Threshold Switching Induced by a Controllable Fragmentation in

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Threshold Switching Induced by a Controllable Fragmentation in Silver Nanowire Networks Tao Wan, Ying Pan, Haiwei Du, Bo Qu, Jia Bao Yi, and Dewei Chu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16142 • Publication Date (Web): 28 Dec 2017 Downloaded from http://pubs.acs.org on December 28, 2017

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ACS Applied Materials & Interfaces

Threshold Switching Induced by a Controllable Fragmentation in Silver Nanowire Networks Tao Wan, Ying Pan, Haiwei Du*, Bo Qu, Jiabao Yi* and Dewei Chu∗ School of Materials Science and Engineering, University of New South Wales, Sydney, NSW 2052, Australia

ABSTRACT Silver nanowire (Ag NW) networks have been widely studied because of a great potential in various electronic devices. However, nanowires usually undergo a fragmentation process at elevated temperatures due to the Rayleigh instability which is a result of reduction of surface/interface energy. In this case, the nanowires become completely insulating due to the formation of random distributed Ag particles with a large distance and further applications are hindered. Herein, we demonstrate a novel concept based on the combination of ultraviolet (UV)/ozone irradiation and a low temperature annealing process to effectively utilize and control the fragmentation behaviour to realize the resistive switching (RS) performances. In contrast to the conventional fragmentation, the designed Ag/AgOx interface facilitate a unique morphology of short nanorod-like segments or chains of tiny Ag nanoparticles with a very small spacing distance, providing conduction paths for achieving tunnelling process between the isolated fragments under electric field. Based on this specific

∗

Author to whom correspondence should be addressed, email: [email protected] (H. Du),

[email protected] (J. Yi), and [email protected] (D. Chu) 1

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morphology, the Ag NW network has a tunable resistance and shows volatile threshold switching characteristics with a high selectivity, which is the on/off current ratio in selector devices. Our concept exploits a new function of Ag NW network, resistive switching, can be developed by designing a controllable fragmentation. KEYWORDS: silver nanowire networks; Rayleigh instability; fragmentation; ultraviolet (UV)/ozone; threshold switching.

1. INTRODUCTION Metal nanowire networks have a great potential for next-generation electronic/optical devices.1-7 As one of the most promising representatives, silver nanowires (Ag NWs) with diverse advantages such as large-scale production, superior mechanical strength, low cost, high transparency, flexibility and high conductivity, have been widely studied over the past decade.8-11 Much effort has been devoted to modulating the conductivity of Ag NW network. On the one hand, the conductivity can be significantly enhanced through various physical and chemical approaches including laser irradiation,12 mechanical pressing,13 plasma14 as well as chemical treatment.15 These methods are focused on reducing the contact resistance by facilitating the localized nanojoining or nanowelding at the NWs junctions. On the other hand, metal nanowires become insulating by a fragmentation process when exposing to high temperatures. This thermal-induced breaking into segments or even spheroidization is caused by Rayleigh instability16 and is normally undesired due to limited applications.

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Inspired by these two opposite designs, between the complete and insufficient fragmentation there should be a critical state with an adjustable conductivity, in which the metal nanowires are already insulating but can be easily switched back to conductive state via applying electric field, especially when the distances between silver nanospheres are very small so that the Ag+ hopping or electron tunnelling effect can be achieved. Based on this insulator-metal transition, a resistive switching (RS) device can be fabricated. However, it is difficult to control this critical state just by the conventional annealing as annealing at a lower temperature is insufficient to achieve fragmentation due to the kinetic limitations while at a higher temperature the Ag NWs transform to aligned nanospheres with a relatively large spacing distance or even are randomly distributed.17-18 In order to better control the fragmentation, advanced technologies or methods have been developed. For example, ion beam bombardment19 can precisely control the particle size and spacing distance, while the required high vacuum and small scale of production limit its practical application. Another idea is core-shell design by encapsulating metal or metal oxide nanowires into the inorganic oxide shells,20-21 afterwards the confined nanochains of metal particles can be fabricated. Given that the driving force of Rayleigh instability is the reduction of total surface/interface energy,22 the fragmentation of Ag NWs can be manipulated through changing the surface energy via a desirable surface treatment on Ag NWs. Here, we report a novel and facile method to (1) modulate the fragmentation of Ag NWs and (2) to control the spacing among Ag fragments. The Ag NW networks were first treated 3



