bluuebunny/crossref_metadata_2025 huggingface dataset:
Voltage Variation of Lithium Transition Metal Oxides Caused by the Migration of Cations
Authors: Chen Zonghai , Li Yan , Qin Yan , Ren Yang , Amine Khalil
Published: February 2020

In the past decade, major effort has been devoted to understanding the structure and the electrochemical benefit of lithium-rich transition metal oxides for high-energy density lithium-ion batteries because of their potentially high specific capacity of up to 250 mAh g-1 when the materials are electrochemically activated. However, a consensus has not been established yet on the local structure of this class of materials. It has been reported that the excess portion of lithium was homogeneously dissolved in the matrix of the transition metal layer (3b sites of space group 166 or R-3m), forming a solid solution. On the other hand, Thackeray and others believed that the excess lithium on 3b sites has a strong tendency to pair with manganese and segregate to form Li2MnO3 domains. Despite the discrepancy in these researchers’ findings on the local structure, both sides agreed that the excess lithium can be electrochemically activated at a potential above 4.5 V vs. Li+/Li to deliver more reversible specific capacity, offering great opportunity for use of these lithium-rich transition metal oxides as high-capacity cathode materials. However, the structural instability of the material after electrochemical activation causes a continuous decrease in its working potential, a phenomenon which we will call voltage fade, that hinders the commercial deployment of this class of high-capacity cathode materials. The migration mechanism of transition metal in the oxygen framework was investigated here to illustrate the voltage decay of lithium-rich transition metal oxides.  For the first time, a voltage increase was observed during the cycling of a lithium manganese oxide spinel within a wide potential window.  It was demonstrated that the voltage variation of these lithium transition metal oxides was caused by the continuous migration of transition metal ions, mostly Mn2+ and Co4+, between the octahedral and tetrahedral sites of oxygen framework, resulting in the ration change between the spinel component and layered component.

Figure 1
Fabrication of Li-Rich Cathodes with Enhanced Stability and Electrochemical Performances for Lithium Ion Battery
Authors: Li Liping , Li Guangshe
Published: February 2020

Li-ion batteries, an ultimate rechargeable energy storage, play more and more important roles in modern people's life and activity. Developing energy storage equipments with high specific energy and stability that can work at very high charge-discharge rate is crucial for efficient use of energy at power-hungry laptops, cameras, mobile phones and electric cars. From the perspective of chemistry, these can be achieved by raising voltage window, controlling the structure transition during charge/discharge proces, and significantly improving the kinetics of lithium ions and electrons in electrode. Here, we will address the our recental finding for enhancing stability and electrochemical performances of Li-rich cathode materials, especially on (1) an simple methode to resolve kinetics problems of ultrafast charging-discharging Li-ion batteries by fabrication of layered LiMO2 (M=Mn, Co, Ni) hexagonal sheet exposing with facets {101}, (2) in situ potassium doping to stabilize the host layered structure by prohibiting the formation of spinel structure during cycling, and (3) optimizing the interactions between the components Li2MnO3 and LiMO2 in the Li-rich oxide (i.e. stabilizing the layered structure through Li2MnO3 and controlling Li2MnO3 activation through LiMO2).

References:

[1]     Luo Dong, Li Guangshe, Fu Chaochao, Zheng Jing, Fan Jianming, Li Qi, Li Liping,
Adv. Energy Mater.
, 2014, 1400062

[2]     Qi Li, Guangshe Li, Chaochao Fu, Dong Luo, Jianming Fan, Liping Li,   
ACS Appl. Mater. Interfaces
, 2014, 6, 10330−10341

[3]     Qi Li, Guangshe Li, Chaochao Fu, Dong Luo, Jianming Fan, Dongjiu Xie, Liping Li,
J. Mater. Chem. A
, 2015, DOI: 10.1039/c5ta00929d

[4]     Qi Li, Guangshe Li, Chaochao Fu, Dong Luo, Jianming Fan, Jing Zheng, Dongjiu Xie, Liping Li,
Electrochim. Acta
154, 2015, 249–258

[5]     Fu Chaochao, Li Guangshe, Luo Dong, Zheng Jing, Li Liping, 
J. Mater. Chem. A
, 2014, 2, 1471-1483

[6]     Luo Dong, Li Guangshe, Guan Xiangfeng, Yu Chuang, Zheng Jing, Zhang Xinhui, Li Liping,
J. Mater. Chem. A
, 2013, 1, 1220-1227

[7]     Zhang Xinhui, Luo Dong, Li Guangshe, Zheng Jing, Yu Chuang, Guan Xiangfeng, Fu Chaochao, Li Liping,
J. Mater. Chem. A
, 2013, 1, 9721-9729J.F. Gieras, Permanent Magnet Motor Technology, 3rd ed., CRC: BocaRaton, 2010, pp. 68-73.
Nanoconfinement of Metal Hydride-Based Li-Ion Battery Anodes
Authors: Huen Priscilla , Jensen Torben René , Ravnsbaek Dorthe Bomholdt
Published: February 2020

Metal hydrides as anode materials for Li-ion batteries are receiving increased attention due to the high theoretical capacities and low cost of many of these materials. Among conversion-type electrodes, metal hydrides have relatively high energy density and low polarization (ΔV ≈ 0.2 V) [1]. Similar to other conversion-type materials, the main challenges of metal hydrides are the large volume change and particle growth during operation, which leads to poor capacity retention.

