Fabrication and characterization of perovskite manganite epitaxial thin films prepared via topotactic fluorination

Jasper Miura,a Akira Chikamatsu,b Takahiro Maruyama,b Kuni Yamada,b Ryosuke Ishigami,b and Tetsuya Hasegawab

aDepartment of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI 02912, United States bDepartment of Chemistry, The University of Tokyo, Bunkyo-ku, Tokyo, 113-0033, Japan

The perovskite manganite LaSrMnO4 (LSMO4) was fabricated using pulsed laser deposition (PLD) on LaAlO3 (LAO) substrates. Multiple samples were fabricated to optimize fabrication conditions, which resulted in successful fabrication in background pressure with a laser fluence of 20 mJ. Following fabrication, thin films were characterized using X-ray diffraction (XRD) analysis, energy-dispersive X-ray spectroscopy (EDS), and stylus profiling. LSMO4 precursor films were fluorinated with polyvinylidene fluoride (PVDF) to form LaSrMnO4Fx and characterized. Initial results suggest fluorination of LSMO4 occurs with reaction times of under an hour, with temperatures at approximately 300ºC.

I. INTRODUCTION

The recent rise and proliferation of a wide range of portable devices requiring energy storage has fostered a need for innovation in battery technologies, including improvements to reduce the memory effect and self-discharge, as well as increasing their energy density. Lithium-ion batteries are currently ubiquitous and prevalent in portable devices today; however, other cations and anions may be used as charge carriers in batteries. The fluoride anion has both a high electronegativity and a high redox potential, making it a promising candidate for use as a mobile and stable ionic charge carrier for rechargeable batteries. Most recent studies investigating the potential of the fluoride anion as an ionic charge carrier use conversion-type reactions in the electrode (transforming metals to metal fluorides and vice versa). However, 123

these conversion-type reactions result in high energy densities, which are detrimental to reversibility, cyclability, and transport kinetics. 45

This produces high overpotentials and results in lower energy efficiencies. 6

Promising alternatives to conversion-type electrode materials are intercalation-based electrode materials, in which ions are inserted into structural vacancies, interstitials, and interlayers within a host crystal lattice. Lithium-ion battery systems have already undergone numerous improvements using intercalation-based reactions, motivating the investigation of

the potential of intercalation-based electrodes in fluoride ion batteries. K2NiF4-type compounds (A2BX4), which consist of alternating layers of perovskite (ABX3) units and rock salt (AX) units, have large interstitial sites between the alternating layers, which can be occupied by anions, including fluoride ions. When every interstitial site (Y) is occupied, the composition becomes A2BX4Y2, meaning that this structure is capable of accepting two fluoride ions per formula unit. In addition to this high theoretical capacity as an electrode material, anion ordering is favorable for fluorinated oxides with this structure. Fluoride ions preferentially occupy the interstitial sites in a K2NiF4-type compounds, which are not directly bonded to the transition metal cations. This anion ordering is likely beneficial for the reversibility of the material. 7

Topotac t i c f luor ina t ion a t low temperatures results in the insertion of F- ions into a host crystalline structure without destroying the initial crystalline framework, either by substitution into the O2- sites (resulting in reduction of host cations), or by insertion into interstitial or interlayer sites (resulting in oxidation of host cations). This method 89

employs the fluorination agent polyvinylidene fluoride (PVDF), which is advantageous for obtaining phase-pure oxyfluorides without metal fluoride impurities, unlike fluorination using other agents such as F2 gas and NH4F. 10

LaSrMnO4 (LSMO4), which has the K2NiF4 structure of alternating perovskite and rock salt layers, has been previously studied for its potential for use as an intercalation-based high voltage cathode material. The extraction of f luoride ions from LSMO4 has been demonstrated, as well as the high discharging capacity and structural stability of LSMO4. 11

Therefore, low-temperature topotact ic fluorination may be suitable for experiments with LSMO4 as a method to introduce fluoride ions into the crystal structure, as the reactivity of thin-film samples with PVDF is much higher than with bulk samples due to the increased surface area-to-volume ratio of thin films relative to bulk samples With the downscaling and increasing popularity of wearable technology, there is a growing incentive to develop small-scale rechargeable batteries, providing motivation to test the viability of topotactically fluorinating thin films as opposed to bulk samples. Fluorination of thin films also has potential applications for thin film batteries, which have a number of benefits over solid-state batteries, including their relative safety due to the lack of a fluid electrolyte, as well as their flexibility, which is useful for wearable technologies. In addition, thin film batteries have a higher average output voltage, a higher energy density, and longer cycling life. In this study, thin films of LSMO4 were fabricated using pulsed laser deposition (PLD), and characterized using x-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), and stylus profiling. Several different conditions were employed to determine the optimum conditions for deposition of epitaxial thin films on the LaAlO3 (LAO) substrate. These films were then topotactically fluorinated, and characterized again to better understand the change in properties that occur due to the

insertion or substitution of fluoride ions into the crystal structure of LSMO4.

II. EXPERIMENTAL

Multiple epitaxial films were fabricated using the PLD physical vapor deposition technique to optimize the fabrication conditions (Table 1). All epitaxial LSMO4 thin films were deposited onto (001) LaAlO3 (LAO, a = b = c = 3.79 Å) single crystal substrates. Two films 12

were fabricated in a single batch; one bare film and one film with a Hall bar mask for characterization of electronic properties. A 248 nm wavelength KrF excimer laser with a repetition rate of 2 Hz ablated the target placed in an ultra-high vacuum chamber. The temperature of the substrates was set to 800ºC. Following deposition, the films were cooled

10 20 30 40 50 60 70 80 90 100

Log.

