Defect-Free Graphene Synthesized Directly at 150 °C via Chemical

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Defect-Free Graphene Synthesized Directly at 150 °C via Chemical Vapor Deposition with No Transfer Byeong-Ju Park,† Jin-Seok Choi,†,‡ Ji-Ho Eom,† Hyunwoo Ha,† Hyun You Kim,† Seonhee Lee,§ Hyunjung Shin,§ and Soon-Gil Yoon*,† †

Department of Materials Science and Engineering, Chungnam National University, Daeduk Science Town, 34134 Daejeon, South Korea ‡ Analysis Center for Research Advancement (KARA), Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, 34141 Daejeon, South Korea § Department of Energy Science, Sungkyunkwan University, 300 CheonCheon-dong, Jangan-gu, 16419 Suwon, Gyeonggi-do, South Korea S Supporting Information *

ABSTRACT: Direct graphene synthesis on substrates via chemical vapor deposition (CVD) is an attractive approach for manufacturing flexible electronic devices. The temperature for graphene synthesis must be below ∼200 °C to prevent substrate deformation while fabricating flexible devices on plastic substrates. Herein, we report a process whereby defect-free graphene is directly synthesized on a variety of substrates via the introduction of an ultrathin Ti catalytic layer, due to the strong affinity of Ti to carbon. Ti with a thickness of 10 nm was naturally oxidized by exposure to air before and after the graphene synthesis, and the various functions of neither the substrates nor the graphene were influenced. This report offers experimental evidence of high-quality graphene synthesis on Ti-coated substrates at 150 °C via CVD. The proposed methodology was applied to the fabrication of flexible and transparent thinfilm capacitors with top electrodes of high-quality graphene. KEYWORDS: direct graphene synthesis via CVD, Ti buffer layer, flexible substrate, synthesis temperature of 150 °C, defect-free monolayer graphene

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molten gallium with graphene nuclei still required transferral to a polycarbonate substrate to accomplish low-temperature graphene synthesis. For practical applications, a system that would allow the reliable synthesis of graphene directly onto insulating substrates is highly desirable. Recently, many studies have explored catalyst-free synthesis of graphene on various substrates.9−13 To this point, however, directly synthesized graphene has featured a localized crystalline morphology with high-defect density, which results in low conductivity and diminished optical performance. Additionally, synthesis temperature has remained above 800 °C because the graphene synthesis is achieved via a surface-limited process, which is not applicable to flexible electronic device applications.

raphene synthesis via chemical vapor deposition (CVD) allows the production of large-area graphene on a variety of metal films such as micrometer-thick Cu, Ni, Ga, and Ge catalytic substrates.1−6 However, large-scale graphene synthesis requires processing temperatures as high as 1000 °C or more, which requires a transfer process. Therefore, a reduction in the graphene synthesis temperature remains a critical challenge for application to electronic devices. To date, ∼300 °C is the lowest temperature that has been recorded during graphene synthesis via oxygen-free atmospheric pressure CVD (APCVD) using benzene on 25 μm thick copper foil, as reported by Jang et al.7 Graphene synthesized on copper foil must be transferred to functional substrates when synthesized at temperatures as low as ∼300 °C. Recently, Fujita et al. reported a reduction of 50 °C during graphene CVD on sapphire and 100 °C on a polycarbonate via dilute methane as the source with molten gallium as a catalyst.8 In this method, however, molten gallium for the graphene nuclei was prepared on a sapphire substrate at 1050 °C for 300 s, which meant © 2018 American Chemical Society

Received: January 2, 2018 Accepted: February 1, 2018 Published: February 1, 2018 2008

DOI: 10.1021/acsnano.8b00015 ACS Nano 2018, 12, 2008−2016

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Figure 1. Thermal stability of TiO2 layer oxidized naturally at air using in situ annealed Ti under ambient hydrogen. (a) XPS Ti 2p core level in TiO2 showing an in situ-annealed Ti was naturally oxidized after exposure to air. (b) Surface roughness of the 10 nm thick-TiO2/glass and TiO2/PET annealed for 2 h at different temperatures under hydrogen. (c) AFM height profiles measured along the lateral distance indicated by a white solid line in the AFM image of the TiO2/glass sample annealed at 150 and 400 °C (inset). (d) Transmittance vs wavelength of the annealed TiO2/glass samples; the inset shows the transmittance of the TiO2/PET samples measured at 550 nm (black, bare PET; blue, asdeposited TiO2 (10 nm)/PET; red, TiO2 (10 nm)/PET annealed at 150 °C). (e) Sheet resistance of bare glass, as-deposited TiO2/glass, and TiO2/glass annealed for 2 h under hydrogen at 400 °C.

graphene synthesis on Ti-coated substrates at 150 °C and investigated the defect-free properties of the resultant graphene. Third, we examined how the Ti underneath a monolayer graphene was changed to TiO2 after exposure to air, whereupon the various functions of graphene were not influenced. Finally, monolayer graphene films were assessed for their utility as the top electrode in transparent and flexible thin-film capacitors.

