Fermi-Level Dependence of the Chemical Functionalization of

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Fermi Level Dependence of Chemical Functionalization of Graphene with Benzoyl Peroxide Dandan Liu, Mengci He, Can Huang, Xiudong Sun, and Bo Gao J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 21 Apr 2017 Downloaded from http://pubs.acs.org on April 25, 2017

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The Journal of Physical Chemistry

Fermi Level Dependence of Chemical Functionalization of Graphene with Benzoyl Peroxide Dandan Liu1, Mengci He1, Can Huang1, Xiudong Sun1,2, Bo Gao1,2* 1. Institute of Modern Optics, Key Lab of Micro-optics and Photonic Technology of Heilongjiang Province, Key Lab of Micro-Nano Optoelectronic Information System Theory and Technology of Ministry of Industry and Information Technology, Department of Physics, Harbin Institute of Technology, Harbin 150001, China. 2. Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 03006, China.

Corresponding Author: Prof. Bo Gao Email: [email protected] Phone: 86-18204505038 Fax: 86-451-86414109

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Abstract The dependence of chemical functionalization of graphene on its Fermi level was investigated based on a heat-initiated solid phase chemical reaction with benzoyl peroxide. The chemical functionalization was performed when gate voltage was applied to shift the Fermi level. Gate voltage-induced variation of the reactant concentration on the graphene surface could not occur due to the neutral intermediate and the solid phase reaction. Raman measurement on functionalized graphene showed that higher Fermi level led to more reacted carbon atoms, which was interpreted by the energy level alignment between graphene and phenyl radical.


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Main text 1. Introduction Chemical functionalization is one simple and versatile way to modify the structural and electronic properties of graphene.1-5 As an electron-rich material, graphene tends to chemically bond with withdrawing-electron reacting species, such as neutral or positive radicals, via the electron transfer mechanism.6 Therein, the energy level alignment between graphene and reacting species may play a crucial role in the electron transfer. Recent studies showed that the underlying substrate remarkably influenced the functionalization of supported graphene.7-9 E.g., graphene on SiO2 and Al2O3 substrates was more reactive towards covalent functionalization than graphene on hexagonal boron nitride (hBN) substrate 7, 8 and on an alkyl-terminated monolayer8. It was partly attributed to larger amplitudes of the substrate-induced electron-hole charge fluctuations10 for graphene on SiO2 and Al2O3 substrates, which were caused by charge impurities in the substrate and polar adsorbates on the surface. The large charge fluctuations resulted in higher effective Fermi level of the n-doped puddles and hence facilitated the electron transfer from the filled states of graphene to the empty states of reacting species.7-9, 11 Besides, substrate-induced Fermi level shift12 was also proposed to account for the different functionalization degree.7 Graphene on the various surfaces displays different extents of overall p-doping. Graphene on SiO2 and Al2O3 substrates are more highly p-doped, compared to graphene on hBN substrate and on alkylterminated monolayers. Unfortunately, the substrate-induced Fermi level shifts are 3 ACS Paragon Plus Environment


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either far less than or comparable to the charge fluctuations for graphene on the above surfaces. Hence, the influence of Fermi level on the functionalization of graphene could not be easily differentiated from the charge fluctuations. So far, there were few studies elaborating the influence of Fermi level of graphene as the sole parameter on the chemical functionalization.7 In one electrochemical functionalization experiment where graphene was electrically doped by an applied gate voltage during reaction, higher reactivity was found at both positive and negative gate voltages, which was because the amount of diazonium cations on the graphene surface was significantly different due to the attraction or repellence by the applied gate voltage.7 At increasingly positive (negative) gate voltages, the Fermi level of graphene shifted up (down), resulting in increasing (decreasing) reactivity toward diazonium functionalization due to the energy level alignment between graphene and diazonium. However, the positively charged diazonium ions were repelled (attracted) by the charged substrate, so that the effective concentration of the diazonium reagent on the graphene surface was lower (higher) than in the bulk solution. Therefore, further exploration of the dependence of chemical functionalization on the Fermi level of graphene is needed by eliminating the concentration variations of reacting species on the graphene surface. Recently, we developed a heat-initiated solid-phase chemical reaction to functionalize graphene by benzoyl peroxide (BPO), in which neutral intermediate phenyl radicals withdrew

