Atomization of reduced graphene oxide ultra thin film for transparent electrode coating

Atomization of reduced graphene oxide ultra thin film for transparent electrode coating

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  XXX-X-XXXX-XXXX-X/XX/$XX.00 ©20XX IEEE Atomizationof Reduced Graphene Oxide Ultra-thin Film for Transparent Electrode Coating Mohd Rofei Mat Hussin Nanotechnology & Adv. Device R&D MIMOS Berhad Kuala Lumpur, Malaysia [email protected] Nik Mohd Razali Mohd Nor Semiconductor Tech. Development MIMOS Semiconductor Sdn Bhd Kuala Lumpur, Malaysia [email protected] Mukter Zaman Faculty of Engineering Multimedia University Cyberjaya, Malaysia [email protected] Siti Aishah Mohamad Badaruddin Process Engineering R&D MIMOS Berhad Kuala Lumpur, Malaysia [email protected] Yuan Piou Choong Mechanical Design Engineering NSW Automation Sdn. Bhd. Penang, Malaysia [email protected] Mohd Hilmy Azuan Hamzah Nanotechnology & Adv. Device R&D MIMOS Berhad Kuala Lumpur, Malaysia [email protected] Hin Yong Wong Faculty of Engineering Multimedia University Cyberjaya, Malaysia [email protected] Abstract— In this research, an atomization process was introduced to deposit carbon-based nanomaterials i.e. graphene oxide (GO) on 200 mm silicon wafer for high volume production of a transparent conductive electrode (TCE). An ultrasonic spray coating process was used to deposit a stable dispersion of graphene oxide in ethanol on silicon and glass substrates. The thickness of the thin films can be controlled within a nanometer- scale with the high uniformity coating process. Functionalization of GO based on thermal reduction in Rapid Thermal Process (RTP) and Plasma Enhanced Chemical Vapour Deposition (PECVD) chambers was performed to make the GO electrically conductive and optically transparent. AFM measurements revealed that the thin film thickness has a strong correlation with the number of spray passes that can be translated into spray volume. It was discovered that the thin films demonstrate a good electrical conductivity with high optical transparency in the visible light. The sheet resistance of the films can be significantly reduced by rapid thermal annealing at a high temperature of up to 1100 ℃ and reach 5.3 kΩ/sq with a light transmittance of 77.1% at 550 nm wavelength. This is a low-cost solution for high volume production of TCE, which have a high potential for scaling-up. Keywords—reduced Graphene Oxide, spray coating, atomization process, transparent conductive electrode, rapid thermal process I. INTRODUCTION Transparent conductive electrodes (TCE) are widely used in various optoelectronic and photovoltaic applications such as solar cells, touch panels, optical communication devices, liquid crystal displays (LCD), etc. The indium tin oxide (ITO) with transparency of over 95% and sheet resistance as low as ~10 Ω/sq has been the dominant source of TCE [1]. However, it has some technical disadvantages such as poor mechanical flexibility, and relatively high deposition temperature that leads to cracking and scarcity issues. The production cost of ITO also has risen rapidly in recent years due to the demand for indium from the explosive growth of LCD based products like smartphone, laptop, and television sets, etc. Therefore, alternatively, the carbon-based nanomaterials have been studied to replace ITO as they have remarkable characteristics, for example, high optical transmittance and outstanding electrical properties. There have been many research activities on graphene, carbon nanotubes, metal nanowires, and hybrid materials to replace ITO in recent years [2-6]. These new materials have excellent mechanical and electrical properties with high chemical stability and low cost of production in comparison with the conventional ITO electrodes. However, the major obstacle to this development is the method of producing good quality of carbon-based nanomaterials and the manufacturing method to meet high volume requirements. Therefore, in this research, we have introduced an atomization method to deposit carbon-based nanomaterials i.e. graphene oxide (GO) on a large substrate for high volume production through ultrasonic spray coating process. The Graphene oxide (GO) is a graphene derivative material that can be viewed as a functionalized analog of graphene [7]. In this work, the functionalization of graphene is part of the nano-engineering in order to tailor their characteristics for TCE application. The oxygen-containing functional groups such as carbonyl, carboxyl, and hydroxyl can be removed from the basal plane and the edges through chemical or thermal reduction process. The reduced graphene oxide (rGO) have similar characteristics to graphene, which is transparent and conductive [8]. Hence, it can be used for transparent conductive coating and the photonic properties of rGO can also be modified by altering the functional groups on the carbon basal plane [8,9,10]. II. EXPERIMENT In this work, there are four main process and characterization steps involved as shown in Fig. 1. The first step is about the material preparation and atomization process, followed by the functionalization process, patterning, and characterization steps. The process details are discussed in the sub-section A, B, C and D below.
