The Preparation of BN-Doped Atomic Layer Graphene via Plasma

Article pubs.acs.org/JPCC

The Preparation of BN-Doped Atomic Layer Graphene via Plasma Treatment and Thermal Annealing Jiao Xu,†,‡ Sung Kyu Jang,†,‡ Jieun Lee,†,‡ Young Jae Song,† and Sungjoo Lee*,†,‡,§ †

SKKU Advanced Institute of Nanotechnology (SAINT), ‡Center for Human Interface Nanotechnology (HINT), and §College of Information and Communication Engineering, Sungkyunkwan University (SKKU), Suwon 440-746, Korea S Supporting Information *

ABSTRACT: We report a new method for the codoping of boron and nitrogen in a monolayer graphene film. After the CVD synthesis of monolayer graphene, BN-doped graphene is prepared by performing power-controlled plasma treatment and thermal annealing with borazine. BN-doped graphene films with various doping levels, which were controlled by altering the plasma treatment power, were found with Raman and electrical measurements to investigate exhibit p-doping behavior. Transmission electron microscopy, electron energy loss spectroscopy, and X-ray photoelectron spectroscopy were used to demonstrate that the synthesized BN-doped graphene films have a sp2 hybridized hexagonal structure. This approach to tuning the distribution and doping levels of boron and nitrogen in monolayer sp2 hybridized BN-doped graphene is expected to be very useful for applications requiring large-area graphene with an opened band gap.

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INTRODUCTION Graphene has attracted a lot of research attention, particularly because of its high carrier mobility and notable electric field effect, since it was first obtained by mechanical exfoliation.1 The carrier mobilities of exfoliated graphene on SiO2/Si samples can be as high as 10 000 cm2 V−1 s−1,1 and chemical vapor deposition (CVD) graphene has been fabricated with a mobility of 4000 cm2 V−1 s−1,2 which means that graphene has potential uses in nano electronic device applications. However, one obstacle to such applications of graphene is its gapless energy band structure. There have been many attempts based on quantum effect theory or graphene lattice symmetry breaking to open the band gap of graphene, such as the fabrication of bilayergraphene with top and back gate field effect transistors,3 graphene nanoribbons,4 and functionalized graphene.5 However, the complexity of the bilayer-graphene double gate FET configuration, the limitations on the size of graphene nanoribbons, and the instability of physically modified graphene mean that these approaches to band gap opening are difficult to implement. The use of chemical doping to open the band gap is on the increase,5−7 especially substitutional doping, which makes it possible to synthesize stable doped graphene on a large scale with CVD. Boron and nitrogen atoms have a similar size to carbon atoms and hexagonal boron nitride (h-BN) has a graphene-like honeycomb structure with a large band gap (∼5 eV), so h-BN is regarded as an ideal dopant for graphene; this approach produces BN-graphene hybridized atomic layer structures.5−9 The outstanding feature of such BN-graphene hybridized structures is that their electrical properties are tunable by varying the dopant concentration, as has been studied both theoretically7 and experimentally.5,6,9,10 In previous studies, BN dopants have been introduced during graphene synthesis,5,6,8 but it has been found that segregation of © 2014 American Chemical Society

the BN domains arises at high BN concentrations because B−N and C−C bonds are stronger than B−C and N−C bonds.7 In this study, we synthesized BN-doped graphene layers by incorporating BN into graphene after the CVD process through plasma treatment and thermal annealing in borazine; the aim was to incorporate the BN dopants at the defect sites that are generated in the CVD graphene by the plasma treatment. Thus, the dimensions and distribution of the BN dopants are those of the generated defect sites, which prevents BN domain segregation and enhances the effects of BN on the graphene band gap. BN-doped atomic layer graphene was found to exhibit a resistance with a semiconductor-like temperature dependence; we determined that one film has a band gap of 14.4 meV, which is useful for optical detector and modulator applications.9

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EXPERIMENTAL SECTION First, samples of pristine graphene on Cu were synthesized with the traditional CVD method2 by using methane as the graphene carbon source. Next, these samples were bombarded with argon plasmas with various plasma powers, 0, 15, 20, and 25 W. Then, the samples were coated with poly(methyl methacrylate) (PMMA) and floated on Cu etchant for etching overnight. The etched plasma-treated graphene samples were transferred to highly doped silicon wafers with 285 nm SiO2, and PMMA was removed with acetone. Finally, the transferred graphene samples were annealed at 1000 °C for 15 min; during the annealing process, 0.2 sccm borazine (B3N3H6) was introduced as the BN source. Received: May 14, 2014 Revised: August 15, 2014 Published: August 28, 2014 22268

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Figure 1. (a) Schematic diagram of our method. (b) Optical microscopy image of a BN-doped graphene layer (plasma 15 W, annealing with 0.2 sccm borazine) (scale bar: 20 μm). (c) AFM results for the BN-doped graphene layer; the thickness is approximately 0.7 nm.

