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Engineering Silver Nanowire Networks: From Transparent Electrodes to Resistive Switching Devices Haiwei Du,‡ Tao Wan,‡ Bo Qu, Fuyang Cao, Qianru Lin, Nan Chen, Xi Lin, and Dewei Chu* School of Materials Science and Engineering, University of New South Wales, Sydney, NSW 2052, Australia S Supporting Information *
ABSTRACT: Metal nanowires (NWs) networks with high conductance have shown potential applications in modern electronic components, especially the transparent electrodes over the past decade. In metal NW networks, the electrical connectivity of nanoscale NW junction can be modulated for various applications. In this work, silver nanowire (Ag NW) networks were selected to achieve the desired functions. The Ag NWs were first synthesized by a classic polyol process, and spincoated on glass to fabricate transparent electrodes. The asfabricated electrode showed a sheet resistance of 7.158 Ω □−1 with an optical transmittance of 79.19% at 550 nm, indicating a comparable figure of merit (FOM, or ΦTC) (13.55 × 10−3 Ω−1). Then, two different post-treatments were designed to tune the Ag NWs for not only transparent electrode but also for threshold resistive switching (RS) application. On the one hand, the Ag NW film was mechanically pressed to significantly improve the conductance by reducing the junction resistance. On the other hand, an Ag@AgOx core−shell structure was deliberately designed by partial oxidation of Ag NWs through simple ultraviolet (UV)-ozone treatment. The Ag core can act as metallic interconnect and the insulating AgOx shell acts as a switching medium to provide a conductive pathway for Ag filament migration. By fabricating Ag/Ag@AgOx/Ag planar structure, a volatile threshold switching characteristic was observed and an on/off ratio of ∼100 was achieved. This work showed that through different post-treatments, Ag NW network can be engineered for diverse functions, transforming from transparent electrodes to RS devices. KEYWORDS: silver nanowires, transparent electrode, mechanical pressing, ultraviolet-ozone treatment, threshold resistive switching
1. INTRODUCTION As an essential component in electronics, transparent conductors (TCs) usually play a critical role in the area of modern electronics, such as touch screens, liquid-crystal displays, solar cells, and organic light-emitting diodes. With high transmittance (>90%) and low sheet resistance (∼10 Ω □−1),1 indium tin oxide (ITO) has dominated the TC market over the past several decades. In addition to the scarcity of indium sources, ITO is inherently incompatible with plastic substrates, because of the high deposition temperature, and its brittle nature also impedes its flexible applications. Large-scale production of ITO is restricted by the relatively low sputtering speed and high cost of conventional sputtering technology as well. Thus, tremendous effort has been devoted to exploring novel materials with comparable electrical and optical performances in order to replace ITO in the future. As an alternative to ITO, metallic nanowire (NW) networks are attracting considerable attention and are regarded as one of the most promising candidates for next-generation transparent electrodes (TEs).2 Especially, silver nanowire (Ag NW) exhibits a great potential for electronic devices,3,4 because of intrinsic low electrical resistivity5 and a flat spectral transmittance. In addition, it can be easily fabricated for large-scale devices by lithography or solution-based deposition approaches.6 To further optimize the performances of Ag NWs-based electrodes, © 2017 American Chemical Society
the researchers have reported novel approaches including tuning the aspect ratio7 by synthesizing very long NWs, designing Ag NWs/metal oxide composites,8 as well as directional arrangement of nanowires.9 However, the as-prepared Ag NW networks still suffer from a relatively higher resistance, inevitably because of the residual surface polyvinylpyrrolidone (PVP) capping layer and contact resistance as the loose contact at the junctions. The junction resistance of Ag NWs deposited by E-beam lithography is even as large as 1 GΩ.10 Moreover, the weak bonding between NWs and the poor adhesion between the NWs and substrates have an adverse effect on mechanical and electrical stability as well. Accordingly, various post-treatments such as solvent washing,11 thermal annealing,12 and chemical coating of gold10 are implemented to further decrease the wire−wire junction resistance. However, those methods still have some disadvantages: (i) a reducing atmosphere is required to avoid oxidation of Ag NWs and thermal annealing (usually >200 °C) is not compatible with the plastic substrates; Received: April 6, 2017 Accepted: May 29, 2017 Published: May 29, 2017 20762
DOI: 10.1021/acsami.7b04839 ACS Appl. Mater. Interfaces 2017, 9, 20762−20770
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a, b) SEM images, (c) length distribution of Ag NWs, and (d) diameter distribution of Ag NWs. (e) XRD pattern, (f) enlarged SEM images, and (g) schematic of a single Ag NW. (h, i) TEM image, (j) HRTEM image, and a selected-area electron diffraction (SAED) pattern (k) taken from a single NW. Inset in panel (k) shows the orientation of the NW, with respect to the electron beam.
