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
NM1.1: Phonon and Thermal Properties of 2D Materials
Monday PM, April 17, 2017
PCC West, 100 Level, Room 106 AB
9:00 AM - *NM1.1.00
Nanoscale Two-Dimensional Electronic and Optoelectronic Devices
Wenjuan Zhu 1 Show Abstract
1 , University of Illinois Urbana-Champaign, Urbana-Champaign, Illinois, United States
9:30 AM - *NM1.1.01
2D Layered Material—From Transistors to Interconnects
H.-S. Philip Wong 1 Show Abstract
1 Department of Electrical Engineering and Stanford SystemX Alliance, Stanford University, Stanford, California, United States
Two-dimensional layered materials have many useful properties that may be useful for future high-performance, energy efficient computing systems technology. Semiconducting 2D layered materials such as MoS2 and black phosphorous (BP) have shown promise as transistor channel materials. I will present our results on making low-resistance electrical contacts to these materials [1, 2]. Graphene, a semi-metal, has very useful properties that makes it particularly suited for extending the use of copper as interconnect wires. These useful properties include serving as a highly effective copper diffusion barrier (thus replacing TaN as the copper liner), and providing enhanced electromigration immunity for a graphene/Cu composite with graphene cladding the copper wire. We will present recent experimental results on these two very promising applications of graphene to complement copper wires [3, 4]. We have also developed a system-level modeling framework that allows us to benchmark the speed performance and energy efficiency of a microprocessor, including contributions from parasitics and wiring interconnects using industry-standard place-and-route optimization tools coupled with compact device models. We will present results that compare 5-nm FinFET CMOS with MoS2 and BP transistors, as well as various wiring options including multilayer graphene and graphene cladded Cu wires .
Acknowledgements: Supported in part by member companies of the Stanford Initiative for Nanoscale Materials and Processes (INMP), member companies of the Stanford SystemX Alliance, the National Science Foundation (EFRI 2-DARE: Energy Efficient Electronics with Atomic Layers (E3AL), Award Abstract #1542883), and also in part by Systems on Nanoscale Information fabriCs (SONIC) and Function Accelerated NanoMaterials Engineering (FAME) Center, two of the six SRC STARnet Centers, sponsored by MARCO and DARPA.
 S. Lee, A. Tang, S. Aloni, H.-S. P. Wong, “Statistical Study on the Schottky Barrier Reduction of Tunneling Contacts to CVD Synthesized MoS2,” Nano Lett., 16 (1), pp 276–281 (2016).
 L. Li, M. Engel, D. B. Farmer, S.-J. Han, H.-S. P. Wong, “High Performance p-Type Black Phosphorus Transistor with Scandium Contact,” ACS Nano, 10 (4), pp 4672–4677 (2016).
 L. Li, X. Chen, C.-H. Wang, J. Cao, S. Lee, A. Tang, C. Ahn, S. S. Roy, M. S. Arnold, and H.-S. P. Wong, “Vertical and Lateral Copper Transport through Graphene Layers,” ACS Nano, 2015, 9 (8), pp 8361–8367.
 L. Li, Z. Zhu, T. Wang, J.A. Currivan-Incorvia, A. Yoon, and H.-S. P. Wong, “BEOL Compatible Graphene/Cu with Improved Electromigration Lifetime for Future Interconnects,” IEEE International Electron Devices Meeting (IEDM), paper 9.5, 2016.
 C.-S. Lee, B. Cline, S. Sinha, G. Yeric, and H.-S. P. Wong, “32-bit Processor Core at 5-nm Technology: Analysis of Transistor and Interconnect Impact on VLSI System Performance,” IEEE International Electron Devices Meeting (IEDM), paper 28.3, 2016.