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RESULTS AND DISCUSSION Figure 1a shows a schematic diagram of this method for the synthesis of BN-doped graphene. When a high power argon plasma is applied to a graphene sample, the kinetic energy of the bombarding argon ions and the energy released by electron−ion recombination generate defects in the sample.11 Carbon atoms at defect sites are more chemically reactive12 and are more likely to bond with BN during the thermal annealing process. Therefore, the BN doping level depends on the density of the defect sites generated by the plasma treatment process. Typical optical microscopy (OM) and atomic force microscopy (AFM) images of a BN-doped graphene layer are shown in Figure 1b,c, which indicate the large scale uniformity and continuity of the BN-doped graphene layer, as is also confirmed by the Raman mapping results in Supporting Information Figures S1 and S2. The thickness of the BN-doped graphene layers on the SiO2/Si substrates is ∼0.7 nm, which is comparable to that of monolayer graphene. To study the effects of doping on the BN-doped graphene layers, we performed Raman analysis on the samples. Figure 2a−d shows the Raman spectra of the graphene samples prepared without thermal annealing and with various plasma powers: 0 W (pristine graphene, no plasma treatment), 15, 20, and 25 W. Figure 2n shows the variation of the ratio of the intensities of the D-peak and G-peak, ID/IG, with plasma power; ID/IG can be used as a measure of the defect density in graphene.13 The very low ID/IG ratio of the 0 W sample is indicative of the high quality of our CVD grown graphene. As the plasma power increases, the ID/IG ratios of the samples rise monotonically, which means that the graphene defect density can be controlled by varying the plasma power,11 as is also confirmed by the ID/IG Raman mapping in Figure S2. Figure 2e−h show the Raman spectra of the samples prepared with plasma treatment (0−20 W) and thermal annealing in the presence of borazine. No significant changes are evident in the Raman images after the thermal annealing process, except for shifts in the G-band, which are shown in the enlarged images of the G-band regions in Figures 2i−l. For the samples prepared with plasma treatment but no annealing, the G-band position fluctuates around 1598 cm−1. In contrast, after annealing in the presence of borazine, the G-band position shifts upward with increases in the plasma power. The variation in the G-band Raman shift with plasma power is shown

in Figure 2m. Further, statistical analyses of the Raman mappings of the BN-doped graphene samples were performed to create G band position histograms (Figure S1) and ID/IG mappings (Figure S2). These results show the uniformity of the BN-doping in the graphene samples. The G-band is due to the bond stretching of sp2 carbon atoms.14 Changes in bond length or electronic structure due to doping can result in changes in the E2g Γ phonon frequency15,16 and thus produce G-peak shifts. For graphene doped with boron or nitrogen, the blue shift of the G-peak increases with the doping level.16 This phenomenon can be understood in terms of the nonadiabatic Kohn Anomaly,15,17 in which the phonon is analyzed with time-dependent perturbation theory. In this approach, dynamic effects are included by making the traditional adiabatic Born−Oppenheimer approximation, and it is found that there is an increasing positive shift in the G-band and that either hole or electron concentration is increased, which is in good agreement with experimental measurements.17 As can be seen in the electrical measurement results in Figure 5, BN-doped graphene exhibits p-type semiconductor behavior, which indicates that the increased hole doping and upward shift in the G band are mainly caused by BN doping, as is consistent with previous results.18 XPS was used to investigate the compositions of the BNdoped graphene films. Figure 3 shows the C 1s XPS spectra of (a) pristine CVD graphene (denoted S1), (b) graphene prepared with plasma treatment followed by thermal annealing without borazine (denoted S2), and (c) plasma treatment followed by thermal annealing with borazine (denoted S3). Samples S2 and S3 were treated with the same plasma power (25 W) and were transferred to SiO2/Si substrates before annealing. Except for the absence of borazine, the other annealing conditions were the same for both samples. The main peak for the three samples is at ∼284.5 eV, which is the graphene C−C sp2 peak. The peaks at 285.6 eV are defect peaks due to carbon atoms that are not in the sp2 configuration.19 For sample S1, the ratio of defects/sp2 is ∼0.21, which could be due to structural disorder at the grain boundary.20 After plasma treatment and annealing without borazine (sample S2, Figure 3b), there is almost no shift in the position of this defect peak, but the ratio of defects/sp2 is increased to ∼0.36; the plasma treatment effect makes the largest contribution to this increase. For sample S3, after plasma treatment and borazine annealing, the defect peak is at a higher 22269

