Low Temperature Growth of Graphene on Glass by Carbon-Enclosed

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Low temperature Growth of Graphene on Glass by Carbon-Enclosed Chemical Vapor Deposition Process and its Application as Transparent Electrode Yu-Ze Chen, Henry Medina, Hung-Wei Tsai, Yi-Chung Wang, Yu-Ting Yen, Arumugam Manikandan, and Yu-Lun Chueh Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm504431d • Publication Date (Web): 09 Feb 2015 Downloaded from http://pubs.acs.org on February 14, 2015

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Chemistry of Materials

Low temperature Growth of Graphene on Glass by Carbon-Enclosed Chemical Vapor Deposition Process and its Application as Transparent Electrode

Yu-Ze Chen, Henry Medina, Hung-Wei Tsai, Yi-Chung Wang, Yu-Ting Yen, Arumugam Manikandan, and Yu-Lun Chueh* Department of Material Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan *E-mail: [email protected]

Abstract- A novel carbon-enclosed chemical vapor deposition (CE-CVD) to grow high quality monolayer graphene on Cu substrate at a low temperature of 500 °C was demonstrated. The quality of the grown graphene was investigated by Raman spectra and the detailed growth mechanism of high quality graphene by the CE-CVD process was investigated in detail. In addition to growth of high quality monolayer graphene, a transparent hybrid few-layer graphene/CuNi mesh electrode directly synthesized by the CE-CVD process on a conventional glass substrate at the temperature of 500 oC was demonstrated, showing excellent electrical properties (~5 Ω/ @ 93.5% transparency) and ready to be used for optical applications without further transfer process. The few-layer graphene/CuNi mesh electrode shows no electrical degradation even after 2 hours annealing in pure oxygen at an elevated temperature of ~300 °C. Furthermore, the few-layer graphene/CuNi mesh electrode delivers an excellent corrosion resistance in highly corrosive solutions such as electroplating process and achieves a good nucleation rate for the deposited film. Findings suggest that the low temperature few-layer graphene/CuNi mesh electrode synthesized by the CE-CVD process is an excellent candidate to replace Indium tin oxide (ITO) as transparent conductive material (TCM) in the next generation. Keywords: low temperature graphene, transparent conductive electrode, corrosion resistance

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Introduction Transparent conductive materials (TCM) are fundamental for the operation of many optoelectronic devices such as solar cells, touch screens, and smart films. Currently, commercial transparent electrodes are usually made of indium tin oxide (ITO) due to its good electrical performance (~10Ω/ @ 90%). However, its brittleness and the limited amount of indium resources, raises concerns. Therefore, alternative materials such as metal mesh and/or metal nanowires (NWs),1-5 carbon nanotubes (CNT),6, 7 and hybrid materials8, 9, have been proposed to replace ITO as the TCM. The use of these alternative materials requires the formation of a porous instead of a continuous film, leading to the light percolated through holes to increase the light transmittance while maintaining good conductivity. Among those possible materials, Cu meshes/nanowires are the best candidates as the TCM due to low cost and excellent conductivity.10-12 However, the scalability goes down to the nanoscale, the increased surface area makes materials to be more reactive. In particular, nanometer thick Cu NWs/meshes are more prone to react with oxygen and humidity when exposed to ambient conditions, resulting in oxidation and leading to severe degradation of electrical performance. Additional coatings on Cu NW such as Ni13 Pt14, and conductive oxides15-17 have been reported to avoid the oxidation. However, the price increases due to the use of expensive metals for coating while enhancing the performance in terms of sheet resistance vs transmittance. Instead, Ag NWs directly offer the better oxidation resistance than the uncoated Cu NWs. Recent works have demonstrated that Ag NWs film can achieve a sheet resistance comparable to ITO.3, 18-20 However, Ag NWs are not still devoid of degradation under harsh conditions, thus decreasing the electrical performance.13, 21 Au coating on polymer fibers22 has shown to be an interesting approach, which is able to achieve an outstanding electrical performance. However, the required transfer process, difficulties to achieve large area, and the use of the expensive Au as a raw material restrict its use for industrial applications. Alternatively, carbon nanomaterials in sp2 bonding such as graphene and carbon 2


