Response of complex networks to compression: Ca, La, and Y aluminoborosilicate glasses formed from liquids at 1 to 3 GPa pressures.

Aluminoborosilicate glasses containing relatively high field strength modifiers (Ca, La, and Y) have been compressed at pressures up to 3 GPa and near the glass transition temperature (Tg) and quenched to room temperature at high pressure followed by decompression. Structural changes were quantified with high-resolution (27)Al and (11)B MAS nuclear magnetic resonance at 14.1-18.8 T. The changes with pressure in Al and B coordinations in the recovered samples are quite large with more than 50% decreases in tetrahedral aluminum ((IV)Al) and 200%-300% increases in tetrahedral boron ((IV)B). Glasses with higher field strength modifiers (La and Y) contain more high coordinated aluminum ((V,V I)Al) at all pressures studied. More high coordinated boron also correlates with higher field strength modifier if all three compositions are compared on an isothermal basis. Although lowering fictive temperature and increasing pressure both increase Al and B coordinations, our study shows that the actual mechanisms for structural changes are most probably different for temperature and pressure effects. Using a rough thermodynamic model to extrapolate to higher pressures, it appears that a simple non-bridging oxygen (NBO) consumption mechanism is not sufficient to convert all the aluminum to octahedral and boron to tetrahedral coordination, suggesting other mechanisms for structural changes could occur at high pressure as NBO becomes depleted.

Self-assembled monolayers of 2-Adamantanethiol on Au{111}: control of structure and displacement.

We have investigated the formation of 2-adamantanethiolate self-assembled monolayers on Au{111} and their displacement by n-dodecanethiol, using scanning tunneling microscopy, X-ray photoelectron spectroscopy, and infrared reflection absorption spectroscopy. Well-ordered 2-adamantanethiolate monolayers undergo rapid and significant molecular exchange upon exposure to n-dodecanethiol solutions, but their structures and intermolecular interactions template the growth of n-dodecanethiolate domains. Annealing 2-adamantanethiolate monolayers at 78 degrees C decreases the density of vacancy islands, while increasing the overall order and the average domain sizes in the films. This results in slower displacement by n-dodecanethiol molecules, as compared to unannealed monolayers. The secondary sulfur position on the adamantyl cage influences the lattice structure and exchange of 2-adamantanethiolate monolayers by alkanethiols.

Molecular flux dependence of chemical patterning by microcontact printing.

We address the importance of the dynamic molecular ink concentration at a polymer stamp/substrate interface during microcontact displacement or insertion printing. We demonstrate that by controlling molecular flux, we can influence both the molecular-scale order and the rate of molecular exchange of self-assembled monolayers (SAMs) on gold surfaces. Surface depletion of molecular ink at a polymer stamp/substrate interface is driven predominantly by diffusion into the stamp interior; depletion occurs briefly at the substrate by SAM formation, but diffusion of molecules into the bulk of the stamp dominates over practical experimental time scales. As contact time is increased, the interface concentration varies significantly due to diffusion, affecting the quality and coverage of printed films. Controlling interfacial concentration improves printed film reproducibility and the fractional coverage of multicomponent films can be controlled to within a few percent. We first briefly review the important aspects of molecular ink diffusion at a stamp interface and how it relates to experimental duration. We then describe two examples that illustrate control over ink transfer during experiments: the role of contact time on monolayer reproducibility and molecular order, and the fine control of fractional monolayer coverage for the displacement printing of 1-adamantanethiolate SAMs by 1-dodecanethiol.

Self-assembly of carboranethiol isomers on Au111: intermolecular interactions determined by molecular dipole orientations.

Self-assembled monolayer (SAM) structures and properties are dominated by two interactions: those between the substrate and adsorbate and those between the adsorbates themselves. We have fabricated self-assembled monolayers of m-1-carboranethiol (M1) and m-9-carboranethiol (M9) on Au[111]. The two isomers are nearly identical geometrically, but calculated molecular dipole moments show a sizable difference at 1.06 and 4.08 D for M1 and M9 in the gas phase, respectively. These molecules provide an opportunity to investigate the effect of different dipole moments within SAMs without altering the geometry of the assembly. Pure and co-deposited SAMs of these molecules were studied by scanning tunneling microscopy (STM). The molecules are indistinguishable in STM images, and the hexagonally close-packed adlayer structures were found to have ((square root of 19) x (square root of 19))R23.4 degrees unit cells. Both SAMs display rotational domains without the protruding or depressed features in STM images associated with domain boundaries in other SAM systems. Differing orientations of molecular dipole moments influence SAM properties, including the stability of the SAM and the coverage of the carboranethiolate in competitive binding conditions. These properties were investigated by dynamic contact angle goniometry, Kelvin probe force microscopy, and grazing incidence Fourier transform infrared spectroscopy.