Research Update: Nanoscale electrochemical transistors in correlated oxides Teruo Kanki and Hidekazu Tanaka Citation: APL Mater 5, 042303 (2017); doi: 10.1063/1.4974484 View online: http://dx.doi.org/10.1063/1.4974484 View Table of Contents: http://aip.scitation.org/toc/apm/5/4 Published by the American Institute of Physics Articles you may be interested in Research Update: Ionotronics for long-term data storage devices APL Mater 5, 042302042302 (2017); 10.1063/1.4974480 High ionic conductivity in confined bismuth oxide-based heterostructures APL Mater 4, 121101121101 (2016); 10.1063/1.4971801 Ultra-low coercive field of improper ferroelectric Ca3Ti2O7 epitaxial thin films APL Mater 110, 042901042901 (2017); 10.1063/1.4974217 Impact of gate geometry on ionic liquid gated ionotronic systems APL Mater 5, 042501042501 (2017); 10.1063/1.4974485 APL MATERIALS 5, 042303 (2017) Research Update: Nanoscale electrochemical transistors in correlated oxides Teruo Kankia and Hidekazu Tanakaa Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan (Received 31 October 2016; accepted 28 December 2016; published online 27 January 2017) Large reversible changes of the electronic transport properties of solid-state oxide materials induced by electrochemical fields have received much attention as a new research avenue in iontronics In this research update, dramatic transport changes in vanadium dioxide (VO2 ) nanowires were demonstrated by electric field-induced hydrogenation at room temperature through the nanogaps separated by humid air in a field-effect transistor structure with planar-type gates This unique structure allowed us to investigate hydrogen intercalation and diffusion behavior in VO2 channels with respect to both time and space Our results will contribute to further strategic researches to examine fundamental chemical and physical properties of devices and develop iontronic applications, as well as offering new directions to explore emerging functions for sensing, energy, and neuromorphologic devices combining ionic and electronic behaviors in solid-state materials © 2017 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/) [http://dx.doi.org/10.1063/1.4974484] I INTRODUCTION Iontronics based on correlated electron systems is an emerging technology that aims to induce a large change of transport properties through the control of ionic behavior by external stimuli In contrast, the action on time-dependence of conductive modulation is slower Despite the slow modulation, the emergence of non-linear, plastic and/or memristive behaviors provides an opportunity to obtain new abilities in information processing, like signal flow in brain, in addition to sensing and energy devices Therefore, iontronics may become an important technology for applications beyond CMOS technology The electronic properties of transition metal oxides are quite sensitive to the orbital occupancy of electrons, and the valence numbers of transition metals are easily changed by redox reactions Thus, electron doping of these materials via a chemical route is a promising approach to realize novel chemical sensors, energy, neuro-mimetic devices, and ion-pumping applications Of the prototypical correlated electron materials, vanadium dioxide (VO2 ) is attractive because it undergoes a metal-insulator transition (MIT) and resistance change over orders of magnitude above room temperature (around 340 K) In VO2 nano- and microstructures,1–9 the coupling of the MIT with mechanical,3–5 optical,9 thermal,8 and electronic properties5,7 can be used to realize tunable resonators, optical switches, and thermo-sensing devices Furthermore, intercalation or desorption of only a few atomic percent of hydrogen or oxygen into/from VO2 dramatically changes its transport properties, equivalent to the changes caused by inducing the MIT.10–16 Conventionally, the amount of hydrogen ions (H+ )/oxygen ions (O2☞ ) in an oxide has been controlled by annealing the sample under redox gas atmosphere,11,12,17,18 in aqueous solution13,14 or with a hydrogen spillover method.10,15 All of these methods require a high temperature of at least 150 ◦ C Recently, electric fields have been used for both hydrogenation and oxidation of oxides at room temperature.16,19–21 For example, a strong electric field in an ionic liquid (IL) gate induces oxygen vacancy formation a Authors to whom correspondence should be addressed Electronic addresses: kanki@sanken.osaka-u.ac.jp and