Directing oxygen vacancy channels in SrFeO2.5 epitaxial thin films

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Directing oxygen vacancy channels in SrFeO epitaxial thin films Amit Khare, Jaekwang Lee, Jaeseoung Park, Gi-Yeop Kim, Si-Young Choi, Takayoshi Katase, Seulki Roh, Tae Sup Yoo, Jungseek Hwang, Hiromichi Ohta, Junwoo Son, and Woo Seok Choi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17377 • Publication Date (Web): 12 Jan 2018 Downloaded from http://pubs.acs.org on January 12, 2018

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ACS Applied Materials & Interfaces

Directing oxygen vacancy channels in SrFeO2.5 epitaxial thin films Amit Khare1,2,3, Jaekwang Lee4, Jaeseoung Park5, Gi-Yeop Kim6, Si-Young Choi6, Takayoshi Katase7, Seulki Roh1, Tae Sup Yoo1, Jungseek Hwang1, Hiromichi Ohta7, Junwoo Son5, and Woo Seok Choi1* 1

2

Department of Physics, Sungkyunkwan University, Suwon 16419, Republic of Korea

Center for Integrated Nanostructure Physics, Institute for Basic Science, Sungkyunkwan University, Suwon 16419, Republic of Korea

3

Department of Physics, Indian Institute of Science Education of Research (IISER) Bhopal – 462 066, India 4

5

Department of Physics, Pusan National University, Pusan 46241, Republic of Korea

Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang 37673, Republic of Korea

6

Materials Modeling and Characterization Department, Korea Institute of Materials Science, Changwon 51508, Republic of Korea

7

Research Institute for Electronic Science, Hokkaido University, Sapporo 001-0020, Japan

ACS Paragon Plus Environment

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KEYWORDS: brownmillerite, epitaxial thin film, electronic structure, crystalline orientation, optical spectroscopy.

ABSTRACT: Transition metal oxides with brownmillerite (BM) structures possess onedimensional oxygen vacancy channels, which play a key role in realizing high ionic conduction at low temperatures. The controllability of the vacancy channel orientation, thus, possess a great potential for practical applications, and would provide a better visualization of the diffusion pathways of ions in transition metal oxides. In this study, the orientations of the oxygen vacancy channels in BM-SrFeO2.5 are stabilized along two crystallographic directions of the epitaxial thin films. The distinctively orientated phases are found to be highly stable and exhibit a considerable difference in their electronic structures and optical properties, which could be understood in terms of orbital anisotropy. The control of the oxygen vacancy channel orientation further leads to modifications in the hydrogenation into the BM-SrFeO2.5 thin films. The results demonstrate strong correlation between crystallographic orientations, electronic structures, and ionic motion in the BM structure.


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INTRODUCTION Ionic conduction in oxygen-deficient transition metal oxides (TMOs) has been attracting increasing scientific attention. In particular, a better understanding of the oxygen-diffusion mechanism has been pursued on a microscopic level, which would remarkably enhance the ability to design and realize functional properties for energy conversion and anion insertion applications.1-11 In this regard, brownmillerite (BM) TMOs are of particular interest, as they accommodate a large number of oxygen vacancies on regular perovskite lattice sites, which are also highly ordered.6,

12-15

The anisotropic BM structure exhibits alternating tetrahedral and

octahedral layers along the orthorhombic b-axis,16,

17

where the oxygen vacancies are fully

ordered within the tetrahedral layer and form one-dimensional oxygen vacancy channels (OVCs) along the orthorhombic a-axis direction.12, 13 In an epitaxial thin film form, the OVCs in the BM structures can be ordered toward different directions with respect to the film surface. Such controllability of the orientation of the crystalline thin films with ordered vacacnies provides important mean to further tailor the ionic functionalities. In principle, the crystallographic orientation can be modified depending on the surface energy, epitaxial strain from the substrate, and crystallographic orientation of the substrate. For example, in the case of La0.5Sr0.5CoO2.5 thin films, it is reported that a compressive strain induces the parallel ordering of oxygen-deficient layers along the growth direction, while a tensile strain stabilizes perpendicular ordering.18,

19

On the contrary, the opposite effect is

observed in the case of strained CaFeO2.5 thin films.3 Theoretically, it has also been demonstrated that the parallel (or perpendicular) ordering of tetrahedral layers is favored under tensile (or compressive) strain for SrFeO2.5 and CaFeO2.5 epitaxial thin films.20


