Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session G8: From Single Molecules to Molecular Assemblies on Surfaces II |
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Sponsoring Units: DCMP Chair: Pengpeng Zhang, Michigan State University Room: 006C |
Tuesday, March 3, 2015 11:15AM - 11:27AM |
G8.00001: Self-Assembly and Dynamics of Organic 2D Molecular Sieves: \textit{Ab Initio} and Molecular Dynamics Studies Alexander St. John, Carlos Wexler Spontaneous molecular self-assembly is a promising route for bottom-up manufacturing of two-dimensional (2D) nanostructures with specific topologies on atomically flat surfaces. Of particular interest is the possibility of selective lock-and-key interaction of guest molecules inside cavities formed by complex self-assembled host structures. Our host structure is a monolayer consisting of interdigitated 1,3,5-tristyrylbenzene substituted by alkoxy peripheral chains containing $n \quad =$ 6, 8, 10, 12, or 14 carbon atoms (TSB3,5-C$n)$ deposited on a highly ordered pyrolytic graphite (HOPG) surface. Using \textit{ab initio} methods from quantum chemistry and molecular dynamics simulations, we construct and analyze the structure and functionality of the TSB3,5-C$n$ monolayer as a molecular sieve. [Preview Abstract] |
Tuesday, March 3, 2015 11:27AM - 11:39AM |
G8.00002: Highly-Oriented Molecular Assembly on Monolayer Graphene for Boosting Photon Harvesting in Bilayer Organic Solar Cells Kilwon Cho A novel approach to dramatically enhance the photon harvesting in organic solar cells was demonstrated by utilizing a graphene-organic heterointerface. A large area, residue-free monolayer graphene was inserted at anodic interface to serve as an atomically thin, transparent and highly conductive epitaxial template for organic crystal growth with specific orientation. The anisotropic nature of optoelectronic properties of organic semiconductor molecules provided a significant enhancement in exciton diffusion length, optical absorption, charge carrier lifetime as well as the energy level alignment at metal-organic and organic-organic interfaces. Especially, the exciton diffusion length increases up to nearly 100 nm, which allows the device thickness to be doubled to yield 5 times higher power conversion efficiency in comparison to conventional planar heterojunction organic photovoltaic cells. Theoretical simulations as well as systematic studies on the film structure and optoelectrical properties were performed to corroborate our new findings. [Preview Abstract] |
Tuesday, March 3, 2015 11:39AM - 11:51AM |
G8.00003: Ultrastrong exciton-photon coupling in single and coupled organic microcavities Bin Liu, Rosemary Bramante, Brent Valle, Kenneth Singer, Tawfik Khattab, Jarrod Williams, Robert Twieg We have demonstrated ultrastrong light-matter coupling in organic planar microcavities composed of a neat glassy organic dye film between two metallic (aluminum) mirrors in a half-cavity configuration. Such cavities are characterized by Q factors around 10. Tuning the thickness of the organic layer enables the observation of the ultrastrong coupling regime. Via reflectivity measurements, we observe a very large Rabi splitting around 1.227 eV between upper and lower polariton branches at room temperature, and we detect polariton emission from the lower polariton branch via photoluminescence measurements. The large splitting is due to the large oscillator strength of the neat dye glass, and to the match of the low-Q cavity spectral width to the broad absorption width of the dye film material. We also study the interaction between excitonic states of neat glassy organic dye and cavity modes within coupled microcavity structures. The high-reflectivity mirrors are formed from distributed Bragg reflectors (DBR), which are multilayer films fabricated using the coextrusion process, containing alternating layers of high (SAN25, n$=$1.57) and low (Dyneon THV 220G, n$=$1.37) refractive index dielectric polymers. Nonlinear optical measurements will be discussed. [Preview Abstract] |
Tuesday, March 3, 2015 11:51AM - 12:03PM |
G8.00004: Single molecule dissociation by tunneling electrons in NO-Co-Porphyrin complex on Au(111): A novel mechanics revealed by scanning tunneling spectroscopy and first-principles thermodynamic simulation Yunhee Chang, Howon Kim, Eui-Sup Lee, Won-Jun Jang, Yong-Hyun Kim, Se-Jong Kahng To microscopically understand the mechanisms of electron-induced NO dissociations, we performed first-principles density-functional theory (DFT) calculations for NO-CoTPP on Au(111). We explain the scanning tunneling microscopy (STM) results that the dissociations of NO were induced by both positive and negative voltage pulses with threshold voltages, $+$0.68 V and 0.74 V, respectively, at 0.1 nA tunneling current, showing power law relations between tunneling current and dissociation yield. To evaluate first-principles thermodynamics of the NO dissociation, we considered not only adsorption-desorption energetics, zero-point energy, and vibrational free energy at experiment temperature from first-principles, but also the chemical potential of NO gas at the cryogenic ultra-high vacuum condition. Using first-principles thermodynamics for the NO dissociation, we argue that the dissociations are induced with inelastic electron tunneling through molecular orbital resonances. [Preview Abstract] |
