Graphene-based Hybrid Nanostructures and their applications

Abstract

There has been significant interest in the synthesis and application of diverse nanostructured materials. In particular, hybridization of various types of nanomaterials can create new types of material systems that exhibit multi-functionality or whose performances are superior to those of the pristine materials. A lot of researches have been paid on recently emerging two-dimensional (2D) nanomaterial of graphene due to its unique properties. In this dissertation, I have focused on hybrid structures that combine 2D graphene with other low-dimensional nanostructured as new type of functional building blocks for futuristic electronics and electrochemical photonics. The three-dimensional (3D), tubular-structured monolayer graphene networks were hybridized with TiO2 nanoparticular layer for futuristic and robust electrode applications. A continuous form of 3D graphene with good carrier mobility provides a direct pathway for electrons to the current collector for a photoanode in dye-sensitized solar cells. This characteristic feature, coupled with its energy level, ensures an enhanced charge collection efficiency. Particular attention was paid to the graphene surface functionalization and the effective loading of TiO2 nanoparticles to improve the light harvesting and minimize electron recombination for a photoanode. The optimal hybrid structure resulted in a 15% enhanced energy conversion efficiency, compared to the TiO2-based analog without graphene. The impedance spectra confirmed that the increase in photovoltaic performance was mainly driven by the efficient charge collection through the 3D, tubular-structured monolayer graphene. This new electrode prototype can serve as a perfect complement to conventional TiO2 nanostructures. In addition, by hybridizing with ultra-thin, Si-rich 2D islands, the electronic property of graphene was modulated. Transmission electron microscopy, atomic force microscopy, and electron and optical spectroscopies confirmed that Si-islands with thicknesses of ~2–4 nm and a lateral size of several tens of nm were bonded to graphene via van der Waals interactions. Field-effect transistors (FETs) based on Gr:Si sheets exhibited enhanced transconductance and maximum-to-minimum current level compared to bare-graphene FETs, and their magnitudes gradually increased with increasing coverage of Si layers on the graphene. The temperature dependent current-voltage measurements of the Gr:Si sheet showed approximately a two-fold increase in the resistance by decreasing the temperature from 250 K to 10 K, which confirmed the opening of substantial bandgap (~2.5 meV) in graphene by coupling with Si islands. Our approaches based on hybridization of graphene with nanomaterials can provide intriguing new possibilities and opportunity to control the electrode designs and electronic band structure. The graphene-based hybrid nanostructures can thus be functional designed as suitable for diverse electrochemical devices and integrated electronic system such as fuel cells, redox flow batteries, capacitors, flexible, wearable, and invisible electronics.