by UV/ozone irradiation to form an oxide layer of AgOx on the surface and subsequently were transformed into well-ordered long chains of silver nanorods after annealing at a lower temperature. It is noted that the presence of derivative oxide layer is very critical to this transformation.17, 22-23 On the one hand, the surface energy of NW networks is determined by the formation of Ag/AgOx interface, and consequently the fragmentation will preferentially occur at the Ag/AgOx interfacial sites. On the other hand, tiny silver islands left from the partial decomposition of AgOx24-25 is also favourable for shortening the spacing distance between silver fragments. The chains of Ag nanorods or nanoparticles transformed from Ag NWs are initially insulating but exhibit a threshold switching (TS) behaviour when applying voltage. Our previous study has carried out the conductance modulation in Ag NWs/Ag@AgOx/Ag NWs planar device with a TS characteristic.26 However, the device exhibits a low selectivity (~100) as the resistance of high resistance state (HRS, OFF state) is low in Ag@AgOx network structure. Additionally, HRS failure is observed after repeated cycles due to the accumulation of Ag at the Ag@AgOx junctions. Therefore, the switching stability needs to be further improved. In order to solve these problems, long chains of Ag fragments along the initial Ag NWs which provide possible conduction paths, are elaborately designed in this work by the combination of UV/ozone and thermal treatment. Since the Ag NWs network is changed to isolated Ag fragments, the device is insulating with a high resistance and the HRS can be well maintained under subsequent switching cycles. Accordingly, a high and stable selectivity (5 × 105) is achieved after the device is switched to low 4


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resistance state (LRS, ON state) by applying voltage bias. Furthermore, the obtained RS characteristics can be tuned in a controllable manner by varying the UV/ozone treatment time. The RS properties and switching mechanism were systematically investigated as well.

2. EXPERIMENTAL SECTION 2.1 Synthesis of Ag NWs Ag NWs were synthesized according to our previous report with a slight modification.26 In a typical synthesis, 0.45 g polyvinglpyrrolidone (PVP) K30 (Mw: 40,000) was dissolved into a beaker containing 30 mL 1,2-propanediol and heated at 120 °C with magnetic stirring for 20 min. Then 3 mL of 1,2-propanediol containing 1 mM NaCl (Mw: 58.4) was added. After stirring for 3 min, 12 mL of 1,2-propanediol containing 0.1 M AgNO3 was added dropwise and the solution was kept heating at 150 °C with magnetic stirring for 65 min. At first the solution was transparent and then the colour of solution transformed from yellow to grey during the last 15 min, indicating the rapid growth of Ag NWs (Figure S1). Afterwards, the beaker was placed in a cabinet at room temperature for 70 days until the Ag NWs precipitation on the bottom was observed. Finally, after removing the organic solvent by pipette, the Ag NWs were washed with deionized (DI) water by centrifugation separation for 5 times, and redispersed in ethanol for further use. The TEM image of as-synthesized Ag NWs was shown in Figure S2.

2.2 Fabrication of Device based on Ag NW Network 5



The glass substrates were cleaned with deionized water and ethanol, and further exposed to UV surface treatment by UV/ozone surface processor with a wavelength of 254 nm (P16-110, Sen Lights Corporation, Japan) for 20 min. As shown in Scheme 1, the Ag NW dispersion was spin-coated onto the pre-cleaned glass substrates at 1500 rpm for 20 s. The spin coating was repeated for 3 times. The obtained samples with an areal mass density of ~ 6 µg cm-2 were further treated by UV/ozone irradiation for 12, 20, 24 and 28 min, respectively. Consequently, the top Ag electrodes were printed on the substrates by a Fujifilm Dimatix DMP-2800 inkjet printer using conductive silver printing ink (purchased from Sigma-Aldrich). The SEM image of Ag top electrodes is shown in Figure S3. After printing, the samples were annealed at 150 °C for 2 h in oven. The devices were named as NW-12, NW-20, NW-24, and NW-28 according to their irradiation time. In addition, the samples treated by UV/ozone were kept transparent before and after annealing (Scheme 1).

Scheme 1. Schematic of the fabrication processes of Ag NW network based resistive switching devices.

2.3 Materials and Device Characterization 6


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Structural analysis of the as-synthesized Ag NWs was carried out using an X-ray diffractometer with Cu Kα radiation (λ = 0.1541 nm). X-ray photoelectron spectroscopy (XPS) measurement was carried out using an ESCALAB250Xi spectrometer. The morphology was observed with transmission electron microscopy, TEM (FEI Tecnai G2 and Phillips CM 200) and scanning electron microscopy, SEM (FEI Nova NanoSEM 450). The RS behaviour of the devices based on Ag NW network was tested by a Keithley 4200 Semiconductor Characterization System connecting with a probe station. The probes were contacted to two adjacent Ag top electrodes. 3. RESULTS AND DISCUSSION Figure 1 shows the morphological evolution of Ag NW network after UV/ozone irradiation and subsequent annealing treatment. As shown in Figure 1a1 and a2, UV/ozone irradiation for 12 min has a slight influence on the Ag NW network and only a few nanowires show fragmentation to shorter segments. With increased UV irradiation time, the nanowire surface starts to become rough (Figure 1b2 and c2), and the surface roughness is more obvious after UV/ozone treatment for 28 min as inhomogeneous formation of AgOx on the nanowire surface can be clearly observed (Figure 1d2). The formation of AgOx on Ag nanoparticles by UV illumination is also reported.27 It is noted that the surface processing used in this work not only provides UV irradiation but also generates the ozone atmosphere.26 First, the heat can be induced by UV/ozone irradiation with a short wavelength (254 nm). Second, the ozone atmosphere generated by UV/ozone irradiation facilitates the surface oxidation 7