h-tanaka@sanken.osaka-u.ac.jp 2166-532X/2017/5(4)/042303/11 5, 042303-1 © Author(s) 2017 042303-2 T Kanki and H Tanaka APL Mater 5, 042303 (2017) in VO2 20 In water-containing gates composed IL or amorphous 12CaO·7Al2 O3 with a nanoporous structure,19,22 H+ can be intercalated under positive electric field Thus, electrochemical gating including water electrolysis has a potential to allow tuning of the doping level of metal oxides at room temperature In this paper, we report a strategy to achieve charge carrier doping by electrochemical ion intercalation/desorption at room temperature We systematically evaluate transport modulation in VO2 nanowires by electrochemical gating via an air gap under humid conditions and establish an ion diffusion model that considers the temporal and spatial evolution of H+ intercalation and diffusion in VO2 II OVERVIEW OF ELECTROCHEMICAL TRANSISTORS In electronics, water is usually regarded as an undesirable material that could damage precision apparatus From the viewpoint of chemistry, water contains the electrochemically active agents H+ and hydroxide ions (OH☞ ) The ionization of water to form H+ and OH☞ occurs when a bias voltage of over 1.23 V is applied at an electric double layer (EDL) on the surface of two metallic electrodes immersed in a water bath and yields molecular oxygen and hydrogen Conversely, reaction of oxygen and hydrogen generates electric power, which is used in fuel batteries As an advanced application, the electric field effect in gates coupled with water can cause redox reactions at room temperature Furthermore, intercalation of H+ into oxide materials induces hybridization with O2☞ , changing the electron occupancy and inducing dramatic variation of the conductivity of correlated electron materials Electron doping via electrochemical gating is a new approach combining chemistry and physics Some electrochemical transistors have been reported.19–22 According to Ji et al.,19 stray water in an IL gate induces large, slow, and hysteretic conductance modulation under applying gate bias This operation allows tuning of the activation energy and changes conductance by an order of magnitude, resulting in hydrogen doping through the electrochemical reaction of water In a more assertive use of water, Ohta et al.22 demonstrated conductance, capacitance, and thermopower modulation over large ranges using water-infiltrated nanoporous glass as the gate insulator on a SrTiO3 channel Regarding redox reactions promoted by IL gating, Jeong et al.20 reported suppression of the MIT in VO2 by inducing oxygen vacancies under an electric field Shi et al.23 demonstrated a colossal change of transport using a solid-state protonic transistor consisting of gate materials combined with the low-temperature proton conductor yttrium-doped barium zirconate (BYZ), yttria-stabilized zirconia with low proton conductivity as a barrier to prevent protons from leaving BYZ, and a SmNiO3 channel These on-chip devices were fabricated by general chemical methods using water baths and represent the foundation of an entirely new field combining iontronics and electronics Compared with these film-based electrochemical devices, there are no reports on nanostructured electrochemical devices In this paper, we explore the functionality of nanostructured electrochemical devices and their mechanism III PLANAR TYPE-ELECTROCHEMICAL TRANSISTOR WITH AN AIR-GAP GATE A Advantages As a suitable device structure to perform transport modulation through electrochemical reactions, we proposed a planar-type field effect transistor with side gates and a nanowire channel separated by air nanogaps (denoted PG-FET), as illustrated in Fig 1(a) The PG-FET structure has the following advantages First, an electric field can be applied under various gas and vapor atmospheres via the air nanogap In this research, air with a variety of humidity levels was used, allowing us to investigate the change of transport behavior with humidity Second, electric fieldinduced ion intercalation and diffusion can be systematically investigated because ions are intercalated from both sides of the channel edges and diffuse from the sides in the VO2 channel This allowed us to estimate ion diffusion length and doping level by measuring electronic transport behavior 042303-3 T Kanki and H Tanaka APL Mater 5, 042303 (2017) FIG (a) Typical device architecture (left) and an atomic force microscopy (AFM) image of the VO2 channel area (right) S, D, and