Research Article www.acsami.org
Transfer-Free Growth of Multilayer Graphene Using Self-Assembled Monolayers Gwangseok Yang,‡ Hong-Yeol Kim,*,‡ Soohwan Jang,§ and Jihyun Kim*,‡ ‡
Department of Chemical and Biological Engineering, Korea University, Seoul 02841, South Korea Department of Chemical Engineering, Dankook University, Yongin 16890, South Korea
§
S Supporting Information *
ABSTRACT: Large-area graphene needs to be directly synthesized on the desired substrates without using a transfer process so that it can easily be used in industrial applications. However, the development of a direct method for graphene growth on an arbitrary substrate remains challenging. Here, we demonstrate a bottom-up and transfer-free growth method for preparing multilayer graphene using a self-assembled monolayer (trimethoxy phenylsilane) as the carbon source. Graphene was directly grown on various substrates such as SiO2/Si, quartz, GaN, and textured Si by a simple thermal annealing process employing catalytic metal encapsulation. To determine the optimal growth conditions, experimental parameters such as the choice of catalytic metal, growth temperatures, and gas flow rate were investigated. The optical transmittance at 550 nm and the sheet resistance of the prepared transfer-free graphene are 84.3% and 3500 Ω/□, respectively. The synthesized graphene samples were fabricated into chemical sensors. High and fast responses to both NO2 and NH3 gas molecules were observed. The transfer-free graphene growth method proposed in this study is highly compatible with previously established fabrication systems, thereby opening up new possibilities for using graphene in versatile applications. KEYWORDS: multilayer, graphene, transfer-free, self-assembled monolayer, chemical sensor
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INTRODUCTION Large-area graphene is commonly produced using the chemical vapor deposition (CVD) method, which is based on the chemical reaction between a carbon source in the gas phase and metal catalysts.1−4 Since a catalytic metal foil is usually employed as a substrate in CVD techniques, an additional step is required for transferring the prepared graphene layers onto the target substrate. A wet-transfer method is conventionally used for fabricating large-area graphene on arbitrary substrates; this method involves the formation of a supporting layer, followed by metal etching, transfer of graphene onto the target substrate, and removal of the supporting layer. However, defects and contaminations such as wrinkles, cracks, and polymer residues can be induced during the polymer-assisted transfer process.5,6 In order to overcome these drawbacks, a rapid thermal annealing process under Ar/H2 atmosphere and state-of-the-art transfer methods such as transfer using pressure sensitive adhesive films and polymer-free biphasic methods have been proposed.7−9 However, these additional processes are time-consuming and can increase the production cost. Bottom-up and transfer-free synthesis of graphene has attracted great attention, as implementation of graphene in microelectronics has been limited by the transfer process.10,11 Transfer-free synthesis of graphene on the desired substrates is challenging because several parameters such as the metal catalyst, carbon source, and the annealing method used in the © XXXX American Chemical Society
process should be carefully chosen. The choice of metal catalyst determines the synthesis mechanism, resulting in carbon precipitation during cooling in the case of carbon-soluble metals and chemical reactions occurring at the surface of the catalyst layer in the case of carbon insoluble metals.12−14 Transfer-free graphene can be produced using various carbon sources including amorphous carbon, polymers, self-assembled monolayers (SAMs), implanted carbon, and graphite powder.15−20 Among the various carbon sources, SAM is very effective and promising since the amount of carbon introduced can be easily controlled. Studies on SAMs regarding various characteristics such as its nature, coating mechanism, and applications have been reported earlier.21−23 In addition, a conformal monolayer coating can be achieved even on textured substrates using SAMs. The length of the carbon channel in SAMs is known to be closely related to the graphene thickness.24 Recently, Weber et al. demonstrated that patterned graphene could be obtained from SAMs by combining electron beam irradiation with a shadow mask.25 In the present study, multilayer graphene was directly grown on various substrates using a phenyl-SAM coating and a subsequent thermal annealing process. While monolayer graphene has shown Received: July 20, 2016 Accepted: September 15, 2016
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DOI: 10.1021/acsami.6b08974 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 1. Schematic of the transfer-free graphene growth procedure illustrating: (a) target substrate, (b) phenyl-SAM coating on the prepared substrate, (c) Cu thin film evaporation on SAM-coated substrate, (d) graphene growth between Cu layer and the target substrate by thermal annealing, (e) selective Cu etching, and (f) graphene-based device after contact metal deposition. (g) Graphene growth temperature as a function of time (the inset is a schematic of the horizontal tube furnace). (h) Schematic of the growth mechanism for multilayer graphene.
