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Facile Synthesis of Silver Nanowires with Different Aspect Ratios and Used as High-Performance Flexible Transparent Electrodes

Facile Synthesis of Silver Nanowires with Different Aspect Ratios and Used as High-Performance... Silver nanowires (Ag NWs) are the promising materials to fabricate flexible transparent electrodes, aiming to replace indium tin oxide (ITO) in the next generation of flexible electronics. Herein, a feasible polyvinylpyrrolidone (PVP)- mediated polyol synthesis of Ag NWs with different aspect ratios is demonstrated and high-quality Ag NWs transparent electrodes (NTEs) are fabricated without high-temperature thermal sintering. When employing the mixture of PVP with different average molecular weight as the capping agent, the diameters of Ag NWs can be tailored and Ag NWs with different aspect ratios varying from ca. 30 to ca. 1000 are obtained. Using these as- synthesized Ag NWs, the uniform Ag NWs films are fabricated by repeated spin coating. When the aspect ratios exceed 500, the optoelectronic performance of Ag NWs films improve remarkably and match up to those of ITO films. Moreover, an optimal Ag NTEs with low sheet resistance of 11.4 Ω/sq and a high parallel transmittance of 91. 6% at 550 nm are achieved when the aspect ratios reach almost 1000. In addition, the sheet resistance of Ag NWs films does not show great variation after 400 cycles of bending test, suggesting an excellent flexibility. The proposed approach to fabricate highly flexible and high-performance Ag NTEs would be useful to the development of flexible devices. Keywords: Ag NWs, Tailorable aspect ratios, Flexible transparent electrodes, Low-temperature welding Background (>80%). But its intrinsic brittleness limits the applica- Flexible transparent electrodes (FTEs) play an import- tions in flexible electronics. Moreover, it requires high ant role in the next generation of flexible electronics temperature deposition process and is challenged by [1–4]. FTEs can be applied to many optoelectronic the scarcity of indium [25–27]. Therefore, several new devices as conductive components, involving touch conductive films with good flexibility and optical screens [5, 6], portable solar cells [7, 8], organic light- transparency, such as metal grids [2, 28, 29], carbon emitting diodes (OLEDs) [9–11], fuel cell electrode nanotubes (CNTs) [30–33], graphene [34–36], Ag [12–17], sensors [18, 19], PM filter [20], transparent NWs [5, 37–41], Cu NWs [42, 43], conductive poly- heaters [21, 22], and wearable electronics [23, 24]. mers [44, 45], and hybrids of these [46–48], have The dominant transparent electrodes (TEs) used been fabricated, striving to replace ITO. Among these currently is indium tin oxide (ITO) owing to the low candidates, Ag NWs films have been investigated sheet resistance (<100 Ω/sq) and high transmittance extensively in both the scientific and industrial insti- tutions, owing to the excellent electrical conductivity and high optical transparency. In addition, Ag NWs * Correspondence: weiwu@whu.edu.cn exhibit outstanding flexibility and stretchability, which Laboratory of Printable Functional Nanomaterials and Printed Electronics, is the one of the appealing advantage to fabricate School of Printing and Packaging, Wuhan University, Wuhan 430072, People’s Republic of China stretchable transparent conductors than fragile ITO Shenzhen Research Institute of Wuhan University, Shenzhen 518057, [49–51]. Moreover, the solution-processed Ag NWs People’s Republic of China © The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Xue et al. Nanoscale Research Letters (2017) 12:480 Page 2 of 12 films are more cost-effective than ITO. All of these employed. It is incompatible with flexible plastic sub- properties make Ag NWs films become promising strates which cannot withstand high temperature, and alternatives to ITO for the applications in flexible hence limits the applications of Ag NWs films in flexible electronics. optoelectronic devices. However, several issues need to be addressed to Herein, a series of Ag NWs with different aspect ra- commercialize Ag NWs films as FTEs. Firstly, Ag NWs tios varying from ca. 30 to ca. 1000 are controllably with different aspect ratios need to be facilely synthe- synthesized and used to fabricate high conductive and sized in controlled manner because the alluring proper- transparent Ag NTEs. First, Ag NWs are prepared by ties of Ag NWs films deeply rely on the dimensions of facile PVP-mediated polyol process where the mixture Ag NWs and a well-designed length and diameter are of of PVP with different average molecular weight can very importance for different applications [52, 53]. efficiently reduce the diameters. Subsequently, the as- Generally, polyol process is the most widely used synthesized Ag NWs with different aspect ratios are method to prepare Ag NWs. Ran et al. [54] synthesized employed to fabricate Ag NWs films without high- thin Ag NWs with aspect ratios larger than 1000 by temperature annealing, respectively. And the corre- using the mixed PVP with the average molecular weight sponding optoelectronic performance are comparative of 58,000 and 1,300,000 as the capping agent. However, investigated. The best sheet resistance and parallel the influence of the aspect ratios on the optoelectronic transmittance can achieve 11.4 Ω/sq and 91.6% when performance of Ag NTEs was not carefully investigated in the aspect ratios reach almost 1000. Moreover, the their work. Although Ding et al. [55] prepared Ag NWs sheet resistance of as-fabricated Ag NWs films is nearly with different diameters varying from 40 to 110 nm and constant after inner-bending and outer-bending tests. fabricated Ag NTEs with a transmittance of 87% and a sheet resistance of ca.70 Ω/sq, many parameters need to Methods be simultaneously adjusted to control the diameters of Ag Materials and Chemicals NWs and the optoelectronic performance of the as- Silver nitrate (AgNO , AR) and anhydrous ethanol obtained Ag NTEs would not be satisfactory. Li et al. [56] (C H OH, AR) were purchased from Sinopharm Chem- 2 5 synthesized thin Ag NWs with diameters of 20 nm ical Reagent Co., Ltd. Copper (II) chloride dehydrate through altering the concentration of bromide. And they (CuCl ·2H O, AR) and PVP (MW≈58,000, marked as 2 2 have fabricated high-quality Ag NWs films with a trans- PVP-58) were purchased from Shanghai Aladdin mittance of 99.1% at 130.0 Ω/sq. Ko et al. [57] developed Reagents Co., Ltd. Ethylene glycol (EG, 98%) and PVP a multistep growth method to synthesize very long Ag (MW≈10,000, 40,000 and 360,000, marked as PVP-10, NWs over several hundred micrometers and the fabri- PVP-40, and PVP-360, respectively) were purchased cated films demonstrated superior transmittance of 90% from Sigma-Aldrich. Deionized water (18.2 MΩ)was with sheet resistance of 19 Ω/sq. The optoelectronic used in the whole experiments. performance of these Ag NWs films are comparable to or even better than those of ITO films. But the minimum as- pect ratio of Ag NWs, which has the ability to fabricate Synthesis of Ag NWs TEs rivaling commercial ITO in terms of sheet resistance Ag NWs with different aspect ratios are prepared by a and transmittance, is still uncertain. Therefore, it is neces- facile one-pot PVP-mediated polyol process. Typically, sary to synthesize Ag NWs with various aspect ratios and 0.170 g of AgNO is dissolved in 10 mL of EG under study their influence on the optoelectronic performance magnetic stirring. Then, 0.15 M of PVP-40 and of Ag NWs films. 