electrons

from

graphene.13

Here

the

dependence

of

chemical 4

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functionalization of graphene on its Fermi level was investigated by performing the heat-initiated solid phase chemical reaction on gated graphene. The chemical functionalization was performed on three graphene samples at distinct Fermi levels which were determined by G band shift due to electron-phonon coupling.14 Gate voltage-induced variation of the reactant concentration on the graphene surface could not occur due to the neutral intermediate and the solid phase reaction. Raman measurement on functionalized graphene showed that when Fermi level of graphene moved higher, more carbon atoms reacted, which presents a novel method to modulate the chemical functionalization of graphene. 2. Experimental methods 2.1 Sample preparation Single-layer graphene was grown on copper foil at temperature of 1000 °C by lowpressure chemical vapor deposition (CVD) using a mixture of 0.15 sccm CH4 and 4.00 sccm H2 at a total pressure of ~100 mTorr. By PMMA-mediated method,15 as-grown graphene was transferred onto Si substrate with 300 nm SiO2 (SiO2/Si), on top of which are 100-nm-thick gold microelectrodes fabricated by photolithography and electron beam deposition. Subsequently, graphene surface was coated with BPO (J&K Chemical, 98%, wetted with ca. 25% H2O) molecules by drop-casting 10 mM BPO’s aqueous solution.



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2.2 Chemical functionalization of gated graphene As schematically shown in Figure 1A, a gate voltage Vg was applied to SiO2/Si substrate to shift the Fermi level of graphene by the electric field effect. Silver paint was glued onto one end of the gold microelectrodes and was connected to external voltage by wires. Following putting the gated graphene samples into a sealed container filled with N2, the chemical functionalization (shown in Scheme 1) was initiated by putting the container into a preheated oven at 75 °C. The reaction time was 30 min, so that all BPO molecules have been decomposed. After functionalization, graphene samples were thoroughly rinsed by immersing into hot acetone for over 30 min, in case of laserinitiated functionalization in subsequent Raman characterization. Scheme 1. Heat-Initiated Chemical Reaction between Graphene and Benzoyl Peroxide O O

75°C

O O

+

N2

+

CO2

2.3 Raman characterization Raman spectroscopy (B&W-Tek) with laser excitation energy of 532 nm (2.33 eV) was used to evaluate the layer numbers, the Fermi level and the functionalization degree of graphene in a backscattering configuration. The spectra were recorded with an acquisition time of 1 s by averaging 10 scans over the range 1200 – 3000 cm-1. The laser spot size is ~1 μm. The spectral resolution is ~2 cm-1. 3. Results and discussion


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The graphene sample was prepared by transferring low pressure CVD-grown singlelayer graphene onto SiO2/Si substrate with gold microelectrodes fabricated by electron beam deposition and photolithography. This method is different from tradition device fabrication process, in which photolithography is done on electronic materials and hence the gold microelectrodes are above the electronic materials. In our process severe contamination by photoresist could be avoided, which has strong adhesion to graphene and could remarkably influence subsequent functionalization. Figure 1B shows the optical image of a typical graphene sample. The blue and yellow strips are graphene and gold microelectrodes, respectively, which are grating structure with period of 10 μm. The size of single-crystal graphene is over 100 μm, ensuring that all graphene sheets contact with gold microelectrodes. To apply gate voltage, silver paint was glued onto one end of the gold grating for connecting wires with external voltage. Single-layer graphene was identified by micro-Raman spectroscopy equipped with a 532 nm laser line. Figure 1C shows a typical Raman spectrum of the pristine graphene samples. There are two characteristic peaks, the G band which is due to the E2g vibrational mode of sp2 bonded carbon atoms and is observed at ~1580 cm-1, and the 2D band at ~2666 cm-1 which is a second-order vibration caused by the scattering of phonons at the zone boundary.16 The height of 2D band is about 2-4 times the height of G band, and the line width of 2D band is as narrow as 22.7 cm-1, indicating a singlelayer graphene. No D band could be observed at ~1340 cm-1, meaning few defects in the pristine graphene samples. 7 ACS Paragon Plus Environment


(A) Silver paint

BPO layer

Graphene

(B)

(C) Intensity (a.u.)