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SrFeO2.5 has a typical BM structure, and shows the anisotropic oxygen-vacancy ordering mentioned above.21 In bulk, it exhibits an insulating ground state and G-type antiferromagnetic ordering with a Neel temperature of ~700 K.22, 23 It has an orthorhombic unit cell with lattice parameters of a = 5.67, b = 15.59, and c = 5.53 Å,22 which can be represented as pseudotetragonal (pt) lattice parameters of apt (cpt) = d101 = 3.960 and bpt = d040 = 3.897 Å. The lattice mismatch between the BM-SrFeO2.5 thin film and SrTiO3 substrate (ac = 3.905 Å) is rather small, i.e., 1.4% (compressive) and −0.20% (tensile) for apt and bpt, respectively, at room temperature. In this study, we employed the delicate balance between the thermodynamic stability and epitaxial strain to control the OVC orientations in SrFeO2.5 epitaxial thin films. In particular, the orientation of the one-dimensional OVC in the BM structure could be converted from an in-plane to an out-of-plane direction by slightly changing a thermodynamic parameter during the epitaxial thin film growth. The successful control of the OVC orientation enabled straightforward access to the anisotropic electronic structures and optical properties of the BM structure, even in the thin-film form. The density functional theory (DFT) calculation implies the anisotropic orbital dependence of the optical transitions, which stems from the layered crystalline structure. Based on the control of the OVC orientation, we also demonstrate the modulation of the hydrogen insertion into the thin film, suggesting a practical application of the approach. EXPERIMENTAL DETAILS Thin film growth and structural characterization. High quality epitaxial SrFeO2.5 thin films (thickness ~18 nm) are grown by pulsed laser epitaxy (PLE) on TiO2-terminated (001) SrTiO3 single crystalline substrates, using a KrF excimer laser (λ = 248 nm; IPEX 864, Lightmachinery) at a repetition rate of 1 Hz and a laser fluence of 1.5 J/cm2 at the target surface. The growth temperature was 700°C, with an oxygen partial pressure P(O2) of 10 mTorr. After the growth, we


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performed post-annealing at the same growth pressure and temperature for 10 min in order to control the OVC orientation. Film thickness of 18 nm was chosen to be thin enough to maintain the substrate induced coherent strain on the films, but thick enough to perform the optical measurements. We observed similar phase formation for thicker films as well (at least up to 55 nm). The crystalline structure, orientations, and the epitaxial relation of SrFeO2.5 thin films were characterized using high resolution x-ray diffraction (XRD) (Smart lab, Rigaku). For a detailed structural and micro structural characterization, we performed scanning transmission electron microscopy (STEM). The specimens were prepared by a focused ion multi-beam system. To protect the films, an amorphous carbon layer was deposited on the top surface before the ion beam milling. Then, a Ga+ ion beam was used to fabricate the thin TEM lamella. To minimize the surface damages induced by the Ga+ ion beam milling, the sample was further milled by an Ar+ ion beam (PIPS II, Gatan) with an acceleration voltage of 100 meV for 4 min. Atomic scale STEM images were taken using a scanning transmission electron microscope (JEM-2100F, JEOL) at 200 kV with a spherical aberration corrector (CEOS GmbH). The optimum size of the electron probe was ~0.9 Å. The collection semi-angles of the high angle annular dark field (HAADF) detector were adjusted from 70 to 200 mrad in order to obtain large-angle elastic scattering electrons for clear Z-sensitive images. The obtained raw images were processed using a band-pass Wiener filter with a local window to reduce the background noise (HREM research Inc.). Reflectance and spectroscopic ellipsometry measurements. Room temperature reflectance spectra were obtained over a wide spectral range (IR-VIS-UV) using an FT-IR spectrometer (Vertex 8v, Bruker) and a monochromatic spectrophotometer (Lambda 950, Perkin-Elmer). Room temperature spectroscopic ellipsometry was performed using variable angle spectroscopic


