Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session M8: From Single Molecules to Molecular Assemblies on Surfaces III |
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Sponsoring Units: DCMP Chair: Jeff Guest, Argonne National Laboratory Room: 006C |
Wednesday, March 4, 2015 11:15AM - 11:51AM |
M8.00001: Towards two-dimensional ferroelectrics and co-crystals Invited Speaker: Axel Enders I will present an experimental study on the self-assembly and electronic properties of the surface-supported hydrogen-bonded organic ferroelectrics, such as croconic acid (CA), rhodizonic acid (RA), 3-hydroxyphenalenone (3-HPLN) and related compounds. Importantly, the polarization of these organics is within the molecular plane, which in principle allows for switchable polarization within 2D molecular sheets. Such 2D sheets of organic ferroelectrics are therefore the focus of this study. I will present an overview over the structural phases of select organic ferroelectrics on various surfaces and demonstrate how ordered 2D polarization states can emerge. I will demonstrate that the co-deposition of CA and 3-HPLN on flat surfaces results in structurally ordered 2D co-crystalline phases wherein various structural isomers and structures of different stoichiometry can be identified. The presented exploratory solvent-free 2D co-crystallization is a departure from existing approaches to co-crystallization, as it could guide the discovery of potentially valuable molecular ferroelectrics, and enable the rational design of organic ferroelectric co-crystals and n-crystals, which quickly become prohibitively complex to investigate using heuristic-guided approaches. [Preview Abstract] |
Wednesday, March 4, 2015 11:51AM - 12:03PM |
M8.00002: Surface Dipole Control of Liquid Crystal Alignment Jeffrey Schwartz, Yuxi Zhao, Alexandra Mendoza, Natcha Wattanatorn, Paul Weiss We investigate the influence of surface dipoles on the alignment of liquid crystals (LCs). Carboranethiol self-assembled monolayers (SAMs) are shown to induce planar anchoring in 4-cyano-4'-pentylbiphenyl LCs at the SAM/nematic interface. We exploit the different dipole moments of carboranethiol structural isomers in order to deconvolve the influence of SAM-LC dipolar coupling from variations in molecular geometry, tilt, and order. The LC director orientation and anchoring energy are measured for devices employing varying caboranethiol isomer alignment layers. By using LC orientation as a probe of interaction strength, we demonstrate that dipolar coupling of SAMs to their environment plays a key role in determining molecular orientations. This understanding may advance the engineering of molecular interactions at the nanoscale. [Preview Abstract] |
Wednesday, March 4, 2015 12:03PM - 12:15PM |
M8.00003: Self-assembly of functionalized indoles on surfaces Fabrizio De Marchi, Daling Cui, Josh Lipton-Duffin, Clara Santato, Jennifer MacLeod, Federico Rosei To predict how a molecule on a surface will interact with its neighbors or with the substrate itself is an intriguing challenge. If overcome, it would allow the design of a pattern by the proper selection of monomers. However, we are far from a complete understanding of self-assembly mechanisms on surfaces, and more insight can be gathered by studying small, simple systems. In nature, small molecules are the building blocks for more complex systems, such as enzymes and DNA; understanding their self-assembly could lead to the ability to encode this kind of complexity and information density into engineered self-assembled molecular structures. We report here on the self-assembly of two simple molecules: indole 2-carboxylic acid (I2CA) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA) over various substrates. DHICA is one of the monomers that forms eumelanin, and brings the possibility of different bonding architectures due to its combination of carboxyl and hydroxyl groups. At surfaces, DHICA forms a number of structures depending on the conditions used to prepare the molecular film. On the other hand, I2CA self-assembles into a simple ordered pattern that is relatively independent of the substrate and preparation conditions. DFT calculations corroborate these observations. [Preview Abstract] |
Wednesday, March 4, 2015 12:15PM - 12:27PM |
