UC San Diego

Since the discovery of graphene in 2004, two dimensional materials (2D) have become the focus of tremendous research owing to their unprecedented properties. Atomically thin nature of 2D materials gives rise to unique physicochemical properties, which makes them attractive for flexible electronics, chemical and biological sensing, energy storage, and solar cells. In this dissertation, sensing and energy related applications of 2D materials are studied.

In chapter 1, graphene based ammonia sensors are presented, in which nano-structuring graphene significantly improves the sensitivity towards ammonia due to the formation of highly reactive edge defects. It was found that sensitivity could be further enhanced by decoration of Pd nanoparticles on the nano-structured graphene.

In chapter 2, hydrogen sensors based on solution processed transition metal dichalcogenides (TMDs) nanosheets-Pd nanoparticles composites are introduced. The sensors can detect hydrogen at room temperature with high sensitivities. The ease of fabrication holds a great potential for low-cost and scalable manufacturing of chemical sensors.

In chapter 3, the fabrication and characterization of graphene/Si heterojunction solar cells are described and various methods to improve the power conversion efficiency (PCE) are presented. A single layer graphene is highly transparent; therefore suitable as a transparent Schottky electrode for solar cells. However, the PCEs of the pristine graphene/Si solar cells are low due to the high sheet resistance of graphene as well as the low Schottky barrier height between pristine graphene and Si. We improved the PCE by a magnitude of order (achieving 9% PCE) with Au nanoparticle decoration followed by a nitric acid treatment owing to the dramatic reduction in the series resistance of the cells and the enhanced Schottky barrier height. Furthermore, we used NiO as a transparent and stable hole doping material for graphene, in which NiO doped cell shows enhanced PCE with high stability.

In chapter 4, MoS2 nanosheet-Ru nanoparticle composite is introduced as a highly efficient catalyst for hydrogen evolution reaction. The MoS2-Ru composite shows much smaller onset potential and Tafel slope compared to pristine MoS2 nanosheets indicating its superior catalytic performance for hydrogen evolution. The fabrication and characterization of the composite is described and its enhanced catalytic performance is discussed.