Pristine single layer graphene (SLG) has exceedingly high mobility, which is∼ 4,000–20,000 cm 2/Vs for typical devices supported on Si/SiO 2 substrates, and may reach as high as 250,000 cm 2/Vs in suspended devices at room temperature.[1] Such high mobilities make graphene an extremely attractive candidate for the next generation electronic materials. However, the absence of a band gap, which is necessary for digital electronics, presents a technological challenge. One effective approach to band gap engineering is the (partial) saturation of the valences of some of the conjugated carbon atoms.[2–17] Nitrophenyl functionalization, in which a fully rehybridized sp 3 carbon atom is created in the lattice, dramatically modifies the electronic and magnetic structure of graphene, with significantly reduced field effect mobility.[18–22] Since this type of functionalization scheme introduces resonant scatters [23] into the graphene lattice, we refer to this as destructive rehybridization.[24] Most approaches for chemical modification of graphene involve the creation of sp 3 carbon centers at the cost of conjugated sp 2 carbon atoms in the graphene lattice. We have recently investigated the application of organometallic chemistry by studying the covalent hexahapto modification of graphitic surfaces with zero-valent transition metals such as chromium.[12, 25] The formation of the hexahapto (η6)-arene− metal bond leads to very little structural reorganization of the π-system. In the reaction of the zero-valent chromium metal with graphene, the vacant d π orbital of the metal (chromium) constructively overlaps with the occupied π-orbitals of graphene, without removing any of the sp 2 carbon atoms from conjugation.[12, 25] Previously we have shown that the formation of such bishexahapto transition metal bonds between the conjugated surfaces of the benzenoid ring systems present in the surfaces of graphene and carbon nanotubes can dramatically change their electrical properties.[12, 24–27] These prior works focus on using the bis-hexahapto-metal bond as an interconnect for electrical transport between the conjugated surfaces, thereby increasing the dimensionality of the carbon nanotube and graphene materials and thus we were concerned with the use of the bishexahapto-metal bond as a conduit for electron transport between surfaces. In contrast, the goal of the present study is to investigate the effect of the hexahapto-bonded chromium atoms on the electronic properties of graphene itself (within the plane of a single layer), by using mono-hexahapto-metal bonds to the graphene surface.

Single layer graphene (SLG) flakes used in this study were extracted from bulk graphite using a standard mechanical exfoliation method and placed on a Si substrate with 300 nm SiO 2. Contacts consisting of 10 nm of Cr and 150 nm of Au were deposited on SLG by e-beam lithography. The devices were then annealed in vacuum by passing a high current for a short time to remove contaminants from the surface.[28] After characterization the devices are immersed in a chromium hexacarbonyl solution for organometallic functionalization. Three different functionalization approaches (see Experimental Section for details) are employed to chemically modify the graphene flakes as shown in Figure 1. In the first method (method A), SLG devices are functionalized in a solution of chromium hexacarbonyl [Cr (CO) 6] in dibutyl ether/tetrahydrofuran under refluxing conditions (140 C, 48 h). In the second method (method B), the SLG devices were immersed in a solution of Cr (CO) 6 as in method A, but in presence of an additional ligand, naphthalene (80 C for 12 h). The naphthalene was …