Deep Jariwala, University of Pennsylvania
Victor Brar, University of Wisconsin–Madison
SungWoo Nam, University of Illinois at Urbana-Champaign
Ursula Wurstbauer, Technical University of Munich
attocube systems AG
CreaTec GmbH, represented by Sentys Inc.
QN03:01: Resistive Memory, Neuromorphic and Other Electronic Devices
Monday PM, April 22, 2019
PCC North, 100 Level, Room 129 A
9:30 AM - QN03.01.01
Novel Synaptic Memory Device Based on 2D TMDC Materials for Neuromorphic Computing
Min-Hyun Lee1,2,Houk Jang2,Renjing Xu2,Minsu Seol1,Chengye Liu2,Donhee Ham2,Hyeon-Jin Shin1,Seongjun Park1
Samsung Advanced Institute of Technology1,Harvard University2Show Abstract
Neuromorphic computing systems using electronic synapses and neurons have been extensively studied in recent years due to its ability to perform analog computing for artificial intelligence. It is also expected to overcome the high energy and various limitations of today’s computing systems. Most of all analog switching device based on the two-terminal memristor, such as conductive bridging random access memory (CBRAM) and oxide based resistive random access memory (RRAM). The memristors are operated by movement of anion vacancies or forming of metal filament, which exhibit relatively good retention and endurance for neuromorphic circuits. However, since the anion vacancies or metal ions must move within the oxide by an electric field, these devices necessarily require high operating voltages of several V or more even few-nm switching oxide. The high operating voltage causes problems such as reduced overall device stability and increased device operation energy.
On the other hand, TMDC (transition metal dichacogenide), a kind of two dimensional (2D) material, has been recently developed with great interest in growth method, process technology, and so on. Especially, the TMDC growth method using the MOCVD technique provides a poly-crystalline layered film from mono-layer to few-layers with layer controllability. In addition, the transfer method using the polymer support has been intensively studied and reached to a level at which almost no defect occurs in the process.
In this presentation, we presented a memristor device using these TMDC materials and show the electrical characteristics of memristor applications. The TMDC materials effectively blocks metal ion transport due to the high bonding energy and the close packing structure of atoms within the single layer. However, the metal ions can be transported through defect sites such as grain boundaries and vacancies, thus enabling the operation of the TMDC memristor. In particular, the bi-layer TMDC, in which the defect site is effectively controlled through transfer process, has the advantage of significantly improving the device-to-device uniformity and endurance. This is because the transfer process with rotation converts most of the defect sites to point defects. In contrast, the mono-layer TMDC has high possibility of line defect such as grain boundary, so that the uniformity and endurance of memristor are not secured.
Finally, we fabricated the memristors of metal / 2L-MoS2 / metal structure using MOCVD technique and rotated transfer method. These devices show bipolar memristor behavior with a lower set/reset voltage of +0.5 V/-0.47 V than the conventional oxide based memristor. The resistance changes of the device from tens of to hundreds of ohms-um^2 is found to occur mainly in 2D materials through external voltage measurement. It also shows the possibility of analog memory whose resistance varies linearly with bias applied from 0.3 V. Also, we confirmed STDP (spike-timing-dependent plasticity) operation in a single device, and tested various applications using a cross-bar structure. This work is an important foundation for developing devices physics to understand the memristive characteristics of 2D material-based memristor and providing neuromorphic computing systems based on such memristors.
9:45 AM - QN03.01.02
Synergistic Gating of Electro-Iono-Photoactive 2D Chalcogenide Neuristors—Co-Existence of Hebbian and Homeostatic Synaptic Metaplasticity
Rohit John1,Arindam Basu1,Zheng Liu1,Nripan Mathews1
Nanyang Technological University1Show Abstract
Emulation of brain-like signal processing with thin-film devices could lay the foundation for building artificially intelligent learning circuitry in future. Encompassing higher functionalities into single artificial neural elements will allow the development of robust neuromorphic circuitry emulating biological adaptation mechanisms with drastically lesser neural elements, mitigating strict process challenges and high circuit density requirements necessary to match the computational complexity of the human brain. Here, 2D transition metal di-chalcogenide (TMDC) (MoS2) neuristors are designed to mimic intracellular ion endocytosis-exocytosis dynamics / neurotransmitter-release in chemical synapses using three approaches: (i) electronic-mode: a defect modulation approach where the traps at the semiconductor-dielectric interface are perturbed, (ii) ionotronic-mode: where electronic responses are modulated via ionic gating and (iii) photoactive-mode: harnessing persistent photoconductivity or trap-assisted slow recombination mechanisms. Exploiting a novel multi-gated architecture incorporating electrical and optical biases, this incarnation not only addresses different charge-trapping probabilities to finely modulate the synaptic weights, but also amalgamates neuromodulation schemes to achieve “plasticity of plasticity-metaplasticity” via dynamic control of Hebbian spike-time dependent plasticity and homeostatic regulation. Co-existence of such multiple forms of synaptic plasticity increases the efficacy of memory storage and processing capacity of artificial neuristors, enabling design of highly efficient novel neural architectures.
10:30 AM - *QN03.01.03
Two-Dimensional Charge-Density-Wave Materials—Unique Properties and Potential Applications
University of California1Show Abstract
The charge density wave (CDW) phase is a quantum state consisting of a periodic modulation of the electronic charge density accompanied by a periodic distortion of the atomic lattice in quasi-1D or quasi-2D metallic crystals. Several layered transition metal dichalcogenides (TMDs) exhibit unusually high transition temperatures to different CDW symmetry-reducing phases [1-2]. For example, crystals of 1T-TaS2 have the CDW transition between the nearly-commensurate (NC-CDW) and the incommensurate (IC-CDW) phases at temperature of 350 K. In this talk, I review our recent experimental results on controlling the CDW phase transitions with an applied electric field and the number of the atomic planes in the quasi-2D films . We have demonstrated a room-temperature voltage controlled oscillator (VCO), which operates on the basis of the NC-to-IC CDW transition in 1T-TaS2 channels, triggered by the applied voltage. We found that the “all-metallic” 1T-TaS2 CDW devices reveal exceptional hardness against X-ray and other radiations . We explained this property of the CDW devices by the high carrier concentration in all their resistive states, and the quasi-2D channel geometry. We have also shown that CDW devices can be used for transistor-less information processing . In the second part of the talk, I will highlight the extension of the quasi-2D van der Waals materials research to the systems of lower dimensionality – quasi-1D van der Waals materials .
This work was supported, in part, by the National Science Foundation (NSF) through the Emerging Frontiers of Research Initiative (EFRI) 2-DARE project: Novel Switching Phenomena in Atomic Heterostructures for Multifunctional Applications (NSF EFRI-1433395), by the Semiconductor Research Corporation (SRC) contract 2018-NM-2796: One-Dimensional van-der-Waals Metals: Ultimately Downscaled Interconnects with Exceptional Current-Carrying Capacity and Reliability, and by the University of California – National Laboratory Collaborative Research and Training Program LFR-17-477237.
 R. Samnakay, D. Wickramaratne, T. R. Pope, R. K. Lake, T. T. Salguero, and A. A. Balandin, Zone-folded phonons and the commensurate-incommensurate charge-density-wave transition in 1T-TaSe2 thin films, Nano Letters, 15, 2965 (2015).
