Nanoscale chemical imaging of single-layer graphene

Electronic properties in different graphene materials are influenced by the presence of defects and their relative position with respect to the edges. Their localization is crucial for the reliable development of graphene-based electronic devices. Graphene samples produced by standard CVD on copper and by the scotch-tape method on gold were investigated using tip-enhanced Raman spectroscopy (TERS). A resolution of <12 nm is reached using TERS imaging with full spectral information in every pixel. TERS is shown to be capable of identifying defects, contaminants, and pristine graphene due to their different spectroscopic signatures, and of performing chemical imaging. TERS allows the detection of smaller defects than visible by confocal Raman microscopy and a far more precise localization. Consecutive scans on the same sample area show the reproducibility of the measurements, as well as the ability to zoom in from an overview scan onto specific sample features. TERS images can be acquired in as few as 5 min with 32 × 32 pixels. Compared to confocal Raman microscopy, a high sensitivity for defects, edges, hydrogen-terminated areas or contaminated areas (in general for deviations from the two-dimensional structure of pristine graphene) is obtained due to selective enhancement as a consequence of the orientation in the electromagnetic field.