Prominent electrochromism through vacancyorder melting in a complex oxide

Transcription

1 Reeived 26 Jul 211 Aepted 22 Mar 212 Published 24 Apr 212 DOI: 1.138/nomms1799 Prominent eletrohromism through vaanyorder melting in a omplex oxide J. Seidel 1,2,3, W. Luo 4,5, S.J. Suresha 6, P.-K. Nguyen 7, A.S. Lee 8, S.-Y. Kim 8, C.-H. Yang 2,9, S.J. Pennyook 4,5, S.T. Pantelides 4,5, J.F. Sott 1 & R. Ramesh 1,2,8 Eletrohromes are materials that have the ability to reversibly hange from one olour state to another with the appliation of an eletri field. Eletrohromi olouration effiieny is typially large in organi materials that are not very stable hemially. Here we show that inorgani Bi.9 Ca.1 FeO 3.5 thin films exhibit a prominent eletrohromi effet arising from an intrinsi mehanism due to the melting of oxygen-vaany ordering and the assoiated redistribution of arriers. We use a ombination of optial haraterization tehniques in onjuntion with high-resolution transmission eletron mirosopy and first-priniples theory. The absorption hange and olouration effiieny at the band edge (blue-yan region) are m 1 and 19 m 2 C 1, respetively, whih are the highest reported values for inorgani eletrohromes, even exeeding values of some organi materials. 1 Materials Sienes Division, Lawrene Berkeley National Laboratory, Berkeley, California 9472, USA. 2 Department of Physis, University of California, Berkeley, California 9472, USA. 3 Shool of Materials Siene and Engineering, University of New South Wales, Sydney, New South Wales 252, Australia. 4 Department of Physis and Astronomy, Vanderbilt University, Nashville, Tennessee 37235, USA. 5 Materials Siene and Tehnology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA. 6 National Center for Eletron Mirosopy, Lawrene Berkeley National Laboratory, Berkeley, California 9472, USA. 7 Department of Materials Siene and Engineering, Rutgers University, Pisataway, New Jersey 8854, USA. 8 Department of Materials Siene and Engineering,University of California, Berkeley, California 9472, USA. 9 Deparment of Physis, and Institute for the NanoCentury, Korea Advaned Institute of Siene and Tehnology (KAIST), Daejeon 35-71, Republi of Korea. 1 Department of Physis, University of Cambridge, Cambridge CB3 HE, UK. Correspondene and requests for materials should be addressed to J.S. ( jbseidel@lbl.gov). nature ommuniations 3:799 DOI: 1.138/nomms Mamillan Publishers Limited. All rights reserved.

2 nature ommuniations DOI: 1.138/nomms1799 Eletri-field-indued phase transitions from a glassy disordered state to an ordered polar state are well known in liquid rystals both with applied DC and AC eletri fields 1,2. In the present work, we disover an analogous proess in oxide dieletris, whih results in a new eletrohromi effet of suffiient strength for ommerial devies. Eletrohromism has been found in both organi and inorgani materials, eah with varying sets of olours. The ability to hange material olours with applied eletri fields has found several ommerial appliations in smart windows, goggles, and displays. Most of these eletrohromi devies involve doped transition metal oxides, suh as and hemially substituted derivatives of this material, in whih an eletrohemial redox reation leads to the hange in olour In reent years, multiomponent transition metal oxides, suh as the perovskite manganites, ferroeletri Pb[Zr x Ti (1 x) ]O 3 and, more reently, the multiferroi BiFeO 3 (BFO) have demonstrated a muh broader range of hemial, strutural and funtional tunability. This arises naturally out of the fat that the two sub-latties in the perovskite struture an be independently tuned through hemial substitutions, leading to signifiant hanges in funtional responses. For example, in the BFO system, ferroeletriity arises primarily from the large polarizability of the 6s eletrons on the Bi site whereas antiferromagneti order arises from the superexhange interations among the Fe-ions mediated through