into Ag NWs to explore novel applications is very interesting. For example, single-metal NW15 or metal NW network16 and crossbar17 with a passivating shell have been found to show a great potential in RRAM or memristor applications. In these devices, the formation of passivating shell is normally achieved by coating SiO218 or post-sulfuration19/oxidation17,20 treatment. Recently, Ni/NiO core−shell NW network20 was reported to show resistive switching (RS) behavior, which was attributed to the Ni filament migration in the surrounding NiO insulating layer. Similarly, as an active electrode, Ag nanoparticles have been widely used in RS devices over the past decade, because they can form Ag filaments during the voltageinduced redox process. Meanwhile, silver oxide (AgOx), the derivative with increased resistance, also can be used as the insulating layer for RRAM function.21 Generally, the RRAM devices with sandwich or planar structure are fabricated through several steps, including film growth and electrode deposition, which is time-consuming and complicated. As the AgOx can be very easily obtained by UV irradiation,22 herein, the RS behavior is expected to be achieved in Ag NW network after deliberately designing an Ag@AgOx layer in the middle of Ag NW film.
(ii) the purification and redispersion of NWs must be repeated many times to dissociate the PVP as much as possible;13 and (iii) a chemical coating of gold also increases the cost, and the residual Cl− on the NW surfaces may influence their chemical properties or change the particle morphology. Thus, it is desired to develop novel processing methods for improving the conductivity of electrode without sacrifice of transmittance and stability. As a very simple technique, mechanical pressing10 can either reduce the resistance of Ag NWs or improve the surface roughness significantly, because of the decreased junction resistance and mechanically compressed Ag NW junctions.14 In addition, it is known that active electrodes, such as Ag, Al, Ni, and Cu, play a very important role in resistive random access memory (RRAM) devices. The formation and migration of metallic ions from these electrodes under applying bias give rise to filamentary conduction inside the insulator layer and, hence, to switching behaviors. Thus, a typical memory device is designed with a metal−insulator−metal (MIM) structure consisting of active electrodes and their derivative oxides or other metal oxides. Based on this point, introducing this idea 20763
DOI: 10.1021/acsami.7b04839 ACS Appl. Mater. Interfaces 2017, 9, 20762−20770
Research Article
ACS Applied Materials & Interfaces
Figure 2. (a−e) Optical images of as-prepared Ag NWs coated on glass substrates with different coating times. (f) Photograph of corresponding TEs on a piece of paper. (g) Optical transmittance and (h) sheet resistance and FOM of Ag NW TEs. Inset in panel (g) shows the linear fitting of optical transmittance at 550 nm as a function of coating time (t). The FOM is determined without background subtraction. 150 °C with magnetic stirring for 70 min. Finally, the as-prepared Ag NWs were obtained by centrifugation separation and redispersed in ethanol and deionized (DI) water for five times. 