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ellipsometry (VASE, J. A. Woollam Co.) for multiple angles (60°, 65°, and 70°). In order to extract physically reasonable dielectric functions from the ellipsometric angles of Ψ and ∆, a simple two-layer model (film/substrate) was used. Density Functional Theory. All calculations are carried out using density-functional theory (DFT) with the plane-wave based Vienna ab initio package (VASP).24, 25 We used the projector augmented wave (PAW) method of Blöchl in the implementation of Kresse and Joubert.26 The generalized gradient approximation (GGA), due to Perdew-Burke-Ernzerhof (PBE) has been used to estimate the exchange-correlation functional. The plane-wave cutoff energy of 500 eV was applied, and the integration of the Brillouin zone was conducted withГ-centered 4×4×2 kpoint meshes for the bulk BM SrFeO2.5. The calculations are converged in energy to 10−7 eV/cell, and the structures are relaxed until the forces are less than 5 × 10−3 eV/Å. In order to improve the description of the Fe-3d states in BM SrFeO2.5, we applied a Hubbard U correction using a rotational invariant scheme. Using Ueff (U – J) =3 eV, we find that the crystalline structures of BM SrFeO2.5 have orthorhombic lattice constants of 5.75, 15.62, and 5.58 Å, which is in good agreement with the experimental values of 5.67, 15.59, and 5.53 Å, respectively. These values correctly reproduce the G-type antiferromagnetic order and the insulating state of BM SrFeO2.5. The imaginary part of the dielectric function is obtained, within the independent particle approximation in the long-wavelength limit q→0, by a summation over empty states using the following equation: ε

1 4 = lim 2 , − Ω ⟶ ,,

,

− ! 〈#,$%&' |#

〉 〈#, $%*' |# , 〉 ,

∗

where the indices v and c refer to the valence and conduction band states respectively, and #,, is the periodic part of the Bloch waveat the k-point k (all-electron Kohn-Sham wave functions


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with band indexn at thek-point kare written as -, = #,, . /∙1 ),ωk are the k-point weights, which are defined such that their sum is 1; the factor 2 accounts for spin degeneracy. The vectors q are unit vectors for the three Cartesian directions and Ω is the volume of the primitive cell. Hydrogenation and dynamic secondary ion mass spectroscopy (D-SIMS) measurements: To effectively inject hydrogens into the samples, nano-sized Pt islands were deposited on the film surfaces using the RF sputtering method.27 The hydrogenation process was performed at various temperatures (100-400°C) with a forming gas (H2(5%)/Ar(95%)) atmosphere for 1h. The structural change caused by hydrogenation was characterized using high resolution x-ray diffraction (XRD) (Discover 8, Bruker). The incorporated hydrogens in BM structure were detected by D-SIMS (IMS 4FE7) depth profiles. RESULTS AND DISCUSSION The control of the OVC orientation in the BM-SrFeO2.5 epitaxial thin films is evidenced by various structural characterizations. Figure 1 summarizes the crystalline structures of BMSrFeO2.5 epitaxial thin films deposited on (001) SrTiO3 substrates. As schematically shown in Figs. 1(a) and 1(b), the OVC along the [101]pt direction of the BM structure can be oriented either along the in-plane (BMip, Figure 1(a)) or the (partial) out-of-plane (BMoop, Figure 1(b)) direction of the epitaxial thin film surface. For BMip (BMoop), the alternate stacking of the octahedral (FeO6) and tetrahedral (FeO4) layers is along the out-of-plane (in-plane) direction. The controlled crystallographic orientation of the epitaxial thin films is revealed by highresolution x-ray diffraction (XRD) θ-2θ scans along the out-of-plane and in-plane directions of the epitaxial thin film. For BMip, half-order reflections resulting from the alternate stacking of octahedral and tetrahedral layers are observed for the out-of-plane XRD measurement, while this feature is absent for the in-plane measurement (Figure 1(c)). On the contrary, for BMoop, half-


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order reflections are observed for the in-plane measurement only (Figure 1(d)). XRD reciprocal space maps (RSM) around asymmetric (103)c reflection (subscript c denotes cubic) of SrTiO3 substrate also show the expected orientation of samples in addition to the coherently strained state of the epitaxial thin films. For BMip, only the (1121)o reflection (subscript o denotes orthorhombic) of the SrFeO2.5 epitaxial thin film is observed near the SrTiO3 (103)c reflection (Figure 1(e)), as expected. On the other hand, (204)o, (343)o, and (402)o reflections are observed for BMoop, indicating that the OVCs in BMoop are actually aligned along the out-of-plane direction (Figure 1(f)). Cross-sectional scanning transmission electron microscopy (STEM) results further confirm the orientations and ordering of OVCs of SrFeO2.5/SrTiO3 epitaxial thin films. The alternate stacking of the octahedral and tetrahedral layers is manifested by the periodic dark stripes shown in the STEM images, representing the expected oxygen-deficient layers (Figures 1(g) and 1(h), for BMip and BMoop, respectively).6 We additionally note that while the rotation of the OVCs is conspicuous between BMip and BMoop, the ordering of the channels within the tetrahedral layer might be different. To date, only few reports experimentally demonstrated rotation of the OVC orientation in BM structures by either imposing different surface symmetry or degrees of epitaxial strain. For example, Inoue et al. reported two different orientations of CaFeO2.5 epitaxial films in the apt and bpt directions using distinctive substrates such as SrTiO3,

