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Reversible Changes in Resistance of Perovskite Nickelate NdNiO Thin Films Induced by Fluorine Substitution 3

Tomoya Onozuka, Akira Chikamatsu, Tsukasa Katayama, Yasushi Hirose, Isao Harayama, Daiichiro Sekiba, Eiji Ikenaga, Makoto Minohara, Hiroshi Kumigashira, and Tetsuya Hasegawa ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00855 • Publication Date (Web): 08 Mar 2017 Downloaded from http://pubs.acs.org on March 9, 2017

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ACS Applied Materials & Interfaces

Reversible Changes in Resistance of Perovskite Nickelate NdNiO3 Thin Films Induced by Fluorine Substitution Tomoya Onozuka,1 Akira Chikamatsu,1* Tsukasa Katayama,1a Yasushi Hirose,1,2 Isao Harayama,3 Daiichiro Sekiba,3 Eiji Ikenaga,4 Makoto Minohara,5 Hiroshi Kumigashira,5 and Tetsuya Hasegawa1,2 1. Department of Chemistry, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan 2. Kanagawa Academy of Science and Technology (KAST), 3-2-1 Sakado, Takatsu, Kawasaki, Kanagawa 213-0012, Japan 3. Tandem Accelerator Complex, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba Ibaraki 3058577, Japan 4. Japan Synchrotron Radiation Research Institute (JASRI)/SPring-8, 1-1-1 Kouto Mikazuki-cho, Hyogo 679-5198, Japan 5. Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), 1-1 Oho Tsukuba, Ibaraki 305-0801, Japan Keywords: pulsed-laser depositions, oxyfluorides, resistance modulations, electronic structures, topotactic reactions

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ABSTRACT

Perovskite nickel oxides are of fundamental as well as technological interest because they show large resistance modulation associated with phase transition as a function of the temperature and chemical composition. Here, the effects of fluorine doping in a perovskite nickelate NdNiO3 epitaxial thin films are investigated through a low-temperature reaction with polyvinylidene fluoride as the fluorine source. The fluorine content in the fluorinated NdNiO3−xFx films is controlled with precision by varying the reaction time. The fully fluorinated film (x ≈ 1) is highly insulating and has a bandgap of 2.1 eV, in contrast to NdNiO3, which exhibits metallic transport properties. Hard X-ray photoelectron and soft X-ray absorption spectroscopies reveal the suppression of the density of states at the Fermi level as well as the reduction of nickel ions (valence state changes from 3+ to 2+) after fluorination, suggesting that the strong Coulombic repulsion in the Ni 3d orbitals associated with the fluorine substitution drives the metal-toinsulator transition. In addition, the resistivity of the fluorinated films recovers to the original value for NdNiO3 after annealing in an oxygen atmosphere. By applying the reversible fluorination process to transition-metal oxides, the search for resistance-switching materials could be accelerated.

INTRODUCTION Transition-metal oxides exhibit a wide variety of electronic and magnetic properties owing to the interplay between the multiple degrees of freedom, such as the charge, lattice, spin, and orbital.1,2 These properties include multiferroicity,3 the Mott transition,4 and high-Tc superconductivity.5