2.3. Fabrication of Ag NW TEs. Before spin coating, glass slides (2 cm × 2 cm, thickness of 1.6 mm) were cleaned by ethanol and DI water, and treated under UV illumination by the UV/ozone surface processor (Sen Lights Corporation, Japan) for 5 min. The Ag NWs were dispersed in ethanol with concentration of 0.8 mg mL−1. Then, 20 μL of the solution was deposited on the glass substrate by spin coating at 1500 rpm for 10 s and 2500 rpm for 20 s, corresponding to one layer. After repeating the coating cycles, the Ag NWs films with desired layers were prepared. 2.4. Fabrication of NW Network RS Device. The Ag NWs were dispersed in ethanol with a concentration of 0.5 mg mL−1 and deposited on glass slides (2 cm × 1.5 cm, thickness of 1.6 mm) by drop coating (five drops, each drop is 20 μL). Then, two glass slides as the mask were covered on the edge of Ag NW film and the film was treated under UV irradiation by the UV/ozone surface processor (Sen Lights Corporation, Japan) for 25 min (power intensity = 15 mW/cm2 wavelength: 254 nm). After UV treatment, the uncovered part became black, which indicated the formation of AgOx while the covered parts were still conductive. 2.5. Materials and Device Characterization. Structural analysis of the as-synthesized Ag NWs was carried out using an X-ray diffractometer with Cu Kα radiation (λ = 0.1541 nm). The microstructures were observed by optical microscopy (Nikon Eclipse
In this work, Ag NWs were synthesized by the well-known polyol process and the as-obtained Ag NWs can be used for diverse applications through two different post-treatments. On the one hand, the NW-based TEs were fabricated by a spincoating method first, and further mechanically pressed to optimize the electrical performances, especially by improving the NW junction resistance significantly. On the other hand, we designed an Ag/Ag@AgOx/Ag planar structure in Ag NW film through the UV treatment. The presence of AgOx not only increases the total resistance but also provides a path for filamentary conduction. As expected, the device showed RS behavior, indicating a potential RS application.
2. EXPERIMENTAL PROCEDURE 2.1. Materials. Silver nitrate (AgNO3, weight-average molecular weight (Mw) = 169.87), polyvinylpyrrolidone (PVP) K30 (Mw = 40 000), sodium chloride (NaCl, Mw = 58.4) and 1,2-propanediol (ρ = 1.036 g/mL). All chemicals purchased from Sigma−Aldrich were used without further purification. 2.2. Synthesis of Ag NWs. The Ag NWs were synthesized using a modified polyol process.23 In a typical synthesis, 30 mL of 1,2propanediol containing 0.45 g PVP was first placed into a 100 mL beaker and heated with magnetic stirring at 150 °C for 45 min and then 3 mL of 1 mM NaCl solution was added. Afterward, 12 mL of 0.1 M AgNO3 solution was added and the solution was kept heating at 20764
DOI: 10.1021/acsami.7b04839 ACS Appl. Mater. Interfaces 2017, 9, 20762−20770
Research Article
ACS Applied Materials & Interfaces
Figure 3. SEM images of Ag NWs coated glass (a) before and (b) after mechanical pressing. (c) Schematic of the Ag NW junction change and (d) the I−V characteristics of Ag NWs before and after pressing. ME600), transmission electron microscopy (TEM) (FEI Tecnai G2), and scanning electron microscopy (SEM) (FEI Nova NanoSEM 450). The transmittance of TEs was determined by a PerkinElmer ultraviolet−visible (UV-vis) spectrometer. The sheet resistance was carried out using a four-point probe resistance tester (Zhuhai Kaivo Optoelectronic Technology Co., Ltd.). The RS characteristics of the Ag/Ag@AgOx/Ag devices were tested by a Keysight B2902A source meter.