 G. Liu, B. Debnath, T. R. Pope, T. T. Salguero, R. K. Lake, and A. A. Balandin, A charge-density-wave oscillator based on an integrated tantalum disulfide–boron nitride–graphene device operating at room temperature, Nature Nanotechnology, 11, 845 (2016).
 G. Liu, E. X. Zhang, C. Liang, M. Bloodgood, T. Salguero, D. Fleetwood, A. A. Balandin, Total-ionizing-dose effects on threshold switching in 1T-TaS2 charge density wave devices,” IEEE Electron Device Letters, 38, 1724 (2017).
 A. G. Khitun, A. K. Geremew, and A. A. Balandin, Transistor-less logic circuits implemented with 2-D charge density wave devices, IEEE Electron Device Letters, 39, 1449 (2018).
11:00 AM - QN03.01.04
Single-Layer Neuromorphic MoS2 Memtransistors Fabricated by Helium Ion Beam Irradiation
Jakub Jadwiszczak1,2,3,Darragh Keane1,2,3,Pierce Maguire1,2,3,Conor Cullen1,2,3,Yangbo Zhou4,Hua Ding Song5,6,Niall McEvoy1,2,3,Zhi-Min Liao5,6,John Boland1,2,3,Hongzhou Zhang1,2,3
Trinity College Dublin1,Centre for Research on Adaptive Nanostructures and Nanodevices2,Advanced Materials and BioEngineering Research Centre3,Nanchang University4,Peking University5,Collaborative Innovation Center of Quantum Matter6Show Abstract
Two-dimensional layered semiconductors have recently emerged as attractive building blocks for next-generation low-power non-volatile memories. However, challenges remain in the controllable sub-micron fabrication of bipolar resistively switching circuit components from these novel materials. Here we report on the scalable experimental realisation of lateral on-dielectric memtransistors from monolayer molybdenum disulfide (MoS2) utilising a focused helium ion beam.
Site-specific localised irradiation with the focused probe of a helium ion microscope (HIM) allows for the creation of charged defects in the MoS2 lattice, confining the damaged region to < 5 nm. The reversible drift of these locally-seeded defects in the applied electric field changes the resistance of the semiconducting channel, opening up versatile memristive functionality due to additional tuning by the field effect. The device can reliably retain its resistance ratios and SET voltages for hundreds of switching cycles at sweep frequencies of up to 2.9 V/s. High-resolution Raman and PL spectroscopy mapping has been employed to reveal the role of mobile defective sites in the switching mechanism of the device by proxy monitoring of adsorbate-enhanced luminescence.
Cryogenic charge transport studies down to 1.5 Kelvin have revealed the impact of defect drift on device operation; parametrised by large differences between the low and high resistance states in: threshold voltages, electron mobility scaling, carrier localisation lengths, Schottky barrier heights, amplitude of 1/f noise and current density saturation. In addition, an early onset metal-insulator transition occurs in the low resistance state of the memtransistor due to the high degree of reversible doping brought about by the drift of mobile n-type donors. Moreover, we demonstrate long term potentiation and depression with sharp habituation that promises rich applications in future neuromorphic architectures, as well as the ability to tune device conductance by cross-terminal stressing with perpendicular contacts.
This work advances the down-scaling progress of memristive devices without sacrificing key performance parameters such as power consumption (< 20 nW) or applicability to synaptic emulation. In addition, we push forward the understanding of resistive switching in atomically thin devices and demonstrate the utility of field-controllable dopant drift in facilitating exotic phenomena in two dimensions.
11:15 AM - *QN03.01.05
Recent Progress on 2D Monolayer Memory Devices
Deji Akinwande1,Rujing Ge1,Myungsoo Kim1,Xiaohan Wu1,Saban Hus1,Jack Lee1
The University of Texas at Austin1Show Abstract
This talk will present the latest research progress on 2D memory materials and devices, otherwise known as atomristors. In particular the talk will highlight our work on fundamental research on switching mechanisms, performance limits, and the new application of zero-power non-volatile radio-freqeuncy (RF) switches. The fundamental understanding is enabled by in-situ scanneling tunneling microscopy studies, materials characterization, and first principles modeling. The monolayer memory devices offer fast switching below 15ns, relatively high retention and the prospects of high endurance with further studies. From an application perspective, the non-volatile RF switches offer superior scalability compared phase-change memory RF switches.
11:45 AM - QN03.01.06
Power Dissipation at Interfaces in Monolayer Transition Metal Dichalcogenides
Akshay Murthy1,Poya Yasaei1,Yaobin Xu1,Roberto dos Reis1,Gajendra Shekhawat1,Vinayak Dravid1
Northwestern University1Show Abstract
Lateral heterogeneities in atomically-thin two-dimensional (2D) materials provide an innovative platform to construct novel optoelectronic and electronic devices. Specifically, heterojunctions and grain boundaries (GBs) have demonstrated large on/off ratios and memristive responses. Despite the unique opportunities these systems offer, lateral heterogeneities also have the potential to induce local temperature rises and thus require careful examination to ensure device performance and reliability. In this study, we probe the temperature rise distribution across these heterogeneities in monolayer transition metal dichalcogenide (TMD) devices using nanoscale characterization techniques, including scanning thermal microscopy and high-resolution S/TEM. Our results directly demonstrate that lateral heterojunctions between MoS2 and WS2 do not appreciably impact the temperature rise distribution, but that GBs in MoS2 clearly localize heat and give rise to non-uniform current densities within the device. S/TEM reveals that the atomic sublattices are stitched nearly flawlessly together across heterojunctions but can be quite defective across GBs. These results suggest that the interfacial atomic structure plays a crucial role in enabling efficient and uniform charge transport without inducing localized heating. Through ongoing work correlating electronic and thermal phenomena with the local atomic and defect structure at 2D interfaces, we aim for a more holistic understanding of charge transport across lateral heterogeneities in 2D TMD systems.
QN03.02: Synthesis and Scalable, Large Area Devices I
Monday PM, April 22, 2019
PCC North, 100 Level, Room 129 A
1:30 PM - *QN03.02.01
Tuning Physicochemical Properties of MoS2 by Mechanical Strain
Stanford University1Show Abstract
Monolayers of 2D materials have ultrahigh mechanical strength, which enables application of very large elastic strains (e.g., ~11% for MoS2 and 25% for graphene). The ability of sustaining such large elastic strain offers unprecedented opportunities to engineer the physicochemical properties by applying mechanical strain. In this talk, I will present two examples on the effect of strain on the properties of MoS2 layer. The first example shows that elastic tensile strain reduces the bandgap of MoS2. When a gradient strain field is applied to MoS2 monolayer, the created bandgap gradient acts as an efficient funnel of photogenerated excitons that leads to enhanced photoluminescence. The second example shows that when elastic strain is applied to the sulfur vacancy on the basal planes of monolayer 2H-MoS2, the strain modifies the local electronic structure and catalytic activity. The proper combinations of S-vacancy and strain allows us to achieve higher intrinsic activity for hydrogen evolution reaction than the edge site of MoS2.
2:00 PM - *QN03.02.02
Towards Unifying Principles in Liquid Exfoliation of Various Layered Crystals
Heidelberg University1Show Abstract
Liquid exfoliation has become an important production technique to give access to large quantities of two-dimensional nanosheets in colloidal dispersion. Importantly, this is a highly versatile technique that can be applied to numerous layered materials. In this talk, recent advancements in the liquid-exfoliation and optical characterization of a range of 2D-materials will be summarized. Unifying principles among various classes of materials in both centrifugation and optical properties were identified recently. Materials under study include various commercially available TMDs, GaS, InSe, h-BN, hydroxides, and more exotic candidates such as RuCl3, PdSe2, LaSeTe2, NiPS3 or even novel synthetic organic sheet stacks (2D polymers and COFs).
