Adsorption of benzene on Si(100) from first principles

Adsorption of benzene on the Si(100) surface is studied from first principles. We find that the most stable configuration is a tetra-$\sigma$-bonded structure characterized by one C-C double bond and four C-Si bonds. A similar structure, obtained by rotating the benzene molecule by 90 degrees, lies slightly higher in energy. However, rather narrow wells on the potential energy surface characterize these adsorption configurations. A benzene molecule impinging on the Si surface is most likely to be adsorbed in one of three different di-$\sigma$-bonded, metastable structures, characterized by two C-Si bonds, and eventually converts into the lowest-energy configurations. These results are consistent with recent experiments.Comment: 4 pages, RevTex, 2 PostScript gzipped figure

Spontaneous magnetization of aluminum nanowires deposited on the NaCl(100) surface

We investigate electronic structures of Al quantum wires, both unsupported and supported on the (100) NaCl surface, using the density-functional theory. We confirm that unsupported nanowires, constrained to be linear, show magnetization when elongated beyond the equilibrium length. Allowing ions to relax, the wires deform to zig-zag structures with lower magnetization but no dimerization occurs. When an Al wire is deposited on the NaCl surface, a zig-zag geometry emerges again. The magnetization changes moderately from that for the corresponding unsupported wire. We analyse the findings using electron band structures and simple model wires.Comment: submitted to PHys. Rev.

Electronic Transport in a Three-dimensional Network of 1-D Bismuth Quantum Wires

The resistance R of a high density network of 6 nm diameter Bi wires in porous Vycor glass is studied in order to observe its expected semiconductor behavior. R increases from 300 K down to 0.3 K. Below 4 K, where R varies approximately as ln(1/T), the order-of-magnitude of the resistance rise, as well as the behavior of the magnetoresistance are consistent with localization and electron-electron interaction theories of a one-dimensional disordered conductor in the presence of strong spin-orbit scattering. We show that this behaviour and the surface-enhanced carrier density may mask the proposed semimetal-to-semiconductor transition for quantum Bi wires.Comment: 19 pages total, 4 figures; accepted for publication in Phys. Rev.

STM and HREELS investigation of gas phase silanization on hydroxylated Si(100)

The gas phase anhydrous reaction of glycidoxypropyldimethylethoxysilane (GPDMES) with a model hydroxylated surface has been investigated using high-resolution electron energy loss spectroscopy (HREELS) and scanning tunneling microscopy (STM).Water dissociation on the clean reconstructed (2 71)-Si(100) surface was used to create an atomicallyflat surface with ~0.5 MLof hydroxyl groups. Exposure of this surface to GPDMES at room temperature under vacuum was found to lead to formation of covalent Si\u2013O\u2013Si bonds although high exposures (6 7108 L) were required for saturation. STM images at the early stages of reaction indicate that the reaction occurs randomly on the surface with no apparent clustering. The STM images together with semiempirical (AM1) calculations provide evidence for hydrogen bonding interactions between the oxygen atoms in the molecule and surface hydroxyl groups at low coverage.Peer reviewed: YesNRC publication: N

Chemical and thermal stability of titanium disilicide contacts on silicon

The suitability of titanium disilicide (TiSi2) contacts for use in electronic transport measurements on chemically modi\ufb01ed Si~100! and Si~111! surfaces prepared in ultrahigh vacuum ~UHV! or via wet chemical approaches has been examined. Rapid thermal annealing at 900 \ub0C of patterned Ti lines was used to produce the C54 phase of TiSi2. Atomic force microscopy, UHV scanning tunneling microscopy, and resistivity measurements were used to characterize the TiSi2 /Si system through different stages of the disilicide formation and subsequent high temperature annealing or wet chemical cleaning and etching. The contacts were found to maintain their integrity after annealing up to 1200 \ub0C or 10 min etching in 40% ammonium \ufb02uoride (NH4F). Under these preparation conditions, the silicon surface in the immediate vicinity of the TiSi2 interface remains free from major defects, thus making titanium disilicide a particularly suitable material for constructing platforms for measurements of electrical transport of silicon surfaces as well as nanostructures fabricated on these surfaces.Peer reviewed: YesNRC publication: Ye

Work function of doped zinc oxide films deposited by ALD

Al-doped ZnO (AZO) is a promising earth-abundant alternative to Sn-doped In2O3 (ITO) as an n-type transparent conductor for electronic and photovoltaic devices. We have deposited AZO films with resistivities as low as 1.1 × 10-3 ω·cm by atomic layer deposition (ALD) using trimethylaluminum (TMA), diethylzinc (DEZ), and water at 200 °C. The work functions of the films were measured using a scanning Kelvin probe (sKP) to investigate the role of aluminum concentration. The work function of AZO films prepared by two different ALD recipes were compared: a "Al-terminated" recipe and a "ZnO-terminated" recipe. As aluminum doping increases, the Al-terminated recipe produces films with a consistently higher work function than the ZnO-terminated recipe. The resistivity of the Al-terminated recipe films shows a minimum at a 1:16 Al:Zn atomic ratio and using a ZnO-terminated recipe, minimum resistivity was seen at 1:19. The film thicknesses were characterized by ellipsometry, chemical composition by EDX, and resistivity by a four-point probe

Electrochemically driven assembly of mixed dithiol bilayers via sulfur dimers

We report on the electroformation of mixed bilayers of rigid (1,4-benzenedimethanethiol) and flexible (1,6-hexanedithiol) dithiols on Au(111) from alkaline aqueous solution. These bilayers are formed by first electrodepositing a monolayer of vertically aligned dithiols. The unreacted dithiols are then oxidatively dimerized with the other dithiols, and a bilayer is formed. ACIS (ac impedance spectroscopy) measurements indicate that the monolayer of the rigid dithiol is a better substrate for dimerization than the flexible dithiol. ACIS also reveals that the electroformation of a bilayer is more complete, and the bilayer has fewer defects than one obtained through chemical incubation. Infrared reflection-absorption spectroscopy provides proof for mixed-bilayer formation of the alkanedithiol with the benzenedimethanethiol having its methylene groups deuterated. The mechanism for bilayer formation via the oxidative dimerization of thiols is supported by high-resolution electron energy loss spectra of bilayers, which show a S-S stretch at 515 cm-1. Our results show that this electrochemical approach is a viable methodology for the formation of complex organic interfaces.Peer reviewed: YesNRC publication: Ye

Electronic structure and STM images of self-assembled styrene lines on a Si(1 0 0) surface

First principles electronic structure calculations are applied to styrene lines on a Si(1 0 0) surface grown via a recently reported \ufffdself-directed\ufffd growth process. It is found that the delocalized ? states of the molecule lie at the bandedge of the valence band, which makes them potentially suitable as carrier channels for hole transport along the styrene line. By contrast, the unoccupied ?* states are situated well above the bandedge of the conduction band. In this case electron transport along the lines seems highly improbable. Finally, scanning tunneling microscope (STM) topographies of occupied states are simulated within a perturbation approach. While the experimental result of vanishing corrugation for the molecule at low bias voltages cannot be accounted for, good agreement is obtained between experiments and simulations for voltages below -2 V.NRC publication: Ye