synthesized NW is single crystalline. The pattern is a result of the double diffraction reflections and can be interpreted as a superposition of square [100] and rectangular [112] zone patterns.24 Figures 2a−e show the optical images of TEs with different coating times, and the density of Ag NWs increases obviously with the increasing coating times, resulting in the formation of a NW network. It has been reported that the area density plays an important role in modulating the conductivity of Ag NW networks.25,26 Here, the transmittance and sheet resistance of the Ag NW TEs are both dependent on the coating times. Increasing coating layers linearly decreases the transparency from 90.73% of bare glass to 73.52% of electrode with 9 layers (inset in Figure 2g), while increasing the conductivity significantly. The sheet resistance decreases sharply from 10 950 Ω □−1 to 15.86 Ω □−1, and then continues to decrease slightly to 4.42 Ω □−1 after coating 9 times (Figure 2h). The enhanced conductivity is due to more NW-NW contacts after the establishment of NW network. In addition, a classic figure of merit (FoM, or ΦTC), defined by Haacke,27 is usually used to evaluate the Ag NW TEs:
3. RESULTS AND DISCUSSION Figure 1 shows the SEM images of Ag NWs and the products are dominated by NWs with relatively uniform length and diameter. The shorter NWs are straight, while the longer NWs are slightly curved. To quantificationally investigate the NW size (Figure S1 in the Supporting Information), the length and diameter distributions are shown in Figures 1c and 1d, in which the mean length and diameter are 19.16 μm and 52.6 nm, respectively, indicating that the aspect ratio is ∼360. The asprepared NWs exhibit a pure face-centered-cubic (fcc) structure (JCPDS File No. 04-0783), as shown in Figure 1e. Figures 1f and 1g show a single Ag NW with a lancelike end and a schematic of a single Ag NW, respectively. Generally, Ag NW with growth along [110] direction exhibits a pentatwinned structure bound by five (100) lateral planes and five triangular (111) facets joined at the tip. Figures 1h−j show the TEM images of as-prepared Ag NWs. It is seen that there is an obvious capping layer on the surface of a single NW (Figure 1j), corresponding to the residual PVP. The PVP layer is difficult to completely eliminate, even after five washes. On the one hand, PVP plays a very important role in the synthesis of Ag NWs, because it is usually used to effectively control the NW formation and disperse NWs by encapsulating the {100} planes of Ag NW. On the other hand, the residual PVP layer adsorbing on the surface has an adverse effect on the conductivity as the PVP layer is an electrically insulating barrier at the junctions. Furthermore, the selected-area electron diffraction (SAED) pattern (Figure 1k) shows that the as-
ΦTC =
T10 Rs
where T is the optical transmittance at 550 nm and Rs is the sheet resistance. Here, the FoM is determined without the background subtraction, since a subtraction might lead to distorted results.28 Thus, the transmittance loss by the glass substrate is also included in the FOM. As shown in Figure 2h, the FoM increases gradually after coating more Ag NWs on the substrate, reaching to the maximum value of 13.55 × 10−3 Ω−1, and then decreases with further deposition. The FoM of asprepared Ag NWs electrode is comparable with previous reports, which are listed in Table S1 in the Supporting Information. An LED lamp connected to the Ag NWs transparent electrode also shows good conductivity (Figure 20765
DOI: 10.1021/acsami.7b04839 ACS Appl. Mater. Interfaces 2017, 9, 20762−20770
Research Article
ACS Applied Materials & Interfaces
Figure 4. (a) Schematic of fabrication of Ag NWs-based resistive switching device, and (b) top view of the device with the SEM images of Ag NW electrode and Ag@AgOx core−shell structure. (c) TEM image and (d) XRD pattern of Ag@AgOx nanowires after UV-ozone treatment for 25 min.