Therefore, a direct synthesis of graphene with no transfer at temperature below 200 °C is required for application to flexible electronic devices. As far as we could ascertain, a method for direct defect-free graphene synthesis on a flexible substrate such as polyethylene terephthalate (PET) at low temperature via chemical vapor deposition has not been reported. Our method was inspired by the frequent use of Ti in dental implants, which is a process that takes advantage of the close affinity of Ti to the carbon in teeth. Herein, we report a micrometer-scale graphene synthesis via CVD at approximately 150 °C using two separate Ti (∼10 nm)-coated substrates: eagle glass (700 μm) and polyethylene terephthalate (130 μm). First, we annealed a 10 nm thick Ti layer at high temperature under hydrogen without breaking the vacuum, after which the deposited Ti was changed to TiO2 following exposure to air. The resultant TiO2 layer exerted no influence on the functions of the substrates. Second, we verified

RESULTS AND DISCUSSION Thermally Stable, Nonconductive, and Optically Transparent TiO2 Buffer Layer. Before investigating direct CVD graphene synthesis on Ti-coated substrates, we examined the properties of ultrathin Ti deposited onto glass and PET substrates under the same conditions as those used for the CVD graphene synthesis (hydrogen atmosphere and temperatures as high as 150 and 400 °C for Ti/PET and Ti/glass, respectively, which revealed the most important parameters in 2009

DOI: 10.1021/acsnano.8b00015 ACS Nano 2018, 12, 2008−2016

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Figure 2. Defect-free graphene synthesized at 150 °C. (a) Raman spectra of graphene synthesized at 150 °C on a 10 nm thick Ti layer reduced for different reduction times under hydrogen at 150 °C. (b,c) I2D/IG and ID/IG intensity ratio and fwhm of 2D- and G-bands of graphene synthesized at 150 °C on Ti reduced for different reduction times under hydrogen at 150 °C. (d) Area of graphene synthesized at 150 °C on Ti reduced for different reduction times under hydrogen at 150 °C. (e,f) Micro-Raman mapping images of the ID/IG and I2D/IG measured in a 20 × 20 μm2 area of graphene synthesized under optimized conditions at 150 °C.

graphene CVD). Ti deposited at room temperature via direct current (dc) sputtering was in situ annealed at different temperatures under hydrogen without breaking the vacuum. In situ-annealed Ti was naturally oxidized by exposure to air, which changed it to TiO2, as illustrated by the XPS Ti 2p core level of a layer etched for 300 s on the surface shown in Figure 1a. Therefore, for the remainder of this paper, Ti-coated substrates are referred to as either TiO2/PET or TiO2/glass after exposure to air before and after graphene synthesis. A 10 nm thick Ti layer deposited via dc sputtering was chosen in this study because it had a smooth morphology and exerted no influence on the transmittance of the substrates (Figure S1).14 To ensure the thermal stability of the Ti layer during graphene synthesis via CVD, we examined the surface roughness, transmittance, and electrical conductivity of a 10 nm thick layer of TiO2 measured under an air atmosphere following the in situ annealment of Ti under hydrogen at 150 and 400 °C (Figure 1b−e). The root-mean-square (rms) roughness revealed no significant changes compared with Ti-coated substrates that had not been annealed (Figure 1b). As shown in Figure 1c, the step-height along a lateral distance of up to 1 μm (inset atomic force microscopy (AFM) image with a white

solid line) showed no noticeable changes compared with that of an as-deposited sample, which indicated an excellent level of thermal stability under these conditions. Regarding transmittance (see the Methods section), the as-deposited samples annealed at 150 and 400 °C for 2 h showed a minute decrease in transmittance (0.4 ± 0.05% at a wavelength of 550 nm) compared with that of a bare glass substrate (Figure 1d). The inset in Figure 1d shows the transmittance (at wavelength of 550 nm) of bare PET, as-deposited, and annealed at 150 °C. No sample showed a change in transmittance. Furthermore, the TiO2/glass displayed a sheet resistance similar to that of glass, and no resistance changes were observed after annealment at 400 °C (Figure 1e). The 10 nm thick TiO2 layer influenced neither the electrical conductivity nor the optical transmittance properties of the substrates, and it showed a predominant thermal stability under annealment under a hydrogen atmosphere at temperatures as high as 400 °C. This experimental observation suggests that Ti could be directly utilized for graphene synthesis in microelectronic devices without the need for additional transfer processes. 2010

