Infrared-transmittance tunable metal-insulator conversion device with thin-film-transistor-type structure on a glass substrate

Infrared (IR) transmittance tunable metal-insulator conversion was demonstrated on glass substrate by using thermochromic vanadium dioxide (VO2) as the active layer in three-terminal thin-film-transistor-type device with water-infiltrated glass as the gate insulator. Alternative positive/negative gate-voltage applications induce the reversible protonation/deprotonation of VO2 channel, and two-orders of magnitude modulation of sheet-resistance and 49% modulation of IR-transmittance were simultaneously demonstrated at room temperature by the metal-insulator phase conversion of VO2 in a non-volatile manner. The present device is operable by the room-temperature protonation in all-solid-state structure, and thus it will provide a new gateway to future energy-saving technology as advanced smart window.

2 Infrared (IR) transmittance tunable metal-insulator conversion was demonstrated on glass substrate by using thermochromic vanadium dioxide (VO2) as the active layer in three-terminal thin-film-transistor-type device with water-infiltrated glass as the gate insulator. Alternative positive/negative gate-voltage applications induce the reversible protonation/deprotonation of VO2 channel, and two-orders of magnitude modulation of sheet-resistance and 49% modulation of IR-transmittance were simultaneously demonstrated at room temperature by the metal-insulator phase conversion of VO2 in a non-volatile manner. The present device is operable by the room-temperature protonation in all-solid-state structure, and thus it will provide a new gateway to future energy-saving technology as advanced smart window. 3 Thermochromic vanadium dioxide (VO2) exhibits a reversible metal-insulator (MI) transition at a critical temperature (TMI) of 68 ○ C due to the structural and electronic structure changes. 1-3 Above TMI, VO2 has a rutile-type tetragonal structure that is a metal and reflective to infrared (IR) due to the free carriers. In the low-temperature phase below TMI, the V ions form a dimer, resulting in a monoclinic structure. 4 This structural transformation accompanies a dramatic change in the 3d-band configuration with appearance of charge gap ~0.6 eV, where the VO2 changes to be an electrical insulator and transparent to IR. 5 Thus VO2 has the potential to demonstrate IR-transmittance tunable MI conversion device on a glass substrate (Fig. 1), which is expected to have a significant contribution on energy saving technology as an advanced smart window: e.g. the device can selectively regulate thermal radiation from sunlight, and function as ON/OFF power switch to control the in-house temperature, which thus greatly reduces energy consumption including light expenses and cooling/heating loads.
For the application to such device, the control of TMI in the boundary of ambient temperature is inevitable to modulate the MI transition charecteristics of VO2. Electron doping by the substitution of aliovalent metal ions, such as tungsten, at V site in VO2 can reduce the TMI down to below room temperature (RT), 6 but it is an irreversible process. The proton doping (protonation) is one of the effective ways to reversibly modulate the TMI; the proton can exist at interstial site in VO2 lattice and acts as shallow donor that donates an electron to V ion, 7 similar to the electrochromic effect in HxWO3. 8 Compared to state-of-the-art modulation techniques, such as electrostatic-charge doping 9,10 and epitaxial strains in thin films, 11 the protonation of VO2 (HxVO2) via a chemical route is the most ideal to switch the MI transition due to the intrinsic non-volatile operations. 4 However, there have been technological difficulties in the protonation of VO2, which required a high-temperature heating with hydrogen source. [12][13][14] The most appropriate route for the RT protonation is electrochemical reaction in an electrolyte, but the uptake amount of proton (H + ) into VO2 was too small to modulate the MI transition, where the proton insertion proceeded only to a value of x = 0.06 in HxVO2, 12 and the liquid-electrolyte likely causes a leakage problem that limits the application in practical use. Then we have recently demonstrated a new approach of water-electrolysis-induced protonation for VO2 epitaxial film grown on sapphire substrate by using a three-terminal TFT-type structure with water-infiltrated nanoporous glass as a gate insulator; 15,16 this device is kind of a pseudo-solid-state electrochemical cell with a nano-gap parallel plate structure composed of VO2 channel and metal gate electrode, where a gate bias application induces water electrolysis in the gate insulator and the produced H + / hydroxyl (OH − ) ion can be used to protonate / deprotonate the VO2 channel, resulting in the reversible MI phase modulation at RT. 16 This RT-protonation approach can be realized in an all-solid-state TFT-type structure, and thus the advantageous feature should lead to a practical IR-transmittance tunable MI conversion device on a glass substrate.
