Session T5: Assembly of Atoms and Molecules in Adsorption Systems

Chirality Recognition and Transition Mechanism of prochiral Molecules on metals

The self-assemble behavior of prochiral species and the induced high-order chirality by 2D confinement on solid surfaces, including (1) QA16C molecules on a Au(111) surface and (2) molecule-metallic (TPA-Fe) complex on Cu(110) as well as their transferring process will be presented. Initial stages of a chiral phase transition in the molecule monolayer on metal surfaces were investigated by scanning tunneling microscopy (STM) at submolecular resolution. The prochiral QA16C molecules form a homochiral lamella phase at low coverages upon adsorption. A transition to a racemate lattice is observed with increasing coverage. Enantiomers of a homochiral lamella line become specifically substituted by opposite enantiomers such that a heterochiral structure evolves. A ``chiral replacement'' model is proposed for the transition: enantiomers replace QA molecules in enantiopure phase, leading to racemic one. Our findings are significant for the understanding and control of chiral phase transitions in related molecular systems like liquid crystals.   

A Library of the Nanoscale Self-Assembly of Amino Acids on Metal Surfaces

The investigation of the hierarchical self-assembly of amino acids on surfaces represents a unique test-bed for the origin of enantio-favoritism in biology and the transmission of chirality from single molecules to complete surface layers. These chiral systems, in particular the assembly of isoleucine and alanine on Cu(111), represent a direct link to the understanding of certain biological processes, specifically the preference for some amino acids to form alpha helices vs. beta-pleated sheets in the secondary structure of proteins. Low temperature, ultra-high vacuum, scanning tunneling microscopy (LT UHV-STM) is used to study the hierarchical self-assembly of different amino acids on a Cu(111) single crystal in an effort to build a library of their two-dimensional structure with molecular-scale resolution for enhanced protein and peptide studies. Both enantiopure and racemic structures are studied in order to elucidate how chirality can affect the self-assembly of the amino acids. In some cases, density functional theory (DFT) models can be used to confirm the experimental structure. The advent of such a library with fully resolved, two-dimensional structures at different molecular coverages would address some of the complex questions surrounding the preferential formation of alpha helices vs. beta-pleated sheets in proteins and lead to a better understanding of the key role played by these amino acids in protein sequencing.   

Surface Patterning of Benzene Carboxylic Acids on Graphite: Influence of structure, solvent, and concentration on molecular self-assembly

Scanning tunneling microscopy (STM) was used to investigate the molecular self-assembly of four different benzene carboxylic acid derivatives at the liquid/graphite interface: pyromellitic acid (1,2,4,5-benzenetetracarboxylic acid), trimellitic acid (1,2,4-benzenetricarboxylic acid), trimesic acid (1,3,5-benzenetricarboxylic acid), and 1,3,5-benzenetriacetic acid. A range of two dimensional networks are observed that depend sensitively on the number of carboxylic acids present, the nature of the solvent, and the solution concentration. We will describe our recent efforts to determine (a) the preferential two-dimensional structure(s) for each benzene carboxylic acid at the liquid/graphite interface, (b) the thermodynamic and kinetic factors influencing self-assembly (or lack thereof), (c) the role solvent plays in the assembly, (e) the effect of \textit{in situ} versus \textit{ex situ} dilution on surface packing density, and (f) the temporal evolution of the self-assembled monolayer. Results of computational analysis of analog molecules and model monolayer films will also be presented to aid assignment of network structures and to provide a qualitative picture of surface adsorption and network formation.   

Distinct Monolayer Structure of DH6T Films Grown on Untreated SiO$_{2}$

Due to the relatively weak interactions between neighboring molecules, thin films of vacuum evaporated organic semiconductors can have different properties in monolayer films as compared to several monolayer and bulk-like films. We have structurally characterized monolayer and several monolayer films of dihexyl-sexithiophene (DH6T) using grazing incidence diffraction and near-edge x-ray absorption fine structure spectroscopy (NEXAFS), and electrically characterized low-coverage FET structures during deposition. The structural data indicate a distinct phase of DH6T in the monolayer which has a distorted unit cell and a distinct NEXAFS signature compared to thicker films. The hole mobility of $\sim$0.8 cm$^{2}$/V-s is approximately an order of magnitude higher than previously reported for vacuum deposited thick films of DH6T on unmodified SiO$_{2}$ surfaces.   