Tuesday, March 3, 2015 12:03PM - 12:15PM |
G8.00005: Growth of Thin, Anisotropic, $\pi $-Conjugated Molecular Films by Step-Wise `Click' Assembly of Molecular Building Blocks: Characterizing Reaction Yield, Surface Coverage, and Film Thickness vs. Addition Step Number Abel Demissie, Greg Haugstad, C. Daniel Frisbie Molecular electronics is an active field of nanotechnology that has gained much interest due to the advent of modern microscopy techniques, and thin film synthesis using click chemistry -- an approach which has enabled scientists to achieve a sub-angstrom control of monolayer length. Among the major challenges to grow oriented, surface-confined wires by click chemistry is development of synthetic routes that yield monodisperse wires, and lack of systematic way to measure the surface coverage of molecules. In this work, we report a comprehensive characterization of $\pi $-conjugated oligophenylene imine (OPI) wires synthesized step-wise by imine condensation click chemistry. OPI wire synthesis began with a self-assembled monolayer (SAM) of 4-formylthiophenol or 4-aminothiophenol on Au, followed by alternate addition of terepthaldehyde or phenylenediamine. OPI wires were characterized after each monomer addition via Rutherford backscattering spectrometry, x-ray photoelectron spectroscopy, cyclic voltammetry, reflection-absorption infra-red spectroscopy, and nuclear reaction analysis. We have determined an average extent of reaction greater than 98{\%} completion for each growth step using five different techniques. Overall, these nanoscale scale surface characterization techniques proved to be an extremely sufficient method for monitoring wire growth and surface coverage. [Preview Abstract] |
Tuesday, March 3, 2015 12:15PM - 12:27PM |
G8.00006: Direct Patterning of Organic Self-Assembled Monolayer (SAM) on GaAs Surfaces via Dip-Pen Nanolithography (DPN) Peng Xiong, Timothy Keiper, Xiaolei Wang, Jianhua Zhao Hybrid structures of functional molecules and solid-state (SS) materials have attracted extensive interest in surface nanoscience and molecular electronics. The formation and micro/nano patterning of organic SAMs on SS surfaces are a key step in fabricating such devices. Here we report realization of high quality MHA SAMs on GaAs and direct formation of micro/nanoscale patterns of MHA SAM on the surface by micro-contact printing ($\mu$ CP) and DPN. The process begins with the preparation of an oxide-free surface of GaAs, for which we employed treatment by an ammonium polysulfide ((NH$_{4})_{2}$S$_{\mathrm{x}})$ solution. The treatment strips native oxides from GaAs creating an atomic layer of sulfur covalently bonded to the fresh surface. Formation of high-quality SAMs of thiol molecules on GaAs then proceeds through exchange of the sulfur and the thiol terminal of the molecules. The effects of the sulfur-passivation and formation of MHA SAM on the treated surface were confirmed by XPS, HRTEM, and DPN. To the best of our knowledge, this is a first realization of direct DPN of nanoscale organic SAM on a semiconductor free of surface oxide. We further evidence the utility of the hybrid platform by demonstrating directed self-assembly of Au nanoparticles onto MHA/ODT SAM templates on GaAs. [Preview Abstract] |
Tuesday, March 3, 2015 12:27PM - 12:39PM |
G8.00007: The S(2p) Core Level Binding Energies for Alternative Adsorption Sites and the Example of Thiol Self Assembly Juanjuan Jia, Vladimir Esaulov, Abdelkader Kara Results of an investigation of the characteristics of thiol SAMs obtained by vacuum evaporative adsorption, useful for reactive substrates, are presented along with core level binding energy (BE) calculations. Thiol ended SAMs of 1,4-benzenedimethanethiol (BDMT) are obtained by evaporation on Au. They display an unconventional BE structure at about 161 eV, which is close to a known BE of an S atom on Au. S(2p) core level BE calculations for molecules chemisorbed on hollow, bridge and atop sites are reported and suggest that the 161 eV peak is indeed due to an alternative adsorption site, which can be associated to an atop configuration. This must therefore not be confused with atomic sulfur and dissociation processes with S-C bond scission. [Preview Abstract] |