of Ag NWs, resulting in the formation of AgOx shell layer and the more distinct surface roughness. With the combination of generated heat and increased surface roughness, thin nanowires are firstly fragmented into nanorods even without further heat treatment because of the facilitated Rayleigh instability. Afterwards, the UV irradiated Ag NW networks were annealed at 150 °C for 2 h in air atmosphere. For NW-12, the fragmentation is only found in a few Ag NWs (Figure 1a3 and a4) and most Ag NWs keep almost unchanged. However, more and more NWs of NW-20 and NW-24 break to segments after annealing (Figure 1b4 and c4). Especially, the NWs of NW-28 transform to chains consisting of many tiny Ag nanoparticles with a very close spacing distance. It is widely accepted that one-dimensional materials such as nanowires are prone to undergo morphological evolution at elevated temperatures, leading to the fragmentation due to the Rayleigh instability. Normally, annealing at 150 °C can only improve the junction resistance by removing PVP from the junctions28 without any morphology changes. To make a comparison, the as-obtained Ag NWs were annealed directly without UV/ozone treatment. As shown in Figure S4, without UV/ozone treatment the morphology of Ag nanowire in terms of diameter and length does not change after the thermal treatment. This indicates that the dominant reason for subsequent fragmentation here is the presence of AgOx shell. After forming inhomogeneous AgOx shell during the UV/ozone treatment, the radius of nanowire shows large fluctuations and the Rayleigh instability is also promoted. Consequently, the fragmentation process can take place at a much lower temperature (150 °C) than its melting point. To further investigate the 8


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morphology evolution of Ag NWs under UV/ozone and thermal treatment, TEM images have been taken, as shown in Figure S5. Under a short UV/ozone exposure time (for example 12 min), only a few AgOx is formed, and the AgOx is easily reduced to Ag with elevated temperature.24-25 Therefore, most of Ag NWs remain unchanged and only a few NWs with smaller diameters are fragmented into short segments, which is similar to previous work.29 Therefore, the device is still conductive. With the increasing UV/ozone exposure time, the surface of Ag nanowires becomes coarse and a thicker insulating layer is further obtained. According to Figure 1c1 and c2, we can clearly see that the generated AgOx layer on the NWs surface is not homogeneous and the layer thickness varies along the NWs. Figure 1e illustrates the mechanism of morphological change. During the UV/ozone irradiation, a surface oxidation takes place gradually with irradiation time due to the UV induced heat and ozone atmosphere generated simultaneously. Compared with the surface of pristine Ag NWs, the surface with AgOx becomes coarse. Additionally, since the AgOx can be decomposed into Ag at high temperature, these Ag/AgOx interfacial sites are more active and can be easily broken during the following heat treatment, resulting in the fragmentation of the nanowires. In addition, after a long time UV/ozone treatment, the surface of the nanowires becomes coarser as more Ag is oxidized to AgOx (as shown in Figure 1d1 and d2). With more AgOx, the NWs can be fragmented into more segments with a shorter length by heat treatment. As shown in Figure 1d3 and d4, the NWs are completely broken into Ag nanoparticles assembled along the nanowire direction, which is very different from 9



the morphologies with a chain of isolated nanoparticles by annealing at high temperature.17, 30 This unique microstructure is resulted from two aspects. On the one hand, more AgOx on the surface induced by prolonging exposure time facilitates the fragmentation. On the other hand, a partial decomposition of AgOx during the heat treatment probably leaves many tiny Ag nanoparticles along the nanowire direction. Thus, the presence of AgOx shell should be responsible for the morphological evolution. Interestingly, this obtained Ag nanoparticles are similar to the microstructure of Ag filaments formed in the memory devices.31 In addition, XPS measurements have been carried out to give a qualitative analysis of the oxidation of Ag NWs by UV/ozone irradiation and reduction of AgOx by a thermal treatment, as shown in Figure S6.

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Figure 1. SEM images of Ag NW networks. After UV/ozone irradiation and before annealing: NW-12 (a1, a2), NW-20 (b1, b2), NW-24 (c1, c2) and NW-28 (d1, d2); after annealing: NW-12 (a3, a4), NW-20 (b3, b4), NW-24 (c3, c4) and NW-28 (d3, d4). Scale bar: 3 µm (longer) and 500 nm (shorter). Figure 1e schematic of fragmentation of Ag NWs.

