Experimental speed distribution measurements and NHC self-assembly on Au(111)

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Authors
Groome, Ryan
Keyword
Evaporation , Effusion , Experimental speed distribution measurements , Velocity selection , Electron-beam evaporation , N-heterocyclic carbenes , Adsorption , Self-assembly , Self-assembled monolayer , Adatom extraction
Abstract
In Part 1, I present experimental measurements of the speed distribution in atomic beams produced by physical vapour deposition. These measurements are still needed in the literature, especially in collisionless beams produced by thermal evaporation. They also have a practical importance for the deposition of high-quality thin film materials, and may provide new strategies for advanced nanostructural control as well. Accordingly, we developed a unique experimental apparatus and procedure to measure this distribution. We used a rotating slotted cylinder velocity selector, or spindle, to mechanically generate velocity-selected atomic beams. Two detectors that operate on different physical principles, a hot-filament ionization gauge and a quartz crystal microbalance, were positioned directly above the spindle to detect transmitted atoms. The distribution of transmission measurements as a function of rotation speed is related to the speed distribution. This work reports on the theory-guided design and testing of the mechanical and control system equipment we built to perform these measurements. The experimental data obtained using an effusive oven, an open effusion cell, and an electron-beam evaporator highlight some interesting and important aspects of evaporation. In Part 2, I present a detailed scanning tunnelling microscopy study of N-heterocyclic carbene (NHC) adsorption and self-assembly on Au(111). Surface modification and functionalization by the formation of a strongly bound self-assembled monolayer (SAM) of organic molecules has received considerable interest as a tool for the protection and stabilization of nanoparticles and the development of lab-on-chip sensors. Although SAMs are most commonly formed by the adsorption of alkanethiols, NHCs have recently attracted considerable interest. NHCs have important advantages compared to thiol-based systems, most notably their chemical tunability and ability to form strongly bound monolayers with improved stability. Given their novelty in the field of materials science, many fundamental properties of NHC-based SAMs are not well understood. This work provides much needed information on the NHC adsorption and self-assembly processes, including factors that control orientation, ordering, mobility, and adatom involvement. Efforts to understand these processes were crucial in advancing the chemistry and applications of thiol-based SAMs, and will similarly offer critical strategies for the design of NHC-based SAMs and their applications.
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