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Supplementary Information
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 SiO2 ............................... 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
1
Supplementary Information
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 SiO2 ............................... 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
1
Supplementary Information
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 SiO2 ............................... 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
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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 SiO2), and higher layer numbers (bilayer, trilayer, etc.) are visible as regions of different
contrast (e.g. darker purple on 300 nm SiO2). 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
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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 SiO2, 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!"𝐴𝐴!"!
=23𝑤𝑤𝐿𝐿
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 nm1 to
3-5 µm,2 and the width of the disrupted lattice region is 1-2 nm from atomically resolved TEM images.1
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Therefore, for L ranging from 125 nm to 2.5 µm, and for w = 2 nm, Agb/Atot = 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 SiO2
Supplementary Figure 3. Reaction scheme of OTS on SiO2. A freshly oxygen plasma cleaned surface with many
–OH terminations is necessary for full coverage of OTS on SiO2. The resulting SAM is covalently linked to the SiO2
substrate, and covalent linkages between molecules also exist. As a result, this SAM layer is very stable.
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Raman spatial map of graphene on hBN flake
Supplementary Figure 4. a, Schematic illustration showing a flake of hBN on SiO2, and a sheet of CVD graphene
covering the hBN and extending onto the SiO2. b, Optical microscope image of a flake of hBN (green region)
deposited on SiO2 (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 SiO2 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 SiO2. The OTS stripes are still clearly visible under the
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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
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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.
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Fitting of spatial D/G profile
The ID / IG ratio profiles of the OTS stripes (Figure 3 in main text) were fit to integrated Gaussian
distributions, D(x):
𝐷𝐷 𝑥𝑥 = 𝑃𝑃 𝑣𝑣 𝑑𝑑𝑑𝑑!
!!
=1
𝜎𝜎 2𝜋𝜋exp −
𝑣𝑣 − 𝜇𝜇 !
2𝜎𝜎!𝑑𝑑𝑑𝑑
!
!!
=121 + erf
𝑥𝑥 − 𝜇𝜇𝜎𝜎 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.
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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
rs = 0.07 nm and ra = 1 nm to account for covalent functionalization sites being less disruptive to the
graphene lattice than ion bombardment sites. The plot of ID/IG vs. LD is shown in Figure S7. Using this
relation, we obtain the following estimates of covalent functionalization site concentrations:
ID / IG LD (nm) σ (cm-2)
SiO2, pristine 0.11 11.9 7.1 x 1011
SiO2, functionalized 1.42 3.1 1.1 x 1013
OTS, pristine 0.12 11.3 7.8 x 1011
OTS, functionalized 0.25 7.8 1.6 x 1012
h-BN, pristine 0.13 10.9 8.4 x 1011
h-BN, functionalized 0.27 7.5 1.8 x 1012
Al2O3 (sapphire), pristine ~0 ~inf ~0
Al2O3 (sapphire),
functionalized
1.16 3.4 8.5 x 1012
Supplementary Table 1. Values of distance between reaction sites (LD) and concentration of reaction sites (σ)
calculated using equation (4) of main text.
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Supplementary Figure 7. Effect of reaction site density on Raman D/G ratio. Plot of ID/IG vs. LD (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 SiO2 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 SiO2 has a bigger
Fermi level shift than on 300 nm SiO2. The carrier density n scales with the inverse of the dielectric
thickness dbg, n = (Vg-V0) ϵϵ0 / (dbg e), where ϵ is the SiO2 dielectric constant, ϵ0 is the permittivity of free
space, Vg is the back gate voltage, and V0 is the voltage at the conductivity minimum. The Fermi level is
EF(n) = ℏ|vF|√(πn), where the Fermi velocity |vF| = 1.1 x 106 m/s.
0 5 10 15 200
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
LD (nm)
I D/I G
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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
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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 SiO2. 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
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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 SiO2/Si. b, Representative Raman spectra for functionalized
graphene on 300 nm SiO2/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).
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