the oxygen ions. The optial properties of suh systems are typially set by the transition metal 3d to O 2p eletroni transitions. With this as the bakground, we are beginning to explore the optial responses of doped perovskites that an be ontrolled through eletri fields. In this paper, we demonstrate a mehanism of intrinsi eletrohromi behaviour, in a doped ferroeletri Bi.9 Ca.1 FeO 3.5 (BCFO), aused by hanges in arrier density beause of the melting of the intrinsi oxygen vaany order and the resulting oxygen vaany redistribution. The observed effet is large ompared with effets in other transition-metal-oxide eletrohromes. Bismuth ferrite has gained interest beause a paper published in 23 showing a large remnant polarization 12. It has an optial band gap of 2.7 ev (ref. 13). The material is multiferroi at room temperature, holds promise for appliations in magneti memory devies, and shows interesting photovoltai properties 14. Previously, studies have been done on domain-inversion and eletrial modulation of ondutane and memristive swithing in multiferroi Ca-doped BFO thin films 15. It was found that the mehanism of this modulation was due to the movement of ompensating oxygen vaanies. It was shown that the alium in BCFO oupies the Bi-lattie position. As Ca 2 + and Bi 3 + differ in valene, the dopant auses the spontaneous formation of oxygen vaanies to maintain balaned valenes. With the appliation of an eletrial bias, these oxygen vaanies diffuse towards the negative eletrode beause of their net positive harge. This migration forms two regions, one that is rih in oxygen vaanies and one that is poor, whih essentially forms n-type and p-type ondutors, respetively. Hene, in the present work, we modulated the ondutane of BCFO films by applying an eletri field to redistribute the oxygen vaanies and explore the film s eletrohromi properties. Results Optial and eletrohromi haraterization. Calium-doped bismuth ferrite thin films of 1 nm thikness were grown epitaxially on strontium titanate (SrTiO 3 ) substrates via pulsed laser deposition with and without strontium ruthenate (SrRuO 3 ) bottom eletrodes, as desribed in previous work 12,15. X-ray diffration was used to verify the high quality of the epitaxial thin films. In addition, marosopi optial absorption measurements were performed. A plot of αe 2 versus photon energy E and the linear extrapolation to αe 2 = indiates a diret gap at 2.5 ev for Bi.9 Ca.1 FeO 3.5, as shown in Fig. 1a. This value is slightly lower than that obtained for pure BFO 13. For the investigation of eletrohromism, we use a home-built optial set-up inorporating a white-light soure (tungsten lamp) and a visible to near-infrared spetrometer (Oean-Optis USB4), for loal optial absorption spetrosopy. Objetive lenses were utilized to obtain a beam spot size of less than 2 µm when foused onto the sample. Mirometre-sized regions of the Ca-doped BFO thin films were poled using a Veeo ondutive atomi fore mirosope, as desribed in previous work and seen in Fig. 1b (ref. 15). Several 3 µm by 3 µm boxes were poled with a bias of 12 V and tip veloity of 3 µm s 1 while the urrent of the region was monitored. The film surfae was also monitored to ensure that there was no damage. Poling was stopped when the urrent reahed about 1 µa. A CCD amera attahed to the aforementioned optial setup was suffiient in finding the poled boxes to align for absorption spetrosopy. The spetra were analysed at various spots outside the written boxes and ompared with that of inside the box. Figure 1b shows the visible hange of the sample surfae as seen through a mirosope after ondutive atomi fore mirosopy (-AFM) poling. Great are was taken to avoid the dieletri breakdown regime during eletrial poling. In the ase of every measured dataset, the inreased eletrial ondutivity and the surfae morphology of the poled area was heked and found pristine after poling. An example is shown in Fig. 1,d. This method produed rather