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Among these materials, rare-earth nickelates (RNiO3, where R represents rare earth elements) have been the subject of long-standing interest in the nascent field of oxide electronics. RNiO3 is known to undergo a first-order metal-insulator transition (MIT), depending on the ionic radius of R.6,7 While the detailed mechanism of the MIT is still a matter of debate, it has been suggested that the charge disproportionation of nickel ions (Ni3+ → Ni(3+δ)+ / Ni(3−δ)) plays a crucial role in the MIT.8–12 The resistance change of one to two orders of magnitude that is associated with the transition has been exploited to develop electronic devices called Mott transistors. Among the various RNiO3 materials being explored, NdNiO3, which exhibits a temperature-driven MIT at ~200 K, is among the most widely studied nickelates. It has been reported that the transition temperature of NdNiO3 can be lowered by gating with an electric double-layer transistor; the application of a negative gate voltage decreased TMI by ~40 K.13,14 However, to be able to electrically control the phase transition in NdNiO3, an ultrathin film with thickness of less than 10 nm is required because a large amount of carriers are present in the metallic state (~1022 cm−3). Chemical modification is an alternative approach for modulating the resistance of this material. A typical example is aliovalent substitution at the Nd site in NdNiO3; the substitution of either divalent (Sr2+ or Ca2+) or tetravalent (Th4+) ions for Nd3+ produces hole or electron carriers, respectively, and subsequently lowers the transition temperature.15 Recently, proton doping at the interstitial sites and the incorporation of oxygen vacancies have also been attracted much attention, because the related chemical reactions, namely, the incorporation/extraction of hydrogen or oxygen, is often reversible, potentially allowing for the reversible switching of the resistance. Ramanathan and co-workers doped protons into RNiO3 (R = La, Nd, Sm, or Eu) using a platinum-catalyzed intercalation reaction16,17 while Wang et al. introduced oxygen vacancies


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into NdNiO3 through annealing in a vacuum.18 In both cases, an increase in resistance by more than six orders of magnitude was observed. The aliovalent substitution of fluoride ions at the oxygen sites would also provide additional electrons to the nickel ions and significantly change the electronic properties of perovskite nickelates. Furthermore, the fluorine substituted at the oxygen sites would directly affect the connectivity of the Ni-anion network, which is responsible for electrical conduction in the nickelates. Thus, fluorine substitution may have as large effect on the electric conductivity as proton or oxygen-vacancy doping. Here we demonstrated fluorine substitution in epitaxial NdNiO3 thin films through low-temperature topotactic fluorination with polyvinylidene fluoride (PVDF)19–21 and investigated the physical properties as well as the electronic structures of the doped thin films. After fluorine incorporation, the films showed highly insulating properties, with the room-temperature resistivity being more than six orders of magnitude higher than that of as-grown NdNiO3. Soft X-ray absorption and hard X-ray photoelectron and spectroscopies (XAS and HAXPES, respectively) revealed that the chemical valence of the Ni ions changed from Ni3+ to Ni2+ with the fluorine substitution In addition, the electronic density of states at the Fermi level (EF) decreased. Moreover, the incorporated fluorine could be removed completely by annealing the thin films in oxygen, leading to conclusion that it should be possible to fabricate resistance-switching devices by controlling the fluorine content in NdNi(O,F)3.

RESULTS and DISCUSSION Figure 1a shows the out-of-plane 2θ–θ X-ray diffraction (XRD) pattern of a precursor NdNiO3 film deposited on a SrTiO3 (STO) (100) substrate as well as those of the thin films reacted with PVDF at 350 °C for 1, 3, 12, and 24 h. The XRD pattern of the precursor film (black line) shows


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the 002 diffraction peak of perovskite NdNiO3 at 2θ = 47.87°. The out-of-plane lattice constant, c, was calculated to be 3.80 Å, which is in good agreement with previously reported values.18,22 After the as-grown oxide films were treated with PVDF, the 002 diffraction peak steadily shifted to lower angles with an increase in the reaction time and was observed at 44.60° in the case of the film treated for 24 h, indicating that out-of-plane lattice expansion occurred after fluorination with PVDF. The anion compositions of the reacted films were observed directly using depthresolved X-ray photoelectron spectroscopy (XPS) measurements, as shown in Figure 1b. It was observed that, with an increase in the reaction time, fluorine atoms were gradually incorporated into the films, confirming that the films were successfully fluorinated and their fluorine content could be controlled by varying the reaction time. Obviously, the entire film, from the surface to the film/substrate interface, was fluorinated; although the topmost surface region had a higher fluorine concentration possibly due to small amounts of surface residues, the fluorine distribution inside the film was rather smooth. In addition, the elastic-recoil detection analysis (ERDA) result for the sample fluorinated for 12 h revealed that the oxygen content, x, and fluorine content, y, in NdNiOxFy were 2.1 and 0.9, respectively, with the uncertainty being 10% (raw data shown in Supplementary Figure S1). Therefore, it can be assumed that the reaction between NdNiO3 and PVDF causes the partial substitution of oxygen anions for fluoride anions, yielding the oxyfluoride NdNiO3−xFx. The change of the lattice constant as a function of the reaction time showed non-linear, sigmoid-like dependence (Supplementary Figure S2a). The fluorine content inside the film was proportional to their lattice constant, obeying Vegard’s law (Supplementary Figure S2b). The reciprocal space maps (RSMs) around the 103+ asymmetric diffraction of the as-grown and fluorinated films were recorded (Figure 1c). The qx value (2.56 nm−1) of the diffraction spot of


