Field-induced water electrolysis switches an oxide semiconductor from an insulator to a metal

Abstract

Water is composed of two strong electrochemically active agents, H⁺ and OH^- ions, but has not been used as an active electronic material in oxide semiconductors. In this study, we demonstrate that water-infiltrated nanoporous glass electrically switches an oxide semiconductor from insulator to metal. We fabricated a field-effect transistor structure on an oxide semiconductor, SrTiO₃ , using water-infiltrated nanoporous glass--amorphous 12CaO·7Al₂ O₃ --as the gate insulator. Positive gate voltage, electron accumulation, water electrolysis and electrochemical reduction occur successively on the SrTiO₃ surface at room temperature. This leads to the formation of a thin (~3 nm) metal layer with an extremely high electron concentration (10¹⁵ -10¹⁶ cm^-2 ), which exhibits exotic thermoelectric behaviour. The electron activity of water as it infiltrates nanoporous glass may find many useful applications in electronics or in energy storage.

ARTICLE

Received 2 Jul 2010 | Accepted 19 Oct 2010 | Published 16 Nov 2010 DOI: 10.1038/ncomms1112

Hiromichi Ohta1,2, Yukio Sato3, Takeharu Kato3, SungWng Kim4, Kenji Nomura4, Yuichi Ikuhara3,5 & Hideo Hosono4

Water is composed of two strong electrochemically active agents, H + and OH ions, but has not been used as an active electronic material in oxide semiconductors. In this study, we demonstrate that water-inltrated nanoporous glass electrically switches an oxide semiconductor from insulator to metal. We fabricated a eld-effect transistor structure on an oxide semiconductor, SrTiO3, using water-inltrated nanoporous glassamorphous 12CaO7Al2O3as the gate insulator. Positive gate voltage, electron accumulation, water electrolysis and electrochemical reduction occur successively on the SrTiO3 surface at room temperature. This leads to the formation of a thin (~3 nm) metal layer with an extremely high electron concentration (10151016 cm 2), which exhibits exotic thermoelectric behaviour. The electron activity of water as it inltrates nanoporous glass may nd many useful applications in electronics or in energy storage.

Field-induced water electrolysis switches an oxide semiconductor from an insulator to a metal

1 Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya 4648603, Japan. 2 PRESTO, Japan Science and Technology Agency, 5 Sanbancho, Chiyoda, Tokyo 1020075, Japan. 3 Nanostructures Research Laboratory, Japan Fine Ceramics Center, 241 Mutsuno, Atsuta, Nagoya 4568587, Japan. 4 Frontier Research Center, Tokyo Institute of Technology, 4259 Nagatsuta, Midori, Yokohama 2268503, Japan. 5 Institute of Engineering Innovation, The University of Tokyo, 21116 Yayoi, Bunkyo, Tokyo 1138656, Japan. Correspondence and requests for materials should be addressed to H.O. (email: [email protected]).

NATURE COMMUNICATIONS | 1:118 | DOI: 10.1038/ncomms1112 | www.nature.com/naturecommunications

2010 Macmillan Publishers Limited. All rights reserved.

ARTICLE

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms1112

Water has not been utilized as an active electronic material though water has been widely applied in industry as a coolant (radiators), solvent (batteries) and pressure

medium (hydroelectric power generation). We have previously aimed to exploit the electrolysis of water1. Although the electrical conductivity of pure water is extremely low (~0.055 S cm 1 at 25 C)2, ionization of H + and OH ions occurs when the bias voltage (>1.23 V) is applied between two metallic electrodes immersed in water, as shown in Figure 1a. The ions are then attracted to the cathode and anode. Finally, H2 and O2 gases are generated on the cathode and anode, respectively, via the electron transfer on the electrode surface. As H + and OH ions, which are strong reducing/oxidizing agents for most oxide semiconductors38, are simultaneously produced in water electrolysis, one may expect that the electrical conductivity of an oxide semiconductor, which is strongly dependent on oxygen non-stoichiometry, can be modulated by utilizing the redox reaction between H + /OH ions and the oxide surface. However, water electrolysis and the redox reaction do not take place, because no electric eld can be applied on the insulating oxide surface in the rst place. Thus, the surface of the oxide must be conductive, as schematically shown in Figure 1b.

