This item is available under a Creative Commons License for non-commercial use only
Low dimensional structures can exhibit unusual properties due to the quantum confinement of electrons. This may manifest itself in any effects that depend on electron behaviour. In this study, the optical and magnetic properties in particular of two types of low dimensional structures are examines. Quantum wires are structures that have macroscopic length scales along one dimension and nanometre length scales in the others. Two Indium induced reconstructions of Silicon surfaces were grown and examined to determine if quantum confinement, a necessary requirement for a quantum wire, did exist for their structures. A UHV compatible Reflectance Anisotropy Spectroscopy (RAS) instrument, that measures the difference in reflectivity of linearly polarised light along orthogonal surface directions, was constructed for this purpose. The Si (111) 4x1-In and Si (001)4x3-In systems were studies. Studies revealed an optical anisotropy of 1.65% for Si (111)4x1-In which, considering the result is from a layer just a monolayer in thickness is very high for a metal-semiconductor system and may be indicative of confinement. Si (111)4x3-In revealed a smaller but still considerable anisotropy of 0.5%. The wires were also studied with Scanning Tunnelling Microscopy (STM) in an effort to conclusively determine the structure. The results gave strong evidence in support of the structural model proposed by Bunk, et al. Scanning-tunnelling spectroscopy provided information on surface states which agree qualitatively with photoemission data. Metallic nanoparticles have been attracting considerable interest due to their novel optical and magnetic properties and as potential components in ‘spintronic’ devices. Iron and cobalt particles were examined by preparing films on Silicon or Graphite (HOPG) substrates. The films were characterised using Atomic Force Microscopy and STM to determine particle coverage, density and distribution. The particles internal composition was examined using Mossbauer spectroscopy and XPS. The films were then examined using a variation of the RAS technique. Magneto Optical Kerr Effect (MOKE) and its time resolved counterpart TRMOKE. Both give an indication of a materials magnetisation by responding to difference in electron spin population. These results were compared to those obtained form SQUID magnetometry. It seems that for monolayers of particles in this size regime. i.e. ~10nm, the optical techniques do not have the required sensitivity to detect a magnetic response, despite their surface specificity. Hence the results at least help to place a lower limit on the techniques’ sensitivity. Iron particles were also examined using Magnetic Force Microscopy, which demonstrated a stronger response over a single particle than a particle cluster. A computational model was generated to explain this interesting effect. This verified the hypothesis that dipolar interactions between the neighbouring particles in a cluster were sufficiently strong to prevent the tip from aligning the particles magnetic moments. In the absence of these interactions, as is the case with an isolated particle, the tip can align the moment.
Pedreschi, F. (2002). Optical and magnetic behaviour of low dimensional structures. Doctoral thesis. Dublin Institute of Technology. doi:10.21427/D7Q60S