All materials can be exfoliated and size-selected in a similar way yielding nanosheet dispersions with well-defined changes in their lateral dimensions and thickness. This allowed us to develop a model to understand the exfoliation in high energy processes in greater depth. In brief, the "exfoliation efficiency" (defined by the combination of layer number and lateral size that can be produced) is dependent on both interlayer and intralayer interactions. In addition, similar to the effects previously observed for MoS2 and WS2, the optical extinction and absorbance spectra show systematic changes across all material classes. This is of great practical importance as it allows for a rapid size determination via optical spectroscopy which can be used for further optimization of the sample preparation. For example, centrifugation can be designed in such a way to enrich dispersions in monolayers without significantly changing their lateral dimensions.
In addition, we show that liquid-exfoliated nanosheets and the fundamental understanding of their optical properties is an ideal foundation to track potential degradation due to reaction with water and oxygen by time and temperature dependent optical measurements of the dispersions. This in turn allows us to elaborate ways to chemically passivate defect sites and quantify the efficiency of various passivation pathways.
3:00 PM - QN03.02.03
Germanium- and Tin Chalcogenides—Growth, Heterostructure Formation, Devices, Nanoscale Light-Matter Interactions
Peter Sutter1,Eli Sutter1
University of Nebraska–Lincoln1Show Abstract
Two-dimensional group IV (Ge, Sn) monochalcogenides – compound analogues of phosphorene – are of fundamental interest due to their anisotropic crystal structure and predicted characteristics such as large exciton binding energies, tunable band offsets and charge separation in heterostructures, selective valley polarization, and multiferroic order in semiconductors with bandgaps in the visible and near-infrared range. To access these predicted properties and pave the way for applications of this family of 2D semiconductors, robust approaches toward single-layer group IV chalcogenides are required. However, realizing monolayers of this class of materials has proven challenging, both by top-down (exfoliation) and bottom-up (growth) approaches.
Here, we discuss results from in-situ low-energy electron microscopy that shed light on the fundamental growth mechanisms and identify the factors that complicate the synthesis of monolayers . We have used this insight to determine avenues for growing ultrathin crystals with thickness down to a single layer, as well as few-layer heterostructures, e.g., between mono- and dichalcogenide phases involving the same metal species. The functional properties of these materials are explored through charge transport in devices, and in particular via measurements of light-matter interactions with nanometer resolution using cathodoluminescence spectroscopy in scanning transmission electron microscopy (STEM-CL) [2,3]. Using a tightly focused electron beam as an excitation source, STEM-CL provides spectroscopy and mapping of light emission far below the diffraction limit to address band-edge luminescence, excitonic effects, charge separation in heterostructures, and confined photonic modes in monochalcogenide mesostructures.
 P. Sutter and E. Sutter, ACS Applied Nano Materials 1, 3026 (2018).
 E. Sutter and P. Sutter, ACS Applied Nano Materials 1, 1042 (2018).
 P. Sutter, C. Argyropoulos, and E. Sutter, Nano Letters, 18, 4576 (2018).
3:15 PM - *QN03.02.04
Electromagnetic Response of Large-Area Graphene Films
Byung Hee Hong1
Seoul National University1Show Abstract
Magnetism of graphene has attracted much attention because of its combined charge and spin manipulation useful for futuristic spintronic devices. In addition, the magnetism of carbon-based materials is particularly important for lightweight actuator/sensor devices and electromagnetic wave shielding in flexible electronics. The previous studies on graphene’s magnetism were mainly focused on paramagnetism and ferromagnetism that are related to electron spins, but the diamagnetism induced by the orbital motion of two-dimensional electron gas has been studied only by theoretical approaches due to the lack of measurable amount of samples. Recent advances in large-scale synthesis of graphene enabled a facile preparation of massive graphene samples by multiple stacking and transfer. Here we report a giant orbital diamagnetism in ultra-thin, lightweight, flexible, and transparent graphene films grown by chemical vapor deposition (CVD), where the diamagnetic susceptibility in perpendicular direction is measured to be ~100 times greater than the strongest diamagnetic materials such as bismuth. As a result, the graphene film also exhibits two orders stronger electromagnetic interference (EMI) attenuation characteristics normalized to the film thickness than conventional EMI shielding or absorbing materials over wide frequency ranges. The EM wave induces an oscillating magnetic moment generated by the orbital motion of moving electrons, which efficiently absorbs the EM energy and dissipate it as a thermal energy. In this case, the mobility of electron is more important than the conductivity, because the EMinduced diamagnetic moment is directly proportional to the speed of electron in an orbital motion. To control the charge carrier mobility of graphene we functionalized substrates with self-assembled monolayers (SAM). As the result, we find that the graphene showing the Dirac voltage close to zero can be more efficiently heated by EM waves. In addition, the temperature gradient also depends on the number of graphene. We expect that the efficient and fast heating of graphene films by EM waves can be utilized for smart heating windows and defogging windshields.
3:45 PM - QN03.02.05
Tailoring Commensurability of hBN/Graphene Heterostructures Through Substrate Morphology and Epitaxial Growth Conditions
Daniel Pennachio1,Chance Ornelas-Skarin2,Nathaniel Wilson1,Elliot Young1,Anthony McFadden1,Tobias Brown-Heft1,Kevin Daniels3,Rachael Myers-Ward3,D. Gaskill3,Charles Eddy3,Chris Palmstrom1
University of California, Santa Barbara1,University of California, Irvine2,U.S. Naval Research Laboratory3Show Abstract
Many of the intriguing properties of single-crystal 2D devices rely on the relative rotational alignment between layers. For instance, in the graphene/hBN system, substantial band structure modulation can occur at specific interlayer alignments , but a misalignment may be beneficial if innate graphene properties are to be examined, as in the case with near-commensurate bilayer graphene showing superconductivity . To allow for scalable graphene/hBN heterostructure formation, this work investigates hBN growth on single-crystal epitaxial graphene (EG) on macrostepped SiC(0001) substrates. The presented results suggest that the EG/SiC(0001) macrosteps influence hBN epitaxy such that a metastable, 30° in-plane hBN/EG rotational alignment is more favorable in certain growth regimes than the fully commensurate hBN/EG alignment, despite their similar crystal structures.
Plasma-enhanced chemical beam epitaxy (PE-CBE), an ultra-high vacuum (UHV) compatible process, was utilized to provide a clean environment for examination of the hBN structural, electrical, and chemical properties via in-situ and in-vacuo characterization methods. To determine the effect of substrate macrostep morphology, SiC (0001) substrates with a 4°-offcut toward <11-20> and nominally on-axis substrates were tested. The alignment of the hBN/EG/SiC(0001) heterostructure was studied by relating in-situ electron diffraction to nuclei edge directions found using ex-situ atomic force microscopy (AFM). Preferential alignment of the hBN nuclei edges to the SiC macrosteps was found in growths with a lower precursor flux (~1 nm/hr. growth rate), while far lower preference was found for higher-flux depositions (16 nm/hr. growth rate). In addition, cross-sectional TEM confirmed the registry and rotational alignment of the hBN to the EG/SiC substrate for both growth conditions, while plan-view TEM showed a single crystal alignment. Energy dispersive X-ray spectroscopy (EDS) during scanning TEM showed the graphene layers remained after the growth process and an atomically sharp hBN/EG interface was present. No hBN/EG intermixing between the layers was detected in either EDS or X-ray photoelectron spectroscopy (XPS). High-resolution topographic measurements and electrical measurements confirming the dielectric integrity of the hBN layer were performed using in-vacuo scanning tunneling microscopy (STM) and conducting AFM. The macrostep-directed epitaxy of hBN on EG highlighted in this work shows how different levels of commensurability can be achieved solely by tuning growth parameters during van der Waals epitaxy, thus reducing the reliance on manual rotation during film transfer and increasing the viability of scalable, single-crystal heterostructure growth.
