Modification of Electronic Properties of Graphene with Self-Assembled Monolayers B. Lee, † Y. Chen, † F. Duerr, † D. Mastrogiovanni, ‡ E. Garfunkel, ‡,§ E. Y. Andrei, † and V. Podzorov* ,†,§ † Department of Physics, ‡ Department of Chemistry, and § Institute for Advanced Materials and Devices, Rutgers University, Piscataway, New Jersey 08854 ABSTRACT Integration of organic and inorganic electronic materials is one of the emerging approaches to achieve novel material functionalities. Here, we demonstrate a stable self-assembled monolayer of an alkylsilane grown at the surface of graphite and graphene. Detailed characterization of the system using scanning probe microscopy, X-ray photoelectron spectroscopy, and transport measurements reveals the monolayer structure and its effect on the electronic properties of graphene. The monolayer induces a strong surface doping with a high density of mobile holes (n > 10 13 cm -2 ). The ability to tune electronic properties of graphene via stable molecular self-assembly, including selective doping of steps, edges, and other defects, may have important implications in future graphene electronics. KEYWORDS Graphene, self-assembled monolayers, graphene edge functionalization, doping of graphene, transport in graphene S elf-assembled monolayers (SAMs) are ultrathin mo- lecular films spontaneously formed at surfaces or interfaces due to chemical or physical interactions of molecules with a substrate, frequently without necessity of high-vacuum or high-temperature processing. 1 SAMs have received considerable attention due to their use in organic electronics as active materials or insulators. 1–4 Recently, it has been demonstrated that electronic properties of small- molecule and conjugated polymer organic semiconductors can be drastically modified by SAMs. 5,6 In this Communica- tion, we report the effect of a self-assembled monolayer of (tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilane (C 8 H 4 F 13 SiCl 3 ), or simply fluoroalkyltrichlorosilane (FTS), 7 on the electronic properties of highly ordered pyrolytic graphite (HOPG) and graphene. The latter system has attracted con- siderable attention due to the massless character of quasi- particles and the related novel mesoscopic transport prop- erties. 8 Our studies using atomic-force microscopy (AFM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Hall effect measurements reveal that a dense, uniform, and stable FTS SAM can be grown at the surface of graphene, inducing an excess of holes with a density of up to n ∼ 1.5 × 10 13 cm -2 . Such a level of doping is unattainable in conventional field-effect transistor (FET) devices. In addition, the SAM-graphene system is found to be very stable (even at elevated temperatures) in high- vacuum and ambient environment. Such robustness and the large electronic effect suggest that integration of SAM with graphene provides a new and reliable method of achieving ultrahigh doping levels in graphene. The samples used in this study were rectangular pieces of multilayer HOPG and single-layer graphene FETs. The HOPG samples had length and width, L ∼ W ) 2-5 mm, and thickness d ) 3-20 μm, comprising 1-6 × 10 4 indi- vidual layers (Figure 1). The electrical contacts to HOPG were prepared by applying colloidal graphite paint to the sides of the samples, thus forming electrical contacts to all the layers. Graphene FETs were prepared on SiO 2 /n-Si wafers using mechanical exfoliation techniques and e-beam lithography (the details can be found elsewhere 9 ). Before the SAM growth, devices were annealed in a flow of ultrahigh purity * To whom correspondence should be addressed, podzorov@physics.rutgers.edu. Received for review: 02/18/2010 Published on Web: 05/26/2010 FIGURE 1. The effect of an FTS self-assembled monolayer on the resistivity of HOPG. (Top) Normalized resistivity R(t) ≡ R(t)/R 0 of multilayer (>10 4 layers) HOPG measured as a function of FTS treatment time (initial values of R vary from sample to sample by as much as 100%). The red arrows indicate the onset of FTS exposure. (Bottom) The corresponding effect on the resistivity of an individual single layer of graphite calculated using eq 1. The sketch shows a Hall-bar sample geometry used throughout this study. pubs.acs.org/NanoLett © 2010 American Chemical Society 2427 DOI: 10.1021/nl100587e | Nano Lett. 2010, 10, 2427–2432