定量探討形成表面合金時之擴散活化能

Determination of the energy barrier of diffusion for surface atoms during the formation of a surface-confined alloy

蔡佩蒼(P. C. Tsay) ； 周棟煥(T. H. Chou) ； 蔡志申(J. S. Tsay)

《東海科學》 4期 (2002/07) Pp. 13-23

摘要

由於表面合金的形成只牽涉到最表面一兩層的原子，古典方法上應用連續流概念處理擴散現象將不再適用。在此我們將報告如何運用蒙地卡羅電腦數值模擬分析配合低能量電子繞射術與歐傑電子能譜術，決定表面原子在表面合金化過程時的活化能。以厚度不超過一個原子層的銀/鉑(111)超薄膜為例，在加熱退火的過程中銀原子混合進入鉑單晶的最表面層而形成二維的表面合金。我們使用蒙地卡羅電腦模擬的方法與實驗所得的數據比較，得到了當形成二維的表面合金結構時，在銀/鉑(111)超薄膜系統中表面原子的擴散活化能為0.95±0.11eV。銅在525K時，開始與鉑(111)基底產生界面擴散作用，此表面合金化現象只限於最表面的兩個原子層。配合蒙地卡羅電腦數值模擬分析與歐傑電子能譜術，可以推算出形成銅/鉑表面合金時表面原子的活化能為1.11 ±0.12eV/atom。

關鍵字

擴散 ； 表面合金 ； 低能量電子繞射術 ； 歐傑電子能譜術 ； 蒙地卡羅電腦數值模擬

並列摘要

Under certain conditions the dissolution of thin absorbed films into an underlying single crystal substrate may be slowed down or even confined to a thin layer near the surface. The result is called a surface alloy. An important parameter for describing the process of surface alloying is the energy barrier of interdiffusion. We report on the determination of the energy barrier of site exchange under the restriction within very few layers near the surface using Monte Carlo simulation combined with lower-energy electron diffraction and Auger electron spectroscopy measurements. Annealing of submonolayer Ag/Pt(111) ultrathin film above 550 K causes formation of a two-dimensional surface alloy. The energy barrier of diffusion for surface atoms can be determined to be 0.95±0.11 eV obtained from data consistent with result of Monte Carlo simulation. Starting with one monolayer of Cu on Pt(111) concentration changes are determined as a function of annealing temperature up to 600 K and annealing time. Comparison of the experimental data with Monte Carlo simulations give the best result for the assumption of a Cu50Pt50 surface alloy yielding an energy barrier for interlayer mixing of 1.11±0.12 eV/atom.

並列關鍵字

diffusion ； surface-confined alloy ； low-energy electron diffraction ； Auger electron spectroscopy ； Monte Carlo simulation