Search query: Electrical contacts to one- and two-dimensional  nanomaterials

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abstract: Existing models of electrical contacts are often inapplicable at the nanoscale because there are significant differences between nanostructures and bulk materials arising from unique geometries and electrostatics. In this Review, we discuss the physics and materials science of electrical contacts to carbon nanotubes, semiconductor nanowires and graphene, and outline the main research and development challenges in the field. We also include a case study of gold contacts to germanium nanowires to illustrate these concepts.
author_names: Francois Leonard

A Alec Talin
title: Electrical contacts to one and two dimensional nanomaterials.
venue: Nature nanotechnology
year: 2011

abstract: DEP is one of promising techniques for positioning nanomaterials into the desirable location for nanoelectronic applications. In contrast, the lithography technique is commonly used to make ultra thin conducting wires and narrow gaps but, due to the limit of patterning resolution, it is not feasible to make electrical contacts on ultra small nanomaterials for a bottom up device fabrication. Thus, integrating the lithography and dielectrophoresis, a real bottom up fabrication can be achieved. In this work, the device with the nanogap in between two nanofinger electrodes is made using electron beam lithography from top down and the ultra small nanomaterials, such as colloidal PbSe quantum dots, polyaniline nanofibers, and reduced graphene oxide flakes, are placed in the nanogap by DEP from bottom up. The threshold electric field for the DEP placement of PbSe nanocrystals was roughly estimated to be about 8.3 x 104 V/cm under our experimental configuration. After the DEP process, several procedures for reducing contact resistances are attempted and measurements of intrinsic electron transport in versatile nanomaterials are performed. It is experimentally confirmed that electron transport in both PbSe nanocrystal arrays and polyaniline nanofibers agrees well with Prof. Ping Sheng's model of granular metallic conduction. In addition, electron transport in reduced graphene oxide flakes follows Mott's 2D variable range hopping model. This study illustrates an integration of the electron beam lithography and the DEP techniques for a precise manipulation of nanomaterials into electronic circuits for characterization of intrinsic properties.
author_names: Yen-Fu Lin

Shao-Chien Chiu

Sheng-Tsung Wang

Sheng-Kai Fu

Chien-Hsiang Chen

Wenguang Xie

Sheng-Hsiung Yang

Chain-Shu Hsu

Jenn-Fang Chen

Xufeng Zhou

Zhaopin Liu

Jiye Fang

Wen-Bin Jian
title: Dielectrophoretic placement of quasi zero one and two dimensional nanomaterials into nanogap for electrical characterizations
venue: Electrophoresis
year: 2012