G indicate the source, drain, and gate electrodes, respectively (b) Optical micrograph of devices over a larger area (left) and a magnified view of a part of a device (right) (c) Cross-sectional AFM image taken at the blue dashed line in (a) (taken from Ref 28) B Fabrication To fabricate PG-FET devices, 35-nm-thick VO2 films were first deposited on Al2 O3 (0001) single-crystal substrates by pulsed laser deposition using an ArF excimer laser at 450 ◦ C under an oxygen pressure of 1.0 Pa X-ray diffraction measurements confirmed that the films were crystalline with b-axis orientation without any impurity phases This is the typical growth direction of VO2 on Al2 O3 (0001).24 The VO2 films were patterned into nanowire channels with planar gates by nanoimprint lithography and etched by reactive ion etching using O2 and SF6 gases After the nanopatterning, we prepared electrode patterns using photolithography with position alignment, and Pt/Cr electrodes were deposited by radio-frequency sputtering Ohmic contacts between the VO2 films and electrodes were confirmed An advantage of this nanoimprint-based fabrication process is that we can obtain over one hundred PG-FETs with nanowire channels at once, as seen in the optical image in Fig 1(b) An atomic force microscopy image of a device is shown on the right of Fig 1(a) and the corresponding height profile is depicted in Fig 1(c) The measured height and gap distance were 35 and 400 nm, respectively Thus, our desired device structures were obtained Temperature dependence of resistance property in our VO2 devices has no changes compared with that in the thin films, showing almost three orders of magnitude changes in resistance both of thin films and nanowire devices, and sharper MIT in nanowire devices due to the appearance of MITs of nanodomains prepared by the same nano-process in Ref Thus our VO2 devices have no damage such as generation of oxygen vacancies and/or diffusion of impurity atoms during nano-fabrication process Humidity-controlled air was supplied using an air compressor into a glovebox introducing an electronic property measurement system from two channels, consisting of flow lines for humidity air through a water bottle and for dry air through a silica gel bottle We can significantly control humid levels between 20% and 80% by adjusting flow rates of the two lines 042303-4 T Kanki and H Tanaka APL Mater 5, 042303 (2017) C Transport properties under humid conditions A feature of the PG-FETs is that the electric field converged at the edges of the VO2 channel Thus, the strongest electrochemical reactions occurred at the channel edges To analyze the electric field distribution in the devices, numerical simulations were performed with the finite element method using AMaze (Advanced Science Laboratory, Inc.) A three-dimensional device geometry was assumed, as shown in Fig 2(a) Figures 2(b) and 2(c) depict cross-sectional potential and electric field maps between a gate and channel, respectively, using typical experimental parameters We quantitatively confirmed that the electric field converged at the channel edges At the channel edge, the magnitude of the field was approximately twice because of a geometric effect, reaching up to 4.5 × 106 V/cm at a gate voltage (V G ) of 100 V despite that of a typical parallel plane only being 2.5 × 106 V/cm Figure 3(a) shows the reversible, non-volatile resistance changes in a VO2 nanowire channel with a width (w) of 500 nm obtained by applying positive and negative V G at 300 K under a humidity of around 50% The normalized resistance (R/R0 , where R and R0 are the measured resistance and resistance of the pristine device before applying a V G at 300 K, respectively) slowly decreased down to the saturation line at roughly R/R0 = 0.75 during the application of V G = +100 V This state was held after the removal of the V G Namely, the device exhibited a non-volatile memory effect The R/R0 increased again with applying V G = ☞100 V The origin of such a slow resistance decrease is sometimes related to mechanical relaxation or slow trapping through a purely electrostatic effect.25–27 In our device operating under dry air conditions in Fig 3(b), such slow changes in resistance were not observed; instead, a steep, small resistance switching of 0.06% occurred, as illustrated in the inset of Fig 3(b) Thus, the origin of the slow decrease in resistance under humid air conditions is probably not caused by mechanical relaxation