Characterisation of the heterojunction microstructure for electrodeposited vertical ZnO nanorods on CVD-graphene

Abstract

A low-cost heterostructure of a wide-bandgap nanostructured semiconductor on top of a transparent conductor (TC) fabricated from abundant sustainable materials is of great interest for optoelectronic devices, such as heterojunction photovoltaic cells. Due to its high optical transparency and electron mobility, graphene is an attractive carbon-based material to replace the costly traditional TCs (i.e. ITO), which pose health hazards and destructive environmental impact thorough their life cycle. However, for the industrial application of graphene as TC it is necessary to interface it with other materials, like oxides, while maintaining the sp2 network that provides its excellent electronic properties. We report the optimized electrodeposition of ZnO on graphene to obtain a textured layer of vertically aligned crystalline semiconducting nanorods (ZVNRs) evenly coating the graphene surface. Lattice distortions and mosaicity in the crystals indicate that the ZVNRs grow under out-of-plane compressive strain. The stability of the sp2 network of graphene during in situ growth of ZnO is demonstrated using Raman spectroscopy from the front and backside of the heterostructure, in a novel approach to investigate junctions of 2D–3D nanomaterials. Despite the structural stability of graphene, its R□ is found to increase by ∼100% with ZVNRs electrodeposition, which is attributed to reduction of mobility due to charge carrier scattering, reduction of carrier concentration due to doping, and/or interfacial strain. The cathodic electrodeposition is demonstrated to be an effective and versatile method to grow crystalline nanostructures in the surface of single layer-graphene while protecting its sp2 hybridization, and it provides relative fast deposition rates, affordability, and low processing temperatures that would allow using polymeric flexible substrates and. These findings on interfacial properties are important to advance the functionality of graphene as TC material and develop nano-heterostructures for myriad technological applications.