Role of Tellurium Ions for Electrochemically Synthesized Zinc Telluride 2D Structures on Nonconductive Substrate

Although electrodeposition has emerged as a promising approach to make metal chalcogenide nanostructures, it has an underlying issue of exfoliating the deposits affixed to a conductive substrate, which is inevitable to transfer electrons for a reduction reaction, for precise characterization and advanced device fabrication. Herein, direct electrodeposition of metal chalcogenides on a silicon dioxide (SiO2) insulator and its device applications for a back‐gated field‐effect‐transistor and a nitrogen dioxide gas sensor are investigated. Tellurium metal nanorods are deposited on SiO2 by the redox reaction of tellurium substances in the electrolyte. Using underpotential deposition, zinc telluride (ZnTe) is propagated onto tellurium sites, which has deposited on SiO2, bridging the microgap electrode on SiO2. The growth mechanisms of ZnTe on the SiO2 are also explored. This finding addresses the major challenge associated with the electrodeposition by the successful deposition of complex chalcogenides on an insulating substrate that expands its applications in fields for advanced electronics.

Over the years, electrochemists have studied electrodeposition on nonconductive materials to overcome this limit. Previous studies are classified into two types based on strategy: one is charge transport through an insulator and the other is propagation from a conductive electrode onto the insulator. Arrington et al. electrochemically deposited clusters of copper dots on the native oxide of a silicon (Si) wafer by modifying its surface with polyamidoamine dendrimers to directly deliver electrons through the native oxide [45] Lee et al. synthesized palladium nanoparticles on SiO 2 using an indirect charge transport method by H-atoms, which is a mediator to deliver the electron through the SiO 2 substrate. [46] A few researchers suggested that a conducting seed material be introduced on the insulating substrate as a place where the deposit starts to develop and laterally propagates to form thin film structures finally. Weng et al. introduced a palladium seed layer to assist copper growth on a glass substrate [47] Chiaki et al. modified the surface of the SiO 2 with a self-assembled monolayer to enhance the lateral growth of gold thin films [48] These resulted in poor thickness Although electrodeposition has emerged as a promising approach to make metal chalcogenide nanostructures, it has an underlying issue of exfoliating the deposits affixed to a conductive substrate, which is inevitable to transfer electrons for a reduction reaction, for precise characterization and advanced device fabrication. Herein, direct electrodeposition of metal chalcogenides on a silicon dioxide (SiO 2 ) insulator and its device applications for a back-gated field-effect-transistor and a nitrogen dioxide gas sensor are investigated. Tellurium metal nanorods are deposited on SiO 2 by the redox reaction of tellurium substances in the electrolyte. Using underpotential deposition, zinc telluride (ZnTe) is propagated onto tellurium sites, which has deposited on SiO 2 , bridging the microgap electrode on SiO 2 . The growth mechanisms of ZnTe on the SiO 2 are also explored. This finding addresses the major challenge associated with the electrodeposition by the successful deposition of complex chalcogenides on an insulating substrate that expands its applications in fields for advanced electronics.

Introduction
Due to their superior and versatile properties, nanostructured metal chalcogenides have been touted as highly promising materials for device applications, such as thermoelectric, [1][2][3][4][5][6] www.advmatinterfaces.de uniformity of deposits due to the imbalance of charge from the lack of charge transport. The uniformity of the deposits was improved by introducing the lithographically patterned photoresist (PR) and nickel patterns on a glass substrate. [49] The nickel pattern was introduced as a conducting electrode to deposit gold nanowire on the glass substrate. Despite the previous attempt to overcome the limits of electrodeposition, only a few studies about the device application of electrodeposited semiconductor nanofilm on the insulator have been reported.