0.111 mM of CuCl ·2H O mixed solution in 10 mL of 2 2 Furthermore, the electronic conductivity of Ag NWs EG is added dropwise to the above solution. Afterwards, films is relatively poor, resulting from the high nanowire the mixture is transferred into Teflon-lined stainless junction resistance [58]. In the polyol synthesis of Ag steel autoclave with a capacity of 50 mL and heated at NWs, PVP, as the surfactant, adsorbs on the surface of 160 °C for 3 h. After cooling down to room temperature Ag NWs, resulting in insulated contact between the naturally, pure Ag NWs are obtained by centrifugation at a wires in the random network [59, 60]. Consequently, speed of 2500 rpm for 5 min and washed three times with different physical and chemical post-processes, involving ethanol and deionized water. Finally, the products are thermal annealing [38, 39, 61, 62], mechanical press dispersed in ethanol for further characterization and appli- [63], nanosoldering with conductive polymers [64], cation. Moreover, the concentration and average molecular plasmonic welding [65], laser nanowelding [66–68], and weight of PVP are very important to control the morph- integration with other materials [60], have been explored ology and size of products. Therefore, different types of to reduce the junction resistance. Among these post- PVP molecules are simultaneously used to regulate the treatments, thermal annealing at almost 200 °C is usually diameters of Ag NWs in the polyol process. Detailed Xue et al. Nanoscale Research Letters (2017) 12:480 Page 3 of 12 experimental parameters are listed in Additional file 1: measured at room temperature by using 4-point probe Table S1, nominated as S1–S13, respectively. resistance tester (FP-001). Fabrication of Ag NTEs Results and Discussion Polyethylene terephthalate (PET) with a thickness of Generally, Ag NWs are synthesized by polyol process in 150 μm is cut to pieces with the dimension of 20 × 20 mm. which PVP is employed as capping agent to ensure the Briefly, the as-prepared Ag NWs are dispersed in ethanol growth of one-dimensional Ag NWs [69, 70]. During the (6 mg/mL), and 50 μL of Ag NWs solution is spin coated synthesis, many parameters such as reaction temperature, at 2000 rpm for 30 s on PET substrate. Finally, the Ag stirring speed, PVP concentration, PVP chain length, addi- NWsfilmsare heated to 140°Cfor 15minwithout any tive agents, and ratio of chemicals can affect the yield and additional post-process treatments. The aspect ratios morphology of synthesized Ag NWs. For example, an of Ag NWs, rotation speed, concentration, and vol- inappropriate reaction temperature less than 110 °C or ume of Ag NWs solution are investigated to fabricate higher than 180 °C allows more Ag atom to form Ag high-quality NTEs. Regarding to the repeated spin nanoparticles (NPs) rather than Ag NWs [70, 71]. The coating, each volume of Ag NWs solution is altered length of synthesized Ag NWs increase as slowing down to 25 μL and the rotation speed is set to 2000 rpm. the stirring speed [72, 73]. In this paper, we mainly investi- A time interval in each spin coating is needed to gate the concentration of PVP and their average molecular volatilize the ethanol. Other parameters are same as weight on the effect of morphology and size of Ag NWs. the aforementioned processes. The corresponding morphology and size distribution of Ag NWs are demonstrated in Fig. 1 and Additional file 1: Characterization and Performance Test Figure S1. Firstly, the concentration of PVP is increased Scanning electron microscopy (SEM) images are from 0.05 M (sample S1, Additional file 1: Figure S1a) to recorded using a cold field-emission SEM (Hitachi S- 0.15 M (sample S2, Fig. 1a). The corresponding morph- 4800). The transmission electron microscopy (TEM) and ology of products is changed from near-spherical Ag NPs the high-resolution TEM (HRTEM) images are obtained to pure Ag NWs with an average diameter of 104.4 nm and by using a JEOL JEM-2100F. The UV-vis absorption length of 12.3 μm. The mixture of Ag NWs and Ag NPs spectra of Ag NWs and the optical transmittance spectra are observed when the concentration of PVP is increased of Ag NWs films are carried out on a Shimadzu UV- to 0.25 M (sample S3, Additional file 1: Figure S1b). 3600 spectrophotometer. The sheet resistance is By further increasing the concentration of PVP to Fig. 1 a, b SEM images of as-synthesized Ag NWs with PVP-40 and PVP-360, respectively. Both the concentration of PVP are 0.15 M. a′ b′ Corresponding statistical distribution of diameter and length. (The insets in a and b are the corresponding SEM images with high magnifi- cation and all the scale bars are 500 nm) Xue et al. Nanoscale Research Letters (2017) 12:480 Page 4 of 12 0.55 M (sample S4, Additional file 1: Figure S1c), a with high average molecular weight in EG solution large number of Ag NPs with different shapes (in- would slow down the growth rate, which are benefit to cluding near-sphere and triangular plate) are formed. form MTPs [76, 77]. As a result, the low average The results indicate that a lower or higher concentra- molecular weight of PVP, like as PVP-10, would not tion of PVP are not beneficial to produce pure Ag efficiently adsorb on the (100) crystal faces to restrict the NWs, further resulting in the absence of Ag NWs. lateral growth. Meanwhile, the small steric effect and low The formation of Ag NPs in the products upon changing viscosity would not prevent the aggregation of silver nano- the concentration of PVP can be attributed to the failure structures. PVP with high molecular weight, like as PVP- of anisotropic growth over the entire surface of multiply 360, possesses strong chemical adsorption on the side twinned nanoparticles (MTPs) [69, 74]. faces to produce long Ag NWs. But the large steric effect In addition, the influence of PVP with different mo- of PVP-360 would lead to the increase of diameter. lecular weight on the morphology and size of Ag NWs is In order to obtain high aspect ratios of Ag NWs, the also discussed. Only Ag NPs and aggregated nanorods adsorption strength and steric effect should be reached are produced when using PVP-10 (sample S5, Additional to a state of balance in the PVP-mediated system. There- file 1: Figure S1d). When employing separately PVP-58 fore, the mixed PVP molecules at different molar ratios (sample S6, Additional file 1: Figure S1e) and PVP-360 are employed as capping agent and the corresponding (sample S7, Fig. 1b), the corresponding morphology and morphology and size distribution of Ag NWs are showed size of products are changed from stubby Ag NWs (with in Fig. 2 and Additional file 1: Figure S2. When mixing average diameter of 235 nm and length of 6.7 μm) to PVP-58 with PVP-40 at the molar ratio of 1:1, Ag NWs high aspect ratio Ag NWs (with average diameter of with average diameter of 47.5 nm and length of 16.1 μm 132.1 nm and length of 69.9 μm). According to the are obtained. While the molar ratio of PVP-40 and PVP- abovementioned results from samples S2, S5, S6, and S7, 58 is adjusted to 1:2 or 2:1, the diameter of Ag NWs is the average molecular weight of PVP not only plays a increased. In addition, the aspect ratios of Ag NWs dra- vital role in the morphology formation of Ag NWs but matically enlarge when mixing PVP-40 with PVP-360 also has a significant influence on the diameter and because the diameters are reduced significantly. When length of Ag NWs products. The influence of PVP with the molar ratio of PVP-40 and PVP-360 is 1:1, the aspect different average molecular weight on the morphology ratios reach almost 1000 and the diameters have a more and size of Ag NWs can be ascribed to three factors: (i) uniform distribution as shown in Fig. 2e. PVP as the capping agent prefers to adsorb on the side The influence of mixed PVP with different chain faces of MTPs [69]. The strong chemical adsorption length on the diameters of Ag NWs could be interpreted promotes the growth of long Ag NWs [75]. (ii) The briefly in Scheme 1a. The long-chained PVP molecules steric effect of PVP capping layer allows silver atoms to can retard the lateral growth of Ag NWs owing to the deposit on the side faces through the gap between adja- strong adsorption to the (100) facets. The large steric cent PVP molecules, further resulting in the formation effect, resulting from the long chains, brings a relatively of thick Ag NWs [54]. (iii) The high viscosity of PVP large distance between adjacent PVP molecules. Ag Fig. 2 SEM images of Ag NWs synthesized using different mixed PVP molecules. a PVP-40:PVP-58 = 2:1, b PVP-40:PVP-58 = 1:1, c PVP-40:PVP-58 = 1:2, d PVP-40:PVP-360 = 2:1, e PVP-40:PVP-360 = 1:1, f PVP-40:PVP-360 = 1:2, respectively. All the total concentration of PVP are 0.15 M, and different PVP molecules are mixed at molar ratio. (The insets in a–f are the corresponding SEM images with high magnification, and all the scale bars are 500 nm) Xue et al. Nanoscale Research Letters (2017) 12:480 Page 5 of 12 Scheme 1 a Schematic illustration of the growth mechanism of Ag NWs using mixed PVP with different chain length. b Different aspect ratio Ag NWs are obtained by the PVP-mediated polyol process atoms can still deposit on the surface of Ag NWs by dif- with the crystal plane spaces for (111) and (200) planes of fusion through the gap between adjacent PVP molecules, face-centered cubic (fcc) Ag. Meanwhile, Ag NWs grow and thick Ag NWs are produced. When using the mixed along the [110] direction, as marked by the white arrow, PVP with different chain length, the short-chained PVP and it is similar to the results in the earlier reports [70, 76]. can fill the gap between long-chained PVP. Therefore, As shown in Fig. 4, the UV-visible absorption spectra the (100) facets can be passivated more efficiently, lead- of as-prepared Ag NWs are different from that of the ing to the formation of smaller Ag seeds and thinner Ag quasi-spherical Ag NPs. The spectra of Ag NWs appear NWs [76]. As shown in Scheme 1b, Ag NWs with double characteristic peaks. A shoulder peak located at typical aspect ratios are obtained in our work. It could around 350 nm could be ascribed to the plasmon reson- be conjectured that higher aspect