10:00 AM - NM1.1.02
In Situ Characterization and Dynamic Control of the Anisotropic Thermal Properties and Phonon Spectra of Black Phosphorus
Joon Sang Kang 1 , Ming Ke 1 , Yongjie Hu 1 Show Abstract
1 Department of Mechanical and Aerospace Engineering, University of California, Los Angeles (UCLA), Los Angeles, California, United States
Two-dimensional van der Waals materials have shown novel fundamental properties and promise for wide applications. Here, we reported, for the first time, an experimental demonstration of the in-situ characterization and control of the anisotropic thermal conductivity of black phosphorus. We develop a novel platform based on ultrafast optical spectroscopy and electrochemical control to investigate the interactions between ions and the 2D lattice. We characterize the anisotropic thermal properties of black phosphorus and discover a strong tenability over thermal conductivity by ionic intercalation. A dramatic reduction of thermal conductivity by up to 6 times from that of the pristine crystal have been observed. This study provides a unique approach to explore the fundamental energy transport involving lattices and ions in the layered structures, and may open up new opportunities in controlling energy transport based on novel operation mechanisms and the rational design of nanostructures.
10:15 AM - *NM1.1.03
Interlayer Phonons in Transition Metal Dichalcogenide Atomic Layers
Rui He 1 Show Abstract
1 , University of Northern Iowa, Cedar Falls, Iowa, United States
Atomically thin ReS2 and NbSe2 crystals are new types of 2D materials that have different crystal structures and electronic properties from common transition metal dichalcogenides (TMDs), e.g. MoS2, MoSe2, WS2, and WSe2. We measured ultralow frequency Raman response of ReS2 and NbSe2 atomic layers. ReS2 has unique distorted 1T structure. We found that the two shear phonon modes in bilayer ReS2 are nondegenerate and clearly resolved in the Raman spectrum, in contrast to the doubly degenerate shear modes in other TMD materials. By carrying out comprehensive first-principles calculations, we can account for the frequency and Raman intensity of interlayer modes and determine the stacking order of bilayer ReS2. Few-layer ReS2 exhibits rich Raman peaks at frequencies below 50 cm-1, where a panoply of interlayer shear and breathing modes are observed. Atomically thin NbSe2 is a metallic layered TMD with novel charge-density-wave (CDW) and superconductive phases. We observed both the interlayer breathing modes and shear modes at frequencies below 40 cm-1 for samples of 2 to 15 layers. Their frequencies, Raman activities, and environmental instability depend systematically on the layer number. We find that, although NbSe2 has different stacking order from MoS2, they share the same crystal symmetry groups and exhibit similar Raman selection rules for interlayer phonons. In addition, the interlayer phonon modes evolve smoothly from T = 300 K to 8 K, with no observable response to the CDW formation in NbSe2. Our results reveal that the interlayer phonons can serve as an effective probe of the interface properties and interlayer interactions in these 2D atomic layers.
10:45 AM - NM1.1.04
Probing Electrical and Thermal Properties in Electrochemically Li-Intercalated MoS2 Nanosheets with Raman Spectroscopy
Feng Xiong 1 2 , Eilam Yalon 2 , Connor McClellan 2 , Aditya Sood 3 , Jinsong Zhang 4 , Jie Sun 4 , Kenneth Goodson 3 , Yi Cui 4 , Eric Pop 2 4 Show Abstract
1 Electrical and Computer Engineering Department, University of Pittsburgh, Pittsburgh, Pennsylvania, United States, 2 Electrical Engineering, Stanford University, Stanford, California, United States, 3 Mechanical Engineering, Stanford University, Stanford, California, United States, 4 Materials Science and Engineering, Stanford University, Stanford, California, United States
Layered two-dimensional (2D) transition-metal dichalcogenides (TMDs) such as MoS2 have shown great promise for nano- and opto-electronics. Recently, intercalation has been shown to be an effective technique to reversibly tune material properties of layered 2D films [1,2]. By inserting guest species (ions, atoms, and molecules) between TMD layers, we were able to engineer their electrical, thermal and optical properties via surface charge transfer, scattering and band structure change, respectively.
For the first time, we use Raman spectroscopy to probe both the electrical (carrier concentration) and thermal transport (thermal conductivity and thermal boundary conductance) in Li-intercalated MoS2 nanosheets. To perform electrochemical Li intercalation, we adopted a planar nanobattery configuration, where lithium iron phosphate (LFP) and exfoliated MoS2 nanosheets were used as counter and working electrodes, respectively. The top transparent window of the device allows Raman measurements during intercalation.
