Abstract
Visualising the distribution of structural defects and functional groups present on the surface of two-dimensional (2D) materials such as graphene oxide challenges the sensitivity and spatial resolution of the most advanced analytical techniques. Here we demonstrate mapping of functional groups on a carboxyl-modified graphene oxide (GO–COOH) surface with a spatial resolution of ≈10 nm using tip-enhanced Raman spectroscopy (TERS). Furthermore, we extend the capability of TERS by measuring local electronic properties in situ, in addition to the surface topography and chemical composition. Our results reveal that the Fermi level at the GO–COOH surface decreases as the ID/IG ratio increases, correlating the local defect density with the Fermi level at nanometre length-scales. The in situ multi-parameter microscopy demonstrated in this work significantly improves the accuracy of nanoscale surface characterisation, eliminates measurement artefacts, and opens up the possibilities for characterising optoelectronic devices based on 2D materials under operational conditions.
Highlights
Visualising the distribution of structural defects and functional groups present on the surface of two-dimensional (2D) materials such as graphene oxide challenges the sensitivity and spatial resolution of the most advanced analytical techniques
In tip-enhanced Raman spectroscopy (TERS), a metallic scanning probe microscopy (SPM) probe placed at the focal point of a laser undergoes localised surface plasmon resonance (LSPR), which together with the lightening rod effect leads to the enhancement and confinement of the electric field at the TERS probe apex
Microchemical analysis of the graphene oxide (GO)–COOH sample used in this study was carried out using Xray photoelectron spectroscopy (XPS) (Supplementary Fig. 1), Fourier-transform infrared (FTIR) spectroscopy (Supplementary Fig. 2a) and Raman spectroscopy (Supplementary Fig. 2b)
Summary
Visualising the distribution of structural defects and functional groups present on the surface of two-dimensional (2D) materials such as graphene oxide challenges the sensitivity and spatial resolution of the most advanced analytical techniques. We present nanoscale surface mapping of structural defects and functional groups on few-layer carboxyl-modified graphene oxide (GO–COOH) flakes with ≈10 nm spatial resolution using TERS.
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