network is also improved, resulting in a reduction of the overall resistance. The effect of mechanical pressing on the conductance was also studied by I−V curves, as shown in Figure 3d. Compared with the as-fabricated electrode, the resistance of pressed electrode is significantly reduced. In addition to the applications of transparent electrode, it is noted that, recently, other potential applications have been expected to be achieved in Ag NW-based composites. For example, the Ag NW/polyaniline layered nanocomposite was designed for a pH self-adjusting system,29 in which the polyaniline acts as a pH sensor and the Ag NWs are used for increasing the conductivity. Here, we designed an Ag/Ag@ AgOx/Ag planar structure by a simple treatment to explore the RS application. Figure 4a shows the fabrication process of a RS device. The Ag NWs were first deposited on the substrate and the film covered with two masks was treated under UV irradiation. After UV irradiation, the Ag NW film was transformed to an Ag/Ag@AgOx/Ag planar structure. The covered parts are still Ag NWs while the uncovered part becomes black and insulating with rough morphology (Figure 4b), indicating the deliberate oxidation at the Ag NW surface by the UV-ozone treatment. The formation of AgOx has been found in Ag nanoparticles and Ag NWs after exposure to light.22,30,31 In this process, a photochemical transformation or a photo-assisted oxidation occurs at the metal/atmosphere interface. During the reaction process, on the one hand, O2 acts an electron acceptor for the photoexcited Ag nanoparticles induced by light exposure;30 on the other hand, it is found that the UV-blue part of the light is mainly responsible for the phototransformation of Ag nanoparticles.22 In this work, we used the UV/ozone surface processor, which not only provides the UV light but also leads to an ozone atmosphere. The generated ozone atmosphere is stronger than O2 as the electron acceptor and can accelerate the AgOx formation. Thus, the formation of AgOx should be the attributed to the UV irradiation and ozone atmosphere. The TEM image (Figure 4c)
S2 in the Supporting Information). Moreover, Ag NWs were spin-coated on polyethylene terephthalate (PET) substrate and the resistance changes slightly after convex and concave bending (Figure S3 in the Supporting Information), indicating that the as-synthesized Ag NWs have promising applications in flexible electrodes. However, the resistance of as-synthesized NWs is still relatively high, because of the larger junction resistance. It is reported that mechanical pressing is an attractive way to reduce the sheet resistance of Ag NW coating.10 With the increasing pressure, the conductance was significantly improved.25 Here, to clearly show the effect of pressing, we selected Ag NWs TE with three coating layers. The electrode was sandwiched between two glass slides and uniaxially pressed at 24.5 MPa for 3 min. According to the previous study,14 the sheet resistance decreased obviously when applying pressure and kept almost unchanged when increasing the pressure up to 25 MPa. The pressure of 25 MPa is also selected as the compaction pressure for making paper-based Ag nanowire electronic circuits.25 Therefore, we chose this pressure only to show the effect of mechanical pressing on conductance. Figures 3a, and 3b shows the comparison of Ag NWs before and after mechanical pressing, and the samples were carbon-coated in order to better observe the details of Ag NW junction. Before pressing, the NW junctions with weak contact were observed, while “fused” NW junctions with strong contact can been seen after pressing. Consequently, the sheet resistance after applying pressure decreases from 172.89 Ω □−1 to 18.60 Ω □−1, and this is improved by almost 1 order of magnitude. Figure 3c shows the schematic of effect of mechanical pressing on NW junctions. Without pressing, a larger contact resistance resulting from a weak nanowire−nanowire connection leads to a relatively lower conductance. After applying pressure, the NW junctions are strongly contacted as the NWs are mechanically compressed to almost the same height. Thus, a bridging effect at junctions becomes more obvious and the interconnection in NW 20766
DOI: 10.1021/acsami.7b04839 ACS Appl. Mater. Interfaces 2017, 9, 20762−20770
Research Article
ACS Applied Materials & Interfaces
Figure 5. (a) Current−voltage (I−V) characteristic with a current compliance (CC) of 1 μA; the sweeping direction is shown by the arrows. (b) The on/off ratio of the Ag/Ag@AgOx/Ag RS device. (c) Schematic of Ag@AgOx junction at the “TS OFF-state” and “TS ON-state”. (d) Proposed mechanism for RS behavior in Ag/Ag@AgOx/Ag planar structure with a critical conductive path (yellow solid line) and a potential path (yellow dashed line).
