Coexistence of memory and threshold switching behaviors in natural milk-based organic memristor

Natural biomaterials have attracted great interest for the fabrication of biocompatible memristors. Here, dense and smooth milk films were deposited on the Pt/SiO2/Si substrate by spin-coating method and resistive switching (RS) devices based milk films with the configuration of Ag/milk/Pt/SiO2/Si are fabricated for the first time. Furthermore, memory RS (MRS) and threshold RS (TRS) effects coexist in the devices, which can be controlled by appropriately setting the compliance current (I cc). The current conduction mechanisms of the devices with MRS and TRS effects are controlled by typical trap-controlled space charge limited current (SCLC) conduction and filamentary conduction mechanism. The good RS performances of the milk-based devices make them promising for sustainable bioelectronics and novel logic device applications.


Introduction
Resistance random access memory (ReRAM), which is based on resistive switching (RS) behavior, as a potential candidate for next-generation nonvolatile memories, has attracted a great deal of attention due to its several advantages, such as fast writing times, high densities, good endurance, long retention, and low operating voltages [1][2][3][4]. A ReRAM cell is generally composed of an switching layer sandwiched between two electron conductors. By applying an electrical field across the electrodes, the resistance can be switched between a high resistance state (HRS) and a low resistance state (LRS). Generally, RS behavior can be classified as two modes: nonvolatile memory RS and volatile threshold RS [5,6]. In memory RS (MRS), both the HRS and LRS can be stably maintained without an external sustaining voltage, which is used for non-volative RRAM applications. While in threshold RS (TRS), only the HRS is stable when no external bias is applied, which can be used as a selector in series with memory cell to subdue the sneak current paths between memory cells [7,8]. Over the past few years, kinds of nonvolatile MRS and volatile TRS behaviors have been observed in diverse switching layer materials [9][10][11][12]. What's more, it is found that the two types of RS behavior can coexist or transform to each other in the switching layer material of a single device, which is in favor of the development of integrated circuits.
Until now, it is demonstrated that diverse switching layer materials, such as binary oxides, semiconductor, halide perovskite, 2D materials and biomaterials, exhibit these RS modes, which can be controlled by appropriately setting the compliance current (I cc ). Among them, natural biomaterials mainly involving protein and carbohydrate, which possess the advantages of widely available, cost-competitive, light-weight and capable of large area fabrication on flexible substrates [13][14][15], have attracted great interest for the fabrication of biocompatible and bio-integrated due to their important potential applications in RS memory devices [16,17]. In 2015, Chen et al reported for the first time the fabrication of naturally silk protein based RS memory device with tunable RS properties [18]. Then, Li et al fabricated the biocompatible memory devices based on natural ferritin, which exhibited reliably inter-convertible RS behaviours [19]. The above reports suggest that the tunable RS properties of protein-based memory devices are derived from formation of conductive filaments and redox reaction in the protein-based switching layer through regulating the magnitude of I cc presets. Thus, amount of protein-rich natural biomaterials have the potential to be employed in RS memory device with tunable RS properties.
Milk is one of the most common biodegradable, bioresorbable, environmentally friendly, natural and abundant liquid biomaterials. It mainly contains, on average, 3.4% protein, 3.6% fat, 4.6% lactose, 0.7% minerals and 87.7% water [20]. Milk is a mixture, which is analogous to egg albumen. Up to date, egg albumen has been used to fabricate various electronic devices and RS memory devices [21]. However, milk has not yet been explored for manufacturing bioelectronics devices. Hence, bovine milk would be tremendously useful in kinds of electronics, especially in RS memory devices. In this work, dense and smooth milk films were fabricated on the Pt/SiO 2 /Si substrate by spin-coating method and we demonstrated for the first time that MRS and TRS behaviors coexist in the Ag/milk/Pt/SiO 2 /Si device through regulating the magnitude of I cc in set process.

Device fabrication
Fresh milk was purchased from a supermarket. Then the milk was centrifuged at 4000 rpm for 10 min and middle part of supernatant was carefully collected by using pipet. After Pt/SiO 2 /Si substrate was cleaned by deionized water and alcohol for several times, the obtained supernatant milk was spin-coated onto the Pt electrode at 500 rpm for 10 s and then 2000 rpm for 30 s. Then the obtained milk films on the Pt/SiO 2 /Si substrates were annealed in the oven at 110°C for 30 min. Finally, circular Ag electrodes with diameter of 200 μm were thermally evaporated through a shadow mask onto the milk films surface as top electrodes.