or slow trapping from an electrostatic effect but by electrochemical reaction with intercalated H+ ,19,22,27 which can substantially lower the resistivity in systems with sensitive 3d orbitals.23 Namely, when applying V G under humid conditions, an EDL of absorbed water is formed on the device With increasing VG , electrolysis of the absorbed water FIG (a) Device geometry used for the static electric field simulation The device area was (x, y, z) = (500 nm, 200 nm, 200 nm), which was divided as: (dx, dy, dz) = (6 nm, 2.5 nm, 2.5 nm) The relative permittivity of Al2 O3 was 8.5 The potential was 100 V at the gate (G) and the channel (C) was grounded (b) Cross-sectional potential map at a gate voltage of 100 V through the 400 nm vacuum gap between G and C on an Al2 O3 substrate under vacuum (c) Cross-sectional electric field map 042303-5 T Kanki and H Tanaka APL Mater 5, 042303 (2017) FIG Time dependence of the normalized resistance (R/R0 , where R0 is the resistance of an untreated VO2 channel at 300 K with applied gate voltage (V G ) values of 100, 0, and ☞100 V) in (a) humid air and (b) dry air The green dashed line in (a) roughly indicates the saturation of R/R0 The inset in (b) is a magnified view (c) V G dependence of R/R0 after applying V G for 20 (d) V G dependence of the current between the gate and source electrodes (IGS ) under a humidity of 60% (blue dotted line) and in dry air (black dotted line) The relative elemental ratios for hydrogen normalized by oxygen in (e) a device after applying V G = 100 V and (f) a pristine device The solid and dashed green lines represent the averages of the hydrogen atom profiles and standard deviations, respectively These data were taken from Ref 28 starts over a threshold V G , and H+ are produced Some H+ are simultaneously intercalated into the VO2 nanowire from edge sides in the cathode.28 The voltage that this process begins at is defined as the threshold voltage (V th ) After the intercalation of H+ , strong H–O bonds are formed and electron transfer occurs from hydrogen onto the oxygen atom, changing the 3d-orbital occupancy of vanadium from V4+ (3d ) to V3+ (3d )10 and resulting in dramatic transport modulation Possibility of generation and/or migration of oxygen vacancies under applying an electric field in VO2 nanowire channels can be eliminated because the resistance in the devices does not change in dry air condition as shown in Fig 3(b) The generation and/or migration of oxygen vacancies by applying biases are independent of humid condition.20 On the other hand, the resistance changes in our devices strongly depend on humidity, which is the evidence that oxygen vacancies not play a main role in the resistance changes Figure 3(c) shows the V G dependence of R/R0 after applying each V G for 20 to investigate the magnitude of the resistance changes for a variety of V G We found that V th was approximately 20 V and the magnitude of the resistance changes was enhanced with increasing V G Along with the resistance changes, the current between the gate and source electrodes (I GS ) suddenly increased at around 20 V, as indicated in Fig 3(d), corresponding to the V th in Fig 3(c) Subtracting the current recorded under humid conditions from that recorded under dry conditions ER ) gives the current generated by the electrolysis of water Accordingly, the density of generated (Igc + H increased with rising V G and the number of intercalated H+ in VO2 increased As a characteristic feature of the PG-FET structure, spatial and local elemental analysis can be easily conducted Figure 3(e) thus shows the spatial mapping of the ratio of hydrogen elements in a device after applying V G = 100 V investigated by time-of-flight secondary ion mass spectrometry (Tof-SIMS) The hydrogen content in the VO2 channel is higher than that in other areas in a device after applying the V G , although the hydrogen content is roughly averaged because the spatial resolution of the measurement was several hundred nanometers, whereas the hydrogen content remained unchanged in a pristine device (Fig 3(f)) These data provide direct evidence for H+ intercalation into the VO2 channel 042303-6 T Kanki and H Tanaka APL Mater 5, 042303 (2017) FIG Time dependence of the normalized resistance (R/R0 ) at humidity levels of (a) 20%, (b) 40%, and (c) 60% (d) Normalized resistance after applying V G for 20 under various humidity levels from 20% to 60% (e) Humidity level dependence of V th Regarding