(LaAlO3)0.3(Sr2AlTaO6)0.7,

LaAlO3, and LaSrAlO4.3 On the other hand, Jeen et al. stabilized the OVC orientations along the in-plane and out-of-plane directions in SrCoO2.5 thin films using different crystallographic orientations of the substrate (Yttria-stabilized ZrO2), and proposed that OVC orientations play a decisive role in catalytic activity.13 In the current study, the control of the orientations of OVC in SrFeO2.5 thin films was realized by simple post-growth kinetics on the same SrTiO3 substrate.


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When the SrFeO2.5 epitaxial thin film was cooled down immediately after the pulse laser epitaxy (PLE) growth, a BMoop phase was obtained. On the contrary, when in-situ post-annealing (10 min at the same temperature and ambient condition) was performed just after the growth, a BMip phase was obtained. The phase could also be controlled using different temperatures and laser energy fluence during PLE. We note that both the BM phases are highly robust and stable with time in terms of oxygen stoichiometry (more than a year without any noticeable change). In particular, it is very difficult to go below x = 2.5 by using only physical means for SrFeOx (See Fig. S1).11 The simple control of the macroscopic crystallographic orientation suggests a very fine balance between the thermodynamic phase stability, surface energy, and epitaxial strain in BM-SrFeO2.5 thin films. We first note that theoretical prediction provides certain insight to this exceptional quality of the BM-SrFeO2.5 thin films on the SrTiO3 substrate. Indeed,Young and Rondinelli explicitly showed that the energy difference between the BMip and BMoop configuration of SrFeO2.5 thin films is the smallest (less than 15 meV/f.u.) when deposited on the SrTiO3 substrate.20 While the energy barrier between the two should also be considered which will be further discussed later, the small energy difference between the BMip and BMoop obviously facilitates the transformation. In addition to the small energy difference, the anisotropic thermal expansion in the BM structure can also be considered.22, 28, 29 The thermal expansion coefficient of SrFeO2.5 along the stacking direction of the octahedral and tetrahedral layers (bpt direction, 2×10-5 Å K-1) is larger than that along the perpendicular direction (apt direction, ~1×10-5 Å K-1). The thermal expansion coefficient of SrTiO3 is ~1×10-5 Å K-1, resulting in an anisotropic temperature evolution of the epitaxial strain. Because the thermal expansion coefficient is similar for the apt of SrFeO2.5 and


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SrTiO3, the epitaxial strain (1.4%) is similar for the entire temperature range. However, the lattice mismatch between the bpt of SrFeO2.5 and SrTiO3 approaches zero as the temperature increases, and at the growth temperature (700°C), the tensile strain reduces to −0.10%. Therefore, the BMoop phase is expected during the growth of the thin film. Although very short, thermal annealing supplies additional energy to the system, and may “relax” structures that are not based on the epitaxial strain, but on the chemical stability at the surface and interface. In particular, while BMoop has 50% of octahedral FeO6 and 50% of tetrahedral FeO4 units, BMip has either fully octahedral or fully tetrahedral units facing the SrTiO3 substrate as well as the surface. Indeed, the ground state of the BM-SrFeO2.5 thin film on the SrTiO3 substrate is with BMip, although the difference is rather small.20 After all, rotation of the OVC orientation is achieved through simple rearrangements of oxygen which would not require large energy, especially considering the high oxygen mobility in SrFeOx system.11,30 The anisotropic electronic structure and optical properties in BM-SrFeO2.5 are readily observed even in the thin film form which merits from the controllability of the crystallographic orientation. Figures 2(a) and 2(b) show in-plane optical conductivities as a function of photon energies (σ1(ω)) for BMip and BMoop-SrFeO2.5 thin films, respectively. The optical band gap is about 2.1 eV for both of the thin films, which is consistent with the reported bulk band gap of SrFeO2.5.31, 32 The optical absorptions above the band gap can be attributed to various charge transfer excitations between O 2p and Fe 3d states within the octahedral and tetrahedral crystal fields. The transitions can be further deconvoluted into five Lorentz oscillators located at ~2.7, ~3.3, ~3.6, ~4.0, and ~4.4 eV. Using the Lorentz oscillators, i.e., 21 =

