Review of Electrochemically Synthesized Resistive Switching Devices: Memory Storage, Neuromorphic Computing, and Sensing Applications

Resistive-switching-based memory devices meet most of the requirements for use in next-generation information and communication technology applications, including standalone memory devices, neuromorphic hardware, and embedded sensing devices with on-chip storage, due to their low cost, excellent memory retention, compatibility with 3D integration, in-memory computing capabilities, and ease of fabrication. Electrochemical synthesis is the most widespread technique for the fabrication of state-of-the-art memory devices. The present review article summarizes the electrochemical approaches that have been proposed for the fabrication of switching, memristor, and memristive devices for memory storage, neuromorphic computing, and sensing applications, highlighting their various advantages and performance metrics. We also present the challenges and future research directions for this field in the concluding section.


Introduction
The key to the development of powerful, energy-efficient neuromorphic computer systems lies in the fabrication of electronic devices that can imitate biological synapses. In this context, the efficiency bottlenecks that occur in the conventional computation of neural algorithms have driven the development of novel devices capable of performing in-memory computing. As a result, energy-efficient neuromorphic systems with resistive memory are expected to eventually replace the prevalent Von Neumann computing architecture [1]. Neuromorphic computing emulates the neural networks of the human brain, utilizing neural/synaptic devices that act as processors and memories to process large amounts of data with low energy requirements. The primary motivation for the development of this technology has been the exponential growth of data in recent decades. Data are regarded as the fuel of the new economy and represent a source of wealth in the knowledge era, particularly because they have been generated more rapidly as technology has advanced in the movement of oxygen vacancies toward the cathode, which acts as an electron acceptor. At the same time, oxygen ions move towards the anode, which switches the device to an LRS by creating oxygen vacancies based on the conductive filaments. On the other hand, the opposite polarity switches the device to an HRS by inserting oxygen ions back into the oxygen sites. The typical VCM-based RS process for an Al/CoFe2O4/FTO device is presented in Figure 2b [59]. The oxygen vacancy channel is formed during electroforming by applying a high voltage between the top and bottom electrodes. Due to the applied electroforming voltage, oxygen anions migrate towards the positively biased top electrode, leaving behind oxygen vacancies. A sufficient number of oxygen vacancies forms conduction filaments between the two electrodes during the SET process. When a reverse bias is applied, the oxygen anions return and fill the oxygen vacancies near the top electrode, producing an HRS [59]. Compared to ECM cells, VCM cells operate at a higher voltage and thus dissipate more power. However, the retention of a VCM cell is much higher than that of an ECM cell. On the other hand, VCM cells generally have a lower ON/OFF ratio than ECM cells [62].  [22]. Copyright © 2023, Elsevier B.V. (b) Schematic representation of the electroforming, SET and RESET process in valance-change memory (VCM) due to the migration of oxygen vacancies. Reprinted with permission from [59]. Copyright © 2023, Springer-Nature. (c) Schematic representation of the SET and RESET process in thermochemical memory (TCM) [63]. Copyright © 2023, De Gruyter. (d) Diagrammatic representation of the amorphous and crystalline state of the cell in phasechange memory (PCM) [61]. Copyright © 2023, Springer-Nature.  [22]. Copyright © 2023, Elsevier B.V. (b) Schematic representation of the electroforming, SET and RESET process in valance-change memory (VCM) due to the migration of oxygen vacancies. Reprinted with permission from [59]. Copyright © 2023, Springer-Nature. (c) Schematic representation of the SET and RESET process in thermochemical memory (TCM) [63]. Copyright © 2023, De Gruyter. (d) Diagrammatic representation of the amorphous and crystalline state of the cell in phase-change memory (PCM) [61]. Copyright © 2023, Springer-Nature.

Valance-Change Memory
In VCM, oxygen vacancies play a vital role in the RS process. The migration of oxygen vacancies and oxygen ions due to the applied electric field is responsible for the SET and RESET process [62]. RS occurs when a positive bias is applied to the anode, which results in the movement of oxygen vacancies toward the cathode, which acts as an electron acceptor. At the same time, oxygen ions move towards the anode, which switches the device to an LRS by creating oxygen vacancies based on the conductive filaments. On the other hand, the opposite polarity switches the device to an HRS by inserting oxygen ions back into the oxygen sites. The typical VCM-based RS process for an Al/CoFe 2 O 4 /FTO device is presented in Figure 2b [59]. The oxygen vacancy channel is formed during electroforming by applying a high voltage between the top and bottom electrodes. Due to the applied electroforming voltage, oxygen anions migrate towards the positively biased top electrode, leaving behind oxygen vacancies. A sufficient number of oxygen vacancies forms conduction filaments between the two electrodes during the SET process. When a reverse bias is applied, the oxygen anions return and fill the oxygen vacancies near the top electrode, producing an HRS [59]. Compared to ECM cells, VCM cells operate at a higher voltage and thus dissipate more power. However, the retention of a VCM cell is much higher than that of an ECM cell. On the other hand, VCM cells generally have a lower ON/OFF ratio than ECM cells [62].

Thermochemical Memory
TCM is generally the least preferred RS mechanism. Typically, TCM devices exhibit unipolar RS, and the formation and rupture of the conduction filaments are dependent on thermal energy and the Joule effect [58]. In the RESET process, an appropriately high current increases the temperature of the conduction filaments due to the Joule heating effect (Figure 2c) [60,62,63]. Russo et al. reported the simulation analysis of a NiO RS device and the effect of the temperature of conductive filaments on RS properties [60].

Phase-Change Memory
PCM is widely used in non-volatile memory storage devices. In PCM cells, the phase of the switching material (i.e., crystalline or amorphous) is tuned by applying external voltage pulses [64,65]. The resistivity of the amorphous phase is higher than the crystalline phase, allowing for data storage in the form of 1 s and 0 s (Figure 2d) [61]. In the RESET process, applying a large electric pulse to the crystalline material for a short time causes a heating effect that increases the temperature of the switching layer; subsequent rapid cooling leads to an amorphous structure. During the SET process, a medium electric pulse is required to heat the material above the crystallization temperature for a relatively long time to switch the device from an HRS to an LRS [64,66,67]. GeSbTe-based devices employ this phase-change mechanism and exhibit a high endurance of 2 × 10 12 cycles [68].
RS memory device is presented in Figure 3a, with its I-V characteristics shown in Figure 3b and its endurance and retention properties summarized in Figure 3c,d, respectively [28].
The technological requirements for RS devices in memory storage applications are summarized in Table 1. In RS memory storage devices, switching the resistance from an HRS to an LRS and vice versa can be used to store data in the form of 0 s and 1 s, respectively [76]. Memory storage devices require low reading and writing voltages, with some RS devices reported to have an operating voltage of 0.5 V with a low power consumption of 0.1 pJ per transition [77]. For practical applications, the endurance of a device should be greater than 10 9 cycles, with many RS devices achieving up to 10 12 cycles [57], and its data retention time should approach 10 years, though a recent report for an RS device shows data retention time higher than this [78]. The switching time is also an essential factor for memory storage devices, with a switching time of fewer than 10 ns recently reported for RS-based devices [79]. In addition, the demand for miniaturized memory devices has grown recently, with Govoreanu et al. fabricating a few nanometersized RS cells [77]. To date, the most frequently researched materials for memory storage applications have been transition metal oxides [80], metal nitrides [81,82], carbon-based materials [83], polymers [84], and chalcogenides [85,86].

