Linyou Cao, North Carolina State University
Bruce Claflin, Air Force Research Laboratory
Thomas Mueller, Vienna University of Technology
Hua Zhang, Nanyang Technological University
Applied Physics Letters | AIP Publishing
NM1.3: Optical Properties and Devices of 2D Materials I
Tuesday AM, April 18, 2017
PCC West, 100 Level, Room 106 AB
12:00 PM - *NM1.3.02
Enhanced Photocarrier Generation at WS2/Graphene Interfaces by Interlayer Coupling
Libai Huang 1 , Long Yuan 1 Show Abstract
1 , Purdue University, West Lafayette, Indiana, United States
Efficient interfacial carrier generation in van der Waals heterostructures is critical for their potential electronic and optoelectronic applications. Here we reveal how interlayer interactions enhance carrier generation in WS2/graphene heterostructures by directly mapping interlayer coupling dependent charge transfer dynamics using ultrafast transient absorption microscopy (TAM). We demonstrate an up to 4-fold enhancement in carrier generation in heterostructures based on graphene and single layer WS2 compared to that in single layer WS2 alone, which is attributed to excitation through interlayer charge transfer transitions. Such interlayer states promote electrons from the graphene layer to the WS2 layer and allow carrier generation with excitation energy well below the WS2 bandgap. High mobility of graphene leads to additional benefits of efficient charge transport, making graphene/2D semiconductor heterostructures highly attractive for applications such as photovoltaics.
12:30 PM - *NM1.3.03
Ultrafast Interlayer Charge Transfer in van der Waals Heterostructures
Hui Zhao 1 Show Abstract
1 , University of Kansas, Lawrence, Kansas, United States
I will introduce our recent results on ultrafast interlayer charge transfer in van der Waals heterostructures formed by two-dimensional materials. In these studies, various van der Waals heterostructures composed of two, three, or four layers were fabricated by manual assembly of exfoliated monolayers or direct synthesis by chemical vapor deposition. Charge transfer in these multilayer structures was monitored by ultrafast pump-probe measurements. Electrons and holes are excited in a targeted layer by a pump pulse with a duration of about 100 femtosecond. Their transfer to another layer was time resolved by measuring transient absorption of a probe pulse tuned to the optical bandgap of that layer. Through these measurements, interlayer change transfer in van der Waals heterostructure with various band alignments was systematically investigated.
NM1.4: Electronic Properties and Devices of 2D Materials I
Tuesday PM, April 18, 2017
PCC West, 100 Level, Room 106 AB
2:30 PM - *NM1.4.01
Electrical Generation and Control of Valley Polarization in 2D Materials
Xiang Zhang 1 , Jun Xiao 1 Show Abstract
1 , University of California, Berkeley, Berkeley, California, United States
Atomically thin layered semiconductors are regarded as promising candidate to resolve the important scaling problem and save Moore’s law in electronics industry. The crystals also reveal unique properties such as valley degree of freedom, which are expected for exotic physics and applications at 2D limit. In this talk, I will present recent progress from my group aiming for realization of 2D electronics and valleytronics. To begin with, I will discuss our work on chemical assembly of heterojunctions using graphene–MoS2–graphene heterostructures1. Coupled with the industrial wafer-scale compatibility of graphene and MoS2 growth, we directly chemically grew these transistors with millimeter-scale coverage. I will then focus on first experimental demonstration of the electrical generation and control of valley polarization in 2D materials2. With unique spin–valley locking property in monolayer WS2, valley carrier injection was achieved via a diluted ferromagnetic semiconductor with efficiency up to 45%. In the last, valley selection rules for nonlinear optical process in monolayer WS2 will be explored3.
1. Zhao, M. et al. Large-scale chemical assembly of atomically thin transistors and circuits. Nat. Nanotechnol. 11, 954–959 (2016).
2. Ye, Y. et al. Electrical generation and control of the valley carriers in a monolayer transition metal dichalcogenide. Nat. Nanotechnol. 11, 598–602 (2016).
3. Xiao, J. et al. Nonlinear optical selection rule based on valley-exciton locking in monolayer WS2. Light Sci. Appl. 4, (2015).
3:00 PM - NM1.4.02
Effect of Ionic Strength on the Electron Mobility of Electrolyte-Gated 2D Field-Effect Transistors
Ming-Pei Lu 1 , Xaio-Yen Dai 2 , Ming-Yen Lu 2 3 Show Abstract
1 , National Nano Device Labs, Hsinchu Taiwan, 2 , Graduate Institute of Opto-Mechatronics, National Chung Cheng University, Chia-Yi Taiwan, 3 , Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu Taiwan
Among several two-dimensional (2D) layered materials, MoS2 material has attracted great attention for future applications in electronic and optoelectronic fields. In this report, we study the effect of ionic strength on the electron mobility of liquid-gated MoS2 field-effect transistors (FETs) for providing insight into the mechanism of mobility scattering in liquid-gated 2D FET systems. We found that the electron mobility decreased as increasing the ionic strength of electrolyte liquids. Interestingly, the electron mobility of 2D FETs under ambient environment was evidently smaller than that under the deionized water condition. Possible mechanism behind experimental observations was also addressed. This report paves a way towards the investigation of mobility scattering in liquid-gated 2D FETs for promising applications of electronics, bioelectronics and nanosensors.
3:15 PM - *NM1.4.03
Two-Dimensional Materials and Their Heterostructures—Exciting Opportunities in Materials Research
Chakrapani Varanasi 1 , Matthew Chin 1 , Sina Najmaei 1 , Madan Dubey 1 Show Abstract
1 , U.S. Army Research Laboratory, Adelphi, Maryland, United States
The Materials Science Division of the US Army Research Office (ARO) seeks to realize unprecedented materials properties by embracing innovative, long-term, high risk, high-payoff basic research opportunities for the US Army. The research area of two dimensional (2D) materials and their heterostructures offer an exciting opportunity to realize novel materials with extraordinary properties. These materials are anticipated to positively impact the development of several next generation Army relevant applications. ARO is the first funding agency in USA to recognize the unique scientific opportunity of 2D materials beyond graphene, and has initiated a long term multidisciplinary basic materials research program in this area. ARO is very actively advancing the basic science by providing support for various 2D materials research areas in processing, characterization, and modeling. In this talk, an overview of some of the major funded projects and their significant impact on the 2D materials research area will be highlighted. Some examples of the current state of research, challenges, and emerging innovations in 2D materials basic research will be discussed. Recent results of the ongoing in-house research activities at the Army Research Laboratory in developing future flexible/foldable nano-electronics based on 2D materials (e.g. transition metal dichalcogenides etc.) will also be discussed.
3:45 PM - NM1.4.04
MoS2 Transistors with 1-Nanometer Gate Lengths
Sujay Desai 1 , Surabhi Madhvapathy 1 , Angada Sachid 1 , Juan Pablo Llinas 1 , Qingxiao Wang 2 , Geun Ho Ahn 1 , Gregory Pitner 3 , Moon Kim 2 , Jeffrey Bokor 1 , Chenming Hu 1 , H.-S. Philip Wong 3 , Ali Javey 1 Show Abstract
1 , University of California, Berkeley, Berkeley, California, United States, 2 , University of Texas at Dallas, Dallas, Texas, United States, 3 , Stanford University, Palo Alto, California, United States
As Si transistors rapidly approach their projected scaling limit of ~ 5 nm gate lengths, exploration of new channel materials and device architectures is of utmost interest. This scaling limit arises from short channel effects like direct source-to-drain tunneling (ISD-LEAK) and the loss of gate electrostatic control on the channel. Heavier carrier effective mass, larger band gap and lower in-plane dielectric constant yield lower direct source-to-drain tunneling current. Uniform and atomically thin semiconductors with low in-plane dielectric constants are desirable for enhanced electrostatic control of the gate. Thus, investigation and introduction of semiconductors that have more ideal properties than Si could lead to further scaling of transistor dimensions with lower OFF-state dissipation power.
Transition metal dichalcogenides (TMDs) are layered 2-dimensional (2D) semiconductors that have been widely explored as a potential channel material replacement for Si, and each material exhibits different band structure and properties. The layered nature of TMDs allows uniform thickness control with atomic level precision down to the monolayer limit. This thickness scaling feature of TMDs is highly desirable for well-controlled electrostatics in ultrashort transistors. Calculations show that MoS2 has more than two orders of magnitude reduction in ISD-LEAK relative to Si mainly because of its larger electron effective mass along the transport direction ( ~ 0.55 for MoS2 versus ~ 0.19 for Si ), with a trade-off resulting in lower ballistic ON-current. Few-layer MoS2 also exhibits a lower in-plane dielectric constant (~ 4) compared to bulk Si (~ 11.7), Ge (~ 16.2) and GaAs (~ 12.9), resulting in a shorter electrostatic characteristic length (λ).
The above qualities collectively make MoS2 a strong candidate for the channel material of future transistors at the sub-5 nm scaling limit. Here, we demonstrate MoS2 transistors with a 1-nm physical gate length using a single-walled carbon nanotube as the gate electrode. These ultrashort devices exhibit excellent switching characteristics with near ideal subthreshold swing ~ 65 millivolts per decade and an ON/OFF current ratio ~ 106. Simulations show an effective channel length of ~ 3.9 nm in the OFF-state and ~ 1 nm in the ON-state. The SWCNT diameter d ~ 1 nm for the gate electrode minimized parasitic gate to source-drain capacitance, which is characteristic of lithographically patterned tall gate structures. The ~ 1 nm gate length of the SWCNT also allowed for the experimental exploration of the device physics and properties of MoS2 transistors as a function of semiconductor thickness (i.e., number of layers) at the ultimate gate length scaling limit. The work here provides new insight into the ultimate scaling of gate lengths for a FET by surpassing the 5 nm limit often associated with Si technology.
Ref: S. B. Desai, et. al., "MoS2transistors with 1-nanometer gate lengths", Science, 354, 99-102, 2016.
4:30 PM - *NM1.4.05
Transition from Quasi-2D to Quasi-1D van der Waals Materials—Electronic Properties of Monoclinic Tantalum Triselenide Capped with Boron Nitride Layers
Alexander Balandin 1 , Guanxiong Liu 1 , Tina Salguero 2 , Sergey Rumyantsev 3 , Michael Shur 3 Show Abstract
1 , University of California, Riverside, Riverside, California, United States, 2 , University of Georgia, Athens, Georgia, United States, 3 , Rensselaer Polytechnic Institute, Troy, New York, United States
Recently discovered unique electrical and thermal properties of graphene stimulated the search for other two-dimensional (2D) atomic crystals with properties distinct from the corresponding bulk. A large number of 2D materials belong to the family of transition metal chalcogenides with weak van der Waals bonding between the structural units. Most research up to now has focused on dichalcogenides, such as MoS2, TaSe2 and others yielding 2D atomic layers. Another class of the transition metal chalcogenides – the trichalcogenides – has quasi-1D crystalline structures. Examples include TiS3, TaSe3 and TaS3. In this presentation, we review our recent experimental results, which show that the mechanical exfoliation approaches developed for 2D materials can be extended to the quasi-1D van der Waals materials such as TaSe3. Electrical measurements established that the quasi-1D TaSe3 – quasi-2D nanowire heterostructures have a breakdown current density exceeding 10 MA/cm2 — an order-of-magnitude higher than that for copper . The atomic chain single crystal nature of TaSe3 results in a low surface roughness and in the absence of grain boundaries; these features potentially can enable the downscaling of these wires to lateral dimensions in the few-nm range. It was found that TaSe3 nanowires have lower levels of low frequency noise compared to carbon nanotubes and graphene. Temperature measurements showed that noise is caused by a random thermally activated process, which complies with Dutta-Horn model. Using this model we determined that the noise activation energy for quasi-1D TaSe3 nanowires exceeds approximately 1.0 eV. Our results suggest that quasi-1D van der Waals metals in combination with 2D h-BN capping layers have potential for applications in the ultimately downscaled local interconnects.
This work was supported, in part, by NSF EFRI 2-DARE project: Novel Switching Phenomena in Atomic MX2 Heterostructures for Multifunctional Applications, and SRC and DARPA through STARnet Center for Function Accelerated nanoMaterial Engineering (FAME). The work at RPI was supported, in part, by the U.S. Army Research Laboratory through the Collaborative Research Alliance (CRA) for Multi-Scale Modeling of Electronic Materials (MSME).
 M.A. Stolyarov, et al., Nanoscale (2016); DOI: 10.1039/c6nr03469a; or https://arxiv.org/abs/1604.03093
5:00 PM - NM1.4.06
DFT Simulation Study for MoS2 Contact Material
Junsen Gao 1 , Manisha Gupta 1 , Dipanjan Nandi 1 , Jiaxin Fan 1 Show Abstract
1 University of Alberta, Department of Electrical and Computer Engineering, Edmonton, Alberta, Canada
Molybdenum disulfide (MoS2) is a two dimensional transition metal dichalcogenide which has graphene like structure. It is an attractive material due to its material properties as it behaves as a direct bandgap material (~1.8 eV) in monolayer state which changes with multiple layers. One of the main challenges of implementing devices with MoS2 is to achieve a good Schottky contact. Here, investigation and evaluation of various types of metal contacts to form high-performance Schottky contact with monolayer or multilayers of Molybdenum disulfide is reported. The simulations are conducted using the Atomistix Toolkit (ATK) and DFT theory. In order to gain a more accurate understanding of the physical characteristics of the 2-D MoS2 material and the variables which demonstrate the main capacities of the devices, the hybrid exchange correlation functional meta-GGA (Meta Generalized Gradient Approximation) with confined c parameter is applied. This will reveal the band structure, density of states, projected density of states, electron distribution and other material properties to help us define the contact performance. Besides the material simulation, the device simulation will also be conducted via ATK and Crosslight. The simulation results will enable determining which of the materials will perform better as Schottky and tunnel barrier under various Schottky contacts. Both mono and multiple layer MoS2 are being studied in this work. MoS2 sheets with different numbers of layers and channel length will be test to extract the influence based on the interface and the barrier heights of Schottky and tunnel barrier. Simulations with different materials like Mo, Ti, Ag, Au, Cu, Al, W, Ni have been conducted and it is observed that Mo forms the best contact with the MoS2. This is due to the strong electron orbital overlap near the interface, which acts as valence bonds instead of Van de Waals force and thus results in a very low Schottky barrier height with a negligible tunnel barrier. We also demonstrate that the scale of the device will have a significant impact on the properties of the interface which will affects the performance of the device.
5:15 PM - *NM1.4.07
Electronic Transport and Device Applications of 2D Materials
Feng Miao 1 Show Abstract
1 , Nanjing University, Nanjing China
During the last decade, tremendous research efforts have been focused on two-dimensional (2D) materials due to their rich physics and great potentials for many applications. Our recent studies on transition-metal dichalcogenides (TMD) with low lattice symmetry will be mainly discussed in this talk. We first studied atomically thin rhenium disulfide (ReS2) flakes exhibiting interesting in-plane anisotropic transport and mechanical properties, as well as excellent optoelectronic properties. We fabricated mono- and few-layer ReS2 field effect transistors, which exhibit competitive performances and record-high anisotropic ratio. We further successfully demonstrated an integrated digital inverter with good performances by utilizing two ReS2 anisotropic field effect transistors, suggesting the promising implementation of large-scale two-dimensional logic circuits.  Our latest results on the ultra-high responsivity phototransistors based on few-layer ReS2 and broadband photovoltaic detectors based on an atomically thin heterostructure will also be presented. [2,3]
The second part of the talk will focus on a predicted type-II Weyl semimetal (WSM) material, tungsten ditelluride (WTe2). We observed notable angle-sensitive negative longitudinal magnetoresistance (MR) and the strong planar orientation dependence which reveal important transport signatures of chiral anomaly. By applying a gate voltage, we further demonstrated that the Fermi energy can be tuned through the Weyl points via the electric field effect; this is the first report of controlling the unique transport properties in situ in a WSM system. 
 Liu, et al. “Integrated Digital Inverters Based on Two-dimensional Anisotropic ReS2 Field-effect Transistors”, Nat. Comm. 6, 6991 (2015).
 Liu, et al. “Ultra-high responsivity phototransistors based on few-layer ReS2 for weak signal detection”, Adv. Func. Mater. 26, 1938 (2016).
 Long, et al. “Broadband photovoltaic detectors based on an atomically thin heterostructure”, Nano Lett. 16, 2254 (2016).
 Wang, et al. “Gate-Tunable Negative Longitudinal Magnetoresistance in the Predicted Type-II Weyl Semimetal WTe2”, Nat. Comm. 7, 13142 (2016).
5:45 PM - NM1.4.08
Fully Flexible and Transparent CVD-Grown Monolayer MoS2 Field-Effect Transistors with all Inkjet-Printed Components
Tae-Young Kim 1 , Jewook Ha 2 , Kyungjune Cho 1 , Younggul Song 1 , Jinsu Pak 1 , Jae-Keun Kim 1 , Seungjun Chung 1 , Yongtaek Hong 2 , Takhee Lee 1 Show Abstract
1 Department of Physics and Astronomy, Seoul National University, Seoul Korea (the Republic of), 2 Department of Electrical and Computer Engineering, Seoul National University, Seoul Korea (the Republic of)
Mechanically flexible and optically transparent atomically thin 2-dimensional (2D) Molybdenum disulfide (MoS2) can be a promising semiconducting material for next-generation flexible and transparent optoelectronic applications. Especially, monolayer MoS2 exhibiting direct band-to-band excitonic transitions have the advantage of high electronic tunability due to its in-plane device form. To make full use of its unique optical and electrical merits in practical applications, a chemical vapor deposition (CVD) technique has been investigated to produce large and uniform monolayer MoS2. To define electrodes and insulating layers on the atomically thin MoS2 channel, conventional electron-beam nanolithography or photolithography techniques have been widely used. Unfortunately, these techniques are not compatible with large-area transparent electronic applications on flexible platforms due to their undesirable procedures such as a photoresist deposition or ultraviolet exposure which can degrade electrical characteristics of MoS2. In this regard, a drop-on-demand inkjet-printing process, which has been proposed for realizing large-area, low-cost, and flexible electronics, has attracted attention for transparent conductive and insulating layer formation due to its abilities using a low-cost, non-vacuum, and low-temperature processing. However, there are no reported results on the combination of these emerging technologies due to its difficulties in high quality monolayer MoS2 synthesis and deposition of electronic components on MoS2 using printing method.
In this presentation, we will report the first demonstration of fully flexible and transparent field-effect transistors (FETs) based on CVD-grown large monolayer MoS2 channel layer with all inkjet-printed components. The large monolayer MoS2 film was synthesized by CVD system under low pressure conditions. The uniform structure of the synthesized MoS2 film was verified by Raman and photoluminescene spectra and mapping. Subsequently, the CVD-grown monolayer MoS2 film was transferred onto a flexible substrate, and then all the other device components including transparent conductive polymer electrodes and dielectric layers were deposited by an inkjet-printing process without any surface treatment under ambient condition. The top-gated FETs have shown reasonable electrical and optical characteristics including high transparency over 70 % in a wide wavelength range and excellent stability under repetitive bending tests. In addition, our devices have shown electronically tunable photoswitching properties including a photoresponsivity of ~ 0.1 W●A-1 and an external quantum efficiency of ~ 8 %, which are comparable to the MoS2 devices fabricated by conventional deposition methods. We believe that the fully transparent and flexible MoS2 FETs with all inkjet-printed components can provide a novel approach to realize next-generation optoelectronic applications of 2D transition-metal dichalcogenide (TMDC) materials.
NM1.5: Poster Session I
Wednesday AM, April 19, 2017
Sheraton, Third Level, Phoenix Ballroom
9:00 PM - NM1.5.01
Nanoscale Thermometer for Different 2D Materials
Xuan Hu 1 , Poya Yasaei 2 , Jacob Jokissari 1 , Amin Salehi-Khojin 2 , Robert Klie 1 Show Abstract
1 Department of Physics, University of Illinois at Chicago, Chicago, Illinois, United States, 2 Mechanical Engineering Department, University of Illinois at Chicago, Chicago, Illinois, United States
Heat dissipation is one of the most significant constraints in the design of modern electronic nano-device. 2-dim materials, such as grapheme and transition metal dichalcogenides (TMDs), have attracted significant interests due to their potential thermal management applications in electronics and optoelectronics. The knowledge of the thermal transport properties in 2-dim materials is, therefore, one of the most important aspects of current research in such materials systems. Most importantly, an understanding of thermal properties of 2-dim materials on the local lever, in particular at interfaces and surface is needed. Until recently, however, the quantification of the temperature gradients at this nm-scale was impossible for 2-dim materials.
In this contribution, we will introduce a method for measuring the local temperature gradients in low dimensional materials using a combination of scanning transmission electron microscope (STEM) and electron energy loss spectroscopy (EELS). More specifically, we will correlate the change in the measured plasmon energies in different TMDs over a range of temperatures. Free-standing 2D materials (graphene, MoS2, MoSe2, WS2, WSe2) are prepared and we find a relationship between the plasmon energy and the sample temperature, which can now be used to map the temperature gradient across interfaces and particle surfaces. We will also perform first-principles modeling of the low-loss EELS signal to correlate the measured changes with the local thermal expansion coefficient.
9:00 PM - NM1.5.02
Synthesis of Transition Metal Dichalcogenide Films by Chemical Transformation of Solid Thin Films
Christopher Chen 1 , Christoph Kastl 1 , Brian Shevitski 1 , Adam Schwartzberg 1 , Tevye Kuykendall 1 , Shaul Aloni 1 Show Abstract
1 , Lawrence Berkeley National Lab, Berkeley, California, United States
The promising properties of transition metal chalcogenides (TMD’s) continue to inspire great deal of research on optical and electronic devices. However, the progress in this field is limited by challenges in materials synthesis and device fabrication. In this work we present a new approach for the synthesis of TMD’s with digital control of layer thickness. This method utilizes chemical transformation of solid thin films of oxides deposited with sub-monolayer precision by ALD. Following their deposition the films are exposed to a chalcogen containing gas resulting in smooth and continuous TMD films whose thickness is defined by the thickness of the ALD deposited oxide film.
Typical experiments involve deposition of metal oxide, for example: WO3 or MoO3, followed by a short conversion procedure involving annealing of the oxide film in presence of a chalcogenation agent, e.g. hydrogen disulfide gas or organochalcogen vapor. Typical composition of the gas phase is equivalent to 1% of H2S in argon. However, precise control of water content in the gas phase composition provides means for controlling the reaction mechanism. At low water vapor content (2-10 ppm) metal oxide films are chalcogenized in place with the thickness of the continuous TMD film defined by the thickness of the oxide layer. The resulting WS2 thin films are nanocrystalline, and moderately luminescent . At higher water concentrations (> 200 ppm) the process is dominated by vapor transport. Under these conditions, the volatility of the oxide species is significantly enhanced, resulting in minimal residual metal disulfide after growth. Under optimized conditions, highly luminescent, triangular monolayer WS2 and MoS2 islands with good island-to-island uniformity can be grown directly on the previously metal-oxide-coated substrate or onto a bare substrate placed downstream of a source. Carefully controlled humidity (~ 100 ppm) consistently produces high quality highly luminescent triangular WS2 and MoS2 islands.
The chemical transformation of solid films by a gas phase precursors offers additional benefits. In addition to precise control of thickness and compatibility with many transition metals, it is also compatible with any substrate that is not adversely affected by the chalcogenation agent. We present deposition of WS2 on variety of substrates including amorphous SiO2 and Si3N4 as well as SiC, TiO2 and GaN. Moreover, we suggest that the use of controlled amounts of water vapor is a new knob by which to tune growth of these materials, and these results demonstrate a route to improved material quality and unprecedented reproducibility of chemical vapor transport of many transition metal dichalcogenides.
9:00 PM - NM1.5.03
2D-Transition Metals Carbides (MXenes) with Outstanding EMI Shielding Properties
Mohamed Alhabeb 1 , Christine Hatter 1 , Babak Anasori 1 , Yury Gogotsi 1 Show Abstract
1 , Drexel University, Philadelphia, Pennsylvania, United States
In todays’ society, our dependence on electronic devices increases as technology advances. This includes TVs, mobile phones, computers, printers, medical devices, etc. These high demands make electronics the fastest growing industry around the globe. However, these electronic gadgets produce electromagnetic interference (EMI) that may have a harmful impact on their performance and operation of other devices in their vicinity.
Currently, thin metal coating or foils are usually used to combat EMI due to their high electrical conductivity. However, density and flexibility of metals sheets is a limitation when it comes to smart wearable and light-weight electronics. 2D transition metal carbides known as MXenes  possess a high electrical conductivity up to 5000 S/cm and high mechanical flexibility.
Here, we report that a micron-thick coating of MXene shows an outstanding EMI shielding performance and outperforms all carbon-based materials of comparable thicknesses and even currently used commercial metal shield made of copper and aluminum.
1. Naguib, M. and Y. Gogotsi, Synthesis of two-dimensional materials by selective extraction. Acc Chem Res, 2015. 48(1): p. 128-35.
2. Shahzad, F., et al., Electromagnetic interference shielding with 2D transition metal carbides (MXenes). Science, 2016. 353(6304): p. 1137-1140.
3. Ling, Z., et al., Flexible and conductive MXene films and nanocomposites with high capacitance. Proceedings of the National Academy of Sciences, 2014. 111(47): p. 16676-16681.
9:00 PM - NM1.5.04
Organic Semiconductor Exfoliated Nanoscale WS2 towards Organic Optoelectronics
Abdus Sarkar 1 , Suman Pal 1 Show Abstract
1 School of Basic Sciences, IIT Mandi, Mandi, Himachal Pradesh, India
Since, the discovery of single layer graphene in 2004, great attention has been paid to other two dimensional (2D) layered materials from the academic to industrial research. Recently, beyond graphene, the emerging 2D transition metal dichalcogenides (TMDCs), such as semiconducting (2H-phase) tungsten sulfide (WS2) have drawn most vibrant areas of nanoscience research because of direct energy gap and fascinating optical and electrical properties in their low dimensional structure. However, most of the preparation (chemical) methods do not preserve the semiconductive properties of WS2. Here, we have prepared a novel two-dimensional semiconductor heterojunction of few layer WS2, which is exfoliated in the presence of an organic semiconductor (OS) in a chloroform solvent and characterized by various experimental techniques. Our AFM, SEM, TEM and Raman spectroscopy results suggested that the few hundred nanometer size, few layer (4-5 layers) thick WS2 sheets were exfoliated in chloroform solvent and grafted with the OS. Hexagonal pattern in HR-TEM image intimate the presearvation of 2H-phase WS2. Insight spectroscopic analysis indicate the possible hole transfer in a few layer single flake WS2 to OS which is lightening the nano-optoelectronic applications. Highly dispersed WS2 nanosheets with prior little aggregation after 180 days.
9:00 PM - NM1.5.05
CVD Growth of Vertically Aligned MoS2 and WS2 Two-Dimensional Nanostructures and Their Anisotropic Properties
Pawan Kumar 1 , Viswanath Balakrishnan 1 Show Abstract
1 School of Engineering, Indian Institute of Technology Mandi, Mandi, HP, India
Growth of two dimensional(2D) semiconducting materials with ability to control the assembly and orientation provides a way to tune their properties to great extent. We present chemical vapor deposition (CVD) growth of MoS2 2D nanostructures with different alignment and their anisotropic wetting behavior by contact angle measurement. Interestingly, vertically aligned MoS2 with hydrophobic surface shows high current density at lower potential and better charge transfer capacity for hydrogen evolution reaction(HER). We have identified CVD process window to precisely control the growth and orientation of MoS2 nanostructures with mechanistic understanding supported by detailed AFM and electron microscope observations. We also extended the growth mechanism to demonstrate vertically alinged growth of WS2 nanostructures. The presented results to control the organization and orientation of semiconducting MoS2 and WS2 nanostructures provides excellent platform to tune the anisotropic properties of 2D materials and are relevant for smart surface, water splitting and energy storage applications.
9:00 PM - NM1.5.06
Selective Patterning of Amorphous Silicon on MoS2 for Enabling Transition-Metal Dichalcogenide Heterostructures
Markus Heyne 1 2 3 , Andy Goodyear 4 , Jean-Francois de Marneffe 3 , Mike Cooke 4 , Iuliana Radu 3 , Erik C. Neyts 2 , Stefan de Gendt 1 3 Show Abstract
1 , KU Leuven, Leuven Belgium, 2 , University of Antwerp, Antwerp Belgium, 3 , imec, Leuven Belgium, 4 , Oxford Instruments Plasma Technology, Bristol United Kingdom
Ultrathin transition-metal dichalcogenide (TMD) layers such as MoS2 and WS2 are promising materials for future transistor channels since they are expected to result in reduced short channel effects compared to silicon-based transistors. Also hetero-structures of those TMDs and others exhibit strong interlayer interaction with interesting properties and enable tunnel field effect transistors (TFET) (1, 2). Proof-of-concept TFET structures are nowadays based on exfoliated and then stacked flakes, or grown on top of each other by chemical vapor deposition (CVD).
We here propose anovel and innovative approach which converts amorphous Si (aSi) into WS2 by means of WF6 and H2S (3). The conversion is done at a temperature of 450 °C and yields stoichiometric, but randomly oriented WS2. Rapid thermal annealing in inert gas at 900 °C crystallizes the layers and yields horizontally aligned WS2 films. By pre-patterning the aSi films, the desired geometry of WS2 structures can be achieved. Arrays of WS2 lines down to 20 nm width can be obtained.
To enable TFET architectures with stacked layers based on this synthesis route, the selective removal of deposited aSi layers on MoS2 by a low damage etch processes is required. To this end, a low power atomic layer etching (ALE) process has been explored, using an Oxford Instruments ICP chamber equipped with an ALE kit to inject short Cl2 pulses into the process chamber(4). The cycles consist of four steps, starting with a chlorine exposure, a purge, a biased Ar plasma etching step, and a final purge step. After optimization of the process conditions, this ALE sequence removed aSi while preserving the underlying MoS2. This achievement could enable TFET fabrication by applying the proposed aSi-to-WS2 conversion on top of MoS2. Although a defectivity reduction by appropriate annealing protocols in sulfur-rich environments need to be addressed in the future, the proposed methods open the way to build TFET structures using VLSI-compatible techniques.
