Silver nitrate (AgNO₃), poly (vinylpyrrolidone) (PVP, K-30), sodium chloride (NaCl), ethylene glycol (EG) and acetone were purchased from Sinopharm Chemical Reagent Co., Ltd. All reagents were of analytical grade and used without further purification.

AgNWs were synthesized by a modified polyol reduction process^[14]. In a typical procedure, 150 ml of 0.15 mol/l PVP EG solution in a three-neck flask was heated to 165 ° C for 1 h under continuous magnetic stirring. Then, 120 µ l of 0.1 mol/l NaCl EG solution was added to the flask. After 10 min, 150 ml of 0.1 mol/l AgNO₃ EG solution was injected to the flask at a rate of 5 ml/min. The obtained solution was cooled naturally to room temperature after 20 min, filtered through 0.8 µ m pore-size filter membrane, and diluted with deionized water (3 times by volume). The AgNWs collected on filter membrane were redispersed in deionized water by sonication. Finally, the silver nanowire solution with a concentration of 0.04 mg/ml in deionized water was obtained.

Fig.1(a) shows the fabrication process of FTCFs with AgNWs on PET substrate. The AgNW film was obtained by vacuum filtration using 0.8 µ m average pore size filter membrane, and then transferred to a flexible substrate by direct PET molding. The PET film was heated at 100 ° C for about 4 h while the filter membrane with the AgNWs was placed on the surface. After cooling to room temperature, the filter membrane was removed forming a silver nanowire film on the PET surface. It can be seen from photograph of FTCFs with different silver nanowire contents in Fig.1(b) that the fabrication process produced optically uniform and transparent, flexible PET with the area of AgNWs coverage defined by the filter membrane diameter (~4 cm). The weight density of the AgNW networks (weight of AgNWs per square meter) on PET films was controlled by adjusting the volume of AgNWs solution during filtering.

Fig.1

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	Fig.1 (a) Schematic of AgNWs on PET film fabrication; (b) photograph of FTCFs with different densities of AgNWs on PET substrate.

The surface morphology of AgNW-PET films was examined by a field emission scanning electron microscope (FE-SEM, Hitachi S-4800, Japan). The optical transmittance of the AgNW/MC films was obtained using a UV-Vis-NIR spectrophotometer (Perkin-Elmer, Lambda 950, USA) with an integrating sphere. The sheet resistance was measured by a four-point probe system (Napson Corp. Cresbox).

As shown in the photograph of Fig.1(b), the films were transparent and the logo of NIMTE was clearly visible when the density was below 60 mg m^-2. Contrarily, the films were opaque and the logo of NIMTE was vague when the density of AgNW was above 200 mg m^-2. Fig.2(a-d) shows the microstructure of FTCFs surface using SEM analysis of AgNW film on PET substrate. The AgNWs were distributed on the PET film uniformly. As the density of AgNWs increased, the AgNW-uncovered space of PET film became narrower, which led to the reducing of the transparence of FTCFs. Fig.2(e, f) shows the distributions of diameter and length by measuring 100 nanowires on the substrate. The silver nanowires present a relatively high aspect ratio (approximately 190) with an average length of 15.1 µ m and average diameter of 79.6 nm.

Fig.2

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	Fig.2 SEM images of FTCFs with different AgNW weight densities: (a) 24 mg m-2, (b) 40 mg m-2, (c) 80 mg m-2 and (d) 200 mg m-2; (e) histogram of diameter size distribution (100 nanowires measured); (f) histogram of length size distribution (100 nanowires measured).

The transmission spectra of FTCFs with different AgNW weight densities are shown inFig.3. The absorption near 380 nm exhibited a surface plasmon resonance of silver nanowires^{[15, 16]}. As the reflecting and scattering light by the AgNWs, the transmittance of films decreased with increasing AgNW weight density. Fig.3(b) shows the difference of the transmittance at a fixed wavelength (550 nm) with AgNW weight density. With the weight density of 16 mg m^-2, the transmittance of the FTCF is slightly reduced from 87.5% (for pure PET film) to 82.3%. As the AgNW weight density increased to 80 mg m^-2, the film became semi-transparent, with the transmittance of 65.6%. The inverse linear relation between transmittance of films and the AgNW weight density is quite similar to the results discussed elsewhere^[17].

Fig.3

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	Fig.3 (a) Transmittance of FTCF with different AgNW weight densities; (b) relationship between AgNW weight densities, transmittance at 550 nm and figure of merit.

Fig.4 shows the electrical sheet resistance of the FTCFs as a function of the nanowire weight density at room temperature. With the AgNW increased from 16 to 60 mg m^-2, the sheet resistance of FTCF was reduced from 177.3 to 4.95 Ω sq^-1 quickly. As the AgNW weight density increased from 100 mg m^-2 to 400 mg m^-2, the sheet resistance of the film decreased from 1.9 to 0.56 Ω sq^-1 slowly.

Fig.4

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	Fig.4 Sheet resistance of FTCF with various AgNW weight densities. Inset: fit of the experimental data to the percolation equation.

The relationship between the experimentally determined sheet resistivity R_s and the area fraction of AgNW A_f in films can be expressed as^[18]:

(1)

where A_fc is the critical area fraction of silver nanowires in FTCF, t is the conductivity exponent, and M is a constant. The area fraction of AgNW A_f can be calculated based on the following equation^[19]:

(2)

where m_s is the weight density of AgNWs and ρ _Ag is the density of silver. < D> is the average diameter of AgNWs, which was confirmed to be 79.6 nm in this report with SEM analysis. As shown in Fig.4 (Inset), the experiment data were fitted well with R² of 0.999 and Pearson's r of -0.999. The critical area fraction A_fc was found to be 1.90%, which was in good agreement with the theoretical value (1.95%) from Excluded Volume Model^[20]. The conductivity exponent was extracted as t = 1.37, which was in agreement with theoretical value of 1.3 for the two-dimensional nanowires network ^[19].

The figure of merit (Φ ) is usually used to describe the overall performance of transparent conductive film, which can be expressed as ^[16]:

(3)

where T is the transmittance excluding PET at a wavelength of 550 nm and R_s is the sheet resistance of the film. According to Table 1, AgNWs played a leading role in the electrical property of the film when the weight density of AgNWs is lower, which improved the figure of merit of our FTCFs. As the weight density of AgNWs became much higher, the transmittance of the films decreased rapidly, which resulted in a poor figure of merit. The maximum figure of merit was obtained to be 31.3 × 10^-3 Ω ^-1 when AgNW weight density was 60 mg m^-2 with T_550nm (excluding PET substrate) of 83.0% and sheet resistance of 4.95 Ω sq^-1. This result was higher than what previous literature reported^{[21, 22]}.

Table.1

Table.1

Table.1 Comparison of the electrical and optical properties of FTCF with different AgNW weight densities

Table.1 Comparison of the electrical and optical properties of FTCF with different AgNW weight densities