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Surfaces, Interfaces, and Applications
Large-Area High-Quality AB-Stacked Bilayer Graphene on h-BN/Pt Foil by Chemical Vapor Deposition Yongteng Qian, and Dae Joon Kang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06862 • Publication Date (Web): 07 Aug 2018 Downloaded from http://pubs.acs.org on August 8, 2018
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Large-Area High-Quality AB-Stacked Bilayer Graphene on h-BN/Pt Foil by Chemical Vapor Deposition Yongteng Qian and Dae Joon Kang* Department of Physics and Institute of Basic Science, Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon-si, Gyeonggi-do 16419, Republic of Korea.
ABSTRACT Large-area, high-quality bilayer graphene (BLG) has attracted great interest, because of its immense potential for many viable applications. However, its growth is still greatly limited owing to its small size and low carrier mobility. In this article, we report the successful growth of large-area, high-quality AB-stacked BLG on hexagonal boron nitride (h-BN)/Pt foil by chemical vapor deposition (CVD). Optical microscopy and scanning electron microscopy observations reveal the formation of uniform and continuous BLG films with sizes of up to 500 µm, which are 4–5 times larger than those reported elsewhere for CVD-grown BLG films. A large carrier mobility of up to 9,000 cm2 V-1 s-1 is observed for the BLG films grown on h-BN/Pt foils in ambient conditions. We also propose a plausible growth mechanism of BLG growth on h-BN/Pt foils. Our findings will contribute to better understanding of the fundamental BLG physics and the development of BLG-based devices.
Keywords: Bilayer graphene, h-BN, Pt foil, large area, chemical vapor deposition, carrier mobility.
*Author to whom correspondence should be addressed:
[email protected] (+82-31-290-5906). 1
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Introduction Monolayer graphene (MLG) has attracted great attention and opened up a new research field in two-dimensional (2D) materials.1–4 This is owing to its unique linear electronic band structure and the existence of massless Dirac fermions. Of equal interest is the AB-stacked bilayer graphene (BLG) film with a band structure different from that of MLG, having massive chiral Dirac fermions, thus behaving differently in terms of the quantum Hall effect.5 Owing to the feasibility of continuously tuning its band gap by applying a transverse electric field, and also by varying the stacking order,6 BLG is more attractive for fundamental studies and potential applications in field effect transistors (FETs), infrared and terahertz light sources, and photo-detectors,7–9 compared to MLG. However, the synthesis of BLG is significantly limited by its small size and low carrier mobility. Hence, it is vital to develop a reliable approach for producing large-area, high-quality BLG films to overcome such limitations. Of the numerous BLG synthetic methods reported so far, chemical vapor deposition (CVD) is considered the most promising, because it allows the growth of high-quality uniform graphene film over a large area.10,11 Hence, much effort has been directed towards the growth of BLG films by CVD. However, such BLG films still suffer from high edge roughness, small size, non-uniform film thickness, etc.12–14 Therefore, extensive efforts have been directed toward developing alternative routes recently, including the use of other new viable substrates and single-crystalline Cu films. Hexagonal-BN (h-BN) is an exceptional dielectric material that not only exhibits the same hexagonal honeycomb crystal structure as graphene, but also has an ultra-flat surface.15-17 Therefore, using h-BN films as substrates for the in-situ growth of BLG is considered promising for producing high-quality BLG films. For example, Wu et al. reported the BLG fabricated on h-BN/Cu foil, which demonstrated that the BLG film can be reach up to ~100 µm, and the carrier mobility was about 772-1060 cm2 V-1 s-1.18 Hence, recently, much effort has been channeled into obtaining large-area, uniform h-BN films using metal catalysts such as Cu or Pt as substrates by 2
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CVD. Here, we employed Pt foil to obtain high-quality, large-area, single layered h-BN as described elsewhere in detail,19 which is subsequently used to grow high-quality large-area BLG film via thermal CVD method. We believe the quality of our single layer h-BN on Pt foil is far better than h-BN on Cu foil. More importantly, the h-BN/Pt foils provide several technical advantages over h-BN/Cu foils as substrates, including (1) Pt foil has higher carbon solubility (Pt foil: ~1.0 atom% at 1,000 °C; Cu foil: ~0.003 atom% at 1,000 °C), (2) Pt foil has ultra-flat surface than Cu foil, (3) recyclable use for successive growth, and (4) more facile transfer of the grown 2D materials onto desirable substrates (Pt foil: 2-5 min; Cu foil: more than 10 h). Thus, we employed the h-BN/Pt foil as a substrate to synthesize high quality large-area BLG.19–24 Despite some reports on the growth of MLG or BLG on Pt foils, there is still no consensus regarding the growth mechanism.19–22 Moreover, the critical aspect of the parameters governing the growth of large-area BLG on h-BN/Pt foils is yet to be unraveled. Therefore, it is vital to investigate a plausible growth mechanism of BLG on h-BN/Pt foils. In this report, we demonstrate a novel approach to obtain large-area uniform BLG films on h-BN/Pt foils via CVD. The film growth was achieved by employing methane as the carbon source and h-BN/Pt foil as the substrate. Optical microscopy and scanning electron microscopy (SEM) observations revealed that continuous BLG films as large as 500 µm could be obtained through this process. The field-effect mobility measurements revealed that the carrier mobility of BLG reaches up to 9,000 cm2 V-1 s-1 at room temperature. These results indicate that our approach is very promising for developing viable electronic and optoelectronic devices.