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the NdNiO3 film was the same as that in the case of the substrate. That indicates that the film underwent epitaxial strain from the substrate, and the in-place lattice was at least partially locked to that of the substrate. While the out-of-plane coordinate, qz, of the diffraction spot decreased after fluorination, the in-plane coordinate, qx, showed almost no shifts, even though the peak itself became broader, which was indicative of a decrease in crystallinity. These observations suggest that the cube-on-cube epitaxial relationships between film and substrate lattice were unchanged during the low-temperature fluorination process. Although the ionic radius of F− (133 pm) is smaller than that of O2− (140 pm),[23] the lattice constants of the films increased after fluorination. This can be attributed to the change in the chemical valence state of the nickel ions; the fluorine substitution process produces additional electrons, which reduce the nickel ions from Ni3+ to Ni2+. These extra electrons enter the antibonding σ* (Ni 3d–O/F 2p) orbitals, increasing the length of the bonds between the nickel atoms and the anions. To confirm the valence associated with fluorination, we performed Ni Ledge XAS measurements, as shown in Figure 1d. It is known24 that the Ni 2p XAS spectrum of Ni3+ is very different from that of Ni2+. Figure 1d also shows the XAS spectra from previous studies as references. Obviously, the spectrum of the as-grown film resembles that of NdNi3+O3. On the other hand, the spectrum changes to that corresponding to Ni2+ after fluorination, supporting the above-mentioned claim that the nickel ions undergo reduction and that their valence state changes from +3 to +2 upon fluorine substitution. Next, we discuss the electrical and optical properties of the NdNiO3−xFx thin films. After fluorination for 3 h, the film sheet resistance increased dramatically, from 3×102 Ω □–1 to ~ 1× 109 Ω □–1. The latter value was comparable to the resistance of the insulating STO substrates, which made it difficult to investigate the intrinsic electrical transport properties of the


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NdNiO3−xFx thin films. A similarly large change in resistivity, of more than six orders of magnitude, was also observed during the annealing of NdNiO3 in an ultra-high vacuum.[18] Figure 2a shows the optical absorption coefficients, α, of the NdNiO3−xFx films as functions of the incident photon energy. For energies lower than 3.0 eV, α decreased monotonically with an increase in the reaction time, suggesting that the carrier density was reduced significantly by fluorine substitution. The linear onset of the (αhν)n–(hν) plot (Tauc plot), where n = 2 (direct transition), was extrapolated as per a previous study;18 this yielded optical bandgap values of 0.84, 0.95, and 2.1 eV for the 3-, 12-, and 24-h fluorinated samples, respectively. These results imply that the fluorination of NdNiO3 opens a finite bandgap of more than 1 eV, a conclusion that is consistent with the abrupt increase observed in the resistivity. The extremely high increases in the resistance resulting from the fluorination process as well as large bandgap opening of 2.1 eV suggest that the process induces drastic changes in the electronic states of the thin films. Thus, in order to investigate the electronic states, we performed bulk-sensitive HAXPES. The Nd 3d core-level spectra are shown in Figure 3a. In the 3d spectra of the rare earth elements, different satellite peaks (marked by triangles in Figure 3a) can be observed; these are attributable to the interactions between the generated core holes and the valence electrons.[25] In the case of the NdNiO3 film, a satellite peak is observed at a binding energy (Eb) smaller than that corresponding to the main peak. This peak is called a “shake-down,” or “well-screened” satellite peak and is characteristic of metallic compounds. After the fluorination of the films, this shake-down peak disappears and a new spectral weight emerges at a larger Eb. The latter, which is called a “shake-up” satellite peak, is characteristic of insulating compounds and appears when the valence electrons are transferred to the 4f levels inside the gap, thus corroborating the claim that NdNiO3 undergoes the MIT driven by fluorine substitution.