ratio Ag NWs may be ance of bulk silver film [70, 78]. The second peak could produced through this experimental route. be attributed to the transverse plasmon mode of Ag The microstructure and morphology of Ag NWs are NWs, and the peak position is related to the dimensions characterized by TEM and demonstrated in Fig. 3a, b. The of silver nanostructures [79]. While the peak at around single nanowire is coated by the thin PVP layer with a 570 nm, resulting from the longitudinal plasmon reson- thickness of ca. 2 nm. Figure 3c shows the HRTEM image ance, is absent in the spectra because the aspect ratios of of Ag NWs with a good crystalline structure. The HRTEM as-prepared Ag NWs are far more than 5 [70, 80]. In image clearly exhibits that the spaces between periodic addition, as marked by the dashed green line, the second fringes are 0.235 and 0.202 nm, in good correspondence peak has a shift to red with the increase of diameters. Xue et al. Nanoscale Research Letters (2017) 12:480 Page 6 of 12 Fig. 3 TEM (a, b) and HRTEM (c) images of Ag NWs synthesized by mixing PVP-40 with PVP-360 (at a molar ratio of 1:1) However, it is noteworthy that there is no obvious peak in the decline of conductivity. In addition, it is noteworthy when the diameters of Ag NWs become larger. For Ag that the sheet resistance significantly decreases to 19.6 Ω/ NWs from sample S6 (average diameter of 235 nm) and sq when using the 8 mg/mL of Ag NWs solution. And it S10 (average diameter of 222.8 nm), the absorption inten- decreases almost fivefold compared with that of using sity maximums locate at the wavelength of 408.5 and 6 mg/mL, which could be attributed to the formation of 406.5 nm, respectively. They are smaller than the peak more efficient conductive percolation routes in the Ag wavelength of Ag NWs with smaller diameters from sam- NWs network, whereas some macroscopic agglomerates ple S7 (average diameter of 132.1 nm, the peak wavelength of Ag NWs appears as the concentration increases to is 412 nm), indicating the detachment of red-shifted ten- 8 mg/mL. Then, the repeated spin-coating process is dency of the right peak wavelength with larger diameters. carried out. As shown in Fig. 5b, both the transmittance It is necessary to optimize the spin-coating process to and sheet resistance decrease as increasing the times of fabricate high-quality Ag NWs films. As shown in Fig. 5a, spin coating. More importantly, when the volume of Ag it is observed that the sheet resistance increases as NWs solution is added from 50 to 75 μL, the sheet resist- increasing the rotational speed because the number of Ag ance dramatically decreases from 98.46 to 11.87 Ω/sq. As NWs clinging on the surface of PET decreases, resulting the volume further increases to 100 μL, the sheet resist- ance decreases to 10.42 Ω/sq with a transmittance of 80.95%. It indicates that the density of nanowires in the nanostructured transparent conducting networks may reach the tipping point where the transition from percola- tion behavior to bulk behavior occurs [81], when the volume is added to 75 μL. Moreover, to evaluate the per- formance of NTEs, the figure of merit (FOM) is calculated that correlates transmittance with sheet resistance. Gener- ally, the transmittance (T ) and sheet resistance (R )ofa λ s thin metallic film satisfy the following Eq. (1): −2 188:5 σ ðÞ λ op T ¼ 1 þ ð1Þ R σ S DC Fig. 4 UV-visible absorption spectra of as-prepared Ag NWs with σ (λ) is the optical conductivity and σ is the direct op DC different diameters current conductivity of the film [37]. The value of σ DC/ Xue et al. Nanoscale Research Letters (2017) 12:480 Page 7 of 12 Fig. 5 a Sheet resistance of Ag NWs films vs the spin-coating speed at different concentration of Ag NWs. b Comparison of optoelectronic performance of Ag NTEs fabricated by different volume of Ag NWs solutions. The concentration of Ag NWs solution is 6 mg/mL, and the volume of each spin coating is 25 μL. The inset is the FOM values of Ag NWs films vs the volume of Ag NWs solution. c–f SEM images of Ag NWs films fabricated by different volumes of Ag NWs solutions, c 25 μL, d 50 μL, e 75 μL, f 100 μL, respectively. All the scale bars are 5 μm σ (λ) are employed as FOM. And a higher value of reduce the junction resistance but also ensure the good op FOM means better optoelectronic performance. The dispersion of Ag NWs in the solvent. On the other hand, inset in Fig. 5b exhibits the FOM values of NTEs fabri- for widthless sticks in two dimensions, the critical num- cated by different volume of Ag NWs solutions. When ber density (N ) of sticks to create a percolation network the volume is added to 75 μL, the Ag NWs has the high- is given by Eq. (2): est FOM value, increasing dramatically from 23.3 to 162.6. It denotes that the balance is achieved between N  L ¼ 5:71 ð2Þ low sheet resistance and high transmittance when implementing three times of spin coating. In addition, L is the length of nanowires [52]. This equation im- Fig. 5c–f shows the SEM images of Ag NWs films on plies that the number density of Ag NWs required for PET with different densities, corresponding to the volume percolation network is inversely proportional to the of Ag NWs solutions for 25, 50, 75, and 100 μl, respect- square of length. Hence, long nanowires tend to build a ively. From the images, it is obvious that the Ag NWs sparse and effective percolation network with a low networks become ever denser and the distribution of Ag number density. It can not only increase the light trans- NWs is more uniform, as increasing the volume of Ag mission but also improve the conductivity through NWs solution. Therefore, the repeated spin-coating building long percolation routes with less nanowire process is available to fabricate uniform Ag nanowire junctions. films with various transmittance and sheet resistance Figure 6a shows the comparison of optoelectronic per- for different applications. formance of NTEs fabricated by Ag NWs with different For application in NTEs, the nanowire junctions have aspect ratios. For samples S2 and S9, the enlargement of a significant influence on the conductivity of random Ag parallel transmittance could be attributed to the smaller NWs network [58]. In polyol process, the as-synthesized diameters which reduced from 104.4 to 47.5 nm because Ag NWs retain a residual insulated PVP layer, resulting nanowires with smaller diameters can scatter less light, in high resistance at junctions and the deterioration of leading to a further decrease in haze. As the aspect conductivity. Lee et al. [59] reported that the repeated ratios exceed 500 (sample S7), Ag NWs films with a solvent washing can reduce the PVP layer from ca. 4 nm parallel transmittance of 81.8% (87.2%) and a sheet to 0.5 nm and allows for room-temperature welding of resistance of 7.4 Ω/sq (58.4 Ω/sq) are obtained. The the overlapping Ag NWs. Similarly, we repeated to wash optoelectronic performance are comparable to those of the as-synthesized Ag NWs for three times with ethyl commercial ITO films (85%, 55 Ω/sq) [5]. Furthermore, alcohol to remove the PVP layer as much as possible. As when the aspect ratios reach almost 1000 (sample S12), Ag the abovementioned results in Fig. 3a, thin PVP layer NWs films show superior transmittance (91.6–95.0%) and with a thickness of 2 nm is left. It can not only efficiently electronic conductivity (11.4–51.1 Ω/sq) than ITO films. Xue et al. Nanoscale Research Letters (2017) 12:480 Page 8 of 12 Fig. 6 a Comparison of optoelectronic performance of NTEs fabricated by Ag NWs with different aspect ratios (AR). b The best FOM valuesof AgNWsfilmsvsthe AR of AgNWs. c The optical transmittance spectra of Ag NWs films fabricated from sample S12. d Percolative figure of merit (П), plotted against conductivity exponents (n). The solid lines are plotted at the given combinations of transmittance (T) and sheet resistance (R ), as calculated from eq. (3). The plotted data of graphene, SWNTs, Cu NWs, Ag NWs are from recently published reports [37, 67, 81]. The star symbol represents the results of Ag NWs films fabricated using sample S12 from this work They sufficiently meet the performance requirements of To further evaluate the optoelectronic performance of TEs in the application of solar cells or touch screens. More- Ag NWs networks, the percolative FOM, П,was over, as shown in Fig. 6b, the biggest FOM value proposed in the Eq. (3) by De et al. [81]: achieves 387, higher than many other reported values "# −2 nþ1 of various TEs [62, 73]. The excellent performance 1 Z T ¼ 1 þ ð3Þ couldbeattributedtothe long andthinAg NWs. In П R addition, it is noteworthy that the FOM value dramat- ically increases from 89 to 224 when the aspect ratios Z is the impedance of free space (377 Ω). T and R 0 s enlarge from 339 (sample S9) to 529 (sample S7). The represent the transmittance and sheet resistance of Ag main reason is probably