DOI: 10.1021/acsnano.8b00015 ACS Nano 2018, 12, 2008−2016

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Table 1. Parameters (I2D/IG, ID/IG, and Full Width at Half-Maxiumum of the 2D- and G-Bands) of Graphene Synthesized under the Most Important Synthesis Parameters of Hydrogen Reduction Time and Working Pressure at 150 °C synthesis parameters

synthesis conditions

H2 reduction time

1h 2h 3h 4h 0.6 Pa 1.3 Pa 6.6 Pa

working pressure

I2D/IG 1.81 1.84 1.92 2.10 2.10 1.70 1.72

± ± ± ± ± ± ±

ID/IG

0.10 0.10 0.06 0.08 0.08 0.15 0.10

0.04 0.03 0.02 0.02 0.02 0.02 0.02

± ± ± ± ± ± ±

0.01 0.01 0.01 0.01 0.01 0.01 0.01

fwhm of 2D-bands 43 37 38 37 37 38 39

± ± ± ± ± ± ±

4 2 1 1 1 3 3

fwhm of G-bands 23 22 22 21 21 23 23

± ± ± ± ± ± ±

3 2 1 2 2 1 2

Figure 3. High-quality graphene synthesized at 150 °C. (a) Transmittance of graphene/TiO2/glass, in which bare TiO2 (10 nm)/glass was used as a reference. To measure the transmittance of only graphene, all parts except a 20 × 20 μm2 graphene area were covered with opaque tape (inset of a). (b) Sheet resistance of monolayer graphene with applied frequency via the Z-theta method. (c,d) AFM image showing graphene synthesized on Ti and the lack of graphene (TiO2 areas) and the height profile of graphene along the blue line (A-B) in c, respectively. Here, ZA and ZB (vertical red line) exhibit the step-height of the no graphene and graphene, respectively, shown in dashed area (c). (e) ADF TEM bright-field image of monolayer graphene. (f) HRTEM image and (g) Fourier transformation of the HRTEM image. The inset in f shows the clear atomic pattern of the small region inside the small yellow circle. (h) XPS Ti 2p core level in TiO2, in which Ti underneath the graphene was naturally oxidized after exposure to air. 2011

DOI: 10.1021/acsnano.8b00015 ACS Nano 2018, 12, 2008−2016

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terminated germanium at 900−930 °C (ID/IG < 0.03).6 The fwhm values of the 2D- and G-bands for all the graphene films synthesized at 150 °C were 37 ± 1 cm−1 and 21 ± 2, respectively, which is comparable to (fwhm values of the 2Dbands; 37 cm−1) graphene synthesized at 600 °C on Cu foil via plasma-assisted thermal CVD.16 The area of monolayer graphene increased abruptly at reduction times above 2 h and was approximately 700 μm2 (square) at 4 h (Figure 2d). This result suggested that large-scale Ti shows promise for largescale monolayer graphene synthesis. Therefore, Ti is considered a strong candidate for direct large-scale graphene synthesis at low temperature on a flexible substrate. The Raman spectra and quality of the graphene synthesized at 150 °C were addressed using CH4/H2 gas (Figures S5 and S6). The highest-quality graphene was synthesized under optimal conditions of H2 reduction time (4 h), working pressure (0.6 Pa), and CH4/ H2 gas ratio (1/10) at 150 °C. Wrinkle-free graphene was confirmed via AFM imaging (Figure S7). The parameters (I2D/ IG, ID/IG, and fwhm of the 2D- and G-bands) that exhibited graphene quality are summarized in Table 1 for synthesized graphene, showing the most important parameters of both hydrogen reduction time and working pressure at 150 °C. Micro-Raman mapping was performed to verify high quality in certain areas of the graphene. The micro-Raman mapping images were measured using a confocal micro-Raman system with a spatial resolution of 500 nm. The mapping images of ID/ IG and I2D/IG measured in a 20 × 20 μm2 area of graphene synthesized under optimized conditions are shown in Figure 2e,f, respectively. The histograms of ID/IG and I2D/IG are shown in Figure S8a,b, respectively. Based on the Raman mapping images, the average intensity ratios of ID/IG and I2D/IG were approximately 0.01 and 2.1, respectively, which clearly demonstrated a defect-free monolayer of graphene. To determine whether the graphene synthesized at 150 °C was optically transparent, transmittance (27 × 27 μm2 area; inset figure of Figure 3a) was investigated with a UV−vis spectrometer. The graphene/TiO2/glass monolayer displayed high transmittance in the visible region (97.5% at a wavelength of 550 nm) (Figure 3a) when bare TiO2 (10 nm)/glass was used as a reference, which agreed with previously reported values for a graphene monolayer.17,18 Next, the sheet resistance of graphene synthesized at 150 °C was measured as a function of frequency via the impedance-phase angle (Z-theta) method (Figure 3b).19,20 Consistent sheet resistances were returned by both a four-point probe and the Z-theta method with transferred graphene and indium−tin-oxide (ITO) films, which confirmed the reliability of the Z-theta method for measuring directly synthesized graphene (Figure S9). The sheet resistance of graphene synthesized at 150 °C was 618 ± 15 Ω/ □, which is comparable to that of Cu-catalyzed graphene.1,21 The carrier concentration, mobility, and resistivity of the graphene were measured using four different samples to ensure reliable results via the van der Pauw four probe method at room temperature: (1.89 ± 0.16) × 1012 cm−2, (5.32 ± 0.59) × 103 cm2 V−1 s−1, and (2.47 ± 0.06) × 10−5 Ω·cm, respectively. The sheet resistance (thickness is ∼0.4 nm) was calculated from the resistivity results observed via the van der Pauw four probes and was consistent with that of graphene measured via the Ztheta method (Figure 3b). Defect-free high-quality graphene was directly synthesized on Ti-catalyzed flexible substrates under optimal conditions for CVD at 150 °C, which confirmed direct applicability to flexible electronic devices.

