Electrochemical gating-induced reversible and drastic resistance switching in VO2 nanowires

Reversible and drastic modulation of the transport properties in vanadium dioxide (VO2) nanowires by electric field-induced hydrogenation at room temperature was demonstrated using the nanogaps separated by humid air in field-effect transistors with planer-type gates (PG-FET). These PG-FETs allowed us to investigate behavior of revealed hydrogen intercalation and diffusion aspects with time and spatial evolutions in nanowires. These results show that air nanogaps can operate as an electrochemical reaction field, even in a gaseous atmosphere, and offer new directions to explore emerging functions for electronic and energy devices in oxides.


Section B. Static Electric Field Analysis using Finite Element Simulations
To analyze electric field distribution in the devices, numerical simulations were performed with a Finite Element Method using AMaze (Advanced Science Laboratory, Inc.). Then a three dimensional device geometry was assumed, as shown in Fig. S2a. Figure S2b shows cross-sectional potential images at the center of a device, forming the basis of Fig. 1c. Figure S1: VO 2 -based planer-gate-type electric field transistor. a and b, Optical micrographs of the devices over a wide area (a) and a magnification of part of a device (b). Section C. Resistive behavior with applied V G =100 V and without V G (=0 V) in 60 % humid air Figure S3 shows time dependence of resistive behavior with V G =100 V and 0 V in humid air condition of 60 %. We confirmed that the resistance in V G =0 V didn't change, while that in V G =100 V drastically reduced in the initial few minutes and gradually continue to decrease.

Section D. Cross-sectional elemental mapping by Tof-SIMS
To investigate the ratios of the V, O and H atoms, ToF-SIMS (time-of-flight secondary ion mass spectrometer) measurements were conducted by a technician at the Foundation for Promotion of Material Science and Technology of Japan (MST). Figure S4b-s shows the negative ion images related to V, O, H, Al and Pt, respectively, for a device with V G =100 V. A pristine device with an area of approximately 37 μm × 37 μm, is shown in Fig. S4a. The spatial resolution and step were roughly 300 nm and 150 nm, respectively.
In the detailed analysis on the relative elemental concentrations, the cross-sectional data across the channel were sampled, indicated by the diagonal red line in Fig. S4a. Figure S2: Device geometry used for the static-electric field simulation. a, The device area of (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 Al 2 O 3 is 8.5. The potential was 100 V at the gate and the channel was grounded. b, Cross-sectional potential map with V G =100 V. Figure S3: Time dependence of resistance with and without V G (100 V and 0 V) in humid air condition. Figure S5a, b, e and f shows the intensities of each element in a device after applying V G =100 V (a and b) and in a pristine device (e and f). Figure S5c, d, g and h shows the normalized ratios between V, H and O in a device after applying V G =100 V (c and d) and a pristine device (g and h). From this it was identified that H increased in the channel after V G =100 V was applied, over the standard deviation (+σ) (Fig. S5d). However, it remained almost unchanged in the pristine device (Fig. S5h). Fig S4. a, b, e and f, Cross-sectional ion maps for VO 2 , VO 3 , V 2 O 5 , VO and AlO 2 in devices after applying V G =100 V and for a pristine device, respectively. c, d, g and h, The relative elemental rates for V and H, normalized by O in a device after applying V G =100 V and for a pristine device, respectively. The solid and dashed green lines in d and h represent the averages of the H atom profiles and the standard deviations (σ), respectively.  Figure S6 shows the resistance behavior in VO 2 nanowires (w=500 nm) as a function of temperature.

Section E. The natural recovery of resistance by the thermal energy
In the resistive measurement without any V G , the resistances between the before and after values correspond to point "A" in the inset of Figure S6. 20 minutes after applying a V G of 100 V the resistance had reduced approximately by half, to "B" in Figure S6. This resistive state remained even following the removal of any V G , which agrees with the behavior in Figure 2a. 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, hydrogen ions are more easily diffusible and removable from VO 2 channel with increasing temperature 17,18,21 . When the temperature returns to 290 K, the suppressed resistive state was recovered to its original state at point "A" in Figure S6.

Section F. Details on the calculations used to obtain Fig 4c and d.
To evaluate the time and spatial evolution of the hydrogen ion concentration in VO 2 according to equations (1) and (2), numerical analysis with the finite difference method was carried out using  Figure S6: Temperature dependence of resistance of a VO 2 channel at V G =0 V after applying V G =100 V for 20 minutes at 290 K under a humidity of 50%. The inset shows the regular temperature vs resistance without V G . The pristine resistance at 290 K is located at point "A" ( +∆ , )−2 2 ( , )− 2 ( −∆ , ) ∆ . Then, x was replaced with iΔx, where 2 and 2 −1 represent n HVO2 ((i-1)Δx,t) and n HVO2 (iΔx,t), respectively. Thus, the numerical calculations could be performed using e=1.602×10 −19 C and ε 0 =8.859×10 −12 . + = × = 0.574 2 × was obtained from Fig. S8, where S inter is the number of VO 2 unit cells at the interface, written as: S inter =2.691×10 6 , which is the cross-sectional area where V G was applied (   Section G. Reproduction for the persisting resistance decrease after removal of the V G in Fig.2a by this simulation. This simulation can reproduce the persisting resistance decrease after removal of the V G . Figure S9a shows resistance drops with time at V G =100 V until 10 min. and at V G =0 V over 10 min. Even following the removal of V G the resistance continuously decreases. This behavior is due to the ion diffusion effect by the first term for concentration gradient in equation (2), though the second term can be eliminated because of V G =0. The simulated diffusion behavior is seen in Figure S9b.