Electrochemical behaviors of tellurium in electrolytes have been fairly studied because of their various oxidation states from +6 to −2, depending on the applied potential and pH of the electrolyte. Danaher et al. reported the reduction behavior of tetravalent state tellurium ions in acidic electrolyte. Telluryl ions (HTeO 2 + , +4) formed from a TeO 2 in an acidic solution were reduced to zero valent tellurium metal (Te, 0), and then tellurium was reduced again to hydrogen telluride (H 2 Te, −2) at a sufficiently negative potential. [50] The redox reaction between HTeO 2 + and H 2 Te, resulting in the formation of zero valent Te has been widely reported in previous studies. These studies observed Te chemically formed by indirect electrical analyses [50,51] or color change of electrolyte near the electrode coming from the black Te particle formation. [52] Although the Te particles were synthesized in the electrolyte homogeneously or on the electrode heterogeneously, chemically generated Te has been the focus of investigations into its deposition mechanisms rather than practical device applications.
In this paper, we suggest a straightforward strategy for the on-chip fabrication of metal tellurides on nonconductive substrates by electrodeposition. To deposit tellurium on the insulator by the redox reaction of tellurium substances, we reconfigured a working electrode consisting of patterned conductive electrode (gold) on a nonconductive substrate (SiO 2 ). We used the propagation deposition of the ZnTe from the electrode onto insulator for the deposition of metal chalcogenides. ZnTe was successfully deposited on the insulator using this redox reaction and propagation deposition (RRPD) strategy. This approach overcomes the limitation of substrate selection in the electrodeposition process and extends the utilization of electrodeposition.

Redox Reaction of Tellurium Substances on SiO 2
In terms of charge transfer, it is challenging to electrodeposit materials on an insulating substrate because a conducting substrate seems necessary to transfer the electrons for the reduction of ions. Although electrodeposition on the SiO 2 seems counterintuitive, it can be explained by the unique role of tellurium ion substances in the redox reaction. Figure 1 schematically represents the redox reaction of tellurium ions. Without an applied potential, tellurium ions (IV) (formed from the dissolution of Figure 1. A schematic illustration of the redox reaction of tellurium ion species. A) Reduction of telluryl ion (IV) to tellurium (0) with a negative potential. B) Overall reduction reactions on gold electrode and redox reaction on SiO 2 with a more negative potential. C) Magnification of reduction reactions on gold electrode in (b). D) Magnification of redox reaction of telluryl ion (IV) and hydrogen telluride (−II) on SiO 2 . Light green, dark gray, and red molecules are telluryl ion (IV), tellurium metal (0), and hydrogen telluride (−II), respectively.

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TeO 2 in the electrolyte) were absorbed on the gold electrode and the SiO 2 . This adsorption is strong on a variety of electrode and oxide surfaces [53][54][55] With the application of a negative potential ( Figure 1A), tellurium ions (HTeO 2 + (IV)) in the electrolyte were reduced to zero-valent tellurium metal (0) on gold electrodes by the following reaction [50,52,[55][56][57][58][59][60] HTeO 3H 4e Te 2H O 2 2 Under an overpotential, HTeO 2 + (IV) in the electrolyte is reduced by accepting four electrons. Figure 1B illustrates the overall situation of the reduction reactions on the gold electrode and redox reaction on the SiO 2 with high negative potential. First, in the enlarged illustration of the reaction at the gold electrode ( Figure 1C), tellurium metal (Te (0)), which is deposited on a gold electrode according to Equation (1), is further reduced to hydrogen telluride (H 2 Te (−II)), corresponding to the following reaction [50,52,[55][56][57][58][59][60] Te H 2e H Te Reactions (1) and (2) occur simultaneously at the gold electrode when a strong negative potential is applied and these reactions have been extensively studied by electrochemical analyses in previous literature. These reactions were confirmed with linear sweep voltammetry (LSV) on the gold electrode in the electrolyte containing 0.2 mm tellurium ion ( Figure S1, Supporting Information), and cathodic peak A (−0.37 mV vs Ag/ AgCl) was ascribed to the reduction of tellurium by Reaction (1). At this potential, a dark-gray film of zero-valent tellurium on the gold electrode was observed. Reaction (2) started at 0.56 V versus Ag/AgCl, indicating the reduction of tellurium metal to hydrogen telluride. Hydrogen telluride is dissolved in water and forms different substances depending on pH, such as H 2 Te, HTe − , and Te 2− [52] Hydrogen telluride generated at the gold electrode is diffused by the concentration gradient from the electrode onto the SiO 2 . Second, at the surface of the SiO 2 ( Figure 1D), H 2 Te (−II) reacts with HTeO 2 + (IV) to form Te (0) on the surface of SiO 2 by transferring four electrons per tellurium atom. This redox reaction of tellurium substances can be expressed as follows [50,51,[55][56][57]60] 2H In this redox reaction, each H 2 Te (−II) is oxidized by losing two electrons per atom and HTeO 2 + is reduced by accepting four electrons per atom, resulting in the deposition of three tellurium atoms on SiO 2 . In the presence of telluryl ions (IV), this redox reaction rapidly occurs due to the instability of Te (−II) substances. [52,59,61] Also, the strong adsorption of telluryl ions (IV) on SiO 2 would help Te (−II) react mainly on the surface of the SiO 2 . As a result of this redox reaction on the SiO 2 , the tellurium metal is deposited on the surface of the SiO 2 insulator without direct current transport.