We acknowledge support from the European Union under Grant Agreement No. 318804 (SNM). M.H. acknowledges the support from Flanders Innovation & Entrepreneurship (VLAIO).
(1) Huo, N.; Kang, J.; Wei, Z.; Li, S.-S.; Li, J.; Wei, S.-H. Adv. Funct. Mater. 2014, 24 (44), 7025–7031.
(2) Chen, H.; Wen, X.; Zhang, J.; Wu, T.; Gong, Y.; Zhang, X.; Yuan, J.; Yi, C.; Lou, J.; Ajayan, P. M.; Zhuang, W.; Zhang, G.; Zheng, J. Nat. Commun. 2016, 7, 12512.
(3) Delabie, A.; Caymax, M.; Groven, B.; Heyne, M.; Haesevoets, K.; Meersschaut, J.; Nuytten, T.; Bender, H.; Conard, T.; Verdonck, P.; Van Elshocht, S.; De Gendt, S.; Heyns, M.; Barla, K.; Radu, I.; Thean, A. Chem. Commun. 2015, 51 (86), 15692–15695.
(4) A. Goodyear, M. Cooke; European patent application EP16187143 (3 September 2015)
9:00 PM - NM1.5.07
A Robust Method for the Synthesis of Colloidal PbS Nanosheets
Liangfeng Sun 1 , Shashini Premathilka 1 , Zhoufeng Jiang 1 , Antara Antu 1 , Joey Leffler 1 , Jianjun Hu 2 , Ajit Roy 2 Show Abstract
1 , Bowling Green State University, Bowling Green, Ohio, United States, 2 , Air Force Research Laboratory, Dayton, Ohio, United States
Two-dimensional nanosheets have the advantage over quantum-dot films since the charge-carrier mobility in the sheet plane is high due to the lack of the charge scattering at the boundary of quantum dots. On the other hand, the two-dimensional structure results in novel properties, including highly efficient carrier multiplication, enhanced optical absorption and radiative recombination, extremely narrow emission spectra and slow Auger recombination.
Ultrathin PbS sheets with lateral-size about hundreds of nanometers have been firstly synthesized through two-dimensional oriented attachment of quantum dots followed by surface reconstruction. However, synthesis of colloidal PbS nanosheets is still at its early stage and the synthesis itself has a low success rate. We conducted a systematic study of the effect of acetic acid and the purity of trioctylphosphine on the formation of the PbS nanosheets. We discovered that there was a threshold of the amount of acetic acid, above which the nanosheets will not form. With the knowledge obtained, we develop a new robust synthesis that can produce pure nanosheets without quantum-dots mixed at nearly 100% success rate. In the new acetate-free synthesis, the purity of trioctylphosphine has no significant effect on the nanosheet growth. We also demonstrate that the thickness of the nanosheets can be tuned by changing the reaction temperature.
9:00 PM - NM1.5.08
Tribological Researching of Atomically Thin TMDCs Using Raman Spectral and AFM
Dameng Liu 1 Show Abstract
1 , Tsinghua University, Beijing China
Tribological properties of atomically thin two-dimensional materials have long been the interests of researching due to their promising applications. In this study, using Raman spectral and atomic force microscope, we explored the friction properties of few-layer MoS2 and other transition metal dichalcogenides (TMDCs), including the friction in tip-sample system and MoS2 homojunctions. In friction force microscope, with improved tips, we suppressed the effect of puckering and revealed the monotonically increasing thicknesses dependent friction of few-layer TMDCs, which is corresponding to the results of density function theory (DFT) calculations. The Raman spectral of twisted multilayer homojunction and related DFT calculations unearth the interlayer van der Waals interactions, which contributes to detecting the interlayer shear and adhesive force in twisted multilayer TMDs.
9:00 PM - NM1.5.09
Characterization of Grain Boundaries and Impact of Plasma-Induced Patterned in 2D Materials
Umberto Celano 1 , Olli Virkki 1 , Daniele Chiappe 1 , Markus Heyne 1 , Ilse Hoflijki 1 , Alexis Franquet 1 , Cedric Huyghebaert 1 , Kristof Paredis 1 , Stefan De Gent 1 , Iuliana Radu 1 , Wilfried Vandervorst 1 Show Abstract
1 , imec, Leuven Belgium
As the electronic devices are rapidly approaching their projected scaling limits, layered two-dimensional (2D) materials such as transition metal dichalcogenides (TMDs) are extensively explored as potential new channel materials and fundamental building blocks of emerging sensors and devices.[1-2] However, their outstanding properties are often degraded in the device fabrication processes including selective growth, patterning and tuning of the electronics properties to name a few.
In this work we use a combination of electrical atomic force microcopy (AFM) and beam analysis techniques to understand the local properties of WS2 and MoS2 comparing pristine material and structures which are selectively grown and patterned. We investigate, in both materials, the local electrical properties of grain boundaries and their transport respectively in pristine and patterned structures for FET devices by conductive atomic force microscopy (C-AFM). Finally, we assess the impact of the plasma-induced damages combining the electrical AFMs and Auger emission spectroscopy. Our results indicate that the ion bombardment in the 2D materials results in a strong decrease of the local material conductivity extending for several tens of nanometers therefore impacting the final device performance.
 G. Fiori, F. Bonaccorso, G. Iannaccone, T. Palacios, D. Neumaier, A. Seabaugh, S. K. Banerjee, and L. Colombo, “Electronics based on two-dimensional materials,” Nat. Nanotechnol., vol. 9, no. 10, pp. 768–779, 2014.
 Desai, S. B., Madhvapathy, S. R., Sachid, A. B., Llinas, J. P., Wang, Q., Ahn, G. H., Javey, A. (2016). MoS 2 transistors with 1-nanometer gate lengths, 354(6308), 2–6.
 Chiappe, D., Asselberghs, I., Sutar, S., Iacovo, S., Afanas’Ev, V., Stesmans, A., … Thean, A. (2016). Controlled Sulfurization Process for the Synthesis of Large Area MoS2 Films and MoS2/WS2 Heterostructures. Advanced Materials Interfaces, 3(4), 1–10.
9:00 PM - NM1.5.10
Interaction of Light with Liquid Suspensions of 2D Nanomaterials—Nonlinear Optics, Thermal Lens Effect and Flow-Induced Alignment
Yanan Wang 1 5 , Yingjie Tang 2 , Feng Lin 1 5 , Peihong Cheng 3 , Xufeng Zhou 4 , Zhuan Zhu 5 , Zhaoping Liu 4 , Dong Liu 2 , Zhiming Wang 1 , Jiming Bao 5 1 Show Abstract
1 Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu China, 5 Electrical Engineering, University of Houston, Houston, Texas, United States, 2 Mechanical Engineering, University of Houston, Houston, Texas, United States, 3 School of Electronic and Information Engineering, Ningbo University of Technology, Ningbo, Zhejiang, China, 4 Institute of Materials Technology & Engineering, Chinese Academy of Sciences, Ningbo, Zhejiang, China
The interaction of light with atomically thin nanomaterials has attracted enormous research interest in order to understand two-dimensional (2D) electron systems and develop novel opto-electronic devices. The observation of spatial self-phase modulation (SSPM) and its multiple diffraction ring patterns in liquid suspension of 2D nanomaterials were believed to be excellent examples of strong laser interaction with 2D nanomaterials and this phenomena have been attributed to their large third-order susceptibility χ(3). By performing a series of control experiments with liquid suspensions of graphene and graphene oxide flakes, we have proved that the self-phase modulation originates from laser-induced local heating and temperature dependent refractive index of the solvents. We further revealed the emergence of the ordered state of flakes and the birefringence patterns from the initial optically isotropic suspensions. Using computational fluid dynamics (CFD), we simulated the evolution of suspension temperature and flow velocity induced by laser heating. The evolution of diffraction rings is well correlated to the transient temperature distribution, and the ordered birefringence pattern arises from the streamline of the fluid and flow-induced flake alignment. Thermal lens effect and subsequent flow-induced alignment are a reflection of the intrinsic shape and optical anisotropy of 2D nanomaterials, but have been previously ignored in nonlinear optical studies of 2D nanomaterials. This work not only clarifies the misinterpretations in SSPM of 2D nanomaterials, but also demonstrates how the thermal effects and flow-induced alignment could influence the optical properties of them. Our method and observations will pave the way for further basic understanding and device applications of 2D nanometerials liquid suspensions.
9:00 PM - NM1.5.11
Liquid Phase Exfoliation and Sensor Properties of Titanium Trisulfide (TiS3)
Alexey Lipatov 1 , Philip Yox 1 , Andrey Lashkov 2 , Jun Dai 1 , Xiao Cheng Zeng 1 , Viktor Sysoev 2 , Alexander Sinitskii 1 Show Abstract
1 , University of Nebraska - Lincoln, Lincoln, Nebraska, United States, 2 , Saratov State Technical University, Saratov Russian Federation
Titanium trisulfide (TiS3) is a layered n-type semiconductor in which chains of sulfur trigonal prisms with Ti4+ centers form a two-dimensional (2D) layer. Monolayer TiS3 was predicted to have an electron mobility over 10000 cm2/Vs . With electronic band gap similar to silicon, it is a very promising material for electronic applications. Recently, we showed few-layer TiS3 field-effect transistors with ON/OFF ratios up to ~104 and demonstrated the material’s compatibility with atomic layer deposition (ALD) technique for top-gate dielectric deposition .
Conventional mechanical exfoliation of TiS3 tends to produce narrow nanoribbons, that are not visible by optical microscopy. Additionally, the low yield of the mechanical exfoliation method is not compatible with large-scale applications. To address these issues, we investigated the liquid phase exfoliation of TiS3 in different solvents. SEM and UV-vis data revealed that ethanol is the most effective solvent for liquid phase exfoliation of TiS3, while TEM confirms the formation of single layers. The data on exfoliation of TiS3 crystals along different crystallographic directions is supported by DFT calculations. We further demonstrate the utility of the liquid-phase exfoliated TiS3 for gas sensor applications. We fabricated TiS3-based gas sensors that can reliably recognize low weight alcohols.
 J. Dai and X.C. Zeng, Angew. Chem., Int. Ed., 54, 2015, p.7572
 A. Lipatov, et. al., Nanoscale 7(29), 2015, p.12291
9:00 PM - NM1.5.12
The First-Principle Investigation on the Phase Change and Band Alignment of Monolayer and Heterostructure Transition Metal Dichalcogenides (TMDs)
Chenxi Zhang 1 , Santosh KC 1 , Yifan Nie 1 , Chaoping Liang 1 , William Vandenberghe 1 , Roberto Longo 1 , Yongping Zheng 1 , Fantai Kong 1 , Suklyun Hong 1 , Robert Wallace 1 , Kyeongjae Cho 1 Show Abstract
1 , University of Texas at Dallas, Richardson, Texas, United States
The boom of research in 2D materials have stimulated a renewed interest in layered crystalline materials with unique electrical and optical properties. Regarded as an outstanding representative, transition metal dichalcogenides (TMDs) ranging from metal, semi-metal to semiconductors have demonstrated their potential significance in the exploration of future nanoelectronic devices.
To utilize the rich electronic properties TMDs, By DFT calculation we investigated the band alignments of 24 monolayer TMDs with respect to the vacuum and made a big map of those as good guiding information for the nanoelctronic devices design.
Hundreds of heterostructures with 2D materials as building blocks have opened a large space to develop new electronic devices for desired applications. In this sense, by DFT simulation we also investigated the band alignment in the TMDs bilayer heterostructures which can be classified into three types: metal-semiconductor, semiconductor-semiconductor and semimetal-semiconductor. Since it is very time consuming to investigate each combination of TMDs bilayer heterostructure, we developed a formula to estimate the highest occupied state evolution before and after stacking by charge equilibrium method (CEM). This formula can give a general understanding on the band alignment variation before and after stacking.
Apart from the applications in the heterostructure devices, TMDs have also been studied in another important application: phase change devices. Among all the TMDs, monolayer MoTe2 and WTe2 are distinguished from other TMDs by the existence of an exceptional semi-metallic distorted octahedral structure (T’) with a quite small energy difference from the semiconducting H phase. In the process of transition metal alloying, an equal stability point of the H and the T’ phase is observed in the formation energy diagram of monolayer WxMo1-xTe2. This thermodynamically driven phase transition enables a controlled synthesis of the desired phase (H or T’) of monolayer WxMo1-xTe2 using a growth method such as chemical vapor deposition (CVD) and molecular beam epitaxy (MBE). Furthermore, charge mediation, as a more feasible method, is found to make the T’ phase more stable than the H phase and induce a phase transition from the H phase (semiconducting) to the T’ phase (semi-metallic) in monolayer WxMo1-xTe2 alloy. This suggests that a dynamic metal-insulator phase transition can be induced which can be exploited for rich phase transition applications in two-dimensional nanoelectronics.
This work was supported in part by the Center for Low Energy Systems Technology (LEAST), one of six centers of STARnet, a Semiconductor Research Corporation program sponsored by MARCO and DARPA.
9:00 PM - NM1.5.13
Machine Learnt Bond Order Potential for Stanene to Probe Thermal Transport Using Molecular Dynamics Simulations
Mathew Cherukara 1 , Badri Narayanan 1 , Alper Kinaci 1 , Kiran Sasikumar 1 , Stephen Gray 1 , Maria Chan 1 , Subramanian Sankaranarayanan 1 Show Abstract
1 , Argonne National Lab, Lemont, Illinois, United States
The growth of single layer tin (stanene) on a Bi2Te3 substrate has engendered a great deal of interest, in part due to stanene’s predicted exotic properties. In particular, stanene has attracted lot of attention owing to its tremendous promise in topological insulation, large-gap 2D quantum spin hall states, lossless electrical conduction, enhanced thermoelectricity, and topological superconductivity. Most of the previous work on stanene has focused on its electronic properties. Atomistic investigations of growth mechanisms (needed to guide synthesis), phonon transport (crucial for designing thermoelectrics), and thermo-mechanical behavior of stanene are scarce. This paucity is primarily due to the lack of inter-atomic potentials that can accurately capture atomic interactions in stanene. To address this, we have developed a bond-order potential (BOP) based on Tersoff’s formalism that can accurately capture bond breaking/formation events, structure, energetics, thermodynamics, phonon frequencies, thermal conductivity, and mechanical properties of single layer tin. We determine the BOP parameters by fitting to a training dataset containing (a) structure, (b) equation of state (energy vs area), (c) elastic constants, and (d) phonon dispersion of stanene obtained from our density functional theory calculations. To optimize this potential, we employed a hybrid global optimization scheme based on genetic algorithms and Nelder-Mead simplex. Finally, we employed our newly developed BOP to study anisotropy in thermal conductivity of stanene sheets and nanotubes, temperature induced rippling, as well as dependence of anharmonicity and thermal conductivity on temperature.
9:00 PM - NM1.5.14
Exploration of the Growth of MoS2 on Naturally Grown Pyrite
Huimin Hao 1 2 , Rusen Yang 2 Show Abstract
1 College of Mechanical Engineering, Taiyuan University of Technology, Taiyuan, Shanxi, China, 2 Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota, United States
Among atomically thin layers of transition metal dichalcogenides, MoS2 attracted more and more research interests due to its unique properties. Although bulk MoS2 is not piezoelectric materials, atomically thin MoS2 with odd numbers of layers exhibits the piezoelectric effect. Piezoelectricity is highly anisotropic, and the piezoelectric constant varies significantly with its crystallographic direction. The current growth of MoS2 has been mainly limited to the silicon wafer, sapphire, glass, and mica, and so on, and the MoS2 thin films are usually random or scattered. To obtain an aligned MoS2 monolayers, we explored a new chemical vapor deposition (CVD) process with natural pyrite as substrates. A cooling system were designed to control the temperature of the pyrite, and sulfur was introduced at desired time. The grown MoS2 was measured with Raman and SEM. MoS2 was successfully deposited on the naturally grown pyrite surface.
9:00 PM - NM1.5.15
Effects of Strain on Structural Rippling in Phosphorene
Oswaldo Sanchez 1 , Ruth Pachter 2 , Ganesh Balasubramanian 1 Show Abstract
1 Mechanical Engineering, Iowa State University, Ames, Iowa, United States, 2 RX, Air Force Research Laboratory, Wright Patterson AFB, Ohio, United States
This presentation focuses on the structural flexibility of phosphorene with defects. Previous analysis on pristine phosphorene demonstrated that it exhibits superior structural flexibility in the armchair direction. This opens the possibility of fabricating devices with complex shapes including folded phosphorene. Here we are interested in modulation of this flexibility upon introduction of vacancy defects in the structure. A computational analysis utilizing molecular dynamics (MD) simulations was applied to investigate the effect of applying a uniaxial strain in the armchair and zigzag directions. The structures measure ~15 nm in length in the strain direction and ~13 nm in the other direction. The vacancies range from mono vacancies (single missing atom) to nano-pores (multiple missing neighboring atoms) of up to seven missing atoms. Structures were found to remain stable despite removal of up to 7 atoms, and the buckling behavior from applied uniaxial strain remains unchanged despite the missing atoms. Additionally, introduction of vacancies presents negligible effects on rippling amplitudes.
9:00 PM - NM1.5.16
Epitaxial Group V 2D Materials—Growth and Electronic Properties
Matthieu Fortin-Deschenes 1 , Olga Waller 1 , Amanda Hebert 1 , Oussama Moutanabbir 1 Show Abstract
1 Génie physique, Polytechnique Montréal, Montréal, Quebec, Canada
Over the last decade, considerable effort has been expended to open the electronic band gap of graphene in order to make it a viable candidate for electronic and optoelectronic applications.In parallel, a variety of novel two-dimensional (2D) materials has been discovered or theoretically predicted since then. Amongst them we find the semi-conducting group V 2D materials, including trigonal and orthorombic phosphorus, arsenic and antimony, with band gaps covering the 0-2.5 eV range [1-3]. So far, only black phosphorus and trigonal antimonene can be produced by exfoliation from bulk crystals [1,4]. It is therefore crucial to develop processes to grow these materials epitaxially in order to make use of their properties, especially of their anisotropic and thickness dependent properties.
In this work, we present our in situ low-energy electron microscopy (LEEM) observations of the nucleation and epitaxial growth of these group V 2D materials with focus on the epitaxial growth of trigonal 2D antimony. These experimental observations are augmented by detailed investigations of the electronic properties and substrate-layer interaction of group V 2D materials on relevant growth substrates. The results presented here are relevant to both fundamental studies on group V 2D material as well as to their integration in electronic and optoelectronic devices.
 Liu, H.; Neal, A. T.; Zhu, Z.; Luo, Z.; Xu, X.; Tomanek, D.; Ye, P. D. Phosphorene: An Unexplored 2D Semiconductor with a High Hole Mobility. ACS Nano 2014, 8, 4033−4041
 Zhang, S., Yan, Z., Li, Y., Chen, Z., & Zeng, H. Atomically Thin Arsenene and Antimonene: Semimetal–Semiconductor and Indirect-Direct Band-Gap Transitions. Angewandte Chemie International Edition 2015, 54(10), 3112-3115.
 Wang, G., Pandey, R., & Karna, S. P. Atomically thin group V Elemental films: theoretical investigations of antimonene allotropes. ACS applied materials & interfaces 2015, 7(21), 11490-11496.
 Ares, Pablo, et al. Mechanical Isolation of Highly Stable Antimonene under Ambient Conditions. Advanced Materials 2016, 28(30), 6332-6336
9:00 PM - NM1.5.17
Mechanisms and Dynamics of Two-Dimensional Black Phosphorus Sublimation
Matthieu Fortin-Deschenes 1 , Pierre Levesque 2 , Richard Martel 2 , Oussama Moutanabbir 1 Show Abstract
1 Génie physique, Polytechnique Montréal, Montréal, Quebec, Canada, 2 Département de chimie, Université de Montréal, Montréal, Quebec, Canada
Black phosphorus is a layered van der Waals material which was first discovered in the bulk form in 1914 . Recently, it has been demonstrated that when thinned down to a few layers, two-dimensional black phosphorus (2D-bP) exhibits interesting properties such as a thickness-dependent band gap and anisotropic transport properties .Black phosphorus thermal stability was studied in the early 20th century , but the atomistic mechanisms governing its decomposition still remain open to interpretation .
Here, we use in situ low-energy electron microscopy (LEEM) to study the thermal stability and decomposition dynamics of 2D-bP under ultra-high vacuum. We find that the thermal decomposition occurs by the successive sublimation of single layers . To gain a deeper understanding of the underlying mechanisms governing 2D-bP sublimation, we compare and test different atomistic models and test them using kinetic Monte-Carlo (KMC) simulations. The KMC simulations are then put in perspective by DFT calculations of the different atomistic processes. We find a good qualitative and quantitative agreement between the LEEM measurements and the KMC simulations and DFT calculations for our sublimation model based on the detachment of phosphorus dimers.
This study provides an important atomistic level understanding of 2D-bP thermal decomposition that is critical to develop reliable material and device processes.
 Bridgman, P. W.; Two New Modifications of Phosphorus. Journal of the American Chemical Society 1914, 36.7 1344-1363.
 Liu, H.; Neal, A. T.; Zhu, Z.; Luo, Z.; Xu, X.; Tomanek, D.; Ye, P. D. Phosphorene: An Unexplored 2D Semiconductor with a High Hole Mobility. ACS Nano 2014, 8, 4033−4041
 Beck, R. P.; Meyer, G.; Smits, A. On Black Phosphorus. I.Koninklijke Nederlandse Akademie van Wetenschappen Proceedings Series B Physical Sciences 1915, 18, 992−1007
 Liu, X., Wood, J. D., Chen, K. S., Cho, E., & Hersam, M. C. In situ thermal decomposition of exfoliated two-dimensional black phosphorus. The journal of physical chemistry letters 2015, 6(5), 773-778.
 Fortin-Deschenes, M., Levesque, P. L., Martel, R., & Moutanabbir, O. Dynamics and Mechanisms of Exfoliated Black Phosphorus Sublimation.The journal of physical chemistry letters 2016, 7(9), 1667-1674
9:00 PM - NM1.5.18
Few-Layer Zinc Oxide Nanosheets Grown at Water-Air and Water-Oil Interface
Xin Yin 1 , Fei Wang 1 , Yanbing Wei 1 , Xudong Wang 1 Show Abstract
1 , University of Wisconsin-Madison, Madison, Wisconsin, United States
Two-dimensional (2D) nanomaterials, in particular when their thickness is just one or a few atomic layers, exhibit physical properties dissimilar to those of their bulk counterparts and other forms of nanostructures. Graphene and transition metal dichalcogenides have epitomized the applications of 2D nanostructures in many electronic, optoelectronic and electrochemical devices. Nonetheless, real-world 2D nanostructures so far have been largely limited to naturally layered materials, that is, the van der Waals solids, synthesized either from top-down or bottom-up. A much larger and diverse portfolio of 2D materials including non-layered compounds are desirable to meet the specific requirements of individual components in various devices.
In our previous work, we reported that ~1 to 2 nm-thick single–crystalline ZnO nanosheets with sizes up to tens of micrometers could be synthesized at the water-air interface, with the surfactant monolayers located at the water-air interface served as the soft templates for the nucleation and growth. Here, through controlled growth in the trough, the adjustment of the headgroup separation could be achieved. The influences of the packing density of the surfactant monolayer on the growth of the nanosheets were studied. When the headgroup separation matched well with the Zn2+ ions separation along the crystal direction <100>, the size of the nanosheets was the smallest while the density of the nanosheets was the highest. More interestingly, when the headgroup separation became larger, the thickness got smaller. Especially when the area per molecule was 0.135 nm2/molecule, the thickness decreased to 1 nm, corresponding to two-layer nanosheets. A relationship between the critical nucleation thickness and the packing density of the surfactant monolayer was proposed to explain these influences.
The growth at the water-oil interface was realized with the toluene as the oil phase, resulting in the polycrystalline triangular nanosheets. Field effect transistor with such synthesized nanosheets as the channel was fabricated. The polycrystalline ZnO showed the p-type transport behavior with a room temperature hole-mobility of 34 cm2V-1s-1, which is two order of magnitude higher than the single-crystalline ZnO. The reason for the improved hole transport properties may originate from the different electronic environment at the grain boundaries that may act as a lateral quantum channel for charge transport.
This mothed, with similar attributes in the processes found in biomineralization, shows great promises as a novel and versatile synthesis paradigm for forming nanosheets from a wide range of inorganic materials including and beyond the van der Waals solids.
9:00 PM - NM1.5.19
Nanoscale Characterization of WSe2 for Solar Cell Applications
Nirmal Adhikari 1 , Avra Bandyopadhyay 1 , Anupama Kaul 1 Show Abstract
1 , University of Texas at El Paso, El Paso, Texas, United States
Two dimensional (2D) thin transition metal dichalcogenides are being widely investigated for optoelectronics applications. Here, we report on the interfacial study of WSe2 with photo-absorber materials for efficient charge transport using Kelvin Probe Force Microscopy (KPFM) for solar cell applications. The WSe2 in these experiments was synthesized using Chemical Vapor Deposition (CVD) with a WO3 powder and Se pellets as the precursors, where the selenium was placed upstream in an Ar carrier gas within the furnace at a temperature zone of 260-270oC. Material analysis using x-ray diffraction and Raman Spectroscopy reveals the presence of WSe2. For the interfacial analysis, nanoscale KPFM measurements show an average surface potential of 300 mV for the CVD synthesized WSe2 flakes. KPFM measurements signify that a thin layer of WSe2 can be used to suppress back recombination of carriers between the electron transport layer (ETL) and the absorber layer. A proper band alignment between ETL and absorber layer helps to increase the overall device performance, which we will elaborate upon in this work.
9:00 PM - NM1.5.20
Phase Engineering of 2D Metal Chalcogenide Crystals
Zafer Mutlu 1 , Ryan Wu 2 , Darshana Wickramaratne 1 , Sina Shahrezaei 1 , Chueh Liu 1 , Selcuk Temiz 1 , Andrew Patalano 1 , Mihri Ozkan 1 , Roger Lake 1 , Andre Mkhoyan 2 , Cengiz Ozkan 1 Show Abstract
1 , University of California, Riverside, Riverside, California, United States, 2 , University of Minnesota, Twin Cities, Minnesota, United States
The ability to grow crystals of desired dimensions is an important aspect of the design of new functional materials. The crystals grown in two dimensions enable successful and efficient modification of a wide range of physical, electronic, optical and structural properties tailored for smaller and more efficient electronics and optoelectronics applications. The control of phases in crystals is also an additional element in the design of functional materials. Two-dimensional (2D) atomic crystals can crystallize in a variety of crystal phases, which possess distinct properties. Therefore, identifying approaches for controlling dimension and crystal phase is essential for tuning the properties of 2D materials to specific applications. This talk discusses the growth and phase engineering of various earth-abundant and non-toxic 2D metal chalcogenide systems including tin sulfides (Sn-S) and iron sulfides (Fe-S) via chemical vapor deposition (CVD). Detailed characterization of each phase of these metal chalcogenide systems is performed using several microscopy and spectroscopy methods, and the results are corroborated by ab-initio density functional theory (DFT) calculations.
9:00 PM - NM1.5.21
Anti-MoS2 Material—2D Nanosheets of Acanthite by Liquid Phase Exfoliation
Neerish Revaprasadu 1 2 , Neerish Revaprasadu 1 2 , David Lewis 2 , Paul Obrien 2 1 , Mohammad Azad Malik 2 1 Show Abstract
1 , University of Zululand, Empangeni, Kwazulu Natal, South Africa, 2 , University of Manchester, Manchester United Kingdom
After the discovery of graphene and the amazing properties associated with it for advanced technological applications, layered 2 dimensional materials have attracted worldwide attention. The bulk layered crystals are composed of stacked layers which have strong covalent bonding within the layers but interconnected by each other by weak Vander Waals force of attraction. The layers can be separated easily by breaking the Vander Waals interaction between the layers. Transition metal dichalcogenides have sandwich like structure in which the transition metal atom is sandwiched between two chalcogenide atoms. Silver sulfide has an interesting structure which is inverse to transition metal dichalcogenides i.e. chalcogen is sandwiched between two silver atoms. It has been observed that the Ag-Ag contacts between two adjacent layers is equal to Vander Waals distance and can be exfoliated. Such sheets with anti-MoS2 structure may exhibit interesting unique properties. Herein, we report the synthesis and characterization of 2D silver sulfide nanosheets. Silver sulfide was synthesized by solventless decomposition of (O-ethyldithiocarbonato)silver(I) complex. The bulk silver sulfide generated was exfoliated using NMP to obtain nanosheets. The size and thickness of the sheets was determined by scanning electron microscopy and atomic force microscopy respectively. We believe that it would be a valuable addition to the 2D materials.
9:00 PM - NM1.5.22
A Versatile Method to Grow Single Crystal 2D Transition Metal Dichalcogenide (MoS2) from Self-Correcting Polymer Template
Xining Zang 1 2 , Minsong Wei 1 2 , Kaiyuan Yao 3 , Liwei Lin 1 2 Show Abstract
1 Mechanical Engineering, University of California, Berkeley, Berkeley, Colorado, United States, 2 , Berkeley Sensor and Actuator Center, Berkeley, California, United States, 3 Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California, United States
In this paper, we synthesize centimeter scale 2D single crystal transition metal dichalcogenide MoS2 with the assistance of gelatin self-assembly. Highly ordered gelatin helical tri-polymer formed 2D scaffold (“Jelly”) complexing with molybdenum ions dissolve in water, which is converted to layer-by-layer chalcogenide crystal by convection during the sulfur annealing process, XPS, Raman, XRD and EDS prove that we synthesis pure uniform multiple layer of MoS2, while the micro-diffraction single crystal XRD prove the single crystalline structure of as-grown MoS2 film. Using diluted solution assisted with surfactant, few layer and monolayer MoS2 is achieved by well-defined self-assembly process. Comparing to the common chemical vapor deposition at 650 oC, we are processing at much lower temperature down to 300 oC. Our method is a much more forgiving process which works at a wide range of precursor concentration and annealing atmosphere. We further pattern a thin film transistor on few layer MoS2.
