Synthesis and Physical Properties of Semi-Transparent Conductive Ag-Nanowire Network

Chiang Mai J. Sci. 2013; 40(6) 985 Chiang Mai J. Sci. 2013; 40(6) : 985-993 http://epg.science.cmu.ac.th/ejournal/ Contributed Paper Synthesis and Physical Properties of Semi-Transparent Conductive Ag-Nanowire Network Tula Jutarosaga*[a,b], Panita Chityuttakan [a,b], Wandee Onreabroy [a,b] and Anuwat Hassadee [a] [a] Department of Physics, Faculty of Science, King Mongkut s University of Technology Thonburi, Bangkok 10140, Thailand. [b] Thin Film Technology Research Laboratory, ThEP Center, CHE, 328 Si Ayutthaya Rd., Ratchathewi, Bangkok 10400, Thailand. *Author for correspondence; e-mail: tula.jut@kmutt.ac.th Received: 11 April 2012 Accepted: 8 August 2012 ABSTRACT A synthesis method of semi-transparent conducting Ag-nanowire network on glass substrates at room temperature using a simple oxidation-reduction reaction of Cu nanoparticles and 0.1M AgNO 3 solution was presented. The morphological, structural, electrical and optical properties of the synthesized Ag nanowire networks were characterized using scanning electron microscopy, X-ray diffraction, 4-point probe technique and UV-Vis spectrophotometry. The synthesized Ag nanowires had FCC structure. The Ag-nanowire network exhibited the semi-transparency up to 36% with the sheet resistance of about 104 Ω/sq. It was suggested that the morphology of the Cu thin film may play an important role in controlling the Ag nanowire density and morphology. Keywords: Ag nanowire network, transparent conductive thin film, oxidation-reduction reaction 1. INTRODUCTION Transparent conductive thin films have been widely used in various applications such as components in electronics displays or photovoltaic cells. Indium tin oxide (ITO) thin films, so far, are probably the most conventional transparent conductive oxide thin films to use in such applications because their high transparency and the high electrical conductivity. Other optional candidates to do the jobs are being investigated, such as graphene [1], other oxide thin films for example doped-zno thin film [2], or carbon-nanotube thin films [3]. Recently, nanowire thin films have shown their acceptable conductivity and transparency to compete with the conventional ITO thin films. In some cases, their cost effectiveness was above the indium tin oxide films which the materials and the required vacuum process are expensive. Ag nanostructure materials are attracted to many researchers due to their unique properties such as the optical, electrical properties [4] and the antibacterial purpose [5]. Especially, Ag nanowires show the promising application for transparent

986 Chiang Mai J. Sci. 2013; 40(6) conductive thin film. Various methods for coating nanowires on the substrates were presented such as roll-coating [6] and airspraying [7]. Liu and Yu showed that the obtained Ag nanowire thin film using the rollcoating method provided the optical transmission spectrum in the visible light region (400nm - 700nm) approximately 74% with the resistivity of 170 Ω/sq [6]. However, as indicated, most reported techniques were methods for transferring nanowires onto the substrates. Groep et al. [8] showed the 2D Ag nanowire network fabricated using electron beam lithography with the nanowire width varied from 45 nm to 100 nm. It was found that the transmission dropped as Ag nanowire width in the network increased. One of the main causes of the drop was attributed to the excitation of the localized surface plasmon resonance (LSPR) of each individual nanowire [8] as the nanowire size increased. Therefore, in our case, we were interested in fabricating self-assemble nanowire network for the application of the transparent conductive thin film in a single simple step and being able to control the sizes of the synthesized nanowires. The reaction of Cu and AgNO 3 is wellknown. However, not many research groups have focused on direct synthesis of Ag nanowire on transparent substrate. In our case, the chosen reaction was an oxidation-reduction reaction of Cu and AgNO 3. Cu(s) + 2AgNO 3 (aq) 2Ag(s) + Cu(NO 3 ) 2 (aq) (1) Ag acted as an oxidizing agent, causing the copper to loose electrons. Cu ions displaced Ag in the AgNO 3, producing an aqueous solution of Cu(NO 3 ) 2. The reduction reaction happened when Ag ions gained electrons from Cu and precipitated out as solid metal. By controlling the size and the arrangement of Cu particles, we expected to control the size and possibly control the density of Ag nanowires. 2. MATERIALS AND METHODS As indicated in the previous section, the presence of Cu was important for the growth of Ag crystal. It was expected that the size of the seed particles may control the diameter of the synthesized Ag crystal. Thin deposition of Cu on substrate was then necessary to obtain the particle sizes in the nanometer range. Also, the film thickness could be easily confirmed by the optical method. Therefore, the chosen thin Cu films were then prepared on glass substrates by a conventional thermal evaporation. Two different evaporation conditions provided two different thicknesses with the difference in morphology. The estimated thicknesses were about 35 nm and 45 nm. The 2.5 2.5 cm 2 Cu films on glasses were then soaked into 10 ml of 0.1M Ag nitrate (AgNO 3 ) solution at room temperature at various times. Only data with soaking times of 1-2 minutes and 15 hours were presented. The films were then rinsed with deionized water and dried. The morphological, structural, electrical and optical properties were then examined using a field emission scanning electron microscope (Hitachi-S4700), X-ray diffractometer, 4-point probe (Signatone Pro4 S-302-4) and UV-Vis spectrophotometer (Avantes AvaSpec). For X-ray diffraction, the 2 glazing angle was conducted using a Bruker AXS D8 Discover using Cu K α with the scan step of 0.05 /s. The crystalline sizes (L) of Cu seed particles and synthesized Ag-nanowire network were then obtained using the following Scherrer formula where B,the peak width, is a function of θ ; K is the Scherrer constant; and λ is the wavelength of Cu K α (0.15 nm).