We have found that water-inltrated nanoporous glass overcomes this problem and switches an oxide semiconductor from an insulator to a metal. Electron accumulation, water electrolysis and redox take place successively on an oxide surface at room temperature (RT), leading to the formation of a thin metal layer on the oxide. We have fabricated a eld-eect transistor (FET) structure with source, drain and gate electrodes on an insulating oxide, using water-inltrated nanoporous glass as the gate insulator. First, the insulating oxide surface becomes slightly conductive by applying the gate voltage because of electrostatic charge accumulation. Then, water electrolysis occurs between the gate and the oxide surface. Finally, a redox reaction takes place between H + /OH ions and the oxide surface. As a result, a thin metal layer is formed on an insulating oxide.

The key material to make the best use of the electron activity of water is nanoporous glass. We chose amorphous 12CaO7Al2O3

(a-C12A7) with a nanoporous structure for this purpose. C12A7 is an abundant and environmentally benign material. Crystalline C12A7 becomes semiconducting9 or metallic10,11 when the clath-

rated free oxygen ions (O2 ), which are incorporated into the cage structure (~0.4 nm in diameter), are removed by chemical reduction treatment. On the other hand, amorphous C12A7 is a good electrical insulator, because amorphous C12A7 does not have a cage structure, we therefore ruled out the possibility of electrical conductivity of an

amorphous C12A7 lm. As C12A7 can be hydrated easily12, it is used commercially as a major constituent of aluminous cement. In 1987, Hosono and Abe13 found that a large amount of bubbles was generated in an a-C12A7 glass, when C12A7 melt was quenched under high oxygen pressure. In the present study, we develop a method to fabricate a-C12A7 lm with nanoporous structure (CAN, hereaer) by pulsed laser deposition.

In this study, we show that water-infiltrated CAN electrically switches an oxide semiconductor from insulator to metal, using a CAN-gated FET structure on a SrTiO3 single crystal (Fig. 2a) as a proof of concept. Although SrTiO3 is a wide bandgap (~3.2 eV) insulator, it becomes n-type conducting SrTiO3 by appropriate reducing treatments5. Conducting SrTiO3, especially when it is a two-dimensional (2D) conductor14,15, is of great importance as

an active material for future electronic devices16, because it has several potential advantages over conventional semiconductor-based electronic materials, such as transparency17, giant magnetoresistance18 and giant thermopower19. On applying positive gate voltage to the CAN-gated SrTiO3 FET, electron accumulation, water electrolysis and electrochemical reduction occur successively on the SrTiO3 surface at RT. This leads to the formation of a thin (~3 nm) metal layer with an extremely high electron concentration (10151016 cm 2), which exhibits exotic thermo-electric behaviour.

ResultsWater-inltrated nanoporous glass (CAN). The trilayer structure composed of Ti (20 nm)/CAN (200 nm)/SrTiO3 is clearly observed in the cross-sectional transmission electron microscopic (TEM) image of the CAN-gated SrTiO3 FET (Fig. 2b). Many brighter contrasts (diameter ~10 nm) are seen throughout the CAN region. Furthermore, several dark contrasts with a diameter < 10 nm are observed in the Z-contrast, high-angle, annular dark-eld scanning TEM (STEM) image of the CAN lm (Fig. 2c). Judging from these TEM/STEM images, high-density nanopores with a diameter of < 10 nm are incorporated into the CAN lm.

We subsequently measured thermal desorption spectrum (TDS) of the CAN lms to detect weakly bonded chemical species in the nanopores. Most of the desorbed species were H2O (m/z = 18, where m/z indicates the molecular mass to charge ratio; Fig. 2d). The amount of H2O up to 400 C was estimated to be 1.4 1022 cm 3, which corresponds to 41%. The bulk density of the CAN lm was ~2.1 g cm3, evaluated by grazing incidence X-ray reectivity (Supplementary Fig. S2), which corresponds to 72% of fully dense a-C12A7 (2.92 g cm3)20. From these results, we judged that moisture

Gate (ex.Ti)

+V +V

e e

e

e

GND

GND

O2 gas H2 gas

OH H+ OH H+

Anode (Pt)

Cathode (Pt)

TiOx

Conductive

MOx-

Insulating MO x

H2O

H2O

In water In water

Figure 1 | Field-induced water electrolysis switches an oxide semiconductor from an insulator to a metal. (a) Simple water electrolysis with two Pt electrodes as the cathode and anode immersed in water. H + and OH ions, which are generated by the electrolysis, become H2 and O2 gases on the anode and cathode, respectively. (b) Water electrolysis with an insulating oxide MOx, with a slightly conductive surface MOx. Similar to a, H + /OH ions are attracted to the MOx, leading to the redox reaction between H + /OH ions and the MOx surface. GND, ground.

