Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session T43: Liquid and Solid Interfaces |
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Sponsoring Units: DCP Chair: Gil Nathanson, University of Wisconsin Room: Hilton Baltimore Holiday Ballroom 2 |
Thursday, March 21, 2013 8:00AM - 8:12AM |
T43.00001: Characterization of critical micelle concentration of ionic liquid on molecular length scale by X-ray surface scattering and spectroscopy study Wenjie Wang, Woongmo Sung, William Lindemann, Ivan Kuzmenko, Doseok Kim, David Vaknin Ionic liquids (ILs) with long alkyl chains tend to form micelles in aqueous solutions once the critical micelle concentration (CMC) is reached, a phenomenon commonly described by the Gibbs isotherm for ionic surfactants. We report synchrotron X-ray measurements at far below, near and above the CMC of each IL of 1-dodecyl-3-methyl-imidazolium halides, [C$_{12}$mim]X, (X$=$Cl,Br,I). Our X-ray reflectivity measurements provide the depth density profiles of the interfacial films formed by the ILs. A liquid state of the alkyl chains can also be identified by grazing incidence X-ray diffraction measurements that reveal the in-plane packing of the IL molecules. The ILs form monolayers on the aqueous surfaces and the cations [C$_{12}$mim]$^{+}$ bind with Cl$^{-}$ and I$^{-}$ ions with different affinity. We discuss our experimental results of surfactants surface enrichment in the context of Gibbs equations. [Preview Abstract] |
Thursday, March 21, 2013 8:12AM - 8:24AM |
T43.00002: Predicting In-Situ X-ray Diffraction for the SrTiO$_{3}$/Liquid Interface from First Principles Kendra Letchworth-Weaver, Deniz Gunceler, Ravishankar Sundararaman, Xin Huang, Joel Brock, T. A. Arias Recent advances in experimental techniques, such as in-situ x-ray diffraction, allow researchers to probe the solid-liquid interface in electrochemical systems under operating conditions. These advances offer an unprecedented opportunity for theory to predict properties of electrode materials in aqueous environments and inform the design of energy conversion and storage devices. To compare with experiment, these theoretical studies require microscopic details of both the liquid and the electrode surface. Joint Density Functional Theory (JDFT), a computationally efficient alternative to molecular dynamics, couples a classical density-functional, which captures molecular structure of the liquid, to a quantum-mechanical functional for the electrode surface. We present a JDFT exploration of SrTiO$_3$, which can catalyze solar-driven water splitting, in an electrochemical environment. We determine the geometry of the polar SrTiO$_3$ surface and the equilibrium structure of the contacting liquid, as well as the influence of the liquid upon the electronic structure of the surface. We then predict the effect of the fluid environment on x-ray diffraction patterns and compare our predictions to in-situ measurements performed at the Cornell High Energy Synchrotron Source (CHESS). [Preview Abstract] |
Thursday, March 21, 2013 8:24AM - 8:36AM |
T43.00003: Alumina(0001)/water interface structure and infrared spectra from first-principles molecular dynamics simulations Eric Schwegler, Tuan Anh Pham, Patrick Huang, Giulia Galli Knowledge of the interaction of water with solid oxide surfaces is of fundamental importance for the stability of solid oxides in aqueous environments. We studied the atomic structure and infrared (IR) spectra of the alumina(0001)/water interface, using molecular dynamics simulations and the Qbox code. We found that the structural properties of the interface, as described within the generalized gradient approximation, are in good agreement with synchrotron X-ray scattering experiments. In addition, a detailed analysis of the computed IR spectra of interfacial water reveals two types of water molecules at the solid-liquid interface: one type participating in strong ``ice-like'' hydrogen bonding with the oxide surface, and one type of water molecules involved in weak ``liquid-like'' hydrogen bonding at the interface. Our results provide a molecular interpretation of the ``ice-like'' and ``liquid-like'' peaks observed in sum-frequency vibrational spectroscopy experiments. [Preview Abstract] |
Thursday, March 21, 2013 8:36AM - 8:48AM |
T43.00004: Structure, Dynamics, and Viscoelasticity of Nanoparticle Thin Films at the Liquid-Air Interface Leandra Boucheron, Jacob Stanley, Yeling Dai, Binhua Lin, Mati Meron, Suresh Narayanan, Alec Sandy, Oleg Shpyrko We experimentally probe the structure and inter-particle dynamics of iron oxide nanoparticle thin films self-assembled at the liquid-air interface. We find that upon deposition on a water substrate, iron oxide nanocrystals coated in oleic acid ligands spontaneously arrange themselves into a hexagonally close-packed configuration. At low particle concentrations, this close-packing results in isolated islands of particles distributed across the liquid surface. Compression in a Langmuir-Blodgett trough and the corresponding increase in surface pressure results in the formation of a uniform quasi-2D monolayer. Using X-Ray Reflectivity (XR) measurements, we were able to quantify the overall change in surface-normal film structure due to an increase in surface pressure. Utilizing X-Ray Photon Correlation Spectroscopy (XPCS), we have measured the characteristic timescale of in-plane particle dynamics. I will discuss these results and their relation to viscoelasticity in quasi-2D self-assembled monolayers. [Preview Abstract] |