Raman spectroscopy was used to analyze the properties of the graphene layer using a diode-pumped solid-state laser with a wavelength of 532 nm under a backscattering geometry (Omicron-Laserage). Micro-Raman mapping was performed over area of 10 × 10 μm2 with a step size of 0.25 μm (1681 points) using a Raman spectroscopy system (Renishaw inVia) with a wavelength of 532 nm laser. The optical transmittance of graphene was investigated using a spectrometer (Cary 5000, Varian) where the substrate was quartz. High-resolution transmission electron microscopy (HR-TEM, G2 F30ST, Tecnai) was employed for examining the cross-sectional structure of the samples. For preparation of TEM samples, as-grown quartz/graphene/Cu samples were directly subjected to focused-ion beam process (Quanta2003, FEI) without Cu etch because Cu layer can act as a protecting graphene layers from ion-beam instead of conventional Pt films. The surface morphology of graphene was characterized using atomic force microscopy (AFM, XE150, Prak system). The sheet resistance of the synthesized graphene was measured using a four-point probe measurement (Desk 205, MS Tech) with a source meter (Keithley 2400). Chemical Gas Sensing Measurement. The prepared transfer-free graphene was fabricated into chemical sensors to explore the applications of our growth method. Electrical properties and sensing performances of the graphene layer were systemically examined. Silver paste was used to form contact pads for electrical measurements, which were conducted using a semiconductor parameter analyzer (4155C, Agilent). To clean the surface of graphene before chemical gas exposure, the graphene sensors were located in a low vacuum chamber and UV light with a wavelength of 365 nm was irradiated via quartz window of vacuum chamber. The sensors were exposed to NO2 (200 ppm, He balance) and NH3 (200 ppm, He balance) gases to assess the sensing performance at room temperature.
outstanding electrical and thermal properties, this one-atom thick material is highly vulnerable to various fabrication processes. Therefore, multilayer graphene is advantageous for practical implementation on electrical and optical device applications. Electrical and optical characteristics as well as chemical sensing performances of the synthesized transfer-free graphene samples were systemically investigated.
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EXPERIMENTAL SECTION Transfer-Free Growth of Graphene. Figure 1 illustrates the schematic of the overall process. The transfer-free growth process was demonstrated in various substrates including SiO2/ Si, quartz, GaN, and textured Si (Figure 1(a)). Target substrates were thoroughly cleaned using the conventional method employing acetone/isopropyl alcohol and piranha solutions (1:3 mixture of hydrogen peroxide and sulfuric acid). The cleaned substrates were immersed in 0.1 M trimethoxy phenylsilane (PhSi(OMe)3, Sigma-Aldrich) solution in hexane overnight followed by thermal annealing at 120 °C for 20 min for preparing the phenyl-SAM/substrate structure (Figure 1(b)). Cu (200 nm) or Ni films (300 nm) were then deposited on top of the SAM using an electron-beam evaporator (Figure 1(c)). The prepared samples were loaded in a low vacuum furnace (600−1000 °C) to form a Cu/graphene/substrate structure (Figure 1(d) and inset of Figure 1(g)). The timedependent growth temperature is shown in Figure 1(g). To determine the optimal growth conditions, the Ar/H2 flow rate and the growth temperature were varied. Finally, the Cu films were wet-etched using 1 wt % ammonium persulfate as shown in Figure 1(e), followed by the device fabrication process (Figure 1(f)). Characterization. The optical and structural characteristics of the synthesized graphene samples were analyzed for determining the effects of different growth conditions. MicroB
