One of the central problems of molecular electronics is to understand electron conduction properties when a functional molecule is interfaced with external electrodes and put under external bias and gate potentials. These properties are influenced by the molecule-electrode interaction as well as by the structure of the functional region of the device. In this thesis, we investigate from first-principles the transport properties of a number of molecular-scale systems, and try to relate the observed features to both the atomic and electronic structure. / We start with a detailed analysis of transport through carbon atomic wires, and find that the equilibrium conductance is sensitive to charge transfer doping, and that the I-V characteristics exhibit negative differential resistance at high bias due to a shift of conduction channels relative to the states of the electrodes. / Using a Sc3N C80 metallofullerene device, we address several general questions about quantum transport through molecular systems and provide strong evidence that transport in such molecular devices is mediated by molecular electronic states which have been renormalized by the device environment. / The possibility of inducing nuclear dynamics in single-molecule Au-C 60-Au transistors via inelastic, resonance-mediated tunneling current is examined using a method based on the combination of a theory of current-triggered dynamics[1] and our nonequilibrium Green's function approach of computing electron transport properties. / We investigate several single molecule field-effect transistors consisting of conjugated molecules in contact with metallic electrodes. The source-drain current is found to be sensitive to the external gate potential and the molecular structure; with modulations of the current as large as several thousand fold. / Given a proposed operation principle, we obtain quantitative results on the rectification properties for an organic molecule rectifying diode. The I-V characteristic shows clear rectification behavior, and is explained from the simple picture of shifting of molecular levels due to substituents and an externally applied bias voltage. / Finally, we report a formulation combining density functional theory with the Keldysh nonequilibrium Green's function, for calculating quantum mechanical forces under external bias and during electron transport. We present an example force calculation consisting of a single atom point contact.

Atomic scale structures are studied using first principles or ab initio simulations. We first review the formalism required for a typical calculation and then proceed to examine a few applications. All the simulations were performed using plane wave based pseudopotential methods and by a direct minimization of the Kohn-Sham total energy functional. / Using this method, we have investigated the effect of impurity atoms like boron, sulphur and phosphorous on the adhesive properties of two aluminium minimum slabs. We found that both sulphur and phosphorous (both substitutional impurities) decreased the adhesive energy and the interfacial strength, while boron (an interstitial impurity) increased the interfacial strength marginally. It was also found that the impurities act as spacers and increase the interfacial spacing. This increase was found to increase with atomic size. These observations are consistent with experimental work. / We have also investigated the possibility of using orbital-free kinetic energy functionals in ab initio simulations. Various functionals, their limiting forms and response properties were studied. The results obtained using this method agree well with full Kohn-Sham calculations. This was demonstrated using a variety of systems (Si clusters, H$ vert$Si(100), c-Si). The excellent scaling properties of the method was illustrated with a simulation of the interactions between a tip and substrate (150 atoms). The normal forces experienced by the small cluster serving as the tip were used to map out the top surface of the substrate. An ad hoc scheme to incorporate pseudopotential non-locality into the orbital-free formalism was also proposed and tested.