As described in Figure 1 and Figure S1 in the Supporting Information, these reactions can be controlled by the applied potential on the gold electrode and Figure 2 shows the electrodeposits achieved at various applied potentials. Figure 2A shows a pristine 200 nm thick gold electrode patterned on SiO 2 . According to the results of Te ion LSV in Figure S1 in the Supporting Information, −0.55 V versus Ag/AgCl was chosen as the potential for the tellurium metal reduction (Reaction (1)). This was enough for the reduction of Te (0) on the gold electrode (peak A in Figure S1, Supporting Information) but not for the reduction of Te (0) to H 2 Te (−II) (peak B in Figure S1, Supporting Information). At an applied potential of −0.55 V versus Ag/AgCl ( Figure 2B), tellurium (Te (0)) was only observed on the gold electrode, not on the SiO 2 . When the tellurium metal was reduced to hydrogen telluride (−II) at a more negative potential (−0.57 V, which is the onset potential of Reaction (2)), Figure 2C reveals that few tellurium nanorods were deposited on the SiO 2 surface due to the redox reaction (Reaction (3)). In the low-and high-resolution transmission electron microscopy (HRTEM) images ( Figure S2, Supporting Information), tellurium nanorods deposited on the SiO 2 showed similar nanorod structures and interplanar spacing values between adjacent lattice fringes agreed with previous literature. [62,63] In the result with the applied potential of −0.60 V versus Ag/AgCl ( Figure 2D), hydrogen telluride was generated more actively, resulting in the formation of the tellurium nanorod clusters on the SiO 2 . These results demonstrate that the redox reaction of tellurium substances plays a key role in the RRPD process to synthesize tellurium www.advmatinterfaces.de metal on the insulator. In Figure S3 in the Supporting Information, the density and length of Te nanorods on the SiO 2 was reduced as the distance from the Au electrode increased. This is not surprising since hydrogen telluride is the only charge mediator to reduce Te (IV) on the SiO 2 and this is affected by the distance from the gold electrode. Tellurium nanodots were observed 20 µm from the gold electrode, meaning that the nuclei would grow with the lowest surface energy structure as a dot due to a lack of hydrogen telluride reactant.