9:00 PM - NM1.5.23
Ab Initio Design of In Situ Composite Morphology through Control of Nucleation Parameters
Rahul Basu 1 Show Abstract
1 Mechanical, Adarsha Institute of Technology, Bangalore, Ka, India
The mass and thermal balance equations for secondary phase formation from a parent matrix are set up based on the diffusive flux.
Solution of the equations depend on solving a moving boundary problem with the appropriate boundary conditions at the surface and presence of heat sources and sinks. Assuming constant thermal properties, with the aid of similarity transformations, error function solutions can be stated which satisfy the boundary conditions for various geometries. Under certain combinations of parameters, one or more morphologies are preferred over the others. This tendency is illustrated for ice ( spheres and plates), over Napthalene ( needles), for the particular case of sublimation and deposition from the vapour phase. Extension of this tendency is possible for solid-solid and liquid- solid transformations where design of nano composites with preferred secondary morphologies may be designed in this way, through the control of boundary conditions, material and thermal parameters. In particular, the planar morphologies preferred by water (snow crystals) and carbon (graphene and graphite) are looked at in terms of the basic macroscopic physical and thermal parameters.
9:00 PM - NM1.5.24
Developing Au@MoS2 Core-Shell Heterostructures with Strong Light-Matter Interactions
Yuan Li 1 , Jeffrey Cain 1 , Xinqi Chen 1 , Vinayak Dravid 1 Show Abstract
1 , Northwestern University, Evanston, Illinois, United States
There are emerging opportunities to harness diverse and complex geometric architectures based on nominal two-dimensional atomically layered structures. The chemical vapor deposition of two-dimensional layered transition metal dichalcogenides (TMDs) has been realized on various flat substrates including oxidized silicon wafer, h-BN, sapphire, graphene and Au foil. However, the direct growth of fullerene-like MoS2 on spherical nanoparticles of noble metals has not yet been achieved. Herein we report synthesis and properties of a new core-shell heterostructure, termed Au@MoS2, where the Au nanoparticle is snugly and contiguously encapsulated by few shells of MoS2 atomic layers. The heterostructures were synthesized by direct growth of multilayer fullerene-like MoS2 shell on Au nanoparticle cores. The Au@MoS2 heterostructures exhibit interesting light-matter interactions due to the structural curvature of MoS2 shell and the plasmonic effect from the underlying Au nanoparticle core. We observed significantly enhanced Raman scattering and photoluminescence emission on these heterostructures. We attribute these enhancements to the surface plasmon-induced electric field, which simulations show to mainly localize within the MoS2 shell. We also found potential evidence for the charge transfer-induced doping effect on the MoS2 shell. The DFT calculations further reveal that the structural curvature of MoS2 shell results in a modification of its electronic structure, which may facilitate the charge transfer from MoS2 to Au. Such Au@MoS2 core-shell heterostructures have potential for future optoelectronic devices, optical imaging, and other energy-environmental applications.
9:00 PM - NM1.5.25
A Single-Molecule-Thin Organic Layer on Black Phosphorous and Transitional Metal Dichalcogenides as Corrosion Inhibitor and Its Characterizations
Cong Su 1 2 , Zongyou Yin 3 , Ju Li 1 Show Abstract
1 Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 Microsystems Technology Laboratories, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
In this work, we have devised a new way of protecting some chemically reactive 2D materials by depositing an ultra-thin (<1nm) organic layer on top of 2D materials. The mothod for depositing the organic materials is simplified into a one step process, where no high vacuum or high temperature is needed (in our case only 130 Celcuis), very easy to be scaled up in industry. As a result, the lifetime of black phosphorous (BP) could be extended with two orders of magnidute exposing to ambient air. A deeper investigation shows that the top layer is connected to the BP by Coulomb interaction with a medium-strength hydrogen bonding. It is worth mentioning that this method could also be applied to transitional metal dichalcogenides, including MoS2, WS2, MoTe2 (1T'), WSe2, and TaS2, for preventing these materials from rapid degradation, but probably with a slightly different bonding mechanism.
9:00 PM - NM1.5.26
Scanning Tunneling Microscopy and Spectroscopy of Air Exposure Effects on Metal Dichalcogenides
Jun Hong Park 1 , Steven Wolf 1 , Suresh Vishwanath 2 , Sarah Eichfeld 3 , Joshua Robinson 3 , Huili Xing 2 , Andrew Kummel 1 , Iljo Kwak 1 Show Abstract
1 , University of California, San Diego, La Jolla, California, United States, 2 , Cornell University, Ithaca, New York, United States, 3 , Pennsylvania State University, State College, Pennsylvania, United States
The effect of air exposure on WSe2/HOPG, MoS2/HOPG and SnSe2/SnSe/HOPG were determined via scanning tunneling microscopy. WSe2 and SnSe2/SnSe/HOPG were grown by molecular beam epitaxy on highly oriented pyrolytic graphite (HOPG), and, afterwards, a Se adlayer cap was deposited in-situ on WSe2/HOPG to prevent unintentional oxidation during transferring from the growth chamber to the STM chamber. Conversely, MoS2 was grown via chemical vapor deposition on HOPG, and transferred into UHV chamber without capping.
After annealing of WSe2 at 773 K to remove (decap) the Se adlayer, STM images show that WSe2 layers nucleate at both step edges and terraces of the HOPG. Exposure to air for 1 week and 9 weeks caused air-induced adsorbates to be deposited on the WSe2 surface; however, the band gap of the terraces remained unaffected and nearly identical to those on de-capped WSe2. The air-induced adsorbates can be removed by annealing at 523 K. Air exposure caused the edges of the WSe2 to oxidize and form protrusions, resulting in a larger band gap in the scanning tunneling spectra (STS) compared to the terraces of air exposed WSe2 monolayers. The preferential oxidation at the WSe2 edges compared to the terraces is likely the result of edge dangling bonds. In the absence of air exposure, the dangling edge bonds had a smaller band gap compared to the terraces and a shift of about 0.73 eV in the Fermi level towards the valence band. However, after air exposure, the band gap of the oxidized WSe2 edges became larger (about 1.08 eV greater) than the band gap of the WSe2 terraces, resulting in the electronic passivation of the WSe2 step edges
In case of CVD grown MoS2 on HOPG, before air exposure, triangular islands of MoS2 were observed on HOPG and a 2.3 eV band gap on the MoS2 monolayer (ML) was observed in STS. However, after air exposure for one day, massive amount of hydrocarbon was observed on MoS2/HOPG, mostly near MoS2 step edges. Although the band gap of MoS2 ML terrace was similar to bare MoS2 ML, the band gap at step edges of MoS2 was mixed with large band gap (oxide formation) and narrow band gap (hydrocarbon) regions. Therefore, when MoS2 is employed for single domain devices, air induce contaminants are only a problem at step edges.
MBE grown SnSe2 layer was not stable in ambient air, contrast to MBE grown WSe2 and CVD grown MoS2. After decapping Se adlayer on SnSe2/HOPG at 250 °C for 15 min, atomically flat SnSe2 layer was observed in the STM images. In STS, band gaps of 1.7 eV on SnSe2 ML and 1.1 eV on SnSe2 BL were observed by STS. However, after air exposure for 1 day, the entire SnSe2 layer appear disordered in STM (decomposed) and STS displayed very narrow band gap. Therefore, SnSe2 requires the capping layer or a passivation method for the device fabrication.
9:00 PM - NM1.5.27
Highly Crystalline CVD-Grown Multilayer MoSe2 Films and their Applications
Na Liu 1 , Heekyeong Park 1 , Seongin Hong 1 , Healin Im 1 , Ok Jin Kim 1 , Young Ki Hong 1 , Sunkook Kim 1 Show Abstract
1 School of Advanced Materials Science & Engineering, Sungkyunkwan University, Suwon Korea (the Republic of)
Layered semiconductors based on transition metal dichalcogenides (TMDs) (MX2, M = Mo, W; X = S, Se, Te) exhibit many distinctive characteristics, including wide band gap (Eg > 1 eV), high carrier mobility (>100 cm2/Vs), large photoresponsivity (~500 A/W), and an outstanding mechanical flexibility. Thus, the TMD materials can serve as high-performance flexible/stretchable multifunctional electronics such as field-effect transistors (FETs), photodetectors, sensors and integrated circuitries. For realizing such electronic devices, reproducible large-area production of two-dimensional TMD materials becomes a key issue.
It has been reported that MoSe2 shows a higher photoresponsivity compared to MoS2 because of the quantum confinement effect during the bad gap transition. On the other hand, multilayer TMDs provide a wider spectral response and higher photoresponsivity than monolayer TMDs. Additionally, FETs based on multilayer TMDs offer a relatively high performance, which are expected to be more suitable for commercial fabrication process than monolayer TMDs. Several groups have successfully synthesized monolayer MoSe2. However, there is hardly to find any research in chemical vapor deposition (CVD)-grown multilayer MoSe2 and its applications.
In this study, a highly crystalline multilayer MoSe2 film was grown by CVD method directly onto SiO2 substrates. Firstly, we used a relatively high pressure (>760 Torr) to form multiple nuclei during the CVD growth, resulting in multilayer MoSe2 film. The fabricated multilayer MoSe2 thin-film transistors (TFTs) exhibit ambipolar behaviors with reasonably large field-effect mobility (~10 cm2/Vs) and high photoresponsivity (93.7 A/W). Secondly, large-grain multilayer MoSe2 was synthesized using a modified CVD method. Polycrystalline compounds of MoSe2 as precursor were directly vaporized onto Mo-coated SiO2 substrates. The multilayer MoSe2 has a relatively large grain size of several hundred micrometers. Transistors based on such MoSe2 single grains show n-type characteristics with field-effect mobility up to 121 cm2/Vs and on/off current ratio higher than 104 on Si wafers or plastic polyimide films. The MoSe2 transistor on polyimide substrate even shows good mechanical flexibility while bending down to a radius of curvature as small as 5 mm. The overall results of CVD-grown multilayer MoSe2 offers high speed, highly photoresponsive transistor for high-performance flexible/stretchable electronics.
9:00 PM - NM1.5.28
Electrical Characterization of Atomically Thin InSe Layers
Himani Arora 1 2 , Tommy Schoenherr 1 , Artur Erbe 1 Show Abstract
1 , Helmholtz-Zentrum Dresden-Rossendorf, Dresden Germany, 2 , Technical University Dresden, Dresden Germany
Two-dimensional (2D) materials have gained enormous attention in recent years owing to their huge potential in future electronics and optics. On the one hand, conventional 2D materials like graphene, MoS2, h-BN are being exhaustively studied, on the other hand, search for novel 2D materials is also at a rapid pace. In this study, we have investigated electrical properties of 2D nanosheets of ultrathin Indium Selenide (InSe), a member of the family of III-VI chalcogenides. These bichalcogenides show very interesting properties and phenomena as compared to other 2D materials. Though many theoretical investigations have been done for these materials, very limited experimental knowledge is available regarding their performance in nano-electronics.
Within the III-VI chalcogenide family, we have focused our study primarily on Indium Selenide (InSe). One of the major reasons is its relatively high stability as compared to other members of the family, which gave us the chance to characterize it over a length of time. Moreover, InSe has lighter electron effective mass (m*=0.143 mo) and shows higher mobility of ~103 cm2V-1s-1 and thus can be used for fast, high performance electronics, where MoS2 has been proved undesirable due to high electron mass (m*=0.45 mo) and low room temperature mobility of 50-200 cm2V-1s-1. In this study, we have characterized electrically atomically thin layers of InSe in order to understand the underlying transport phenomena. The InSe layers prepared via micromechanical cleavage of its bulk crystal were deposited on Silicon substrate covered with the SiO2 layer serving as back gate for the fabricated FETs. On performing electrical measurements on these devices, bilayer InSe-based FETs showed n-type conductance and a calculated mobility of 2.1x10-4 cm2V-1s-1. The mobility obtained is found to be very low due to high contact resistance between metal electrode and InSe layer or due to the degradation of InSe surface in the ambience. Currently, we are attempting to improve the mobility of the InSe thin layers while optimizing the metal-InSe interface. In future, these electrical measurements will be performed in an Argon-filled Glove Box in order to avoid the contamination of the surface of InSe. This will definitely improve the conduction through the layers and give us a chance to study their unaltered properties in detail.
9:00 PM - NM1.5.29
Large Scale Density Functional Theory Calculations of Homogeneous and Heterogeneous Two-Dimensional (2D) Materials
Mahesh Neupane 2 1 , Decarlos Taylor 1 , Edward Byrd 1 , Terrance O'Regan 2 Show Abstract
2 , US Army Research Lab, Adelphi, Maryland, United States, 1 , US Army Research Laboratory, Aberdeen Proving Ground, Maryland, United States
Two-dimensional (2D) van-der-Waal (vdW) materials such as graphene, transition metal dichalcogenides (TMDCs) and hexagonal boron-nitride (h-BN) have garnered surging interest due to their unique electronic and optical properties. In particular, these properties exhibit a strong dependency on the number of stacking layers and patterns, mainly due to vdW interaction and out-of-plane quantum confinement. Theoretical validations of the experimentally observed layer-dependent properties are generally provided by ab initio density functional theory plane wave (PW) methods such as VASP and Quantum Espresso (QE). Due to the relatively large number of basis function per atom, the PW-based calculations for larger, experimentally observed stacking order and thickness in these 2D materials become prohibitively challenging. An alternative approach that supports linear scaling and is capable of handling larger systems is based on the contracted and localized basis sets in the real-space . One of the tools utilizing localized basis sets which leads to exponentially localized density matrices in real-space is CP2K . Though CP2K shows promise of scaling to nearly 1 million atoms, its accuracy in predicting layer-dependent structural and electronic properties of 2D materials has never been tested. Motivated by these factors, we perform a systematic study of the layer-dependent structural and electronic properties of homogeneous and heterogeneous vdW systems using CP2K, and compare and contrast our results to the popular PW tools (VASP and QE). Our findings pave the way to a more extensive use of the CP2K method in the study of complex homogeneous and heterogeneous vdW 2D materials in the experimentally observed sizes and scales.
1. W. Hu, L. Lin and C. Yang, The Journal of Chemical Physics, 143, 124110 (2015)
2. J. VandeVondele, J. Hutter, The Journal of Chemical Physics, 127, 114105 (2007).
Linyou Cao, North Carolina State University
Bruce Claflin, Air Force Research Laboratory
Thomas Mueller, Vienna University of Technology
Hua Zhang, Nanyang Technological University
Applied Physics Letters | AIP Publishing
NM1.8: Controlled Scalable Synthesis of 2D TMDC Materials and Heterostructures II
Thursday AM, April 20, 2017
PCC West, 100 Level, Room 106 AB
9:30 AM - *NM1.8.01
Processing and Applications of Monodisperse Two-Dimensional Nanomaterial Inks
Mark Hersam 1 Show Abstract
1 , Northwestern University, Evanston, Illinois, United States
Two-dimensional nanomaterials have emerged as promising candidates for next-generation electronics and optoelectronics , but advances in scalable nanomanufacturing are required to exploit this potential in real-world technology. This talk will explore methods for improving the uniformity of solution-processed two-dimensional nanomaterials with an eye toward realizing dispersions and inks that can be deposited into large-area thin-films . In particular, density gradient ultracentrifugation allows the solution-based isolation of boron nitride , montmorillonite , and transition metal dichalcogenides (e.g., MoS2, WS2, ReS2, MoSe2, WSe2) [5,6] with homogeneous thickness down to the atomically thin limit. Similarly, two-dimensional black phosphorus is isolated in organic solvents  or deoxygenated aqueous surfactant solutions  with the resulting phosphorene nanosheets showing field-effect transistor mobilities and on/off ratios that are comparable to micromechanically exfoliated flakes. By adding cellulosic polymer stabilizers to these dispersions, the rheological properties can be tuned by orders of magnitude, thereby enabling two-dimensional nanomaterial inks that are compatible with a range of additive manufacturing methods including inkjet , gravure , screen , and 3D printing . The resulting printed two-dimensional nanomaterial structures show promise in several applications including photodiodes , anti-ambipolar transistors , gate-tunable memristors , and heterojunction photovoltaics .
 D. Jariwala, et al., ACS Nano, 8, 1102 (2014).
 E. B. Secor, et al., Journal of Physical Chemistry Letters, 6, 620 (2015).
 J. Zhu, et al., Nano Letters, 15, 7029 (2015).
 J. Zhu, et al., Advanced Materials, 28, 63 (2016).
 J. Kang, et al., Nature Communications, 5, 5478 (2014).
 J. Kang, et al., Nano Letters, DOI: 10.1021/acs.nanolett.6b03584 (2016).
 J. Kang, et al., ACS Nano, 9, 3596 (2015).
 J. Kang, et al., Proc. Nat. Acad. Sci. USA, 113, 11688 (2016).
 E. B. Secor, et al., Journal of Physical Chemistry Letters, 4, 1347 (2013).
 E. B. Secor, et al., Advanced Materials, 26, 4533 (2014).
 W. J. Hyun, et al., Advanced Materials, 27, 109 (2015).
 A. E. Jakus, et al., ACS Nano, 9, 4636 (2015).
 D. Jariwala, et al., Proc. Nat. Acad. Sci. USA, 110, 18076 (2013).
 D. Jariwala, et al., Nano Letters, 15, 416 (2015).
 V. K. Sangwan, et al., Nature Nanotechnology, 10, 403 (2015).
 D. Jariwala, et al., Nano Letters, 16, 497 (2016).
10:00 AM - NM1.8.02
Growth of Monolayer TMDs in the 1H and 1T' Phase and Possible Applications
Carl Naylor 1 , A.T. Charlie Johnson 1 Show Abstract
1 , University of Pennsylvania, Philadelphia, Pennsylvania, United States
Monolayer transition metal dichalcogenides (TMDs) are of interest due to their unique electrical and optical properties. Reproducible growth methods are key for scientist to study these materials. I will detail a reproducible and controlled growth method for chemical vapor deposition of monolayer, single-crystal TMD flakes of 1H and 1T’ phase. Atomic force microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and transmission electron microscopy confirm the composition and structure of the TMD flakes. Electrical measurements were performed on the TMDs and variable temperature magnetotransport shows weak antilocalization (WAL) at low temperatures for the monolayer 1T’ flakes. We also studied the decaying mechanism in the 1T' TMDs. Our growth approach provides a pathway to systematic investigation of monolayer, single-crystal 1H and 1T’ TMD and implementation in next-generation nanoelectronic devices.
I will also detail the incorporation of these TMDs into novel applications such as biosensors. By using the 1H TMDs, we were able to develop scalable and accurate biosensors for endorphins. We incorporated the biosensors onto flexible substrates for everyday usage. Finally by stacking materials and creating out of plane heterostructures we were able to make a universal biosensor compatible for all 2D materials.
10:15 AM - NM1.8.03
CVD Growth of Few Layer MoTe2 in the 2H, 1T’ and 1T Phases—Tunable Properties of MoTe2 Films
Thomas Empante 1 , Yao Zhou 2 , Velveth Klee 1 , Ariana E. Nguyen 1 , I Hsi Lu 1 , Michael Valentin 1 , Sepedeh Naghibi Alvillar 1 , Edwin Preciado 1 , Sarah Bobek 1 , Adam Berges 1 , Miguel Isarraraz 1 , Evan Reed 2 , Ludwig Bartels 1 Show Abstract
1 , University of California Riverside, Riverside, California, United States, 2 , Stanford University, Stanford, California, United States
Chemical vapor deposition (CVD) allows the preparation of few-layer films of MoTe2 in three distinct structural phases depending on the growth quench temperature: 2H, 1T’ and 1T. We present experimental and computed Raman spectra for each of the phases and utilize transport measurements to explore the properties of the novel 1T MoTe2, which has not been reported on experimentally to the best knowledge of the authors. Density Functional Theory (DFT) modeling predicts a (semi-) metallic character with gap opening under compressive strain. Our experimental 1T films affirm the former and show facile μA-scale source-drain currents. Variation of the growth method allows the formation of hybrid films of mixed phases that exhibit susceptibility to gating and significantly increased conductivity at the same time.
10:30 AM - NM1.8.04.1
Record Current Density in Monolayer P-Type WSe2 with Ultrathin MoO3 Hole Doping Layers
Connor McClellan 1 , Lili Cai 2 , Eilam Yalon 1 , Xiaolin Zheng 2 , Eric Pop 1 3 Show Abstract
1 Electrical Engineering, Stanford University, Stanford, California, United States, 2 Mechanical Engineering, Stanford University, Stanford, California, United States, 3 Material Science and Engineering, Stanford University, Stanford, California, United States
Transition-Metal Dichalcogenides (TMDs) have recently been explored for a range of opto- and nanoelectronic applications. For all such applications, stable doping techniques compatible with standard fabrication must be introduced. In particular, doping shoule be used to “dial in” the position of the Fermi level, or to reduce access and contact resistance.
Here, we present electrical and optical characterization of heavy p-type doping in WSe2 obtained by ultrathin MoO3 capping layers. High quality MoO3 layers ranging from 10 nm to 40 nm thick are deposited using a flame deposition technique  on monolayer (1L) and few-layer WSe2 field-effect transistors (FETs). The flame-deposited MoO3 nucleates on WSe2 and scanning electron microscopy confirms complete coverage of the WSe2 channel. The large work function of MoO3 allows hole charge transfer from MoO3 to WSe2.
Raman spectroscopy shows ~1 cm-1 red-shift of the A1g peak, as well as peak splitting of the (otherwise degenerate) E2g and A1g peaks. This red shift in the A1g peak indicates a doping effect , whereas the lack of E2g peak shift indicates no strain from the MoO3 layer, which is consistent with the high quality deposition of MoO3 on WSe2 .
We also characterize the MoO3 doping effect using electrical measurements of back-gated WSe2 FETs. Devices are fabricated by e-beam lithography, with Pt contacts, and channel lengths from 300 nm to 1 µm. Prior to MoO3 deposition, the WSe2 FETs showed ambipolar conduction, with relatively symmetric electron and hole currents. The MoO3 doped WSe2 FETs exhibit heavy p-type doping behavior with relatively weak gate dependence, carrying up to 800 µA/µm in few-layer WSe2 FETs, and record current density of 500 µA/µm in 1L WSe2. Transfer length method (TLM) is used to extract a low sheet resistance of ~5 kΩ/sq. and record low hole contact resistance of 800 Ω●µm for few-layer WSe2.
This work highlights the ability to use high workfunction oxides for stable p-type doping of 2D materials, achieving high carrier densities and low contact resistance. Our results pave the way towards the design of better, technologically relevant devices based on 2D materials.
10:45 AM - NM1.8.04.2
Frequency Dependent Thermoelectric Properties of MoS2 in Hopping Regime
Krishna Valavala 1 , Sunphil Kim 1 , Arend van der Zande 1 , Sanjiv Sinha 1 Show Abstract
1 , University of Illinois at Urbana-Champaign, Urbana, Illinois, United States
A wide array of atomically thin 2D materials are being investigated as possible candidates for various electronic and optoelectronic devices. Atomic layers of MoS2, in particular, have been studied extensively due to their promising electronic performance demonstrated early on  and its availability due to mature CVD growth process . One of the biggest challenges in using MoS2 in electronic devices is its high electrical resistivity, which is the result of hopping transport mechanism through localized states in its band structure. It is therefore important to understand the hopping mechanism and estimate the density of states of the localized states to engineer the material for electronic applications. Here, we performed frequency dependent electrical and thermopower measurements on CVD grown monolayer MoS2 at various gate voltages and temperatures. Using the frequency dependence of AC conductivity, we observed varying AC hopping regimes such as multiple hopping, pair hopping and nearly constant hopping at various temperatures and gate voltages. Information about density of states of localized states is also inferred from AC conductivity and thermopower data .
1. Radisavljevic, Branimir, et al. "Single-layer MoS2 transistors." Nature nanotechnology 6.3 (2011): 147-150.
2. van der Zande, Arend M., et al. "Grains and grain boundaries in highly crystalline monolayer molybdenum disulphide." Nature materials 12.6 (2013): 554-561.
3. Davis, E. A., and Mott, N.F., "Conduction in non-crystalline systems V. Conductivity, optical absorption and photoconductivity in amorphous semiconductors." Philosophical Magazine 22.179 (1970): 0903-0922.
11:30 AM - *NM1.8.05
Synthesis and Properties of Monolayer WS2—Manipulating the Photoluminescence
Berend Jonker 1 Show Abstract
1 , Naval Research Laboratory, Washington, District of Columbia, United States
Single monolayer transition metal dichalcogenides such as WS2 exhibit exceptionally strong photoluminescence (PL) dominated by a combination of distinct neutral and charged exciton contributions. The dielectric screening is very low due to their 2D character. Thus their properties are strongly effected by their immediate environment, and can be modified and controlled by variations in local charge density due to adsorbates or electrostatic gating. For example, we have shown how one can optically and reversibly prepare WS2 so that PL originates selectively from either the trion or neutral exciton . The PL polarization arising from spin-valley coupling is further effected by intervalley scattering . We discuss here how the PL properties are effected by growth procedures, and both intrinsic and extrinsic mechanisms.
Monolayer WS2 films are prepared so that the PL is from either the neutral exciton or the negatively charged trion. While the neutral exciton exhibits zero polarization at 300K, we observe optical polarization in excess of 40% for the trion at 300K. The trion emission has a unique, non-monotonic temperature dependence which enables us to determine that intervalley scattering, electron-hole radiative recombination, and Auger processes are the dominant mechanisms at work in this system . Because this dependence involves trions, one can use gate voltages to modulate the PL polarization (or intensity). We modulate the electron density with a gate voltage, and subsequently the polarization of the trion emission continuously from 20-40% . Both polarization and emission energy monotonically track the gate voltage.
Large area high quality materials are essential for technological applications, and enable research requiring larger sample areas such as at high magnetic fields . The introduction of hydrogen to the argon carrier gas during CVD growth dramatically improves the optical quality and increases the growth area of WS2, resulting in films exhibiting mm2 coverage and PL linewidths below 40 meV . The choice of growth substrate (SiO2, Al2O3, silica) and transfer substrate also strongly impact these properties .
Finally, we consider the challenges and opportunities provided by integration of these 2D materials with different substrates to form hybrid 2D / 3D heterostructures .
This work was supported by core programs at NRL and AFOSR AOARD 14IOA018-134141.
 Currie, Hanbicki, Kioseoglou, Jonker, Appl. Phys. Lett. 106, 201907 (2015).
 Kioseoglou, Hanbicki, Currie, Friedman, Jonker, Sci. Rep. 6, 25041 (2016).
 Hanbicki, Kioseoglou, Currie, Hellberg, McCreary, Friedman, Jonker, Sci. Rep. 6, 18885 (2016).
 Hanbicki, McCreary, Kioseoglou, Currie, Hellberg, Friedman, Jonker, AIP Adv. 6, 055804 (2016).
 Stier, McCreary, Jonker, Kono, Crooker, Nature. Commun. 7, 10643 (2016).
 McCreary et al, Sci. Rep. 6, 19159 (2016); Sci. Rep. 6, 35154 (2016).
 Li, McCreary and Jonker, under review.
12:00 PM - NM1.8.06
In Situ TEM Observation of Dynamic Change in Atomic-Layers MoS2
Kuo-Lun Tai 1 , Chun-Wei Huang 1 , Tsung-Chun Tsai 1 , Guan-Min Huang 1 , Yi-Tang Tseng 1 , Jui-Yuan Chen 1 , Wen-Wei Wu 1 Show Abstract
1 , National Chiao Tung University, Hsinchu Taiwan
Two-dimensional materials of transitional metal dichalcogenide (TMD) family, such as MoS2, have attracted significant attention due to its potential to take the place of silicon for the next-generation nanoelectronic device application. However, growth of large area and high quality MoS2 is still challenging. Additionally, the actual dynamic observation of atomic layered MoS2 remains a lot more to explore.
Here, we report the direct vapor phase synthesis of atomic layers MoS2 via sulfurization of MoO3 under low-pressure chemical vapor deposition. The influences of substrate temperature, substrate pre-treatment and total pressure on the growth were systematically investigated. The large area growth results and intriguing morphology were analyzed by scanning electron microscopy (SEM) and optical microscopy (OM). The thickness of the layer and the self-stacked structure was confirmed by Raman and atomic force microscopy(AFM), respectively.
Furthermore, we directly observed the structure evolution in atomic layers MoS2 via In-situ transmission electron microscopy (TEM). The structure evolution visualized the dynamic process of the atomic motions and defect movements . This study not only supplied a distinct method to explore the growth mechanisms but also exhibited the potential application of MoS2 in nanoscale.
12:15 PM - NM1.8.07
Lower Temperature 2D MoS2 Deposition Utilizing Plasma Processing
Philip Campbell 1 2 , Johannes Chiu 3 , Eric Snyder 4 , Atul Gupta 3 , Hunter Ray 1 , Hang Chen 5 , Kevin Wenzel 4 , Brent Wagner 2 , W. Jud Ready 2 , Eric Vogel 1 Show Abstract
1 Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States, 2 Electronic Systems Laboratory, Georgia Tech Research Institute, Atlanta, Georgia, United States, 3 , MKS Instruments, Inc., Wilmington, Massachusetts, United States, 4 , MKS Instruments, Inc., Andover, Massachusetts, United States, 5 Institute for Electronics and Nanotechnology, Georgia Institute of Technology, Atlanta, Georgia, United States
There is significant interest in single layer transition metal dichalcogenides, including MoS2, due to their potential applications in novel electronic and optical devices. For example, their unique band structure (direct-to-indirect bandgap transition) enables an extremely high on/off current ratio in field-effect transistor applications. A number of synthesis methods for large-area MoS2 films have been developed, including chemical vapor deposition and reaction of a thin metal film with sulfur. Despite the large focus on synthesis, many established synthesis methods require high temperatures and direct synthesis on device substrates is not possible within the thermal budget of a CMOS process. Alternatively, films of MoS2 are grown on a sacrificial substrate and subsequently transferred to the desired device substrate, introducing residues which are difficult to remove from the MoS2 film and which degrade device performance.