clearly shows that AgOx with a thickness of ∼80 nm was formed on the surface of the Ag nanowire. Meanwhile, three diffraction peaks at ∼32.2°, 34.1° and 37.1°, corresponding to a secondary phase of AgO (JCPDS File No. 89-3081) are also detected in the Ag@AgOx XRD pattern (Figure 4d). The AgO diffraction peaks become more obvious when prolonging the UV illumination time to 1 h (Figure S4 in the Supporting Information). These characterizations also prove that Ag NWs have become Ag@AgOx core−shell structures, because of a surface oxidation by UV-ozone treatment. Meanwhile, it is noted that the nanowire joining, welding, or sintering by UV irradiation cannot be ignored. The photonic sintering in previous studies32,33 is mainly intended to increase the conductance of Ag NWs by lowering the junction resistance between adjacent Ag NWs, since it effectively provides localized heating at the NW junctions, rather than bulk heating. Thus, the photonic sintering by UV irradiation indeed improves the contact between nanowires, and, consequently, the better contact at nanowire junction is beneficial for further electrical tests. However, in this work, an insulating part in the middle of the Ag NW network was deliberately designed by UV-ozone treatment, in order to achieve the RS properties. Therefore, the photonic sintering in this work shows a relatively minor effect on RS behavior of the Ag/Ag@AgOx/Ag device. To study the RS characteristics, we tested the I−V curves under the voltage sweep, as shown in Figure 5a. The device with an initial high resistance state (HRS) changes to a low resistance state (LRS) when applying a positive voltage at a threshold voltage (Vth). This is a result of Ag filament formation in the Ag@AgOx structure. As mentioned above, insulating AgOx shell layer can act as the switching media for Ag filament migration (Figures 5c and 5d). Interestingly, when the voltage returns to zero, the device changes back to HRS, indicating the rupture/dissolution of unstable filaments. The same phenomenon can be observed by applying a negative voltage as well. Generally, RS can be divided to two modes: nonvolatile memory switching (MS) and volatile threshold
switching (TS).34,35 The difference is that (i) the MS has bistable resistance states at lower bias, while the TS only has a monostable resistance state; and (ii) in MS mode, the RS is originated from a continuous conductive pathway, whereas, in TS mode, the conductive filament consists of isolated metal nanoparticles.35 Here, the I−V characteristics clearly demonstrate that the RS behavior is independent of the voltage polarity, corresponding to a typical unipolar TS characteristic,34,36 rather than a memory RS, since there is only one stable resistance state at zero voltage.34 To avoid confusion, the HRS and LRS in this work are renamed as “TS OFF-state” and “TS ON-state”. It is found that the induced Joule heating37,38 could be the main reason for the switching from TS ON-state to TS OFF-state, since the filaments become unstable at higher temperature induced by Joule heat, resulting in consequent dissolution of filaments and the instability of the ON states. Regarding to the Ag@AgOx network, Joule heat is generated when a large current passes through the filament in the network, and then a fast rupture of filament occurs simultaneously (see Figure 5d). Therefore, the device is switched to the OFF-state. Unlike the MS mode with continuous conductive filament, the TS behavior is usually observed in multimetal-island systems.39,40 Similarly, the designed Ag@AgOx core−shell in the present work can be considered as a network with multimetal islands. As shown in Figure 5d, the AgOx shell formed by UV-ozone treatment is insulating but can facilitate Ag ions migration. The device is initially in TS OFF-state; when applying voltage, the migration and reduction of Ag ions lead to the formation of Ag filaments in the connecting AgOx shells between the Ag cores. Accordingly, the junctions with high initial resistance are transformed to the conductive bridges, because of the presence of the Ag filaments. Consequently, the conductive junctions could be regarded as isolated metal-like islands, giving rise to the TS behavior, as previously reported. During the different cycles the device shows obvious TS performance; however, the Vth and Vhold parameters seem have 20767
DOI: 10.1021/acsami.7b04839 ACS Appl. Mater. Interfaces 2017, 9, 20762−20770
Research Article
ACS Applied Materials & Interfaces
Figure 6. Current−voltage (I−V) characteristics with a current compliance (CC) of (a−d) 1 μA (from two test points), (e) 5 μA, and (f) 10 μA for several cycles.
which is significantly larger than that of the device with higher area density (∼2 V). Meanwhile, the hold voltage (Vhold) increases as well, because of the higher Joule heating by a very high voltage. As mentioned previously, the Ag filament can only migrate through the Ag@AgOx nanowire junctions, which are the isolated “islands”, and, consequently, the designed Ag@ AgOx core−shell structure can be considered as a network with multimetal islands. When the area density is decreased, the number of Ag@AgOx nanowire junctions obviously decreases, resulting in the decreased number of “metal islands”. Thus, the Vth for the transition from a high resistance state (HRS) to a low resistance state (LRS) significantly increases as the probability of the formation of conductive pathways decreases. Therefore, the TS behavior is probably related to the number of Ag@AgOx nanowire junctions, which can be modulated by tuning the area density. When the device operates for more than 10 times, the TS behavior is going to disappear (Figure S6f in the Supporting Information), which is due to a very high voltage. Thus, a higher area density of Ag NW is needed in order to reduce the Vth and Vhold values and further lower the power consumption. This work shows that, through different post treatments, the Ag NW network can achieve a functional shift from the conventional transparent electrode to the threshold RS device, especially by designing an insulating AgOx layer inside the Ag NW network.