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Note that the Nd 3d5/2 main peak was observed around 983 eV for all the samples, indicating that the Nd ions remained trivalent throughout the fluorination process. The Ni 2p core-level photoelectron spectra are shown in Figure 3b. Each spectrum is characterized by 2p3/2–2p1/2 spinorbit separation and satellite peaks. The Eb value of the Ni 2p peaks is influenced by both the chemical valence of Ni and the counter anions (O2− versus F−). That is to say, the reduction of Ni3+ to Ni2+ in the oxides lowers the Eb value,26–29 while the isovalent substitution of NiO for NiF2 increases the Eb29 value. As can be seen in Figure 3b, the spectral weight shifts to a higher Eb after reductive fluorine substitution, indicating that the effect of the counter anions and not the change in the chemical valence is the dominating factor in the case of NdNiO3−xFx. Figure 4 shows the valence-band HAXPES spectra of the NdNiO3 and NdNiO3−xFx films. In the spectrum of the precursor NdNiO3 film (black curve), the features located at lower binding energies (features a and b) can primarily be attributed to the contribution of the Ni 3d orbital, while those at larger binding energies (features c and d) are mainly owing to the O 2p orbital.30 A finite density of states is observed at EF; this is consistent with the experimentally observed metallic conduction at room temperature. Further, after fluorination, features (a) and (b) disappeared, and the density of states at EF reduced to zero with a bandgap opening, suggesting that the perovskite nickel oxyfluorides are electrically insulating. This change indicates that the insulating characteristics of the NdNiO3−xFx thin films are induced by a Mott-type gap opening and not by rigid-band shifting. In addition, a new spectral structure (e) emerges at an Eb value higher than those of the O 2p-nature states. Considering that the electronegativity of fluorine is higher than that of oxygen, we believe that this feature originates from the F 2p-derived states. Indeed, its relative intensity increased with the fluorine content, while the oxygen-related peaks were suppressed, confirming the substitution of fluorine for oxygen.


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Finally, we discuss the reversibility of the fluorination process. Figure 5a shows the 2θ–θ patterns of the as-grown NdNiO3 film, the 24-h fluorinated NdNiO3−xFx film, and a NdNiO3−xFx film annealed in oxygen at 105 Pa for 5 h at 450 °C. Notably, the lattice constant of the annealed film reverted to that of the as-grown NdNiO3 film, suggesting that fluoride ions were released during the annealing of the film in oxygen while the cation framework was maintained. In fact, a negligible amount of fluorine was observed within this film by energy dispersive X-ray spectroscopic (EDS) analysis (Supplementary Figure S3). The reversible nature of the phase transformation seems surprising because possibly stronger Ni–F bonds are replaced by Ni–O bonds during oxygen annealing. We speculate that the change in the chemical oxidation state of nickel ions plays an important role in the formation of Ni-F or Ni-O bonds. During the reductive fluorination, electrons are added to the Ni3+ ions, promoting the Ni–O bond breaking and the formation of Ni–F bonds. On the other hand, during the oxygen annealing, loss of electrons from the Ni2+ ions breaks the Ni–F bonds. In situ monitoring of the film resistance during the oxygen annealing process (see inset in Figure 5b) revealed an exponential decrease in the film resistivity down to a value comparable to that of the as-grown film. Figure 5b plots the resistivity versus temperature curves for the as-grown and oxygen-annealed films. Both curves indicate that the MIT occurs at ~200 K and the curves overlap very well. Note that the resistivity of the fluorinated film was higher than 104 Ω cm (~ 1.0 GΩ □–1). As shown in the inset of Figure 5a, the black color of the NdNiO3 film faded and the film became transparent after fluorination for 24 h. However, the film became black again after the oxygen-annealing process. These observations confirm that topotactic reversible fluorination is an effective method for modulating electrical conduction in metal-oxide thin films and thus can be used for synthesizing films suitable for resistance-switching applications.