In order to improve the compatibility of MoS2 synthesis with CMOS processing, lower temperature synthesis methods are necessary. This work is aimed at reducing the synthesis temperatures for MoS2 films through the use of plasmas. Two main approaches are used to study plasma-assisted synthesis: (1) conversion of thin MoOx film to MoS2, using S and H radicals generated via remote plasma and direct plasma, and (2) vapor phase reaction between MoO3 and an H2S plasma. For both approaches, a range of process conditions are used to understand the factors that most influence the synthesis process. The results demonstrate that the efficiency of conversion of a MoOx to MoS2 depends strongly on the flow rate of H2S used, with successful synthesis of large area films at high flow rates and a substrate temperatures of 450° C. Our results indicate that the reduction of MoOx film with H radicals is a crucial pre-step in the sulfurization of the film leading to high quality MoS2 formation at low temperatures. For the vapor phase process, MoS2 synthesis is possible at temperatures of 400 °C or lower, and synthesis is demonstrated for both crystalline and amorphous substrates. These results identify two promising methods to enable direct synthesis of MoS2 within a CMOS fabrication process.
12:30 PM - NM1.8.08
Synthesis of Few Layer MoSe2 on Homogeneous Single Layer Epitaxial Graphene on Si-Face 6H-SiC
Luke Nyakiti 1 , Zachary Robinson 2 , Rachael Myers-Ward 3 , Marc Currie 3 , Jennifer Hite 3 , Karthik Sridhara 1 , Charles Eddy 3 , Edward Clancy 3 , D. Kurt Gaskill 3 Show Abstract
1 , Texas A&M University, Galveston, Texas, United States, 2 , State University of New York at Brockport, Brockport, New York, United States, 3 , US Naval Research Laboratory, Washington, District of Columbia, United States
Molybdenum Diselenide (MoSe2) is a semiconducting transition metal dichalcogenide (TMD) formed when Mo is sandwiched between Se atoms to form either a trigonal prismatic or octahedral atomic structure. Theoretical studies of TMDs show exceptional optical band structure dependence on layer thickness, a property that makes the 2D material system a promising candidate for high performance, low cost tunable materials for flexible electronics, high electron mobility transistors and field tunneling transistors. Most of the reported 2D TMDs reported in the literature are primarily investigated from sub-micron size flakes produced by mechanical exfoliation process followed by transfer onto SiO2/Si substrate. This technique is only useful for laboratory-based, proof-of-concept studies that cannot be scaled for industrial production. It is therefore prudent that a direct approach for the formation of integrated 2D heterostructure with tunable properties be studied. This presentation will highlight the nucleation and evolution of MoSe2, effects of underlying Epitaxial Graphene (EG) layer on MoSe2 lateral growth and corresponding optical as well as composition variation across the step-bunched terrace widths of the templated layers of EG/6H-SiC(0001). µ-Raman spectroscopy, AFM and Photoluminescence (PL) will be used to extract composition, presence, layer thickness, surface morphology variations and optical band structures of the heterostructure. The synthesis process of MoSe2 on a template of monolayer EG/6H-SiC(0001) was carried out in horizontal CVD reactor at 700 – 950oC for 10 - 30 min in a flowing UHP Ar/H2 ambient of 80/20 sccm and 10-100 mbar. The initial Raman Spectroscopy characterization of MoSe2/EG/6H-SiC heterostructure shows a strong planar and axial acoustic active modes of the TMDs material system. In addition, the out-of-plane vibrations of Se atoms and in-plane vibrational modes of Mo and Se atoms observed. The Raman spectrum also show EG Raman 2D mode post MoSe2 synthesis, suggesting preservation of underlying EG. The successful synthesis of wafer-scale formation of MoSe2/EG/SiC heterostructures offers the opportunity for a number of applications with significant advantages over the conventional top-down exfoliation and transfer process
12:45 PM - NM1.8.09
First-Principles Kinetic Monte Carlo Simulation of the Atomic Growth of Transition Metal Dichalcogenides
Yifan Nie 1 , Chaoping Liang 1 , Pil-Ryung Cha 2 , Luigi Colombo 3 , Robert Wallace 1 , Kyeongjae Cho 1 Show Abstract
1 , University of Texas at Dallas, Richardson, Texas, United States, 2 , Kookmin University, Seoul Korea (the Republic of), 3 , Texas Instruments Incorporated, Dallas, Texas, United States
The two dimensional (2D) transition metal dichalcogenides (TMDs) has been under the spotlight of multi-discipline research since its re-discovery. Not only do they possess unique electrical and optical properties that contribute to the establishment of new disciplines of fundamental physics, their low dimension and a finite bandgap make them the potential solution of the scaling crisis faced by the silicon-based electronic device technology. The controlled bottom-up synthesis of the crystalline solid of a material is critical for its thorough investigation and application. One of the challenges which the community is current facing is that the conventional growth model and theory of the three-dimensional (3D) crystals do not always fit for the growth of layered materials. In addition, the experiment alone is inefficient to the searching of the optimal growth conditions, because it is highly time consuming and the controllable experimental parameters often convolute with each other. Simulation tools, especially those in the atomistic scale, can significantly speed up the parameter optimization, as they are able to identify the key microscopic processes and the experimental parameters related to them.
In order to address the key issues in the development of controlled synthesis route of the TMDs, we have established a first principles kinetic Monte Carlo (KMC) simulation model. This model can help understand the underlying growth mechanisms of the 2D materials. Key factors controlling many quality criteria for a growth method are identified. The criteria include sticking coefficient, domain morphology, (homogeneous) nucleation density, grain size, layer number, etc. The model is based on the competition of different diffusion paths of adatoms, which plays the pivotal role in the determination of almost all the quality factors above. The inputs of the KMC simulation are based on the ab-initio calculated datasets. Single parameters can be tuned to see how they steer the growth result. The full functionalities of our KMC model includes: metal and chalcogen adsorption/desorption/diffusion on substrate and grown TMD surface, TMD stacking sequence, chalcogen/metal ratio, flake edge diffusion and vacancy diffusion.
The mechanism described in the current model is in quantitative agreement with the molecular beam epitaxy (MBE) process. It has been used to assist the large grain, few to monolayer growth of WSe2 flakes. With proper modification of the model, it can also be used to simulate the chemical vapor deposition method.
This work was supported by the Center for Low Energy Systems Technology (LEAST), one of six centers supported by the STARnet phase of the Focus Center Research Program (FCRP), a Semiconductor Research Corporation Program sponsored by MARCO and DARPA.
NM1.9: Electronic Properties and Devices of 2D Materials II
Thursday PM, April 20, 2017
PCC West, 100 Level, Room 106 AB
2:30 PM - *NM1.9.01
2D Carbides and Nitrides (MXenes)—Synthesis, Properties and Applications
Babak Anasori 1 2 , Maria Lukatskaya 1 2 , Yury Gogotsi 1 2 Show Abstract
1 Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania, United States, 2 Drexel Nanomaterials Institute, Drexel University, Philadelphia, Pennsylvania, United States
The family of two-dimensional (2D) transition metal carbides and nitrides, MXenes, has been expanding rapidly since the discovery of Ti3C2 in 2011. They are currently made by selective etching and exfoliation of layered ternary carbides. This wet synthesis method adds hydroxyl or oxygen terminations and hydrophilicity to MXene surfaces. Very recently, it was also shown that ultrathin carbide crystals can be synthesized by chemical vapor deposition. About 20 different MXenes have been synthesized, and the structures and properties of more than 30 other MXenes have been theoretically predicted. The availability of solid solutions on M and X sites, control of surface terminations, and a recent discovery of multi-element layered MXenes offer a potential for synthesis of dozens of new distinct structures rendering MXenes the largest known family of 2D materials. MXenes’ versatile chemistry renders their properties tunable for different applications, such as energy storage, electromagnetic interference shielding, reinforcement for composites, water purification, chemical, photo- and electro-catalysis, bio- and gas-sensors, lubrication, etc. Attractive electronic, optical, plasmonic and thermoelectric properties have also been predicted. The synthesis, structure, properties and applications of MXenes followed by an outlook for the future research will be presented.
3:00 PM - NM1.9.02
Thickness Control of Few-Layer Black Phosphorus and Its Device Applications
Geonyeop Lee 1 , Yongbeom Kwon 1 , Jinho Bae 1 , Suhyun Kim 1 , Jihyun Kim 1 Show Abstract
1 , Korea University, Seoul Korea (the Republic of)
Black phosphorus (BP) receives great attention for a substitute of graphene. It is reported that direct band gap of BP can be controlled from ~0.3 eV (bulk) to ~2.0 eV (mono layer) depending on its thickness and extremely high carrier mobility (~20000 cm2/V s) is expected theoretically. However, it is difficult to obtain ultrathin BP flakes due to their low probability of mechanical exfoliation and effective growth techniques have not been developed. Therefore, study of thinning methods to fine-control the thickness of BP flakes is important in order to improve the performances of BP-based devices.
In our study, thickness of BP flakes were controlled by SF6 plasma using reactive ion etching (RIE) system. BP flakes were mechanically exfoliated onto SiO2/Si substrate with back-gate electrode (Ti/Au) in Ar-filled glove box to prevent degradation. After that, we fabricated two electrodes (Cr/Au) using photolithography to measure the electrical properties. BP flakes were chemically etched by SF6 plasma without damages of internal BP structure. We placed substrate with the face down in the RIE chamber to avert direct ion bombardments. Property changes of thinned BP flakes such as morphological, optical and electrical properties were measured using atomic force microscope, Raman spectroscopy, XPS and semiconductor parameter analyzer. Field-effect transistors based on thickness-controlled BP showed improved device performance. The details of our experiment conditions and results will be presented at the meeting.
3:15 PM - NM1.9.03
Te-Doped Black Phosphorus Field-Effect Transistors
Zhongming Zeng 1 2 , Fusheng Wen 1 , Jianyong Xiang 1 , Zhongyuan Liu 1 Show Abstract
1 , State Key Laboratory of Metastable Materials Science and Technology, Qinghuangdao China, 2 Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, Jiangsu, China
Two-dimensional (2D) crystals are a class of materials for applications in future electronic technologies. Few-layer black phosphorus (BP), a newly identified 2D elemental material, has been demonstrated to be an appealing candidate material owing to its exotic physical properties such as thickness-dependent tunable band gap and high carrier mobility. However, practical implementation has been limited by fast ambient degradation of few-layer BP. Here we show that by doping with Te, performance of BP devices is markedly improved and environmental stability is significantly enhanced. For example, while only ~4.5% of 22 tested undoped BP devices exhibit carrier mobilities of ~1,000 cm2V-1s-1, ~38% of 45 Te-doped ones possess values over ~1,000 cm2V-1s-1 with the maximum up to 1,850 cm2V-1s-1. After three week exposure to air and moisture, un-doped devices approached to failure with non-detectable mobility, while Te-doped devices retain mobilities of over 200 cm2V-1s-1 and ON/OFF current ratios of ~500. Our results demonstrate a peculiar promising pathway of doping to improve performances and suppress ambient degradation of BP devices.
3:30 PM - *NM1.9.04
Multifunctional Few Layered Black Phosphorus Composite
Qingyu Yan 1 , Zhongzhen Luo 1 , Yu Zhang 1 Show Abstract
1 , Nanyang Technological University, Singapore Singapore
Delamination of black phosphorus (BP) into monolayer or few-layer, dubbed phosphorene, and manipulation of its newly discovered properties that are unattainable by its bulk structure have been a recent scientific breakthrough. As an elemental analogue of graphene, researchers intended to exploit its structural similarity to substitute graphene as anode for lithium ion batteries (LIBs) as one of the dominant power sources for portable electronic devices. In addition, theoretical studies have revealed that Li diffusion along zigzag directions in phosphorene is hundreds times faster than that in MoS2 or graphene.
We will present our recent activities on synthesis of few layered black phosphorus nanosheets through diferent methods and study their proerties in thermoeelctric, electrocatalyst and Li/Na storage applications. For thermoeelctric applications, with teh aid of surface nanoparticle attachment, we could significantly decrease the thermoconductivity. Especially, we try to improve their chemical stability by forming composite structures with the aid of spark plasma sintering (SPS) process. It shows that excellent air stability of SPS-processed black phosphorus can be achieved over the 60 days observation in maintaining its high Li storage properties.
4:30 PM - NM1.9.05
Atomically Thin Two-Dimensional Organic-Inorganic Hybrid Perovskites
Letian Dou 1 , Peidong Yang 1 , Dandan Zhang 1 Show Abstract
1 Department of Chemistry, University of California, Berkeley, Berkeley, California, United States
Intense research efforts have focused on two-dimensional (2D) materials in the last decade. Layered organic-inorganic hybrid perovskites, which were discovered more than 20 years ago and have proved to be promising semiconductor materials, have not been made into atomically thin sheets and investigated so far. In this talk, we report the solution-phase growth of single and few unit-cell-thick single crystalline 2D hybrid perovskites of (C4H9NH3)2PbBr4 with well-defined square shape and large size. In contrast to other 2D materials, the hybrid perovskite sheets exhibit an unusual structural relaxation, and this structural change leads to a band gap shift compared to the bulk crystal. The high quality 2D crystals exhibit efficient photoluminescence, and color tuning could be achieved by changing sheet thickness as well as composition via the synthesis of related materials. This study opens up the possibility of fundamental research into the synthesis and properties of atomically thin 2D hybrid perovskites, and introduces a new family of 2D semiconductors for nanoscale optoelectronic devices.
4:45 PM - NM1.9.06
Low-Frequency Current Fluctuations in the Charge-Density-Wave Quasi-2D 1T-TaS2 Thin Films
Guanxiong Liu 1 , Tim Pope 2 , Tina Salguero 2 , Alexander Balandin 1 Show Abstract
1 , University of California, Riverside, Riverside, California, United States, 2 , University of Georgia, Athens, Georgia, United States
The charge-density-wave (CDW) phase is a macroscopic quantum state consisting of periodic modulation of the electronic charge density accompanied by lattice distortion in quasi-1D and layered quasi-2D metallic crystals. The phase transition can be affected by temperature, pressure, electrical bias and film thickness . We have recently demonstrated that the nearly commensurate (NC) to incommensurate (IC) CDW phase transition in quasi-2D 1T-TaS2 films can be triggered by applying an electrical bias . This effect was utilized to build a compact high-frequency CDW voltage-controlled oscillator operating at room temperature (RT). In this study, we report results of the low-frequency electronic noise measurements in 1T-TaS2 CDW device operating near the phase transition point. The devices were fabricated with the h-BN capping layer to prevent oxidation and degradation of quasi-2D TaS2 channels. In the temperature and voltage dependent noise measurements we observed sharp increase of the noise spectral density near the CDW phase transition. In the voltage dependent measurements, an additional noise peak occurs before the threshold voltage of the NC-IC CDW transition. The voltage at the second noise peak coincides with the voltage value, at which the current-voltage characteristics deviate from the linear dependency. There exists the 1/f2 component in the noise spectra at this bias. We interpret it as the CDW de-pinning from the lattice in the presence of the electrical field. The obtained results are important for understanding the nature of the electron transport and electrically-induced phase transitions in the quasi-2D CDW systems.
This work was supported, in part, by NSF EFRI 2-DARE project: Novel Switching Phenomena in Atomic MX2 Heterostructures for Multifunctional Applications.
 R. Samnakay, D. Wickramaratne, T. R. Pope, R. K. Lake, T. T. Salguero and A. A. Balandin, Nano Letters, 15, 2965 (2015).
 G. Liu, B. Debnath, T. R. Pope, R. K. Lake, T. T. Salguero and A. A. Balandin, Nature Nanotechnology, 11, 845 (2016).
5:00 PM - NM1.9.07
Plasma-Enhanced Atomic Layer Deposition of Uniform Hexagonal Boron Nitride Films
Hamin Park 1 , Tae Keun Kim 1 , Sung Woo Cho 2 , Hong Seok Jang 2 , Sang Ick Lee 2 , Sung-Yool Choi 1 Show Abstract
1 , KAIST, Daejeon Korea (the Republic of), 2 , DNF Co., Ltd., Daejeon Korea (the Republic of)
Hexagonal boron nitride (h-BN) has attracted a lot of interest because of its extraordinary characteristics such as a wide band-gap (5.5-6.0 eV) and inherently flat surface that does not contain dangling bonds or charged impurities. h-BN is potentially suitable for dielectric layers in electronic devices, however it has been previously manufactured using mechanical exfoliation and chemical vapor deposition methods, which make the large-scale synthesis of uniform h-BN films very challenging. In this study, we produced highly uniform and scalable h-BN films by plasma-enhanced atomic layer deposition (PE-ALD), which were characterized by various techniques including atomic force microscopy, transmission electron microscopy, Raman spectroscopy, and X-ray diffraction. The film composition studied by X-ray photoelectron spectroscopy and Auger electron spectroscopy corresponded to a B:N stoichiometric ratio close to 1:1, and the band-gap value (5.65 eV) obtained by electron energy loss spectroscopy was consistent with the dielectric properties. We expect the PE-ALD technology for large-scale and uniform h-BN films as a platform for 2-D material based electronics.
5:15 PM - NM1.9.08
Large Area Synthesis of Black Arsenic-Phosphorus—Next Generation Two-Dimensional Infrared Semiconductors
Eric Young 1 2 , Junsoo Park 2 , Ryan Deblock 2 , Mike Lange 1 , Jesse Tice 1 , Bruce Dunn 2 , Vidvuds Ozolins 2 , Dwight Streit 2 , Vincent Gambin 1 Show Abstract
1 , Northrop Grumman, Redondo Beach, California, United States, 2 Materials Science and Engineering, University of California, Los Angeles, Los Angeles, California, United States
Herein we report the large-scale synthesis of few-layered black arsenic phosphorus (AsP) alloys via solid-source molecular beam epitaxy (MBE). Our method affords precise synthetic control of AsP thin films, resulting in a range of tunable compositions with varying electronic and optical properties. Post-growth hermetic passivation and subsequent high-temperature rapid thermal annealing results in crystalline films as verified by Raman spectroscopy and structural metrology. Composition variation results in a tunable bandgap in the range of 0.15 eV to 0.3 eV for thin films. Further reduction of the film thickness causes bandgap increases beyond 1.6 eV. We simulate crystal structure, bandgap and Raman spectra from first-principle DFT-based computations and compare to our experimental results. Bandgap tunability, as well as uniquely strong electronic, thermal, and optical anisotropy, makes AsP a promising candidate for near- to mid-wavelength IR devices. Our wafer-scale growth technique enables the development of next-generation black AsP devices for optoelectronic, digital and RF applications.
5:30 PM - NM1.9.09
Resonant Bonding Driven Giant Phonon Anharmonicity and Low Thermal Conductivity of Phosphorene
Guangzhao Qin 1 , Ming Hu 1 Show Abstract
1 , RWTH Aachen University, Aachen Germany
Two-dimensional (2D) phosphorene, which possesses fascinating physical and chemical properties distinctively different from other 2D materials, calls for fundamental understanding of thermal transport properties for its rapidly growing applications in nano-/opto-electronics and thermoelectrics. However, even the basic phonon property, for example, the exact value of the lattice thermal conductivity of phosphorene reported in literature, can differ unacceptably by one order of magnitude. More importantly, the fundamental physics underlying its unique properties such as strong phonon anharmonicity and unusual anisotropy remains largely unknown. Herewith, based on the analysis of electronic structure and lattice dynamics from first-principles, we report that the giant phonon anharmonicity in phosphorene is associated with the soft transverse optical (TO) phonon modes and arises from the long-ranged interactions driven by the orbital governed resonant bonding. We also provide a microscopic picture connecting the anisotropic and low thermal conductivity of phosphorene to the giant directional phonon anharmonicity and long-ranged interactions, which are further traced back to the asymmetric resonant orbital occupations of electrons and characteristics of the hinge-like structure. The unambiguously low thermal conductivity of phosphorene obtained consistently by three independent ab initio methods confirms the phonon anharmonicity to the large extent and is expected to end the confusing huge deviations in previous studies. This work further pinpoints the necessity of including van der Waals interactions to accurately describe the interatomic interactions in phosphorene. To the best of our knowledge, it is for the first time proposed in 2D material that resonant bonding leads to low thermal conductivity, despite that it was originally found in 3D thermoelectric and phase change materials. Our study offers new insights into phonon transport from the view of orbital states, which would be of great significance to the design of emerging phosphorene based nano-devices.
5:45 PM - NM1.9.10
Thermodynamic Calculation and Practical Pathways in Controlled Synthesis of Two-Dimensional Transition Metal Dichalcogenides
Hamed Simchi 1 , Timothy Walter 1 , Suzanne Mohney 1 Show Abstract
1 , The Pennsylvania State University, State College, Pennsylvania, United States
Two-dimensional Transition Metal Dichalcogenides (2D TMDs) have attracted a lot of interest in recent years due to their unique electrical, optical, and thermal properties resulting from their distinctive structure. Still, there is a huge potential in finding new functions by creating heterostructures and superlattices, or alloying/doping of TMDs. In this study, thermodynamic calculations of the sulfidation of selected transition metals (Mo, W, Re, V, Nb, Ta) were employed to provide insight into the environmental conditions (e.g. pressure, temperature) under which their respective sulfides are favored to form. This theoretical analysis is supported by experiments to provide a practical pathway in controlled synthesis of TMDCs. For this purpose, metal seed layers were deposited by DC magnetron sputtering and sulfidized under atmospheric pressure at 750 oC. Surface chemistry and morphology of the films before and after the sulfidation were investigated using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), respectively. It was found that all of the as deposited films were oxidized during the storage or handling in the lab environment. XPS analysis revealed, after treatment with sulfur (p (S2) = 1–10 Torr), Mo, W, and Re films were transformed to MoS2, WS2, and ReS2 phases. In contrast, V2O3 was only partially transformed to a combination of sulfide and sub-stoichiometric oxide or oxysulfide. Furthermore, sulfidation of Nb and Ta films was challenging, and Nb2O5 and Ta2O5 remained the predominant components. AFM analysis of the surface topographies revealed very divergent morphologies after the sulfidation reaction, showing a combination of horizontal and vertical growth for the MoS2 and WS2, but ribbons for ReS2 and flakes for sulfidized V. Isobaric and isothermal stability diagrams, generated via thermodynamic calculations, showed that for p (O2) < 10-10 bar, Mo, W, and Re are favored to convert to sulfides under p (S2) as low as 10-2 bar at 750 oC. In contrast, due to very high stability of Nb2O5 and Ta2O5, a calculated sulfur partial pressure exceeding 1 bar by many orders of magnitude is required to make NbS2 and TaS2, respectively. At convenient pressures, sulfidation is favored under H2S (g), which is recommended instead. These results will guide future experiments to design new synthesis processes and structures for next-generation devices.
NM1.10: Poster Session II
Friday AM, April 21, 2017
Sheraton, Third Level, Phoenix Ballroom
9:00 PM - NM1.10.01
Bulk Electronic Structure and Topologically Trivial Fermi-Arcs in WTe2
Flavio Bruno 1 , Anna Tamai 1 , Quan Sheng Wu 2 , Irene Cucchi 1 , Celine Barreteau 1 , Alberto de la Torre 1 , Siobhan McKeown Walker 1 , Sara Ricco 1 , Zhiming Wang 3 , Timur Kim 4 , Moritz Hoesch 4 , Ming Shi 3 , Nick Plumb 3 , Enrico Giannini 1 , Alexey Soluyanov 2 , Felix Baumberger 1 Show Abstract
1 Department of Quantum Matter Physics, University of Geneva, Geneva Switzerland, 2 , ETH, Zurich, Zurich Switzerland, 3 , Paul Scherrer Institute, Geneva Switzerland, 4 , Diamond Light Source, Didcot United Kingdom
The remarkable electronic properties of WTe2, including non-saturating magnetoresistance and pressure induced superconductivity, have attracted much attention in the past few years. In addition, very recent theoretical work predicts that WTe2 may be an example of a new class of Weyl semimetal. If this is the case, the presence of Weyl points near the Fermi level should be reflected in novel transport phenomena. The starting point to discuss the origin of the extraordinary properties of this material must be a comprehensive description of its electronic structure. In this work, we report a unifying picture of the surface and bulk electronic structure of WTe2. This is achieved by combining laser and synchrotron based angle resolved photoemission spectroscopy (ARPES) measurements with surface band structure calculations. We demonstrate the presence of two distinct surface band structures in the ab-plane of WTe2, which we associate with the inequivalent top and bottom surfaces of the non-centrosymmetric bulk structure. We identify arc-like surface states on both surfaces and use surface electronic structure calculations to show that their presence is independent of the existence of type-II Weyl points in the bulk. We thus demonstrate that the observation of arc-like surface states alone cannot be used to identify WTe2 as a type-II Weyl semimetal. Finally, we present a full description of the controversial bulk Fermi surface of WTe2 where we find three-dimensional electron and hole pockets whose areas are in good agreement with those found by quantum oscillations studies. Thus, we stablish a comprehensive view of the electronic structure of WTe2 which facilitates greater understanding of the extraordinary properties of WTe2.
9:00 PM - NM1.10.02
Layer-Dependent Measurements of Electronic Band Alignment for Individual MoS2, WS2, and MoSe2 Flakes Supported on SiO2 Using Photoemission Electron Microscopy (PEEM) with Deep Ultraviolet Illumination
Morgann Berg 1 , Kunttal Keyshar 5 3 , Ismail Bilgin 2 3 , Fangze Liu 2 3 , Hisato Yamaguchi 3 , Robert Vajtai 4 , Calvin Chan 1 , Gautam Gupta 3 , Swastik Kar 2 , Pulickel Ajayan 4 , Taisuke Ohta 1 , Aditya Mohite 3 Show Abstract
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States, 5 , Applied Materials, Inc., Santa Clara, California, United States, 3 , Los Alamos National Laboratories, Los Alamos, New Mexico, United States, 2 , Northeastern University, Boston, Massachusetts, United States, 4 , Rice University, Houston, Texas, United States
Tailoring band alignment layer-by-layer using heterojunctions of two-dimensional (2D) semiconductors is an attractive prospect for producing next-generation electronic and optoelectronic devices that are ultra-thin, flexible, and efficient. 2D layers of transition metal dichalcogenides (TMDs) in laboratory devices have already demonstrated properties favorable for electronic and optoelectronic applications. Despite these strides, a systematic understanding of how band alignment evolves from monolayer to multilayer TMDs is still missing owing to the lack of a suitable experimental approach. Quantitative determination of the electronic band alignment necessitates that measurements be performed in a controlled environment (such as vacuum) using a substrate that interacts minimally with the overlying TMDs to suppress the electronic influence of supporting substrates (preferably insulating) and prevent chemical modification of TMDs due to adsorbates (primarily water).
Here we report on the local band alignment of monolayer, bilayer, and tri-layer flakes of MoS2, WS2, and MoSe2 supported on SiO2 substrates, measured using a new approach to probe the occupied electronic states based on photoemission electron microscopy with deep ultraviolet excitation. The spatially-resolved, simultaneous measurements of the vacuum level and the valence band edge at the Brillouin zone center, combined with known information regarding the optical band gaps and predicted energy differences between Γ- and K-points, enable us to construct absolute band diagrams for MoS2, WS2, and MoSe2 as a function of layer thickness. Where WS2 and MoS2 display type-I band alignment across lateral 1ML-2ML and 2ML-3ML junctions, homojunctions made of MoSe2 are shown to exhibit staggered (type-II) alignment. This result for MoS2 differs from some earlier reports based on Kelvin probe and scanning photocurrent microscopies, and highlights the strong influence of environmental effects on the band alignment in MoS2 homojunctions. We measured a monotonic decrease in the energy separations between the vacuum level and the valence band maximum from MoS2, WS2, to MoSe2, for monolayer specimens. This result implies that a hetero-bilayer constructed from any combination of these materials is expected to display type-II band alignment, advantageous for hosting long-lived interlayer or indirect excitons and for efficient charge separation in photovoltaic applications. This comprehensive band alignment picture of TMDs provides vital information for layer-by-layer band alignment engineering of 2D heterojunctions.
This work was performed at CINT (DE-AC04-94AL85000), and is supported by Sandia LDRD, US DOE EERE SunShot Initiative BRIDGE (DE-FOA-0000654 CPS25859), and Army Research Office MURI (W911NF-11-1-0362). SNL is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Co., for the US DOE NNSA (DE-AC04-94AL85000).
9:00 PM - NM1.10.03
Dual Gated Bilayer Graphene-on-MoS2 Phototransistor with Single Photon Detection Capability
Kallol Roy 2 , Tanweer Ahmed 2 , Harshit Dubey 2 , T Sai 2 , Ranjit Kashid 2 , Shruti Maliakal 1 , Kimberly Hsieh 2 , Saquib Shamim 2 , Arindam Ghosh 2 3 Show Abstract
2 Department of Physics, Indian Institute of Science, Bangalore, Karnataka, India, 1 Physics, Indian Institute of Science Education and Research, Mohali, Punjab, India, 3 Centre for Nano Science and Engineering, Indian Institute of Science, Bangalore, Karnataka, India
2D van der Waals heterostructures allow fabrication of atomically sharp junctions and scope to design new type of physical systems where novel physical phenomena can be studied1–3. Heterostructures combine properties of dissimilar materials to achieve better device performances and hence can be applied to multiple fields4–6.
Here we show a novel design of a heterostructure, made of van der Waals materials, which can be used to detect extremely low intensity light consisting of single or few photons. The main constituent elements of the structure are bilayer graphene and few atomically thick MoS2. Bilayer graphene allows opening of a bandgap with the application of an external vertical electric field between the layers. Presence of bandgap in the conduction channel of the field effect transistor (FET) allow reducing the dark current significantly. Previously reported graphene-MoS2 planer-photodetectors suffer from high dark current (~0.5 mA)7 which limits the device performance by introducing electrical noise8–11. Low dark current (< 10 nA) helps attaining extremely low noise equivalent power (NEP < 10-22 W Hz-1/2) and high specific detectivity (D* > 1018 cm Hz1/2 W−1). The photoresponsivity of these devices remain high (> 109 A W-1) with low operational source-drain bias (50 mV). Even when these detectors are operated in the low frequency regime (0.1
1. Gorbachev, R. V. et al. Strong Coulomb drag and broken symmetry in double-layer graphene. Nat. Phys. 8, 896–901 (2012).
2. Britnell, L. et al. Resonant tunnelling and negative differential conductance in graphene transistors. Nat. Commun. 4, 1794 (2013).
3. Kou, L. et al. Robust 2D Topological Insulators in van der Waals Heterostructures. ACS Nano 8, 10448–10454 (2014).
4. Dean, C. R. et al. Boron nitride substrates for high-quality graphene electronics. Nat. Nanotechnol. 5, 722–726 (2010).