abstract: The continuously increasing concentration of carbon dioxide (CO2) in the atmosphere is universally accepted as the most prominent cause of global warming. The ability to harness the power of CO2 on a large scale and integrate it back into the utilization cycle as a sustainable form of energy production is highly desirable. Among various renewable projects, semiconductor photocatalysis has gained considerable interdisciplinary research fascination for their diverse potentials in energy and environmental applications. In this regard, photocatalytic reduction of CO2 with H2O vapor into useful hydrocarbons under the visible light irradiation is a compelling solution to circumvent both energy and environmental crisis to mimic the natural photosynthesis in plants. Among the developed photocatalysts, the focus in this study has been on the recently researched TiO2 with exposed {001} facets (TiO2 001) and graphitic carbon nitride (g C3N4) in the solar energy conversion due to their high chemical stability, corrosion resistant and environmentally benign nature. However, the rapid recombination of photogenerated charge carriers during the reaction process and low electrical conductivity greatly impair the efficiency of TiO2 001 and g C3N4. Consequently, the state of the art technology is far from being optimal before it can be considered as a viable process. Thus, various modifications have been employed to enhance the performance of the photocatalysts. This project focused on the use of a continuous, gas phase heterogeneous photocatalytic system, which was performed under the irradiation of visible light. In the initial phase, the incorporation of graphene with high energy {001} facets of nitrogen doped TiO2 (N TiO2 001/GR) composite was designed for the conversion of CO2 into CH4. The N TiO2 001/GR sample exhibited a total CH4 evolution of 3.70 umol/gcatalyst, which was 11 fold higher activity compared to the TiO2 001 sample. The presence of {001} facets of TiO2 possessed a higher surface energy than the thermodynamically stable and low surface energy of {101} facets, rendering it to be more reactive for the photoactivity. Meanwhile, the nitrogen doping and graphene increased the light absorbance of the samples and reduced band gap energies. Importantly, graphene acted as an electron storage channel for efficient charge separation to inhibit the charge recombination. In the subsequent phases of this study, as an alternative to the conventional UV active TiO2 photocatalyst, visible light responsive pristine g C3N4 was modified to enhance the photocatalytic activity by coupling with other semiconductors such as silver halides (AgX, X Br or Cl) forming Type I or Type II heterojunction. The types of heterojunction were influenced by the electronic band potentials of g C3N4, AgBr and AgCl. Upon the light irradiation, Ag metal was formed, which promoted the photocatalytic activity due to its surface plasmon resonance (SPR) effect. The well matched energy bands of Ag, AgBr and g C3N4, which exhibited Type II heterojunction, resulted in enhanced photoactivity for the CO2 reduction to CH4 by 34.1 and 4.2 times higher compared to those of bare AgBr and g C3N4, respectively due to the efficient migration of charge carriers in space apart from each semiconductor. In a separate study, noble metal Pt was incorporated with g C3N4 to form a binary nanocomposite photocatalyst. As such, 2 wt% of Pt metal clusters demonstrated the highest CH4 evolution of 13.02 umol/gcatalyst after 10 h of reaction. Pt served as reduction co catalysts for electron trapping sites to suppress the recombination of charge carriers and also displayed broad absorption in the visible light region. It is highly in demand in the search for metal free photocatalytic systems for the large scale applications. In another study, a 2D graphene was employed as a reduction co catalyst with g C3N4 to form 2D/2D heterointerfaces. Two synthesis approaches to form graphene modified g C3N4 were carried out, namely (1) a one pot impregnation thermal reduction strategy and (2) a novel surface charge promoted self assembly technique. For the former approach, the sublimation amount of urea (precursor of g C3N4) was difficult to be controlled during the annealing process and the main challenge still revolved in the development of an intimate interfacial contact between graphene and g C3N4. Therefore, to overcome this bottleneck, further improvements were subsequently conducted by altering the surface charge of g C3N4 via acid pretreatment of HCl to fabricate protonated g C3N4 (pCN) for the ease of coupling with rGO to form rGO/pCN sample via electrostatic attraction. Interestingly, the rGO/pCN photocatalyst exemplified the highest CH4 production of 13.93 umol/gcatalyst compared to pCN (2.58 umol/gcatalyst) after 10 h of reaction due to its exceptional 2D/2D heterojunction interface. Overall, this research accentuates the scientific aspects of photocatalytic process and fundamental understanding behind the enhancement of each carbon based photocatalytic system. As a whole, the as developed hybrid nanocomposite is anticipated to be a robust means to address various energy and environmental related issues via photocatalytic process for a sustainable energy future.
author_names: Wee-Jun Ong
title: Design and development of visible light responsive two dimensional carbon based nanomaterials for photocatalytic reduction of carbon dioxide into methane
year: 2016

abstract: Abstract Although transparent conductive oxides (TCOs) including indium tin oxide (ITO) fluorine tin oxide (FTO) and Aluminum doped Zinc oxide (AZO) have been commercialized, and they can be found in many consumer electronics, TCOs are mechanically brittle and this issue actually limits them to the application of future flexible solar cell devices. Therefore, one dimensional (1D) metallic materials such as nanowires (NWs) have widely studied as transparent conductors because they exhibit excellent electrical and optical properties due to the quantum confinement effect that occurs in such structural one dimensionality. More importantly, they are ductile as compared to traditional TCOs. Since demand for modern electronic devices have been escalating, a great variety of 1D materials such as bimetallic or core shell metallic NWs, metal oxide, metal CNTs/Graphene and metal polymer composite 1 D nanomaterials have been developing owing to the fact that their electrical, optical, and mechanical performance becomes several order of magnitude higher than the pure metal NWs, especially under extreme mechanical and chemical conditions. Derived from those metal NW researches, advanced two dimensional (2D) metallic materials such as nanomesh and nanogrid are designed as potential alternative to replace either conventional TCO films or random metal NW based films due to their ample carriers (electrons) can transport consistently through this solid metallic network and are collected by the contact terminals without significantly compromising the optical transmittance. Moreover, metal nanomesh and nanogrid architectures possesses excellent flexibility, which can withstand even more harsh mechanical conditions. Therefore, they can provide a great potential for the emerging super flexible and stretchable devices. Despite the aforementioned advantages, scalable fabrication of 1D and 2D nanomaterials is still a challenging research topic, in terms of the yield and quality. To integrate nanoscale building blocks into high performance devices further requires more sophisticated designs. In this review, we emphasize on the current development of 1D and 2D nanomaterials in the application of transparent conductors in renewable energy devices. Computational simulation methods for property prediction and device design will also be reviewed. Additionally, we also discuss the promising fabrication methods for those nanomaterials that are considered to be scalable, applicable, and budget wise to this solar industry.
author_names: Po-shun Huang

