DOI: 10.1038/NCHEM.1421 Understanding and Controlling the Substrate Effect on Graphene Electron-Transfer Chemistry via Reactivity Imprint Lithography Qing Hua Wang, Zhong Jin, Ki Kang Kim, Andrew J. Hilmer, Geraldine L.C. Paulus, Chih-Jen Shih, Moon-Ho Ham, Javier D. Sanchez-Yamagishi, Kenji Watanabe, Takashi Taniguchi, Jing Kong, Pablo Jarillo-Herrero, and Michael S. Strano Contents CVD graphene sample characterization: optical images and Raman spectra... 2 Grain boundary area contribution... 3 Octadecyltrichlorosilane (OTS) self-assembled monolayer (SAM) formation on SiO 2... 4 Raman spatial map of graphene on hbn flake... 4 OTS stripes under graphene... 5 Diazonium reagent stability... 6 Fitting of spatial D/G profile... 7 Estimation of nitrobenzene coverage on graphene... 8 Electrochemical functionalization... 10 References... 13 NATURE CHEMISTRY www.nature.com/naturechemistry 1
CVD graphene sample characterization: optical images and Raman spectra The graphene used in this work was grown by chemical vapour deposition (CVD) on Cu foils. The Cu foil was approximately 3 inches x 1 inch, and after CVD growth the graphene-on-cu was cut to smaller sample sizes using scissors before transferring the graphene onto the various substrates. The same large piece of graphene-on-cu was used to generate all the samples used in this work, and all the samples are single continuous sheets of polycrystalline monolayer graphene with some bilayer and multilayer islands. The optical images in Figure S1a show graphene after transfer onto the various substrates. Monolayer graphene appears as a uniform change in colour on most of the substrates (e.g. slightly purple on 300 nm SiO 2 ), and higher layer numbers (bilayer, trilayer, etc.) are visible as regions of different contrast (e.g. darker purple on 300 nm SiO 2 ). For Raman mapping, care was taken to choose uniform monolayer graphene areas with no visible tears or multilayer islands. Supplementary Figure 1. Characterization of CVD-grown graphene samples on various substrates. a, Optical microscope images with white light illumination of CVD monolayer graphene on the different substrates. The NATURE CHEMISTRY www.nature.com/naturechemistry 2
square indicates a 10 µm x 10 µm area in which Raman mapping was conducted. Care was taken to choose uniform monolayer graphene areas with no visible tears or multilayer islands. A bilayer island is indicated by a red arrow, and a wrinkle in the graphene that formed during the transfer process is indicated by a blue arrow. b, Raman spectra from Raman map of graphene on plasma-cleaned SiO 2, taken from locations 0.5 µm apart between adjacent spectra, showing uniform monolayer graphene (due to shape of 2D peak and 2D/G intensity ratio) and low defect concentration (due to lack of D peak). c, Raman spectra from Raman map of the same sample as (b) after diazonium functionalization. A significant increase in the D peak indicates a high degree of reaction has occurred. Grain boundary area contribution The CVD-grown graphene used in our work is polycrystalline. To determine the contribution of grain boundaries, which may behave differently under diazonium functionalization conditions and may have different contributions to the overall Raman spectrum, we geometrically estimated the fractional area of grain boundaries within the overall graphene sample area, assuming that spectral contributions are proportionate to the area. In this calculation, we assume closely packed hexagonal grains of side length L with a region of width w at each boundary that may have a different behavior than the bulk graphene lattice, as shown in Figure S2. The fractional area of the grain boundary is: AA!" = 2 ww AA!"! 3 LL Supplementary Figure 2. Grain boundary contribution. Grains are approximated as hexagons of side length L, and a region of width w around each grain boundary that has behavior that may differ from that of the bulk graphene crystal. Typical grain sizes for CVD graphene grown on Cu foil from the literature range from 250 nm 1 to 3-5 µm, 2 and the width of the disrupted lattice region is 1-2 nm from atomically resolved TEM images. 1 NATURE CHEMISTRY www.nature.com/naturechemistry 3
