Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session Y32: Computational Discovery and Design of Novel Materials XIII |
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Sponsoring Units: DMP DCOMP Chair: Darshana Wickramaratne, University of California, Santa Barbara Room: 295 |
Friday, March 17, 2017 11:15AM - 11:27AM |
Y32.00001: Improving thermal conductivities of nanotube composites: by end functionalization Timothy Burt, Matthew Houck, Kieran Mullen, Daniel Glatzhofer Several computational models of heat transport within functionalized/non-functionalized carbon nanotube-polymer composites were developed. On-lattice random walk simulations were used to predict the effect of interfacial or Kapitza resistance on the heat flow for different orientations of the SWCNTs which were randomly dispersed throughout the polymer. Functionalization, in the sense of adding certain molecules to the ends of a SWCNT which lowers the thermal boundary resistance, increases the thermal conductivity k in the simulations as expected. From our results we find that k $\propto$ N, where N is the number of tubes, for nearly all physically realizable simulations up to $\approx$ 20\% volume fraction of SWCNTs with no apparent phase transitions. The effect of changing the aspect ratio of the tubes (L/D) is presented. Results will be compared with measurements of carbon nanotubes functionalized with a variety of polymers and functionalization groups, notably PMMA and amine-terminated SWCNT. Electrophoresis will be used to orient the tubes in a particular direction and k will be directly measured. [Preview Abstract] |
Friday, March 17, 2017 11:27AM - 11:39AM |
Y32.00002: Carbon nanotube conditioning: ab initio simulations of the effect of defects and doping on the electronic properties of carbon nanotube systems. Matias Soto, Enrique Barrera Using carbon nanotubes for electrical conduction applications at the macroscale has proven to be a difficult task, mainly, due to defects and impurities present, and lack of uniform electronic properties in synthesized carbon nanotube bundles. Some researchers have suggested that growing only metallic armchair nanotubes and arranging them with an ideal contact length could lead to the ultimate electrical conductivity; however, such recipe presents too high of a cost to pay. A different route and the topic of this work is to learn to manage the defects, impurities, and the electronic properties of carbon nanotubes present, so that the electrical conduction of a bundle or even wire may be enhanced. We used density functional theory calculations to study the effect of defects and doping on the electronic structure of metallic, semi-metal and semiconducting carbon nanotubes in order to gain a clear picture of their properties. Additionally, using dopants to increase the conductance across a junction between two carbon nanotubes was studied for different configurations. Finally, interaction potentials obtained via first-principles calculations were generalized by developing mathematical models for the purpose of running simulations at a larger length scale using molecular dynamics. [Preview Abstract] |
Friday, March 17, 2017 11:39AM - 11:51AM |
Y32.00003: Development of a multi-space constrained density functional theory approach and its application to graphene-based vertical transistors Han Seul Kim, Yong-Hoon Kim We have been developing a multi-space-constrained density functional theory approach for the first-principles calculations of nano-scale junctions subjected to non-equilibrium conditions and charge transport through them [1,2]. In this presentation, we apply the method to vertically-stacked graphene/hexagonal boron nitride (hBN)/graphene Van der Waals heterostructures in the context of tunneling transistor applications. Bias-dependent changes in energy level alignment, wavefunction hybridization, and current are extracted. In particular, we compare quantum transport properties of single-layer (graphene) and infinite (graphite) electrode limits on the same ground, which is not possible within the traditional non-equilibrium Green function formalism. The effects of point defects within hBN on the current-voltage characteristics will be also discussed. [1] Kim, H. S. and Kim, Y.-H. Bulletin of the American Physical Society \textbf{60}, BAPS.2015.MAR.T23.15. [2] Kim, H. S. and Kim, Y.-H. Bulletin of the American Physical Society \textbf{61}, BAPS.2016.MAR.K31.5. [Preview Abstract] |
Friday, March 17, 2017 11:51AM - 12:03PM |
Y32.00004: Theoretical and Experimental Studies of Functionalized Carbon Nanotubes for Improved Thermal Conductivity Alexander Kerr, Timothy Burt, Kieran Mullen, Daniel Glatzhofer, Matthew Houck, Paul Huang The use of carbon nanotubes (CNTs) to improve the thermal conductivity of composite materials is thwarted by their large thermal boundary resistance. We study how to overcome this Kapitza resistance by functionalizing CNTs with mixed molecular chains. Certain configurations of chains improve the transmission of thermal vibrations through our systems by decreasing phonon mismatch between the CNTs and their surrounding matrix. Through the calculation of vibrational normal modes and Green's functions, we develop a variety of computational metrics to compare the thermal conductivity ($\kappa$) of our systems. We show how different configurations of attached chains affect the samples' $\kappa$ values by varying chain identity, chain length, number of chains, and heat driver behavior. We vary the parameters to maximize $\kappa$. To validate and optimize these metrics, we perform molecular dynamics simulations for comparison. We also present experimental results of composites enhanced with CNTs and make comparisons to the theory. We observe that some composites are thermally improved with the inclusion of CNTs, while others are scarcely changed, in agreement with theoretical models. [Preview Abstract] |
