Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session R1: Van der Waals Bonding in Advanced Materials IV |
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Sponsoring Units: DMP DCOMP Chair: Alexandre Tkatchenko, Fritz Haber Institute Room: 260 |
Thursday, March 16, 2017 8:00AM - 8:12AM |
R1.00001: Decomposition mechanisms in metal borohydrides and their ammoniates Evan Welchman, Timo Thonhauser We present results from \textit{ab initio} calculations to explain the mechanisms that cause ammoniation to lower decomposition temperatures in metal borohydrides ($\mathcal{M}$(BH$_4$)$_x$, MBs) with $\mathcal{M}$ of low electronegativity ($\chi_p < 1.6$), but raise decomposition temperatures of high-$\chi_p$ MBs ($\chi_p > 1.6$). Results include an examination of materials' dihydrogen bond networks, energy of formation calculations, and transition state searches for potential decomposition mechanisms. We find that ammoniation always causes thermodynamic destabilization, helping to explain lower decomposition temperatures for low-$\chi_p$ MBs. For high-$\chi_p$ MBs, we find that ammoniation blocks B$_2$H$_6$ formation, the preferred decomposition mechanism in these MBs. We describe two ways that B$_2$H$_6$ formation can be blocked, which are each valid in one of the ammoniated MBs ($\mathcal{M}$(BH$_4$)$_x\cdot$$y$NH$_3$, MBAs) studied. We also consider why these materials release either H$_2$ or NH$_3$ gas upon decomposition. We find that NH$_3$ is much more strongly coordinated with higher-$\chi_p$ metals, and that direct H$_2$ formation becomes easier in higher-$\chi_p$ materials, contributing to why higher-$\chi_p$ MBAs are more likely to produce H$_2$ upon decomposition. [Preview Abstract] |
Thursday, March 16, 2017 8:12AM - 8:24AM |
R1.00002: H4-alkanes: A new class of hydrogen storage material? David Harrison, Evan Welchman, Timo Thonhauser The methane-based material (H$_2$)$_4$CH$_4$, also called H4M for short, is in essence a methane molecule with 4 physisorbed H$_2$ molecules. While H4M has exceptionally high hydrogen storage densities when it forms a molecular solid, unfortunately, this solid is only stable at impractically high pressures and/or low temperatures. To overcome this limitation, we show through simulations that longer alkanes (methane is the shortest alkane) also form stable structures that still physisorb 4 H$_2$ molecules per carbon atom; we call those structures H4-alkanes. We further show via molecular dynamics simulations that the stability field of molecular solids formed from H4-alkanes increases remarkably with chain length compared to H4M, just as it does for regular alkanes. From our simulations of H4-alkanes with lengths 1, 4, 10, and 20, we see that e.g.\ for the 20-chain the stability field is doubled at higher pressures. While even longer chains show only insignificant improvements, we discuss various other options to stabilize H4-alkanes more. Our proof-of-principle results lay the groundwork to show that H4-alkanes can become viable hydrogen storage materials. [Preview Abstract] |
Thursday, March 16, 2017 8:24AM - 8:36AM |
R1.00003: First-principles study of van der Waals driven molecular sponges as carriers for small molecules Majid Mortazavi, Alexandre Tkatchenko Molecular sponges are crystal materials with molecular building blocks glued together by van der Waals (vdW) interactions acting as hosts to incorporate guests within their pores. A recent example is Tetrakis(dimethloxyphenyl)adamantane (TDA) with branched adamantane scaffolds demonstrating a remarkable ability to capture/release a wide range of small molecules [1]. While various TDA-based inclusion complexes have been synthesized, the underlying principles of formation of these complexes remain puzzling [1]. Using vdW-inclusive DFT calculations we study the formation of inclusion complexes with several small molecules via full structural relaxation and stability analysis, and subsequently proposing several possible formation mechanisms for the inclusion compounds. Based on formation energies we predict a selective molecular capture depending on the class of TDA hosts. The selective response is closely correlated to the structural features of TDA crystals, such as lattice energies and pore sizes. The insights gained in our work are beneficial for engineering TDA-based sponges for diverse practical applications including storage of hazardous and explosives materials, and potential uses in medicine and materials science. [1] Angew. Chem. 128/13910 (2016). [Preview Abstract] |
