Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session F36: 2D Materials - van der Waals Heterostructures I |
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Sponsoring Units: DCMP Chair: Matthew Rosenberger, U.S. Naval Research Laboratory Room: LACC 410 |
Tuesday, March 6, 2018 11:15AM - 11:27AM |
F36.00001: Super-Planckian electron cooling in a van der Waals stack Alessandro Principi, Mark Lundeberg, Niels Hesp, Klaas-Jan Tielrooij, Frank Koppens, Marco Polini Radiative heat transfer (RHT) between macroscopic bodies at separations that are much smaller than the thermal wavelength is ruled by evanescent electromagnetic modes and can be orders of magnitude more efficient than its far-field counterpart, which is described by the Stefan-Boltzmann law. In this Letter we present a microscopic theory of RHT in van der Waals stacks comprising graphene and a natural hyperbolic material, i.e. hexagonal boron nitride (hBN). We demonstrate that RHT between hot carriers in graphene and hyperbolic phonon-polaritons in hBN is extremely efficient at room temperature, leading to picosecond time scales for the carrier cooling dynamics. |
Tuesday, March 6, 2018 11:27AM - 11:39AM |
F36.00002: Giant Tunneling Magnetoresistance in Spin-Filter van der Waals Heterostructures Tiancheng Song, Xinghan Cai, Wei-Yuan Tu, Xiaoou Zhang, Bevin Huang, Nathan Wilson, Kyle Seyler, Michael McGuire, David Cobden, Di Xiao, Wang Yao, Xiaodong Xu Magnetic multilayer devices that exploit magnetoresistance are the backbone of magnetic sensing and modern data storage technologies. However, van der Waals heterostructures with magnetic information storage and processing functionalities are still largely unexplored due to the lack of atomically thin 2D magnets. Recent breakthroughs in 2D magnetic materials offer a timely opportunity to explore these key functionalities in the atomically thin limit. Here we report novel multiple-spin-filter magnetic tunnel junctions (sf-MTJs) based on van der Waals (vdW) heterostructures in which atomically thin chromium triiodide (CrI3) acts as a spin-filter tunnel barrier. We demonstrate drastically enhanced tunneling magnetoresistance with increasing CrI3 layer thickness. Based on magnetic circular dichroism measurements, we attribute these effects to the intrinsic layer-by-layer antiferromagnetic ordering of atomically thin CrI3. The realization of such vdW heterostructure sf-MTJs could thus stimulate the study of novel 2D magnetic interface phenomena and spintronics, such as spin current sources and magnetoresistive random-access memory (MRAM). |
Tuesday, March 6, 2018 11:39AM - 11:51AM |
F36.00003: Magnetoresistance in Vertical Metal-Graphene-Metal Junctions Enrique Cobas, Olaf Van T Erve, Saujan Sivaram, Berend Jonker Predictions of spin filtering at graphene interfaces have prompted various experimental efforts to measure magnetoresistance and spin polarization of currents at graphene-metal interfaces. However, different experimental geometries and fabrication methods have resulted in different observed behaviors, from metallic transport to tunneling and from tunnel magnetoresistance to spin splitting. We review these results and present measurements and modeling of our own high-quality graphene junctions. We fabricate graphene junctions on highly crystalline, lattice-matched substrates by chemical vapor deposition and in-situ electron-beam evaporation of the top contact metal, followed by wafer-scale device patterning methods. We compare the behavior of graphene junctions with three different ferromagnetic bottom-top electrode combinations: NiFe-Fe, NiFe-Co and NiFe-Ni and elucidate the top interface transport mechanism by inserting a copper spacer layer between the graphene and ferromagnetic film. |
Tuesday, March 6, 2018 11:51AM - 12:03PM |
