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communications B-doped Graphene

Synthesis of Boron-Doped Graphene Monolayers Using the Sole Solid Feedstock by Chemical Vapor Deposition Huan Wang, Yu Zhou, Di Wu, Lei Liao, Shuli Zhao, Hailin Peng,* and Zhongfan Liu* Graphene, a single layer of sp2-bonded carbon atoms arranged in a honeycomb crystal lattice, has been attracting much attention since the first isolation in 2004.[1] Based on its prominent electronic properties such as high carrier mobility and room-temperature quantum Hall effect, graphene is expected to become a rising star in the future nanoelectronics.[2] Pristine graphene is a zero-bandgap semiconductor with its Fermi level (EF) located at the Dirac point, which exhibits an ambipolar transport behavior. For practical applications such as optoelectronic devices,[3] solar cells,[4] lithium batteries,[5] and even supercapacitors,[6] substitutional doping with B or N atoms is an efficient way to tailor the electronic property of graphene to p- or n- type behavior, respectively.[7] It has been theoretically predicated that the EF of graphene can be largely tuned by 0.65 or 0.59 eV in the case of 2 atom% B- or N-substitution, respectively.[7b] In comparison with the existing diverse methods of large-area N-doped graphene preparation,[8] the synthesis of B-doped graphene is still in its infancy. Several approaches such as reactive microwave plasma[9] and oxygen reduction reaction[10] were used to prepare B-doped graphene. However, it is still a challenge to avoid introducing overabundance defects and control the layers as well as microstructure of B-doped graphene. On the other hand, the growth of graphene via chemical vapor deposition (CVD) on copper foil is particularly popular due to its distinct advantages such as large scale, easy transfer and low cost.[11] The catalytic growth mechanism on Cu foil surface in the CVD process allows using different carbonaceous materials as the carbon source of graphene.[12] In addition, the doped graphene can be synthesized by mixing dopant precursors into the forming gas or carbonaceous source during CVD growth.[8b,13] Thus, the designing and choosing of CVD growth precursor are of great significance for the production of uniform and high-quality

H. Wang, Y. Zhou, D. Wu, L. Liao, S. Zhao, Dr. H. L. Peng, Prof. Z. F. Liu Center for Nanochemistry Beijing National Laboratory for Molecular Sciences State Key Laboratory for Structural Chemistry of Unstable and Stable Species College of Chemistry and Molecular Engineering Peking University Beijing 100871, PR China E-mail: [email protected]; [email protected] DOI: 10.1002/smll.201203021

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graphene. Recently, the CVD growth of B-doped graphene on Cu foil was achieved using methane and boron powder as carbon and boron precursors, respectively.[13a] However, the use of two precursors may give rise to the uncontrollability of B-doped graphene growth because of the difference between the source of carbon and boron, such as thermal properties and decomposition rate as well as the reaction with the Cu foil and hydrogen. In contrast, the use of a sole growth precursor that contains both carbon and boron elements can simultaneously introduce chemical species in a controlled manner for the large-scale growth of uniform and high-quality B-doped graphene. To this end, we developed a CVD growth method for B-doped graphene by using phenylboronic acid (C6H7BO2) as the sole precursor. The substitutionally B-doped graphene is shown to be a large-area and homogeneous monolayer, exhibiting a p-type transport behavior with a considerably high carrier mobility of about 800 cm2 V−1 s−1 at room temperature. A schematic representation of the B-doped graphene growth is illustrated in Figure 1a while the detailed growth procedure is presented in the experimental section. Basically, in a low-pressure CVD system, the sole source phenylboronic acid sublimated upstream and transported by carrier gas to the copper foil at high temperature, where it decomposed into carbonaceous, boron and oxygen species under hydrogen atmosphere. B-doped graphene film was gradually grown on the copper surface via the graphitization of adsorbed carbonaceous and boron species, while the oxygen species were removed by the formation of H2O or COX. After transferred from the copper foil onto a SiO2/Si substrate, the graphene sample was shown to be a large-area, continuous and homogenous film from the optical (Figure 1b) and scanning electron microscopy (SEM) (Figure 1c) images. The sample is predominantly of monolayer coverage due to the suppression of graphene adlayers on copper[11] although wrinkles and small dark islands were discernible in the SEM image. The thickness of the graphene film was further determined by atomic force microscopy (AFM). As shown in Figure 1d, AFM image and height profile of the graphene film reveal a very flat surface with a uniform thickness of about 0.86 nm. The histogram of thickness distribution in Figure 1e shows the thickness ranges from ∼0.8 to ∼1.2 nm, consistent with that of a monolayer graphene. Moreover, wafer-sized B-doped graphene monolayer was successfully grown (Figure 1f), which may be further scaled up for batch production of B-doped graphene by using the roll-to-roll process,[14] or larger vessel

