Magnetic and Photoluminescent Coupling in SrTi0.87Fe0.13O3âÎ´

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Magnetic and Photoluminescent Coupling in SrTi0.87Fe0.13O3−δ/ZnO Vertical Nanocomposite Films Chen Zhang,*,†,‡ Dong Hun Kim,† Xiaohu Huang,§ Xue Yin Sun,† Nicolas M. Aimon,† Soo Jin Chua,*,‡,§ and Caroline A. Ross*,† †

Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States ‡ Singapore-MIT Alliance, National University of Singapore, 4 Engineering Drive 3, Singapore 117576 § Institute of Materials Research and Engineering, 2 Fusionopolis Way, Singapore 138634 S Supporting Information *

ABSTRACT: Self-assembled growth of SrTi0.87Fe0.13O3−δ (STF)/ZnO vertical nanocomposite films by combinatorial pulsed laser deposition is described. The nanocomposite films form vertically aligned columnar epitaxial nanostructures on SrTiO3 substrates, in which the STF shows room-temperature magnetism. The magnetic properties are discussed in terms of strain states, oxygen vacancies, and microstructures. The nanocomposites exhibit magneto-photoluminescent coupling behavior that the near-band-edge emission of ZnO is shifted as a function of magnetic field. KEYWORDS: vertical nanocomposite, self-assembly, three-dimensional heteroepitaxy, vertical interface, strain, magneto-photoluminescent coupling

1. INTRODUCTION A wide range of useful materials properties are influenced by the presence of interfaces between dissimilar oxide thin-film materials.1,2 Self-assembled vertical nanocomposite films, in which two codeposited oxides grow as columnar structures with interfaces oriented normal to the plane, present a convenient geometry for studying interface effects.3−5 Most studies of these nanocomposites have focused on perovskite and spinel phases codeposited on a perovskite substrate, in particular BiFeO3/ CoFe2O46−10 grown on SrTiO3 (STO), but other combinations of oxides have been explored, including perovskite/wurtzite (e.g., ZnO11−18), perovskite/fluorite (e.g., CeO219−23), perovskite/rocksalt (e.g., MgO24,25 or NiO26,27), and perovskite/ monoclinic (e.g., Sm2O312,28−31) structures. These nanocomposites show coupled properties, a prominent example being the magnetoelectric coupling seen in multiferroic BaTiO3/CoFe2O4, BiFeO3/CoFe2O4, and others,3,32−34 in which an electric field affects the magnetic properties and a magnetic field affects the polarization. This cross-coupling is mediated via strain transfer between the magnetoelastic and piezoelectric phases at the vertical interfaces. Nanocomposites also exhibit enhancement of the physical properties of one of the oxide phases, including low-field magnetotransport or magnetoresistance,12−14,19 ferroelectric Curie temperature,28 ionic conductivity,20 and reduced dielectric loss and leakage current.4,12,29 However, the inherent potential of vertical nanocomposite structures for effective coupling of two physical parameters has © 2017 American Chemical Society

not yet been widely realized. In contrast to conventional lateral multilayers with in-plane (IP) interfaces, vertical nanocomposites possess advantages including growth to large thicknesses without the propagation of misfit dislocations, a large interfacial area, and the absence of interfacial strain clamping from the substrate. The study of strain-coupling phenomena in this film architecture is still in its infancy. In this work, we demonstrate a new magnetophotoluminescent coupling effect that is enabled in a vertical nanocomposite structure. The nanocomposite in this work combines a magnetic perovskite SrTi0.87Fe0.13O3−δ (STF)35,36 and a luminescent wurtzite ZnO codeposited on STO. STF exhibits useful magnetooptical,37 electrochemical,38 and photocatalytic39 properties and can be epitaxially grown on Si substrates by the use of buffer layers, potentially enabling device integration. ZnO, due to its direct band gap and large exciton binding energy, is an important photonic material for applications such as lighting40−42 and photovoltaics.43 Recent progress has shown that the electronic band structures of ZnO nanowires can be effectively tuned by an elastic strain gradient,44−47 making the self-assembled vertical nanocomposite structure an ideal model system that enables in situ control of optical properties of ZnO via magnetic means. This article describes the microstructures, Received: June 17, 2017 Accepted: August 30, 2017 Published: August 30, 2017 32359