 M. Yankowitz, et al., Nat. Phys. 8, 382 (2012).
 Y. Cao, et al., Nature, 43, 556 (2018)
4:00 PM - QN03.02.06
Controlled Vapor Growth and Nonlinear Optical Applications of Large Area 3R Phase Transition Metal Dichalcogenides Atomic Layers
Xiao Wang1,Zhouxiaosong Zeng1,Danliang Zhang1,Anlian Pan1
Hunan University1Show Abstract
Two-dimensional (2D) layered 3-rhombohedral (3R) phase transition metal dichalcogenides (TMDs) have received significantly increased research interest in nonlinear optical applications due to their unique crystal structures and the broken inversion symmetry. However, controlled growth of 2D 3R phase TMDs still remains a great challenge. In this work, we report a universal growth method for large area TMDs atomic layers with controllable crystal phases via a developed temperature selective physical vapor deposition route. Large area triangular 3R phase TMDs (e.g. WS2 and WSe2) layers are synthesized at a lower deposition temperature. Steady state and time-resolved photoluminescence (PL) spectroscopy and Raman spectroscopy have been used to study the unique properties of 3R phase layers due to different layer stacking and interlayer coupling. More importantly, with broken inversion symmetry, 3R phase layers show a quadratically increased second harmonic generation (SHG) intensity with respect to layer numbers. Furthermore, by polarization-resolved SHG, we observe a uniform polarization preference in bilayer and trilayer 3R phase WS2, which could be a benefit for practical applications. Our results not only contribute to the controlled growth of 2D TMDs layers with different phases but also pave the way to the promising nonlinear optical devices.
4:15 PM - QN03.02.07
Chemical Vapor Deposition Synthesis and Characterization of Ultra-Thin Single-Crystal Metallic Molybdenum Dioxide Nanosheets
Amey Apte1,Sandhya Susarla1,Anand Puthirath1,Pulickel Ajayan1
Rice University1Show Abstract
Two-dimensional materials such as graphene and transition metal dichalcogenides (TMDCs) show intriguing properties compared to their bulk counterparts. Most of these materials crystallize in hexagonal systems with covalent bonding in two dimensions and weak van der Waals' bonding in the out-of-plane direction. Recently, two-dimensional transition metal oxides have attracted enormous attention due to their unique differences compared to the other chalcogenides in terms of structure, properties, and chemistry which are important for potential applications. Most syntheses so far have been achieved via exfoliation and sol-gel methods which result in good yield but lack size and quality control. In this work, we explore the synthesis of ultra-thin molybdenum dioxide (MoO2) nanosheets using chemical vapor deposition. Unlike Mo-based chalcogenides, MoO2 crystallizes in a monoclinic structure with anisotropy. We achieve single-crystal flakes with lateral size of tens of microns and thickness of ~5 nm as confirmed via atomic force microscopy. The crystal structure is verified with high resolution transmission electron microscopy and grazing incidence X-ray diffraction whereas Raman, FTIR, and X-ray photoelectron spectroscopies reveal the intricate changes in the bond vibration and binding energy shifts. The flakes show strong optical absorption at ~550 nm and high metallic conductivity as verified by transport measurements. The structural anisotropy of the as-synthesized MoO2 flakes was verified with polarized Raman spectra for vibration modes at 205 cm-1 and 745 cm-1. Experiments for hydrogen evolution reaction showed good catalytic properties of the as-synthesized MoO2 nanoflakes at -0.4V. The mechanical properties of the flakes were investigated with nanoindentation and nanoscratch experiments. The work shows a facile way of synthesizing metallic nanosheets with potential opticelectronic, catalytic, and mechanical applications.
4:30 PM - QN03.02.08
Oligothiophene-Bridged Conjugated Covalent Organic Frameworks
Niklas Keller1,Derya Bessinger1,Stephan Reuter1,Mona Calik1,Laura Ascherl1,Fabian Hanusch1,Florian Auras1,2,Thomas Bein1
LMU Munich1,University of Cambridge2Show Abstract
2-Dimensional covalent organic frameworks (2D-COFs) are crystalline porous materials comprising aligned columns of π-stacked building blocks. Regarding potential applications of COFs in organic electronics and optoelectronics, access to oligothiophene-based COFs would be of great interest. However, the realization of such materials has remained a challenge, in particular concerning the laterally conjugated imine-linked COFs. We have developed a new building block design, implementing an asymmetric modification on an otherwise symmetric backbone that allows us to obtain a series of highly crystalline quaterthiophene (4T)-derived COFs with tunable electronic properties.1 Studying the optical response of these materials, we have observed for the first time the formation of a charge transfer state between the COF subunits across the imine bond.
In this work, we have developed the first quaterthiophene-based 2D covalent organic frameworks comprising ordered π-stacked columns of 4T and pyrene moieties. Applying an asymmetric functionalization strategy of the otherwise C2-symmetric 4T backbone allowed us to incorporate alkyl chains for optimized solubility while still retaining the ability of the building blocks to stack in close-packed face-on thiophene columns. We also demonstrate that this approach provides a facile route for modifying the electronic properties of the 4T backbone via incorporation of electron-deficient subunits, thus forming donor-acceptor type chromophores. The absorption and emission spectra confirm that the 4T-based building blocks are electronically integrated into the framework. Spectral features below the energy of the π-π* transition and the analysis of the corresponding emission decay time traces reveal the fast and efficient formation of a charge transfer state between the imine-linked pyrene and quaterthiophene subunits. We believe that our new asymmetric building block design provides a general strategy for the synthesis of well-ordered COFs from various extended building blocks. This will greatly expand the range of applicable molecules for realizing frameworks with tailor-made optoelectronic properties.
 Keller, N.; Bessinger, D.; Reuter, S.; Calik, M.; Ascherl, L.; Hanusch, F. C.; Auras, F.; Bein, T., Oligothiophene-Bridged Conjugated Covalent Organic Frameworks. J. Am. Chem. Soc. 2017, 139, 8194-8199.