Nanoconfinement, a bottom-up approach for preparing nano-sized materials, effectively limits the size of particles to the pore size of the nano-scaffold. In addition, nano-scale diffusion distances facilitate transport of Li-ions and enhance the kinetics of the conversion reactions.

In this study, we present the improved electrochemical performance of sodium aluminum hydride, NaAlH4 through nanoconfinement in mesoporous carbon scaffolds with different pore sizes. NaAlH4 has a theoretical capacity of 1985 mAh g-1. Although NaAlH4 is shown to be electrochemically active, large capacity fade is observed already during the first charge with a typical reversible capacity of only 30 % [2]. In contrast, nanoconfined NaAlH4 shows a 70 % recovery of capacity during the initial discharge-charge cycle.

We have investigated the reaction mechanism of the conversion reaction by operando synchrotron radiation powder X-ray diffraction (SR-PXD), and found important differences between nano-confined and non-confined samples. The mechanism and electrochemical performance are further investigated by detailed electroanalysis and conductivities of NaAlH4 electrodes are examined by electrochemical impedance spectroscopy (EIS). The electrochemical reactivity of empty carbon scaffold and its contribution to the battery performance will also be discussed.

This work is supported by European Marie Curie Actions under ECOSTORE grant agreement n° 607040.

References

[1] Y. Oumellal, A. Rougier, G. A. Nazri, J-M. Tarascon, L. Aymard, Nat. Mater., 2008, 7, 916.

[2] J. A. Teprovich, J. Zhang, H. Colón-Mercado, F. Cuevas, B. Peters, S. Greenway, R. Zidan, M. Latroche, J. Phys. Chem. C, 2015, 119, 4666.
Morphology-Tuned Synthesis of NiCo2O4-Coated 3D Graphene Architectures As Binder-Free Electrode for Lithium Ion Battery
Authors: Zhang Chunfei , Yu Jong-Sung
Published: February 2020

Nanostructured NiCo2O4 is directly grown on the surface of three-dimensional graphene-coated nickel foam (3D-GNF) by a facile electrodeposition technique and subsequent annealing. The resulting NiCo2O4 possesses a distinct flower or sheet morphology tuned by potential or current variation electrodeposition, which is used as a binder-free lithium ion battery anode for the first time. Both samples exhibit high lithium storage capacity profiting from unique binder-free electrode structures. The flower-type NiCo2O4 demonstrates high reversible discharge capacity (1459 mAh g–1 at 200 mA g–1) and excellent cyclability with around 71% retention of the reversible capacity after 60 cycles, which are superior to sheet-type NiCo2O4. Such superb performance can be attributed to high volume utilization efficiency with unique morphological character, a well-preserved connection of active materials to the current collector, a short lithium ion diffusion path, and fast electrolyte transfer in binder-free NiCo2O4-coated 3D graphene structure. The simple preparation process and easily controllable morphology make binder-free NiCo2O4/3D-GNF hybrid a potential material for commercial application.
Restoration of Nickel Rich Electrodes from Spent Lithium-Ion Batteries
Authors: Sloop Steven E
Published: February 2020

This paper describes performance capabilities of refurbished LiNixMnyCozO2 lithium-ion chemistries with x:y:z from 1:1:1 to 6:2:2. The process to refurbish these electrodes from spent batteries is known as “Direct Recycling”. The vertical integration of advanced battery recycling into battery manufacturing will significantly reduce material costs and address long term economic sustainability for the battery industry.

Figure 1
Strengthening of Lightweight Thin-Film Electrodes with Metal-Filaments for Renewable Energy Applications
Authors: Yang Yang
Published: February 2020

Metal-filaments strengthened thin-films were rationally designed to serve as freestanding and defective electrodes for renewable energy applications. A transformative nanomanufacturing process was developed to fabricate these thin-film electrodes (mixed oxides of iron group metals) with nanoporous structure and controllable composition in a facile and scalable manner. More specifically, electrodeposition and anodic treatments were employed to produce freestanding and defective NiFe oxides nanoporous layers (NPL). These NPL can be directly used as flexible and additive-free electrodes for renewable energy generation (water splitting) and storage (supercapacitor) applications without using binders, current collector and other additives. Significantly enhanced electrochemical performances were therefore achieved due to the unique merits of these NPL: i) highly porous structure considerably increase the electrode/electrolyte interface, which facilitates the electrochemical reaction; ii) these defective NPL provide a large number of active sites (defects) for electrochemical reactions; iii) residual metal-filaments in the NPL form an interconnected conductive framework, which drastically improves the flexibility and conductivity of the electrode.
Ab-Initio approach to Model Impedance and Charge-Discharge Characteristics of Electrical Double Layer Capacitors
Authors: Hasyim Muhammad Risyad , Ma Danhao , Rajagopalan Ramakrishnan , Randall Clive A.
Published: February 2020

Electrical double layer capacitors (EDLCs) are energy storage devices which primarily utilize ionic adsorption and high surface area electrodes to store electrostatic charge at electrode/electrolyte interface. In order to describe and correlate electrode architecture to the energy and power performance of capacitors, first principle models that can explain both the ac and dc response of the capacitor need to be developed. In this work, we describe an ab initio mathematical model that take into account both the double layer and interfacial resistance contributions at the electrode/electrolyte interface. The model was tested, and validated using experimental data of various carbon-based EDLCs using both electrochemical impedance spectroscopy (EIS) and galvanostatic discharge data. Physical parameters of the model were obtained through non-linear regression of frequency-domain EIS data, which was then transformed into time domain using numerical inverse Laplace transform to predict the potential-time discharge response of the capacitor. The experimental data was in excellent agreement with the model predictions.

