Direct Synthesis of Graphene with Tunable Work Function on

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Direct Synthesis of Graphene with Tunable Work Function on Insulators via In-situ Boron Doping by Nickel-Assisted Growth Wen-Chun Yen, Henry Medina, Jian-Shiou Huang, Chih-Chung Lai, YuChuan Shih, Shih-Ming Lin, Jian-Guang Li, Zhiming Wang, and Yu-Lun Chueh J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp508365h • Publication Date (Web): 04 Oct 2014 Downloaded from http://pubs.acs.org on October 14, 2014

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The Journal of Physical Chemistry

Direct Synthesis of Graphene with Tunable Work Function on Insulators via In-situ Boron Doping by Nickel-Assisted Growth Wen-Chun Yen1, Henry Medina1, Jian-Shiou Huang1, Chih-Chung Lai1, Yu-Chuan Shih1, ShihMing Lin1, Jian-Guang Li1, Zhiming M. Wang2 and Yu-Lun Chueh1* 1

Department of Materials Science and Engineering, National Tsing Hua University, 30013,

Taiwan. 2

Engineering Research Center for Semiconductor Integrated Technology, Institute of

Semiconductors, Chinese Academy of Sciences, Beijing 100083, China. ABSTRACT- Work function engineering, a precise tuning of the work function, is essential to achieve devices with the best performance. In this study, we demonstrate a simple technique to deposit graphene on insulators with in situ controlled boron doping to tune the work function. At a temperature higher than 1000 °C, the boron atoms substitute carbon sites in the graphene lattice with neighboring carbon atoms, leading to the graphene with a p-type doping behavior. Interestingly, the involvement of boron vapor into the system can effectively accelerate the reaction between nickel vapor and methane, achieving a fast graphene deposition. The changes in surface potential and work function at different doping levels were verified by Kelvin probe force microscopy, for which the work function at different doping levels was shifted between 20 to 180 meV. Finally, the transport mechanism followed by the Mott variable-range hopping model was found due to the strong disorder nature of the system with localized charge-carrier states. KEYWORDS: graphene, boron-doped, Ni vapor-assisted, variable-range hopping, work function

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Introduction Doping is one of the most significant aspects of device fabrication into electronic industry to control the carrier concentration and transport. For doping of graphene, it is required not only to form PN junctions but also to provide an effective path to open band-gap1-7 and change work functions8-11 in order to improve the performance in device applications such as organic and optical electronics.1, 8, 12 Many different methods have been suggested to achieve graphene doping into n-type and p-type behaviors such as electro-bias,2, 8, 13 mechanical force14, chemical species.9, 15-20

and CVD processes.21-22 To achieve wide range selectivity/variability and simple device

structure, chemical doping is considered as the most effective method to integrate doped graphene into devices, such as transistors, amplifiers, sensors, interconnects, and electrodes. Basically, chemical doping can be accomplished by simply decorating graphene with different molecules, for example, F4-TCNQ,15 TCNE,16 BPO,17 TPA,18 Br2, I2,19 C3N6H6,22 HNO3, AuCl4, and HCl.20 In addition, interaction with different substrates such as silicon oxide23 and silicon carbide1 shows electrostatic doping on graphene caused by charge deviation due to the difference of work function. Only few works have sought to increase the work function using physical adsorption doping as anode material in photovoltaics.9, 24 Nevertheless, most of chemical dopants are linked with graphene by physical adsorption only and are easily removed from the graphene surface, shortening the lifetime of the device. The incorporation of other atomic elements on the graphene hexagonal lattice by strong chemical bonding, namely substitutional doping, provides a way to resolve the stability issue of the doped graphene. Unfortunately, most of synthetic efforts have been focused on n-type doping graphene to low the work function25 and no effort on the substitutional p-doping of graphene with precise tuning in work function has been achieved. In this regard, we demonstrate an effective method to in-situ synthesize substitutional boron doping graphene by the Ni vapor-assisted CVD process, inherently avoiding wrinkles, scratches, and polymer related contaminants during the transfer

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process. The bonding information of the graphene was confirmed by X-ray spectrometer (XPS). Compared with the physical adsorption doping, our substitutional boron doping can be precisely controlled to achieve different work functions, changing from 20 to 180 meV measured by Kelvin probe force microscopy via adjusting the graphene deposition rate. The detailed optical and electrical properties of the doped graphene were measured and reported. By controlling different temperatures, the carrier density can be controlled precisely. Our approach gives the graphene a step onwards for its utilization in applications such as photovoltaics, optical, and organic electronics that require a precise control of the work function for device performance enhancement.

