Organometallic Hexahapto Functionalization of Single Layer Graphene as a Route to High Mobility Graphene Devices

Abstract

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][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] Nitrophenyl functionalization, in which a fully rehybridized sp 3 carbon atom is created in the lattice, dramatically modifi es the electronic and magnetic structure of graphene, with signifi cantly reduced fi eld effect mobility. [18][19][20][21][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 modifi cation 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 modifi cation of graphitic surfaces with zero-valent transition metals such as chromium. [ 12 , 25 ] The formation of the hexahapto ( η 6 )-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) fl akes 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...

Catalyst Residue and Oxygen Species Inhibition of the Formation of Hexahapto-Metal Complexes of Group 6 Metals on Single-Walled Carbon Nanotubes

“…Yield: 69.6%. 6 , (C 7 H 8 )M(CO) 3 , or (C 6 H 6 )Cr(CO) 3 This protocol was adapted from the literature [40]. In a general procedure, SWCNTs (0.04 g) were placed in a Schlenk flask equipped with a stir bar and were heated at 100 • C under vacuum overnight.…”

“…The wafer was diced up into 1.0" × 0.5" coupons. Exfoliated graphite, (CH 3 CN) 3 M(CO) 3 , and (C 7 H 8 )M(CO) 3 (M = Cr, W) were prepared according to the methods presented in the literature [39,40]. All solvents were distilled (unless they were obtained from the solvent purification system) and degassed.…”

Stabilizing graphene-based organometallic sandwich structures through defect engineering

“…In fact, a large increase in D-band intensity was indeed observed experimentally [6] upon formation of the Bz|M|Gr complexes, possibly indicating complex-formation at the defect sites. Sarkar et al [6,7] also reported a decrease in conductance upon complex formation for a related structure, Gr|M|(CO) 3 , that is isoelectronic to the sandwich structures of interest here. Although the aforementioned experimental works did not report the transport properties of the Bz|M|Gr complexes themselves, the reported loss of conductivity for the related structures is also consistent with the complex-formation at defect sites.…”

“…However, experimentally, it has proven harder to create such complexes [10]. There is only a single report for these specific complexes [6], although some related structures were recently reported [7,11]. Some of the difficulties encountered in creating these structures stem from: (a) the high mobility of TMs on graphene, and (b) the high cohesive energies of the TMs [12].…”

“…To this end, several different avenues are being explored (see, for example, the review article by Terrone et al [3] and the articles therein), such as creating vacancies, doping graphene with heteroatoms such as nitrogen, or creating more elaborate organometallic complexes [4][5][6][7].…”