Article Cite This: ACS Appl. Electron. Mater. 2019, 1, 1275−1281
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Convertible Insulator−Conductor Transition in Silver Nanowire Networks: Engineering the Nanowire Junctions Yanzhe Zhu,† Junhong Chen,† Tao Wan,*,† Shuhua Peng,‡ Shihao Huang,† Yifeng Jiang,† Sean Li,† and Dewei Chu† †
School of Materials Science and Engineering and ‡School of Mechanical and Manufacturing Engineering, The University of New South Wales, Sydney, Australia 2052
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ABSTRACT: Achieving a switchable conductance in metallic nanowire networks is of great importance for applications in wearable electronics, selectors, information storage, and neuromorphic computing. However, to realize the electricfield-induced switchable conductance, an insulating characteristic in a conductive network should be enabled first to allow a desirable switch window, and in turn, metal/oxide core−shell nanowires and nanowire−polymer composites have been proposed. Here, a novel strategy to tune the electrical conductance of Ag nanowire networks through engineering the interface of nanowire junctions is developed, without introducing an additional insulating layer. The initial device with a percolating network structure shows high resistivity due to the loose wire-to-wire contact, but it can be reversibly switched to a lower resistance state under an electric field. The observed switchable behavior is attributed to the Ag migration between the wire junctions. The device is also turned into a good conductor by formation of an intimate contact between nanowires. With further increasing the heating temperature, the junction resistance dominated network is then determined by individual Ag nanowires as a result of nanowire fragmentation, and a different threshold switching behavior is demonstrated. Furthermore, the overall conductance of the network is closely related with the nanowire density, and the electrical properties of the devices reveal a similar trend to those of the junction-modified samples. KEYWORDS: Ag nanowire networks, nanowire junctions, Ag migration, switching, density
1. INTRODUCTION One-dimensional nanostructures, such as metallic nanowires (NWs) with a high aspect ratio, show unique properties that are unattainable with conventional bulk counterparts, making them attractive for applications in electronic devices.1,2 The applications of the NW network that have been recently pursued range from transparent conductors, in which stable high conductivity is preferred to provide reliable performance, to sensors and information processing, in which tunable resistance triggered by external stimuli is utilized to implement complex tasks.3−5 To realize a switchable resistance in a metallic NW network, the structure of the NWs and the network needs to be rationally designed. Many types of metal/oxide core−shell structures, such as Ag@TiO2,6 Ag@AgOx,7 Au@Ga2O3,8 Ge@ GeOx,9 and Ni@NiO10 have been developed by various methods, including solvothermal treatment, high temperature annealing, and oxidation reactions, where the insulating metal oxide shell around the metallic core acts as a switching layer for defect migration. The redistribution of these defects, including native mobile ions (e.g., oxygen ions or oxygen vacancies) and foreign metal ions (e.g., Ag, Cu, and Ni ions), are realized under the influence of the applied electric field, giving rise to a © 2019 American Chemical Society
high resistance state (HRS or off-state) and a low resistance state (LRS or on-state). These conductance states correspond to different memory states and define the memory window, which can be used for information storage and processing.11,12 In a simple crossbar device with two crossed NWs, the observed controllable resistance is related to the diffusion of the mobile ions at the NW contacted interface.13,14 A random NW network consists of numerous crossbars or junctions, and in turn, the overall conductance of the network can be tuned through the engineering of these wire-to-wire junctions.4,15,16 It is also reported that a connectivity path with the lowest formation energy is preferable for the current flow in a NW network, which determines the memory state of the device.4 In addition to the structural modification of the NWs, the Ag NW network can be embedded in an insulating polymer, and the formation and rupture of the Ag filaments connecting adjacent NWs upon a threshold voltage (Vth) and a hold voltage (Vhold) lead to the observed threshold switching (TS) behavior, which can be further used as a current limiter.5,17 Received: April 8, 2019 Accepted: June 13, 2019 Published: June 13, 2019 1275
DOI: 10.1021/acsaelm.9b00218 ACS Appl. Electron. Mater. 2019, 1, 1275−1281
Article
ACS Applied Electronic Materials In this work, a switchable conductance in a bare Ag NW percolating network was demonstrated, unlike conventional devices, which need post-treatments to introduce an additional insulating layer. The intrinsic property of the network originates from the loosely connected NWs, which are common in devices based on Ag NWs because of the organic residues during the polyol reduction process.18,19 Though the network with high initial resistance has been considered as a major drawback in conductor applications where high conductivity is desired, here it can be electrically activated to realize the tunable resistance. The device showed typical TS behavior, which depends on the history of the applied voltage. As the overall device conductivity is closely associated with the NW structure, including NW junctions and their morphology, our strategy is to use low temperature thermal treatments to tune the electrical properties. The electrical activation of the network became much easier with increasing annealing temperature because of the closer NW contact, whereas higher voltage is required to induce the resistance change of the broken network after 250 °C annealing, and thus, different TS is observed. On the other hand, the effect of NW density on the device conductance was also studied.
Figure 1. (a) TEM image of the Ag NWs and (b) TEM image of two contacted Ag NWs. The arrow indicates the organic residue.