Upon intercalation, we observed linewidth broadening and softening of the out-of-plane mode with increasing Li ion concentration. The mode exhibited much weaker dependence but still showed slight softening, possibly due to strain in the lattice as a result of the Li influx. All these effects were fully reversible upon de-intercalation. We performed Hall measurements to characterize the carrier concentration at different stages of Li intercalation and their corresponding Raman frequency shifts. For bilayer MoS2, the mode frequency decreased by as many as 18 cm-1, at a carrier concentration of 6.5×1013 cm-2. The mode decreased by 3 cm-1 at this doping level, corresponding to ~1.7 % strain according to .
We also performed Raman thermometry measurement on our 2L MoS2 devices before and after Li intercalation, using a similar approach to the one described by Judek et al. . We found that the TBC between 2L MoS2 and SiO2 showed a significant reduction upon Li intercalation.
Our work highlights how electrochemical intercalation could be utilized for controlled doping in 2D TMDs and modulation of their thermal transport. Our findings provide a convenient method to measure the impurity concentration and thermal boundary conductance in 2D materials using Raman spectroscopy, which are crucial for electronic applications involving 2D materials.
 W. Bao et al., Nat. Commun. 5, 4224 (2014).
 F. Xiong et al., Nano Lett. 15, 6777 (2015).
 C. Rice et al., Phys. Rev. B 87, 081307 (2013).
 J. Judek et al., Sci. Rep. 5, 12422 (2015).
11:30 AM - *NM1.1.05
Devices, Nanofunctions, and Unconventional Applications of 2D Materials
C. English 1 , Kirby Smithe 1 , Ning Wang 1 , Michal Mleczko 1 , Connor McClellan 1 , Saurabh Suryavanshi 1 , Eric Pop 1 Show Abstract
1 Electrical Engineering, Stanford University, Stanford, California, United States
Two-dimensional (2D) materials have unusual and anisotropic electrical and thermal properties. This talk will present recent highlights from our atoms-to-systems research on graphene, BN, and transition metal dichalcogenides (TMDs).
We have studied graphene from basic transport measurements and simulations, to the recent wafer-scale demonstration of analog dot product nanofunctions for neural networks . We are also growing and evaluating the electrical and thermal properties of TMDs including MoS2, HfSe2, and WTe2 [2,3]. Recent results include low-resistance contacts , 10-nm scale transistors, and high-field transport studies including velocity saturation. We have also examined the anisotropic thermal conductivity of these materials, for unconventional applications to thermal switches and thermal routing. If time permits, I will discuss “bottom up” thermal management starting at dimensions comparable to the electron and phonon mean free paths (~100 nm), where quasi-ballistic heat flow effects dominate [5,6].
Our studies reveal fundamental limits and new applications that could be achieved through the co-design and heterogeneous integration of 2D nanomaterials.
 N.C. Wang, S.K. Gonugondla, I. Nahlus, N.R. Shanbhag, E. Pop, "GDOT: A Graphene-Based Nanofunction for Dot-Product Computation," IEEE VLSI Tech. Symp., Jun 2016, Honolulu HI
 K.K.H. Smithe, C.D. English, S.V. Suryavanshi, E. Pop, "Enhanced Electrical Transport and Performance Projections of Synthetic Monolayer MoS2 Devices," arXiv:1608.00987 (2016)
 M.J. Mleczko, R.L. Xu, K. Okabe, H.-H. Kuo, I.R. Fisher, H.-S.P. Wong, Y. Nishi, E. Pop, "High Current Density and Low Thermal Conductivity of Atomically Thin Semimetallic WTe2," ACS Nano 10, 7507-7514 (2016)
 C.D. English, G. Shine, V.E. Dorgan, K.C. Saraswat, E. Pop, "Improved Contacts to MoS2 Transistors by Ultra-High Vacuum Metal Deposition," Nano Lett. 16, 3824-3820 (2016)
 M.-H. Bae, Z. Li, Z. Aksamija, P.N. Martin, F. Xiong, Z.-Y. Ong, I. Knezevic, E. Pop, "Ballistic to Diffusive Crossover of Heat Flow in Graphene Ribbons," Nature Comm. 4, 1734 (2013)
12:00 PM - NM1.1.06
Effects of Defects on the Temperature Dependent Thermal Conductivity of Suspended Monolayer MoS2 Grown by Chemical Vapor Deposition
