Direct Room Temperature Welding and Chemical Protection of Silver

Nov 29, 2017 - Silver nanowire (Ag-NW) thin films have emerged as a promising next-generation transparent electrode. However, the current Ag-NW thin f...
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Direct Room Temperature Welding and Chemical Protection of Silver Nanowire Thin Films for High-Performance Transparent Conductors Yongjie Ge, Xidong Duan, Meng Zhang, Lin Mei, Jiawen Hu, Wei Hu, and Xiangfeng Duan J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b07851 • Publication Date (Web): 29 Nov 2017 Downloaded from http://pubs.acs.org on November 29, 2017

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Direct Room Temperature Welding and Chemical Protection of Silver Nanowire Thin Films for High Performance Transparent Conductors Yongjie Ge,‡, † Xidong Duan,‡, † Meng Zhang,† Lin Mei,† Jiawen Hu,*, † Wei Hu,*, § Xiangfeng Duan*,†, & †

State Key Laboratory for Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China § School of Physics and Electronics, Hunan University, Changsha 410082, China & Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, USA KEYWORDS: Ag nanowires, transparent electrode, NaBH4 treatment, polyvinylpyrrolidone, dodecanethiol

ABSTRACT: Silver nanowire (Ag-NW) thin films have emerged as a promising next-generation transparent electrode. However, the current Ag-NW thin films are often plagued by high NW-NW contact resistance and poor long-term stability, which can be largely attributed to the ill-defined polyvinylpyrrolidone (PVP) surface ligands and non-ideal Ag-PVP-Ag contact at NW-NW junctions. Herein we report a room temperature direct welding and chemical protection strategy to greatly improve the conductivity and stability of the Ag-NW thin films. Specifically, we use a sodium borohydride (NaBH4) treatment process to thoroughly remove the PVP ligands and produce a clean Ag-Ag interface that allow direct welding of NW-NW junction at room temperature, thus greatly improving the conductivity of the Ag-NW film, outperforming those obtained by thermal or plasmonic thermal treatment. We further show that, by decorating the as-formed Ag-NW thin film with a dense, hydrophobic dodecanethiol layer, the stability of the Ag-NW film can be greatly improved by 150 times compared with that of PVP-wrapped ones. Our studies demonstrate that a proper surface ligand design can effectively improve the conductivity and stability of Ag-NW thin films, marking an important step towards their applications in electronic and optoelectronic devices.

INTRODUCTION Transparent conductors (TCs) are one of the key components in diverse optoelectronic devices, such as display,1 organic light-emitting diode (OLED),2 and photovoltaic devices.3 With excellent electronic properties and optical transparency, thin films of indium tin oxide (ITO) are the most widely used TCs. However, many industrial applications are limited by their brittleness, high production cost, and the scarcity of indium resource. To address these challenges, several alternative TC materials have been investigated, such as conducting polymers,4 carbon nanotubes,5-6 graphene,7-8 and Ag nanowires (Ag-NWs).9-12 Amongst, Ag-NWs are particularly attractive for their outstanding mechanical, optical, electrical properties and potentially low fabrication cost. Ag-NWs can be chemically synthesized using the wellestablished polyol reduction method, in which AgNO3 is reduced by ethylene glycol in the presence of polyvinylpyrrolidone (PVP).13-17 During the synthesis of Ag-NWs, PVP plays two important roles: serving as structure-directing agent and stabilizer. In general, PVP can selectively adsorb on the [100] facets of Ag nuclei, restraining their growth kinetics and leading to the formation of Ag-NWs with desired length and diameter.18 Meanwhile, the long-chain PVP ligands offer steric

stabilization to the Ag nuclei and NWs, preventing them from aggregation and precipitation. During post-solution processing of Ag-NWs into thin films, the PVP ligands also facilitate the storage, separation, cleaning, and manipulation of Ag-NWs. However, the residual PVP ligands forms an insulating layer around the Ag-NWs, creating high contact resistance at NWNW junctions that largely limits the current delivering capability of the Ag-NW thin films.19-20 A number of strategies have been developed to improve the conductivity of the Ag-NW thin films, including mechanical pressing,21 plasmonic welding,22 thermal annealing,23-25 hybrid films of graphene sheets and Ag-NWs,26-33 nanowire assembly,34 photochemical welding,35-36 and capillary force-induced welding.37 Very recently, we also found that light illumination can effectively weld adjacent Ag and Au nanoparticles assembled at water-air interface via Ostwald ripening mechanism.38 However, these strategies either need harsh conditions or are inefficient enough for large area fabrication. For example, thermal annealing has the risk to damage the flexible host substrate (e.g., polyethylene glycol terephthalate, PET) because of the high temperature and relatively long duration applied (typically, 200 oC 20 min).23 A recent study also reported a relatively mild humidassisted annealing strategy (60-85 oC, 50 min)39 for improving

