Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session Y36: van der Waals effects in low dimensional systemsFocus

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Sponsoring Units: DMP Chair: Alexander Balandin, University of California, Riverside Room: LACC 410 
Friday, March 9, 2018 11:15AM  11:27AM 
Y36.00001: Firstprinciples studies of earlystage CO_{2} capture in diamineappended metal organic frameworks JungHoon Lee, Jeffrey Neaton mmenM_{2}(dobpdc) (M = Mg, Mn, Fe, Co, Ni, Zn) metalorganic frameworks (MOFs) are phasechange adsorbents with significant potential for CO_{2} capture due to their high working capacities and strong selectivity resulting from a novel cooperative adsorption mechanism. Despite prior work on this system, our understanding of the early stages of CO_{2} capture in these MOFs is still lacking. Herein, we investigate CO_{2} capture in mmenM_{2}(dobpdc) MOFs with firstprinciples density functional theory (DFT) and abinitio molecular dynamics (MD) calculations. Our van der Waalscorrected DFT calculations show that CO_{2} prefers to bond to the free N ion of the mmen ligand (in a socalled ‘tail’ geometry) instead of the metalbonded N ion (or ‘head’ geometry). We find that tail geometry is more stable then head geometry by about 100 kJ/mol and even more stable than a previouslyproposed earlystage CO_{2} capture geometry (the socalled ‘pair’ geometry). In addition, our abinitio MD calculations show that the tail geometry can undergo a transition to a chain geometry, the known ground state structure above a threshold CO_{2} partial pressure. 
Friday, March 9, 2018 11:27AM  11:39AM 
Y36.00002: THz Spectroscopy of 2,4,6trinitrotoluene Molecular Solids from First Principles Ido Azuri, Anna Hirsch, Anthony M. Reilly, Alexandre Tkatchenko, Shai Kendler, Oded Hod, Leeor Kronik We present a computational analysis of the THz spectrum of the monoclinic and orthorhombic polymorphs of 2,4,6trinitrotoluene. Very good agreement with experimental data is found when using density functional theory that includes TkatchenkoScheffler pairwise dispersion interactions. Furthermore, we show that for these polymorphs the theoretical results are only weakly affected by manybody dispersion contributions. Absence of dispersion interactions, however, causes sizable shifts in vibrational frequencies and directly affects the spatial character of the vibrational modes. Mode assignment allows a distinction between contributions of the monoclinic and orthorhombic polymorphs and shows that modes in the range of 03.3 THz are comprised of both inter and intramolecular vibrations, with the former dominating below 1.5 THz. Intramolecular contributions primarily involve the nitro group. Finally, we present a prediction for the THz spectrum of 1,3,5trinitrobenzene, showing that a modest chemical change leads to a markedly different THz spectrum. 
Friday, March 9, 2018 11:39AM  11:51AM 
Y36.00003: Importance of van der Waals correction for energetics in transition metal monoxides Haowei Peng, John Perdew Density functional theory (DFT) is in principle exact for the groundstate properties of all materials, even for the strongly correlated materials such as the transition metal monoxides. However, DFT with various approximations to the exchangecorrelation predict the zincblende or wurtzite structure ground state for MnO, instead of the experimental rocksalt phase. The correct groundstate phase has been obtained only by highlevel random phase approximation and diffusion Monte Carlo (DMC). Here we propose and test for MnO, FeO, CoO, and NiO that a semilocal density functional can solve this problem by properly including both selfinteraction and van der Waals corrections. The importance of the latter was previously unanticipated. The MnO structural energy difference from SCAN+rVV10+U agrees very well with that from DMC (SCAN is the Strongly Constrained and Appropriately Normed metaGGA, rVV10 is the revised Vydrovvan Voohris nonlocal correlation functional, and the onsite U is determined from linear response). 
