Spectral density analysis of thin gold films : Thickness and structure dependence of the optical properties
Spectral density analysis of thin gold films : Thickness and structure dependence of the optical properties
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金薄膜的光谱密度分析：光学特性的厚度和结构依赖性
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发表时间：
2013
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通讯作者：
G. Niklasson
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作者：
P. Lans;E. Tuncer;I. Valyukh;H. Arwin;C. Granqvist;G. Niklasson
本文研究了用有效介质理论描述金薄膜光学性质的可行性。用直流磁控溅射法在不加热的玻璃衬底上沉积了质量厚度为1.4 ~ 9.2nm的金膜。在0.25 ~ 2 μ m范围内用椭圆偏振光谱法进行了光学测量，并测定了各膜的有效复介电函数。金膜被模拟为金和空气的混合物，并且使用谱密度函数（SDF）的一般有效介质描述来描述它们的光学性质。数值反演的实验介电函数给出了一个广泛的，而没有特征的SDF，与一些叠加的峰值，无论是岛状结构和叠层薄膜。广泛的背景在性质上类似于Bruggeman模型的预测（14）。1.引言金基纳米颗粒和薄膜在绿色纳米技术中的应用具有很大的兴趣（1）。它们的光学性质已经用涉及表面磁化率的岛膜理论（2）或用有效介质理论（3）把它们当作金和空气的混合物来描述。最近对岛状和网状薄膜的研究表明，这种薄膜可以用有效介电函数以及与标称厚度相同数量级的有效厚度来表示（4，5）。岛膜理论似乎具有一些优势，关于由纳米颗粒组成的膜的理论和实验之间的比较（5），而有效介质理论（EMT）通常用于模拟纳米复合材料的光学性质。然而，EMT对膜的实际纳米结构非常敏感。膜的结构可以由谱密度函数（SDF）（6，7）严格表示，其可以通过实验数据的数值反演（8{10）获得。本文测定了质量厚度dmass在1.4 ~ 9.2nm范围内的纳米金膜和网状金膜的介电函数。我们从实验数据中获得SDF，并讨论了SDF形状的物理含义。
In this paper we study the feasibility of representing the optical properties of ultrathin gold fllms by efiective medium theories. Gold fllms with mass thicknesses in the range of 1.4 to 9.2nm were deposited by DC magnetron sputtering onto non-heated glass substrates. Optical measurements in the range 0.25 to 2"m were carried out by spectroscopic ellipsometry, and the efiective complex dielectric function of each fllm was determined. The gold fllms were modelled as a mixture of gold and air, and a general efiective medium description using the spectral density function (SDF) was used to describe their optical properties. Numerical inversion of the experimental dielectric function gave a broad and rather featureless SDF, with a few superimposed peaks, both for island structures and percolating fllms. The broad background is qualitatively similar to predictions of the Bruggeman model (14). 1. INTRODUCTION Gold-based nanoparticles and thin fllms are of large interest for applications in green nanotech- nology (1). Their optical properties have been described either by the island fllm theory involving surface susceptibilities (2) or by treating them as a mixture of gold and air using efiective medium theories (3). Recent studies of island and network fllms have shown that such fllms can be repre- sented by an efiective dielectric function together with an efiective thickness of the same order of magnitude as the nominal thickness (4,5). The island fllm theory seems to have some advantages regarding the comparison between theory and experiments for fllms consisting of nanoparticles (5), while efiective medium theories (EMTs) are commonly used to model the optical properties of nanocomposites. However, EMTs are very sensitive to the actual nanostructure of the fllm. The structure of the fllms can be rigorously represented by a spectral density function (SDF) (6,7), which can be obtained by numerical inversion of experimental data (8{10). In the present paper we determine the dielectric function of gold nanoparticle fllms and network gold fllms with mass thickness dmass ranging from 1.4nm to 9.2nm. We obtain the SDF from the experimental data and discuss the physical implications of the shape of the SDF.