4:45 PM - QN03.02.09
Bottom-Up Synthesis of Ultrathin PdSe2 Crystals with High Electron Mobility
Yiyi Gu1,2,3,Yiling Yu2,Chenze Liu4,Hui Cai2,Akinola Oyedele2,Anna Hoffman4,Yu-Chuan Lin2,Alexander Puretzky2,Gerd Duscher4,Philip Rack4,2,Christopher Rouleau2,David Geohegan2,Kai Xiao2
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences1,Oak Ridge National Laboratory2,University of Chinese Academy of Sciences3,The University of Tennessee, Knoxville4Show Abstract
PdSe2 is a high-mobility 2D material that has an unusual pentagonal structure, leading to buckled and puckered layers. Its strong interlayer coupling results in highly interesting and asymmetric optoelectronic properties and makes it difficult to exfoliate. However, to date it has not been directly synthesized on substrates. In this work, we report the bottom-up, vapor transport growth of few-layer PdSe2 crystals that are rectangular and tens of microns in size. The high quality of the PdSe2 crystals were confirmed by low-frequency Raman spectroscopy, atomic force microscopy (AFM), and high resolution annular dark field scanning transmission electron microscopy (ADF-STEM). Second-harmonic generation (SHG) mapping analysis of the PdSe2 flakes was used to characterize their strong optical anisotropy. Field-effect transistors made from the few-layer PdSe2 crystals revealed tunable ambipolar charge carrier conduction with a high electron mobility of ~78 cm2V-1s-1, which is comparable to that of exfoliated PdSe2, indicating the promise of this anisotropic 2D material for electronics.
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.
Deep Jariwala, University of Pennsylvania
Victor Brar, University of Wisconsin–Madison
SungWoo Nam, University of Illinois at Urbana-Champaign
Ursula Wurstbauer, Technical University of Munich
attocube systems AG
CreaTec GmbH, represented by Sentys Inc.
QN03.03: Electronic Properties and Devices I
Tuesday AM, April 23, 2019
PCC North, 100 Level, Room 129 A
10:30 AM - *QN03.03.01
From Epitaxy to Science and Processing Technologies of Two-Dimensional InSe van der Waals Crystals
University of Nottingham1Show Abstract
The continuous miniaturization of electronic devices has propelled modern technologies to higher performance and efficiency, but future progress requires a shift from traditional semiconductors towards new multifunctional systems and integration technologies. The development of two-dimensional (2D) van der Waals (vdW) crystals has led to the discovery of new phenomena and the realization of several functional devices. Among these vdW crystals, the metal chalcogenide InSe compound represents a promising semiconductor [1-6]. Our recent demonstration of fast broad-band photodiodes , field effect transistors (FETs) with electron mobility higher than in Si-FETs , and quantum devices [4, 6] has revealed the great potential of this material. From the growth and fundamental studies to the demonstration of prototype devices, this talk will describe how these systems can provide a platform for scientific investigations and new routes to 2D electronics and optoelectronics.
 G.W. Mudd et al., Advanced Materials 25, 5714 (2013); ibidem 27, 3760 (2015).
 G.W. Mudd et al., Scientific Reports 6, 39619 (2016).
 D.A. Bandurin et al., Nature Nanotechnology 12, 223, (2017).
 Z.R. Kudrynskyi et al., Physical Review Letters 119, 157701 (2017).
 N. Balakrishnan et al., 2D Materials 4, 25043 (2017); ibidem 2D Materials 5, 035026 (2018).
 J. Hamer et al., Nano Letters 18, 3950 (2018).
11:00 AM - QN03.03.02
Hexagonal Boron Nitride as a Buffer Layer in Monolayer Molybdenum Disulfide Transistors
Alexander Mazzoni1,2,Sina Najmaei2,Katherine Price3,2,Robert Burke2,Michael Valentin4,2,Matthew Chin1,2,Madan Dubey2
University of Maryland1,U.S. Army Research Laboratory2,Duke University3,University of California, Riverside4Show Abstract
One current problem with the fabrication of devices using two-dimensional (2D) materials as the semiconducting channel is the formation of a high-quality semiconductor-dielectric interface. Seeding for atomic layer deposition (ALD) films is notoriously difficult on van der Waals (vdW) layers due to the absence of dangling bonds. One method to improve the seeding on vdW materials is to use plasma-enhanced atomic layer deposition (PEALD). However, PEALD often damages the top-most layers which prevents the use of PEALD on monolayer materials.
Here we investigate the use of few-layer hexagonal boron nitride (h-BN) as an interfacial buffer layer transferred on top of monolayer molybdenum disulfide (MoS2) before deposition of the top dielectric. Top-gated monolayer MoS2 transistors with various dielectric stacks are fabricated and the h-BN layers’ impact is characterized via optical and electrical measurements.
11:15 AM - QN03.03.03
What Limits the Intrinsic Carrier Mobility of Two-Dimensional Metal Dichalcogenides?
The University of Texas at Austin1Show Abstract
Two-dimensional (2D) metal dichalcogenides (MX2) are the most common type of 2D semiconductors and have shown great potential for a wide range of chemical and physical applications. However, they are limited by a low electron/hole mobility, which has been recognized as one of the major challenges impeding their further developments, and urges efforts to understand the mobility-limiting factors and discovery of higher-mobility alternatives. Here using density functional perturbation theory and Wannier interpolation of the electron–phonon matrix to study a wide range of MX2, we find that the intrinsic carrier mobility, in contrast to common belief, neither correlates with the effective mass nor can be assessed by the widely used deformation potential theory; instead it is limited by the longitudinal optical (LO) phonon scattering for most MX2, while for MoS2 and WS2, the mobility is limited by the longitudinal acoustic (LA) phonon scattering. Furthermore, we find that the LO scattering strength is strongly correlated with the magnitude of the Born effective charge, suggesting that the carrier transport is greatly affected by the electric polarization change induced by the atomic vibration. This finding enables us to use the Born effective charge to rapidly screen the 2D MX2 database for high-mobility semiconductor candidates. Our work reveals the underlying factors governing the intrinsic carrier mobility of 2D MX2, and offers a practical descriptor for discovering high-mobility candidates.
Ref: L. Cheng, Y. Liu, JACS, 2018, DOI: 10.1021/jacs.8b07871
QN03.04/QN01.02/QN02.02: Keynote: Joint Session: Novel Two-Dimensional Materials from High-Throughput Computational Exfoliation
Tuesday PM, April 23, 2019
PCC North, 100 Level, Room 129 A
11:30 AM - *QN03.04.01/QN01.02.01/QN02.02.01
Novel Two-Dimensional Materials from High-Throughput Computational Exfoliation
We have performed an extensive high-throughput screening of known inorganic materials, in order to identify those that could be exfoliated into novel two-dimensional monolayers and multilayers . The screening protocol first identifies bulk materials that appear layered according to a simple and robust chemical definition of bonding, determining then for all of these the binding energies of the respective monolayers, and their electronic state (metallic vs insulating), magnetic configuration (ferro-,ferri- or antiferro-magnetic), and phonon dispersions (to evaluate mechanically stability). Such protocol identifies a portfolio of close to 2,000 inorganic materials that appear either easily or potentially exfoliable, to be investigated further for promising properties. First focus has been on the determination of the effective masses and mobilities (from the full solution of the Boltzmann transport equation) for electronic applications; of topological invariants; of superconductivity and charge-density waves; and of photocatalytic parameters for water splitting. Thanks to the use of the AiiDA (http://aiida.net) materials' informatics platform, all the high-throughput calculations can be performed and streamlined in fully searchable and reproducible ways, they are stored in a database with their full provenance tree of all parent and children calculations, and can be shared with the community at large in the form of raw or curated data via the Materials Cloud (http://www.materialscloud.org) dissemination portal.
 Nicolas Mounet, Marco Gibertini, Philippe Schwaller, Davide Campi, Andrius Merkys, Antimo Marrazzo, Thibault Sohier, Ivano Eligio Castelli, Andrea Cepellotti, Giovanni Pizzi and Nicola Marzari, Two-dimensional materials from high-throughput computational exfoliation of experimentally known compounds, Nature Nanotechnology 13, 246–252 (2018).