Neuromorphic Computing
Neuromorphic computing seeks to emulate brain functions using electronic devices. The human brain is composed of billions of neurons, with synapses controlling their connections. At a basic level, memory and learning are controlled by pre-neurons, synapses, and post-neurons. Typically, a signal is transferred as an electrical spike from a pre-neuron to a post-neuron via a synapse. When the pre-neuron receives the signal, transmitters are generated in the vesicles, and depolarization of the pre-neuron opens the channel, through which the transmitters reach a receptor in the post-neuron by passing through a 20-40 nm long synaptic cleft [1]. In the case of artificial synapses, the properties of a biological synapse are emulated using metal/insulator/metal RS devices, with the top and bottom electrodes acting as the pre-neuron and post-neuron, respectively, and the switching layer or insulator acting as the synapse. For the more accurate mimicking of synaptic properties, analog RS-based devices are generally used, with metal oxides [1], perovskites [87], polymers [88], organic materials [89], and 2D nanomaterials [90] being widely used for RS-based neuromorphic computing applications. A schematic representation of biological and electrical synapses based on RS devices is presented in Figure 3e [86].
Synaptic plasticity is an essential property of the human brain that is associated with modifications in the synaptic weight, which is the strength between two neurons. Spiketiming-dependent plasticity (STDP) and spike-rate-dependent plasticity (SRDP) are the two main variants of synaptic plasticity [1]. Positive and negative values for the synaptic weight represent potentiation and depression, respectively, which can further be classified into short-term potentiation (STP), long-term potentiation (LTP), short-term depression (STD), and long-term depression (LTD) [91], all of which facilitate information storage and learning. STD involves a temporary rise in the synaptic weight followed by an abrupt decline to the original state. In LTD, a permanent change in the synaptic weight occurs following repeated input pulses [91]. A temporal decay in the negative synaptic value indicates STD, while LTD is identified by a permanently low negative synaptic weight [1,91]. The potentiation-depression and antisymmetric Hebbian learning rules of a Ag/a-BN/Pt memristor device are displayed in Figure 3f,g, respectively [82].
The fabrication of electronic devices to mimic synaptic properties (such as STD and LTD) remains challenging. Recently, many electronic devices have been reported that mimic basic and advanced brain functionalities, including RS-based memory devices [92], transistors [93], and ferroelectric junctions [94]. However, the current electronic synaptic devices generally have high power/energy consumption, unstable potentiation and depression dynamics, and unstable conduction states. In addition, the non-linearity and fewer resistive states of artificial synaptic devices have become bottlenecks, affecting the Nanomaterials 2023, 13, 1879 8 of 45 realization of high-performance neuromorphic computing systems. Therefore, further research is required to overcome these challenges and produce efficient artificial synapses. The technological requirements for RS devices in neuromorphic computing applications are summarized in Table 1.

Sensor Applications
RS devices can be employed in both sensors and detectors [75,95]. Most resistancebased sensing devices operate based on a change in resistance induced by a change in the properties of the constituent materials. In RS-based electrochemical sensors, the oxidation and reduction of constituent materials in accordance with the applied voltage and presence of the analyte are monitored [96]. In this context, the change in the resistance from an LRS to an HRS (or vice versa), switching voltage, hysteresis area, and memory window are important physical parameters for RS-based sensors. In particular, when an RS device comes into contact with a target analyte, one or more of these physical parameters will generally change, and monitoring this change allows the device to be employed as a sensor. A representative RS-based sensor is presented in Figure 3h-j [75]. In this case, the change in the current over time and in response to the concentration of the analyte when applying an appropriate voltage shows that resistive materials can respond to the analyte and switch to a different resistive state [75]. The analyte can thus be analyzed quantitatively or qualitatively based on the gap in the voltage or the change in RS properties [49]. The technological requirements for RS devices in sensor applications are summarized in Table 1.

Other Applications
Other applications that can employ RS memory characteristics include thermoelectric [21,22] and optical devices [97]. These are applications for solving the trade-off between conductivity and transmittance [98,99] (Figure 3k-m).  Reprinted with permission from [28].  Reprinted with permission from [28].

Fabrication Techniques for Resistive Switching Devices
The method employed to fabricate a device will affect its RS properties [100][101][102], with the same material potentially demonstrating different RS behavior when synthesized using different approaches [103,104]. Thus, various factors such as industrial practicality, cost-effectiveness, and RS performance need to be considered when fabricating an RS device. Techniques such as sputtering [105][106][107], lithography [108], spin coating [109], spray pyrolysis [110], and electrodeposition [104] have previously been reported for the synthesis of the active switching layer in RS devices, while hydrothermal [111], dip coating [112], SILAR [26], electrospinning [113], and many other solution-processable techniques [114,115] have also been developed for the same purpose. For the top and bottom electrodes, physical deposition techniques are utilized; thus, this section focuses on the options for the fabrication of the active layer.
The advantages and disadvantages of switching layer synthesis techniques are summarized in Table 2 [116][117][118][119][120]. Physical fabrication techniques such as sputtering, atomic layer deposition, and pulsed layer deposition are precise and suitable for industrial applications. However, they are costly and require many steps to fabricate cost-effective RS devices, while their high operating temperatures, requirement for a vacuum, and inability to control the morphology are also limitations. For example, as presented in Figure 4a, the fabrication process for a Pt/HfO 2 /Ti RRAM device involves two-step lithography, etching, and deposition processes that are quite complex [100]. Similarly, chemical vapor deposition requires an inert atmosphere and high temperatures. In contrast, spin coating, hydrothermal fabrication, and spray pyrolysis represent cost-effective and easy fabrication approaches. However, spin-coated and hydrothermally synthesized thin films have lower adhesion and durability than physical vapor deposition and are thus not suitable for large-scale deposition. Figure 4b presents the process for the preparation of a spincoated Cu/HfO 2 /p++ Si device [27]. The high synthesis temperature required for these techniques is also a disadvantage. Figure 4c presents the steps involved in the fabrication of a Ag/CuO/SiO 2 /p-Si RS device, which involves the deposition of a CuO thin film on a SiO 2 substrate using spray pyrolysis and the deposition of the top electrode using e-beam evaporation [101].   Figure 4d presents an anodized aluminum oxide (AAO) template-based electrochemical approach for RS device fabrication [102]. Electrochemical synthesis techniques generally involve a simple fabrication process and are industrially feasible, inexpensive, environmentally friendly, do not require high temperatures, and produce consistent growth over a large surface area. Moreover, they allow precise control of the film thickness and achieve high switching quality through adjustment of the synthesis parameters. In addition, the adherence and durability of electrochemically synthesized thin films are significantly higher than those with other solution-processable techniques. Electrochemical approaches can also be used to fabricate the entire metal-insulator-metal device, while template-assisted deposition and nanostructured devices can be achieved with electrochemical synthesis. Some reports have also suggested that electrochemically synthesized devices have superior properties compared with sol-gel and spin-coated devices [103,104]. The advantages and disadvantages of the synthesis methods are summarized in Table 2 [116][117][118][119][120]. The thickness of the switching layer can also affect the RS properties of the end device, and electrochemical synthesis allows the deposition of a very thin switching layer with controlled morphology. For example, the minimum thickness of a switching layer achieved using electrodeposition is 15-20 nm [121].   Various criteria should be considered when comparing the RS performance of memory devices made using diverse fabrication technologies, such as the thicknesses of the switching and top and bottom electrode layers, the choice of top and bottom electrodes, and the synthesis methods because they all have an influence on the RS characteristics. We analyzed the RS performance of various copper oxide-based devices by considering the previously mentioned factors while focusing on industrially applicable techniques that are cost-effective with a simple synthesis process, as summarized in Table 2. From the table, it is evident that electrochemically synthesized copper oxide-based devices are switched at a low voltage (0.7 V), similar to RS devices fabricated using a sputtering process (V SET : 0.7 V and V RESET : −0.5 V). In addition, devices made using these techniques exhibited endurance greater than 10 3 cycles and retention up to 10 3 s, while devices fabricated using other methods achieved relatively poor RS performance [104][105][106][107][108][109][110][111][112][113][114][115][116][117][118][119][120]. These results indicate that the electrochemical method can provide excellent RS performance, similar to sputtering and other physical synthesis procedures.

