Reproduction de : Thèse de doctorat : Toxicologie : Metz : 1978. / Titre provenant de l'écran-titre. Notes bibliographiques. Index.

There are many advantages to the solution phase synthesis of semiconductor nanowires, the most notable of which are the ease of scalability and the production of nanowires in higher yields than those typically obtained in chemical vapor deposition (CVD) based processes. The solution phase synthesis of high quality, high aspect ratio (>100) narrow diameter semiconductor nanowires depends sensitively on three parameters: the diameter of the nanocrystals utilized to promote (seed) nanowire growth, molecular precursor decomposition kinetics and the choice of solvent in which the nanowires are grown. Bismuth is a low melting point (270 °C) semimetal and thus an ideal candidate for the solution-liquid-solid (SLS) growth of nanowires. A bismuth nanocrystal synthesis was developed that affords nanocrystals with average diameters from 4 - 20 nm. The nanocrystal diameter is controlled by varying the capping ligand (TOPO) to bismuth molar ratio. The synthesis of Au2Bi nanocrystals was also studied as it also affords small diameter (~ 2 nm) nanocrystals that are suitable for SLS nanowire growth. Molecular precursor decomposition kinetics can have a significant impact on nanowire yield and quality. Precursors that decompose too quickly can produce relatively large diameter nanowires, while precursors that decompose too slowly can produce nanowires with a highly tortuous morphology as a result of a high density of crystallographic defects. The choice of molecular precursor for the synthesis of III-V and II-VI nanowires was investigated. The solvent in which nanowires are grown can also have a significant effect on nanowire yield, quality and morphology. Coordinating solvents such as alkylphosphine oxides and alkylamines can interact with the atoms, or atomic complexes, that constitute nanowires and thus mediate the nanowire growth rate. In some instances, for example InAs nanowires grown in TOPO, this interaction can completely quench nanowire growth. This solvent effect has been investigated for the growth of III-V and II-VI nanowires. Solvents can also affect nanowire morphology. Branched ZnSe nanowires, i.e. hybrid nanostructures in which ZnSe nanorods grow epitaxially from the surface of ZnSe nanowires, are synthesized in trioctylamine whereas TOPO suppresses this branched growth. Finally, a mechanism which allows for the synthesis of narrow diameter nanowires seeded by much larger diameter nanocrystals is investigated. Bismuth nanocrystals with an average diameter of ~ 20 nm are utilized to promote the growth of narrow diameter (~ 6 nm) CdSe.₀₈Te.₉₂ nanowires. / text

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This study uses clusters formed in a UHV-compatible cluster apparatus, which was built and commissioned during this thesis. The design and operation of the cluster deposition system is described. This system is optimised for high clus- ter fluxes and for the production of cluster assembled nanoscale devices. One key feature of the system is a high degree of flexibility, including interchangeable sputtering and inert gas aggregation sources, and two kinds of mass spectrome- ter, which allow both characterisation of the cluster size distribution and deposi- tion of mass-selected clusters. Another key feature is that clusters are deposited onto electrically contacted lithographically defined devices mounted on an UHV- compatible cryostat cold finger, allowing deposition at room temperature as well as at cryogenic and at elevated temperatures. The electrically contacted nanoscale cluster devices were fabricated using a novel template technique. Hereby, clusters are placed between two electrodes separated only by ∼100 nm. The width of the cluster ensemble is in the order of a few cluster diameters, which means that the assembled clusters form a cluster wire bridging the electrode separation. During this thesis, the design and layout has been optimised to be able to measure electrical properties of the cluster devices and in particular to investigate the interaction between the cluster ensemble and the contact electrodes. In-situ electrical characterisation of cluster assembled nanoscale devices are performed in the temperature range 4.2 K to 375 K. The samples are provided with a backgate, which in principle allows modification of the conduction through the cluster ensemble by applying a gate voltage. However, no change in conduc- tion with changes in gate voltages was seen. The main focus of the electrical measurements is on the current voltage char- acteristics. It was noticed that the nanoscale bismuth (and antimony) cluster devices exhibited non-linear current voltage characteristics, which were in stark contrast to the linear current voltage characteristics measured for cluster films previously. Investigations into the causes of this non-linearity suggests that tun- nelling conduction occurs between the cluster ensemble (wire) and the contact electrodes. The non-linear current voltage characteristics were fitted using three models of tunnelling conduction and appear to be best fitted using a model in- volving fluctuation-assisted tunnelling through barriers of different heights. Further, measurements of the temperature dependent resistance are performed showing an increase of resistance with decreasing temperature for bismuth and antimony assembled cluster devices. The temperature dependence of bismuth as- sembled cluster wires can be explained by the decrease of the carrier concentration in bismuth for decreasing temperature. Annealing of the cluster ensemble and the cluster contact connection resulted in an increase in conduction. This increase of conduction can be explained due to the current flow through the cluster wire. Locally, at the bottlenecks, the current flow causes resistive heating and subsequently coalescence of two (or more) clusters.