onsistent results, and visible spetra data was aquired. Furthermore, annealing the samples at 15 C for 4 h aused the boxes to visibly disappear ompletely. This is due to an inrease in diffusivity of the oxygen vaanies allowing them to spread throughout the material bak to their equilibrium, thereby lowering the effetive free arrier density. Figure 1e shows the transmission hange after poling of the material as a funtion of wavelength. The referene spetrum T is taken to be the transmittane of the unoloured region of the BCFO film outside of the eletrially poled box shown in Fig. 1b. The minimum transmission for 1% Ca doping ours at 2.55 ev (485 nm), whih is approximately the band gap, E g, of BCFO. The hange in visible transmittane spetra at this wavelength is 4%. Additionally, there is a small minimum at about 67 nm (1.85 ev), whih is assoiated with oxygen vaanies in the sample. From the transmission hange, T = T/T, the absorption oeffiient and olouration effiieny (CE) are alulated. The absorption oeffiient is evaluated from: ( ) = a l 1 1 ln t T where t is the film thikness. At 48-nm wavelength, α is alulated to be m 1. The CE is evaluated from: CE ( l) a = Q (1) (2) where Q is the volumetri harge density. To alulate Q, we must take some assumptions ompared with a typial eletrohromi devie beause harge is not injeted into the system, but rather formed intrinsially through harge separation. We neglet all diffusion gradients and assume that there are two half spaes to the film: an n-type and a p-type region. We also assume that all oxygen vaanies have fully diffused, and the number of oxygen vaanies per unit volume, n Vo, after doping and before -AFM, is onstant, eah ontributing two eletron arriers when diffused. As we know only the p-type region ontributes to the olouration hange 15, we make a onservative estimate that the absorption hanges in the p-type region equals to the measured effetive absorption hange. For the p-type region, eah missing oxygen vaany produes two holes making the arrier density produed in the volume equal to 2n Vo. These assumptions allow us to find the theoretial lower bound on CE for the measured α at a given wavelength. In our nature ommuniations 3:799 DOI: 1.138/nomms Mamillan Publishers Limited. All rights reserved.

3 nature ommuniations DOI: 1.138/nomms1799 ARTICLE a 14 b µm αe 2 (a.u.) 8 6 E gap =2.5 ev d 4 E va =1.85 ev 2 e T/T E (ev) 1.1 f % Ca % Ca 5% Ca 2% Ca Wavelength (nm) Mo.8 W.992 O 3 5 WO Nb 2 O 5 3 NiO NiO Rh TiO 2CoO, Nb2O5 Ta 2 O 2 O 5 5 IrO x Bi2 O Wavelength (nm) Figure 1 Optial and eletrohromi haraterization. (a) Optial bandgap E g = 2.5 ev (red dashed line) of Bi.9 Ca.1 FeO 3.5 evaluated from absorption measurement (blak). An additional absorption band originating from oxygen vaanies is visible around E va = 1.85 ev. Data for BFO is shown for omparison (blue). (b) Shemati of ondutive AFM poling of the sample at 12 V, and optial mirosope image ( 4) of a 3-µm by 3-µm poled region in a 1-nm thik film. () Condutive AFM image of a double box poled with 12 V/ + 12 V. Sale bar, 1 µm. (d) Topography image of the same area after poling showing no morphology hange. Sale bar, 1 µm. (e) Transmission hange versus wavelength for different Ca-doping levels (T transmission inside poled box, T transmission outside poled box). (f) Colouration effiieny versus wavelength for Bi.9 Ca.1 FeO 3.5 (red). Maximum CE is 19 m 2 C 1 at 48 nm (2.55 ev, pink area). Values for known inorgani eletrohromi materials are added for omparison; absolute values are shown (data points adapted from ref. 5). Colouration effiieny (m 2 C 1 ) MoO 3 previous work, we found that the initial oxygen vaany density n Vo for Bi.9 Ca.1 FeO 3.5 is m 3 (ref. 15) implying that the volumetri harge density, Q, is m 3 C. At 48-nm wavelength, CE is alulated to be 19 m 2 /C (Fig. 1f). This value is higher than CEs found in most transition-metal-oxide eletrohromes; it even surpasses CE values reported