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nanotubes (CNT) has been also proposed as TCM due to their better corrosion resistance compared with metals whose sheet resistance is comparable with that of ITO.6, 7, 23-25 In order to take advantage of the electrical conductivity of metal structures such as metal nanowires, graphene has been suggested as a coating layer to improve the corrosion resistance and the current capacity.26, 27 However, for the use of graphene as corrosion resistance coating, two issues, including the synthesis of high quality graphene layer with defect-free and the selective growth of few layer graphene have to be addressed. Although some methods to grow graphene at low temperature have been proposed, the graphene layer shows the poor quality with small domains.28 Instead, high quality graphene layers was achieved by CVD method on copper (Cu) foil as catalyst while the actual polymer and thermal release assisted transfer methods from the Cu foil to an insulating substrate in large area raise concerns due to formation of wrinkles, scratches, and polymer residues, inducing several defects and scratches on graphene lattice and losing its coating properties.29, 30 Despite the large domain size achieved by graphene growth on Cu catalyst, the grain boundaries of monolayer graphene cannot retard the diffusion of oxygen molecules, leading to the oxidation of the metal substrate.31 Instead, few-layer graphene seems to be the best choice, offering a superior anticorrosion coating.27 In this work, we propose a novel CVD method to grow high quality monolayer graphene using the carbon-enclosed process (CE-CVD), for which the synthesis temperature for graphene growth on Cu foil as catalyst can be significantly reduced from 1000 to 500 °C while maintaining the quality of the graphene confirmed by Raman spectra. The detailed growth mechanism of the high quality graphene synthesized at low temperature by the CE-CVD process was proposed and investigated. Furthermore, uniform and direct growth of few-layer graphene on a CuNi (cupronickel) alloy grid on the conventional glass, exhibiting very high transmittance behavior, can be also demonstrated by the CE-CVD process at the grown temperature of 500 oC. The excellent electrical performance was tested under different harsh environments. In addition, the excellent electrical performance, offering an excellent nucleation rate of the deposited film during 3



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electrochemical deposition without current degradation was tested. Experimental Section Chemical vapor deposition (CE-CVD). Cu foils were placed on graphite plate and then inserted into the furnace equipped with 3-inch carbon-enclosed quartz tube for the following growth process. The length of heating zone in our system is about 46 cm and the temperature mapping matches a Gaussian distribution. For the CE-CVD process, the temperature of the center area was steadily ramped to ~1000 °C under 50 sccm hydrogen (H2) flow during 60 min at a constant pressure of 9 x 10-1 torr. Subsequently, 5 sccm methane (CH4) flow was introduced along with 5 sccm H2 for 30 min. After growing 30 min, furnace was rapid cooled to room temperature.

Characterizations. The quality and uniformity of graphene synthesized via CE-CVD were characterized by Micro-Raman spectroscopy (HORIBA, LabRAM, HR800) with 514 nm lasers. The samples for TEM observation was prepared by focus ios beam (FIB). The cross-sectional structure of graphene was revealed by high resolution transmission electron microscopy (HRTEM, JEOL, JEM-3000F). The optical transmittance was measured by the spectrophotometer (Hitachi, U-4100 UV-visible-NIR Spectrophotometer). Electrical property measurement was carried out by using parameter analyzer (KEITHLEY, 4200-SCS). Field emission scanning electron microscopy (FESEM, JEOL, JSF-6500F) was applied in order to observe the coverage of graphene.

Fabrications of graphene Field-Effect Transistors. The graphene grown on Cu foils was transferred by PMMA-lifted method onto a 50 nm SiO2/Si substrate. Then they were immersed in acetone solution to remove the PMMA. After using photolithography to define the electrode and channel, the Cr with 5 nm and Au with 50 nm as electrodes were deposited by electron beam evaporators, following by lift-off process.

Fabrications of metal mesh shielded with graphene. First, Ni with 20 nm as a adhesion layer 4


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and Cu with 80 nm were deposited by electron beam evaporators on commercial glass after defining meshes by photolithography. Second, the metal mesh was placed onto the graphite plate and then inserted into the furnace equipped with carbon-coated quartz tube. The position of samples were at the end of heating zone, and its corresponding temperature was 500 ͦC calibrated by K-type thermocouple. When process temperature was reached its setting point and stabilized, the methane with 500 sccm was introduced. Finally, after 5 min for deposition of graphene, the furnace was rapid cooled to room temperature. Results and Conclusion Figure 1(a) depicts schematics of the low temperature CE-CVD process for the growth of the monolayer graphene on a Cu foil as the catalyst. Contrary to previous methods for growing graphene, a graphite plate was used as a holder for the substrate during the CE-CVD process. The corresponding Raman spectrum of the holder is shown in Figure S1. The Raman spectrum of the holder shows the typical features of graphite: D, G and 2D bands.32 The G peak shows the strongest intensity and is located at ~1580 cm-1 and the 2D signal located at ~2700 cm-1 is approximate half of the intensity of the G band, which is expected for bulk graphite. A relatively large D band located at ~1350 cm-1 is attributed to defective sites in the material probably caused during the manufacturing process.33 Furthermore, a one-closed end tube was designed to fit inside the tube furnace in order to increase the local concentrations of hydrocarbon during the annealing process. Then, the samples with the underlying graphite plate were placed inside the one-closed end tube and together inserted into the tube furnace for following process. Note that after a several rounds, a noticeable carbon coating occurs in the middle of the heating zone (Figure 1a). We suggest that the graphene synthesis on the Cu foil can be enhanced by the surrounding carbon coating, which was intentionally left in the center of the quartz tube. The temperature distribution along the quartz tube was calibrated within an interval of 1 cm by inserting K-type thermocouple as shown in Figure 1(b). The position of the sample was carefully calibrated by a thermocouple and placed at the left end of the heating area, for which the corresponding temperature of the 5