Results and discussion Optical micrographs and Raman spectroscopic measurements Optical microscopy was employed to investigate the morphological features of the BLG films. Figure 1a shows the optical images of the BLG film transferred onto a 300-nm-thick SiO2/Si substrate. The surface of the BLG is continuous, and its size is 3
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as large as ~500 µm. Raman spectroscopy was used to further investigate the structure, uniformity, and quality of the BLG films. Figure 1b shows a typical Raman spectrum of the BLG film transferred onto a 300-nm-thick SiO2/Si substrate; two typical characteristic peaks are observed (G and 2D) at 1,585 and 2,682 cm-1, respectively. The full-width at half-maximum (FWHM) of the 2D peak is ~51 cm-1 and the 2D to G peak intensity ratio, I2D/IG is ~1, indicating AB-stacked BLG.25–27 Further, the I2D/IG ratio is greater than 1 in some regions, suggesting the formation of twisted BLG. As Raman spectrum of the 2D band reveals a stacking type of BLG film,28 we performed a Raman spectroscopic study to determine the stacking type of the BLG film (Figure S1). The 2D band could be deconvoluted into four Lorentzian components: P11, P12, P21, and P22, located at 2,675, 2,690, 2,720, and 2,738 cm-1, respectively. These correspond to the four permissible photon transition processes in a characteristic BLG film. The FWHM values of the four peaks are 35, 31, 30, and 32 cm-1, respectively. Based on the double resonance theory, these four Lorentzian peaks with FWHM values ranging from 30–35 cm-1 indicate the formation of AB-stacked BLG.26,29,30 The quality of the BLG film over a large area was also verified by Raman mapping of the I2D/IG and estimating the FWHM of the 2D band over an area of 75 µm × 75 µm, and the results are shown in Figures 1c and d. The I2D/IG ratio (Figure 1c) is close to 1, while the FWHM of the 2D peak (Figure 1d) ranges from 48 to 52 cm-1, indicating the ultrahigh uniformity of the grown BLG over an area of 75 µm × 75 µm. For comparison, the Raman spectra of MLG and multilayer graphene are also shown with respect to BLG in Figures S2a and S2b. For a more comprehensive evaluation, we also collected the Raman spectra at 200 different spots from a 1 mm × 1 mm area of the transferred BLG film. From these distributions, we conclude that 80% of the BLG film has AB stacking, while the remaining part has twisted stacking.
