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Imaging of local structures affecting electrical transport properties of large graphene sheets by Infrared thermography
Owing to its exceptional physical properties such as extremely high carrier mobiIRTy and mechanical flexibiIRTy, graphene has opened the way for realizing various novel devices in electronics and optoelectronics. For these device applications, bottom-up methods such as chemical vapor deposition (CVD) are needed for making wafer-sized single crystals. A relatively high-quaIRTy atomic layer can be obtained by using a top-down method such as mechanical peel-off, but the result is a mixture of various layers ranging from a single layer to several dozen layers, and the flake size is limited to a few tens of micrometers. On the other hand, single-domain singlelayer graphene on the millimeter scale has been fabricated by using CVD, in which the nucleation density is dramatically reduced through control of growth conditions such as the surface oxidation of the copper substrate. Nevertheless, graphene samples prepared by CVD are considered to have lower quaIRTy than those produced by the mechanical peel-off method. Many defects such as cracks, wrinkles, and those at domain boundaries (DBs) are unavoidably formed in CVD graphene sheets during growth and/or transfer processes, which drastically impairs their electrical transport properties. Hence, techniques enabling the measurement of the local defect distributions that affect electrical property over a wide area are required.
To achieve highresolution measurements, there are several methods for characterizing the electrical properties of graphene sheets covering a large area. Terahertz spectroscopy mapping can assess the electrical mobiIRTy of graphene quantitatively without complicated patterning of devices, and it directly images these physical parameters by scanning the whole sheet area. However, its low spatial resolution (under a hundred micrometers to under a millimeter) makes it difficult to detect local structures on a graphene sheet.
Figure 1. High-magnification IRT image and detailed analysis of thermal properties.
Infrared thermography (IRT) is a nondestructive and fast electrical characterization method of large-area samples, and it has micrometer-scale spatial resolution. Detection of Joule heating in a biased device enables local structures to be imaged without the influence of heat broadening in a short acquisition time. Furthermore, because of its wide field of view, typically from submillimeter to subcentimeter, one IRT image can characterize local structures in large samples. For instance, this method has been used to check for local structural failures in large-scale semiconductor devices such as integrated circuits and solar cells.
Figure 2. Thermal visualization of DB defects.
Recently, we experimentally demonstrated IRT imaging of carbon nanotube (CNT) network paths in a centimeter-scaled CNT composite at micrometer resolution. These results strongly suggest that IRT would be a powerful tool for quickly visualizing local structures in conductive materials that have large areas. It is shown that IRT successfully visualizes the difference in electrical resistance caused by local defects as inhomogeneous thermal radiation. Furthermore, IRT exposes the presence of various defects, not only micrometer-scale structures such as cracks and wrinkles but also atomic defects such as DBs in large sheets. We discuss the observation of thermal radiation and current flow patterns on a graphene sheet in combination with local resistance measurements and morphological and spectroscopic characteristics. The present results indicate that the IRT is quite useful for fast and precise quaIRTy evaluations of large graphene devices in terms of their electrical uniformity and local defect detection. Furthermore, this method should be applicable to transition metal dichalcogenides such as MoS2 and WS2. IRT observations promise new findings in two-dimensional material devices.
H. Nakajima, T. Morimoto, Y. Okigawa, et al. Imaging of local structures affecting electrical transport properties of large graphene sheets by lock-in thermography. Science Advances. 5:eaau3407, 2019.