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Figure 2. (a−d) Raman spectra of the samples prepared without annealing and with plasma treatment (0, 15, 20, 25 W, respectively). (e−h) Raman spectra of the samples prepared with plasma treatment (0, 15, 20, 25 W) and annealing in the presence of borazine. (i−l) G-band comparisons. (m) G-band peak shift as a function of plasma power. (n) Raman ID/IG ratio as a function of plasma power.

Figure 4a shows a transmission electron microscopy (TEM) bright field image of a BN-doped graphene film (prepared with 15 W plasma treatment and thermal annealing in the presence of borazine). Figure 4f shows a higher magnification image of this sample edge area, which clearly shows that the BN-doped graphene has a monolayer structure, as is also confirmed by the presence of one set of hexagonal spots in the fast Fourier transform (FFT) pattern (inset). Figure 4b−d show the corresponding carbon, nitrogen, and boron electron energy loss spectroscopy (EELS) mapping images, which demonstrate the random and dispersive distributions of boron and nitrogen in the BN-doped graphene film. In the UV−vis spectra of the BN-doped graphene films (Figure S3) prepared with various plasma powers (0−25 W) and thermal annealing in borazine, there are no h-BN UV absorbance peaks (at ∼200 nm). These results imply that the segregation of BN incorporated in graphene observed in previous studies5,6 is suppressed by our doping method. Figure 4e shows the EELS spectrum of the point indicated in Figure 4a. In the K-shell EELS spectra for the elements B, C, and N, the first sharp peaks (the stronger π peak for B and the weaker π peaks for C and N) originate from the 1s−π* antibonding orbit and the wider peaks originate from the 1s−σ* antibonding orbit.6 These results indicate the presence of B and N, and show that

position, 286.0 eV, and the ratio is still around 0.36, as estimated from the peak area. This shift is probably due to C−N bonding (∼286.2 eV).21 Hence, the peak at ∼286.0 eV in the C 1s XPS spectrum of sample S3 could be due to persistent carbon defects and C−N bonding. The other peaks marked C1 and C2 are due to C−O bonding.22 Figure 3d,e shows the N 1s and B 1s XPS spectra of the BN-doped graphene sample (S3). As shown in Figure 3d, the peak at ∼398 eV is the N−B peak.21 The shoulder emerging at ∼400.3 eV is due to bonding between nitrogen and carbon,21,23 and the other shoulder is due to oxidized nitrogen.24 In the B 1s XPS spectrum, only B−N bonding is observed. Similar results have previously been reported,5 and were explained as due to the formation of the thermodynamically more favorable configuration, the B−N−C structure. When the BN domain is mainly terminated with nitrogen atoms, it is difficult to detect the B−C bonding signal in the B 1s spectrum. With increases in the plasma power, the BN concentration increases. The BN concentration in our plasma-treated and postannealed samples is estimated from the B 1s and N 1s XPS peak areas. The atomic percentages of B and N are estimated to be 6.3% and 7.5% (15 W plasma treatment and thermal annealing), 8.8% and 8.9% (20 W plasma treatment and thermal annealing), and 10.7% and 12.0% (15 W plasma treatment and thermal annealing), respectively. 22270

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Figure 3. XPS C 1s spectra of (a) pristine graphene, (b) graphene prepared with plasma treatment but no annealing, and (c) graphene prepared with plasma treatment and annealing in the presence of borazine. (d) XPS N 1s and (e) B 1s spectra of the BN-doped graphene sample.

