覆鈀鎢針上成長金字塔結構及相關原子運動能障之研究

Abstract

在這個研究工作中，我們觀察到覆鈀鎢針上成長的單原子尖端金字塔結構，並瞭解其熱穩定性，以及能夠重覆再生的特性。利用場離子顯微鏡，我們也同時觀察了與金字塔成長相關的原子過程，包括Pd吸附原子在鎢表面(112)、(110)、(111)平台上的擴散運動和下行運動；在鎢(111)平台的上行運動及台階邊緣的脫離運動，以及鎢(111)表面自由能造成的能量梯度。實驗的結果顯示出：Pd吸附原子在鎢(112)、(110)、(111)平台的擴散活化能分別為0.32±0.02，0.51±0.03，1.02±0.06eV；在台階邊緣的額外反射位障分別為0.29±0.03，0.27±0.02，0.61±0.01eV，由實驗觀察結果發現其下行運動的機制，應為跳躍的模式；Pd在W(111)台階發生上行運動時所需克服的能量位障＝1.84±0.07eV，在台階邊緣的脫離位障＝1.72±0.07eV，並且鎢(111)平台造成的能量梯度為0.013eV/4.47Å。由這些結果，對於Pd成長的金字塔結構能夠有更基本的瞭解。我們也嚐試著將鎢表面成長的單原子金字塔結構，運用在掃描穿隊顯微鏡的掃描探針中，這一方面的工作，仍有待繼續研究。

The growth of a single-atom sharp pyramidal structure on the Pd covered W tip was observed in this study. Its pro-perties such as thermal stability and repeated regeneration processes are found. Using Field Ion Microscopy, we also investigated such atomic processes in relation to the pyramidal facet growth, including terrace diffusion and descending diffusion of Pd adatoms on W(112), (110), and (111) surfaces, ascending diffusion of Pd adatoms onto W(111) surface, detachment of Pd adatoms from the kink sites of W(111) boundary and the free energy anisotropy of W(111) surfaces. The activation barriers for the terrace diffusion of Pd adatoms on W(112), (110) and (111) surfaces are determined to be 0.32±0.02, 0.51±0.03, 1.02±0.06eV, respectively. And the extra reflective barriers for the descending diffusion are determined to be 0.29±0.03, 0.27±0.02, 0.61±0.01eV, respectively. The mechanism of des-cending diffusion for Pd adatoms on W surfaces should be hopping. The activation energy for the ascending diffusion for Pd adatoms at W(111) steps is derived to be Ea＝1.84±0.07eV and for the detachment from the kink sites is derived to be Edet＝1.72±0.07eV. The free energy anisot-ropy near the W(111) surface is also derived to be 0.013eV/4.47Å. The investigation of the mechanism and energetics of the atomic processes gives us more information for the growth of pyramidal structure. We also try to use such a W tip with single-atom sharp apex as the scanning probe of Scanning Tunneling Microscopy. The work is currently in progress.