for organi eletrohromes, suh as indigo blue, whih usually show higher CE values than inorgani materials, but are less hemially stable 5. For a omparison of olouration effiienies of oxide eletrohromes, see Fig. 1f. Transmission eletron mirosopy studies. The absorption hange for 2% Ca is smaller than that for 1% Ca, as seen in Fig. 1e. If only the absolute amount of oxygen vaanies present determined the effet, one would intuitively expet the opposite to be the ase. Clearly, there must be some mirosopi differene in both types of samples that leads to the observed behaviour. To gain more insight, we performed high-resolution transmission eletron mirosopy (TEM) studies of both 1 and 2% Ca-doped BFO samples in the as-grown and poled states. The results are seen in Fig. 2. Figure 2a shows the as-grown state of a sample ontaining 2% Ca. The oxygen-vaany order parallel to the sample surfae with a period of eight unit ells is seen. An image of the -AFMpoled state after appliation of 12 V an be seen in Fig. 2b. The appliation of an eletri field does not yield hanges in the oxygen-vaany ordering. The results for the same type of measurements for a sample ontaining 1% Ca are shown in Fig. 2,d. The as-grown state shows an oxygen-vaany ordering with a period of 2 3 unit ells under an angle of 45 to the sample surfae. After the appliation of a bias of 12 V in the -AFM poling, the oxygen-vaany ordering is ompletely melted. In this area, the overall vaany onentration is lowered after poling, whereby the area beomes p-type 15. A lose inspetion of the individual ion positions for the poled and unpoled state, by high-resolution TEM, is presented in Fig. 2e,f. From these images, Fe-ion position oordinates were extrated as shown in Fig. 2g. Histogram plots show that the average Fe offentring is redued for the poled state as ompared with the unpoled state. A shemati representation an be seen in Fig. 2h. In addition, at high temperature, the measured band gap of BFO dereases as the deviation from the ubi struture dereases 16,17. Both studies indiate that the optial properties of undoped BFO are sensitive nature ommuniations 3:799 DOI: 1.138/nomms Mamillan Publishers Limited. All rights reserved.

4 nature ommuniations DOI: 1.138/nomms1799 a b e f Unpoled Poled d g Unpoled Fe x off-entring (Å) Fe y off-entring (Å) Poled h Fe Bi Figure 2 TEM haraterization of the oxygen-vaany ordering in Ca-doped BFO. (a) Ca 2% as-grown, (b) Ca 2% poled, () Ca 1% as-grown, (d) Ca 1% poled; sale bars, 5 nm, insets show loal diffration patterns. (e,f) High resolution TEM showing ion positions for poled and unpoled state for Ca 1% (sale bars,.4 nm). (g) Histogram analysis of Fe ion position. Dashed yellow lines indiate the entre of the unit ell. (h) Shemati of Fe-ion displaement using data of (g). to the polar order parameter and the strutural symmetry. In our ase, the strong eletrohromi olouration in the eletrially poled state is aompanied by a relative shift of the Fe ion with respet to the unit ell entre also indiating a hange of the polar order parameter. First priniples alulations. To understand the effets of alium doping and oxygen vaanies on the optial properties of BFO, for both the ompensated and p-type ase, we have also performed first-priniples density-funtional theory 18,19 (DFT) alulations, using approahes previously used for BFO 2 25 (Methods). After the strutural and eletroni ground states have been determined, the frequeny-dependent real part ε 1 and imaginary part ε 2 of the dieletri funtion are alulated 26. The usual expression for the optial absorption oeffiient α(ω) is, w a ( w ) = e 2 ( w) n where ω is the energy of the absorbed photon, n is the refrative index, and is the speed of light in vauum. The optial absorption oeffiient α(ω) an also be expressed in terms of the real and imaginary parts of the dieletri funtion, 2w / a ( w ) = e + e e (3) (4) The alulated eletroni density-of-states and optial