The device with Ag NWs was then fabricated, and the Ag electrodes with a diameter of about 65 μm were printed onto the NWs, which can be seen in Figure 2(a). The adjacent electrodes were used to measure the electrical property of the nearby Ag NWs. The optical microscope image in Figure 2(b) reveals randomly distributed Ag NWs. Figure 2(c,d) shows the typical I−V curves of the Ag NW network. The observed behavior corresponds to TS, as the on-state can hardly be maintained after removal of the applied voltage,23 and a selectivity (on/off ratio) of 102 is achieved. Specifically, during the positive voltage sweep, the current gradually increased, and then, abrupt change of the current was observed at about 2.5 V, which is termed Vth. While the voltage was swept back to 0 V, the device was switched to the off-state, and the corresponding voltage is called Vhold and vice versa under negative voltage sweep. Similar phenomena have been reported in previous studies, and specific microstructures such as a Ag@AgOx NW network7 and a Ag NWs−PDMS composite5 have been deliberately fabricated. Here the switching behavior occurs between the nanoscale junctions of the bare Ag NWs. Because of the high wire-to-wire contact resistance,24−26 current and joule heating agglomerate at the junctions, enhancing the Ag migration as well as the formation of Ag filaments, leading to abrupt conductance increase. However, the filament bridging the adjacent nanowires is fragile and unstable, and it tends to break up to minimize the interfacial energy after removal of voltage, resulting in TS performance.5,23 It has been recently reported that the atomic surface diffusion caused by the gradient of surface atomic vacancy concentration led to the rupture of the nanoscale filaments in the systems with Ag or Cu active species.27 Additionally, strong filaments or partial fusing can be formed between the NWs by electrowelding, resulting in an irreversible change of the resistance.25,26 Figure 2(e,f) shows a repeatable bidirectional TS and related cumulative probability of the Vth and Vhold, indicating that the junction resistance can be reversibly modulated. A high voltage (e.g., 4 V) was usually needed to set the device on during the first cycle, and subsequent Vth under negative voltage became smaller (e.g., −2.5 V). This positive voltage sweep process can be referred to as electroforming, in which larger voltage facilitated the Ag filament generation to form a conductive path with the lowest energy connectivity. The activated pathway is subsequently degraded as a result of the ruptured filament, but it can be easily reconnected by smaller electrical stimuli. Furthermore, a general reduced trend of Vth from ∼3 to ∼1 V during positive sweep was observed under repeated 160 cycles because of the accumulation of Ag within the junctions. This is similar to the remembering behavior in a biological brain, where information can be easily picked up
2. EXPERIMENTAL SECTION 2.1. Fabrication of Ag NW Network Device. Ag NWs were synthesized according to a previously reported method.20 Before the deposition of Ag NWs, slide glass substrates were cleaned with deionized water and absolute ethanol and further irradiated by UV light (Senlights, 110 W, 325 nm) for 20 min. Then, the prepared Ag NW dispersion with a concentration of 0.2 mg mL−1 was spin-coated onto the substrate at 1500 rpm for 20 s, and this procedure was repeated two times. Consequently, the Ag electrodes were printed onto the sample by an inkjet printer (DMP-2800, Fujifilm Dimatix) using conductive silver printing ink (Sigma-Aldrich, 30−35 wt % in triethylene glycol monomethyl ether) and dried at 80 °C in a preheated oven (Forced Convection Oven DKM300, Yamato Scientific) for 1 h. To investigate the effect of heating temperature on electrical performance, the NW sample was cut into four pieces, which were annealed at 100, 150, 200, and 250 °C for 1 h in the preheated oven, respectively, before the inkjet process. These devices were denoted as NW-80, NW-100, NW-150, NW-200, and NW-250 according to the highest heating temperature. In addition, devices with a higher density of Ag NWs were prepared by repeating spincoating three and four times, individually, and were denoted as NW80-3 and NW-80-4, respectively. 2.2. Materials and Device Characterization. The morphologies of the devices were checked with optical microscopy (Nikon Eclipse ME600 Microscope), transmission electron microscopy (TEM, Phillips CM 200), and scanning electron microscopy (SEM, FEI Nova NanoSEM 450). The electrical properties of the devices were measured with a Keithley 4200 Semiconductor Characterization System connected with a probe station. During measurements, probes are contacted to two adjacent Ag top electrodes.
3. RESULT AND DISCUSSION Figure 1 shows the TEM images of the Ag NWs. The addition of the PVP during polyol reduction enables the good dispersity of the NWs, where most of the PVP can be removed after the subsequent washing process with ethanol.21 However, complete removal of the residual PVP is difficult due to its chemical bonding with NWs.22 As shown in Figure 1(b), organic residue is clearly seen between the washed nanowires, which in turn affects the NW connection as well as the electrical property. 1276
DOI: 10.1021/acsaelm.9b00218 ACS Appl. Electron. Mater. 2019, 1, 1275−1281
Article
ACS Applied Electronic Materials
Figure 2. (a) SEM image of the Ag electrodes. (b) Optical microscope image of the NW-80. Typical I−V curve of NW-80 in (c) linear scale and (d) log scale. The arrows and numbers indicate the voltage sweep direction. (e) 161 DC sweep cycles and (f) the corresponding cumulative probability of Vth and Vhold without the first cycle.
Figure 3. Log−log scale I−V curves under (a) positive and (b) negative voltage sweep, respectively.
further control the contact geometry between the NWs as well as the junction resistance, as shown in Figure 4. The curve loop under both positive and negative voltage sweep gradually decreased with increasing temperature to 200 °C, changing from hysteresis loops to straight lines. This corresponds to the transition from insulating contact to intimate contact between the NWs. Compared with NW-80, NW-100 can easily reach the compliance current (Icc) at a lower applied voltage (