Optimization of Micrometer-Scale Graphene Synthesis at 150 °C via thermal CVD on Ti-Buffered Substrates. In this study, we demonstrated the feasibility of direct graphene synthesis at low temperatures via CVD on Ti with no transfer. In our synthesis process, once the Ti-coated substrates were removed from the sputtering chamber, they were then transferred to the CVD chamber for graphene synthesis (see the Methods section). During the transfer of the Ti-coated substrate between the two chambers, the Ti was oxidized naturally to TiO2 via exposure to air. The reduction process (Figure S2a) and graphene synthesis on reduced Ti (Figure S2b) are described in detail in the Methods section. To investigate the synthesis mechanisms of the directly synthesized graphene on the titanium buffer layer, we used density functional theory (DFT) calculations to study the influence that the naturally formed TiO2 layer exerted on the energetics of the formation of graphene. The calculated binding energy of a carbon atom on a Ti(0001) layer suggests that the strong chemical interaction between them stabilizes the carbon sources on a Ti-coated substrate. The formation energy, Eform, of a monolayer of graphene on a Ti(0001) lattice from separately adsorbed carbon atoms was negative (−0.06 eV/C atom, −2.22 eV/nm2), confirming that the formation is significantly thermodynamically driven (Figure S3a). Considering the local geometry of a monolayer of graphene formed on Ti(0001), we believe that a large portion of this negative Eform can be attributed to the formation of a well-confined carbon nucleus on Ti(0001) (yellow highlighted nuclei, Figure S3a). For subsequent carbon atom diffusion along the titanium sites of the Ti(0001) surface for monolayer graphene synthesis, we calculated the low-diffusion energy barriers of 0.26 eV, demonstrating the synthesis of monolayer graphene. It is noteworthy that in the presence of the oxidized surface of Ti atoms, a carbon atom continued to preferentially bind to the Ti rather than to the oxidized Ti atoms. The calculated Eform of a monolayer of graphene on the partially oxidized Ti(0001) lattice was highly positive, (Eform = 0.22 eV/C atom, 8.54 eV/nm2), which showed that the formation of a monolayer of graphene is thermodynamically unfavorable in the presence of TiO2 (Figure S3b). Based on our DFT calculation results, we experimentally studied the correlation between the degree of the hydrogen reduction of naturally formed TiO2 (under hydrogen (5.3 × 10−3 Pa) at 150 °C in the CVD chamber) and the area of the graphene layer. The direct graphene synthesis at 150 °C on a 10 nm thick TiO2 layer was not confirmed via various experiments using different synthesized parameters (Figure S4), which clearly supported the theoretical expectations (Figure S3b). The reduction process was performed for 1−4 h under hydrogen at 150 °C using TiO2 (10 nm)/PET substrates. The Raman spectra of the graphene synthesized at 150 °C on Ticoated PET that was reduced for different times at 150 °C revealed a defect-free monolayer of graphene (Figure 2a). The I2D/IG and ID/IG intensity ratios and the fwhm of the 2D- and G-bands were improved with increases in the reduction time (Figure 2b,c, respectively). The I2D/IG and ID/IG intensity ratios observed with a reduction time of 4 h were 2.1 ± 0.08 and 0.02 ± 0.01, respectively, which indicated that the quality of these monolayers was superior to that of the graphene synthesized directly on SiO2 substrates via CVD at 800 °C, which had a relatively high-defect density (ID/IG of 0.3).15 The ID/IG ratio of our defect-free graphene was comparable to that of defectfree single-crystalline graphene synthesized on hydrogen2012

