Charactrisation of Functionalised Graphene using Vibrational Sum-Frequency Generation Spectroscopyn

Abstract

Vibrational sum-frequency generation (SFG) spectroscopy is a powerful technique to gain vibrational information about various chemical bonds at a surface or interface. This second order non-linear spectroscopy technique utilises two laser beams spatially and temporally overlapped on the target sample. SFG is only allowed in non-centrosymmetric environments, and one of the most surface-specific and sensitive techniques. Grapheneisnowwidelyusedinvariousapplications,buttheabsenceofbandgapin pristinegraphenelimitsitsusesinopto-electronicdevicesandsemiconductors. Oneway to open the band gap in graphene and modifying its properties is by functionalisation. In this project, SFG was first used to characterise chemically modified CVD graphene on gold substrates. A vibrational band at 3064 cm−1 was observed and assigned to an aromatic C–H stretch of the phenyl group covalently attached to the graphene sheet. A 3-7% coverage of functionalisation was estimated and ∼1.3◦ tilt angle of the phenyl group was calculated with respect to the graphene surface. Secondly, SFG was used to characterise hydrogenated graphene on gold. Four substrates were hydrogenated for different times using the Birch reduction method, and the recorded SFG spectra showed a change in signal intensity as a function of hydrogenation time. By comparing these results with the ones obtained from densityfunctional theory (DFT) calculations, the hydrogen atoms were found to be covalently attached to the graphene as dimers in the ortho and para configurations. Finally, SFG was used to compare the reactivity of graphene’s terraces vs. edges towards functionalisation. SFG spectra of phenyl functionalised CVD graphene from the graphene’s terrace and the edge were recorded. An increase in the intensity of the phenyl aromatic C-H stretch was observed at the edges, suggesting higher reactivity of graphene’s edges than the terraces.