Two-Dimensional Nitrogen-Enriched Carbon Nanosheets with Surface-Enhanced Raman Scattering

We have fabricated two-dimensional nitrogen-enriched carbon nanosheets (2D-NECNs) through the pyrolysis of cross-linked poly(4-vinylpyridine) homopolymers as a platform for detecting physically absorbed dye molecules using Raman-scattering spectra. Upon pyrolysis, a polymeric layer consisting of pyridinic rings was converted into a carbonized nanosheet enriched with pyridinic nitrogen (N₆), pyrrolic nitrogen (N₅), graphitic nitrogen (GN), and nitrogen oxide (NO) groups, the fractions of which were finely controlled through pyrolysis at temperatures selected in the range of 430-550 °C. The effects of temperature on the formation of nitrogen- and carbon-containing species in 2D-NECN were examined by XPS, which showed that N₆ and N₅ were the dominant species over GN and NO at 430 °C. Increasing the temperature of pyrolysis produced carbonized nanosheets containing more GN and NO generated at the expense of pyridinic groups. Using rhodamine 6G (R6G) and crystal violet (CV) molecules as probes for Raman measurements, we found that the Raman enhancement on 2D-NECN is due to a chemical mechanism (CM) and that the observed enhancement of the Raman intensity of molecules adsorbed on 2D-NECN hence shows a clear dependence on the nitrogen configuration of the four types. Among the nitrogen species, GN dominates the large enhancement. The chemical Raman enhancement is ascribed to the ability of GN to improve the π-conjugated domains and narrow the energy gap in 2D-NECN.