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the sheet conductivity of Ag-NW thin films, which could pose less or even negligible damage to the most common plastic substrate such as PET. However, this process requires a rather long treatment time of about 50 min, which could become the critical limiting step in a continuous (e.g., roll-to-roll) fabrication line, thereby dramatically increasing the overall production cost. Additionally, Ag-NW films have poor long-term stability because they are easily oxidized in humid air due to their interactions with moisture, oxygen, and especially sulfurcontaining coumpounds.40 To address this problem, a variety of materials, e.g. graphene oxide,41 graphene,29 and conductive ion-gel36 have been exploited as the protective layer. However, it remains a challenge to develop a highly efficient approach that can be applied under mild conditions over large scale to simultaneously improve both the conductivity and stability of the Ag-NW thin films. In general, the ill-defined Ag-PVP-Ag interface represents the key limiting part that dictates the high contact resistance at the NW-NW junctions. However, the complete removal of PVP has been proven challenging. Inspired by the successful surface ligand removal strategies developed, e.g., chemical reduction,42 ligand displacement,43-44 and electrochemical reduction,45 that enable removal of strong ligands (e.g., thiols) from surface, herein, we report a direct room temperature welding and chemical protection approach to improve both the conductivity and stability of the Ag-NW films using a rapid and scalable solution process. Our approach features several desirable characteristics. From the outset, by employing a NaBH4 treatment process to completely remove surface PVP ligands, our approach produces an atomically clean Ag-Ag interface to ensure direct welding of NW-NW junction at room temperature, thus greatly increasing the conductivity of the Ag-NW thin films. Furthermore, by using a subsequent room temperature surface decoration with a hydrophobic dodecanethiol (DT) layer, our approach significantly retards the moisture diffusion and surface oxidation of the Ag-NWs to greatly improve the long-term stability of the AgNW films. Lastly, by replacing long-chain ill-defined surface ligands (PVP) with short-chain densely package ligands (DT), our approach greatly decreases the contact resistance at the Ag-NW film/semiconductor layer interface, thereby improving the carrier injection/collection efficiency to/from other active layer of optoelectronic devices.

Fabrication of Ag-NW film: Ag-NWs (diameter: ~ 70 nm; length: ~ 25 µm) were synthesized following the polyol reduction method.14-15 After cleaning by 4 cycles of centrifugation and re-dispersing in ethanol, the PVP-wrapped Ag-NWs were dispersed in ethanol, forming a stock NW dispersion (0.8 mg·mL-1). Then, the dispersion (100 µL) was spin coated on a clean PET substrate (35 mm × 14 mm). After drying in air, PVP-wrapped Ag-NW films were obtained, whose sheet resistance and transmittance can be tuned by the surface density of the NWs. To remove PVP, the PVP-wrapped Ag-NW films were immersed in 0.5 M NaBH4 solution (mixed solvent of water and ethanol, 1:1 in volume) for 30 s, followed by rinsing with water and ethanol and drying in N2 blow. The obtained surface-cleaned Ag-NW films were then immersed in 0.5 M DT in ethanol for 210 s, followed by rinsing with ethanol and drying in N2 blow. For comparison, HT- and ODT-decorated Ag-NW films were also prepared in a similar way. Construction of hole-only device: To evaluate the performance of the Ag-NW film, sandwiched hole-only devices were constructed using the film as bottom electrode. Briefly, PVP-wrapped Ag-NW films were fabricated in a rectangular area (5 mm × 15 mm) defined on a clean PET substrate using a 3M tape, followed by sequentially immersing in NaBH4 solution for 30 s and in DT solution for 210 s. Then, a layer of copper (II) phthalocyanine (CuPC, p-type organic semiconductor) with 80 nm thickness was thermally deposited onto the DT-decorated Ag-NW films. Finally, an Ag film (top electrode) with 60 nm thickness was thermally deposited onto the CuPC film through a mask. For comparison, hole-only devices without DT decoration were also constructed from PVPwrapped and surface-cleaned Ag-NW films using the same protocols described above. Characterizations: Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were obtained on a ∑IGMA microscope (Zeiss, Germany) and a JEM-2100F microscope (JEOL, Japan), respectively. Energy dispersive X-ray spectroscopy (EDS) mappings were performed on a spectrometer equipped with the SEM microscope. UV-Vis extinction spectra of the Ag NWs and transmission spectra of the Ag-NW films were obtained on a UV-1800 spectrometer (Shimadzu, Japan). Fourier-transform infrared (FT-IR) spectra for pure DT and Ag-NWs were measured on an IRTracer-100 spectrometer (Shimadzu, Japan), using a liquid IR cell and KBr pellet, respectively. Sheet resistance of the Ag-NW film was measured on a RTS-9 four-probe instrument (Guangzhou Four-probe Scientific Co., Ltd, China). The three-phase contact angle of the Ag-NW film was measured on a JC2000C contact angle measurement instrument (Shanghai Zhongchen Digital Technology Equipment Co., Ltd, China). To minimize the influence of host substrate on the contact angle, a denser Ag-NW film with a transmittance smaller than 50 % at 550 nm was fabricated on the PET substrate. The CuPC layer and top Ag electrode were deposited using a JSD300 vacuum evaporator (Anhui Jiashuo Vacuum Science and Technology Co., Ltd, China). Current-voltage (I-V) responses for the Ag-NW films and the hole-only devices were monitored using a B2912A precision source/measure unit (Keysight, America).