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CONCLUSIONS We investigated the effects of fluorine doping of NdNiO3 thin films using PVDF. After reacting with PVDF, NdNiO3 epitaxial thin films grown on SrTiO3 substrates transformed into films of NdNiO3−xFx, where the fluorine content, x, could be controlled by varying the reaction time. The fluorination of the films resulted in a significant increase in the electrical resistance of more than six orders of magnitude. Furthermore, XAS and HAXPES measurements suggested that a Motttype gap opening is the cause of the extremely high resistance increase associated with the fluorination process. We also found that the resistance of the fluorinated thin films returned to the original value when the films were annealed in an oxygen atmosphere. The reversible resistance modulation of NdNiO3 may lead to device applications, e.g., in thermal or atmospheric sensors. Since it can be performed without expensive setups, e.g., ultra-high vacuum or microfabrication, the reversible fluorination/oxidation of transition-metal oxides would pave the way for the discovery of new physical properties in various transition metal oxides.

EXPERIMENTAL SECTION Epitaxial NdNiO3 thin films with a thickness of ~70 nm were deposited as precursors onto SrTiO3 (100) (STO) substrates (Shinkosha Co., Ltd.) by pulsed laser deposition. The deposition conditions were as follows: substrate temperature of 650 °C, oxygen partial pressure of 13 Pa, laser (KrF excimer, 248-nm wavelength) fluence of 2.0 J cm–2, and repetition rate of 5 Hz. The fabricated precursor films were then reacted with polyvinylidene fluoride (PVDF) pellets (Fluorochem Ltd.) to introduce fluorine. The precursor film was heated with 0.1 g of PVDF in an argon gas flow of 70.0 mL min–1. In keeping with the method previously developed by our group, the film was tightly wrapped in Al foil, and PVDF pellets were placed on the foil (see


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Supplementary Figure S4). Wrapping was used in order to prevent the adhesion of the charcoallike residue produced by the decomposition of PVDF.20 The reaction temperature was fixed at 350 °C, while the reaction time was varied (3–24 h). The crystal structures of the films were characterized by X-ray diffraction (XRD) analysis (Bruker AXS D8 Discover) performed using Cu-Kα radiation. The film thicknesses were measured by the X-ray reflectivity method (Bruker AXS D8 DISCOVER). The chemical compositions of the films were measured using X-ray photoelectron spectroscopy (XPS, ULVAC-PHI PHI5000 VersaProbe), which was performed using an Al-Kα X-ray source and Ar+ sputtering; scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS, JEOL JSM-7100F); and elastic recoil detection analysis (ERDA), which was performed using a 38.4 MeV Cl7+ ion beam generated by the 5 MV tandem accelerator at Micro Analysis Laboratory, Tandem accelerator (MALT), the University of Tokyo.31 The optical gap values were determined using an UV–visible–near-infrared (UV–Vis–NIR) spectrometer (Jasco V670DS). The HAXPES spectra of the core levels and valence bands were measured with an electron energy analyzer (VG SCIENTA Scienta R-4000), which had an energy resolution of 0.27 eV, at a photon energy of 7.94 keV at beamline BL47XU at the SPring-8 facility. The XAS spectra were measured by the total-electron yield method using beamline BL-2A at the Photon Factory, KEK. All the spectra were measured at 300 K. Finally, the electrical resistivities of the samples were measured in a cryostat (Quantum Design PPMS) using the four-wire geometry.