Propagation Deposition of ZnTe on Tellurium Nanorods
The propagation deposition of ZnTe on the tellurium nanorods utilizes a unique reaction called underpotential deposition (UPD). This is the electrodeposition of species at a potential less negative than the equilibrium potential for the reduction of this metal. Figure 3 represents the deposition of ZnTe on the SiO 2 by the RRPD process. For the propagation deposition step in RRPD, 0.05 m zinc nitrate for the supply of zinc ions was simply added to the tellurium electrolyte used in the previous section. At a potential of −1 V versus Ag/AgCl (which is an insufficient potential for the direct reduction of zinc ions to zinc metal) the UPD of ZnTe and the generation of hydrogen telluride occur so that the ZnTe was reduced on the gold electrode according to Reaction (4) [64][65][66] Zn Cit Te 4H 2e ZnTe 2H Cit In Reaction (4), the tellurium site plays a key role in the UPD reaction, resulting in an insufficient potential for the direct reduction of zinc ion to zinc metal. Morphological changes of the ZnTe deposits on the SiO 2 were indicated as a function of the deposition time. At the beginning of the deposition (40 s), needle-like Te nanoparticles with a size of less than 50 nm were observed on the SiO 2 ( Figure 3A). The columnar growth of the Te nanoparticles produced by Reaction (3) seemed to appear randomly on the surface with a preferred growth direction. After 80 s ( Figure 3B), the lengths of the needle-like nanoparticles approximately doubled, showing an average length of 100 nm and diameter of 10 nm. As their length increased, the nanorods were linked to each other, allowing physical connections. Once the tellurium nanorods were connected to the gold electrode, they served as an electron channel. After time passed (160 s), tellurium nanorods acted as an extended current path of the electrons supplied from the gold electrode, followed by the UPD of ZnTe, Reaction (4), at the tellurium site deposited on SiO 2 . Due to the deposition of ZnTe, the surface morphology of the nanorods became rough and no longer maintained sharp tips ( Figure 3C). After 300 s ( Figure 3D), the SiO 2 surface was fully covered by the ZnTe electrodeposit consisting of nanosized grains. It seems the nanorods served as the current path and the frame to form thin films on the SiO 2 surface. Figure 3E schematically illustrates the RRPD process of ZnTe on SiO 2 ,

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indicating the redox reaction of the tellurium nanorods for the extended current path and the propagation deposition of the ZnTe thin film by the UPD on the tellurium nanorod sites. According to transmission electron microscope (TEM) images of the deposits close to the gold electrode ( Figure 3F), the ZnTe forming core-shell structure (ZnTe/Te) was the result of the UPD reaction on tellurium nanorods sites. The ZnTe deposits gradually propagated onto other tellurium nanorods on the SiO 2 . The thickness of the ZnTe shell of the tellurium nanorod became thinner as the nanorods were farther from the gold electrode ( Figure S4, Supporting Information). This propagation process of ZnTe suggests that the tellurium with a sufficiently low resistivity as a semi-metal can be the path for electrons. It seems the gold electrode, which already covered ZnTe consuming tellurium sites, and the tellurium nanorods offer abundant sites for the reaction of UPD, resulting the rapid propagation of ZnTe onto tellurium nanorods on SiO 2 . Even tellurium nanorods were covered by ZnTe layer, the underpotential deposition of the ZnTe could be continued by the additional electrodeposition of the tellurium on the ZnTe layer. In a word, this RRPD process represents a simple one-step electrochemical method to deposit a semiconductor material on a nonconductive substrate by utilizing various oxidation states of ion substances in the electrolyte and propagation of ZnTe onto the tellurium sites on SiO 2 .

Characterization and Device Applications of ZnTe on SiO 2
