以掃描式穿透顯微鏡研究NaxCoO2單晶表面(x = 0.84)

Abstract

我們以掃描式探針顯微鏡(scanning tunneling microscopy, STM)研究NaxCoO2的表面結構，觀察濃度x = 0.84之單晶樣品，在超高真空系統中剝離出新的表面進而在STM下觀察其表面結構，在室溫下觀察到表面Na形成排列結構p(√7 x √7) hexagonal phase和p(3 x 3) kagome phase，以及p(2√3 x 2√3) pinwheel phase。同時也觀察到Na的排列結構隨著時間會有明顯的變化，如流體般與固態之間的漲落；並以觀察到的實驗結果進而建構以Na原子形成trimmer之模型來描述Na原子在表面的排列結構。我們也觀察到類一維的p(6 x 1)的條紋相(stripe phase)，其結構週期在表面低能電子繞射(low energy electron diffraction, LEED)和在單晶X-ray Laue繞射中都被觀察到，說明了條紋相結構不僅存在表面上同時也存在整個單晶塊材之中。在STM下也觀察到三個方向條紋相的分佈邊界(domain boundary)，以及Na排列相隨著條紋相之週期排列起伏的情況。在LEED實驗中，些微加熱或降低溫對於條紋相p(6 x 1)排列結構有增進的效果。但在低溫的STM實驗中，僅少數可觀察到kagome phase的Na排列，大部分觀察到的Na排列結構較室溫下所觀察的無序。

We report the first direct real-space scanning tunneling microscopy (STM) study of NaxCoO2 surface (x = 0.84), as prepared by in-situ cleaving in ultra high vacuum. Two categories of ordering phenomena were discovered. First, three Na ordered patterns of Na0.84CoO2 surface, with p(√7 x √7), p(3 x 3), or p(2√3 x 2√3) unit cells, were resolved. These patterns are ascribed to Na ordering. Second, a new phase of one-dimensional stripe was identified. This stripe phase exists on the surface and in the bulk crystal. We also observed fluctuations of the Na ordered phases at room temperature. The ordering of the stripe phase can be slightly improved by slight annealing followed by cooling, as evidenced by low energy electron diffraction (LEED). However, preliminary low temperature STM (~100K) studies showed the Na is less ordered and only occasional small area of the kagome phase was observed.