Electrodeposition
Electrochemical synthesis is an elegant and versatile technique that allows the controlled deposition of an electrolyte onto conducting material with the application of an electric current or voltage across two electrodes [122]. Electrochemical synthesis can be classified as cathodic and anodic electrodeposition [123]. In cathodic electrodeposition, charged species, clusters, and nanoparticles are deposited on the cathode. Depending on the deposited charged species, charged particles, and applied power, electrodeposition can be classified as electroplating, electrophoretic, or electroless deposition. In electroplating, metal cations in the electrolyte move towards the cathode due to an external electric field and are reduced on the surface of the cathode (Figure 5a). Most semiconductors, chalcogenides, and oxide-based thin films can be fabricated using this technique.
In electrophoretic deposition, similar reaction mechanisms can occur, but it does not involve any chemical reaction, i.e., the reduction of ionic species. Instead, the driving force for this technique depends on the charge and electrophoretic mobility of the particles involved. The electrolyte is replaced by a suspension of charged particles in an organic solvent [123]. This technique can be used to deposit charged particles, nanomaterials [124,125], and carbonbased materials [126]. Figure 5b presents the general mechanisms for electrophoretic deposition. Dispersed charged particles in the organic solvent are attracted or deposited on the oppositely charged conducting electrode following the application of an external DC voltage/current. Electrophoretic deposition can be both cathodic and anodic [127].
In electroless deposition, deposition occurs without any external electric supply. This is an autocatalytic process in which redox reactions occur due to reducing agents that act as a source of electrons ( Figure 5c) [123,128]. The electrode is dipped in an electrolyte containing reducing agents and metal precursors. The autocatalytic redox reactions deposit metal atoms on the electrode by removing the metal atoms on the electrode surface. This deposition technique provides a simple and cost-effective path for the decoration of metals such as Sn, Au, and Ni or nanoparticles on metal-oxide thin films [129,130] and is highly selective.
Electrochemically prepared thin films can be deposited in a potentiostatic or galvanostatic manner, in which the current density varies with a constant voltage and the electrode potential varies with a constant current, respectively. Chronoamperometry, chronopotentiometry, and voltammetric deposition are also common thin-film synthesis techniques [104]. In electrochemical fabrication, several parameters should be considered for the uniform deposition of thin films, including the electrolyte concentration, applied electric field, pH, deposition time, deposition temperature, and distance between the two electrodes [131,132]. The thickness of the deposited film is controlled by tuning the deposition time and the applied electric field, while pH and the deposition temperature are responsible for com-positional changes in the deposited materials [38]. In addition, the distance between the two electrodes can determine the structure and morphology of the thin film.

Anodization
Anodization is a widely used technique for the preparation of high-quality thin films for the fabrication of electronic devices due to its large-scale fabrication capabilities and simplicity [131][132][133]. In the anodization process, the metal anode is electrochemically oxidized. Figure 5d presents the metal anode as a reservoir of infinite metal cations (M z+ /N y+ ) with an electrolyte reservoir of infinite anions (O 2− /A n− ) [132]. The applied electrical potential oxidizes the cations on the surface of the anode, which forms a thin oxide film on the surface of the anode. The thickness of the oxide layer is directly proportional to the applied electric field, which triggers the ionic conductivity of the oxide on the surface of the metal. Three theories have been proposed to explain the rate-determining steps: Verwey's theory, which proposes that ion transfer within the oxide film is responsible for the metaloxide thin film [134], Mott-Cabrera's theory, which posits that ion transfer across the metal-oxide interface and determines the thickness of the film [135], and Dewald's theory, which highlights the influence of ion transfer across the oxide-electrolyte interface [136]. The high field mechanism explains the exponential dependency of the current density (i) on the electric field strength (E), as given in Equation (1)

Anodization
Anodization is a widely used technique for the preparation of high-quality thin films for the fabrication of electronic devices due to its large-scale fabrication capabilities and simplicity [131][132][133]. In the anodization process, the metal anode is electrochemically oxidized. Figure 5d presents the metal anode as a reservoir of infinite metal cations (M z+ /N y+ ) with an electrolyte reservoir of infinite anions (O 2− /A n− ) [132]. The applied electrical potential oxidizes the cations on the surface of the anode, which forms a thin oxide film on the surface of the anode. The thickness of the oxide layer is directly proportional to the applied electric field, which triggers the ionic conductivity of the oxide on the surface of the metal. Three theories have been proposed to explain the rate-determining steps: Verwey's theory, which proposes that ion transfer within the oxide film is responsible for the metal-oxide thin film [134], Mott-Cabrera's theory, which posits that ion transfer across the metal-oxide interface and determines the thickness of the film [135], and Dewald's theory, which highlights the influence of ion transfer across the oxide-electrolyte interface [136].
The high field mechanism explains the exponential dependency of the current density (i) on the electric field strength (E), as given in Equation (1) [132].
where N is the surface density of mobile ions, v is the vibrational frequency of mobile ions, q is the charge associated with the mobile ions, W is the activation energy, αa is the activation distance, k B is the Boltzmann constant, and T is the absolute temperature. The applied electric potential equations can be derived by assuming that the ions gain sufficient potential energy via thermal agitation. Figure 5e presents the potential energy of ions in relation to the distance with and without an electric field. The former is assumed to reduce the height of the barrier from W to (W − qE), while the latter increases the barrier for ions moving against the field from W to (W + qE) [132]. A strong electric field during anodization can cause the metal cations to migrate towards the metal-oxide-electrolyte interface, while O 2− or anions are inserted into the metal oxide and reduced at the metal-metal-oxide interface. The oxidation of metal cations occurs within the metal oxide, at the metal-metal-oxide interface, or at the metal-oxideelectrolyte interface. Anodization parameters such as the applied voltage and anodization time influence the thickness and crystallinity of the resulting oxide film, while the choice of the electrolyte affects the composition of the oxide layer and the pH of the solution and etching agents determine the morphology of the oxide thin film.

Electrodeposited Resistive Switching Materials for Memory Storage Applications
Minimal research has been conducted on electrochemically synthesized thin films for memory storage applications. However, the most frequently researched electrodeposited materials for RS device fabrication are carbon-based materials, metal oxides, and chalcogenides. The following subsections discuss the use of these materials.

Carbon-Based Resistive Switching Devices
There are very few reports available on carbon-based RS devices [137]. In recent years, graphene oxide [138,139], diamond-like carbon [140], carbon quantum dots [141], single-walled carbon nanotube (SWCNT)@TiO 2 core-shell wires [142], graphene oxide-TiO 2 [143], self-generated amorphous carbon [144], graphene-integrated devices [145], and carbon nanotubes have been explored for use in RS memory applications [146], but very few of these have been electrochemically synthesized. Carbon materials have numerous advantageous properties related to their morphology, defects, percentage of oxygen groups, and sp 2 and sp 3 carbon atoms, and these properties can be utilized to tune the RS effect of a device [35,137].
Russo et al. reported a new electrochemically deposited carbon material for RS devices with free bipolar RS properties [35]. They generated carbon nanowalls (CNWs) via electrophoretic deposition from arc-discharged polyyne electrolytes and demonstrated that the defects in and the orientation of the CNWs affected their electrical resistivity, with the presence of oxygen vacancies and a perpendicular orientation leading to an LRS in an Al/CNW/FTO device. Additionally, trapping and de-trapping electrons in oxygen vacancies with a positive and negative bias switches the device between an LRS and an HRS, respectively [35]. In 2019, Russo et al. reported the electrophoretic deposition of carbon material for memory application [137]. They employed a two-step synthesis approach that combined the electrophoretic deposition of the carbon material with subsequent electrochemical oxidation to produce an active switching layer (Figure 6a) [137]. They reported that the content of oxygen and carbonyl groups (C=O) in the electrochemically oxidized carbon material controlled the RS properties. The anodically oxidized carbon material exhibited three-level memory behavior and a higher ON/OFF ratio than non-oxidized carbon materials. Figure 6b,c present the cyclic stability and retention of the Al/oxidized carbon structures@FTO device [137], with the degree of oxidation well controlled by the anodic oxidation parameters. The oxidized carbon material exhibited RS behavior within a ±2 V range. While a higher percentage of carbonyl groups (C=O) did not provide a higher ON/OFF ratio, the amount of sp3 and sp2 carbon atoms and oxygen-containing carbon groups did play a key role in the Al/OC@FTO device [137]. Liu and co-workers also reported that the electrophoretic deposition of graphene oxide could potentially be used in memristive devices [147]. This is because the oxygenation levels of graphene oxide can affect the properties that are tuned using electrophoretic parameters. Their fabricated Al/GO/ITO device had an ON/OFF ratio of more than 10 and a retention time of 100 s [147]. Table 3 summarizes the electrochemically prepared carbon-based materials for memory applications. This summary highlights a considerable research gap in terms of electrochemically synthesized carbon materials for RS-based memory devices.
Nanomaterials 2023, 13,1879 18 of 51 memristive devices [147]. This is because the oxygenation levels of graphene oxide can affect the properties that are tuned using electrophoretic parameters. Their fabricated Al/GO/ITO device had an ON/OFF ratio of more than 10 and a retention time of 100 s [147]. Table 3 summarizes the electrochemically prepared carbon-based materials for memory applications. This summary highlights a considerable research gap in terms of electrochemically synthesized carbon materials for RS-based memory devices.