Figure 1
(Invited) Tuning Molecular Singlet Fission – Strong Versus Weak Electronic Coupling
Authors: Guldi Dirk M.
Published: February 2020

Innovative strategies are necessary to overcome the Shockley-Queisser limit, which places the maximum solar conversion efficiency for a single p-n junction to slightly more than 30%. One such strategy, singlet fission (SF), allows high energy, singlet excited state photons (S1) to be down-converted into up to twice as many low-energy, triplet excited state photons (T1). As such, SF is a fascinating many-electron problem from a physical chemistry perspective. In an optimal SF scenario, a chromophore in a singlet excited state (S1) interacts with a nearby ground state (S0) chromophore to yield up to two, non-interacting T1s. Doubling the number of excited states, thus, goes hand in hand with sacrificing half or more of the overall energy per excited state. The energetic requirement for SF is E(S1) ≥ 2(E(T1)), and the free reaction enthalpy for the S1 ® 2(T1) conversion governs the energy loss in SF. The SF rate depends on the free reaction energy and the electronic coupling matrix element between one chromophore in its S1 and another in its S0 ground state. A strict definition of the SF rate implies the formation rate of the two individual triplets (T1 + T1), not that of the correlated triplet pair, 1(T1T1). In (T1 + T1), the two triplets should have lost electronic coherence owing to interactions with the environment, but can retain spin coherence on much longer timescales. In a nutshell, electronic coupling between the chromophores should be sufficient to drive SF, while at the same time keeping the coupling sufficiently weak to allow spin decoherence leading to two independent triplet states.

The systematic variation / comprehensive understanding of electronic coupling between the individual pentacenes constitutes one of the grand challenges in the field of molecular SF, especially when experiments are conducted in solution. By means of tailoring the synthesis of pentacene dimers, trimers, and tetramers we will span our investigations from a weakly coupled, charge transfer to a strongly coupled, charge separated regime. In turn, we are confident to gain control over the rates and yields of intramolecular SF and, subsequent, inter- and intramolecular charge extraction. In particular, we plan to investigate pentacene materials, in which the energy of the charge transfer state and that of the charge separated state is tuned in the range around twice of the triplet excited state, and to explore the impact on the formation of pairs of independent triplets (T1 + T1).
High-Performance Supercapacitors of N-Doped Graphene Aerogel and Its Nanocomposites
Authors: Sawangphruk Montree
Published: February 2020

Nitrogen-doped reduced graphene oxide aerogel (N-rGO aerogel) with high porosity and ionic conductivity were synthesized by a hydrothermal reduction of graphene oxide with hydrazine and following freezing-dry method. N-rGO aerogel was spray-coated on carboxyl-modified carbon fiber paper with a hydrophilic surface and used as the supercapacitor electrode. Not only can the N-rGO aerogel electrode accelerate the diffusion of the electrolyte but also it can store electronic charge via a surface redox reaction due to the N-containing groups. Among the electrolytes studied, the ionic liquid-based supercapacitor of the N-rGO aerogel provides a wide working potential of ca. 4.0 V and rather high specific capacitance of 765 F/g at 1 A/g with a capacity retention of 86% over 3000 charge–discharge cycles. The device prototype fabricated in a single coin cell shape (CR2016) can supply electricity to red LED over 17 min. In addition, high-performance supercapacitors of N-rGO aerogel composites with other materials will be discussed in my presentation.
General Synthetic Method for N-Doped Macroporous Graphene-Encapsulated Mesoporous Metal Oxides and Its Application As New Anode Materials for Sodium-Ion Hybrid Supercapacitors
Authors: Kim Min Su , Lim Eunho , Lee Jinwoo
Published: February 2020

Employing graphene as an electrode material is advantageous for energy storage devices because it is flexible, highly electrically conductive, chemically stable, and has large theoretical surface area. However, graphene generally suffers from serious agglomeration and re-stacking due to its strong pi-pi stacking and van der Waals interactions between graphene nanosheets during charge-discharge process. To fully use the potential merits of graphene and improve the kinetics of electrode materials in energy storage devices, composites of graphene and metal oxides have been tested as electrode. Metal oxides in the composites, prevent the re-stacking of graphene. Graphene layers in the composites also suppress the volume change and agglomeration of metal oxide and provide a highly conductive matrix for metal oxide. We report a general method to synthesize mesoporous metal oxide@N-doped macroporous graphene composite by heat-treatment of electrostatically co-assembled amine-functionalized mesoporous silica/metal oxide composite and graphene oxide, and subsequent silica removal to produce mesoporous metal oxide and N-doped macroporous graphene simultaneously. Four mesoporous metal oxides (WO3-x, Co3O4, Mn2O3 and Fe3O4) were encapsulated in N-doped macroporous graphene. Used as an anode material for sodium-ion hybrid supercapacitors (Na-HSCs), mesoporous reduced tungsten oxide@N-doped macroporous graphene (m-WO3-x@NM-rGO) gives outstanding rate capability and stable cycle life. Ex situ analyses suggest that the electrochemical reaction mechanism of m-WO3-x@NM-rGO is based on Na+ intercalation/de-intercalation. To the best of our knowledge, this is the first report on Na+ intercalation/de-intercalation properties of WO3-x and its application to Na-HSCs.
Raman Spectroscopy Studies of Large Area Graphene and Graphene Molecules
Authors: Dervishi Enkeleda , Sykora Milan , Ji Zhiqiang , Doorn Stephen K.
Published: February 2020