Milad Yarali 1 , Tushar Gupta 2 , Xufei Wu 3 , Lixin Xie 4 , Shuo Chen 4 , Tengfei Luo 3 , Nikhil Koratkar 2 , Anastassios Mavrokefalos 1 Show Abstract
1 Department of Mechanical Engineering, University of Houston, Houston, Texas, United States, 2 Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 3 Aerospace and Mechanical Engineering, University of Notre Dame, Notre Dame, Indiana, United States, 4 Department of Physics and TcSUH, University of Houston, Houston, Texas, United States
Unique electronic and optical properties of atomically thin molybdenum disulfide (MoS2) highlight it as a promising two-dimensional material for future advanced microelectronic devices. From both fundamental and technological points of view studying heat transport in such material sheds lights on its potentials especially for the applications where thermal management is critical. We utilize a micro-device with integrated resistance thermometers to report for the first time the thermal conductivity of suspended monolayer MoS2 grown by chemical vapor deposition (CVD) in the temperature range of 10-475K. Transmission Electron Microscopy (TEM) is used to characterize the material structure on the measured sample to reveal how defects in the lattice atomic arrangement affect thermal transport. We observe that thermal conductivity monotonically increases with increasing temperature which is in contrast with trends exhibited in exfoliated single layer MoS2 obtained by Raman Spectroscopy . High resolution TEM shows that these CVD grown samples are actually polycrystalline in nature with low angle grain boundaries, that we believe is the source of this monotonic trend. It is worth mentioning that the measured room temperature thermal conductivity in this work, ~ 34 Wm-1K-1, is well matched with that reported for exfoliated single layer and 1.5 times smaller than that of 11 layers MoS2 prepared by CVD . First-principles lattice dynamics simulations are carried to describe the phonon transport mechanism of our measured samples to quantify the effects of these atomic scale defects to the phonon scattering rates.
 Yan et al. ACS Nano 8(1), 986 (2014).
 Sahoo et al. J. Phys. Chem. C 117(17), 9042 (2013).
12:15 PM - *NM1.1.07
Probing Interlayer Coupling of Two-Dimensional Layered Materials beyond Graphene by Raman Spectroscopy
PingHeng Tan 1 Show Abstract
1 , Institute of Semiconductors, Chinese Academy of Sciences, Beijing China
The fast progress of graphene research, fueled by the unique properties of this two-dimensional material, paved the way to experiments on other layered materials (LMs). Transition metal dichalcogenides (TMDs), represented by MX2 (M = Mo, W; X = S, Se, Te), have received extensive researches on their distinctive electronic/optical properties, such as, indirect-to-direct bandgap transition, valley polarization and enhanced excitonic effect, and broad applications in optical/electronic devices. The advances of TMDs further accelerate the experiments on general LMs. Actually, there are different types of LMs including semiconductors, insulators, topological insulators and superconductors, which needs further exploration.
The van der Waals (vdW) interlayer interactions in layered materials are much weaker than the strength of the covalent bonds between the atoms within each layer, however, such weak vdW interactions makes their physical properties layer-dependent.[3-6] On the one hand, the interlayer interactions can be directly probed by the ultralow-frequency shear (S) and layer breathing (LB) modes, which are due to relative motions of the planes, either perpendicular or parallel to their normal. There are N-1 S and N-1 LB modes in N-layer in-plane isotropic LMs. The S modes in anisotropic LMs are non-degenerate and there are 2(N-1) S modes in N-layer anisotropic LMs. The weak interlayer interaction also affects the frequency of the high frequency optical modes in LMs, which results in the observation of Davydov splitting for the high frequency optical modes. 
