collaborating with scientists and engineers from IMEC in Belgium. The 

collaboration requires that two CHiMaD students (Grant 

Garner and Robert Seidel) be placed at IMEC facilities in 

Belgium, under the supervision of Dr. Roel Gronheid, for assembly-line development and testing of new materials. De 

Pablo and Nealey are also collaborating with Dr. Younkin 

from Intel on development of X-ray based metrology tools 

and development of software for industrial design of DSA 

processes.

RSoXS of Block Copolymer Directed Self Assembly

Collaborators: Joseph Kline

Materials Science and Engineering Division, NIST

RSoXS characterization of the in-film structure of block copolymer directed self assembly. Scattering experiments 

were performed at Advanced Light Source by Jiaxing Ren 

(Nealey group) and Joseph Kline.

Directed Self-Assembly of Block Copolymers Films for Lithographic Applications

Group Researchers and Their Roles 

Name Position

Institution/

Affiliation Project Title (PI)

Paul Nealey PI UC/IME

Juan de Pablo PI UC/IME

Stephen Sibener PI UC/CHEM

Luping Yu PI UC/CHEM

Heinrich Jaeger PI UC/CHEM

Ian Foster PI UC/CI

Jiaxing Ren Graduate Student UC/IME RSoXS of block copolymer directed self-assembly (Nealey)

Fanny Simoes Postdoctoral Researcher ANL/APS Intermediate Energy X-ray beamline setup (Nealey)

John Colson Postdoctoral Researcher UC/IME Directed self-assembly of block coplymer polyelectrolytes (Nealey)

Qianqian Tong Graduate Student UC/CHEM (Sibener)

Jonathan Raybin Graduate Student UC/CHEM (Sibener)

Hyung Ju Ryu Postdoc UC/CHEM (Sibener)

Gurdaman Khaira Student UC/IME Evolutionary algorithms for materials design in 

DSA (DePablo)

Brandon Peters Graduate Student UC/IME Directed self-assembly of copolymers (De Pablo)

Grant Garner Graduate Student UC/IME Directed self assembly (De Pablo/Nealey)

Roselyne Tchoua GraduaCIte Student UC/CI Machine learning environment for automated creation of advanced 

databases (Foster)

Katherine Oosterbaan Undergraduate Student UC/CHEM (Sibener)

Michelle Chalupnik Undergraduate Student UC/CHEM (Sibener)

Jay Prakadan Undergraduate Student UC/CHEM (Sibener)

Mladen Rasic Undergraduate Student UC/CHEM Web-based software for polymer property prediction (Freed/de 

Pablo)
YEAR IN REVIEW 2014 | 37

Block Copolymer Directed Self Assembly 

Collaborators: Jack Douglas

Materials Science and Engineering Division, NIST

Sibener and de Pablo research groups are collaborating 

with Jack Douglas at NIST for directed self assembly of 

block copolymers. 

PUBLICATIONS

Brandon L. Peters, Ben Rathsack, Mark Somervell, Takeo 

Nakano, Gerard Schmid and Juan J. de Pablo (2015), 

“Graphoepitaxial assembly of cylinder forming block copolymers in cylindrical holes,” J. of Polymer Science B, Polymer Physics, Article first published online: 16 DEC 2014 | 

DOI: 10.1002/polb.23652. CHiMAD primary support.

Khaira, Gurdaman S.; Qin, Jian; Garner, Grant P.; Nealey, 

Paul F., Jaeger, H., and de Pablo, Juan J., (2014) “Evolutionary Optimization of Directed Self-Assembly of Triblock 

Copolymers on Chemically Patterned Substrates,”, ACS 

Macro Letters, 3, 747-752. CHiMAD primary support. 

Tong, Q.; Malachosky, E.W.; Raybin, J.; Guyot-Sionnest, 

P.; Sibener, S.J., (2014), “End-to-End Alignment of Gold 

Nanorods on Topographically Enhanced Cylinder-Forming 

Templates and their Surface Enhanced Raman Properties.” 

Journal of Physical Chemistry C, 118 (33), 19259-19265. 

Partial CHiMaD support/primary CHiMaD support for DSA 

components of this project.

DATABASES

Polymer Design Data Base

System of interest: Characteristic properties for design of 

polymer blends and copolymers for engineering applications, including DSA. The database is now being populated by resorting to automated techniques that extract target 

content from the literature. Human intervention is then required to verify relevance and accuracy of computer-generated entries. The data base will be tested through a formal 

course, to be taught at the University of Chicago in Spring 

2015, with collaborators from NIST (Debra Audus and Jack 

Douglas). The data base also includes a thermodynamic 

property prediction set of modules based on state-of-theart molecular models.

Methodology: Theory and experiment

Distribution: Not released yet

Collaborators: Juan de Pablo, Ian Foster, Paul Nealey, and 

Heinrich Jaeger

Prediction of Phase behavior of polymers and 

polyelectrolytes 

System of interest: Polymer blends, polyelectrolytes, 

copolymers 

Methodology: Computational and experimental 

Distribution: Under development 

Collaborators: Juan de Pablo, Ian Foster, Paul Nealey, 

Matthew Tirrell, Heinrich Jaeger, Jack Douglas, Carl Freed

SOFTWARE

Simulation Software 

Authors: Juan de Pablo, Gurdaman Khaira

Type: Licensed

Software for simulation of block copolymer directed self assembly was released to INTEL in November 2014. 
38 | CENTER FOR HIERARCHICAL MATERIALS DESIGN

BIOMATERIALS

Matthew Tirrell (lead)1, Juan J. de Pablo1, Monica Olvera de la Cruz2, Erik Luijten2,3,4

1

Institute for Molecular Engineering, University of Chicago

2

Department of Materials Science and Engineering, Northwestern University

3

Department of Engineering Sciences and Applied Mathematics, Northwestern University

4

Department of Physics and Astronomy, Northwestern University (by courtesy)

The Biomaterials Use-Case Group is developing new materials based on multi-valent ionic interactions. This direction is significant because it can yield new types of self-assembled structures that: (a) do not depend on typical 

and well-studied hydrophobic interactions to create the assembly; (b) yield materials with useful gelation, surface 

coating and self-healing properties; (c) are biocompatible; (d) can be used to sequester solid particles and nucleate 

the formation of inorganic phases, as in biomineraliztion; and, (e) afford routes to numerous design innovations. 

Knowledge of the as-yet unexplored phase behavior of polyelectrolyte complexes, which we are creating, is scientifically significant and technologically relevant. We are learning when and, more importantly why, polyelectrolyte 

complexes may be solid or liquid. We are building a database of phase behavior to comprehend the stability of 

complexes with respect to salt, with a variety of salt valences.

DESIGN GOALS 

The knowledge foundation we are creating, via experiment, 

theory and simulation, will enable the design of new materials based on multi-valent ionic interactions is several domains of materials development: coatings for biomaterials, 

encapsulants for particles, self-healing hydrogels as biomaterials, and biomineralization with applications to bone and 

dental repair.

SIGNIFICANT ACCOMPLISHMENTS 

The principal accomplishments have been (a) the development of a new liquid state capable of both predicting and 

interpreting experimental phase behavior, leading to (b) a 

better understanding of the interfacial tension property at 

the interface between polyelectrolyte complexes and water; 

(c) simulations that shed light on how stereo-regularity and 

–sequences influence whether a complex is liquid or solid.

RESEARCH ACCOMPLISHMENTS 

A mixture of oppositely charged polyelectrolyte chains in 

solution can associate and form various kinds of complexes, collectively known as polyelectrolyte complexes (PECs). 