Structural characterizations of the BLG film To investigate the structural characteristics of the CVD-grown BLG, field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), 4
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and high-resolution transmission electron microscopy (HRTEM) were employed. Figures S3a and S3b show representative FE-SEM micrographs of the BLG films. The surfaces of the BLG films are continuous over an area as large as ~500 µm, which agrees with the optical images. Note that the size of the BLG films fabricated in this study is 4–5 times larger than those reported elsewhere for CVD-grown BLG films.13,31–33 FE-SEM images of the BLG grown on h-BN/Pt foil and Pt foil for different growth times are presented to reveal the continuous large-area BLGs obtained (Figures S4a–f and S5a–f). Figure 2a shows the AFM images of our CVD-grown BLG samples. The thickness of the film is ~0.9 nm, which is characteristic of BLG.34,35 Figure 2b shows the HRTEM image of our CVD-grown sample. A bilayer structure is evident from the two clear lines marked with arrows, suggesting the distance between two layer is approximately 0.235 nm. Six randomly selected area electron diffraction (SAED) patterns are shown in Figure S6, revealing two hexagonal crystalline structures of graphene with different rotation angles (from 3.6 to 30.5°). This indicates that our BLG film is polycrystalline in nature with twisted stacking.18,36,37 To better understand the distribution of the twisted structure in our BLG, we recorded the Raman spectral I2D/IG ratio over an area of 1 mm × 1 mm (Figure S7). The result reveals that the proportion of the twisted structure of the BLG film is less than 11%. Growth mechanism of the BLG film on the h-BN/Pt foil To unravel the growth mechanism of the BLG film on h-BN/Pt foil, we systematically studied the growth characteristics at different growth times. Figures 3a–f and 3g–l show the optical images of BLG films grown on h-BN/Pt and Pt foils, respectively. As shown in Figures 3 a–f, during the first 20 min of growth, uniform monolayer graphene films are formed on h-BN/Pt foils. When the growth time is increased to 40 min, small patches of the BLG films (10–20 µm) are formed (Figure 3b). At 60 min, the sizes of the BLG films increase to 150–200 µm (Figure 3c). Further increase in the growth time up to 80–120 min, yields BLG films as large as 500 µm (Figures 3d–f). In contrast, Figures 3g–l show the optical images of the BLG 5
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films grown on Pt foils. When the growth time is 20 min, continuous monolayer graphene films are observed in a similar manner. As the growth time is increased to 40 and then to 120 min, BLG films start to appear on the Pt foils (Figures 3h–l). However, the grown BLG films have some defects, unlike those grown on h-BN/Pt foils. For example, when the growth time is 80 min, the surface of the BLG film on Pt foils is found to be discontinuous (Figure 3j). When the growth time is increased to 120 min, numerous particulates are observed on the surface of the BLG film (Figure 3l). Based on the above optical images, the correlation between the size of the BLG film and growth time on h-BN/Pt and Pt foils are presented in Figure 4a. The size of BLG grown on h-BN/Pt foils can reach 500 µm (Figure 4a) after 120 min of growth, which is three times that of the BLG film grown on Pt foils. The Raman spectral I2D/IG ratio and FWHM of the 2D peak of the BLG film grown on h-BN/Pt foil at different growth times are shown in Figures 4b–d. When the growth time is 20 min, the FWHM of the 2D peak is ~ 36 cm-1 and the I2D/IG ratio is ~ 2, suggesting the formation of MLG films.27 As the growth time increases from 20 to 120 min, the FWHM of the 2D peak (from 47 to 52 cm-1) and the I2D/IG ratio (from 0.9 to 1.2) become approximately 50 cm-1 and 1, respectively, indicating the formation of BLG.38,39 The Raman spectra and I2D/IG ratio of BLG grown on Pt foils at different growth times are shown in Figures S9a and S9b, for comparison. Based on these observations, we propose a growth mechanism of BLG on h-BN/Pt and Pt foils, as depicted in Figures 5a and 5b, respectively. When h-BN/Pt foils are used as substrates, BLG can be grown on the h-BN film over the entire Pt foil. h-BN and graphene have similar hexagonal honeycomb structure. Their lattice constants are also similar: 2.504 for h-BN and 2.456 Å for graphene, respectively.40 Therefore, during the first 20 min of growth, continuous graphene films can be easily formed on h-BN/Pt foils (Figure 3a). When the growth time is increased, carbon atoms start to diffuse into the Pt foils, because Pt foils have relatively high carbon solubility. Note that H2 not only acts as a co-catalyst for the dehydrogenation of CH4, but also as an etching agent that erodes the h-BN film during the course of the BLG 6
