- 學位論文
自組裝奈米結構的研究從皺化結構的相變化到氧化層的熱脫附
The study of self-assembled nanostructures from the phase transition of faceted structures to the thermal desorption of oxide films
摘要
利用新發展出的「控溫低能電子繞射(TPLEED)」配合控溫歐傑電子能譜儀(TPA)以及可程式控溫熱脫附(TPD)之分析，我們研究了鈀在鎢(111) 表面皺化/反皺化之相變。和近期的理論模擬結果[C. Oleksy, Surf. Sci. 549, 246 (2004)] 一致，我們發現加熱到接近開始反皺化相變的溫度之下退火是可以製備出最大的皺化金字塔的方法。另一方面，當皺化相變之曲線會隨著降溫速率增加而有延遲的現象，反皺化相變之曲線卻不被升溫速率所影響，即使升溫速率相差到64倍。另一值得注意的報告是當經過足夠長的退火時間，剩餘Pd足以引起整個表面皺化，然而表面卻分裂成反皺化和皺化的區域。這使我們提出Pd/W(111)系統的反皺化相變主要是起因於鈀的損失而非熱擾動。而表面因熱脫附損失的鈀可以有效的從鄰近3-D鈀島(Pd 3d islands)來補充。也就是說鈀的損失及補充可以在很短的時間內達到平衡，而這正是反皺化相變之曲線不被升溫速率所影響的原因。 另外，我們研究了鎳鋁合金(110)上之一維條狀氧化物的熱脫附行為。被氧化的鎳鋁合金表面最有趣的特性就是表面形成窄長的一維條狀氧化物而這些一維條狀氧化物的邊界被觀察到是多種金屬奈米群(metal nanoclusters)的良好晶核位置。真正的奧秘之處當然是在這層氧化薄膜，然而有關鎳鋁合金高溫氧化及氧化層熱脫附行為的研究卻是很有限的。從我們可程式控溫熱脫附(TPD)的研究可知，被氧化的鎳鋁合金表面的主要脫附物是Al2O。令人驚訝的是，不管氧化層的厚度為何Al2O的熱脫附行為是零冪次脫附(zero-order desorption)。當在1400K的高溫氧化時，表面形成模糊的C(√2×3√2)R45°及條狀氧化物的共存態，這告訴我們表面有部分區域是沒有氧化物覆蓋的。然而卻有大量的氧化物(Al2O)從此表面脫附，這暗示著在此高溫下氧化將造成大量的氧鑽進塊材內部，同時氧化物在表面聚集、增厚和脫附。
並列摘要
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.
參考文獻
延伸閱讀
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