DOI: 10.1021/acsami.6b08974 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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with high hydrophobicity were observed by increasing the coating duration. For the sample prepared overnight, a contact angle of ∼81.2° was obtained, which is in good agreement with the previous reports.24,26 In this paper, the overnight coating process was used to form closely packed SAMs. Various substrates were used for demonstrating the transferfree growth method. At first, the direct growth of multilayer graphene was performed on a SiO2/Si substrate. Figure 3 shows the temperature dependence of transfer-free graphene growth on the SiO2/Si substrate. The Ar flow rate was fixed at 50 sccm while examining the effects of the growth temperature. Three graphene-related Raman peaks, corresponding to the D (∼1360 cm−1), G (∼1595 cm−1), and 2D (∼2700 cm−1) bands were observed, as shown in Figure 3(a). The G band corresponds to the E2g phonon vibration, and the 2D band is related to momentum conservation by two phonons. On the contrary, the D band can be activated by the defects present in graphene.27,28 As the growth temperature is increased from 800 to 1000 °C, sharper Raman bands (D, G, and 2D) are observed and the relative intensity of the 2D band increases, indicating that high quality graphene samples are formed at higher growth temperatures. At low growth temperatures (800 and 900 °C), a thick graphene layer containing cracks was observed (Figure 3(b,c)). However, a uniform graphene layer without cracks is formed at 1000 °C, as shown in Figure 3(d). The results obtained from the optical images are in good agreement with the Raman analysis results. In order to compare the effects of the metal catalyst on graphene growth, Ni was also used as a catalyst instead of Cu. As mentioned earlier, the solubility of carbon in the metal determines the synthesis mechanism of graphene. Since the carbon solubility of Ni is 3 orders of magnitude higher than that of Cu at 1000 °C,29,30 carbon precipitation is an important factor in the graphene growth process. For the sample prepared at 1000 °C, a weak 2D band and broad D and G bands are observed from the Ni/SAM/SiO2/Si structure (Figure S1(a,b)
Chemical gases were introduced for 2 min followed by exposure of the sensors to air for 6 min. A constant voltage of 1 V was applied during the sensing experiments. To examine the stability of the fabricated graphene sensors, the sensitivity was measured again after 100 days under the same gas exposure condition. Alternative exposure of NO2 gas (30 s) and air (30 s) was repeated for 50 times within 3000 s. In addition, the sensitivity was continuously measured as the NO2 concentration was varied from 25 to 200 ppm in order to determine the detection limit of transfer free graphene sensors. The surface of graphene was cleaned by UV irradiation after exposure of NO2.
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RESULTS AND DISCUSSION The formation of a conformal coating of SAMs on the target substrate is a critical step for the growth of high quality graphene because the carbon density in the SAMs strongly affects the number of graphene layers. To confirm the coating of SAM, the contact angles of the coated samples were investigated using deionized (DI) water (Figure 2). Samples
Figure 2. Contact angle of DI water on SAM-coated SiO2/Si substrate with different coating durations: (a) bare SiO2/Si, (b) overnight coating, and (c) the contact angles obtained with different coating durations.
Figure 3. (a) Evolution of Raman spectra of graphene directly grown on SiO2/Si substrate depending on the growth temperature. Optical images of the samples grown at (b) 800 °C, (c) 900 °C, and (d) 1000 °C. C
DOI: 10.1021/acsami.6b08974 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 4. Evolution of Raman spectra of graphene directly synthesized on SiO2/Si substrate depending on (a) Ar flow rate and (b) ratio of Ar:H2 gas. Optical images of the graphene directly grown on SiO2/Si substrate with different Ar flow rate of the following: (c) 10 sccm and (e) 500 sccm. Raman mapping images of ID/IG with different Ar flow rate of the following: (d) 10 sccm and (f) 500 sccm.