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1200

1600

2000

2400

2800

Raman shift (cm-1)

Figure 1. (A) Schematic illustration of the gated graphene sample for chemical functionalization. From top to bottom: BPO layer, graphene, gold microelectrodes, SiO2, Si. (B) Optical image of a typical pristine graphene sample. The scale bar is 10 μm. (C) A typical Raman spectrum of pristine graphene. To determine the Fermi level of graphene samples at specific gate voltages, Raman measurements were performed at a series of gate voltages before depositing BPO molecules layer, 14, 17-20 which also confirmed the devices working perfectly. Figure 2A shows the G band spectra of No.1 graphene (Gra-1) sample at gate voltages from -80 V to 80 V at step of 20 V at 75 °C. It can be found that the G band frequency is dependent on the gate voltage due to the interaction of the G phonons with Dirac fermions,14, 17-19 which also has similar effect on 2D band (see Figure S1 in Supporting Information). Figure 2B shows that G band frequency ωG as a function of gate voltage. It can be seen that the pristine graphene without gate voltage is overall p-doping, which is believed to be originated from the oxygen adsorption from atmosphere and the oxygen in SiO2/Si substrate.21 ωG exhibits nearly symmetric changes relative to the value of Vg ~20 V, indicating the Fermi level is located at Dirac point when the gate voltage is ~20 V. This value is in agreement with previous study.14 The G band was found to be blue-shifted by 7.4 cm-1 when the gate voltage decreased from 20 V to -80 8 ACS Paragon Plus Environment

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V. It is noted that we could not see the reported change of the G band line widths (see Figure S2 in Supporting Information).14 It is because our Raman measurement was done at 75 °C, and hence the hot phonons broadened the line widths and hid the effect from the electron-phonon coupling. (A) Intensity (a.u.)

80 V 60 V 40 V 20 V 0V -20 V -40 V -60 V -80 V 1540

1560

1580

1600

1620

Raman shift (cm-1) 1590

G band frequency (cm-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(B)

1587

1584

1581

1578 -80

-40

0

40

80

Gate voltage (V)

Figure 2. (A) G band spectra of No.1 graphene (Gra-1) sample at gate voltages from 80 V to 80 V at step of 20 V. (B) G band frequency ωG of Gra-1 as a function of gate voltage Vg. It is reported that for single-layer graphene, the absolute value of the Fermi energy |𝐸𝐹 | with respect to the Dirac point has a linear relation to the G band frequency shift in high carrier density, irrespective of the temperature: 14

G  G0 

Auc D 2 EF , 2π G MvF2

(1)



where 𝜔𝐺 is G band frequency, 𝜔𝐺0 is G band frequency at the charge-neutral Dirac point, Auc is the area of the graphene unit cell, D is the electron-phonon coupling strength, 12.6 eV/Å, 𝜈𝐹 is the Fermi velocity, 106 m/sec, M is the carbon atom mass, 1.993 x 10-26 kg. Then we obtain: EF  meV   23.2  meV cm1   G  G0  cm1  .

(2)

Therefore, the Fermi level occurs at 172 meV below Dirac point when the gate voltage is -80 V (Indicated by black dashed line in Figure 3A).

(A)

Gra-3(80V) Gra-2(0V) Gra-1(-80V)

K

(B)

63meV

Gra-3 (80 V)

-9meV

Gra-2 (0 V)

Intensity (a.u.)

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Gra-1 (-80 V)

-172meV 1500

2000

2500 -1

Raman shift (cm )

Figure 3. (A) Schematic illustration of relative Fermi level of Gra-1 (black), Gra-2 (red) and Gra-3 (blue) samples at gate voltage of -80 V, 0 V and 80 V, respectively. (B) Raman spectra of Gra-1 (black), Gra-2 (red) and Gra-3 (blue) samples functionalized at Vg ~ -80 V, 0 V and 80 V, respectively. The three spectra are normalized by the height of G band. After knowing the Fermi level dependence on gate voltage, the graphene surface was coated with BPO molecules by drop-casting 10 mM BPO’s aqueous solution. Subsequently, the sample was put into a sealed container filled with N2 and reacted with 10 ACS Paragon Plus Environment