Herein, we demonstrate an IR-transmittance tunable MI conversion device by extending the three-terminal TFT-type structure with water-leakage-free gate insulator to a large-area VO2 film prepared on a glass substrate. Figure 2(a) schematically illustrates the device structure, which has a typical three-terminal TFT geometry composed of an active VO2 layer, a gate insulator, and source-drain-gate electrodes. The E1−E4 electrodes were used for the characterization of electronic property for the VO2 layer. Since there have been a few papers on the MI transition characteristics of VO2 5 polycrystalline film, we have examined the material properties and examined the device characteristics using the VO2 film on a glass substrate. The device structure with the 2.0-mm-square VO2 channel was fabricated on an alkaline-free glass substrate (Corning ® EAGLE XG ® , substrate size: 10100.7 mm 3 ) by pulsed laser deposition (PLD) using stencil masks. 17 A KrF excimer laser (wavelength of 248 nm, repetition rate of 10 Hz) was used to ablate ceramic target disks. The details of device fabrication are summarized in Supplementary Material. In order to fabricate the transparent device structure, which is essential to realize IR-transmittance modulation, all the films used in this study were selected from the wide-gap oxide materials. A nickel oxide/indium tin oxide (NiO/ITO) bilayer film was used as the transparent counter/top gate electrode, and F-doped SnO2 film (SnO2:F) was used as the source-drain and E1−E4 electrodes. The gate insulator consists of an amorphous 12CaO·7Al2O3 thin film with nanoporous structure (Calcium Aluminate with Nanopore, CAN); 15,18 since the 12CaO·7Al2O3 is a hygroscopic material, water vapor in air is automatically absorbed into the CAN film via the capillary action. Thus, a positive gate voltage (Vg) application between the gate and source electrodes induces electrochemical reactions such as protonation of the cathodic VO2 layer (VO2 + xH + + xe -→ HxVO2) and hydroxylation of the anodic NiO layer (NiO + OH -→ NiOOH + e -) 19 (Fig. 2(b)). As a result, alternative positive and negative Vg applications induce the reversible protonation / deprotonation of VO2 channel, modulating it from IR-transparent insulator to IR-opaque metal.  Fig. 2(a). The opto-electronic properties were characterized immediately after each Vg application, where the sheet resistance (Rs) was measured by the DC four probe method in the van der Pauw configuration and optical transmittance was measured by Ultraviolet-VIS/Near-IR microscope (Lamda 900s, PerkinElmer) and Fourier-transform IR spectrometer (FT-IR 660Plus, JASCO) with the light irradiation area of 0.2 × 0.2 cm 2 . 7 We first evaluated the MI-phase modulation by applying +Vg (protonation). Figure   3 plots the Rs of VO2 channel as a function of applied +Vg, where the retention time at each +Vg was set for 10 seconds. The Rs was largely modulated from the virgin state where M is molar mass of VO2, ρ is the film density, t is the film thickness, and F is Faraday constant, respectively). This result suggests that almost all the provided electrons at Vg up to +8 V were used for electrochemical protonation of the VO2 channel, obeying Faraday's laws of electrolysis, and that the device operation can be controlled by the current density. It should be noted that the Q continued to increase gradually at Vg ≥ +8 V, while the Rs was unchanged, suggesting that the Q observed at 8 Vg ≥ +8 V originates from the gas formation by water electrolysis at the surfaces of cathodic VO2 / anodic NiO layers. The protonated HxVO2 channel was stable under ambient conditions at RT after the +Vg application; the Rs of HxVO2 channel was unchanged for several days. Although it is necessary to test the retention-time dependence of the Rs for HxVO2 channel kept under the several conditions, the result basically supports the non-volatility of device operation due to the electrochemical protonation.

Figures 4(a) and 4(b)
sumarize the opto-electronic properties of the device. The temperature dependence of Rs ( Fig. 4(a)) was measured before and after applying Vg of +12 V (protonation) and -30 V (deprotonation) for 10 seconds alternately at RT in air.
The Rs variation with respect to negative Vg application is shown in Supplementary Fig.   S1. The Rs-T curves were measured up to 90 ○ C during the heating runs because the deprotonation of HxVO2 was previously confirmed to occur at T = 100-150 ○ C. 16 The inset plots the temperature derivative curves of d(log Rs) / dT to clearly visualize the TMI.
At the initial state, the MI transition was observed at TMI = 70 ○ C, which is defined as the peak position in d(log Rs) / dT versus T, while it disappeared by applying Vg = +12 V, indicating that the VO2 channel changes from an insulator to a metal at RT, because of the decrease of TMI below RT by the protonation. The Rs-T were reversibly modulated and recovered to initial state by applying Vg = -30 V, where two orders of magnitude modulation of Rs was observed at RT by the MI-phase conversion of VO2.