Thursday, March 21, 2013 8:48AM - 9:00AM |
T43.00005: Ab initio molecular dynamics study of liquid Li surfaces exposed to deuterium Mohan Chen, Junchao Xia, Ilgyou Shin, Emily Carter We investigate the structure of liquid Li and its interactions with deuterium atoms using PROFESS (PRinceton Orbital-Free Electronic Structure Software) [1]. This linear-scaling orbital-free density functional theory method is a very fast quantum mechanics technique that allows one to perform ab initio molecular dynamics of metals for a large number of atoms and fairly long times. We adopt the WGC99 kinetic energy density functional that is very accurate for simple metals [2]. We use well validated bulk-derived local pseudopotentials [3] to describe the electron-ion interactions. Key properties of liquid Li will be presented and discussed, such as its bulk and surface structures, etc. Time permitting, we will discuss predictions related to adsorption and absorption of deuterium atoms into Li. This work provides new insights into understanding the surface structure of liquid Li using large-scale ab initio molecular dynamics methods. [1] L. Hung, C. Huang, I.Shin, G. Ho, V. L. Ligneres, and E. A. Carter, Comput. Phys. Comm., 181, 2208 (2010). [2] Y. A. Wang, N. Govind, and E. A. Carter, Phys. Rev. B, 60, 16350 (1999). Erratum: Phys. Rev. B, 64, 089903-1 (2001). [3] C. Huang and E. A. Carter, Phys. Chem. Chem. Phys., 10, 7109 (2008). [Preview Abstract] |
Thursday, March 21, 2013 9:00AM - 9:12AM |
T43.00006: The effect of support on the characteristics of Pt Nanoparticles Ghazal Shafai Erfani, Talat S. Rahman We have carried out density functional theory calculations within the projector augmented wave scheme (PAW) and the pseudopotential approach to evaluate the effect of the support ($\gamma$-alumina and titania) on geometric and electronic structural properties of Pt22, Pt33, Pt44, Pt55 nanoparticles (NPs) with the shape previously characterized by extended X-ray absorption fine structure spectroscopy (EXAFS) [1]. We are in particular interested in the electronic structural changes of the perimeter atoms, as we expect them to play a major role in catalysis. We find stabilization of the NP on the substrate to depend critically on the existence of oxygen vacancies on the surface and the effect to be more prominent for titania than for alumina. On both substrates the average bond-length (first nearest-neighbor distance) expands (1 to 3\%) as compared to that of unsupported NPs. We present results for the charge transfer and local density of states of the atoms at the interface and make comparisons with available experimental data on the propensity of these atoms to be chemically active.\\[4pt] [1] Roldan Cuenya et. al. Phys. Rev. B 84, 245438 (2011). [Preview Abstract] |
Thursday, March 21, 2013 9:12AM - 9:24AM |
T43.00007: Optical properties of TiO$_2$ nanoclusters Matti Alatalo, Sami Auvinen, Matti Lahti, Heikki Haario, Erik Vartiainen, Juho Jalava, Ralf-Johan Lamminm\"aki The structural, electronic and optical properties of TiO$_2$ nanoclusters have been investigated using first principles calculations. The shape of the clusters is shown to affect the optical properties more than the cluster size in the ultra small particles. We show that the first principles results for the optical properties can be extended towards larger clusters by using the generalized oscillator model, fitted to the first principles data. This allows us to bridge the gap between the atomistic regime, addressable by quantum mechanical calculations up to a few nanometers, and the size region of tens of nanometers, relevant for UV applications. This method provides an extension of the turbidity spectum method, used earlier for determining the size distribution of larger TiO$_2$ nanoparticles. We also discuss the electronic structure of the clusters. In particular, we provide an explanation for the gap states observed in stoichiometric clusters. [Preview Abstract] |
Thursday, March 21, 2013 9:24AM - 9:36AM |