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BPO at 75 °C for 10 min while applying gate voltage. After rinsing thoroughly, multiple Raman spectra in different locations were collected to evaluate the functionalization degree. The black curve in Figure 3B is the Raman spectra of Gra-1 sample functionalized at Vg ~ -80 V, which corresponds to graphene at Fermi level of -172 meV relative to Dirac point. It can be seen that after chemical modification, a strong D band appeared at 1338 cm-1 with line width of 24 cm-1 and a weak D’ band appeared at 1615 cm-1 with line width of 15 cm-1, which corresponds to the rehybridization of carbon atoms from sp2 to sp3 induced by the covalent attachment of phenyl groups in graphene.22 Meanwhile, the height ratio of 2D to G band decreased to 1.7, and the line width of 2D band increased to 30 cm-1. Another two graphene samples, No. 2 graphene (Gra-2) and No. 3 graphene (Gra-3), were functionalized at gate voltages of 0 V and 80 V, respectively. As indicated by red and blue dashed lines in Figure 3A, the Fermi level of Gra-2 and Gra-3 at gate voltages of 0 V and 80 V are located at 9 meV below Dirac point and 63 meV above Dirac point by analyzing the dependence of G-band spectra and ωG on gate voltage, respectively (See Figure S3 and S4 in Supporting Information). The red and blue curves in Figure 3B are Raman spectra of Gra-2 and Gra-3 samples functionalized at Vg ~ 0 V and 80 V, respectively. The three spectra are normalized by the height of G band for making the changes of other bands easily observed. It can be seen from Figure 3B that as the Fermi level of graphene moved up from -172 meV to 63 meV, the intensity of both D



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and D’ bands increased, while the intensity of 2D band decreased, indicating more phenyl groups covalently attached to graphene22. The integrated intensity ratio of D and G band, ID/IG, is a key parameter to quantitatively characterize the density of reacted carbon atoms in the form of23 rA2  rS2   ID   πrS2  e  π rA2  rS2    CS 1  e  πrS2  ,  CA 2 2 e IG  rA  2rS 

(3)

where σ is the density of reacted carbon atoms, CA=4.2, CS=0.87, rA=1.0, rS=0.07.7, 23 Figure 4 shows ID/IG and the percentage of reacted carbon atoms as a function of Fermi level relative to Dirac point, which is indicated by black and red squares, respectively. Averaging from five different spots in each of the above three samples, the values of ID/IG are 0.83±0.045, 1.42±0.18 and 1.69±0.13, respectively, when the relative Fermi level is -172 meV, -9 meV and 63 meV. Calculating with Equation (3)23 and the reported values7, 23, the corresponding percentage of reacted carbon atoms is 0.011%, 0.35% and 0.44%, respectively. It can be found that the density of reacted carbon atoms are positively correlated with the Fermi level of graphene, which agrees well with the speculation in the chemical functionalization of supported graphene.7 One previous study about the reaction between graphene and diazonium7 showed that, when Fermi level shifted downward, ID/IG first linearly decreased towards zero, and then kept at zero when Fermi level was below a critical value. After linearly fitting ID/IG (black line in Figure 4), we obtained the relation between ID/IG and relative Fermi level in the form of


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ID =3.7 103 EF  meV   1.46 . IG

(4)

From Equation 4, we can know that the critical value of relative Fermi level is -395 meV in our reaction. No reaction is assumed to occur if the relative Fermi level is shifted lower than -395 meV. It is noted that ID/IG and percentage of reacted carbon atoms fluctuated among the five different spots. It is probably due to surface strain originated from surface roughness or impurities on SiO2/Si substrate,24, 25 or the PMMA residues during the graphene transfer process.26

Figure 4. Integrated intensity ratio of D band and G band ID/IG (black squares) and percentage of reacted carbon atoms (red squares) as a function of Fermi level relative to Dirac point. Black line is the linear fitting of ID/IG vs Fermi level. The behavior of the gate voltage dependent chemical functionalization can be explained by the gate voltage induced shift of Fermi level in graphene, which is schematically shown in Figure 5. The absolute Fermi level of intrinsic graphene 𝑬𝟎𝑭 is 4.57 eV.27 The phenyl radical has a half-wave potential of 0.05 eV,28 corresponding to a value to 4.75 13 ACS Paragon Plus Environment