Friday, March 9, 2018 11:51AM  12:27PM 
Y36.00004: van der Waals interactions in lowdimensional nanostructures (2D, 1D, 0D) and layered solids Invited Speaker: John Dobson This talk will discuss the influence of dimensionality, size, and electronic energy gap on the dispersion interaction. The ability of various physical and chemical computational schemes to deal with this physics will be considered. 
Friday, March 9, 2018 12:27PM  12:39PM 
Y36.00005: Layer dependent electronic and optical properties of Silicane, Germanane and Stanane. Jose Eduardo Padilha, Rosana Rabelo Mançano, Luiz Cótica, Rafael Da Silva, Roberto Miwa, Renato Pontes The successful isolation of graphene by Geim and Novoselov, totally changed our minds. Folowed by graphene, new 2D structures were studied e.g. hexagonal boron nitride (hBN), transition metal dichalcogenides (TMDC), black and blue phosphorus, among others. Looking into the periodic table, in the same column of carbon, we can find silicon, germanium and tin. Unlike carbon, these other groupIV elements do not have a layered structure similar to graphite. Their monolayer structures are obtained if grown on metallic substrates. Recently, the successful manufacture of fully hydrogenated germanene, the hydrogenation of silicene on Ag(111) and formation of halfsilicane were reported. Such achievements bring new opportunities for the applications of 2D structures of group IV in nanoelectronics. In this work, by means of the stateoftheart in first principles calculations, we study the electronic and optical properties of hydrogenated group IV  Silicane, Germanane and Stanane. We show that it is possible to control the energy gap of the systems through the number of layers. Moreover, we accurately determined the alignment of the energy levels (band offsets) for all the structures. 
Friday, March 9, 2018 12:39PM  12:51PM 
Y36.00006: Effect of Point Defects on Optical Properties of Graphene Fluoride: A FirstPrinciples Study LingYi Huang, Xu Zhang, Mingliang Zhang, Gang Lu The experimental optical gap of graphene fluoride has been measured between 3.1 and 3.8 eV, which is much smaller than the corresponding theoretical predictions (∼5.1 − 5.7 eV). To resolve this discrepancy, we have examined optical properties of graphene fluoride with several possible point defects (vacancies and substitution atoms). We employ a firstprinciples method for largescale calculations of electronic excitations in solids based on timedependent density functional theory (TDDFT) with optimally tuned and rangeseparated hybrid (OTRSH) functionals. The firstorder perturbation theory is applied to the solution of the OTRSH functional Hamiltonian, from which the single and twoparticle excitation energies can be calculated. The method is validated for lithium fluoride, graphene fluoride, and phosphorene with excellent agreement to previous computational and experimental results. We reveal that the optoelectronic properties of graphene fluoride can be influenced profoundly by a small amount of fluorine vacancies and exciton binding energy in graphene fluoride can be doubled by a small concentration of oxygen substitutional defects. These point defects are believed to be responsible for the discrepancy in the optical gaps between the theory and experiments. 
Friday, March 9, 2018 12:51PM  1:03PM 
Y36.00007: Modeling NonReactive MoleculeSurface Systems on Experimentally Relevant Time and Length Scales: Dynamics and Conductance of Polyfluorene on Au(111) Zhi Li, Alexandre Tkatchenko, Ignacio Franco We propose a computationally efficient strategy to accurately model nonreactive moleculesurface interactions that adapts densityfunctional theory calculations with the TkatchenkoScheffler scheme for van der Waals interactions into a simple classical force field. The resulting force field requires just two adjustable parameters per atom type that are needed to capture shortrange {and polarization} interactions. The developed strategy allows for classical molecular dynamics simulation of molecules on surfaces with the accuracy of highlevel electronic structure methods but for system sizes (10^{3}10^{7} atoms) and timescales (~ microseconds) that go well beyond what can be achieved with firstprinciples methods. Parameters for H, sp^{2} C and O on Au(111) are developed and employed to atomistically model experiments that measure the conductance of a single polyfluorene on Au(111) as a continuous function of its length. The simulations capture both the gross and fine features of the observed conductance decay during junction elongation, and lead to a revised atomistic understanding of the experiments. 