Figure 4. (a) TEM bright field image; EELS mapping images for (b) C, (c) N, and (d) B; (e) the EELS image of the point marked in (a). (f) Higher magnification TEM image of a sample edge, showing its single-layer structure (inset: the selective area electron diffraction pattern).

heavily doped substrates (resistivity ∼ 0.2 Ω cm) as the back gates. Source and drain electrodes (Au/Ti = 45 nm/7 nm) were patterned onto the BN-doped graphene films by using photolithography, electron-beam evaporation, and a metal

the BN-doped graphene film has a sp2 hybridized hexagonal structure.25 Back-gate field effect transistors were fabricated to investigate the electrical properties of the BN-doped graphene films by using 22271

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Figure 5. Id−Vg characteristics of the FETs based on BN-doped graphene prepared with plasma powers of (a) 0 W, (b) 15 W, (c) 20 W, and (d) 25 W. (e) Resistances R as functions of temperature. (f) ln(R) as functions of 1/T.

treated with 15, 20, and 25 W) are 6.04, 10.7, and 14.4 meV respectively, that is, the band gap increases as the BN doping concentration increases.

lift-off process. The channel dimensions of the fabricated transistors are a width of 3 μm and a length of 8 μm. Figure 5a−d shows the drain currents of the FETs as functions of the gate voltage. All samples exhibit ambipolar behavior, as does pristine graphene. For the sample prepared without plasma treatment (Figure 5a), the Dirac point is located very close to Vg = 0 V, which indicates that the charge doping induced by the impurities during device fabrication is suppressed in the fabricated transistors.11,26 For the BN-doped graphene samples prepared with various plasma powers (15, 20, and 25 W), the Dirac points are shifted upward (Figure 5b,c,d), which confirms the hole doping of graphene, as is consistent with previous results.18 Note also that as the plasma power (BN doping level) increases, the Dirac points shift to more positive values, which indicates enhancement of the hole doping effect. The field effect mobilities of the BN-doped graphene transistors were determined by using μ = [L/W × 1/CoxVds] × [dIds/dVg], where Cox = 1.2 × 10−4 F m−2 is the capacitance between the channel and the back gate per unit area (Cox = ε0εr/d; εr = 3.9; d = 285 nm), and found to be in the range 33−64 cm2/(V s). The resistances of the samples were measured at various temperatures with four-probe measurements. (The device configuration is shown in Figure S4.) Figure 5e shows the resistance (normalized to the minimum resistance) versus temperature curves of the devices. The resistances of the doped samples increase with decreases in temperature, whereas that of graphene decreases linearly with temperature. Good linear fits are possible for all the plots of the converted ln(R/Rmin)−1/T data points for the doped samples, so our BN-doped graphene samples exhibit typical semiconducting characteristics. The variation in the resistances with temperature conform to R(T) ∝ exp(ΔE/kBT), where kB is the Boltzmann constant, T is the temperature, and ΔE is the band gap. Hence, ΔE can easily be extracted from the slopes of the ln(R)−1/T curves. The calculated band gaps of the BN-doped graphene films (plasma

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CONCLUSION In conclusion, we have demonstrated that BN-doped atomic layer graphene can be synthesized by performing plasma treatment and thermal annealing in the presence of borazine. Our results show that boron and nitrogen are distributed uniformly in the synthesized BN-doped graphene films and that a sp2 hybridized hexagonal structure is formed. By controlling the active sites in CVD graphene for BN bonding, which are generated by the plasma treatment, the doping level of BN, which is introduced by the thermal annealing process, can be controlled. The highest bandgap obtained in our graphene samples is 14.4 meV, which is calculated based on the resistance− temperature measurement. Compared to the pristine graphene, the mobilities of BN-doped graphene samples are decreased, which are in the range of 33−64 cm2/(V s).

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ASSOCIATED CONTENT

S Supporting Information *

The Raman mapping data, UV spectroscopy results, device configuration, and characterization method are discussed in detail in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 22272

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ACKNOWLEDGMENTS This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Korean government (MSIP) (Grant Numbers: 2009-0083540 and 2012R1A1A2020089) and the Global Frontier Program through the Global Frontier Hybrid Interface Materials (GFHIM) (2013M3A6B1078873) of the NRF funded by the MOSIP, Korea.

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