It has been reported that the protonation of VO2 (HxVO2) is thermodynamically favorable, 20 where hydrogen in VO2 tends to form an strong O-H bond with the closest oxygen 12,21 and electron transfer from hydrogen onto the oxygen atom effectively reduce the electronegativity of the phase and makes it thermodinamically stable than 9 that of pure VO2 phase. Actually, deprotonation needed thermal annealing at higher temperature than that of protonation. 7 Therefore, the difference between the +Vg (protonation) and −Vg (deprotonation) should originate from the negative free Gibbs energy and activation barrier for the surface reaction, i.e. the in-diffusion and out-diffusion of H + transport have different interfacial resistances. Compared to the device of VO2 epitaxial film on sapphire substrate, 16 the present device is operable by smaller DC voltage and shorter Vg application time, suggesting that the polycrystalline surface of VO2 film, shown in Fig. 2(c), enlarges the surface area with respect to that of the VO2 epitaxial film and enables the effective protonation of VO2 channel layer.
Then the optical transmission spectra (Fig. 4(b)) were measured. The initial device is transparent except for the weak absorption due to the transitions between the V 3d bands with crystal-field splitting at the wavelength (λ) > 500 nm. 22,23 and also due to the thin-film interference. By applying +12 V, it shows an abrupt transmittance decrease in the IR region, where the transmittance modulation ratio (ΔT) at λ = 3000 nm was 49 %, while almost no change is seen in the VIS region. These results indicate that the modulation from IR transparent insulator to IR opaque metal was successfully demonstrated by RT protonation.
We then measured the thermopower (S) of VO2 channel protonated and deprotonated at each ±Vg in order to characterize the electronic-structure change resulting from carrier doping (protonation). 24 Since the S basically reflects the energy differential of the density of states (DOS) around the Fermi level (EF), , its value changes significantly due to the electronic-structure reconstruction across the TMI. The S was measured at RT by giving a temperature difference (ΔT) up to ~4 K using two Peltier devices, where the actual temperatures of both sides of VO2 channel layer were monitored by two tiny thermocouples with tip diameter of 150 m. The schematic measurement setup for thermopower was reported in Ref. 18. The thermo-electromotive force (ΔV) and ΔT were simultaneously measured, and the S were obtained from the linear slope of the ΔV-ΔT plots. Figure 5(a) shows the relationship between S and 1/Rs, where the positive Vg up to +12 V in a +1 V step was first applied for protonation and then negative Vg up to -30 V in a -3V step was applied for deprotonation. The S were always negative, indicating that the HxVO2 layer is an n-type conductor. The |S| linearly decreased from 420 μV K -1 to 30 μV K -1 with logarithmic increase in 1/Rs was reversibly observed for the application of ±Vg, suggesting that protonation of the VO2 channel provides electrons to the conduction band, and the energy derivative of DOS near the EF becomes moderate, resulting in the consequent reduction of |S|.
Here we like to compare the present results with another electron-doping system, (V1-yWy)O2 polycrystalline films grown on glass substrates. Supplementary Figs. S2 and S3 summarize the opto-electronic properties of (V1-yWy)O2 films, where the MI transition was also modulated by W doping and the TMI was suppressed below RT at y = 0.06 (Fig. S2). The |S| of (V1-yWy)O2 film decreased with increasing y and became constant at small |S| of 30 μV K -1 (Fig. S3), which is the same with the present metallic HxVO2 film (30 μV K -1 ).
Lastly, in order to analyse the device operation, a simple bi-layer model of thermopower was applied to estimate the thickness (d) of metallic HxVO2 layer ( Fig.   5(b) In summary, we have demonstrated the IR-transmittance tunable MI conversion device, which has a three-terminal TFT geometry consisting of transparent oxide thin films of VO2 active channel, water-leakage-free CAN gate insulator, NiO counter layer / ITO gate electrode, and SnO2:F source-drain electrodes, on a glass substrate. At initial state, the device was insulator and transparent in the IR region. For +Vg application, the The present IR-transmittance tunable MI conversion device has several advantages.
The device can be fabricated on a glass substrate, which is suitable for the application to glass window; the device fully transmits IR in the OFF state, whereas it does not transmit in the ON state. Meanwhile, the device can function as ON/OFF power switch for electronic device to control the in-house temperature. Moreover, the device can be operated by RT-protonation without sealing thanks to the water-leakage-free CAN gate 12 insulator; the all-solid-state structure can resolve the liquid-leakage problem, which is a beneficial point compared to the liquid-electrolyte gated devices. 26 Although the demonstration of power saving by this device should be necessary to show the suitability for the practical application, the present device concept provides a potential gateway to a new functional device for future energy saving technologies such as advanced smart windows.