ZnO-Based Resistive Switching Devices
In recent years, transition metal oxides have attracted significant attraction due to their CMOS compatibility and ease of synthesis [148]. ZnO is the most common metal oxide used in electrochemical synthesis. As with carbon materials, the orientation and morphology of ZnO can affect the RS properties. Jing et al. synthesized ZnO-based RS devices, and Figure 7a presents the structure of their electrodeposited ZnO thin-film-based device [36]. They produced nanoflake, agglomerated nanoparticle, and nanorod morphologies by tuning the deposition temperature and found that the Au/ZnO nanorod/FTO device had strong RS properties, while the ZnO nanoflakes and nanoparticles did not (Figure 7b). The ZnO nanorod-based device exhibited an endurance of 2000 cycles and retained data for up to 1800 s (Figure 7c,d, respectively) [36]. In another study, Yusoff et al. synthesized ZnO nanowires using two-step electrodeposition and thermal oxidation. They varied the deposition time to investigate the effect of the nanowire size, the thickness of the film, and the RS properties of the device [149].  [147] morphologies by tuning the deposition temperature and found that the Au/ZnO nanorod/FTO device had strong RS properties, while the ZnO nanoflakes and nanoparticles did not (Figure 7b). The ZnO nanorod-based device exhibited an endurance of 2000 cycles and retained data for up to 1800 s (Figure 7c,d, respectively) [36]. In another study, Yusoff et al. synthesized ZnO nanowires using two-step electrodeposition and thermal oxidation. They varied the deposition time to investigate the effect of the nanowire size, the thickness of the film, and the RS properties of the device [149]. A cyclic voltammetric deposition is an essential form of electrochemical deposition. In this technique, an applied voltage is swept between specific voltage ranges, producing a thin film with superior surface coverage compared with conventional electrodeposition [104]. Sun et al. fabricated a Au/ZnO/AZO structure in which ZnO thin film was cathodically electrodeposited in a zinc nitrate solution [150]. The electrochemical deposition was carried out potentiostatically in a conventional three-electrode cell at −0.9 V with a constant temperature of 353 K and a deposition time of 1000 s. The RS properties were tuned A cyclic voltammetric deposition is an essential form of electrochemical deposition. In this technique, an applied voltage is swept between specific voltage ranges, producing a thin film with superior surface coverage compared with conventional electrodeposition [104]. Sun et al. fabricated a Au/ZnO/AZO structure in which ZnO thin film was cathodically electrodeposited in a zinc nitrate solution [150]. The electrochemical deposition was carried out potentiostatically in a conventional three-electrode cell at −0.9 V with a constant temperature of 353 K and a deposition time of 1000 s. The RS properties were tuned by simply adding nitric acid to the electrolyte. They reported that the reaction of nitric acid with OH-ions in the electrolyte decreased oxygen availability, leading to more oxygen vacancies and n-type conductivity in ZnO. The ON/OFF ratio significantly decreased with an increase in the nitric acid concentration. A low concentration of nitric acid led to few oxygen vacancies while a high concentration did not induce the RS effect. When the carrier concentration was higher than 10 19 cm -3 , the materials did not exhibit any RS properties [150].
Fuzi et al. also fabricated a flexible Au/ZnO/ITO/PET RS device using electrodeposition. They reported that deposition time is an essential parameter when tuning the RS properties [148]. Figure 7e presents the semi-logarithmic I-V curves for electrochemically prepared ZnO thin films deposited for 15, 30, 60, and 120 s. Above 60 s, the device did not exhibit any notable I-V characteristics, while the R OFF /R ON ratio decreased with increasing deposition time (Figure 7f). The combination of electrochemical with other techniques assisted in the oxidization of Zn, leading to stronger RS properties than electrochemically deposited ZnO. In particular, thermal oxidation is commonly used in conjunction with electrochemical synthesis. For example, Quintana et al. synthesized faceted and spongy single-crystal ZnO using a combination of electrodeposition and hydrothermal techniques [151]. The faceted and spongy Zn rods were synthesized galvanostatically and potentiostatically, respectively, followed by hydrothermal treatment in distilled water to produce ZnO rods. The end device exhibited good I-V and retention properties.
Many research groups have synthesized composite, doped, bilayer, and decorated semiconductor materials to improve RS properties. Recently, Younis et al. synthesized a Tidoped ZnO thin film from Zn(NO 3 ) 2 ·6H 2 O via electrodeposition on ITO [152]. The Au/Ti@-ZnO/ITO device exhibited electroforming-free and current compliance-free memory effects and typical bipolar RS properties with good SET (2.9 V) and RESET (−2.7 V) voltages. The device was stable for up to 200 cycles and retained data for up to 2000 s. The memory margin of the device with a 2% Ti@ZnO structure was much higher than that of the control device (pure ZnO) [152].
Bilayered and trilayer devices have also been shown to have tunable RS properties. For example, Fauzi et al. electrodeposited ZnO on a copper substrate followed by thermal oxidation to produce a Au/ZnO-Cu 2 O-CuO/Cu RS device. Figure 8a,b present the electrodeposition setup and device structure, respectively [153]. A trilayer structure was observed due to thermal oxidation. The active layer thickness (ZnO-Cu 2 O-CuO) was approximately 81 nm. The authors reported that the HRS/LRS ratio was directly proportional to the thickness of the thin film (Figure 8c,d); as the thickness increased with longer deposition times, the difference between the resistance of the LRS and HRS and the HRS/LRS ratio tended to decrease. In 2018, Fauzi et al. deposited Ti and Zn on a Cu substrate using dilute electrodeposition and subsequently oxidized it thermally at 600 • C. They observed that the Au/TiO 2 -Cu 2 O-CuO/Cu device had a higher HRS/LRS difference than Au/ZnO-Cu 2 O-CuO/Cu. They suggested that metal ion formation was responsible for the RS effect in their device [154].
The decoration of ZnO with quantum dots can also enhance RS properties. Younis et al. electrochemically synthesized ZnO nanorods decorated with CeO 2 quantum dots (∼5 nm; Figure 8e) [155]. The average diameter and length of the ZnO nanorods were ∼20 and ∼200 nm, respectively. The fabricated Au/CeO 2 −ZnO/FTO device exhibited stronger memristive properties than the Au/ZnO/FTO device (Figure 8f). In terms of the temperature dependence of the RS characteristics, the CeO 2 −ZnO nanocomposite showed no change in the memory window. However, the memory window of the ZnO nanorod-based device tended to decrease as the temperature increased. Figure 8g,h depict the endurance and retention properties of the Au/CeO 2 −ZnO/FTO device. Their results suggested that the decoration of CeO 2 quantum dots improves the RS characteristics of electrochemically synthesized ZnO nanorods [155]. The electrochemical approach is also useful for tuning the charge transport within a device. Ocampo et al. fabricated nanostructured porous Si-metal-oxide (ZnO and VO 2 ) composites, with the porous Si-ZnO device exhibiting better memristive properties than the Si-VO 2 -based RS device due to the higher porosity of the ZnO, which facilitated charge transport within the active switching layer [156]. Table 4 summarizes the electrochemically synthesized ZnO-based RS devices that have been reported to date. The electrodeposition technique has frequently been used for material synthesis, but further research is warranted on the synthesis of anodizationbased ZnO RS materials. It is important to note that electrodeposited ZnO RS devices require higher switching voltages than physically deposited RS devices, while their memory window, endurance, and retention properties are also lower. Given this, research should focus on improving the non-volatile memory performance of electrochemically synthesized ZnO RS devices.

