Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session X36: 2D Materials - van der Waals Bonding, Thermal Properties and FrictionFocus
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Sponsoring Units: DMP Chair: Dario Rocca, Univ Henri Poincare-Nancy/LPMI Room: LACC 410 |
Friday, March 9, 2018 8:00AM - 8:12AM |
X36.00001: Properties of Graphene/h-BN Superlattices Measured with Active Angle Control of Crystal Alignment Changjian Zhang, Rebeca Ribeiro-Palau, Kenji Watanabe, Takashi Taniguchi, James Hone, Cory Dean Van der Waals (vdW) heterostructures have drawn wide interest as an excellent platform to investigate material properties subjected to a periodic potential. In graphene/hexagonal boron nitride lattice scale mismatch gives rise to naturally occurring superlattice potential in the form of a moiré pattern. This results in significant modification of the band structure of graphene yielding new local mini-gaps and a gapped charge neutrality point. These new properties have a critical dependence on the rotational alignment of the layers, yet to date experimental studies of these effects have been limited by the uncontrollability of the relative crystallographic orientation. Here we present optical, electronic and mechanical properties of h-BN/graphene superlattices measured on the same device owing to the ability of actively controlling the crystal lattice alignment. Combining a dry transfer process, to assemble high quality heterostructures, and the use of an AFM in contact mode, we achieved a precise control of ~0.2 degree of the relative crystallographic orientation of the h-BN/graphene superlattice. We demonstrate results consistent with previous works and report new observations by taking advantage of the on-demand angle control of the h-BN/graphene superlattice. |
Friday, March 9, 2018 8:12AM - 8:24AM |
X36.00002: X-ray Electron Density Investigation of van der Waals Gap Interaction in TiS2 Hidetaka Kasai, Kasper Tolborg, Mattia Sist, Jiawei Zhang, Venkatesha Hathwar, Mette Filsoe, Simone Cenedese, Kunihisa Sugimoto, Jacob Overgaard, Eiji Nishibori, Bo Iversen Van der Waals (vdW) gap interaction is decisive for many extraordinary properties in vdW gap materials. However, it is a strong challenge for theory to model the vdW gap interaction since vdW force is not described in ground states. The experimental observation of the vdW gap interaction can be useful to understand the vdW gap and for theory to develop functionals that accurately model the long-range electron correlations. In this study, we observed the vdW gap bonding in layered TiS2 using accurate experimental electron density (ED). The ED was determined from high-resolution X-ray diffraction data using the Hansen-Coppens multipole model. The experimental ED was evaluated quantitatively by Bader's topological analysis. The obtained ED and the topological measures were compared with DFT calculations using vdW functionals, e.g. SCAN+rVV10. Quantitative agreement is observed for the covalent Ti-S interaction. On the other hand, significant difference is observed for the vdW gap interaction with experiment revealing a more covalent bonding situation than the DFT. Therefore, the present work provides an experimental benchmark for testing theoretical models of vdW gap iteraction. |
Friday, March 9, 2018 8:24AM - 8:36AM |
X36.00003: Tailoring Thermal Expansion Coefficient of Transition Metal Dichalcogenides via Alloy Engineering Xuan Hu, Baharak Sayahpour, Serdar Ogut, Amin Salehi-Khojin, Robert Klie Transition metal dichalcogenides (TMDs) have shown intriguing properties which make them suitable as candidates for future electronics. For example, the thermal expansion coefficient (TEC) dramatically increases when the thickness is reduced to a monolayer. However, the TEC mismatch in 2-dim materials is a significant problem in the design of the electronic nano-device. Therefore, an effective method for controlling the TEC of TMDs and an understanding of the related mechanism are needed. |
Friday, March 9, 2018 8:36AM - 9:12AM |
X36.00004: Van der Waals Bonded Materials: From Quasi-2D to Quasi-1D Invited Speaker: Alexander Balandin The advent of graphene stimulated interest to other two-dimensional (2D) van der Waals materials. In the first part of my talk, I will describe interesting phenomena emerging in 2D transition metal dichalcogenides (TMDs), which reveal charge density wave (CDW) phases [1-2]. Bonding and integrating three different 2D materials, 1T-TaS2, graphene and h-BN, in a way that exploited the unique characteristics of each, we were able to demonstrate a voltage controlled oscillator (VCO) functioning at room temperature [2]. In the second part of my talk, I will explain why the research of 2D materials should be extended to the one-dimensional (1D) van der Waals materials. Using transition metal trichalcogenides (TMTs) such as TaSe3 we demonstrated prototype “atomic wire” interconnects with extraordinary current density [3-4]. I will present results, which show that integration of quasi-2D and quasi-1D van der Waals materials can lead to new functionality, previously not achievable. |
Friday, March 9, 2018 9:12AM - 9:24AM |
X36.00005: A New Classification for Axis Dependent Conduction Polarity Bin He, Mike Scudder, Yaxian Wang, Maxx Arguilla, Nicholas Cultrara, Joshua Goldberger, Wolfgang Windl, Joseph P Heremans Axis dependent thermopower polarity has been observed in metals. It is usually explained as a multi-band effect with mobility variation of two carrier. Yet the signature of a multi-band system is a large Nernst coefficient, because the Nernst coefficient in multiband systems has a term that expresses the difference between the thermopowers of the two bands. We observe the anisotropic thermopower in single-crystal NaSn2As2, with very small Nernst coefficient, ruling out the two-band explanation. Here we propose a new classification, which is a single-band induced anisotropy, due to a particular shape of the Fermi surface. This model successfully explains the anisotropic behavior of NaSn2As2. Re4Si7 is revisited, which has a positive thermopower along a, b direction and negative thermopower along c-direction. Four coefficients (thermopower, resistivity, Hall coefficient and Nernst coefficient) along 3-orientations are measured from 100 K to 400 K. The Hall coefficients along the three axes are showing the same sign and similar concentration, while the Nernst coefficients are negligibly small compared to the thermopower. This again suggests that the thermopower anisotropy of Re4Si7 should be a single-band effect. |