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  Fig. 1. Theoverall process flow for the experiment of transparent conductive electrode coating. A. Material Preparation and Atomization Process The first step in producing transparent conductive electrode was the production of graphene oxide (GO) solution via modified Hummer's method. Graphite powder was used and mixed with Sodium Nitrate (NaNO3) and concentrated sulphuric acid (H2SO4). The solution was stirred at a constant speed and potassium permanganate (KMnO4) was then gradually added prior to dilution process with distilled water. This process leads to the formation of active species; diamanganese heptoxide (Mn2O7), which has the ability to selectively oxidize the unsaturated double bonds graphitic structure and reaction pathways during oxidation [11,12]. Exfoliation process was then carried out by sonicating the GO dispersion in ambient temperature for 20 minutes to destroy the forces between layers. Finally, the GO was fully exfoliated to single or few layers. The GO was dispersed in ethanol with the concentration of 1 mg/ml. Sonication is needed prior to spray coating process. The atomization process used an ultrasonic atomizer system to spray coat the nano-thin film of a graphene layer on glass slides and 200 mm silicon wafer substrate. The solution was spray coated using NSW-MIMOS Mi-Atomizer 3.0 system. The thickness of the GO film was controlled by the number of spray passes of the GO solution. For the purpose of this experiment, we fixed the number of the spray passes to 1, 2, 3 and 4 passes. B. Functionalization Process In this work, two functionalization methods were used to make the GO film electrically conductive and yet transparent. For making the GO film electrically conductive, thermal reduction process was introduced to the film. The first method is based on the Rapid Thermal Process (RTP) with temperature of up to 1100°C and continuous flow of N2 gas in the RTP chamber. The AMAT Centura RTP system was used for this experiment. This reduction process is to reduce the oxygen-containing functional groups from the basal plane and the edge of the GO film on silicon dioxide wafer substrate and the glass slide. This reduced graphene oxide (rGO) film on silicon dioxide wafer is needed for the measurement of sheet resistance by Four Point Probe system to study the uniformity of the thin film coated on a large substrate (200mm wafer diameter). The second method is based on the thermal annealing in plasma enhanced CVD chamber with the temperature of up to 700°C with continuous flow of N2 gas. The Oxford Instrument Nanofab-700 PECVD System was used for the reduction process. The maximum temperature for the system is 700°C. C. Patterning Process The rGO transparent electrode was then patterned using standard lithography and plasma etching process, which includes the coating of photoresist on the rGO film, the photoresist exposure and developing step, and the etching of unwanted rGO film area using plasma etching in PECVD system. D. Characterization Methods The transparency percentage (% Transparency) of the rGO film on the glass slides were then measured using the Agilent Cary 7000 Universal Measurement Spectrophotometer (UV- Vis & UV-Vis-NIR Systems). The sheet resistance of rGO film on SiO2/Si wafers was measured using KLA Tencor Prometrix RS75 Four Point Probe system. The Bruker Innova Atomic Force Microscope (AFM) is also being used for the measurement of the rGO film thickness coated on the silicon wafer. The results were then analyzed to study the spray passes and the functionalization effects. III. RESULTS AND DISCUSSION The number of spray passes can control the thickness of thin films. The number of spray passes can be translated into the volume of the GO solution. Four samples with different spray passes, which the spray volume varies from 500 μL, 1000 μL, 1500 μL, and 2000 μL were prepared to study the correlation between process parameters and the physical characteristics of the thin films. Table I showed the sheet resistance and the uniformity of rGO films for different functionalization methods (RTP and PECVD). From the analysis, the RTP based reduction method clearly outperformed the PECVD method. The RTP samples have shown much better performance, which one decade lower sheet resistance as compared to PECVD based reduction method. Therefore, the RTP samples were chosen for further analysis to study their characteristics. However, there is no value of sheet resistance for the sample with only one spray pass regardless of their functionalization method. This is because of the rGO film was not properly formed on the substrate and the rGO flakes were not fully connected. The results also indicated that the sheet resistance and the non- uniformity of the thin films decreased with an additional number of spray passes demonstrating that the thickness of the rGO thin films increases with increasing spray passes. The uniformity of the thin film is also improved when the thickness of the thin film increased. In order to achieve 90% uniformity of the rGO thin films, at least 3-spray passes are required to deposit the nanomaterial on silicon substrate. PECVD samples are not stable and there is no data available for the non-uniformity of these samples. TABLE I. SHEET RESISTANCE AND UNIFORMITY OF RGO (RTP & PECVD) Reduction Method Sheet Resistance, Rs (kΩ/sq) & Non-Uniformity (%) 1 Pass 2 Pass 3 Pass 4 Pass RTP N/A Rs: 12.3 Rs: 5.3 Rs: 3.3 N/A NU: 14.22% NU: 9.38% NU: 6.98% PECVD N/A 700 160 70 N/A N/A N/A N/A
