Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session L13: 2D Materials (General) -- New Materials and Emerging PropertiesFocus
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Sponsoring Units: DMP DCOMP Chair: Yuanyue Liu, University of Texas at Austin Room: BCEC 153B |
Wednesday, March 6, 2019 11:15AM - 11:27AM |
L13.00001: Oxide Nanosheet Dielectrics for 2D Devices Samantha Smiley, Andrew O’Hara, Sokrates T Pantelides Two-dimensional materials provide a platform for small-scale and ultrathin electronics and the continued scaling of devices like field effect transistors (FETs). However, it is still necessary to develop high-k gate-dielectric materials that possess suitable band alignments and minimize electron leakage. 2D oxide nanosheets such as Ca2Nb3O10 exhibit both high dielectric constants and good thermal stability, suggesting their potential for use in 2D-material-based FETs. In this work, we perform quantum mechanical calculations using density functional theory in order to model interface and defect properties of Ca2Nb3O10. Calculations of interface band alignments of Ca2Nb3O10 and various candidate 2D semiconductors allow for the determination of suitable material pairings for p- or n-type transistors. Atomic defects that arise during material fabrication and preparation, like oxygen vacancies, can create defect levels within the oxide band gap. Therefore, we calculated the position of these defect energy levels and how they align relative to the semiconductor band edges to provide insights into potential sources of electron leakage and device degradation in the proposed structures. |
Wednesday, March 6, 2019 11:27AM - 11:39AM |
L13.00002: Unified Dielectric Nature of Two-Dimensional Materials Elton Santos Dielectric constant, which defines the polarization of the media, is a key quantity in condensed matter. It describes to a large degree the electron-electron interaction, which has a crucial effect on band gaps, optical excitations, and screening. Here we show that instead of the dielectric constant ε, the 2D polarizabilty α correctly captures the dielectric nature of a 2D material for both in-plane and out-of-plane polarizations. We reveal that the long-sought universal dielectric-scale relationships in the 2D world: the in-plane polarizability α|| is inversely proportional to the minimal bandgap Eg, while the out-of-plane polarizability α⊥ is directly related to intrinsic thickness of the 2D material. An analytical quantum-mechanical model is developed which give a sound background to the dielectric-scale relationships, which is supported by a broad high-throughput screening over thousands of materials. Moreover, such relations unify the dielectric properties between the 2D materials and their 3D counterparts in a natural manner, which ultimately pushes the boundary of the understanding of electronic screening in both dimensions. |
Wednesday, March 6, 2019 11:39AM - 11:51AM |
L13.00003: Synthesis and structural characterization of the single chain limit of van der Waals materials Thang Pham, Sehoon Oh, Scott Meyer, Brian Shevitski, Kyunghoon Lee, Jeffrey Cain, Chengyu Song, Peter Ercius, Christian F. Kisielowski, Marvin L Cohen, Alex Zettl The successful isolation of a single layer of van der Waals (vdW) materials, such as graphene and transition metal dichalcogenides (TMDs), and the renewed interests in their emergent properties in the atomically thin limit have motivated the exploration of other vdW materials. Transition metal trichalcogenides (TMTs), such as niobium triselenide (NbSe3), are closely related quasi one-dimensional (1-D) vdW materials consisting of trigonal prismatic chains binding together by weak vdW interaction. The bulk TMTs abound with peculiar properties, such as sliding charge density wave and superconductivity. |
Wednesday, March 6, 2019 11:51AM - 12:27PM |
L13.00004: Revealing the Full Spectrum Layered Materials with Super-Human Predictive Abilities Invited Speaker: Evan Reed We have utilized data mining approaches to elucidate over 1000 2D materials and several hundred 3D materials consisting of van der Waals bonded 1D subcomponents, or molecular wires. We find that hundreds of these 2D materials have the potential to exhibit observable piezoelectric effects, representing a new class of piezoelectrics. A further class of layered materials consists of naturally occurring vertical hetero structures, i.e. . bulk crystals that consist of stacks of chemically dissimilar van der Waals bonded layers like a 2-D super lattice. We further combine this data set with physics-based machine learning to discover the chemical composition of an additional 1000 materials that are likely to exhibit layered and two-dimensional phases but have yet to be synthesized. This includes two materials our calculations indicate can exist in distinct structures with different band gaps, expanding the short list of two-dimensional phase change materials. We find our model performs five times better than practitioners in the field at identifying layered materials and is comparable or better than professional solid-state chemists. Finally, we find that semi-supervised learning can offer benefits for materials design where labels for some of the materials are unknown. |
Wednesday, March 6, 2019 12:27PM - 12:39PM |