Friday, March 9, 2018 9:24AM - 9:36AM |
X36.00006: Heat Transport through van der Waals Atomic Layers Shi En Kim, Joonki Suh, Fauzia Mujid, Chibeom Park, Jiwoong Park Atomically thin layered materials demonstrate unique potential for nanoscale thermal transport in two ways: 1) they possess anisotropic transport characteristics and 2) they can be vertically stacked to generate artificial films with novel interlayer transport properties. We present the thermal conductivities of artificially-stacked MoS2 films measured in the temperature range of 77–350 K using time-domain thermoreflectance. These films are generated by the layer-by-layer assembly of large-area MoS2 monolayer films in vacuum with precisely controlled film thickness and pristine interlayer interfaces. One unique property of our stacked films is the layer misalignment, which is expected to considerably suppress the out-of-plane thermal conductivities compared to those measured from exfoliated single crystal MoS2. The integration of atomically thin films into electronic devices will boost current heat management technologies via the realization of electron-transmitting, phonon-blocking material systems, opening up new possibilities in thermoelectrics. |
Friday, March 9, 2018 9:36AM - 9:48AM |
X36.00007: Seebeck Coefficient of a Single van der Waals Junction in Twisted Bilayer Graphene Phanibhusan Mahapatra, Kingshuk Sarkar, Hulikal Krishnamurthy, Subroto Mukerjee, Arindam Ghosh In this work, we have measured the thermoelectric transport across the van der Waals gap formed in twisted bilayer graphene (tBLG). We show that the thermoelectric properties are entirely determined by the out of plane ZO’/ZA2 phonons which can manifest as an effective “interlayer phonon drag” through inter-layer electron-phonon scattering. The gate-voltage and temperature-dependence of Seebeck coefficient differs dramatically from the expected Landauer-Buttiker formalism in conventional tunnel junctions, along with a violation of Mott formula, which indicate possible signature of lattice anisotropy on thermoelectric transport. The power factor-temperature (PFT) product calculated from measured Seebeck coefficient and cross-plane resistance, can be as high as 0.3 W/mK which, together with the possible low inter-layer thermal conductance due to phonon filtering in cross-plane transport may lead to a high figure of merit thermoelectric system. |
Friday, March 9, 2018 9:48AM - 10:00AM |
X36.00008: Interlayer Electron-Phonon Interaction and Phonon Chirality at Two-Dimensional Material/Silicon Dioxide Interface Chen Chen, Xiaolong Chen, Yuchuan Shao, Qiushi Guo, Bingchen Deng, Sung Min Lee, Chao Ma, Je-Guen Park, Shengxi Huang, Fengnian Xia In this work, we report the emerging phonon emissions arising from the strong electron-phonon interaction at the few-layer two-dimensional (2D) material/silicon dioxide (SiO2) interfaces. The new phonon modes, which are originally Raman silent, are robust at the interfaces with various 2D materials coupled to the surface of SiO2. We demonstrate that through electron-phonon coupling, the intrinsic properties of the electronic band structures in the coupled 2D materials are revealed by the novel chirality and in-plane anisotropy of the phonon modes. Our observations provide a deep insight into the microscopic mechanism of electron-phonon interaction at the interfaces between 2D materials and dielectrics. |
Friday, March 9, 2018 10:00AM - 10:12AM |
X36.00009: The origin of diverse lattice dynamics in the graphene family Gour Das, Amrita Bhattacharya
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Friday, March 9, 2018 10:12AM - 10:24AM |
X36.00010: A thermokinetic model of friction in molybdenum disulfide (MoS2) Adam Hinkle, John Curry, Brendan Nation, Minh Pham, Nicolas Argibay, Michael Chandross Molybdenum disulfide (MoS2) is a lamellar compound with a long history of use as a solid lubricant due to its ability to significantly reduce friction. The compound’s lamellar structure, with weak inter-lamellar bonding, can result in both commensurate and incommensurate contacts that significantly affect the sliding properties of MoS2, and directly affect the friction. We present results of detailed molecular dynamics calculations of energy barriers to sliding and rotation, and discuss how these barriers relate to macroscopic friction coefficients. |
Friday, March 9, 2018 10:24AM - 10:36AM |
X36.00011: Friction Scaling at the Interface of 2D van der Waals Heterostructures Kursti DeLello, Rebeca Ribeiro-Palau, Tarun Chari, Kenji Watanabe, Takashi Taniguchi, James Hone, Kenneth Shepard, Cory Dean Much of tribology research in the past has been hindered by the fact that friction measurements are overwhelmingly dominated by surface roughness, where a significant experimental limitation has been designing experiments with a well-controlled interface. Two-dimensional (2D) heterostructures present a fantastic new avenue in this regard. Atomically ordered and pristine interfaces can be realized over micron length scales, providing unprecedented opportunity to mix and match material properties, without being limited by issues inherent to the growth process of conventional semiconductor heterostructures, such as interfacial chemistry and lattice matching. To date, the interfacial interactions of these new structures remains relatively poorly understood. Here we investigate the area and temperature scaling of friction as a function of angle between two atomically flat 2D materials with different lattice constants, graphite and h-BN, using atomic force microscopy. We show both the area and temperature scaling of friction varies with the angle of relative alignment, with a stronger dependence close to alignment, suggesting a relationship between the friction force and the presence of a moiré superlattice. |
Friday, March 9, 2018 10:36AM - 10:48AM |
X36.00012: Abstract Withdrawn |
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