F36.00004: Electrochemical intercalation of discrete van der Waals heterostructures Mehdi Rezaee, D. Kwabena Bediako, Shu Yang Frank Zhao, Takashi Taniguchi, Kenji Watanabe, Tina Brower-Thomas, Philip Kim Engineering van der Waals hetero interface for controlling electronic and ionic charge is a challenging problem. In electrochemical energy storage systems, mobile ions reversibly associate with a host lattice by adsorption or insertion/intercalation into vacant sites, such as the interface gaps of van der Waals materials. In this presentation, we will discuss the electro-intercalation of lithium at the level of vdW heterostructures layers comprised of deterministically stacked hexagonal boron nitride (hBN), graphene, and molybdenum dichalcogenide (MoCh2; Ch = S, Se) layers, enabling the direct resolution of intermediate stages in the intercalation of discrete heterointerfaces and the extent of charge transfer to individual layers. Using magnetoresistance and spectroscopic data as probes of reaction progress coupled with low-temperature quantum magneto-oscillation measurements, these studies at well-defined mesoscopic electrodes show that the creation of intimate vdW heterointerfaces is a powerful knob for modulating intercalation potentials and tuning ion capacities of 2D interfaces. |
Tuesday, March 6, 2018 12:03PM - 12:15PM |
F36.00005: Thermoelectric and Thermionic Transport across van der Waals Heterostructures Nirakar Poudel, Steve Cronin, Li Shi, David Choi, Liang Shijun, Lay Kee Ang The thermoelectric voltage generated at an atomically abrupt interface has not been studied exclusively due to lack of established measurement tools and techniques. We report a novel technique and device structure to probe the thermoelectric transport across Au/h-BN/graphene heterostructures. An indium tin oxide (ITO) transparent electrical heater is patterned on top of this heterostructure, enabling Raman spectroscopy and thermometry to be obtained from the graphene top electrode in situ under device operating conditions. An AC voltage V(w) is applied to the ITO heater and the thermoelectric voltage across the Au/h-BN/graphene heterostructure is measured at 2w using a lock-in amplifier. We report the Seebeck coefficient for our thermoelectric structure to be -215 μV/K. The Au/graphene/h-BN heterostructures enable us to explore thermoelectric transport on nanometer length scales. The thermoelectric voltage generated at the graphene/h-BN interface is due to thermionic emission rather than bulk diffusive transport. As such, this should be thought of as an interfacial Seebeck coefficient rather than a Seebeck coefficient of the constituent materials. |
Tuesday, March 6, 2018 12:15PM - 12:27PM |
F36.00006: Photoresponse of Natural van der Waals Heterostructures Akm Newaz, Alexander Yore, Tong Mou, sauraj jha, Kirby Smithe, Bin Wang, Eric Pop Van der Waals heterostructures (vdWH) consisting of 2D materials offer a platform to obtain materials by design and are very attractive owing to unique electronic states. Research on 2D vdWHs has so far been focused on fabricating individually stacked atomically thin unary or binary crystals including graphene, h-BN, and 2D semiconductors. Here we present our study of the optoelectronic properties of a naturally occurring vdWH, known as franckeite, which is a layered crystal composed of lead, tin, antimony, iron, and sulfur. We found that franckeite (60 nm < d < 100 nm) behaves as a narrow band gap semiconductor demonstrating a wide-band photoresponse. We have observed the band-edge transition at ∼1500 nm (∼830 meV). Laser-power and temperature-resolved photocurrent measurements reveal that the photocarrier generation and recombination are dominated by continuously distributed trap states within the band gap. We calculated the optical absorption properties via density functional theory. Moreover, the device has a fast photoresponse with a rise time as fast as ∼1 ms. Our study provides a fundamental understanding of the optoelectronic behavior in a complex naturally occurring vdWH, and may pave an avenue toward developing nanoscale optoelectronic devices with tailored properties. |
Tuesday, March 6, 2018 12:27PM - 12:39PM |
F36.00007: Resonant Photon-Assisted Tunneling in Van der Waals Heterostructures Woo-Ram Lee, Wang Kong Tse We present a theory for electron tunneling in a graphene/h-BN/graphene van der Waals heterostructure under strong optical illumination. Using the Keldysh-Floquet Green’s function formalism, we have obtained the tunneling current between the graphene layers when both layers are driven periodically by an optical field. Our theory predicts that photon-assisted anti-resonance peaks are periodically developed in the tunneling current-voltage characteristic, on top of the contribution due to the dark tunneling current. We find that, while the latter originates from interband tunneling processes, the anti-resonance peaks originate from intraband tunneling processes between the Floquet subbands in different graphene layers. |