DOI: 10.1021/acsami.7b08741 ACS Appl. Mater. Interfaces 2017, 9, 32359−32368

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Figure 1. (a) XRD θ−2θ scan of single-phase ZnO film, single-phase STF film, and STF/ZnO nanocomposite on (001) STO substrates. Magnified XRD θ−2θ scan of (b) single-phase STF and STF/ZnO nanocomposite near the STF (002) diffraction peak and (c) single-phase ZnO and STF/ ZnO nanocomposite near the ZnO (112̅0) diffraction peak. In (b), the stronger peak is from STO and the weaker peak is from STF.

3. RESULTS AND DISCUSSION Figure 1a shows the XRD θ−2θ scan of single-phase ZnO, single-phase STF, and nanocomposite STF/ZnO. In the nanocomposite, distinct peaks from both phases can be seen in addition to the substrate peaks. The films are highly textured with STF showing (00l) peaks and ZnO showing (1120̅ ) peaks. Figure 1b displays the high-resolution XRD (HRXRD) spectra near the STF (002) peak from single-phase STF and from an STF/ZnO nanocomposite. The lattice parameter of bulk stoichiometric STF (i.e., without oxygen deficiency) is expected to be smaller than that of the STO substrate due to the smaller ionic radius of Fe4+ compared to Ti4+. However, STF films typically grow with an oxygen deficiency and the Fe is present as mixed-valence Fe3+ and Fe2+, yielding a larger lattice parameter49 and explaining why the single-phase STF grown epitaxially on STO exhibits an out-of-plane (OP) lattice parameter larger than that of STO. The STF in the composite exhibits a smaller out-of-plane lattice parameter compared to that of single-phase STF. Figure 1c shows the HRXRD spectra near the ZnO (1120̅ ) peaks from ZnO and STF/ZnO. The ZnO (112̅0) peak in the nanocomposite is in the same position and has the same width as that of the single-phase film on STO, and there is no evidence of peaks from parasitic Fe2O3 or ZnFe2O4 (spinel) phases. The in-plane orientations, lattice parameters, and strain states of STF/ZnO nanocomposites were studied using RSM. The structural parameters of the STF/ZnO nanocomposites and single-phase STF and ZnO are summarized in Table 1. The RSM asymmetric scans of the STF in the nanocomposite, the single-phase STF film, the ZnO in the nanocomposite, and the single-phase ZnO film are shown in Figure 2a−d, respectively. In both nanocomposite and single-phase films, the STF is tetragonally strained with c/a ratio >1, where c is the out-ofplane and a is the in-plane lattice parameter. In the single-phase STF film, the in-plane lattice parameter matches that of the STO substrate, and the film is tetragonally distorted with outof-plane tensile strain due to the Poisson effect. For STF in the nanocomposite, both the in-plane and out-of-plane lattice

strain states, and magnetic properties of ZnO/STF nanocomposites as a function of deposition conditions and demonstrates magnetic-field-dependent photoluminescence (PL).