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substrate is measured to be ~500 °C and more detailed information has been mentioned in experimental part in supplementary information. Finally, the chamber was rapidly cooled. In the typical graphene CVD process, graphene uniformly grows on the both sides of the Cu foil. However, the Raman spectra of the graphene grown on both sides of the Cu foil by the CE-CVD show different characteristics. The Raman spectrum obtained from the front side (facing gas) of Cu foil reveals a strong intensity of G and D signal accompanied with a relative low intensity of 2D signal as shown in Figure 1(c), indicating the formation of defective graphite-like carbon materials. We suggest that the formation of graphene on the front side of the Cu foil is caused by the thermal decomposition of the methane (CH4) under the high temperature zone. Notably, the decomposed hydrocarbons drifted for a long distance (~23 cm) until reaching the Cu foil. As for the rear side (facing the graphite plate) the quality of the film shows surprising results, as shown in Figure 1(d). The width of the 2D signal is around ~35 cm-1 and the ratio of I2D/IG is about ~2. More importantly, the D signal, which is the feature of defects in graphene, is non-detectable, suggesting the formation of high-quality and monolayer graphene34 on the Cu foil at the very low temperature (500 °C) compared with the normal the CVD process carried out at temperatures > 1000 °C. To our knowledge, this is the lowest temperature ever be demonstrated to achieve a non-defective (no D band) and continuous graphene films on Cu by the conventional CVD process (Table 1). We suggest that the formation of the graphene in such low temperature is greatly enhanced due to an enhanced catalytic behavior from the graphite holder. In order to prove this concept, Figure 2a shows the plot of Raman I2D/IG as the function of the substrate temperatures from the front and rear sides of the Cu foil. For the growth of the monolayer graphene from the front side of the Cu foil, the I2D/IG rapidly decreases due to the poor quality of the deposited graphene due to the insufficient catalytic behavior of the Cu foil at the substrate temperature of 500 oC. On the other hand, for the growth of the monolayer graphene from the rear

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side of Cu foil, the I2D/IG shows a very slight change, indicating that the good quality of the monolayer graphene can be indeed achieved even at the substrate temperature of 500 °C. The corresponding Raman mapping images for the monolayer graphene grown on the rear side of the Cu foil at different substrate temperatures reveal the excellent uniformity (Figure S2). We expect that a further reduction in the growth temperature might be still possible. However, the substrate temperature of 500 °C is the lowest temperature to have a reliable process in a relatively large area (4 cm2) due to the large temperature gradient in our furnace system. The results suggest that the graphite holder significantly enhances the catalytic behaviors for the growth of monolayer graphene. To confirm the monolayer graphene growth on the rear side of the Cu foil, a high-resolution transmission microscopy (HR-TEM) image of the monolayer graphene from the cross-sectional structure was taken. Obviously, the graphene grown on the rear side of the Cu foil was transferred to a quartz substrate by the PMMA-lifted method. In order to avoid a possible damage caused by Ar ions during the TEM sample preparation by focus ion beam (FIB) cutting process, the graphene/quartz sample was first capped with a 100 nm-thick SiO2 layer deposited by electron beam evaporator. The corresponding HRTEM image is shown in Figure 2(b), for which the only monolayer graphene at the interface between the SiO2 capped layer and the quartz substrate could be observed. Furthermore, in order to confirm the monolayer graphene synthesized at 500 °C, the optical properties of the CE-CVD graphene deposited at various temperatures were examined as shown in Figure 2(c). Obviously, the light transmittance of graphene deposited at different temperatures was around 97 % at 550 nm wavelength, confirming that the monolayer graphene in average no matter how different substrate temperatures were applied.35 After systematically conducting a series of experiments and analyses, the monolayer graphene deposited at such low temperature by the CE-CVD was experimentally demonstrated. However, several questions associated with the formation mechanism still remain. For a better