67 87 2 2 ∑7 , 2 ∗ 4 9 72 − 2 : + 872 2


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we obtained excellent fits of σ1(ω), as shown in Figures 2(a) and 2(b). Here, e and m* represent the electronic charge and the effective mass, respectively, and nj, γj, and ωj are the carrier density, the scattering rate, and the resonant frequency of the j-th oscillator, respectively. In particular, the optical feature at ~2.7 eV is mainly attributed to a low-lying charge transfer transition between O 2p and Fe 3d states. The optical absorptions at energies ~3.3, ~3.6, ~4.0 and ~4.4 eV can be thought to have mainly O 2p → Fe 3d t2goct, O 2p → Fe 3d egtetra, O 2p → Fe 3d egoct and O 2p → Fe 3d t2gtetra characters, respectively, based on the comparison with the DFT calculation as discussed below.11 The fitting parameters are summarized in Table 1. While the positions (ωj) and the widths (γj) of each oscillator do not change significantly, BMip and BMoopSrFeO2.5 thin films exhibit noticeable differences in their spectral weight. The spectral weight transfers to the peaks located at lower energy as the orientation of the thin film changes from BMip to BMoop. In addition, the phonon spectra are cleary distinguished for the different orientations, although a detailed understanding of the origin of each phonon lines is a topic for another study. The anisotropic optical properties can be understood in terms of the orbital-selective anisotropic electronic structure. Figures 2(c)-2(e) show the orbital-dependent electronic density of states (DOS) for the SrFeO2.5, obtained from the DFT calculation. The DOS for SrFeO2.5 shows complicated features that result from both an octahedral (Fe 3do) and tetrahedral (Fe 3dt) crystal field for Fe electronic states. Most of the Fe 3d states are strongly hybridized with O 2p states for both occupied and unoccupied states. Our theoretical and optical observations are also consistent with the reported O K-edge x-ray absorption spectra, where the degree of hybridization between Fe and oxygen is reported.33 Based on the anisotropic crystalline structure,


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orbitlas with in-plane (dxy and dx2−y2) and out-of-plane (dyz, dzx, and d3z2−r2) components would dominate the optical transitions for BMip and BMoop-SrFeO2.5 thin films, respectively. Both the occupied and unoccupied states move together for the in-plane and out-of-plane orbitals, which makes the differences in the energy states rather small. Nevertheless, the in-plane orbitals are more extended toward a larger separation, which demonstrates the increased optical absorption at higher energy in BMip-SrFeO2.5. Moreover, for BMoop-SrFeO2.5, inter-site charge transfer transitions between Fe 3dt and Fe 3do states may also be allowed, which could enhance the lowlying excitations, e.g., ωj = 3.3 and 3.6 eV. Similar inter-site transitions are not possible for BMip-SrFeO2.5 thin films because the oscillations of the electric field are only along the plane with the same structural unit. From the imaginary part of the frequency-dependent dielectric function, ε2(ω) (Figure 2(f)), we can further confirm that in-plane and out-of-plane orbitals actually dominate the optical transitions for BMip and BMoop-SrFeO2.5 thin films, respectively, which qualitatively captures the optical characteristic observed experimentally (Figure 2(g)). By controlling the OVC orientation, we could observe a drastic contrast in the hydrogenation into the SrFeO2.5 epitaxial layers. In order to effectively inject hydrogen, we annealed the epitaxial layers with Pt nano islands in forming gas ambient using hydrogen spillover methods at various temperatures between 100 and 400οC. Without hydrogen (in vacuum or oxygen ambient), both the BMip and BMoop-SrFeO2.5 thin film structures were highly stable against thermal treatment in various reducing ambient conditions (see Figure S1). Figures 3(a) and 3(b) show the XRD results of the hydrogenation process for the BMip-SrFeO2.5 thin films. A systematic expansion in the out-of-plane lattice parameters was observed with an increased annealing temperature, as shown in the inset of Figure 3(a), indicating a gradual insertion of hydrogen into the sample and hydrogen-induced chemical expansion. Figures 3(c) and 3(d) show the D-SIMS