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growth.32 Furthermore, because Pt foils have a higher catalytic activity than h-BN for graphene growth, with increasing growth time, graphene would form on top of the Pt foil while the monolayer h-BN film would be slowly etched away by the hydrogen atoms flown into the tube to facilitate the graphene growth on top of the Pt foil.18,21,41 Therefore, during the growth process, with continuous erosion of the h-BN film, carbon atoms will diffuse into the interface between the h-BN film and the first layer of graphene to form the second layer of graphene film on the Pt foil. Finally, uniform and large-area BLG films are formed over the Pt foil. To corroborate the speculations made above, we performed X-ray photoelectron spectroscopy (XPS) studies on BLG grown for different times. As shown in Figure S8, when the growth time is below 60 min (Figures S10a–c), the XPS results reveal that h-BN film is not completely etched. However, when the growth time is 80 min (Figure S9d), the XPS results reveal that the h-BN film is completely etched away. This implies that the complete etching of h-BN takes ~80 min. For a longer growth time, the XPS data indicates only carbon atoms (Figures S9e and S9f). Based on these XPS results, we conclude that the h-BN film is etched away by hydrogen flown into the growth tube during the growth of graphene. In contrast, it should also be noted that the BLG films are grown directly on Pt foils, they exhibit some defects, including a non-uniform and discontinuous surface (Figure 3l). This is because Pt foils have relatively high carbon solubility and the surface of the bulk Pt foils has a high nucleation site density, resulting in the random growth of graphene on Pt foils (Figure 5b). Thus, it is very difficult to control the number of graphene layers on Pt foil only. However, compared to Pt foil, h-BN/Pt foil as a substrate provides several advantages. For instance, the carbon atoms can easily diffuse on the h-BN surface because the h-BN film has an ultra-flat surface and the same hexagonal honeycomb crystal structure as graphene; thus, a continuous graphene film can be easily obtained and higher growth rates can be expected. h-BN film not only serves as an underlayer template, but also as a sacrificial layer (i.e., being etched away), allowing the formation of another graphene layer on top of the Pt foil. Therefore, when the 7
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h-BN/Pt foil is used as a substrate, uniform and continuous BLG film can be easily grown on the Pt foil underneath h-BN. Electrical transport characterization To clarify the electrical characteristics of BLG, back-gated (i.e. single-gated) and dual-gated BLG FETs were fabricated (a schematic of the device is shown in Figure S10). These back-gated FETs were fabricated employing the BLG grown on h-BN/Pt foils, BLGs transferred onto h-BN, and BLGs grown on Pt foils. Figure 6a shows that the resistivity of BLG grown on h-BN/Pt foils is lower than those of the other samples, indicating that our approach is very effective in obtaining high-quality BLGs. Figure 6b reveals that the mobility at room temperature can reach up to 9,000, 6,700, and 4,100 cm2 V-1 s-1, respectively, for BLG on h-BN/Pt foil, BLG transferred onto h-BN, and BLG grown on Pt foil. Note that the mobility of BLG grown on h-BN/Pt foils is two times that of BLG grown on Pt foils. The mobility of our BLG film grown on h-BN/Pt foil is much higher than those of CVD-grown BLGs reported elsewhere.18,25,31,33,37 As shown in Figure 6b, the histogram of the carrier mobility of 100 FETs based on BLG grown on h-BN/Pt foils exhibits much higher mobility of up to 9,000 cm2 V-1 s-1, compared to BLGs transferred onto h-BN and grown on Pt foils. We also measured the resistance at the charge neutrality point (RDirac) for the dual-gated FET as a function of the top gate voltage (Vtg). By sweeping Vtg from −10 to 10 V at a fixed Vbg value in the range of −50 to 40 V, the tunability of the bandgap by the electric field was verified, as shown in Figure 6c. It is clear that, with increasing Vbg in both positive and negative directions, the RDirac increases. This indicates that the bandgap of BLG can be modulated by increasing the Vbg.26,34,41 Figure S11a shows the contour plot of the resistance as functions of Vtg and Vbg biases at fixed Vbg values (−40 to 40 V) while Vtg is swept from −10 to 10 V. This result also shows that the lower resistance is observed on the bottom-left and top-right region, indicating that BLG has a tunable bandgap. Figure S11b shows the top-gate Dirac point (Vtg-Dirac) vs. Vbg extracted from Figure S11a. As shown, Vtg-Dirac linearly increases with Vbg. To determine the transfer characteristics of the dual-gated devices, 8
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we examined Ids vs. Vtg characteristics (Figure 6d). It shows a gate voltage swing, demonstrating current modulation in the dual-gate configuration.41 Figure 6e shows the transconductance (gm = dIds/dVtg) of the dual-gated device derived from Figure 6d. The dual-gated device has a maximum gm of up to ~1.45 mS. This value is ~5 times higher than that of the back-gated device (See Figure S12). Our results clearly indicate that the BLG film grown on h-BN/Pt foil using CVD might be a feasible alternative for realizing high-performance graphene-based FETs.
