Growth of Organic Semiconductor Crystals on Graphene with Controlled Charge Carrier Density

Abstract

Incorporation of graphene and organic semiconductors (OSCs) promises a new generation of opto-electronic devices that can inherit the best qualities of these two materials for advanced capabilities. Such incorporation usually requires deposition of organic semiconductor films from either solution phase or vapor phase onto the graphene’s surface. It is especially advantageous because graphene has a very large surface area of sp2 carbon network, which provides an excellent template for the growth of novel nano-microstructures. However, full control of the growth of OSC crystals on graphene surface remains a challenging task. Graphene’s surface properties strongly depend on external factors such as the supporting solid substrate or the electrical field. In particular, the wettability of a graphene layer can be changed simply by doping charge carriers into it or switching the underlying substrate. Therefore, development of graphene growth templates for OSC crystals should consider this factor and its related effects, but these considerations have not been investigated. The thesis aims to contribute towards these considerations and use them to tackle the challenges in graphene-OSCs applications. In Chapter 2, the growth of pentacene crystals on doping-controlled graphene is investigated. For this goal, numerous pentacene thin films grown on graphene with finely-tuned charge-carrier densities ng are analyzed to reveal the correlation between growth dynamics and the doping states of the graphene template. Increasing the ng in graphene strengthens the interactions between pentacene ad-molecules and graphene surface; this results in a columnar growth mode. Keeping graphene electronic states undoped favors the layer-by-layer growth mode of pentacene; this mode produces high crystallinity that can boost the opto-electronic properties of pentacene thin films. In Chapter 3, the correlations between the growth of fullerene (C60) crystals and the charge transfer from graphene to C60 molecules are revealed. The adsorption of organic semiconductor molecules on graphene surface is mostly governed by the van der Waals interactions. However, the methods introduced in chapter 2 allow tuning of graphene’s Fermi energy level (EF) during molecular adsorption, so the adsorption of molecules, especially those with low lowest unoccupied molecular orbital levels (LUMO) on graphene surface becomes complicated. In particular, the adsorption of C60, which has LUMO ~4.5 eV, on graphene is accompanied by electron transfer from graphene. Such charge transfer varies the molecular interactions and growth dynamics of C60 ad-molecules, so that the final thin film possesses very small grains with poor crystalline order. When the charge transfer between graphene and C60 is suppressed, the van der Waals interaction between them is dominant, so growth proceeds with layer-by-layer mode, and yields crystals with large grain size and high crystalline order.