L13.00005: A Defect-Enabled Reconfigurable Surface Benjamin Katz, Vincent Henry Crespi A novel class of surfaces holds the possibility of reversible reconfiguration into dramatically distinct, stable shapes. This property stems in part from their defects–they have equal numbers of pentagon and heptagon disclinations. Exploring these surfaces with an example constructed out of a graphene monolayer with the disclinations arranged in a kagome-like superlattice, we model its mechanical response with semiclassical molecular dynamics. The pentagon disclinations form cones with a two-fold degree of freedom in their up/down orientation, yielding a reconfigurable surface with a large number of stable shapes. Enumerating a complete 'zoo' of such shapes for a small patch of this material reveals that not only are the interactions between these degrees of mechanical freedom long-range enough to produce a gaussian-like 'density of states' for given cone orientations, but also that the surface possesses other hidden degrees of freedom in certain orientations–further increasing the number of stable shapes it can hold. These shapes cover a broad range of physical forms and a scale comparable to important biomolecules, raising the possibility of biological applications. |
Wednesday, March 6, 2019 12:39PM - 12:51PM |
L13.00006: Fabrication of atomically thin nanowire for mesoscopic transport study Abin Joshy, Yun Ling, Xue Liu, Andrew Steely, Jinyu Liu, Liubov Yu. Antipina, Pavel B. Sorokin, Ana M. Sanchez, Zhiqiang Mao, Jiang Wei Layered transition metal chalcogenides(TMC) have attracted tremendous attention owing to their weak van der Waals(vdW) interactions between chalcogens, allowing easy mechanical exfoliation of single atomic layer from the bulk. We extend this idea of “exfoliating layers” to “exfoliating atomic chains” in Ta2(Pd or Pt)3Se8, which is a vdW bonded stack of molecular strands. In this work, we succeeded to achieve Ta2Pd3Se8 single and few molecular thin (0.7nm, 1.3nm, 2nm) wires through micromechanical exfoliation. High-resolution TEM confirms 1D chain like morphology of these wires. In addition, we also demonstrated that this method can be applied to other isostructures, such as Ta2Pt3Se8. We also discovered that intrinsic semiconducting properties can be largely preserved in the Ta2Pd3Se8 nanowire transistor as evidenced by high on/off ratio (up to 104) and mobility (up to 80 cm2V-1s-1). Finally, our work provides a new strategy for obtaining air-stable and strictly 1D material system, which offers excellent opportunities for the study of 1D physics. |
Wednesday, March 6, 2019 12:51PM - 1:03PM |
L13.00007: Linking interlayer twist angle to geometrical parameters of self-assembled folded graphene structures Dawei Zhai, Johannes Rode, Christopher Belke, Sung J. Hong, Hennrik Schmidt, Nancy Patricia Sandler, Rolf J. Haug Previous studies have shown the possibility of obtaining folded bilayer graphene ribbons through spontaneous self-tearing and peeling from a substrate [1]. However, the effect of interlayer twist angle has been neglected. Here we investigate the morphology of spontaneously self-grown nanoribbon structures using AFM. Data reveal similar twist angle dependence of the width and interlayer separation, as well as a width-dependent fold radius. As the self-growth involves bilayer formation, bending, tearing and substrate peeling processes, these observations are well described by an energy minimization model that includes the bilayer adhesion energy density as represented by a distance dependent Morse potential. We obtain an explicit expression for the radius-width dependence that predicts a renormalized bending rigidity and stand in good agreement with experimental observations. The newly found relations between these geometrical parameters suggest a mechanism for tailored growth of folded twisted bilayer graphene- a platform for many intriguing physics phenomena. |
Wednesday, March 6, 2019 1:03PM - 1:15PM |
L13.00008: First principles prediction of 2D lattice coherency in van der Waals heterostructures Benoit Van Troeye, Jean-Christophe Charlier, Xavier Gonze, Simon Dubois, Aurélien Lherbier Beginning with graphene-hBN van der Waals (vdW) heterostructures [1], many Moiré patterns between 2D materials have been observed [2,3]. Still, predicting the coherent, semi-coherent (Moiré) or incoherent matching of 2D lattices at their interface is a challenge for Density Functional Theory (DFT), due to the very large size of the supercells needed in such studies. We introduce a first-principles-based model that bypass the need for large supercells. It generalizes the well-known Frenkel-Kontorova model [4] by including physical effects present in real materials, as derived from a perturbative approach to the problem. In particular, a mean-field modification of the 2D lattice parameters and elastic constants appears, even if the matching of the lattices is incoherent. The results are compared to plain DFT computations and to experimental observations of lattice accommodation in vdW-heterostructures. Then, we predict lattice (in)coherency for a set of 36 vdW-heterostructures based on graphene, phosphorene and different transition metal dichalcogenides. |
Wednesday, March 6, 2019 1:15PM - 1:27PM |