Characterization of the ZnTe thin film on the SiO 2 by the RRPD process was investigated via scanning transmission electron microscopy (STEM), HRTEM, selective area electron diffraction (SAED), and X-ray photoelectron spectroscopy (XPS). The crosssectional STEM of the ZnTe film showed Zn and Te through the energy-dispersive X-ray spectroscopy (EDS) mapping analysis ( Figure 4A). As shown in the EDS mapping, the Zn and Te elements were deposited uniformly on the SiO 2 and the intermetallic compound ZnTe was confirmed in the following analyses. At the bottom part of the film on the SiO 2 , the interplanar spacing values between adjacent lattice fringes were composed of two different crystallites of Te and ZnTe ( Figure 4B). Unlike the film deposited at the bottom part, the interplanar spacing values of the top part of the film were estimated to be 0.21 and 0.35 nm, which correspond to the (220) and (111) crystal planes of the ZnTe film ( Figure 4C). The differences in components depend on the region of the support of the RRPD process for

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ZnTe growth on the SiO 2 substrate. As described in Figure 3, the tellurium nanorods, which act as the current path, were formed on the surface of the SiO 2 only. Thus, tellurium was observed at the bottom part of the film on SiO 2 . Once the ZnTe covered the tellurium nanorods, the UPD reaction of the ZnTe on the tellurium site was controlled by the mass transport of tellurium (IV) due to the dilute concentration of tellurium ion in the electrolyte, resulting no more production of tellurium in a zero-valent metal state. Figure 4D shows the SAED pattern of ZnTe thin film on SiO 2 and the SAED pattern was captured from the entire thickness of ZnTe covering both the top and bottom part. The imperfect ring patterns corresponding to the Te crystallite revealed the formation of Te nanorods at the beginning of the deposition process on the SiO 2 substrate, as discussed in the previous sections. The chemical nature of the surface of the electrodeposited film was examined by the XPS after Ar-sputtering to remove the oxide layer on the top surface of ZnTe thin films ( Figure 4E,F) [64,67] From the XPS spectrum, the peaks at 572.56 and 582.95 eV correspond to binding energy of Te 2− (3d) and the peaks at 1044.69 and 1021.67 eV corresponded to Zn 2+ (2p), confirming the formation of intermetallic Zn-Te on the SiO 2 substrate. [68] Furthermore, zero-valent tellurium metal peaks detected at the initial state were from the redox reaction of tellurium (Reaction (3)). The film electrodeposited on the Au electrode was crystalline with a dense structure ( Figure S5, Supporting Information). The interplanar spacing values between adjacent lattice fringes were estimated to be 0.21 and 0.35 nm, corresponding to the (220) and (111) crystal planes of the ZnTe film. The agreement between the interplanar spacing values of the thin films on the Au and the SiO 2 are due to the propagation of the ZnTe thin film onto the SiO 2 through tellurium nanorods. The SAED pattern of the film also demonstrated diffraction patterns associated with its crystallinity. Clear diffraction peaks only from the electrodeposit on the SiO 2 were obtained by an electrochemical etching process. In this process, an anodic potential (+0.6 V) applied to the Au to dissolve the ZnTe thin film to zinc and tellurium ions, retaining the ZnTe on the SiO 2 only. The ZnTe film on SiO 2 remained due to a disconnected electrical path during the anodic dissolution process. Before the disconnection, the top part of the ZnTe thin film was slightly dissolved. The X-ray diffraction (XRD) of the ZnTe on the SiO 2 was analyzed ( Figure S6, Supporting Information), obtaining the diffraction peaks from the ZnTe film on the SiO 2 . In the XRD data, the most prominent peak represented the ZnTe (111) direction, which was confirmed in SAED patterns, and tellurium diffraction was also detected, in agreement with the previous characterization of the ZnTe thin film with the tellurium nanorods on the SiO 2 .
ZnTe was prepared on a designed working electrode ( Figure 5A) to investigate device applications of 2D ZnTe thin films on SiO 2 . The designed working electrode was constructed with the Au microelectrodes (3 µm gap) patterned by conventional lift-off lithography (step 1) and covered with PR with a window for the deposition of ZnTe thin films (step 2). Using the RRPD process, a 2D ZnTe thin film was deposited on both the gold electrode and SiO 2 (step 3) ( Figure S7A, Supporting Information), followed by the removal of PR for the device application (step 4) ( Figure S7B, Supporting Information).
