Electronic and optical properties of nanostructured materials

Abstract

Three types of nanostructured materials have been investigated: C60 on InP (100), Indium clusters on InP (100) and luminescent alkylated-Si quantum dots. The growth model and electronic structureOf C60molecules adsorbed on InP (100) were studied by XPS and UPS as a function of coverage and annealing temperature. The C Is, P 2p, In 4d core levels and the valence band spectra point to the presence of a localized covalent bond between C60 molecules and the substrate. No filling of the lowest unoccupied molecular orbit derived bands was observed. The absence of any change in the surface components of In 4d core level upon C6o adsorption indicated that the chemisorption bond exists between the fullerene molecules and phosphorus atoms rather than between C60 molecules and indium atoms. This assertion is supported by the simultaneous desorption of bothC6o and P upon annealing to 640 K and above. The evolution of clean, In-terminated InP (100)-(2 x 4) surfaces is investigated by SRPES as a function of annealing temperature. As-prepared InP (100)-(2 x 4) surface are found to be free of metallic indium, and the In 4d core level shows two clear surface components. A third, indium-cluster-related component appears after annealing above 360 ± 10 OC, due to phosphorous desorption, and is accompanied by a corresponding reduction in intensity in the In-P surface component. Further annealing leads to a decrease in binding energy of the indium cluster related peak due to increased metallicity and hence core-hole screening in the clusters. The increasingly metallic nature of the indium clusters is also revealed by the appearance and growth of a Fermi edge in valence band spectra. During the course of illumination with 145 eV photons we have monitored the evolution of the Si 2p core level, and observed in real time a splitting and growth of a new Si 2p component assigned to the Si4+ ionic state of Si. This new peak is attributed to in situ oxidation of Si quantum dots caused by photo-induced reaction with water, multilayers of which are present on the surface of the as-introduced quantum dots. X-ray excited optical luminescence (XEOL) reveals that two bands are active upon soft X-ray photon excitation. Surprisingly the 390 nm band (blue light) is the most intense, which is quite different to the result for UV photoexcitation, where the 600-700 nm band is the most prominent one (orange light). The orange light originated from Si-Si bond, blue light from Si-C bond. The ageing phenomenon of photoluminescence is observed but it is reversible.