Engineering of graphene properties via nanostructure and edge-defect design

Abstract

Nanoscale edge-structures in graphene play an important role in determining its physical and chemical properties. However, conventional patterning for graphene nanostructure via lithography and etching causes C atoms at the edges to become disordered and contaminated with residues. The resulting graphene loses their intrinsic characteristics, thereby restricting the advancement of engineering and analysis of defects in graphene. In this dissertation, I have focused on the engineering of graphene properties by nanostrucutre and edge-defect design without degradation of intrinsic properties.

A new strategy to fast synthesis of edge-defined graphene flakes on single-crystal Cu foil was suggested to control the edge-defect chirality. Polycrystalline Cu foils were first converted to single-crystal Cu foils through high-temperature annealing processes. Then, a sodium chloride (NaCl) additive was introduced during chemical vapor deposition growth of graphene on the Cu foils. Investigation of early stage growth in this process suggests that the NaCl additive enhances nucleation events and depresses dendritic growth, producing high-density graphene flakes (GFs) with crystallographically well-defined edges: (i) rectangular-shaped GFs on Cu(100) and (ii) hexagonal-shaped GFs on Cu(111).

In another way for controlling the graphene edge-structures, I introduced the direct patterned synthesis of graphene meshes on Cu foils, that use self-assembled silica sphere arrays as growth masks. We then explored edge-defect sites in the palladium-decorated graphene meshes to enable highly sensitive and fast response to hydrogen gas. Compared with continuous graphene-based sensors, the graphene mesh sensors exhibited a faster response to hydrogen gas with sensitivity as high as ~5% at a low concentration of 10 ppm H2/air, even at room temperature. The enhanced H2 detection characteristic of the graphene mesh sensor is attributed to the existence of edge-defect which led to lowered transport barrier for charge carriers.

The nanostructured graphene meshes provide not only the edge-defects but also empty spaces for more sufficient electric-field transmission. By taking advantage of this, gate-tunable Schottky junction solar cells (SJSCs) based on graphene mesh electrodes on n-type Si were fabricated and the effect of the external gate voltage (Vg) on the photovoltaic characteristics was investigated. I found that the power conversion efficiency (PCE) continuously increased with increasing absolute values of Vg. The finite element simulation highlighted the benefits of the graphene mesh electrodes from the non-uniform and dynamic modulation of potential distributions driven correlatively by a work function change in the graphene regions and electric-field penetration through the hole regions.

My approaches provide the basis for understanding the intrinsic characteristics of nanostructured graphene and graphene edge-defects. Furthermore, it could open the way for exploiting the engineered nanostructures and edge-defects in atomically thin materials to control their electrical and optical properties.