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understanding of the influence of the surrounding carbon on the formation of graphene utilizing the graphite plate as a substrate holder, we methodically conduct a series of experiments to ascertain the enhanced catalytic behaviors of the carbon gaseous molecules during the CE-CVD process in order to figure out the possible mechanisms. Raman spectroscopy and scanning electron microscopy (SEM) were used to record the uniformity as well as the microstructure of the graphene growth utilizing the underlying graphite plate and the surrounding carbon molecules in comparison to the CE-CVD process using a pure quartz plate as a holder without the surrounding carbon molecules deposited on the wall of the tube as shown in Figures 3(a) and 3(b), respectively. Clearly, the uniformity and quality of graphene substantially decrease owing to the obvious noise confirmed by low intensity and non-uniformity of I2D/IG Raman mapping results. Figure 3(c) shows the SEM image of the graphene grown on the rear side of the Cu foil, indicating a poor coverage of the monolayer graphene being only ~70 %. Oppositely, Figure 3(d) and 3(e) show major improvement on the intensity and uniformity of I2D/IG Raman mapping results. Likewise, the SEM results revealed an improved coverage of the monolayer graphene with full coverage ~ 100 % as shown in Figure 3(f). Moreover, a plot of coverage of graphene deposited with and without the graphite plate versus the deposited temperatures is presented in the Figure 3(g) in order to summarize the results, for which the coverage percentage was estimated by evaluating the presence of the 2D band from Raman mapping results. It is obvious that the coverage of the graphene deposited with underlying graphite plate is indeed much higher than that without the graphite plate. The coverage of the graphene deposited without the graphite plate abruptly decreases with the increase of the substrate temperature. In the case for the growth of the graphene using the graphite plate, a slight decrease in the coverage at the high temperature was also found. The coverage reduction with the increasing temperature is caused by the evaporation of Cu atoms resulted from the reductive hydrogen atmosphere at the high temperature. In order to prove this assumption, a Cu thin film was deposited on the quartz and subsequently exposed to high temperature under the reductive hydrogen atmosphere. The changes 8


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in the thickness, defined as evaporation rate, were recorded as displayed in Figure 3(g) at right axis. According to the relation between the evaporation rates of Cu atoms as the function of the substrate temperatures, it is expected that evaporated Cu atoms will not only interfere with the landing of hydrocarbon but also hamper the expansion of graphene seeds, finally resulting in intrinsic poly-grains of the graphene. Although it is a well-known physical phenomenon that can explain the reduction of the coverage with the increased temperature, the large difference between the two conditions cannot be well explained yet. A potential explanation for this difference is attributed to the extra carbon feedstock provided by the carbon plate and surrounding carbon. To demonstrate this statement, we made an attempt to synthesize graphene on the Cu foil without introduction of methane while using the underlying graphite plate under the same growth condition. Surprisingly, the graphene was still formed on the Cu foil confirmed by the Raman as shown in Figure S3. Hence, this result implies that carbon feedstock could be also provided by the underlying graphite plate. However, the quality of the film grown by this means is not comparable with those grown with the introduction of additional methane. Based on the aforementioned discussion, a possible mechanism for the growth of the high quality monolayer graphene at the low temperature by the CE-CVD was proposed as shown in Figure 4. Normally, the thermal decomposition of the methane requires high temperature of ~ 1500-2000 K.36 It means that methane cannot be successfully decomposed without reaching temperatures close to 1020 °C at the midpoint of the heating zone. Hence, methane is first partially decomposed into methyl-radical molecules, including CH, CH2, and CH3 when passing through the center of the furnace. This partially decomposed methyl-radical molecules flow through the carbon-coated zone around the quartz tube until landing on the Cu foil. The methyl-radical molecules are further decomposed into carbon feedstock to trigger the monolayer graphene by the graphitization process at the temperature of 500 oC This decomposition of hydrocarbons by the catalytic properties of carbon films has been previously observed and confirmed at temperatures as low as 400 K.36-39 Meanwhile, the underlying graphite plate is not 9