T43.00008: Narrowing of band gap in thin films and linear arrays of ordered TiO$_{2}$ nanoparticles Yu Liu, James Taing, Cheng-Chien Chen, Adam Sorini, Ming H. Cheng, Alexandria Margarella, Hendrik Bluhm, Zhi Liu, Thomas Devereaux, John Hemminger Utilizing ambient pressure synchrotron x-ray spectroscopies, we report the properties of thin films and linear arrays of ordered TiO$_{2}$ nanoparticles under in situ water vapor exposure and heating. Our nondestructive depth profiles indicates an enhancement of the density of states (DOS) near the Fermi level due to surface Ti$^{3+}$ states and oxygen vacancies caused by heating isolated TiO$_{2}$ nanoparticles. In contrast, introducing water on the TiO$_{2}$ interface eliminates oxygen vacancies and increase Ti$^{4+}$ configurations, thereby suppressing the DOS enhancement. Our results suggest that the TiO$_{2}$ band gap can be tuned reversibly under water exposure and heating, and isolated TiO$_{2}$ nanoparticles can potentially enhance solar absorption efficiency and the life time of electron-hole pairs for photocatalysis. [Preview Abstract] |
Thursday, March 21, 2013 9:36AM - 9:48AM |
T43.00009: Modification of the wettability of TiO$_2$ surfaces with ion bombardment. Oscar Rodriguez de la Fuente, Beatriz Martinez, Juan Rojo Tailoring the affinity of a surface towards water adsorption is crucial for a number of physicochemical processes. Many applications depend on its proper control, such as those related to cell adhesion, some catalytic phenomena or the development of hydrophobic textiles. TiO$_2$ is a most interesting material, especially for its enhanced hydrophilicity when illuminated with light. In this work, we have modified rutile TiO$_2$(110) surfaces with ion bombardment and studied their composition, structure and interaction with water with contact angle measurements (static and dynamic), optical microscopy, AFM, Auger electron spectroscopy, LEED and IRAS. We show how the density of water nucleation centers and the shape of the microdroplets, when the surface is exposed to water vapor, depend on the morphological and chemical state of the surface. In general, we observe that the affinity for water is larger for the flat, non-bombarded surfaces. Indeed, and contrary to most observations reported in the literature, the contact angle of both microscopic and macroscopic droplets is higher for the defective surfaces. We attribute such behavior to the special structure of the first adsorbed molecular water layers, which is strongly influenced by surface defects and the hydrogen bond network. [Preview Abstract] |
Thursday, March 21, 2013 9:48AM - 10:00AM |
T43.00010: Rotational Tunneling of CH$_{2}$D$_{2}$ Monolayers on MgO(100) Andrew Hicks, John Larese Understanding the detailed nature of the interactions governing physisorption is a central topic in surface science, with wide ranging energy applications in heterogeneous catalysis, gas separation, and hydrogen storage. For systems with a strong interaction potential relative to the rotational constant of the adsorbate, adsorbed molecules are constrained to minima in the rotational potential. Adsorbed molecules may then tunnel through the rotational barrier between potential minima. Rotational tunneling spectra (RTS) are extremely sensitive to changes in the symmetry and strength of the rotational potential and are unmatched in their ability to probe the electrostatic potentials associated with adsorption sites. Furthermore, RTS can be clearly observed using inelastic neutron scattering. Building upon previous work of CH$_{4}$ on MgO (see J.Z. Larese, \textit{Physica B}, 1998), RTS of CH$_{3}$D and CH$_{2}$D$_{2}$ are interpreted using the pocket state (PS) formalism developed by \textit{H\"{u}ller et al}. The ground librational state of the adsorbate is split into twelve ``pockets'', each localized around one of twelve minima in the rotational potential. We report recent RTS of single monolayers of CH$_{3}$D and CH$_{2}$D$_{2}$ adsorbed on the MgO(100) surface using \textbf{BASIS} at the SNS at ORNL. These pioneering measurements represent the highest resolution investigation available for this (or any other) RTS. The discussion will include challenges in reconciling the transitions predicted by PS theory and the features observed in the experimental data. [Preview Abstract] |
Thursday, March 21, 2013 10:00AM - 10:12AM |
T43.00011: Concentration of point defects at metal-oxide surfaces: case study of MgO (100) Norina Richter, Sergey Levchenko, Matthias Scheffler, Sabrina Sicolo, Joachim Sauer We calculate from first principles the concentration of neutral and charged oxygen vacancies on a doped MgO (100) surface at realistic ($T$, $p_{\rm O_2}$) conditions. Vacancy formation energies are computed using hybrid density-functional theory with parameters of the exchange-correlation functional adjusted according to a basic consistency requirement on the Kohn-Sham and $G_0W_0$ defect transition levels. The parameters are validated by CCSD(T) calculations of formation energies for neutral vacancies using embedded cluster models. Gibbs free energies of formation are obtained using the {\em ab initio} atomistic thermodynamics approach.\footnote{K. Reuter and M. Scheffler, Phys. Rev. B \textbf{65}, 035406 (2001); C. M. Weinert and M. Scheffler, Mat. Sci. Forum \textbf{10-12}, 25 (1986); M. Scheffler and J. Dabrowski, Phil. Mag. A \textbf{58}, 107 (1988)} We demonstrate that the concentration of surface vacancies is significantly increased due to band bending and Fermi level pinning at the surface, resulting in lower formation energies of charged vacancies. [Preview Abstract] |