Broadly, these complexes can be classified as solid phase 

PEC precipitates, or fluid phase complexes known as coacervates. The basic physics of why complexes form is well-understood: compensation of charges on one polyelectrolyte chain 

in solution by charges of opposite sign that are intrinsically entropy-deprived by virtue of being on another polymer chain, 

while maintaining electro-neutrality, thereby liberating entropy-rich small ions into a co-existing aqueous solution favors 

formation of PECs. Thus, polymer concentration in the complex phase is presumably governed by a balance between attractive interactions from electrostatics and repulsive excluded 

volume interactions. However, more detailed, and especially 

quantitatively predictable physical understanding of polyelectrolyte complex formation remains elusive. We envisage these 

studies are essential for the expansion of PECs as platforms 

for next generation designer materials.

The phase behavior of polyelectrolyte blends is described by 

incorporating local ionic correlations via liquid state (LS) theory. This theory accurately describes strongly enhanced phase 

separation that is experimentally observed in polyelectrolyte 

mixtures. In addition to strongly enhanced macrophase separation between the two polymer types, liquid-liquid phase separation within the charged polymer is found at high electrostatic 
YEAR IN REVIEW 2014 | 39

interaction strengths. Above a threshold value, which depends on the number of mers of polymer, the charge fraction 

of the charged polymer, and the Flory-Huggins interaction χ, 

the charged polymer separates into regions of ion-rich and 

ion-poor regions, forming ion clusters within the charged 

polymer phase. This is indicated by multiple spinodal regions 

shown for Г of 17.28 kT and 23.43 kT in Figure 1.

We have illustrated the phase determining role of chirality of 

chain backbone in polyelectrolyte complexation in Figure 2. 

Systematic atomic-level simulations of short poly(glutamic 

acid) and poly(lysine) chains revealed that achiral chains 

formed unstructured complexes. In contrast, antiparallel stable ß-sheets were formed between homochiral polypeptides. 

We attribute the changes in chirality along the peptide backbone to cause “kinks” in the backbone, thus inhibiting extended ß-sheet formation. We provide new insights that 

allow for facile morphology control in polyelectrolyte complexes, and will be useful in developing of simple yet strong 

bio-inspired polyelectrolyte based hybrid materials.

Biomineralization is another area of research activity and accomplishment in our multi-valent ionic materials use-case. 

Calcium phosphate (CaP) is an important material in biology 

and biomedical applications. Moreover, calcium phosphate 

nanoparticles has been used as stable, biocompatible, nontoxic and cheap inorganic materials in cosmetics and pharmaceutical applications. A particularly interesting design

Figure 1. Spinodal lines for a symmetric polymer blend system with N = 40, charge fraction fq = 0.1, 

and Г= 0.24, 4.02, 9.23, 12.07, 17.28, 23.43 from top to bottom.

Figure 2. Chiral polymers (left) can conform to one another in a beta sheet arrangement leading to 

solid complexes; racemic polymers (right) cannot.
40 | CENTER FOR HIERARCHICAL MATERIALS DESIGN

goal for CaP consists of development of 

injectable, self-setting cements for bone 

regeneration. Such cements would offer 

significant advantages over conventional, pre-shaped bone substitutes. During 

minimally invasive bone regenerative surgery, CaP based cement materials can 

be directly injected into irregular bone 

defects and act as an osteoconductive 

material. Importantly, the performance 

of the cement predominantly relies on 

the structural characteristics of the CaP 

nanoparticles that constitute the cement. 

In our biomaterials use-case, we have adopted a biomimetic approach to synthesize CaP nanoparticles with controlled 

size (Fig. 3a) and surface properties for 

formulation of a new class of injectable 

materials (Fig. 3b). The approach includes 

precipitation of CaP from aqueous solution in the presence of different well-characterized polyelectrolytes. Subsequently, 

we exploit interparticle interactions to induce self-assembly of the CaP nanoparticles into an integrated gel (Fig. 3c and 

3d). Our approach requires precise control of multiple interactions between calcium and phosphate ions, polyelectrolyte 

additives and other precipitating phases 

that are being modeled, characterized, and optimized. 

Important parameters such as pH, temperature, polymer concentration and polymer molecular weight all 

play a significant role in the proposed process.

Finally, work by Luijten investigates how self-assembly involving biological and synthetic components is 

affected by nonuniform dielectric properties. As a prototypical model system, we consider dielectrically heterogeneous Janus particles—spherical colloids with a 

dielectric mismatch between their two hemispheres. 

When such particles are suspended in solution, this 

mismatch leads to different polarization charges induced on the two hemispheres. Employing a new and 

efficient computational approach that dynamically 

and spatially resolves the polarization charge distribution within molecular dynamics simulations, we found 

that dielectric effects can significantly alter the ion 

distribution around dielectric Janus colloids. More importantly, large-scale molecular dynamics simulations 

Figure 3. (a) Effect of concentration on the size of CaP nanoparticles stabilized by two different polyelectrolytes. (b) 

Gel extrusion of CaP_1 + CaP_2 by a conventional medical syringe. (c) Effect of initial concentration on the aggregation of the CaP_1 + CaP_2 systems. (d) Representative CaP_1 + CaP_2 aggregation. 

Figure 4. Self-assembly of colloids with anisotropic dielectric properties (red and blue surfaces), bound by charged nanoparticles. Depending on the solvent selected, the nanoparticles 

preferentially bind (electrostatically) to surfaces with either low or high dielectric constant, 

thus opening the possibility of solvent-dependent structures.
YEAR IN REVIEW 2014 | 41

have shown that within self-assembled aggregates these Janus particles can have various orientational preferences, controlled by 

the surrounding medium. We envisage that 

this property can be exploited to create nanostructures that are responsive to their dielectric environment.

COLLABORATIONS

SANS investigations of copolyelectrolyte 

micelles and hydrogels

Collaborators: Vivek Prabhu

Chemical Engineer, Materials Science and 

Engineering Division, NIST

Srivastava, Tirrell and de Pablo are collaborating with Prabhu on synthesis of deuterated 

copolyelectrolytes and conducting subsequent SANS experiments on copolyelectrolyte based micelles and hydrogels. The aim 

of the work is to illustrate the structural hierarchy in the hybrid materials, thus enabling 

intelligent design of drug carrying and tissue 

scaffold materials.

This active collaboration initiated in October 2014. Two postdocs from the Tirrell

group visited NIST in December 2014 to conduct SANS experiments. A joint proposal at 

CNMS, ORNL has also been accepted for 

synthesis of deuterated polymers.

Inorganic Hierarchical Materials 

Collaborators: Sheng Lin-Gibson

Biosystems and Biomaterials Division, NIST

Qazvini, de Pablo and Tirrell are collaborating with Lin-Gibson on biomineralization and 

biomimetic approaches to generate hierarchical structures based on inorganic materials such as CaP and complex coacervates.

Modeling of polymeric materials

Collaborators: Jack F. Douglas

Materials Science and Engineering Division,

NIST

The De Pablo research group is collaborating with Douglas in studying transport properties of polymer blends and solutions, polymer films and 

glasses. A postdoctoral researcher, jointly advised by de 

Pablo and Douglas, will start at NIST in March 2015 to further the collaborative work on polyelectrolyte biomaterials. 