of the Supporting Information), indicating that low quality graphene is formed compared to the sample grown in the presence of Cu. Crumpled and agglomerated carbon clusters are observed in the corresponding optical images (Figure S1(c,d)). Since carbon atoms can be easily precipitated at the grain boundary of Ni than inside the Ni grain,31 maze-like carbon clusters are formed. To further optimize the growth conditions, the effects of Ar and H2 flow rates on the quality of graphene were investigated. Raman spectra analysis can be used for determining the optimal Ar flow rate. The intensity ratios of the D to G band (ID/IG) and 2D to G band (I2D/IG) were analyzed since ID/IG and I2D/ IG are well-known indicators for evaluating the quality and thickness of graphene.1,27,32,33 ID/IG decreases from 0.95 to 0.42 and I2D/IG increases from 0.39 to 0.48 upon increasing the Ar flow rate from 10 to 500 sccm (Figure 4(a)). In addition, a uniform and continuous graphene layer was obtained at an Ar flow rate of 500 sccm (see Figure S2). These results indicate that a high Ar flow rate is important for the optimal growth of
graphene in our growth conditions. To evaluate the uniformity of the synthesized graphene, map of ID/IG ratio of graphene grown under Ar flow rate of 10 and 500 sccm are represented over area of 10 × 10 μm2 in Figure 4(d,f), which are associated with optical images shown in Figure 4(c,e) respectively. ID/IG ratio of graphene grown under Ar flow rate of 10 sccm is in the range of 1.02 and 1.32 while that of 500 sccm is in the range of 0 to 0.5, which indicates that high quality graphene was grown at Ar flow rate of 500 sccm. The distinct color difference of graphene grown under Ar flow rate of 10 sccm indicates nonuniform graphene growth compared with that of 500 sccm. The Raman mapping images confirm the importance of high Ar flow rate for high quality graphene growth. The effects of H2 flow rates were also examined. By keeping the total flow rate of Ar and H2 at 50 sccm, the ratio Ar:H2 was varied as 1:0, 46:4, and 25:25. In our experiments, H2 gas is shown to have adverse effects on the graphene growth, as indicated in Figure 4(b). Broad Raman peaks and decreased 2D peaks are observed when H2 was introduced during the growth D
DOI: 10.1021/acsami.6b08974 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 5. (a) Raman spectra of graphene directly grown on quartz, GaN, and textured Si substrates. (b) Optical transmittance of multilayer graphene directly grown on quartz, (c) photograph of bare quartz and graphene/quartz, and (d) cross-sectional HR-TEM image of graphene grown between quartz substrate and Cu.
where carbon atoms exist between the Cu metal and the substrate. As noncatalytic growth of graphene using SAMs was reported earlier,36 the Cu metal acts not only as a catalyst for surface-mediated graphene growth but also as an encapsulation layer to prevent carbon out-diffusion. We believe that low carbon solubility of Cu may give rise to multilayer graphene growth. We also performed the growth on GaN/sapphire substrates at 900 °C because GaN has a lot of applications in power and optical devices. As GaN cannot endure thermal annealing at 1000 °C, the growth temperature was lowered down to 900 °C. An annealing temperature of 900 °C was employed with textured Si for reducing Cu agglomeration, where textured Si has applications for solar cells. The quality of the transfer-free graphene grown on GaN and textured Si was not as good as the one on SiO2/Si and quartz substrates (Figure 5(a)). The Cu agglomeration occurring at the textured surface and the thermal annealing performed at a low temperature might deteriorate the quality of graphene. Further optimization of growth conditions is necessary to enhance the quality of graphene on different substrates. To explore the applications of graphene produced by our transfer-free growth method, graphene chemical sensors were fabricated. The schematic diagram of the graphene sensor is shown in Figure 6(a). The sensing performance to NO2 and NH3 gas molecules was examined. The sensitivity of the fabricated graphene sensor to a chemical gas is evaluated by using the following equation: sensitivity(%) = [(R(0)−R(t))/ R(0)] × 100%, where R(0) and R(t) correspond to the initial resistance and the resistance at a time of t. Fast responses with good reproducibility were obtained with NO2 and NH3 sensing, as shown in Figure 6(b) and exact response times for NO2 and NH3 are estimated in Figure S5. The opposite responses obtained with NO2 and NH3 are attributed to the electronic properties of each molecule. Since NO2 strongly withdraws
process. In addition, nonuniform layers with spotty surfaces are observed in the optical images (see Figure S3). AFM images of the samples obtained at Ar:H2 flow rates of 50:0 and 25:25 are shown in Figure S4(a,b), respectively. The root-mean-square (RMS) roughness of the samples obtained with Ar:H2 flow rates of 50:0 and 25:25 are 8.027 and 5.490 nm, respectively, which can be explained by vertical growth of graphene under Ar ambient. Although the surface roughness of graphene was reduced with the introduction of H2, insufficient graphitization occurred when H2 was introduced in the growth process, as can be seen from the Raman spectra (Figure 4(b)). After determining the optimal growth conditions (1000 °C, Ar 500 sccm), transfer-free graphene synthesis was demonstrated on a variety of substrates including GaN, quartz, and textured Si (Figure 5(a)). To investigate the optical properties of the transfer-free graphene samples, graphene was directly grown on quartz substrates. Figure 5(c) shows a photograph of bare quartz and graphene/quartz substrates. The entire area of the graphene/quartz substrate is uniformly dark compared to the bare quartz substrate. The sheet resistance of graphene on quartz is ∼3500 Ω/□. The optical transmittance of the graphene is 84.3% at a wavelength of 550 nm (Figure 5(b)). Since individual graphene layers can absorb 2.3% of the incident light at 550 nm,34 the number of graphene layers is estimated to be ∼7. TEM was employed to obtain further insights on the number of graphene layers. The thickness of graphene is less than 5 nm (Figure 5(d)) indicating that multilayer graphene was grown on the quartz substrate. The growth mechanism of multilayer graphene is illustrated in Figure 1(h). The transfer-free growth mechanism of multilayer graphene proposed in this work comprises two elements: (i) surface-mediated reaction and (ii) noncatalytic reaction. Catalytic growth on the Cu surface relates to the synthesis of mono- to bilayer graphene,1,35 while noncatalytic growth of graphene relates to a graphene growth reaction E
DOI: 10.1021/acsami.6b08974 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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for large substrates and the feasibility of fabricating graphene on various substrates.