the humidity dependence of V th , Figures 4(a)–4(c) show the resistive behavior of the devices under 20%, 40%, and 60% humidity, respectively, at a variety of V G At 20% humidity, R/R0 decreases more slowly and the magnitude of modulation is smaller even at a high V G of 75 V than those at higher humidity levels The R/R0 values after applying V G for 20 under different humidity are plotted in Fig 4(d) We determined V th , as indicated by the arrows, and found that V th decreases with increasing humidity level Figure 4(e) clearly indicates this tendency D Mechanism on H+ intercalation and diffusion by electrochemical gating The amount and diffusion of intercalated H+ strongly affect the resistive behavior of a device Based on empirical estimations, it is known that 1% hydrogen intercalation per VO2 unit cell causes resistivity to decrease by almost one order of magnitude.13,14 The external H+ react stochastically at the VO2 –air interface The reaction rate depends on the external H+ concentration (nH+ ) generated by ER , promoting the formation electrolysis of the absorbed water The nH+ tends to increase with rising Igc + of HVO2 Meanwhile, intercalated H are partially removed from VO2 The mechanisms of ion intercalation and diffusion are illustrated in Fig In the PG-FETs, intercalation proceeds from the channel edge under applying V G and H+ diffuse into the VO2 channel Assuming the bidirectional FIG Schematic of ion intercalation at the interface between air and VO2 resulting from chemical kinetics and the diffusion area (gray region), which was found using Fick’s diffusion model under an electric field (E(x)) derived by the Poisson equation The simulated channel resistance was calculated using the parallel resistor model for the intercalated resistivity (ρIH ) and nonintercalated resistivity (ρ0 ) and by taking each width into consideration, as shown on the right side (Taken from Ref 28) 042303-7 T Kanki and H Tanaka APL Mater 5, 042303 (2017) reaction of intercalation and desorption of H+ , the dependence of the concentration of intercalated ions inside VO2 at the VO2 –air interface (ninter ) can be written as a differential equation with respect to time (t), dninter = k1 nH+ − k2 ninter , (1) dt where k and k are the forward and reverse reaction rate constants, respectively, and depend on the activation energy at the VO2 –air interface and temperature Next, we consider how the intercalated ions diffuse in VO2 As a general theory of ion diffusion, the ionic fluxes likely arise from the gradients of ion concentration and electric field in solid-state materials.29 Thus, as nHVO2 is the H+ concentration in VO2 , the H+ flux (JHVO2 ) can be described as follows: JHVO2 = −D∇nHVO2 + µEnHVO2 , (2) where D is the diffusivity, µ is the mobility, and E is the internal electric field in VO2 The first and second terms represent ion diffusion induced by the ion concentration gradient and electric field, respectively The E resulting from V G is screened by mobile electrons in VO2 according to the Poisson equation, given as a function of the distance (x) from the VO2 –air interface (x = 0), namely, (x0 − x), where e is the elementary charge, N is the carrier density in VO2 , and ε r and ε E (x) = εeN εr are the relative permittivity of VO2 and permittivity of a vacuum, respectively x can be expressed as εr VG a function of V G as εeN , where d is the distance between the gate and channel N and ε r depend d on the magnitude of intercalated H+ , while N0 /ε r can be treated as a constant because the change rate of ε r is roughly proportional to that of N 30 Thus, the length of x would be determined only by the magnitude of V G ; E(x) decreases linearly as a function of x and becomes zero at x , as shown in Fig As evidence for this bidirectional intercalation and desorption of H+ , Fig indicates the resistance behavior in VO2 nanowires (w = 500 nm) as a function of temperature In the resistance measurement without V G , the resistances between the values before and after correspond to point “A” in the inset of Fig In contrast, after applying a V G of 100 V for 20 min, the resistance approximately halved to “B” in Fig This lower resistive state remained even following the removal of V G , consistent with the memory effect observed in Fig 3(a) Thereafter, as the temperature increased to 380 K over the transition temperature without V G , the resistance decreased accompanied by the MIT In this process, H+ diffuses