Tongchuan Gao
title: Current development of 1D and 2D metallic nanomaterials for the application of transparent conductors in solar cells: Fabrication and modeling
year: 2017

abstract: An efficient hole transporting layer (HTL) based on functionalized two dimensional (2D) MoS2 poly(3,4 ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) composites has been developed for use in organic solar cells (OSCs) Few layer, oleylamine functionalized MoS2 (FMoS2) nanosheets were prepared via a simple and cost effective solution phase exfoliation method; then, they were blended into PEDOT:PSS, a conducting conjugated polymer, and the resulting hybrid film (PEDOT:PSS/FMoS2) was tested as an HTL for poly(3 hexylthiophene)[6,6] phenyl C61 butyric acid methyl ester (P3HT:PCBM) OSCs. The devices using this hybrid film HTL showed power conversion efficiencies up to 3.74% which is 15.08% higher than that of the reference ones having PEDOT:PSS as HTL. Atomic force microscopy and contact angle measurements confirmed the compatibility of the PEDOT:PSS/FMoS2 surface for active layer deposition on it. The electrical impedance spectroscopy analysis revealed that their use minimized the charge transfer resistance of the OSCs, consequently improving their performance compared with the reference cells. Thus, the proposed fabrication of such HTLs incorporating 2D nanomaterials could be further expanded as a universal protocol for various high performance optoelectronic devices.
author_names: Madeshwaran Sekkarapatti Ramasamy

Ka Yeon Ryu

Ju Won Lim

Asia Bibi

Hannah Kwon

Jieun Lee

Dong Ha Kim

Kyungkon Kim
title: Solution Processed PEDOT:PSS/MoS2 Nanocomposites as Efficient Hole Transporting Layers for Organic Solar Cells
venue: Nanomaterials
year: 2019

abstract: Graphene, a two dimensional (2D) carbon nanomaterial, has received great attention owing to its advantageous characteristics, including superior electrical, chemical and physical properties. In particular, exfoliation of graphite with strong oxidants has been explored to prepare a suspension of individual 2D graphene oxide (GO) nanosheets dispersed in various types of solvents. This colloidal suspension can readily be employed in a wide range of promising potential applications. However, one major challenge is to assemble such individual nanosheets on defined areas of surfaces forming specific structures. Here, we developed a simple and robust one step strategy to create highly regular ordered nanostructures composed of chemically reduced graphene oxide (rGO) nanosheet building blocks on SiO2/Si or glass substrates over large areas. An aqueous suspension of rGO nanosheets dispersed in a volatile solvent was subjected to a confined geometry to induce a spontaneous formation of rGO patterned arrays with unprecedented regularity by utilizing both a controlled coffee ring effect and consecutive stick slip motions of rGO solutions during the drying process. As a result, the generation of ultrathin nanotextured films in the form of micropatterns (i.e. concentric gradient rGO coffee rings) was effectively tailored. As a biological approach, to fully utilize patterned rGO associated with the nanoscale surface topography, we used these patterned rGO arrays as a biomimetic in vitro architecture to study interfacial interactions of living cells such as fibroblasts, myoblasts, and neuronal cells. The observed results show that the patterned rGO arrays can be used as manipulated cellular responsive templates to form living cell assemblies and related patterns that are useful for regenerative tissue engineering.
author_names: Seok Hee Kang

Yong Cheol Shin

Eun Young Hwang

Jong Ho Lee

Chang-Seok Kim

Zhiqun Lin

Seung Hyun Hur

Dong-Wook Han

Suck Won Hong
title: Engineered "coffee rings" of reduced graphene oxide as ultrathin contact guidance to enable patterning of living cells
year: 2019

abstract: The long term stability and superior device reliability through the use of delicately designed metal contacts with two dimensional (2D) atomic scale semiconductors are considered one of the critical issues related to practical 2D based electronic components. Here, we investigate the origin of the improved contact properties of alloyed 2D metal semiconductor heterojunctions. 2D WSe2 based transistors with mixed transition layers containing van der Waals (M vdW, NbSe2/WxNb1 xSe2/WSe2) junctions realize atomically sharp interfaces, exhibiting long hot carrier lifetimes of approximately 75,296 s (78 times longer than that of metal semiconductor, Pd/WSe2 junctions) Such dramatic lifetime enhancement in M vdW junctioned devices is attributed to the synergistic effects arising from the significant reduction in the number of defects and the Schottky barrier lowering at the interface. Formation of a controllable mixed composition alloyed layer on the 2D active channel would be a breakthrough approach to maximize the electrical reliability of 2D nanomaterial based electronic applications.
author_names: Yonghun Kim

