Dissertation

Scanning tunnelling microscopy studies of nonlocal atomic manipulation and molecular kinetics on the si(111)-7x7 surface

Lock, Duncan

2015

Overview

Molecule-by-molecule construction has for years offered the tantalising prospect of atomic-scale devices. Current transistor fabrication techniques have for decades moved toward progressively smaller dimensions. This has driven interest in ‘bottom up’ routes to nanofabrication, where atoms and molecules are arranged into a desired structure. The scanning tunnelling microscope (STM) offers such a route. Nanofabrication wth STM lithography can reach sub-nanometre resolution. To achieve such control over individual molecules, a sound understanding of the behaviour of molecules on the surface is fundamentally important. In particular, the behaviour of aromatic organic molecules on surfaces is important for many technologies. Part II of this thesis presents a study of thermally activated movement of aromatic molecules on Si(111)-7 × 7, as recorded by STM. There are distinct variations in the measured rates of thermal displacement and desorption, determined by the different binding sites within the Si(111)-7 × 7 unit cell. The variation in the rates is a consequence of differences in both the energy barriers and Arrhenius prefactors. We reveal site-preference in the adsorption site of an aromatic molecule as it enters the surface from the gas phase via a physisorbed precursor. The complexity of the adsorption mechanism cannot be properly captured by the Langmuir isotherm. A Monte-Carlo simulation that takes into account a mobile physisorbed precursor accurately describes the adsorption process, highlighting the importance of molecular kinetics during adsorption. Part III of this thesis presents STM induced manipulation of aromatic molecules on Si(111)-7 × 7. A high voltage pulse at a point of the Si(111)-7 × 7 surface can desorb benzene, toluene, or chlorobenzene upwards of ten nanometres away from the tunnel junction. Increasing the current or time of the pulse, and hence the number of electrons, causes more molecules to be desorbed. This creates a depopulated region around the pulse site. For benzene, toluene, or chlorobenzene, the threshold bias to the effect is 2.0 V, below which no desorption occurs. Electrons travel across the surface isotropically before inducing desorption in a single electron process (1.18 ± 0.24 e−). Analysis of the effect at different temperatures and pulse voltages reveals that the injected electrons undergo diffusive motion between tip & molecule. The radius of the depopulated region decreases with decreasing temperature, but is constant for pulse voltages between 2.0 − 2.8 V. Drawing comparisons between 2 photon photoemission studies of the Si(111)-7 × 7 surface reveals a common electronic state at 2.0 V through which the electrons propagate the surface. The final chapter describes precise current injections made directly into toluene molecules adsorbed on Si(111)-7 × 7. Injecting directly into the molecule reveals the threshold to desorption of 1.4 eV. The difference in thresholds between locally and nonlocally induced desorption makes it clear that the 2.0 V threshold is a property of the surface, not the molecule.

• Publisher: ProQuest Dissertations & Theses
• Subject: Adsorption / Binding sites / Hydrocarbons