A case study: effect of defects in CVD-grown graphene on graphene enhanced Raman spectroscopy

Abstract

Graphene-enhanced Raman spectroscopy (GERS) is a technique to increase the Raman scattering of adsorbed probe molecules on graphene. Here we systematically explore the effect of the method used to transfer the CVD-grown graphene onto another substrate on Raman scattering. We have found that graphene transferred using poly methyl methacrylate (PMMA) produces 6 times the Raman scattering signal increase of that produced by graphene transferred using thermal release tape. The reason for this is that PMMA-transferred graphene contains a larger amount of defects such as carboxyl and hydroxyl groups that help the attachment of probe molecules to the graphene surface, leading to improved π-π* interactions and thus easier charge transfer between the probe molecules and graphene. Our results indicate the need for a much closer look at the functional groups of graphene which are different for the two transfer methods. 2 detection properties due to a high surface area, easy functionalization and stability in ambient environments. Graphene can be synthesized using epitaxial growth 1 , mechanical exfoliation 2 , chemical methods 3 and chemical vapor deposition (CVD) 4 . Among them, the CVD technique is extensively used to produce thin graphene films since it provides a relatively friendly synthesis route and also produces high quality graphene. In general, graphene films obtained by CVD needs to be transferred to desired substrates for further applications. Several transfer methods such as polymer-assisted 5 (PMMA, PDMS, thermal release tape), polymer-free 6 , electrochemical delamination 7 have been used to transfer the graphene films. Polymer-assisted transfer has been widely used due to its easy processing steps. Aside from surface-enhanced Raman scattering (SERS) 8,9 which is one of the most efficient detection tools utilizing metal substrate for a variety of common molecules, graphene-enhanced Raman spectroscopy (GERS) 10 is another efficient technique to increase the Raman scattering of adsorbed probe molecules adsorbed on graphene.The concept of GERS has been widely applied to graphene oxide 3b, 11 , hydrogen terminated graphene 12 , nano-mesh graphene 13 and nitrogen-doped graphene 14 . It has been reported that GERS depends on various factors such as the number of graphene layers 15 , the density of the probe molecules 16 , the space between graphene and the probe molecule, the Fermi level of graphene, which changes with doping 17 , an interference effect from the substrate, and molecular alignment 18 . Further, in GERS the contribution of the electromagnetic-enhanced plasmons is in the terahertz range, and Raman signals are solely due to the chemical mechanism (CM) which is closely related to the charge transfer between probe molecules and the graphene substrate. Here, we report the CM properties of graphene grown by CVD can be affected by the methods of transfer (i) PMMA-and (ii) TRT-assisted, by comparing their GERS of R6G probe molecules. To date there are, to the best of our knowledge, no comprehensive reports o...

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