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 AM, 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.