TiO 2 -Based Resistive Switching Devices
Memory storage devices based on transition metal oxides have been well studied in recent years. In particular, TiO 2 has been the most widely researched material for RS memory applications. In 2010, Miller et al. synthesized titanium dioxide using anodization for memory storage applications and investigated the memristive properties of both annealed and nonannealed devices. The nonannealed anodized TiO 2 exhibited a pinched hysteresis loop, while the annealed films did not demonstrate any significant RS properties. There is no need for the post-deposition annealing of anodized films due to the inherent presence of oxygen vacancies at the Ti/TiO 2 interface [37]. There have been very few reports available on the fabrication of TiO 2 nanotubes using electrodeposition for memristor applications, but it is clear that electrodeposition can produce TiO 2 nanotubes for RS applications [157]. Power consumption is an important aspect of RS devices that depends on the operating voltage. Therefore, there is a need to fabricate devices with a lower switching voltage. In this respect, anodically prepared Au/TiO 2 /Ti devices can be operated within ±0.3 V [158].
The anodization of Ti foil in different electrolytes leads to the formation of TiO 2 nanotubes with different shapes, leading to devices with different RS properties.

TiO2-Based Resistive Switching Devices
Memory storage devices based on transition metal oxides have been well studied in recent years. In particular, TiO2 has been the most widely researched material for RS memory applications. In 2010, Miller et al. synthesized titanium dioxide using anodization for memory storage applications and investigated the memristive properties of both annealed and nonannealed devices. The nonannealed anodized TiO2 exhibited a pinched hysteresis loop, while the annealed films did not demonstrate any significant RS properties. There is no need for the post-deposition annealing of anodized films due to the inherent presence of oxygen vacancies at the Ti/TiO2 interface [37]. There have been very few reports available on the fabrication of TiO2 nanotubes using electrodeposition for memristor applications, but it is clear that electrodeposition can produce TiO2 nanotubes for RS applications [157]. Power consumption is an important aspect of RS devices that depends on the operating voltage. Therefore, there is a need to fabricate devices with a lower switching voltage. In this respect, anodically prepared Au/TiO2/Ti devices can be operated within ±0.3 V [158].
The anodization of Ti foil in different electrolytes leads to the formation of TiO2 nanotubes with different shapes, leading to devices with different RS properties.  Similarly, Yin et al. fabricated a Pt/TiO 2 /Ti device in which TiO 2 was deposited via anodization in an H 2 SO 4 electrolyte. Interestingly, four stable resistance states were observed in the prepared device. The eightwise and counter-eightwise I-V curves indicated the coexistence of ion-and electron-dominated RS mechanisms [160]. The choice of the top electrode in an anodic TiO 2 -based RS device influences its RS properties. Given this, Yu et al. reported an anodic TiO 2 -based RS device with Ag and Au as the top electrodes. The Au-based device did not exhibit any notable I-V characteristics; however, with the Ag top electrode, improved I-V characteristics were observed. They also reported that anodizing for only 5 min produced a TiO 2 device with more suitable properties than the anodization times of 1, 3, and 10 min. Thus, the deposition time is also a very sensitive parameter for electrochemically synthesized RS devices [161].
Tao et al. fabricated a Cu/TiO 2 /Ti nanopore array membrane device using twostep anodization [162]. First, Ti foil was anodized in an NH 4 F-based ethylene glycol electrolyte and washed in an ultrasonicator. It was then anodized in the same electrolyte under an ice bath at 20 V, 40 V, and 60 V. The devices anodized at 20 V and 40 V did not require electroforming steps. RS occurred within a ±1 V window, and the R HRS /R LRS ratio increased as the anodization voltage increased. Generally, electrochemical metallization was observed in the device with a Cu top electrode, but the Cu/TiO 2 /Ti device exhibited Ohmic conduction. This suggested that the oxygen vacancies led to the formation of conduction filaments in the TiO 2 during the SET process. The device was composed of Cu nanocones and Ti nanopores, with a highly concentrated electric field between the two, which enabled the migration of oxygen vacancies [162]. Chen et al. also fabricated coreshell copper nanowire-TiO 2 nanotubes (TNTs) via anodization and electrodeposition [163]. The TNTs were synthesized using two-step anodization, while the Cu nanowires were grown on the TNTs using electrodeposition. The RS properties were investigated for both Au/TNTAs/Ti and Au/Cu nanowire-TNTAs/Ti devices, with the Cu nanowire-TNTAbased device exhibiting both analog and digital RS properties. The device was stable for 10 4 cycles and was able to retain data for 10 4 s [163]. Table 5 summarizes the previously reported electrochemically prepared TiO 2 -based RS devices [37,[157][158][159][160][161][162][163][164][165][166]. Unlike ZnO, anodization was frequently used for the synthesis of TiO 2 for RS memory applications. All of the electrochemically prepared TiO 2 RS devices have demonstrated suitable switching voltages. However, improvements in the memory window, endurance, and retention properties of electrochemically prepared TiO 2 RS devices are required.