Therefore, for L ranging from 125 nm to 2.5 µm, and for w = 2 nm, A gb /A tot = 0.09% to 1.8%. Thus the overall contribution of the grain boundaries to the Raman spectroscopy results is expected to be quite low, and the spectra are dominated by the contribution of the bulk graphene lattice. Octadecyltrichlorosilane (OTS) self-assembled monolayer (SAM) formation on SiO 2 Supplementary Figure 3. Reaction scheme of OTS on SiO 2. A freshly oxygen plasma cleaned surface with many OH terminations is necessary for full coverage of OTS on SiO 2. The resulting SAM is covalently linked to the SiO 2 substrate, and covalent linkages between molecules also exist. As a result, this SAM layer is very stable. NATURE CHEMISTRY www.nature.com/naturechemistry 4
Raman spatial map of graphene on hbn flake Supplementary Figure 4. a, Schematic illustration showing a flake of hbn on SiO 2, and a sheet of CVD graphene covering the hbn and extending onto the SiO 2. b, Optical microscope image of a flake of hbn (green region) deposited on SiO 2 (purple-grey background) with CVD graphene covering the entire area. The marked square is approximately 10 µm x 10 µm. c, Raman spatial map of the ratio of graphene D peak intensity to G peak intensity after diazonium functionalization, taken in the area indicated by the square in (b). The region of low functionalization in the upper right corner corresponds to graphene that is over the hbn flake, while the remaining areas of high functionalization over the SiO 2 substrate. OTS stripes under graphene Supplementary Figure 5. Atomic force microscopy (AFM) images of graphene that has been transferred to a substrate with narrower stripes of OTS and wider gaps of SiO 2. The OTS stripes are still clearly visible under the NATURE CHEMISTRY www.nature.com/naturechemistry 5
graphene, showing the graphene conformally covering the patterned substrate. The thin lines across the sample are wrinkles in the graphene sheet that form during the graphene transfer process. Diazonium reagent stability The stability of the 4-nitrobenzenediazonium reagent in aqueous solution with sodium dodecyl sulfate (SDS) was studied as a function of time under our reaction conditions by NMR and optical absorbance spectroscopy. For the NMR spectra, deuterium oxide was used as the solvent, and for the optical absorbance spectra, deionized water was the solvent. The solutions were kept at the same reaction temperature (35 C) as was used for graphene functionalization. In Figure S6a, the NMR spectrum of the initial solution shows four peaks (or two pairs of peaks) associated with the protons in the diazonium ion. After 1.5 hr and 24 hr, the peaks have not significantly changed, indicating the diazonium is fairly stable over that time. However, after 3 days (72 hr), the four initial peaks have become significantly weaker and several additional peaks have appeared at lower chemical shifts. We attribute these changes to the formation of azo dye molecules from two diazonium molecules joining together, as well as some possible short-chain oligomers. In Figure S6b, optical absorbance spectra are shown. The optical absorbance spectra show a prominent peak at about 255 nm and a smaller peak at about 310 nm. Over the duration that was used for the graphene functionalization reactions, the spectra are fairly constant. At the longest time, an additional broad but weak peak near 400 nm appears as an azo product. These spectra are consistent with time- and surfactant-dependent spectra reported in the literature. 3 NATURE CHEMISTRY www.nature.com/naturechemistry 6
Supplementary Figure 6. 4-nitrobenzenediazonium stability over time. Nuclear magnetic resonance (NMR) spectra and optical absorbance spectra (UV-vis) were obtained for the 4-nitrobenzenediazonium tetrafluoroborate in aqueous solution with sodium dodecyl sulfate at the graphene functionalization reaction concentrations as a function of time. Between each spectrum, the solution was kept at the same reaction temperature in the same location in which the graphene samples were reacted. a, NMR spectra of proton chemical shift showing four peaks from the four protons in the 4-nitrobenzenediazonium cation. After 1.4 hr and 24 hr, the peaks have not significantly changed. After 3 days (72 hrs) the main diazonium peaks are significantly reduced, and additional side product peaks have appeared. b, Optical absorbance spectra of the diazonium solution over time, normalized to the peak at 255 nm, showing the diazonium reagent is fairly stable over time. At the longest solution aging times, a broad feature around 400 nm corresponding to azo product formation can be observed. NATURE CHEMISTRY www.nature.com/naturechemistry 7