Nucleation of C$_{60}$ on ultrathin SiO$_2$

We utilize scanning tunneling microscopy to characterize the nucleation, growth, and morphology of C$_{60}$ on ultrathin SiO$_2$ grown at room temperature. C$_{60}$ thin films are deposited in situ by physical vapor deposition with thicknesses varying from $<$0.05 to $\sim $1 ML. Island size and capture zone distributions are examined for a varied flux rate and substrate deposition temperature. The C$_{60}$ critical nucleus size is observed to change between monomers and dimers non-monotonically from 300 K to 500 K. Results will be discussed in terms of recent capture zone studies and analysis methods. Relation to device fabrication will be discussed. doi:10.1016/j.susc.2011.08.020   

Scanning Tunneling Spectroscopy of Self-assembled Nanoribbons of C$_{60}$-Diamantane Hybrid Molecules

As transistors approach the nanoscale, single molecules become viable alternatives for macroscopic devices. In terms of utilizing carbon, diamondoids -- single cages of the bulk diamond lattice -- have recently become available as the smallest units to explore these structures. We use low temperature scanning tunneling microscopy to perform electron mapping studies on self-assembled monolayers of novel hybrid molecules consisting of a single C$_{60}$ fused with the double diamond cage called diamantane. Unlike standard C$_{60}$ self-assembled monolayers, these hybrid molecules tend to form strips or nanoribbon assemblies as opposed to large-scale single sheets. We find spectroscopic differences between these hybrid molecules and the ``parent'' molecule C$_{60}$, and utilize spatial mapping to find electronic differences in the edge states of these ribbons. We discuss these results in terms of rectification and the potential of these hybrid molecules for molecular electronics.   

Pentacene pinwheels: chiral heterostructures between C$_{60}$ and pentacene

Pentacene and C$_{60}$ are archetypal molecules for optically active acceptor-donor molecular heterojunctions and for the study of self-assembly on surfaces. Using UHV STM, we demonstrate that these molecules - despite their high degree of symmetry - can self-assemble into {\it chiral} heterostructures when C$_{60}$ is deposited on a 'random-tiling' [1] of pentacene (Pn) on Cu(111). Two different chiral heterostructures with a central C$_{60}$ surrounded by 6 Pns in a 'pinwheel' formation are identified: one with interlocking Pns of mixed chirality and one with shared Pns of the same chirality. The latter is observed in single chirality domains that include hundreds of pinwheels. These complex structures are solved unambiguously through the analysis of the STM images and an understanding of Pn adsorption on Cu(111) [2]. Chiral light-absorbing acceptor-donor heterojunctions may provide a powerful platform for detecting and manipulating charge, spin, and optical polarization; we will discuss the electronic properties of these structures, and the prospect for studying the interaction of these types of systems with light using our laser-STM approach.\\[4pt] [1] J. A. Smerdon et al., Phys. Rev. B {\bf 84}, 165436 (2011).\\[0pt] [2] J. Lagoute et al, Phys. Rev. B {\bf 70}, 245415 (2004).   

Interaction of CO$_{2}$ with Oxygen Adatoms on Rutile TiO$_{2}$(110) Surface

On TiO$_{2}$(110), oxygen vacancies (V$_{O}$'s) act as the primary catalytic sites and as such they have been extensively investigated. However, only a few studies have been reported about the interactions of adsorbates with oxygen adatoms (O$_{a}$'s) that are created by O$_{2}$ dissociation in V$_{O}$'s. Here, we report a combined scanning tunneling microscopy (STM) / density functional theory (DFT) study of CO$_{2}$ on bare and O$_{a}$ covered TiO$_{2}$(110). STM images of TiO$_{2}$(110) surfaces obtained before and after in-situ dose at $\sim $50 K show that CO$_{2}$ molecules preferentially adsorb next to O$_{a}$'s forming CO$_{2}$/O$_{a}$ complexes. Temperature dependent studies further reveal that the CO$_{2}$ binding energy next to O$_{a}$'s is similar to that on V$_{O}$'s. Additional CO$_{2}$ molecules are found to diffuse rapidly along the Ti row between two CO$_{2}$/O$_{a}$ complexes. Due to the slow STM sampling rate the images display a time average of all CO$_{2}$ binding configurations on the Ti rows and reveal differences in the populations found on ideal Ti sites and Ti sites next to V$_{O}$'s.   

Adsorption of CO$_{2}$ in porous MCM-41 and MCM-48 using small angle scattering

Adsorption of CO$_{2}$ onto the surface of nanopores in organic rich materials, such as shale and coals, is of great interest for understanding the processes associated with geological sequestration. These natural materials have complex pore structures which make the interpretation of experimental sorption measurements complicated. MCMs are synthetic materials with a well-defined regular porous structure that provides an ideal substrate to evaluate the models for the adsorption of gases (CO$_{2})$ into nanopores. Samples of MCM-41 and MCM-48 were synthesized at Indiana University and were characterized by nitrogen adsorption isotherms and Small Angle X-ray Scattering (SAXS). SANS studies were carried out on MCMs with different pore sizes as a function of pore filling and the results are interpreted in terms of layer growth models.   

