Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session K1: Van der Waals Bonding in Advanced Materials IFocus
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Sponsoring Units: DMP DCOMP Chair: Valentino R. Cooper, Oak Ridge National Laboratory Room: 260 |
Wednesday, March 15, 2017 8:00AM - 8:36AM |
K1.00001: Atomistic mechanisms of van der Waals epitaxy and property optimization of layered materials Invited Speaker: Jin-Ho Choi Since the first isolation of graphene from graphite by mechanical exfoliation, atomically-thin or layered materials have been occupying the central stage of today's condensed matter physics and materials sciences because of their rich and exotic properties in two dimensions (2D). Many members of the ever-expanding 2D materials family share the distinct commonality of possessing relatively-weak van der Waals (vdW) interlayer coupling, whereas each member may invoke its own fabrication approaches, and is characterized by its unique properties. In this presentation, we first discuss the major atomistic processes and related morphological evolution in the epitaxial growth of graphene, such as nucleation, diffusion, feedstock dissociation, and grain boundaries, and further review the current status of the vdW epitaxy of newly discovered 2D materials. The review covers the vdW epitaxy of both monolayered 2D systems and their lateral or vdW-stacked heterostructures, emphasizing the vital importance of the vdW interactions in these systems. We also briefly highlight on some of the recent advances in the property optimization and functionalization of 2D materials, using examples from the fields of optics, electronics, spintronics, and catalysis. Work done in collaboration with Ping Cui, Wei Chen, Jun-Hyung Cho, and Zhenyu Zhang. [Preview Abstract] |
Wednesday, March 15, 2017 8:36AM - 8:48AM |
K1.00002: A first principle study of the naturally aligned bilayer two dimensional (2D) materials Decarlos Taylor, Mahesh Neupane, Edward Byrd, Supeng Ge, Roger Lake Semiconducting layered transition metal dichalcogenides (TMDC) exhibit layer stacking and alignment dependent electronic properties resulting from the inter-layer vdW interactions. Independent of the growth conditions, these interactions also contribute to the inter-layer misorientation between the layers during layer-by-layer growth of a multilayer system. Therefore, in order to maintain the commensurability, the size of the supercell must grow exponentially with decreasing misorientation angle and increases from a few atoms for higher angles to several thousand atoms for the smaller angles. This exponential growth in the supercell size limits theoretical study of these systems using first principles methods such as those implemented in VASP and Quantum Espresso. Motivated by these factors, we have conducted a large-scale, first principles study of the electronic properties of the misoriented bilayer 2D materials using the scalable CP2K software package with hybrid basis sets. Using CP2K, the material and device related properties such as band levels, band offsets and work-function as a function of the misorientation angles are calculated and analyzed. The results for the larger misorientation angles with smaller super cell sizes are verified against the results of VASP and Quantum Espresso. A correlation between the misorientation angles, super cell size, and band gaps is established. [Preview Abstract] |
Wednesday, March 15, 2017 8:48AM - 9:00AM |
K1.00003: Friction Forces Between Atomically Flat 2D Materials Kursti DeLello, Rebeca Rebeiro-Palau, Tarun Chari, Kenji Watanabe, Takashi Taniguchi, James Hone, Ken Shepard, Cory Dean The ability to fabricate layered heterostructures from the assembly of 2D crystals has emerged as an exciting new opportunity in materials synthesis. Held together by van der Waals (wdW) forces, virtually any of the materials that can be exfoliated to the single atom limit can be integrated with one another into heterogeneous structures. This allows unprecedented opportunity to mix and match material properties, unbounded by the challenges inherent to the growth process of conventional semiconductor heterostructures, such as interfacial chemistry and lattice matching. However, to date surprisingly little is known about the interfacial interactions of these new structures. Here we investigate the angular dependence of friction between two atomically flat 2D materials with different lattice constants, (eg. graphene, \textit{h}-BN, WSe$_2$, WS$_2$, etc) using atomic force microscopy, and propose it as a method to probe vdW interactions. We show that the friction signal increases close to alignment, and is much lower for large angles, despite no direct atomic overlap, suggesting a relationship between the friction force and the presence of a moir\'e superlattice. [Preview Abstract] |
Wednesday, March 15, 2017 9:00AM - 9:12AM |
