Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session M16: Focus Session: Designed Function in Reduced Dimensional Materials and Clusters |
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Sponsoring Units: DMP Chair: Raghunathan Ramakrishnan, University of Basel, Switzerland Room: 101AB |
Wednesday, March 4, 2015 11:15AM - 11:27AM |
M16.00001: Topologically nontrivial electronic bands and tunable Dirac cones in graphynes with spin-orbit coupling Vladimir Juricic, Guido van Miert, Cristiane Morais Smith Graphynes represent an emerging family of carbon allotropes that differ from graphene by the presence of the triple bonds (-C$\equiv$C-) in their band structure. They have recently attracted much interest due to the tunability of the Dirac cones in the band structure. I will show that the spin-orbit coupling in $\beta$-graphyne could produce various effects related to the topological properties of its electronic bands. Intrinsic spin-orbit coupling yields high- and tunable Chern-number bands, which may host both topological and Chern insulators, in the presence and absence of time-reversal symmetry, respectively. Furthermore, Rashba spin-orbit coupling can be used to control the position and the number of Dirac cones in the Brillouin zone [1]. Finally, I will also discuss the electronic properties of $\alpha-$ and $\gamma-$graphyne in the presence of the spin-orbit coupling within recently developed general theory of spin-orbit couplings in graphynes [2].\\[4pt] [1] G. van Miert, C. Morais Smith, and V. Juricic, Phys. Rev. B {\bf 90}, 081406(R) (2014).\\[0pt] [2] G. van Miert, V. Juricic, and C. Morais Smith, Phys. Rev. B {\bf 90}, 195414 (2014). [Preview Abstract] |
Wednesday, March 4, 2015 11:27AM - 11:39AM |
M16.00002: Chemical Trends for Transition Metal Compound Bonding to Graphene Bjoern Lange, Volker Blum Transition metal compounds are of interest as catalysts for the hydrogen evolution reaction (HER). However, a perfect candidate to replace expensive platinum has not yet been identified. To tailor a specific compound, several properties come into play. One is the bonding to the underlying substrate, for which $\pi$-bonded carbon nanostructures are promising candidates. Here we analyze the bonding of small transition metal compound nanoclusters to a graphene layer for a range of chemical compositions: M$_x$A$_y$ (M = Mo, Ti; A = S, O, B, N, C). The clusters are generated by an unbiased random search algorithm. We perform total energy calculations based on density functional theory to identify lowest energy clusters. We calculate binding energies using the PBE and HSE functionals with explicit van der Waals treatment[1] and benchmark those against RPA cluster calculations. Our results indicate that molybdenum-carbides and -nitrides tend to bond tightly to graphene. Mo-oxides and -sulfides show small binding energies, indicating van der Waals bonding.\\[4pt] [1] Tkatchenko, A., Scheffler, M., PRL \textbf{102}, 073005 (2009) [Preview Abstract] |
Wednesday, March 4, 2015 11:39AM - 11:51AM |
M16.00003: Density-Functional Theory Study of Nucleation and Growth of Metallic Nanoparticles on MoS2(001) Wissam A. Saidi The dispersion of metallic Pt nanoparticles (NPs) on MoS$_2$ monolayers is systematically analysed using first-principles density functional theory calculations. The nucleation of the NPs is followed step-by-step where we find that $n = 5$ is the cluster size where the growth of the NPs transforms from 2-dimensional (2D) to 3D. Owing to the topography of MoS$_2$(001), the 2D NPs mostly attach to the support via a direct bonding with Mo atoms that sit in the troughs of the surface, while the 3D NPs are bonded to the sulfur atoms that are more extended in the vacuum region. Furthermore, we find that Pt is sufficiently mobile on the surface where the number of hopping events per second is $\approx 10^3$~s$^{-1}$ along [$10\overline{1}$] and $\approx 10$~s$^{-1}$ along [$1\overline{1}0$] at room temperature. The somewhat large mobility suggests that monomer diffusion is not likely to be the rate-limiting step for Oswald ripening, and that Pt sputtering on MoS$_2$(001) will result in large particles rather than a fine dispersion. The existence of a fast diffusion channel along [$10\overline{1}$] suggests that the morphology of the NPs is anisotropic. [Preview Abstract] |
Wednesday, March 4, 2015 11:51AM - 12:03PM |