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eV for Fermi level 𝑬𝐏𝐫 𝑭 , which is ~180 meV lower than intrinsic graphene. The energy level alignment is schematically shown in Figure 5. It can be seen that, when graphene is positively gated at 80 V, Fermi level will move up, which is 63 meV above 𝑬𝟎𝑭 and highly above the Fermi level of the phenyl radicals by 243 meV. Electrons in the filled states of graphene have a high tendency to flow from the filled states of graphene into the empty states of phenyl radical, meaning a faster reaction and hence a higher percentage of reacted carbon atoms. When graphene is negatively gated at -80 V, Fermi level will move down, which is 172 meV below 𝑬𝟎𝑭 and slightly above the Fermi level of the phenyl radicals by 8 meV. Electrons in the filled states of graphene could still flow into the empty states of phenyl radical, but the tendency is lower than that for positively gated graphene, indicating a slower reaction and hence a lower percentage of reacted carbon atoms.

Evac 4.75eV

4.57eV

63meV 180 meV

Graphene (gated at 80V)

Phenyl radical

172meV

Graphene (gated at -80V)

Figure 5. Schematic diagram for the energy level alignment between gated graphene and phenyl radical. 𝑬𝟎𝑭 is the Fermi level of intrinsic graphene. 𝑬𝐏𝐫 𝑭 is the Fermi level of phenyl radical. Evac is the vacuum level. 14 ACS Paragon Plus Environment

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From the fitting of ID/IG, we have already known that no reaction is assumed to occur if the relative Fermi level of graphene is shifted lower than -395 meV, which is 215 meV below the Fermi level of phenyl radical. Considering the electron-hole charge fluctuation of graphene on SiO2 substrate,10, 29 only when the Fermi level of graphene is shifted until the fluctuated electrons are below Fermi level of phenyl radical, could the electrons in the filled states of graphene never flow into the empty states of phenyl radical. Therefore, the charge fluctuation in our graphene samples is ~215 meV, which is larger than reported values on the same substrate.10, 29 We speculate that the carbon atoms around small amount of defects in graphene may have high reactivity and hence contribute a lot to D band in Raman spectra. Given the thin dielectric layer, the biggest gate voltage which could be applied is 80 V. Hence, we could not move the Fermi level of graphene far below that of phenyl radical to prohibit the functionalization. But our experiment did not show an abnormal result as found in the electrochemical functionalization7, because the phenyl radical is neutral and the reaction is solid phase. The gate voltage did not have a static interaction with the neutral phenyl radical and could not alter the concentration of phenyl radical on the graphene surface. 4. Conclusions The dependence of chemical functionalization of graphene on its Fermi level was investigated by performing the heat-initiated solid phase chemical reaction on gated graphene. The chemical functionalization was performed on three graphene samples at Fermi levels of -172 meV, -9 meV and 63 meV relative to Dirac point, respectively. It 15 ACS Paragon Plus Environment


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is found that, when Fermi level moved up from -172 meV to 63 meV, defect-related D band and D’ band increased, meaning more carbon atoms reacted, which can be interpreted by the energy level alignment between graphene and phenyl radical. This study experimentally presents a novel method to modulate the chemical functionalization of graphene, which has potential applications in fabricate graphene patterns and devices.

Supporting Information Available: 2D band spectra of No. 1 graphene (Gra-1) sample, Line width of 2D band at gate voltages for Gra-1, G band spectra of No.2 graphene (Gra-2) sample and No.3 graphene (Gra-3) sample at gate voltages from -80 V to 80 V at step of 20 V, G band frequency ωG of Gra-2 and Gra-3 as a function of gate voltage Vg. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Nos. 21203046, 21473046 and 11374074) and the New Faculty Start-up Funds from Harbin Institute of Technology.

AUTHOR INFORMATION

Corresponding Author 16 ACS Paragon Plus Environment

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*E-mail: [email protected].

Author Contributions

B.G. conceived and designed the experiments. D.D.L., M.C.H. and C.H. carried out the experiments. D.D.L. and B.G. wrote the manuscript. X.D.S. and B.G. supervised the project. All authors contributed to data analysis and scientific discussion.

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TOC Graphic

Gra-3(80V) Gra-2(0V) Gra-1(-80V)

K

63meV

Gra-3 (80 V)

-9meV

Gra-2 (0 V)

Intensity (a.u.)

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Gra-1 (-80 V)

-172meV 1500

2000

2500

Raman shift (cm-1)


Fermi-Level Dependence of the Chemical Functionalization of

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