K. Vinodgopal (North Carolina Central University), X. You (University of North Carolina, Chapel Hill), E. Laws, S. Sendlinger, N. Sciortino, G. Dai (North Carolina Central University), and J. Atkin (University of North Carolina, Chapel Hill)

The goal of carbon based electronics can be realized by creating reproducible and controllable band gaps in the normally “metallic” graphene.¹ Functionalization of the graphene surface via the formation of hexahapto(η⁶-metal) bonds yield structures that should not show any significant structural distortion or rehybridization as is the case with organic functionalization of graphene. Several approaches are possible to achieve such a “subtle” covalent functionalization. Haddon et. al. have developed a wet-chemistry synthetic approach by refluxing exfoliated graphene with either Cr(CO)₆ or (η⁶-benzene)Cr(CO)₃ in a high boiling solvent under an inert atmosphere.^2-4

Our approach to achieve this same objective of functionalizing the graphene surface with zero-valent transition metal carbonyl precursors is to use chemical vapor deposition (CVD) methods. CVD enables us to grow high quality single-layered graphene on copper as well as carry out the subsequent functionalization of the graphene surface. The essential requirement for such a process is that the carbonyl compound must have high vapor pressure which Cr(CO)₆ satisfies. In fact, Anpo et. al. have used a simple CVD process to functionalize the organosilica framework of phenylene (-C₆H₄-)-bridged hybrid mesoporous materials (HMM-ph) with Cr(CO)₆ and Mo(CO)₆.⁵ The CO vibration provides a convenient handle to confirm the functionalization. The IR spectral peak for Cr(CO)₆ was observed at 1991 and 2020 cm^-1 which were assigned to the T_1u and E_g vibrational modes, respectively. After functionalization, the IR spectral peaks were slightly red shifted to 1981 cm^-1 and a broad peak at 1940-1840 cm^-1 corresponding to a₁ (1981 cm^-1) and e (1911, 1884 cm^-1) vibrational mode of physically absorbed C₆H₆Cr(CO)₃ on the amorphous SiO₂ surface.

Our CVD procedure involves a) preparation of mostly single layered graphene on a copper substrate by CVD and b) subsequent functionalization by vapor deposition of the Cr(CO)₆ at 80°C at atmospheric pressure under a flow of Ar/H₂ to yield graphene. However, given the difficulties of obtaining IR spectra on the copper substrate, we have focused on Raman imaging using a 473 nm laser and correlated Raman AFM mapping. The correlated Raman-AFM mapping shown below indicates fairly uniform deposition of the chromium carbonyl on the surface and red shifts in the CO vibration consistent with formation of a zero- valent complex on the graphene surface. Additional confirmation of the formation of the hexa-hapto complexes of is provided by Scanning Electron Microscopy with EDX.

Figure Caption: A) AFM image of the CVD functionalized graphene-Cr(CO)₃ surface on copper; B) correlated Raman map of the same region obtained with 473 nm laser excitation and C) representative Raman spectrum obtained from one of the bright green spots displayed on the correlated Raman map. These bright green spots in Panel B correspond to the region defined by the green spectral limits shown in the spectrum in panel C.

References:

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2. Sarkar, S.; Moser, M. L. ; Tian, X.;Zhang, X., Al-Hadeethi, Y.F.; Haddon, R. C., “Metals on Graphene and Carbon Nanotube Surfaces: From Mobile atoms to Atomtronics to Bulk Metals to Clusters and Catalysts.” Chem.Mater. 2013 DOI: 10.1021/cm4025809.
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