Ah Ra Kim

Jin Ho Yang

Kyoung Eun Chang

Jung-Dae Kwon

Sun Young Choi

Jucheol Park

Kang eun Lee

Dong-Ho Kim

Sung Mook Choi

Kyu H Lee

Byoung Hun Lee

Myung Gwan Hahm

Byungjin Cho
title: Alloyed 2D Metal Semiconductor Heterojunctions: Origin of Interface States Reduction and Schottky Barrier Lowering.
venue: Nano letters
year: 2016

abstract: Recently, investigating the unique electrical properties of low dimensional (One and two dimensional) materials as alternatives to silicon has become popular among researchers. In order to observe the intrinsic properties and device performance, it is essential to elucidate the electron transport at the electrode/nanomaterial interface. This study reviews various current approaches used to evaluate the contact resistance of electronic devices based on the most representative low dimensional nano materials such as carbon nanotubes, nanowires, graphene and molybdenum disulfide. Various analytical factors that have generally not been considered in conventional electronics are introduced to define the contact resistance within the nano meter scale. Additionally, a comparison of three different methods for determining the contact resistance to interpret experimental data is conducted. Finally, several attempted efforts to reduce the contact resistance are presented.
author_names: Jae Young Park

Jinsoo Cho

Seong Chan Jun
title: Review of contact resistance analysis in nano material
year: 2018

abstract: Carbon nanotubes (CNTs) are an interesting allotrope of carbon which can have a wide range of applications as they have extraordinary mechanical, electrical and thermal properties. All this makes CNTs an interesting nanomaterial for different applications ranging from mechanical sensors to electrical microelectrodes and even biological applications due to their biological compatibility. Beside the excellent material properties, the use of special structuring of these materials is of great importance. The random orientation of CNTs cannot be controlled which usually leads to irreproducible material which is not suitable for real world applications. Therefore, the controlled growth of vertically aligned carbon nanotubes (VACNTs) is considered in this work. VACNTs have been grown on Si/SiO2 substrates using a water assisted chemical vapor deposition technique. By this technique highly crystalline, pure, low layer multiwalled CNTs with a vertical orientation to the substrate are obtained. The parameters for the growth are optimized and even structuring of the VACNTs is possible obtaining VACNTs with different heights in one synthesis step. This structuring is the used to construct a nano microstructured artificial hair cell type sensor as an example for a mechanical sensor which can measure three dimensional forces by the changing contact resistance between neighboring CNT bundles of different heights. Because to the excellent electrical properties together with the highly increased surface area due to the vertical alignment of VACNTs, they are compared to randomly oriented CNTs for microelectrode applications. In this, the advantage of vertical alignment becomes clear in the dramatic decrease in impedance and enormous increase in capacity. These microelectrodes are then tested for biological applications for which the compatibility and growth pattern of cortical neurons on VACNTs is studied. While CNTs represent one dimensional systems, also two dimensional materials related to carbon such as graphene oxide and reduced graphene oxide are studied and used for gas adsorption as well as liquids absorbents in combination with bacterial cellulose in the form of aerogels. Finally, phosphorene as an example of a two dimensional material closely related to graphene is synthesized. Phosphorene, although structurally similar to graphene, it has a band gap in contrast to graphene which makes it an interesting material field effect transistor devices as shown in this work.
author_names: Sandeep Kumar Yadav
title: Group 14 and 15 elements as building blocks for low dimensional functional nanostructures
year: 2017

abstract: Recent advancements in the integration of nanoscale, single crystalline semiconductor 'X' on substrate 'Y' (XoY) for use in transistor and sensor applications are presented. XoY is a generic materials framework for enabling the fabrication of various novel devices, without the constraints of the original growth substrates. Two specific XoY process schemes, along with their associated materials, device and applications are presented. In one example, the layer transfer of ultrathin III V semiconductors with thicknesses of just a few nanometers on Si substrates is explored for use as energy efficient electronics, with the fabricated devices exhibiting excellent electrical properties. In the second example, contact printing of nanowire arrays on thin, bendable substrates for use as artificial electronic skin is presented. Here, the devices are capable of conformably covering any surface, and providing a real time, two dimensional mapping of external stimuli for the realization of smart functional surfaces. This work is an example of the emerging field of "translational nanotechnology" as it bridges basic science of nanomaterials with practical applications.
author_names: Morten Hannibal Madsen

Kuniharu Takei

Rehan Kapadia

Hui Fang

Hyunhyub Ko

Toshitake Takahashi

Alexandra Caroline Ford

Min Hyung Lee

Ali Javey
title: Nanoscale semiconductor "X" on substrate "Y" processes, devices, and applications.
venue: Advanced materials
year: 2011