Since morphologies of most NWs can be maintained well after UV/ozone irradiation for a short time, the NW-12 is still conductive, as shown in Figure 2a. However, the I-V characteristics with applied voltages of NW-20, NW-24 and NW-28 11



show significant changes. During the first voltage sweep (Figure 2b), the current is initially below 0.1 nA, indicating that the devices are insulating. Then an abrupt increase in current is observed and the devices turn to metallic state at a critical voltage, which is called “training” voltage (VT).32 It should be noted that this is not the conventional electroforming step in memory devices since there is no stable filament formed. Furthermore, to investigate the training process, the devices were measured at 100 test points under a positive sweep. Figure S7a-c show the corresponding I-V curves. Figure 2c and Figure S7d show the cumulative probability and the mean value of VT, respectively. The VT of NW-20 is the lowest while NW-28 shows a much higher VT. It is known that the training process is the first switching transition from insulating state to conductive state and is also associated with the initial conductance barrier. For NW-20, it is easily switched since there are still a lot of Ag segments which are conductive. However, NW-28 has been transformed to chains of tiny Ag particles, in which the gaps between particles act as transport barriers. Therefore, a higher voltage is needed to switch on the device. Further prolonging the UV/ozone irradiation time to 1 h, the device is completely insulating and cannot be switched even under an applied voltage up to 50 V (Figure S8).

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Figure 2. I-V characteristic of (a) NW-12. Typical first positive sweeps of (b) NW-20, NW-24 and NW-28, which are the “training process”. The current compliance (Icc) is 10 µA. The cumulative probability of (c) VT from different test points. I-V characteristics under 100 positive and negative voltage sweeps: (d) NW-20, (e) NW-24 and (f) NW-28. The cumulative probability of Vth and Vhold: (g) NW-20, (h) NW-24 and (i) NW-28. ∆V = Vth – Vhold. Subsequently, each device is measured by applying positive and negative voltage sweeps for 100 cycles at the same testing point. As shown in Figure 2d-f, all the devices show bidirectional volatile switching with the selectivity up to 5 × 105. Since the devices have been “trained”, the voltage now at which transition from OFF state to ON state takes place is threshold voltage (Vth). When the bias is swept back to 0 V, however, the device is switched to the initial HRS at the holding voltage (Vhold), 13



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indicating that the LRS is unstable and cannot be maintained. This switching process is the typical characteristic of the volatile unipolar threshold switching (TS), which has a great potential in selector applications. For the typical RS devices using active Ag

electrode,

the

electrochemical

reaction

of

Ag

ions

and

subsequent

electro-migration are responsible for the switching behaviour.33 However, due to the long distance (~ 70 µm, as shown in Figure S3) between the planar Ag electrodes, it is difficult for Ag ions to migrate from one electrode to another one in this study. To verify the role of Ag electrodes, the device without Ag top electrodes is tested and the TS behaviour can be achieved as well, as shown in Figure S9. This indicates that the symmetrical bidirectional volatile switching is related to the designed specific morphology, and the role of Ag top electrodes used here is more likely to improve the device stability rather than to induce the TS. Nevertheless, there are lots of possible conduction paths, which may lead to large variation and asymmetrical characteristics of I-V curves. Therefore, the devices were tested for 100 cycles. Despite the small variation of Vhold, an obvious variation of Vth is found, which can be ascribed to the network system and the large distance between the electrodes. Meanwhile, it should be noted that the specific microstructure of nanowires between electrodes (e.g., density and fragmentation degree of nanowires) cannot be the identical even under the same experimental conditions, thus large variations of the Vth are found during the sweeps. Furthermore, the reduced Vth is observed after long-time repeating switching cycles due to the inevitable accumulation of Ag between adjacent Ag segments. Since the network is still broken, 14


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the resistance of HRS exhibits no obvious change and HRS failure is avoided. Therefore, all the devices show stable selectivity. As a shorter UV/ozone treatment time can leads to a small amount of segments and a few silver nanoparticles (NW-20), the device can be easily switched, since the segments are actually the conductive short Ag nanorods. Differently, the devices with more Ag fragments by prolonging the UV/ozone irradiation time need a higher Vth to switch on. This is mainly attributed to the increased number of barrier gaps. Specifically, in the completely broken network, a large number of Ag nanoparticles are generated and the gaps between Ag nanoparticles act as conducting barriers. Therefore, a higher Vth is required to enable the switching process. Moreover, it was reported that the width of I-V curve (∆V = Vth – Vhold) usually influenced the appropriate voltage range of a selector or even the overall voltage for operating the 1 selector + 1 memory (1S1R) device.34 As shown in Figure 2g-i, the value of ∆V increases gradually from NW-20 to NW-28, showing the dependence on morphologies of the devices. Thus, the voltage range can be simply controlled by varying the UV/ozone irradiation time. In RS devices, Icc is usually needed to set to avoid the device breakdown. Furthermore, the applied Icc usually plays another important role in controlling the switching behaviour, especially in the devices exhibiting TS behaviour. In most filamentary switching based devices,31,

35-36

completed filaments connecting two

electrodes cannot be formed under a relatively low Icc (e.g. ≤ 10 µA), leading to the unstable switching or even volatile TS performance. When the Icc is increased to 15



sufficiently facilitate the formation of stable conductive filaments, the transition from TS to memory switching takes place as it has been shown elsewhere.31 Figure 3a-d shows the NW-20 device operated under different Icc values. The RS behaviour is still in TS mode after increasing the Icc or even without set Icc during the tests, indicating that the RS of the devices based on Ag NW networks is independent on the Icc value, which differs from the previous works.31, 36-37 Therefore, the switching mechanism may not be associated with the filamentary conduction.