The magnetic properties of amorphous Fe$ sb{ rm x}$T$ sb{ rm 100-x}$ (T = Zr, Hf, Sc; 89 $ le$ x $ le$ 93) alloys have been systematically studied mainly by Mossbauer spectroscopy and magnetization measurements. Two magnetic transitions have been observed in Fe$ sb{92.5}$Hf$ sb{7.5}$ (the only FeHf alloys studied here) and FeZr alloys with Fe content between 90 and 93 at.%. We have ruled out the cluster models which attribute the second transition either to the ordering of AFM clusters or to the freezing of FM clusters, and confirmed that the second transition is due to the homogeneous freezing of the transverse spin components as a result of exchange frustration. On varying the frustration level, the evolution process from a ferromagnetic to a spin glass ordering has been observed. Fe$ sb{89}$Zr$ sb{11}$ essentially exhibits conventional ferromagnetic behavior, while Fe$ sb{90}$Zr$ sb{10}$ shows two magnetic transitions with a noncollinear ground state. As we increase the Fe content further (and thus raise the frustration level), the second transition temperature T$ sb{xy}$ and the noncollinearity of the ground state increase, whereas the first transition temperature T$ sb{c}$ drops. T$ sb{xy}$ and T$ sb{c}$ are expected to meet at x $ simeq$ 94.5, suggesting that Fe$ sb{94.5}$Zr$ sb{5.5}$ (which can not be made by melt-spinning) would be a spin glass. Fe$ sb{91}$Sc$ sb9$ is the most frustrated system studied here and it exhibits a single transition to a spin glass. Extrapolation of the magnetic properties of all systems gives a common limit as the Fe concentration approaches 100%, and suggests that amorphous Fe is a spin glass with the spin freezing temperature about 100K and an average iron moment of $ sim$1.6$ mu sb{B}$. Below the second transition, the transverse spin components are strongly correlated on a nearest neighbor scale in Fe$ sb{92}$Zr$ sb7$Sn, while the correlation length of the longitudinal components shows no detectable change on the local scal

A spectrometer was built for the purpose of performing Depth Selective Conversion Electron Mossbauer Spectroscopy (DSCEMS) on surfaces containing iron nuclei. The spectrometer was based on a Retarding Field Analyzer design by van Noort and van Gorkum$ sp1$. It has an acceptance angle of 5% of 4$ pi$, and is capable of energy resolutions of between.8% and 4.3% of the pass energy. The spectrometer is suited for samples with a diameter as large as 10 mm. The designing and dimensioning processes, construction, operation, and performance of the spectrometer are presented here.

In November 2002 a Microfabrication Lab was established in the physics department of McGill University to support research in nanoscience and technology. At the same time, I arrived at McGill to begin my graduate study. So I was assigned to do research on microfabrication, especially bulk wet etching of silicon using TetraMethyl Ammonium Hydroxide (TMAH). / The content of microfabrication is quite broad, and also very useful in both industry and academic. Since our fab is a newly built one and I had no experience in this area before, this thesis mainly included some basic processes in microfabrication, such as the photolithography, wet etching, reactive ion etching, and soon. Also it compared the wet etching with dry etching. Some results of TMAH wet etching were showed in the thesis, which agreed well with that of the other groups. A simulation program was developed to predict the etching result of TMAH and it appeared to work well. Finally, based on the knowledge and experience acquired, processes in making cantilever and tip structures, which are critical in the scanning probe microscopes, were developed. Silicon oxide cantilevers with length of 100-200 mum, width of 30-50 mum, and thickness of 1 mum were obtained. Pyramid like silicon tips were also fabricated using the wet etching.

In this thesis we study the thermal and inertia effects on atomic friction. The Friction Force Microscope (FFM) is simulated with the Tomlinson model. We introduce a method to calculate atomic friction including thermal effects. A novel scaling form is proposed and tested. We also investigate the inertial effects of the bulk and the tips. We find if the bulk is weakly damped, there is hysteresis in the velocity versus driving force curve. When the tips are weakly damped, the friction force shows large deviations. / In the first half of the thesis, we study the atomic friction between an FFM tip and a solid substrate at nonzero temperatures. Through theoretical analysis, we find the frictional force follows an universal scaling form F ∝ const---T2/31n 2/3(T/v), where T is temperature and v is scanning velocity. We test the theoretical results with numerical simulations. The FFM is simulated with the Tomlinson model. A tip is connected to a support with a spring. The support is pulled with a constant scanning velocity. We model the substrate with two different systems: one is a sinusoidal surface potential function for rigid substrate; the other is a ball-spring model for an elastic substrate. We find the friction force follows the scaling form well, with both kinds of substrates. This confirms the universality of our findings. The results also compare well to recent experimental work, permitting the quantitative extraction of microscopic parameters. / In the second half of the thesis, we study the inertial effects on atomic friction with a multiple-tip Tomlinson model. The model consists of an array of tips mounted on a common bulk support. We find if the bulk is weakly damped and driven by an external force, there is hysteresis in the velocity versus driving force curve. The relative amount of hysteresis decreases with the number of tips for incommensurate systems. We also find that if the tips are weakly damped and experience longjumps, then when the bulk is pulled with a constant velocity, the total friction force shows a large deviation. The deviation exists even for incommensurate systems and cannot be averaged out by incorporating more tips. However, the deviation decreases with increasing temperatures. These findings provide new insights on the influence of the inertia of the bulk and the tips. They also shed some light on future multiple-tip FFM experiments.