1. Experimental Section 1.1. Synthesis of as-deposited intrinsic/ B-doped graphene Ni ingots and B powder were evaporated into vapor state after increasing from room temperature to target temperatures (1000~1150 ºC, 25~30 ºC/min) with a mixed gas of 20 sccm H2 and 100 sccm Ar in the 1 inch diameter quartz tube. When the temperature was reached to the target temperature, the working pressure was increased to 60 torr by reducing the pumping rate. 50 sccm CH4 was then introduced into the tube and reacted with Ni and B vapor for 5 minutes and as-deposited B-doped graphene can be obtained on arbitrary substrates. After deposition finished, the CH4 and H2 was turned off.

1.2. Characterization Confocal Micro-Raman Spectroscopy (LABRAM HR 800 UV) with 2 µm spot size 514 nm laser was used to provide the point Raman spectrum and Raman mapping image. Meanwhile, the depth profile and the interfacial bonding state were examined by X-ray photoelectron spectroscopy, which was calibrated by Pt bottom electrode (XPS, Perkin Elmer Phi 1600 ESCA system, operated at 25 mA/15kV). The microstrucutres were examined by field emission transmission electron microscopy (FE-TEM, JEM-3000F, JEOL operated at 300 kV with point-

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to-point resolution of 0.17 nm) and element mapping was achieved by electon energy loss spectrum (EELS). The electrode for as-deposited B-doped graphene used to measure the electronic behavior was prepared by e-gun deposition with 3 nm Cr and 50 nm Au, respectively. Angilent 1500 B analyzer was used to measure I-V behavior and R-T relationship from 150 to 300K. Scanning probe microscope system (Bruker, Model : Dimension ICON, XY noise : < 0.15 nm；Z sensor noise : < 0.35 Å) is used to measure the surface potential and work function by tapping mode (Kelvin probe force microscopy)

2. Results and discussion A conventional heat furnace was used to directly deposit graphene on insulators by the Ni vapor-assisted method as we had reported previously, with which Ni ingots were placed upstream of a quartz tube used to provide Ni gaseous vapor during the annealing process.26-27 Subsequently, the insulating substrate was positioned in the middle of the heating zone along with the B powder (purity~99. 99 %) as depicted in Figure 1 (a). Argon and hydrogen in 5 to 1 ratio were introduced into the system during the whole process to provide a reductive ambient, preventing oxidation of nickel gaseous vapor at a fixed pressure of 1~2 torr. Subsequently, a flow of 50 sccm CH4 was introduced to the system as a carbon source for direct growth of in-situ doped graphene with pressures and growth temperatures fixed at 50~100 torr and 1000-1150 °C, respectively. Simultaneously, the boron powder underneath the substrate was partially evaporated into the gas phase and mixed with the carbon gases. The carbon radicals would form sp2 bonds with each other in the presence of Ni vapor and self-assemble into a graphene hexagonal lattice after cooling process. Figure 1 (b) shows the Raman spectra acquired by a 514 nm wavelength laser for the as-deposited B-doped graphene grown at 1100 °C for 5 min. The Raman spectra indicates typical defective/doped graphene features containing six distinct peaks located at 1356, 1602, 2450, 2706, 2942, and 3237 cm-1, which are related to D, G, D+D´´, 2D, D+D´, and 2D´ band,