absorption oeffiients of both the unompensated and the ompensated Ca-doped BFO are shown in Fig. 3a,b. It is well known that density funtional theory alulations underestimate the band gap for most materials, and inorporation of onsite Coulomb interation with Hubbard U parameter improves the band gap value. It has also been shown that oxygen vaanies have a role in the optial properties of BFO 25. With the speifi hoie of onsite Coulomb interation parameters (U = 4 ev, J = 1 ev), the optial band gap is alulated to be 1.95 ev for undoped BFO. Assuming 1/8 of Ca doping and omplete ompensation of holes by oxygen vaanies (Ca 1/8 Bi 7/8 FeO 3 1/16 in Fig. 3), the optial band gap is redued by.1 ev from the undoped material. This result is onsistent with our experimental observations, whih show that Ca doping redues the band gap of BFO from 2.7 (ref. 13) to 2.5 ev (Fig. 1a). Beause of the omplete harge ompensation, the Fermi level lies in the band gap. Ca-doped BFO without harge ompensation from oxygen vaanies shows a muh sharper rise in the density-of-states of the ondution band, whih results in an enhaned optial absorption near the band edge (Ca 1/8 Bi 7/8 FeO 3 in Fig. 3a). The differene between the ompensated and the unompensated Ca-doped BFO, in the density-of-states of the ondution bands, beomes apparent by looking at the eletron density of the lowest ondution states, as shown in Fig. 3,d. The lowest ondution states in unompensated Ca-doped BFO are mostly extended Fe 3d states, similar to undoped BFO. In ontrast, the lowest ondution states in ompensated Ca-doped BFO loalize at Fe sites near the ompensating oxygen vaanies. The defet-like loalized states in ompensated BCFO orrespond to the small shoulder at the ondution band edge in the eletroni density-of-states, resulting in the redued density-of-states of the extended ondution band states just above it (Fig. 3a) and the lower optial absorption oeffiient near the band edge. At 1.95 ev, the differene in alulated absorption oeffiient is m 1, whih is in very good agreement with the measured hange of m 1 at 48 nm. Disussion The theoretial alulations and the TEM haraterization point to the following mehanism of olossal eletrohromism in 1% Ca-doped BFO. In the as-grown sample, oxygen vaanies ompensate the Ca dopants and form ordered strutures. On appliation of an eletri field, the vaany ordering is melted, and vaany-rih (n-type) and poor (p-type) regions are formed owing to the resulting redistribution of vaanies. The large olouration originates from the inreased optial absorption at the band edge of the p-type region. In ontrast, oxygen-vaany ordering in 2% Ca-doped nature ommuniations 3:799 DOI: 1.138/nomms Mamillan Publishers Limited. All rights reserved.

5 nature ommuniations DOI: 1.138/nomms1799 ARTICLE a b 25 8 Density of states (a.u.) Absorption oeffiient α (1 6 m 1 ) Coloration effiieny (m 2 C 1 ) Energy (ev) Energy (ev) d Energy (ev) Bi V ö Ca Fe O Figure 3 First-priniples alulations. (a) Calulated eletroni density-of-states near the bandgap (blue lines) for BFO with alium doping Bi 7/8 Ca 1/8 FeO 3 (red) and BFO with alium doping and oxygen vaany ompensation Ca 1/8 Bi 7/8 FeO 3 1/16 (blue). (b) Optial absorption oeffiients α as a funtion of photon energy. The alulations are performed using GGA + U method with U = 4 ev and J = 1 ev for BFO with alium doping Bi 7/8 Ca 1/8 FeO 3 (red) and BFO with alium doping and oxygen-vaany ompensation Ca 1/8 Bi 7/8 FeO 3 1/16 (blue). Calulated CE is also shown (grey). () Isosurfae eletron density of the lowest lying ondution band states in oloured Bi 7/8 Ca 1/8 FeO 3, and (d) in oxygen-vaany ompensated Ca 1/8 Bi 7/8 FeO 3 1/16 (V ö denotes position of oxygen vaany (Kröger Vink notation)). BFO remains almost unhanged under an eletri field, whih is onsistent with the small hange in absorption. DFT alulations were arried out with superells ontaining uniformly distributed Ca dopants and oxygen vaanies. The