M8.00004: Stereoelectronic Switching in Single-Molecule Junctions Haixing Li, Timothy Su, Michael Steigerwald, Colin Nuckolls, Latha Venkataraman We demonstrate the first single-molecule switch that operates through a stereoelectronic effect in silicon-based molecular backbones terminated with methyl-sulfide linker groups. We utilize the subangstrom level of control in a scanning tunneling microscope-based break-junction (STM-BJ) technique to manipulate the conformation of these single-molecule junctions formed with silanes. We show that we can increase conductance by elongating the molecular junction and decrease conductance by compressing the junction. The switching that we see is binary and is faster than the microsecond time resolution of the STM. Theoretical calculations support the existence of molecular junction conformations that differ in their electronic character, and provide evidence that the strong conjugation in these silicon chains, comparable to that of conjugated carbon chains, enables this stereoelectronic switching. [Preview Abstract] |
Wednesday, March 4, 2015 12:27PM - 12:39PM |
M8.00005: Transport through Self-Assembled Monolayer Molecular Junctions: Role of In-Plane Dephasing Jonatan Dubi Self-assembled-monolayer (SAM) molecular junctions (MJs) constitute a promising building block candidate for future molecular electronic devices. Transport properties of SAM-MJs are usually calculate using either the phenomenological Simmons model, or a fully-coherent transport theory, employing the SAMs periodicity. As I will show, the standard theory seems to have some discrepancy with experimental observations. To overcome these dicrepancies, I suggest that dephasing plays an important role in determining the transport properties of SAM-MJs. I will present an approach for calculating the transport properties of SAM-MJs that inherently takes into account in-plane dephasing in the electron motion as it traverses the SAM plane. The approach describes well the two hallmarks of transport through SAM-MJs, namely the exponential decay of current with molecular chain length and the reduction of the current per molecule as compared to single-molecule junctions. Specifically, I will show that dephasing leads to an exponential decay of the current as a function of molecular length, even for resonant tunneling, where the fully coherent calculation shows little or no length-dependence of the current. The dephasing is also shown to lead to a substantial reduction of the current in a [Preview Abstract] |
Wednesday, March 4, 2015 12:39PM - 12:51PM |
M8.00006: SERS detection of vibrational Stark effect using PCBM-based molecular junctions Yajing Li, Peter Doak, Pavlo Zolotavin, Jeffrey Neaton, Leeor Kronik, Douglas Natelson Understanding the interplay of local electric field and vibrational degrees of freedom of molecules are of interest. We fabricate gold bowtie structures with nanometer inter- electrode spacing using controllable electromigration. Those gold nanostructures support highly localized plasmons and have proven to be suitable SERS substrates with single-molecule sensitivity, which enable the study of molecular vibrational and electronic physics. By measuring the Raman emission from the electrically biased PCBM-containing junctions, we observed strong linear shifts on the vibrational energies of PCBM. We will present the experiments as well as preliminary theoretical expectations obtained by DFT calculations. We compare the field driven change of vibrational energies of PCBM with those observed in C60 junctions, which have been reported to exhibit quadratic change of vibrational energies due to bias induced charge variation. [Preview Abstract] |
Wednesday, March 4, 2015 12:51PM - 1:03PM |
M8.00007: Graphene Template for Epitaxial Growth of Pentacene and C$_{60}$ Thin Film Kwanpyo Kim, Elton J.G. Santos, Tae Hoon Lee, Yoshio Nishi, Zhenan Bao The study and reliable control of molecular packing structures at the graphene-molecule interface are of great importance for various applications. We utilize suspended graphene as an assembly template to investigate thin-film epitaxial growth of various organic molecules. Thin-film packing structures of pentacene and C$_{60}$ on graphene are investigated using transmission electron microscopy. For pentacene thin-film, we observe an unusual polymorph growth on graphene, which shows significant strain along the c-axis of pentacene crystals. Moreover, the strained film exhibits a specific molecular orientation and a strong azimuthal correlation with underlying graphene lattice. For C$_{60}$ crystals, we observe large grain sizes and somewhat strong azimuthal correlation with respect to underlying graphene lattice direction. Utilizing \textit{ab initio} electronic structure calculations with van der Waals interactions, we understand the observed molecular growth behavior mainly with graphene-molecule interaction. [Preview Abstract] |