Zhenwei Yao1, Monica Olvera de la Cruz1,2,3,4

DYNAMICS OF VACANCIES IN TWO-DIMENSIONAL 

LENNARD-JONES CRYSTALS

RESEARCH HIGHLIGHT

1

Department of Materials Science and 

Engineering, Northwestern University, 

Evanston, IL 60208-3108 USA

2

Department of Chemistry, Northwestern 

University, Evanston, IL 60208-3108 USA

3Department of Chemical and Biological 

Engineering, Northwestern University, 

Evanston, IL 60208-3108 USA

4

Department of Physics, Northwestern 

University, Evanston, IL 60208-3108 USA

SCIENTIFIC ACHIEVEMENT

Vacancies represent an important class of 

crystallographic defects, and their behaviors can be strongly coupled with relevant 

material properties. In this work, we study 

the dynamics of generic n-point vacancies in two-dimensional Lennard-Jones 

crystals in several thermodynamic states. 

Simulations reveal the spectrum of distinct, 

size-dependent vacancy dynamics, including the nonmonotonously varying diffusive 

mobilities of one-, two- and three-point 

vacancies, and several healing routines of 

linear vacancies. Specifically, we numerically observe significantly faster diffusion 

of the two-point vacancy that can be attributed to its rotational degree of freedom.

The high mobility of the two-point vacancies opens the possibility of doping 

two-point vacancies into atomic materials to enhance atomic migration. The rich 

physics of vacancies revealed in this study 

may have implications in the engineering of 

defects in extensive crystalline materials for 

desired properties.

SIGNIFICANCE

The dynamics of crystallographic defects 

influences many aspects of crystalline 

materials. The two dimensional LennardJones crystal has been extensively used 

to describe the physical properties of 

materials such as metals and nanoparticles. This study shows that it also serves 

to perform fundamental researches on the 

defect dynamics. Counterintuitive results 

emerge in the two dimensional LennardJones crystal: two-point vacancies diffuse 

significantly faster than one-point and 

three-point vacancies. In addition, rich 

dynamics of vacancies with different sizes 

are found in simulations. These results 

may be useful as well with more complex 

and realistic three-dimensional structures.

CITATION INFORMATION

Zhenwei Yao and Monica Olvera de la 

Cruz, Phys. Rev. E, 90, 062318 (2014)

ACKNOWLEDGEMENTS

M.O. thanks the financial assistance of the 

U.S. Department of Commerce, National 

Institute of Standards and Technology 

as part of the Center for Hierarchical 

Materials Design (CHiMaD) award 

70NANB14H012. Z.Y. thanks the Office 

of the Director of Defense Research and 

Engineering (DDR&E) and the Air Force 

Office of Scientific Research (AFOSR) 

under Award No. FA9550-10-1-0167 for 

financial support. We thank Alexander 

Patashinski, Chandler Becker, and Carrie 

Campbell for stimulating discussions..
42 | CENTER FOR HIERARCHICAL MATERIALS DESIGN

DATABASES

Prediction of Phase behavior of polymers and 

polyelectrolytes 

System of interest: Polymer blends, polyelectrolytes, 

copolymers

Methodology: Computational and experimental

Distribution: Under development

Collaborators: Juan de Pablo, Ian Foster, Paul Nealey, 

Matthew Tirrell, Heinrich Jaeger, Jack Douglas, Carl Freed

Polymer Design Database

System of interest: Characteristic properties for design of 

polymer blends and copolymers for engineering applications, including DSA. The database is now being populated by resorting to automated techniques that extract 

target content from the literature. Human intervention is 

then required to verify relevance and accuracy of computergenerated entries. The database will be tested through a 

formal course, to be taught at the University of Chicago in 

Spring 2015, with collaborators from NIST (Debra Audus and 

Jack Douglas). The database also includes a thermodynamic property prediction set of modules based on state-of-theart molecular models. 

Methodology: Theory and experiment 

Distribution: Not released yet 

Collaborators: Juan de Pablo, Ian Foster, Paul Nealey, and 

Heinrich Jaeger 

Biomaterials

Group Researchers and Their Roles 

Name Position

Institution/

Affiliation Project Title (PI)

Matthew Tirell PI UC/ME Biomaterials

Juan de Pablo PI UC/IME Biomaterials

Monica Olvera de la Cruz PI NU/MSE Biomaterials

Erik Luijten PI NU/MSE Biomaterials

Samanvaya Srivastava Postdoctoral Researcher UC/IME Biomaterials (Tirell)

Nader Taheri Qazvini Postdoctoral Researcher UC/IME Biomaterials (Tirell/de Pablo)

Sanaz Sadati Postdoctoral Researcher UC/IME Biomaterials (de Pablo)

Andres Cordoba Uribe Postdoctoral Researcher UC/IME Biomaterials (de Pablo)

Zhenwei Yao Postdoctoral Researcher NU/MSE Biomaterials (de la Cruz)

Kyle Hoffman Graduate Student UC/IME Biomaterials (de Pablo)

Joshua Lequieu Graduate Student UC/IME Biomaterials (de Pablo)

Huanxin Wu Graduate Student NU/MSE Biomaterials (Luijten)

Development and application of strategies for 

incorporating nonuniform dielectric media in particlebased simulations

Collaborators: Zhenli Xu

Department of Mathematics, Institute of Natural Sciences, 

Shanghai Jiao Tong University

The Luijten research group works with an Applied Mathematics group at the Institute of Natural Sciences at Shanghai 

Jiao Tong University, aiming to develop efficient algorithms 

for solving the Poisson equation in inhomogeneous dielectric 

media. Prof. Xu’s group has expertise in the image-charge 

method while the Luijten research group is specialized in 

fast boundary-element-based algorithms. By combining the 

advantages of both methods, we will have the capability to 

simulate dynamic systems with full dielectric effects.

This collaboration focuses on algorithms for the Poisson 

equation. Luijten and graduate student Huanxin Wu visited 

Shanghai Jiao Tong University in December 2014.

PUBLICATIONS

K.Q. Hoffmann, S.L. Perry, L. Leon, D. Priftis, M. Tirrell, J.J. 

de Pablo (2014), “A Molecular View of the Role of Chirality in 

Charge-driven Polypeptide Complexation” Soft Matter, DOI: 

10.1039/C4SM02336F. Primary CHiMaD support.

J. P. Lequieu, D. M. Hinckley, J.J. de Pablo (2014), “HighPrecision Characterization of DNA-Nanoparticle Association 

Energies” Soft Matter, DOI: 10.1039/C4SM02573C. Primary 

CHiMaD support.

Zhenwei Yao and Monica Olvera de la Cruz, “Dynamics 

of vacancies in two-dimensional Lennard-Jones crystals,” 

Phys. Rev. E, 90, 062318 (2014). Partial CHIMAD Support.
YEAR IN REVIEW 2014 | 43

ORGANIC BULK HETEROJUNCTION 

POLYMER SOLAR CELLS

Luping Yu (lead)1, Juan de Pablo2, Giulia Galli2, Tobin Marks3

1

Department of Chemistry, University of Chicago

2

Institute for Molecular Engineering, University of Chicago

3

Department of Materials Science and Engineering, Northwestern University

Bulk heterojunction organic solar cells (OSCs) represent an alternative solar energy harvesting system. This use-case 

group is addressing critical issues in material design and synthesis, morphological control and influence on OSC 

performances, device optimization, data collection on material and device characteristics and structure/property 

relationship. New polymers exhibiting low absorption band gaps are synthesized, which act as either electron donors 

or acceptors. New polymers exhibiting proper energy levels match are also developed for use as charge relay or 

energy harvesting antenna in ternary OSCs. New theoretical methods were developed to investigate the electronic 

processes (electronic excitation, charge separation and transport). Large scale modeling approach are being developed to investigate the morphological formation and control. These studies are also closely coupled with NIST 

scientists to fully utilize their expertise in structural and morphological investigation. These new materials and basic 

knowledge gained will deepen our understanding in structure/property relationship and push the performance of 

OSCs towards commercial application.