Tuesday, March 6, 2018 12:39PM - 12:51PM |
F36.00008: Nonlinear carrier dynamics in layered material heterojunctions Han Wang, Damien West, Vincent Meunier, Shengbai Zhang The hole transfer across MoS2/WS2 heterojunction was observed to be within 50fs at experiments.1 The nonlinear dynamics of hole transfer process in MoS2/WS2 heterojunction was systematically studied with a two-level model. The critical phenomenon of maximal hole oscillation across MoS2/WS2 is proved to be dependent on multiple parameters, not limited to dipole transition matrix element Mz as shown in our previous work2. Also, due to the periodic boundary condition, the previous model implicitly requires that all excitons are formed at the same time, however, this work shows that multiple excitons created in sequence by continuous wave laser will synchronize and reach the maximal hole transfer point at the same time. The critical phenomena of the nonlinear hole dynamics were explained with the theory of attraction of fixed point lines. Our work sheds light on a new direction of nonlinear carrier dynamics in two-dimensional material physics. |
Tuesday, March 6, 2018 12:51PM - 1:03PM |
F36.00009: Imaging Bulk and Edge Transport near the Dirac Point in Graphene Moiré Superlattices Ziwei Dou, Sei Morikawa, Alessandro Cresti, Shu-Wei Wang, Charles Smith, Christos Melios, Olga Kazakova, Kenji Watanabe, Takashi Taniguchi, Satoru Masubuchi, Tomoki Machida, Malcolm Connolly Van der Waals structures formed by aligning graphene monolayers with hexagonal boron nitride exhibit a moiré superlattice and broken sublattice symmetry that opens a gap at the Dirac point. The electrical conductivity is thermally activated at high temperature and saturates at low temperature indicating the existence of subgap states [1]. Transport via such states both in the bulk [2] and at the edges [3] have been suggested. We present a scanning gate microscopy study of moiré superlattice devices with similar gap size but different charge disorder. In the device with high charge impurity (~1010 cm-2) and low saturated resistivity (~10 kΩ) at the Dirac point we observe a clear response at the edges. Combined with simulations, we interpret the response as a result of enhanced edge doping. In addition, a device with low charge impurity (~109 cm-2) and high resistivity (~100 kΩ) shows bulk response only, consistent with the absence of edge-state shunting [3]. Our results provide microscopic insight into edge conduction that can be helpful in understanding transport in gapped Dirac systems. [1] Hunt et al. Science. 21, 1427-1430 (2013) [2] Gorbachev et al. Science. 24, 448-451 (2014) [3] Zhu et al. Nat. Commun. 8, 14552 (2017) |
Tuesday, March 6, 2018 1:03PM - 1:15PM |
F36.00010: Pressure effects on bilayer graphene (BLG)/WSe2/boron nitride (BN) device Zhisheng Lin, Mark Lohmann, Tang Su, Everardo Molina, Kenji Watanabe, Takashi Taniguchi, Jing Shi
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Tuesday, March 6, 2018 1:15PM - 1:27PM |
F36.00011: Electric Field Dependence of the Weak Anti-Localization of Bilayer Graphene Proximity Coupled to WSe2. Mark Lohmann, Bowen Yang, Everardo Molina, Jing Shi As is the case for single layer graphene, the intrinsic spin-orbit coupling (SOC) in bilayer graphene is extremely weak, but it can be increased by orders of magnitude when the graphene is proximity coupled to transition metal dichalcogenides (TMDs). We investigated the electric field dependence of bilayer graphene/WSe2 heterostructures, monitoring the change in the SOC through the weak anti-localization signature in our magnetoconductance measurements. In this study we chose to use thin exfoliated WSe2 because it shows the largest induced SOC in graphene as compared to other TMDs [1]. With both top and back gates, we carried out magnetoconductance measurements at a fixed carrier density in the valence band under different electric fields. We observed a systematic increase in the induced SOC when the electric field is tuned from 0.2 V/nm to -0.5 V/nm. By comparing areas of the same bilayer graphene channel which are both uncovered and covered by WSe2, we also examined the predicted electric field induced insulating behavior in these heterostructures due to the proximity coupling to WSe2. |
Tuesday, March 6, 2018 1:27PM - 1:39PM |