NATURE COMMUNICATIONS | 1:118 | DOI: 10.1038/ncomms1112 | www.nature.com/naturecommunications

2010 Macmillan Publishers Limited. All rights reserved.

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms1112

ARTICLE

Gate (Ti)

5

104 105 106 107 108 109 1010 1011

105 106 107 108 109 1010 1011 1012

Gate insulator "CAN"

Source (Ti)

(001) SrTiO3 single crystal

4

Drain current, I d(A)

Drain (Ti)

Drain current, I d(A)

3

2

1

3

Gate current, I g(nA)

20

10

0

10

g (pA)

800

400

400

2

1

Gate current, I g(pA)

20 10

0 10 20

0I

15 5

10 0 5 10 15

Vg(V)

4

CAN CAN

Ti CAN

5

Vhigh Vlow

Ilow

Ihigh

Capacitance (pF)

150

100

50

0

500 m

Capacitance (pF)

200 150 100

50

800 m

400 m

400 m

SrTiO3 SrTiO3

SrTiO3

0 10 0 10 20

30 20 10 0 10 20 30

Gate voltage, Vg (V) Gate voltage, Vg (V)

Intensity (a.u.)

Water conc. in the film (%)

50

40

30

20

10

00

Figure 3 | Electron transport properties of the CAN-gated SrTiO3 FET at RT. (a) Id versus Vg, (b) Ig versus Vg and (c) C versus Vg, Id versus Vg (d), Ig

versus Vg (e), and C versus Vg curves (f) of the dense a-C12A7-gated SrTiO3 FET are also shown for comparison. Both the channel length, L, and the channel width, W, are 400 m. The gate voltage sweeps were performedin numerical order (for example: 1: 5 V0 V + 5 V0 V5 V). (a) Id

versus Vg curves (Vd = + 2 V) show large anticlockwise hysteresis, although very small clockwise hysteresis is seen in the dense one (d, Vd = + 1 V).

(b) Ig increases exponentially up to 20 nA with Vg, which i104 greater than that of the dense a-C12A7-gated SrTiO3 FET (e, red: observed,grey: smoothed). (c) The C versus Vg curve shows a large anticlockwise hysteresis loop. The maximum C (frequency: 20 Hz) of the nanoporous a-C12A7-gated SrTiO3 FET i160 pF, ~76% of that of the dense a-C12A7-gated SrTiO3 FET ((f) ~210 pF).

10 20 30 40 50

Porosity (%)

100 200 300 400 500 600 700 Temperature, T (C)

Figure 2 | The CAN-gated SrTiO3 FET. (a) A schematic illustration of the CAN-gated SrTiO3 FET. (b) Cross-sectional TEM image of the CAN-gated

SrTiO3 FET (left panel). Scale bar is 100 nm. Trilayer structure composed of Ti/CAN/SrTiO3 is observed. Large amount of light spots are seen in the whole CAN region. Broad halo is observed in the selected area electron diffraction pattern of CAN, and diffraction pattern from SrTiO3 single crystal is also shown below (right panel). (c) Z-contrast, high-angle, annular dark-eld STEM image of the CAN/SrTiO3 interface. Scale bar is 20 nm. Nanopores with diameter < 10 nm appear dark. (d) TDS spectrum of water (m/z = 18 H2O) in the CAN lm (blue). The amount of H2O upto 400 C was estimated to be 1.4 1022 cm 3 ( = 0.41 g cm 3, ~41%).

TDS spectrum in the dense a-C12A7 lm is shown for comparison (red, 0.009 g cm 3). The water concentration increases monotonically with an increase in the porosity of the CAN lms (the inset).

in the air (humidity 4050%) would inltrate the CAN lm most likely due to the capillary eect, hence, nanopores in the CAN lm were lled with water.