DESIGN GOALS 

Organic Bulk Heterojunction Polymer Solar Cells use-case 

group explored the development of ternary OSCs system, 

which can further enhance the power conversion efficiency from the binary OSCs system. Efforts involving synthesis, characterization and device evaluation were pursued. 

New polymers with varied band gaps are synthesized and 

evaluated for OPV applications. The group aims to describe 

the morphology of the PTB7/PCBM light-absorbing layer in 

organic photovoltaic devices (see Figure 1 for structures). 

Of particular interest is the interface between crystalline 

PTB7 and amorphous PCBM regions and its relevance to 

the charge transfer through the film. To that end, molecular dynamics models to capture the behavior of these molecules are in development. The group aims to carry out large 

simulations of these systems on GPU-enabled computing 

clusters.

SIGNIFICANT ACCOMPLISHMENTS 

Bulk heterojunction (BHJ) polymer solar cells (PSCs) are a 

popular research subject currently pursued by many groups 

around world.1,2 The state-of-the-art PSCs are composed of 

a polymer donor and a fullerene acceptor as the active layer and their overall photovoltaic performance is dependent 

on many factors, such as the electrical and optical properties of donor polymers, device architectures and interfacial 

layers used.3-13 Among them, the nature of donor polymer 

is without doubt the determining factor in power conversion efficiency (PCE) of PSCs. In this work, we report for the 

first time the study of the influence of polymer polydispersity 

(PDI, ratio between weight-averaged molecular weight and 

number-averaged molecular weight)) on the performance of 

polymer solar cells (PSCs) composed of PTB7 and PC71BM 

as the active layer materials. It was found that polymers exhibiting large polydispersity index contain structural defects 
44 | CENTER FOR HIERARCHICAL MATERIALS DESIGN

Figure 1. (a), Chemical structures of PTB7, PBB3 and PC71BM.

(b), Current-voltage characteristics of PTB7 solar cells with different Ð values. (c), UV-vis absorption spectra of PTB7 with 

different Ð values.

that play the role of energy transfer and charge trapping/

recombination centers. The results highlighted the importance of controlling PDI of donor polymers for polymeric solar cells.

RESEARCH ACCOMPLISHMENTS

The synthetic chemistry for these polymers exhibits side reactions and generally resulted in polymers exhibiting large 

PDI. We found that PDI of PTB7 could dramatically influence 

its photovoltaic performance. Detailed studies led us to conclude that side reactions introduces structural defects that 

could act as energy transfer and trapping centers in the solar cell devices that hinder the charge generation process 

and facilitate charge recombination in BHJ organic photovoltaic (OPV) cells (Figure 

2). Model reaction confirmed this conclusion.

Chemical structures of 

PTB7 and PC71BM are 

shown in Figure 1. Three 

batches of PTB7 used in 

this work have PDI values at 2.1, 3.5 and 4.3 

with similar molecular 

weight values at 100, 118 

and 103 kg/mol, respectively. When these polymers were used as the 

donor materials in solar 

cells under AM 1.5 G illumination at 100 mW/cm2, 

the corresponding current 

Figure 2. Reaction formula for normal polymerization and model reaction.

Figure 3. Structures of previously developed thiophene imide semiconductors and the new sulfonamide semiconductors developed in this study.

density versus voltage (J-V) characteristics of the solar cells 

are shown in Figure 1b, which indicate dramatic differences 

in the solar cell parameters for these devices. The external 

quantum efficiency (EQE) of the three solar cells showed the 

same trend and was found to extend to longer wavelength 

and external quantum efficiency (EQE) values decreased 

dramatically from 300 nm to 700 nm as PDI increases. Since 

Tunneling electronic microscopy (TEM) studies indicated 

that morphology is almost identical in these samples, the effect of structural defects is mainly electronic and manifested 

in deterioration of almost every aspect of associated physical properties in OSCs, ranging from exciton and charge 

dissociation to charge transport. The mobility decreased 

from 5.87×10-4 cm2 V-1 s-1 (PDI 2.1) to 4.39×10-4 cm2 V-1 

s-1 (PDI 3.5) and 3.44×10-4 cm2 V-1 s-1 (PDI 4.3) for PTB7 

hole only devices. The bimolecular recombination of charge 

carriers increased. The structural defects was shown to act 

as trapping centers in the devices. 

Recently, our group initiated synthesis of donor polymers 

and developed the bithiopheneimide (BTI) building block, 

which has been successfully used for the synthesis of a 
YEAR IN REVIEW 2014 | 45

number of high performing organic semiconducting materials including n- and p-type semiconductors for OFET and 

donor materials for OPV (Figure 3, top). Under the CHiMaD 

program we have now developed, based on DFT molecular 

orbital calculations, a new acceptor unit for organic semiconductors based on a cyclic sulfonamide, bithiophenesulfonamide (BTSA) (Figure 3, bottom).

In an initial effort we developed an optimized method for the 

BTSA core synthesis allowing for the multi-gram scale synthesis of this building block (Figure 4). Next, we studied several alkyl-functionalized derivatives of the BTSA unit so that 

to tune the solubility, aggregation, and processing parameters of the corresponding polymers. Thus, several synthetic methodologies for the functionalization were explored, 

including simple alkylation and more sophisticated procedures based on catalytic alkylation. Furthermore, due to the 

asymmetric reactivity of this core, it was important to develop selective methods for the mono- and di-bromination of 

the BTSA unit, which is critically important for the ultimate 

synthesis of the specifically-designed polymeric and molecular semiconductor targets.

Several BTSA molecular and polymeric systems were synthesized. Figure 4 shows only two examples. The BTSA 

Figure 4. Synthesis of the bithiophenesulfonamide (BTSA) core, halogenated derivatives, and two representative molecular and polymeric semiconductors.

based small molecule SM1 was prepared, and single 

crystal analysis of 

SM1 revealed interesting packing features due to close 

contacts between 

the SO2 group and 

H atoms for adjacent molecules. 

Furthermore, we 

synthesized and 

fully characterized 

a series of 12 new 

BTSA polymers, 

with the BTSA-dithiophene copolymer P1 being an 

example (Figure 

4, bottom). All the 

polymers were fully characterized in 

regard to structure, 

purity, and molecular mass. Currently their implementation 

into OFETs and OPV is in progress. Preliminary results are 

promising with OFET mobilities approaching 0.1 cm2/Vs and 

OPV power conversion efficiencies > 5%. Figure 5 shows 

representative FET characteristics for two polymers and 

OPV data for P1.

On the simulation front, current generic molecular dynamics force fields such as Optimized Potentials for Liquid Simulations (OPLS) and Assisted Model Building with Energy 

Refinement (AMBER) do not provide parameters to fully 

describe these aromatic molecules. Daniel Reid (de Pablo 

group), is using and developing methods to generate these 

missing parameters, specifically the bond, angle, dihedral, 

and partial charge parameters for atoms in and near aromatic regions.