QN03.05: Photonic Properties and Devices I
Tuesday PM, April 23, 2019
PCC North, 100 Level, Room 129 A
1:30 PM - QN03.05.01
Enhancement and Control of Circularly Polarized Emission in Monolayer Heterogeneous WS2 with a Plasmonic Chiral Metasurface
Wei-Hsiang Lin1,Pin Chieh Wu1,George. R. Rossman1,Nai-Chang Yeh1,Harry Atwater1
California Institute of Technology1Show Abstract
Controlling the circular polarization of light is a key aspect for the development of functional nanophotonics for emerging applications. Monolayer transition metal dichalcogenides (TMDCs) are excellent candidates for spintronic, valleytronic and optoelectronic devices because the two inequivalent valleys in the Brillouin zone can give rise to valley-polarized photoluminescence (PL) under excitation with circular polarized light (CPL). However, the atomic monolayer thickness is a significant challenge for WS2 photoluminescence emission due to its weak light-matter interaction. We investigated the feasibility of enhancing the light-matter interaction in monolayer WS2 by means of the exciton-plasmon interaction with spin-orbit coupling of light. This objective was achieved by designing and fabricating plasmonic spiral rings with subwavelength dimensions on a hybrid substrate for WS2, and by synthesizing high-quality WS2. Specifically, we developed controlled growth of heterogeneous domains in CVD-grown monolayer WS2 single crystal on SiO2/Si substrate using tungsten oxide and sulfur precursors at T=850 degree C. Spatially resolved PL, Raman, X-ray photoelectron spectroscopy and Kevin probe force microscopy images revealed the formation of homojunctions in these single crystals which imply a direct correlation between the chemical stoichiometry and optoelectronic heterostructure. The WS2 were integrated with a plasmonic spiral ring metasurface with subwavelength sized elements, designed to enhance the light-matter interaction. Our plasmonic chiral metasurface consisted of a gold back reflector, a 20-nm-thick SiO2 dielectric layer followed by a monolayer heterogeneous WS2 layer on which we fabricated a gold spiral array. By optically pumping the plasmonic chiral metasurface/WS2 heterostructure with CPL and measuring the resulting spatially resolved CP emission (Pcirc) at room temperature and low temperature (80K), we found the optical chirality of WS2 was enhanced by more than 10 times relative to WS2 layers. Additionally, by proper designs of the dimensions of the chiral metasurface structure, a linearly-polarized incident light can be converted to circularly-polarized light. These results suggest a new pathway of manipulating the valley-polarized PL emission in 2D materials via plasmonic chiral metasurfaces, which may be further applied to the development of valley-polariton optoelectronic devices.
1:45 PM - QN03.05.02
Photoluminescence Enhancement at Heterojunction in WS2-MoS2 Lateral Heterostructures Revealed by Tip Enhanced Optical Spectroscopy
Andrey Krayev1,Sourav Garg2,Seongsin M. Kim2,Patrick Kung2
Horiba Scientific1,The University of Alabama2Show Abstract
2D semiconductors such as transition metal dichalcogenides (TMDC) recently attracted significant attention of the research community due to large number of fascinating optoelectronic and photocatalytic properties of these materials originating from their 2D nature. Defect engineering in these materials and defect-bound excitons are of particular importance due to their potential use as qubits for quantum computing.
Lateral heterostructures of different TMDCs is one of possible ways of creating controlled defects.
Tip enhanced optical spectroscopy (TEOS) which encompasses tip enhanced Raman scattering (TERS) and tip enhanced photoluminescence (TEPL) is a fast developing method that allows spectroscopic characterization of 2D materials with nanoscale spatial resolution and cross-correlation of this optical information with various properties revealed by scanning probe microscopy.
Here we report observation of strongly enhanced PL from WS2 at the grain boundary in CVD-grown monolayer of WS2–MoS2 lateral heterostructures transferred onto gold. Scanning Kelvin probe microscopy revealed strongly profiled saw-tooth-like junction between the MoS2 and WS2 sub-crystals with typical size of the features being from few hundreds to over 1000 nm. TEPL measurements showed that PL peak corresponding to WS2 gets strongly enhanced within approximately 100nm next to the heterojunction. TERS spectra suggested alloying within few tens of nanometers away from formal boundary between MoS2 and WS2. We argue that observed strongly localized nanoscale enhancement of the WS2 PL is associated with defects located at heterojunction and subsequently excitons bound to these defects. We’ll discuss prospective use of such structures as single photon emitters with prospective application as qubits for quantum computing.
2:00 PM - *QN03.05.03
Optically Active Defects in Tunable 2D Materials
Technical University-Munich1Show Abstract
Atomically thin two-dimensional layered materials receive great interest because of their unique physical properties. Particularly, monolayers of semiconducting transition metal dichalcogenides, such as MoS2, excel due to their strong light-matter interaction that is dominated by exciton phenomena [1-3]. Key to the integration of monolayer 2D materials into atomistic circuitries is the possibility to tune and engineer their properties on demand and on-chip e.g. by defects, dielectric environment and local doping [4-8]. We will introduce a methodology based on helium ion microscopy (HIM) to controllably realize single optically active emission centers in MoS2, which show clear indications of quantum dot-like behavior. Our results demonstrate the potential of nanoscopically focused ion beams to engineer the optical properties of semiconducting 2D materials at the nanometer scale [5,9].
We thank J. Klein, A. Kuc, F. Sigger, F. Merbeler, J. Wierzbowski, M. Altzschner, F. Kreupl, K. Müller, M. Kaniber, M. Florian, M. Lorke, M. Knap, R. Schmidt, J.J. Finley, and U. Wurstbauer for a fruitful collaboration and the DFG via NIM, eConversion, and project HO3324/9-1 for financial support.
 U. Wurstbauer, et al. J. Phys. D: Appl. Phys., 2017, 50, 173001.
 S. Funke, et al., J. Phys.: Condens. Matter, 2016, 28, 385301.
 B. Miller, et al., Nano Lett., 2017, 17(9), 5229.
 S. Diefenbach, et al., J. Phys. Chem. C, 2018, 122 (17), 9663.
 J. Klein, et al., 2D Materials, 2018, 5, 011007.
 J. Wierzbowski, et al., Nature Scientific Reports, 2017, 7, 12383.
 M. Florian, et al., Nano Lett., 2018, 18, 2725.
 E. Parzinger, et al., Nature 2D materials, 2017, 1, 40.
 J. Klein, et al. 2019.
3:15 PM - *QN03.05.05
Light Emitting Optoelectronic Devices Based on van der Waals Heterostructures
Yonsei University1Show Abstract
Two-dimensional (2D) materials are promising due to unique optical and electrical properties. New physics observed only in 2D materials allow for development of new-concept devices by using their valleys, tunneling effect, engineered band offset, and strong light-matter interaction. Recently, van der Waals heterostructures (vdWH) have been achieved by putting these 2D materials onto another, in the similar way to build Lego blocks. Assembled 2D blocks provide a big playground for engineers and physicists to investigate unprecedented properties of 2D materials and fabricate multi-functional optoelectronic devices. In this talk, I introduce our recent achievement in optoelectronic devices based on van der Waals heterostructures. By engineering band alignment of the stacked 2D materials and utilizing work function tunability of graphene, light emitting devices with exceptional multi-functions were fabricated. The electrically tunable light emitting transistors and tunnel devices can emit a strong light with modulation of charge (or trion) density. So, wavelength and intensity of emitted light can be controlled by electric field. Further, light emission from interlayer exciton generated in the stacked 2D semiconductors was demonstrated in device operation for the first time. Our work shows a great promise of van der Waals heterostructures for optoelectronic applications. I will also introduce our recent progress in high integration of graphene devices (or graphene leads). We developed a novel patterning technique by using XeF2 gas and fluorographene via contacts for graphene interconnects embedded in a van der Waals heterostructure. Our via contacts can be useful for 3D integration of 2D devices for future 2D electronics.