Field-induced water electrolysis. We then measured the electron transport properties of the CAN-gated SrTiO3 FET at RT. Figure 3

summarizes (a) drain current (Id) versus gate voltage (Vg) curves,

(b) gate current (Ig) versus Vg curves, and (c) capacitance (C) versus Vg curve of the CAN-gated SrTiO3 FET. Corresponding properties of the dense a-C12A7-gated SrTiO3 FET21 (d, e, f) were also measured for comparison. Both the channel length, L, and the channel width, W, were 400 m. Dielectric permittivity (r) of a-C12A7 was 1221. The gate voltage sweeps were performed in numerical order (for example: 1: 5 V0 V + 5 V0 V5 V). The Id versus Vg curves (a) show large anticlockwise hysteresis, indicating movement of mobile ions22, though very small clockwise hysteresis (~0.5 V) is seen in the dense a-C12A7-gated SrTiO3 FET (d).

Although very small, Ig (~2 pA) is observed in the dense a-C12A7-gated SrTiO3 FET (e), whereas the Ig of the CAN-gated SrTiO3 FET (b) increases exponentially up to 20 nA with Vg, suggesting that mobile ions transport electronic charge. The C versus Vg curve (c) of the CAN-gated SrTiO3 FET shows large anticlockwise hysteresis loop. The maximum C is ~160 pF, ~76% of the value for the dense a-C12A7-gated SrTiO3 FET ((f) ~210 pF), consistent with the

fact that the volume fraction of dense a-C12A7 part in the CAN lm is ~72%.

We also observed a clear pinch-o and saturation in Id at low Vg region (see Supplementary Fig. S3), indicating that the operation of this FET conformed to standard FET theory at low gate voltage. Thus, we concluded that rst the insulating SrTiO3 surface became slightly conductive with gate voltage because of the

NATURE COMMUNICATIONS | 1:118 | DOI: 10.1038/ncomms1112 | www.nature.com/naturecommunications

2010 Macmillan Publishers Limited. All rights reserved.

ARTICLE

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms1112

A B

C D E

Current, I dor I g(A)

102

103

104

105

106

107

108

109

1010

108

107

106

105

104

Sheet resistance, R xx ()

S

Rxx

Vg = +10 V

+20 V

+30 V

+40 V

30 V

Thermopower, S (VK1 )

500

400

300

200

100

Id

5.3 1014 cm2

9.3 1014 cm2

4.7 1015 cm2

2.8 1016 cm2

8.9 1016 cm2

0 0 10 20 30 40

Gate voltage, Vg (V)

Ig

0 1 2 3 Retention time (103 s)

0 0.5

Figure 5 | Thermopower and sheet resistance as a function of applied Vg.

Novel thermopower behaviour: V-shaped turnaround of S is seen, though Rxx (circle: before S measurement, square: after S measurement) decreases gradually as Vg increases, probably due to transition of electronic nature from 3D to 2D in the interface region.

105

104

103

Sheet resistance, R xx ()

106

105

104

103

102

A

B

C

D

E

A

Sheet resistance, R xx ()

B

C

D E

1014 1015 1016 1017 Ion density (cm2)

101 102

Temperature, T (K)

Figure 4 | A redox reaction switches an insulating SrTiO3 to metal. (a) Retention time dependences of Id and Ig for the CAN-gated SrTiO3 FET at several Vg at RT (Vd = + 2 V). The Id increases gradually with retention time at constant Vg. Ion density, which is the retention time integral of Ig, reaches ~9 1016 cm 2 when Vg = + 40 V is applied. (b) Rxx versus ion density for the state AE marked in a at RT. The grey line (slope = 1) indicates

Rxx = (enxx) 1, where nxx and are ion density, and FE (0.8 cm2 V 1 s 1), which was obtained as in Supplementary Figure S4b. (c) Temperature dependence of Rxx for the state AE marked in Figure 4a.

electron charge that was accumulated at the SrTiO3 surface by pure electrostatic eect.