DOI: 10.1021/acsnano.8b00015 ACS Nano 2018, 12, 2008−2016

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Figure 4. Flexible and transparent thin-film capacitors using a graphene top-electrode. (a) Schematic diagram of BMNO thin-film capacitor with graphene top electrodes. (b) Raman spectrum of graphene synthesized on 200 μm diameter Ti disks with 10 nm thickness. The insets show the graphene area (red color) on the Ti disk measured by Raman mapping. (c) Dielectric properties vs applied frequency and (d) leakage current density vs applied voltage of the thin-film graphene/TiO2/BMNO/ITO/PET capacitors. The dielectric and leakage properties of the graphene/TiO2/BMNO/ITO/PET capacitors after four bending cycles are shown in c and d, respectively.

application as the top electrodes in transparent and flexible thin-film capacitors. Bi2Mg2/3Nb4/3O7 (BMNO) pyrochlore thin films with a thickness of 200 nm were deposited onto copper-clad laminate and Pt/TiO2/Si substrates and revealed a high dielectric constant of 40−60 at 100 kHz and a low leakage current density of ∼10−8 A cm−2 at 10 V in amorphous films synthesized at room temperature.26−29 BMNO materials were chosen to produce transparent and flexible thin-film capacitors. Although ITO electrodes are not good candidates for flexible electronic devices, this study used commercial ITO-coated PET substrates with a thickness of 130 nm for flexible bottom electrodes. Based on the BMNO capacitor structure shown in Figure 4a, a graphene top electrode was synthesized on 10 nm thick and 200 μm diameter Ti disks on top of a BMNO/ITO/ PET stack under a hydrogen atmosphere at 150 °C. The presence of graphene was confirmed by Raman spectroscopy, indicating a defect-free graphene monolayer (Figure 4b). At a wavelength of 550 nm (Figure S10), the transmittance of the graphene/TiO2/BMNO(200 nm)/ITO/PET capacitors was 77.4 ± 0.2% (similar to that of BMNO(200 nm)/ITO/PET) compared with that of a commercial ITO/PET version (81.2 ± 0.2%). The dielectric constants for the studied capacitors were between 48 and 46 at a frequency of 100 kHz, which showed slight dielectric dispersion with increases in the frequency (Figure 4c). The dielectric loss (dissipation factor) of the capacitors was maintained at 0.04 ± 0.003 at 100 kHz. To examine the flexibility of the capacitors, bending tests were performed at the radius of a curvature (0.3 × 10−2 m) to assess the dielectric (Figure 4c) and leakage (Figure 4d) properties of the graphene/TiO2/BMNO/ITO/PET capacitors (see Methods and Figure S11). The electrical properties of three identical samples were determined after four bending cycles, and the bottom ITO electrodes did not withstand severe bends. Minimal decreases in the dielectric constant (a decrease of

AFM images of graphene synthesized on Ti at 150 °C after hydrogen reduction for 4 h at 150 °C revealed a clear contrast with TiO2 (Figure 3c). The graphene was not synthesized on unreduced TiO2. The height profile along the blue line (A-B) in Figure 3c shows a step-height of 0.41 ± 0.03 nm (Figure 3d), which is similar to the value reported for a monolayer of graphene (∼0.33 nm). 22 The graphene monolayer is remarkably illustrated by the annual dark-field (ADF) transmission electron microscopy (TEM) images in Figure 3e. Additionally, a high-resolution TEM image (Figure 3f) of the monolayer graphene flakes synthesized at 150 °C and its Fourier transformation (Figure 3g) revealed an atomically thin graphene lattice. The graphene synthesized at 150 °C via CVD revealed a polycrystalline nature, and after exposure to air, the Ti underneath the graphene was changed to TiO2 when oxygen penetrated the grain boundary of the polycrystalline graphene (Figure 3h). That result showed that the various functions of graphene were not influenced, and that the graphene synthesized directly at 150 °C on 10 nm thick Ti-coated PET substrates required no transfer process. Flexible and Transparent Thin-Film Capacitors Using Directly Grown Graphene Top Electrodes. There have been many recent attempts to miniaturize electronic devices in order to imbed passive components (80% of electronic components) such as capacitors into printed circuit boards at low temperatures (