Conclusion Our work presents a new synthesis method for growing BLG on Pt foils by CVD. Our results demonstrate that the h-BN/Pt foil as a substrate is not only an ideal platform for producing high-quality large-area BLG films, but also leads to significant improvement in the electrical characteristics of the BLG film. Optical micrographs and SEM revealed that continuous and homogeneous BLG films, as large as 500 µm in size could be obtained, thus demonstrating the great potential for scale-up of the process. Moreover, the field-effect mobility measurements showed that the carrier mobility of these BLG films reached up to 9,000 cm2 V-1 s-1 at room temperature. This suggests that the high-quality BLG grown on reusable Pt foils by CVD can be a promising approach to develop high-performance graphene-based electronics. We also proposed a plausible growth mechanism of BLG on h-BN/Pt foils. We believe that the proposed mechanism will contribute to a better understanding of the fundamental BLG physics and device applications of BLG. These superior results demonstrate that our BLG is an excellent candidate for a wide range of applications, including transistor (field effect transistor and quantum tunneling transistor), light emitting diodes (LED), electronics, photonics, as well as energy storage devices etc.
Materials and Methods CVD growth of h-BN films on Pt foils. The growth of h-BN was achieved by placing Pt foil as a substrate in the middle of a one-zone tube furnace, and introducing 9
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methane as the carbon precursor. A monolayer h-BN was grown on the Pt foil purchased from Good Fellow Cambridge Ltd., UK via CVD, following our previously developed synthetic method reported elsewhere.21,22 The as-obtained monolayer h-BN/Pt foil was then employed for the subsequent growth of BLG. Optical images, Raman spectra, and atomic force micrographs of the monolayer h-BN film are shown in Figure S13. CVD growth of a BLG film on h-BN/Pt foil. The typical growth procedure is as follows: The h-BN/Pt foil was first placed at the center of the one-zone tube furnace. The furnace was heated up from room temperature to 1,000 °C in 30 min under an Ar gas flow of 200 sccm. After that, Ar gas flow was turned off and H2 and CH4 gases at 50 and 20 sccm, respectively, were allowed to flow for the film growth. The optimal growth time was determined to be 120 min. After the completion of the growth, H2 and CH4 gases were turned off, and the furnace was cooled naturally to room temperature. The BLG films were grown directly on Pt foils, following the same procedure, to serve as a control. Bubbling method for the transfer of BLG. Our bubbling method has been proven to be very effective in transferring high-quality, contamination-free 2D films onto any target substrate, as described elsewhere.21,22 The details of the involved bubbling transfer method are as the follows: First, the BLG/Pt foil was spin-coated with polyvinyl alcohol (PVA) and then with poly methyl methacrylate (PMMA) layers. The sample was subsequently placed under vacuum for 24 h. After one-day of vacuum
storage,
PMMA/PVA/BLG
the film
bubbling onto
method
was
300-nm-thick
employed Si/SiO2
to
transfer
substrates.
the The
PMMA/PVA/BLG/Pt foils and a high-quality Pt foil were used as the cathode and anode, respectively. A 1.0 M NaOH aqueous solution at room temperature was used as the electrolyte. The bubbling transfer method was performed at a stable current of 1 A (corresponding to the electrolytic voltage of 5–10 V) for 3–8 min. After that, the PMMA/PVA/BLG film was peeled off from the Pt foil and washed thrice with deionized water to remove residual NaOH. It was then transferred onto a 10