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respectively.28-29 The intensity of the D band is usually related to defect-induced activation of the breathing mode from the hexagonal carbon ring. Note that the large intensity of the D band for the boron doped graphene is not only connected to the defective and small ordering domain nature during the Ni vapor assisted grown, but also due to the substitutional boron atoms in graphene lattice as defective sites.30 The relative sharp G band in the Raman spectra is the footprint of the graphitization process from graphene. Boron doped graphene also shows another distinct peak at 2942 cm-1 denoted as the D+D´ near the 2D band, which is non-visible in nondefective graphene. Dissimilar from the D band, the existence of the D+D´ band can be referred to graphene doped with heterogeneous atoms or molecular through substitutional doping.28 By tuning deposition temperatures in a range between 1000 to 1150 °C for 20 min, the nickel/boron evaporation and methane decomposition rates can be consistently manipulated to achieve different boron doping levels. The respective Raman spectra under different temperatures are presented in Figure 1 (c). Obviously, the representative D+D´ band can be observed as the annealing temperature >1000 ºC, while the D band and the G band upshift from 1322 and 1597 to 1356 and 1602 cm-1, respectively, compared to intrinsic graphene, indicating doping of graphene,30 Note that the blueshift of the 2D band observed in figure S1 confirms the increased ptype behavior of the boron doping as the annealing temperature increases. Furthethe, almost linear dependence of the 2D shift with temperature in the range between 1050 to 1150 ºC to probe the controllability of the doping. However, the 2D band is relatively unclear for the graphene grown at 1000 °C, which is expected that the momentum conservation cannot be satisfied with the two phonons process required for the 2D band because of only 1~2 graphene layers and interruption of graphene crystallinity due to the short deposition time of ~5 min and the substitutional bonding of graphene with boron atoms. Interestingly, the ID/IG ratio decreases when the deposition temperature increases, especially for temperatures higher than 1100 °C, indicating reduction of defective sites, namely, the enhancement of the crystallinity of the p-doped graphene

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due to the large graphene grain size achieved under high temperature annealing. The uniformity of the film grown at 1100oC is supported by the Raman mapping of I2D’/IG shown in Figure S2. In addition, the Raman mapping of ID+D’/IG (Figure S2b) is used to prove the substitutional doping along the film. X-ray Photoelectron Spectroscopy (XPS) was used to confirm the atomic bonding between carbon and boron in the as-deposited B-doped graphene. Figures 2 (a) to 2 (d) show XPS results for the sample in Figure 1 (b), which was deposited at 1100 °C for 5 min. The strong peak located at 284.2 eV (C2) is the signature of the sp2 bonding while the small peak located at 285 eV (C3) corresponds to sp3 carbon due to grain boundaries and defects in the lattice structure.31-32 The peak located at 283.5 eV (C1) is normally assigned to B-C bonding.31 The peaks located at 286 eV (C4), 287 eV (C5), and 288 eV(C6) are attributed to hydroxyl groups and boron oxycarbides, respectively.32 The presence of boron into graphene lattice is confirmed by the B 1s from the XPS spectra as shown in Figure 2 (b). The diminished peak located at 186.7 eV (B1) is attributed to elemental boron.32 The peaks located at 187.7 eV (B2) and 188.5 eV (B3) are attributed to B4C and substitutional boron in a graphitic position.31 The latter two peaks located at 190 eV (B4) and 191 eV (B1) are normally associated to boron oxycarbides.32 Interestingly, the strongest peak in the B 1s spectra (B3) shows substitutional boron atoms in the graphitic position. We also suggest that the oxide formation is most likely due to contamination after exposing to air. The XPS depth concentration profiles were achieved by using Ar ion beam to dig through the as-deposited Bdoped graphene until the substrate as shown in Figures 2(c) and 2(d). The Carbon 1s signal remains unchanged for different depths of the film until the SiO2 substrate. On the other hand, the boron concentration illustrates a striking growth in deeper sites compared to 3.2 at% on the surface and 8.4 at% at 1.7 nm in depth (Figure 2c). Besides, the peak is located near 190 eV, indicating the partial oxidation of boron (Figure 2d).32 This characteristic is attributed to the high diffusivity nature of boron atoms whose the diffusivity of boron atoms in the SiO2 is described by