Characterizations
The morphology analyses were evaluated with TEM (FEI TecnaiTM F30). The SEM image was taken using scanning electron microscopy (SEM, Hitachi S-4800). The AFM images were scanned in tapping mode by atomic force microscope (AFM, MFP-3D, Asylum Research). Current−voltage characteristics were investigated by a Keithley 2400 source measurement unit.

Result and discussion
Fresh milk was purchased from a supermarket (figure 1(a)). Milk films were deposited on the Pt/SiO 2 /Si substrate by spin-coating method and as a proof of concept, milk-based RS devices with the configuration of Ag/milk/Pt/SiO 2 /Si were fabricated ( figure 1(b)). The fabrication processes are described in the Supporting Information. The surface smoothness and flatness of the switching layer films have an important influence on the RS behaviors of the devices. Hence these two properties of milk films were characterized via scanning electron microscopy (SEM) and atomic force microscope (AFM). As shown in figure 1(c), milk films surface are smooth and flat. The cross-sectional SEM profile of the milk films is shown in the right inset of figure 1(c), which reveals that the milk films deposited on the substrate with a thickness around 310 nm are highly uniformity and compactness. Figure 1(d) depicts the AFM topography image of the milk films, and the root-mean-square roughness is 5.1 nm.
The RS characteristics of the Ag/milk/Pt/SiO 2 /Si device clarified by current-voltage (I-V ) curve in the atmosphere were then investigated. During I-V measurements, the DC voltage was applied from the top Ag electrodes while keeping Pt bottom electrodes grounded and then swept with sweeping direction and sequence in the figures. Figure 2(a) shows the typical I-V characteristics of the milk-based RS device, which was examined under I cc of 1 μA. Initially, when the applied voltage sweeps from 0 V to threshold voltage (V th ) along the stage 1, the RS device exhibits low current conductive state (OFF state). With the voltage exceeding the V th (0.92 V), the current suddenly increases to I cc , indicating the device transition from OFF state to high current conductive state (ON state). Before the subsequent voltage sweeping back to hold voltage (V hold ) (0.44 V), the device remains the ON state (stage 2). As the voltage reduces to less than V hold , the device switches back to OFF state. Then the applied voltage sweeps from 0 V to −1 V, the device transition from OFF state to ON state (stage 3 to stage 4).  From the aforementioned results, by controlling the magnitude the I cc in the SET process, the RS behaviors of the Ag/milk/Pt/SiO 2 /Si device can transform from the volatile TRS effect to the non-volatile MRS effect. When the I cc of the device in set process is 1 μA, the TRS effect is observed. When the I cc is increased to 100 μA and 10 mA, the device exhibits a non-volatile MRS effect, which corresponds to non-volatile bipolar RS and WORM effect.
For purpose of further exploring the interesting RS behaviors of the milk-based device, the mechanisms of the RS effects at different I cc should be comprehended. It has been demonstrated that when a positive voltage is applied to the active metal top electrode, anodic dissolution of the active metal top electrode occurs according to this reaction equation where M is the active metal. Then active metal ions migrate into switching layer under the applied electric field between top and bottom electrode. During active metal ions migration process, metal ions are reduced to metal atoms and further form metal clusters [22,23]. When the top electrode is Ag, Ag clusters are formed in the switching layer during the resistance transition, which contribute to form conductive filaments. Chen et al proved that Ag clusters are formed in the silk protein-based RS device during the RS processes and no metallic conductive filaments are formed during the Set and Reset processes [20]. Instead, conductive filaments are formed resulting of hopping of electrons between Ag atoms during charge trapping and detrapping processes. Thus current conduction mechanism of the protein-based RS device with Ag as electrode may be controlled by typical trap-controlled SCLC conduction and filamentary conduction mechanism To understand the conduction mechanisms of the milk-based device, the positive parts of I-V curves in TRS effect tested at the low set I cc of 10 μA are replotted in double-Ln scale as shown in figures 3(a) and (b). In OFF state, when the applied voltage is very low, the current in the switching layer is nearly a constant. As the voltage increases, the slope of ln(I)-ln(V ) curve varies from 1.1 to 1.9, which indicates that the conduction mechanism conforms to the Ohmic conduction and SCLC conduction behaviors [24]. Then the slope increases to 9.9 with the voltage increase. When the applied voltage is beyond V th , the device completes the SET process, indicating filaments are formed in the switching layer [25]. Subsequently, in the process of applying negative bias, the device switches from ON state to OFF state at V hold (0.44 V), and the slope changes from about 2.1 to 1.1. According to the above analysis, a schematic illustration is represented to explain the conduction mechanism as shown in figures 3(c)-(f). Local Joule heating induced by the current in the RS process could