Cu 2 O-Based Resistive Switching Devices
Cu 2 O has been frequently employed for various applications, including memory [167][168][169], photocatalysis [170], solar cells [171], supercapacitors [172], and gas sensing [173]. However, Cu 2 O has not been well explored for RS applications. Cu 2 O has the potential to be compatible with integrated circuit fabrication processes. Yazdanparast et al. reported unipolar RS properties for electrodeposited Cu 2 O on a Au-coated glass substrate in a highly alkaline tartrate bath. The RS of their device was attributed to the formation and rupture of Cu filaments, with an initial resistance of 6.5 × 10 6 Ω. The LRS of the device demonstrated a linear decrease in resistance with decreasing temperature, suggesting the formation of Cu filaments. The calculated diameter (70 nm) and resistance temperature coefficient (1.57 × 10 −3 K −1 ) in an LRS confirmed the formation of Cu filaments, which were nearly equal to Cu nanowires. The authors also calculated the temperature (798 ± 55 K) of the Cu filaments during the RESET process, which caused the rupture of the filaments due to Joule heating [174]. In another study, the same research group reported the effect of various electrodeposited parameters on RS properties. Electrodeposition voltage can be used to tune the size of the electrodeposited microstructure and RS properties, and their study found that decreasing the applied negative voltage increased the grain size of Cu 2 O. Additionally, the grain size increased with an increase in temperature. They observed that the forming voltage decreased with an increase in the electrodeposited negative potential and an increase in temperature. Furthermore, the SET/RESET voltage was dependent on the grain size of the material. In addition, the LRS current did not depend on the grain size or the surface area of the top electrode, which suggested that filamentary conduction is the dominant mechanism in Au/Cu 2 O/Au-Pd devices [175].
Fabrication of high-density memory storage devices is a major challenge that can be achieved by scaling down the size of the device and developing cross-point array structures, which require precise, large-scale, and selective area deposition methods. In this respect, electrochemical deposition can be used to precisely fabricate active switching layers. Recently, Han et al. fabricated a memory device by electrodepositing Cu 2 O (70 nm) within the holes of a SiO 2 substrate on a Pt bottom electrode. The top Pt electrode was deposited using an e-beam evaporator (Figure 10a) [167]. The size of the cell is presented in Figure 10b [167]. The device exhibited bipolar RS within ±1 V and a forming voltage within ±1 V, representing a low power consumption (Figure 10c) [167]. The fabricated device also demonstrated good retention ( Figure 10d) and endurance (Figure 10e) [167]. In another study, the same research group fabricated Cu/CuO x /Cu devices using the electrodeposition of Cu, CuO x , and Cu on an AAO template (Figure 10f) [168]. The active layer thickness and pore diameter of the AAO template influenced the SET/RESET voltage of the devices. The device structure and I-V characteristics of the electrochemically deposited Cu/CuO x /Cu devices are presented in Figure 10g [168]. The RS mechanisms of these devices depended on the top and bottom electrode material because the ionization energy of Cu is lower than that of Cu 2 O. The authors suggested that, when a positive bias was applied to the bottom electrode, the Cu induced the diffusion of Cu + ions into the Cu 2 O layer. Furthermore, the Cu ions drifted from the top electrode to the bottom electrode and formed Cu conduction filaments in the switching layer. The filaments were ruptured by applying a negative bias to the bottom Cu electrode (Figure 10h) [168]. Patil et al. also fabricated Al/Cu 2 O/FTO memristive devices based on an electrochemical approach, showing that the space chargelimited current (SCLC) mechanism was responsible for conduction, while RS occurred due to the filamentary effect [176]. Flexible memory storage devices are the future of ubiquitous computing systems. To fabricate a flexible memory device, a flexible substrate is employed to support the entire device. Recently, Park et al. electrodeposited Ni/CuO x /Ni on a flexible polyethylene terephthalate (PET) substrate, producing a device with reliable RS and endurance. The fabricated device exhibited strong performance under flat, tensile, and compression stress [177]. Kim et al. also fabricated a self-compliance-based bilayered memory device with a Pt/CuO x /CuO x /Pt structure. The electrical properties of the CuO x layers were tuned by controlling the pH of the electrolyte. In their device, one CuO x layer acted as an internal resistor while the other operated as an RS layer. Due to the internal resistor, the LRS current was controlled and reproducible RS properties were observed without current compliance [178]. Electrochemical approaches can also be extended to the fabrication of metal sulfide-based RS devices. For example, Yan and co-workers fabricated Ag/Cu 2 S/Ag and Ag/Cu 2 S/Cu memristive devices and recorded good RS performance [179]. Table 6 presents previously reported electrochemically synthesized Cu 2 O-based RS devices. The RS performance of these devices is not yet satisfactory. More attention should be focused on improving their memory window, endurance, and retention properties. Furthermore, electrodeposition has frequently been employed to synthesize Cu 2 O-based RS devices; thus, other electrochemical approaches should be researched to achieve highperformance RS memory devices. The choice of the electrolyte in electrochemical synthesis affects the morphology and composition of the active layer material, which impacts the RS properties of the end device. For Cu 2 O-based RS devices, CuSO 4-based electrolytes have been preferred in combination with acid and NaOH. Lactic acid, sulfuric acid, and tartaric acid have been the most preferred choices of researchers for the electrochemical synthesis of Cu 2 O-based RS devices. Therefore, RS properties could be improved further by testing other electrolytes during the synthesis of Cu 2 O.

Other Metal-Oxide-and Composite-Based Resistive Switching Devices
Transition metal oxides are promising switching layer options for RS devices. Electrochemically synthesized ZnO, TiO 2 , and CuO are transition metal oxides that are frequently used for RS memory applications, while NiO, MnO 2 , WO 3 , Ta 2 O 5 , ZrO 2 , and Co 3 O 4 have also been reported for RS memory applications [180][181][182][183][184][185]. Nanotemplate-based synthesis has recently been reported to be a promising technique for RS devices. Recently, Kim et al. reported NiO nanowire-based devices fabricated via electrodeposition with AAO membranes. The fabricated device exhibited similar RS properties to those of NiO thin films. The authors reported that the operating voltage of the device was directly proportional to the length of the NiO nanowires [186]. In the same vein, Song et al. fabricated AAO nanotemplates based on Ni/NiO/Ni devices with cell sizes of less than 100 nm. They were able to produce NiO nanodots with a high density (1 × 10 10 /cm 2 ) and uniformity. The fabricated device exhibited unipolar RS characteristics. Interestingly, NiO doping with phosphorous switched the Au/NiO/Au device from unipolar to bipolar when fabricated using AAO template-based electrodeposition. The fabricated device operated within ±1 V [187]. AAO nanotemplate-based nanopatterning techniques thus have significant potential for use in high-density memory applications [88]. Valve metal oxides and valve metal alloy oxides can also be tailored electrochemically to boost the performance of RS devices. Sophocleous et al. demonstrated a TaO x -based device with good memristive properties. A switching layer with a thickness of 10 nm was fabricated using electrochemical anodization, and a high ON/OFF ratio was obtained. The device was low-cost and environmentally friendly and exhibited better RS performance with possible CMOS compatibility [188]. Zaffora et al. also reported a Ta/Ta 2 O 5 /Pt-based RS device with the active switching layer fabricated using anodization in a borate buffer electrolyte. The device had a stable LRS and HRS for up to 10 6 cycles and a retention of up to 10 4 s. The SET and RESET times for the device were 20 ns and 165 ns, respectively [189]. In 2019, the same research group fabricated a device based on the anodization of Ta and an Al alloy for RS applications. The device had similar properties to an anodic Ta 2 O 5 -based RS device. They also fabricated a mixed Hf-Nb oxide via the anodization of Hf and Nb. Their results suggested that this electrochemical approach has significant potential for the production of high-quality thin films for RS devices [132].
Flexibility and transparency can expand the range of applications for RS devices. Interestingly, WO 3 -based RS devices have been reported to show both characteristics. Ji et al. fabricated a Cu/WO 3−x /ITO-based flexible RS device using anodization. The device had a high I ON /I OFF ratio of~10 5 and a stable retention of up to 5 × 10 5 s. They also reported outstanding uniformity over 10 3 bending cycles [190]. Qian et al. fabricated a highly transparent ITO/WO 3 /ITO RS device using cathodic electrodeposition (Figure 11a) [180]. The device had high optical transmittance (Figure 11b), a low operating voltage (Figure 11c), and a long retention time (>10 4 s) [180]. Electrochemically deposited Mn 3 O 4 devices can also exhibit RS characteristics. Koza et al. deposited Mn 3 O 4 potentiostatically in an acetate electrolyte with a potential range of 0.20-0.35 V. The Mn 3 O 4 thin film has unipolar RS properties, and the SET and RESET times of the device were 2 and 50 ns, respectively [43]. An electrodeposited δ-Bi 2 O 3 -based RS device has also been reported to be unipolar (Figure 11d) [181]. The authors concluded that the formation and rupture of Bi filaments were responsible for switching in a Pt/δ-Bi 2 O 3 /Au memory cell (Figure 11e) [181]. Recently, electrodeposited VO 2 has also been reported for RS applications [191,192]. An electrodeposited P-type CO 3 O 4 nanosheet-based device exhibited bipolar RS behavior and a stable RS process over 200 cycles, with data retained for up to 10 4 s. [193]. Oxidized MoS 2 is also an attractive material for RS applications due to its promising memristive properties. Jamilpanah et al. reported an electrodeposited MoS 2 -MoO 2 -MoO 3 -based RS device with ECM and VCM mechanisms when using Ag and Au top electrodes, respectively. Interestingly, the annealing of the composite created pure-phase MoS 2 and MoO 3 in Ar and air, respectively, which did not demonstrate memristive properties [194].
As with programmable solid-state metallization cells, liquid-state metallization cells produce good results in aqueous systems. Han et al. fabricated a device by stabilizing the aqueous growth of Cu in polycarbonate nanopores with the use of shock electrodeposition. The device exhibited considerable I-V characteristics and endurance for up to 500 cycles with a retention time of <10 s [195]. Polymer-based composites can also exhibit useful RS properties. For example, Xu et al. fabricated a Co-nanoparticle dispersed polypyrrole (PPy) based on an electrochemical approach for RRAM applications (Figure 11f) [182]. Metal nanoparticles were composited with a polymer, which enhanced the spontaneous oxidation of the conducting polymer. The RS mechanism of the device was based on oxygen ion mobility, which was able to be tuned by the metal nanoparticle concentration. The Cu/Co−PPy composite/Pt device exhibited stable bipolar RS (Figure 11g), which was due to oxygen ion movement within the switching layer (Figure 11h) [182]. Electrodeposited Prussian blue [196], AAO template-based electrodeposited Bi 1−x Sb x [197], and electrodeposited copper tetracyanoquinodimethan [198] have also been reported for RS memory applications.  [180]. Copyright © 2023, American Chemical Society. (d) I-V curves for a Pt/δ-Bi2O3/Au cell during the FORMING, SET, and RESET steps at a rate of 0.7 V/s. (e) Schematic illustration of the electroforming, formation, and rupture of a conducting filament during the SET and RESET steps in a Pt/δ-Bi2O3/Au device. Reprinted with permission from [181]. Copyright © 2023, American Chemical Society. (f) Electrochemically deposited Co−PPy-composite memory device structure. (g) I-V characteristics for a bipolar RS Co−PPy-L sample. (h) Schematic representation of the RS mechanism during the SET and RESET processes in a Co−PPy-composite device. Reprinted with permission from [182]. Copyright © 2023, American Chemical Society.
As with programmable solid-state metallization cells, liquid-state metallization cells produce good results in aqueous systems. Han et al. fabricated a device by stabilizing the aqueous growth of Cu in polycarbonate nanopores with the use of shock electrodeposition. The device exhibited considerable I-V characteristics and endurance for up to 500 cycles with a retention time of <10 s [195]. Polymer-based composites can also exhibit useful RS properties. For example, Xu et al. fabricated a Co-nanoparticle dispersed polypyrrole (PPy) based on an electrochemical approach for RRAM applications (Figure 11f) [182]. Metal nanoparticles were composited with a polymer, which enhanced the spontaneous oxidation of the conducting polymer. The RS mechanism of the device was based on oxygen ion mobility, which was able to be tuned by the metal nanoparticle concentration. The Cu/Co−PPy composite/Pt device exhibited stable bipolar RS (Figure 11g), which was due to oxygen ion movement within the switching layer (Figure 11h) [182]. Electrodeposited Prussian blue [196], AAO template-based electrodeposited Bi1−xSbx [197], and electrodeposited copper tetracyanoquinodimethan [198] have also been reported for RS memory applications.  Table 7 summarizes the electrochemically fabricated metal oxides, composites, and polymer-based RS devices reported to date. There is a broad scope for synthesizing oxideand polymer-based RS materials using electrochemical approaches, leading to greater RS memory performance than existing devices. Many important properties such as surface area, power consumption, switching uniformity, and reliability need to be investigated further to fabricate high-performance memory devices. Importantly, flexible and physically transient switching layers have rarely been synthesized using electrochemical approaches. In the future, these devices will play an essential role in many applications.