2. EXPERIMENTAL METHODS Single-phase STF and ZnO and STF/ZnO nanocomposite films were grown on single-crystal (001) STO substrates by pulsed laser deposition (PLD) at 650 °C in 3 × 10−6 Torr vacuum ambient, using a KrF laser at an operating wavelength of 248 nm. The pulse fluence was 2.6 J/cm2 with repetition rates of 2−10 Hz. The target-tosubstrate distance was 8 cm. The thickness of the nanocomposite films ranges from 150 to 200 nm. Self-assembled STF/ZnO nanocomposites were developed by a combinatorial method,48 by which STF and ZnO targets were successively ablated, creating “layers” with thickness less than one monolayer for each ablation cycle. The nanocomposite structures were formed via self-assembled phase separation, as schematically shown in Figure S1. All of the nanocomposite films (STF)x/(ZnO)1−x analyzed in this work were of ∼50:50 molar ratio (x ≈ 0.5), determined using wavelengthdispersive X-ray spectroscopy (WDS), except for the sample used for transmission electron microscopy (TEM) characterization, for which an STF-rich sample is used to conveniently distinguish the two phases. X-ray diffraction (XRD) was carried out by a Bruker D8 Discover high-resolution triple-axis X-ray diffractometer (Cu Kα radiation), with a linear position sensitive detector for collecting reciprocal space maps (RSMs). The quantification of lattice parameters from RSM was calibrated by STO substrate position and film tilt. Cross-sectional TEM was conducted on a JEOL 2010F field emission microscope, with the TEM sample prepared by focused ion beam technique in Helios NanoLab 600. Atomic force microscopy (AFM) was carried out using a NanoScope Dimension IV microscope. The composition of the targets and films was measured by wavelength-dispersive X-ray spectroscopy. The magnetic properties of the films were measured by vibrating sample magnetometry (VSM) (ADE model 1660) and a quantum design superconducting quantum interference device (SQUID) (MPMS 5S Quantum Design magnetometer). X-ray photoelectron spectroscopy (XPS) was carried out using a PHI VersaProbe II X-ray photoelectron spectrometer. The binding energies were calibrated by the C 1s peak at 284.8 eV. The optical properties of the films were studied using micro-PL using a Renishaw Ramanscope 2000 with a He−Cd laser at an excitation wavelength of 325 nm. 32360


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Figure 4a displays magnetic hysteresis loops of the nanocomposite measured by VSM at room temperature. The film shows in-plane magnetic anisotropy with a saturation magnetization of ∼1.2μB/Fe and coercivity of ∼100 Oe. For comparison, measurements of bare STO substrates annealed in the PLD chamber at the same temperature as the growth temperature showed negligible magnetic moment. The inset in Figure 4a shows the zero-field-cooled (ZFC) and field-cooled (FC) magnetization versus temperature curves. The ZFC and FC curves diverge at ∼60 K, which has been attributed to the coexistence of ferromagnetic and antiferromagnetic interactions induced by the mixed-valence Fe ions.51 Transition metal ions, such as Fe3+ in the B sites of perovskites, are usually antiferromagnetically coupled, such as in BiFeO3 , but ferromagnetic coupling can arise between Fe ions of mixedvalence states via double exchange. Considering the interdiffusion of ions in the nanocomposites, the solubility of Fe3+ and Fe4+ in ZnO is reported to be very small, 0.05−0.5 atom % for Fe3+, but the solubility of Fe2+ is higher.52 Therefore, we cannot exclude the presence of some Fe2+ in the ZnO. This would not be detectable from XRD because Fe2+ substituting for Zn2+ leads to only a small increase of the ZnO lattice parameter of ∼0.01 Å for ∼10 atom % Fe2+ substitution.53 We note that any Fe present in the ZnO could contribute to the magnetization of the nanocomposite, but the reported magnetization of Fe in ZnO varies widely.54−56 The hysteresis loops of the single-phase STF film are given in Figure 4b as a comparison. This indicates an out-of-plane easy axis and a larger coercivity of 2.5 kOe. The reported coercivity of single-phase STF films having the same Fe content as this work ranges from ∼100 Oe35 to ∼2.6 kOe,36 with the difference in coercivity attributed to factors such as film thickness.35 We now discuss the magnetic anisotropy in nanocomposites and single-phase STF films. For single-phase STF films grown on STO substrates under in-plane compressive strain, the perpendicular magnetic anisotropy is attributed to magnetoelasticity arising from Fe2+ and Fe4+ ions located at octahedral sites.36 For Fe4+ (d4), stabilization originates from the splitting of the eg orbital degeneracy (Jahn−Teller effect), whereas for Fe2+ (d6), stabilization originates from the t2g state. No magnetoelastic effects are expected from high-spin Fe3+ (d5) in octahedral sites due to the half-filled 3d shell. In nanocomposites, strain plays an even more important role in magnetic properties far beyond critical thickness.57 In this work, the XRD data in Figure 1b show that the STF peak shifts to the right in nanocomposites, compared to the single-phase STF film. Assuming constant oxygen deficiency in the nanocomposites and single-phase STF films, the reduced c-lattice parameter in nanocomposites is solely attributed to the ZnOinduced vertical compressive strain. This reduced vertical tensile strain in STF phase can be responsible for the inplane magnetic anisotropy in the nanocomposites. Shape anisotropy plays a minor role in the net anisotropy because the magnetic moment is small, and magnetocrystalline anisotropy does not contribute because the ⟨100⟩ in-plane and out-of-plane directions are equivalent. To further examine the magnetism in the nanocomposites, samples deposited at different laser repetition frequencies (2, 5, and 10 Hz) were compared. The XRD θ−2θ data (Figure S3) show that all of the films consist of a similar texture of STF and ZnO phases. The in-plane lattice parameter of STF, as indicated by RSM in Figure S4, matched that of the substrate,