Wednesday, March 4, 2015 1:03PM - 1:15PM |
M8.00008: Study of the phase separation of organic molecules from solution on Si(111) substrates Miriam Cezza, Colin Qualters, Raymond Phaneuf Understanding the science behind assembly of small organic molecules into domains is important for numerous applications, among which organic solar cells are especially noteworthy. An important process on which organic solar cells depends is the phase separation of organic molecules. The formation of a morphology during phase separation from a solvent-based, bimolecular solution onto a substrate depends on several parameters: relative molecular concentrations, solubilities of each type of molecule in the solvent, solvent evaporation rate, and annealing conditions. We carry out studies on molecular mixtures consisting of tetranitro zinc-phthalocyanine (tn-ZnPc) and PCBM in chloroform, and native oxide-covered Si(111) substrates. We investigate the role that solvent evaporation rate during deposition, followed by solvent vapor annealing (SVA), plays on the formation of phase separated mixtures and their crystallization and phase transformation. We also investigated the relative concentration of individual molecules in mixtures. We found that PCBM molecules alone undergo several phase transformations as the solvent evaporation rate decreases, while tn-ZnPc is very stable. Moreover, the concentration of tn-ZnPc in mixtures highly affects the PCBM crystallization. [Preview Abstract] |
Wednesday, March 4, 2015 1:15PM - 1:27PM |
M8.00009: Atmospheric Effects on diF TESADT Thin-Film Transistors Brad Conrad, Cortney Bougher, Shawn Huston, Jeremy Ward, Abdul Obaid, Marsha Loth, John Anthony, Oana Jurchescu Crystalline organic semiconductors often display carrier mobilities that vary with environmental conditions and fabrication parameters. Additionally, the electrical properties of organic thin-film devices are highly dependent on film structure, crystallinity, and molecular packing. In solution-deposited polycrystalline thin-films, the regions between crystals often affect the overall device performance, as molecular ordering and crystal structure may differ significantly from neighboring regions. Device characterization and Kelvin Probe Force Microscopy (KPFM) is used to analyze the electrical properties of grain boundaries, electrodes, and crystalline regions within 2,8-difluoro-5,11-triethysilylethynyl anthradithiophene (diF TESADT) thin-film transistor surfaces. The influence of both atmospheric dopants and exposure time is examined and explained in the context of device characterization and interfacial effects. [Preview Abstract] |
Wednesday, March 4, 2015 1:27PM - 1:39PM |
M8.00010: 2d Assembly and solvation of supramolecular ionic polymers Janice Reutt-Robey, Qian Shao, Levan Tskipuri, Daisuke Takajo Supramolecular polymers are important building blocks for functional nanomaterials. The structural fidelity of soft (non-covalent) species during transfer from the solution phase to a solid substrate is an important issue for material design. We report on the 2d structures of supramolecular ionic polymer chain structures consisting of the ionic solutes C186H244B3IrN6O12 ($+$3) (twin bowl) and IrN6C30H24 (-3). In solution, these ionic solutes assemble into supramolecular chain-like structures, with lengths averaging 20 nm. Deposition onto an Ag(111) substrate by a liquid microaerosol source yields 2d islands of the ionic polymers embedded in a thin film of the solvent, CH3Cl. Molecularly resolved UHV-STM images reveal in tact transfer of the supramolecules with size distributions comparable to the solution phase. Solute ion attachment/detachment from the supramolecular chain ends occurs, facilitated by the bounding solvent layer. Solvent-solute islands adopt striking geometric shapes and these structures are discussed in terms of 2-d solvation energies. This work was supported by the National Science Foundation under CHE-MSN Grant CHE1310380. [Preview Abstract] |
Wednesday, March 4, 2015 1:39PM - 1:51PM |
