Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session L53: Focus Session: Molecular Adsorption and Metal and Oxide Film Growth |
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Sponsoring Units: DMP Chair: Shirley Chiang, University of California, Davis Room: Mile High Ballroom 2C |
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L53.00001: Diffusion of anthracene derivatives on Cu(111) studied by STM and DFT Jonathan Wyrick, Ludwig Bartels, Theodore Einstein Substituted anthracenes have drawn attention due to their ability to diffuse uniaxially on a Cu(111) surface. We compare anthracene to three of its derivatives whose 9,10 hydrogens are replaced by elements of the chalcogen group that act as linkers binding the molecules to a Cu(111) substrate. DFT calculations shed light on STM imaging and diffusion studies on the three substituted species. We present an analysis of the DFT results in which energetic contributions to the diffusion barriers are partitioned among the Kohn-Sham orbitals, allowing us to make assignments as to how each orbital affects diffusion for each species and draw comparisons between them. [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L53.00002: Highly Ordered Organic Molecular Thin Films on Silicon Studied by STM and LEED Sean Wagner, Pengpeng Zhang Achieving growth of long-range ordered organic molecular thin films on inorganic substrates continues to be a significant challenge for organic electronics applications. Here, we report the growth of highly ordered zinc phthalocyanine (ZnPc) thin films both in-plane and out-of-plane on the deactivated Si(111) surface by scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). By adjusting the substrate temperature during deposition, the anisotropic step-flow growth mode can be accessed causing a reduction in the substrate symmetry which allows for the long-range in-plane ordering as well as the decrease of grain boundary density [1] [2]. Additionally, the ZnPc molecules are able to maintain a highly ordered configuration in multi-layers despite a gradual decrease in the molecule-substrate interaction, which is attributed to the strong interlayer $\pi$-$\pi$ interaction [2]. \newline \newline [1] S. R. Wagner, R. R. Lunt, and P. P. Zhang, \textit{Phys. Rev. Lett.} \textbf{110}, 086107 (2013).\newline [2] S. R. Wagner and P. P. Zhang, (Submitted). [Preview Abstract] |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L53.00003: Adsorption geometry of ZnTPP molecules on Au(111): self-assembly and surface interaction Charles Ruggieri, Sylvie Rangan, Robert Bartynski, Elena Galoppini The interaction between Zinc Tetraphenylporphyrin (ZnTPP) molecules and a Au(111) surface, from initial adsorption sites to monolayer organization, is investigated using scanning tunnel microscopy with a particular emphasis on registry of the overlayer and surface atomic structure. At low coverages ZnTPP decorates step edges. With further deposition, ZnTPP molecules form self-organized islands of flat-lying macrocycles having a well-defined registry with, and dimensions bounded by, the underlying Au(111) herringbone reconstruction. At monolayer coverage, the herringbone reconstruction persists, enabling the relationship between the geometry of the self-organized molecular layer and that of the Au(111) surface to be established. Surface annealing generates a more complex self-assembled structure characterized by Au step edges that strictly align with ZnTPP molecular rows. The underlying mechanisms for this behavior will be discussed. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L53.00004: Scanning Tunneling Microscopy Analysis of a Pentacene/Graphene/SiC(0001) system Andrew Yost, Ozgun Suzer, Joseph Smerdon, TeYu Chien, Jeffrey Guest A complete understanding of the structure of molecular assemblies, as well as an understanding of donor-acceptor interactions is crucial in the development of emergent molecular electronics technologies such as organic photovoltaics. The pentacene (C$_{22}$H$_{14})$ is a good electron donor in Pentacene-C60 system, which is a model system of an organic photovoltaic cell.. Here we present scanning tunneling microscopy studies of the pentacene(Pn) molecule on Graphene(G) that is epitaxially grown on SiC(0001). In addition to the morphologies reported in literature, several new structures of Pn on on G/SiC(0001) were observed with different periodicity and registry both in monolayer and bilayer coverages of molecules on the surface. Preliminary scanning tunneling spectroscopy of the molecular system is also discussed; well-isolated states and a large HOMO-LUMO gap indicate the Pn is weakly coupled to the grapheme and underlying substrate. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L53.00005: STM and DFT examination of self-assembled 5,6,7-trithiapentacene-13-one on vicinal gold (788) Amanda Larson, Jeremiah van Baren, Jeremy Kintigh, Jun Wang, Jian Ming Tang, Glen P. Miller, Karsten Pohl The novel pentacene derivative 5,6,7-trithiapentacene-13-one (TTPO) is a robust electron donor candidate for use in high temperature photovoltaic devices. STM imaging has revealed interesting nanoscale surface structures of TTPO molecular chains as well as an ordered self-assembled monolayer on 3.9nm wide gold (788) surface terraces. TTPO is a polar species of pentacene with centered oxygen and sulfur bridge substituents. It is along this sulfur bridge that TTPO arranges itself laterally with a small cant angle between the molecule and the gold surface. This lateral assembly varies from the common flat-lying and standing-up phases of pentacene on surfaces. Combining imaging with density functional theory calculations allows for classification of these self-assembled structures with particular interest being directed toward the interaction between TTPO and gold at this organic-metallic interface. Understanding the structure of organic-metal interfaces with molecular precision potentially allows for the tailoring of those interfaces in order to maximize charge carrier transport. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L53.00006: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L53.00007: Combination of Two Nanoscale Quantum Systems by Controlled Manipulations Yang Li, Andrew DiLullo, Brandon Fisher, Saw-Wai Hla Modifying properties of materials at the nanoscale is an important ability for bottom-up designs of new materials, nanodevices, which might lead to new applications in the future. One method to achieve this goal is to put two nanoscale systems together and then study how they influence the properties of each other. Scanning tunneling microscopes (STMs) are ideal tools for these manipulations. Here, we report one way to modify electronic properties of two nanoscale systems, vacancies and molecules, by a novel process of STM manipulation. By these manipulations, performed near 6 K, surface vacancies were controllably created on a noble metal surface. Molecules were selectively moved into the created vacancies. Scanning tunneling spectroscopy was used to measure the change of the electronic structures of this new vacancy and molecule complex. It was found that the energy spectrum of the vacancy-molecule complex was a combination of the vacancy electronic structure and signature molecular orbitals. This work demonstrates the controlled combination of two nanoscale quantum systems which resulted in definite overlap of the electronic states of the constituent parts. By this process it is possible to design and form a new class of nanoscale systems for future research. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L53.00008: Templated quasicrystalline molecular layers Joe Smerdon, Kirsty Young, Michael Lowe, Sanger Hars, Thakur Yadav, David Hesp, Vinod Dhanak, An-Pang Tsai, Hem Raj Sharma, Ronan McGrath Quasicrystals are materials with long range ordering but no periodicity. We report scanning tunneling microscopy (STM) observations of quasicrystalline molecular layers on five-fold quasicrystal surfaces. The molecules adopt positions and orientations on the surface consistent with the quasicrystalline ordering of the substrate. Carbon-60 adsorbs atop sufficiently-separated Fe atoms on icosahedral Al-Cu-Fe to form a unique quasicrystalline lattice whereas further C$_{60}$ molecules decorate remaining surface Fe atoms in a quasi-degenerate fashion. Pentacene (Pn) adsorbs at tenfold-symmetric points around surface-bisected rhombic triacontahedral clusters in icosahedral Ag-In-Yb. These systems constitute the first demonstrations of quasicrystalline molecular ordering on a template. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L53.00009: Spin dynamics and quantum states in 3d atomic chains on Cu3N-Cu(110) molecular network Oleg Stepanyuk, Dmitry Bazhanov, Valeri Stepanyuk Based on first-principle calculations we studied the magnetic state and exchange coupling of transition metal atomic chains of Mn, Fe and Co deposited on a self-corrugated Cu3N-Cu(110) molecular network. We considered various atomic sites for adsorption on the corrugated Cu3N layer. By calculating the ground state magnetic configurations it was shown, that the magnetic order, anisotropy and exchange coupling within atomic chains depend sensitively on their chemical composition and adsorption sites on Cu3N network. We have found that exchange coupling in nanowires could be ferromagnetic and anti-ferromagnetic depending on the position of the chain on the surface. The spin-dynamics is investigated by means of kinetic Monte Carlo method based on transition-state theory. Using ab-initio determined exchange parameters and spin moments we apply the irreducible tensor operator technique to evaluate the Heisenberg-Dirac-Van Vleck quantum spin Hamiltonian for calculation of magnetic susceptibility of atomic chains. Using this value as a macroscopic entanglement witness we demonstrate that in antiferromagnetic chains of different length the entanglement temperature can be as much as 30-40 K. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L53.00010: Spin Crossover Transition in Molecular Adsorbates Xin Zhang, Sumit Beniwal, Axel Enders, Peter A. Dowben, Tatiana Palamarciuc, Patrick Rosa, Jean-Fran\c{c}ois L\'etard, Jing Liu, Eduardo V. Lozada, Fernand Torres, Luis G. Rosa, Bernard Doudin The occupied and unoccupied electronic structure of ultra thin films of the spin crossover [Fe(H$_{\mathrm{2}}$B(pz)$_{\mathrm{2}})_{\mathrm{2}}$(bipy)] complex (with H$_{\mathrm{2}}$B(pz)$_{\mathrm{2}}=$bis(hydrido)bis(1H-pyrazol-1-yl)borate and bipy $=$ 2,2'-bipyridine) was investigated by ultraviolet photoelectron spectroscopy (UPS), inverse photoemission (IPES) and X-ray absorption spectroscopy (XAS). The XAS spectra clearly shows the change of iron L edge spectra associated with thermal induced spin crossover. Generally changes occurring for the iron coordination, across the spin crossover transition, are seen to be very similar. The spin crossover transition, and certainly the unoccupied electronic structure, is influenced by the polarization direction of molecular ferroelectric poly(vinylidene fluoride -- trifluoroethylene) substrates at temperatures in the vicinity of the thermally driven spin cross-over transition. Combining the STM studies with the thickness dependent IPES results of the molecular adsorbate on gold substrates, we understand that the molecular thin film spin-states may also be affected by thickness of the Fe(H$_{\mathrm{2}}$B(pz)$_{\mathrm{2}})_{\mathrm{2}}$(bipy) film. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L53.00011: Sub-molecular electronic structure of self-assembled metal-organic nano-chains on a noble metal surface Agustin Schiffrin, Martina Capsoni, Adam Shaw, Sarah Burke Complexes composed of organic ligands coordinated with transition metal atoms exhibit broad absorption bands from the ultraviolet to the near-infrared. These are the result of the intrinsic molecular electronic properties, which include intra-ligand excitations and metal-to-ligand charge transfer. When adsorbed on a surface, these compounds are relevant for photovoltaic applications. In order to ensure a hierarchical transfer of function from the nano- to the macro-scale, electronic characterization at the single molecule level is essential. We present a low-temperature scanning tunneling spectroscopy study on the local electronic structure of one-dimensional self-assembled metal-organic nanostructures formed on a noble metal surface. The nano-chains consist of terpyridine-based ligands coordinated with iron (Fe) adatoms. We map the local density of electronic states of the system with sub-molecular spatial resolution. Energy-broadened highest-occupied molecular orbitals are dominated by metal states, whereas sharp resonances above Fermi are mainly related to the organic moiety. Coordination between the ligand and Fe induces energy shifts and a break of spatial symmetry of the unoccupied states, pointing to an electron transfer from the metal atom to the terpyridine groups. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L53.00012: Induced Nanoscale Surface Vacancies and their Local Electronic Characteristics Andrew DiLullo, Yang Li, Danda Pani Acharya, Noboru Takeuchi, Saw-Wai Hla Nanoscale surface topological variations effect local electrochemical properties. We directly alter nanoscale surface corrugation by local probe manipulations using a scanning tunneling microscope and report here the procedure and resulting changes in surface electrochemistry. Tunneling resonances, found at certain probe-sample biases, are found by analysis of spatial height-differential mapping (dz/dV). These resonances result from field emission where the emitted electron has greater energy than the local surface potential at the probe lateral position. We extract, by fitting to Gundlach's equation, the tip work function, sample work function at probe position, and absolute tip height from the sample. The difference in the extracted work function at the surface vacancies and the surface terraces demonstrates a significantly altered electronic character. It is important to be able to understand nanoscale variations in the local work function, as this surface potential can play a large role in determining the outcome of attempted surface electrochemistry. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L53.00013: Mg(0001): Electronic structure features controlling the limit of and reactivity in the thin-film regime, stacking fault of Mg adislands and adatom self-diffusion Marisol Alcantara Ortigoza, Maral Aminpour, Talat S. Rahman We analyze the electronic structure of the Mg(0001) surface as a function of slab thickness to reveal the features that control chemical reactivity of films of less than 17 layers. The thickness dependence of the oxidation rate of Mg thin films is directly related to the in-plane-PLDOS(E) of the first- and second-layer atoms around the Fermi level. Regarding the origin of the stacking fault - which is attached to a Friedel-oscillation-driven charge density pocket at the fcc site, we find that the role of the charge-density pocket is that of strengthening the substrate bonds since adsorption at the fcc site charge is distributed among surface atoms enhancing their mutual binding. Charge-density analyses, however, are only indirect evidence that the stacking fault is caused by Mg Friedel oscillations. To strengthen our arguments, we thus test an additional material: Be(0001) -- another hcp sp- and nearly-free-electron metal that is also strongly influenced by Friedel oscillations. Comparison of Mg(0001) and Be(0001) shows that the charge density enhancement at the fcc site for Be(0001) is dramatically larger than that found for Mg(0001). Most importantly, Be(0001) provides more evidence that the stacking fault preference is driven by the Friedel oscillations. Namely, the Be monomer on Be(0001) not only also prefers the fcc stacking fault site than the hcp one but the stacking fault energy is strikingly large: 44 meV. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L53.00014: The Physical Character of the Au (001) Surface Reconstruction in the Presence of CO and O2 Andrew Loheac, Michael S. Pierce, Andi Barbour, Vladimir Komanicky, Chenhui Zhu, Hoydoo You The interaction of carbon monoxide and oxygen on Au (001) single crystal facets has been investigated using synchrotron based surface x-ray diffraction and scattering techniques. Preliminary experiments confirm the quasi-hexagonal surface reconstruction can be influenced by exposure to CO and O, and indicate that oxidation may be present. Subsequent surface x-ray scattering experiments included a residual gas analyzer (RGA) with isotopic CO to tag the chemical species. Both CO (by itself) and O (dissociated from molecular $\mathrm{O}_2$ by the x-rays) are capable of lifting the hexagonal surface reconstruction resulting in a disordered bulk truncated surface. A wide range of pressures (1 mTorr - 10 Torr) and temperatures (300 K - 900 K) have been explored. We have also adapted a system of coupled partial differential equations to model the absorption kinetics and surface reconstructions. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L53.00015: When is one layer complete? Using simultaneous in-situ RHEED and x-ray reflectivity to map layer-by-layer thin-film oxide growth M.C. Sullivan, M.J. Ward, H. Joress, A. Gutierrez-Llorente, A.E. White, A. Woll, J.D. Brock The most popular tool for characterizing \textit{in situ} layer-by-layer growth is Reflection High-Energy Electron Diffraction (RHEED). X-ray reflectivity can also be used to study layer-by-layer growth, as long as the incident angle of the x-rays is far from a Bragg peak. During layer-by-layer homoepitaxial growth, both the RHEED intensity and the reflected x-ray intensity will oscillate, and each complete oscillation indicates the addition of one layer of material. However, it is well documented, but not well understood, that the maxima in the RHEED intensity oscillations do not necessarily occur at the completion of a layer. In contrast, the maxima in the x-ray intensity oscillations do occur at the completion of a layer, thus the RHEED and x-ray oscillations are rarely in phase. We present our results on simultaneous \textit{in situ} x-ray reflectivity and RHEED during layer-by-layer growth of SrTiO$_3$ and discuss how to determine the completion of a layer for RHEED oscillations independent of the phase of the RHEED oscillation. [Preview Abstract] |
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