Fitting of spatial D/G profile The I D / I G ratio profiles of the OTS stripes (Figure 3 in main text) were fit to integrated Gaussian distributions, D(x): DD xx =!!! = 1 σσ 2ππ PP vv dddd!!! exp vv μμ! 2σσ! = 1 2 1 + erf xx μμ σσ 2 where P(x) is the Gaussian distribution function, µ is the mean, σ is the variance, and erf is the error function. After fitting the profiles, we obtain σ = 0.85 µm for the OTS stripes, and σ = 0.76 µm for the BN edge. dddd NATURE CHEMISTRY www.nature.com/naturechemistry 8
Estimation of nitrobenzene coverage on graphene The surface concentration of nitrobenzene groups on graphene after functionalization was estimated using equation 4 of the main text based on the work of Lucchese et al, 4 with the change of r s = 0.07 nm and r a = 1 nm to account for covalent functionalization sites being less disruptive to the graphene lattice than ion bombardment sites. The plot of I D /I G vs. L D is shown in Figure S7. Using this relation, we obtain the following estimates of covalent functionalization site concentrations: I D / I G L D (nm) σ (cm -2 ) SiO2, pristine 0.11 11.9 7.1 x 10 11 SiO2, functionalized 1.42 3.1 1.1 x 10 13 OTS, pristine 0.12 11.3 7.8 x 10 11 OTS, functionalized 0.25 7.8 1.6 x 10 12 h-bn, pristine 0.13 10.9 8.4 x 10 11 h-bn, functionalized 0.27 7.5 1.8 x 10 12 Al 2 O 3 (sapphire), pristine ~0 ~inf ~0 Al 2 O 3 (sapphire), functionalized 1.16 3.4 8.5 x 10 12 Supplementary Table 1. Values of distance between reaction sites (L D ) and concentration of reaction sites (σ) calculated using equation (4) of main text. NATURE CHEMISTRY www.nature.com/naturechemistry 9
5 4.5 4 3.5 3 I D /I G 2.5 2 1.5 1 0.5 0 0 5 10 15 20 L D (nm) Supplementary Figure 7. Effect of reaction site density on Raman D/G ratio. Plot of I D /I G vs. L D (distance between covalent reaction sites) from equation (4) in manuscript, with distances and constants modified for covalent functionalization rather than physical damage from ion bombardment. Electrochemical functionalization To further test the implications of our substrate-dependent graphene reactivity results and model, we conducted electrochemical functionalization experiments where the graphene was electrically doped by an applied back gate voltage. The samples consisted of sheets of CVD-grown graphene ~5 mm x 5 mm in area transferred to Si substrates with both 100 nm and 300 nm SiO 2 layers, with silver paste applied at the graphene boundaries as grounding contacts (see schematic diagram in Figure S8a). One sample was used for reaction at each value of back voltage. By using these two thicknesses of the dielectric layer, we can apply the same back gate voltages but achieve different amounts of Fermi level change in graphene. That is, for a given back gate voltage, the graphene on 100 nm SiO 2 has a bigger Fermi level shift than on 300 nm SiO 2. The carrier density n scales with the inverse of the dielectric thickness d bg, n = (V g -V 0 ) ϵϵ 0 / (d bg e), where ϵ is the SiO 2 dielectric constant, ϵ 0 is the permittivity of free space, V g is the back gate voltage, and V 0 is the voltage at the conductivity minimum. The Fermi level is E F (n) = ħ v F (πn), where the Fermi velocity v F = 1.1 x 10 6 m/s. NATURE CHEMISTRY www.nature.com/naturechemistry 10
The reactant solution was an aqueous mixture of 10 mm 4-nitrobenzenediazonium tetrafluoroborate in 0.5 wt% sodium dodecyl sulfate, and was deposited onto the graphene samples in a droplet. The solution droplet was in contact with the graphene and grounding contacts, but not the back contact. The effect of positive and negative back voltages on both the Fermi level of graphene and the charged diazonium ions is schematically illustrated in Figure S8b. At increasingly positive (negative) back gate voltages, the Fermi level of graphene shifts up (down), resulting in increasing (decreasing) reactivity toward diazonium functionalization due to the change in overlap of states between graphene and diazonium. However, the positively charged diazonium ions are repelled (attracted) by the charged substrate, so that the effective concentration of the diazonium reagent at the graphene surface is lower (higher) than in the bulk solution. The presence of a charged electrode also introduces the electrochemical reduction of diazonium ions to diazonium radicals, in addition to the change in concentration. Raman spectroscopy was used to verify the pristine graphene samples before reaction, and to evaluate the degree of covalent functionalization after reaction. For each sample, the back gate voltage was applied for 1 min while the other two electrodes were kept at ground (0 