Research

Abstract

The controlled modification of graphene's electronic band structure poses serious challenges. In the present work, we study the effect of sp² 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 sp² clusters within the sp³ 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 sp²/sp³ 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). Sp² 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 sp²/sp³ 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 sp²/sp³ ratio-induced lattice structure evolution on photodetection.

Abstract

Even after the passage of three decades since the commercialization of first lithium-ion batteries (LIBs) and development of several other battery systems, LIBs continue to remain the battery of choice for increasingly expanding consumer electronics market. With the new found impulsion for developing electric and hybrid electric vehicles to minimize the dependence over fossil fuels and reduce carbon footprint of transport sector, efforts are required to enhance the energy and power delivery capabilities of LIBs. Silicon is a potential anode offering high theoretical capacity, surpassing that of even lithium. However, its pulverization due to cycling induced large volumetric fluctuations and limited electronic conductivity, drastically reduce its cycling stability. Graphene/reduced graphene oxide (rGO) with high electronic conductivity and flexibility, when composited with silicon is expected to overcome these challenges. Further, electrode structure and synthesis routes also significantly affect overall performance of the cell. Here, we review the progresses made in silicon/graphene nanocomposite chemistry for LIBs in the last five years. Additionally, different structural innovations such as core–shell, yolk–shell, porous, etc., for improving the electrode properties are also reviewed.