掠入射荧光XAFS研究Pt超薄膜的局域结构

Local structures of Pt thin films studied by grazing incidence fluorescence XAFS

利用荧光 X 射线吸收精细结构（XAFS）技术研究了厚度为 3—105 nm 范围的分子束外延生长的 Pt 金属薄膜的局域结构。结果表明，30 nm 和 105 nm Pt 薄膜的 XAFS 结果与多晶 Pt 箔的相似，在径向结构函数图中的 2.61、3.80、4.63、5.43 ? 处出现第一、二、三和四配位壳层的配位峰，保持着 Pt 金属的面心立方结构特征。30 nm Pt 薄膜样品的结构参数：配位数 N＝11.9，键长 R＝2.77 ?，无序度σ2＝0.0055 ?2。3 nm 和 10 nmPt 超薄膜的径向结构函数曲线中的第一配位峰形状与 105 nm Pt 厚膜的相似，但强度明显降低，其结构参数分别为: N=10.1, R= 2.78 ?, σ2=0.0077 ?2；N=11.3, R= 2.77 ?, σ2=0.0061 ?2；同时其高壳层的配位峰近于消失，说明 3 nm 和 10 nm Pt 超薄膜的中程有序结构遭到较大影响，无序度明显增加。在 350—800oC 温度范围内生长的 105 nm Pt 薄膜，其局域结构近似于 Pt 金属晶体。

The local structures of Pt thin films grown by molecular beam epitaxy with the thickness range from 3 to 105 nm have been studied by the grazing incidence fluorescence XAFS technique. The results indicate that the local structures of Pt thin films of 30 and 105 nm are similar to that of Pt foil. Four peaks located at 2.61, 3.80, 4.63, 5.43 ? appear in the radial structural function (RSF) of 105 nm Pt thin film, which are corresponding to the first, second, third and fourth Pt-Pt shells, respectively. For the first shell of 30 nm Pt thin film, the coordination number N is 11.9, the bond length R is 2.77 ?, and the Debye-Waller factor σ2 is 0.0055 ?2. With the thickness decreasing from 105 to 3 nm, the magnitude of the first RSF peak decreases obviously for the 3 nm Pt thin film, despite their peak shapes are similar. The N, R and σ2 are 10.1, 2.78 ?, 0.0077 ?2 and 11.3, 2.77 ?, 0.0061 ?2 for the Pt thin films of 3 and 10 nm, respectively. Moreover, the RSF peaks in higher shells almost disappear for the 3 and 10 nm Pt thin films. These results show that the medium-range order around Pt atoms is largely destroyed for the Pt thin films. The XAFS results imply that the local structures of 105 nm Pt thin films grown at 350—800 °C are close to that of the Pt crystal.