that the longer Ag NWs from NWs films, respectively. High values of П mean low sampleS7 form amoreeffective percolation network sheet resistance and high transmittance. Percolative with a smaller number of nanowires, leading to much FOM (П) and conductivity exponent (n) in this work are more light transmission through the Ag NWs network. calculated to be 89.8 and 1.50 by using Eq. (3), respect- It indicates that the long Ag NWs strategy is a facile ively. The percolative FOM value is higher than other and effective way to obtain NTEs with promising reported values of various TEs (shown in Fig. 6d). It optoelectronic performance, when the thin Ag NWs could be attributed to two reasons: The thin PVP with a diameter less than 20 nm are not synthesized layer (ca. 2 nm) can effectively reduce the nanowire successfully [52, 67]. Figure 6c demonstrates optical junction resistance. On the other hand, the long Ag transmittance spectra of Ag NWs films fabricated NWs (ca. 71.0 μm) form long conductive routes in from sample S12. The spectra show a wide flat region the percolation networks, resulting in the decrease of from visible light to near infrared wavelength, which number of junctions. Interestingly, the value of n is a non- can improve the utilization range of light and is ad- universal exponent which has been related to the presence vantageous for display and solar cell applications, of a distribution of nanowire junction resistance [82–84]. while the transmittance of ITO films displays dramatic Lee et al. [67] used a laser nano-welding process to reduce fluctuation over the region of visible light [7]. the nanowire junction resistance, and the value of n is Xue et al. Nanoscale Research Letters (2017) 12:480 Page 9 of 12 calculated to be 1.57. The value is close to that in our work. It further suggests that the thin PVP layer and long Ag NWs are efficient to allow low-temperature welding of Ag NWs network. Figure 7a exhibits optical photographs of the uniform Ag NWs film on PET. The film is highly transparent as the school badge in the background can be clearly seen through the film. Figure 7b, Additional file 1: Figure S3 and Additional file 2: Video S1 show that Ag NWs film on PET turn on the LED bulb when applying a low voltage. It indicates that the whole surface of Ag NWs film is highly conductive. In addition, The Ag NW film is very flexible as shown in Fig. 7c. The mechanical stability of the fabricated Ag NTEs on PET substrate is evaluated by a bending test. As shown in Fig. 8, the bending test consists of 100 cycles of inner Fig. 8 The bending test, including inner bending and outer bending and 300 cycles of outer bending with a bending bending. Both the bending radios are 1.5 cm. The inset shows the radio of 1.5 cm. No visible defects, such as cracking or bent Ag NTEs is still conductive over the whole surface. (R and R represent the sheet resistance of films before and after bending tearing of the surface, are observed even after more test, respectively) than 400 cycles of bending test. And Ag NTEs exhibit a stable electronic performance with little change of sheet resistance. Its property to tolerate hundreds of mechanical bending test could be attributed to the and electronic conductivity (7.4–58.4 Ω/sq), compar- flexibility of long Ag NWs and the benign adhesion able to those of commercial ITO films (85%, 45 Ω/sq). to the substrate. Furthermore, high-performance Ag NTEs with a trans- mittance of 91.6% and a sheet resistance of 11.4 Ω/sq Conclusions are obtained, as the aspect ratios exceed 1000. The long In summary, Ag NWs with different aspect ratios vary- nanowires and thin PVP layer lead to less number of ing from ca. 30 to ca. 1000 are prepared via a facile nanowire junctions and reduced junction resistance, PVP-mediated polyol process and are applied to the respectively. It allows low-temperature sintering of Ag fabrication of high-performance Ag NTEs with low- NWs network, which is advantageous for the applica- temperature sintering. In the polyol process, the diame- tions in the flexible plastic substrates. Moreover, Ag ters of Ag NWs are strikingly reduced and the aspect NTEs show excellent flexibility against the bending test. ratios reach almost 1000 when employing mixed PVP We believe that the ability to synthesize Ag NWs with as the capping agent. Additionally, when the aspect different aspect ratios and fabricate high-performance ratios exceed 500, the optoelectronic performance of NTEs with low-temperature welding are very valuable Ag NWs films show good transmittance (81.8–87.2%) to the development of flexible electronic devices. Fig. 7 a Optical image of as-fabricated Ag NWs films on PET. b Ag NWs film is connected in an electric circuit in which an LED is lit. c Optical image of the flexible Ag NWs film Xue et al. Nanoscale Research Letters (2017) 12:480 Page 10 of 12 Additional Files temperature and its application for fully stretchable polymer light-emitting diodes. ACS Nano 8(2):1590–1600 11. Lee J, A K, Won P, Ka Y, Hwang H, Moon H, Kwon Y, Hong S, Kim C, Lee C, Additional file 1: Table S1. Reaction parameters of Ag NWs with Ko SH (2017) A dual-scale metal nanowire network transparent conductor different concentrations of PVP and mixed PVP molecules at different for highly efficient and flexible organic light emitting diodes. Nano 9(5): mole ratios. Herein, silver nanoparticles and silver aggregated nanorods 1978–1985 are abbreviated to Ag NPs and Ag ANRs, respectively. Figure S1. SEM 12. Chang I, Lee J, Lee Y, Lee YH, Ko SH, Cha SW (2017) Thermally stable Ag@ images of Ag NWs under different reaction conditions: (a) 0.05 M PVP, ZrO core-shell via atomic layer deposition. Mater Lett 188:372–374 (b) 0.25 M PVP, (c) 0.55 M PVP, (d) PVP-10, (e) PVP-58, respectively. (f) 13. Chang I, Park T, Lee J, Lee HB, Ji S, Lee MH, Ko SH, Cha SW (2014) statistical size distribution of Ag NWs synthesized using PVP-58. Performance enhancement in bendable fuel cell using highly conductive Figure S2. Statistic sizes distribution of Ag NWs synthesized using Ag nanowires. Int J Hydrog Energy 39(14):7422–7427 different mixed PVP molecules. (a) PVP-40:PVP-58 = 2:1, (b) PVP-40:PVP- 14. Chang I, Park T, Lee J, Lee HB, Ko SH, Cha SW (2016) Flexible fuel cell using 58 = 1:1, (c) PVP-40:PVP-58 = 1:2, (d) PVP-40:PVP-360 = 2:1, (e) PVP-40:PVP- stiffness-controlled endplate. Int J Hydrog Energy 41(14):6013–6019 360 = 1:1, (f) PVP-40:PVP-360 = 1:2, respectively. Figure S3. Ag NWs film 15. Chang I, Park T, Lee J, Lee MH, Ko SH, Cha SW (2013) Bendable polymer is connected in an electric circuit, being applied a low voltage. electrolyte fuel cell using highly flexible Ag nanowire percolation network (DOCX 2653 kb) current collectors. J Mater Chem A 1(30):8541–8546 Additional file 2: The video of Ag NWs flexible transparent electrodes. 16. Park T, Chang I, Jung JH, Lee HB, Ko SH, O'Hayre R, Yoo SJ, Cha SW (2017) (AVI 9706 kb) Effect of assembly pressure on the performance of a bendable polymer electrolyte fuel cell based on a silver nanowire current collector. Energy 134:412–419 Acknowledgements 17. Park T, Chang I, Lee HB, Ko SH, Cha SW (2017) Performance variation of This work was supported by the NSFC (51471121), Basic Research Plan bendable polymer electrolyte fuel cell based on Ag nanowire current Program of Shenzhen City (JCYJ20160517104459444, collector under mixed bending and twisting load. Int J Hydrog Energy JCYJ20170303170426117), Natural Science Foundation of Jiangsu Province 42(3):1884–1890 (BK20160383), and Wuhan University. 18. Hong S, Yeo J, Lee J, Lee H, Lee P, Lee SS, Ko SH (2015) Selective laser direct patterning of silver nanowire percolation network transparent Authors’ Contributions conductor for capacitive touch panel. J Nanosci Nanotech 15(3):2317–2323 QWX completed all the experiments and wrote the manuscript. WJY, JL, QYT, 19. Kim KK, Hong S, Cho HM, Lee J, Suh YD, Ham J, Ko SH (2015) Highly sensitive LL, MXL, QL, and RP assisted with the manuscript preparation. WW and stretchable multidimensional strain sensor with prestrained anisotropic conceived the study, revised the manuscript, and supervised the work. All metal nanowire percolation networks. Nano Lett 15(8):5240–5247 authors read and approved the final manuscript. 20. Jeong S, Cho H, Han S, Won P, Lee H, Hong S, Yeo J, Kwon J, Ko SH (2017) High efficiency, transparent, reusable, and active PM2.5 filters by hierarchical Competing Interests Ag nanowire percolation network. Nano Lett 17(7):4339–4346 The authors declare that they have no competing interests. 21. 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Facile Synthesis of Silver Nanowires with Different Aspect Ratios and Used as High-Performance Flexible Transparent Electrodes

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Springer Journals
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Copyright © 2017 by The Author(s).