Given the long timescales necessary to see polymer crystallization occur, we would like to hold bonds, angles, and 

dihedrals with high vibrational frequencies rigid, as this will 

allow for a larger simulation timestep. A frequency analysis 

was performed on PTB7 using Gaussian. It was found that 

vibrational modes of roughly 450 cm-1 or greater occurred 

within the rings while modes of 175 cm-1 or less occurred 

around bonds connecting rings. This large difference and 
46 | CENTER FOR HIERARCHICAL MATERIALS DESIGN

Figure 5. Top: Examples of OFET (bottom-gate, top-contact) performance for the indicated BTSA-bithiophene copolymers. CF means 

the polymers were processed from CHCl3

; CB means processed from chlorobenzene. Bottom: Example of OPV J-V and EQE characteristics for a P1-based inverted OPV cell.

high frequency of the ring vibrations 

suggests that we can perform simulations with the aromatic rings held 

rigid without significant loss in realism, allowing us to use a much larger 

time step and sample longer timescales. The frequency analysis was 

done using b3lyp/6-311g on a single 

monomer.

Since the rings are fairly stiff, the parameters governing the bonds connecting the aromatic rings will be key 

in reproducing the polymer’s properties. To calculate the relevant bond, 

angle and dihedral parameters, we 

use electronic structure calculations to find the energy at points in 

each parameter’s stretch, bend or 

twist, then employ error minimization techniques to assign relevant 

values. Several methods and basis sets were used, including HF/6-

311g, b3lyp/6-311g**, and mp2/

cc-PVDZ. Unfortunately, each pairing of method and basis set gives an 

appreciably different result. Highertheory methods such as b3lyp and 

mp2 agree on equilibrium configurations, though show different energy 

barriers to rotation. For the time being, these equilibrium configurations 

will be used with placeholders for 

more refined force constants.

The delocalized π structure of the 

aromatic rings persists over multiple monomers. We find that the dihedrals between groups of rings 

vary with polymer length until the chain consists of roughly 

3 monomers. Since the electronic structure methods used 

scale with either n4 or n5, where n is the number of electrons, this limits the level of theory that can be accessed for 

these more relevant longer chains.

To fully specify these molecular dynamics force fields, partial charges for atoms in and near the aromatic regions must 

also be determined, as OPLS does not provide values for atoms in these situations. Purely electronic structure-driven 

approaches such as natural bonding orbitals give results inconsistent with OPLS in test cases. We achieve better results 

by modifying and using a method called RESP, restrained 

electrostatic potential fitting. This method attempts to minimize the error between the electric potentials generated 

from electronic structure calculations and those generated 

by a set of point charges (such as one for each atom). Error 

between these potentials is minimized at a series of coordinates formed in a shell around the molecule. The standard 
YEAR IN REVIEW 2014 | 47

method adds a quadratic restraint on partial charges to prevent the values of highly shielded charges from becoming 

too large or otherwise unphysical. This method closely reproduces values given by OPLS in test cases without significant shielding.

To make this method more robust in cases with shielded 

atoms, we have modified it to weight regions along the potential-fitting shell such that each atom is of nearly equal 

importance in the fitting. We have also allowed the shell to 

become thicker in regions near shielded atoms. These modifications give more physical partial charges for molecules 

such as TPD, where atoms such as nitrogen are buried.

The Galli group is performing electronic property modeling 

on polymer solar cells. He has focused research on the proper tools for identifying the band gap size and location of donors and acceptors in the ternary blend synthetized by Lu et. 

al. For this work, a cascade of energy levels was measured, 

and it was suggested that such a cascade improves charge 

separation and prevent charge recombination. The proposed 

Figure 6. HOMO and LUMO energy levels of the ternary mixture described in ref. [1] (experiment), compared with DFT, periodic calculations (Theory).

Table 1. Fundamental and optical band gaps for PTB7, PID2, and PC71BM from theory and experiment

cell consists of poly-3-oxothieno[3,4-d]isothiazole-1,1-dioxide/benzodithiophene (PID2) and polythieno[3,4-b]-thiophene/benzodithiophene (PTB7), along with the common 

electron acceptor [6,6]-phenyl C71 butyric acid methyl ester 

(PC71BM). The experimental values, along with our best theoretical estimates, are presented in Figure 6. In our calculations PTB7 and PID2 are approximated as periodic systems 

using plane wave Density Functional Theory (DFT) (B3LYP for energy levels and PBE for geometries) with a single 

monomer in the periodic cell, neglecting the 2-ethyl hexyl 

side chains, so as to reduce computational cost. Our preliminary results support the ordering of the band edge positions presented.

A summary of our findings for the band gaps of all systems of Figure 6 is presented in Table 1. We use long-range 

corrected ωB97X/def2-TZVP for the TD-DFT estimates 

of the optical gap and B3LYP/def2-TZVP for estimates of 

the fundamental gap. All isolated system geometries are 

PBE-D3BJ-gCP/def2-TZVP, which differs minimally from B3LFigure 7: Relevant dihedrals of PTB7

Figure 8: Relevant dihedrals of PID2
48 | CENTER FOR HIERARCHICAL MATERIALS DESIGN

Figure 10: PES along dihedrals of PID2

Figure 9: PES along dihedrals of PTB7

Figure 11: The HOMO/LUMO levels of PTB7 versus structural deformation along dihedral angles

Figure 12: The HOMO/LUMO levels of PID2 versus structural deformation along dihedral angles

YP-D3BJ-gCP/def2-TZVP geometries.

Our band gaps for periodic systems 

systematically overestimate experimental values, which is surprising as we 

were expecting the opposite error, with 

respect to experiments, and the reasons of this overestimate is under investigation. However we note that the trend 

of increasing band gap size reported is 

reflected in our calculations. The band 

gap of the isolated monomers of PTB7 

and PID2 are overestimated as well, 

which is to be expected in the case of 

π-conjugated systems. Efforts to model 

these polymers in the past have varied, 

somewhat inconsistently, in their treatment of the flexible dihedrals contained 

within the polymer chains [2, 3, 4, 5], 

which are visualized in Figures 7(PTB7) 

and 8(PID2). We have performed our 

own potential energy scans along the 

dihedrals and chosen the optimal conformers for our own use in Figure 6 and 

Table 1. The potential energy surfaces along these dihedrals are presented in Figures 9 and 10, and the HOMO/

LUMO separation is presented in Figures 11 and 12. Barriers of 3-6 kcal/mol 

are present in these dihedral rotations. 

The band gap has significant variation 

with the rotation along these dihedral 

angles, especially when π-conjugation 

is severely disrupted, overall suggesting that structural deformation must be 

accounted for in order to reproduce the 

band gap accurately. Work is in progress to fully understand these issues.

In short, we have modeled the band gap 

size and location for the ternary blend of 

Lu, et al., supporting the cascade alignment of the HOMO energy levels. Ongoing work is focused on refining our 

models, in particular, on the investigation of the connection between structure and energy level alignment.
YEAR IN REVIEW 2014 | 49

Organic Bulk Heterojunction Polymer Solar Cells 

Group Researchers and Their Roles 

Name Position

Institution/

Affiliation Project Title (PI)

Luping Yu PI UC/Chem OPV solar cells

Juan de Pablo PI UC/IME Simulation of OPV Materials

Giulia Galli PI UC/IME

Tobin Marks PI NU/MSE

Ferdinand Melkonyan Postdoctoral Researcher UC/Chem Novel Materials for Optoelectronic Application

Zhenxu Cai Postdocoral Researcher UC/Chem Novel Materials for OPV Application (Yu)

Mathew Goldey Postdoctoral Researcher UC/IME Simulation of OPV Materials (Galli)

Luyao Lu Graduate Student UC/Chem Novel Materials for OPV Application (Yu)

Daniel Reid Graduate Student UC/IME Simulation of OPV Materials (de Pablo)