a larger variation. These are probably due to the existence of a large number of junctions in the complex nanowire network, depending on device geometry, which are difficult to control well. Furthermore, the variation of threshold voltage was even observed in a relatively simple Ag/Ag2S/Ag nanowire transistor.41 Specifically, the different localized degree of oxidation and numerous Ag@AgOx junctions/interconnects may randomly influence the conductive path, resulting in the fluctuant RS behavior. On the other hand, the rupture of filament by different localized Joule heating cannot be completely avoided, thus resulting in fluctuation of the Vth or Vhold parameters. The on/off ratio is ∼100 (Figure 5b), which is similar to the result of the device based on a Ag nanoparticle− graphene composite.42 To study the repeatability, the RS properties have been tested for many cycles with different CC values (Figure 6). As shown in Figures 6e and 6f, increasing the CC value by an order of magnitude hardly changes the threshold RS behavior. After testing ∼70 cycles, shrinking I−V curves can be observed (see Figure S5 in the Supporting Information), indicating that TS behavior is disappearing after working many times. This can be ascribed to the HRS failure by the inevitable accumulation of Ag precipitates at the junctions under repeated switching cycles, as reported in the previous work with using active electrodes.43 Another factor probably influencing the Ag NW network is the Ag NW area density. In the previous study,29 the switching ratio of the Ag NW/polyaniline (PANI) pH self-adjusting switching system is enhanced by the addition of Ag NWs, because of the enhanced electrical property of the PANI. In this work, the middle part of the Ag NW network is intentionally oxidized to form an insulating Ag@AgOx core−shell structure. For comparison, the device with a lower Ag NW area density is fabricated by only one drop (20 μL). As seen in Figure S6 in the Supporting Information, the device with the lower area density of Ag NWs still shows TS behavior in I−V curves, and a similar switching ratio can also be realized under the same current compliance. However, it is found that the Vth is >20 V,
4. CONCLUSIONS In this work, silver nanowires (Ag NWs) with aspect ratio of ∼360 were synthesized by a polyol process and then spincoated on a glass substrate to fabricate the transparent electrodes. The maximum and comparable figure of merit (FOM, or ΦTC) was 13.55 × 10−3 Ω−1. The conductance of Ag NW film was further increased by mechanical pressing, because of the reduced junction resistance by a strong junction contact. In addition, by taking advantage of the Ag NW network, we reported a novel and simple method to fabricate a resistive switching (RS) device just through UV-ozone treatment. After 20768
DOI: 10.1021/acsami.7b04839 ACS Appl. Mater. Interfaces 2017, 9, 20762−20770
Research Article
ACS Applied Materials & Interfaces
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UV illumination, a part of Ag NWs became Ag@AgOx core− shell structure, which was confirmed by SEM, TEM, and XRD characterizations. As expected, the insulating AgOx shell provided a conductive pathway for Ag filament migration under the applied voltage. A threshold RS characteristic, as well as the on/off ratio of ∼100, was achieved in Ag/Ag@AgOx/Ag planar structure. This work showed that, through different posttreatments, metal NW networks can be designed not only for transparent electrodes but also for RS applications.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b04839. An SEM image of Ag NWs with diluted concentration; a table listing the comparison of the electrical and optical properties; a photograph of Ag NWs transparent electrode; the resistances of Ag NW coated on a PET substrate; the XRD pattern of Ag@AgOx nanowires after UV-ozone treatment for 60 min; I−V characteristics of the RS device tested after 70 cycles; and the I−V characteristics of the RS device with lower area density (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Dewei Chu: 0000-0003-4581-0560 Author Contributions ‡
These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is funded by the Australian Research Council Project (Grant No. FT140100032). The authors would like to thank Ms. Katie Levick and Mr. Yin Yao for assistance with TEM and SEM measurements. H. Du thanks the China Scholarship Council (CSC) for financial support (No. 201406410060).
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DOI: 10.1021/acsami.7b04839 ACS Appl. Mater. Interfaces 2017, 9, 20762−20770