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Materials Science; Nanotechnology; Nanotechnology and Microengineering; Nanoscale Science and Technology; Nanochemistry; Molecular Medicine
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1931-7573
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10.1186/s11671-017-2259-6
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Abstract

Silver nanowires (Ag NWs) are the promising materials to fabricate flexible transparent electrodes, aiming to replace indium tin oxide (ITO) in the next generation of flexible electronics. Herein, a feasible polyvinylpyrrolidone (PVP)- mediated polyol synthesis of Ag NWs with different aspect ratios is demonstrated and high-quality Ag NWs transparent electrodes (NTEs) are fabricated without high-temperature thermal sintering. When employing the mixture of PVP with different average molecular weight as the capping agent, the diameters of Ag NWs can be tailored and Ag NWs with different aspect ratios varying from ca. 30 to ca. 1000 are obtained. Using these as- synthesized Ag NWs, the uniform Ag NWs films are fabricated by repeated spin coating. When the aspect ratios exceed 500, the optoelectronic performance of Ag NWs films improve remarkably and match up to those of ITO films. Moreover, an optimal Ag NTEs with low sheet resistance of 11.4 Ω/sq and a high parallel transmittance of 91. 6% at 550 nm are achieved when the aspect ratios reach almost 1000. In addition, the sheet resistance of Ag NWs films does not show great variation after 400 cycles of bending test, suggesting an excellent flexibility. The proposed approach to fabricate highly flexible and high-performance Ag NTEs would be useful to the development of flexible devices. Keywords: Ag NWs, Tailorable aspect ratios, Flexible transparent electrodes, Low-temperature welding Background (>80%). But its intrinsic brittleness limits the applica- Flexible transparent electrodes (FTEs) play an import- tions in flexible electronics. Moreover, it requires high ant role in the next generation of flexible electronics temperature deposition process and is challenged by [1–4]. FTEs can be applied to many optoelectronic the scarcity of indium [25–27]. Therefore, several new devices as conductive components, involving touch conductive films with good flexibility and optical screens [5, 6], portable solar cells [7, 8], organic light- transparency, such as metal grids [2, 28, 29], carbon emitting diodes (OLEDs) [9–11], fuel cell electrode nanotubes (CNTs) [30–33], graphene [34–36], Ag [12–17], sensors [18, 19], PM filter [20], transparent NWs [5, 37–41], Cu NWs [42, 43], conductive poly- heaters [21, 22], and wearable electronics [23, 24]. mers [44, 45], and hybrids of these [46–48], have The dominant transparent electrodes (TEs) used been fabricated, striving to replace ITO. Among these currently is indium tin oxide (ITO) owing to the low candidates, Ag NWs films have been investigated sheet resistance (<100 Ω/sq) and high transmittance extensively in both the scientific and industrial insti- tutions, owing to the excellent electrical conductivity and high optical transparency. In addition, Ag NWs * Correspondence: weiwu@whu.edu.cn exhibit outstanding flexibility and stretchability, which Laboratory of Printable Functional Nanomaterials and Printed Electronics, is the one of the appealing advantage to fabricate School of Printing and Packaging, Wuhan University, Wuhan 430072, People’s Republic of China stretchable transparent conductors than fragile ITO Shenzhen Research Institute of Wuhan University, Shenzhen 518057, [49–51]. Moreover, the solution-processed Ag NWs People’s Republic of China © The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Xue et al. Nanoscale Research Letters (2017) 12:480 Page 2 of 12 films are more cost-effective than ITO. All of these employed. It is incompatible with flexible plastic sub- properties make Ag NWs films become promising strates which cannot withstand high temperature, and alternatives to ITO for the applications in flexible hence limits the applications of Ag NWs films in flexible electronics. optoelectronic devices. However, several issues need to be addressed to Herein, a series of Ag NWs with different aspect ra- commercialize Ag NWs films as FTEs. Firstly, Ag NWs tios varying from ca. 30 to ca. 1000 are controllably with different aspect ratios need to be facilely synthe- synthesized and used to fabricate high conductive and sized in controlled manner because the alluring proper- transparent Ag NTEs. First, Ag NWs are prepared by ties of Ag NWs films deeply rely on the dimensions of facile PVP-mediated polyol process where the mixture Ag NWs and a well-designed length and diameter are of of PVP with different average molecular weight can very importance for different applications [52, 53]. efficiently reduce the diameters. Subsequently, the as- Generally, polyol process is the most widely used synthesized Ag NWs with different aspect ratios are method to prepare Ag NWs. Ran et al. [54] synthesized employed to fabricate Ag NWs films without high- thin Ag NWs with aspect ratios larger than 1000 by temperature annealing, respectively. And the corre- using the mixed PVP with the average molecular weight sponding optoelectronic performance are comparative of 58,000 and 1,300,000 as the capping agent. However, investigated. The best sheet resistance and parallel the influence of the aspect ratios on the optoelectronic transmittance can achieve 11.4 Ω/sq and 91.6% when performance of Ag NTEs was not carefully investigated in the aspect ratios reach almost 1000. Moreover, the their work. Although Ding et al. [55] prepared Ag NWs sheet resistance of as-fabricated Ag NWs films is nearly with different diameters varying from 40 to 110 nm and constant after inner-bending and outer-bending tests. fabricated Ag NTEs with a transmittance of 87% and a sheet resistance of ca.70 Ω/sq, many parameters need to Methods be simultaneously adjusted to control the diameters of Ag Materials and Chemicals NWs and the optoelectronic performance of the as- Silver nitrate (AgNO , AR) and anhydrous ethanol obtained Ag NTEs would not be satisfactory. Li et al. [56] (C H OH, AR) were purchased from Sinopharm Chem- 2 5 synthesized thin Ag NWs with diameters of 20 nm ical Reagent Co., Ltd. Copper (II) chloride dehydrate through altering the concentration of bromide. And they (CuCl ·2H O, AR) and PVP (MW≈58,000, marked as 2 2 have fabricated high-quality Ag NWs films with a trans- PVP-58) were purchased from Shanghai Aladdin mittance of 99.1% at 130.0 Ω/sq. Ko et al. [57] developed Reagents Co., Ltd. Ethylene glycol (EG, 98%) and PVP a multistep growth method to synthesize very long Ag (MW≈10,000, 40,000 and 360,000, marked as PVP-10, NWs over several hundred micrometers and the fabri- PVP-40, and PVP-360, respectively) were purchased cated films demonstrated superior transmittance of 90% from Sigma-Aldrich. Deionized water (18.2 MΩ)was with sheet resistance of 19 Ω/sq. The optoelectronic used in the whole experiments. performance of these Ag NWs films are comparable to or even better than those of ITO films. But the minimum as- pect ratio of Ag NWs, which has the ability to fabricate Synthesis of Ag NWs TEs rivaling commercial ITO in terms of sheet resistance Ag NWs with different aspect ratios are prepared by a and transmittance, is still uncertain. Therefore, it is neces- facile one-pot PVP-mediated polyol process. Typically, sary to synthesize Ag NWs with various aspect ratios and 0.170 g of AgNO is dissolved in 10 mL of EG under study their influence on the optoelectronic performance magnetic stirring. Then, 0.15 M of PVP-40 and of Ag NWs films. 