Friday, March 9, 2018 1:03PM  1:15PM 
Y36.00008: Ab initio study of ptype origin of SnSe and the alloying behavior of SnSe_{1x}S_{x} Jaekwang Lee Recently, SnSe single crystal has been found to exhibit excellent thermoelectric performance with an extremely high ZTvalue of 2.6. Although this high ZT value has attracted considerable attention, the microscopic origin of ptype character of SnSe has yet to be clearly understood. Here, by combining scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, we find that the most dominant Sn vacancies [1] move the Fermi energy inside the dispersive valence band and produce extra holes throughout the system. On the other hand, other intrinsic vacancies create a nondispersive donor level and generate immobile electrons localized near the vacancy site. In addition, we first directly probed the microscopic structures of SnSe_{1x}S_{x} alloys and their structural evolution at the atomic level by combining STM experiments with DFT calculations [2]. [1] Appl. Phys. Lett., 110, 262106 (2017). [2] Phys. Chem. Chem. Phys., 19, 21648 (2017). 
Friday, March 9, 2018 1:15PM  1:27PM 
Y36.00009: Quantitative agreement between electronoptical phase images of WSe2 and simulations based on electrostatic potentials that include bonding effects Matthieu Verstraete, Sven Borghardt, Zeila Zanolli, Florian Winkler, juri barthel, Amir Hossein Tavabi, R. E. DuninBorkowski, Beata Kardynal The quantitative analysis of electronoptical phase images recorded using offaxis electron holog raphy often relies on the use of computer simulations of electron propagation through a sample. However, simulations that make use of the independent atom approximation are known to over estimate experimental phase shifts by approximately 10%, as they neglect bonding effects. Here, we compare experimental and simulated phase images for fewlayer WSe2. We show that a com bination of pseudopotentials and allelectron density functional theory calculations can be used to obtain accurate mean electron phases, as well as improved atomicresolution spatial distribution of the electron phase. The comparison demonstrates a perfect contrast match between experimental and simulated atomicresolution phase images for a sample of precisely known thickness. The low computational cost of this approach makes it suitable for the analysis of large electronic systems, including defects, substitutional atoms and material interfaces. 
Friday, March 9, 2018 1:27PM  1:39PM 
Y36.00010: Abinitio Study of Polymorphic Phase Transitions of Two Dimensional MoS_{2} Sera Jeon, Jaekwang Lee MoS_{2} has been widely studied in recent times because of its intriguing electrical and optical properties and much functionality with other materials. In particular, one of the most interesting properties is that MoS_{2} can exist in different structural forms with distinct electronic properties from semiconducting 1HMoS_{2}, T’’MoS_{2}, to metallic 1TMoS_{2}, 1T’MoS_{2} and 1T’’MoS_{2} phases. 
Friday, March 9, 2018 1:39PM  1:51PM 
Y36.00011: Stacking Effects on Graphene and Dichalcogenide Hybrid Structures Abdulrhman Alsharari, Mahmoud Asmar, Sergio Ulloa Topological properties of heterostructures of graphene (G) and transition metal dichalcogenides (TMDs) have been the center of recent interest [1]. Here we study bilayers and trilayers of G and TMD and show that stacking and ordering is crucial to the resulting band structure of the system and its consequent topology. To analyze the topological properties of these systems we adopt a tight binding formalism that allows a complete topological characterization, including Chern numbers and Z_{2} index, and helps to explicitly confirm the existence of edge states in these systems. A GTMD heterostructure displays a phase transition that allows for the appearance of gapless edge states [1]. However, a Bernal stacked bilayer graphene deposited on a TMD monolayer is found to not host gapless edge states. We show that the crucial factor determining the appearance of edge states in such systems is the relative strength of the diagonal Zeemanlike spinorbit term inherited from the TMD and the staggered potential on graphene generated also by proximity. 
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