COLLABORATIONS

Morphological studies of Ternary OPV systems

Collaborators: Dean M. DeLongchamp

Materials Science and Engineering Division, NIST

Materials Synthetic Design and Evaluation 

Collaborators: Antonio F. Facchettia, Wei Zhaob

aDepartment of Chemistry, Northwestern University

bPolyera Corporation

Polymeric Materials Microstructural Characterization 

by Synchrotron Grazing Incidence Wide Angle

X-ray Scattering

Collaborators: Lin X. Chen

Department of Chemistry, Northwestern University

Chemical Sciences and Engineering Division

Argonne National Laboratory

Molecular Materials Microstructural Characterization 

by Thin Film X-ray Diffraction

Collaborators: Michael Bedzyk

Department of Materials Science and Engineering 

Northwestern University

Materials Electronic Structure Computation

Collaborators: Rocío Ponce Ortiz

Departamento Química Física

Universidad de Málaga, Spain 
50 | CENTER FOR HIERARCHICAL MATERIALS DESIGN

Consonant with NIST’s unique role in the MGI, a major thrust of CHiMaD will be designing and 

prototyping tools that enable materials design. CHiMaD has identified two major thrust areas: 

database development and tools for computational design. 

DATABASES

In collaboration with NIST scientists, CHiMaD aims to make 

significant advances in data-driven informatics to accelerate 

material discovery, assessment, research and innovation. 

We are moving on a number of fronts in this area:

■CALPHAD protodata databases (Olson, Voorhees, 

Wolverton, Campbell, Kattner): Assessment of phase 

diagrams and the resulting free energy functions 

provide essential information for materials design. 

Unfortunately, the data upon which these assessments 

rely are frequently lost following the assessment 

making it extremely difficult to easily modify existing 

ones. We are developing publicly accessible CALPHAD 

protodata databases that contain fundamental phaselevel raw data consisting of, for example, measured 

phase relations (tie-lines), calorimetric measurements, 

DFT-predicted quantities such as elastic constants of 

existing and new compounds. Furthermore, with the goal 

of creating a community wide pre-Calphad protodata 

database, we will also be inviting researchers with 

other existing databases to contribute. As examples, 

we are creating these databases for the alloy systems 

under investigation in the CHiMaD use-case groups, 

specifically: Si-alloys for structural applications and 

Co-based alloys for high-temperature applications. 

Please see Precipitation-Strengthened Alloy and Si 

Alloys use-case group for more information.

■NanoMine (Brinson, Chen): This database contains 

processing, structure and property parameters for 

polymer nanocomposite systems. The system employs 

the new NIST Materials Data Creator system. Detailed 

templates and material data structures (“schemas”) 

have been developed to account for processing, 

structure and property information of material systems. 

Please see Polymer Matrix Materials use-case group 

for more information.

■Polymer Design (de Pablo, Foster, Nealey, Jaeger): 

This database will contain characteristic properties 

for design of polymer blends and copolymers for 

engineering applications, including directed selfassembly. Please see Directed Self Assembly of Block 

Copolymers Films for Lithographic Applications usecase group for more information.

■Prediction of Phase Behavior of Polymers and 

Polyelectrolytes (de Pablo, Foster, Nealey, Tirrell, 

Jaeger, Douglas, Freed): This database will employ 

both data from both simulations and experiments 

on polymer blends, polyelectrolytes, copolymers, 

focusing on phase behavior to comprehend the stability 

of complexes with respect to salt, with a variety of salt 

valences. Please see Biomaterials use-case group for 

more information.

■Materials Data Facility (Foster, Choudhary, de

Pablo, Voorhees): With the advent of techniques that 

can measure the three-dimensional micro- and nanostructure of a material as well as its temporal evolution, 

the materials community is facing the challenge of 

handling, storing, and sharing large datasets. We are 

developing a large-data storage and sharing system to 

address this challenge. 

SOFTWARE TOOLS

CHiMaD is also developing computational tools that can be 

used to follow the evolution of the micro- and nano-structure 

of both soft and hard matter: 

■Community Standard Phase Field Methods

(Voorhees, Warren): In the hard matter area, one of the 

key computational materials approaches is the phase 

field method. While the method has been used with 

TOOL DEVELOPMENT
YEAR IN REVIEW 2014 | 51

great success in a research environment, its utility as 

a materials design tool is hampered by the lack of a 

community standard phase field code. The objective of 

this work will be to create such a community standard 

by convening workshops and facilitating interactions 

between the many groups working in this area.

■Fatigue Modeling using Phase Field Methods:

Umantsev (FSU) is developing phase field methods 

specifically for fatigue modeling. This code, in 

collaboration with the nonlocal mechanics methods 

developed by Liu (NU), will be used in the PrecipitationStrengthened Alloys use-case group to follow 

the evolution of fatigue. Please see PrecipitationStrengthened Alloys use-case group for more 

information.

■Locally Adaptive Time Stepping for Parallel 

Architectures: Chopp (NU) developed a modified 

2D parallel implementation of a recently introduced 

algorithm called LATS (Locally Adaptive Time 

Stepping). This method, while considering a domain 

subdivided into multiple subdomains, allows each 

subdomain to advance according to its own time step 

size. In context of solving a parallel architecture, such 

as a phase-field type model, allowing each subdomain 

to progress with a different time step leads to being 

able to concentrate the computational effort where it is 

needed, for example, at the interfaces. 

■Theoretically Informed Coarse Grained Simulations

(de Pablo, Olvera de la Cruz, Luijten): The primary 

computational tool for prediction of structural, 

thermodynamic, and dynamic properties of functional 

polymers and block polymers must rely on a coarsegrained modeling approach capable of describing the 

structure and dynamics of soft matter, including block 

polymers during solvent mediated assembly, both in the 

bulk and in films. Our proposed strategy will be based 

on the “Theoretically Informed Coarse Grained” (TICG) 

simulation formalism, developed by de Pablo. TICG 

simulations rely on a particle-based representation, 

thereby providing a means to describe large, threedimensional systems in the presence of fluctuations 

and their corresponding dynamics. A beta version of 

this software has been released to INTEL. Please see 

Directed Self Assembly of Block Copolymers Films for 

Lithographic Applications use-case group for more 

information.

EXPERIMENTAL TOOLS

Experimental data is required in a number of the databases 

that we are developing. These data are being created using 

both standard approaches and new tools that we are developing. These new tools are:

■Rapid Assessment of Phase Relations: Co-alloy

team, part of the Precipitation-Strengthened Alloys 

use-case group, identified the highest priority alloying 

elements for thermodynamic and mobility database 

development. This then formed the basis for thin-filmbased initial high-throughput experiment design by 

Chung and Bedzyk of Northwestern University that 

employs the Advanced Photon Source characterization 

facilities at the Argonne National Laboratory. Please 

see Precipitation-Strengthened Alloys use-case group 

for more information.

■Resonant Soft X-ray Scattering (Nealey): (RSoXS) 

The Nealey group is collaborating with the Argonne 

National Laboratory to commission a new Intermediate 

X-ray Energy beamline at Sector 29 of the Advanced 

Photon Source that will allow access to a larger area 

of reciprocal-space than standard two-and three-circle 

diffractometers typically found in RSoXS endstations. 