5. Levendorf, M. P. et al. Graphene and boron nitride lateral heterostructures for atomically thin circuitry. Nature 488, 627–632 (2012).
6. Geim, a K. & Grigorieva, I. V. Van der Waals heterostructures. Nature 499, 419–425 (2013).
7. Roy, K. et al. Graphene–MoS2 hybrid structures for multifunctional photoresponsive memory devices. Nat. Nanotechnol. 8, 826–830 (2013).
8. Lin, Y. M. & Avouris, P. Strong suppression of electrical noise in bilayer graphene nanodevices. Nano Lett. 8, 2119–2125 (2008).
9. Pal, A. N. & Ghosh, A. Resistance Noise in Electrically Biased Bilayer Graphene. Phys. Rev. Lett. 102, 126805 (2009).
10. Pal, A. N. et al. Microscopic Mechanism of 1/ f Noise in Graphene: Role of Energy Band Dispersion. ACS Nano 5, 2075–2081 (2011).
11. Kumar, C., Kuiri, M., Jung, J., Das, T. & Das, A. Tunability of 1/ f Noise at Multiple Dirac Cones in hBN Encapsulated Graphene Devices. Nano Lett. 16, 1042–1049 (2016).
9:00 PM - NM1.10.04
Metal-Organic Chemical Vapor Deposition of Hexagonal Boron Nitride
Anthony Rice 1 , Andrew Allerman 1 , Mary Crawford 1 , Thomas Beechem 1 , Taisuke Ohta 1 , Douglas Medlin 1 , Catalin Spataru 1 , Jeffrey Figiel 1 , Michael Smith 1 Show Abstract
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States
Hexagonal Boron Nitride (hBN) is a wide bandgap (~6 eV) semiconductor of interest both for use in traditional optoelectronic devices as well for use as a substrate and encapsulant for graphene. However, high performance hBN based 2D heterostructures currently utilize hBN exfoliated from hBN single crystals a few millimeters in size, which limits scalability. Consequently, we are investigating the use of metal-organic chemical vapor deposition (MOCVD) at high temperature as a means to produce hBN at the wafer scale with control over film thickness.
In this study, hBN epitaxial films were deposited on c-plane sapphire wafers in a cold-walled, rf-heated MOCVD system using NH3 and TEB as precursors with N2 diluent at 50 Torr total pressure. Deposition temperatures were varied from 1100 °C to 1800 °C, measured by optical pyrometry of the susceptor, and NH3 flows were varied from 10 sccm to 10 slm to produce NH3 partial pressures of 30 mTorr to 30 Torr. Characterization by Raman spectroscopy showed a peak at ~1369 cm-1, consistent with BN of hexagonal symmetry. The deposition rate of hBN, measured by in situ reflectance, was found to vary linearly with TEB flow rate for films deposited with NH3 flows less than 1 slm to a maximum deposition rate of ~700 nm/hr. In contrast, the deposition rate of hBN was greatly reduced and was not linearly dependent with TEB flow rate for films deposited with NH3 flows greater than 1 slm. Characterization by room temperature photoluminescence showed spectra with near band edge emission that varied greatly with deposition condition. For films deposited with NH3 flows less than 1 slm or at temperatures less than 1500°C, several broad peaks at ~220 nm and longer wavelengths were observed and are thought to be the emission of defect-bound excitons and point defects, respectively . However, for films deposited with NH3 flows more than 1 slm and at temperatures more than 1500°C, a sharp emission peak at 215 nm was observed and is thought to be the optical phonon replica of free excitons . To our knowledge, this is the first observation of room temperature free-exciton-related emission in hBN films deposited by MOCVD. These results suggest that hBN films of high optical quality can be produced by MOCVD at high temperature.
 A. Pierret et al., PRB 89, 035414 (2014);  G. Cassabois et al., Nature Photonics 10, 262 (2016). Sandia National Laboratories is a multi-mission laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
9:00 PM - NM1.10.05
Interaction Mechanism and Pseudocapacitance of Polar and Non-Polar Polyfluorenes with 2D Titanium Carbide (MXene)
Muhammad Boota 1 , Mariacecilia Pasini 2 , Francesco Galeotti 2 , William Porzio 2 , Mengqiang Zhao 1 , Yury Gogotsi 1 Show Abstract
1 , Drexel University, Philadelphia, Pennsylvania, United States, 2 Istituto per lo Studio delle Macromolecole, Consiglio Nazionale delle Ricerche, Milano Italy
Two-dimensional (2D) materials have garnered tremendous interest due to their unprecedented physical and chemical properties. Recently, a large family of 2D metal carbides and nitrides (MXenes) have been discovered which exhibits metallic conductivity, hydrophilicity and several other attractive properties. MXenes have already shown a great promise in broad range of applications such as energy conversion and storage, water purification, catalysis, antibacterial agent, transparent conductive electrodes, electromagnetic shielding and others. One way to expand the MXenes applications is to tune their physical, chemical and electrochemical properties by mixing them with polymers to synthesize functional nanocomposites. Therefore, it is necessary to first understand the interaction mechanisms of polymers with 2D MXene layers.
Here, we provide a mechanistic insight into interaction of π-conjugated polymers with titanium carbide, Ti3C2Tx (MXene), using polyfluorene derivatives (PFD) having the same conjugated backbone but different lateral chain from apolar to polar. Three PFD having no polar, with polar nitrogen, and with charged nitrogen functionality were synthesized via the Suzuki polycondensation reaction. Detailed microscopic and spectroscopic results demonstrated that polar polymers with charged nitrogen ends tend to strongly interact with the Ti3C2Tx layers, causing an increase in interlayer spacing and large shifts in spectroscopic peaks. When optimized composites were tested as pseudocapacitive electrodes, improved capacitance values and excellent capacitance retention were observed. Our results provide a valuable insight into exploring new organic materials capable of intercalation between the layers of Ti3C2Tx, and other MXenes for energy storage applications and beyond.
9:00 PM - NM1.10.06
Solution-Phase Production and Application of Two-Dimensional Metal Diborides
Ahmed Yousaf 1 2 , Qing Hua Wang 3 , Alexander Green 1 2 Show Abstract
1 The Biodesign Institute, Arizona State University, Tempe, Arizona, United States, 2 School of Molecular Sciences, Arizona State University, Tempe, Arizona, United States, 3 , Arizona State University, Tempe, Arizona, United States
Allotropes of boron are considered companion compounds of carbon allotropes and have attracted considerable interest due to their exceptional electrical and mechanical properties. However, large-scale synthesis of low-dimensional boron compounds has remained a significant challenge. Recent efforts to synthesize two-dimensional boron-based materials have led to the discovery of borophene but its challenging synthesis hinders further application. To overcome these problems, we applied liquid-phase exfoliation to produce a new class of boron-based two-dimensional materials derived from bulk metal diborides. These two-dimensional metal diborides, termed metal boridenes in analogy with graphene, contain sheets of boron with metal atoms sandwiched between them. Extensive characterization of these materials using aberration-corrected transmission electron microscopy, scanning tunneling microscopy, atomic force microscopy, electron energy-loss spectroscopy, and optical absorbance spectroscopy confirm their two-dimensional structure. Furthermore, we find that these solution-processed metal boridenes can be dispersed in solution at high concentrations in excess of 1 mg/mL, enabling their use in macroscopic forms. We demonstrate that metal boridenes, such as chromium boridene, can be incorporated directly from solution-phase dispersions into polymers for structural reinforcement. Tensile testing of chromium boridene/polyvinyl alcohol composites demonstrates increases in ultimate tensile strength and the elastic modulus of 87% and 57%, respectively, compared to polyvinyl alcohol polymer alone. These substantial improvements in mechanical properties exceed those obtained from graphene, MoS2, and BN prepared using similar methods. We anticipate that metal boridenes will open up a range of novel applications for boron-based two-dimensional materials and also yield new low-temperature means of processing bulk metal diborides.
9:00 PM - NM1.10.07
Ultraflat Stanene on Cu(111) Surface
Aidi Zhao 1 , Jialiang Deng 1 , Bing Wang 1 Show Abstract
1 , University of Science and Technology of China, Hefei China
In search of two-dimensional materials with intriguing electronic properties, heavy group-IV elemental analogues of graphene have been predicted to be 2D topological insulators owing to the honeycomb structures and enhanced spin-orbital coupling effect. Thin layers of buckled silicene, germanene and stanene have recently been synthesized by molecular beam epitaxy and characterized with local probe microscopy. However, unlike graphene, the unambiguous identification of their atomic structures remains challenging due to the buckling nature. The direct observation of the full atomic lattice of the honeycomb has not been achieved, the lack of atomically resolved edge structure also prohibits further investigation of possible topological edge states. Here, we demonstrate the synthesis of ultraflat single-atomic-layer stanene on Cu(111) surface by molecular beam epitaxy. High-resolution scanning tunneling microscopy shows that the stanene flakes possess an unprecedented planar honeycomb structure, making a tin counterpart of graphene. The electronic structure of the planar stanene was studied by angle-resolved photoemission spectroscopy combined with first-principles calculations. The synthesis of high-quality stanene will expedite discovery of more stanene-related 2D materials with nontrivial electronic properties.
9:00 PM - NM1.10.08
MoS2−Graphene Heterostructure Based Field-Effect Transistor for Sensing of Volatile Organic Compounds
Tung Pham 1 , Pankaj Ramnani 1 , Youngwoo Rheem 1 , Ashok Mulchandani 1 Show Abstract
1 , University of California, Riverside, Riverside, California, United States
In this work, we investigate the electrical properties of MoS2−Graphene heterostructure synthesized by physical stacking of single-layer MoS2 over single-layer graphene grown using chemical vapor deposition (CVD). We fabricate field-effect transistors (FET) using the hybrid material as the channel to overcome the limitations of individual materials, i.e. zero band gap in graphene and low electrical mobility of MoS2. Due to the difference in the work-functions and MoS2 being a n-type (electron-rich) semiconductor and as-prepared graphene being significantly p-doped (electron-depleted), there is a significant transfer of electrons from MoS2 to graphene. This charge transfer leads to shifting of the charge neutrality point (Vnp) by as much as 30 V in the FET transfer characteristics (Id v/s Vg). The interaction between the individual MoS2 and graphene layers is further confirmed by quenching of photoluminescence intensity in MoS2-graphene hybrid by 50% as compared to single-layer MoS2 and shifting of peaks in Raman spectroscopy. Additionally, the MoS2-graphene hybrid shows stability in ambient air with negligible shifting of Vnp, otherwise observed for graphene-based FET transfer characteristics. Finally, we investigate potential real world applications of MoS2-graphene FET devices for sensitive and selective detection of volatile organic compounds (VOCs) such as acetone and toluene. We compare the performance of the hybrid material to single-layer MoS2 and single-layer graphene using chronoamperometric and transfer characteristic measurements. We conclude that for acetone the sensitivity and signal-to-noise ratio for MoS2-graphene hybrid is much higher compared to single-layer MoS2 and graphene, while for toluene the response of singe-layer graphene is higher compared to the hybrid or single-layer MoS2.
9:00 PM - NM1.10.09
Fabrication of Transition Nickel Ditellurides for Highly Conductive Transparent Electrodes
Sung Hyuk Lee 1 , Jae Im Jeong 1 , Jeon Taik Lim 1 , Seok Yong Seo 1 , Suk Jun Kim 1 , Taewon Yuk 1 Show Abstract
1 Energy, Materials and Chemical Engineering, Korea University of Technology and Education, Chungcheongnam-do Cheonan-si Korea (the Republic of)
Fabrication of thin films of nickel ditelluride was attempted by two ways(Compound target, Co-sputtering). 9 nm~11nm-thickness thin film exhibited an electrical resistivity of 82~118μΩcm with transparency of 46%~53%. Both of fabricated thin film crystal structure is Ni1Te2, Melonite, 98-004-3293. our results prove that nickel ditelluride are one of promising candidates for transparent electrodes(existing electrode-200 μΩcm,80%). First, 2 inch-compound targets were prepared by fabricating the intermetallics followed by SPS. thin film(thickness 9nm) was deposited by Radio frequency sputtering(PVD) using the Intermetallic Target. Second, thin film(11nm) was deposited by co-sputtering a Ni and a Te target at Ni:Te=DC:RF. For two method, we controlled sputtering power and deposition time and substrate heating conditions to maximize their electrical conductivity. Through TEM analysis proved that more grains with c-axis. Therefore, by after annealing and chemical exfoliation, Nickel ditellurides thin film grain size can be growth coarse and make more thin layer.
Correspondence to : Prof. Suk Jun Kim (firstname.lastname@example.org)
9:00 PM - NM1.10.10
Rinsing Phosphorus Oxide Bubbles on Surface of Black Phosphorus by 1,2-Ethanedithiol Treatment
Dohyun Kwak 1 , Hyun-Soo Ra 1 , Min-Hye Jeong 1 , A-Young Lee 1 , Jong-Soo Lee 1 Show Abstract
1 , DGIST, Daegu Korea (the Republic of)
Black phosphorus (BP) is one of two dimensional semiconductor materials that have weak interlayer interaction by Van der Waals force and strong interaction by covalent bonding in plane. Few-layer BP has a thickness tunable bandgap of 0.3 eV (Bulk) to ~2 eV (Monolayer) in addition to highly anisotropic properties.1 Its high hole mobility of ~ 1000 cm2V-1s-1 makes it attractive for high performance electronic devices.2 Although BP has favourable potential for electronic applications, formation of phosphorus oxide bubbles on BP surface in the presence of oxygen and water degrades electronic properties.3
We demonstrated that 1,2-ethanedithiol (EDT) treatment on BP with annealing process can not only remove phosphorus oxide bubbles on BP surface, but also recovery its electronic properties degraded by oxygen and water. Moreover, the EDT treated BP field effect transistor shows n-type doping behavior, which presents a potential for practical semiconductor devices, such as logic gates, photodiodes, and solar cells.
S. Das et al. Nano Lett. 14, 5733-5739 (2014)
L. Li et al. Nat. Nanotech. 9, 372-377 (2014)
M. Hersam et al. Nano Lett. 14, 6964-6970 (2014)
9:00 PM - NM1.10.11
Wearable Thermoelectric Generators by Chemically Exfoliated Transition Metal Dichalcogenide Nanosheets
Sun Woong Han 1 , Tae Hoon Ki 1 , Tae Il Lee 2 , Hong Koo Baik 1 Show Abstract
1 , Yonsei University, Seoul Korea (the Republic of), 2 , Gachon University, Gyeonggi-Do Korea (the Republic of)
We demonstrated the potential of chemically exfoliated 1T-Transition Metal Dichalcogenide Nanosheets(TMDC NSs) as an emerging thermoelectric material group for wearable thermoelectric generators that operate near room temperature. The electric power generation from a prototype wearable thermoelectric generator that was woven into a wristband fitted on a real human body. In particular, the film consisting of the TMDC NSs, which are stacked with weak van der Waals interactions, has a repeatable plug-in connection between TMDC NSs. These films showed high crack resistant thermoelectric performance under repeatable bending and stretching. 2.5 μV of electric potential was generated by our wearable generator from the dynamic movement of a human wrist. Although the power level of the 1T-TMDC device is lower than that of Bi2Te3-based devices, this work is the first trial that demonstrates using chemically exfoliated 1T-TMDC in a thermoelectric generator.
 Oh, J. Y. et al. Energy Environ. Sci., 2016, 9, 1696-1705
9:00 PM - NM1.10.12
Doping and Interface Modification of TMDCs-Based Nanoelectronics
Siyuan Zhang 1 , Christina Hacker 1 , Sujitra Pookpanratana 1 Show Abstract
1 , National Institute of Standards and Technology, Gaithersburg, Maryland, United States
Doping and interface modification through the deposition of ultrathin layers of molecules (sub-monolayer, monolayer, and few-layer coverage) can play an important role in application of two-dimensional (2D) transition metal dichalcogenides (TMDCs) crystals. It is a powerful tool for modifying the electrical and optical properties, leading to dramatically increased conductivities and decreased barriers for charge-carrier injection or extraction. The overall goals of this research are to examine the effects of doping and interface modification of 2D materials (especially MoS2, MoTe2 and WSe2), specifically from the following interrelated aspects: 1) apply different approaches to form organic monolayers (or sub-monolayers) which can modify the overlying or underlying 2D materials layers, strong redox-active molecules are used as the dopants; 2) fully characterize the resulting ultrathin layers of compounds on the surface, understand in detail the mechanisms of the binding, and examine the electrical consequences of different modification methods; 3) explore the device applications of molecular functionalized surfaces in nanoelectronics. The 2D films are prepared through the gold-mediated exfoliation method. A systematic and statistically relevant transport study to determine the impact of these organic monolayers on the TMDC device, as well as the electronic, chemical and optical structure of the TMDC probed by UPS, XPS and Raman spectroscopies.
9:00 PM - NM1.10.13
Raman and PL Characterization of 2D Metal Chalcogenide Crystals
Antonio Cruz 1 , Zafer Mutlu 1 , Selcuk Temiz 1 , Cengiz Ozkan 1 , Mihri Ozkan 1 Show Abstract
1 , University of California, Riverside, Riverside, California, United States
The study of two-dimensional (2D) materials has been among the most exciting materials science topics in recent years. This interest has grown in large part because of the surprising, intimate connections between nano-scale dimensions and material properties such as strength, carrier mobility and thermal conductivity. Today, the 2D-materials research landscape is populated by studies of the transition-metal dichalcogenides (TMDs), which include the familiar MoS2 and WS2; and work on these material systems continues steadily. However, there remain less well-studied 2D material systems both inside and outside of the TMD group. In an effort to better understand some of these less common materials, and to further our knowledge of 2D material phenomena in general, this work investigates hafnium chalcogenides. We perform Raman and photoluminescence spectroscopies on exfoliated samples of HfS2 and HfSe2 to characterize their electronic properties.
9:00 PM - NM1.10.14
Excitons in WSe2 with a Single Se Vacancy
Jie Jiang 1 , Ruth Pachter 1 , Shin Mou 1 Show Abstract
1 , Air Force Research Laboratory, Dayton, Ohio, United States
In this work, we aim to confirm that a single Se vacancy in monolayer WSe2 results in defect-induced emissions that are consistent with experiment, specifically by applying many-body non-self-consistent G0W0 and G0W0-BSE methods. First, for pristine WSe2, we show that the calculated band gap, including spin-orbit coupling, of 1.92 eV, is consistent with the measured band gap of 2.21 eV for monolayer WSe2. Experimentally, a relatively large exciton binding energy was estimated in this case (0.46 eV for an optical gap of 1.75 eV; the A exciton has been measured in a range of 1.65-1.75 eV). G0W0-BSE calculations were found to be in agreement with measurements, with excitations at 1.59 and 2.04 eV for A and B excitons, respectively. At the same time, for example for the monolayer A exciton at 1.75 eV, excitations related to defects that were not observed at higher temperatures, were observed at 1.7 and. 1.67 eV. Indeed, G0W0-BSE results demonstrate that due to the Se vacancy defect, several new defect exciton states redshifted from the A exciton emerge.
9:00 PM - NM1.10.15
Chemical Exfoliation of Black Phosphorus for Inkjet Printing
Misook Min 1 , Gustavo Saenz 1 , Peide Ye 2 , Anupama Kaul 1 Show Abstract
1 , University of Texas at El Paso, El Paso, Texas, United States, 2 , Purdue University, Lafayette, Indiana, United States
Two-dimensional (2D) materials including graphene, MoS2, hexagonal boron nitride (h-BN) and black phosphorus have attracted significant interest in electronic, optoelectronic devices and sensors. Among the various 2D materials, black phosphorus is a semiconductor with a thickness-dependent, direct band gap ranging from ~0.3 eV in the bulk to ~1.5 eV in the monolayer limit. Furthermore, mechanically exfoliated black phosphorus possesses high ON/OFF ratios (~104 - 105) and room temperature mobilities (~200 - 1000 cm2 V-1 s-1). Therefore, black phosphorus has great potential for various applications such as inkjet printing. Black phosphorus can be obtained by several different approaches such as mechanical exfoliation, chemical vapor deposition, and liquid-phase exfoliation.
Here we present a scalable method for preparing black phosphorus membranes via direct liquid-phase exfoliation of the bulk crystal in organic solvents and the performance comparison with the counterpart low yield synthesis method, the micromechanical cleavage. The liquid-phase exfoliation method is potentially appropriate for the large scale production of 2D materials. The black phosphorus sheets are characterized by Raman spectroscopy, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). In addition, we developed a simple and efficient inkjet printing technology by using black phosphorus dispersions. Our results shed light on the important aspects of forming printed patterns on a wide range of substrates, as well as the ink characteristics of black phosphorus dispersions.
9:00 PM - NM1.10.16
Spatially-Resolved Investigation of Transport Properties of Transition Metal Dichalcogenide Single-Layer Devices
I Hsi Lu 1 , Miguel Isarraraz 1 , Edwin Preciado 1 , Velveth Klee 1 , Sahar Naghibi 1 , Gretel Von Son 1 Show Abstract
1 , University of California, Riverside, Riverside, California, United States
We demonstrated the direct chemical vapor deposition (CVD) growth of single layer molybdenum disulfide (MoS2) directly onto SiO2 and LiNbO3 substrates. We then utilize spatially resolved excitation measurement such as scanning photocurrent microscopy (SPCM) and surface acoustic wave spectroscopy (SAW) to characterize the photoconductive natures of the device. MoS2 FET devices on SiO2 are fabricated and measured by SPCM to explore the band-bending effects that are associated with contact resistance in such systems. MoS2 Hybrid FET-SAW device are fabricated on LiNbO3 substrates so as to gain contact-free information on charge carrier mobility following local optical excitation. Because of the contact-independent nature of this approach, complementary data can be obtained. Application to different TMD materials highlight their difference and provides a vista onto the majority charge carriers in each material irrespective of contact formation.
9:00 PM - NM1.10.17
Large Scale Growth of Vertical MoS2 Using Organic Promoter for High-Performance Hydrogen Evolution Reaction
Mina Kang 1 , Sung Myung 1 , Ki-Seok An 1 Show Abstract
1 , Korea Research Institute of Chemical Technology, Daejeon, SE, Korea (the Republic of)
A facile method was demonstrated for conventional thermal chemical vapor deposition (TCVD) growth of large-scale vertically oriented molybdenum disulfide nanoflakes on a basal MoS2 thin film (v-MoS2 on MoS2-TF) utilizing an organic promoter layer, which catalyzed the synthesis and allowed large scale growth of v-MoS2 on MoS2-TF. Here, the growing density of the vertical MoS2 flakes was manipulated by adjusting the growth temperature and the thickness of the organic promoter layers. The v-MoS2 on MoS2-TF is even transferrable to arbitrary flexible substrates using the PMMA wet-transfer method without inducing noticeable amount of defects. A thin-film transistor was also fabricated to determine the electrical property of the v-MoS2 on MoS2-TF film. Significantly, the vertically grown MoS2 was also applicable as the catalyst for hydrogen evolution reaction, which demonstrated a decreased overpotential by 1/3 of those by horizontally grown MoS2 or other CVD-grown crystalline MoS2 thin films previously reported.
9:00 PM - NM1.10.18
Radiatively Dominated Charge Carrier Recombination in Black Phosphorus
Prashant Bhaskar 1 , Alexander Achtstein 1 , Martien Vermeulen 1 , Laurens Siebbeles 1 Show Abstract
1 , Delft University of Technology, Delft Netherlands
Black Phosphorus, a van der Waals solid, consists stacked 2D sheet-like structure with a tunable direct bandgap from 0.3 eV for bulk to 1.6 eV for a monolayer. We show that black phosphorus is a highly efficient infrared emitter. To study the carrier dynamics, excess electron-hole pairs were generated in bulk black phosphorus by irradiation with 3 MeV electron pulses. The transient microwave conductivity due to excess charges was measured as a function of time for different initial charge densities at temperatures in the range 203-373 K. A new global analysis scheme, including the treatment of intrinsic carriers is provided, which shows that the recombination dynamics in black phosphorus, a low bandgap semiconductor, is strongly influenced by the presence of intrinsic carriers. The temperature dependence of the charge mobility and charge carrier decay via second-order radiative recombination is obtained from modeling of the experimental data. The combined electron and hole mobility was found to increase with temperature up to 250 K and decrease above that. Auger recombination is negligible for the studied densities of excess electron-hole pairs up to 2.5×1017 cm-3. For this density the major fraction of the excess electrons and holes undergoes radiative recombination. It is further inferred that for excess charge densities of the order of 1018 cm-3 electrons and holes recombine with near unity radiative yield. The latter offers promising prospects for the use of black phosphorus as efficient mid infrared emitter in devices.
9:00 PM - NM1.10.19
Optical, Charge and Resonance Transfer in Nanoparticle/2D Material Hybrids
Paul Atkin 1 2 , Torben Daeneke 1 , Kourosh Kalantar-zadeh 1 , Ivan Cole 2 Show Abstract
1 , RMIT University, Melbourne, Victoria, Australia, 2 , CSIRO, Melbourne, Victoria, Australia
In this project, we aim to explore the electronic and optical interactions of nanoparticles or quantum dots in close proximity to the surface of two dimensional (2D) transition metal dichalcogenides (TMDs). In the most recently completed study of the project, 2D tungsten disulphide (WS2) nanoflakes were synthesised and hybridised with carbon dots (CDs) using a facile two-step method of exfoliation of bulk WS2 followed by microwave irradiation of nanoflakes in a solution of citric acid. A range of precursor concentrations was investigated leading to optimized reaction conditions which supress independent nucleation and favour CD growth on the basal planes of the 2D nanocrystals. Physicochemical characterisation indicated that the hybrid consists of graphitic CDs with diameters of approximately 2-5 nm, attached to monolayer WS2 via electrostatic attraction forces. This synthesised hybrid material was investigated for photocatalytic applications. We found that within one hour approximately 30% more of the model organic dye was photodegraded by the hybrid material compared with the pristine 2D WS2. This enhancement was associated to the affinity of the CDs to the organic dye rather than heterojunctioning. Comparisons of the photocatalytic efficacy of this hybrid material with those of recently reported 2D TMDs and their hybrids showed a significantly higher turnover frequency. Additionally, the presented microwave-based synthesis method for developing hybrids of 2D WS2 and CDs, without making significant changes to the base 2D crystal structure and its surface chemistry, has not been demonstrated before. Altogether, the hybrid 2D material provides great potential for photocatalysis applications.
9:00 PM - NM1.10.20
Investigation of Charge Transport in Confined-Yet-Coupled 2D Semiconductors
Adam Woomer 1 , Tyler Farnsworth 1 , Scott Charles 1 Show Abstract
1 , University of North Carolina, Durham, North Carolina, United States
Innovation in technologies from solar cells, to medical sensors, to integrated circuits require new strategies to control semiconductor properties. Semiconductors prepared as nanomaterials – i.e. 0D quantum dots and 2D MoS2, WSe2, or Phosphorene – have radically improved properties due to quantum confinement. Their ultrathin dimensions, however, can make them impractical in many applications, e.g. when total light absorption is necessary. When restacked into 3D aggregates, these materials retain their quantum-confined optoelectronic properties. This result is unexpected given the electronic coupling between adjacent materials, and has inspired a significant research effort into ‘confined-yet-coupled’ materials. To this point, the extent of electronic coupling and charge transport has been extensively studied in quantum dot solid systems, but has been relatively unexplored for 2D semiconductor aggregates. Here we study charge transport in 3D films built from 2D semiconductors with well-defined lateral dimensions and thickness. We use uniaxial pressure and temperature dependent electrical measurements to extract the activation energy of charge transport in films of MoS2 and Phosphorene. We find that conductivity increased with compression, due to improved flake-to-flake physical contact. From selective centrifugation of liquid exfoliated 2D semiconductors, we vary flake lateral dimension and flake thickness to control charging energy. Finally, we supplement our study with first principles modeling of charge transport in simple 2D semiconductor structures as a function of rotation and interlayer distance. We find that rotational and positional disorder will preserve the electronic structure and quantum confined properties of 2D semiconductors. This fundamental investigation on electronic coupling in aggregated 2D semiconductors will enable the construction of 3D materials with designed optoelectronic properties to meet the specific requirements of any application.
9:00 PM - NM1.10.21
CVD Growth and Characterization of 2D TMD Films, Alloys and Heterostructures Thereof
David Barroso 1 , Velveth Klee 1 , Edwin Preciado 1 , Natalie Duong 1 , Michael Gomez 1 , Ingrid Liao 1 , Cindy Merida 1 , Ludwig Bartels 1 Show Abstract
1 Materials Science and Engineering, University of California, Riverside, Riverside, California, United States
The field of optoelectronic materials has grown in the past decade with the introduction of single layer transition metal dichalcogenides (TMDs). TMDs in the form MX2 (M = Mo, W; X = S, Se) have been found to offer exciting optical and electronic properties such as a thickness/composition dependent direct/indirect band gap (transition), spin split valence band, valleytronics potential, etc.. For many envisioned device applications, TMD heterojunctions are required or advantageous. Here we show the use of a 1-step chemical vapor deposition (CVD) process that results in lateral heterostructures of 2D TMDs. Alternatively, through the use of organic precursors, alloys of these TMDs were grown and studied, showing tunable electronic and optical properties. We have characterized our materials optically and with regards to electric transport.
9:00 PM - NM1.10.24
Toward Selenene and Tellurene—Two-Dimensional Topological Insulators
Elisabeth Bianco 1 Show Abstract
1 Chemistry, Rice University, Houston, Texas, United States
The discovery of graphene and the unique electronic/optoelectronic phenomena that arise in two-dimensions, such as the room-temperature quantum Hall effect, ushered in a new era in materials research. Consequently, several two-dimensional (2D) materials beyond graphene have been synthesized from other layered crystals in recent years. Motivated by similar unique structure-property relationships, our goal is to grow a completely novel class of 2D materials by imposing new, unnatural crystal structures on elemental chalcogens (Se and Te) via different growth methods including molecular beam epitaxy, pulsed laser deposition, and physical vapor deposition. Density functional theory calculations indicate that selenene and tellurene can be grown in a stable, 2D lattice configuration. Moreover, these materials could possess attractive electronic structures/properties including a single Dirac cone and, when considering spin-orbit coupling, potentially topological insulating behavior. Additionally, selenene and tellurene are predicted to have stable van der Waals layered 3D analogs. Initial efforts toward growing selenene by physical vapor deposition have resulted in nm-thicknesses of crystals that appear in preliminary transmission electron microscopy studies to have the desired square lattice. X-ray photoelectron spectroscopy suggests this material is stable in air for several days. Topological insulators have potential for application in quantum logic and spintronics, and the ballistic transport afforded by spin-locked states could allow for extremely high mobility devices. The results of these experiments will provide further insight to how well we can model and predictively determine the behavior of 2D topological insulators for computationally-guided materials by design.