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Synthesis of Boron-Doped Graphene Monolayers

Figure 1. (a) Schematic diagram of CVD growth of boron-doped graphene on Cu surface with phenyboronic acid as the carbon and boron sources. The red, grey, yellow, and green spheres represent boron, carbon, oxygen and hydrogen atoms, respectively. (b) Optical micrograph of a monolayer boron-doped graphene transferred onto SiO2/Si substrate. The arrow points to blank SiO2/Si substrate. (c) SEM image of the borondoped graphene film transferred onto SiO2/Si substrates. The inset shows the low-magnification SEM image of the same sample. (d) AFM image of the region pointed by black arrow in panel (b) with a z-scale of 20 nm. (e) Histogram of thickness distribution from AFM height images. (f) Contrast enhanced photograph of the B-doped graphene sample on 4-inch Si/SiO2 substrate. (g) UV-vis transmittance spectra of the boron-doped graphene and the reference intrinsic graphene on quartz substrate. The intrinsic graphene monolayer was CVD grown on copper using methane and then transferred on the quartz substrate. Inset: the photograph of boron-doped graphene monolayer on a quartz substrate.

and copper foil. On the other hand, optical transparency is important for integrating doped graphene films into optoelectronics devices, where transmission or harvesting of light through the graphene film is critical. We have conducted the ultraviolet-visible spectroscopy (UV-vis) to reveal the transparency of the B-doped graphene films transferred onto a quartz substrate (Figure 1g inset). Taking the same blank quartz substrate as the reference, the UV-vis spectrum small 2013, 9, No. 8, 1316–1320

shows an optical transmittance of ∼98.1% at 550 nm for the B-doped monolayer (Figure 1g), comparable with that of the intrinsic graphene monolayer (∼97.6% at 550 nm). X-ray photoelectron spectroscopy (XPS) was performed to determine the chemical composition and bonding form of the B-doped graphene films. As shown in Figure 2a, the B 1s peak of B-doped graphene at ∼190.9 eV, lower than that of the phenylboronic acid precursor (Figure S2), which evidently indicates that the boron atoms do exist in the graphene sample. Substitutionally doping of boron in the graphene skeleton is further confirmed by the slight asymmetry of B 1s peak, which suggests that the main bonding form of boron atoms should be BC3.[15,13b] The prominent C 1s peak at 284.8 eV corresponds to the graphite-like sp2 hybridized state. By taking the area ratio of the B 1s and C 1s peaks after considering the atomic sensitivity factors, the content of boron was estimated to be 1.5 atom%. The other two small peaks at 286.5 eV and 288.9 eV in the C 1s spectrum (Figure 2b) were assigned to C-O species, which are mainly caused by the physisorption of oxygen on the doped graphene. A similar phenomenon was also observed in the N-doped graphene.[8] Transmission electron microscopy (TEM) was used to examine the microstructure of the boron-doped graphene. As shown in Figure 3a, B-doped graphene sample exhibits a continuous film with a uniform thickness, on which residue nanoparticles can be preliminary recognized by contrast under high magnification with a sufficient defocus. Selected area electron diffraction (SAED) reveals a single set of hexagonal symmetry, demonstrating the single-crystalline nature of the domains. Along the arrows in Figure 3b, the peak intensity profile of diffraction pattern was obtained. Labeled by Bravais-Miller indices, the intensity of inner peaks from plane {1–100} is stronger than that of outer peaks from {1–210}, further indicating that the film is monolayer graphene.[16] Raman spectroscopy is a powerful technique to identify the numbers of layers, doping, structural disorder of graphene.[17] It is well known that graphene is very sensitive to charged impurity scatting and extrinsic doping. In additional to substitutional dopants, impurities introduced during the transfer process can also cause doping of CVD grown graphene. For example, charge impurities such as H2O and O2 trapped in the substrate or at the graphene/substrate interface during the transfer process could cause p-type doping of graphene from charge neutrality.[18] To minimize the transfer-related impurities at the graphene-substrate interface, a 'dry transfer' procedure of graphene films was employed according to a previously reported method.[19] Both B-doped and intrinsic (non-doped) graphene films were transferred under dry conditions onto the SiO2 substrate for Raman measurements (Figure 4a). For intrinsic graphene, the prominent G and 2D Raman bands centered at ∼1586 and ∼2686 cm−1 correspond to the E2g vibration mode of sp2 bonded carbon and the second order vibration caused by scattering of phonons, respectively.[17] The D band (∼1351 cm−1), which corresponds to defect level, is negligible in the intrinsic graphene. In contrast, the B-doped graphene exhibits a pronounced D band (∼1351 cm−1) and an accompanied D' band (∼1620 cm−1) with an intensity ratio of the D