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300-nm-thick Si/SiO2 substrate. Finally, the sample was dipped in acetone for 10 min to remove PMMA and then in hot water (100 °C) for 15 min to remove PVA. Characterization. Optical micrographs were acquired by optical microscopy (Olympus, Olympus DX51). The surface morphology was determined using a FE-SEM (JEOL JSM7401F) and HRTEM (JEOL 2100F, 200 kV). The film thickness and the surface topography were determined by AFM (Veeco, Dimension 3100). Raman spectra were obtained by micro Raman microscopy (Renishaw, InVia Basic) with a 532 nm wavelength laser. XPS (Thermo Scientific, ESCALAB 250Xi) was performed with an Mg Kα X-ray source. Fabrication of back-gate and dual-gate FETs BLGs were transferred onto Si substrates using our previously developed PMMA/PVA method described elsewhere.22 A heavily B-doped p-type Si substrate (sheet resistance of 0.005 Ω cm) was employed as the back gate with a thermally oxidized 100-nm-thick SiO2 top layer as the gate oxide layer. Multiple electrodes were patterned on the BLG film by a conventional photolithography process. Subsequently, contact electrodes were deposited using a home-made electron-beam evaporator to form Ohmic contacts. A 20-nm-thick Cr layer was first evaporated, followed by a 50-nm-thick Au layer (Cr/Au: 20/50 nm). The back-gate bias was applied using a Si back-gate, with SiO2 as the dielectric layer. For dual-gate field effect transistors (FETs), 50-nm-thick Al2O3 was deposited as the top-gate dielectric layer through a thermal atomic layer deposition system, using trimethyl aluminum as the Al source and water as the oxidizer, at 200 °C. Top-gate electrodes were patterned, and metal films were then deposited (Cr/Au: (20/50) nm) using a home-made electron beam deposition system. These devices were finally annealed at 200 °C for 150 min with H2/Ar ((50/50) sccm). Electrical characterizations of back-gate FETs Field-effect mobility (µ) was determined to evaluate the electrical characteristics of the back-gated FETs in vacuum using a probe station with a Keithley SCS-4200 11
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system. To extract the µ of BLG, the total resistance of the device (Rtotal) was estimated using the following equation:30,39 Rtotal = 2 R contact + R channel = 2 R contact +
L , W ne µ
(1)
where, Rcontact is the contact resistance between the Au drain and source electrodes and BLG, Rchannel is the resistance of the BLG channel, L is the channel length, W is the channel width, e is the electron charge, µ is the carrier mobility, and n is the carrier concentration in the BLG channel region, which can be approximated with the following equation:
n = n02 + nbg2 = n02 +[Cbg (Vbg -VDirac )/ e]2 , where,
(2)
n0 is the residual carrier concentration representing the density of carriers at
the Dirac point, nbg = Cbg × (Vbg − VDirac ) / e is the carrier concentration induced by the back-gate bias away from the Dirac point; and Cbg is the back-gate capacitance ( ε0ε SiO2 dSiO2 ≈ 11.5 nF/cm2).40 To calculate the carrier mobility, we define the following: −
1 2
y = a + b × (1+ c × x) , where
y = Rtotal , a = 2Rcontact , b =
(3)
L n2 × e2 2 , c = 0 2 , and x = (Vbg − VDirac ) to fit WCbg µ C bg
the measured data. Thus, the carrier mobility can be calculated by the following equation:
µ=
L . W × b × Cbg
12
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(4)
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Supporting Information: See the Supplement for additional detail of Raman, Optical images, SEM, TEM, XPS, and device structure. Author Information Corresponding Author: E-mail:
[email protected] ORCID Yongteng Qian: 0000-0002-4738-1598 Dae Joon Kang: 0000-0002-4030-4071 Note The authors declare they have no competing financial interest.
Acknowledgements This work was supported by the grants (2017R1D1A1B03034847) of the National Research Foundation of Korea funded by the Korean government. Y. Qian wishes to thank
the
financial
support
through
China
Scholarship
201808260016).
13
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(CSC,