Table 1. Structural Parameters for STF/ZnO Nanocomposites and Single-Phase STF and ZnO Films

out-of-plane lattice parameter (Å) in-plane lattice parameter (Å) unit cell volume (Å3) c/a ratio

STO substrate

STF film

STF in composite

3.905

3.946

3.937

3.905 59.55 1.000

3.905 60.17 1.011

3.898 59.82 1.010 ZnO in composite

bulk ZnO out-of-plane lattice parameter (Å) in-plane lattice parameter (Å) unit cell volume (Å3) c/a ratio

ZnO film

3.253

3.257

3.258

5.213, 5.634 47.77 1.603

5.352, 5.544 48.32 1.643

5.351, 5.547 48.35 1.642

parameters are smaller than those of single-phase STF, whereas the ZnO was under tensile strain. This is attributed to an auxetic-like effect (negative Poisson ratio), proposed by MacManus-Driscoll et al.,50 in which the vertical epitaxy in the nanocomposite films between the two phases dominates the out-of-plane strain state and in-plane relaxation is inhibited. The crystallographic matching of the STF, ZnO, and the substrate is shown schematically in Figure 2e. The STF had a cube-on-cube growth on the STO substrate, whereas the ZnO grew with the (112̅0) prismatic planes parallel to the substrate and its c axis in plane. The ZnO phase forms two possible inplane variants related by a 90° in-plane rotation: ZnO (0001)∥STO (110) and ZnO (1̅100)∥STO (110). The orientation relationship of ZnO/STF is the same as that of ZnO/La0.7Sr0.3MnO3 (LSMO) in ref 14, with the ratio of wurtzite/perovskite lattice planes in the vertical direction of 6:5. Unlike ZnO/LSMO in ref 14, the wurtzite ZnO is in out-ofplane tension and the perovskite STF is in compression, which is consistent with the XRD measurements. Figure 3a displays a cross-sectional TEM image of an STF/ ZnO nanocomposite film, in which columnar grains of the two phases are visible. Unlike the previous samples, which had about 50 vol % of each phase, the TEM was done on an STFrich nanocomposite so that phase identification would be easier. The selected-area electron diffraction (SAED) (inset) reveals the orientation of the ZnO (112̅0) and STF (002) planes. The epitaxial interface between the two phases perpendicular to the substrate was observed from a highresolution TEM (HRTEM) image (Figure 3b). The scanning transmission electron microscopy (STEM) Z-contrast imaging is a composition-sensitive technique with the lighter contrast indicating regions with heavier elements, that is, the STF phase. Figure 3c shows that the columns have average diameters of ∼10 nm. No clustering of the Sr, Ti, Fe, and Zn in the columns was observed by STEM elemental mapping (see Figure S2). The AFM phase-contrast image in Figure 3d indicates a mazelike structure on the top surface. This highly resembles the nanomaze structure demonstrated by Chen et al. in the LSMO/ ZnO nanocomposite system, which shows rectangular nanodomains that tend to orthogonally align with each other.14 It possibly suggests a common growth mechanism in the perovskite/wurtzite self-assembly, which links to a pseudospinodal mechanism with characteristics of both spinodal decomposition and nucleation and growth.1 32361