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to ensure that most impurities such as the water and oxygen were removed.[22] The curve of drain current versus gate voltage (Figure 4d) reveals a typical p-type conductive behavior for the boron-doped graphene device, with the neutrality point at about 30 V. For comparison, intrinsic graphene device was fabricated and measured using the same procedure, which exhibits the charge neutrality at nearly 0 V. The shift of charge neutrality points (Dirac points, approximately 30 V) corresponds to a hole doping concentration of Figure 2. (a) XPS B 1s spectra of the B-doped and intrinsic graphene films. (b) XPS C 1s nd ∼ 2 × 1012 cm−2 in B-doped graphene. spectrum of the B-doped graphene. The C 1s peak can be split to three Lorentzian peaks at 284.8, 286.5 and 288.9 eV, which were labeled by violet, cyan, and green lines, respectively. Furthermore, transport in the B-doped graphene device exhibits a strong asymmetry in electron and hole conduction. and G bands about 1.9, which can be explained by the elas- The conductance of hole carrier is preserved, while the electically scattered photo-excited electron generated by hetero- tron conductance decreases, consistent with the p-doping atoms embedded in the graphene lattice and intravalley effect.[23] We further extracted carrier mobility near the Dirac double resonance scatting processes, respectively.[17] Similar point from the curve. Significantly, the room temperature carfeatures have also been reported in p-type graphene films rier mobility value of the B-doped graphene film is about with AuCl3 doping.[20] Compared to that of the intrinsic gra- 800 cm2 V−1 s−1 (Figure S3), comparable to the best values of phene, both G band (1592 cm−1) and 2D band (2695 cm−1) of reported B-doped graphene samples.[13b] We believe that the the B-doped graphene show upshifts of 6 cm−1 and 9 cm−1, controllable CVD synthesis using the sole solid source could respectively. Meanwhile, the intensity ratio of the 2D and G yield larger crystalline grain and uniform boron-substitution bands (I2D/IG) decreases. The emergence of D and D′ bands, in the monolayer B-doped graphene, which is of importance the observed shifts in G and 2D band position as well as the for the high-quality graphene devices. Further investigations decrease in I2D/IG ratios are consistent with the expected on the detailed growth mechanism and device physics are p-type doping effect in B-doped graphene films.[21] still desired. Electronic transport measurements of the B-doped graWe have successfully synthesized B-doped graphene via phene were further performed under vacuum to reveal the a simple and efficient CVD method, and demonstrated that doping effect and carrier mobility. To fabricate graphene the use of phenylboronic acid as a sole precursor could yield field-effect transistors, the graphene film was transferred wafer-scale, uniformly monolayer and high quality B-doped from copper foil onto a SiO2/Si substrate via the dry transfer graphene. XPS analysis verified that boron atoms were procedure,[19] followed by standard electron-beam litho- embedded into the graphene lattice. The B-doped graphene graphy, oxygen plasma etching, and thermal evaporation of exhibits a p-type doping behavior with a considerably high metal contacts. Figure 4b shows a typical optical image of carrier mobility of about 800 cm2 V−1 s−1, confirmed by electhe B-doped graphene device. To confirm the uniformity of trical transport measurements. This new method of borondoping effect, micro-Raman mapping was conducted on the doping will enable the modulation-doped growth of mosaic same B-doped graphene device. As shown in Figure 4c, the graphene with p-i or p-n junctions,[24] and hold promising for map of D band intensity has a uniformly distributed color, the applications in next-generation optoelectronics, fuel cell indicating the high homogeneity of the B-doping within and lithium-ion batteries. the Raman spatial resolution of ∼1 μm. Transport measurements were carried out after the devices were kept in the vacuum chamber (about 10−7 mbar) for more than 40 hours Experimental Section

Figure 3. (a) Low-resolution TEM image of a boron-doped graphene film suspended on a Cu grid. (b) Corresponding SAED pattern. Inset shows the intensity profile of diffraction pattern along the arrows.