EXPERIMENTAL SECTION Materials: AgNO3 (AR), PVP (K-30), CuCl2 (AR), acetone (AR), hexanethiol (HT, 96%), dodecanethiol (DT 98%), octadecanethiol (ODT, 98%) and NaBH4 (98%) were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Ethylene glycol (AR), ethanol (AR), and 0.125 mmthick PET film were purchased from Hengyang Kaixin Chemical Reagent Co., Ltd (Hengyang, China), Shanghai Titan Scientific Co., Ltd (Shanghai, China), and Shanghai Feixia Rubber and Hardware Co., Ltd (Shanghai, China), respectively. All the materials were used as received without further purification. Ultrapure water (≥18.2 MΩ·cm) purified from a RS2200QSS-PURIST ultrapure water system (Rephile Bioscience, Ltd. Shanghai, China) were used throughout the experiments.

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zation of a colloid dispersion is achieved either by the steric repulsion from the long chain surface ligands or by the electrostatic repulsion amongst charged colloidal particles, in which the former can withstand a high ionic strength while the latter cannot.48 Clearly, the above comparison indicates that the addition of NaBH4 leads to two effects: removal of the PVP ligands and reducing in electrostatic repulsion, while the addition of NaClO4 could only reduce the electrostatic repulsion. We have further examined the ligand removing kinetics using UV-vis spectroscopy. Figure 2D shows the evolution of peak wavelength (λmax) shift for the transversal plasmon peak of Ag-NWs dispersed in a 0.05 M NaBH4 solution (mixed solvent of water and ethanol, 1:1 in volume). The λmax continuously blue-shifts because removal of the PVP ligands gradually changes the dielectric environment of the Ag-NWs.49 Figure 2E shows the plots of the λmax versus treatment time in 0.05, 0.10, and 0.50 M NaBH4 solutions. The PVP removing rate PVP accelerates with increasing NaBH4 concentration. In particular, the entire removal process can be accomplished within 70 s if a high NaBH4 concentration (0.50 M) is used.

RESULTS AND DISCUSSION The overall sheet resistance of Ag-NW films is generally dictated by the contact resistance at NW-NW junctions characterized with a Ag-PVP-Ag interface, which represents the critical limiting part of the thin film.19-21, 24 Therefore, a direct welding of the NW-NW junction is essential for ensuring efficient electron transport across the NW-NW junctions and thereby for improving the overall conductivity of the Ag-NW thin films. Previous studies have shown that a thermal23 or plasmonic thermal treatment22 can allow removal of PVP layer at NW-NW junction to effectively weld NWs, which is, however, generally limited by the need of high temperature treatment or intense light illumination with long duration. Here we report an alternative chemical approach to completely remove interfacial PVP ligands and produce intimate Ag-Ag contacts, which can allow a direct chemical welding of Ag-NWs at room temperature. Figure 1 schematically shows our protocol to remove PVP from the Ag-NW thin film by NaBH4 treatment, followed by the surface decoration with a dense, hydrophobic DT protection layer. The PVP ligand cannot be easily removed using common washing process because its rich carbonyl group chemically binds to the Ag surface, forming a strong Ag-O bond with a binding energy of 50.9 kCal/mol.46 During NaBH4 treatment, NaBH4 decomposes and produces abundant hydride ion, which has a binding energy to Ag of 81.71 kCal/mol, much higher than that of the Ag-O bond.47 As a result, the carbonyl group can be displaced by hydride, thereby enabling a complete removal of PVP to produce desired clean Ag-Ag interface at the NW-NW junctions. This removal mechanism means that the NaBH4 can be replaced by other strong reducing agents or electrochemical methods that can produce a similar hydrogen absorption layer.

