No description available.

No description available.

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.

No description available.

none

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 organometallic chemistry, metal complexes bearing unsaturated hydrocarbon ligands are of extensive interest, especially the C3H3-M system which includes propargyl (HC¡ÝCCH2-M), allenyl (H2C=C=CH-M), and acetylide (H3CC¡ÝC-M) forms. To study the chemistry of these species on metal surfaces, we used proprargyl bromide (HC¡ÝCCH2-Br) as precursor to produce C3H3(ad) on Ag(111) under ultrahigh vacuum (UHV) conditions. The thermal reactions pathway was investigated by Temperature-Programmed Desorption (TPD), and Reflection-Absorption Infrared Spectroscopy (RAIRS). In addition, density functional theory (DFT) calculations were conducted to obtain the optimized geometry for the adsorbates, and the computed IR spectra facilitated the vibrational mode assignments. TPD spectra showed that hydrogenation products C3H4 evolved at 310 K and 475 K. However, the desorption peak at 310 K was broad, indicating that more than one species were encompassed. Besides the hydrogenation product, a coupling product C6H6 (2,4-hexadiyne) was also unveiled as part of the desorption feature at 475 K. The identity of the possible C3H4 hydrogenation products (propyne and/or allene) was not discriminable by the mass spectrometry. The problem was circumvented by using £\,£\-dimethyl-substituted propargyl chloride because this dimethyl-substituted species also resulted in hydrogenatioin products around 310 K and 475 K, respectively; and the corresponding allenic and acetylenic end-products are distinguishable by the mass spectrometry. The results indicated that the broad feature at 310 K, in fact, contained both allene (lower temperature) and propyne (higher temperature), whereas the hydrogenation product at 475 K was propyne. The RAIR spectrum at 200 K showed that all C3H3(ad) on Ag(111) readily took on the allenyl form after the C-Br bond scission. It is thus obvious that allene at 310 K was generated by adding one hydrogen to the £\-carbon of the surface allenyl. RAIR spectroscopy revealed a drastic change after annealing the surface to 250 K, where the spectrum was almost identical to that obtained from using propynyl iodide (H3C-C¡ÝC-I) as a direct source for methylacetylide (H3C-C¡ÝC-Ag). Consequently, the products of propyne and 2,4-hexadiyne could be reasoned out.

We investigated the reactivity of difluorovinylidene groups (C2F2) on Cu(111) under ultrahigh vacuum conditions. Difluorovinylidene moieties bonded to surface were generated by the dissociative adsorption of 1,1-dibromodifluoroethylene. Temperature Programmed desorption (TPD) and reflection-adsorption infrared spectroscopy (RAIRS) revealed the thermal reaction pathways, and a variety of intermediates were identified or inferred. The major desorption product, hexafluoro-2-butyne (C4F6), was detected at 445 K. It invokes a step of fluoride addition to difluorovinylidene to render the intermediacy of C2F3. However, differences exist when the vibration data from F + C=CF2 were compared with those from C−CF3 and CF=CF2 in previous literature, implying that the form is neither ethylidyne nor vinyl. Based on the concept of fluorine hyperconjugation, density function theory (DFT) calculations were utilized to obtain two transition states, quasi-vinyl and -ethylidyne, which can account for the differences present in the IR spectra. The relative thermal stability follows the trend of vinyl > quasi-ethylidyne > quasi-vinyl > vinylidene > ethylidyne suggested by IR and DFT calculations. Finally, the end product, CF3C¡ÝCCF3, might be formed by coupling of two quasi-ethylidyne species via the partial allenic forms.

The decomposition of formic acid proceeds via two principal reaction pathways: dehydration and dehydrogenation. Mechanisms and reaction ratios depend on the nature of the catalysts used. This work provides mechanistic insight into the decomposition of formic acid on Co(0001) and a highly stepped cobalt surface. The catalytic systems were studied in ultra-high vacuum by XPS and temperature programmed desorption. On both surfaces, an overall reaction (1) was observed: 2 HCOOH→H_2 O+CO+H_2+CO_2 (1) The surfaces had differing reaction intermediates, reaction temperatures, and activation energies. On Co(0001), formate, carbon, and hydroxyl are intermediates and the reaction has an activation energy of 44.3 ± 0.6 kJ/mol, pre-exponential factor of 0.7 ± 0.05 mbar/s. On highly stepped cobalt, formate and formyl are intermediates and the reaction has an activation energy of 147.2 ± 2.0 kJ/mol and pre-exponential factor of 1011.3 ± 0.2 mbar/s. Desorption energies of observed species and mechanisms of observed reactions are reported. A detailed description and proof of concept of a PM-IRRAS reactor designed for this thesis is also presented.

This thesis deals with the development of scanning probe microscopes. Mechanical requirements for microscopes using measuring methods of scanning tunneling microscopy (STM) and atomic force microscopy (AFM) under enviroments of an ultrahigh vacuum (UHV) and variable temperatures are specified. Mechanical designs of two microscopes are discussed and their control electronics described. A special chapter is devoted to description of linear piezo manipulators and mechanical design of these prototypes.