Self-Assembly of Metal Phthalocyanine on Silicon Studied by Scanning Probe Microscopy

Integration of molecular electronics into modern electronics requires an improved understanding and control of hetero-interfaces between organic molecules and semiconductor surfaces. Supramolecular assembly provides flexibility in building up complex and functional materials with nanometer scale precision over large surface areas. To study behavior at the hetero-interface, we perform supramolecular assembly of Zinc Phthalocyanine (ZnPc) on deactivated Si(111)B $\surd $3x$\surd $3 and on Si(111) 7x7. Using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) methods both the morphology and electronic structures of the self-assembled monolayer are investigated. Molecule-molecule and molecule-substrate interactions will be discussed.   

Decomposition of NH$_{3}$ and H$_{2}$ on ZrB$_{2}$ (0001) surface

Group III nitride semiconductors (AlN, GaN, InN, and their alloys) are important materials for applications in solid-state lighting, optoelectronics, and photovoltaics. However, the lack of lattice--matched substrates for their growth results in less than optimal material quality. In the last decade, zirconium diboride (ZrB$_{2})$ has been demonstrated as a promising substrate for GaN growth because of its similar lattice constant and thermal expansion properties when compared to the nitride. Moreover, the high electrical conductivity of ZrB$_{2}$ makes it desirable for many GaN-based device applications. In this talk, we present results of density functional theory calculations for the reactivity of the ZrB$_{2}$(0001) surface towards the N precursor, NH$_{3}$, and the carrier gas, H$_{2}$, commonly used in metal organic chemical vapor deposition and molecular beam epitaxy of nitrides. Two different terminations of ZrB$_{2}$(0001) surface, the Zr and B terminations, are considered and assessed in terms of their catalytic properties toward NH$_{3}$ and H$_{2}$ decomposition. The theoretical results are analyzed in connection with our recent XPS and RAIRS measurements.   

Morphology of monolayer films on quasicrystalline surfaces from the phase field crystal model

We present a computational study of the morphology of adsorbed monolayers on quasicrystalline surfaces with five- and seven-fold symmetry. The Phase Field Crystal model is employed to first simulate the growth of the quasicrystal surfaces and a two-dimensional film is then coupled elastically to the substrate. We find several different pseudomorphic phases for different types of surfaces and monolayer/substrate interactions, and quantify them by computing local order parameters. In agreement with recent experiments using colloids in quasiperiodic light fields, we find that the formation of quasicrystalline order is greatly inhibited on the seven-fold surfaces.   

Depolarization and bonding in quasi-one-dimensional Na structures on Cu(001)

The formation of quasi-one-dimensional (Q1D) $p(n\times2)$-Na/Cu(001) structures is addressed by density-functional theory investigations for adsorbate coverage from low to the saturation one. A general dependence of the dipole moment on the given configuration is deduced by extending that for uniform distributions, and is greatly affecting the energetics of the Na overlayer. Larger stability for Q1D arrangements aligned along $[110]$ and $[1\overline{1}0]$ holds at coverage larger than $0.2$~ML, in agreement with low-temperature He scattering experiments, and can be explained by a reduced dipole-dipole repulsion for the $p(4\times2)$ with respect to hex-like distributions. Interatomic bonding charge displacements along zig-zag rows of Na atoms further support the Q1D structure and contribute significantly to the surface corrugation as seen by the He probe.   

Adsorption of In atoms on In/Si(111)-8$\times $2 studied by variable-temperature STM

We have investigated adsorption of In atoms on In/Si(111) surface by using variable-temperature scanning tunneling microscope. At room temperature, the adsorbed In atoms on 4$\times $1 are invisible due to its high mobility on a 4$\times $1 phase. As the temperature decreases below the phase transition temperature, additional In atoms begin to appear being fixed on a 8$\times $2 phase. The In atoms are mostly adsorbed as an isolated atoms (monomers) and paired ones (dimers) on In chains. The adsorbed In atom induces a local phase-flipping in the vicinity, resulting in a phase shift along the chain direction. Occasional hopping of isolated In atoms along the chain direction are observable at 80K. The hopping occurs preferentially in the direction of the local phase-flipping side. Formation of dimers in relation to the preferential hopping direction will be discussed