The clean surface of a bare working electrode consisting of a Au microelectrode with a 3 µm gap was confirmed by scanning electron microscope (SEM) images (top and cross-section view) in Figure 5B,C, indicating that no residual photoresist remained after step 2. The reliability of the insulating properties of SiO 2 under high voltage was confirmed by a breakdown test ( Figure S8, Supporting Information). Accordingly, the thermally grown SiO 2 (thickness of 100 nm) has enough insulating characteristics to prevent the tunneling of electrons through SiO 2 under the deposition voltage range (≤ 1 V vs Ag/ AgCl) used in this study. After the RRPD process, the ZnTe thin film was synthesized on both the Au electrode and SiO 2 , covering the entire gap of SiO 2 and bridging the Au electrodes ( Figure 5D). As shown in the cross-sectional view ( Figure 5E), the electrodeposited film was dense and uniform throughout the surface of Au electrodes and SiO 2 without cracks and disconnections. There was variation in the thickness. Specifically, the part of ZnTe film grown on the conductive Au was thicker (200 nm) than the other part grown on the SiO 2 substrate (<100 nm). This originated from the difference in growth rate based on the different deposition mechanisms. Operation tests for an electric device and a sensing device were conducted. For the electric device, the performance of a back-gate field effect transistor (back-gated FET) was analyzed by simply utilizing the Au electrodes and Si substrate as the source/drain and back gate, respectively. Without the additional process for depositing electrodes, the electrodeposit prepared by the RRPD process could be utilized for electric and sensing device applications. Back-gated FET performance of the ZnTe on the SiO 2 was characterized by measuring source/drain current (I DS ) at a constant source/drain voltage (V DS ) of 0.5 V as a function of gate voltage (V g ) ( Figure 5F,G). As-deposited ZnTe on the SiO 2 behaved as a p-type semiconductor, having an on/off ratio of 1.97 and a hole mobility of 3.27 × 10 −3 cm 2 V −1 s −1 . After annealing at 300 °C in an Ar atmosphere for 1 h, the mobility (11.8 cm 2 V −1 s −1 ) and on/ off ratio (1.27 × 10 4 ) increased because of the improvement of the crystallinity of the ZnTe. The ZnTe thin film demonstrated sensing response signals to NO 2 gas. The adsorption of NO 2 , which involves acceptance of the electrons from the valence band of p-type semiconductor to dangling bonds of the top surface of the film, decreases the resistance of sensing materials by increasing carrier density. The response time was defined as the time required for the sensor to reach 90% of its steady state and the recovery time was defined as the period required to recover 90% of its maximum response without target gas. The electrodeposited ZnTe thin films ( Figure 5H,I) detected NO 2 gas with a sensitivity of 55-60% which is related to its p-type semiconducting properties [69] For sensitivity (142.8%/ppm), response time (0.2 min), and recovery time (17.0 min) with a low concentration of NO2 (0.4-3 ppm), the ZnTe thin films outperformed the conventional Te film NO 2 sensors films [69][70][71][72] and Te thin films were fabricated by the RRPD process ( Figure S9 and Table S1, Supporting Information). This result was studied with a computational quantum mechanical model using density-functional theory (DFT) ( Figure 7J,K), calculating the binding energies between NO 2 and Te dangling bond on the Te and ZnTe. Based on this, the top surface Te atoms of the ZnTe thin film (−0.51-−0.76 eV) had a lower binding energy with NO 2 than that of only Te itself (−0.99-−1.64 eV), leading to an easier www.advmatinterfaces.de detachment of NO 2 gas from the ZnTe thin films. Therefore, the short recovery time of the ZnTe thin film is because of an easier detachment of NO 2 gas from the top surface Te atoms of ZnTe thin film. p-Type semiconductor properties of ZnTe thin films improved sensing performance, while Te is classified as a semi-metal with a weak semiconducting property. The ZnTe thin film presented a high selectivity to NO 2 gas compared to other gases, such as ammonia (NH 3 ), water vapor (H 2 O), methanol (CH 3 OH), and toluene (C 6 H 5 CH 3 ) ( Figure S10, Supporting Information). The NH 3 gas showed positive sensitivity due to its electron-donating properties, which increased the resistance of the ZnTe thin film. With very low sensitivities of 0.1% to water vapor, methanol, and toluene, ZnTe exhibited outstanding performance and selectivity to NO 2 .

Conclusion
In conclusion, we demonstrated the electrodeposition of 2D ZnTe thin films on a SiO 2 substrate via the RRPD process. Deposition on the SiO 2 by a redox reaction was inevitably affected by the rereduction potential of tellurium metal (0) to H 2 Te (−II), and the propagation deposition of ZnTe was conducted after tellurium nanorods were physically connected to the gold electrode. Although it is impossible with conventional electrodeposition methods, we confirmed the operation of an electrodeposited ZnTe back-gated FET device and a sensor device as direct on-chip conditions without additional film transfer. The fabricated ZnTe thin film device showed intriguing sensing performance toward NO 2 gas with reliable electrical properties. In addition to the SiO 2 substrate, the RRPD process was also available on other nonconductive substrates (Al 2 O 3 and polyimide), and various metal chalcogenides were also synthesized (Figures S11 and S12, Supporting Information). This strategy overcomes the fundamental limit of electrodeposition in that it cannot be used on nonconductive substrates required for various device applications. This concept permits the direct preparation of valuable materials by electrodeposition on insulating substrates and simultaneous characterization of electrical and sensing performance.

