Shearer Cameron J, Slattery Ashley D, Stapleton Andrew J, Shapter Joseph G, Gibson Christopher T
Centre for NanoScale Science and Technology, School of Chemical and Physical Sciences, Flinders University, Bedford Park, South Australia, 5042, Australia.
Nanotechnology. 2016 Mar 29;27(12):125704. doi: 10.1088/0957-4484/27/12/125704. Epub 2016 Feb 19.
Graphene has emerged as a material with a vast variety of applications. The electronic, optical and mechanical properties of graphene are strongly influenced by the number of layers present in a sample. As a result, the dimensional characterization of graphene films is crucial, especially with the continued development of new synthesis methods and applications. A number of techniques exist to determine the thickness of graphene films including optical contrast, Raman scattering and scanning probe microscopy techniques. Atomic force microscopy (AFM), in particular, is used extensively since it provides three-dimensional images that enable the measurement of the lateral dimensions of graphene films as well as the thickness, and by extension the number of layers present. However, in the literature AFM has proven to be inaccurate with a wide range of measured values for single layer graphene thickness reported (between 0.4 and 1.7 nm). This discrepancy has been attributed to tip-surface interactions, image feedback settings and surface chemistry. In this work, we use standard and carbon nanotube modified AFM probes and a relatively new AFM imaging mode known as PeakForce tapping mode to establish a protocol that will allow users to accurately determine the thickness of graphene films. In particular, the error in measuring the first layer is reduced from 0.1-1.3 nm to 0.1-0.3 nm. Furthermore, in the process we establish that the graphene-substrate adsorbate layer and imaging force, in particular the pressure the tip exerts on the surface, are crucial components in the accurate measurement of graphene using AFM. These findings can be applied to other 2D materials.
石墨烯已成为一种具有广泛应用的材料。石墨烯的电子、光学和机械性能受到样品中层数的强烈影响。因此，石墨烯薄膜的尺寸表征至关重要，尤其是随着新合成方法和应用的不断发展。存在多种确定石墨烯薄膜厚度的技术，包括光学对比度、拉曼散射和扫描探针显微镜技术。特别是原子力显微镜（AFM）被广泛使用，因为它提供三维图像，能够测量石墨烯薄膜的横向尺寸以及厚度，进而确定存在的层数。然而，在文献中，AFM已被证明不准确，报道的单层石墨烯厚度测量值范围很广（在0.4至1.7纳米之间）。这种差异归因于针尖 - 表面相互作用、图像反馈设置和表面化学。在这项工作中，我们使用标准和碳纳米管修饰的AFM探针以及一种相对较新的AFM成像模式，即峰值力轻敲模式，来建立一种协议，使用户能够准确确定石墨烯薄膜的厚度。特别是，测量第一层时的误差从0.1 - 1.3纳米降低到0.1 - 0.3纳米。此外，在此过程中我们确定，石墨烯 - 衬底吸附层和成像力，特别是针尖施加在表面上的压力，是使用AFM准确测量石墨烯的关键因素。这些发现可应用于其他二维材料。