effets of extended oxygen vaanies suh as vaany lusters or planes of ordered vaanies, observed in other transition metal oxides, have not been explored theoretially in the urrent study beause of the high omputational demand. However, a reent study has shown that oxygen vaany lustering in SrTiO 3 indues loalized states at.6 ev below the ondution band minimum 27, whereas the single vaany states are muh loser to the ondution band minimum. The observed weak absorption peak at 1.85 ev in our measurements ould therefore also originate from lustering of oxygen vaanies. Our experimental and theoretial studies have revealed a new kind of eletrohromi phenomenon. In omparison with earlier work, eletrohromi work on usually involves samples with nanorystals embedded in a glassy matrix, and oupled ation and eletron transport ours over large distanes and is onfined to the non-rystalline matrix 28 ; and in rystalline SrTiO 3 (ref. 29), ioni transport requires exoti valene states of Ti (and/or Sr). Of partiular interest, from the eletrohromis point of view, is that the present system involves loal oxygen ordering within a rystalline state, whih is unlikely to require exoti valenes while still ahieving large intrinsi eletrohromi effets. The ability to reversibly ahieve a large hange in visible transmittane in the blue-yan region (2.55 ev) is so far unmathed in transition metal oxides. Our observations support the notion that suh behaviour aompanied by a strutural transition as seen in a hange of Fe-ion displaement is a generi feature akin to hemially driven phase hanges that are now well established in omplex oxide systems suh as manganites, uprates, and relaxor materials 3. Furthermore, the observation of strong absorption hange and olouration, that is ahieved solely by redistribution of oxygen vaanies within the material itself, by melting the vaany-ordered state, should motivate a searh for similar ontrol in other related omplex oxide systems, thus making this intrinsi effet potentially attrative for eletrohromi devie appliations. Methods Materials X-ray haraterization. High-resolution X-ray reiproal spae mapping studies were ompleted on a Bi.9 Ca.1 FeO 3.5 (BCFO) film (~3 nm in thikness) grown on (1) SrTiO 3 substrates using a Panalytial X Pert MRD Pro 4-irle diffratometer (Fig. 4). We measured the (23) peak to investigate the in-plane lattie parameter as well as the out-of-plane lattie parameter. The (23) diffration peak of the film is loated near the dashed line indiating the film has been grown oherently and, thus, the in-plane lattie parameter of the film is nearly idential to that of the substrate. However, the shape of the peak is somewhat diffused along the transverse san diretion as a result of partial strain relaxation whih is natural in suh a thik (~3 nm) film. Normal BFO films show the split of the (23) peak arising from monolini distortion of the unit-ell; however, this BCFO film does not show any peak split indiating that the unit ell is ideally tetragonal. From the (23) peak position, we an alulate the lattie parameters of the unit-ell to be a = 3.96 Å and = Å. The strutural analysis onfirms that the BCFO film has been grown epitaxially on (1) SrTiO 3 substrate, and the stabilized phase an be desribed by a tetragonal unit ell. Transmission eletron mirosopy. High-angle annular dark-field sanning transmission eletron mirosopy was arried out using the aberration-orreted TEAM.5 mirosope (a modified FEI Titan 8 3 quipped with a Shottkytype high-brightness field-emission eletron soure and an improved hexapoletype spherial aberration (CS) orretor) loated at the National Center for Eletron Mirosopy (NCEM). The TEAM.5 mirosope was operated at 3 kv. The probe semi-onvergene angle was set to 16.5 mrad, whih yields a alulated probe size of.63 Å. Although a smaller probe size is in priniple feasible on the nature ommuniations 3:799 DOI: 1.138/nomms Mamillan Publishers Limited. All rights reserved.

