Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Q53: Surfaces, Interfaces, and Thin Films: Molecules on Surfaces |
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Sponsoring Units: DMP Chair: Chenggnag Tao, Virginia Polytechnic Institute and State University Room: Mile High Ballroom 2C |
Wednesday, March 5, 2014 2:30PM - 2:42PM |
Q53.00001: End Functional Hydrogen Bonding Modulates Odd-Even Effect in Alkanethiol Monolayer Assembly Kshitij Jha, Yeneneh Yimer, Mesfin Tsige Hydroxyl (-OH) terminated n-alkanethiols on gold show a two-dimensional zig-zag nature for the top hydrogen bonded network. We observe transfer of packing characteristics from top network to the buried ad-atom (sulfur) distribution quantified as occupancy percentages for atop, hollow (fcc and hcp), and bridge sites. Employing validated metal potentials and all-atom molecular dynamics, we also quantify the dynamic correlation between the top layer (end-functional) and ad-layer (thiol) through variance in distribution peaks for nearest neighbors as a function of temperature. The hydrogen bond network and packing of the monolayer increases in strength with chain length for –OH terminated n-alkanethiols. Shorter chain lengths lead to better transfer of packing, for a given network strength. Odd chain lengths, compared to even, have a lower lattice spacing by an average of 0.04 {\AA}. The transfer effect of the top network is not observed, as expected, for methyl (-CH$_3$) terminated n-alkanethiols since there is no hydrogen bonding. Trends in packing and transfer for monolayer assembly could provide design principles for polymer based nanoactuators and sensors. [Preview Abstract] |
Wednesday, March 5, 2014 2:42PM - 2:54PM |
Q53.00002: Imaging the wave functions of adsorbed molecules using angle-resolved photoemmision data Daniel L\"uftner, Thomas Ules, Eva Maria Reinisch, Georg Koller, Serguei Soubatch, F. Stefan Tautz, Michael G. Ramsey, Peter Puschnig The frontier electronic orbitals of molecules are the prime determinants of the respective compounds' chemical, electronic, and optical properties. Although orbitals are very powerful concepts, experimentally only the electron densities and energy levels are directly observable. As has been shown in recent publications, angle-resolved photoemission (ARPES) intensity maps of organic molecular layers are related to the absolute value of the Fourier transform of the initial state molecular orbital. However, the lost phase information impedes the back-transformation of the orbital into real space. Here, we show how molecular orbital images as well as the absent phase information can be retrieved by applying an iterative procedure which takes experimental ARPES maps as input and only assumes spatial confinement of the orbital. The method is demonstrated for several molecular orbitals of two proto-typical pi-conjugated molecules: the LUMO, HOMO, and HOMO-1 of pentacene, and the LUMO and HOMO of PTCDA [1]. The technique is simple and robust and further emphasizes the capabilities of ARPES looking at spatial distributions of wave functions of adsorbed molecules thereby complementing data obtained from scanning probe methods. [1] D.L\"uftner et al., PNAS (accepted) [Preview Abstract] |
Wednesday, March 5, 2014 2:54PM - 3:06PM |
Q53.00003: Tuning gap states at organic-metal interfaces by quantum size effects Meng-Kai Lin, Yasuo Nakayama, Chin-Hung Chen, Chin-Yung Wang, H.-T. Jeng, Tun-Wen Pi, Hisao Ishii, S.-J. Tang Organic-metal interfaces are key elements to the organic-based electronics. The energy level alignment (ELA) between metal Fermi level and molecule orbital levels determines the injection barriers for the charge carriers at the interfaces, which are crucial for the performance of organic electronic devices. Dipole formation at the interfaces has been regarded as the main factor for ELA and several models were proposed for the mechanism of it in the context of the interface between organic molecules and bulk metal crystal surface, at which surface states (SS) were mostly used to probe the interfacial properties. We show that when the bulk metal crystal is replaced by a uniform metal thin film, another 2-dimensional electronic state, quantum well states , will not only be able to probe but also modify the interfacial electronic structures such as gap states, which don't have the counterpart at the organic-bulk crystal interface. Moreover, thickness-dependent quantum size effects of metal thin films provide a new method for engineering the organic electronic devices. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q53.00004: Single Molecule Characterization of Conjugated Oligomers Formed through Radical Cyclization at a Surface Hsin-Zon Tsai, Alexander Riss, Sebastian Wickenburg, Liang Tan, Patrick Gorman, Dimas Oteyza, Yen-Chia Chen, Aaron Bradley, Miguel Ugeda, Grisha Etkin, Steven Louie, Felix Fischer, Michael Crommie Conjugated polymers have gained considerable attention due to their potential industrial applications and interesting fundamental properties. Real-space imaging their chemical bonds and understanding their electronic structures at the nanoscale could lead to enhanced control in the synthesis of these polymers for the potential applications in the nanoelectronics. Here, we present the synthesis and characterization of poly-acetylene derivatives resulting from cyclizations of enediyne molecules on an Au(111) surface. We performed non-contact atomic force microscopy (nc-AFM) with sub-molecular resolution to determine the precise chemical structure of cyclized monomers and chemically linked molecular chains. Additionally, STM measurements provide insight into the corresponding electronic structure and reveal a 1D conducting channel along the backbone of the conjugated oligomers, consistent with theoretical predictions. This work demonstrates the unique insight that can be gained by combining nc-AFM and STM to study the chemical and electronic structure of molecular assemblies at surfaces. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q53.00005: Scanned Probe Characterization of Atmospheric Effects on diF TESADT Thin-Film Transistors Cortney Bougher, Shawn Huston, Jeremy Ward, Abdul Obaid, Marsha Loth, John Anthony, Oana Jurchescu, Brad Conrad Single crystal organic semiconductors have been shown to exhibit carrier mobilities comparable to the silicon currently used in photovoltaics. However, during solution deposition of common organic semiconducting materials the resultant thin-film is often polycrystalline. Device performance and electrical properties of organic thin-film transistors are highly dependent on crystal structure and molecular packing. In polycrystalline thin-films, boundary regions between crystal grains can affect the overall performance of devices, as crystal structure and packing may differ from that of the surrounding crystal regions. These boundary regions may also serve as defect sites, allowing environmental factors, such as oxygen content and humidity, to alter local charge transport through devices. We utilize Kelvin Probe Force Microscopy (KPFM) to characterize how grain boundaries alter local conductivity and device performance as a function of doping in 2,8-difluoro-5,11-triethysilylethynyl anthradithiophene (diF TESADT) thin-film transistor surfaces. Device voltage drops at grain boundaries are characterized as a function of both atmospheric dopants and transition time between dopants. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q53.00006: Interfacial energy level shifts in few-molecule clusters of the organic semiconductor PTCDA Sarah Burke, Katherine Cochrane, Agustin Schiffrin, Tanya Roussy Detailed knowledge of the local electronic structure of organic semiconductors near interfaces is crucial for the understanding of a variety of electronic and optoelectronic applications of these emerging materials. However, organic molecules are highly sensitive to the local environment, which abruptly changes at an interface. Here, we present a study on the prototypical organic semiconductor PTCDA by scanning tunneling microscopy and spectroscopic mapping. Nanoscale clusters of varying size and geometry were probed on a bilayer NaCl film on Ag(111). The molecular states, while decoupled from the underlying metal surface, are relatively delocalized within these monolayer islands. Depending on the size of the cluster and arrangement of molecules within the cluster, edge molecules exhibit varying energy level shifts relative to the central molecules, both of which differ from the isolated molecule. For well ordered islands, this leads to a type-1 heterojunction, with a larger band gap at the edge of the cluster differing by as much as 0.5eV. In considering nanoscale structures within multicomponent device architectures, such internal heterostructures established by differences in the local environment are an important consideration, and could even be exploited. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q53.00007: The Observation of an Unoccupied Shockley-Type Surface State at the H$_2$-phthalocyanine/Ag(111) Interface Benjamin Caplins, David Suich, Alex Shearer, Charles Harris A free-electron interface state has been observed at the H$_2$-Phthalocyanine/Ag(111) interface using time- and angle-resolved two-photon photoemission. Energetically the interface state is located $\sim$0.21 eV above the Fermi level and angle-resolved measurements yield an effective mass of 0.5 $m_e$. These measurements, in conjunction with density functional theory calculations allow us to assign the interface state as being a metal/molecule hybrid state derived from the Shockley state of the clean Ag(111) surface. Time-resolved measurements of two different crystalline phases of H$_2$-Phthalocyanine monolayers reveal that the lifetime of the interface state is sensitive to the bonding geometry of the molecule. The results of this study add to a mounting body of evidence that suggests that the Shockley surface state is robust and persists after deposition of organic $\pi$-conjugated materials in the form of an uplifted interface state, instead of being `quenched'. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q53.00008: Few-layer and symmetry-breaking effects on the electrical properties of ordered CF$_3$Cl phases on graphene Josue Morales-Cifuentes, Yilin Wang, Janice Reutt-Robey, T.L. Einstein An effective pseudopotential mechanism for breaking the inherent sub-lattice symmetry of graphene has been studied using DFT calculations on hexagonal boron nitride.\footnote{Gianluca Giovannetti et al.\ , PRB 76, 073103 (2007)} Electrical detection of CF$_3$Cl phase transitions on graphene shows the existence of a commensurate ordered phase in which this can be tested.\footnote{Yilin Wang et al.\ , APL 103, 201606 (2013)} We study the electronic properties of this phase using VASP ver 5.3.3, with ab initio van der Waals density functionals (vdW-DF1 and vdW-DF2).\footnote{Ji\v{r}\'{i} Klime\v{s} et al.\ , PRB 83, 195131 (2011) } \footnote{Kyuho Lee et al.\ , PRB 82, 081101(R) (2010) }Consistent with a physisorbed phase, binding energies and charge transfer per CF$_3$Cl molecule are calculated to be on the order of 280meV and 0.01e, respectively. By exploring different coverages and orientations of this ordered phase we are able to open a band gap in some configurations; said gap is in the range of 8 to 80meV depending on the strength of the effective pseudopotential. Furthermore, we calculate the screening of these effects in bi-layer and tri-layer graphene. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q53.00009: Covalent Attachment to GaP(110) -- Engineering the Chemical Functionalization of a III-V Semiconductor A.J. Bradley, M.M. Ugeda, Wenjun Liu, Min Yu, T. Don Tilley, Rub\'en P\'erez, Jeffrey B. Neaton, M.F. Crommie With its 2.3 eV bulk bandgap, relatively high conduction band edge, and low chemical reactivity, the (110) surface of GaP is an excellent candidate for many UV and visible light applications, such as photo-catalysis and light-induced chemical reduction. However, the reconstruction and resulting charge transfer of the surface makes it difficult to covalently attach the required molecules. Indeed, very little work has been done to understand either covalent functionalization or passivation of this surface. Here we report on a Staudinger-type, thermally-driven covalent attachment of perfluorophenyl azide (pfpa) to GaP(110). We have studied the adsorption of pfpa molecules by means of high-resolution scanning tunneling microscopy and spectroscopy in combination with first principles calculations. We show a progression from a physisorbed state at room temperature to a covalently attached state after exposure to slightly higher temperatures ($\sim$ 50$^{\circ}$C). The developed approach is expected to be valid for various other functional groups attached to the azide, as well as other III-V semiconductors. \\[4pt] [1] J.L.A. Alves, et. al.\textit{Phys. Rev. B.}, 6188-6198.\\[0pt] [2] M.M. Ugeda, et. al.\textit{J. Phys. Chem. C.