Extending electronic devices beyond the limitations of current micro-electronics manufacturing will require detailed knowledge of how to make contacts to semiconductor surfaces. In this work, we investigated several methods by which such connections to silicon surfaces could be achieved. Scanning tunneling microscopy (STM) was our main experimental tool, allowing direct imaging of the surfaces at the atomic level. First, the growth of self-forming linear nanostructures of organic molecules on silicon surfaces offers a possibility of creating devices with hybrid organic-silicon functionality. We have studied the growth of many different molecules on a variety of hydrogen-terminated silicon surfaces: H-Si(100)-2x1, H-Si(100)-3x1, and H-Si(111)-1x1. We found molecular growth patterns affected by steric crowding, by sample doping level, or by exposure to ion-pump created radicals. We formed the first contiguous "L-shaped" molecular lines, and used an external electric field to direct molecular growth. We attempted to study a novel method for nanoscale information transfer along molecular lines based on excitation energy transfer. The second part of the work focuses on the development and use of a new multiple-probe STM instrument. The design and the custom STM control software written for it are described. Connections to Si surfaces were achieved with a combination of lithographically defined metal contacts and STM tips. Two-dimensional surface conductivity of the Si(111)-7x7 surface was measured, and the effect of modifying the surface with organic molecules was investigated. A novel method, scanning tunneling fractional current imaging (STFCI), was developed to further study surface conductance. This method allowed us to determine, for the first time, that the resistance of steps on the Si(111)-7x7 surface is significantly higher than that of the surface alone.

Our group has developed a family of single molecules termed nanocars, which are aimed at performing controllable motion on surfaces. In this work, a series of light-activated motorized nanomachines incorporated with a MHz frequency light-activated unidirectional rotary motor were designed and synthesized. We hope the light-activated motor can serve as the powering unit for the nanomachines, and perform controllable translational motion on surfaces or in solution. A series of motorized nanovehicles intended for scanning tunneling microscopy (STM) imaging were designed and synthesized. A p-carborane-wheeled motorized nanocar was synthesized and monitored by STM. Single-molecule imaging was accomplished on a Cu(111) surface. However, further manipulations did lead to motor induced lateral motion. We attributed this result to the strong molecule-surface interactions between the p-carborane-wheeled nanocar and the Cu(111) surface. To fine-tune the molecule-surface interactions, an adamantane-wheeled motorized nanocar and a three-wheel nanoroadster were designed and synthesized. In addition, the STM substrates will be varied and different combinations of molecule-surface interactions will be studied. As a complimentary imaging method to STM, single-molecule fluorescence microscopy (SMFM) also provides single-molecule level resolution. Unlike STM experiment requires ultra-high vacuum and conductive substrate, SMFM experiment is conducted at ambient conditions and uses non-conductive substrate. This imaging method allows us to study another category of molecule-surface interactions. We plan to design a fluorescent motorized nanocar that is suitable for SMFM studies. However, both the motor and fluorophore are photochemically active molecules. In proximity, some undesired energy transfer or interference could occur. A cyanine 5- (cy5-) tagged motorized nanocar incorporated with the MHz motor was designed and synthesized in order to minimize the potential energy transfer or interference between the motor and the fluorophore. The SMFM study of this cy5-tagged motorized nanocar is currently undergoing. The design of light-activated motorized nanocar inspired the design of nanosubmarines. We used fluorescence quenching and fluorescence correlation spectroscopy (FCS) to study the diffusion of single molecules. The fluorescence quenching experiments of Ru(bpy)3+2 by a quenching nanosubmarine was conducted, but no motor induced acceleration of the molecule were observed. Another fluorescent nanosubmarine was monitored by FCS, and no increase of diffusion coefficient was found. Finally, a 1-D channel approach was adopted for decreasing the effects of Brownian motion, and acceleration of nanosubmarine was observed.

This work investigates the electronic properties of very small gold and semiconductor particles using Scanning Tunneling Microscopy/Spectroscopy (STM/STS) and Ballistic Electron Emission Spectroscopy (BEES). Complementary theoretical works were also performed. The first theoretical work was to calculate the quantized states in the CdS/HgS/CdS quantum-well-quantum-dot nanocrystals. An eight-band envelope function method was applied to this system. This method treats exactly the coupling between the conduction bands, the light-hole bands, the heavy-hole bands, and the spin-orbit split bands. The contributions of all other bands were taken into account using second order perturbation theory. Gold nanocrystals with diameters of 1.5 nm have discrete energy levels with energy spacings of about 0.2 eV. These values are comparable to the single electron charging energy, which was about 0.5 eV in our experimental configuration. Since bulk gold doesnt have an energy gap, we expect the electron levels both below and above the Fermi level should be involved in the tunneling. Measured spectroscopy data have rich features. In order to understand and relate these features to the electronic properties of the nanocrystals, we developed a tunneling model. This model includes the effect of excited states that have electron-hole pairs. The relaxation between discrete electron energy levels can also be included in this model. We also considered how the nanocrystals affect the BEES current. In this work an ultra-high vacuum and low-temperature STM was re-designed and rebuilt. The BEEM/BEES capabilities were incorporated into the STM. We used this STM to image gold nanocrystals and semiconductor nanocrystals. STS and BEES spectra of gold nanocrystals were collected and compared with calculations.

Elastic and inelastic interactions are studied in tunnel junctions of a superconducting nanoelectromechanical setup and in response to resent experimental superconducting scanning tunneling microscope findings on a paramagnetic molecule. In addition, the electron density of molecular graphene is modeled by a scattering theory approach in very good agreement with experiment. All studies where conducted through the use of model Hamiltonians and a Green function formalism. The nanoelectromechanical system comprise two fixed superconducting leads in-between which a cantilever suspended superconducting island oscillates in an asymmetric fashion with respect to both fixed leads. The Josephson current is found to modulate the island motion which in turn affects the current, such that parameter regions of periodic, quasi periodic and chaotic behavior arise. Our modeled STM setup reproduces the experimentally obtained spin excitations of the paramagnetic molecule and we show a probable cause for the increased uniaxial anisotropy observed when closing the gap distance of tip and substrate. A wider parameter space is also investigated including effects of external magnetic fields, temperature and transverse anisotropy. Molecular graphene turns out to be well described by our adopted scattering theory, producing results that are in good agreement with experiment. Several point like scattering centers are therefore well suited to describe a continuously decaying potential and effects of impurities are easily calculated.

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This thesis presents surface science studies, investigating several aspects of titanium dioxide at the atomic scale. The greater part of this work is devoted to the preparation by chemical vapor deposition (CVD) of titanium(IV) tetraisopropoxide (TTIP) of ultrathin TiO2 or TiOx films on Au(111). Four ordered structures were growth and characterized. It was also demonstrated how the morphology of the film (wetting film vs island) can be tailored. The acquired knowledge about the CVD process was exploited to load nano porous gold with titania, enhancing its catalytic activity. The reactivity towards water adsorption of the titania structures on Au(111) was also investigated. Finally, part of this work concerned the studying of the behavior of water on the stoichiometric rutile TiO2(110) surface, combining the experiments with density-functional theory (DFT) calculations and (kinetic) Monte Carlo simulations. The main experimental techniques used in this work are low-energy electron diffraction (LEED), scanning tunneling microscopy (STM) and photoelectron spectroscopy (PES).

No description available.