Figure 2. Stability tests and removal kinetics in NaBH4 solution of mixed water and ethanol (1:1 in volume). (A-C) Optical images for the PVP-wrapped Ag-NWs in 0.5 M NaClO4 solution and 0.5 M NaBH4 solution for different duration. (D) Time-dependent λmax for the PVP-wrapped Ag-NWs in 0.05 M NaBH4 solution. (E) Plots of λmax versus time for the PVP-wrapped Ag-NWs treated in NaBH4 solution with different concentrations.

Figure 1. Schematic showing the removal of PVP ligand from Ag-NW film and subsequent decoration with a DT layer, to enable direct Ag-Ag contact and complete protection by a dense hydrophobic DT layer.

The complete removal of PVP ligands from the Ag-NW surface can be further directly confirmed using SEM and TEM studies. Prior to SEM/TEM measurements, the samples were negatively stained in 1 wt% Na3O40PW12 solution for 5 min because the PVP layer has a low electron density and cannot be directly resolved by electron microscope. Figure 3A and 3B show the negatively stained SEM images for multiple AgNWs before and after NaBH4 treatment, respectively. A thin layer coating of ~ 4-12 nm thickness with lighter contrast is clearly seen on the edge of the PVP-wrapped Ag-NWs (Figure 3A), while this feature is absent for the NaBH4-treated sample (Figure 3B). The same phenomena can also be observed from the corresponding TEM images (Figure 3C and 3D). The

On the basis of stabilizing role of PVP ligand for Ag-NW dispersion, we first examined its removal using simple dispersion stability tests. Figures 2A, 2B and 2C show the optical images for the Ag-NWs dispersed in 0.5 M NaBH4 solution for different duration, along with those dispersed in 0.5 M NaClO4 solution for comparison. With increasing duration, the Ag-NWs dispersed in NaBH4 solution gradually aggregate and finally settle down within 60 min. In contrast, the Ag-NWs dispersed in NaClO4 solution don’t show apparent aggregation and precipitation at the same time scale. In general, the stabili-

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HRTEM image of the PVP-wrapped Ag-NW clearly shows the existence of a non-crystalline area next to the Ag bulk crystalline structure (Figure 3E). In contrast, such noncrystalline area is completely absent after NaBH4 treatment (Figure 3F), confirming the complete removal of PVP ligands. To probe the chemical composition information and the molecular structural information of the removed surface ligands, we have also conducted energy dispersive X-ray spectroscopy (EDS) and infrared (IR) studies. EDS elemental mapping shows that the carbon content on the surface of Ag-NWs is dramatically reduced after NaBH4 treatment (Figure 3G and 3H). Additionally, the IR studies show the characteristic stretching vibration peak of the carbonyl group at 1650 cm-1 observed in PVP-wrapped Ag-NWs disappeared in the NaBH4-treated Ag-NWs. (Figure 3I). All these observations further support that the PVP ligands are effectively removed upon a solution-based NaBH4 treatment process.

It is evident that the sheet resistance continuously decreases with increasing concentration and then saturates at ~0.5 M. Additionally, the sheet resistance further decreases with increasing treatment time in 0.5 M NaBH4 solution (Figure 4D). In either case, the transmittance does not show obvious change with the concentration of NaBH4 solution or the treatment time. Notably, for a Ag-NW film with a 92.6% transmittance, the sheet resistance can be decreased from an initial value of 81.5 to 35.0 ohm/sq upon immersing in 0.5 M NaBH4 solution for just 30 s. Importantly, the final sheet resistance value (35.0 ohm/sq, 92.6 T%) compares well with that of the state-of-art Ag-NW films with NW-NW junctions welded using thermal annealing (68 ohm/sq, 92.5 T%),23 plasmonic treatment (580 ohm/sq, 95 T%),22 or by selective nucleation and growth of silver nanoparticles (31.7 ohm/sq, 91.5 T%),35 implying that the formation of atomically clean Ag-Ag contact without surfactant at the interface can enable direct room temperature welding at the NW-NW junction without additional process. Furthermore, a series of PVP-wrapped Ag-NW films with different initial transmittance were also fabricated. In all cases, the removal of the PVP ligand can effectively reduce the sheet resistance whiles not affecting their transmittance (inset in Figure 4D).