To clarify the role of water in the electrical transport properties, we measured the Id versus Vg characteristics of the CAN-gated

SrTiO3 FET at 0 C using a Peltier cooler, because H + and OH ions cannot move through ice (Supplementary Fig. S4). The device does not show such large anticlockwise hysteresis at 0 C. Although the maximum C increased from ~160 pF (25 C) to ~240 pF (0 C), Id

at Vg = + 10 V decreased from ~5 A (25 C) to ~1 A (0 C), most likely because of the fact that water in the CAN acts as a simple gate dielectric at 0 C (r 25 C ~78, r 0 C ~88)23. The eld-eect mobility (FE) value of the FET at 0 C is ~0.8 cm2 V 1 s 1, which is comparable to that of the dense a-C12A7-gated SrTiO3 FET (~2 cm2 V 1 s 1), obtained from FE = gm((C Vd)/(W L)) 1, where gm was the trans-

conductance Id/Vg. We also measured Hall mobility (Hall) of a CAN-gated SrTiO3 FET aer several Vg applications (metallic state) and obtained Hall values of 2.32.5 cm2 V 1 s 1, which is approximately

three times larger than the FE value (~0.8 cm2 V 1 s 1). These results clearly indicate that H + and OH ions in the CAN are the main contributors to electron transport at RT.

Insulator-to-metal switching. We then observed the electrochemical redox reaction of SrTiO3. Figure 4a shows the changes of Id and

Ig during Vg sweep from + 10 to + 40 V, as a function of retention time. The Id gradually increases with retention time. The rate of Id increase grows with Vg. The Ig also increases with Vg. The Id reaches ~1 mA at Vg = + 40 V. The Id decreases drastically when negative Vg of 30 V is applied.

The sheet resistance (Rxx) at several points (AE) was plotted as a function of ion density, which was obtained by Ig t (Fig. 4b). The observed values were close to the grey line, which is Rxx = (enxx) 1,

where nxx and are ion density, and FE (0.8 cm2 V 1 s 1) was obtained as in Supplementary Figure S4b, suggesting that carrier electrons were generated as a result of the electrochemical reduction of SrTiO3. Although samples AC exhibit insulating Rxx T behaviour,

D and E exhibit metallic Rxx T behaviour (Fig. 4c).

Exotic thermopower. We have also shown that the metal layer on the SrTiO3 in the FET exhibits novel thermoelectric behaviour: V-shaped turnaround of S is seen, although Rxx decreases gradually as Vg increases (Fig. 5). As we have reported previously, |S| value enhancement of electron-doped SrTiO3 can be observed when the conducting layer thickness is < ~3 nm19, probably because of 2D eect24, where the thermal de Broglies wavelength of conduction electrons in SrTiO3 is ~6 nm25. We therefore expected that 2D |S|

can be observed at high Vg, because the layer thickness may become thinner.

When the Vg was <+ 26 V, |S| value gradually decreased with Vg. Similar |S| behaviour, which can be analysed using simple three-dimensional (3D) electron diusion theory, was also observed in the dense a-C12A7-gated SrTiO3 FET, as shown in Supplementary Figure S5. Thus, we used the 3D electron diusion theory26

to analyse the layer thickness. The 3D electron concentrations (n3D) at Vg = + 16, + 20 and + 26 V were estimated to be ~1.5 1019, 1.5 1020 and ~1.1 1021 cm 3, respectively. The nxx values at Vg = + 16, + 20 and + 26 V, which can be calculated using Rxx and FE (0.8 cm2 V 1 s 1), are 1.4 1013, 9.0 1013 and 3.3 1014 cm 2, respectively, suggesting that the SrTiO3 thicknesses (~nxx/n3D) are ~9, 6 and 3 nm, respectively, conrming that the observed S obeys simple 3D electron diusion theory. On the contrary, when the Vg>26 V was applied, the |S| value increased. The conducting layer thickness

NATURE COMMUNICATIONS | 1:118 | DOI: 10.1038/ncomms1112 | www.nature.com/naturecommunications

2010 Macmillan Publishers Limited. All rights reserved.

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms1112

ARTICLE

may be < 3 nm, the observed upturn at Vg > 26 V is most likely due to 2D eect19,24.