Thursday, March 16, 2017 8:36AM - 8:48AM |
R1.00004: First-principles studies of small-molecule adsorption and mechanical properties of amine-appended metal-organic frameworks Jung-Hoon Lee, Jeffrey B. Neaton Metal-organic frameworks (MOFs) are a class of highly-porous solids, consisting of metal centers (M) connected by organic ligands, that are of interest for gaseous storage and carbon capture. Recently, a new class of amine-appended MOFs, mmen-M$_{\mathrm{2}}$(dobpdc), have been shown to have exceptional CO2 adsorption properties via a novel phase transition with CO2 concentration. Here, we study this cooperative effect, as well as the mechanical properties of such MOFs with M$=$Mg, Mn, and Zn, using first-principles density functional theory (DFT) calculations with van der Waals corrections. We find excellent agreement with measured CO$_{\mathrm{2}}$ heats of adsorption, and demonstrate that amine ligands enhance CO$_{\mathrm{2}}$ binding energies (by about 30 kJ/mol) and selectivity under humid conditions, in agreement with multicomponent adsorption measurements. We further calculate that the polycrystalline Young's modulus of mmen-Mn$_{\mathrm{2}}$(dobpdc) increases by 70{\%} due to the amine ligands, a dramatic enhancement. All together, our calculations suggest that amine-based ligands can be used to optimize both small gas separation and mechanical properties of MOFs. [Preview Abstract] |
Thursday, March 16, 2017 8:48AM - 9:00AM |
R1.00005: Zero and finite temperature adsorption energies in zeolites: A random phase approximation study Anant Dixit, Michael Badawi, Tomas Bucko, Sebastien Lebegue, Janos Angyan, Dario Rocca Zeolites are widely used in industry as sorbents and catalysts due to their porous structures that can host various ions and molecules [1]. Adsorption of molecules in zeolites is typically determined by van der Waals (vdW) forces. This represents a challenge for ab-initio calculations, since semi-local approximations based on the density functional theory (DFT) fail to include vdW interactions. The random phase approximation (RPA) is an ab-initio scheme to build a fully non-local correlation functional that accurately captures vdW forces. The RPA can today describe realistic systems with a few hundreds of electrons and generate accurate benchmarks. Using the RPA as implemented in Refs. 2-3 we compute adsorption energies of methane and carbon dioxide in siliceous chabazite at 0 K. To evaluate temperature effects on the interaction energy we average over multiple configurations generated by molecular-dynamics simulations at 300 K. These results are compared with those obtained using DFT approximations with semi-empirical corrections.[1] S. Auerbach, K. Carrado, P. Dutta, Handbook of Zeolite Science and Technology, CRC Press (2003) [2] D. Rocca, J. Chem. Phys. 140, 18A501 (2014) [3] A. Dixit, J. Angyan, D. Rocca, J. Chem. Phys. 145, 104105 (2016) [Preview Abstract] |
Thursday, March 16, 2017 9:00AM - 9:12AM |
R1.00006: Interlayer Binding of Bilayer Low-dimensional $sp$,$sp^2$- Hybridized Carbon Networks : Quantum Monte Carlo Study Hyeondeok Shin, Jeongnim Kim, Anouar Benali, Yongkyung Kwon We have performed the quantum Monte Carlo(QMC) method to study interlayer binding of a bilayer $\alpha$-graphyne. A $sp$,$sp^2$-hybridized $\alpha$-graphyne have been received a great deal of attention due to its unique electronic properties compared with that of graphene. However, since the most stable mode in the DFT framework was varied along applied vdW-corrected correlation functionals, the most favored mode for a bilayer $\alpha$-graphyne was not confirmed.[1] Our QMC calculations confirmed the most favored mode for a bilayer $\alpha$-graphyne and revealed that vdW-corrected DFT binding energies significantly underestimate interlayer bindings of $sp$,$sp^2$-hybridized carbon network systems while overestimations within corresponding DFT functionals were observed in a graphene in recent QMC studies.[2] Among vdW-corrected DFT functionals, the rVV10 functional described the most consistent interlayer geometries with QMC ones for low-dimensional carbon allotropes, however, completely misestimated charge density distribution within the rVV10 yields significant quantitative discrepancy of interlayer binding energies between QMC ones. [1] O. Leenaerts {\it et al.}, Appl. Phys. Lett. {\bf 103}, 013105 (2013). [2] E. Mostaani {\it et al.}, Phys. Rev. Lett. {\bf 115}, 115501 (2015). [Preview Abstract] |