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depth profile for after and before the hydrogenation, respectively. The signature of hydrogen shown as the black line manifests the actual presence of hydrogen in the BMip-SrFeO2.5 thin film. When the hydrogens were incorporated into the SrFeO2.5, hydrogens donate electrons to reduce the Fe3+ to form Fe2+. The systematic lattice expansion caused by hydrogen incorporation also corresponds to the increase in the ionic radii of iron ions in ferrites during the reduction from Fe3+ to Fe2+.34-36 We further note a recent study on hydrogenation of the BM structure emphasizing the hydrogen insertion and corresponding valence state change of the cations in TMO thin films.37 For example, in a hydrogenated SrCoO2.5 thin film, increased optical transparency and band gap were reported. While optical measurements on hydrogenated SrFeO2.5 thin films were not plausible due to the deposited Pt nanocatalyst, we can expect similar optical properties as well. In contrast to the BMip, the BMoop-SrFeO2.5 thin films did not show any discernible structural change (at least along the out-of-plane direction) (not shown). This result seems to be rather counter-intuitive because OVCs are usually identified as the channels for ionic conduction, e.g., for oxygen ions in BM-SrCoO2.5.38 In the same context, it would also be natural to consider more fluent hydrogen conduction along the OVC direction. Because BMoop has open OVCs onto the surface, hydrogen will be more easily incorporated into the sample. However, if the OVCs can be used for the hydrogen conduction, the easily incorporated hydrogen will be able to escape the BMoop-SrFeO2.5 thin films more easily as well. On the contrary, it will be more difficult for hydrogen to be incorporated into the BMip-SrFeO2.5 thin films because the OVCs are along the in-plane direction only. However, once incorporated, it will be much more likely for hydrogen to be captured within the thin film. While the detailed understanding of the hydrogenation is beyond the scope of the present work, the highly anisotropic hydrogenation along the OVCs in


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BM-SrFeO2.5 exemplieds the importance of crystallographic orientation engineering in epitaxial thin films.

CONCLUSION In summary, we studied the thermodynamic orientation control of the oxygen vacancy channels along the in-plane and out-of-plane directions within brownmillerite SrFeO2.5 epitaxial thin films. The orientation control could be achieved via the delicate balance between the thermodynamic phase stability and epitaxial strain in brownmillerite SrFeO2.5 thin films. The orientation of the oxygen vacancy channels influences the optical properties significantly via orbital selective anisotropic optical transitions. Furthermore, anisotropic hydrogenation was demonstrated for the BM-SrFeO2.5 thin films, highlighting the role of the anisotropic atomic structure in realizing ionic transport and storage. Our results offer a comprehensive understanding on the oxygen vacancy channel orientations and design principles for the brownmillerite structure.


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Figures

Figure 1. Crystal structures and orientation control of brownmillerite SrFeO2.5 epitaxial thin films. Schematic representations of FeO6 octahedra (light green squares) and FeO4 tetrahedra (green triangles) arrangements for (a) BMip and (b) BMoop-SrFeO2.5 epitaxial thin films on SrTiO3 substrates. The thick arrows in the coordinate system represent the directions of the oxygen vacancy channels. In-plane and out-of-plane x-ray diffraction (θ-2θ) patterns for epitaxial (c) BMip and (d) BMoop-SrFeO2.5 epitaxial thin films on SrTiO3 substrate. Reciprocal space maps of


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(e) BMip and (f) BMoop-SrFeO2.5 thin films around the (103)c Bragg reflection of the SrTiO3 substrates. Cross-section scanning transmission electron microscopy (STEM) images of (g) BMip and (h) BMoop-SrFeO2.5 thin films. Arrows show tetrahedral layers. The scale bar represents 1

2

a

BMip

2

b

BMoo p

3

-1

-1

σ1(ω) (10 Ω cm )

nm.

1

1

0 0.00 0.04 0.08

2

0 0.00 0.04 0.08

4

ω (ev)

2

4

ω (ev) o

3 c

Fe 3 d

DOS (states per ev f.u.)