Here, we will report how to explore the possibility and ability by the S and LB modes to probe the interlayer coupling in multilayer TMDs.[1-6] We also discuss provides a direct evidence from Raman spectroscopy of how the nearest vdW interactions significantly affect the frequency of the high-frequency intralayer phonon modes in multilayer MoTe2.. Our work shows that Raman spectroscopy is an ideal tool to probe the fundamental physics and potential applications of these various LMs, particularly when they are reduced down to monolayers or multilayers.
 Zhang, X.; Han, W. P.; Wu, J. B. et al. Phys. Rev. B, 2013, 87, 115413.
 X. Zhang, X.-F. Qiao, W. Shi, J.-B. Wu et al., Chem. Soc. Rev., 2015, 44, 2757-2785.
 H. Zhao, J.-B. Wu, H. X. Zhong et al., Nano Research, 2015, 8(11), 3651-3661
 X. Zhang, Q.-H. Tan, J.-B. Wu et al., Nanoscale, 2016, 8, 6435-6450.
 Q. J. Song, Q.-H. Tan, X. Zhang et al., Phys. Rev. B, 2016, 93, 115409.
X. –F. Qiao, J. –B. Wu, L. –W. Zhou et al., Nanoscale, 2016, 9, 8324-8332.
12:45 PM - NM1.1.08
Large Area van der Waals Epitaxy of MoS2 on III-Nitride Substrates
Cheyu Liu 1 3 , Kyooho Jung 1 , Wonsik Choi 1 , Jeongdong Kim 1 , Munho Kim 1 , Xiaochen Ge 2 , Haochung Kuo 3 , Weidong Zhou 2 , Xiuling Li 1 Show Abstract
1 , University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 3 , National Chiao Tung University, Hsinchu Taiwan, 2 , University of Texas at Arlington, Arlington, Texas, United States
Ideal two-dimensional layered crystals (2D materials) such as MoS2 and other transition metal dichalcogenides (TMDCs) display no dangling bonds. vdW epitaxy normally refers to the deposition of 2D on 2D materials in a commensurate fashion, forming homo or heterostructures with only weak van der Waals (vdW) like forces across the 2D/2D interface. One distinct feature of vdW epitaxy is that it lacks strain accommodation, composition gradients, and structural defects at the interface, making structurally “perfect” interfaces. 2D/III-V vdW structures are largely unexplored, yet they offer potential for creating new types of heterojunctions with difference band alignment, relaxed lattice matching requirements, and doping requirements for more diverse optoelectronic devices. They can also serve as a model system to explore the physical and transport properties at the interface between vdW and non-vdW surfaces.
We have previously reported InGaAs nanowire growth on graphene, MoS2, and BN surfaces, which confirms the epitaxial nature of III-V growth on 2D van der Waals surfaces. In particular, due to the < 0.5% lattice mismatch between InAs and graphene, spontaneous phase segregation of InxGa1-xAs into InAs and InyGa1-yAs (y
In this work, we show the epitaxial growth of aligned MoS2 triangles on the InxGa1-xN (x = 0 - ~ 0.15) layers. We demonstrate the growth evolution of MoS2 from micron size triangles to a continuous sheet over > centimeter in size. The well-behaved electrical characteristics of a MoS2/InGaN p-n junction diode confirms the interface quality formed monolithically through the vdW epitaxy.
NM1.2: Controlled Scalable Synthesis of 2D TMDC Materials and Heterostructures I
Monday PM, April 17, 2017
PCC West, 100 Level, Room 106 AB
2:30 PM - *NM1.2.01
Possibilities for Growing and Scaling CVD Graphene and hBN, and Progress on Diamane
Rodney Ruoff 1 Show Abstract
1 Center for Mulitdimensional Carbon Materials (Institute for Basic Science), Ulsan National Institute of Science and Technology (UNIST), Ulsan Korea (the Republic of)
We have invented methods for converting polycrystalline metal foils to single crystal over large area. Several of the metals are lattice matched to graphene and h-BN. I update on our progress in this area and use of such foils to grow graphene and h-BN. I then provide an update on our research on trying to convert AB-stacked large area bilayer and few-layer graphene to diamane. This research was supported by the Institute for Basic Science IBS-R019-D1.
