(Invited) Graphene Molecules: Synthesis, Electronic Properties and Applications

Nanographene structures called Graphene Molecules (GMs) have been recently generating a growing interest. Theory predicts that these graphene fragments, comprising fewer than 300 carbon atoms (< 5 nm in size), are subject to quantum confinement effects similar to those observed in inorganic semiconductor quantum dots, but with different scaling laws, excited state properties and effects of functionalization. Predictions of unique size-dependent properties of GMs suggest the possibility that the studies of these structures may yield observation of interesting new phenomena and potentially development of new technologies. However, experimental studies of the electronic structure and demonstration of applications of GMs have so far been limited. This is in part due to challenges associated with the preparation of well-defined structures and in part due to the challenges in applying traditional methods in studies of GMs.

In the first part of my talk I will describe new method for a bottom up synthesis of well-defined GMs on metal oxide surfaces that we recently developed in our laboratory. The method, performed at ambient conditions, yields surface adsorbed GMs structurally and electronically equivalent to those synthesized in solution. The approach reduces the challenges associated with the tendency of GMs to aggregate and provides a convenient path for integration of GMs into optoelectronic applications. In the second part of my talk I will discuss results of our studies of charge transfer properties of GMs with optical band gaps tuned into visible wavelengths, facilitated by the new preparation method. I will show how an electrochemical potential can be used to inject one or more charges into GMs with a high level of control. I will show that the injection of charges leads to dramatic changes in GM absorption properties, quantitatively explain how the two are related and present insights into the kinetics of the charge injection. I will discuss how the basic understanding of the GM charging processes can be effectively exploited in a practical application, such as electrochromic device, wherein the graphene nanostructure is for the first time used as the optically active component. Finally, I will present the results of our studies of GMs as light harvesting components in sensitized solar cells (SSCs). In these studies we have found that mesoscopic TiO₂/GM films can be used prepare stable SSCs with efficiencies of η ~1%. I will discuss the limitations and advantages of the GM-based SSC architectures.