In this study, we present a detailed Raman analysis of large-area graphene, nano-graphene sheets (>25 nm) and graphene molecules with synthetically controlled lateral dimensions between 0.76 to 1.39 nm. The integrated area ratios of D and G bands increase with the size of the molecules, when excited using two different wavelengths (532 nm and 405 nm). The D and G band positions were also found to depend on the size of the graphene molecules. The intensities of the higher-order bands (e.g., 2D, D+G, and 2G), increase for both wavelengths, as the size of the graphene molecules increases. For complete spectroscopic studies of nano- and micro-size graphitic structures, we also probed nano-graphene sheets (>25 nm) and large area graphene using the same excitation wavelengths. The reaction/synthesis time was varied to control size and crystallinity of the graphene molecules, and their corresponding spectral features including integrated area band ratios and the band positions, were thoroughly investigated. These results provide a potential route for using Raman spectroscopy as a diagnostic tool for structural characterization and understanding the reaction dynamics of graphene molecules smaller than 2 nm. Lastly, this work presents a complete representation of the spectroscopic behavior nano- and micro-sized structures (ranging from 0.76 nm to 1cm), which is currently missing from the studies of the present graphitic materials generated via ion-bombardment and other techniques.
Simultaneous Electrochemical Exfoliation and Functionalization of Graphene By Diazonium Compounds
Authors: Aynalem Andinet Ejigu , Dryfe Robert AW , Kinloch Ian A
Published: February 2020

Covalent functionalisation of graphene is considered as one method for enhancing the processability of graphene for various applications. The majority of the study to-date has focused on the functionalization of pre-made graphene.1-3 In this contribution, we describe single stage simultaneous electrochemical exfoliation and functionalisation of graphene using diazonium compounds. Caesium salt (dissolved in dimethyl sulfoxide) as intercalating ions in combination with diazonium salt (either 4-nitrobenzenediazoniumtetrafluoroborate, 4-bromobenzenediazonium tetrafluoroborate or anthraquinone-1-diazonium chloride) as functionalisation moieties were used. Simultaneous exfoliation and functionalisation is beneficial for a number of reasons. First, the presence of diazonium salt in the exfoliation solution not only plays a role in functionalisation but also facilitate the exfoliation process through the generation of N2 gas which assists the separation of the functionalised graphene layers. Second, this process is selective for edge functionalisation and finally as both functionalisation and exfoliation occurs at the same time, monolayer or few layer graphene can be functionalised in situ before they aggregate. Significantly, functionalisation enhanced the dispersibility of graphene in aqueous solution and its solubility was two orders of magnitude larger than the non-functionalised electrochemically exfoliated graphene. As also shown in Figure 1, the functionalisation introduced surface active redox reactions when used as a supercapacitor electrode. The redox reactions enhanced the charge storage capacity of graphene and the specific capacitance found to be highly dependent on the degree of graphene functionalization.

1. J. R. Lomeda, C. D. Doyle, D. V. Kosynkin, W.-F. Hwang and J. M. Tour, J. Am. Chem. Soc., 2008, 130, 16201-16206.

2. E. Bekyarova, M. E. Itkis, P. Ramesh, C. Berger, M. Sprinkle, W. A. de Heer and R. C. Haddon, J. Am. Chem. Soc., 2009, 131, 1336-+.

3. J. Greenwood, T. H. Phan, Y. Fujita, Z. Li, O. Lvasenko, W. Vanderlinden, H. Van Gorp, W. Frederickx, G. Lu, K. Tahara, Y. Tobe, H. Uji-i, S. F. L. Mertens and S. De Feyter, ACS Nano, 2015, 9, 5520-5535.

Figure 1. Cyclic voltammograms recorded at 100 mV s-1 in 6.0 M KOH (aq) using symmetrical coin cells constructed from electrochemical exfoliated restacked graphene and graphene functionalised insitu with 4-nitrobenzenediazoniumtetrafluoroborate (G-NBD). The voltage was swept between 0.0 V to 1.0 V

Figure 1
A New Form of Electrochemical Kinetics, at the Single Layer Graphene – Electrolyte Interface
Authors: Bandaru Prabhakar R , Narayanan Rajaram , Yamada Hidenori , Lipomi Darren , Marin Brandon
Published: February 2020

The “all-surface” characteristic of single–layer graphene (SLG) offers a remarkable paradigm for the exploration of novel physical and chemical phenomena. However, in many cases it is unclear as to how a graphene surface manifests clearly different chemical characteristics compared to any other surface. In this work, we report on a characteristic, experimentally demonstrated, that is unique to the two-dimensional nature of SLG in terms of a completely new type of electrokinetics in stark contrast to the accepted Marcus-Hush-Chidsey (MHC) formalism.

We will indicate that the specific consideration of the unusual linear variation of the two-dimensional density of states (DOS) in SLG, would cause a deviation from both Arrhenius activation relation based Butler-Volmer kinetics as well as the MHC kinetics, which assumes a constant DOS. Consequently, we propose a new lower-dimensional DOS based reaction rate constant and experimentally show that aspects related to a non-ideal DOS were explained through electron and hole puddles formed in wrinkled graphene.