9:00 PM - NM1.10.25
Optimization of Light-Matter Interactions for Enhancements in Photoluminescence and Raman Spectroscopy of MoSs Using Single Au Tapered Nanoresonators
Edgar Palacios 1 , Spencer Park 1 , Lincoln Lauhon 1 , Koray Aydin 1 Show Abstract
1 , Northwestern University, Evanston, Illinois, United States
Interactions between nanoresonators and 2D-dimensional transitional metal dichalcogenides (2D-TMDCs) have been studied extensively within the past couple years to demonstrate interesting nanoscale phenomena and high performance applications. In particular, MoS2 monolayers have been receiving immense attention for emitter and absorber based devices due to their direct bandgap which lies in the visible regime. In an effort to improve the applicability of these materials, groups have used single and periodic arrays of plasmonic materials to increase the intensity of incident electric fields within 2D-materials. In the case of single plasmonic nanoantennas interfaced with 2D-TMDCs, however, groups have utilized elements with discrete resonances which does not present a complete picture of optimal requirements when interfaced with plasmonic elements to provide photoluminescence (PL) enhancement and metal/TMDC interactions on a single MoS2 flake. In this work, we combine a single Au tapered plasmonic nanoantenna which exhibits optical resonances that extend above and below the bandgap. Using this design we are able to demonstrate PL and raman enhancements over a broad range of plasmonic resonances in order to provide a more comprehensive picture on light-matter interactions in MoS2 monolayers.
9:00 PM - NM1.10.26
Significant Field Enhancement by MoS2 Flake—Enhanced Intensities of Fluorescence and Raman Spectrum of Dye Molecule
Masanori Sakamoto 1 , Ken-ichi Saitow 1 2 Show Abstract
1 , Department of Chemistry, Graduate School of Science, Hiroshima University, Higashi Hiroshima Japan, 2 , Natural Science Center for Basic Research and Development (N-BARD), Higashi-hiroshima Japan
Two-dimensional transition metal dichalcogenides (2D-TMDs) has attracted much attention as alternative layered materials beyond the graphene. Recently, 2D-TMDs have been significantly studied and expected as a material such as light harvesting, batteries, photovoltaics, solid lubricants, photodetector, and so on. Molybdenum disulfide (MoS2) is one of the most studied systems in 2D-TMDs, because it is abundant as a mineral (molybdenite) in the earth and unusual properties. Recently, several studies on fluorescence intensity enhancement and surface-enhanced-Raman-scattering have been reported. However, the enhanced mechanism due to MoS2 has not been clarified from the viewpoint of field enhancement yet. Here, we show the significant field enhancement using MoS2 flakes, prepared by liquid-phase-exfoliation method. The field enhancement was evaluated by the fluorescence and Raman intensity of dye solution as well as the calculation with finite-difference time-domain (FDTD) method. MoS2 flakes were also analyzed by optical microscope, scanning electron microscope, atomic force microscope, and Raman spectra. As a result, it was found that the Raman and fluorescence intensities of dye solution are enhanced up to 500- and 30-folds, respectively, by MoS2 flakes. The field enhancement was also observed at the surface of MoS2, according to the result of FDTD calculation, and it locally affects the excitation process of the dye molecule at around the surface.
9:00 PM - NM1.10.27
Bulk to Monolayer Properties of the Platinum Dichalcogenides
Protik Das 1 , Mahesh Neupane 2 , Darshana Wickramaratne 3 , Roger Lake 1 Show Abstract
1 Electrical and Computer Engineering, University of California-Riverside, Riverside, California, United States, 2 Sensors and Electron Devices Directorate, U.S. Army Research Laboratory, Adelphi, Maryland, United States, 3 Materials Department, University of California, Santa Barbara, Santa Barbara, California, United States
The platinum dichalcogenides PtS2, PtSe2 and PtTe2 are a new class of layered two-dimensional transition metal dichalcogenides (TMDC). Recent studies have identified novel type-II Dirac fermions and local Rashba spin polarizations in PtSe2 . However, existing studies are limited to the monolayer structures of these materials. The evolution of the electronic structure in this class of materials as a function of film thickness remains unexplored.
Using first-principles hybrid functional calculations we investigate the electronic properties of PtS2, PtSe2 and PtTe2 as a function of film thickness. We find each of the monolayer Pt dichalcogenides are semiconducting. However, a semiconductor-to-metal transition occurs at a thickness of two monolayers in PtTe2 and four monolayers of PtSe2. PtS2 remains semiconducting as the film thickness increases towards the bulk limit. Effective masses and the energy differences between satellite valleys are evaluated for each of the semiconducting structures. Our calculations also identify a Mexican hat dispersion in the valence band from one monolayer of PtS2 and two monolayers of PtSe2 which was earlier observed in few layer III-VI materials . The energy iso-surface of the valence band maxima of such Mexican hat is a ring that exhibits a concentric spin texture. The magnitude and direction of the spin texture can be tuned with the application of a vertical electric field. We investigate the net spin polarization as a function of vertical electric field for monolayers of each of the Pt-dichalcogenides.
 H. Huang, S. Zhou and W. Duan, "Type-II Dirac fermions in the PtSe2 class of transition metal dichalcogenides," Physical Review B, vol. 94, p. 121117, 2016.
 D. Wickramaratne, F. Zahid and R. K. Lake, "Electronic and thermoelectric properties of van der Waals materials with ring-shaped valence bands," Journal of Applied Physics, vol. 118, p. 075101, 2015.
Acknowledgement: This work is supported in part by FAME, one of six centers of STARnet, a Semiconductor Research Corporation program sponsored by MARCO and DARPA. The computational resource was provided by the Extreme Science and Engineering Discovery Environment (XSEDE).
9:00 PM - NM1.10.28
Investigating Anisotropic Optical Property and Thermal Conductivity of Few-Layer Td-WTe2 via Micro Raman Spectroscopy
Yu Chen 1 , Jingzhi Shang 1 , Jiadong Zhou 2 , Zheng Liu 2 , Yu Ting 1 3 Show Abstract
1 Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore Singapore, 2 Centre for Programmed Materials, School of Materials Science and Engineering, Nanyang Technological University, Singapore Singapore, 3 Department of Physics, Faculty of Science, National University of Singapore, Singapore Singapore
The layered transition metal dichalcogenide (TMD) of WTe2 has attracted tremendous interest since the discovery of its extremely large non-saturating magnetoresistance. Due to the distinct Td structure with two non-equivalent W atoms in unit cell, larger interlayer spacing and semimetal band structure, two dimensional (2D) Td-WTe2 is expected to have distinguished properties from other 2D TMDs. Here, we report the linearly polarized Raman measurements on Td-WTe2 layers with different thicknesses, the observed intensity of Raman peak shows the clear angular dependence, which is in accordance with the C2ν point group symmetry. Moreover, the thermal conductivities along zigzag and armchair directions from few layer to thick Td-WTe2 samples were measured by temperature-dependent Raman spectroscopy. The prominent Raman peak at around 216 cm-1, which is assigned to A21, is most sensitive to tempearture change with the first-order thermal coefficient of -0.012 cm-1K-1 comparing to that (i.e. -0.008 cm-1K-1) of A51 mode at around 167 cm-1. The estimated anisotropic difference along zigzag and armchair directions is ∼ 10%. Such anisotropic in-plane thermal conductivity derived from mode A21 opens up many opportunities for further fundamental studies and the next-generation thermoelectric device applications.
9:00 PM - NM1.10.29
Schottky Barrier Heights at Lateral Two-Dimensional Metallic and Semiconducting Transition-Metal Dichalcogenide Interfaces
Adiba Zahin 1 , Shanshan Su 1 , Darshana Wickramaratne 1 , Roger Lake 1 Show Abstract
1 , University of California, Riverside, Riverside, California, United States
Low-resistivity metal contacts on 2D semiconductors is a great challenge for present devices. However, high contact resistances with the monolayer TMDs significantly degrade the performance of TMD transistors . Metal deposition on single-layer planar surfaces such as TMDs or graphene may exhibit different degrees of clustering , which may induce inefficient electron and phonon transport  across the metal TMD interfaces. One approach to alleviate the poor contact resistances could be creating seamless high-quality in-plane heterojunctions between metallic and semiconducting TMDs. Lateral heterojunctions could also lead to exciting new physics and applications. A pristine lateral metal TMDC interfaces could offer a route to more transparent contacts. So, here we are investigating monolayer horizontal heterojunctions in which the heterojunction is formed by in-plane covalent bonds. We are working with TaS2-MoSe2 and NbSe2-WSe2 combinations which have lattice mismatch less than 5%. We are considering two geometries, an armchair and a zigzag heterojunction. Two different strain configurations were also considered, one in which the semiconducting TMDs are strained to match the lattice constant of metallic TMDs and the other in which metallic TMDs are strained to match the lattice constant of semiconducting TMDs. The energy versus momentum (E-k) relations were calculated using ab initio density-functional theory (DFT) calculations with the PBE functional including spin-orbit coupling. The result indicates that the valence band of MoSe2 lies at the Fermi level of TaS2, similarly the valence band of WSe2 lies at the Fermi level of NbSe2 in these lateral heterojunction configuration providing a zero-barrier contact for holes. Which suggests that there are opportunities for low resistance contacts to monolayer TMDs using lateral metallic-semiconducting TMDs.
9:00 PM - NM1.10.30
Anisotropic Etching of Hexagonal Boron Nitride Crystal on Cu-Ni Substrate
Yijing Stehle 1 , Ivan Vlassiouk 1 , Panos Datskos 1 Show Abstract
1 , Oak Ridge National Lab, Oak Ridge, Tennessee, United States
Hydrogen plays duel role during the CVD growth of graphene and hexagonal boron nitride as promote growth while etch the weak bond away. The understanding of etching role of hydrogen will not only help with grow high quality material, but also provide a way to form nano structure on the materials. This paper introduce and discuss the hydrogen induced etching behavior of hexagonal boron nitride and etching pattern development. The formation of triangular shaped anisotropic etching patterns with the same morphology, orientation, and location as hBN single crystals synthesized at the same condition were observed for the first time. These results demonstrate that the growth and etching kinetics of hBN share common physics at a fundamental level. This observation helped us correlate the etched domain and hBN single crystals, which can be used to etch target sites of hBN in controllable etching processes for nanostructure fabrication and in plane heterostructure growth.
9:00 PM - NM1.10.31
The Effect of Stoichiometry on Carrier Mobility of Pulsed Laser Deposited Few-Layer Tungsten Disulfide Thin Films
Urmilaben Rathod 1 , Jitendra Kumar Jha 1 , Jincheng Du 1 , Nigel Shepherd 1 Show Abstract
1 Materials Science and Engineering, University of North Texas Denton, Denton, Texas, United States
Transition metal dichalcogenides (TMDs) such as tungsten disulfide have generated considerable interest for a variety of device applications due to attractive properties like high mobility, in-plane carrier confinement, and tunable bandgaps. However, significant knowledge gaps related to the processing-structure-property relationships of TMDs remain, and growing these materials with the high structural quality required for devices is a key technical challenge. The majority of reports use chemical vapor deposition (CVD) variants, and growth by physical vapor deposition (PVD) is less explored. Few-layer WS2 films were deposited onto quartz substrates by pulsed laser deposition, and afterward annealed in sulfur vapor to determine the impact on stoichiometry, crystallinity and electrical properties. X-ray diffraction and Raman spectroscopy confirmed enhanced crystallinity after annealing, where a doubling of the grain size was obtained. X-ray photoelectron spectroscopy showed an increase in the W/S ratio from 1.5 to 2 after annealing, which indicates improved stoichiometry. Ultraviolet photoelectron spectroscopy showed reduced W in the form of W3+ in the as-deposited specimens, which was eliminated by annealing. This confirmed that the as-deposited films are sulfur deficient with W3+ forming in response sulfur vacancies in order to maintain charge neutrality. Hall characterization of the films revealed an improvement in carrier mobility from 5 cm2v-1s-1 to 900 cm2v-1s-1 with annealing, which we ascribe to improved crystallinity and stoichiometry i.e., reduced incoherent scattering. In addition, with annealing the conduction characteristics changed from n-type to p-type. We assign the n-type conduction in the as-deposited films to native donors associated with sulfur vacancies according to: WS2 → 2Vs..+ Wxw+ 4e′+ S2(g). We propose that the annealed WS2 films showed p-type conduction due to sulfur in-diffusion and removal of sulfur vacancies according to:2Vs..+Wxw→S2(g) → 2Sxs +Wxw + 4h.
9:00 PM - NM1.10.32
Exploring Dimensionality in Hybrid Perovskites
Aditya Sadhanala 1 , Satyawan Nagane 2 , Shraddha Chhatre 2 , Baodan Zhao 1 , Richard Friend 1 Show Abstract
1 , University of Cambridge, Cambridge United Kingdom, 2 , CSIR-National Chemical Laboratory, Centre of Excellence in Solar Energy, Physical and Materials Chemistry Division, Maharashtra India
Solution processable hybrid perovskite semiconductor materials have been demonstrated to immense potential in various optoelectronic applications. Dimensionality in perovskites is an important aspect as this influences a range of photo-physical and optoelectronic properties in diverse ways. Such diverse photo-physical properties have various optoelectronic applications. We would present snapshot of these diverse optoelectronic properties with change in dimensionality.
9:00 PM - NM1.10.33
Transparent Solar Cell Utilizing p-n Heterojunction of Two-Dimensional GaTe and InGaZnO
Ah-Jin Cho 1 2 , Kyung Park 2 , Solah Park 1 2 , Min-Kyu Song 1 2 , Kwun-Bum Chung 3 , Jang-Yeon Kwon 1 2 Show Abstract
1 School of Integrated Technology, Yonsei University, Incheon Korea (the Republic of), 2 , Yonsei Institute of Convergence Technology, Incheon Korea (the Republic of), 3 Division of Physics and Semiconductor Science, Dongguk University, Seoul Korea (the Republic of)
The wide-spread of solar energy harvesting system is often hampered by the limitation of installation space. In order to overcome this hurdle, solar cell in transparent and light-weight form, which can be integrated into the building envelop is getting attention. Transparent solar cell has inherent limitation in terms of efficiency, but such limitation can be offset as it can be installed in a larger-scale without consideration for aesthetic and spatial aspect. When building integrated photovoltaic system is achieved, the total power generation will surpass that of the conventional bulky solar cell.
2D materials are composed of atomic layers which are very weakly bonded to each other, and which can be easily separated by mechanical exfoliation. 2D semiconductors tend to maintain their characteristics even when scaled down to nanometer thickness and also tend to absorb more sunlight than bulk semiconductors under the same thickness. Due to its thinness, multi-layer 2D materials are highly transparent. Such unique properties make 2D material highly advantageous to be utilized as active layer of transparent solar cell.
GaTe is one of 2D semiconductors, showing direct bandgap of 1.7eV and p-type transfer characteristic. In a multi-layer form, it can transmit most of the visible light due to its thinness. IGZO is n-type oxide semiconductor, which is transparent due to its wide bandgap of 3.4eV. By joining multi-layer GaTe/IGZO p-n heterojunction with ITO electrode, we have fabricated the first fully transparent solar cell using 2D material. The device shows high transparency of ~90% and efficiency of 0.73% with fill factor of 37%. In addition, our photovoltaic device exhibits instantaneous generation of photo-carriers under periodic pulse of 400 and 550nm light. Further analysis on the operating mechanism was conducted by studying the band structure with Kelvin probe force microscopy measurement (KPFM). Based on the contact potential difference of GaTe and IGZO measured by KPFM and spectroscopic ellipsometry data, we have concluded that our GaTe/IGZO is type II staggered-gap heterojunction. Our research is meaningful in that we have suggested 2D semiconductor as a new candidate to fabricate transparent solar cell and have shown its feasibility.
9:00 PM - NM1.10.34
Ab Initio Calculation of Layer Dependent Raman Mode Evolution of Few Layer Stanene
Tonmoy Kumar Bhowmick 1 Show Abstract
1 , University of California Riverside, Riverside, California, United States
We report Raman scattering of Stanene layers comprising of single, two, three and five layers, showing a strong dependence on the layer thickness. Similar to graphite and graphene, the atoms within each layer in Stanene are joined together by covalent bonds, while van der Waals interactions keep the layers together. This makes the physical and chemical properties of 2D hexagonal Stanene layer dependent. Here, we discuss the basic lattice vibrations of monolayer, multilayer, and bulk Stanene including high-frequency optical phonons, the Raman selection rule, layer-number evolution of phonons and Raman modes. The interlayer vibrational modes are used in rapid and substrate-free characterization of the layer number of multilayer Stanene. The success of Raman spectroscopy in investigating Stanene nanosheets paves the way for experiments on other similar hexagonal 2D crystals and related van der Waals heterostructures.
9:00 PM - NM1.10.35
Layer-by-Layer Assembly of a Hybrid of Phosphorene and Molybdenum Disulphide
Harneet Kaur 1 Show Abstract
1 , CSIR National Physical Laboratory, Delhi India
Probing alternative approaches for producing thin films of emerging two dimensional (2D) layered materials on substrates is necessary to reveal their unlocked applications. Recently, hybrid p-n junctions has been realised based on these 2D materials but on lab scale. To fully utilize the potential of these van der Wall hybrids, it is necessary to develop a versatile method that is amenable for large area applications. In this regard, we demonstrate the layer-by-layer assembly of a novel hybrid p-n junction based on phosphorene and molybdenum disulphide (MoS2). Langmuir- Blodgett technique has been employed to systematically deposit the free-standing nanosheets of MoS2 onto the phosphorene nanosheets. Various microscopic and spectroscopic techniques revealed the presence of perfect overlapped hybrid structures of phosphorene/MoS2. The gate tunability of this large area p-n diode has been explored. Our results holds promise for solar energy applications.
9:00 PM - NM1.10.36
2D Layered PtSe2 Lateral Heterojunction Device by Thickness Modulation
Yuda Zhao 1 , Yang Chai 1 Show Abstract
1 Applied Physics, The Hong Kong Polytechnic University, Kowloon Hong Kong
The two-dimensional layered transition metal dichalcogenides (TMDs) are promising candidates as channel materials. The TMDs FET can overcome short channel effect, and exhibit large on/off ratio and small subthreshold swings. The electronic transport behavior of group-6 TMDs have been extensively studied. Recently, the unique layer-dependent properties of group-10 TMDs have been reported. The strong interlayer interaction and the dramatically layer-dependent bandgap evolution in PtSe2 is suitable for the heterojunction device.
We find that the group-10 TMD, PtSe2 exhibits the layer-dependent semiconductor to semimetal transition from monolayer to the bulk counterpart. With Ti/Au contact, it shows electron-dominated transport behavior. The lateral junction devices with asymmetric thickness PtSe2 shows current rectification behavior due to the dramatic change of absolute VBM and CBM energy level with the layer number. The large tunability of the electronic bandstructure of PtSe2 provides the possibilities for constructing different types of functional electronic devices. We acknowledge the support from the Research Grant Council of Hong Kong (Grant No.: PolyU 152145/15E), and the Hong Kong Polytechnic University (grant Nos.: G-SB53 and 1-ZVGH).
9:00 PM - NM1.10.37
Thermal Analysis of Isotope-Doped Phosphorene
Oswaldo Sanchez 1 , Ganesh Balasubramanian 1 Show Abstract
1 Mechanical Engineering, Iowa State University, Ames, Iowa, United States
Extensive studies on thermal transport have been conducted for graphene, both experimental and computational, but new graphene analogous materials have emerged that require similar investigations. Here the thermal conductivity of phosphorene is investigated, with an emphasis on isotope substitution effects. These results are compared to existing results for similar investigations on graphene. We apply molecular dynamics (MD) simulations to evaluate the effect of the isotope substitution on thermal transport in phosphorene.
9:00 PM - NM1.10.38
Single and Few-Layer MoS2: CVD Synthesis, Transference, and Photodetection Application
Gustavo Saenz 1 , Carlos Francisco de Anda Orea 1 , Anupama Kaul 1 Show Abstract
1 , University of Texas at El Paso, El Paso, Texas, United States
Two-dimensional layered materials, materials with weak out-of-plane van der Waals bonding and strong in-plane covalent bonding, have attracted special attention in recent years since the isolation and characterization of monolayer graphite, the graphene. The electrical bandgap in Transition Metal Di-Chalcogenides (TMDCs), non-existent in graphene, make them a good alternative family of materials for novel electronic and optoelectronic applications. 2H-MoS2, one of the most stable TMDCs, has been extensively studied, including the synthesis methods, and its potential applications in photodetection. The chemical vapor deposition (CVD) synthesis method has increased its potential over the years. The advantages of this method are scalability compared to micromechanical exfoliation, common process used in research laboratories, and the maintenance of the quality and intrinsic properties of the material compared to the liquid exfoliation methods. In this work, we synthesized high quality pristine 2H-MoS2 via atmospheric pressure chemical vapor deposition (APCVD) by vapor phase reaction of MoO3 and S powder precursors. The samples were characterized via Raman and photoluminescence (PL) spectroscopy and compared to mechanically exfoliated MoS2 crystal by measuring the full-width half maxima (FWHM) of monolayer and few-layer mesoscopic flakes. In addition, the CVD synthesized single and few-layered MoS2 domains were transferred to different substrates using a high yield process, including a flexible substrate, preserving the quality of the material. Finally, and MoS2-graphene heterostructure broadband two-terminal photodetector was designed, fabricated, and measured the intrinsic optoelectronic characteristics, including the electrical transport, photocurrent, and photoresponsivity. Demonstrating thus the capability of heterostructure fabrication and the quality of our synthesis and device fabrication process.
9:00 PM - NM1.10.39
Effect of Optical and Thermal Excitation on Phonon Dynamics in Monolayer Tungsten Diselenide
Avra Bandyopadhyay 1 , Chandan Biswas 1 , Gustavo A. Saenz 1 , Anupama Kaul 1 Show Abstract
1 , University of Texas at El Paso, El Paso, Texas, United States
Monolayer Tungsten diselenide is one of the most promising 2-D materials in optoelectronic device applications due to its direct bandgap (~1.6 eV). The integrated optoelectronic devices based on monolayer WSe2 have the potential for extremely high degree of integration, and hence heat transfer and phonon behavior are very important for heat management of nanointegrated devices. At the same time, Raman spectroscopy is a powerful tool to characterize the optical phonon behavior of nanomaterials. The position and width of the Raman scattering peak can reflect vibrational frequency and dynamics of optical phonons, respectively, while the latter is directly related to the heat diffusion rate. In nanoelectronic device applications, it is very important to understand the effect of phonons because the self-heating of the device can significantly affect the performance. In this work, a comprehensive analysis of the effect of optical and thermal excitation on the behavior of phonon dynamics in monolayer WSe2 is studied. Monolayer WSe2 flakes from natural WSe2 crystals were transferred onto Si/SiO2 (270nm) substrates by mechanical exfoliation. The flakes were observed under an optical microscope. Raman and Photoluminescence spectroscopy were done in a wide range of temperature (79K-573K) at different intensities of optical power source. The phonon lifetime was found from the change in Raman peak widths at different temperatures (at a fixed optical power) and at different optical powers (at room temperature). Additionally, the temperature coefficients of the major Raman peaks were calculated based on the Peak shifts due to varying temperature. Finally, the concentration of the phonons was calculated from the change in slope at the low energy edges of the PL spectra due to different thermal and optical excitations.
9:00 PM - NM1.10.40
Two-Dimensional Few-Atom-Thick PbS/CdS Core/Shell Colloidal Nanosheets
Liangfeng Sun 1 , Simeen Khan 1 , Zhoufeng Jiang 1 , Shashini Premathilka 1 , Antara Antu 1 , Jianjun Hu 2 , Andrey Voevodin 2 , Paul Roland 3 , Randy Ellingson 3 Show Abstract
1 , Bowling Green State University, Bowling Green, Ohio, United States, 2 , Air Force Research Laboratory, Dayton, Ohio, United States, 3 , University of Toledo, Toledo, Ohio, United States
Two-dimensional few-atom-thick PbS/CdS core/shell nanosheets are synthesized using a cation-exchange method. A significant blue-shift of the photoluminescence is observed, indicating a stronger quantum confinement in the PbS core as its thickness is reduced to eight atomic layers. High resolution transmission-electron-microscopy images of the cross-sections of the core/shell nanosheets show atomically sharp interfaces between PbS and CdS. Accurate analysis of the thickness of each layer reveals the relationship between the energy-gap and the thickness in the extremely one-dimensionally confined nanostructure. Photoluminescence lifetime of the core/shell nanosheets is significantly longer than the core-only nanosheets, indicating better surface passivation.
9:00 PM - NM1.10.41
A Two-Dimensional Polymer Synthesized through Topochemical [2+2]-Cycloaddition on the Multigram Scale
Ralph Lange 1 , Gregor Hofer 1 2 , Thomas Weber 2 , Nils Juergensen 3 4 , Uli Lemmer 4 , Gerardo Hernandez-Sosa 3 4 , A. Dieter Schlueter 1 Show Abstract
1 Laboratory for Polymer Chemistry, ETH Zurich, Zurich Switzerland, 2 X-Ray Platform, ETH Zurich, Zurich Switzerland, 3 , InnovationLab, Heidelberg Germany, 4 Light Technology Institute, Karlsruhe Institute of Technology, Karlsruhe Germany
There is ample research on inorganic two dimensional materials, centering loosely around the groundbreaking work of Geim und Novoselov on graphene. In comparison, knowledge on organic two-dimensional polymers (2DP) is limited: this is the third ever case in which a single-crystal-to-single-crystal transformation and subsequent liquid-phase exfoliation yielding 2DPs could be achieved.
We here report the convenient, inexpensive and large scale synthesis of a pyrylium based triolefinic tetrafluoroborate salt as a novel monomer for the topochemical synthesis of an organic two-dimensional polymer. The structure of the monomer crystals and their quantitative conversion into layered polymer crystals is confirmed by XRD, IR and 13C CP/MAS NMR.
The novel system provides outstanding possibilities:
> Easy accessibility (35 g prepared after single runs of a 4 step synthesis)
> Crystals grow in sizes of up to 5 mm
> Single and multi layers of polymer by liquid-phase exfoliation within a few days.
The 2DP may be regarded as a positively charged, planar and crystalline honeycomb-mesh. The negative counter-ions are supposed to encase the sheet from both sides. Properties of these organic materials are intrinsically different from their inorganic 2D counterparts, e.g. pores are rather large (16 – 28 Å) and the polymerization reaction is reversible at elevated temperatures (>150 °C). Therefore, spatially resolved post-modifications under mild conditions are conceivable.
The obtained sheets have a size on the order of several microns and are thus smaller than the crystals from which they are obtained. AFM measurements show apparent heights of 1.5 – 2.5 nm for the thinnest features, to which we tentatively assign monolayer character. Current research is directed towards improving the dispersity, whereby the final goal is to obtain fractions of homogenously thick sheets (ideally monolayers). Ultracentrifugation may prove a valuable tool here.
First trials have shown the polymer to be semi-conductive and electroluminescent. Prototype organic light-emitting devices have been manufactured by drop-casting a 2DP dispersion.
 a) J. Sakamoto, J. van Heijst, O. Lukin, A. D. Schlüter, Angew. Chem. Int. Ed. 2009, 48, 1030-1069; b) P. Payamyar, B. T. King, H. C. Ottinger, A. D. Schlüter, Chem. Commun. 2016, 52, 18-34.
 R. Z. Lange, G. Hofer, T. Weber, A. D. Schlüter, A two-dimensional polymer synthesized through [2+2] cycloaddition on the multigram scale, submitted.
 a) P. Kissel, D. J. Murray, W. J. Wulftange, V. J. Catalano, B. T. King, Nat. Chem. 2014, 6, 774-778; b) M. J. Kory, M. Wörle, T. Weber, P. Payamyar, S. W. van de Poll, J. Dshemuchadse, N. Trapp, A. D. Schlüter, Nat. Chem. 2014, 6, 779-784.
 a) K. Novak, V. Enkelmann, G. Wegner, K. B. Wagener, Angew. Chem. Int. Ed. 1993, 32, 1614-1616; b) K. Hesse, S. Hünig, Liebigs Ann. Chem. 1985, 715-739.
 Collaboration with Prof. Helmut Cölfen, Univesity of Konstanz
9:00 PM - NM1.10.42
Fabrication of Orderly Arrayed 2D Porous Carbon Nanosheets
Junlong Huang 1 , Yanhuan Lin 1 , Yiwei Sun 1 , Yeru Liang 1 , Yongming Chen 1 , Dingcai Wu 1 , Ruowen Fu 1 Show Abstract
1 , Sun Yat Sen University, Guangzhou China
Currently, there is an urgent need for design and construction of advanced structures that enable carbon nanosheets to array in a layer-by-layer manner and minimize their unfavorable restacking to retain abundant surfaces/interfaces. Here we report a versatile method for preparing an unprecedented class of orderly arrayed 2D porous carbon nanosheets intrinsically interconnected with porous carbon spacers based on gradient crosslinking of lamellar block copolymers. Benefiting from the robust support of interlayered porous carbon spacers, the porous carbon nanosheets demonstrate an ordered layer-by-layer array characteristic. Furthermore, the intrinsic porous structure of porous carbon spacers maximizes the utilization of well-developed surfaces/interfaces of porous carbon nanosheets. The as-prepared orderly arrayed 2D porous carbon nanosheets have a high Langmuir surface area of up to 1052 m2 g-1. These finding are expected to boost the development of highly porous yet orderly arrayed 2D nanomaterials for a spectrum of challenging applications including energy, catalysis, adsorption and environment.
