published_or_final_version / Computer Science / Doctoral / Doctor of Philosophy

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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.

In the last few years, two major contributions to the asteroid database have been the eight-color and thermal radiometric surveys. The former consists of broad-band photometric measurements through eight filters spanning the 0.3 to 1.1 μm wavelength range. The latter consists of thermal flux measurements at 10 and/or 20 μm, and when combined with measures of the reflected light, can yield reliable estimates of their geometric albedos. Visual display of the eight-color survey data can be simplified by reducing the dimensionality of the problem. A principal components analysis was performed to accomplish this task. The analysis shows that 95 percent of the information contained in the seven independent color indices is contained in two principal components. This result is due to the fact that most asteroid spectra can be explained in terms of two absorption features, one at ultraviolet and the other at near-infrared wavelengths. The photometric and radiometric data sets were also used, along with cluster analysis techniques, to produce an improved asteroid taxonomic system. Seven major classes are now recognized and are designated A, C, D, E, M, P, and S. Three interesting minor classes are also identified: B, F, and G. Marginal evidence for an eighth major class, here called T, exists in the data, but the reality of this class awaits confirmation by further observations of potential members. Three asteroids do not fall into any of the above classes and are assigned unique designations: R (349 Dembowska), Q (1862 Apollo), and V (4 Vesta). Four E-class asteroids are now known to exist in the main belt, yet nearly twice this number exist in or near the Hungaria region. Twenty eight D-class asteroids have been identified in the outer belt, where they represent a significant fraction of the population. Five D asteroids exist in the main belt, which one lying near the inner edge of the belt, which is dominated by S-class asteroids. Two of the interesting minor classes are associated with particular dynamical families. The Nysa family, with the single exception of Nysa itself, consists entirely of class F asteroids, while the B asteroids are found almost exclusively in the Themis family. The earth-approaching population is represented by at least two objects similar to Vesta and Dembowska, which are as many as are in the entire main belt, while most of the earth-approachers are of class S.

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