Thesis (Honors)--College of William and Mary, 2008. / Includes bibliographical references (leaves 78-79). Also available via the World Wide Web.

Thesis (Ph. D.)--Lehigh University, 1999. / Includes vita. Bibliography: leaves 110-115.

Thesis (Ph. D.)--Columbia University, 1930. / Vita. Bibliography: p. 21.

Thesis (Ph. D.)--University of Texas at Austin, 2008. / Vita. Includes bibliographical references.

Continuous wave thermal propagation experiments were made with two single crystals of bismuth at frequencies up to 7 kHz. The experiments were performed at temperatures close to 4 K (i. e. close to the dielectric-like thermal conductivity peak). Accurate phase shift measurements were made in order to permit the detection of small departures from diffusive propagation. Attenuation measurements were also made. A summary of some microscopic theories of time-dependent thermal propagation in dielectric crystals is given. It is concluded that, for dielectric crystals in both the "hydrodynamic" and "ballistic" phonon gas regimes, the initial deviations from diffusive propagation will be described by a modified heat equation of the Vernotte type: [formula omitted] with appropriate identifications of the relaxation time. The possibility that the small numbers of charge carriers present in bismuth might lead to different forms of deviation is explored. Several types of thin-film insulating layers and superconducting alloy thermometers were investigated. Kodak Photo-Resist was found to be the most useful insulating material. This was used in conjunction with constantan heater films and Pb-In alloy thermometer films. The heat wave detection system employed a radio frequency thermometer bias current, a radio frequency tuned circuit, an envelope detector and phase-sensitive detection of the audio frequency heat wave signals. Heat wave phase lags were measured with a precision of 1°, using the phase-sensitive detector as a null detector. The measurements were analyzed in terms of a thermal transmission line model based on the modified heat equation given above. The electrical analogue of τ in such a model is L/R. A thermal leakage conductance term ⩋(electrical analogue G/C) was included in the model. The results at low frequencies were in excellent agreement with those expected on the basis of the transmission line model under conditions of diffusive propagation at high attenuations. Values of the apparent diffusivity obtained from these measurements were in reasonable agreement with the results of D. C. experiments made by other workers on comparable specimens. The quantity ⩋/ω was shown to be small at all frequencies used. Phase lag measurements at higher frequencies indicated significant departures from diffusive propagation in both crystals. (The crystals had different orientations.) The measurements in this range suggested a harmonic-wave-like mode of propagation. This mode appeared to break down at the highest frequencies examined. Evidence is presented to show that the observed deviations reflected thermal properties of the bismuth crystals rather than properties of the thin films, or spurious electrical effects. The apparent wave velocities were lower, and the corresponding relaxation times were longer than those predicted on the basis of the microscopic theories and from the diffusivity values obtained at low frequencies. In view of these numerical discrepancies, it is suggested that the wave-like mode could be a mode peculiar to the bismuth system, rather than the "second sound" mode predicted for ideal dielectrics. Some further experiments are suggested. / Science, Faculty of / Physics and Astronomy, Department of / Graduate