3:00 PM - *NM1.2.02
Defect Engineering in Monolayer MoSe2 through Controlled Synthesis and Doping
Kai Xiao 1 , Xufan Li 1 , Masoud Mahjouri-Samani 1 , Ming-Wei Lin 1 , Liangbo Liang 1 , Akinola Oyedele 1 , Mengkun Tian 2 , Alexander Puretzky 1 , Juan-Carlos Idrobo 1 , Mina Yoon 1 , Bobby Sumpter 1 , Gerd Duscher 2 , Christopher Rouleau 1 , David Geohegan 1 Show Abstract
1 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 2 , University of Tennessee, Knoxville, Tennessee, United States
The functionality of 2D materials is significantly affected by dopants and defects. Therefore, a detailed understanding of their fundamental physics and chemistry is mandatory for the creation of new game-changing electronic devices. Here I will discuss how to control the defects during growth and processing and how those defects determine the electronic and optical properties of emerging 2D materials that are promising for optoelectronic applications. Through isoelectronic doping in monolayer of MoSe2, the Se vacancies are effectively suppressed and photoluminescence is significantly enhanced due to the decrease of defect-mediated non-radiative recombination. In addition, we demonstrate the non-equilibrium, bottom-up synthesis of single crystalline monolayers of 2D MoSe2−x with controllable levels of Se vacancies far beyond intrinsic levels. Both substitutional dopants and vacancies were shown to significantly alter the carrier properties and transport characteristics within a single monolayer (e.g., n- to p-type conduction in W-doped MoSe2 and in Se-deficient MoSe2-x). These measurements indicate that defects engineering through growth and processing is a promising approach to control the heterogeneity of 2D TMDs to realize the scalable production of high performance optoelectronic and electronic devices.
Acknowledgment: Synthesis science was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences (BES), Materials Sciences and Engineering Division and characterizations were performed at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.
3:30 PM - NM1.2.03
Van der Waals Epitaxy and Layer by Layer Nanoscale Patterning of 2D Transition Metal Dichalcogenides
Bruce Parkinson 1 Show Abstract
1 , University of Wyoming, Laramie, Wyoming, United States
We have used molecular beam epitaxy to deposit 2D transition metal dichalcogenides (TMDs) on other 2D materials resulting in interfaces with no broken covalent bonds. Lattice mismatches between substrate and epilayer do however produce interesting Moiré patterns that can be imaged with a scanning tunneling microscope (STM). We discovered that continuous scanning of either an STM or atomic force microscope (AFM) resulted in the nucleation of holes, presumably at single surface atom vacancies, that grew with subsequent scanning until a complete layer was removed only from the scan area. We report the possibility of building up layers of conductive TMDs such as NbSe2, insulating TMDs such as ZrS2 and semiconducting TMDs such as WSe2 with subsequent nanoscale patterning to produce precisely patterned single layer electronic devices, such as diodes and transistors.
4:15 PM - *NM1.2.04
High-Resolution Probes of Metal Chalcogenide Semiconductors—Growth, Electronic Structure, Light-Matter Interactions
Peter Sutter 1 Show Abstract
1 , University of Nebraska–Lincoln, Lincoln, Nebraska, United States
Layered metal chalcogenides have received attention as 2D materials beyond graphene. Semiconducting chalcogenides are particularly attractive since they may enable next-generation low-dimensional (opto) electronics benefiting from carrier confinement and other unique characteristics, such as a thickness dependent electronic structure and strong light-matter interactions. Here, I discuss the use of novel high spatial resolution probes to study the properties of 2D semiconductors and their variations on the nanometer scale.
Real-time microscopy provides insight into the microscopic mechanisms governing the synthesis and transformation of 2D semiconductors. Novel approaches for measuring the band structure of 2D materials with high spatial resolution are used to establish the thickness dependent electronic properties of transition metal dichalcogenides, as well as other key aspects such as the dependence of the interlayer interaction on the relative orientation of the layers in few-layer stacks. Finally, I present nanometer-scale measurements of light-matter interactions in 2D semiconductors, which offer a way to probe and manipulate optical excitations in heterogeneous systems near defects, edges, interfaces, etc.
