Understanding and controlling the substrate effect on graphene electron-transfer chemistry via reactivity imprint lithography
Qing Hua Wang , 1
Zhong Jin , 1
Ki Kang Kim , 2
Andrew J. Hilmer , 1
Geraldine L. C. Paulus , 1
Chih-Jen Shih , 1
Moon-Ho Ham , 3
Javier D. Sanchez-Yamagishi , 4
Kenji Watanabe , 5
Takashi Taniguchi , 5
Jing Kong , 2
Pablo Jarillo-Herrero 4
& Michael S. Strano 1
30 January 2012
27 June 2012
12 August 2012
Graphene has exceptional electronic, optical, mechanical and thermal properties, which provide it with great potential for use in electronic, optoelectronic and sensing applications. The chemical functionalization of graphene has been investigated with a view to controlling its electronic properties and interactions with other materials. Covalent modification of graphene by organic diazonium salts has been used to achieve these goals, but because graphene comprises only a single atomic layer, it is strongly influenced by the underlying substrate. Here, we show a stark difference in the rate of electron-transfer reactions with organic diazonium salts for monolayer graphene supported on a variety of substrates. Reactions proceed rapidly for graphene supported on
SiO and 2 Al (sapphire), but negligibly on alkyl-terminated and hexagonal boron nitride (hBN) surfaces, as shown by Raman spectroscopy. We also develop a model of reactivity based on substrate-induced electron–hole puddles in graphene, and achieve spatial patterning of chemical reactions in graphene by patterning the substrate. O 2 3
Figures at a glance
Figure 1: Chemical reactivity of graphene supported on different substrates.
a, Reaction scheme of covalent chemical functionalization of graphene by 4-nitrobenzenediazonium tetrafluoroborate. b, Representative Raman spectra of CVD-grown graphene deposited on different substrate materials before and after diazonium functionalization, normalized to the G peak height. These substrates are, from bottom to top, 300-nm-thick SiO on 2 silicon, SiO functionalized by an 2 OTS SAM, single-crystal hBN flakes deposited on SiO and single-crystal α-Al 2 O 2 ( 3 c-face sapphire). The SiO substrate here was plasma-cleaned. 2 c, Change in intensity ratio of Raman D and G peaks ( I / D I ) after diazonium functionalization (difference between functionalized and unfunctionalized ratios) plotted as a function of water contact-angle of the substrate before graphene deposition. The dashed line is an exponential fit of the data. Raman spectra were taken with a laser excitation wavelength of 633 nm. G
Figure 2: Raman spectroscopy peak parameter analysis.
Spatial Raman maps were collected for graphene supported on each substrate for the same 10 µm × 10 µm regions before and after diazonium functionalization, with 121 spectra in each map.
a, Histograms of I / D I ratios before and after functionalization. A low degree of covalent functionalization (small increase in G I / D I ) is seen for G OTS and hBN, and a much higher degree for SiO (plasma-cleaned) and 2 Al. O 2 3 b– e, Scatter plots of Raman peak parameters with data points adapted from pristine, mechanically exfoliated graphene doped by electrostatic gating; dashed lines added to guide the eye are included to aid comparison . 37 , 41 b, G peak full-width at half-maximum (FWHM, Γ ) versus G peak position ( G ω ). Comparison data from ref. G 41 are shifted up to fit the higher FWHM of CVD graphene. Before reaction, graphene follows the doping trend, but highly functionalized samples significantly deviate above the curve. c, 2D peak position ( ω ) versus G peak position ( 2D ω ), with additional data points adapted from ref. G 41 for distinguishing n-doped and p-doped exfoliated monolayer graphene, shifted to account for the dependence of ω on excitation laser wavelength 2D . Diazonium-functionalized graphene in our experimental data is p-doped, but deviates left from the trend of pristine, gated graphene. 56 d, 2D peak FWHM (Γ ) versus 2D peak position ( 2D ω ), showing clearly distinguished clusters for each substrate before and after functionalization. Increasing Γ 2D values before functionalization reflect inhomogeneous broadening due to electron–hole charge fluctuations. 2D e, I / 2D I intensity ratio versus G peak position ( G ω ), with comparison data adapted from ref. G 37 showing the doping trend. Raman spectra were taken at 633 nm laser excitation wavelength.
