Scanning near-field microwave microscopy of VO₂ and chemical vapor deposition graphene

Near-field scanning microwave microscopy (SMM) is a near-field technique, which enables probing local electric properties of materials, i.e., complex permittivity. Recently, this technique was incorporated into a commercially available atomic force microscope (AFM), providing a new powerful imaging mode in the suite of AFM techniques. AFM probe-surface distance control allows routine acquisition of near-field microwave images with a lateral resolution better than 100 nm, which was previously unattainable. In this paper, work performed with an AFM-based SMM system at the Center for Nanophase Materials Sciences at ORNL is reviewed. As an introduction, a brief general overview of the near-field microwave microscopy is provided followed by a description of the SMM system. Application of the technique to studies of metal-insulator phase transition in single-crystalline nanoplatelets of vanadium dioxide is illustrated. Further, the capabilities of SMM in its application to imaging of conductivity inhomogeneities in single- and few-layer graphene samples grown via different chemical vapor deposition (CVD) routes is demonstrated. The imaging of graphene illustrates the specific nature of contrast in the SMM, where the signal is described by complex numbers. To facilitate the interpretation of the contrast, a simple graphical scheme inspired by standard Nyquist plots is proposed.