and leaves the VO2 channel more easily with increasing thermal energy.13,14,19 When the temperature was returned to 290 K, the suppressed resistance state recovered to its original state at point “A” in Fig The recovery of resistance induced by thermal energy indicates the desorption of H+ from VO2 This experimental observation qualitatively follows Eqs (1) and (2) For a more quantitative understanding, it is necessary to conduct an unsteady state analysis ∂JHVO ∂nHVO According to Fick’s second law for a one-dimensional system, namely, ∂t = − ∂x , the time FIG Temperature dependence of resistance in a VO2 channel at zero gate voltage (V G ) after applying V G = 100 V for 20 at 290 K under a humidity of 50% The inset shows the regular temperature plotted against resistance without V G The initial resistance at 290 K is located at point “A.” 042303-8 T Kanki and H Tanaka APL Mater 5, 042303 (2017) and spatial evolutions of ion concentration under unsteady states can be predicted and the following equation can be obtained: ∂nHVO2 ∂ nHVO2 eµN0 ∂ nHVO2 − (x0 − x) = −D nH2 + ∂t εr ε0 ∂x ∂x (3) To evaluate the time and spatial evolution of the H+ concentration in VO2 according to Eqs (1) and (3), numerical analysis with the finite difference method was carried out Before the calculations, Eq (3) was changed to i ∂nHVO ∂t through =D i+1 i i−1 nHVO − 2nHVO + nHVO 2 following ∆x transformations: nHVO (x+∆x,t)−2nHVO (x,t)−nHVO (x−∆x,t) + eµN0 εr ε0 ∂nHVO2 (x,t) ∂x xC i xC i−1 + 1+i− − i nHVO n ∆x ∆x HVO2 = nHVO2 (x,t)−nHVO2 (x−∆x,t) ∆x and (4) ∂2 nHVO2 (x,t) ∂x i−1 nHVO2 rep- i 2 = Then, x was replaced with i∆x, where nHVO and ∆x 2 resent nHVO2 ((i − 1)∆x, t) and nHVO2 (i∆x, t), respectively The calculation procedure and parameters used are described in detail in Ref 28 The simulated curves of H+ concentration behavior as a function of time were derived using Eq (4) As an example, Fig 7(a) shows the results calculated at 30 V G = 38 V The equations derived from nHVO (t) to nHVO (t) were obtained The H+ concentration at 2 the VO2 –air interface, nHVO (t), quickly increased as soon as V G was applied, and the ions slowly diffused into VO2 , propagating to a depth of approximately nm after 10 Ion intercalation into VO2 proceeded from both edges, so a parallel resistor model shown on the right of Fig was used to determine the resistivity in the intercalated (ρIH ) and nonintercalated (ρ0 ) parts in Fig Assuming 1% hydrogen intercalation per VO2 unit cell induces a decrease of resistivity by one order of magnitude,13,14 R/R0 could be evaluated using the following equation: −1 m ρ0 R x0 + (w − 4x ) = 4 × , (5) i=0 ρi R0 ρ0 w m IH where ρiIH is the part of the spatially divided resistivity in the diffuse area using 2x to consider ion −ni diffusion by the ion concentration gradient, which is divided into i, and is given as ρ0 × 10 HVO2 , where ρ0 is the initial resistivity without V G at room temperature The experimental result in the inset i FIG (a) Simulation of the time evolution in each nHVO (i = to 30) defined as the H+ concentration in VO2 at a gate voltage (V G ) of 38 V (b) Time dependence of the simulated normalized resistance (R/R0 ) at different V G for VO2 channels with a width of 500 nm The inset shows the experimental data obtained under a humidity of 60% at 300 K 042303-9 T Kanki and H Tanaka APL Mater 5, 042303 (2017) FIG Spatiotemporal evolution of the hydrogen ion concentration at gate voltages (V G ) of (a) 27 V and (b) 38 V x = indicates the VO2 –air interface (taken from Ref 28) Time dependence of (c) resistance and (d) hydrogen ion diffusion at V G = 100 and V of Fig 7(b) showing the time dependence of R/R0 at a variety of V G under 60% humidity is reasonably reproduced by the simulation in Fig 7(b) with k , k , and µ as fitting constants However, better fitting requires more consideration of the setting parameters, for example, considering the dependence of k and k on V G , which enhances the decrease of resistivity with increasing V G , and more precise resistive simulation, which could be realized using a random resistor network Nevertheless, this simulation includes the important elements of ion diffusion behavior Spatiotemporal evolution of ion diffusion in VO2 is shown in Figs 8(a) and 8(b) for V G = 27 and 38 V, respectively, where x = indicates a channel edge H+ concentration increases FIG (a) Time dependence of the simulated normalized resistance (R/R0 ) values for VO2 channels with a