Our experiments and related fitting of the data considerably supplements the presently used standard models to analyze the variation of the reaction rate constants and has significant implications wherever nanoscale electrodes are utilized in energy storage and transduction.
Quantum Chemical Predictions for Alkaline Earth (AE)-Doped Graphene: A Density Functional Theory (DFT) Based Investigation for a Novel Class of Carbon-Based Two-Dimensional Nanomaterials Toward Electrochemical, Catalytic and Electronic Applications
Authors: Serraon Ace Christian Feraren , Padama Allan Abraham Bustria , del Rosario Julie Anne Dalmacio , Cervera Rinlee Butch , Ocon Joey Duran
Published: February 2020

Graphene doping is a known route towards increasing the reactivity of graphene, particularly for the oxygen reduction reaction in fuel cells and metal-air batteries. The most prominent dopants in graphene for ORR are non-metals near to carbon in the periodic table. While alkaline-earth elements, such as beryllium, magnesium, calcium, strontium and barium are relatively abundant in the Earth’s crust, graphenes modified with these elements have not been fully explored.

Graphene systems, which were substitutionally doped with alkaline-earth elements, were investigated through density functional theory (DFT) calculations to elucidate its energetics and electronic properties. A localized ionic bonding between alkaline-earth elements and the graphene substrate was observed, with greater charge transfer as inferred by Bader analysis for Be and Mg. The localized nature of the charge transfer from the dopant to the adjoining carbon atoms in the substrate is a novel property of AE-doped graphene. Semi-metallic properties due to strongly localized states near the Fermi level have been observed for all AE-doped graphenes except for Be. For Be, p-type semiconductor properties were observed consistent with previous studies on Be doped graphene.

This will provide the groundwork for further study towards the use of alkaline-earth metal dopants in an alternative precious-metal free cathode material for metal-air battery and fuel cell applications. The basic and exploratory nature of this scientific study is also expected to open a path towards other emergent applications for the catalysis of other reactions, as well as in electronics and other domains. Observable trends between different alkaline-earth doped graphenes have also been investigated.

Figure 1
(Invited) Strongly-Bound Interlayer Exciton States in Twisted Bilayer Materials
Authors: Graham Matt W
Published: February 2020

When two sheets of a 2D material such as graphene are stacked at a twisted angle, the 2p orbitals rehybridize, giving angle-tunable absorption resonances. By comparing the ultrafast intra-band and multi-photon transient absorption spectra, our results agree best with a recent theoretical model that predict that overlapping interlayer 2p orbitals interfere to produce symmetrized bound exciton states that are decoupled from lower lying continuum states. We further map out the excited state manifold of exciton fine states using 2-photon photoluminescence microscopy. Our spectral analysis suggests the observed photoluminescence emission is a bright exciton state that is thermally populated by a lower-lying, long lived dark-exciton state. For this dark state, both our ultrafast transient absorption and two-photon photoluminescence studies support a large binding energy of the order of ~0.6 eV. If resonantly excited, we believe twisted bilayer graphene is the first 2D metallic material that can form stable, strongly bound excitons. Our results support that twisted bilayer graphene and similar materials are novel example of 2D hybrid material, where resonantly excited excitons are strongly-bound and quasi-stable from inherently weak coupling to the continuum metallic states. The formation mechanism of this dark state is very general, and holds exciting promise for other twisted stacked 2D materials.

[1] R. W. Havener, Y. Liang, L. Brown, L. Yang, J. Park, Van Hove Singularities and Excitonic Effects in the Optical Conductivity of Twisted Bilayer Graphene, Nano Letters, 14, 3353-3357 (2014).

[2] Y. Liang, R. Soklaski, S. Huang, M. W. Graham, R. Havener, J. Park, L. Yang, Strongly Bound Excitons in Gapless Two-dimensional Structures, Phys Rev. B, 90, 115418-3 (2014).

[3] H. Patel, R. Havener, L. Brown, Y. Liang, L. Yang, J. Park, M.W. Graham, Tunable Optical Excitations in Twisted Bilayer Graphene Form Strongly Bound Excitons, Nano Letters, 15, 5932-7 (2015).

Figure 1
Classification of Graphene Islands Via Confocal Raman Spectra Cluster Analysis
Authors: Blades Michael , Ignatova Tetyana , Ta Huy Q. , Bachmatiuk Alicja , Rummeli Mark Hermann , Lee Young Hee , Rotkin Slava V.
Published: February 2020

Since it was first isolated in 2004, monolayer graphene has been studied for its unique optical, electrical, and thermal properties. Recently, interest is growing in the rich physics of twisted bilayer graphene, especially with optical tools such as Raman spectroscopy[1]. In this work, we study the micro-Raman spectra of bi- and tri-layer graphene islands, and develop methods to classify features of these spectra.

Graphene islands were grown on monolayer graphene via the CVD method[2], and then transferred to a Si/SiO2 substrate. Micro-Raman hyperspectral maps were taken from bi- and tri- layer islands with a wide variety of shapes and sizes. Using homemade code we fit the spectra from each pixel in the maps and extract the peak parameters for all of the major graphene Raman features. These pixels define a coordinate manifold, with the peak parameters building a hyperdimensional space over these coordinates. Principal Component Analysis was used to find the vectors where the spectra vary most widely, and data clusters were identified along these principal vectors. These clusters were further decomposed using this method iteratively.