9:00 PM - NM1.10.44
Modulation Doping of 2D Layered MoS2 by Transition Metal Oxide Deposition
Kang Xu 1 , Yi Wang 1 , Yuda Zhao 1 , Yang Chai 1 Show Abstract
1 , The Hong Kong Polytechnic University, Hong Kong China
For typical TMDs materials, the pristine carrier type is predominated by their intrinsic characteristics. 1 The control of carrier type and carrier density provides a way to tune the physical properties of two-dimensional (2D) semiconductors, which is crucial for building complementary logic circuits in the future. Various methods have been developed to dope 2D layered TMDs, including substitutional doping during growth, 2 ion implantation, 3 plasma treating, 4 etc. Although these doping methods have been demonstrated to be effective, they inevitably result in the distortion of TMDs crystal structure, introduce ionized impurity scattering center, and degrade the charge mobility. Modulation doping in a hetero-structure can effectively inject carrier into or extract carrier from the 2D semiconductors, and eliminate the adverse effect from the ionized dopants.
Here we construct a hetero-structure with transition metal dichalcogenides (TMDs) and transition metal oxide. By choosing the oxide with different charge neutrality level (CNL), we demonstrate effective electron injection into MoS2 by TiO2 doping, and electron extraction from MoS2 by MoO3 doping. The TiO2 doped MoS2 exhibit trion-dominant photoluminescence, and the increase of electron density. In contrast, the MoO3 doped MoS2 shows exciton-dominant photoluminescence, and the decrease of electron density. Based on electrical characterizations, the doping levels of TiO2 and MoO3 are comparable (~0.7×1012 cm-2). Our theoretical calculations show good agreement with our experimental results. The modulation doping with transition metal oxide is compatible with conventional Si processing and highly air-stable. This method can be also extended for the controllable doping of other 2D materials.
(1) C. Zhou, Y. Zhao, S. Raju, Y. Wang, Z. Lin, M. Chan, Y. Chai, Adv. Funct. Mater. 2016, 26, 4223.
(2) J. Suh, T.-E. Park, D.-Y. Lin, D. Fu, J. Park, H. J. Jung, Y. Chen, C. Ko, C. Jang, Y. Sun, Nano Lett. 2014, 14, 6976.
(3) A. Nipane, D. Karmakar, N. Kaushik, S. Karande, S. Lodha, ACS nano 2016, 8 (10), 10808.
(4) M. Chen, H. Nam, S. Wi, L. Ji, X. Ren, L. Bian, S. Lu, X. Liang, Appl. Phys. Lett. 2013, 103, 142110.
9:00 PM - NM1.10.45
Two-Step CVD Growth of Anisotropic ReS2 and WS2 Heterostructures
Bin Chen 1 , Kedi Wu 1 , Aslihan Suslu 1 , Sijie Yang 1 , Aliya Yano 1 , Emmanuel Soignard 1 , Toshihiro Aoki 1 , Sefaattin Tongay 1 Show Abstract
1 , Arizona State University, Tempe, Arizona, United States
ReS2 attracts attentions as a newly discovered semiconductor in two dimensional (2D) transition metal dichalcogenides (TMDs) family, owing to its unique structural anisotropic behaviors. Building heterojunctions of 2D materials is an effective way to maximize their potentials. However, heterostructures comprising anisotropic and isotropic of 2D TMDs layers are rarely explored. Here, we demonstrate the two-step chemical vapor deposition method for growing ReS2/WS2 heterostructure. Raman spectroscopy shows that ReS2 grows along edges and scatters on surface of WS2. Electron energy loss and optical absorption spectroscopy reveal that ReS2 grown on edges overlaps underlying WS2, which is later confirmed by high resolution scanning transmission electron microscopy images. Despite the structural difference of the two materials, Re chain  direction in ReS2 always aligns with zigzag  direction of WS2. Being able to synthesize and characterize such a representative structure not only provides insights into its growth mechanism but also opens the world for a broader range of other possible 2D based aniso/isotropic heterostructures for potential applications.
9:00 PM - NM1.10.46
Effect of Passivation Layer of the Thermal Stability of a Few-Layer Phosphorene/AZO Heterostructure
Sushil Pandey 1 , Nezhueyotl Izquierdo 1 , Stephen Campbell 1 Show Abstract
1 , University of Minnesota, Minneapolis, Minneapolis, Minnesota, United States
Two-dimensional phosphorene has attracted considerable interest due to its superior properties for future optoelectronics and nanoelectronics applications. The thermal stability of the film and its heterostructures is a limiting factor in device fabrication, especially in the formation of low-resistance ohmic contacts. Here we report the effect of the passivation layer composition on thermal stability as measured by the Raman spectra of a phosphorene/AZO heterostructure. Few-layer phosphorene was formed using micromechanical exfoliation on an Al-doped ZnO film/Si substrate. Samples were passivated by Al2O3, Si3N4 or an Al2O3/Si3N4 stack deposited by atomic layer deposition. Atomic force microscopy was carried out on these samples to measure the phosphorene thicknesses which were 8 to 16 nm prior to anneal. When these samples were annealed at different temperatures (100 to 550 oC), the nitride-only structure was found to have highest thermal sustainability, demonstrating clear phonon peaks of out-of-plane (A1g) and in-plane (B2g and A2g) modes up to the highest temperature tested (550 oC). The two samples with an Al2O3 coating retained A1g, B2g, and A2g phonon peaks up to 450 oC (oxide only) and 500 oC (oxide/nitride). For anneals up to 200 oC the nitride-only passivated phosphorene film was found to have a significant red shift in A1g, B2g and A2g phonon peaks. This is attributed to the effects of tensile strain. At annealing temperatures higher than 200 oC, the spectra blue shifted and attained closer value to bulk peak position, suggesting strain relaxation in film at higher temperature. This relaxation was also evident in a reduction of the full-width at half maximum of the Raman phonon peaks at higher annealing temperature. There are two possible reasons for the relaxation at higher temperature: (1) out-diffusion of Zn atoms from Al-ZnO film may help release stress in phosphorene, and (2) anneals may reduce the stress in the capping layers indirectly reducing the stress in the phosphorene layer. This demonstration of low-strain phosphorene layers at the higher annealing temperature enabled by Si3N4 passivation may accelerate the further development of high-performance phosphorene/ZnO heterojunction devices for optoelectronic and electronic applications.
9:00 PM - NM1.10.47
Two-Dimensional Silicon Carbide as a Potential Thermoelectric Material
Srilok Srinivasan 1 , Ganesh Balasubramanian 1 Show Abstract
1 , Iowa State University, Ames, Iowa, United States
Bulk SiC is a wide band gap material with mechanical and thermal properties desirable for high temperature applications. Recent first principle calculations demonstrating the stability of atomically thin SiC structures, and the successful synthesis of two dimensional SiC and SiC2 have spurred the interest in transport properties of low-dimensional silicon carbide structures. Here, we computationally investigate the potential for 2D SiC as a thermoelectric material using density functional theory (DFT) and molecular dynamics (MD) simulations. With regard to thermal conductivity (k), we show that 2D SixC1-x can be considered as a stoichiometric hybrid of graphene and silicene. The variation of k of 2D SixC1-x with “x” can be interpreted using a mean field model developed based on the mass disorder, for example, isotope doping in graphene or silicene lattice. We find that thermal conductivity is significantly reduced compared to graphene or silicene while preserving the electrical conductivity making 2D SixC1-x a potential thermoelectric material. The thermoelectric figure of merit (ZT) for 2D SixC1-x structures were computationally calculated using electrical conductivity and the Seebeck co-efficient obtained from Boltzmann Transport Equation (BTE). The dependence of density of states and dispersion curves of phonons and electrons on the relative composition of Si and C was studied in order to determine the optimum value of “x”. Furthermore, to investigate the role of defects in phonon scattering we perform MD simulations with vacancy defects. The interatomic potential used in MD simulations were validated using the results from DFT. We show that the thermal conductivity decreases significantly with defects until it reaches a saturation value of k. Our findings from this computational study qualitatively matches with collaborative experimental results.
9:00 PM - NM1.10.48
Novel Hybrid Perovskite-Based Nanosheets via Rapid Microwave-Assisted Reactions
Sara Akbarian-Tefaghi 1 , Treva Brown 1 , Paul Renquet 1 , Taha Rostamzadeh 1 , Clare Davis-Wheeler 1 , John Wiley 1 Show Abstract
1 Department of Chemistry and Advanced Materials Research Institute, University of New Orleans, New Orleans, Louisiana, United States
Oxide nanosheets, attainable via liquid exfoliation of layered perovskites, are interesting 2D building blocks that can form novel assemblies and heterostructures. While such nanosheets offer similar characteristics to 2D transition metal dichalcogenides, phosphorene, and graphene family members, they can also have the advantage of being readily prepared with important variations in composition, slab thickness and surface groups. Here, a novel series of double- and triple-layered Dion-Jacobson perovskites (HPrNb2O7, HCa2Nb2FeO9, and HLaCaNb2MnO10) were examined for the production of functionalized oxide nanosheets. Rapid microwave-assisted reactions were used to fabricate organic-inorganic nanostructures – protonated perovskites were first exfoliated in an aqueous solution of tetra(n-butyl)ammonium hydroxide and then their reactivity with different organics containing hydroxyl or amine functional groups was studied in order to tailor the nanosheet surfaces. The rapidity of exfoliation and surface modification afforded by these techniques allows for effective screening and tuning of novel nanosheets with directed properties. Grafting reactions involving double-layered perovskite nanosheets readily occurred with a variety of alcohols and amines, while those from triple-layered hosts were relatively limited. The structure and properties of these various nanosheets as a function of layer thickness and surface groups were characterized with atomic force and electron microscopies, X-ray diffraction, and vibrational and diffuse-reflectance UV-Visible spectroscopies. Variations in the reactivity of the different hosts as a function of composition and layer thickness for the oxide series will be presented and the role that metal-oxide layer surface charge plays in their reactivity discussed.
9:00 PM - NM1.10.49
Giant Magnetism in Hydrogenated Silicene Nanoflakes and Their Potential Application as Spin Devices
Sadegh Mehdi Aghaei 1 , Irene Calizo 1 2 Show Abstract
1 Electrical and Computer Engineering, Florida International University, Miami, Florida, United States, 2 Mechanical and Materials Engineering, Florida International University, Miami, Florida, United States
Silicene, the silicon version of graphene, has drawn tremendous attention due to its unique characteristics and inherent compatibility with silicon based nanotechnology. The electronic properties of a 2D silicene sheet are expected to significantly change in low dimensional forms due to quantum confinement and edge effects. The 0D nanostructures derived from 2D sheets such as silicene nanoflakes (SiNFs) are of great interest since they have the potential of increased performance with miniaturization. In this study, the electronic and magnetic properties of SiNFs are investigated by using density functional theory calculations. It is found that hexagonal SiNFs exhibit non-magnetic semiconducting behavior, while the triangular SiNFs are magnetic semiconductors. Hydrogenation is applied as an effective means to tune the electronic and magnetic properties of SiNFs. It is discovered that a small-gap semiconductor to half-metal transformation occurs by half-hydrogenation of SiNFs. It is observed that, for the first time, the half-hydrogenated SiNFs offer a giant spin moment which is directly proportional to the square of the flakes size (n). The calculated total magnetic moment (M) for hexagonal and triangular half-hydrogenated SiNFs are determined to be M = 3n2 and M = (n2+5n)/2, respectively. The strong induced spin magnetizations in these nanoflakes align parallel and show a substantial collective behavior by long range magnetic coupling, which makes them a potential candidate for spintronic circuit devices. It is also revealed SiNFs become nonmagnetic semiconductors with a large band gap by full hydrogenation. Finally, simple models for spin switches are proposed to uncover the capability of tuning transport properties by controlling the coverage of hydrogenation on SiNFs. These results may open new gates in exploring silicon based spin devices and trigger further experimental studies.
9:00 PM - NM1.10.50
Black Phosphorus Field Effect Transistor with Hexagonal Boron Nitride as a Dielectric/Passivation Layer
A-Young Lee 1 , Hyun-Soo Ra 1 , Dohyun Kwak 1 , Jong-Soo Lee 1 Show Abstract
1 , DGIST, Daegu Korea (the Republic of)
Black phosphorus, or BP is a two dimensional material with tunable band gap (0.3–1.5 eV) has recently been attracting attention for application in high performance electronic and optoelectronic devices. However, surface oxidation of BP under the air with humidity forms spikes on BP layer, which provides a degradation of device performance including hysteresis, mobility, and current on-off ratio within one day. In this letter, we first demonstrate top-gate BP field effect transistors (FETs) using hexagonal boron nitride (h-BN) as a passivation and dielectric layer (ε = 3). 4 different structures of BP devices will be introduced and compared. Then, try to analyze the factors that cause the change of BP. Finally, high-mobility BP FETs with low operation voltage and near-zero hysteresis are demonstrated using h-BN as a gate dielectric materials.
9:00 PM - NM1.10.51
Trap-Mediated Electronic Transport Characteristics of Gate-Tunable Pentacene/MoS2 p-n Junctions
Jae-Keun Kim 1 , Kyungjune Cho 1 , Tae-Young Kim 1 , Jinsu Pak 1 , Jingon Jang 1 , Younggul Song 1 , Youngrok Kim 1 , Seungjun Chung 1 , Takhee Lee 1 Show Abstract
1 , Seoul National University, Seoul Korea (the Republic of)
Transition metal dichalcogenide (TMDC) materials have emerged as promising semiconductors for future nanoelectronic devices due to their ultrathin nature and favorable electronic properties. TMDCs have pristine surfaces free of dangling bonds due to van der Waals (vdW) bonding between the layers of the TMDCs, which enables vertical staking of other materials that do not have similar lattice constants to those of TMDCs. The vdW heterostructures which are composed of this dangling bond-free materials have been studied because of its novel physical phenomena and possibility of device integration. Organic materials also can be a candidate of vdW heterostructure with a lack of dangling bonds, which suggests another form of vdW heterosturcture. A few previous studies demonstrated the gate-tunable electronic and optoelectronic characteristics in vdW organic/TMDC p-n heterostructures. However, understanding of the electrical transport properties of the organic/TMDC structures is still limited because organic materials exhibit the presence of chemical and structural defects because of imperfect crystallinity, which can lead to charge trap densities on the order of 1018/cm3.
In this presentation, we will report the fabrication of the heterostructure devices made with n-type molybdenum disulphide (MoS2) and p-type organic pentacene and their electrical properties at different gate voltages and different temperatures. We observed that the the pentacene/MoS2 p–n heterostructure devices were gate-tunable and were strongly affected by trap-assisted tunnelling through the vdW gap at the heterojunction interfaces between MoS2 and pentacene. The pentacene/MoS2 p-n heterostructure diodes had gate-tunable large ideality factor, which could be attributed to trap-mediated conduction nature of devices. Also, the evidences of interfacial trap states were observed with transmission electron microscopy and atomic force microscopy. From the temperature-variable current-voltage characterization, the gate-tunable electrical characteristics of the devices were explained by a space-charge-limited conduction at all the measured temperature range. Also, the electrical characteristics were explained by a variable range hopping conduction at low temperature and thermally activated transport at high temperature. This study will help the understanding of the role of traps in the electrical properties of organic/TMDC materials vdW heterostructure devices.
9:00 PM - NM1.10.52
Optoelectronic Properties of Pristine and Deterministically Thinned 2H-MoTe2
Tobias Octon 1 , Karthik Nagareddy 1 , Iddo Amit 1 , Saverio Russo 1 , Monica Craciun 1 , C. David Wright 1 Show Abstract
1 , University of Exeter, Exeter United Kingdom
Transition Metal Dichalcogenides (TMDCs) are 2D materials which have semiconducting to superconducting phases. One member of this family of materials is semiconducting 2H-MoTe2 with a band gap similar to silicon (0.9 - 1.1 eV) making it suitable for optoelectronics under NIR and visible light. We have measured the photoresponsivity and photo-response times for few-layer MoTe2 on SiO2 finding it to be a fast and high responsivity photodetector at 685 nm. Compared to other TMDCs, MoTe2 performs well as a photodetector under visible light due to its lower band gap. Through the use of different dielectrics (such as h-BN) we observe how the mechanisms of electronic and optoelectronic properties change due to modifying the availability of trap states which occur between the conduction and valence bands of 2H-MoTe2.
Acquiring flakes of few-layer thickness MoTe2 is non-trivial on a mass scale. However, to facilitate this we have devised a deterministic layer by layer laser-thinning technique. Laser thinning was achieved using a 514 nm laser while an in-situ Raman spectroscopy measurement verifies that layers can be sublimated from the flake one at a time. We find that laser-thinned MoTe2 devices have improved electronic properties and expect that, and will report whether, optoelectronic properties be also be enhanced in a similar fashion.
9:00 PM - NM1.10.53
Scalable Growth of Monolayer Transition Metal Dichalcogenides Thin Film with Large Domain Size on Oxide Substrates via Chemical Vapor Deposition
Shanghuai Feng 1 , Jianyong Xiang 1 , Zhongyuan Liu 1 , Fusheng Wen 1 , Yongjun Tian 1 Show Abstract
1 State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinghuangdao, Hebei, China
Two dimensional transition metal dichalcogenides (TMDCs) offers great opportunities in designing miniature and atomically thin (opto)electronic devices owing to their unique electronic and optical properties. The availability of continuous monolayer film formed with large domains is key to practical application of TMDCs in opto(electronics). We demonstrated here an economic and scalable growth of continuous monolayer WS2 film on SiO2/Si substrate (10 x 10 mm 2) via a chemical vapor deposition (CVD) method. The as-grown monolayer WS2 film is composed of triangular single domains with the averaged lateral dimension of over 300 µm, demonstrating a very low density of grain boundary. From the low energy electron diffraction and polarized Raman measurements, we show that the single domains are grown in random orientitions over the substrate. The field effect electron mobility has a narrow distribution and is determined to be ~10 cm2/Vs averaged over 50 top-gated devices. We find that persisting a high partial pressure of WO3 vapor plays a key role for the continuous growth of monolayer WS2, while a sufficiently high substrate temperature allows for the growth of small WS2 domain into large area single domain. Moreover, by using this method, we succeed in preparing continuous film of MoS2/WS2 heterostructure .
9:00 PM - NM1.10.54
Exciting Multiple Electrons by One Photon in 2D Semiconductors for Photovoltaics
Laurens Siebbeles 1 , Aditya Kulkarni 1 Show Abstract
1 , TU Delft, Delft Netherlands
Absorption of sufficiently energetic photons in a bulk semiconductor leads to hot electrons and holes that usually cool to the band edge by thermal relaxation. In semiconductor nanomaterials this cooling can be intercepted by excitation of additional electrons across the band gap. In this way, one photon generates multiple electron-hole pairs via a process known as Carrier Multiplication (CM), which is of interest for the development of highly efficient solar cells and photodetectors.
We studied charge carrier photogeneration, CM, charge mobility and decay in: a) films of PbSe nanocrystals coupled by organic ligands, b) 2D superlattices of directly coupled PbSe nanocrystals in a square or honeycomb pattern, and c) PbS nanosheets. The studies were performed using ultrafast pump-probe spectroscopy with optical or terahertz conductivity detection.
The nanogeometry of the material was found to have enormous effects on CM and the charge mobility. In 2D superlattices of nanocrystals coupled by atomic bonds CM occurs in a step-like fashion with threshold near the minimum energy of twice the band gap. In these superlattices the CM efficiency and charge mobility are much higher than for films of QDs that are coupled by organic ligands. When the thickness of PbS nanosheets is reduced, the CM efficiency becomes higher.
The factors governing the efficiency of CM and the impact on the efficiency of photovoltaic devices will be discussed.
9:00 PM - NM1.10.55
Photogeneration and Mobility of Charge Carriers in Atomically Thin Colloidal InSe Nanosheets Probed by Ultrafast Terahertz Spectroscopy
Jannika Lauth 1 , Aditya Kulkarni 1 , Frank C. M. Spoor 1 , Nicolas Renaud 1 , Ferdinand Grozema 1 , Arjan J. Houtepen 1 , Juleon M. Schins 1 , Sachin Kinge 2 , Laurens Siebbeles 1 Show Abstract
1 , Delft University of Technology, Delft Netherlands, 2 , Toyota Motor Europe, Zaventem Belgium
The implementation of next generation ultrathin electronics by applying highly promising dimensionality-dependent physical properties of two-dimensional (2D) semiconductors is ever increasing. In this context, the van der Waals layered semiconductor InSe has proven its potential as photodetecting material with high charge carrier mobility. We have determined the photogeneration charge carrier quantum yield and mobility in atomically thin colloidal InSe nanosheets (inorganic layer thickness 0.8 – 1.7 nm, mono/double-layers, ≤5 nm including ligands) by ultrafast transient terahertz (THz) spectroscopy. A near unity quantum yield of free charge carriers is determined for low photoexcitation density. The charge carrier quantum yield decreases at higher excitation density, due to recombination of electrons and holes, leading to the formation of neutral excitons. In the THz frequency domain we probe a charge mobility as high as 20 ± 2 cm2/Vs. The THz mobility is similar to field-effect transistor mobilities extracted from unmodified exfoliated thin InSe devices. The current work provides the first results on charge carrier dynamics in ultrathin colloidal InSe nanosheets.
9:00 PM - NM1.10.56
Controlled Nucleation and Growth of Monolayer Tungsten Diselenide (WSe2) Films on Sapphire via Metalorganic Chemical Vapor Deposition
Xiaotian Zhang 1 , Tanushree Choudhury 2 , Bhakti Jariwala 1 , Kehao Zhang 1 2 3 , Yu-Chuan Lin 1 2 , Baoming Wang 4 , Fu Zhang 1 , Sarah Eichfeld 1 2 , Nasim Alem 1 2 , Aman Haque 4 , Joshua Robinson 1 2 3 , Joan Redwing 1 2 Show Abstract
1 Department of Materials Science and Engineering, Pennsylvania State University, University Park, Pennsylvania, United States, 2 2D Crystal Consortium, Center for 2-Dimensional and Layered Materials, Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania, United States, 3 Center for Atomically Thin, Multifunctional Coatings, The Pennsylvania State University, University Park, Pennsylvania, United States, 4 Department of Mechanical Engineering, The Pennsylvania State University, University Park, Pennsylvania, United States
Monolayer two-dimensional (2D) materials, particularly the family of transition metal dichalcogenides (TMDCs) have been a focus of intense interest since the discovery of graphene in 2004. Compared to conventional semiconducting thin films, monolayer semiconducting TMDCs such as MoS2 and WSe2 exhibit good carrier mobility as well as mechanical flexibility, which would greatly benefit flexible electronics technology. Prior studies demonstrated that chemical vapor deposition (CVD) and metalorganic CVD (MOCVD) can provide large-scale uniform films of TMDCs. However, films synthesized from these methods on amorphous substrates such as SiO2/Si substrate are polycrystalline which limits the electrical performance. In order to obtain monolayer single crystal TMDCs films, a single crystal substrate is needed as a template for epitaxial growth and controlled gas source concentration is desired to form initial oriented nuclei and to suppress continued nucleation during later lateral growth. Our prior studies showed the use of annealed sapphire to get oriented growth of WSe2. In this work, we demonstrate a two-stage growth technique to achieve uniform monolayer single crystal WSe2 films by MOCVD using tungsten hexacarbonyl (W(CO)6) and hydrogen selenide (H2Se) as precursors on annealed sapphire substrates in a cold wall vertical reactor. The growth is carried out at 700 Torr and 800oC for 30 to 60 minutes. The H2Se flow rate remains constant at 7 sccm and ultra-high purity hydrogen is used as the carrier gas. We utilize a two-stage W(CO)6 flow process to control nucleation and lateral growth. Initially W(CO)6 is introduced at a higher flow rate (~1.2×10-3 sccm) to drive nucleation and control initial nuclei density. Subsequently, the flow rate is then reduced (~4.2×10-4 sccm) to limit further nucleation and promote lateral growth resulting in coalescence to form a continuous monolayer film while minimizing secondary layer nucleation. High resolution transmission electron microscopy (TEM) and selected area diffraction were used to study domain orientation and grain boundaries in films removed from the growth substrate. The results demonstrate the potential to obtain monolayer TMDC films over large substrate areas via controlled nucleation in MOCVD growth.
9:00 PM - NM1.10.57
WS2 and h-BN Nanodispersions for Inkjet Printing Applications
Jay Desai 1 , Chandan Biswas 1 , Anupama Kaul 1 Show Abstract
1 , University of Texas at El Paso, El Paso, Texas, United States
Since the discovery of graphene, various layered materials have been studied to observe the change in physical, chemical and electrical properties when three-dimensional bulk materials are exfoliated to their two-dimensional counterparts. In this work, we demonstrate optical and electrical transport properties of spin coated chemically exfoliated WS2 in dimethyl formamide (DMF), and dielectric properties of spin coated chemically exfoliated h-BN in isopropyl alcohol (IPA) using different sonication times. High electrical conductivity of WS2 nanodispersions was observed when appropriate amount of voltage was applied indicating their semi-conductive behavior. The effect of sonication time on amount of defects present and degree of exfoliation was studied using temperature dependent raman spectroscopy. Surface morphology of different WS2 and h-BN nanodispersions were studied using optical microscopy. Optical bandgap for both WS2 and h-BN nanodispersions were determined from optical absorbance spectrum. Inkjet printing was used to demonstrate good printability of our dispersions. These dispersions indicate the potential of WS2 and h-BN in various optoelectronic applications.
9:00 PM - NM1.10.58
Mohs Hardness—How a Simple Tool Can Be an Effective Means for Discovering 2D Materials
Tyler Farnsworth 1 , Eleanor Brightbill 1 , Patrick O'Brien 2 , Kaci Kuntz 1 , Adam Woomer 1 , Scott Warren 1 3 Show Abstract
1 Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States, 2 Department of Physics, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States, 3 Department of Applied Physical Sciences, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States
Two-dimensional (2D) materials are often championed as components for novel technologies due to the extreme change in properties that accompanies their transition from bulk to a quantum-confined 2D state. Despite their primise, the current library of 2D materials has remained primarily limited to layered van der Waals (vdW) crystals . Past efforts to add to the library of 2D materials have relied on chemical intuition regarding interlayer binding forces and bond stability, which can be limited, at best. More recent attempts to expand the 2D material library have used computer-based techniques but these also have been focused primarily on vdW systems with high crystallographic symmetry1 or specific compositions2. In this work, we introduce a new set of criteria for identifying layered precursors that may be exfoliated into 2D form. Our approach harnesses two important properties: Mohs hardness and melting point. These properties enable a rapid and effective method to identify candidates for exfoliation. For layered crystals, we propose that the Mohs hardness scale provides a detailed physical description of inter-layer (out-of-plane) interactions while the melting point measures intra-layer (in-plane) bond strength. Based on this model, we evaluated hundreds of naturally-occurring minerals and identified > 230 layered precursors that meet our criteria (Mohs < 3) for producing a stable, 2D material. Using this list, we use the Scotch-tape and liquid exfoliation methods to demonstrate the successful synthesis of several novel 2D materials, including an ionic layered crystal, as well as the exfoliation of layered crystals with melting temperatures as low as 320 °C. Our results are promising – revealing a simple, yet powerful, tool to expand the library of 2D materials and enhance the rapidly growing field of nanotechnology.
1. Lebègue, S. et.al. Phys. Rev. X. 2013, 3, 031002
2. Revard, B. C. et.al. Physical Review B. 2016, 93, 054117
9:00 PM - NM1.10.60
One-Dimensional Electron Gas in Lateral Heterostructures of Single Layer Materials
Oleg Rubel 1 Show Abstract
1 Materials Science and Engineering, McMaster University, Hamilton, Ontario, Canada
Confinement of the electron gas along one of the spatial directions opens an avenue for studying fundamentals of quantum transport along the side of numerous practical electronic applications, with high-electron-mobility transistors being a prominent example. A heterojunction of two materials with dissimilar electronic polarization can be used for engineering of the conducting channel. Extension of this concept to single-layer materials leads to one-dimensional electron gas (1DEG). MoS2/WS2 lateral heterostructure is used as a prototype for realization of 1DEG. The electronic polarization discontinuity is achieved by straining the heterojunction taking advantage of dissimilarities in the piezoelectric coupling between MoS2 and WS2. A complete theory that describes an induced electric field profile in lateral heterojunctions of 2D materials is proposed and verified by first principle calculations.
9:00 PM - NM1.10.61
Hexagonal Boron Nitride Crystal Growth from Ni-Cr Flux—Experiment and Simulation
Song Liu 1 Show Abstract
1 , Kansas State University, Manhattan, Kansas, United States
Over the past ten years, new applications of hexagonal boron nitride (hBN) are being envisioned, which will utilize the optical, phononic, nuclear, atomically flat surface, flexoelectricity, and electronic properties. Examples include hBN deep ultraviolet light emitters (i.e., light emitting diodes and laser diodes), neutron detectors, nanophotonics, and quantum computers. For these, control of the thickness, area, crystal perfection, crystallographic orientation and purity of the hBN crystals is essential. We are producing high purity, low defect density hBN crystals at atmospheric pressure by cooling a molten Ni-Cr flux saturated with boron and nitrogen from 1550 °C to 1500 °C at a slow rate (1 °C/h). Single crystals up to 4 mm2 in area, some of the largest ever produced, were grown. With the goal of growing larger crystals, theoretical methods, including density functional theory (DFT) and reactive molecular dynamics (rMD) simulations, were performed to understand and control the growth process. DFT calculations show that the Ni surface steps are responsible for initial nucleation. Elemental B diffuse on surface and in the sublayer, but elemental N only diffuses on the surface. Large scale rMD simulations revealed a full scheme of hBN evolution from the linear configuration, to the branched, then to the hexagonal geometry, which is consistent with the paths predicted by DFT calculations. Different temperatures (900 – 1500 K) have been considered to investigate the thermal influence on hBN quality. As a result, the formation of a continuous hBN network was only observed at 1300 K and above.