F36.00012: Lateral force microscopy study of a mixed-layer graphene structure JEE SOO CHANG, Sunghyun Kim, Jaekyeong Yeon, Suenne Kim Using a commercial (Park Systems, NX-10) atomic force microscope (AFM), we produced a mixed-layer graphene structure composed of one- to four-layer domains by folding a monolayer graphene. After, we obtained several lateral force microscopy (LFM) friction images of the fabricated structure in order to verify its atomic configurations under ambient conditions. LFM friction images show that the topmost layers of the respective domains have different in-plane crystallographic orientations. Based on a statistical analysis, we will show that the cut edges of the mixed-layer graphene tend to have zigzag or armchair directions while the folded lines do not reveal the tendency. In addition, certain strips have also been observed concurrently in the LFM images of graphene; it can result in the serious misinterpretation of the LFM images of atomic scale friction. We will discuss about the cause of the phenomenon and suggest a simple method to avoid it. |
Tuesday, March 6, 2018 1:39PM - 1:51PM |
F36.00013: Fabry-Perot Cavity-Enhanced Optical Absorption in Ultrasensitive Tunable Photodiodes Based on Hybrid 2D Materials Qixing Wang, Andrew Wee
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Tuesday, March 6, 2018 1:51PM - 2:03PM |
F36.00014: Moire patterns and electronic structure of hBN encapsulated graphene Nicolas Leconte, Han Kihyuk, Jeil Jung Hexagonal boron nitride layers are barrier materials for graphene and other 2D materials used to prepare high electrical quality devices. Their surface interaction with graphene becomes most apparent at energy ranges accessible through back-gating in the limit of long period moire patterns in nearly aligned structures. Here we examine the electronic structure of graphene encapsulated by nearly aligned hBN layers that give rise to two moire patterns, one per each G/BN interface, that can add up constructively or cancel out destructively depending on the relative sliding displacement and rotation between the hBN layers. Most of the electronic properties of hBN encapsulated graphene can understood from a linear superposition of the moire pattern effects stemming from each one of the interfaces. By examining the band gaps, density of states, and band structures for different relative arrangements of the top/bottom hBN layers we conclude that an appropriate coupling of graphene with the two hBN interfaces can greatly enhance the electronic structure features of both primary and secondary Dirac points, and influence the degree of particle-hole asymmetry in the system. |
Tuesday, March 6, 2018 2:03PM - 2:15PM |
F36.00015: Direct observation and in-situ control of interfaces between heterostructures of chemically and structurally distinct 2D materials Zachariah Hennighausen, Ioana Buda, Christopher Lane, Ismail Bilgin, Colin Casey, Kevin Mendez, Monika Eggenberger, Pradeep Waduge, Daniel Rubin, Abdelkrim Benabbas, Hirohito Yamazaki, Paul Champion, Meni Wanunu, Arun Bansil, Swastik Kar We show that when monolayer Bi2Se3 is CVD-grown on monolayer TMDs (MoS2, MoSe2, or WS2), they prefer to grow either rotationally aligned with respect to the TMD crystal, or stabilize at certain specific angles. They form Moiré superlattices, which lead to dramatic changes in the optical and electronic properties, including the >99% quenching of the TMD’s bright photoluminescence (PL). When the heterostructure (HS) is exposed to a laser or an electron-beam treatment (LT or ET), the Bi2Se3 breaks into smaller domains and rotates into new Moiré patterns. It is possible to tunably increase and decrease the PL; visually change their color; and modify the population of neutral and charged excitonic species (A- and A excitons), as well as their recombination energies with submicron spatial resolution. Thermal annealing reverses these changes and drives them back into stable Moiré configurations. We will also present in-situ observation of large (>100nm) Bi2Se3 domains rotating, splitting-up, and recombining, as well as improved (annealing) of the TMD crystallinity, using only perturbations from an SAED TEM electron-beam. Parallel first-principles electronic structure computations are presented on the resulting HSs in order to gain insight into the experimental observations. |
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