Effect of Gold Nanoparticles and Unwanted Residues on Raman Spectra of Graphene Sheets

We report the effect of gold nanoparticles (AuNPs) and unwanted sodium citrate residues (UnR) left after deposition of AuNPs by drop-casting method on the Raman spectra of graphene sheets (GS). The AuNPs solution was deposited on three different substrates: 5.0 wt% Yb3+-doped (Q5) phosphate glass, silica glass (S1), and Si/SiO2-300 nm (S2) substrates. For Q5 substrate, a slight increase in intensity of the G peak was observed, mostly for thinner layers, which can be attributed to a weak SERS effect shielded by UnR. The combination of the following aspects: a blue shift of the G band position, a slight increase in the FWHM (Full Width at Half Maximum), and a slight increase of the Raman intensities of both G and 2D bands in other GS without UnR supports the argument of shielded SERS effect. On the other hand, the effects of UnR on the S1 and S2 substrates produce a decrease on the Raman intensities of G and 2D bands, opposite to the effect produced by the AuNPs; this result was found more intense for the S2 substrate in relation to S1. This is possibly caused by the greater amount of UnR accumulated on the Si/SiO2 substrate, due to its higher hydrophilicity in relation to other samples. Additional Raman measurements reveal that the Raman intensity of GS in all substrates is unaffected by the presence of a possible humidity on GS, revealing the effect of UnR. Hence, it is vital to understand how residues influence the salient features of GS/AuNPs.