Discussion

The present CAN-gated SrTiO3 FET has several merits compared with established methods such as thermal reduction5, ion irradiation17,27, thin lm growth14,15,17,18 and simple FET21,28, which are oen utilized

to switch an insulating SrTiO3 to a metal. This is because the current method allows a metal layer to be easily fabricated with extremely low energy. First, required electricity is extremely less (~50 W cm 2 for

D in Fig. 4) compared with that of thermal reduction (~kW) and ion implantation (~W cm 2). Second, an extremely thin (~3 nm) metal layer, which exhibits exotic thermopower (Fig. 5), can be fabricated. Normally, such thin layers must be fabricated through complicated vapour phase epitaxy methods, such as pulsed laser deposition and molecular beam epitaxy. Third, our CAN-gated SrTiO3 FET exhibits a nonvolatile metal or highly conductive/insulator transition (Figs 3a and 4a and Supplementary Fig. S6), because a reversible redox reaction is utilized in addition to the eld eect.

Although several efficient gating methods using liquid electrolytes have been proposed very recently2831, we would like to argue that the present water-inltrated nanoporous glass CAN is truly superior to the liquid-based gate dielectrics. Liquid electrolytes including gel would be very useful to electrostatically accumulate carriers at the transistor channel by applying rather low Vg (a few volts) using their huge capacitance. However, they would not be suitable for practical applications without sealing because of liquid leakage problem. Our CAN is a chemically stable rigid glassy solid with a higher decomposition voltage, showing excellent adhesion with chemically robust oxide such as SrTiO3 surface and no liquid (water) leakage occurs.

Carrier injection/discharge can be controlled by Vg as redox reaction, which occurs at the interface of H + or OH /semiconductor, is utilized for carrier injection/discharge, though rather high Vg (several ten volts) should be required for Redox reaction. Further, we observed novel thermopower behaviour: V-shaped turnaround, probably due to transition of electronic nature from 3D to 2D in the interface region, as the FET could be operated at high Vg. These clearly indicate the eectiveness of our CAN.

In summary, we have demonstrated that water, when it inltrates nanoporous glass, can switch an insulating oxide to metal. As an example, we have built a FET on an insulating oxide, SrTiO3, using water-inltrated nanoporous glass, amorphous 12CaO7Al2O3 with nanoporous structure CAN as the gate insulator. First, the insulating SrTiO3 surface became slightly conductive with gate voltage because of electrostatic charge accumulation. Then, H + /OH ions were generated due to water electrolysis occurring between the gate and the SrTiO3 surface. Subsequently, a redox reaction took place between H + /OH ions and the SrTiO3 surface. As a result, a thin metal (~3 nm) layer with extremely high electron concentration of 10151016 cm 2 was formed on the insulating SrTiO3. The electron activity of water, as it inltrates nanoporous glass CAN, may nd many useful applications in electronics or energy storage.

Methods

Fabrication of the CAN-gated SrTiO3 FET. The FET structures (Fig. 2a) were fabricated on the (001)-face of SrTiO3 single crystal plate (10 10 0.5 mm,

SHINKOSHA), treated in NH4F-buered HF (BHF) solution32. First, 20-nm-thick metallic Ti lms, used as the source and drain electrodes, were deposited through a stencil mask by electron beam evaporation (base pressure ~104 Pa, no substrate heating/cooling) onto the SrTiO3 plate. Ohmic contact between the Ti and SrTiO3

surface was conrmed by conventional IV characteristics (output characteristics), as shown in Supplementary Figure S3b. Then, a 200-nm-thick CAN lm was deposited through a stencil mask by pulsed laser deposition (KrF excimer laser, uence ~3 J cm2 per pulse) at RT using dense polycrystalline C12A7 ceramic as target. During the CAN deposition, the oxygen pressure in the deposition chamber was kept at 5 Pa. Finally, a 20-nm-thick metallic Ti lm, used as the gate electrode, was deposited through a stencil mask by electron beam evaporation.

Analysis of the CAN lms. The bulk density and thickness of the CAN lms were evaluated by grazing incidence X-ray reectivity (ATX-G, Rigaku). Microstructures of the CAN lms were observed by using TEM (JEOL JEM-2010, acceleration voltage = 200 kV) and aberration-corrected STEM (JEOL JEM-2100F, acceleration voltage = 200 kV). Water concentration in the CAN lms was analysed by TDS measurements (TDS1200, ESCO). The TDS measurements were carried out in a vacuum chamber with the background pressure of ~10 7 Pa at varied temperatures from 60 to 700 C at a heating rate of 60 C per min.