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  The rGO thinfilms pattern on glass substrate were shown in Fig. 2 and Fig. 3. These all are the RTP samples that have lower sheet resistance as compared to PECVD samples. The images clearly showed that the rGO films were successfully patterned on SiO2/Si substrate and glass slides by using standard lithography process and plasma etching. The darkness of the rGO thin films increased with increasing spray passes as clearly shown in Fig. 2. There is a strong correlation between the densities of rGO flakes with the number of spray passes. In Fig. 3, the microscope images showed the density of rGO flakes increased with increasing spray passes. Fig. 2. rGO films pattern on glass slides for different spray passes. Fig. 3. Optical microscope images for rGO film coated on a glass substrate. The thickness of the rGO thin film for different spray passes was measured using AFM analysis. Fig. 4 showed the 3D model of AFM measurement for a single spray pass, which points out that the thickness of the rGO thin film is about 4.631 nm. According to Shi et al. [3], the thickness of a single layer rGO is about 1.2 nm. This means that a single spray pass may result in a thin film deposition consist of a few layers of rGO flakes. The correlation study of thin film thickness and the number of spray passes (Fig. 5) showed the linear correlation with the R-squared (R2 ) about 0.9547. It is based on the average thickness of rGO thin film measured at several locations on the silicon substrate. In average, the thickness of the rGO thin films for 1, 2 and 3-spray passes are 5 nm, 9 nm, and 11 nm respectively. Fig. 4. Atomic Force Microscopy (AFM) measurement for a single spray pass. The value of the average thickness is much larger than that of pristine graphene, which was reported as about 0.3 nm [13]. However, the average thickness of the rGO thin film is comparable to a few-layers rGO, which is about 1.3 nm in ref. [14] and 1.2 nm in ref. [15]. The rGO is thicker than pristine graphene is because of the existence of residual covalently bonded reducing agent i.e. oxygen-containing functional groups such as carbonyl, carboxyl, and hydroxyl groups [14]. Fig. 5. Correlation study of rGO films thickness with spray passes. Theoretically, pristine graphene has excellent optoelectronic properties such as high conductivity (Rs < 102 Ω/sq) at transparency higher than 90% for optical transmission at 550 nm wavelength [6]. However, for the graphene derivative materials, these optoelectronic properties are normally degraded and very much dependent on the preparation and functionalization methods. Fig. 6 showed the GO and rGO transparency percentage for a various number of spray passes. The percentage of transparency of GO thin films is higher than rGO thin films. Increasing the number of spray passes would also reduce the transparency percentage. This is because of the thickness of the thin films become thicker when more GO flakes drop on the substrate. There is a trade-off between the optical conductivity and electrical conductivity. Therefore, optimization of the reduction process is needed to ensure good quality of rGO thin films formed on the substrate, which can produce low sheet resistance and high light transmittance.
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  Fig. 6. GOand rGO light transmittance percentage at different wavelength with different spray passes. Table II below summarized the percentage of transparency of the GO and rGO at 550 nm wavelength optical transmission. The GO thin films showed degradation of %Transparency with an increasing number of spray passes but the reduction of light transmittance percentage is much lower as compared to rGO thin film materials. All GO samples with 1, 2 and 3 spray passes have more than 95% transparency but when the layer went through the reduction RTP process, then the degradation rate becomes more severe resulted the rGO thin films with 3 spray passes to reduced down to 77% transparency. In comparison with other types of materials as reported by Hofmann et al. [16], the %Transparency of rGO in this study is comparable to the ITO, silver nanowire (AgNWs), single-wall nanotubes (SWNTs) and polystyrene sulfonate (PEDOT:PSS) but with one decade higher sheet resistance. However, it is noted that the FOM (Figure-of- Merit) of the rGO thin films from this study is higher than the previously reported rGO by Shi et al. [3], which the transmittance (%) is only about 68.7% to 79.6% with the sheet resistances between 58 kΩ/sq to 3140 kΩ/sq. TABLE II. TRANSPARENCY PERCENTAGE OF GO AND RGO (RTP) #Pass % Transparency Graphene Oxide, GO Reduced Graphene Oxide, rGO (RTP) 1 98.6% 91.6% 2 97.1% 84.9% 3 96.2% 77.1% For the transparent conductive electrode (TCE) to be considered for industrial application, it should have a sheet resistance of less than 100 Ω/sq at more than 90% transparency [17]. However, in a real application, the requirements on the TCE strongly depends on the specific application [18]. IV. CONCLUSION Ultrasonic atomization process was introduced as one of the methods to deposit nanomaterials coating on large diameter substrate i.e. silicon/glass wafer for the transparent conductive electrode. The thickness of the thin films can be controlled within a nanometer-scale with high uniformity coating process. This is one the low-cost solution for high volume production of TCE and it has high potential for scaling-up production. As a proof-of-concept, we have successfully fabricated the transparent electrode on 200 mm silicon wafers and glass slides with good transparency percentage (>77%). RTP reduction method with temperature of up 1100 ℃ has shown better performance in comparison with PECVD reduction method. ACKNOWLEDGMENT The authors acknowledge funding from the Government of Malaysia under the 11MP program (P11-Establishment of National Nanoelectronics Shared Infrastructure). This work was performed with the collaboration between MIMOS Berhad, NSW Automation Sdn. Bhd, and Faculty of Engineering, Multimedia University. REFERENCES [1] D. S. Hecht, L. Hu, and G. 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