M16.00004: Li Storage Properties of Disordered Single- and Bi-Layer Graphene H. Yildirim, Alper Kinaci, Zhi-Jian Zhao, Maria Chan, Jeffrey P. Greeley Due to the limited capacity of the traditional intercalation-type graphite materials (373 mAh/g, LiC$_{6})$, much effort has been made to explore new anode materials to meet the increasing demand for batteries of high energy density. Among them, graphene has much attracted attention as an ideal platform for higher Li storage capacity, and for obtaining fundamental understanding of Li-C interaction. In this respect, we performed extensive first-principles calculations to model Li adsorption and intercalation in single- and bi-layer graphene, which are activated by defects for Li adsorption. For a wide range of Li coverages, the calculations predict that defect-free single layer graphene is not thermodynamically favorable compared to bulk metallic Li. However, graphene activated by defects are generally found to bind Li more strongly, and the interaction strength is sensitive to both the nature of defects and their densities. A rigorous thermodynamic analysis establishes the theoretical Li storage capacities of the defected graphene, and in some cases, these capacities are found to approach, although not exceed, those of bulk graphite. We will provide a performance comparison between defected single- and bi-layer graphene and bulk-graphite for Li storage capacities. A detailed analysis of the effect of the van der Walls (vdW) interactions will also be presented. [Preview Abstract] |
Wednesday, March 4, 2015 12:03PM - 12:15PM |
M16.00005: Structure and electronic properties of alkali and alkaline-earth metals on graphene Jian Zhou, Shunhong Zhang, Qian Wang, Qiang Sun, Purusottam Jena A thorough search of the monolayer structure of Li, Na, K, and Ca atoms on graphene, based on a synergistic combination of density functional theory and particle swarm optimization algorithm, yielded unusual deposition patterns. For Li atoms, we show that they prefer to cluster on graphene, irrespective of their concentration. We further show that an external electric field applied vertically to the graphene surface or doping with boron can prevent this clustering, leading to the homogeneous growth of Li. For larger atoms Na, K, and Ca, they distribute uniformly when their coverage ratio M:C is 1:6, but the Na and Ca atoms self-assemble to form parallel quasi-one-dimensional chains when their coverage is reduced to 1:8. Electron-phonon coupling calculations further show that the NaC$_{6}$ is a superconductor with critical temperature of 5.8 K. At low concentration (M:C $=$ 1:8) and depending on metal species, well-aligned atomic metal chains interact with graphene with varying intensity, making it possible to achieve either rigid or non-rigid band doping in graphene. [Preview Abstract] |
Wednesday, March 4, 2015 12:15PM - 12:27PM |
M16.00006: The effect of depolarization fields on the electronic properties of two-dimensional materials Young-Han Shin, Hye Jung Kim, Mohammad Noor-A-Alam Graphene is a two-dimensional semimetal with a zero band gap. By weakening the $sp^2$ covalent bonding of graphene with additional elements such as hydrogen or fluorine, however, it is possible to make it insulating. We can expect that the band gap converges to that of a three-dimensional analogue by repeating such two-dimensional layers along the normal to the layer. If we control the position of additional elements to make a dipole monolayer, the system will have an intrinsic internal field decreases as the number of layers increases. But, for two-dimensional bilayers, depolarization field is so strong that its electronic properties can be much different from its monolayer analogue. In this presentation, we show that the internal fields induced by dipole moments can change electronic properties of two-dimensional materials such as graphene-like structures and complex metal oxides. [Preview Abstract] |
Wednesday, March 4, 2015 12:27PM - 12:39PM |
M16.00007: First-principles studies of hematite nanoribbons Prabath Wanaguru, Jiao An, Qiming Zhang A study of two types of hematite nanoribbons, based on density functional theory has been performed. Geometry and magnetic order of these nanoribbons were optimized using DFT$+$U method implemented in VASP suite of software. It is found that the band gaps decreases from the value of $\sim$ 2.0eV to $\sim$ 1.7eV as the widths increase from 6{\AA} to more than 40{\AA}. Despite the bulk hematite is indirect in band gap nature, one type of nanoribbons show direct band gap nature in several widths. Cleaving energies are positive with respect to the hematite sheet and as width increases it is decreasing. Moving from smaller width to a larger width, nanoribbons were showing more of a sheet like character. Other properties of these nanoribbons will also be discussed. [Preview Abstract] |
