Reversible synthesis of GO: Role of differential bond structure transformation in fine-tuning photodetector response

The controlled modification of graphene's electronic band structure poses serious challenges. In the present work, we study the effect of sp ² cluster size variation on the electronic band gap and photoconductive properties of reduced graphene oxide (RGO). This is achieved by performing reversible functionalization of RGO with oxygen species. The reversible functionalization of RGO results in its partial transformation to graphene oxide (GO) so that the size of the sp ² clusters within the sp ³ matrix varies, thereby affecting the π-π* band structure and photoconductive properties. The study reveals: (1) incremental creation/elimination of oxygenated surface bonds' related energy states within the π-π^* band; (2) customized tuning of the sp ²/sp ³ ratio; (3) the presence/absence of oxygenated states impacts the optical transition processes both from band-to-band and oxygenated states; and (4) the incremental addition/depletion of surface states in a tunable manner directly influences the carrier transport in the photoconductive device. Experiments show a two-stage transformation of RGO electronic properties with changing oxygen functionalities: oxidation (Stage I) and decomposition or erosion (Stage II). Sp ² cluster size variation induced bandgap change was analyzed by Raman and photoluminescence studies, indicating the possibility for photodetection in a specific band encompassing NIR to UV, depending on the sp ²/sp ³ ratio. Energy-dispersive x-ray spectroscopy and Fourier transform infrared studies confirm the surface oxygenation/de-oxygenation during plasma treatment, and XRD confirms partial transformation of RGO to GO and its amorphization at higher plasma exposure times. In addition, the photodetector performance is optimized in terms of carrier generation-recombination and carrier-lattice scattering. Thus, manipulating better photoconductive response is possible through suitable handling of the parameters involved in the plasma treatment process. This is the first study on the influence of the sp ²/sp ³ ratio-induced lattice structure evolution on photodetection.