Electron transport properties. Electrical properties of the FETs were measured by using a semiconductor device analyser (B1500A, Agilent). The capacitance of the CAN layer on the FETs was measured using an LCR meter (4284A, Agilent). The thermopower (S) values were measured using two Peltier devices under the FET to give a temperature dierence between the source and drain electrodes (Supplementary Fig. S7). Two thermocouples (K-type) located at both ends of the channel were used for monitoring the temperature dierence (T, 05 K). S values were measured aer each Vg sweep (for example: Vg application 0 V + 16 V0 VS

measurements).

References

1. Trasatti, S. 17991999: Alessandro Voltas Electric pile two hundred years, but it doesnt seem like it. J.Electroanal.Chem. 460, 14 (1999).

2. Light, T. S., Licht, S., Bevilacqua, A. C. & Morash, K. R. The fundamental conductivity and resistivity of water. Electrochem.Solid-StateLett. 8, E16E19 (2005).

3. Zunger, A. n-type doping of oxides by hydrogen. Appl.Phys.Lett. 81, 7375 (2002).

4. Van de Walle, C. G. Hydrogen as a cause of doping in zinc oxide. Phys.Rev. Lett. 85, 10121015 (2000).

5. Frederikse, H. P. R., Thurber, W. R. & Hosler, W. R. Electronic transport in strontium titanate. Phys.Rev. 134, A442A445 (1964).

6. Breckenridge, R. G. & Hosler, W. R. Electrical properties of titanium dioxide semiconductors. Phys.Rev. 91, 793801 (1953).

7. Hosono, H., Kikuchi, N., Ueda, N., Kawazoe, H. & Shimidzu, K. Amorphous transparent electroconductor 2CdOGeO2: conversion of amorphous insulating cadmium germanate by ion implantation. Appl.Phys.Lett. 67, 26632665 (1995).

8. Miyakawa, M., Hayashi, K., Hirano, M., Toda, Y., Kamiya, T. & Hosono, H. Fabrication of highly conductive 12CaO7Al2O3 thin lms encaging hydride ions by proton implantation. Adv.Mater. 15, 11001103 (2003).

9. Hayashi, K., Matsuishi, S., Kamiya, T., Hirano, M. & Hosono, H. Light-induced conversion of an insulating refractory oxide into a persistent electronic conductor. Nature 419, 462465 (2002).

10. Matsuishi, S. etal. High-density electron anions in a nanoporous single crystal: [Ca24Al28O64]4+(4e). Science 301, 626629 (2003).

11. Kim, S.- W. etal. Metallic state in a lime-alumina compound with nanoporous structure. NanoLett. 7, 11381143 (2007).

12. Hayashi, K., Hirano, M. & Hosono, H. Thermodynamics and kinetics of hydroxide ion formation in 12CaO7Al2O3. J.Phys.Chem. B109, 1190011906 (2005).

13. Hosono, H. & Abe, Y. An oxygen-eervescent aluminate glass. J.Am.Ceram. Soc. 70, C38C39 (1987).

14. Ohtomo, A. & Hwang, H. Y. A high-mobility electron gas at the LaAlO3/SrTiO3 heterointerface. Nature 427, 423426 (2004).

15. Kozuka, Y., Kim, M., Bell, C., Kim, B. G., Hikita, Y. & Hwang, H. Y. Two-dimensional normal-state quantum oscillations in a superconducting heterostructure. Nature 462, 487490 (2009).

16. Mannhart, J. & Schlom, D. G. Oxide interfacesan opportunity for electronics. Science 327, 16071611 (2010).

17. Reagor, D. W. & Butko, V. Y. Highly conductive nanolayers on strontium titanate produced by preferential ion-beam etching. Nat.Mater. 4, 593596 (2005).

18. Brinkman, A. etal. Magnetic eects at the interface between non-magnetic oxides. Nat.Mater. 6, 493495 (2007).

19. Ohta, H. etal. Giant thermoelectric Seebeck coefficient of a two-dimensional electron gas in SrTiO3. Nat.Mater. 6, 129134 (2007).