4:45 PM - *NM1.2.05
Paper and Circuits Only Atoms Thick
Jiwoong Park 1 Show Abstract
1 , University of Chicago, Chicago, Illinois, United States
2D layered materials are like paper: they can be colored, stitched, stacked, and folded to form integrated devices with atomic thickness. In this talk, I shall discuss how different 2D materials can be grown with distinct electrical and optical properties (coloring), how they can be connected laterally to form patterned circuits (stitching), how their properties can be controlled by interlayer rotation (twisting), and how they can be folded to generate novel 3D structures and devices (folding).
5:15 PM - *NM1.2.06
Two-Dimensional Layered Transitional Metal Dichalcogenides—Growth Dynamics and Electron Device
Yang Chai 1 Show Abstract
1 Applied Physics, The Hong Kong Polytechnic University, Hong Kong Hong Kong
A microscopic understanding on growth pathways of two-dimensional (2D) materials is important for controllable synthesis of functional nanostructures. It is a fundamental interest of many scientists to control the orientation and grain size of 2D materials. Here, we reveal distinct growth dynamics of MoS2 flakes from the thermolysis of ammonium thiomolybdates using in-situ transmission electron microscopy. Our studies show the direct observations of MoS2 growth as the evolution of temperature, and shed light on the controllable orientation and grain size of 2D materials.1
The crystal structure and physical properties of two-dimensional transition metal dichalocogenides (TMDs) are substantially dependent on the filling of d orbitals of transition metal. Group-10 TMDs have been revealed with widely tunable bandgap, high mobility and good stability.2 We demonstrate layer-dependent semiconductor-to-semimetal evolution of 2D PtSe2. Few-layer PtSe2 field-effect transistor shows high room-temperature mobility (~210 cm2V-1s-1) in a back-gated configuration on SiO2/Si, comparable to that of BP. Bulk PtSe2 device exhibits the metallic-like conductivity (6.64×105 S/m). The characteristics of widely tunable bandgap of PtSe2 allows it to be effectively response to near-infrared light. Furthermore, our results showed that PtSe2 has much better air-stability (over 1 year) than BP.
1. Nature Communications, 2016, 7, 12206-1-7
2. Advanced Materials, 2016, 28(12), 2399-2407
5:45 PM - NM1.2.07
Lateral Integration of n-(p-) Type TMDs and Its Alloy
Chen Wang 1 , Yu Huang 1 2 , Xiangfeng Duan 3 2 Show Abstract
1 Department of Materials Science and Engineering, University of California, Los Angeles, Los Angeles, California, United States, 2 California Nanosystems Institute, University of California, Los Angeles, Los Angeles, California, United States, 3 Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, California, United States
Two-dimensional layered materials (2DLMs), especially transition metal dichaolcogenide (TMD) have recently emerged as a central focus of materials research. The weak van der Waals interactions between each atomic layers in 2DLMs makes it feasible to isolate, mix, match and combine highly disparate atomic layers to create a wide array of van der Waals heterostructures (vdWHs). Additionally, the 2D essence of TMD endows one more unique freedom of lateral integration. Here we report lateral epitaxial growth of n-(p-) type TMDs and its alloy. Using a home-built in situ source switch and dual source CVD system, we conducted systematic investigations of lateral integration of p-type TMDs after n-type TMD or alloy of n-type TMD with p-type TMD. By using the in situ source switch CVD system, p-type WSe2 can be epitaxial grew after the n-type WS2 single crystal formation. Further characterization using Raman mapping, photoluminescence mapping, EDX mapping directly verified the spatial separation of two different type TMD. More precise STEM SAED and EDS line scan give direct evidence of atomically sharp interline. Further lateral heterojunction device was made to observe the photovoltaic effect. In addition, by controlling the vapor pressure of n-type WS2 and p-type WSe2 in dual source system, TMD alloy with different composition can be synthesized. Continuous Raman mode, photoluminescence peak and electrical properties evolution can be observed. Our study thus defines a general strategy to preparing lateral heterojunction or alloy and opens up a new pathway to tailoring the electronic properties of 2D materials for functional electronics and optoelectronics.
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-g