9:00 PM - NM1.10.62
Jettible Functional Materials for Applications in Printed Electronics
Robert Ionescu 1 , Jarrid Wittkopf 1 , Helen Holder 1 , Ning Ge 1 Show Abstract
1 , HP, Palo Alto, California, United States
Atomically thin semiconductor materials such as the transition metal chalcogenides of the form of MX2 have great potential in applications for next generation electronics. One current issue for optoelectronics is poor energy efficiency in this can be greatly improved by novel 2D materials with direct bandgap in the visible light spectrum. In combination with graphene and boron nitride we can revolutionize the way we use our optoelectronic devices. Unlike the indirect band-gap of silicon, 2D layered materials possess a direct band-gap single-layer. A direct band gap is essential for light emission and light based devices. A variety of applications include, but not restricted to, field effect transistors (FETs), photodetectors, photosensors, and photovoltaic devices. A variety of transistors based on 2D layered materials have already shown that they can deliver high electron mobility, high on/off ratio, and move us towards transparent ultra-thin devices. Numerous methods of synthesizing these materials have revolved around a chemical vapor deposition (CVD) processes. Since industry has been trying to move away from powder synthesis CVD processes, this new printing method using inks incorporating 2D materials allows for new fast processing of electronics. Printing of these new functional materials can move electronics prototyping away from extended fab processes and allow for quick development of a multitude of different functional devices.
9:00 PM - NM1.10.64
Layered Transition Metal Dichalcogenides Bulk Heterojunctions as Photoanodes in Photoelectrochemical Cells
Federico Pesci 1 , Maria Sokolikova 1 , Chiara Grotta 1 , Cecilia Mattevi 1 Show Abstract
1 , Imperial College London, London United Kingdom
Group VI-of layered transition metal dichalcogenide (TMDs) have been known for decades in their bulk form and renewed interest arose in the last decade because of their unique electrical and optical properties when their thickness is reduced to monolayer. In particular, their band-gap presents a transition from and indirect-to-direct type when the thickness is reduced to monolayer and MoS2 and WS2 with a direct band gap of 1.9 and 2 eV respectively have been predicted to potentially enable water oxidation due to the favourable position of their valance band and conduction band levels.
Here we report the use of n-type WS2 and MoS2 nanosheets as photoanodes for water splitting in a complete PEC cell. The materials are chemically exfoliated in water, forming a colloidal suspension of mono and few-layered nanosheets. Both materials show positive photocurrents at low applied overpotentials and under simulated solar radiation, demonstrating their capability to oxidize water. The incident-photon-to-current efficiency (IPCE) greatly enhances up to one order of magnitude when MoS2 and WS2 are mixed together to form a randomly structured van der Waals bulk heterojunction consisting of percolating networks of the two materials. We attribute this enhanced IPCE to the formation of a type II heterojunction which leads to a more efficient separation of charge carriers. Transient absorption and transient photocurrent studies in a complete PEC cell have been carried out to elucidate the dynamics of the photogenerated charge carriers.
 Pesci, F. M. et al., Manuscript under revision.
9:00 PM - NM1.10.65
Synthesis and Characterization of Two-Dimensional Nickel Selenide Nanoflakes by Chemical Vapor Deposition
Fang-Chi Hsu 1 , Chiu-Yen Wang 1 Show Abstract
1 , National Taiwan University of Science and Technology, Taipei Taiwan
Recently the study of two-dimensional (2D) nanostructure have been developed rapidly due to it exhibit many unique physical and chemical properties when compared to their bulk counterparts. The purpose of this work is to study 2D material grows on silicon oxide substrate and control its orientation and morphology. This study controlled the synthesis of nickel selenide (NiSe) nanoflakes (NFs) by using chemical vapor deposition (CVD) method with 30 mm inner diameter quartz tube in a horizontal tubular furnace. Nickel chloride hexahydrate (NiCl2．6H2O) and selenium powder were used as precursors. NiSe NFs grow on a silicon oxide substrate (80 nm SiO2). This study observed that the growth temperature, system pressure, carrier gas flow rate, substrate position, and growth time influence the growth condition of NiSe NFs. The morphologies and crystal structures of the 2D NiSe NFs were characterized by using optical microscopy (OM), field-emission scanning electron microscope (FE-SEM), X-Ray Diffraction (XRD), and transmission electron microscopy (TEM). The composition was analyzed with energy dispersive x-ray spectroscopy (EDS) attached the SEM. Based on the results, the 2D NiSe NFs were fabricated by CVD method successfully. The next step of the investigation will focus on apply to the electronic devices with suitable NiSe NFs in future.
9:00 PM - NM1.10.66
Low Temperature Synthesis of Atomically Thin MoS2 Using Atomic Layer Deposition and Plasma Sulphurization
Akhil Sharma 1 , Ravi Sundaram 2 , Vincent Vandalon 1 , Martijn Vos 1 , Erwin Kessels 1 , Ageeth Bol 1 Show Abstract
1 , Eindhoven University of Technology, Eindhoven Netherlands, 2 , Oxford Instruments Plasma Technology, Bristol United Kingdom
The synthesis of 2-D MoS2 with a combined atomic layer deposition (ALD) and plasma sulphurization (PS) approach offers several key advantages over the conventional techniques used for MoS2 synthesis. The combined approach basically conjoins the inherent advantages related to ALD and PS enabling large area synthesis of high quality and uniform MoS2 films with sub-monolayer control over the thickness. These features are vital for nano - electronic applications and are difficult to achieve with techniques such as CVD. Furthermore, both ALD and PS can be performed at low temperatures (~200°C) allowing synthesis on a wide range of temperature sensitive substrates.
In this contribution, the synthesis of 2-D MoS2 was carried out using the combined ALD + PS approach in the temperature range between 200°C - 450°C and the synthesized films were extensively characterized. Ultrathin MoO3 films ranging from 0.5 nm - 5 nm were deposited by PE-ALD using halide-free chemistry on thermal SiO2 substrates at 200°C with a growth rate of 0.8 Å/cycle . The TEM analysis revealed that the ALD grown MoO3 films as thin as down to ~ 2 nm were closed and uniform in nature. The MoO3 films were plasma sulphurized (H2S:Ar = 1:9) at a low pressure (200 mTorr) in the temperature range between 200°C - 450°C in an ALD reactor equipped with remote ICP type plasma system. This transformed the ALD grown MoO3 ultrathin films to mono or few layered MoS2 films.
The number of layers in the MoS2 films was investigated by Raman spectroscopy which showed the frequency difference values (Δk) between two vibrational modes ranging from 20.6 cm-1 - 25 cm-1 corresponding to a monolayer or few-layered film respectively. The uniformity of the films was verified by Raman mapping measurements. Furthermore, a clear correlation was found between the thickness of MoO3 and the resulting MoS2 films which demonstrates an excellent control over the thickness of the MoS2 films using this combined approach. The photoluminescence spectroscopy results were in line with Raman analysis showing a strong peak at ~1.9 eV corresponding to the direct band gap transition associated with a monolayer of MoS2. Moreover, XPS analysis was conducted to investigate the chemical composition of the films. Finally, second - harmonic generation spectroscopy will be used to probe the electronic structure and orientation of MoS2 crystals. To summarize, low-temperature, thickness controlled and large area synthesis of 2-D MoS2 thin films has been demonstrated by our combined approach which paves the path towards future nano-electronics and catalysis applications.
1 Vos et al., J. Vac Sci Tech. A, 2016, 34, (1), pp. 01A103, DOI: 10.1116/1.4930161
9:00 PM - NM1.10.67
Ab Initio Simulations of Stability and Electronic Properties of Cu(111)/Transition Metal Dichalcogenide Interfaces
Benjamin Helfrecht 1 2 , David Guzman 1 2 , Alejandro Strachan 1 2 Show Abstract
1 School of Materials Engineering, Purdue University, West Lafayette, Indiana, United States, 2 , Birck Nanotechnology Center, West Lafayette, Indiana, United States
As transition-metal dichalcogenide (TMD) materials rise as potential candidates for next generation electronics, understanding the physics governing the metal/TMD interface has become crucial in circuit device design and ultra-scaled interconnect technology. Using first-principles calculations based on dispersion-corrected density functional theory, we characterize the structural and electronic properties as well as the thermodynamic stability of monolayer semiconducting and metallic TMDs of the form MX2 (M = Mo, W, Nb, Ti, V; X = S, Se, Te) adsorbed on Cu(111). For all compounds, we study structures having a trigonal prismatic (H) and a distorted octahedral (T) coordination. We find that the T phase is stable on Cu(111) only if this is the TMD ground state; conversely, the H phase is stable on Cu(111) regardless of the ground state of the TMD.
We find that the Cu/TMD interfacial binding strength is reduced going from Mo to W and increased going from S to Te in the case of semiconductor TMDs. For metallic TMDs, the binding strength is reduced going from S to Te. The binding energies of the hybrids range from 1.3 Jm-2 to 2.3 Jm-2, compared with the binding energies of Cu/Graphene (0.72 Jm-2) and Cu/SiO2 (1.4 – 4 Jm-2).
The electronic properties of the Cu/TMD hybrids are characterized through the band alignment of the freestanding Cu and TMD monolayers. We find that the potential step at the interface is positive for semiconducting TMDs and negative for metallic TMDs. The magnitude of the potential also indicates the degree of hybridization in the Cu/TMD system: a larger potential step suggests stronger hybridization. The potential step created at the Cu/TMD interface increases in magnitude from S to Se to Te in the semiconducting TMDs and decreases in magnitude from S to Se to Te for the metallic TMDs. These trends are the same as those observed with the interfacial binding energies. Taken together, these results suggest that metallic TMDs hybridize more strongly with Cu than semiconducting TMDs.
9:00 PM - NM1.10.68
Heterostructures of Atomically Thin Materials for Tunnel Field Effect Transistors (TFETs)
Selcuk Temiz 1 , Zafer Mutlu 1 , Mihrimah Ozkan 1 , Cengiz Ozkan 1 Show Abstract
1 , University of California, Riverside, Riverside, California, United States
Atomically thin van der Waals (vdW) materials are the dominant research concentration in multidisciplinary nanomaterials field. These materials consist of atoms that covalently bonded to each other in plane, and weak vdW bonds formed between the layers of atoms. Interesting properties such as dangling bond free interfaces, energy independent step-like density of states, high electrical mobility, etc. are stem from this unique construction of atoms and their interactions with each other. The heterostructures of these layered materials are the excellent platforms for the new generation nanoscale device applications in low power electronics, specifically TFETs in which the band to band tunneling enables us to use low power and obtain fast switching while operating devices. Herein, we synthesize near broken band gap heterostructures of few layers WSe2 and SnSe2 crystals by employing dry transfer technique. The atomically thin crystals are micro-mechanically separated from their bulk-form-single crystals, and the heterostructures are vertically constructed with the homemade transfer tool. The interface optimization is achieved through in situ annealing of samples, and heterostructures are investigated by optical and chemical characterization tools such as photoluminescence (PL), Raman spectroscopy, scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), X-ray photoelectron spectroscopy (XPS), etc.
9:00 PM - NM1.10.69
Direct Observation of the Electrically Tunable Quasi-Particle Bandgap and Exciton Binding Energy in Monolayer MoS2
Kaiyuan Yao 1 2 , Nicholas Borys 1 , Aiming Yan 3 , Salman Kahn 3 , Aslihan Suslu 4 , Yufeng Liang 1 , Edward Barnard 1 , Sefaattin Tongay 4 , Alex Zettl 3 , P James Schuck 1 Show Abstract
1 Molecular Foundry, Lawrence Berkeley National Lab, Berkeley, California, United States, 2 Department of Mechanical Engineering, University of California, Berkeley, Berkeley, California, United States, 3 Department of Physics, University of California, Berkeley, Berkeley, California, United States, 4 Department of Material Science and Engineering, Arizona State University, Tempe, California, United States
Optical properties of monolayer MoS2 are dominated by tightly-bound exciton states . These excitons give rise to a series of weakening optical resonances with increasing energy that eventually merge into a continuum of free-carrier excitations at the so-called ‘quasiparticle (QP) bandgap’. While quantification of critical properties such as the exciton binding energy depends on precise determination of the QP bandgap, the oscillator strength of this continuum of transitions is significantly weaker than that of the excitons , and most previous work either relied on model-based inference [3, 4], or were limited to conducting substrates which impose significant additional screening effects . Direct experimental identification of the QP bandgap of monolayer MoS2 and how it renormalizes with carrier concentration [6, 7] has been elusive.
In this work, bandgap renormalization of monolayer MoS2 is directly observed by performing photoluminescence excitation (PLE) spectroscopy on back-gated devices. Compared with reflectivity/absorption measurements, PLE preferentially probes states within the K/K’valleys which have a higher quantum yield than the strongly absorbing states in the band nesting region [7, 8, 9, 10]. With this increased selectivity, the PLE spectroscopy reveals a step-like feature at ~2.7eV in the photo-excitation spectrum at 77K. This feature is attributed to the onset of the continuum, which is based on the observed reduction of bound-exciton emission above the step-edge, as well as the expected trend of renormalization with carrier concentration. As doping levels are tuned from the high residual levels formed during growth to a low, nearly neutral regime, the QP bandgap shows a nonlinear increase from ~2.57 eV to ~2.70 eV. By combining gate-dependent PLE and optical reflectivity measurements to track carrier-induced changes to the QP bandgap and exciton (and trion) resonances, respectively, the binding energy of the lowest-lying excitonic state is found to renormalize from 780 meV in the low-doping regime to 640 meV in the higher residual doping conditions. The observed large renormalization of QP bandgap and exciton binding energy provide important information for the design of exciton-based optoelectronic devices.
 Mak et al., Nature materials 12 (2013): 207-211.
 Qiu et al., Physical review letters 111.21 (2013): 216805.
 Chernikov et al., Physical review letters 115.12 (2015): 126802.
 Liu et al., Advanced Materials 28 (2016): 6457-6464.
 Zhang et al., Nano letters 14 (2014): 2443−2447.
 Liang et al., Physical review letters 114.6 (2015): 063001.
 Zhou et al., Nano letters 16.5 (2016): 3148-3154
 Kozawa et al., Nature communications 5 (2014): 4543.
 Hill et al., Nano letters 15.5 (2015): 2992-2997.
 Borys, Yao et al., submitted.
9:00 PM - NM1.10.71
Unusual Vibrational and Optical Characteristics in Pseudo-1D TiS3 Nano-Whiskers
Kedi Wu 1 , Engin Torun 2 , Hasan Sahin 2 , Bin Chen 1 , Xi Fan 1 , Anupum Pant 1 , David Wright 1 , Toshihiro Aoki 1 , Francois Peeters 2 , Emmanuel Soignard 1 , Sefaattin Tongay 1 Show Abstract
1 , Arizona State University, Tempe, Arizona, United States, 2 Physics, University of Antwerp, Antwerpen Belgium
Layered titanium trisulfide (TiS3) is a member of transition metal trichalcogenides (TMTCs) family which has attracted research interests due to the strong in-plane anisotropy. In our work, the unusual vibrational and optical properties of TiS3 whiskers have been established by high pressure and angle-resolved Raman spectroscopy, density functional theory (DFT), and finite displacement analysis techniques. Individual TiS3 layers are made of weakly interacting quasi-1D chains extending along the b-axis, and as a result, unique rigid 1D chain vibration and sulfur-sulfur molecular oscillation modes are observed. High-pressure Raman studies reveal that (1) the sulfur-sulfur molecular oscillation mode has an unconventional negative pressure dependence (dω/dP<0) whereas other peaks stiffen (dω/dP>0) as anticipated; (2) some of the vibrational modes are doubly degenerate in ambient but the degeneracy is lifted at high pressures. Angle-resolved Raman studies further show the intensity of different peaks goes through maxima and minima depending on the polarization angle and configuration, which are associated with the particular Raman tensors specific to this material system. These results not only mark the first high-pressure studies on pseudo-1D TiS3 systems, but also the experiments together with detailed theoretical analysis have largely advanced the understanding of vibrational and optical characteristics of TMTCs materials.
9:00 PM - NM1.10.72
The Gas-Sensing Performance of the Bi2S3-Bi2O3Heterojunction to Ammonia at Room Temperature
Hao Kan 1 , Qian Liu 1 , Zhilong Song 1 , Baohui Zhang 1 , Guangzu Zhang 1 , Jingyao Liu 1 , Shenglin Jiang 1 , Huan Liu 1 , Yaodong Guan 1 Show Abstract
1 , Huazhong University of Science and Technology, Wuhan China
Owing to the unique characteristics of the heterojunction interface, heterojunction nanomaterials have attracted much attention in gas sensing. In this work, Bi2S3 nanosheets were synthesized via a facile solvothermal process. We then obtained Bi2S3-Bi2O3nanocomposites were formed by oxidating as-prepared Bi2S3 nanosheets at 300°C for 3 h. Unlike the pristine Bi2S3 nanosheets-based gas sensor which exhibited a high response to nitrogen dioxide (NO2) with a negligible response to ammonia (NH3), the nanocomposites-based gas sensor possessed a sensitive and selective response towards NH3without response to NO2 at room temperature We conducted XRD and XPS analysis were to investigate the different sensing behaviors of the Bi2S3-Bi2O3 nanocomposites compared to pristine Bi2S3 nanosheets-based gas sensor. The improvement of performance towards ammonia may be ascribed to the synergetic effects of composite materials heterojunction. The understanding of the gas sensing mechanism of Bi2S3-Bi2O3 heterojunction may offer a new design freedom to sensitive and selective gas sensors.
9:00 PM - NM1.10.73
Room-Temperature Colloidal Synthesis of Bi2Te3 Nanosheets
Maria Sokolikova 1 , Cecilia Mattevi 1 Show Abstract
1 Department of Materials, Imperial College London, London United Kingdom
Bi2Te3 and its alloys are known to be among the best thermoelectric materials performing at room temperature. It has been theoretically predicted that producing Bi2Te3 in a form of ultra-thin films can significantly increase the thermoelectric figure of merit due to a strong quantum confinement of charge carriers . Until now, atomically thin Bi2Te3 nanosheets have been produced mainly by mechanical exfoliation, while chemical exfoliation in liquid phase from bulk powders has been marginally explored and normally led to few-layered nanosheets with lateral sizes smaller than 1 micron. Colloidal synthesis can be significantly advantageous to enable processing at low temperatures and structure and morphology control of the material, and so far has been employed for the synthesis of Bi2Te3 nanoparticles . In this work we demonstrate room-temperature colloidal synthesis of Bi2Te3 nanosheets with lateral size up to 0.7 micron and uniform thickness of 5 nm. The growth was achieved upon reaction between in situ generated bismuth oleate and trioctylphosphine telluride in a non-coordinating solvent at temperatures as low as 35oC. High-resolution transmission electron microscopy characterization shows nanosheets with high crystal quality. We elucidate the growth mechanism which proceeds via a 2D-coordinating chemistry achieved in solution, which is key to enable the 2D expansion of the Bi2Te3 lattice and to hinder the formation of isotropic or multi-branches structure.
1. Hicks, L. D. & Dresselhaus, M. S. Effect of quantum-well structures on the thermoelectric figure of merit. Phys. Rev. B 47, 12727–12731 (1993).
2. Son, J. S. et al. n-Type nanostructured thermoelectric materials prepared from chemically synthesized ultrathin Bi2Te3 nanoplates. Nano Lett. 12, 640–647 (2012).
9:00 PM - NM1.10.74
Origins of Ripples in As-Grown Few-Layered MoS2 Structures under Applied Strain at Atomic Scales
Jin Wang 1 , Raju Namburu 2 , Madan Dubey 3 , Avinash Dongare 1 Show Abstract
1 Materials Science and Engineering and Institute of Materials Science, University of Connecticut, Storrs, Connecticut, United States, 2 Computational and Information Sciences Directorate, U.S. Army Research Laboratory, Aberdeen Proving Ground, Maryland, United States, 3 Sensors and Electron Devices Directorate, U.S. Army Research Laboratory, Adelphi, Maryland, United States
The 2-dimensional (2D) structures of MoS2 exhibit significantly different electronic band structure from bulk MoS2 crystals, and therefore show significant promise for applications in field effect transistors, optoelectronics, photo transistors and detectors, and chemical catalysts. The chemical vapor deposition (CVD)-grown two-dimensional molybdenum disulfide (MoS2) structures comprise of flakes of few layers wherein the top layers are relatively smaller in size than the bottom layers, resulting in the formation of edges/steps across adjacent layers. The strain response of such few-layered structures is likely to be different as compared to the thin film counterpart without any edges. In this study, the strain response of as-grown few-layered MoS2 is investigated at the atomic scales using classic molecular dynamics (MD) simulations. MD simulations suggest that the as-grown structures are able to relax the applied in-plane strain through the nucleation of ripples at the various edges that propagate inwards under both tensile and compressive loading conditions. The origins of such ripples are attributed to the presence of edges as MD simulations for the case of MoS2 thin films showed no ripple formation under the same loading conditions. The ripples nucleated during the compression have amplitudes of ~10 Å, whereas those observed under tension have amplitudes of ~ 0.1 Å. Moreover, the strain relaxation of the as-grown MoS2 structures, and in-turn ripple formation is observed to be size-dependent. Research was sponsored by the Army Research Laboratory and was accomplished under Cooperative Agreement Number W911NF-14-2-0059.
9:00 PM - NM1.10.75
Interlayer Excitons in 2D/Organic van der Waals Heterostructures
Tong Zhu 1 , Long Yuan 1 , Libai Huang 1 Show Abstract
1 , Purdue University, West Lafayette, Indiana, United States
Fabrication of van der Waals (vdW) heterostructures based on two-dimentional materials (graphene, insulating hexagonal boron nitride and transition-metal dichalcogenides) has become a burgeoning area of research for the applications in optoelectronic devices, photovoltaics and light emitting devices. Molecular and polymeric organic solids are also free of dangling bonds, offering potential to be integrated with 2D materials to form vdW heterostructures. For 2D/organic vdW heterostructure which involves a 2D material and layers of small organic molecular thin film, the interface between the two different types of materials is more complex and less constrained compared to all-2D heterostructures. The intriguing yet less exploited questions for 2D/organic vdW heterostructures include the optoelectronic behaviors such as charge transfer and energy transfer across the interface and how these processes are affected by dielectric screening and interlayer coupling. In this study, we designed a type II heterojunction by choosing WS2 and tetracene to fabric the 2D-organic vdW heterostructure with the conduction band minimum lying in tetracene and valence band maximum resides on WS2. Photoluminesce imaging and transient absorption microscopy technique are employed to study the charge-transfer and energy-transfer dynamics between the two different layers. We observed emission from the spatially indirect charge-transfer exciton, indicating efficient separation of electron-hole on different layers. We extracted ultrafast charge transfer and energy transfer rates by performing thickness dependence study by integrating different numbers of layers of WS2 with tetracene thin film, revealing strong electronic coupling at the interface. Such ultrafast charge and energy transfer in the VdW heterostructures formed by integrating TMDs and organic molecules have considerable potential to enable novel 2D devices for optoelectronics and light harvesting.
9:00 PM - NM1.10.76
Controlling the Energetics of 2D MoS2 with Surface Dipoles
Elisa Miller-Link 1 , Eric Benson 1 , Sanjini Nanayakkara 1 , Jeffrey Blackburn 1 Show Abstract
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
The usefulness of 2D materials is still being discovered because the fundamental properties are beginning to be tuned for various applications, such as vertical solar cells, catalysis, field effect transistors, solar fuels, etc. We show that 2D-MoS2 energetics can be controlled with surface dipoles. Solution-exfoliated MoS2 is modified with diazonium salts containing different functional groups. The workfunction of the MoS2 surface depends on the functional group, and there is a correlation between the electron withdrawing/donating character (Hammett parameter) of the functional group and the workfunction, which is measured via X-ray photoelectron spectroscopy. Furthermore, the functional groups affect the electrochemistry of the MoS2 surfaces. The functionalized MoS2 surfaces with different electron donating and withdrawing groups varied slightly in the onset potential for hydrogen reduction but have more dependence on the activity. Specifically, different exchange currents as well as Tafel slopes are observed for the various modified surfaces. Controlling surface energetics will be broadly useful for applications involving 2D MoS2.
9:00 PM - NM1.10.77
Experimental Demonstration of an Electride as a 2D Material
Daniel Druffel 1 , Kaci Kuntz 1 , Adam Woomer 1 , Frank Alcorn 1 , Jun Hu 1 , Scott Warren 1 Show Abstract
1 , University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States
Crystalline ionic solids that possess loosely bound anionic electrons—materials called electrides—offer useful physical properties in chemical synthesis and electronics. For example, electrides can serve as reducing agents or catalysts for chemical syntheses, and in electronics as low-temperature electron emitters, transparent conductors, and battery electrodes. In these applications, nano-sized electrides offer advantages because of their high surface areas. For example, monolayer flake of Ca2N has a theoretical specific surface area of 1460 m2/g, which far exceeds the highest reported specific surface area (~20 m2/g) of other electrides. However, until now, electrides have not been studied experimentally as nanomaterials. We show that layered electrides, in which layers of atoms are separated by layers of a 2D electron gas (2DEG), can be exfoliated into two-dimensional (2D) flakes using liquid exfoliation. We performed electron microscopy and elemental analysis on the 2D flakes to confirm that their crystal structure and stoichiometry match that of the parent 3D layered electride. Our 2D flakes remain crystalline in a nitrogen atmosphere or in select organic solvents for at least one month. The 2D flakes exhibit metallic character, evidenced by ultraviolet photoelectron spectroscopy, and an optical response in the visible and infrared spectrum that agrees with our DFT calculations. Together these findings suggest that the 2DEG is preserved in the 2D material. We have combined the exotic properties of electrides with the high surface area of 2D materials. For example, our experiments show that a 10-nm thick film would transmit 97% of light, while also having a sheet resistance of just 4 Ω/■. This work introduces 2D electrides as an emerging class of 2D materials.
9:00 PM - NM1.10.78
Water-Assisted Synthesis of Large-Area WS2 Monolayers with High Optical Quality
Pawel Palczynski 1 , Francesco Reale 1 , Cecilia Mattevi 1 Show Abstract
1 Department of Materials, Imperial College London, London United Kingdom
Like with many TMDs and other novel 2D materials the chemical vapour deposition (CVD) method has been a most promising method of growing large scale monolayer WS2. Intensive research efforts are now aiming to achieve high quality material extended over wafer size areas. The core issue of the synthesis is the choice of the W precursor, which is required to readily and homogenously evaporate and to undergo a complete reaction with sulphur without leaving impurities in the grown materials . Here we show that the use of hydrated tungstenoxide (H2WO4) in presence of chlorine precursors in Ar atmosphere enables the growth of monolayer WS2 as continuous films extended over 700 µm in lateral size at temperatures as low as 750°C. The as-achieved material exhibits room temperature field effect mobility of ~45 cm2 V-1 s-1 onto SiO2/Si substrates and superior optical qualities, with photoluminescence (PL) peak FWHM of 36 meV, when compared to its counterpart grown at higher temperatures with standard WO3 precursor . The PL peak position approaches 2eV and it consists of an individual component attributable to excitons recombination as elucidated by low temperature PL characterization. The use of volatile non-oxide W precursors species enables sulphidization processes different from what occurrs in oxide precursors, and these can lead to improved crystal quality and controllable sulphur vacancies.
9:00 PM - NM1.10.79
Complex and Non-Centrosymmetric Stacking of Layered Chalcogenide Materials Created by Screw Dislocations
Melinda Shearer 1 , Leith Samad 1 , Yi Zhang 1 , Alexander Puretzky 2 , Kevin Eliceiri 1 , John Wright 1 , Robert Hamers 1 , Song Jin 1 Show Abstract
1 , University of Wisconsin-Madison, Madison, Wisconsin, United States, 2 , Oak Ridge National Lab, Oak Ridge, Tennessee, United States
Layered transition metal dichalcogenides are currently being studied for a wide variety of applications based on their interesting and highly tunable properties, which heavily depend on the phase and layer stacking of these materials. A plethora of complex stacking arrangements with corresponding unique properties can be generated through artificial stacking of monolayers or the creation of heterostructures. Here, we demonstrate that screw dislocations are another way to greatly influence the layer stacking of WSe2 nanostructures, and subsequently the properties of this material. WSe2 nanoplates containing screw dislocations with variation in the number, type, and shape of the dislocation spiral have been studied via a one-to-one correlation of atomic force microscopy and low-frequency Raman spectroscopy, revealing a complex and diverse range of layer stacking patterns. The shape of the dislocation spiral plays a key role in determining the layer stacking; plates with hexagonal dislocation spirals form the standard 2H layer stacking arrangement, while plates with triangular dislocation spirals form a non-centrosymmetric stacking that gives rise to strong second harmonic generation and enhanced photoluminescence. Plates containing a mixture of these shapes will contain mixed properties, creating a spectrum of unique layer stackings of WSe2. These new properties and stacking changes have not yet been observed for WSe2, and could be interesting for spintronics and valleytronics applications.
9:00 PM - NM1.10.80
Synthesis of SnS Nanoplates—A Potential 2D Material
Nancy Trejo 1 , Anne Hunter 1 , My Nguyen 1 , Cody Wrasman 1 , Eray Aydil 1 Show Abstract
1 Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota, United States
Two dimensional (2D) layered materials such as graphene, metal dichalcogenides and black phosphorus are of increasing interest because of their unique optical and electronic properties. Tin monosulfide (SnS) has the same crystal structure as black phosphorus. SnS has potential applications in photovoltaics, photocatalysis, thermoelectrics, and batteries. Furthermore, 2D SnS can be used in electrodes, conducting films, and near infrared sensors. We have synthesized 3-60 nm thick and up to approximately 10 micron wide SnS nanoplates via decomposition of tin(IV) diethyldithiocarbamate upon hot injection into oleylamine (300-340 oC). The reaction products are characterized using a combination of electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction (XRD), atomic force microscopy and Raman spectroscopy. All characterization techniques confirm that the final reaction product is orthorhombic SnS (e.g., after 60 minutes at 340 oC). At low temperatures (300 oC) and short synthesis times (3 minutes) we also observe the presence of tin disulfide (SnS2) which suggests that decomposition of tin(IV) diethyldithiocarbamate first produces SnS2. SnS2 is subsequently reduced, likely by oleylamine, to SnS. Orthorhombic SnS grows preferentially as plates, with  direction normal to the plate surfaces. In fact, when plates are large (>1 micron) XRD from films cast from colloidal dispersions in toluene show predominantly the (040) diffraction.