K1.00004: Twisted Bilayer Graphene - Topological Change of Fermi Surface and Lift of Layer Degeneracy Pilkyung Moon, Mikito Koshino, Youngwook Kim, Patrick Herlinger, Jurgen H. Smet, Takashi Taniguchi, Kenji Watanabe Van Hove singularities (vHs) play a diverse role for the electronic properties of crystals, but typical materials require unfeasibly large electron density to reach vHs. When two atomic layers are stacked in an incoherent way, however, the order of the electron density to reach vHs becomes several order smaller. This is because the moir\'{e} interference between the lattices makes a new class of superlattices with an exceptionally long periodic potential.\footnote{P. Moon and M. Koshino, Phys. Rev. B 85, 195458 (2012), Phys. Rev. B 87, 205404 (2013); C. R. Dean et al., Nature 497, 598 (2013); B. Hunt et al., Science 340, 1427 (2013).} In this talk, we systematically investigate the magnetotransport of twisted bilayer graphene. A topological phase transition at vHs is disclosed in the abrupt conversion of electrons to holes, a loss of a non-zero Berry phase and distinct sequences of integer quantum Hall states above and below the singularity. Moreover, we reveal the origin of the anomalous sequence of the Hall conductivity.\footnote{Y. Kim et al., Nano Lett. 16, 5053 (2016)} [Preview Abstract] |
Wednesday, March 15, 2017 9:12AM - 9:24AM |
K1.00005: Direct Measurement of Layer Adhesion in 2D Materials: Graphene on HOPG Arthur P. Baddorf, Jun Wang, Dan C. Sorescu, Seokmin Jeon, Alexei Belianinov, Sergei V. Kalinin, Petro Maksymovych The interest in mechanical properties of layered and 2D materials has reemerged in light of device concepts that take advantage of flexing, adhesion and friction. We report an experimental determination of the nanoscale adhesion of a graphene sheet on highly ordered pyrolytic graphite based on the effects of neon intercalation. Low energy ion implantation leads to local “blisters” in the top-most layer of the HOPG. Analysis of atomically resolved scanning tunneling microscopy images coupled with density functional theory is used to construct a strain map within the deformed graphene sheet. Adhesion energy is estimated using an analytical model originally devised for macroscopic deformations of graphene. This model yields an adhesion energy of 0.221 ± 0.011 J/m$^{2}$, which is in excellent agreement with reported experimental and theoretical values. This implies that macroscopic mechanical properties of graphene scale down to a few nanometers length. The simplicity of this method enables analysis of elastic mechanical properties in 2D layered materials and enable investigation of the nanoscale variability of mechanical properties. [Preview Abstract] |
Wednesday, March 15, 2017 9:24AM - 9:36AM |
K1.00006: On demand angle control in van der Waals heterostructures Rebeca Ribeiro-Palau, Tarun Chari, Kenji Watanabe, Takashi Taniguchi, James Hone, Kenneth Shepard, Cory Raymond Dean Fabricating layered heterostructures from the assembly of layered 2D crystals (so called, van der Waals (vdW) materials) has emerged as a new paradigm in nano-structured materials. One intriguing feature of these hybrid systems is that device characteristics often depend critically on the relative crystallographic orientation between adjacent layers. Experimental studies of these effects however have remained significantly constrained, owing to the inability to precisely control the rotation order. Here we present a new device architecture where the crystallographic alignment between layers can be manipulated in situ, while characterizing the electronic response. Variation in the rotational orientation to better than 0.5 degree is demonstrated, enabling for the first time a systematic experimental exploration in twisted layered structures. The angular dependence in a number of interesting van der Waals heterostructures will be discussed. [Preview Abstract] |
Wednesday, March 15, 2017 9:36AM - 9:48AM |
K1.00007: First-principles investigation of the interlayer coupling in chromium-trichloride-a layered magnetic insulator Santosh KC, Michael A. McGuire, Valentino R. Cooper The crystallographic, electronic and magnetic properties of layered CrCl$_{3\, }$were investigated using density functional theory. We use the newly developed spin van der Waals density functional (svdW-DF) in order to explore the atomic, electronic and magnetic structure. Our results indicate that treatment of the long-range interlayer forces with the svdW-DF improves the accuracy of crystal structure predictions. The cleavage energy was estimated to be 0.29 J/m$^{2}$ suggesting that CrCl$_{3}$ should be cleavable using standard mechanical exfoliation techniques. The inclusion of spin in the non-local vdW-DF allows us to directly probe the coupling between the magnetic structure and lattice degrees of freedom. An understanding of the link between electronic, magnetic and structural properties can be useful for novel device applications such as magnetoelectric devices, spin transistors, and 2D magnet. [Preview Abstract] |
Wednesday, March 15, 2017 9:48AM - 10:00AM |