Tuesday, March 3, 2015 12:39PM - 12:51PM |
G8.00008: Spatial Arrangement of Organic Compounds on a Model Mineral Surface: Implications for Soil Organic Matter Stabilization Haile Ambaye, Loukas Petridis, Sindhu Jagadamma, Michael Kilbey, Valeria Lauter, Bradley Lokitz, Melanie Mayes Stability of organic carbon compounds in soil is important for global climate futures which could be affected by the complexity of the mineral-organic carbon interfaces. We examined the nanoscale structure of model interfaces by depositing films of organic carbon compounds of contrasting chemical character, hydrophilic glucose, deuterated-amphiphilic stearic acid (SA) and Natural Organic Matters (NOM) onto a soil mineral analogue (Al$_{2}$O$_{3})$. The NOM was separated into its constituent components such as NOM-Philic and NOM-Phobic when it is deposited onto the soil mineral. We used Neutron Reflectivity technique to understand the depth organization of the thin films. The result indicates that glucose molecules reside in a layer between Al$_{2}$O$_{3}$ and stearic acid and SA self-assembles. No self-assembly of SA was observed when SA and NOM-Phobic was deposited on the mineral soil. Molecular dynamics simulations reveal the thermodynamic driving force behind glucose partitioning on the mineral interface. [Preview Abstract] |
Tuesday, March 3, 2015 12:51PM - 1:03PM |
G8.00009: Sliding friction of thick and thin oxygen layers on spin crossover materials Zachary B. Fredricks, K. M. Stevens, Daniel Dougherty, Jacqueline Krim Friction at the nanoscale is known to encompass phononic, electrostatic, conduction electronic and magnetic effects [1], with relatively little known about magnetic contributions to friction [2]. To probe such effects we have employed a quartz crystal microbalance technique to record the sliding friction associated with thin and thick films of solid and liquid oxygen, a paramagnetic material, atop nanoscale films of the spin-crossover material Fe[(H$_{2}$Bpz$_{2})_{2}$bpy], which is diamagnetic at cryogenic temperatures and paramagnetic at room temperature. Previously these systems have been shown to be frictionally sensitive to the application of small fields, for Pb(111) substrates [3]. We observe changes in dissipation as well for Fe[(H$_{2}$Bpz$_{2})_{2}$bpy], in response to externally applied magnetic fields. We will report our efforts to model the frictional interaction, which is reduced in the presence of a weak applied magnetic field, and is also observed to be temperature dependent. [1] I. Altfeder and J. Krim, J. Appl. Phys. (2012)~ [2] B. Wolter et al., PRL (2012) [3] M. Highland et al., PRL (2006) [Preview Abstract] |
Tuesday, March 3, 2015 1:03PM - 1:15PM |
G8.00010: Imaging Surface Reactions of Formaldehyde on TiO$_{2}$ Zhenrong Zhang, Miru Tang, Zhi-Tao Wang, Zhu Ke, Yaobiao Xia, Kenneth Park, Igor Lyubinetsky, Zdenek Dohn\'alek, Qingfeng Ge Formaldehyde is involved in many surface catalytic and photo-catalytic reactions on metal oxides. We studied surface reactions of formaldehyde on reduced TiO$_{2}$(110) surfaces using variable-temperature scanning tunneling microscopy (STM) and density functional theory (DFT). STM images taken from a same area at various temperatures clearly show that formaldehyde preferentially adsorbs on the bridge-bonded oxygen vacancy (V$_{\mathrm{O}})$ defect sites. Bias-dependent STM images suggest the bonding configurations of the Ti-bound CH$_{2}$O and the V$_{\mathrm{O}}$-bound CH$_{2}$O. The isothermal time dependent images show the rotation of V$_{\mathrm{O}}$-bound CH$_{2}$O and the two diffusion channels of formaldehyde at different temperatures. We also directly observed the formation of formaldehyde dimmer. [Preview Abstract] |
Tuesday, March 3, 2015 1:15PM - 1:27PM |
G8.00011: Water adsorption on non polar ZnO surfaces: from single molecules to multilayers Stephane Kenmoe, P. Ulrich Biedermann The interface between water and ZnO plays an important role in many domains of technological relevance. Following the vital role of adsorbed water on substrate properties and the fascinating properties of interfacial water, there is a great interest in characterizing this interface. We use DFT to study the possible aggregation regimes that can form on the ZnO non-polar low-index {\$}(1010){\$} and {\$}(1120){\$} surfaces. We study the adsorption of water monomers, small water clusters like water dimers, water chains, ladder-like water structures, water thin films and water multilayers. Based on this, trends in binding energy as well as the binding mechanisms are analyzed to understand the driving forces and the nature of the fundamental interactions that stabilize the adsorbed layers. [Preview Abstract] |
Tuesday, March 3, 2015 1:27PM - 1:39PM |