Electrochemically Synthesized Resistive Switching Materials for Neuromorphic Computing Applications
In neuromorphic computing applications, electronic devices generally mimic biological synaptic properties. In biological synapses, the pre-neuron transfers an electrical/chemical signal to the post-neuron via the synaptic cleft and modulates the signal amplitude and shape of the post-synaptic spike. This process involves different biological mechanisms and is referred to as synaptic learning [199][200][201][202][203][204]. Figure 12a presents a typical biological synapse of the human brain [205]. Due to structural and mechanistic similarities, RS devices are generally used to mimic bio-synaptic properties. Artificial or electronic synaptic devices can mimic some synaptic rules and properties, including STDP, SRDP, potentiation, and depression [1]. Several electronic devices can operate as artificial synaptic devices, such as RS/memristors [92], transistors [93], and ferroelectric junctions [94]. Of these, RS/memristor devices are the most suitable candidates due to their tunable electrical properties.

Electrochemically Synthesized Resistive Switching Materials for Neuromorphic Computing Applications
In neuromorphic computing applications, electronic devices generally mimic biological synaptic properties. In biological synapses, the pre-neuron transfers an electrical/chemical signal to the post-neuron via the synaptic cleft and modulates the signal amplitude and shape of the post-synaptic spike. This process involves different biological mechanisms and is referred to as synaptic learning [199][200][201][202][203][204]. Figure 12a presents a typical biological synapse of the human brain [205]. Due to structural and mechanistic similarities, RS devices are generally used to mimic bio-synaptic properties. Artificial or electronic synaptic devices can mimic some synaptic rules and properties, including STDP, SRDP, potentiation, and depression [1]. Several electronic devices can operate as artificial synaptic devices, such as RS/memristors [92], transistors [93], and ferroelectric junctions [94]. Of these, RS/memristor devices are the most suitable candidates due to their tunable electrical properties.  Reprinted with permission from [200]. Copyright © 2023, American Chemical Society.
Electrochemically synthesized devices can also be employed as artificial synaptic devices, but limited research has been conducted in this area. In particular, electrochemically prepared metal oxides, chalcogenides, and polymers can mimic bio-synaptic properties. Recently, Park et al. reported electrochemically synthesized reduced graphene oxide (rGO) as a wearable textile for neuromorphic applications. They electrodeposited rGO on stainless conductive yarns following the electrochemical reduction of GO (Figure 12b,c) [205]. The fabricated device exhibited an excitatory post-synaptic current (EPSC) and pairedpulse facilitation (PPF) (Figure 12d). By controlling the amplitude of the applied pulse, the device could switch from STP to LTP. The device also exhibited stable neuromorphic properties under bending (Figure 12e,f), highlighting its suitability for integrated textile applications [205].
An anodic oxide is a promising option for neuromorphic applications, but only two reports of electrochemically fabricated devices containing this material are available in the literature. Chen et al. reported Pt/TiO 2−x /Tibased electronic synapse devices with a switching layer that was fabricated via anodization of a Ti substrate [206]. The electrochemical setup for anodization is depicted in Figure 12g, with the fabricated device exhibiting RS for up to 4500 switching cycles (Figure 12h). A rapid switching time (90 s) and multistate conductance were also observed for this device. PPF, paired-pulse depression, and the applied pulse waveform are presented in Figure 12i,j. The device also mimicked the potentiation and depression properties of biological synapses (Figure 12k). Choi et al. also fabricated a Pt/TaO y /nanoporous TaO x /Ta crossbar array structure via the anodization of the Ta layer in an aqueous sulfuric acid electrolyte [207]. The fabricated crossbar array structure minimized the undesired leakage current through neighboring cells. The device exhibited high non-linearity (10 4 ), low synapse coupling (4.00 × 10 −5 ), and good endurance and retention. The authors also mimicked basic and advanced synaptic properties such as LTP, LTD, and STDP. They reported that the device had an 89.08% recognition accuracy after 15 training epochs. As discussed earlier, very few reports are available in the literature related to electrochemically synthesized artificial synaptic devices; thus, they require further investigation. Table 8 summarizes the synaptic learning properties of electrochemically fabricated RS devices for neuromorphic computing applications.

Electrochemically Synthesized Resistive Switching Materials for Sensing Applications
Due to rapid industrialization, pollution has emerged as a major global challenge. Therefore, there is a need to identify specific pollutants and physical and biological entities in drinking water, food, and the air before they can harm human health, while bio-sensors are needed for the early diagnosis of health issues. In this context, several sensor technologies have been investigated to reduce costs, increase portability and sensitivity, and improve recycling capabilities. In particular, in recent years, many researchers have looked to fabricate sensors that utilize the RS effect, including those for the detection of ascorbic acid [75], drug levels [49], glucose/saliva [208], heavy metals [209,210], gas [211,212], and radiation [213]. For example, Figure 13a presents a memristive aptasensor that can sense drugs based on RS [49]. The change in electrical characteristics such as the voltage gap is a response to the change in the drug concentration and human serum (Figure 13b,c) [49]. Based on this, the drugs are analyzed both quantitatively and qualitatively.