Chiang Mai J. Sci. 2013; 40(6) 987 B (2) The electrical properties of the films were reported in term of sheet resistance. When the film became thinner, the resistivity is a strong function with film thickness. The sheet resistance has the same unit as resistance. However, it is specified as ohm/square (Ω/sq) not to confuse with the resistance. 3. RESULTS AND DISCUSSION Figure 1(a, b) show the scanning electron micrographs of as-deposited 35-nm and 45-nm Cu thin film on glass substrate using two different evaporation conditions of Cu metal. Based on the simple observation, the 35-nm Cu film showed the island-like structure. Most particles are isolated. However, in the case of 45-nm Cu film, the smaller Cu particles were observed. We expected that the Cu particles impinged and subsequently formed continuous thin film. This would be later confirmed by the electrical measurement which we found that there was electrical conductivity in the 45-nm Cu thin film, but not in the 35-nm. The different in the Cu seed sizes was possibly a result of the evaporation conditions. However, we would not focus on how to obtain this different morphology of Cu thin film. We were more interested in the effect of size of the Cu particles on the morphological and structural of the synthesized materials. Figure 1. Scanning electron micrographs of (a) 35-nm Cu/glass and (b) 45-nm Cu/glass. Therefore, the simple calculation based on the correlation length concept [9] was applied on the scan profile of the SEM micrographs, instead of using AFM profile image, in order to extract the correlation length. As shown in figure 2(a) and 2(b), the scan profiles were obtained from SEM micrographs using Image J [10]. The roughness of the film was not directly measured from the images. However, the profile implied that the roughness of 35-nm Cu/glass was higher than 45-nm Cu/glass. Note that all images were taken at the similar depth of field setting. Our analysis showed that the correlation length in 35-nm Cu film and 45-nm Cu were approximately 50 nm. The correlation length is relatively similar to particle size of the observed SEM images. However, in the case of 35-nm Cu film, the correlation length should be relatively larger than that of 45-nm Cu, but the analysis showed a very similar correlation length because, when looked closely at the image, the Cu islands in figure 1(a) consisted of smaller Cu particles. Without the calculation, the observed particle size might be misleading. The obtained correlation length corresponded to the observed particle size in the SEM micrographs.

988 Chiang Mai J. Sci. 2013; 40(6) Figure 2. Scan profiles of the (a) 35-nm Cu/glass and (b) 45-nm Cu/glass. Figure 3(a, b) show the synthesized materials after soaking in 0.1M AgNO 3 solution for 1 minute. The synthesized structures in both cases were wire-like structure. The density of the wire from 35-nm Cu/glass starting materials was higher than that of 45-nm Cu/glass. Also, figure 3(a) clearly show that the observed wire formed network, while in figure 3(b) the lower density wire with sparse needle-like structures was observed on the glass substrate. It is suggested that the morphology of Cu nanoparticles probably strongly affected the synthesized products. The nanowire diameter of figure 3(a) was varied from approximately 40 nm to about 450 nm with the average of 140 nm ± 80 nm. Figure 3. Scanning electron micrographs of (a) wire-like nanostructure from 35-nm Cu/ glass and (b) that from 45-nm Cu/glass after soaking in 0.1M AgNO 3 solution for 1 minute. Figure 4(a, b) show the X-ray diffraction patterns of as-deposited 35-nm and 45-nm Cu thin films. The figure confirmed that as-deposited films are Cu. The films are face center cubic structure of Cu (JCPDS file 04-0836). As expected, the high intense peak for FCC, (111) plane, are observed in both cases. In the case of FCC Cu, (111) plane is at 43.297. Also, the thicker films showed the higher intensity than the thinner one and the (200) and (220) planes are presented in the diffraction pattern of 45-nm Cu thin film. Figure 4(c, d) show the X-ray diffraction pattern of as-synthesized Ag nanostructure from 35-nm and 45-nm Cu thin films, respectively. Overall, the spectra indicated the synthesized materials are FCC Ag in both cases corresponding to the JCPDS file no 89-3722. It is interesting to point out that the (111) plane intensity of the