When all reasonable clusters are identified, the clusters can be remapped to the individual islands, allowing one to classify distinct regions on the islands with similar spectral features. Correlations between the peak parameters can be extracted from each cluster, and then fitted to determine the relationships between them. These relationships reflect the unique physics of the few-layer graphene system, such as biaxial strain or doping level. By classifying regions with similar behavior, we can determine how this physics is influenced by the geometry of the system.

Acknowledgement: This work was supported by: NSF ECCS-1509786

References:

[1]Ado Jorio, Luiz Gustavo Cançado, Raman spectroscopy of twisted bilayer graphene, Solid State Communications, Volumes 175–176, December 2013, Pages 3-12, ISSN 0038-1098, http://dx.doi.org/10.1016/j.ssc.2013.08.008.

[2]Huy Q. Ta, David J. Perello, Dinh Loc Duong, Gang Hee Han, Sandeep Gorantla, Van Luan Nguyen, Alicja Bachmatiuk, Slava V. Rotkin, Young Hee Lee, and Mark H. Rümmeli, Stranski–Krastanov and Volmer–Weber CVD Growth Regimes To Control the Stacking Order in Bilayer Graphene, Nano Letters 2016 16 (10), 6403-6410, DOI: 10.1021/acs.nanolett.6b02826

Figure 1
Bright Multicolor Bandgap Fluorescent Carbon Quantum Dots for Electroluminescent Light-Emitting Diodes
Authors: Fan Louzhen
Published: February 2020

Themonochrome light-emitting diodes (LEDs) from blue to red that directly use multicolor bandgap fluorescent carbon quantum dots (MCBF-CQDs) as the active emission layer will be reported. The MCBF-CQDs were synthesized by a facile solvothermal method and exhibited emission color from blue to red with a quantum yield (QY) up to 75%. Even without using hole transport layer, the maximum luminance (Lmax) of blue LEDs reached about 136 cd/m2, a record for CQDs-based monochrome electroluminescent LEDs. We further demonstrated the distinctive advantage of such LEDs, the remarkably stable and voltage-independent emission color, which enabled the BF-CQDs great opportunities to achieve high-performance LEDs. Also presented are white LEDs (WLEDs) fabricated by using BF-CQDs blended poly(N-vinyl carbazole) as the emissive layer, which showed a high-performance even comparable to semiconductor QDs-based LEDs.
(Invited) A First-Principles Study on Photoinduced Hole Injection into Organic Molecule on Metal Halide Perovskite Surface
Authors: Sugimoto Manabu
Published: February 2020

The perovskite solar cell (PSC) is one of the emerging photovoltaic devices with rapid enhancement of the energy conversion efficiency. For further improvement, versatile efforts have been made to uncover important factors determining to the device performance. In particular, carrier injection across material interfaces in the device is considered to be the most critical electronic process.

In this presentation, we will report our recent computational study on photo-induced hole injection rate at the interface between the metal halide perovskite (PVK) and organic hole transport materials (HTM). In this study, we use the density functional theory (DFT) for geometry optimization and excited state calculations. The rate constant is evaluated by using the Marcus theory in which several electronic excited states assigned to the local excitation in PVK are taken into account as initial state for the carrier injection process. The final state is assumed to be the lowest excited state assigned to a charge (electron) transfer state from HTM to PVK, corresponding to the hole injection from PVK to HTM. In evaluation of the electronic coupling matrix element, we apply by the generalized Mulliken-Hush model by Newton and Cave.

As HTMs, we herein focus on 4,4'-dimethoxy triphenylamine (DMeOTPA) which is a partial structure of spiro-OMeTAD (N2,N2,N2’,N2’,N7,N7,N7’,N7’-octakis(4-methoxyphenyl)-9,9’-spirobi[9H-fluorene]-2,2’,7,7’-tetramine). For comparison, we also study on 4,4'-dimethlthio triphenylamine (DMeSTPA) and 4,4'-dimethlseleno triphenylamine (DMeSeTPA). On the basis of the computational analysis, we will suggest a compound with higher efficiency in the photo-energy conversion, and will discuss important electronic properties of the materials in the interfacial hole transfer in PSC.
Electrocatalysis of Oxygen Reduction with Elctropolymerized Metallo-Corroles
Authors: Elbaz Lior , Friedman Ariel , Gross Zeev
Published: February 2020

In recent years we have shown very high electrocatalytic activity with metallo-corroles, specifically with cobalt- and iron-2,3,7,8,12,13,17,18-octabromo-5,10,15-tris(pentafluorophenyl) corrole [M(tpfcBr8)]. Both showed activity comparable with some of the stat-of-the-art molecular catalysts for ORR in acid and alkaline solutions. These very promising results encouraged us to continue developing other corrole-based catalysts in order to enhance the overall activity and shift the reaction mechanism towards the desirable direct 4-electron mechanism. In this work we have taken the bio-mimetic approach of joining catalytic centers together in order to obtain synergistic effects which could be translated to improved activity, selectivity and stability. Cobalt tris (4-aminophenyl) corrole (CoTAPC) were synthesized. The aminophenyl substituents on the corrole ring allowed us to polymerize these corroles using electrochemistry. The CoTAPC was electropolymerized at various polymerization conditions directly on glassy carbon electrodes. We have shown that we can control the growth, structure and density of this polymer by changing the conditions at which the polymerization reaction takes place. The final product was fully characterized using, FTIR, Raman spectroscopy and microscopic techniques. In agreement with previous studies with amino-substituted porphyrins, the obtained polymerization mechanism is similar to that of polyaniline. Indeed, FTIR-ATR analysis shows transition of the primary amine substitutes to secondary and tertiary amine, indicating the formation of dihydrophenazine and phenazine unit linkage respectively.