Experimental Section
Electrodeposition of ZnTe Thin Films: Electrolyte for ZnTe electrodeposition contains 0.2 mm tellurium dioxide (TeO 2 ), 50 mm zinc nitrate hexahydrate (Zn(NO 3 ) 2 · 6H 2 O), 0.2 m citric acid anhydrous www.advmatinterfaces.de (C 6 H 8 O 7 ), and 0.3 m sodium citrate dehydrate (C 6 H 6 Na 3 O 9 ). Nitric acid (HNO 3 ) was used to adjust pH of the electrolytes to pH 4 ± 0.01. All chemicals used in this work were purchased from Sigma-Aldrich. Three-electrode system was used for electrochemical experiments. For the working electrode, microelectrodes (20 nm Ti/200 nm Au) were fabricated on Si substrate with a thermally grown 100 nm SiO 2 by a conventional lift-off process (Figure 5a, step 1). As a mask to guide for ZnTe electrodeposition on the desire location, the photoresist pattern was fabricated by photolithography (step 2). The microelectrodes patterned on SiO 2 substrate were attached to the glass plate by Al tape, and contact between the microelectrodes and Al tape was made by silver paste. Pt-coated Ti plate (20 × 130 mm 2 ) and Ag/AgCl electrode (Fisher Scientific, 13-620-53) were used as counter and reference electrodes, respectively. The electrodeposition was conducted using the ZnTe electrolyte with a constant potential of −1 (V vs Ag/AgCl) (Step 3). Electrochemical deposition system was installed in a jacketed beaker (inner diameter: 45 mm, outer diameter: 75 mm, height: 90 mm) at 80 °C. The temperature of the system was controlled by using a thermostatic circulator (JEIO tech., RW-0525G). Constant potentials for electrodeposition of ZnTe were applied by potentio/galvanostat (VersaSTAT3, AMETEK Co.). Finally, the photoresist was removed by acetone in ultrasonic to investigate further characterization and analysis (Step 4).
Characterization of Material and Electrical Properties: The morphology and chemical composition of electrodeposited ZnTe thin films was studied by SEM (MIRA3, TESCAN), TEM (JEOL, JEM-2100F) and STEM, and crystallinity was investigated by SAED. Crystal structural was characterized by XRD (Rigaku, D/MAX-2500) using Cu K α radiation (λ = 1.5406 Å) in a Bragg-Brentano (θ-2θ) configuration. Chemical bindings of elements were examined by XPS (VG SIENCTA, R3000). Before XRD and XPS analysis, an anodic stripping process was performed with a constant potential of +0.6 (V vs Ag/AgCl) after deposition to exclude a signal that came from ZnTe on Au electrodes. Electrical properties of the ZnTe thin films were characterized by measuring I-V and I 1/2 -V plots using KEYTHLY 2636A system source meter. Carrier mobility was calculated by the following equations: where m lin is the slope of linear region, L and W are the length and width of the channel, and C i is the gate insulator capacitance per unit area. Measurement of Gas Sensing Performance: To investigate the sensing performance of electrodeposited thin films on insulator, a gas sensing experiment was carried out with a customized quartz chamber, having an inlet and outlet for the gas flow of NO 2 gas. All measurements for the sensing properties (i.e., resistance changes, sensitivity, and selectivity) were performed at room temperature. NO 2 gas was carried by dry air and concentrations were controlled by flow rates using a mass flow controller from 0.4 to 3 ppm. The resistance of the sensor devices was measured by using a two-point probe method applying a voltage of 0.1 V. The sensitivity (S) of sensors was defined as: S = (R 0 − R g )/R 0 , where R g and R 0 are the electrical resistance of the sensor with and without target gas, respectively. The response and recovery time were defined as the time for the sensor to reach 90% of its maximum resistance and recover 90% of its steady-state. The selectivity of the sensor was tested by measuring the sensing performance toward various target gases (NH 3 , H 2 O, methanol, and toluene).

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.