}(accepted November 2013). [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q53.00010: Ferrocenes on Calcite: Single-electron tunneling detected at room temperature Philipp Rahe, Ryan Steele, Clayton Williams We present the assembly of a functionalized ferrocene derivative on a truly insulating support, namely the calcite (10\={1}4) surface, and investigate the transfer of single electrons between the molecules and the conductive tip of an atomic force microscope in the absence of a macroscopic tunneling current. Molecules on insulating surfaces attract currently increasing attention [1], stimulated by promising applications in the fields of surface functionalization and, especially, in the context of molecular (opto-)electronics. For isolated atoms and single molecules adsorbed on thin insulating films, the manipulation and storage of single charges has been induced by a tunneling current [2,3]. Our approach, however, is based on single-electron tunneling force microscopy methods [4,5] combined with Kelvin-probe force microscopy. By using this combination of methods we present the measurement and control of the charge state of the ferrocene molecules by injecting and extracting charge on the order of single electrons.\\[4pt] [1] Rahe et al.; Adv. Mater. 2013, 25, 3948;\\[0pt] [2] Gross et al.; Science 2009, 324, 1428;\\[0pt] [3] Leoni el al.; Phys. Rev. Lett 2011, 106, 216103;\\[0pt] [4] Bussmann et al.; Appl. Phys. Lett. 2004, 85, 2538;\\[0pt] [5] Bussmann et al.; Appl. Phys. Lett. 2006, 88, 263108 [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q53.00011: Identifying individual chemical bonds in single-molecule chemical reaction products using nc-AFM Sebastian Wickenburg, Dimas G. de Oteyza, Yen-Chia Chen, Alexander Riss, Hsin-Zon Tsai, Zahra Pedramrazi, Aaron J. Bradley, Miguel M. Ugeda, Patrick Gorman, Grisha Etkin, Duncan J. Mowbray, Alejandro Perez, Angel Rubio, Michael F. Crommie, Felix R. Fischer Determining reaction pathways and products is an integral part of chemical synthesis. Ensemble measurements are commonly used, but identifying products of complex reactions at surfaces presents a significant challenge. Here we present a non-contact AFM (nc-AFM) study to directly address this issue[1]. We followed the change of the chemical structures, from reactants to products of enediyne cyclization reactions on metal surfaces. Thermal annealing of enediynes induced a series of cyclization cascades leading to radical species and the formation of dimers. Atomically resolved nc-AFM images reveal the precise chemical structure and the formation of chemical bonds between single molecular units. With the support of DFT calculations, we identified the underlying chemical pathways and barriers, demonstrating the potential of this atomically resolved AFM technique to study unknown reaction products in surface chemistry at the single-molecule level. [1] D. G. de Oteyza et al., Science 340, 1434 (2013) [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q53.00012: Electronic Polarization at Pentacene/Polymer Dielectric Interfaces: Imaging Surface Potentials and Contact Potential Differences as a Function of Substrate Type, Growth Temperature, and Pentacene Microstructure Yanfei Wu, Greg Haugstad, C. Daniel Frisbie Interfaces between organic semiconductors and dielectrics may exhibit interfacial electronic polarization, which is equivalently quantified as a contact potential difference (CPD), an interface dipole, or a vacuum level shift. Here we report quantitative measurements by Scanning Kelvin Probe Microscopy (SKPM) of surface potentials and CPDs across ultrathin (1-2 monolayer) crystalline islands of the benchmark semiconductor pentacene thermally deposited on a variety of polymer dielectrics (e.g., poly(methyl methacrylate), polystyrene). The CPDs between the pentacene islands and the polymer substrates are in the range of -10-$+$50 mV, they depend strongly on the polymer type and deposition temperature, and the CPD magnitude is correlated with the dipole moment of the characteristic monomers. Surface potential variations within 2 monolayer (3 nm) thick pentacene islands are approximately15 mV and may be ascribed to microstructure (epitaxial) differences. Overall, the microscopy results reveal both strong variations in interfacial polarization and lateral electrostatic heterogeneity; these factors ultimately should affect the transport properties of these interfaces in devices. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q53.00013: Tunable Charge Transfer Dynamics at Tetracene/LiF/C60 Interfaces Dmitry Yarotski, Siddharth Sampat, Aditya Mohite, Brian Crone, Anton Malko, Antoinette Taylor, Sergei Tretiak Organic conducting polymers offer an attractive alternative to regular semiconductors in both photovoltaic and optoelectronic applications due to their low cost and improved processability. Although current organic devices suffer from relatively low electro-optical conversion efficiency, novel polymer nanocomposites, both organic/organic and organic/inorganic, should provide better control over the material properties and improve the device performance. In such nanocomposites, physical and electronic structures of the interfaces govern carrier generation and transport characteristics yet underpinning mechanisms are still poorly understood. Here, we apply ultrafast optical spectroscopy to observe an interfacial charge transfer dynamics in Tc/LiF/C60 multilayered heterostructures, where charge separation processes compete with parasitic radiative and non-radiative charge transfer exciton recombination. Our studies reveal that the tunneling barrier created by LiF buffer layer between donor and acceptor materials provides means for independent control over the rates of direct and diffusion-induced charge transfer exciton formation and recombination dynamics. These findings might have implications for development of more efficient organic photovoltaic and light-emitting devices. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q53.00014: Tuning energy level alignment at organic/semiconductor interfaces using a built-in dipole in Chromophore-Bridge-Anchor compounds Sylvie Rangan, Alberto Batarseh, Keyur Chitre, Andrew Kopecky, Elena Galoppini, Robert Bartynski We report a new approach, using chromophore-containing molecules designed to contain internal dipoles, for controlling at the molecular level the energy level alignment between an organic chromophore and a transition metal oxide surface. The approach employs a chromophore-bridge-anchor molecular architecture where the three components are electronically decoupled. By introducing electron donor (D) and acceptor (A) groups to the bridge, an intramolecular dipole is introduced between the chromophore and the anchor. When a monolayer of such molecules is bonded to a metal oxide surface, the resulting dipole layer establishes a potential difference that shifts the chromophore levels with respect to those of the substrate. This concept is demonstrated using a chromophore (ZnTPP)-bridge (substituted with an electron donating (NMe2) and electron withdrawing (NO2) groups to create a built-in dipole)-anchor (Isophtalic acid) architecture. Shifts of the chromophore's HOMOs on the order of plus or minus 0.1 eV with respect to the ZnO valence band edge have been observed, without altering the photoabsorption properties of the chromophore or the HOMO-LUMO gap. An important strength of this concept is that it provides a general design applicable to a large number of anchoring functional groups, built-in dipole bridges, and redox-active centers. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q53.00015: Vibrational properties of an adamantane monolayer on a gold surface Yuki Sakai, Giang D. Nguyen, Rodrigo B. Capaz, Sinisa Coh, Ivan V. Pechenezhskiy, Xiaoping Hong, Michael F. Crommie, Feng Wang, Susumu Saito, Steven G. Louie, Marvin L. Cohen We study the vibrational properties of an adamantane monolayer on a Au(111) surface. The IR spectrum of a self-assembled monolayer of adamantane on Au(111) is measured by a newly developed infrared scanning tunneling microscopy (IRSTM) technique. We analyze the IR spectrum of this system by a density functional theory and find that the IR spectrum is severely modified by both adamantane-gold and adamantane-adamantane interactions. One of three gas-phase C-H bond stretching modes is significantly red-shifted due to the molecule-substrate interactions. The intermolecular interactions cause a suppression of the IR intensity of another gas-phase IR peak. The techniques used in this work can be applied for an independent estimate of molecule-substrate and intermolecular interactions in related diamondoid/metal-substrate systems. [Preview Abstract] |
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