The study of self-assembled nanostructures from the phase transition of faceted structures to the thermal desorption of oxide films
Date Issued
2007
Date
2007
Author(s)
Liao, Yu-Wen
DOI
en-US
Abstract
Utilizing a new developed technique called “Temperature Programmed Low Energy Electron Diffraction (TPLEED), together with temperature programmed Auger (TPA) and Temperature programmed thermal desorption (TPD), we have studied the faceting/defaceting phase transitions of Pd/W(111). In apparent agreement with results of recent theoretical simulation [C. Oleksy, Surf. Sci. 549, 246 (2004)], we find the effective way to create the largest facets is to anneal at a temperature right below the temperature the defaceting transition occurs. On the other hand, while the paths of faceting transitions show normal retardation as the cooling rate is increased, the paths of defaceting transitions show negligible dependence on the heating rate even if increased by 64-fold. Another notable observation is a phase separation of the surface into defaceted and faceted regions after long annealing time while there is more than enough Pd remaining to induce faceting of the whole surface. This leads us to the proposal that instead of thermal disorder, the observed defaceting transition of the Pd/W(111) system is mainly driven by local loss of Pd, which is due to thermal desorption. Such desorption loss could be effectively replenished via surface diffusion at the vicinity of the Pd 3d islands. The observed independence of the defaceting transition path on the heating rate is rationalized as the consequence of a balance in between the loss and the supply of Pd, which can establish very quickly as the temperature rises.
In addition, the thermal desorption and oxidation behavior of one-dimensional self-aligned oxide stripes on NiAl(100) surface have been investigated. The most interesting property of oxidized NiAl(100) surface is that very long and narrow one-dimensional stripes form on the oxidized NiAl(100) surface and the straight boundaries between these stripes have been shown to be good nucleation sites for the growth of a variety of metal nanoclusters. But the real mystery, of course, is in the thin oxide layer. Only limited research studied about the behavior of the oxidation at high temperature and desorption behavior from the oxide layer of NiAl(100). From the TPD study, the dominant desorption species from oxidized NiAl(100) surface is Al2O. Surprisingly, the Al2O desorption follows a zero-order desorption kinetics regardless of the thickness of oxide layer. In addition, when adsorbing oxygen at 1400K the surface forms the coexistence structure of faint C(√2×3√2)R45° spots and oxygen stripe phase, which is inferred to be due to part of the surface becoming clean. However, a largest desorption amount of Al2O is observed from the structure which is proposed that much oxygen diffuses into bulk and meanwhile causes the surface agglomeration, thicker and desorption.
Subjects
皺化
熱脫附
faceting
zero-order desorption
Type
thesis
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