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only to provide additional catalytic properties but also to supply additional carbon feedstock, resulting in the defect-free monolayer graphene on the rear side of the Cu foil. It is reasonable to believe that the catalysis for methane decomposition assisted by Cu is diminished because of insufficient thermal energy at the temperature of ~500 °C. Contrary to previous studies using Cu atoms from the Cu foil as catalyst for the decomposition of the methane and the formation of the monolayer graphene, the CECVD provides new thoughts toward the growth of the defect-free monolayer graphene at the annealing temperatures of ~500 °C. Interestingly, this process can be extended for the growth of high quality graphene to other metals such as Ni foil at the low temperature of ~500 oC as confirmed by Raman spectra in Figure S4. In addition, the low temperature CE-CVD process can also allow us to grow the graphene on the surface of Cu nanowires without melting Cu NWs as demonstrated in Figure S5. The electrical performance of graphene grown by the CE-CVD process was also acquired to estimate the quality of graphene. The corresponding sheet resistance of graphene deposited at 500 °C, 550 °C, 600 °C, 750 °C, 800 °C and 1000 °C (calibrated by K-type thermocouple) were measured by 4-probe measurements as shown in Figure S6. Fundamentally, the sheet resistance increases by increasing of deposition temperatures. The lowest resistance can be measured to be ~150~200 Ω/□ at the deposited temperature 500 °C. A possible reason for these results can be attributed to the slight reduction of the coverage of graphene with the increased growing temperature caused by the larger Cu evaporation during the high temperature process as shown in Figure 3(g). In addition, the extracted field effect mobility ranged from 1k~2k cm2/V.s was measured by back-gate field effect transistors, which is better than other related works of the graphene synthesis at the low temperature as listed in Table 1.40-49 Opposite to previous works that rely on complicated facilities, the process of the CE-CVD was simple and fast without sacrificing the quality of the graphene. In order to display the great potential application of the CE-CVD process at the lowest

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growth temperature for the growth of the graphene as the excellent electrode, we made an attempt to synthesize graphene on a thin metal grid deposited directly on commercial optical glass with the melting temperature below 600 oC as a supporting substrate. It is important to note that the annealing temperature over 600 °C are usually restricted to the commercial glass to avoid the softening of the glass. Initially, a predefined mesh pattern followed by depositing 20/80 nm Ni/Cu was carried out using conventional photolithography methods on the glass in order to increase the transparency directly. Note that during the annealing process, the two layers can form a Cu-Ni alloy film, which is preferred over the use of pure Cu layer to allow the formations of few-layer graphene in order to provide an effective corrosion protection compared to the monolayer graphene.27,

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After the low temperature CE-CVD process, the few layer graphene can be

synthesized within 5 min while the metal mesh does not suffer any visible change as shown in the optical micrograph image in Figure 5(a) and its corresponding Raman mapping image of 2D band is shown in Figure 5(b). In addition, Figure S7 obviously displays the Raman spectra of the glass substrate and the graphene grown on Cu-Ni alloy mesh obtained from the Raman mapping image in Figure 5(b), distinctly confirming the growth of the few-layer graphene on the Cu-Ni mesh. Note that the Graphene/carbon signal can be sensed on the glass. The high resolution Raman spectrum of the graphene grown on the Cu-Ni mesh using 30 sec integration time in order to confirm the absence of D band as shown in Figure 5(c). Therefore, these results indeed confirm the formation of the defective-free few-layer graphene synthesized on the Cu-Ni mesh using the commercial optical glass as the substrate, showing good uniformity in terms of the ratio of I2D/IG and the vanished D band. In addition, the optical transmittance was also measured via UV-vis spectroscopy. The transmittance at 550 nm is about 93.5 % with very low sheet resistance of 5 Ω/, which satisfies the criteria for use as transparent conductive electrode as shown in Figure 5(d). Note that the optical transmittance does not suffer a major change before and after the growth of the few-layer graphene, which can be attributed to the selective formation of the

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graphene only on the top of the metal grid confirmed by the Raman mapping as shown in the inset of the optical image in Figure 5(d). To further confirm the number of graphene layers grown on the Cu-Ni metal mesh deposited on commercial glass, TEM observation of the cross-section view was carried out as shown in Figure 5(e). Obviously, ~10 graphene layers were successfully synthesized and conformal coated on the Cu/Ni alloy. It is worth mentioning that the deposited thickness of catalytic metal is much thinner compared with other reference works for graphene films synthesized on metal thin films because of the low deposition temperature in the present study.42, 50 This thickness reduction clearly provides a reduction of the raw material used for graphene synthesis, implying a large decrease of the graphene production cost. Furthermore, the electrical performances at different optical transmittance measurements of the graphene/Cu-Ni mesh and other reports using Cu NWs,8, 11, 12, 22 AgNws,18-20 CNT,25 and Graphene23,25 were plotted as shown in Figure 5(f), obviously indicating our excellent performance including the higher transparency of > 90 % and the lower sheet resistance of

Low Temperature Growth of Graphene on Glass by Carbon-Enclosed

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