Electrochemically Synthesized Resistive Switching Materials for Sensing Applications
Due to rapid industrialization, pollution has emerged as a major global challenge. Therefore, there is a need to identify specific pollutants and physical and biological entities in drinking water, food, and the air before they can harm human health, while biosensors are needed for the early diagnosis of health issues. In this context, several sensor technologies have been investigated to reduce costs, increase portability and sensitivity, and improve recycling capabilities. In particular, in recent years, many researchers have looked to fabricate sensors that utilize the RS effect, including those for the detection of ascorbic acid [75], drug levels [49], glucose/saliva [208], heavy metals [209,210], gas [211,212], and radiation [213]. For example, Figure 13a presents a memristive aptasensor that can sense drugs based on RS [49]. The change in electrical characteristics such as the voltage gap is a response to the change in the drug concentration and human serum (Figure 13b,c) [49]. Based on this, the drugs are analyzed both quantitatively and qualitatively. To the best of our knowledge, there have been no reports of electrochemically prepared RS materials being used for sensing applications, but the working principle of many sensors is based on electrochemical sensing. For example, a pH sensor has been developed that utilizes the RS effect at the electrolyte-oxide-semiconductor interface [214]. The device was fabricated with Pt as the bottom electrode and SiO2 as the switching layer. For electrochemical characterization, Pt was employed as the counter electrode and Ag/AgCl as the reference electrode. The authors reported that the threshold voltage of the device increased linearly with an increase in the pH from 1 to 12 with high linear sensitivity (46.3 mV/pH). The contact between the electrolyte and SiO2 layer formed an electric double To the best of our knowledge, there have been no reports of electrochemically prepared RS materials being used for sensing applications, but the working principle of many sensors is based on electrochemical sensing. For example, a pH sensor has been developed that utilizes the RS effect at the electrolyte-oxide-semiconductor interface [214]. The device was fabricated with Pt as the bottom electrode and SiO 2 as the switching layer. For electrochemical characterization, Pt was employed as the counter electrode and Ag/AgCl as the reference electrode. The authors reported that the threshold voltage of the device increased linearly with an increase in the pH from 1 to 12 with high linear sensitivity (46.3 mV/pH). The contact between the electrolyte and SiO 2 layer formed an electric double layer that led to the aggregation of ions at the interface. Under forward bias, positive ions were injected into the SiO 2 layer to form conduction filaments, and the device switched to an LRS. Under a reverse bias, the device switched to an HRS by extracting the ions from the SiO 2 layer. The change in the concentration of positive ions in an electrolyte changes the RS properties, allowing the pH of the electrolyte to be determined [214]. Overall, electrochemically prepared materials have been employed in many sensing applications. Combining the advantages of these materials with RS holds promise for the development of novel sensor designs.

Future Outlook
The present literature review has shown that the electrochemical synthesis of RS devices can reliably produce high-performance memory, synaptic, and sensor devices. However, electrochemical synthesis has not been fully explored compared to other solutionprocessable and physical deposition techniques. Very few electrochemically fabricated devices are available, and various synthesis techniques, nano/micromaterials, and device structures need to be tested to maximize the potential of this area of research. The effects of electrochemical parameters on RS properties have also not been well explored in past studies. Therefore, future research on various electrochemically synthesized materials and corresponding engineered devices is required in the form of well-planned experiments to realize high-performance RS devices for use in memory, neuromorphic computing, sensor, and security applications.
Most previous research has employed electrochemical techniques only for the fabrication of switching layers, while standard physical deposition techniques are used for the top and bottom electrodes. However, it is possible to synthesize an entire RS device using electrochemical fabrication, which would help to reduce production costs and simplify the fabrication process. One such device has already been reported by Han et al., utilizing a bottom-up approach [168]. Working on similar lines, all-electrochemically fabricated RS devices could revolutionize non-volatile memory and neuromorphic computing. In particular, electrochemically synthesized carbon nanomaterials have significant potential for use in various fields, particularly due to their strong RS behavior. One of the advantages of electrochemical synthesis is that it allows for the uniform deposition of carbon materials on conducting substrates; thus, graphene [215], graphene oxide [216], and CNTs [217] can be used as an active switching layer and top/bottom electrodes in RS devices via electrophoretic deposition or electrodeposition. While ZnO, TiO 2 , and CuO are the most commonly electrochemically synthesized materials for RS applications, the electrochemical approach also supports the synthesis of CMOS-compatible materials such as HfO 2 , TaO x , and Ta 2 O 5 . Therefore, it is recommended to synthesize these materials using a variety of electrochemical synthesis techniques.
Tuning the electrochemical parameters (such as the electrolyte precursor, electrolyte concentration, pH, deposition time, and deposition temperature) can also be used to enhance RS properties. Moreover, the morphology and/or orientation of the material can be adjusted to influence the RS properties and mechanisms [35]. Doping, metal decoration, composites, and bilayered structures have proved effective in this respect for various applications. In particular, it has been demonstrated that the decoration of metal oxides with metal [218], the use of core-shell structures [219], and the fabrication of bilayered structures can improve the RS properties of a device. Multilayered electrodeposited devices can also exhibit stronger RS properties. In addition to metal oxides, polymers [220] and metal-organic materials [221] can be deposited using electrochemical synthesis and are suitable for neuromorphic and sensing applications. However, few studies reported on electrochemically synthesized polymer-based RS devices. Therefore, further exploration of electropolymerization is recommended, as this technique has the potential to achieve impressive RS properties. In addition, PCM is an emerging RS device, and its electrochemical fabrication may pave the way for the future development of RS applications.
Most commonly employed phase-change memory materials (i.e., Ge-Sb-Te) can be electrochemically deposited onto a conducting substrate in a nonaqueous electrolyte using a three-electrode system (conducting substrate: working electrode, Ag/AgCl: reference electrode, and Pt: counter electrode Cl was added to the CH 2 Cl 2 as a supporting electrolyte, with Ge-Sb-Te ultimately electrodeposited at −1.75 V vs. Ag/AgCl. Similar to this approach, binary and ternary chalcogenides can be deposited on conductive substrates for RS applications. In addition, given that most top and bottom electrodes are either metal or conducting materials, it is possible to exploit electrodeposition to fabricate and/or pattern top and bottom electrodes. Considering the previously identified research gaps in the literature, we believe that the electrochemical approach holds significant promise for the fabrication of state-of-the-art RS devices.

Conclusions
This review outlined recent advances in electrochemically synthesized memory, synaptic, and sensor devices, with particular emphasis on their established RS mechanisms and the major challenges associated with their manufacturing processes, including material matching, electrode caps, and layers of insulator components. Research and development in recent years have focused on revolutionizing metal-oxide materials, which have been used as core components of conventional metal-insulator-metal memristors. The possible applications, prospects, and future development of various materials and their synthesis routes were outlined in this paper. Industrial manufacturing is necessary for the commercialization of RS devices, and electrochemical approaches have several features that make them commercially viable, including their ease of fabrication and low-temperature processing environment. The precise control of the thickness of the switching layer and the composition of materials by tuning the electrochemical parameters can lead to better switching voltages, endurance, retention, and switching times. The bottom-up synthesis of entire RS devices using electrochemical techniques can also be employed to develop CMOS-compatible memory, synaptic, and sensor devices. In addition to this, RS devices can be miniaturized using template-based electrodeposition techniques. Oxides, polymers, organic materials, and composite-based devices can be fabricated using electrochemical synthesis techniques. Therefore, this research area needs to be explored further for the production of high-performance RS devices that can be employed in various applications. In particular, an understanding of RS and charge transport mechanisms will help to optimize future RS devices, so new models and simulation techniques will be needed in the coming years. Finally, device design and the engineering of the device structure are some issues that require close attention. If these research directions are followed, multifunctional RS devices have the potential to be the future of electronics.