3:45 PM - *QN03.05.06
Advanced Photonic Devices Based on Layered Materials and Heterostructures
University of Cambridge1Show Abstract
Graphene is an ideal material for optoelectronic applications. Its photonic properties give several advantages and complementarities over Si photonics. I will show that graphene-based integrated photonics could enable ultrahigh spatial bandwidth density, low power consumption for next generation datacom and telecom applications. Heterostructures based on layers of atomic crystals have a number of properties often unique and very different from those of their individual constituents and of their three dimensional counterparts. I will show how these can be exploited in novel light emitting devices, such as single photon emitters, and tuneable light emitting diodes
4:15 PM - *QN03.05.08
Double Indirect Interlayer Exciton in a MoSe2/WSe2 van der Waals Heterostructure
Matthew Rosenberger1,Berend Jonker1,Aubrey Hanbicki1,Hsun-Jen Chuang1,C. Stephen Hellberg1,Saujan Sivaram1,Kathleen McCreary1,Igor Mazin1
Naval Research Laboratory1Show Abstract
Van der Waals heterostructures (vdWhs) are an emerging class of semiconductor heterostructures formed by stacking discrete monolayers of 2D materials such as the transition metal dichalcogenides (TMDs). In these structures, it is possible to create interlayer excitons (ILEs), spatially indirect, bound electron-hole pairs with the electron localized in one TMD layer and the hole in an adjacent layer. MoSe2/WSe2 forms a bilayer vdWh with a type II band alignment, such that the top of the valence band is formed predominantly by W states and the bottom of the conduction band by Mo states. Photoluminescence (PL) from the ILE state is expected to occur in the range of 1.35 - 1.4 eV, well separated from the room-temperature emission energies of the isolated MoSe2 (1.55 eV) and WSe2 (1.65 eV) monolayers. We fabricate MoSe2/WSe2 heterostructures from individual monolayer components synthesized by chemical vapor deposition and transferred and flattened using state-of-the-art preparation techniques  to insure intimate interlayer contact. We indeed observe a strong ILE feature at room temperature comparable to the emission arising from the constituent monolayers . We are able to clearly resolve two distinct emission peaks in this ILE feature at low temperature, separated by 24 meV. These peaks have nearly equal intensity, indicating they are of common character, and have opposite circular polarizations when excited with circularly polarized light. Ab initio calculations successfully account for these observations – they show that both emission features originate from excitonic transitions that are indirect in momentum space and derive from the spin-orbit splitting of the conduction band, in contrast with previous work that attributes one feature to a direct transition. In addition, the electron is strongly hybridized between both the MoSe2 and WSe2 layers, with significant weight in both layers, contrary to the commonly assumed model. Thus, the transitions are not purely interlayer in character. We find that including interlayer hybridization is essential to theoretically determine the ILE character. The hybridized electron eigenstates are superpositions of both spin states, which tilts the electron spins, so that both ILEs are optically bright with opposite polarizations. This work represents a significant advance in our understanding of the properties of TMD heterostructures, with implications for future electronic and valleytronic applications.
 M.R. Rosenberger et al, ACS Appl. Materials & Inerfaces 10, 10379-10387 (2018). DOI: 10.1021/acsami.8b01224
 A.T. Hanbicki et al, ACS Nano 12, 4719-4726 (2018). DOI: 10.1021/acsnano.8b01369
QN03.06: Poster Session I: 2D Materials-Tunable Physical Properties, Heterostructures and Device Applications
Tuesday PM, April 23, 2019
PCC North, 300 Level, Exhibit Hall C-E
5:00 PM - QN03.06.01
Sub 10nm Localized Thinning of Atomic Layers WS2 Through In Situ STEM/TEM
Yi-Tang Tseng1,Kuo Lun Tai1,Pu-Wei Wu1,Wen-Wei Wu1
National Chiao Tung University1Show Abstract
Two dimensional transitional metal dichalcogenides (TMDs) have been widely investigated in recent years for their electronic, optical and catalytic properties as well as specific mechanical properties. In addition, the characteristics of TMDs would be determined by the layer numbers or layer stacking configurations. Thus, it is essential to design a method for developing new 2D-architecture by tailoring the properties of the material. In this work, we provided a sub 10nm localized thinning technique of atomic layers WS2 via in-situ scanning/transmission electron microscopy (STEM/TEM). The entire process was visualized at atomic scale and conducted at high temperature. In the beginning, we shortly presented a methodical CVD system to synthesize the crystalline atomic-layers WS2, followed by transferring the as-grown samples onto specialized TEM chips. Subsequently, evidenced by the TEM results, we successfully shaped the WS2 layer by partially peeled off each WS2 monolayer. Additionally, during the thinning process, the exclusive 2D-behaviors such as nanolayers migration, coalescence and etching were observed. Finally, we utilized atomic resolution STEM images to identify layer variation and structural configuration, which helped us deduce the exact atomic stacking sequence. Further EDS/EELS analysis provide elemental information to elucidate the sulfur depletion mechanism. This sculpting technique allows for sub-10nm thinning features while preserving the crystallinity, and also paves the way for the rational design of WS2-based quantum and optoelectronic devices.
5:00 PM - QN03.06.02
Solution Processed Transition Metal Dichalcogenides for Printed Electronics Applications
The University of Manchester1Show Abstract
Semiconducting 2D materials, such as the Transition Metal Dichalcogenides (TMDs), have been widely explored for use in the fabrication of devices such as; field-effect transistors (FETs), sensors, and light emission devices, due to their attractive and size tuneable electrical properties. In particular, FETs manufactured around single mechanically exfoliated flakes with mobilities greater than 700 cm2.V-1.s-1 have been widely reported. TMDs can also be handled as liquid phase dispersions, enabling the formulation of inks for use in high throughput and low cost printed electronic devices. Liquid dispersions of TMDs therefore represent an attractive alternative to current organic semiconductor technology, as mobility and on/off ratio enhancements may be possible.
However, current literature demonstrates that FETs manufactured using solution processed TMDs suffer from low mobilities and on/off current ratios. The reason for the observed shortcomings in such devices is unknown, but possible reasons can be broadly narrowed down to contamination/oxidation of the flakes, surface defects, or poor flake-flake contacts within a film of solution processed TMD material.
Here, we will present results which elucidate possible mechanisms for the poor electronic performance of solution processed TMD thin film devices. First, we have developed new methods of thin film assembly from solution processed few layer 2D material dispersions including; liquid/liquid interfacial assembly, and Langmuir-Blodgett assembly via a Pickering emulsion intermediate phase. This has resulted in average film thicknesses of less than 5 nm. Subsequently, photo and field-effect transistors were assembled around the resultant thin films. Considerable improvements to the electrical properties were achieved by the application of dielectric polymer passivation layers and careful separation of monolayer enriched material from the liquid dispersions. Alternate exfoliation routes such as chemical and electrochemical exfoliation and the effects on resulting thin film transistors have also been explored.