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CONCLUSIONS We report a simple transfer-free growth method for producing multilayer graphene samples on various substrates using a SAM (PhSi(OMe)3) as the solid-state carbon source. The metal/ SAM/substrate structure was converted to metal/graphene/ substrate after subjecting the samples to thermal annealing at temperatures as low as 800 °C. High quality graphene was produced on SiO2/Si and quartz substrates, although further optimization of growth conditions is required on GaN and textured Si. Samples exhibiting 84.3% optical transmittance at 550 nm and a sheet resistance of ∼3500 Ω/□ were achieved. Graphene, grown directly on quartz substrates, showed excellent chemical sensitivity toward NO2 and NH3 molecules. The transfer-free growth method for graphene, proposed in this work, opens up new possibilities in industrial graphene-based device applications.
Figure 6. (a) Schematic of the graphene sensor. Sensing performance of graphene grown using the transfer-free method: (b) time-resolved sensitivity toward NH3 and NO2 gas molecules. Sensitivity with increasing NO2 concentration: (c) time-resolved change of sensitivity and (d) maximum sensitivity after 120 s gas exposure and 120 s UV irradiation with increasing concentration.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b08974. Detailed Raman analysis, optical, and AFM images according to the growth conditions (PDF)
electrons from the graphene, it is considered as a typical p-type dopant. However, NH3 is known as an n-type dopant as it donates electrons to graphene.37,38 Note that the graphene exposed to ambient air generally shows p-type behavior due to the water vapor and oxygen molecules. Therefore, the electrical conductivity of graphene is enhanced when NO2 molecules are adsorbed, while the electrical conductivity is reduced due to the compensation effect when NH3 molecules are adsorbed on the graphene surface. Since the long-term and operational stabilities are important factors for sensing applications, those of transfer-free graphene sensors were characterized as shown in Figure S6. The measured sensitivity of the same device after 100 days kept under air atmosphere from the last measurement was not degraded (Figure S6(a)). The device still operated after 50 cycles of NO2 gas exposure although the variation in the sensitivity decreased due to the accumulated NO2 molecules on the graphene surface (Figures S6(b)). However, it was recovered after cleaning by UV irradiation. It is also important to know the minimum concentration that our graphene sensor can detect. The NO2 gas concentration was changed from 25 to 200 ppm, where 25 ppm is the minimum concentration that can be managed. Approximately one-fourth of the sensitivity for 25 ppm of NO2 gas was achieved compared to that for 200 ppm of NO2 and the sensitivity increased with increasing concentrations. It is noticeable that our graphene sensors properly operate at a low concentration of gas as low as 25 ppm. We compared the sensitivity of our transfer-free graphene sensors for NO2 to that of other graphene-based and oxidebased sensors listed in Table S1. Even though the sensitivity of the samples used in this study is lower than commercialized oxide-based sensors, the results were comparable to that of other graphene-based sensors. The excellent performances of our graphene chemical sensors indicate that the growth method for the production of transfer-free graphene can be useful in various electrical and optical device applications. We believe that our growth method exhibits promising industrial feasibility owing to its scalability
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (H.-Y.K.). *E-mail:
[email protected] (J.K.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the financial support from the LG Innotek-Korea University Nano-Photonics Program and the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Korea (No. 20163010012140).
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DOI: 10.1021/acsami.6b08974 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