G8.00012: Adsorption, vibration and diffusion of oxygen on Ag(110) Takat Rawal, Sampyo Hong, Aki Pulkkinen, Matti Alatalo, Talat Rahman We have performed density functional theory calculations for the adsorption, vibration and diffusion of oxygen on Ag(110). At low coverage, O$_2$ adsorbs at the four-fold hollow (FFH) with the molecular axis aligned along the $[1\bar{1}0]$ direction. The dissociation of O$_2$ is easier along the [001] direction than along the $[1\bar{1}0]$ direction. For O$_2$ species in FFH aligned along the [001] the O-O intra-molecular stretching mode is coupled with the substrate vibration and thus its dissociation can be induced by surface phonon. In addition, O diffusion barrier from FFH to next FFH along the $[1\bar{1}0]$ is small (~0.07 eV only) but is by far larger (~0.4 eV) along [001]. On the other hand, O species in the short-bride (SB) site prefers to diffuse along the [001] (to FFH) rather than along the $[1\bar{1}0]$ direction (to next SB). Finally, the preference of atomic oxygen to form O-Ag-O complex on Ag(110) is responsible for disordering of the surface by means of substantial lateral and vertical displacements of Ag atoms in the topmost layer. In fact, such disordering phase of Ag(110) may act as a precursor of the reconstructed phase of Ag(110). [Preview Abstract] |
Tuesday, March 3, 2015 1:39PM - 1:51PM |
G8.00013: Carbon dioxide and water adsorption on highly epitaxial Delafossite CuFeO2 thin film S. Rojas, T. Joshi, P. Borisov, M. Sarabia, D. Lederman, A.L. Cabrera Thermal programmed desorption (TPD) of CO2 and H2O from a 200 nm thick CuFeO2 Delafossite surface was performed in a standard UHV chamber, The CuFeO2 thin film grown using Pulsed Laser Deposition (PLD) over an Al2O3 (0001) substrate with controlled O2 atmosphere resulted with highly epitaxial crystal structure. The adsorption/desorption of CO2 and H2O process was also monitored with X-ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES). Our results revealed that carbon dioxide interacts with CuFeO2 forming Fe carbonates compounds on its surface. Hydroxides were also formed on the surface due to water presence. Using TPD data, Arrhenius plots for CO2 and water desorption were done and activation energy for desorption was obtained. [Preview Abstract] |
Tuesday, March 3, 2015 1:51PM - 2:03PM |
G8.00014: Onboard Catalysis of Formic Acid for Hydrogen Fueled Vehicles Altaf Karim, Muhammad Mamoor Metal hydrides are used as a medium of hydrogen storage in hydrogen powered vehicles. Such hydride materials cannot store hydrogen more than 10 wt{\%}. The bottleneck in this issue is the reversible storage of hydrogen at ambient temperature and pressure. Alternatively formic acid is becoming more popular medium for the onboard hydrogen production for these vehicles. Its decomposition on metal surfaces and nanostructures is considered to be a potential method to produce CO-free hydrogen at near ambient temperatures. We applied Density Functional Theory (DFT) based Kinetic Monte Carlo (KMC) simulations as our tool to study the reaction kinetics of hydrogen production from formic acid on different catalytic surfaces and nano structures (Au, Pd, Rh, Pt). Our results show that nanostructures and artificially engineered bimetallic catalysts give higher rate of hydrogen production then their monometallic counter parts under various temperature and pressure conditions. [Preview Abstract] |
Tuesday, March 3, 2015 2:03PM - 2:15PM |
G8.00015: First Principles Investigation of Reversible Adsorption/Desorption and Molecular Dissociation of $CO_{2}$ on Ferroelectric $PbTiO_{3}$-supported $ZnO$ Thin Films Babatunde Alawode, Alexie Kolpak Applying an electric field across a ferroelectric changes its polarization direction. When epitaxial layers are grown on such materials, the polarization may induce new atomic configurations at the interface. In this paper, we use density functional theory calculations to demonstrate that this effect can be used to reversibly modify the surface chemistry of thin $ZnO$ grown on ferroelectric $PbTiO_{3}$. We show that both the substrate polarization direction and the thickness of the $ZnO$ film have a significant effect on the adsorption energy of $CO_{2}$ and methanol, indicating that dynamic polarization switching could be used to minimize reaction barriers and/or enhance selectivity. As an example, we demonstrate a pathway for thermal $CO_{2}$ dissociation over a single monolayer of $ZnO$ on $PbTiO_{3}$ with a predicted reaction rate several orders of magnitude higher than that over unsupported $ZnO$. Our results suggest that ferroelectric-supported $ZnO$, and dynamically tunable catalysts in general, could enable lower-energy approaches for $CO_{2}$ conversion. [Preview Abstract] |
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