This new capability will be used to follow the evolution of 

polymer nanostructures for the directed self-assembly 

use-case group. Please see Directed Self Assembly of 

Block Copolymers Films for Lithographic Applications 

use-case group for more information.
52 | CENTER FOR HIERARCHICAL MATERIALS DESIGN

ASM Materials Genome ToolKit 

One of the objectives of CHiMaD is to expand the knowledge of current and future generation of researchers and 

scientists by providing opportunities for them to explore 

and learn core computational materials engineering tools 

and methods. To facilitate this objective, CHiMaD has partnered with ASM International to provide a number of qualified undergraduate level Materials Science and Engineering 

programs in United States with ASM Materials Genome 

Toolkit at a substantially reduced license fee. The toolkit 

provides a bundle of the ThermoCalc, DICTRA and PRISMA

software of general applicability across materials classes. 

Building on our unique successes in metals applications, 

the bundle includes a first set of thermodynamic and mobility databases for metallic alloy systems. As a part of this 

initiative, the selected programs are expected to actively 

participate in the ASM Undergraduate Design Competition.

Workshop, Symposium and Short Courses

Paul Nealey, Ricardo Ruiz and Juan de Pablo (UChicago) co-organized the SPIE Short Course on Directed Self 

Assembly at the SPIE 3-beam Conference, San Jose, California, USA. (02/14).

Gregory Olson (NU) hosted the 30th Annual Review Meeting for the Materials Technology Laboratory/Steel Research 

Group at Northwestern University, Evanston, Illinois, USA. 

(03/14)

Carelyn Campbell (NIST) organized the NIST/CHiMaD 

CALPHAD Data Workshop: Building a Data Infrastructure 

for Phse-Based Materials Data at NIST. During the workshop, plans were made for the pre-CALPHAD protodata repository to be hosted at NIST, and the metal alloy project 

was selected as the first to begin populating this database 

using a specified DSpace format. (04/14)

Juan de Pablo (UChicago), Jack F. Douglas (NIST) , Frederick R. Phelan Jr. (NIST) were co-organizers of the Workshop on Coarse-Grained Modeling of Polymers and Soft 

Materials for the Materials Genome Initiative at the National Institute of Standards and Technology, Gaithersburgh, 

Maryland, USA. (09/14)

Wei Chen (NU) acted as the review coordinator and special 

session organizer of Multiscale Materials and Product Design, ASME International Design Technical Conference, Design Automation Conference 2014, Buffalo, New York, USA. 

(09/14)

Juan de Pablo (UChicago), Joseph Kline (NIST), Roel 

Gronheid (IMEC), Philip Hustad (Dow Electronic Materials)

co-organized symposia on Directed Self-Assembly for 

Nanomanufacturing, at Materials Research Society Fall 

Meeting, Boston, Massachusetts, USA. (12/14)

Erik Luijten (NU) taught a 10-hour course on Computational Modeling of Soft Matter at the International Winter School 

on Soft Matter at Shanghai Jiao Tong University, Shanghai, 

China. This course was attended by ~40 undergraduates 

and beginning graduate students from Shanghai Jiao Tong 

University and other institutions in China and Singapore. 

(12/14)

Vacuum Science and Macroscopic AFM Workshop, University of Chicago, Chicago, Illinois, USA. This workshop 

featured science demos (macroscopic AFM for examining 

corrugated surfaces) for visiting elementary school parents 

and teachers, and was organized by the Directed Self-Assembly of Block Copolymers Films for Lithographic Applications Use-case Group.

I

n its first year CHiMaD has initiated several outreach programs including the dissemination of the 

ASM Materials Genome toolkit with the aim to encourage educational institutions to incorporate 

materials design into their curricular, and the MGI Seminar Series, which hosted ten international researchers in 2014. Our researchers also organized numerous workshops and short-courses around 

the world as well as symposia in prestigious conferences. Many of the events are organized in conjunction with NIST researchers, strengthening CHiMaD-NIST collaborations.

OUTREACH
YEAR IN REVIEW 2014 | 53

Seminars

The following seminars were hosted as a part of the MGI 

Seminar Series in 2014. 

Field Simulations of Phase Transformations in Materials

Alexander Umantsev

Professor, Fayetteville State University, USA.

May 14 (Questek), May 20 (ANL), May 21 (NU), 2104 

The Exascale Co-design center for Materials in 

Extreme Environments (ExMatEx)

James Belak

Co-Director, DOE/ASCR Exascale CoDesign Center for Materials in Extreme Environments Lawrence Livermore National Laboratory, USA.

May 27, 2014

A Computational Approach to Materials Design

Shengyen Li

Postdoctoral Researcher, Materials Science and Engineering Division, National Institute of Standards and Technology

June 2, 2014

Computational Materials Design for High Plastic 

Formability of Mg Alloys

Motohiro Yuasa

Researcher, National Institute of Advanced Industrial Science and Technology, Japan 

August 21, 2014

Introduction of Thermodynamic-Fluctuation-Based 

Nuclea-tion to Phase-Field Model

Machiko Ode

Senior Researcher, Computational Materials Science Unit, 

National Institute for Materials Science, Tsukuba Ibaraki, 

Japan 

August 28, 2014

Martensitic Transformations in Engineering Materials: 

A Mesoscale Study

Hemantha Yeddu

Postdoctoral Researcher, Los Alamos National Laboratory

October 6, 2014

An Insider View on Scientific Publishing

Baptiste Gault

Journal Publisher, Materials Science, Elsevier Ltd.

October 9, 2014

The Materials Genome Initiative and the Data 

Revolution

Carelyn Campbell

Materials Science and Engineering Division, National Institute of Standards and Technology, USA

October 28, 2014

Integrated Computational Materials Engineering 

and the Materials Genome Initiative: Activities at the 

Hero-m Center

Annika Borgenstam

Materials Science and Engineering, KTH, Sweden

December 4, 2014

Architectured Steel

Toshihiko Koseki

Professor, Department of Materials Engineering, University 

of Tokyo, Japan

December 11, 2014

Other Activities

CHiMaD organized the CHiMaD Inaugural Meeting, bringing 

together more than 100 researchers from its partner institutions. (05/14)

CHiMaD website went live at chimad.northwestern.edu. 

(05/14)

Illinois Science & Technology Council newsletter, Catalyst, 

highlighted CHiMaD in their October issue with an article authored by CHiMaD co-director Peter Voorhees and associate director Begum Gulsoy entitled Illinois Demonstrates 

the Materials Genome Initiative: Center for Hierarchical Materials Design. (10/14)

Wei Chen, serves as the Director of Predictive Science & 

Engineering Design (PS&ED) Cluster at Northwestern University. The current focus of the PS&ED cluster is on research topics that address predictive science, design, and 

manufacturing of emerging materials and structure systems. 

Several PS&ED supported cluster projects in 2014-2015 are 

focused on Materials Genome Initiative. 

Science Club, Andrew Carnegie School, Chicago, Illinois, 

USA. A weekly club for middle school students was organized by the members of the Directed Self-Assembly of 

Block Copolymers Films for Lithographic Applications 

Use-case Group for interactive science demonstrations. 
54 | CENTER FOR HIERARCHICAL MATERIALS DESIGN

HONORS AND AWARDS

L. Catherine Brinson

Department of Materials Science and Engineering

Department of Mechanical Engineering

Northwestern University

Nadai Medal, American Society of Mechanical Engineering

Mark C. Hersam

Department of Materials Science and Engineering

Northwestern University

MacArthur Fellowship, 2014

American Association for the Advancement of Science 

(AAAS) Fellow, 2014

Wing K. Liu

W. P. Murphy Professor

Department of Mechanical Engineering

Northwestern University

President of the International Association for 

Computational Mechanics

Chair of the US National Committee on Theoretical and 

Applied Mechanics within the National Academies

2014 highly cited researcher in the field of Computer 

Science and listed as a member of the World’s Most 

Influential Scientific Minds by Thompson Reuters for the 

period 2002 through 2012.