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Arrhenius form, namely DB = 3.96×10-2 exp(-3.65[eV]/KT) (cm2/s) by the thermal equilibrium33 and drives the boron atoms into deeper sites, even into SiO2 during the high temperature process. Despite that the diffusivity of carbon is also high, the carbon atoms tend to segregate onto the SiO2 surface rather than staying inside the SiO2 as confirmed by XPS when the carbon signal suddenly vanishes and the Ar ion beam reached the SiO2 substrate.34 On the contrary, the high diffusivity feature of boron can be understood, particularly inside the SiO2. The B 1s spectra confirm that the boron concentration keeps increasing even when Ar ion beam penetrates deeper into the substrate. Figure 3 shows the cross-section high-resolution (HR) TEM images of the B-doped graphene deposited at 1100oC for 5 min. The SiO2 layer at the bottom of the image is the substrate used for graphene deposition and the SiO2 on the top was deposited by electron beam evaporation for sample protection during the focus ion beam (FIB) process. As can be seen in Figure 3 (a), under a low magnification, the as-deposited B-doped graphene is not really clear due to flaws in the lattice so that the electrons diffract unevenly showing several dark and clear areas while the layered structure becomes clear at higher magnification. The thickness of the as-deposited Bdoped graphene is about 4~5 nm, corresponding to 7~8 layers (Figure 3b). Interestingly, the distance between layer and layer slightly expanded to 0.41 nm compared with intrinsic graphene (0.35 nm) by the same method.26 The increase in the interlayer distance is attributed to the partial insert of boron atoms between graphene layers. Notably, the contrast of SiO2 on both sides of graphene is different. The upper SiO2 (deposited after the process) is much brighter compared with the SiO2 substrate due to the doping effect of boron atoms. Electron energy loss spectrum (EELS) mapping for carbon and boron as shown in Figures 3(c) and 3(d) are consistent with our assumption. Understandably, the carbon signal in the as-deposited B-doped graphene from EELS mapping is not stronger than that in the SiO2 substrate, supporting the premise that carbon particles do not tend to diffuse into the SiO2 substrate, especially when the C-C bonds are

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formed.34 On the other hand, for the boron mapping as shown in Figure 3(d), the as-deposited Bdoped graphene region shows a boron less feature and most of the boron atoms were diffused into the SiO2 substrate during the high temperature annealing. This phenomena takes place due to the high diffusivity nature of B atoms in the SiO2 substrate hence the B atoms without carbon bonding would easily penetrate into the SiO2 substrate. Notably, the SiO2 used as a protective layer at top side also shows the boron doping. As a result, we suggest that the boron diffusion into the top layer is caused by the thermal effect of ion or electron beam caused by FIB process or TEM analysis respectively. Optical and electrical characteristics of the as-deposited B-doped graphene are demonstrated in Figures 4(a) and 4(b). The transmittance at 550 nm decreases from 97 % to 38 % when the deposition temperature increases from 1000 to 1150 ºC at a fixed deposition time of 5 min. The low transmittance at higher temperatures is caused by the increased thickness of the graphene because of enhanced catalytic effect between Ni and methane due to an increase of Ni vapor at higher temperatures. In comparison with the intrinsic as-deposited graphene, the transmittance remains the same at a synthesis temperature of 1000 ºC. It is expected that at 1000 ºC, the quantity of boron vapor and/or reactive energy are insufficient to form carbon-boron bonds. At temperatures larger than 1000 ºC, the transmittance largely drops when the boron vapor was involved in the system compared to the pristine graphene growth without the boron doping. As the temperature increases, the deviation (∆) of transmittance goes from 2 % at 1050 ºC to 23 % at 1150 ºC, compared with the pristine case (Figure 4a). Furthermore, Van der Pauw measurements were used to measure Hall mobility and sheet carrier density for the as-deposited B-doped graphene at different magnetic flux densities from 1 to 5 kG as shown in Figure 4 (b) (Figure S3). The sheet carrier density decreases with an increase of the annealing temperature, indicating that the concentrations of boron atoms in the graphene decrease. In addition, from the transmittance data, it was found that the involvement of boron vapor into the system would tend to accelerate