K1.00008: Role of van der Waals Interactions in Alleviating Epitaxial Strain in WS$_{2}$/WSe$_{2}$ Lateral Heterojunctions Lijie Tu, Ka Un Lao, Saien Xie, Junteng Jia, Alvaro Vazquez-Mayagoitia, Jiwoong Park, Robert A. DiStasio Jr. Novel 2D transition metal dichalcogenides (TMD) materials are emerging as promising candidates for high-performance mechanical, electrical, optical, and magnetic devices with tunable parameters. In this work, we will consider a WS$_{2}$/WSe${_2}$ (4\% lattice mismatch) lateral heterostructure that was grown with epitaxial interfaces on an SiO$_2$ substrate and showed periodic ripples in the WSe${_2}$ monolayer. To explain these experimental findings, we have investigated the subtle energetic balance between epitaxial strain and van der Waals (vdW) interactions with the underlying substrate in this system. To obtain a quantitative theoretical estimate of the bending and stretching energy components of the WSe$_2$ monolayer, we extended current bulk continuum mechanical theory to atomically-thin nanofilms. For the vdW interactions, we considered different first-principles based approaches that account for both pairwise-additive dispersion interactions and the many-body dispersion (MBD) expansion of the long-range correlation energy. We will also briefly discuss our computational strategy, which utilizes novel algorithmic developments with high-performance computing resources, to explore the flat-rippled phase space in this realistic 2D material containing $\approx$150,000 atoms. [Preview Abstract] |
Wednesday, March 15, 2017 10:00AM - 10:12AM |
K1.00009: Observation of Interlayer Electron-Phonon Coupling in WSe2/hBN Heterostructures Chenhao Jin, Jonghwan Kim, Joonki Suh, Zhiwen Shi, Bin Chen, Xi Fan, Matthew Kam, Kenji Watanabe, Takashi Taniguchi, Sefaattin Tongay, Alex Zettl, Junqiao Wu, Feng Wang Van der Waals heterostructure of atomically thin two-dimensional (2D) crystals is an emerging class of material where the coupling between the adjacent layers can lead to new quantum phenomena that are completely different from the individual constituents. In condensed matter physics, the two most important many-body interactions are electron-electron coupling and electron-phonon coupling. Up until now, only electron-electron interactions between adjacent 2D layers have been extensively studied, and they have given rise to many fascinating physical behaviors. Here we report extraordinary interlayer electron-phonon interaction in WSe2/hBN heterostructure, where optically silent hBN phonons emerge in Raman spectra with surprisingly strong intensity through resonant coupling to WSe2 electronic transitions. Excitation spectroscopy reveals the double-resonance nature of such enhancement, and identifies the two resonant states to be the A-exciton transition of monolayer WSe2 and a new ``hybrid'' state present only in WSe2/hBN heterostructures. Our first observation of the interlayer electron-phonon interaction is important for fundamental understanding of van der Waals heterostructures, and can open up new ways to engineer electrons and phonons for novel device applications. [Preview Abstract] |
Wednesday, March 15, 2017 10:12AM - 10:24AM |
K1.00010: Temperature dependence of hot electron interlayer transport through a WSe2/MoSe2 heterostructure Maxwell Grossnickle, Fatemeh Barati, Shanshan Su, Roger Lake, Vivek Aji, Nathaniel Gabor By synthesizing two-dimensional van der Waals heterostructures from carefully chosen materials, we may engineer semiconductors with remarkable functionality. However, electron transport through the interface between two different atomically thin semiconductors is not well understood. We report on the temperature dependence of the current-voltage characteristics in a tungsten diselenide / molybdenum diselenide heterostructure device. We observe asymmetric current-voltage characteristics and negative differential conductance that changes dramatically as a function of increasing temperature. We develop a detailed model of interlayer electron transfer that fully captures the temperature dependence and accounts for highly efficient interlayer impact excitation by hot carriers. [Preview Abstract] |
Wednesday, March 15, 2017 10:24AM - 10:36AM |
K1.00011: Layered materials as platforms for quantum technologies Carmen Palacios-Berraquero, Dhiren M. Kara, Alejandro R.-P. Montblanch, Matteo Barbone, Pawel Latawiec, Duhee Yoon, Anna K. Ott, Marko Loncar, Andrea C. Ferrari, Mete Atature Quantum emitters (QEs) recently seen in tungsten diselenide (WSe$_{\mathrm{2}})$, member of the 2-dimensional transition metal dichalcogenides (2d-TMDs), present potential for quantum information technology (QIT) hosted in a silicon-compatible platform. QEs in 2d-TMDs until now have appeared randomly and with unstable emission properties. We demonstrate a method to deterministically create large-scale QE arrays in LMs. We present results showing QE arrays of tens of microns square and more than 100 QEs, in both WSe$_{\mathrm{2}}$ and WS$_{\mathrm{2}}$. We do this through strain-engineering at the nanoscale, placing monolayers onto nanopatterned substrates. The quality of the QEs is equal or superior to those appearing randomly. We present results on the susceptibility of the deterministic QEs to pillar height, TMD material source and fabrication method. We study their excitonic and optical properties through micro-resolved photoluminescence and charge-controlled experiments for creating optically active spin qubits using hybrid and 2d-heterostructure devices. This work places layered materials as potential key players in QIT -- enabling a more efficient study of these QEs and advancing towards real quantum circuit architectures. [Preview Abstract] |
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