扫描电子显微镜法测定金属衬底上石墨烯薄膜的覆盖度

Coverage measurement of graphene film on metallic substrate using scanning electron microscopy

利用化学气相沉积法生长在金属衬底上的石墨烯薄膜,由于其尺寸的可控性和转移的便利性,被广泛用作各种透明电极.石墨烯薄膜的方块电阻是衡量其品质的重要指标之一,而石墨烯覆盖完全是保证薄膜拥有优良导电性能的基本前提.本文通过研究评估不确定度的分量,提出利用扫描电子显微镜像素计算微区和宏观覆盖度的方法.考虑到石墨烯薄膜覆盖区域与未覆盖区域边界的确定,以及晶畴数目的选取这两个因素对覆盖度测定造成的误差.通过微区有效扫描电子显微镜图像的确定、宏观石墨烯薄膜有效扫描电子显微镜图像的测量数目以及宏观石墨烯薄膜覆盖均匀性的表达,系统研究了化学气相沉积法生长在金属衬底上的石墨烯薄膜的微区覆盖度、宏观覆盖度和覆盖均匀性.该方法通过获得有限次微区扫描电子显微镜图像,不仅可以计算宏观石墨烯薄膜的覆盖度,还可以给出覆盖均匀性,既节省了测量时间,同时也能保证测量有效性.

Graphene films grown on metallic substrates by chemical vapor deposition have wide potential applications, such as serving as transparent electrodes, transistors, sensors, etc. The coverage of graphene on metal surface can influence many performance parameters, such as square resistance and transparence, after it has been transferred to other substrates. As most of the performance parameters cannot be measured while graphene is still on the metal, it is very useful to evaluate the coverage of graphene before further actions. In this paper, we present a method to measure the coverage of graphene on metal by using scanning electron microscopy and image processing software. We also calculate and measure the uncertainty of the measured coverage. There are two main factors, namely the determination of the boundary between the covered areas and the uncovered areas, and the number of the graphene islands or vacancy islands in view, which can bring uncertainty to the coverage. The former factor raises the uncertainty of the coverage while the number of graphene （vacancy） islands in view is higher, because the more the islands in view, the smaller the islands are, therefore the total boundaries become more. The latter factor reduces uncertainty with the number of islands increasing, because of the quantum fluctuation. The uncertainty of the latter factor is proportional to 1/√N, where N is the number of islands in view. As we can see, the number of islands in view is the key parameter to balance the two factors. We measure the graphene coverage with different graphene islands in view, and also measure the uncertainty by using the statistics knowledge. Meanwhile, we also build a model to calculate the uncertainty under different numbers of islands in view. The experiments and the calculations accord with each other reasonably well. By these carefully modeling and experimentations, we optimize and balance the two faces and suggest the number of islands in view to reduce the uncertainty of the measured coverage to a lowest value. The use of these measured data can ensure the accuracy of the graphene coverage measurement with minimal time cost.