Figure 3: Spatial control of reactivity of graphene on patterned substrates.
a, Schematic illustration of RIL. The SiO substrate is patterned by a PDMS stamp inked with 2 OTS. Graphene is transferred over the OTS-patterned substrate and reacted with 4-NBD tetrafluoroborate. b, AFM topographic image of the OTS stripes (narrower raised regions) on SiO before graphene deposition. 2 c, Raman spatial map of I / D I intensity ratio after diazonium functionalization. The narrow, mildly functionalized stripes correspond to the regions over the G OTS pattern and the wide, strongly functionalized stripes correspond to the regions over the SiO gaps. 2 d, Spatial profile of I / D I for the stripe pattern (blue curve) along the line A–B in the Raman map (inset), and a fit to an integrated Gaussian function with a variance of 0.85 µm. G e, A spatial Raman map (lower left inset) was measured for a region of graphene covering both SiO and a flake of hBN (white box in optical image in upper right inset). The 2 I / D I spatial profile along the line C–D is shown together with the integrated Gaussian fit, which has a variance of 0.76 µm. G
Figure 4: Patterning of proteins on graphene.
a, Schematic illustration of protein-attachment chemistry. The graphene is covalently functionalized with 4-carboxybenzenediazonium tetrafluoroborate, then NTA–NH. Reaction with 2 NiCl causes Ni 2 ions to complex to the covalently attached structures, and link to polyhistidine (His)-tagged EGFP. (Image of EGFP is taken from the RCSB PDB ( 2+ www.pdb.org) from ref. 57.) b, ATR–IR spectra of pristine CVD graphene (blue curve) and CO H-diazonium functionalized CVD graphene (red curve), showing O−H and C=O vibrations from the carboxyl groups. 2 c, Confocal fluorescence microscope image of EGFP attached to graphene resting on a substrate with alternating stripes of bare SiO and 2 OTS patterned on graphene. The bright green stripes, indicating a higher concentration of EGFP attachment, corresponds to graphene resting on bare SiO, and the darker stripes correspond to graphene resting on 2 OTS-patterned regions where very little EGFP was able to attach. Inset: intensity profile of fluorescence along the white line indicated in c.
Figure 5: Modelling of substrate-influenced reactivity.
a, Schematic of the role of electron–hole charge fluctuations in graphene reactivity. Solid curves indicate spatial variation of the local Fermi level in charge puddles, and the dashed lines indicate the average Fermi level. The green curve (left) represents graphene on a substrate that causes it to be mildly p-doped with small charge fluctuations, and the red curve (right) represents higher p-doping and large charge fluctuations. According to electron-transfer theory, n-doped puddles have a higher reactivity towards diazonium functionalization and the p-doped puddles have very low reactivity. b, Experimental data from graphene on various substrates are plotted together with the curve from the electron-transfer model for the initial graphene Fermi level ( E ) and change in F I / D I ratio after diazonium functionalization. The experimental average G E values are calculated from the F I / 2D I ratios before functionalization G . Each experimental point is the average value for a particular sample taken from 121 Raman spectra in a map, and the error bars represent standard deviation. The average doping for all samples is p-type, and does not agree with the model. 37 c, Average E values are offset by considering the FWHM of 2D peaks, which reflects inhomogeneous broadening due to electron–hole charge fluctuations, to reflect the maximum n-doping. F d, Resulting I / D I ratio changes measured after electrochemical functionalization experiments at different applied gate voltages for samples on 100 nm and 300 nm G SiO dielectric layers, showing the effect of Fermi level shifts and field-induced diazonium concentration change on overall reactivity. 2