6 nature ommuniations DOI: 1.138/nomms1799 q (Å 1 ) q (Å 1 ) SrTiO 3 (23) Bi.9 Ca.1 FeO 3-.5 (23) Figure 4 X-ray diffration strutural haraterization. X-ray reiproal spae map of an epitaxially grown Bi.9 Ca.1 FeO 3.5 film. expenses of a redued depth of field, the hosen setting allows for a suffiiently large depth of field, whih enhanes the ontrast of the atomi olumns. The annular semidetetion range of the high-angle annular dark-field detetor was about mrad. Atomi fore mirosopy. Loal eletrial poling under ambient onditions was ahieved using an AFM-based set-up. Measurements were arried out on a Digital Instruments Nanosope-IV Multimode AFM equipped with a ondutive AFM appliation module. The investigations were arried out with ommerially available nitrogen-doped diamond-oated Si tips (NT-MDT). Typial san rates were 3 µm s 1. First-priniples alulations. We perform first-priniples density-funtional theory alulations using the projetor-augmented-wave method as implemented in the Vienna ab-initio software pakage ode. Density-funtional theory with generalized-gradient approximation plus onsite Coulomb interation (GGA + U) is used with parameters U = 4 ev and J = 1 ev, whih is within the range of values used in the literature. We use a 5-eV plane-wave utoff energy for both strutural optimization and dieletri funtion alulations. The lattie of bulk BFO has rhombohedral symmetry with G-type AFM order. Superells ontaining or pseudo-ubi perovskite units are used for simulating Ca-doping and oxygen vaanies. The ioni positions are relaxed so that the fore on every ion is less than 1 mev Å 1. The frequeny dependent dieletri funtion is alulated after the strutural and eletroni ground states have been determined. Referenes 1. Handshy, M. A., Clark, N. A. & Lagerwall, S. T. Field-indued first-order orientation transitions in ferroeletri liquid rystals. Phys. Rev. Lett. 51, (1983). 2. Chowdhury, A., Akerson, B. J. & Clark, N. A. Laser-indued freezing. Phys. Rev. Lett. 55, (1985). 3. Deb, S. K. Optial and photoeletri properties and olour entres in thin films of tungsten oxide. Philos. Mag. 27, (1973). 4. Granqvist, C. G. Handbook of Inorgani Eletrohromi Materials (Elsevier, Amsterdam, 1995). 5. Monk, P. M. S., Mortimer, R. J. & Rosseinsky, D. R. Eletrohromism and Eletrohromi Devies (Cambridge, UK, 27). 6. Granqvist, C. G. Eletrohromi materials: Out of a nihe. Nature Mater. 5, 89 9 (25). 7. Behinger, C. et al. Photoeletrohromi windows and displays. Nature 383, (1996). 8. Huiberts, J. N. et al. Yttrium and lanthanum hydride films with swithable optial properties. Nature 38, (1996). 9. Somani, P. R. & Radhakrishnan, S. Eletrohromi materials and devies: present and future. Mater. Chem. Phys. 77, (22). 1. Zhi, Y., Liu, D., Zhou, Y., Chai, Z. & Liu, L. Eletrohromism aompanying ferroeletri domain inversion in ongruent RuO 2 :LiNbO 3 rystal. Opt. Express 13, (25). 11. Xi, Q., Liu, D., Zhi, Y., Luan, Z. & Liu, L. Reversible eletrohromi effet aompanying domain-inversion in LiNbO 3 :Ru:Fe rystals. Appl. Phys. Lett. 87, (25). 12. Zhao, T. et al. Eletrial ontrol of antiferromagneti domains in multiferroi BiFeO 3 films at room temperature. Nature Mater. 5, (26). 13. Xu, X. S. et al. Optial properties and magnetohromism in multiferroi BiFeO 3. Phys. Rev. B 79, (29). 14. Yang, S.- Y. et al. Above-bandgap voltages from ferroeletri photovoltai devies. Nat. Nanotehnol. 5, (21). 15. Yang, C.- H. et al. Eletri modulation of ondution in multiferroi Ca-doped BiFeO 3 films. Nature Mater. 8, (29). 