The electrocatalytic oxygen reduction reaction activity of the polyCoTAPC was studied in both acidic and alkaline conditions. In acidic solution, the number of electrons participating in the reaction (n) was improved by 0.3 electrons and the half-wave potential (E1/2) shifted by 120 mV more positive when compared to the monomer (CoTAPC), while in alkaline solution the n increased by 0.43 electrons and the E1/2 shifted by 200 mV more positive compared to CoTAPC. These shifts are attributed to the unique polymeric structure which allows both sharing of electrons and tandem electrocatalysis by at least two catalytic sites. Another outcome of these unique structures, is the catalytic site density which can possibly compensate for the inherently low turn-over frequency usually associated with non-precious metal catalysts in general.
Phase Field Modeling of Metal Oxidation Kinetics
Authors: Wen Youhai
Published: February 2020

The oxidation kinetics of metals/alloys at elevated temperatures is essential to the corrosion-life prediction of the structural materials in advanced energy systems. The growth of oxide film on a metal surface typically involves coupling of chemical reaction and diffusion of charged point defects. A phase-field model is developed to address the essential physics during metal oxidation over a continuous range of time and film thickness. The model is able to reproduce the classic Deal-Grove model with respect to the linear-parabolic transition. In particular, the model reproduces Wagner’s theory in the thick film limit and predicts a deviation from the Wagner-parabolic growth when the oxide film thickness is on the order of Debye length. The essential characteristics of electric field developed in an oxide film during high temperature metal oxidation and some other progresses in terms of realistic oxidation modeling will also be discussed.
A Novel Fabrication of Vertically Stacked Gate-all-around Silicon Nanowire Arrays
Authors: Meng Lingkuan , Xu Qiuxia , Yan Jiang
Published: February 2020

It is well known that the top-down fabricated gate-all-around (GAA) silicon nanowire transistor with vertically stacked arrays is promising candidates to replace Finfet devices in sub-10nm nodes. The structure offers optimal electrostatic control, thereby enabling ultimate CMOS device scalability. Silicon nanowire (SiNW) transistors have shown promising potential to revolutionize the area of electronic, optical, chemical, and biological device applications. However, the fabrication of vertically stacked SiNW arrays in a controllable manner remains challenging due to complex topography and geometry characteristics.

Top-down controlled nanowire patterning methods have been developed mostly by Bosch-based process combined with successive stress-limited oxidation. Recently, a more controllable method has been demonstrated to fabricate stacked horizontal nanowires using an epitaxial growth such as Si/SiGe/Si/SiGe structures, but the process requires careful heteromaterial interface handling.

In this work, we propose a simple top-down method to fabricate vertically stacked nanowires arrays on bulk silicon substrates by originally developed process based on a conventional etch tool. Then, the stress-limited oxidation process can be introduced to achieve a uniform shape in a controllable fashion from a bulk silicon wafer. And also, we will discuss some challenges and innovations associated to such structure fabrications.

Figure 1
Copper Oxide Nanowire Plasma-Assisted Growth Dynamics and Role of Underlying Oxide Layers
Authors: Cvelbar Uros , Baranov Oleg , Filipič Gregor
Published: February 2020

The plasma-assisted growth of nanowires is gaining importance in recent years due to many potential applications in harvesting renewable energy including nanowire arrays embedded on electrodes for solar fuel harvesting. This growth of metal oxide nanowires on electrodes can be done in plasma by either top-down approach or bottom-up one. However, when depositing or growing nanowires, one has to take into account what is occurring with underlying surface layers. The underlying or interfacial layers between electrode and nanowires have to be as thin as possible in order not to present a barrier during operation. In case of top down approach like in plasma enhanced chemical vapor depositions (PECVD) this is not that big issue, since interfacial layers are reasonably thin. This becomes more important in bottom-up approach, when nanowires are grown directly on the metallic surface or by so-called direct nanowire growth on the substrate. The direct plasma growth process is rather fast and yields large arrays of nanowires on surface of metallic substrate, but the growth and the role of oxide layer created during the plasma process is unknown. The process that involves oxygen plasma and surface oxidation doesn’t only produce nanowires, but also certain oxide layers beneath the nanowire arrays. Herein, the role of underlying oxide layer to growth of nanowires as well as its own dynamics are unrevealed.

The special interest of the presented research are copper oxide nanowires due to their applications in solar fuel cells for water molecule splitting. For this reason, copper samples were treated in low-pressure RF ICP oxygen-argon plasma in order to grow copper oxide nanowires. The result of this time resolved growth is observation of oxide layer saturation mode. These results differ from the copper oxide nanowire growth by thermal oxidation methods where a parabolic kinetics describe all stages of the NW growth. To meaningfully explain these observations a theoretical model was developed. The model explained this phenomenon and revealed a mechanism of the oxide layer growth with respect to the formation of the oxide nanowire arrays. The results of the investigation were confirmed by the experiments and vice versa. The results demonstrated that the density of the ion flux extracted from the plasma to the sample is one of key parameters to tailor the number density of the nanowires and is directly related to underlying oxide growth.
Photopatternable, Electrochemically Plated Conductive Fabrics
Authors: Taushanoff Stefanie , Dubin Val M
Published: February 2020

Standard off-the-shelf fabrics can be turned into conductive fabrics using electrochemical metallization processes. Entire sheets of metalized fabrics can be easily generated using any one of a number of sensitization-activation-plating processes [1, 2] but selective metallization of fabrics, or generating metallized patterns, is far less common. Here we present photopatterning of metal features on commercially available fabric by combining Nano3D Systems eLOCOS™ electrochemical plating solutions with photopatternable sensitization processes (Fig, 1, 2).