Friday, March 17, 2017 12:03PM - 12:15PM |
Y32.00005: Hepta-graphene: tunable band gap in a graphitic layer Alejandro Lopez-Bezanilla, Ivar Martin, Peter Littlewood Creating an electronic band gap in graphene has been a major challenge limiting its applications. We provide theoretical evidence that chemical modification of graphene can transform its unit cell from hexagonal to rectangular, and lead to band gap opening upon additional application of shear stress. The new material is called hepta-graphene, an elastic interlock of 7-membered C rings in a rectangular lattice and dynamical stability. [Scientific Reports 6, 33220 (2016)] [Preview Abstract] |
Friday, March 17, 2017 12:15PM - 12:27PM |
Y32.00006: A first step towards Understanding CZTS $\Sigma $3 (112) family grain boundaries Man Hoi Wong, Kin Fai Tse, Junyi Zhu CZTS is a promising low cost, earth abundant solar cell absorber material for thin film solar cell. Grain boundary properties are crucial in the performance enhancement of the solar cell device, yet not well studied. This work is dedicated to the characterisation of $\Sigma $3 grain boundaries with and interfaces, which are also found to be abundant experimentally. DFT calculations with GGA exchange functional is used to calculate the structure of those grain boundaries. Using modified wedge method, formation energies of different reconstructed grain boundary and relevant surfaces were calculated from various slabs. According to our calculations, we found a repulsive grain boundary based on anion-anion terminations with low formation energy. Such problematic grain boundaries may widely distributed in the device and hinder the performance. In addition to the structure studies, Electronic structures of low energy grain boundaries were calculated to understand their impact on device efficiency. Our results match very well with observations from previous experimental works. It provided a solid foundation for future studies of grain boundary engineering, which could be crucial to the success of CZTS. [Preview Abstract] |
Friday, March 17, 2017 12:27PM - 12:39PM |
Y32.00007: Investigating Cu Grain Boundary Oxidation via Density Functional Theory, Reactive Force Fields, and Experiment Matthew Curnan, Wissam Saidi, Christopher Andolina, Judith Yang The presence of grain boundaries in metals is expected to significantly alter metal oxidation behavior due to O diffusion and segregation towards GBs, leading to distinct preferential Cu oxide nucleation behavior on GBs under different thermodynamic conditions. A comprehensive understanding of which O diffusion and oxide nucleation processes occur under particular thermodynamic conditions and over broad sets of GBs is not available. To resolve relationships between stable structures and O diffusion energetics, we develop a methodology based on a Cu/O reactive force field (RFF) trained with Density Functional Theory (DFT) and experimental data. Using this DFT-RFF approach, we determine the O diffusion activation barriers, segregation energetics, and relative GB energy vs. tilt angle profiles of different Cu-O GB systems as functions of temperature. Given that the relative energetics of different GB tilt angles can be accurately predicted and tuned with temperature, the inclusion of these energetics in Cu/O RFFs can help predict experimentally observed GB angle distributions at particular temperatures. When trained with experimental and DFT energetics and structures, these RFFs can be expanded to study the tuning of GB properties for Cu/O and related doped systems. [Preview Abstract] |
Friday, March 17, 2017 12:39PM - 12:51PM |
Y32.00008: Screw-Dislocated Level Structure of Graphene Potential Wells Daniel Walkup, Joseph Stroscio We investigate the linearized graphene Hamiltonian with a radially-symmetric power-law potential in the two-dimensional parameter space of mass and magnetic field. At zero mass the quasibound eigen-resonances, denoted \textbar n,m\textgreater (where n and m are the radial and azimuthal quantum numbers) form distinct ladders for each m, and each such ladder has the property that for massless particles the energies and eigenstates are discontinuous at a critical field B$_{c}$. Turning on a mass bridges this discontinuity, but negative and positive masses connect different eigenstates, producing a screw dislocation in the eigenstate spectrum. We numerically propagate the wavepacket of an \textbar n,m\textgreater eigenstate in a slowly-evolving Hamiltonian whose path encloses B$_{c}$, and verify that a closed adiabatic loop conveys the particle to the \textbar n$+$/-1,m\textgreater state, demonstrating the existence of a screw dislocation in the spectrum. We explain the dislocation in terms of the Berry phase acquired while encircling the critical magnetic field B$_{c}$. [Preview Abstract] |
Friday, March 17, 2017 12:51PM - 1:03PM |