width of 400, 1500, and 3000 nm at a gate voltage (V G ) of 100 V The left and right insets show a magnified view of (a) and the simulation results, respectively (taken from Ref 28) (b) Dependence of R/R0 on channel width (w) The blue line and red squares indicate the curve calculated using Eq (5) and the saturated R/R0 points for w = 400, 1500, and 3000 nm in (a), respectively 042303-10 T Kanki and H Tanaka APL Mater 5, 042303 (2017) with both time and V G Within the framework of this model, H+ accumulates in an inner area of VO2 , as clearly observed in Fig 8(b) at V G = 38 V This is caused by the continuous non-equilibrium states of ion intercalation and diffusion under an electric field In more detail, the H+ accumulation is induced by the slower ion-diffusion rate in the inner part of VO2 because of the decrease of internal electric field with increasing x As an example of this scenario, following the removal of any V G , the accumulated H+ fades away over time because of the ion diffusion induced by the concentration gradient Finally, the H+ concentration becomes homogeneous and the equilibrium state is reached, as shown in Fig 8(d) In the process of approaching a homogenous ion concentration after removing V G , resistance continues to decrease moderately, as seen in Fig 8(c), reproducing the experimental resistance behavior observed in the region without V G in Fig 3(a) Thus, this device behaves as a kind of proton pump using solid-state materials activated under non-equilibrium states by applying a gate bias E Dramatic resistance modulation in narrower nanochannel The model introduced in Section III D predicts that the magnitude of the resistance decrease will be enhanced with decreasing w because VO2 –air interface diffusion caused by the electrochemical gating from the channel sides becomes more prominent as w decreases Figure 9(a) shows the time dependence of R/R0 for devices with a variety of wire widths (w = 400, 1500, and 3000 nm) at V G = 100 V and 300 K The saturation values for R/R0 increased with decreasing w and the sharpness of the resistance decrease in the initial process differed between the three devices, as seen in the left inset of Fig 9(a), which is a magnified view of Fig 9(a) from to This behavior is in agreement with the simulation depicted in the right inset of Fig 9(a) Figure 9(b) shows the dependence of R/R0 on w The curve calculated from Eq (5) clearly indicates that a narrower w enhances the modulation effect The experimental data follow this calculated curve A sufficiently narrow channel can thus provide perfect electrochemical gating to induce the MIT over the whole channel area IV CONCLUSIONS Our results indicated that an air nanogap can work as an electrochemical reaction field even in a gaseous atmosphere The intercalated ions originating from the atmosphere reversibly changed the physical properties of VO2 The electrochemical effect from the channel edge was enhanced as the channel width decreased This work offers a new way to investigate the fundamental chemical and physical relationships between ion diffusion and electron transport through their effects on intercalation and non-equilibrium ion diffusion under an applied gate bias The method used here will be applicable to a wide range of materials Regarding future applications, devices combining iontronics and electronics will offer new directions to explore emerging functions for sensing, energy, and neuromorphologic devices ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research A (No 26246013), a Grant-inAid for Challenging Exploratory Research (No 16K14387) from the Japan Society for the Promotion of Science (JSPS), and the Nanotechnology Platform Project (Nanotechnology Open Facilities in Osaka University) from the Ministry 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MATERIALS 5, 042303 (2017) Research Update: Nanoscale electrochemical transistors in correlated oxides Teruo Kankia and Hidekazu Tanakaa Institute of Scientific and Industrial Research, Osaka University,... occupancy and inducing dramatic variation of the conductivity of correlated electron materials Electron doping via electrochemical gating is a new approach combining chemistry and physics Some electrochemical. .. gating including water electrolysis has a potential to allow tuning of the doping level of metal oxides at room temperature In this paper, we report a strategy to achieve charge carrier doping by electrochemical