鉑或銥原子團在鉑表面之擴散研究

Date
2008
Authors
高玉娟
Kao Yu Jan
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Abstract
鉑或銥原子團在鉑表面之擴散研究 使用場離子顯微鏡(FIM)在超高真空(UHV)的環境下,觀察鉑或銥原子團在鉑表面上的擴散運動。當溫度加熱到135K,從三角形場離子影像的方向判斷,單顆鉑在鉑(111)面上只會吸附在FCC site。接著利用觀察時所繪出原子位置地圖(site mapping),分析原子擴散的步數,並代入擴散的波茲曼分佈(Boltzmann distribution),得到鉑在鉑(111)面的擴散活化能為0.33±0.01eV。再根據單顆鉑出現在FCC site 與HCP site的次數與波茲曼分佈的關係,計算出鉑吸附在FCC site的束縛能比在HCP site的束縛能大了0.04eV以上。 其次,觀察中也發現七顆銥原子團(Ir7)在鉑(111)面上,以正六角形的形貌擴散的機率是35/40。統計40次擴散事件,得到擴散活化能為1.41 0.03 eV。綜合比較Ir7/Ir(111)[6]、Ir7/Pt(111)、Pt7/Pt(111)[3]系統,發現原子團在Pt (111)面上,Ir7比Pt7更難擴散,而Ir7在Ir(111)面上卻比Pt(111)面上更難擴散。影響上述擴散難易程度的因素包括吸附原子團內部的束縛能、原子團與基底的交互作用。
study of Pt or Ir atoms and clusters diffusion on Pt surfaces Using the field ion microscope in ultra high vacuum, we observe directly the diffusion behavior of Pt or Ir atoms on Pt surfaces. After annealing at 135K, single Pt occupies at FCC site exclusively on Pt(111). This phenomenon is decided by the apex of the image triangle. By site mapping, we analyze how many steps Pt diffuses, and then put the mean square displacements into the Boltzmann distribution. The diffusion activation energy of Pt on Pt(111) is 0.33±0.01 eV. According to the frequency of Pt occupation at FCC (HCP) site, the binding energy of Pt at FCC site is 0.04 eV greater than HCP site at least. The probability of Ir7 on Pt(111) as close-packed hexagon is 35/40. During 40 diffusion events, we get the diffusion activation energy of Ir7 on Pt(111) is 1.41±0.03 eV. Comparing Ir7/Pt(111) with Pt7/Pt(111)[3], Ir7 on Pt(111) has greater diffusion energy than Pt7 on the same substrate. Comparing Ir7/Pt(111) with Ir7/Ir(111)[6], Ir7 on Ir(111) has greater diffusion activation energy than on Pt(111). Finally, We discuss the factors of the diffusion activation energy, including the binding energy of the adsorbed clusters, and the interaction between the adsorbed clusters and the substrate.
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Keywords
場離子顯微鏡, 鉑(111), 擴散, 面心立方晶格位置, 七顆銥原子團, FIM, Pt(111), diffusion, FCC site, Ir7
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