Electroless plating of nickel and copper on fabric using eLOCOS solutions creates metallized fabric with resistances comparable to those of commercially available conductive fabric. Sheets of metalized fabrics are readily available and have applications in electromagnetic shielding and static dissipation. There is interest in being able to pattern metallic paths onto fabric for fabric-printed circuit boards, as well as smart textiles and wearable technology where ease of use and a seamless integration of electronics into fabric is essential for widespread acceptance of new technology. Methods for achieving circuit-like patterns or isolated conductive areas on fabric include ink jet printing, weaving and screen printing conductive polymers. The advantages of using photolithography to generate patterns include cost, scalability, and improved resolution, but using established photolithographic methods on fabric substrates presents new challenges. Overcoming these challenges allows for the application of established selective metallization techniques to be applied to textile-based technologies using methods similar to those used for the manufacture of circuit boards on rigid substrates.

References

Lu, Yinxiang. Electroless Copper Plating on 3-Mercaptopropyltriethoxysilane Modified PET Fabric Challenged by Ultrasonic Washing. Applied Surface Science 255 (2009) pp. 8430-8434

Simor, Marcel et.al. Atmospheric-Pressure Plasma Treatment of Polyester Nonwoven Fabrics for Electroless Plating. Surface and Coatings Technology 172(1) pp1-6 (2003).

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All-Solution-Processing of Nanoporous and Wrinkled Electrodes on Polymer Substrates for Use in Electrochemical Biosensing
Authors: Gabardo Christine , Adams-McGavin Chris , Fung Barnabas , Yang Jie , Soleymani Leyla
Published: February 2020

Multi-lengthscale electrodes that combine features from the nanometer to millimeter scale are of tremendous interest for use in electrochemical biosensors due to their increased analytical sensitivity and their enhanced interaction with biomolecular analytes. In this paper, we present a benchtop rapid prototyping approach based on all-solution-processing for creating multi-lengthscale electrodes on polymer substrates.

Multi-lengthscale electrodes are created by combining xurography, nanoparticle self-assembly, electroless deposition, and polymer-induced thin film wrinkling. Xurography is used to develop a mask for implementing the electrode configuration on polymeric substrates with micrometer resolution. A nanoparticle seed layer is deposited with the aid of molecular linkers to direct the electroless deposition processes to desired locations on the substrate. Electroless deposition is used to create a uniform and conductive thin film on a shrinkable polymer substrate. Thin film wrinkling induced using a shape memory polymer is then used to develop tunable wrinkles with minimum feature sizes in the micro/nanoscale.

It is observed that the structural parameters of the wrinkled film such as pore density, wrinkle wavelength, and wrinkle peak-to-valley values are controllable by varying the electroless deposition duration. This structural variability is then used to tune the functional parameters of the electrodes such as sheet resistance and electroactive surface area. The combination of electroless deposition with polymer-induced thin film wrinkling results in thin films having a sheet resistance of 0.39±0.04 Ω/□, which is comparable to films of the same thickness deposited by sputtering. We used the electrodes developed here for enzyme-free glucose detection due to their high surface area induced by the combination of porosity and wrinkling. The results demonstrate that the sensitivity of these electrodes can be tuned by varying the electroless deposition duration. The optimal porous and wrinkled electrodes demonstrate a sensitivity of 23 µA/mM.cm2 for enzyme free glucose detection, which is 18 times larger than the sensitivity measured with planar electrodes.

In sum, an all-solution-processing method is developed, which allows electrodes of various configurations, with tunable features from the nanometer to millimeter scale to be created in a matter of hours. The structural tunability offered by this fabrication method, allows the functional parameters of the electrodes such as sheet resistance, electro-active surface area, and analytical sensitivity to be tuned. The application of the porous and wrinkled electrodes created in this work to enzyme-free glucose detection is demonstrated.
(Invited) Redox Switchable Organic Conjugated Materials for Flexible Electrochromic Devices
Authors: Salamone Matteo , Ruffo Riccardo , Mari Claudio Maria , Beverina Luca
Published: February 2020

Organic π−conjugated compounds have experienced over the last decades a tremendous interest as versatile and highly performing active components in a large variety of devices and applications. Two peculiar characteristics of organic materials are key for their technological exploitation. Firstly, most organic conjugated materials can be manufactured by low cost, solution based deposition techniques. Moreover, the intrinsic flexibility of both their electrochemical and optical properties holds promises for custom designed active materials. On this respect, the formulation of general structure-property relationships is a key requirement.

This talk will address the main characteristics connected with reversible redox behaviour in conjugated materials: charge transport capabilities and tuning of optical properties. The discussion will focus on the structural design required for the preparation of preferentially n or p dopable semiconducting polythiophenes derivates highlighting the role of heteroaromatic compounds as intrinsically electron excessive or electron deficient building blocks. The role of solid state interactions in both single and multi component thin layers will then describe, focusing on recent examples of organic-based electrochromic devices developed in the frame of the EELICON FP7 project.