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Figure 2. Asymmetric RSM scan of (a) STF in a nanocomposite film about the STO (103) peak, (b) single-phase STF film, (c) ZnO in a nanocomposite film about the STO (103) and ZnO (112̅2), (202̅0) reflections, and (d) single-phase ZnO film. (e) Three-dimensional crystallographic matching of the two phases. 32362


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Figure 3. (a) Cross-sectional TEM and SAED (inset) images of STF-rich STF/ZnO nanocomposite, (b) HRTEM image of the nanocomposite interface, (c) STEM Z-contrast image of the nanocomposite, and (d) AFM phase-contrast image of the nanocomposite surface.

perovskite oxides commonly leads to an expansion of the crystal lattice due to underbonding.60−64 In addition to oxygen vacancies, the out-of-plane tensile strain in the nanocomposites is governed by additional factors. For example, the lower deposition rate produced larger feature sizes, resulting in less vertical interface area, which would impose a different out-ofplane strain in the STF. A detailed analysis of such effects can be found in ref 5, as well as the demonstration of nanoscaffolddensity-tuned vertical heterointerface area and strain in thick nanocomposite films which can be found in ref 57. We now discuss the coupling of the magnetic and optical properties, which was investigated by measuring PL with an applied magnetic field. The magnetic field was produced by a setup with rare-earth permanent magnets and had a magnitude of 2000−3000 Oe across the sample. In the micro-PL measurement, an excitation wavelength of 325 nm (3.815 eV) and backscattered geometry were used. The same PL excitation fluence was used in each measurement to rule out possible heating effects, and the luminescence was collected at multiple positions on the film. We exclude any potential photoluminescence contributions from the STF (or Fe-doped STO) film. No photoluminescence signal was observed from pure STF films at the same excitation energy, as shown in Figure S6. In fact, due to the small binding energy and the short radiative lifetime of self-trapped excitons, single-crystal STO shows no photoluminescence at room temperature.65 The radiative recombination can be enhanced in doped STO by localizing the e−h pairs at defects or confinement. The defect-

but the out-of-plane lattice parameter was larger for higher deposition rate. All of the samples exhibited in-plane magnetic easy axes, as indicated by the hysteresis loops in Figure S5. In Figure 4c, the out-of-plane tensile strain of the STF with respect to the pseudocubic lattice parameter of the single-phase STF increased with increasing deposition rate, whereas the saturation magnetization decreased from 0.8 to 1.4μB/Fe. The XPS Fe 2p core-level spectra of the nanocomposites and singlephase STF films are depicted in Figure 4d. The binding energies of Fe 2p3/2 and 2p1/2 doublet of the nanocomposites shifted to lower energy compared to that of the single-phase STF film. All of the nanocomposite samples showed evidence of Fe3+ and Fe2+ satellite peaks, which are more intense at lower deposition rate, which suggests a lower-average Fe valence as the deposition rate decreases in the nanocomposites. The mixed-valence Fe commonly formed under reducing conditionsto charge-balance the oxygen vacancies.51 The lower-average Fe valence state implies a larger oxygen vacancy concentration in the films, which are deposited at lower deposition rate and show higher saturation magnetization. The trend between the oxygen vacancy concentration and saturation magnetization is broadly consistent with the density functional theory electronic structure calculations for Co-58 and Fe59-doped STO, which indicates a larger magnetic moment for Co or Fe ions locally adjacent to an oxygen vacancy. We further discuss the dependence of the out-of-plane tensile strain and magnetization on deposition rate, as shown in Figure 4c. The presence of oxygen vacancies in both bulk and thin-film 32363


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Figure 4. (a) Magnetic hysteresis loops of STF/ZnO nanocomposites measured by VSM. The inset shows FC and ZFC curves measured by SQUID. (b) Hysteresis of single-phase STF. (c) Saturation magnetization and out-of-plane strain of the nanocomposites as a function of laser repetition frequency. (d) XPS spectra of Fe 2p for single-phase STF and STF/ZnO nanocomposites synthesized with different laser repetition frequencies.

Figure 5. PL spectra of (a) STF/ZnO nanocomposite and (b) single-phase ZnO film under 0 applied field, in-plane (IP) magnetic field, and out-ofplane (OP) magnetic field.

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contribute. First, the magnetostrictive effect in STF originates from the Fe mixed-valence ions located at octahedral sites, in which the Fe4+ and Fe2+ are magnetoelastic but Fe3+ is not. The Fe3+ ions are primarily found in single-phase STF, but the presence of Fe2+ is significant in STF in nanocomposites as indicated by the XPS spectra in Figure 4d. The magnetostriction of STF in nanocomposites is therefore expected to be significantly larger than that of the single-phase STF, leading to greater magnetic-field-induced strain. Second, all of the reported strain-induced luminescence shifts were measured from ZnO nanowires uniaxially strained along c axis or bent ZnO nanowires with an approximation of uniaxial strain along the c axis. In this work, the ZnO was under uniaxial stress perpendicular to the c axis, which results in reduction of the crystal symmetry from C6v to C2v by the uniaxial distortion of the lattice76 and induces significant change in the NBE emission of ZnO. The optical response to uniaxial stress perpendicular to the c axis has not been reported due to the difficulty in synthesizing nonpolar ZnO nanostructures. Third, the strain-induced shift of the luminescence is influenced by the piezoelectric field in ZnO if the depletion layer is comparable to that of the length scale of the nanostructure.77 Also, ions can be driven by the strain-induced piezoelectric field across the interface78 and account for donor- or acceptor-bound exciton or donor−acceptor pair emissions at lower photon energies.42,79

induced photoluminescence in doped STO lies in the visible range.66,67 Indirect band-to-band radiative recombination involving phonon emission in doped STO was observed only at low temperatures and intense excitation power.68 The PL spectra of the near-band-edge (NBE) emission of a nanocomposite film are shown in Figure 5a. A red shift of 5 meV of the PL peak is observed when the magnetic field is applied out of plane (hard axis). There is no observable PL shift for applying the magnetic field in plane compared to the zerofield case. Because the in-plane remanence of the nanocomposites is small, this suggests the nanocomposite undergoes a net demagnetization, but the magnetization of individual regions of STF remains in plane. Therefore, any magnetic coupling, such as magnetoelastic strain, imposed on the ZnO by the STF is equivalent for zero-field and in-plane saturation. To exclude any possible contribution from magnetophotoluminescence of ZnO itself, a single-phase ZnO film was characterized as a control. In Figure 5b, no shift for the NBE emission peaks is observed from single-phase ZnO film under applied magnetic field. The PL shift is therefore not related to changes in excitons caused directly by the magnetic field, such as the Zeeman splitting of the bound exciton line. Prior work has shown that high magnetic fields modify the optical spectra in films and nanostructures of dilute magnetic semiconductors. For example, Hou and Zou et al.69 demonstrated the correlation between magnetic interactions and optical emission in Mn-doped ZnSe nanoribbons via identification of exciton magnetic polarons. Makino et al.70 described the red-shifted PL in a magnetic field of ≤20 T and predominant Zeeman splitting of the bound exciton lines in a magnetic field ranging from 24 to 50 T in MgxZn1−xO/ZnO heterojunctions. Lin et al.71 reported that in Co-doped ZnO nanorods the PL intensity decreased when the magnetic field was increased up to 14 T and a diamagnetic shift in the PL peaks was observed in a magnetic field ≥4 T. In this work, by considering the much smaller magnitude of the magnetic field and low concentration of Fe ions incorporated in ZnO, it is reasonable that the direct influence of field on the radiative recombination transitions in ZnO is minimal. As a result, the red-shifted recombination transitions in ZnO are attributed to the out-of-plane tensile strain exerted by the magnetostrictive STF along the vertical interfaces. The tensile strain along the STF/ZnO interface caused by magnetizing the film out of plane is in agreement with a positive magnetostriction that we found previously for single-phase STF films.35 The photoluminescence−strain relationship is also consistent with the blue-shifted PL in nonpolar heteroepitaxial (Zn,Mg)O quantum wells under anisotropic in-plane compressive strain reported by Chauveau et al.72 In addition, Liu et al.73 demonstrated magnetization switching caused by photostriction in SrRuO3/CoFe2O4 vertical nanocomposites. The magnetic-field-induced optical effects observed here provide a complement to these light-induced magnetic changes. Quantitatively, on the basis of the reported strainluminescence gradient for ZnO nanowires, a uniaxial or bending tensile strain of 0.25,44,45,74 0.18,69 0.2,47 or 0.125%46 would result in 5 meV red shift of the NBE (at room temperature),74,75 donor bound exciton,45,47 or free exciton (at low temperatures)46 peaks. Given the reported magnetostriction constants of STF are 2.1 × 10−6,35 the magnetoelastic strain alone is not large enough to account for the PL shift. The cause of the enhanced effect we observe is not fully understood, but there are several factors that may

4. CONCLUSIONS In conclusion, we have demonstrated the three-dimensional heteroepitaxy of self-assembled STF/ZnO vertical nanocomposites. The STF columns had a cube-on-cube growth on the STO substrate, whereas the ZnO grew with the (1120̅ ) prismatic planes parallel to the substrate and its c axis in plane. The STF columns had average diameters of ∼10 nm, separations of ∼5 nm, and a room-temperature magnetic moment of 0.75−1.45μB/Fe atom, increasing at lower deposition rates. In contrast to the out-of-plane easy magnetic axis in single-phase STF films, STF/ZnO nanocomposite films show in-plane magnetic anisotropy due to the vertical compressive strain imposed by the ZnO phase. The photoluminescence of the nanocomposites can be controlled by applying a magnetic field along the hard axis. A red shift of photon energies of ∼5 meV was induced by applying a magnetic field of 2000−3000 Oe out of plane, whereas singlephase ZnO films showed no such effect. The origin of the magneto-luminescent effect is attributed to strain imposed by the magnetostrictive STF along the vertical STF/ZnO interfaces.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b08741. Schematics of self-assembled growth of STF/ZnO nanocomposite; XRD θ−2θ scans; reciprocal space maps; hysteresis loops of STF/ZnO nanocomposites synthesized at different deposition frequencies; photoluminescence of pure STF film (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.Z.). 32365


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ACS Applied Materials & Interfaces *E-mail: [email protected] (S.J.C.). *E-mail: [email protected] (C.A.R.).

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ORCID

Chen Zhang: 0000-0003-2498-4019 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was financially supported by the Singapore-MIT Alliance and by the Solid-State Solar-Thermal Energy Conversion Center (S3TEC), Department of Energy award number DE-SC0001299.

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NOTE ADDED AFTER ASAP PUBLICATION This paper was published on the Web on September 11, 2017. Additional text corrections were implemented throughout the document, and the corrected version was reposted on September 12, 2017.

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