Thursday, March 16, 2017 9:12AM - 9:24AM |
R1.00007: Non-Covalent Interactions and Thermal Effects in the Quest for Selective Atomic Layer Deposition Jonathan Wong, Taewon Suh, Ka Un Lao, James R. Engstrom, Robert A. DiStasio Jr. In the manufacturing of nanoscale devices, the Atomic Layer Deposition (ALD) process has several desirable characteristics that allow aluminum oxide films to be grown (on dielectric or metallic substrates) with a precise thickness and intricate three-dimensional features. Furthermore, the gas-phase synthetic nature of ALD is very compatible with present manufacturing lines. However, the ALD process lacks control over substrate selectivity at the exceedingly small length scales required for the manufacturing of high-density transistor units that would extend the applicability of Moore's law. In this work, we utilized van der Waals (vdW) inclusive density functional theory (DFT) and high-level quantum chemical methods to study substrate selectivity in this process via the formation of Lewis acid-base complexes (or adducts) with the trimethylaluminum ALD precursor. We will demonstrate how non-covalent interactions and thermal effects modify both the kinetics and thermodynamics of the ALD process. In this context, we will also discuss several promising yet unexplored avenues towards increasing the efficiency of the ALD process and obtaining substrate selectivity. [Preview Abstract] |
Thursday, March 16, 2017 9:24AM - 9:36AM |
R1.00008: Critical wetting instabilities of light gases on graphene Adrian Del Maestro, Sanghita Sengupta, Nathan Nichols, Valeri Kotov The formation of liquid films on electronically neutral substrates is dominated by the van der Waals interaction which creates an effective repulsion between the liquid-gas and liquid-substrate boundaries. We investigate wetting phenomena within the Lifshitz theory for light gases composed of hydrogen, helium and nitrogen in three different geometries where graphene is either affixed to an insulating substrate, submerged or suspended. We have discovered that the presence of graphene has a significant effect in all configurations. In particular, in the suspended case where graphene is able to wet on only one side, we find that film growth becomes arrested at a critical thickness which may trigger liquid film surface instabilities and pattern formation analogous to spinodal dewetting. We discuss experimental consequences of this novel phenomenon and potential applications in the field of two-dimensional materials-based technologies. [Preview Abstract] |
Thursday, March 16, 2017 9:36AM - 9:48AM |
R1.00009: Diffusion Monte Carlo method for evaluating Hamaker constants Ryo Maezono, Kenta Hongo We evaluated Hamaker's constants for Si$_6$H$_{12}$ (CHS) to investigate its wettability, which is industrially useful but no references available. The constant is fundamental for wettability, but not directly accessible by experiments. Ab initio estimations are therefore in demand, and surely give an impact for broader fields such as tribology where the wettability plays an important role. The evaluation of binding curves itself could be a big challenge if it is applied to a practical molecule such as CHS, because highly accurate descriptions of electron correlations in vdW bindings get tough for such larger sizes with anisotropy. We applied DMC to overcome this difficulty, showing a new direction for wettability issues. Since ab intio estimations rely on simple assumptions such as additivity (and hence we denote it as $A_{add}$), it would include biases. Taking a benzene as a benchmark, we compared $A_{add}$ evaluated from several available binding curves with other reported $A_L$ (estimations based on Lifshitz theory). By the comparison, we get trends of biases in $A_add$ due to non-additivity and anisotropy because $A_L$ is expected to capture these effects to some extent in macroscopic manner. The expected trends here surprisingly well explain the series of results for CHS. [Preview Abstract] |
Thursday, March 16, 2017 9:48AM - 10:00AM |