be influenced the mobility of the Ag ions. In low voltage regime of the TRS effect, a few of dissolving Ag ions migrate into milk switching layer and form Ag clusters, which could act as trapping centers. Meanwhile, these traps are unoccupied and the injected excess carriers are dominated by the thermally generated carriers ( figure 3(c)). With the voltage increase, these traps are gradually filled by the inject electrons, which resulting in the SCLC model ( figure 3(d)). Then electrons hopping between Ag atoms lead to current increase suddenly to I cc , which indicates the formation of filaments in the milk switching layer. Because of low I cc , filaments formed in the switching layer are tiny and slight and rupture of conductive filaments occurs under the reverse voltage sweeping, as shown in figure 3(f). Thus, milk-based RS device exhibits TRS effect. Figures 4(a) and (b) show the double-Ln plots of I-V curves in nonvolatile bipolar RS effect examined at the set I cc of 100 μA. In OFF state, the slope of ln(I)-ln(V ) curve is 0.96 at the beginning of applied voltage sweeping from 0 V to higher value, which indicates the Ohmic conduction. Then, the slope of ln(I)-ln(V ) curve increases from 1.53 to 2.09 before voltage exceeds Set voltage, which indicates that the conduction mechanism is dominated by typical trap-controlled SCLC conduction. When the applied voltage exceeds the Set voltage, the current suddenly increase to I cc , which indicates the device switches from OFF state to ON state. In ON state, the slope of ln(I)-ln(V ) maintains around 1, which indicates the Ohmic conduction. For the negative bias region ( figure 4(b)), when the applied voltage is beyond reset voltage, the slope switches to 1.83. Then the slope decreases to 1.12 with voltage sweeping. These conduction behaviors indicate that the conduction mechanisms conform to the Ohmic conduction and trap-controlled SCLC conduction. Then, a schematic illustration is represented to explain the conduction mechanism of set and reset processes. Because the I cc is increases to 100 μA in nonvolatile bipolar RS, more Ag ions migrate into milk switching layer and form more Ag clusters. During the voltage sweeps below the set voltage, the conduction mechanisms are similar to that of TRS behaviors. Due to more Ag atoms exist in the switching layer, more and thicker filaments generate in milk switching layer when the applied voltage is beyond SET voltage. In the negative bias region, when the voltage is beyond reset voltage filaments rupture because of the Coulomb repulsion effect [26,27]. The formation and rupture of filaments during applied voltage sweeping contribute to transformation between ON state and OFF state, which bring about MRS effect in milk -based RS device with a higher I cc at 100 100 μA.
Double-Ln plots of I-V curves in WORM effect examined at the set I cc of 1 mA are shown in figures 5(a) and (b). In the OFF state, as shown in figure 5(a), the slope of ln(I)-ln(V ) curve is 0.96, indicating the Ohmic conduction. With the applied voltage increasing, the slope varies from 0.96 to 2.23, which suggests that conduction mechanism is dominated by trap-controlled SCLC conduction. Interestingly, current occurs violently fluctuation in the rectangle marked with red-dotted lines with voltage increasing. When the applied voltage exceeds the switching voltage, the device switches from OFF state to ON state. In ON state, as shown in figure 5(b), the slope of ln(I)-ln(V ) curve is around 1, indicating the Ohmic conduction. According to the front elaboration, a schematic illustration is displayed to explain the conduction mechanism of the RS processes. When the slope varies from 0.96 to 2.23, Ag ions migrate into switching layer and are reduced to form Ag atoms acting as trapping centers. In this region, the conduction mechanisms are still similar to that of TRS behaviors. Because the I cc is up to 1 mA, amount of Ag atoms form in the switching layer, which results in the formation of robust Ag filaments with localized high density. Hence, with the voltage increasing, current begin to appear fluctuation due to formation and rupture of the filaments and then these filaments maintain conducting, which switches the device from OFF state to ON state. Furthermore, these conducting filaments could not rupture in the subsequent voltage sweeping, which attributes to the device is constantly in ON state.

Conclusions
In summary, dense and smooth milk films were deposited on the Pt/SiO 2 /Si substrate by spin-coating method. We have demonstrated for the first time that the Ag/milk/Pt/SiO 2 /Si device exhibits MRS and TRS effects by regulating the magnitude of I cc in set process. Through analyzing the RS behaviors of the milk-based device, magnitude of I cc in set process controls the formation, concentration and distribution of Ag atoms in the switching layer. The current conduction mechanisms of the devices with MRS and TRS effects are controlled by typical trap-controlled SCLC conduction and filamentary conduction mechanism, which are depict in the physical model we proposed. The good RS performances of the milk-based devices make them promising for sustainable bioelectronics and novel logic device applications.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).

Conflicts of interest
There are no conflicts to declare.