Showing papers by "Peter H. Beton published in 2006"

Bimolecular networks and supramolecular traps on Au(111).

Abstract: We demonstrate the formation of intermixed phases and self assembled molecular templates on the Au(111) surface. The templates are stabilized by hydrogen bonding between melamine molecules with trigonal symmetry and linear PTCDI (perylene tetra-carboxylic di-imide) molecules. When annealed, these molecules spontaneously form either a chiral intermixed phase or a honeycomb arrangement in which vertexes and edges correspond respectively to melamine and PTCDI molecules. We also observe minority phases with more complex intermolecular junctions. The use of these networks as templates is demonstrated by the controlled capture of fullerenes within the pores of the network to form dimers, hexamers, and heptamers. Our results confirm that bimolecular templates can be realized on a range of substrates.

Hydrogen-bonded PTCDA-melamine networks and mixed phases.

Abstract: A stable hydrogen-bonding junction is formed between 3,4,9,10-perylene-3,4,9,10-tetracarboxylic-dianhydride (PTCDA) and 1,3,5-triazine-2,4,6-triamine (melamine). This bimolecular system was studied on the Ag−Si(111) × R 30° surface at sub-monolayer coverage, and two distinct phases are observed. A hexagonal lattice is formed that is stabilized by hydrogen bonding between PTCDA and melamine. This phase, in which melamine acts as a 3-fold vertex, is a close analogue to the 3,4,9,10-perylene-3,4,9,10-tetracarboxylic-diimide−melamine network reported recently. To our knowledge this hydrogen-bonding junction has not been previously observed and might not be expected due to lone pair repulsion. However we confirm that this combination is stable using ab initio methods. In the second intermixed phase parallel rows of PTCDA molecules coexist with an array of melamine molecules, and we propose a model for this structure.

Experimental and theoretical identification of adenine monolayers on Ag-terminated Si(111)

Abstract: We discuss the formation of ordered adenine monolayers on a Ag-Si (111)root 3 x root 3R30 degrees surface held at room temperature. Adenine forms a hexagonal network, which is stabilized by hydrogen bonding. By comparing the ordering observed using scanning tunneling microscopy with molecular superstructures calculated using ab initio density functional theory, we suggest that adenine forms an array of irregular hexagons through the combination of three distinct hydrogen-bonded adenine dimers. The dimensions of the ordered monolayer are commensurate with the surface with root 3 x root 7 periodicity relative to the Ag-Si (111) root 3 x root 3R30 degrees reconstruction, in agreement with calculated molecular dimensions. An analysis of double-domain regions leads us to conclude that there are two domains with different chiralities.

Dianhydride-amine hydrogen bonded perylene tetracarboxylic dianhydride and tetraaminobenzene rows.

Abstract: We have investigated the coadsorption of perylene tetracarboxylic dianhydride (PTCDA) and tetraaminobenzene (TAB) on the Ag/Si(111)-square root(3) x square root(3) R30 degree surface using scanning tunneling microscopy. At room temperature, PTCDA islands with square and herringbone ordering are formed which, on exposure to TAB, are converted into an intermixed phase in which PTCDA and TAB form alternating rows. From our images, we determine the relative placement of TAB and PTCDA molecules and conclude that the row structure is stabilized by hydrogen bonding between dianhydride and diamine groups. We confirm that this hydrogen bonding junction is stable using ab initio calculations and show that the proposed geometry is consistent with calculated intermolecular dimensions.

Manipulation of C-60 on the Si(001) surface: Experiment and theory

Abstract: We extend our previous discussion [D. L. Keeling et al., Phys. Rev. Lett. 94, 146104 (2005)] of molecular rolling on reactive surfaces. Experimental results showing long-range quasiperiodic tip responses with periods of two, three, and four lattice constants are presented. In addition, we show systematic change of the repeating wave form which we attribute to a slow variation of the tip-sample junction at an atomic level. Using ab initio simulations, we have reexamined the translation of ${\mathrm{C}}_{60}$ across the surface and confirmed our proposed pivoting mechanism via which the molecule rolls by sequential breaking and forming bonds with the surface. A complex character of the molecular displacement, which is accompanied by a substantial deformation of the molecule and the surface, is revealed by careful analysis of the atomic geometry and the electron density redistribution along the path. A large variety of observed tip traces along the trough are then explained by analyzing in detail all possible transition paths between the known stable adsorption sites. Detailed models for the periodic traces with two and three lattice constants are also suggested based on a modified pivoting mechanism. In addition, we predict and discuss possible along-the-row manipulation paths.

Kinetic instabilities in the growth of one dimensional molecular nanostructures.

Abstract: We develop a theory for the growth of one dimensional (1D) chains stabilized by anisotropic interactions, such as hydrogen bonding. Molecular chains are nucleated, grow, and may then undergo a kinetically driven transition to a two dimensional morphology. Kinetic Monte Carlo simulations show that extended 1D growth occurs between two temperature limits determined by two distinct kinetic instabilities. The limiting temperatures depend on interaction strength and deposition rate and, for a certain parameter range, 1D growth is completely suppressed.

Lateral translation of covalently bound fullerenes

Abstract: Lateral manipulation of fullerenes on clean silicon surfaces may be induced by either an attractive or repulsive interaction between adsorbed molecules and the tip of a scanning probe microscope, and can result in a complex response arising from molecular rolling. The model for rolling is supported by new results which show that manipulation is suppressed for adsorbed functionalized fullerenes due to the presence of phenyl sidegroups. The influence of varying the dwell time of the tip during manipulation is also reported. By reducing this time to a value which is less than the response time of the feedback control loop it is possible to induce manipulation in a quasi-constant height mode which is accompanied by large increases/decreases in current.