R1.00010: Fabrication of functional ultrathin single-crystal nanowires from quasi-one dimensional van der Waals crystals Ta$_{\mathrm{2}}$(Pd or Pt)$_{\mathrm{3}}$Se$_{\mathrm{8}}$ Xue Liu, Jinyu Liu, Jin Hu, Chunlei Yue, Zhiqiang Mao, Jiang Wei, Yibo Zhu, Ana Sanchez, Liubov Antipina, Pavel Sorokin Micromechanical exfoliation or wet exfoliation of two-dimensional van der Waals materials has triggered an explosive interest in 2D material research. In our work, we extend this idea to 1D van der Waals material. By using micromechanical exfoliation or wet exfoliation, 1D nanowire with size as small as six molecular ribbons can be readily achieved in the Ta$_{\mathrm{2}}$(Pd or Pt)$_{\mathrm{3}}$Se$_{\mathrm{8}}$ system. The semiconducting properties of exfoliated Ta$_{\mathrm{2}}$Pd$_{\mathrm{3}}$Se$_{\mathrm{8}}$ nanowires show n-type, whereas Ta$_{\mathrm{2}}$Pt$_{\mathrm{3}}$Se$_{\mathrm{8}}$ nanowires are p-type. Our electronic band structure calculation for Ta$_{\mathrm{2}}$Pd$_{\mathrm{3}}$Se$_{\mathrm{8}}$ nanowire reveals that from multi-ribbon to single-ribbon the band gap evolves from indirect 0.5eV in bulk to direct 1eV in single-ribbon. A functional ``NOT'' gate consisting of field-effect transistors based on these two types of complementary nanowires has also been successfully realized. Moreover, the photocurrent response of Ta$_{\mathrm{2}}$Pd$_{\mathrm{3}}$Se$_{\mathrm{8}}$ nanowire transistors has been studied as well. Ta$_{\mathrm{2}}$(Pd or Pt)$_{\mathrm{3}}$Se$_{\mathrm{8}}$ system, as an intrinsic quasi-1D material, provides a viable platform for the study of low dimensional condensed matter physics. [Preview Abstract] |
Thursday, March 16, 2017 10:00AM - 10:12AM |
R1.00011: Two-dimensional van der Waals p-n junction of InSe/Phosphorene Jose Eduardo Padilha, Roberto Hiroki Miwa, Antonio Jose Roque da Silva, Adalberto Fazzio We investigate the energetic stability, the structural and electronic properties of van der Waals heterostructures composed by a combination of single layer InSe, bilayer phosphorene (BP), and graphene. We found that a single layer of InSe stacked on graphene (InSe@G) present a n-type Schottky barrier, which can be tuned by applying an external electric field perpendicularly to the InSe/G interface ($E^{\rm ext}_{\perp}$). Upon further increase of $E^{\rm ext}_{\perp}$ we may promote a n-type doping of the InSe layer. This is in contrast with the other semiconductor/metal vdW heterojunction, BP@G, where the BP sheet becomes p-type doped as a function of $E^{\rm ext}_{\perp}$. By considering a semiconductor/semiconductor vdW system, namely BP@InSe, we found that lowest (highest) unoccupied (occupied) states lie on the InSe (BP) layers. The BP/InSe interface presents a type-II band alignment. Exploiting the electron-hole separation in BP@InSe, and the formation of ohmic contacts in InSe@G and BP@G, tuned by $E^{\rm ext}_{\perp}$, we propose a p-n junction composed by p-type BP and n-type InSe, with the graphene acting as electrodes and also as a source of electrons/holes in the n-type/p-type InSe/BP heterostructure. [Preview Abstract] |
Thursday, March 16, 2017 10:12AM - 10:24AM |
R1.00012: Long-Range Repulsion Between Spatially Confined van der Waals Dimers Mainak Sadhukhan, Alexandre Tkatchenko It is an undisputed common wisdom that non-retarded vdW interactions between two objects in vacuo are inherently attractive. The universality of vdW attraction is attributed to the dipolar coupling between fluctuating electron charge densities. Here we demonstrate that the long-range interaction between spatially confined vdW dimers becomes repulsive when accounting for the full Coulomb interaction between charge fluctuations. Our analytic results are obtained by using the Coulomb potential as a perturbation over dipole-correlated states for two quantum harmonic oscillators embedded in spaces with reduced dimensionality, however the long-range repulsion is expected to be a general phenomenon for spatially-confined quantum systems. The emergence of repulsive van der Waals interaction due to spatial confinement rationalizes recent observations of anomalously strong screening of the lateral vdW interactions between aromatic hydrocarbons adsorbed on metal surfaces [1] as well as the surprising increase of water flow-rate inside carbon nanotubes with the decreasing tube radius [2]. [1] C. Wagner et al, Phys. Rev. B. \textbf{81}, 035423 (2010)[2] A. Michaelides, Nature \textbf{537}, 171 (2016). [Preview Abstract] |
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