0.111 mM of CuCl ·2H O mixed solution in 10 mL of 2 2 Furthermore, the electronic conductivity of Ag NWs EG is added dropwise to the above solution. Afterwards, films is relatively poor, resulting from the high nanowire the mixture is transferred into Teflon-lined stainless junction resistance [58]. In the polyol synthesis of Ag steel autoclave with a capacity of 50 mL and heated at NWs, PVP, as the surfactant, adsorbs on the surface of 160 °C for 3 h. After cooling down to room temperature Ag NWs, resulting in insulated contact between the naturally, pure Ag NWs are obtained by centrifugation at a wires in the random network [59, 60]. Consequently, speed of 2500 rpm for 5 min and washed three times with different physical and chemical post-processes, involving ethanol and deionized water. Finally, the products are thermal annealing [38, 39, 61, 62], mechanical press dispersed in ethanol for further characterization and appli- [63], nanosoldering with conductive polymers [64], cation. Moreover, the concentration and average molecular plasmonic welding [65], laser nanowelding [66–68], and weight of PVP are very important to control the morph- integration with other materials [60], have been explored ology and size of products. Therefore, different types of to reduce the junction resistance. Among these post- PVP molecules are simultaneously used to regulate the treatments, thermal annealing at almost 200 °C is usually diameters of Ag NWs in the polyol process. Detailed Xue et al. Nanoscale Research Letters (2017) 12:480 Page 3 of 12 experimental parameters are listed in Additional file 1: measured at room temperature by using 4-point probe Table S1, nominated as S1–S13, respectively. resistance tester (FP-001). Fabrication of Ag NTEs Results and Discussion Polyethylene terephthalate (PET) with a thickness of Generally, Ag NWs are synthesized by polyol process in 150 μm is cut to pieces with the dimension of 20 × 20 mm. which PVP is employed as capping agent to ensure the Briefly, the as-prepared Ag NWs are dispersed in ethanol growth of one-dimensional Ag NWs [69, 70]. During the (6 mg/mL), and 50 μL of Ag NWs solution is spin coated synthesis, many parameters such as reaction temperature, at 2000 rpm for 30 s on PET substrate. Finally, the Ag stirring speed, PVP concentration, PVP chain length, addi- NWsfilmsare heated to 140°Cfor 15minwithout any tive agents, and ratio of chemicals can affect the yield and additional post-process treatments. The aspect ratios morphology of synthesized Ag NWs. For example, an of Ag NWs, rotation speed, concentration, and vol- inappropriate reaction temperature less than 110 °C or ume of Ag NWs solution are investigated to fabricate higher than 180 °C allows more Ag atom to form Ag high-quality NTEs. Regarding to the repeated spin nanoparticles (NPs) rather than Ag NWs [70, 71]. The coating, each volume of Ag NWs solution is altered length of synthesized Ag NWs increase as slowing down to 25 μL and the rotation speed is set to 2000 rpm. the stirring speed [72, 73]. In this paper, we mainly investi- A time interval in each spin coating is needed to gate the concentration of PVP and their average molecular volatilize the ethanol. Other parameters are same as weight on the effect of morphology and size of Ag NWs. the aforementioned processes. The corresponding morphology and size distribution of Ag NWs are demonstrated in Fig. 1 and Additional file 1: Characterization and Performance Test Figure S1. Firstly, the concentration of PVP is increased Scanning electron microscopy (SEM) images are from 0.05 M (sample S1, Additional file 1: Figure S1a) to recorded using a cold field-emission SEM (Hitachi S- 0.15 M (sample S2, Fig. 1a). The corresponding morph- 4800). The transmission electron microscopy (TEM) and ology of products is changed from near-spherical Ag NPs the high-resolution TEM (HRTEM) images are obtained to pure Ag NWs with an average diameter of 104.4 nm and by using a JEOL JEM-2100F. The UV-vis absorption length of 12.3 μm. The mixture of Ag NWs and Ag NPs spectra of Ag NWs and the optical transmittance spectra are observed when the concentration of PVP is increased of Ag NWs films are carried out on a Shimadzu UV- to 0.25 M (sample S3, Additional file 1: Figure S1b). 3600 spectrophotometer. The sheet resistance is By further increasing the concentration of PVP to Fig. 1 a, b SEM images of as-synthesized Ag NWs with PVP-40 and PVP-360, respectively. Both the concentration of PVP are 0.15 M. a′ b′ Corresponding statistical distribution of diameter and length. (The insets in a and b are the corresponding SEM images with high magnifi- cation and all the scale bars are 500 nm) Xue et al. Nanoscale Research Letters (2017) 12:480 Page 4 of 12 0.55 M (sample S4, Additional file 1: Figure S1c), a with high average molecular weight in EG solution large number of Ag NPs with different shapes (in- would slow down the growth rate, which are benefit to cluding near-sphere and triangular plate) are formed. form MTPs [76, 77]. As a result, the low average The results indicate that a lower or higher concentra- molecular weight of PVP, like as PVP-10, would not tion of PVP are not beneficial to produce pure Ag efficiently adsorb on the (100) crystal faces to restrict the NWs, further resulting in the absence of Ag NWs. lateral growth. Meanwhile, the small steric effect and low The formation of Ag NPs in the products upon changing viscosity would not prevent the aggregation of silver nano- the concentration of PVP can be attributed to the failure structures. PVP with high molecular weight, like as PVP- of anisotropic growth over the entire surface of multiply 360, possesses strong chemical adsorption on the side twinned nanoparticles (MTPs) [69, 74]. faces to produce long Ag NWs. But the large steric effect In addition, the influence of PVP with different mo- of PVP-360 would lead to the increase of diameter. lecular weight on the morphology and size of Ag NWs is In order to obtain high aspect ratios of Ag NWs, the also discussed. Only Ag NPs and aggregated nanorods adsorption strength and steric effect should be reached are produced when using PVP-10 (sample S5, Additional to a state of balance in the PVP-mediated system. There- file 1: Figure S1d). When employing separately PVP-58 fore, the mixed PVP molecules at different molar ratios (sample S6, Additional file 1: Figure S1e) and PVP-360 are employed as capping agent and the corresponding (sample S7, Fig. 1b), the corresponding morphology and morphology and size distribution of Ag NWs are showed size of products are changed from stubby Ag NWs (with in Fig. 2 and Additional file 1: Figure S2. When mixing average diameter of 235 nm and length of 6.7 μm) to PVP-58 with PVP-40 at the molar ratio of 1:1, Ag NWs high aspect ratio Ag NWs (with average diameter of with average diameter of 47.5 nm and length of 16.1 μm 132.1 nm and length of 69.9 μm). According to the are obtained. While the molar ratio of PVP-40 and PVP- abovementioned results from samples S2, S5, S6, and S7, 58 is adjusted to 1:2 or 2:1, the diameter of Ag NWs is the average molecular weight of PVP not only plays a increased. In addition, the aspect ratios of Ag NWs dra- vital role in the morphology formation of Ag NWs but matically enlarge when mixing PVP-40 with PVP-360 also has a significant influence on the diameter and because the diameters are reduced significantly. When length of Ag NWs products. The influence of PVP with the molar ratio of PVP-40 and PVP-360 is 1:1, the aspect different average molecular weight on the morphology ratios reach almost 1000 and the diameters have a more and size of Ag NWs can be ascribed to three factors: (i) uniform distribution as shown in Fig. 2e. PVP as the capping agent prefers to adsorb on the side The influence of mixed PVP with different chain faces of MTPs [69]. The strong chemical adsorption length on the diameters of Ag NWs could be interpreted promotes the growth of long Ag NWs [75]. (ii) The briefly in Scheme 1a. The long-chained PVP molecules steric effect of PVP capping layer allows silver atoms to can retard the lateral growth of Ag NWs owing to the deposit on the side faces through the gap between adja- strong adsorption to the (100) facets. The large steric cent PVP molecules, further resulting in the formation effect, resulting from the long chains, brings a relatively of thick Ag NWs [54]. (iii) The high viscosity of PVP large distance between adjacent PVP molecules. Ag Fig. 2 SEM images of Ag NWs synthesized using different mixed PVP molecules. a PVP-40:PVP-58 = 2:1, b PVP-40:PVP-58 = 1:1, c PVP-40:PVP-58 = 1:2, d PVP-40:PVP-360 = 2:1, e PVP-40:PVP-360 = 1:1, f PVP-40:PVP-360 = 1:2, respectively. All the total concentration of PVP are 0.15 M, and different PVP molecules are mixed at molar ratio. (The insets in a–f are the corresponding SEM images with high magnification, and all the scale bars are 500 nm) Xue et al. Nanoscale Research Letters (2017) 12:480 Page 5 of 12 Scheme 1 a Schematic illustration of the growth mechanism of Ag NWs using mixed PVP with different chain length. b Different aspect ratio Ag NWs are obtained by the PVP-mediated polyol process atoms can still deposit on the surface of Ag NWs by dif- with the crystal plane spaces for (111) and (200) planes of fusion through the gap between adjacent PVP molecules, face-centered cubic (fcc) Ag. Meanwhile, Ag NWs grow and thick Ag NWs are produced. When using the mixed along the [110] direction, as marked by the white arrow, PVP with different chain length, the short-chained PVP and it is similar to the results in the earlier reports [70, 76]. can fill the gap between long-chained PVP. Therefore, As shown in Fig. 4, the UV-visible absorption spectra the (100) facets can be passivated more efficiently, lead- of as-prepared Ag NWs are different from that of the ing to the formation of smaller Ag seeds and thinner Ag quasi-spherical Ag NPs. The spectra of Ag NWs appear NWs [76]. As shown in Scheme 1b, Ag NWs with double characteristic peaks. A shoulder