Asymmetrically functionalized graphene for photodependent diode rectifying behavior.

As an atomically thin sheet of carbon atoms packed in a twodimensional (2D) honeycomb lattice with excellent electronic, thermal, and mechanical properties, graphene has shown great potential for a wide range of applications. Examples include the use of graphene and its derivatives as transparent conductive electrodes or active materials in solar cells, counter electrodes in dye-sensitized solar cells, electrocatalysts for oxygen reduction in fuel cells, high-performance electrodes in supercapacitors, batteries, actuators, and sensors. Of particular interest, Guo et al. reported a significant advancement in the development of layered graphene/quantum dots for highly efficient solar cells. Stoller et al. produced graphene-based supercapacitors free from any conducting filler with a specific capacitance of 135 Fg 1 in aqueous electrolytes. We also demonstrated that N-doped graphene could act as a metal-free electrode with a much better electrocatalytic activity, long-term operational stability, and tolerance to crossover effect than platinum for oxygen reduction in alkaline fuel cells. By using graphene as a superior dimensionally compatible and electrically conductive component, Guo et al. further constructed a smart graphene-based multifunctional biointerface for cell growth as well as in situ selective and quantitative molecular detection. There is now a pressing need to integrate graphene sheets into multidimensional and multifunctional systems with spatially well-defined configurations, and hence integrated systems with a controllable structure and predictable performance. This requires controlled functionalization of graphene sheets at the molecular level, which is still a big challenge. The recent availability of solution-processable graphene by exfoliation of graphite into graphene oxides (GOs), followed by solution reduction, has allowed the functionalization of graphene sheets through various solution reactions. As far as we are aware, however, there is still no report on the asymmetric functionalization of graphene sheets by attaching different chemical moieties to their two opposite surfaces. The asymmetric functionalization, if realized, should significantly advance the self-assembling of graphene sheets into many new multidimensional and multifunctional systems with molecular-level control. Herein, we report for the first time a simple but effective asymmetric modification method for functionalizing the two opposite surfaces of individual graphene sheets with different nanoparticles (NPs) in either a patterned or nonpatterned fashion. The resultant asymmetrically modified graphene sheets with ZnO andAuNPs on their two opposite surfaces were demonstrated to show a strong photodependent diode rectifying behavior. We have previously developed a polymer masking technique for asymmetric functionalization of carbon-nanotube sidewalls by sequentially masking vertically aligned carbon nanotubes twice, with only half of the nanotube length being modified each time. In the present study, we used a new polymer masking technique for sequentially masking individual graphene sheets twice with only one side of the surface being modified each time. In a typical experiment, an aqueous dispersion of chemically derived graphene sheets was firstly prepared by the method described in the literature. We then deposited a dilute aqueous solution (0.05 mgmL ) of the well-dispersed graphene nanosheets (Figure 1a and Figures S1 and S2 in the Supporting Information) onto a silicon substrate by spin-coating (900 rpm, Spin Coater KW4A, Chemat Technology). Individual graphene sheets on the substrate were then treated by an acetic acid plasma to introduce carboxylic groups on the exposed surface of each of the silicon-supported graphene sheets (Figure 2a), while their opposite surface was protected by the silicon substrate to be free from the plasma treatment. Thereafter, spherical ZnO NPs (ca. 10 nm in diameter, Figure 1b and Figure S3 in the Supporting Information) were attached to the plasma-treated graphene surface (Figures 1c and 2b) through the specific interaction of carboxylic groups with oxide particles according to previously published procedures. This was followed by spin-coating (2000 rpm, Spin Coater KW-4A, Chemat Technology) a thin layer of poly(methyl methacrylate) (PMMA) from a CHCl3 solution (10 wt% PMMA) to mask the ZnO-attached graphene surface (Figure 2c). The PMMA-coated, ZnO-attached graphene sheets were then separated from the silicon substrate by immersion in a HF aqueous solution (ca. 5 wt%) to [*] Dr. D. Yu, E. Nagelli, Prof. L. Dai Department of Macromolecular Science and Engineering and Department of Chemical Engineering Case Western Reserve University 10900 Euclid Avenue, Cleveland, OH 44106 (USA) and Interdisciplinary School of Green Energy Ulsan National Institute of Science and Technology (UNIST) Ulsan 689-798 (South Korea) Fax: (+1)216-368-3016 E-mail: [email protected]