20. Kim, S.- W., Toda, Y., Hayashi, K., Hirano, M. & Hosono, H. Synthesis ofa room temperature stable 12CaO7Al2O3 electride from the melt and its application as an electron eld emitter. Chem.Mater. 18, 19381944 (2006).

21. Ohta, H. etal. Field-modulated thermopower in SrTiO3-based eld-eect transistors with amorphous 12CaO7Al2O3 glass gate insulator. Appl.Phys.Lett. 95, 113505 (2009).

22. Sze, S. M. SemiconductorDevices:PhysicsandTechnology 2nd edn. (John Wiley & Sons, 2002).

23. Buchner, R., Barthel, J. & Stauber, J. The dielectric relaxation of water between 0C and 35C. Chem.Phys.Lett. 306, 5763 (1999).

24. Hicks, L. D. & Dresselhaus, M. S. The eect of quantum well structures on the thermoelectric gure of merit. Phys.Rev.B 47, 1272712731 (1993).25. Mune, Y., Ohta, H., Koumoto, K., Mizoguchi, T. & Ikuhara, Y. Enhanced Seebeck coefficient of quantum-conned electrons in SrTiO3/SrTi0.8Nb0.2O3

superlattices. Appl.Phys.Lett. 91, 192105 (2007).

NATURE COMMUNICATIONS | 1:118 | DOI: 10.1038/ncomms1112 | www.nature.com/naturecommunications

2010 Macmillan Publishers Limited. All rights reserved.

ARTICLE

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms1112

26. Vining, C. B. A model for the high-temperature transport properties of heavily doped n-type silicon-germanium alloys. J.Appl.Phys. 69, 331341 (1991).

27. Kan, D. etal. Blue-light emission at room temperature from Ar+ irradiated SrTiO3. Nat.Mater. 4, 816819 (2005).

28. Ueno, K. etal. Electric-eld-induced superconductivity in an insulator. Nat.Mater. 7, 855858 (2008).

29. Yuan, H., Shimotani, H., Tsukazaki, A., Ohtomo, A., Kawasaki, M. & Iwasa, Y. High-density carrier accumulation in ZnO eld-eect transistors gated by electric double layers of ionic liquids. Adv.Funct.Mater. 19, 10461053 (2009).

30. Kergoat, L. etal. A water-gate organic eld-eect transistor. Adv.Mater. 22, 25652569 (2010).

31. Heller, I. etal. Charge noise in grapheme transistors. NanoLett. 10, 15631567 (2010).

32. Kawasaki, M. etal. Atomic control of the SrTiO3 crystal surface. Science 266, 15401542 (1994).

Acknowledgments

We thank D. Kurita, A. Yoshikawa, T. Mizuno for technical assistance and R. Asahi for discussions. H.O. is supported by MEXT (22360271, 22015009), Y.S. by Research Fellowships of JSPS for young scientists. The Research at Tokyo Tech. is supported by JSPSFIRST Program.

Author contributions

H.O. performed the sample fabrication, measurements and data analysis. Y.S., T.K., and Y.I. performed TEM/STEM analyses. S.W.K. supplied dense C12A7 ceramics. S.W.K., K.N. and H.H. contributed to water analyses. All authors discussed the results and commented on the manuscript. H.O. planned and supervized the project.

Additional information

Supplementary Information accompanies this paper on http://www.nature.com/ naturecommunications

Competing nancial interests: The authors declare no competing nancial interests.

Reprints and permission information is available online at http://npg.nature.com/ reprintsandpermissions/

How to cite this article: Ohta, H. etal. Field-induced water electrolysis switches an oxide semiconductor from an insulator to a metal. Nat.Commun. 1:118 doi: 10.1038/ncomms1112 (2010).

License: This work is licensed under a Creative Commons Attribution-NonCommercial-Share Alike 3.0 Unported License. To view a copy of this license, visit http:// creativecommons.org/licenses/by-nc-sa/3.0/

NATURE COMMUNICATIONS | 1:118 | DOI: 10.1038/ncomms1112 | www.nature.com/naturecommunications

2010 Macmillan Publishers Limited. All rights reserved.