Revealed herein are examples of FET devices manufactured around thin films of TMDs assembled utilising various simple, low cost, and highly scalable assembly methods. This work provides insights into the manufacturability of solution processed 2D FETs by elucidating and resolving some of the failure modes associated with solution processing methods. Finally, a robust and simple all-solution processed manufacture technique for high performance thin film devices made from dispersions of 2D TMD material is demonstrated.
5:00 PM - QN03.06.03
Deterministic Folding of 2D Materials for Electronic Device Applications
Huan Zhao1,Beibei Wang1,Xiaodong Yan1,Haozhe Wang2,Jing Kong2,Aiichiro Nakano1,Rajiv Kalia1,Han Wang1
University of Southern California1,Massachusetts Institute of Technology2Show Abstract
Atomically-thin two dimensional (2D) layered materials have exceptional mechanical flexibility. Therefore, 2D nano-sheets can be strained, stretched and folded into various origami structures and superlattices. 2D material origami structures possess unprecedented physical properties emerged from the folded edge, the interface, and the rotational angle of crystal lattices induced by the highly controllable folding direction.
In this talk, we will introduce a deterministic folding technique for creating controllable 2D material origami structures. This technique allows us to accurately fold a 2D atomic sheet at any position and direction pre-defined by standard electron beam lithography. 2D origami structures of layer-by-layer stacked heterostructure of 2D materials and large-scale CVD monolayer array will be demonstrated as examples of the scalability and controllability of this technique. We also looked deep into the molecular-level dynamics to understand the fundamental interactions underlying the folding process. Through simulation, we found the free energy of graphene increased after folding while the entropy of graphene decreased.
In addition, we will demonstrate various applications of our origami technique. 2D superlattice with controlled rotational angle can be prepared using the folding approach, which can be a powerful platform for exploring a rich array of physical phenomenon. For example, we found a pronounced Raman enhancement in a twisted graphene sample prepared by folding, suggesting the existence of Van Hove singularities. In addition, we were able to reconfigure the functionalities of 2D material electronics through folding. I will introduce a novel flash memory device built with this approach.
In summary, we developed a technique to deterministically fold 2D materials. Not only can this technique serve as a platform for studying nano-mechanics and exploring emerging physical phenomenon, but also it could be promising for realizing foldable and adaptive electronics, nano-actuating, and bio-nano interfaces.
Acknowledgment: This work is supported by the AFOSR FATE MURI program (Grant no. FA9550-15-1-0514).
5:00 PM - QN03.06.04
Graphene–Si–Graphene Bipolar Junction Transistor with Tunable Gain
Zhe Liu1,Wenzhe Zang1,Dehui Zhang1,Audrey Rose Gutierrez1,Zhaohui Zhong1
University of Michigan–Ann Arbor1Show Abstract
Traditional silicon bipolar junction transistor (BJT) is widely used in current amplification and high power circuits. With its compatibility to planar process, BJTs with different parameters such as doping level and base width are dedicatedly designed and fabricated to achieve large desired gains. For traditional silicon BJT, gain is determined intrinsically by these parameters and nearly impossible to tune in active region. Here we report a tunable graphene-Si-graphene BJT device with graphene functioning as both emitter and collector, and a thin silicon channel in between acting as the base. An additional layer of Cr/Au is placed over the emitter and collector regions with a thin layer of top dielectrics to function as the top gate. During device operation, electrons in emitter overcome the graphene/Si Schottky barrier to diffuse into the base under a positive voltage, and are subsequently collected by collector under a large reverse bias. Uniquely, with a top gate voltage, the doping and thus Fermi level of emitter graphene could be further tuned, leading to a tunable gain of graphene BJT. Further details of device operation mechanism and device characteristics will be discussed during the conference. The tunability demonstrated here for graphene-Si-graphene BJT can also be readily applied to other type of two dimensional material–Si hybrid devices for better control and performance.
5:00 PM - QN03.06.06
An Investigation of Carrier Mobility in MoS2 Grown by Chemical Vapour Deposition in a 300mm Reactor
Emma Coleman1,Paul Hurley1,Scott Monaghan1,Jun Lin1,Farzan Gity1,Michael Schmidt1,James Connolly1,Lee Walsh1,Karim Cherkaoui1,Katie O Neill2,Niall McEvoy2,Cormac O'Coileain2,Colm O'Dwyer3,Georg Duesberg4,Ian Povey1
Tyndall National Institute1,CRANN, Trinity College Dublin2,University College Cork3,Universität der Bundeswehr München4Show Abstract
Two dimensional (2D) van der Waals bonded materials exhibit a range of electronic properties spanning from semi-metals through to wide bandgap semiconductors. Potential applications include electronics, sensors and display technologies . In relation to practical device applications, research is focussed on large area growth [2 - 6], stable approaches to doping and contacting [7,8]. We report on the structural properties and carrier mobility in thin films (2-10nm) of MoS2 grown in a commercial 300mm atomic layer deposition reactor. The MoS2 films are grown on sapphire and SiO2/Si substrates at temperatures in the range of 350oC to 550oC by a chemical vapour deposition process using Mo(CO)6 and H2S precursors.
Analysis of the films with Raman, X-ray photoelectron spectroscopy and electron backscattered diffraction all confirm the films as MoS2. The carrier concentration, carrier type and carrier mobility are studied with Hall measurements. Excellent ohmic behaviour is achieved on MoS2 (10nm, 550oC with no post grown annealing) deposited on sapphire and a-Al2O3/sapphire substrates. Room temperature Hall analysis of the MoS2 films indicates that the non-intentionally doped MoS2 films are n-type with very low carrier concentrations of ~1014cm-3. Comparison of the free carrier concentrations in the grown films (~1014cm-3 ) to Hall analysis of natural and synthetic MoS2 crystals (1x1016 to 1x1017 cm-3) , is consistent with the presence of electrically active defect states in the energy gap of the MoS2 for the polycrystalline CVD grown films . MoS2 films grown at 350oC and subsequently annealed at 550oC in a H2S/H2 ambient indicate a significant increase in the electron concentration (~8x1016 cm-3).
The electron mobility is in the range of 3 to 17 cm2/V.s for films grown at 550oC (with comparable values for MoS2 films grown at 350oC and subsequently annealed at 550oC in a H2S/H2). Electron mobility values up to ~ 17 cm2/Vs for a MoS2 grain size in the 5nm to 20nm range in this work, compared to values in the range of 25 to 30 cm2/Vs reported by K. Kang et al.,  for monolayer MoS2 with a grain size around 1µm. This motivates the need to investigate scattering mechanisms and the influence of grain size on electron mobility in CVD grown MoS2, and preliminary results will be shown on the temperature dependence of electron concentration and electron mobility in the CVD grown MoS2 films.
 Geim,A.K.& Grigorieva,I.V. Van der Waals Heterostructures. Nature 499,419–425 (2013).
 Lin,Y.-C.etal. Wafer-scale MoS2 thin layers prepared by MoO3 sulfurization. Nanoscale 4, 6637–6641(2012).
 Kang et al., Nature, 2015, 520, 656–660
 S. M. Eichfeld, L. Hossain, Y. C. Lin, A. F. Piasecki, B. Kupp, A. G. Birdwell, R. A. Burke, N. Lu, X. Peng, J. Li, A. Azcatl, S. McDonnell, R. M. Wallace, M. J. Kim, T. S. Mayer, J. M. Redwing, and J. A. Robinson,