L13.00009: Modeling mechanical relaxation in misaligned 2D heterostructures Ziyan Zhu, Stephen Carr, Shiang Fang, Steven Torrisi, Paul Cazeaux, Mitchell Luskin, Efthimios Kaxiras Two-dimensional van der Waals layered materials (e.g., twisted bilayer graphene) provide a platform to study correlated many-body physics and have potential device applications. However, these layered systems are computationally challenging to model by conventional methods due to their large supercells. Here, we present a multi-scale model to efficiently calculate the mechanical relaxation pattern in incommensurate van der Waals heterostructures at arbitrary twist angles and lattice mismatch. We adopt a continuum model to describe lattice relaxation and a generalized stacking fault energy, computed from the density functional theory, to account for interlayer couplings. We obtain the optimized structure by minimizing the total energy. Our model extends the computationally accessible regime to layered systems with relatively small twist angles and large moiré patterns. This model can be applied to a wide range of materials, including those with no empirical interlayer coupling potential available, such as graphene and the transition metal dichalcogenides. |
Wednesday, March 6, 2019 1:27PM - 1:39PM |
L13.00010: Evidence of Moiré Excitons in van der Waal(vdW) Heterostructure Kha Tran, Galan Moody, Fengcheng Wu, Xiaobo Lu, Junho Choi, Jiamin Quan, Akshay Singh, Jacob S Embley, André Zepeda, Marshall Campbell, Kyounghwan Kim, Amritesh Rai, Travis Autry, Daniel Sanchez, Takashi Taniguchi, Kenji Watanabe, Nanshu Lu, Sanjay Banerjee, Emanuel Tutuc, Suenne Kim, Li Yang, Kevin Silverman, Allan MacDonald, Xiaoqin (Elaine) Li We report spectroscopy evidence of interlayers excitons confined by a moire superlattice in hBN encapsulated MoSe2/WSe2 heterostructures with small twist angles. Low temperature photoluminescence(PL) measurement shows that interlayer exciton splits into multiple peaks with alternating circular polarization. We assign these peaks to the ground state and excited state excitons localized within a Moiré supercell and explain how the spatial variation of optical selection rule can give rise to multiple peaks with alternative circular polarization. Temperature dependence PL, twist angle dependence, and recombination dynamics all agrees with the localized exciton picture. Our results suggest the feasibility of engineering artificial excitonic crystal using vdW heterostructures for nanophotonics and quantum information applications. |
Wednesday, March 6, 2019 1:39PM - 1:51PM |
L13.00011: ABSTRACT WITHDRAWN
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Wednesday, March 6, 2019 1:51PM - 2:03PM |
L13.00012: First-Principles Study of Chemical doping in WSe2 Maohua Du, Dan Han, Wenmei Ming, Haixuan Xu, Shiyou Chen 2D transition metal dichalcogenides (TMDs) are promising nano-electronic materials. The ability to dope 2D TMDs both n- and p-type with high carrier concentration and mobility is essential to the development of MOSFET and CMOS technologies. WSe2 has been shown to exhibit both p- and n-type conductivities after chemical doping; however, the n-type doping is much less efficient than the p-type doping. We have carried out density functional theory calculations of a wide range of transition metal dopants in WSe2 with the goal to understand the different dopant behaviors and to design new n-type dopants with improved doping efficiency. We have also studied in details the native defect properties since defects can potentially compensate the dopants. The calculated trend of dopant formation energies is consistent with the available experimental results. We will discuss the underlying mechanisms that are responsible for the different dopant formation energies based on the local symmetries of the dopants in WSe2 and in competing secondary phases, which affect the crystal field splitting and the relative chemical stability of dopants in different crystal environments. We will also suggest new dopants based on our calculations. |
Wednesday, March 6, 2019 2:03PM - 2:15PM |
L13.00013: Ab Initio Calculations of Bonding and Charge Transfer in Borophene Sheets on a Cu(111) Substrate Stephen Eltinge, Rongting Wu, Ilya K. Drozdov, Percy Zahl, Ivan Bozovic, Adrian Gozar, Sohrab Ismail-Beigi Borophene, the two-dimensional hexagonal lattice form of boron, is predicted to have technological uses in flexible electronics and as a precursor to metallic boron nanotubes. Boron’s electron deficiency leads to borophene sheets that have both hexagonal and triangular bonding, and different arrangements of these motifs lead to a wide range of nearly isoenergetic phases. Previous work has grown boron on Ag(111) substrates and reported the striped β12 and χ3 borophene phases, but more recent growth on a Cu(111) substrate by our team describes a more complex sheet structure with a much larger unit cell. We describe ab initio density functional theory simulations of borophene sheets on the Cu(111) surface to understand and explain this observed lattice structure. We discuss the bonding structure within the borophene sheet, paying special attention to electron doping via charge transfer from the Cu substrate. In addition, we compare our results to calculations of free-standing borophene sheets and discuss implications for practical growth of borophene on substrates. |
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