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B-doped Graphene Growth and Transfer: B-doped graphene films were grown inside a 12-inch horizontal tube furnace (Lindberg/ Blue M) equipped with a 1-inch-diameter quartz tube. Copper foil (99.8% purity, 25 μm thick, Alfa Aesar) was loaded in the hot center of the furnace, while phenylboronic acid powder (C6H7BO2, 98% purity, J&K Scientific) was placed upstream at a location ∼35 cm away from center. A schematic representation of experimental setup was shown in Figure S1. After the copper foil was annealed at 1030 °C under 10 sccm H2 with a pressure of 120 mbar for 30 min, the furnace was cooled down to 950 °C. Phenylboronic acid powder was heated with a heating tape to 130 °C for the gradual sublimation. 10 sccm H2 was used as the carrier


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Figure 4. (a) Typical Raman spectra of the boron-doped (red) and intrinsic (black) graphene transferred on SiO2/Si substrate by a dry transfer procedure. (b) Optical micrograph of a boron-doped graphene device. (c) Raman map of D band intensity the channel region of the boron-doped graphene device shown in (b). (d) Source-drain current (Ids) vs back gate voltage (Vg) with Vds = 0.1 V of the boron-doped (red) and intrinsic (black) graphene device, respectively.

gas to transport the vapor of phenylboronic acid downstream to the copper foil. After 20 min growth, the furnace was cooled down to room temperature under 10 sccm H2. B-doped graphene films grown on copper were transferred onto SiO2/Si with a dry transfer procedure to avoid the p-type doping caused by the adsorbed H2O and O2. Briefly, the graphene samples with polymethyl methacrylate (PMMA) films detached from the copper foil were cleaned by deionized (DI) water, isopropanol and then dried in air for 6 h before the graphene/PMMA films were placed onto the target substrates. Characterization of B-doped Graphene: SEM images were obtained on a Hitachi S4800 field-emission scanning electron microscope. AFM was carried out on a Vecco IIIa nanoscope using the tapping mode. Optical transmittance spectra were collected on a Perkin-Elmer Lambda 950 UV-vis spectrophotometer. XPS analysis was conducted on a Kratos Axis Ultra spectrophotometer with the monochromatic Al X-ray at low pressures of 5 × 10−9 to 1 × 10−8 Torr. Raman spectra and 2D Raman mapping were performed on a Horiba HR800 Raman system with 514.5 nm excitation. The laser spot size was about 1 μm. TEM images were obtained using a Tecnai F20 TEM equipment operating at 200 kV. A lacey carbon film supported on copper grids was used for TEM characterization. Transport Measurements: To fabricate graphene FETs, a continuous graphene film transferred onto a silicon substrate with 300 nm SiO2 as back gate was firstly etched into Hall-bar configurations by O2 plasma after electron beam lithography (EBL). Metal small 2013, 9, No. 8, 1316–1320

contacts (30 nm Cr and 70 nm Au) were thermally evaporated onto the area defined by another step of EBL. The devices were placed in a homemade vacuum chamber equipped with a turbo station (Pfeiffer, Hicube 80 Eco). Electrical transport properties were measured using a Keithley 4200-SCS semiconductor system after the pressure was pumped down to 10−7 mbar. Field-effect mobilities were extracted by μ = dd IVds WC L V , where Cox (11.2 nF cm−2) g ox ds is the capacitance of the dielectric per unit area, and L (12.3 μm), W (2.3 μm) are the channel length and width, respectively. Hole C V doping concentration was estimated by nd = ge g .

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements The work was ﬁnancially supported by the National Natural Science Foundation of China (nos. 51121091, 51072004, 21173004, 21222303 and 51290272) and the National Basic Research


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Program of China (nos. 2013CB932603, 2012CB933404, 2011CB921904 and 2011CB933003), the Program for New Century Excellent Talents in universities and the Scientiﬁc Research Foundation for Returned Overseas Chinese Scholars, the State Education Ministry (SRF for ROCS, SEM).

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Received: December 4, 2012 Published online: March 6, 2013

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