V). The samples were rinsed in ultrapure water before Raman spectroscopy, during which multiple spectra in different locations were taken. Figures S9a-b show representative Raman spectra of the samples after electrochemical functionalization, and Figure S9c shows the D/G ratios plotted as a function of gate voltage. In general, the effect of the change in diazonium ion concentration due to the electric field from the biased substrate is quite strong, but we are still able to see the effect of the Fermi level shift. In the results of Figure S9, at zero applied bias the amount of reaction is very small. At positive bias, the Fermi level of graphene shifts up, implying more reactivity, but the charged substrate repels the diazonium cations for a reduced concentration at the surface. The combined effect is an increased amount of reaction compared to 0 V, and higher reactivity at 40 V than at 20 V, suggesting that the shift in the Fermi level has allowed more functionalization to occur, even though the diazonium concentration is lower. The overall reactivity for 100 nm oxide samples being lower than that on 300 nm oxide is likely explained by the thinner oxide screening the electric field less strongly, so that the diazonium cations are repelled more and the surface concentration is decreased. At negative bias, the Fermi level of graphene shifts down, implying less reactivity, but the charged substrate here attracts the diazonium cations for an increased concentration at the surface. The Raman data shows that the amount of reaction is much higher, suggesting that the higher diazonium concentration plays a bigger role than the Fermi level. However, for the 100 nm oxide samples, the +40 V sample is less reacted than the +20 V, suggesting that the Fermi level has shifted farther down at +40 V and the influence of the Fermi level on reactivity has begun to overcome the effect of the increased diazonium concentrating. We also note that the minimum conductivity point of our graphene is likely NATURE CHEMISTRY www.nature.com/naturechemistry 11
between 30-50 V, so all our data points are likely within the hole conduction (p-doped) region. However, because of electron-hole charge puddles, there are local regions that are n-doped, and the increasing positive bias makes these regions more n-doped and shifts the Fermi level of other regions enough to be n-doped. These experiments also show that electrochemical control of the reactivity of graphene is possible but rather complex, so that the ability to control the reactivity via substrate engineering as shown in the main text is valuable. Supplementary Figure 8. Electrochemical functionalization of graphene. a, Schematic illustration of sample and electrode setup for experiments. Pieces of CVD graphene ~5 mm x 5 mm were transferred to Si substrates with 100 nm or 300 nm SiO 2. Contacts were made from silver paint along two opposite edges of the graphene sample. A droplet of diazonium solution was placed over the sample and contacts. The contacts and solution were kept at ground (0 V) while different back gate voltages were applied to different samples for 1 min each. b, Schematic illustrations of Fermi level modulation in graphene with positive and negative gate voltage (top row) and ionic NATURE CHEMISTRY www.nature.com/naturechemistry 12
displacement in the solution (bottom row). The diazonium concentration at the surface is decreased or increased depending on the sign and magnitude of the applied back gate voltage. Supplementary Figure 9. Results of electrochemical functionalization of graphene. a, Representative Raman spectra for functionalized graphene on 100 nm SiO 2 /Si. b, Representative Raman spectra for functionalized graphene on 300 nm SiO 2 /Si. c, D/G intensity ratio from Raman spectra of functionalized graphene as function of gate voltage. All samples were reacted for 1 min. References 1 Huang, P. Y. et al. Grains and grain boundaries in single-layer graphene atomic patchwork quilts. Nature 469, 389-392, (2011). 2 Kim, K. et al. Grain Boundary Mapping in Polycrystalline Graphene. ACS Nano 5, 2142-2146, (2011). 3 Blanch, A. J., Lenehan, C. E. & Quinton, J. S. Dispersant Effects in the Selective Reaction of Aryl Diazonium Salts with Single-Walled Carbon Nanotubes in Aqueous Solution. J. Phys. Chem. C 116, 1709-1723, (2011). 4 Lucchese, M. M. et al. Quantifying ion-induced defects and Raman relaxation length in graphene. Carbon 48, 1592-1597, (2010). NATURE CHEMISTRY www.nature.com/naturechemistry 13