We use an atomic force microscope to perform nanolithography by local oxidation on two-dimensional electron gases. We study the oxidation process under different environmental conditions. It is found that oxide growth is hindered at high temperature and affected by ozone. These findings help us to understand the role of the water film present on the surface, and lead us to propose a mechanism that would avoid its widening effect. We develop a new lithography technique, diamond scribe scratching, and use it to make coarse mesoscopic structures. Then, we combine diamond scribe scratching and local oxidation to fabricate working mesoscopic structures, namely quantum point contacts.

We investigated the feasibility of using X-rays to study self-assembled monolayers on inorganic surfaces. Variable-temperature X-ray powder diffraction measurements of metal alkylphosphonate salts measured the contraction and expansion of planar inorganic layers as the material passed through a series of phase transitions when heated and cooled. Small-angle X-ray scattering measurements of alkanethiolate-capped gold nanoclusters allowed the determination of the average nanocluster size and the average distance between nanoclusters. These techniques have been demonstrated to contribute useful information which, when combined with results from other probes, lead to a detailed model of the materials' structures and properties.

Thermal conductivity is a powerful probe of electronic quasiparticles, especially at low temperatures. At higher temperatures it gives very useful information on the behavior of quasiparticles and phonons, however both contributions are more difficult to identify precisely. / We carried out a comparative study of the thermal conductivity in the hole and electron-doped cuprates, focussed on Bi2Sr2CaCu 2O8 (hole-doped) and Pr1.85Ce0.15CuO 4 (electron-doped). After a brief review of the literature it was clear that these families show very different physical properties, although they present similar features in their structure and phase diagram. / We detected the presence of a residual normal fluid in Bi2Sr 2CaCu2O8, in rather good agreement with the theory for d-wave superconductors, and showed its absence in Pr 1.85Ce0.15CuO4, firm indication of a nodeless gap. At higher temperatures we observed for the first time a peak below Tc in the thermal conductivity of Pr1.85Ce 0.15CuO4.

This thesis is dedicated to understanding the mechanism of the in situ synthesis of magnetic nanocomposites based on cellulosic substrates and characterizing the products of this synthesis. The preparation of uniform membranes containing large quantities of magnetite particles (Fe$ sb2$O$ sb3$) of defined site was our specific objective. For that purpose, cellulosic substrates of two different kinds: one of bacterial origin bacterial cellulose--BC), the other (Lyocell) derived from dissolving-grade wood-pulp through a dissolution/coagulation process (film-casting) were used as never-dried gel membranes. BC has also been used in the form of a suspension of open fibrillar pellets which were dried to a parchment after the in situ treatment. Characterization of the resulting magnetic materials was performed using transmission electron microscopy (TEM) in imaging and diffraction mode, X-ray diffraction (XRD), vibrating sample magnetometry (VSM) and Mossbauer spectroscopy. In BC membranes, needle-like lepidocrocite ($ gamma$-FeOOH) formed along the cellulose fibrils, using the crystalline surface as a nucleation site. Spherical magnetite particles subsequently formed around the needles. The less swollen Lyocell substrates produced needle-like feroxyhite ($ delta$-FeOOH) concentrated at the membrane surface while spherical magnetite particles formed within the membrane after several cycles of treatment. The treated BC and Lyocell membranes were both superparamagnetic at room temperature. The BC suspensions had no space constraint hence the treatment conditions were more critical for controlling the morphology of synthesized ferrites. These conditions could be defined to yield homogeneous membranes containing magnetite particles uniform in size, too large however to behave superparamagnetically at room temperature.