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References (1) Novoselov, K. S.; Fal'ko, V. I.; Colombo, L.; Gellert, P. R.; Schwab, M. G.; Kim, K. A Roadmap for Graphene. Nature 2012, 490, 192-200. (2) Withers, F.; Del Pozo-Zamudio, O.; Mishchenko, A.; Rooney, A. P.; Gholinia, A.; Watanabe, K.; Taniguchi, T.; Haigh, S. J.; Geim, A. K.; Tartakovskii, A. I.; Novoselov, K. S. Light-Emitting Diodes by Band-Structure Engineering in Van der Waals Heterostructures. Nat. Mater. 2015, 14, 301-306. (3) Wang, F.; Peng, L.; Feng, Y.; Li, Y. Two-Dimensional Fluorinated Graphene: Synthesis, Structures, Properties and Applications. Adv. Sci. 2016, 3, 1500413. (4) Chen, J. H.; Jang, C.; Xiao, S. D.; Ishigami, M.; Fuhrer, M. S. Intrinsic and Extrinsic Performance Limits of Graphene Devices on SiO2. Nat. Nanotechnol. 2008, 3, 206-209. (5) Nonoselov, K. S.; Mc Cann, E.; Morozov, S. V.; Fal’ko, V. I.; Katsnelson, M. I.; Zeitler, U.; Jiang, D.; Schedin, F.; Geim, A. K. Unconventional quantum Hall effect and Berry’s phase of 2π in bilayer graphene. Nat. Phys. 2006, 2, 177-180. (6) Zhang, Y.; Tang, T. T.; Girit, C.; Hao, Z.; Martin, M. C.; Zettl, A.; Crommie, M. F.; Shen, Y. R.; Wang, F. Direct Observation of A Widely Tunable Bandgap in Bilayer Graphene. Nature 2009, 459, 820-823. (7) Xia, F.; Farmer, D. B.; Lin, Y. M.; Avouris, P. Graphene Field-Effect Transistors with High on/off Current Ratio and Large Transport Band Gap at room Temperature. Nano Lett. 2010, 10, 715-718. (8) Ju, L.; Shi, Z.; Nair, N.; Lv, Y.; Jin, C.; Velasco, J., Jr.; Ojeda-Aristizabal, C.; Bechtel, H. A.; Martin, M. C.; Zettl, A.; Analytis, J.; Wang, F. Topological Valley Transport at Bilayer Graphene Domain Walls. Nature 2015, 520, 650-655. (9) Ci, L.; Song, L.; Jin, C.; Jariwala, D.; Wu, D.; Li, Y.; Srivastava, A.; Wang, Z. F.; Storr, K.; Balicas, L.; Liu, F.; Ajayan, P. M. Atomic Layers of Hybridized Boron Nitride and Graphene Domains. Nat. Mater. 2010, 9, 430-435. (10) Bointon, T. H.; Barnes, M. D.; Russo, S.; Craciun, M. F. High Quality Monolayer Graphene Synthesized by Resistive Heating Cold Wall Chemical Vapor Deposition. Adv. Mater. 2015, 27, 4200-4206. (11) Li, Q.; Chou, H.; Zhong, J. H.; Liu, J. Y.; Dolocan, A.; Zhang, J.; Zhou, Y.; Ruoff, R. S.; Chen, S.; Cai, W. Growth of Adlayer Graphene on Cu Studied by Carbon Isotope Labeling. Nano Lett. 2013, 13, 486-490. (12) Fang, W.; Hsu, A.; Song, Y.; Birdwell, A.; Amani, M.; Dubey, M.; Dresselhaus, M.; Palacios, T.; Kong, J. Asymmetric Growth of Bilayer Graphene on Copper Enclosures using Low-Pressure Chemical Vapor Deposition. ACS Nano, 2014, 8, 6491-6499. (13) Yan, Z.; Liu, Y.; Ju, L.; Peng, Z.; Lin, J.; Wang, G.; Zhou, H.; Xiang, C.; Samuel, E. L.; Kittrell, C.; Artyukhov, V. I.; Wang, F.; Yakobson, B. I.; Tour, J. M. Large Hexagonal Bi- and 14
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Trilayer Graphene Single Crystals with Varied Interlayer Rotations. Angewandte Chemie. 2014, 53, 1565-1569. (14) Kim, S. M.; Hsu, A.; Park, M. H.; Chae, S. H.; Yun, S. J.; Lee, J. S.; Cho, D. H.; Fang, W.; Lee, C.; Palacios, T.; Dresselhaus, M.; Kim, K. K.; Lee, Y. H.; Kong, J. Synthesis of Large-Area Multilayer Hexagonal Boron Nitride for High Material Performance. Nat. Commun. 2015, 6, 8662. (15) Qian, Y.; Van Ngoc, H.; Kang, D. J., Growth of Graphene/h-BN Heterostructures on Recyclable Pt Foils by One-Batch Chemical Vapor Deposition. Sci. Rep. 2017, 7, 17083. (16) Li, X.; Lu, X.; Li, T.; Yang, W.; Fang, J.; Zhang, G.; Wu, Y. Noise in Graphene Superlattices Grown on Hexagonal Boron Nitride. ACS Nano 2015, 9, 11382-11388. (17) Babenko, V.; Murdock, A. T.; Koos, A. A.; Britton, J.; Crossley, A.; Holdway, P.; Moffat, J.; Huang, J.; Alexander-Webber, J. A.; Nicholas, R. J.; Grobert, N. Rapid Epitaxy-Free Graphene Synthesis on Silicidated Polycrystalline Platinum. Nat. Commun. 2015, 6, 7536. (18) Wu, Q.; Jung, S. J.; Jang, S. K.; Lee, J.; Jeon, I.; Suh, H.; Kim, Y. H.; Lee, Y. H.; Lee, S.; Song, Y. J. Controllable Poly-Crystalline Bilayered and Multilayered Graphene Film Growth by Reciprocal Chemical Vapor Deposition. Nanoscale. 2015, 7, 10357-10361. (19) Kim, G.; Jang, A. R.; Jeong, H. Y.; Lee, Z.; Kang, D. J.; Shin, H. S. Growth of High-Crystalline, Single-Layer Hexagonal Boron Nitride on Recyclable Platinum Foil. Nano Lett. 2013, 13, 1834-1839. (20) Gao, L.; Ren, W.; Xu, H.; Jin, L.; Wang, Z.; Ma, T.; Ma, L. P.; Zhang, Z.; Fu, Q.; Peng, L. M.; Bao, X.; Cheng, H. M. Repeated Growth and Bubbling Transfer of Graphene with Millimetre-Size Single-Crystal Grains using Platinum. Nat. Commun. 