SUPPLEMENTARY INFORMATION...Raman spatial map of graphene on hBN flake Supplementary Figure 4. a,...

NATURE CHEMISTRY | www.nature.com/naturechemistry 1

SUPPLEMENTARY INFORMATIONDOI: 10.1038/NCHEM.1421

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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

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Understanding and Controlling the Substrate Effect on Graphene

Electron-Transfer Chemistry via Reactivity Imprint Lithography



Michael S. Strano

Contents










References ................................................................................................................................................... 13

1


Understanding and Controlling the Substrate Effect on Graphene

Electron-Transfer Chemistry via Reactivity Imprint Lithography



Michael S. Strano

Contents










References ................................................................................................................................................... 13

© 2012 Macmillan Publishers Limited. All rights reserved.



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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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