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the self-assemble reaction of graphene by Ni vapor and methane, leading to a faster graphene deposition rate. Although the total amount of boron vapor increase, the boron concentration decreases due to the accelerated deposition rate of the graphene, leading to a less boron concentration in the deposited film. As a result, the Hall mobility of the as-deposited B-doped graphene is much lower than that of intrinsic graphene owing to two possible effects, including the localized charge-states due to the in-situ boron doping and the small domains nature of the vapor-assisted deposition process. Both effects would raise the charge scattering, leading to the drop of the hole mobility, which explains that the hole mobility remains unchanged in a range from 45 to 58 cm2V-1s-1. In addition, it is most likely that the domain size is not the major contributor to the mobility since Raman spectra show an increment in the domain size as the temperature is raised. Instead, the result implies that the main contribution of the hole scattering is due to long range dipole interactions induced by charge asymmetry (Coulomb impurities) substitutional because of boron doping.35 Therefore, the hole mobility remains in almost the same value for different thicknesses of the as-deposited B-doped graphene. To shed light on the tunable work function by the boron doping, Kelvin probe force microscopy (KPFM) was used to obtain the amplitude modulation of work function by measuring the contact potential difference between the probe and materials in nano-scale as shown in Figures 5 (a) to (f) for the graphene with and without doping, respectively. Figures 5 (a), (c), and (e) display the surface potential difference from the tip and Figures (b), (d), and (f) show the potential gradient at the surface of each sample. Note that the work function of the material can be given by Φm = Φtip – eVdc, in which Φm and Φtip are the work functions of the material and the tip whereas Vdc is the record surface potential. The local variation of work function can be expressed by ∆Φm = –e∆Vdc, which is independent of the Φtip.36 Figures 5 (a) and (b) show the intrinsic as-deposited graphene at 1100 ºC with 50 sccm methane flow for 5 min while Figures 5 (c) and (d) show the as-deposited B-doped graphene with the same deposition condition. The

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difference between the average work function of intrinsic and as-deposited B-doped graphene is near 25~30 meV and both distributions of surface potential are uniform without any obvious aggregation of boron. By reducing the methane flow from 50 to 10 sccm and prolonging the deposition time from 5 to 25 minutes in order to fix the amount of graphene, the doping concentration of heavily B-doped graphene can be achieved, resulting significantly increased work function near 170 to 180 meV as shown in Figure 5 (e). However, the surface potential distribution becomes uneven due to the partial aggregation of boron (Figure 5f). The electronic behavior and transport mechanism of the as-deposited B-doped graphene grown at 1100 ºC for 5 min were also elucidated by using the variable temperature I-V measurements between 150 and 300 K as shown in Figure 6 (a). Note that four-probe measurements were used to avoid the contact resistance problem. I-V curves of the B-doped graphene remain a linear behavior during all temperature regions, showing an Ohmic behavior. As expected for the B-doped graphene, the conductivity decreases as the temperature decreases. By fitting the conductivity and the temperature measurements as shown in inset, a linear relationship can be obtained between ln(S) and T-1/3. This relationship between S and T indicates that the carrier transport mechanism follows the classical Mott variable-range hopping, which can be described as37 lnσ = lnσ0 - (T0/T)1/(d+1) were d represents the dimension of the system and can be substituted by 2 for the 2D or quasi-2D system in graphene.38 The equation then can be transformed into lnσ = lnσ0 - (T0/T)(1/3) for asdeposited B-doped graphene. Mott’s model describes systems with strongly disorder and localized charge carrier-states, especially for the low temperature carrier transportation.37 The phonons inside the as-deposited B-doped graphene stimulate both electrons and holes to the tunneled transportation in localized states. Figure 6 (b) shows the I-V behaviors for 1050, 1100

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and 1150 ºC at room temperature by fixing the deposition time of 5 min. Clearly, all the I-V behaviors show the linear dependent features and the relationship between sheet resistance and temperature are shown in the inset of Figure 6 (b). The sheet resistance for 1050 ºC is ~5 kΩ/sq, which is much higher compared with the intrinsic as-deposited graphene since the graphene lattice is damaged by substitutional boron although the carrier concentration is increased. This trade-off between lattice ordering and carrier concentration often arises in semiconductors, especially in the substitutional doping case. After increasing the deposition temperature, the sheet resistance largely drops from ~2300 Ω/sq at 1050 ºC to 190 Ω/sq at 1150ºC, which is most likely due to the fact that graphene thickness increase at higher temperatures. Although the partial pressure of the boron vapor in certain temperature is difficult to be regulated, this as-deposited technique allow us to change the methane flow to control the deposition rate of graphene and therefore controlling the boron concentration into graphene. In addition, this method provides a much simpler process (only one step), clean (without any chemical pollutions), and atomically flat (without no wrinkles and cracks) to deposit our tunable p-type doped graphene on insulators and has a great potential to apply on anode electrode for photovoltaics, optical, and organic electronics applications. Furthermore, this process can be extended to other doping species to induce different in-situ p- or n-type doping. In-situ nitrogen doping is expected to induce n-type doping of graphene, leading to a reduction of the work function, which is suitable for complementary cathode electrodes.