Wednesday, March 4, 2015 12:39PM - 12:51PM |
M16.00008: Getting the Most from 2-D Materials: the Role of Device Dimensionality Huashan Li, David Strubbe, Jeffrey Grossman While the control of material dimensionality has been widely used as an important design means, the device dimensionality, which relates to the regularity of the material ensemble rather than the material itself, has received far less attention. Recently, both vertical [1,2] and lateral [2] heterojunctions based on 2-D materials have been successfully synthesized, which provides an unprecedented opportunity to renew our understanding of the concept of ``dimensionality.'' In this study, we propose a design strategy of controlling device dimensionality by computationally investigating a ``1.5-dimensional'' solar cell device made of a 2-dimensional graphene based material. According to the predicted optical properties and charge dynamics, this prototype system has the potential to achieve desirable characteristics of robustness against defects, efficient polaron pair dissociation, broad tunability with surface functionalization and the possibility to form tandem cells. In addition, the optimization of correlated light harvesting procedures simultaneously becomes attainable in such 1.5-d solar cell due to the extra degree of freedom to manage the flux of mass and energy.\\[4pt] [1] Geim, A. et al., Nature, 2013, 499, 419-425\\[0pt] [2] Gong, Y. et al., Nat. Mater. 2014 [Preview Abstract] |
Wednesday, March 4, 2015 12:51PM - 1:03PM |
M16.00009: Structure Matters More than Size: Tuning the Electronic Properties of (TiO$_{2})_{\mathrm{n}}$ Clusters Noa Marom, Saswata Bhattacharya, Luca Ghiringhelli To design (TiO$_{2})_{\mathrm{n}}$ clusters with desired properties we implemented a suite of three genetic algorithms (GA) tailored to optimize for low total energy (EGA), high vertical electron affinity (VEA-GA), and low vertical ionization potential (VIP-GA). The property-based GAs are an extension of the cascade GA reported in [1]. Analysis of the structures found by the VEA-GA and the VIP-GA vs. the EGA reveals structure-property relations. A high VEA is correlated with the presence of several dangling-O atoms (typically 3-4), rather than the previously suggested tri-coordinated Ti atom [2]. A low VIP is correlated with low bond connectivity (typically 2) between two dangling-O atoms. We show that the electronic properties of (TiO$_{2})_{\mathrm{n}}$ clusters with n up to 20 correlate more strongly with the presence of these structural features than with size. We further suggest that the presence of dangling-O atoms on TiO$_{2}$ clusters or surfaces may be associated with enhanced catalytic activity and that these O atoms may serve as active sites. The process of optimization for a target property reveals the underlying structure-property relations and the structural features that may serve as active sites for catalysis. This generally applicable approach may provide valuable physical insight and design rules for better nanocatalysts.\\[4pt] [1] S. Bhattacharya \textit{et al.} PRL 111, 135501 (2013); N. J. Phys, in press (2014)\\[0pt] [2] N. Marom \textit{et al.} PRL 108, 106801 (2012). [Preview Abstract] |
Wednesday, March 4, 2015 1:03PM - 1:15PM |
M16.00010: Electronic and optical properties of ultrathin silicon nanomembranes: A first-principles investigation Woosun Jang, Su-Hyun Yoo, Aloysius Soon Owing to its unique and exotic physical and chemical properties, there has been a lot of effort undertaken to explore and study ultrathin low-dimensional nanostructures (e.g. graphene and MoS2). Of late, two-dimensional (2D) nanomembranes of silicon - a well-known prototypical bulk semiconductor - have attracted much attention, and has found its potential in niche nanodevice applications e.g. field effect transistors (FET) and secondary battery anodes. In this work, after considering various nanomembranes of Si with varying thicknesses, we study geometric and electronic structures using first-principles density-functional theory calculations (and beyond). Here, we consider both bulk-terminated pristine Si nanomembranes as well as surface-reconstructed ones, as motivated by available experimental and theoretical reports. To understand the influence of growth conditions on these Si nanomembranes, we have also studied the role of surface-passivation (e.g. with O, H, and OH) on their electronic and optical properties. Namely, we carefully investigate their thickness-dependent electronic band structure (i.e. both their fundamental and optical band gap energies), so as to elucidate their intrinsic structure-property relations for designing future technologically important nanodevices. [Preview Abstract] |