Figure 3. I-V characteristics of Ag-NW-20 with and without Icc: (a) 10 µA, (b) 50 µA, (c) 100 µA and (d) without Icc.

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In order to understand the switching mechanism, a Fowler-Nordheim plot35 from a typical I-V curve (Figure 4a) is shown in Figure 4b, in which the field emission and direct tunnelling are observed at high and low bias respectively. As shown in Figure 4c, a linear relationship with a negative slope between ln(I/V2) and 1/V is observed, indicative of the field emission. In addition, ln(I/V2) is linearly dependent on the ln(1/V), which is corresponding to the direct tunnelling.38 It is reported that electron tunnelling probability is low at low bias due to the existence of the barrier. In this case, the resistance keeps almost unchanged even with further increasing voltage. However, a sharp increase in current is obtained when a certain voltage (Vth) is reached since the direct tunnelling is changed to field emission. Accordingly, the rectangular tunnelling barrier is deformed to triangular one, giving rise to the reduced barrier width and increased tunnelling probability.38 Similar results have been reported in other different systems.39-40 For example, in Ag/SiO2/Pt planar and vertical devices, the observed TS behaviour is also ascribed to the tunnelling transport mechanism based on isolated Ag nanoparticles, which are formed during the electroforming process.31 The gaps between the adjacent nanoparticles act as potential barrier, and tunnelling process takes place when a sufficient voltage is applied.41

17



Figure 4. A typical I-V curve of the NW-20 device (a). Fowler-Nordheim plot of the I-V curve (b). Enlarged Fowler-Nordheim plot at high bias (c). The ln (1/V2) vs ln (1/V) curve at low bias (d).

Here, the Ag nanorods or particles formed from nanowires can act as isolated metal islands or clusters and play a critical role in the TS behaviour. It is reported that the turn-on voltage in boron nitride nanotubes functionalized with gold quantum dots depends on the number of gaps between the metal quantum dots.41 After UV/ozone treatment with different time, different degree of fragmentation of Ag NWs is achieved as well as Vth, as shown in Figure 1 and 2. This indicates that the value of Vth is closely related to the numbers of metal islands or the gaps between metal islands 18


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which can be simply controlled since the morphology evolution is strongly dependent on the UV/ozone treatment. Figure 5 depicts conductive paths in different devices to explain the mechanism of TS switching under voltage bias. When the voltage is applied to the electrodes, a potential drop is created across the Ag fragments. However, the conductivity of the device would not be decreased significantly if nanowires are insufficiently broken (NW-12) because the remained nanowires are still connected with each other, leaving the nearly intact conductive network. In this case, no switching behaviour is observed. For the nanowires with sufficient fragmentation (NW-20, NW-24 and NW-28), conductive network is broken and electric field is needed to be applied in order to enable the tunnelling process, and hence to realize the insulator-metal transition. Compared with NW-20 and NW-24, NW-28 exhibits a much higher Vth due to the formation of chains of Ag fragments, leading to more barrier gaps after longer treatment. In this case, it is difficult to switch the devices on. Hence, a high voltage is needed to overcome the barrier.

Figure 5. Schematic of proposed mechanism of TS switching in device based on Ag NW networks.

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In order to study the device stability, NW-20 device is selected to be tested for more than 200 cycles. Figure 6a shows the 101st-200th I-V curves of NW-20, which was tested under the positive bias sweep only. Except a slight fluctuation in the HRS during very few tests, the device overall shows a very good stability with a stable selectivity of 5 × 105. Figure 6b shows the corresponding cumulative probability of the Vth and Vhold. The cumulative probability of Vhold keeps almost unchanged and is similar to the previous results. Interestingly, the Vth shows a narrow distribution and a much smaller variation, indicating a more uniform switching compared with the results of first 100 cycles (Figure 2g). The lowered Vth implies that the device can be easily switched on due to the accumulation of Ag after first 100 cycles. In addition, since the applied Vth is small (≤ 1 V), the accumulation of Ag between the adjacent Ag segments slows down under subsequent 100 cycles, leading to the improved switching behaviour. Moreover, the low switching voltage (Vth ≤ 1 V) and the low switching current (10 µA) is not only critical for effectively avoiding the permanent damage but also for a low energy and low power consumption system operated at sub 1 V.42

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Figure 6. The 101st-200th I-V curves (a) of NW-20 under the positive bias sweep only (0 → 2V). The cumulative probability (b) of the Vth and Vhold. 4. CONCLUSIONS In summary, we reported a novel method to control the fragmentation of Ag NW network by the combination of UV/ozone irradiation and low temperature annealing. Because of the heat and ozone atmosphere generated by UV/ozone irradiation, an inhomogeneous surface oxidation on Ag NWs took place and the thermal-induced fragmentation could preferentially occur at the Ag/AgOx interfacial sites. It was found that prolonging the UV/ozone treatment time led to more AgOx and Ag NWs underwent a unique morphological evolution from nanowires to short segments or even chains of tiny Ag nanoparticles. Because of this unique morphology, the devices show volatile threshold switching characteristics with a high and stable selectivity of 5 × 105 when applying voltage. Meanwhile, the dominant switching mechanism is attributed to tunnelling process between the Ag fragments. Moreover, the threshold voltage (Vth) can be modulated simply by morphology control. This work demonstrated that the resistive switching application of Ag NW network can be explored by a controllable fragmentation. ACKNOWLEDGEMENTS This work is funded by the Australian Research Council Project (Grant No. FT140100032). The authors would like to thank Mr. Yin Yao for carbon coating.

ASSOCIATED CONTENT 21



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Supporting Information A photograph of colour evolution during the polyol process, a TEM image of as-synthesized Ag NWs, a SEM image of Ag top electrodes, SEM images of pristine Ag NWs before and after annealing at 150 °C for 2h, TEM images of Ag NW after UV/ozone irradiation and thermal treatment, XPS Ag 3d5/2 spectra of pristine Ag NW, Ag NW after UV/ozone for 28 min, and Ag NW after UV/ozone and 150 °C, I-V characteristics of NW-20, NW-24 and NW-28 from different tests points during the first positive sweep and mean value of VT, I-V characteristics of device after UV irradiation for 1 h, I-V curve of the device without planar Ag top electrodes.

REFERENCES (1) Ye, S.; Rathmell, A. R.; Chen, Z.; Stewart, I. E.; Wiley, B. J. Metal Nanowire Networks: The Next Generation of Transparent Conductors. Adv. Mater. 2014, 26 (39), 6670-6687. (2) Liu, S.; Han, C.; Tang, Z.-R.; Xu, Y.-J. Heterostructured Semiconductor Nanowire Arrays for Artificial Photosynthesis. Mater. Horiz. 2016, 3 (4), 270-282. (3) Weng, B.; Liu, S.; Tang, Z.-R.; Xu, Y.-J. One-Dimensional Nanostructure Based Materials for Versatile Photocatalytic Applications. RSC Adv. 2014, 4 (25), 12685-12700. (4)

Liu,

S.;

Tang,

Z.-R.;

Sun,

Y.;

Colmenares,

J.

C.;

Xu,

Y.-J.

One-Dimension-Based Spatially Ordered Architectures for Solar Energy Conversion. Chem. Soc. Rev. 2015, 44 (15), 5053-5075. (5) Sun, Y.; Wang, Z.; Shi, X.; Wang, Y.; Zhao, X.; Chen, S.; Shi, J.; Zhou, J.; Liu, D. Coherent Plasmonic Random Laser Pumped by Nanosecond Pulses Far from the Resonance Peak of Silver Nanowires. J. Opt. Soc. Am. B 2013, 30 (9), 2523-2528. (6) Netzer, N. L.; Qiu, C.; Zhang, Y.; Lin, C.; Zhang, L.; Fong, H.; Jiang, C. Gold– Silver Bimetallic Porous Nanowires for Surface-Enhanced Raman Scattering. Chem. Commun. 2011, 47 (34), 9606-9608.