Tobin J. Marks

Department of Materials Science and Engineering

Department of Chemistry

Northwestern University

2013 Alan G. MacDiarmid Medal, University 

of Pennsylvania 

2014 Sir Geoffrey Wilkinson Medal, UK Royal Society 

of Chemistry

2013 Bayer Lecturer, Cornell University

In 2014, CHiMaD researchers were recognized for contributions to their fields 

and were the recipients of numerous prestigious honors and awards. CHiMaD 

graduate and undergraduate students were also recognized among their peers 

for their work regarding CHiMaD related research projects. 

2013 Bayer Lecturer, University of Pittsburgh 

2014 M.D. Rausch Lecturer, University of Massachusetts, 

Amherst

2014 Xingda Lecturer, Peking University 

2014 Frontiers in Molecular Sciences Lecturer/Professor, 

Chinese Academy of Sciences

2014 Distinguished Clean Energy Lecturer, University 

of Washington

2014 R.B. King Lecturer, University of Georgia

Gregory B. Olson

W. P. Murphy Professor

Department of Materials Science and Engineering,

Northwestern University

Honorary Doctorate, Royal Institute of Technology (KTH), 

Sweden

Evanston/Northwestern Catalyst Award for 

Entrepreneurship 

David N. Seidman

W. P. Murphy Professor 

Department of Materials Science and Engineering,

Northwestern University

ASM International Edward DeMille Campbell Memorial 

Lectureship, 2015

AIME Honorary Member Award—nominated by the TMS 

Fellow, American Association for the Advancement 

of Science

Steven J. Sibener

Professor

Department of Chemistry

University of Chicago

Two-day “Science at the Interface” SibenerFest Symposium 

with 150 attendees, March 28–29 2014. This conference 

brought together a diverse collection of scientists from 
YEAR IN REVIEW 2014 | 55

around the world whose common focus is understanding 

the complex chemistry and physics that take place at 

interfaces while emphasizing the intuitive approach that 

has often characterized Steve’s many contributions to the 

field. In addition to the Scientific Conference, there was 

also a Pedagogical Tutorial for students and postdoctoral 

researchers.

Wei Xiong 

Research Associate (Olson Research Group)

Materials Science and Engineering

Northwestern University

TMS Young Leader Award

Gurdaman Khaira 

Graduate Student (De Pablo Research Group)

University of Chicago

First Prize — Best Paper Award, 2014 International 

TECHON Conference

Luayo Lu

Graduate Student (Yu Research Group)

Department of Chemistry

University of Chicago

Selected speaker for “Excellence in Graduate Polymer 

Research,” American Chemical Society symposium

Qianqian Tong

Graduate Student (Sibener Research Group)

Department of Chemistry

University of Chicago

Yang Cao-Lan-Xian “Best Thesis Award” in the 

Department of Chemistry

Jonathan Raybin 

Graduate Student (Sibener Research Group)

Department of Chemistry

University of Chicago

First Prize Graduate Student Poster Presentation Award 

at the 2014 Prairie Chapter Meeting of the American 

Vacuum Society

Joan Shiu Award for Student Service

Hongyi Xu1

, Ruoqian Liu2, Alok Choudhary1, 

and Wei Chen2

1

Department of Electrical Engineering and 

Computer Science

2Department of Mechanical Engineering 

Northwestern University

Best Paper Award, 40th ASME Design Automation 

Conference, “A Machine Learning-Based Design 

Representation Method for Designing Heterogeneous 

Microstructures,” IDETC2014-34570, August 17-20, 

Buffalo, New York, 2014.

R. DeBlock, K. Jorgensen, C. Lynsky, E. Pang, 

S. Plain, P. J. Santos, R. Wang 

Materials Design (MAT390) class undergraduate students

(Olson, PI)

3rd Prize, ASM 2014 undergraduate design competition 
56 | CENTER FOR HIERARCHICAL MATERIALS DESIGN

PUBLICATIONS

De Pablo, J. J., Jones, B., Kovacs, C. L., Ozolins, V., & Ramirez, A. P. (2014). The Materials Genome Initiative, the interplay of experiment, theory and computation. Current Opinion in Solid State and Materials Science, 18(2), 99-117.

Hoffmann, K. Q., Perry, S. L., Leon, L., Priftis, D., Tirrell, M., & de Pablo, J. J. (2015). A molecular view of 

the role of chirality in charge-driven polypeptide complexation. Soft matter, 11(8), 1525-38.

Jariwala, D., Sangwan, V. K., Seo, J. W. T., Xu, W., Smith, J. N., Kim, C. H., Lauhon, & L. J., Hersam, M. 

C. (2014). Large-area, low-voltage, anti-ambipolar heterojunctions from solution-processed semiconductors. Nano letters.

Khaira, G. S., Qin, J., Garner, G. P., Xiong, S., Wan, L., Ruiz, R., Jaeger, M. H., Nealey P. F., & de Pablo, 

J. J. (2014). Evolutionary Optimization of Directed Self-Assembly of Triblock Copolymers on Chemically 

Patterned Substrates. ACS Macro Letters, 3(8), 747-752.

Kim, I. S., Sangwan, V. K., Jariwala, D., Wood, J. D., Park, S., Chen, K. S., Shi, F., Ruiz-Zepeda, F., 

Ponce, A., Jose-Yacaman, M., Dravid, V. P., Marks, T. J., Hersam, M. C., & Lauhon, L. J. (2014). Influence of Stoichiometry on the Optical and Electrical Properties of Chemical Vapor Deposition Derived 

MoS2. ACS nano, 8(10), 10551-10558.

Lequieu, J. P., Hinckley, D. M., & de Pablo, J. J. (2015). A Molecular View of DNA-conjugated Nanoparticle Association Energies. Soft Matter, Advance Article. DOI: 10.1039/C4SM02573C.

Peters, B. L., Rathsack, B., Somervell, M., Nakano, T., Schmid, G., & de Pablo, J. J. (2014). Graphoepitaxial assembly of cylinder forming block copolymers in cylindrical holes. Journal of Polymer Science 

Part B: Polymer Physics. DOI: 10.1002/polb.23652.

Tong, Q., Malachosky, E. W., Raybin, J., Guyot-Sionnest, P., & Sibener, S. J. (2014). End-to-End Alignment of Gold Nanorods on Topographically Enhanced, Cylinder Forming Diblock Copolymer Templates and Their Surface Enhanced Raman Scattering Properties. The Journal of Physical Chemistry C, 

118(33), 19259-19265.

Xu, H., Liu, R., Choudhary, A., and Chen, W., “A Machine Learning-Based Design Representation Method for Designing Heterogeneous Microstructures”, IDETC2014-34570, August 17-20, Buffalo, New 

York, 2014. 

Yao, Z., & de la Cruz, M. O. (2014). Dynamics of vacancies in two-dimensional Lennard-Jones crystals. 

Physical Review E, 90(6), 062318.
YEAR IN REVIEW 2014 | 57

Center for Hierarchical Materials Design

O.T. Hogan Biological Sciences Building

2205 Tech Drive, Suite 1160

Evanston, IL, 60208

(847)-467-NAIS(E) | (847)-467-6247

chimad@northwestern.edu

chimad.northwestern.edu

CONTACT
IN PARTNERSHIP WITH 

CHiMaD is supported by U.S. Department of Commerce, 

National Institute of Standards and Technology, 

under financial assistance award number 70NANB14H012.