Wednesday, March 4, 2015 1:15PM - 1:27PM |
M16.00011: What is the work function of a small nanocrystal? Lingyuan Gao, Jaime Souto, Alex Demkov, James Chelikowsky The work function is defined as the difference between the electrostatic potential energy (-e$\phi )$ of an electron in the vacuum near the metal surface and the metal's Fermi energy. For a single crystal metal, the measured work function typically depends on the orientation of the metal surface. This seems counterintuitive, as the Fermi energy is the same across the metal sample, and the vacuum energy is also expected not to depend on the direction. The problem becomes even more interesting for a metallic nanocrystal, where facets of different orientation meet. We investigate this problem using the real space first-principles method PARSEC and consider aluminum nanocrystals as a test system. The real space nature of the code doesn't require periodic boundary conditions and enables calculations of nanocrystals with realistic dimensions. We compare our nanocrystal results for (001), (110) and (111) Al surfaces with those obtained from standard slab calculations and available photoemission and electrical data. We acknowledge supports from SciDAC program, Department of Energy, Office of Science, Advanced Scientific Computing Research and Basic Energy Sciences. This work is supported through grant DE-SC0008877 [Preview Abstract] |
Wednesday, March 4, 2015 1:27PM - 1:39PM |
M16.00012: Effects of Interaction Range and Strength on the Phase Behavior of Small Clusters of Colloidal Particles Ray Sehgal, Dimitrios Maroudas We report the findings of a computational study of the phase behavior of thermodynamically small assemblies (clusters) of colloidal particles interacting via a potential that includes electrostatic repulsion and depletion-based variable-ranged attraction. We applied the data mining technique of diffusion mapping to determine the dimensionality of an appropriate coarse-grained description of the phase behavior and to assess the suitability of chosen order parameters. The results of this technique indicate that two coarse variables, which represent metrics of assembly density and crystallinity, are required to describe the phase behavior of these colloidal assemblies. We generate free-energy landscapes (FELs) in this well-justified coarse-variable space using Monte Carlo umbrella sampling. We constructed these FELs over a range of interparticle interaction strength and range and obtained a comprehensive picture regarding the possible stable configurations of such colloidal assemblies at equilibrium and the phase changes observed between them. In particular, we observe an order-to-disorder transition between crystalline and fluid-like phases as well as a polymorphic transition between relaxed face-centered cubic and hexagonal close-packed structures. [Preview Abstract] |
Wednesday, March 4, 2015 1:39PM - 1:51PM |
M16.00013: Stabilization of Small Boron Cage by Transition Metal Encapsulation Lijun Zhang, Jian Lv, Yanchao Wang, Yanming Ma The discovery of chemically stable fullerene-like structures formed by elements other than carbon has been long-standing desired. On this aspect significant efforts have centered around boron, only one electron deficient compared with carbon. However, during the past decade a large number of experimental and theoretical studies have established that small boron clusters are either planar/quasi-planar or forming double-ring tubular structures. Until recently, two all-boron fullerenes have been independently discovered: B$_{38}$ proposed by our structure searching calculations[1] and B$_{40}$ observed in a joint experimental and theoretical study. Here we extend our work to the even smaller boron clusters and propose an effective routine to stabilize them by transition metal encapsulation. By combining swarm-intelligence structure searching and first-principles calculations, we have systematically investigated the energy landscapes of transition-metal-doped MB$_{24}$ clusters (M = Ti, Zr, Hf, Cr, Mo, W, Fe, Ru and Os). Two stable symmetric endohedral boron cages, MoB$_{24}$ and WB$_{24}$ are identified. The stability of them can be rationalized in terms of their unique 18-electron closed-shell electronic structures. [1] J. Lv, Y. Wang, L. Zhu, and Y. Ma, Nanoscale 6, 11692 (2014). [Preview Abstract] |
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