peak located at typical aspect ratios are obtained in our work. It could around 350 nm could be ascribed to the plasmon reson- be conjectured that higher aspect ratio Ag NWs may be ance of bulk silver film [70, 78]. The second peak could produced through this experimental route. be attributed to the transverse plasmon mode of Ag The microstructure and morphology of Ag NWs are NWs, and the peak position is related to the dimensions characterized by TEM and demonstrated in Fig. 3a, b. The of silver nanostructures [79]. While the peak at around single nanowire is coated by the thin PVP layer with a 570 nm, resulting from the longitudinal plasmon reson- thickness of ca. 2 nm. Figure 3c shows the HRTEM image ance, is absent in the spectra because the aspect ratios of of Ag NWs with a good crystalline structure. The HRTEM as-prepared Ag NWs are far more than 5 [70, 80]. In image clearly exhibits that the spaces between periodic addition, as marked by the dashed green line, the second fringes are 0.235 and 0.202 nm, in good correspondence peak has a shift to red with the increase of diameters. Xue et al. Nanoscale Research Letters (2017) 12:480 Page 6 of 12 Fig. 3 TEM (a, b) and HRTEM (c) images of Ag NWs synthesized by mixing PVP-40 with PVP-360 (at a molar ratio of 1:1) However, it is noteworthy that there is no obvious peak in the decline of conductivity. In addition, it is noteworthy when the diameters of Ag NWs become larger. For Ag that the sheet resistance significantly decreases to 19.6 Ω/ NWs from sample S6 (average diameter of 235 nm) and sq when using the 8 mg/mL of Ag NWs solution. And it S10 (average diameter of 222.8 nm), the absorption inten- decreases almost fivefold compared with that of using sity maximums locate at the wavelength of 408.5 and 6 mg/mL, which could be attributed to the formation of 406.5 nm, respectively. They are smaller than the peak more efficient conductive percolation routes in the Ag wavelength of Ag NWs with smaller diameters from sam- NWs network, whereas some macroscopic agglomerates ple S7 (average diameter of 132.1 nm, the peak wavelength of Ag NWs appears as the concentration increases to is 412 nm), indicating the detachment of red-shifted ten- 8 mg/mL. Then, the repeated spin-coating process is dency of the right peak wavelength with larger diameters. carried out. As shown in Fig. 5b, both the transmittance It is necessary to optimize the spin-coating process to and sheet resistance decrease as increasing the times of fabricate high-quality Ag NWs films. As shown in Fig. 5a, spin coating. More importantly, when the volume of Ag it is observed that the sheet resistance increases as NWs solution is added from 50 to 75 μL, the sheet resist- increasing the rotational speed because the number of Ag ance dramatically decreases from 98.46 to 11.87 Ω/sq. As NWs clinging on the surface of PET decreases, resulting the volume further increases to 100 μL, the sheet resist- ance decreases to 10.42 Ω/sq with a transmittance of 80.95%. It indicates that the density of nanowires in the nanostructured transparent conducting networks may reach the tipping point where the transition from percola- tion behavior to bulk behavior occurs [81], when the volume is added to 75 μL. Moreover, to evaluate the per- formance of NTEs, the figure of merit (FOM) is calculated that correlates transmittance with sheet resistance. Gener- ally, the transmittance (T ) and sheet resistance (R )ofa λ s thin metallic film satisfy the following Eq. (1): −2 188:5 σ ðÞ λ op T ¼ 1 þ ð1Þ R σ S DC Fig. 4 UV-visible absorption spectra of as-prepared Ag NWs with σ (λ) is the optical conductivity and σ is the direct op DC different diameters current conductivity of the film [37]. The value of σ DC/ Xue et al. Nanoscale Research Letters (2017) 12:480 Page 7 of 12 Fig. 5 a Sheet resistance of Ag NWs films vs the spin-coating speed at different concentration of Ag NWs. b Comparison of optoelectronic performance of Ag NTEs fabricated by different volume of Ag NWs solutions. The concentration of Ag NWs solution is 6 mg/mL, and the volume of each spin coating is 25 μL. The inset is the FOM values of Ag NWs films vs the volume of Ag NWs solution. c–f SEM images of Ag NWs films fabricated by different volumes of Ag NWs solutions, c 25 μL, d 50 μL, e 75 μL, f 100 μL, respectively. All the scale bars are 5 μm σ (λ) are employed as FOM. And a higher value of reduce the junction resistance but also ensure the good op FOM means better optoelectronic performance. The dispersion of Ag NWs in the solvent. On the other hand, inset in Fig. 5b exhibits the FOM values of NTEs fabri- for widthless sticks in two dimensions, the critical num- cated by different volume of Ag NWs solutions. When ber density (N ) of sticks to create a percolation network the volume is added to 75 μL, the Ag NWs has the high- is given by Eq. (2): est FOM value, increasing dramatically from 23.3 to 162.6. It denotes that the balance is achieved between N  L ¼ 5:71 ð2Þ low sheet resistance and high transmittance when implementing three times of spin coating. In addition, L is the length of nanowires [52]. This equation im- Fig. 5c–f shows the SEM images of Ag NWs films on plies that the number density of Ag NWs required for PET with different densities, corresponding to the volume percolation network is inversely proportional to the of Ag NWs solutions for 25, 50, 75, and 100 μl, respect- square of length. Hence, long nanowires tend to build a ively. From the images, it is obvious that the Ag NWs sparse and effective percolation network with a low networks become ever denser and the distribution of Ag number density. It can not only increase the light trans- NWs is more uniform, as increasing the volume of Ag mission but also improve the conductivity through NWs solution. Therefore, the repeated spin-coating building long percolation routes with less nanowire process is available to fabricate uniform Ag nanowire junctions. films with various transmittance and sheet resistance Figure 6a shows the comparison of optoelectronic per- for different applications. formance of NTEs fabricated by Ag NWs with different For application in NTEs, the nanowire junctions have aspect ratios. For samples S2 and S9, the enlargement of a significant influence on the conductivity of random Ag parallel transmittance could be attributed to the smaller NWs network [58]. In polyol process, the as-synthesized diameters which reduced from 104.4 to 47.5 nm because Ag NWs retain a residual insulated PVP layer, resulting nanowires with smaller diameters can scatter less light, in high resistance at junctions and the deterioration of leading to a further decrease in haze. As the aspect conductivity. Lee et al. [59] reported that the repeated ratios exceed 500 (sample S7), Ag NWs films with a solvent washing can reduce the PVP layer from ca. 4 nm parallel transmittance of 81.8% (87.2%) and a sheet to 0.5 nm and allows for room-temperature welding of resistance of 7.4 Ω/sq (58.4 Ω/sq) are obtained. The the overlapping Ag NWs. Similarly, we repeated to wash optoelectronic performance are comparable to those of the as-synthesized Ag NWs for three times with ethyl commercial ITO films (85%, 55 Ω/sq) [5]. Furthermore, alcohol to remove the PVP layer as much as possible. As when the aspect ratios reach almost 1000 (sample S12), Ag the abovementioned results in Fig. 3a, thin PVP layer NWs films show superior transmittance (91.6–95.0%) and with a thickness of 2 nm is left. It can not only efficiently electronic conductivity (11.4–51.1 Ω/sq) than ITO films. Xue et al. Nanoscale Research Letters (2017) 12:480 Page 8 of 12 Fig. 6 a Comparison of optoelectronic performance of NTEs fabricated by Ag NWs with different aspect ratios (AR). b The best FOM valuesof AgNWsfilmsvsthe AR of AgNWs. c The optical transmittance spectra of Ag NWs films fabricated from sample S12. d Percolative figure of merit (П), plotted against conductivity exponents (n). The solid lines are plotted at the given combinations of transmittance (T) and sheet resistance (R ), as calculated from eq. (3). The plotted data of graphene, SWNTs, Cu NWs, Ag NWs are from recently published reports [37, 67, 81]. The star symbol represents the results of Ag NWs films fabricated using sample S12 from this work They sufficiently meet the performance requirements of To further evaluate the optoelectronic performance of TEs in the application of solar cells or touch screens. More- Ag NWs networks, the percolative FOM, П,was over, as shown in Fig. 6b, the biggest FOM value proposed in the Eq. (3) by De et al. [81]: achieves 387, higher than many other reported values "# −2 nþ1 of various TEs [62, 73]. The excellent performance 1 Z T ¼ 1 þ ð3Þ couldbeattributedtothe long andthinAg NWs. In П R addition, it is noteworthy that the FOM value dramat- ically increases from 89 to 224 when the aspect