4. CONCLUSIONS A novel and simple approach to deposit B-doped graphene on insulating substrates without any transferred process or chemical treatment was achieved. By introducing the Ni and boron gaseous vapors at the same time, the CH4 gas decomposed and self-assembled into B-doped graphene, creating B-C bonds inside the graphene lattice. The doping level of as-deposited graphene by boron can be precisely controlled by annealing temperature and time, respectively,

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leading to tunable work functions upshifting between 20 to 180 meV at different B-doping levels. Mott’s model was used to describe the carrier transport mechanism in the B-doped graphene, indicating that the dominant carrier transport is due to a variable range hopping as expected for the strong lattice disorder with localized charge carrier-states nature. The doping approach gives the graphene a step onwards for its utilization in applications such as photovoltaics, optical, and organic electronics that require a precise control of the work function for device performance enhancement.

ACKNOWLEDGMENT The research was supported by the Ministry of Science and Technology through grants no 1012112-M-007-015-MY3, 101-2218-E-007-009-MY3, 102-2633-M-007-002, and the National Tsing Hua University through Grant no. 102N2022E1. Y.L. Chueh greatly appreciates the use of facility at CNMM, National Tsing Hua University through Grant no. 102N2744E1.

ASSOCIATED CONTENT Supporting Information 2D band shift of as-deposited B-doped graphene as the function of the deposited temperatures; Raman mapping images for as-deposited B-doped graphene deposited at 1100 °C for 5 mins with 50 sccm methane flow; Hall measurements of as-deposited Bdoped graphene deposited at 1100 °C for 5 mins with 50 sccm methane flow at different magnetic fields. This information is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION 12


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Corresponding Author *Tel.: +886 3 5715131 ext 33965; e-mail: [email protected] Notes The authors declare no competing financial interest. REFERENCES

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Figure Captions Figure 1. (a) A schematic for growth of the B-doped graphene. Ni ingot was placed at the upstream of the quartz tube and the substrate was located on a quartz boat with boron powder at the end of tube. CH4/H2/Ar mixed gas was introduced into the system. The inset shows the corresponding optical image. (b) Raman spectra for the B-doped graphene deposited at 1100 ºC for 5 min growth. (c) Raman spectra for B-doped graphene deposited at different temperatures for 5 min growth. Figure 2. XPS spectra for B-doped graphene deposited at 1100 ºC. (a) and (b) are C 1s and B 1s spectra. (c) and (d) are the XPS depth profile for carbon and boron related peak respectively. Figure 3. (a) A low magnification TEM image of B-doped graphene deposited at 1100 ºC for 5 min growth. (b) A high resolution TEM image of B-doped graphene taken from white rectangular area in (a). (c) and (d) are EELS mapping images for carbon and boron elements, respectively. Dark regions denote element less feature. Figure 4. (a) Transmittance of intrinsic and B-doped graphene at different deposition temperatures with an incident light with wavelength of 550 nm, respectively. The inset shows optical images from left to right sides are the real image for pure quartz, 1050 ºC intrinsic graphene, 1050 ºC B-doped graphene, and 1150 ºC B-doped graphene, respectively. All the samples were deposited for 5 minutes. (b) Hall measurements for Bdoped graphene at different deposition temperatures. Figure 5. KPFM images for 1 × 1 µm2 size. (a)(c)(e) show the voltage difference between the tip and the sample surface and (b)(d)(f) are the variation of the surface potential on each sample. (a) and (b) display the intrinsic graphene without B-doping. (c) and (d) B-doped

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graphene deposited at 1100 ºC with 50 sccm CH4 for 5 minutes. (e) and (f) B-doped graphene deposited at 1100 ºC with 10 sccm CH4 for 25 minutes denoted as heavily Bdoped graphene. The scale is the same for all the images. Figure 6. (a) Variable temperature I-V measurements from 150 to 300 K. The B-doped graphene was deposited at 1100 ºC with 50 sccm CH4 for 5 minutes. The inset shows the curve fitting for conductance (S) and temperature (T) using the classical Mott variable-range hopping behavior. (b) I-V measurements at Room temperature for different B-doped graphene grown at temperatures. The inset shows the relationship between sheet resistance (R/sq) and deposition temperature (T).

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 6

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TOC

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Direct Synthesis of Graphene with Tunable Work Function on

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