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(7) Wang, Z.; Yu, R.; Wen, X.; Liu, Y.; Pan, C.; Wu, W.; Wang, Z. L. Optimizing Performance of Silicon-Based p–n Junction Photodetectors by the Piezo-Phototronic Effect. ACS Nano 2014, 8 (12), 12866-12873. (8) Van De Groep, J.; Spinelli, P.; Polman, A. Transparent Conducting Silver Nanowire Networks. Nano Lett. 2012, 12 (6), 3138-3144. (9) Huang, G.-W.; Feng, Q.-P.; Xiao, H.-M.; Li, N.; Fu, S.-Y. Rapid Laser Printing of Paper-Based Multilayer Circuits. ACS Nano 2016, 10 (9), 8895-8903. (10) Li, R.-Z.; Hu, A.; Zhang, T.; Oakes, K. D. Direct Writing on Paper of Foldable Capacitive Touch Pads with Silver Nanowire Inks. ACS Appl. Mater. Interfaces 2014, 6 (23), 21721-21729. (11) Liu, S.; Weng, B.; Tang, Z.-R.; Xu, Y.-J. Constructing One-Dimensional Silver Nanowire-Doped Reduced Graphene Oxide Integrated with CdS Nanowire Network Hybrid Structures Toward Artificial Photosynthesis. Nanoscale 2015, 7 (3), 861-866. (12) Nian, Q.; Saei, M.; Xu, Y.; Sabyasachi, G.; Deng, B.; Chen, Y. P.; Cheng, G. J. Crystalline Nanojoining Silver Nanowire Percolated Networks on Flexible Substrate. ACS Nano 2015, 9 (10), 10018-10031. (13) Tokuno, T.; Nogi, M.; Karakawa, M.; Jiu, J.; Nge, T. T.; Aso, Y.; Suganuma, K. Fabrication of Silver Nanowire Transparent Electrodes at Room Temperature. Nano Res. 2011, 4 (12), 1215-1222. (14) Garnett, E. C.; Cai, W.; Cha, J. J.; Mahmood, F.; Connor, S. T.; Christoforo, M. G.; Cui, Y.; McGehee, M. D.; Brongersma, M. L. Self-Limited Plasmonic Welding of Silver Nanowire Junctions. Nat. Mater. 2012, 11 (3), 241-249. (15) Lu, H.; Zhang, D.; Cheng, J.; Liu, J.; Mao, J.; Choy, W. C. Locally Welded Silver Nano-Network Transparent Electrodes with High Operational Stability by a Simple Alcohol-Based Chemical Approach. Adv. Funct. Mater. 2015, 25 (27), 4211-4218. (16) Rayleigh, L. On the Instability of Jets. Proc. London Math. Soc. 1878, 1 (1), 4-13. 23



Page 24 of 27

(17) Karim, S.; Toimil-Molares, M.; Balogh, A.; Ensinger, W.; Cornelius, T.; Khan, E.; Neumann, R. Morphological Evolution of Au Nanowires Controlled by Rayleigh Instability. Nanotechnology 2006, 17 (24), 5954. (18) Langley, D.; Lagrange, M.; Giusti, G.; Jimenez, C.; Bréchet, Y.; Nguyen, N. D.; Bellet, D. Metallic Nanowire Networks: Effects of Thermal Annealing on Electrical Resistance. Nanoscale 2014, 6 (22), 13535-13543. (19) Lian, J.; Wang, L.; Sun, X.; Yu, Q.; Ewing, R. C. Patterning Metallic Nanostructures by Ion-Beam-Induced Dewetting and Rayleigh Instability. Nano Lett. 2006, 6 (5), 1047-1052. (20)

Qin,

Y.;

Lee,

S.-M.;

Pan,

A.;

Gösele,

U.;

Knez,

M.

Rayleigh-Instability-Induced Metal Nanoparticle Chains Encapsulated in Nanotubes Produced by Atomic Layer Deposition. Nano Lett. 2008, 8 (1), 114-118. (21) Liu, L.; Lee, W.; Scholz, R.; Pippel, E.; Gösele, U. Tailor-Made Inorganic Nanopeapods: Structural Design of Linear Noble Metal Nanoparticle Chains. Angew. Chem. Int. Ed. 2008, 47 (37), 7004-7008. (22) Peng, H.; Wang, N.; Shi, W.; Zhang, Y.; Lee, C.; Lee, S. Bulk-Quantity Si Nanosphere Chains Prepared from Semi-Infinite Length Si Nanowires. J. Appl. Phys. 2001, 89 (1), 727-731. (23) Toimil Molares, M.; Balogh, A.; Cornelius, T.; Neumann, R.; Trautmann, C. Fragmentation of Nanowires Driven by Rayleigh Instability. Appl. Phys. Lett. 2004, 85 (22), 5337-5339. (24) Tominaga, J. The application of Silver Oxide Thin Films to Plasmon Photonic Devices. J. Phys. Condens. Matter 2003, 15 (25), R1101–R1122. (25) Kolobov, A.; Rogalev, A.; Wilhelm, F.; Jaouen, N.; Shima, T.; Tominaga, J. Thermal Decomposition of a Thin AgOx Layer Generating Optical Near-Field. Appl. Phys. Lett. 2004, 84 (10), 1641-1643. (26) Du, H.; Wan, T.; Qu, B.; Cao, F.; Lin, Q.; Chen, N.; Lin, X.; Chu, D. Engineering Silver Nanowire Networks: from Transparent Electrodes to Resistive Switching Devices. ACS Appl. Mater. Interfaces 2017, 9, 20762-20770. 24