Chiang Mai J. Sci. 2013; 40(6) 989 figure 4(c) is stronger than that of figure 4(d) while other peak heights are relatively similar. This observation indicated that the Cu seed thickness caused the preferred orientation of the synthesized materials. In addition, in both as-deposited Cu thin film and Ag-nanowire network, oxide had not been observed. Figure 4. X-ray diffraction spectra of (a) 35-nm and (b) 45-nm Cu thin film/glass substrates and (c, d) Ag nanowires after the reaction between Ag nitrate and 35-nm and 45-nm Cu, respectively. Table 1 shows the calculated crystalline size of Cu thin film and Ag-nanowire network using Scherrer formula. For the Cu thin film, the crystalline sizes are approximately 14 and 15 nm for 35-nm and 45-nm Cu thin films, respectively. As shown in the table, after processing 35-nm and 45-nm Cu thin films, the crystalline sizes of the synthesized Ag nanowires are approximately 24 nm and 22 nm, respectively. From equation 1, the reaction of Cu(s) and AgNO 3 (aq) caused the precipitation of Ag(s) and Cu(NO 3 ) 2 (aq). At the initial stage, the Ag crystalline size would be controlled by the Cu crystalline size. However, the crystal grew in both diameter and length directions when the process proceeded. The growth caused the increase of the crystalline size in case of figure 3(a). However, we observed less nanowire in figure 3(b) possibly because of the smaller size of the starting Cu seeds and also possibly the adhesion of the Cu film to the substrate. Unlike the 35-nm Cu, 45-nm Cu films may compose of layers of smaller Cu particle size. The precipitate Ag particles possibly went from the substrate back to the solution as colloid during the reaction. Therefore, mostly needlelike Ag was observed as shown in figure 3(b).

990 Chiang Mai J. Sci. 2013; 40(6) Table 1. The crystalline sizess of Cu particles and Ag nanowires calculated from the X-ray diffraction pattern using Scherrer formula. Film condition As-deposited 35-nm Cu/glass As-deposited 45-nm Cu/glass 35-nm Cu/glass soaked in AgNO 3 for 1 min 45-nm Cu/glass soaked in AgNO 3 for 1 min Crystalline size (nm) Cu Ag 14-15 - - 24-22 Table 2 shows the sheet resistance of the as-deposited Cu/glass and Ag-nanowire thin film. From the table, 35-nm as-deposited Cu/ glass film showed no electrical conductivity which was possibly a result of the isolation of Cu islands (figure 1(a)). In contrast, the sheet resistance of 45-nm Cu/glass film was about 2.8±0.3 Ω/sq, indicated that the film are continuous. After soaking in AgNO 3 solution for 1 min and 15 hours, the sheet resistance changed. For the 35-nm Cu/glass substrate, the sheet resistance became detectable. As shown in figure 3(a), the connection among nanowires provided electrical conduction paths. However, in the case of a sample from 45-nm Cu/glass, the sheet resistance could not be measured. From figure 3(b), lower density of the synthesized nanowires was observed which provided no electrical conduction paths. However, after soaking the solution for 15 hours, the sheet resistance could be measured in both cases. The values are approximately 38.5 and 104.2 Ω/sq, respectively. It could be inferred that the conduction paths were created after soaking the sample in AgNO 3 solution long enough. Table 2. The sheet resistance of the as-deposited Cu/glass and Ag-nanowire network on glass. Film condition As-deposited Cu/glass Soaked in AgNO 3 for 1 min Soaked in AgNO 3 for 15 hours Sheet resistance (Ω/sq) 35 nm Cu/glass NA 25.5 ± 3.5 38.5 ± 7.6 45 nm Cu/glass 2.8 ± 0.3 NA 104.2 ± 19.0 The resistivity of bulk Cu and bulk Ag metal at 20 C is 1.69 10-6 Ω cm and 1.59 10-6 Ω cm [11], respectively. Based on the values and the sheet resistance in table 2, the calculated equivalent thickness of Cu thin film with the sheet resistance of 2.8 Ω/sq was 6 nm, while those of Ag thin films are 0.6 nm, 0.4 nm and 0.15 nm for the sheet resistance of 25.5 Ω/sq, 38.5 Ω/sq and 104.2 Ω/sq, respectively. The equivalent thickness of Cu thin film was a lot less than our estimation from the evaporation because the film was not completely continuous. The observed high resistivity of the film was normal for thin metallic film with this similar structure [12]. Also, the equivalent Ag thickness was in the sub nanometer range possibly indicating that the quality of the nanowire network was inferior than the continuous film due to the problem of nanowire contacts causing the inferior electron transport. The further improvement was then needed to be investigated.