ratios Z is the impedance of free space (377 Ω). T and R 0 s enlarge from 339 (sample S9) to 529 (sample S7). The represent the transmittance and sheet resistance of Ag main reason is probably that the longer Ag NWs from NWs films, respectively. High values of П mean low sampleS7 form amoreeffective percolation network sheet resistance and high transmittance. Percolative with a smaller number of nanowires, leading to much FOM (П) and conductivity exponent (n) in this work are more light transmission through the Ag NWs network. calculated to be 89.8 and 1.50 by using Eq. (3), respect- It indicates that the long Ag NWs strategy is a facile ively. The percolative FOM value is higher than other and effective way to obtain NTEs with promising reported values of various TEs (shown in Fig. 6d). It optoelectronic performance, when the thin Ag NWs could be attributed to two reasons: The thin PVP with a diameter less than 20 nm are not synthesized layer (ca. 2 nm) can effectively reduce the nanowire successfully [52, 67]. Figure 6c demonstrates optical junction resistance. On the other hand, the long Ag transmittance spectra of Ag NWs films fabricated NWs (ca. 71.0 μm) form long conductive routes in from sample S12. The spectra show a wide flat region the percolation networks, resulting in the decrease of from visible light to near infrared wavelength, which number of junctions. Interestingly, the value of n is a non- can improve the utilization range of light and is ad- universal exponent which has been related to the presence vantageous for display and solar cell applications, of a distribution of nanowire junction resistance [82–84]. while the transmittance of ITO films displays dramatic Lee et al. [67] used a laser nano-welding process to reduce fluctuation over the region of visible light [7]. the nanowire junction resistance, and the value of n is Xue et al. Nanoscale Research Letters (2017) 12:480 Page 9 of 12 calculated to be 1.57. The value is close to that in our work. It further suggests that the thin PVP layer and long Ag NWs are efficient to allow low-temperature welding of Ag NWs network. Figure 7a exhibits optical photographs of the uniform Ag NWs film on PET. The film is highly transparent as the school badge in the background can be clearly seen through the film. Figure 7b, Additional file 1: Figure S3 and Additional file 2: Video S1 show that Ag NWs film on PET turn on the LED bulb when applying a low voltage. It indicates that the whole surface of Ag NWs film is highly conductive. In addition, The Ag NW film is very flexible as shown in Fig. 7c. The mechanical stability of the fabricated Ag NTEs on PET substrate is evaluated by a bending test. As shown in Fig. 8, the bending test consists of 100 cycles of inner Fig. 8 The bending test, including inner bending and outer bending and 300 cycles of outer bending with a bending bending. Both the bending radios are 1.5 cm. The inset shows the radio of 1.5 cm. No visible defects, such as cracking or bent Ag NTEs is still conductive over the whole surface. (R and R represent the sheet resistance of films before and after bending tearing of the surface, are observed even after more test, respectively) than 400 cycles of bending test. And Ag NTEs exhibit a stable electronic performance with little change of sheet resistance. Its property to tolerate hundreds of mechanical bending test could be attributed to the and electronic conductivity (7.4–58.4 Ω/sq), compar- flexibility of long Ag NWs and the benign adhesion able to those of commercial ITO films (85%, 45 Ω/sq). to the substrate. Furthermore, high-performance Ag NTEs with a trans- mittance of 91.6% and a sheet resistance of 11.4 Ω/sq Conclusions are obtained, as the aspect ratios exceed 1000. The long In summary, Ag NWs with different aspect ratios vary- nanowires and thin PVP layer lead to less number of ing from ca. 30 to ca. 1000 are prepared via a facile nanowire junctions and reduced junction resistance, PVP-mediated polyol process and are applied to the respectively. It allows low-temperature sintering of Ag fabrication of high-performance Ag NTEs with low- NWs network, which is advantageous for the applica- temperature sintering. In the polyol process, the diame- tions in the flexible plastic substrates. Moreover, Ag ters of Ag NWs are strikingly reduced and the aspect NTEs show excellent flexibility against the bending test. ratios reach almost 1000 when employing mixed PVP We believe that the ability to synthesize Ag NWs with as the capping agent. Additionally, when the aspect different aspect ratios and fabricate high-performance ratios exceed 500, the optoelectronic performance of NTEs with low-temperature welding are very valuable Ag NWs films show good transmittance (81.8–87.2%) to the development of flexible electronic devices. Fig. 7 a Optical image of as-fabricated Ag NWs films on PET. b Ag NWs film is connected in an electric circuit in which an LED is lit. c Optical image of the flexible Ag NWs film Xue et al. Nanoscale Research Letters (2017) 12:480 Page 10 of 12 Additional Files temperature and its application for fully stretchable polymer light-emitting diodes. ACS Nano 8(2):1590–1600 11. Lee J, A K, Won P, Ka Y, Hwang H, Moon H, Kwon Y, Hong S, Kim C, Lee C, Additional file 1: Table S1. Reaction parameters of Ag NWs with Ko SH (2017) A dual-scale metal nanowire network transparent conductor different concentrations of PVP and mixed PVP molecules at different for highly efficient and flexible organic light emitting diodes. Nano 9(5): mole ratios. Herein, silver nanoparticles and silver aggregated nanorods 1978–1985 are abbreviated to Ag NPs and Ag ANRs, respectively. Figure S1. SEM 12. Chang I, Lee J, Lee Y, Lee YH, Ko SH, Cha SW (2017) Thermally stable Ag@ images of Ag NWs under different reaction conditions: (a) 0.05 M PVP, ZrO core-shell via atomic layer deposition. Mater Lett 188:372–374 (b) 0.25 M PVP, (c) 0.55 M PVP, (d) PVP-10, (e) PVP-58, respectively. (f) 13. Chang I, Park T, Lee J, Lee HB, Ji S, Lee MH, Ko SH, Cha SW (2014) statistical size distribution of Ag NWs synthesized using PVP-58. Performance enhancement in bendable fuel cell using highly conductive Figure S2. Statistic sizes distribution of Ag NWs synthesized using Ag nanowires. Int J Hydrog Energy 39(14):7422–7427 different mixed PVP molecules. (a) PVP-40:PVP-58 = 2:1, (b) PVP-40:PVP- 14. Chang I, Park T, Lee J, Lee HB, Ko SH, Cha SW (2016) Flexible fuel cell using 58 = 1:1, (c) PVP-40:PVP-58 = 1:2, (d) PVP-40:PVP-360 = 2:1, (e) PVP-40:PVP- stiffness-controlled endplate. Int J Hydrog Energy 41(14):6013–6019 360 = 1:1, (f) PVP-40:PVP-360 = 1:2, respectively. Figure S3. Ag NWs film 15. Chang I, Park T, Lee J, Lee MH, Ko SH, Cha SW (2013) Bendable polymer is connected in an electric circuit, being applied a low voltage. electrolyte fuel cell using highly flexible Ag nanowire percolation network (DOCX 2653 kb) current collectors. J Mater Chem A 1(30):8541–8546 Additional file 2: The video of Ag NWs flexible transparent electrodes. 16. Park T, Chang I, Jung JH, Lee HB, Ko SH, O'Hayre R, Yoo SJ, Cha SW (2017) (AVI 9706 kb) Effect of assembly pressure on the performance of a bendable polymer electrolyte fuel cell based on a silver nanowire current collector. Energy 134:412–419 Acknowledgements 17. Park T, Chang I, Lee HB, Ko SH, Cha SW (2017) Performance variation of This work was supported by the NSFC (51471121), Basic Research Plan bendable polymer electrolyte fuel cell based on Ag nanowire current Program of Shenzhen City (JCYJ20160517104459444, collector under mixed bending and twisting load. Int J Hydrog Energy JCYJ20170303170426117), Natural Science Foundation of Jiangsu Province 42(3):1884–1890 (BK20160383), and Wuhan University. 18. Hong S, Yeo J, Lee J, Lee H, Lee P, Lee SS, Ko SH (2015) Selective laser direct patterning of silver nanowire percolation network transparent Authors’ Contributions conductor for capacitive touch panel. J Nanosci Nanotech 15(3):2317–2323 QWX completed all the experiments and wrote the manuscript. WJY, JL, QYT, 19. Kim KK, Hong S, Cho HM, Lee J, Suh YD, Ham J, Ko SH (2015) Highly sensitive LL, MXL, QL, and RP assisted with the manuscript preparation. WW and stretchable multidimensional strain sensor with prestrained anisotropic conceived the study, revised the manuscript, and supervised the work. All metal nanowire percolation networks. Nano Lett 15(8):5240–5247 authors read and approved the final manuscript. 20. Jeong S, Cho H, Han S, Won P, Lee H, Hong S, Yeo J, Kwon J, Ko SH (2017) High efficiency, transparent, reusable, and active PM2.5 filters by hierarchical Competing Interests Ag nanowire percolation network. Nano Lett 17(7):4339–4346 The authors declare that they have no competing interests. 21. 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Published: Aug 7, 2017

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