Y32.00009: Band Structure and Topological Properties of Graphene in a Superlattice Spin Exchange Field H.A. Fertig, Luis Brey, A.R. Carvalho We analyze the energy spectrum of graphene in the presence of spin-orbit coupling and a unidirectionally periodic Zeeman field, focusing on the stability and location of Dirac points it may support. It is found that the Dirac points at the $K$ and $K'$ points are generically moved to other locations in the Brillouin zone, but that they remain present when the Zeeman field $\vec{\Delta}(x)$ integrates to zero within a unit cell. A large variety of locations for the Dirac points is shown to be possible: when $\vec\Delta \parallel \hat{z}$ they are shifted from their original locations along the direction perpendicular to the superlattice axis, while realizations of $\vec\Delta(x)$ that rotate periodically move the Dirac points to locations that can reflect the orbit of the rotating electron spin as it moves through a unit cell. When a uniform Zeeman field is applied in addition to a periodic $\vec\Delta \parallel \hat{z}$ integrating to zero, the system can be brought into a metallic, Dirac semimetal, or insulating state, depending on the direction of the uniform field. The latter is shown to be an anomalous quantum Hall insulator. [Preview Abstract] |
Friday, March 17, 2017 1:03PM - 1:15PM |
Y32.00010: Structure prediction of Cu-Pt nanocatalysts using a cluster expansion Chenyang Li, Tim Mueller Electrochemical reduction of CO/CO$_2$ to hydrocarbon fuels can lead to a more sustainable fuel cycle. Copper and its alloys have been shown to have relatively high catalytic activities for CO/CO$_2$ reduction. To better understand the atomic structure and properties of Cu nanoalloys, we have generated a cluster expansion for Cu-Pt nanoparticles that enables us to rapidly predict nanoparticle energies as a function of composition, particle size and temperature. To validate and refine this approach, we have compared the predictions made by the cluster expansion to experimental characterization of Cu-Pt nanoparticles. We present a comparison of the computational and experimental results for properties including the lattice parameter, shape, and surface composition. [Preview Abstract] |
Friday, March 17, 2017 1:15PM - 1:27PM |
Y32.00011: Structural evolution, growth and stability of metal titanium clusters. Hasani Chauke, Tshegofatso Phaahla, Phuti Ngoepe, Richard Catlow The transition metals clusters such as titanium have received a significant attention due to their excellent physical and chemical properties and great technological application in many fields. A survey of small Ti clusters was performed using interatomic potentials and computational methods based on density functional theory; and the knowledge led master code with a genetic algorithm to generate the lowest energy geometries of Tin (n $=$ 2-32) clusters. The all electron spin-unpolarized generalized gradient approximation is used to determine the ground state structures, binding energy and electronic properties. The structural evolution of titanium clusters, which favors the icosahedron structure growth pattern is observed. The energy for the ground state configurations is found to increase monotonically with the clusters size. Their relative stability results predict clusters with 5 and 7 as more stable. The energy difference for clusters n$\ge $24 is very small, suggesting that the larger clusters could be stable at moderate temperatures. In addition to the magic numbers that are often reported i.e. Ti7 and Ti13; clusters 5, 9, 14, 17 and 26 have extra stability. [Preview Abstract] |
Friday, March 17, 2017 1:27PM - 1:39PM |
Y32.00012: Interplay between chemical and electronic properties of native oxides at photoelectrochemical interfaces Tuan Anh Pham, Xueqiang Zhang, Brandon Wood, Sylwia Ptasinska, Tadashi Ogitsu Native surface oxidation of photoelectrodes in aqueous solutions is known to play a critical role in the performance and stability of photoelectrochemical cells for solar water splitting; however, probing atomistic details of such process under operating conditions presents significant challenges. Here, we use high-level first-principles simulations of GaP/InP photoelectrodes in contact with water to accurately predict the influence of surface oxide species on the semiconductor band edges. Theoretical results were then combined with band edge measurements to predict the surface chemistry under realistic environments, suggesting that dissociated and non-dissociated adsorbed water coexist on the GaP/InP surfaces, as well as a more rigid hydrogen-bond network at the GaP-water interface. Ambient pressure X-ray photoelectron experiments validate the theoretical predictions, while providing detailed information of surface oxide species. [Preview Abstract] |
Friday, March 17, 2017 1:39PM - 1:51PM |
Y32.00013: Tuning the structural and electronic properties of heterogeneous chalcogenide nanostructures Federico Giberti, Marton Voros, Giulia Galli Heterogeneous nanostructures, such as quantum dots (QDs) embedded in solid matrices, are promising platforms for solar energy conversion. Unfortunately, there is scarce information on the structure of the interface between the dots and their embedding matrix, thus hampering the design of functional materials with desired optoelectronic properties. Here, we developed a hierarchical computational strategy to obtain realistic models of semiconductor QDs embedded in matrices using enhanced sampling classical molecular dynamics simulations and predicted their electronic structure using first-principles electronic structure methods. We investigated PbSe/CdSe systems which are promising materials for solar cell applications and found a favorable quasi-type-II band alignments both for PbSe QDs in CdSe matrices and for CdSe embedded in PbSe. However, in the former case, we found the presence of detrimental intra-gap states, while in the latter no defect states are present. Hence we predict that embedding CdSe in PbSe leads to a more efficient platform for solar energy conversion. In addition, we showed that the structure of CdSe QD and in turn its band gap might be tuned by applying pressure to the PbSe matrix, providing a way to engineer the properties of new functional materials. [Preview Abstract] |
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