Chiang Mai J. Sci. 2013; 40(6) 991 Beside the electrical properties, in order to use this network as transparent electrodes, we were interested in the optical transmission. The films were analyzed using UV-Vis spectrophotometer to obtain the relationship between the percent transmission (%T) and the wavelength. The transmittance data were taken and compared to the glass substrate. Figure 5(a) and 5(b) show the optical transmission spectra of 35-nm Cu/glass film and their products as well as 45-nm Cu/glass film and their products, respectively. The transmission of Cu thin film in figure 5(a) was higher than that of figure 5(b) due to the thickness effect. In both figures, the transmittance (%T) of the Cu thin films showed the similar characteristics. The percent transmission rised as the incident wavelength increased and then peaked at 580 nm, following by the decrease of the transmission. At the wavelength below 580 nm is where the interband transition took place [13]. Figure 5. The optical transmission spectra of (a) 35-nm Cu/glass film and their products as well as (b) 45-nm Cu/glass film and their products. After the reaction, 35-nm Cu thin film converted to Ag nanowires. The overall transmission dropped. Due to the interband absorption edge of Ag is at 4 ev, the transmission increased and peaked at 320 nm. Then, the transmission decreased sharply again as a result of the high reflectivity of Ag in the visible wavelength [14]. In case of 45-nm Cu thin film, after converting to Ag nanowires, the transmission spectra showed the similar characteristics to the Ag nanowires from 35-nm Cu thin film.

992 Chiang Mai J. Sci. 2013; 40(6) However, the overall percent transmission was higher due to the lack of the presence of nanowire networks. Table 3 shows the average percentage of the transmission of the Cu thin film and Agnanowire films in the visible region (400 nm to 700 nm). As expected thicker Cu films transmitted lower percentage than the thinner one. The 35-nm Cu film transmitted approximately 41%, while the 45-nm Cu film transmitted approximately 25%. In case of 35-nm Cu film, after soaking in AgNO 3 solution for 1 minute and 15 hours, the transmittance reduced to 28% and 27%, respectively. The reduction of the transmittance was possibly a result of the formation nanowire network. In contrast, the transmittance of samples for 45-nm Cu seed increased from 25% to 46% after soaking in AgNO 3 solution for 1 minute. The increases of the percent transmission corresponded to the SEM micrographs in figure 3(b). After soaking for 15 hours, the percent transmission then reduced to 36%. We started to see the shiny materials on the substrate. Further investigation on the effect of the particle size, the reaction temperature, the addition of other additive and the concentration of the reactants are necessary to be studied in order to obtain the high quality transparent Ag-nanowire thin film. Table 3. Average percent transmission (400 nm to 700 nm) of the as-deposited Cu/glass and Ag-nanowire thin film. Film condition As-deposited Cu/glass Soaked in AgNO 3 for 1 min Soaked in AgNO 3 for 15 hours 35 nm Cu/glass 41 28 27 Transmittance (%) 45 nm Cu/glass 25 46 36 4. CONCLUSION The Ag-nanowire thin films were fabricated using a simple oxidation-reduction reaction of Cu seed and AgNO 3 solution at room temperature. The preliminary semitransparent conducting nanowire films were fabricated with the percent transmission of 36% and the sheet resistance of 104 Ω/sq. ACKNOWLEDGEMENTS The work has been supported in part by the funding from the Faculty of Science, King Mongkut s University of Technology Thonburi. We would like to thank Dr. Chanwit Chityuttakan for providing the Cu thin films. REFERENCES [1] Geng J., Liu L., Yang S.B., Youn S.C., Kim D.W., Lee J., Choi J. and Jung H., A Simple Approach for Preparing Transparent Conductive Graphene Films Using the Controlled Chemical Reduction of Exfoliated Graphene Oxide in an Aqueous Suspension, J. Phys. Chem. C., 2010; 114: 14433-14440. [2] Ma Q., Ye Z., He H., Hu S., Wang J., Zhu L., Zhang Y. and Zhao B., Structural, Electrical, and Optical Properties of Transparent Conductive ZnO:Ga Films Prepared by DC Reactive Magnetron Sputtering, J. Crystal Growth, 2007; 304: 64-68.

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