Page 25 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(27) Grillet, N.; Manchon, D.; Cottancin, E.; Bertorelle, F.; Bonnet, C.; Broyer, M.; Lermé, J.; Pellarin, M. Photo-Oxidation of Individual Silver Nanoparticles: a Real-Time Tracking of Optical and Morphological Changes. J. Phys. Chem. C 2013, 117 (5), 2274-2282. (28) Lagrange, M.; Langley, D.; Giusti, G.; Jiménez, C.; Bréchet, Y.; Bellet, D. Optimization of Silver Nanowire-Based Transparent Electrodes: Effects of Density, Size and Thermal Annealing. Nanoscale 2015, 7 (41), 17410-17423. (29) Sun, Y.; Mayers, B.; Xia, Y. Transformation of Silver Nanospheres into Nanobelts and Triangular Nanoplates through a Thermal Process. Nano Lett. 2003, 3 (5), 675-679. (30) Huang, X.; Zhan, Z.; Wang, X.; Zhang, Z.; Xing, G.; Guo, D.; Leusink, D. P.; Zheng, L.; Wu, T. Rayleigh-Instability-Driven Simultaneous Morphological and Compositional Transformation from Co Nanowires to CoO Octahedra. Appl. Phys. Lett. 2010, 97 (20), 203112. (31) Sun, H.; Liu, Q.; Li, C.; Long, S.; Lv, H.; Bi, C.; Huo, Z.; Li, L.; Liu, M. Direct Observation of Conversion Between Threshold Switching and Memory Switching Induced by Conductive Filament Morphology. Adv. Funct. Mater. 2014, 24 (36), 5679-5686. (32) Kim, J.; Ko, C.; Frenzel, A.; Ramanathan, S.; Hoffman, J. E. Nanoscale Imaging and Control of Resistance Switching in VO2 at Room Temperature. Appl. Phys. Lett. 2010, 96 (21), 213106. (33) Waser, R.; Dittmann, R.; Staikov, G.; Szot, K. Redox-based Resistive Switching Memories-nanoionic Mechanisms, Prospects, and Challenges. Adv. Mater. 2009, 21 (25-26), 2632-2663. (34) Seok, J. Y.; Song, S. J.; Yoon, J. H.; Yoon, K. J.; Park, T. H.; Kwon, D. E.; Lim, H.; Kim, G. H.; Jeong, D. S.; Hwang, C. S. A Review of Three-Dimensional Resistive Switching Cross-Bar Array Memories from the Integration and Materials Property Points of View. Adv. Funct. Mater. 2014, 24 (34), 5316-5339.

25



(35) He, L.; Liao, Z.-M.; Wu, H.-C.; Tian, X.-X.; Xu, D.-S.; Cross, G. L.; Duesberg, G. S.; Shvets, I.; Yu, D.-P. Memory and Threshold Resistance Switching in Ni/NiO Core-Shell Nanowires. Nano Lett. 2011, 11 (11), 4601-4606. (36) Wang, H.; Du, Y.; Li, Y.; Zhu, B.; Leow, W. R.; Li, Y.; Pan, J.; Wu, T.; Chen, X. Configurable Resistive Switching between Memory and Threshold Characteristics for Protein-Based Devices. Adv. Funct. Mater. 2015, 25 (25), 3825-3831. (37) Zhao, X.; Liu, S.; Niu, J.; Liao, L.; Liu, Q.; Xiao, X.; Lv, H.; Long, S.; Banerjee, W.; Li, W.; Si, S.; Liu, M. Confining Cation Injection to Enhance CBRAM Performance by Nanopore Graphene Layer. Small 2017, 13, 1603948. (38) Araidai, M.; Tsukada, M. Theoretical Calculations of Electron Transport in Molecular Junctions: Inflection Behavior in Fowler-Nordheim Plot and Its origin. Phys. Rev. B 2010, 81 (23), 235114. (39) Lee, G.-H.; Yu, Y.-J.; Lee, C.; Dean, C.; Shepard, K. L.; Kim, P.; Hone, J. Electron Tunneling through Atomically Flat and Ultrathin Hexagonal Boron Nitride. Appl. Phys. Lett. 2011, 99 (24), 243114. (40) Hourani, W.; Rahimi, K.; Botiz, I.; Koch, F. P. V.; Reiter, G.; Lienerth, P.; Heiser, T.; Bubendorff, J.-L.; Simon, L. Anisotropic Charge Transport in Large Single Crystals of π-Conjugated Organic Molecules. Nanoscale 2014, 6 (9), 4774-4780. (41) Lee, C. H.; Qin, S.; Savaikar, M. A.; Wang, J.; Hao, B.; Zhang, D.; Banyai, D.; Jaszczak, J. A.; Clark, K. W.; Idrobo, J. C.; Li, A.-P.; Yap, Y. K. Room-Temperature Tunneling Behavior of Boron Nitride Nanotubes Functionalized with Gold Quantum Dots. Adv. Mater. 2013, 25 (33), 4544-4548. (42) Yoon, J. H.; Zhang, J.; Ren, X.; Wang, Z.; Wu, H.; Li, Z.; Barnell, M.; Wu, Q.; Lauhon, L. J.; Xia, Q.; Yang, J. J. Truly Electroforming-Free and Low-Energy Memristors with Preconditioned Conductive Tunneling Paths. Adv. Funct. Mater. 2017, 27, 1702010.

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Graphic Abstract

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Threshold Switching Induced by a Controllable Fragmentation in

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