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FIGURES

Figure 1. (a) XRD 2θ–θ patterns of precursor NdNiO3 and fluorinated films. S and F denote diffraction peaks related to SrTiO3 substrate and film, respectively. (b) Depth-dependence of fluorine concentration as measured by XPS with Ar+ sputtering. Film–substrate interface was determined as the location at which the intensities of the Nd, Ni, Sr, and Ti peak change abruptly. (c) RSMs around the 103+ asymmetric diffraction for the as-grown film and those fluorinated for 3 and 24 h. NNO, STO, and NNOF represent NdNiO3, SrTiO3, and NdNiO3−xFx, respectively. (d) XAS spectra of as-grown and 24 h-fluorinated films. Dashed plots are taken from literature.24


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Figure 2. (a) Optical absorption coefficients, α, of NdNiO3 and NdNiO3−xFx thin films prepared using different reaction time. (b) Tauc ((αhν)2 versus hν) plots of insulating films assuming a direct transition. Strong absorption at energies greater than 3 eV is owing to the STO substrate (bandgap: 3.2 eV).


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Figure 3. (a) Nd 3d and (b) Ni 2p HAXPES spectra of NdNiO3 and NdNiO3−xFx films measured at 300 K. Annotations “s.d.,” “s.u.,” in (a) and “sat.” in (b) stand for “shake-down” satellite, “shake-up” satellite, and satellite, respectively.


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Figure 4. Valence band HAXPES spectra of NdNiO3 and NdNiO3−xFx thin films measured at 300 K.


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Figure 5. (a) XRD 2θ–θ patterns of as-grown, 24-h fluorinated, and oxygen-annealed (450 °C for 5 h) films. (b) Electrical resistivity versus temperature plots of as-grown film and oxygenannealed film after fluorination. Inset shows electrical resistance measured in situ during oxygen annealing process.


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ASSOCIATED CONTENT Supporting Information. Raw data and detailed analysis of ERDA measurements; Relationships between the lattice constants, the reaction time, and fluorine contents; SEM-EDS spectra of as-grown, fluorinated, and oxidated samples; Schematic image of the fluorination setup. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. T. O. and A. C. conceived and designed the study. T. O., A. C., and T. K. performed the synthesis, XRD, XPS, and electrical resistivity measurements. A. C., T. O., and E. I. performed the synchrotron HAXPES measurements. A. C., T. O., M. M. and H. K. performed the synchrotron HAXPES measurements. T.O., Y. H., I. H., and D. S. conducted the ERDA measurements. T. O., A. C., and T. H. wrote the manuscript, with contributions and feedback from all the authors. Present Address a) Laboratory for Materials and Structures, Institute for Innovative Research, Tokyo Institute of Technology, 4259, Nagatsuta, Midori-ku, Yokohama, 226-8503 Japan ACKNOWLEDGMENT


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We thank Prof. Hiroyuki Matsuzaki of the University of Tokyo for his assistance with the ERDA measurements. The synchrotron radiation experiments were performed at the Proton Factory, KEK, with the approval of the Photon Factory Program Advisory Committee, KEK (Proposal Nos. 2015G577 and 2015S2-005), and at BL47XU of the SPring-8 facility with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2016A1221). This work was supported by the Kurata Memorial Hitachi Science and Technology Foundation, Core Research for Evolutionary Science and Technology of the Japan Science and Technology Agency (JST-CREST), and Grants-in-Aid for Scientific Research (Nos. 15H05424 and 16H06441) from the Japan Society for the Promotion of Science (JSPS). The depth-resolved XPS measurements were performed at Research Hub for Advanced Nano Characterization, the University of Tokyo; this center is supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. One of the authors (TO) was supported by the Japan Society for the Promotion of Science through the Program for Leading Graduate Schools (MERIT). REFERENCES

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