First-principles study on atomic and electronic structures of graphene for design of the electronic devices

Abstract

Graphene, a two-dimensional monolayer of sp2-bonded carbons, has inspired a wide variety of research activities, from practical materials engineering to pure scientific studies of electron gas in low dimensions. Among them, the feasibility of graphene as a transparent conductor in electronic devices is particularly intriguing. To elucidate the features of graphene as an electronic material from practical point of view, I studied the effect of surface defects on the work function of graphene using density-functional theory. I found that in-plane geometrical deformations (such as Stone–Thrower–Wales defects, carbon vacancies, and hydrogenated edges) have only a marginal effect. (Ch. 3) Fermi level shift (ΔEF) of graphene with respect to the charge neutrality level is investigated at the contact with three different metal species to consider the doping effect by metal contact. To confirm the validity of my calculation, the electric conductance of graphene field-effect transistors (GFETs) fabricated with selected metal contacts is experimentally measured through transfer length measurement (TLM) technique, and measured contact properties are compatible with our results from DFT calculation. (Ch. 4) I also performed density-functional theory calculations to study the electronic structures at the interfaces between graphene and organic molecules that have been used in organic light-emitting diodes. In terms of work function, graphene itself is not favorable as either anode or cathode for commonly used electron or hole transport molecular species. However, the formation of charge transfer complex on the chemically inert sp2 carbon surface can provide a particular advantage. (Ch. 5)