M8.00011: High-resolution imaging of interfacial water: from water monomer to two-dimensional ice Ying Jiang, Jing Guo, Xiangzhi Meng, Ji Chen, Jinbo Peng, Jiming Sheng, Limei Xu, Xinzheng Li, Enge Wang Water-solid interactions are of broad importance both in nature and technology. The hexagonal bilayer model based on the Bernal-Fowler-Pauling ice rules has been widely adopted to describe water structuring at interfaces. Recently, we made a breakthrough in achieving submolecular-resolution imaging of individual water molecules using a scanning tunneling microscope (STM) [1]. Such a technique opens up the possibility of determining the detailed topology of H-bonded networks at water/solid interfaces with atomic precision. Thanks to the high-resolution STM imaging, we discover a new type of two-dimensional (2D) ice-like bilayer structure built from cyclic water tetramers on an insulating NaCl(001) film, which is completely beyond the conventional bilayer picture [2]. A novel bridging mechanism allows the interconnection of water tetramers to form chains, flakes and eventually a 2D extended ice bilayer containing a regular array of Bjerrum D-type defects. Ab initio density functional theory calculations substantiate this bridging growth mode and reveal a striking proton-disordered ice structure. [1] J. Guo, X. Z. Meng, J. Chen, J. B. Peng, J. M. Sheng, X. Z. Li, L. M. Xu, J. R. Shi, E. G. Wang*, and Y. Jiang*, Nature Materials 13, 184 (2014). [2] J. Chen, J. Guo, X. Z. Meng, J. B. Peng, J. M. Sheng, L. M. Xu, Y. Jiang*, X. Z. Li*, E. G. Wang, Nature Communications 5, 4056 (2014). [Preview Abstract] |
Wednesday, March 4, 2015 1:51PM - 2:03PM |
M8.00012: Amino Acid Immobilization of Surface Diffusion on Copper Nathan Guisinger, Erin Iski, Andrew Mannix, Brian Kiraly, Brandon Fisher, Mark Hersam The 2D-scale study of relevant biomolecules, like amino acids, is pertinent for a variety of applications from the origin of biological homochirality and the amplification of surface chirality to the examination of noncovalent supramolecular interactions. The need for pristine molecular resolution of these systems requires the use of ultra-high vacuum scanning tunneling microscopy (UHV STM) as the primary technique for these studies. Through the detailed examination of the self-assembly behavior of five amino acid molecules on a Cu(111) single crystal, a fascinating and unexpected phenomena was discovered. All of the amino acids assisted in the immobilization of copper atoms on the surface. The energetic landscape of the surface as mediated by temperature and molecular coverage facilitated the growth of copper islands. The growth and size fluctuation of the islands offered an interesting snapshot of metal nanocluster diffusion that often occurs at time scales beyond the resolution of a given experimental technique. The presence of $\sim$ 1 ML of molecules on the surface effectively trapped the metal atoms into localized islands. Elevated temperatures ($\le $ 350 K) were used to promote the further diffusion, coalescence, and extinction of the islands for a more detailed understanding of the coarsening and ripening mechanisms. [Preview Abstract] |
Wednesday, March 4, 2015 2:03PM - 2:15PM |
M8.00013: Metal-Organic Chains with Single-Site Pt(II): Insights from first principles simulations Duy Le, Talat S. Rahman Creation, stabilization, characterization and control of single atom transition metal sites on surfaces may lead to significant advancement of the next-generation catalyst. Motivated by the experimental results of Skomski eu al. [1], we have performed density functional theory calculations of Pt-dipyridyltetrazine complexes on the reconstructed Au(100) surface. Results of our simulations show that the Pt-dipyridyltetrazine complexes form 1-dimensional chains aligned 45$^o$ with respect to the Au(100) reconstruction row with the molecule-molecule distance of 6.93 \AA. More importantly, Bader analysis shows that Pt atoms are cationic with +0.75 charge. This amount of charge is in accord with the charge on Pt in PtO determined by the same analysis indicating that the oxidation states of the Pt atoms in the Pt-dipyridyltetrazine network on the reconstructed Au(100) surface are closer to that of Pt atoms in PtO, which is +2, than in Pt$_3$O$_4$ or PtO$_3$. This result agrees extremely well with experimental XPS data [1]. [1] D. Skomski, C.D. Tempas, K.A. Smith, and S.L. Tait, J. Am. Chem. Soc. 136, 9862 (2014). [Preview Abstract] |
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