0 dxy dx -y

-3 3

2

dyz dz

2

dzx

2

t

d

Fe 3d

0 -3 6

dxy dx -y 2

dyz dz

2

dzx

2

e

O 2p

3 0 -3 -6 -4

-3

-2

-1

0

2px

2py

2pz

1

2

3

Energy (eV) 8

Theory

f

6

ε2(ω)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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BMpi BMo op

4

4

Experiment

g

2

2 0

1

2

3

ω (eV)

4

5

0

1

2

3

4

5

ω (eV)

Figure 2. Optical properties and electronic structures of anisotropic SrFeO2.5 epitaxial thin films. σ1(ω) for (a) BMip and (b)BMoop-SrFeO2.5epitaxial thin films. Low(high) energy spectra are obtained by infra-red spectroscopy (spectroscopic ellipsometry). Orbital selective partial density


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of states for Fe (c) 3do, (d) Fe 3dt, and (e) O 2p for the BM-SrFeO2.5. (f) Theoretically calculatedand (g) experimental ε2(ω) for BMip and BMoop-SrFeO2.5 epitaxial thin films.

Lattice constant (Å)

Intensity (arb. units)

a BMip

b

3.970 3.965 3.960 0

100 200 300 400 Annealing T (°C)

400°C 350 300 250 200 100 Pristine

10

20

30

40

45

2θ (degrees)

46

47

2θ (degrees)

6

Intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10

after hydrogenation

before hydrogenation

5

10

4

10

3

10

2

10

1

10

Ti Fe Sr

H O

0

20

40

60

Depth (nm)

80

c

d 0

20

40

60

80

100

Depth (nm)

Figure 3. Hydrogenation of BMip-SrFeO2.5 epitaxial thin film. (a) and (b) x-ray diffraction θ-2θ patterns for BMip-SrFeO2.5 epitaxial thin film annealed in hydrogen atmosphere at different temperatures. On hydrogenation, the intensity of half-order peak decreases and disappears at 400°C, inducing disorders. Lattice parameter (out-of-plane) increases with the increasing hydrogenation temperature (Inset). D-SIMS depth profile for Sr, Fe, Ti, H, and O ions for the BMip-SrFeO2.5 thin films (c) after and (d) before the hydrogenation.


17


Page 18 of 25

Table 1. Peak position, width, and spectral weight for BM SrFeO2.5/SrTiO3 thin films

Peak

BMip

BMip

BMoop

BMoop

position

Peak width

Spectral

Peak width

Spectral

[eV]

[eV]

weight

[eV]

weight

[eV2]

[eV2]

2.7

0.62

1.68

0.55

1.11

3.3

0.70

3.48

0.75

5.17

3.6

0.66

3.97

0.57

3.71

4.0

0.65

4.87

0.62

3.56

4.4

1.30

6.58

1.35

6.58

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Confirmation of phase stability by annealing of BMip and BMoop-SrFeO2.5 thin films in different ambient Corresponding Author *Prof. Woo Seok Choi, E-mail: [email protected]


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Author Contributions A.K., H.O., and W.S.C. conceived and designed the experiments. A.K. fabricated the samples. A.K. and T.K. carried out the XRD measurements and structural analyses. A.K. and T.S.Y. performed the room temperature ellipsometry, J.L. performed the DFT calculations, and G.-Y.K. and S.-Y.C. performed the STEM measurements. S.R. and J.H. preformed IR-spectroscopy. J.P. and J.S. performed the hydrogenation and D-SIMS measurements, A.K. and W.S.C. wrote the manuscript with the input from all the authors. W.S.C. supervised the project. ACKNOWLEDGMENT We thank insightful discussion with J. Yu, J. Young, and J. Rondinelli. This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (NRF2017R1A2B4011083). A.K. also acknowledges the DST, India for the INSPIRE Faculty Award (DST/INSPIRE/04/2015/001736). H.O. was supported by JSPS-KAKENHI (No. 17H01314, 25106007). This work was also supported in part by the Network Joint Research Center for Materials and Devices. REFERENCES (1) Sengodan, S.; Choi, S.; Jun, A.; Shin T. H.; Ju, Y. –W.; Jeong, H. Y.; Shin, J.; Irvine, J. T. S.; Kim, G. Layered oxygen-deficient double perovskite as an efficient and stable anode for direct hydrocarbon solid oxide fuel cells. Nat. Mater. 2015, 14, 205-209. (2) Mueller, D. N.; Machala, M. L.; Bluhm, H.; Chueh, W. C. Redox activity of surface oxygen anions in oxygen-deficient perovskite oxides during electrochemical reactions. Nat. Commum. 2015, 6, 6097.


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Table of Contents


25

Directing oxygen vacancy channels in SrFeO2.5 epitaxial thin films

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