Thursday, March 21, 2013 10:12AM - 10:24AM |
T43.00012: Tuning the Electronic and Chemical Properties of Monolayer MoS$_2$ Adsorbed on Transition Metal Substrates Wei Chen, Elton Santos, Wenguang Zhu, Efthimios Kaxiras, Zhenyu Zhang Using first-principles calculations within density functional theory, we investigate the electronic and chemical properties of a single-layer MoS$_2$ adsorbed on Ir(111), Pd(111), or Ru(0001), three representative transition metal substrates having varying work functions but each with minimal lattice mismatch with the MoS$_2$ overlayer. We find that for each of the metal substrates, the contact nature is of Schottky type, and the dependence of the barrier height on the work function exhibits a partial Fermi-level pinning picture. Using hydrogen adsorption as a testing example, we further demonstrate that the introduction of a metal substrate can substantially alter the chemical reactivity of the adsorbed MoS$_2$ layer. The enhanced binding of hydrogen, by as much as about 0.4 eV, is attributed in part to a stronger H-S coupling enabled by the transferred charge from the substrate to the MoS$_2$ overlayer, and in part to a stronger MoS$_2$-metal interface by the hydrogen adsorption. These findings may prove to be instrumental in future design of MoS$_2$-based electronics, as well as in exploring novel catalysts for hydrogen production and related chemical processes. [Preview Abstract] |
Thursday, March 21, 2013 10:24AM - 10:36AM |
T43.00013: Modifying the Photoluminescence of Monolayer MoS2 by Metal Deposition DeZheng Sun, YuMeng You, Kin Fai Mak, Fan Zhang, James Hone, Ludwig Bartels, Tony Heinz Monolayer MoS2 exhibits strong photoluminescence (PL) due to its direct band gap located at K point. Because of its monolayer thickness, light emission from MoS2 is known to be strongly influenced by interactions with surrounding media [1]. In this study, we have investigated the effect on the photoluminescence of exfoliated monolayers of MoS2 induced by the deposition of gold atoms. The PL from the sample was recorded as a function of amount of gold deposited, up to an effective thickness of about 1 nm. Atomic force microscopy revealed that the gold forms isolated island structures on the surface. A progressive increase in quenching was seen with increasing gold coverage. Deposition of gold on suspended MoS2 samples led to quenching of the PL by more than a factor of 100. Given the low reactivity of gold, we attribute the PL quenching primarily to energy transfer of the photogenerated excitons to the metal clusters. The observed changes in the shape and intensity of emission spectra will be discussed in terms of this mechanism and possible effects of doping induced by the gold deposition.\\[4pt] [1] K. F. Mak, C. Lee, J. Hone, J. Shan and T. F. Heinz, PHYSICAL REVIEW LETTERS, 105, 136805 (2010), [Preview Abstract] |
Thursday, March 21, 2013 10:36AM - 10:48AM |
T43.00014: Adsorption and Dynamics behaves of Platinum Atoms on Si(111)-7x7 Surface Studied with Scanning Tunneling Microscopy and First principles Calculation Che-Fu Chou, Hsun-Ta Tu, Chou-Min Yang, Wan-Sheng Su, Mon-Shu Ho In this study, behaves of platinum atoms on Si (111) surface were study in use of ultrahigh vacuum scanning tunneling microscope (STM). The surface morphologies of platinum atoms adsorbed on Si (111) surface were observed. Dynamic study showed how the platinum atoms adsorbing and hopping on Si (111) surface. Activation energy was also calculated by fitting the experimental data. A first principle calculation was then performed to establish the adsorption sites, hopping path and the activation energy in the experiment. [Preview Abstract] |
Thursday, March 21, 2013 10:48AM - 11:00AM |
T43.00015: An Extremely Simple Route to Large-Area Microchannels and Inorganic Stripes Wei Han, Bo Li, Xukai Xin, Zhiqun Lin Microchannels were yielded in an extremely simple route by freely evaporating PS latex particle suspension on a rigid substrate, due to the capillary stress generated during the evaporation process that fractured the thin film and the cracks progressed towards the center of the evaporating suspension. The simple tailoring of the upper surface of the imposed confined geometry (i.e., parallel plates or vertical slide) directed the formation of parallel microchannels in a precisely controllable manner over large areas. Quite intriguingly, these prepared microchannel patterns may be served as templates to craft ordered Au stripes with unprecedented regularity. This facile approach opens a new avenue for producing macroscopic patterns and developing microelectronics or microfluidic-based biochips in a simple and controllable manner. [Preview Abstract] |
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