16. Palai, R. et al. β phase and γ-β metal-insulator transition in multiferroi BiFeO 3. Phys. Rev. B 77, 1411 (28). 17. Catalan, G. & Sott, J. F. Physis and appliations of bismuth ferrite. Adv. Mater. 21, (29). 18. Hohenberg, P. & Kohn, W. Inhomogeneous eletron gas. Phys. Rev. 136, B864 B871 (1964). 19. Kohn, W. & Sham, L. J. Self-onsistent equations inluding exhange and orrelation effets. Phys. Rev. 14, A1133 A1138 (1965). 2. Blöhl, P. E. Projetor augmented-wave method. Phys. Rev. B 5, (1994). 21. Kresse, G. & Furthmüller, J. Effiient iterative shemes for ab initio total-energy alulations using a plane-wave basis set. Phys. Rev. B 54, (1996). 22. Perdew, J. P., Burke, K. & Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 77, (1996). 23. Anisimov, V. I., Aryasetiawan, F. & Liehtenstein, A. I. First-priniples alulations of the eletroni struture and spetra of strongly orrelated systems: the LDA+ U method. J. Phys. Condens. Matter 9, (1997). 24. Ederer, C. & Spaldin, N. A. Influene of strain and oxygen vaanies on the magnetoeletri properties of multiferroi bismuth ferrite. Phys. Rev. B 71, (25). 25. Ju, S. & Cai, T. Y. First-priniples studies of the effet of oxygen vaanies on the eletroni struture and linear optial response of multiferroi BiFeO 3. Appl. Phys. Lett. 95, (29). 26. Gajdos, M. et al. Linear optial properties in the projetor-augmented wave methodology. Phys. Rev. B 73, (26). 27. Cuong, D. D. et al. Oxygen Vaany Clustering and Eletron Loalization in Oxygen-Defiient SrTiO 3 : LDA+U Study. Phys. Rev. Lett. 98, (27). 28. Solis, J. L., Hoel, A., Lantto, V. & Granquist, C. G. Infrared spetrosopy study of eletrohromi nanorystalline tungsten oxide films made by reative advaned gas deposition. J. Appl. Phys. 89, (21). 29. Blan, J. & Staebler, D. L. Eletrooloration in SrTiO 3 : Vaany Drift and Oxidation-Redution of Transition Metals. Phys. Rev. B 4, (1971). 3. Dagotto, E. Nanosale Phase Separation and Colossal Magnetoresistane (Springer, New York, 23). Aknowledgements This researh is supported by the US Department of Energy, Offie of Siene, Division of Materials Siene and Engineering (S.J.P., S.T.P., and W.L.), and under ontrat No. DE-AC2-5CH11231 (R.R.), and by the Basi Siene Researh Program through the National Researh Foundation of Korea (NRF) funded by the Ministry of Eduation, Siene and Tehnology ( ). Computations were performed at the National Energy Researh Sientifi Computing Center. We aknowledge support from the National Center for Eletron Mirosopy, Lawrene Berkeley National Laboratory. J.S. aknowledges support from the Alexander von Humboldt Foundation. Author ontributions J.S. and P.-K.N. designed the experiment, performed spetrosopy and analysed optial properties. W.L. performed first-priniples alulations. S.J.S. arried out TEM measurements. C.-H.Y., A.S.L. and S.-Y.K. synthesized the films. C.-H.Y. measured X-ray diffration and analysed the rystal struture of the materials. J.S. and S.-Y.K. haraterized eletri swithing properties using ondutive atomi fore mirosopy. S.J.P., S.T.P., J.F.S. and R.R. ontributed material and analysis. J.S. and W.L. wrote the manusript with the help and disussion of all the other authors. Additional information Competing finanial interests: The authors delare no ompeting finanial interests. Reprints and permission information is available online at reprintsandpermissions/ How to ite this artile: Seidel, J. et al. Prominent eletrohromism through vaanyorder melting in a omplex oxide. Nat. Commun. 3:799 doi: 1.138/nomms1799 (212). nature ommuniations 3:799 DOI: 1.138/nomms Mamillan Publishers Limited. All rights reserved.