2012, 3, 699. (21) Gao, J. H.; Sagisaka, K.; Kitahara, M.; Xu, M. S.; Miyamoto, S.; Fujita, D. Graphene Growth on A Pt(111) Substrate by Surface Segregation and Precipitation. Nanotechnology. 2012, 23, 055704. (22) Gao, T., Xie, S., Gao, Y., Liu, M., Chen, Y., Zhang, Y., Liu, Z. Growth and Atomic-Scale Characterizations of Graphene on Multifaceted Textured Pt Foils Prepared by Chemical Vapor Deposition. ACS Nano 2011, 5, 9194-9201. (23) Karamat, S.; Sonuşen, S.; Çelik, Ü.; Uysallı, Y.; Özgönül, E.; Oral, A. Synthesis of Few Layer Single Crystal Graphene Grains on Platinum by Chemical Vapour Deposition. PNS: MI. 2015, 25, 291-299. (24) Van Ngoc, H.; Qian, Y.; Han, S. K.; Kang, D. J. “PMMA-Etching-Free Transfer of Wafer-scale Chemical Vapor Deposition Two-dimensional Atomic Crystal by a Water Soluble Polyvinyl Alcohol Polymer Method”. Sci. Rep. 2016, 6, 33096. (25) Yan, K.; Peng, H.; Zhou, Y.; Li, H.; Liu, Z. Formation of Bilayer Bernal Graphene: Layer-by-Layer Epitaxy via Chemical Vapor Deposition. Nano Lett. 2011, 11, 1106-1110. (26) Malard, L. M.; Pimenta, M. A.; Dresselhaus, G.; Dresselhaus, M. S. Raman Spectroscopy in Graphene. Phys. Rep. 2009, 473, 51-87 15
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(40) Lee, K. H.; Shin, H. J.; Lee, J.; Lee, I. Y.; Kim, G. H.; Choi, J. Y.; Kim, S. W. Large-Scale Synthesis of High-Quality Hexagonal Boron Nitride Nanosheets for Large-Area Graphene Electronics. Nano Lett. 2012, 12, 714-718. (41) Liao, L.; Bai, J.; Qu, Y.; Lin, Y. C.; Li, Y.; Huang, Y.; Duan, X. High-k Oxide Nanoribbons as Gate Dielectrics for High Mobility Top-Gated Graphene Transistors. Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 6711-6715.
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Figure 1. Optical images and Raman spectra of BLG transferred onto a 300-nm-thick SiO2/Si substrate. (a) Optical images of the BLG film. (b) Raman spectrum of BLG. (c) Raman mapping of I2D/IG. (d) Raman mapping of FWHM of the 2D peak. (e) Raman mapping of ID/IG.
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Figure 2. Structural characterization of the BLG film. (a) AFM image of the BLG film transferred onto a 300-nm-thick SiO2/Si substrate, and (b) HRTEM image of the CVD-grown sample showing a bilayer structure.
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Figure 3. Optical images of BLG transferred onto a 300-nm-thick Si/SiO2 substrate. (a)–(f) Optical images of BLG grown on a h-BN/Pt foil with growth times of 20, 40, 60, 80, 100, and 120 min, respectively. (g)–(l) Optical images of BLG grown on a Pt foil with growth times of 20, 40, 60, 80, 100, and 120 min, respectively. The scale bar is 50 µm.
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Figure 4. (a) Relationship between the size of the BLG film and the growth time on different substrates. (b) Raman spectra of the BLG shown in Figures 3 (a)–(f). (c) I2D/IG ratio of BLG grown on the h-BN/Pt foil with different growth times. (d) FWHM of the 2D peak of BLG grown on the h-BN/Pt foil with different growth times.
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Figure 5. A schematic representation of a plausible growth mechanism: (a) BLG film growth on the h-BN/Pt foil and (b) BLG film growth on Pt foil.
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Figure 6. Electrical transport properties of BLG. (a) The longitudinal resistivity against the applied gate voltage Vg, for BLG grown on h-BN/Pt foil, transferred onto h-BN, and grown on a Pt foil. (b) Histogram of the carrier mobility distribution of BLG grown on the h-BN/Pt foil, BLG transferred onto h-BN, and BLG grown on the Pt foil in FETs. (c) Curves of R as a function of Vtg at fixed Vbg ranging from −50 to 40 V with a 10 V step. (d) Transfer characteristics at Vds = 2 V for the dual-gated device. (e) gm as a function of Vtg.
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