Figure 3. Characterizing the removal of the PVP ligand from AgNWs. SEM images for multiple Ag-NWs (A and B), TEM images for a single Ag-NW (C and D), HRTEM images for a single AgNW (E and F), EDS mappings for Ag-NWs (G and H) and IR spectra (I) for PVP (curve a) and Ag-NWs (curves b and c) before (A, C, E, G and curve b in panel I) and after (B, D, F, H and curve c in panel I) treatment in 0.5 M NaBH4 solution for 30 s.

Figure 4. Influence of residual PVP ligands on the physical properties of the Ag-NW film. (A) SEM images of the PVP-wrapped Ag-NW film before and after (inset) treatment in a 0.5 M NaBH4 solution for 30 s. (B) Corresponding optical images for the PVPwrapped Ag-NW film before (left) and after (right) NaBH4 treatment. (C and D) The variation of relative sheet resistance (left, Y axis) and transmittance (at 550 nm, right Y axis) for the PVPwrapped Ag-NW film as a function of NaBH4 concentration (C, for 1 s duration) and durations (D, in a 0.5 M NaBH4 solution). Inset in D: the variation of transmittance and sheet resistance for the Ag-NW thin films with different initial transmittances before (a) and after (b) treatment in a 0.5 M NaBH4 solution for 30 s.

To probe the impact of NaBH4 solution treatment on the electronic and optical properties of the Ag-NW thin films, we have prepared a series of Ag-NW thin films on PET substrates, and treated them with NaBH4 solution for various durations. In general, the treatment does not alter the overall structure, as shown in the SEM images of NW network before and after the treatment (Figure 4A and its inset). As a result, the optical transparency of the Ag-NW film is not significantly affected by NaBH4 treatment, as confirmed by the photograph shown in Figure 4B. To quantitatively probe the effect on electronic and optical properties, we have measured the sheet resistance and optical transmittance of the NW thin films after treatment in NaBH4 solution with various concentrations for various durations. Figure 4C shows the relative variation of the sheet resistance and the transmittance of a typical Ag-NW thin film with initial transmittance of 92.6% after one-second treatment in the NaBH4 solution with increasing concentration.

Apart from sheet resistance and transmittance, the longterm stability of Ag-NW film is another critical issue for practical applications because Ag-NWs are susceptible to degradation when exposed in ambient environment. Considering that the thiol group of DT shows great affinity to silver metal and DT can easily form a dense monolayer on Ag surface,50 we exploited a self-assembled DT layer as the protective layer to prevent Ag from oxidization. Different from other methods, decorating the Ag-NW film with DT is relatively straightforward because it can be performed after the fabrication of the

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Ag-NW film. Figure S1 shows the sheet resistances and transmittance spectra for the PVP-wrapped Ag-NW film (initial transmittance of 92.1% at 550 nm), surface-cleaned AgNW film, and DT-decorated Ag-NW film, all of which are just fabricated and exposed in summer air (average relativity humidity: >90%, average temperature: >25 oC). The DTdecorated Ag-NW thin film shows a sheet resistance (32.5 ohm/sq) and transmittance (92.0%) comparable to that of the surface-cleaned Ag-NW film (30.0 ohm/sq and 92.0 T%). These observations indicate that DT decoration itself does not affect the intimate Ag-Ag contacts at the NW-NW junctions and the network structure of the NW thin film. Importantly, the functionalization of Ag-NW thin film with DT can have profound impact to the stability of the Ag-NW thin film and its sheet resistance. In particular, the sheet resistances for PVP-wrapped, surface-cleaned, and DT-decorated Ag-NW films increase 450, 10, and 3 times, respectively, when exposed in summer humid air for 10 days (Figure 5A). Evidently, the DT-decorated Ag-NW thin films show much improved long-term stability when compared with the other two types of film. The same conclusion can be reached while these Ag-NW films were exposed for the same duration in autumn air (average relativity humidity: