Topotactic fluorination of strontium iron oxide thin films using polyvinylidene fluoride

We report herein the topotactic fluorination of SrFeO3 d thin films (d 0, 0.5, 1) with polyvinylidene fluoride (PVDF). SrFeO3 xFx epitaxial thin films were obtained by fluorination at 150–270 C, which is substantially lower than the reaction temperature for polycrystalline bulk samples prepared with PVDF. The fluorine content (x) of the film was widely varied by controlling the PVDF-treatment temperature and/or the amount of oxygen vacancies in the precursor film. The higher reactivity of the SrFeO2 and SrFeO2.5 thin films can be reasonably explained by a fluorine-diffusion mechanism via oxygen vacancies.


Introduction
Since the discovery of superconductivity (transition temperature of T c ¼ 46 K) in non-ordinary oxyuorides Sr 2 CuO 2 F 2+x , 1 the replacement of O 2À by F À in transition-metal oxides has attracted a great deal of attention as a chemical technique that can considerably modify the electronic properties of their mother compounds.Among these compounds, uorinesubstituted iron oxides have been a subject of intense study owing to their unique magnetic properties.For example, the partially F-substituted hexagonal perovskite, 15R-BaFeF 0.2 O 2.42 , shows drastically enhanced antiferromagnetic ordering with a Néel temperature (T N ) of z 700 K, which is close to the highest values ever reported for iron oxides. 2 Such a high T N can be explained by local changes of the Fe-X-Fe (X ¼ O, F) bond angle, bond length, as well as chemical reduction of iron.
Iron oxyuorides are oen metastable and decompose at higher temperatures, making the development of low-temperature synthetic routes desirable.One promising method is a topotactic reaction employing a uorination agent, polyvinylidene uoride (PVDF), which is stable in air, and has a melting point of $170 C. 3 Fluorination using PVDF is advantageous for obtaining phase-pure oxyuorides without metal uoride impurities, which are frequently formed by other uorinating agents such as F 2 gas and NH 4 F. 3 In addition, PVDF is a non-oxidizing reagent, 4 in contrast to highly oxidizing F 2 gas; therefore, metal ions are not oxidized during the uorination reaction, but rather, reduced.][7][8][9][10][11][12][13] For example, a perovskite oxide SrFeO 3Àd (with Fe 3+/4+ ) can be transformed to SrFeO 2 F (with Fe 3+ ) by annealing with PVDF. 5 It is expected that the reactivity of thin-lm samples with PVDF should be much higher than that of bulk samples because thin lms have larger surface areas and smaller volumes than bulk samples.More recently, the uorination technique using PVDF was applied to SrFeO 3Àd thin lms, where a PVDF solution was spin-coated on the lm and the lm/polymer bilayer was annealed at 600 C, though the SrFeO 3Àa F g lms thus obtained had relatively low uorine contents (g < 1) despite higher synthesis temperature than that for bulk SrFeO 2 F. 14 Another important factor governing the reactivity of thin-lm samples with PVDF is the amount of oxygen vacancies in the precursor oxides, because the incorporation of uorine ions into the oxygen-vacancy sites would be faster than the actual replacement of oxygen by uorine.In fact, bulk SrFeO 2 F can be obtained at 150 C by uorinating innite-layer SrFeO 2 with XeF 2 . 15Herein, we performed topotactic uorine doping of SrFeO x (2 # x < 3) thin lms using PVDF.As a result, we succeeded in fabricating SrFeO 3Àx F x epitaxial thin lms, in which the uorine content (x) was controllable over a wide range (0.8 # x # $1.5) by means of the heat-treatment temperature.The uorination reaction was conducted at 150-270 C, which was much lower than the 400 C reaction temperature reported for the bulk.We also found that the value of x is dependent on the amount of oxygen vacancies present in the precursor SrFeO x lm.
lms were grown on SrTiO 3 (001) (STO, Shinkosha Co.) substrates by a pulsed-laser deposition (PLD) technique, with a SrFeO 3Àd ceramic pellet (20 mm in diameter and 5 mm in thickness, TOSHIMA Manufacturing Co.) used as a PLD target.The fourth harmonic of a Nd-YAG laser (wavelength l ¼ 266 nm) with an energy of 0.3 J cm À2 per shot and a repetition rate of 10/3 Hz was employed for ablation.The substrate temperature and oxygen partial pressure were kept at 700 C and 7 Â 10 À5 mbar, respectively, during each deposition run, and coherent growth of the precursor SrFeO 2.5 lms on STO (001) substrates was conrmed.Oxidized SrFeO x (x z 3) lms with perovskite structure were fabricated by annealing SrFeO 2.5 lms at 700 C in air for 2 h.Additional reduced SrFeO x (x z 2) lms with innite-layer structures were obtained by heating SrFeO 2.5 lms with CaH 2 (Wako Pure Chemical Industries, Ltd.) at 280 C for 24 h in evacuated Pyrex tubes as described in ref. 16.
The obtained precursor SrFeO x (x z 2, 2.5, and 3) lms were further subjected to uorination with 0.1 g of PVDF (Fluorochem Ltd.) at temperatures (T f ) ranging from 100 to 450 C for 24 h under an Ar gas ow of 70 cm 3 min À1 (with the lms covered by Al foil so as not to contact with PVDF directly).Typical lm thickness, as measured using a stylus surface proler, was $80 nm.

Characterization
Crystal structures of the SrFeO 3Àx F x lms were obtained with an X-ray diffractometer employing Cu-Ka radiation and a transmission electron microscope (TEM).The chemical composition of the lms was analysed by energy dispersive X-ray spectrometry (EDS) equipped with a scanning electron microscope in which the electron accelerating voltage was set at 2.5 keV to reduce the background signal from the substrate.The amount of uorine was evaluated by nuclear reaction analysis (NRA) using the 19 F(p,ag) 16 O resonant nuclear reaction at 902 keV.In NRA measurements, a CaF 2 single crystal was used as a reference for uorine.The EDS and NRA results included experimental errors of $20%.The depth proles of the chemical compositions were evaluated by X-ray photoemission spectroscopy (XPS) with Ar + -ion sputtering.The core levels of iron were also observed by XPS.The surface morphology was characterized by atomic force microscopy (AFM).

Results and discussion
Crystal structure analysis   Notably, the q x value of the SrFeO 3Àx F x (103) peak coincides with that of STO (103).This implies that the a-axis of the SrFeO 3Àx F x lm was completely locked to the STO lattice, even aer treatment with PVDF.In other words, the perovskite-like cation network was maintained during the topotactic uorination reaction promoted by PVDF.
Fig. 1(c) shows the plots of lengths of the a-and c-axes for the SrFeO 3Àx F x lms as a function of T f .The c-axis length increased from 3.989 to 4.022 Å upon increasing T f from 150 to 270 C in a nonlinear manner, whereas the a-axis length was essentially independent of T f .In the case of bulk SrFeO 2 F, it was reported that the cell volume was greatly increased upon uorination of precursor SrFeO 3Àd because of the simultaneous occurrence of uorine insertion and substitution. 5,17Therefore, in this case, the increase in the c-axis length suggests that similar uorine insertion and substitution reactions take place.

Compositional analysis
To verify uorine doping and to investigate the relationship between the doped uorine content and T f , EDS measurements were performed for O Ka, F Ka, and Fe La. 1.This implies that the uorine content of the SrFeO 3Àx F x lms can be controlled by T f .NRA measurements were conducted so as to quantitatively determine the uorine content of the lms.Fig. 4(a) shows the NRA spectrum of the SrFeO 3Àx F x lm uorinated at 250 C. The g-ray emitted by the nuclear reaction of 19 F(p,ag) 16 O was clearly observed in the SrFeO 3Àx F x lm, and the x value was determined to be 0.92 AE 0.18.Fig. 4(b) shows the correlation between the c-axis length and x values in SrFeO 3Àx F x , where the x values were evaluated from EDS and NRA measurements independently, referred to as x(EDS) and x(NRA), respectively.For the evaluation of x(EDS), the relationship x ¼ 3 Â S F /(S F + a Â S O ) was used in an assumption of O : F ¼ 3 À x : x, where the relative sensitivity factor (a) was estimated to be 0.989 based on a Monte Carlo simulation of electron trajectory in solids. 18As seen in Fig. 4(b), both x(EDS) and x(NRA) are in good agreement with each other, which indicates that the assumption mentioned above (O : F ¼ (3 À x) : x) is reasonable, although we cannot deny the possibility that a certain amount of oxygen vacancies remains in the lm (in other words, the doped uorine atoms were substituted for the oxygen sites of the perovskite lattice).Notably, the c-axis length of the SrFeO 3Àx F x lm increased as the value of x increased, despite the fact that F À has a smaller ionic radius than O 2À .This can be rationalized by taking the chemical reduction of the Fe ions into consideration.The shrinkage of cell volume associated with F À substitution is overwhelmed by cell expansion upon the reduction of Fe ions. 5,17The x value of the SrFeO 3Àx F x lm uorinated at 270 C was $2, which is twice that of bulk SrFeO 2 F, though we cannot deny the possibility that the uorine contents were overestimated because carbon impurities on the surface may adsorb uorine ions during the PVDF treatment.Because uorination proceeds at the lm surface, the population of uorine ions is potentially higher closer to the surface.Fig. 5 shows the uorine depth prole of the SrFeO 3Àx F x (x z 1) lm obtained at 250 C, measured by XPS with Ar + -ion sputtering, where the peak area of F 1s relative to that of O 1s, A F /A O , at the surface (0 nm) was set to 1. Near the surface (0-15 nm) the A F /A O decreased with increasing depth, reaching $0.8 at 15 nm, suggesting the presence of impurities containing uorine on the surface.The A F /A O value was virtually constant at 15-80 nm, and slightly increased near the interface of the lm and the substrate.These results suggest that uorine ions diffused not only in the vicinity of the surface, but also over the entire lm.On the other hand, in the STO substrate region (>90 nm), uorine was not detected, indicating that the diffusion of uorine into the STO substrate is negligible.

Valence of iron and surface morphology
Fig. 6 depicts the Fe 2p core-level XPS spectra of the SrFeO 3Àx F x lms uorinated at 150, 250, and 270 C. Each spectrum showed Fe 2p 3/2 and 2p 1/2 peaks, and a satellite peak located between the Fe 2p 1/2 -Fe 2p 3/2 doublet.Notably, the locations of the satellite peaks, which are known to be very sensitive to the oxidation state of Fe, differ from one sample to another.The satellite peaks in the SrFeO 3Àx F x lms uorinated at 150 and 250 C were located at an E b of $719 and $718 eV, respectively, which are equivalent to the peak in LaFeO 3 with Fe 3+ (E b ¼ 718.7 eV). 19That is, the valences of the Fe ions in the lms are almost trivalent.Meanwhile, the satellite peak of the lm uorinated at 270 C was located at an E b of 712-719 eV, between the doublet peaks, suggesting that the Fe ion has a mixed valence state of Fe 2+ /Fe 3+ . 20The Fe 2+ /Fe 3+ ratio was further evaluated by comparing the XPS data of the SrFeO 3Àx F x lm uorinated at 270 C and the SrFeO 2.5 lm reported in ref. 21.The area-intensity of the Fe 3+ satellite peak relative to the Fe 2p 3/2 main peak for the uorinated lm was approximately half that for the SrFe 3+ O 2.5 lm, implying that $50% of Fe exists as Fe 3+   in the SrFeO 3Àx F x lm.Thus, we can roughly deduce the uorine content (x) to be x z 1.5.Fig. 7 shows the AFM images of the SrFeO 2.5 precursor lm and the SrFeO 3Àx F x lms uorinated at 250 and 270 C. The root mean square values of surface roughness were found to be 0.35, 0.64, and 0.81 nm, respectively, indicating that the uorination process does not cause severe surface roughening.

Dependence on oxygen vacancies of precursor lms
The relationship between the uorine content (x) and oxygen vacancies in the precursor lms is another important consideration.Fig. 8(a) and (b) compare the XRD patterns of the oxidized SrFeO x (x z 3) and reduced SrFeO x (x z 2) precursor lms uorinated at 150 C for 24 h.The oxidized and reduced precursor lms show the (002) diffraction peaks of perovskitetype and innite-layer structures with c ¼ 3.835 and 3.490 Å, respectively.The c-axis length of the oxidized SrFeO x (x z 3) lm is close to that of the strained SrFeO 3 lm on the STO substrate, 3.823 Å. 22 Aer treatment with PVDF, the lm prepared from the SrFeO x (x z 3) precursor showed two (002) diffraction peaks corresponding to c ¼ 3.871 and 3.955 Å, whereas the lm prepared from SrFeO x (x z 2) exhibited one peak with c ¼ 4.002 Å. Fig. 8(c) shows a plot of the c-axis lengths of the SrFeO x (x z 2, 2.5, and 3) precursor and uorinated lms.As seen in Fig. 8(c), the c-axis length of the lm uorinated at 150 C becomes longer as the oxygen content in the precursor SrFeO x lm is decreased from x z 3 to 2. Because the c-axis lengths of the uorinated lms reect the uorine content, this result suggests that precursor lms containing more oxygen vacancies tend to incorporate more uorine ions into the lm.
To investigate the origin of the two (002) peaks, the uorine and oxygen depth proles of the SrFeO 3Àx F x lm obtained from the oxidized SrFeO x (x z 3) precursor were measured in detail (Fig. 9).As seen from the gure, the A F /A O vs. depth plot shows two plateaus: $0.8 at 5-40 nm and $0.4 at 40-80 nm.These uorine-rich and uorine-poor regions correspond to the two (002) diffraction peaks at 2q z 45.9 and 47.0 , respectively (Fig. 8(a)).These results imply that the diffusion of uorine ions is considerably slowed down as the concentration of oxygen vacancies decreases.

Comparison with the diffusion mechanism
As stated above, the SrFeO 3-x F x lms were obtained at much lower temperatures (150-270 C) than the bulk sample (400 C). 5 Now, we will discuss the difference in the reactivity between the thin lm and bulk based on the diffusion equation. 23The uorine ions were not diffused into the STO substrates (Fig. 5).In such a case, the relative uorine concentration, C(x), is given by eqn (1): where x is the distance from the surface, t is the time, D is the diffusion coefficient, k is the surface exchange coefficient, l is the thickness of the lm, and L ¼ lk/D. 23The b n values are the positive roots of the equation: b n tan b n ¼ L. Approximately, the value of D describes the overall shape of the C(x) curve, while k determines the C(x) value at x ¼ 0. Fig. 10 shows the depth dependence of C(x) for a thin lm with l ¼ 80 nm, and for a bulk sample with l / N at t ¼ 24 h, where D was set to 3 Â 10 À15 cm 2 s À1 , so as to reproduce the uorine depth prole measured by XPS (Fig. 5).In the case of the bulk sample, C(x) decreases with x in an exponential manner, with the diffusion length of 3 Â 10 2 nm.That is, only uorine diffuses into the surface regions, which means that the higher uorine contents experimentally observed in thin lms are attributable to smaller grain sizes, representing the maximum length of the diffusion path.

Conclusions
We have reported the successful synthesis of SrFeO 3Àx F x epitaxial thin lms on STO substrates via topotactic uorination of SrFeO 3Àd precursor lms using PVDF.The SrFeO 3Àx F x thin lms were obtained at a lower temperature than polycrystalline bulk samples.Furthermore, the uorine content (x) in the SrFeO 3Àx F x lms was controllable by adjusting the uorination temperature and/or the amount of oxygen vacancies in the precursor lm.The higher uorination reactivity in the SrFeO 3Àd precursor lm, compared with that observed in bulk samples, can be rationalized by taking smaller grain sizes, being the maximum length of the diffusion path, into account within the framework of the uorine-diffusion model via oxygen vacancy.
Fig. 10 Relative fluorine concentration (C(x)) vs. depth (x) curves at t ¼ 24 h, calculated for a thin film with l ¼ 80 nm and for a bulk sample with l / N. D and k were assumed to be 3 Â 10 À15 cm 2 s À1 and 1 Â 10 À10 cm s À1 , respectively.

Fig. 1 (
Fig. 1(a) shows the 2q-q X-ray diffraction (XRD) patterns of lms obtained by uorination of the SrFeO 2.5 precursor lm at T f ¼ 100-450 C for 24 h.The XRD pattern of the SrFeO 2.5 precursor lm has also been included in the gure for comparison.The lm treated with PVDF at 100 C showed a diffraction peak at 2q z 45.8 , corresponding to the (002) reection of the SrFeO 2.5 structure, which means that PVDF did not react with the SrFeO 2.5 lm below 100 C. The lms uorinated at 150-270 C exhibited only the (002) diffraction peaks of the perovskite structure, indicating that SrFeO 3Àx F x can be obtained at 150-270 C. Additionally, the position of the (002) peak was shied to the lower-angle side (from 45.5 to 45.1 ) on increasing T f from 150 to 270 C. At 300 C, the diffraction peak of perovskite SrFeO 3Àx F x disappeared, and a peak assignable to SrF 2 (002) appeared.Above 350 C, the peaks corresponding to SrF 2 (002) and Fe 3 O 4 (002) evolved, reecting the complete decomposition of perovskite SrFeO 3Àx F x into SrF 2 and Fe 3 O 4 .

Fig. 1
Fig. 1 (a) 2q-q X-ray diffraction patterns of the SrFeO 2.5 precursor film and films fluorinated at 100-450 C with PVDF for 24 h.(b) Logarithmic contour mapping in reciprocal space for asymmetric (103) peaks of the SrFeO 3Àx F x film on the SrTiO 3 substrate fluorinated at 250 C. (c) Lengths of aand c-axes of SrFeO 3Àx F x films as a function of fluorination temperature.

Fig. 1 (
Fig.1(b) shows an XRD reciprocal space map for asymmetric (103) diffraction of the SrFeO 3Àx F x lm uorinated at 250 C. Notably, the q x value of the SrFeO 3Àx F x (103) peak coincides with that of STO (103).This implies that the a-axis of the SrFeO 3Àx F x lm was completely locked to the STO lattice, even aer treatment with PVDF.In other words, the perovskite-like cation network was maintained during the topotactic uorination reaction promoted by PVDF.Fig.1(c)shows the plots of lengths of the a-and c-axes for the SrFeO 3Àx F x lms as a function of T f .The c-axis length increased from 3.989 to 4.022 Å upon increasing T f from 150 to 270 C in a nonlinear manner, whereas the a-axis length was essentially independent of T f .In the case of bulk SrFeO 2 F, it was reported that the cell volume was greatly increased upon uorination of precursor SrFeO 3Àd because of the simultaneous occurrence of uorine insertion and substitution.5,17Therefore, in this case, the increase in the c-axis length suggests that similar uorine insertion and substitution reactions take place.Fig.2(a)shows a wide-view cross-sectional TEM image of the SrFeO 3Àx F x lm uorinated at 270 C. Neither segregated impurities nor amorphous phases were recognized from the TEM observations.Fig.2(b) is a magnied view of the same image.The image clearly indicates a tetragonal perovskite structure with lattice constants of a $ 3.9 Å and c $ 4.0 Å, which is consistent with those obtained from XRD (a ¼ 3.908 Å

Fig. 2 (
Fig.1(b) shows an XRD reciprocal space map for asymmetric (103) diffraction of the SrFeO 3Àx F x lm uorinated at 250 C. Notably, the q x value of the SrFeO 3Àx F x (103) peak coincides with that of STO (103).This implies that the a-axis of the SrFeO 3Àx F x lm was completely locked to the STO lattice, even aer treatment with PVDF.In other words, the perovskite-like cation network was maintained during the topotactic uorination reaction promoted by PVDF.Fig.1(c)shows the plots of lengths of the a-and c-axes for the SrFeO 3Àx F x lms as a function of T f .The c-axis length increased from 3.989 to 4.022 Å upon increasing T f from 150 to 270 C in a nonlinear manner, whereas the a-axis length was essentially independent of T f .In the case of bulk SrFeO 2 F, it was reported that the cell volume was greatly increased upon uorination of precursor SrFeO 3Àd because of the simultaneous occurrence of uorine insertion and substitution.5,17Therefore, in this case, the increase in the c-axis length suggests that similar uorine insertion and substitution reactions take place.Fig. 2(a) shows a wide-view cross-sectional TEM image of the SrFeO 3Àx F x lm uorinated at 270 C. Neither segregated impurities nor amorphous phases were recognized from the TEM observations.Fig. 2(b) is a magnied view of the same image.The image clearly indicates a tetragonal perovskite structure with lattice constants of a $ 3.9 Å and c $ 4.0 Å, which is consistent with those obtained from XRD (a ¼ 3.908 Å and c ¼ 4.022 Å).

Fig. 3 (
a) depicts the EDS spectra near the O Ka and F Ka peaks of the SrFeO 2.5 precursor and SrFeO 3Àx F x lms uorinated at 150, 250, and 270 C, where the spectral intensity was normalized by the area of the Fe La peak.As seen in the gure, the peak area of F Ka increased with increase in T f , while that of O Ka showed a tendency to decrease.Fig.3(b)shows the plots of the areas of the F Ka and O Ka peaks, S F and S O , respectively, against T f .The S F /S O ratio of the lm uorinated at 250 C is 1 : 2, whereas that of the lm uorinated at 270 C is enhanced to 2 :

Fig. 2
Fig. 2 Cross-sectional transition electron microscopy images of the SrFeO 3Àx F x film fluorinated at 270 C with (a) a wide-range view and (b) a magnified view.

Fig. 3
Fig. 3 Energy dispersive X-ray spectra (EDS) near (a) O Ka, F Ka, and Fe La peaks of the SrFeO 2.5 precursor film and SrFeO 3Àx F x films fluorinated at 150, 250, and 270 C. (b) EDS peak area of F Ka and O Ka as a function of fluorination temperature.

Fig. 4
Fig. 4 (a) Nuclear reaction analysis (NRA) spectra of the SrFeO 3Àx F x film fluorinated at 250 C. (b) c-Axis length dependence of x in the SrFeO 3Àx F x film, estimated from energy dispersive X-ray spectra (EDS) and NRA measurements.Different samples were used for NRA and EDS measurements.

Fig. 5
Fig. 5 Fluorine depth profile of the SrFeO 3Àx F x (x z 1) film fluorinated at 250 C, obtained from the SrFeO 2.5 precursor film, measured by Xray photoemission spectroscopy with Ar + -ion sputtering.

Fig. 7
Fig. 7 Atomic force microscopy images of the (a) SrFeO 2.5 precursor film and SrFeO 3Àx F x films fluorinated at (b) 250 and (c) 270 C.

Fig. 8 X
Fig. 8 X-ray diffraction patterns of (a) oxidized SrFeO x (x z 3) and (b) reduced SrFeO x (x z 2) precursor films, and films fluorinated at 150 C with PVDF for 24 h.(c) c-Axis lengths of SrFeO x (x z 2, 2.5, and 3) precursor and fluorinated films.

Fig. 9
Fig. 9 Fluorine depth profile of the SrFeO 3Àx F x film fluorinated at 150 C obtained from the oxidized SrFeO x (x z 3) precursor film by X-ray photoemission spectroscopy with Ar + -ion sputtering.