Inte

nsity

(arb

. uni

t)

a

b

* *

* *

* *

**

004

006

006

#1136

#300

#303

#1136

#300

#303

#306

#308

004

Figure 1: 2θ 1-dimensional X-ray diffraction patterns of the LaSrMnO4 precursor film. (a) LSMO4 XRD pattern for the bare thin film. (b) LSMO4 XRD pattern for the Hall bar thin film. Sample #1136 (oxygen atmosphere) is depicted in red, #300 (argon atmosphere) in blue, and #303 (argon atmosphere, high laser fluence) in green. *XRD peaks representing the substrate material, LAO.

Table 1: Experimental Conditions for Thin Film Fabrication Optimization

Sample Atmosphere Energy Result

#1136 O2 / 300 mTorr 10 mJ Undesirable phase

#296 Ar / 300 mTorr 10 mJ No deposition

#300 Ar / 10-4 Torr 10 mJ Too thin

#303 Ar / 10-4 Torr 20 mJ LSMO4 peaks 004 and 006 observed

#306 Background 20 mJ LSMO4 peaks 004 and 006 observed

#308 O2 / 10-4 Torr 20 mJ LSMO4 peak 006 broad and low intensity

down to ambient temperatures at a rate of 20ºC/min. These conditions approximate those of Vafaee et al., who previously successfully produced epitaxial thin films of LSMO4 by PLD. 13

Topotactic fluorination was conducted in a tube furnace in an argon environment, in conditions ranging from 200ºC to 300ºC. Films were wrapped in aluminum foil to prevent direct contact with the PVDF. Typical film thickness was 20 nm, as measured by a stylus profiler. Crystal structures of the LSMO4 films and the fluorinated films were obtained with an both one-dimensional and two-dimensional X-ray diffractometer. Rocking curves were also collected to evaluate the crystallinity of several of the thin film samples. The chemical

compositions of the films were measured using EDS.

III. RESULTS AND DISCUSSION

The first sample (#1136) was fabricated in an oxygen atmosphere of 300 mTorr with a laser fluence of 10 mJ. While this deviates from the argon atmosphere used in Vafaee et al., an oxygen atmosphere was used because it was thought that it would have little effect on the already-oxidized state of LSMO4. However, one-dimensional XRD analysis of the resulting thin film failed to produce the peaks representative of LSMO4 (Figure 1), and the only peak did not have the lattice parameter of the desired material. The second sample (#296) was fabricated in an argon atmosphere of 300 mTorr with a laser fluence of 10 mJ, as delineated by Vafaee et al. This experiment failed to fabricate a thin film, likely because the plasma plume failed to reach the substrate. To rectify this problem, the atmospheric pressure was decreased to 10-4 Torr for the third sample

Table 2: Lattice Parameters Calculated from XRD Patterns

Sample Thin Film 2θ [º] c [Å] Peak

#1136 Bare 45.622 11.7510 –

#1136 Hall bar 44.281 12.0824 –

#300 Bare 40.631 13.0965 LSMO4 (006)

#300 Hall bar 40.737 13.0645 LSMO4 (006)

#303 Bare 40.269 13.2072 LSMO4 (006)

#303 Hall bar 40.205 13.2270 LSMO4 (006)

#306 Bare 40.099 13.2335 LSMO4 (006)

#308 Bare 41.482 12.8193 LSMO4 (006)?

X-Ra

y In

tens

ity (a

rb. u

nits

)

17 18 19 20 21 22 23

a

b

FWHM = 0.096018º

FWHM = 0.10172º

Figure 3: Rocking curves collected for sample #303 (a) and sample #306 (b). Sample #303 had an FWHM of 0.096018º, and sample #306 had a slightly larger FWHM of 0.10172º. Both of these values are relatively low, suggesting high crystallinity of the epitaxial thin films.

LSMO105

LSMO103

LAO104

Figure 2: Two-dimensional XRD pattern collected for sample #306. The a-axis lattice constant from this analysis was 3.8135 Å, and the LSMO4 film appears as a clear spot peak.

(#300). This experiment produced identifiable peaks for LSMO4 in the one-dimensional XRD patterns (Figure 1), the lattice parameter of which are reported in Table 2. However, the film produced was too thin for further characterization. The fourth sample (#303) increased the laser fluence to 20 mJ. The XRD patterns also produced identifiable (004) and (006) peaks representing LSMO4. However, the (008) peak reported in Vafaee et al., was not observed. To further optimize the fabrication conditions, two more samples were fabricated, with an argon atmosphere in background pressure (#306) and in an oxygen atmosphere at 10-4 Torr (#308). The XRD pattern of sample #306 was very similar to that of sample #303, while the 006 peak of sample #308 was broad with low intensity, suggesting that the oxygen has a detrimental effect on thin film crystallinity. Two-dimensional XRD analysis of sample #306 indicated an a-axis lattice constant of 3.8315 Å, which is similar to the LAO a-axis lattice constant of 3.79 Å and suggests successful fabrication of the thin film onto the substrate with little strain. In addition, the LSMO4 was observed as a clear spot peak, with no deviation around the chi-axis, suggesting that the film was of good quality and single crystallinity (Figure 2). Quality assessment of several samples revealed a full width at half maximum (FWHM) for rocking curves collected for samples #303 and sample #306 (Figure 3). The FWHM for sample #303 was 0.096018º, and 0.10172º for sample #306, both displaying high crystallinity. All fluorination experiments were conducted on thin films fabricated under the conductions of sample #306, with a thickness of approximately 20 nm as measured by a stylus profiler (Table 3). The first experiment was conducted for 3 hours at 200ºC, and resulted in XRD pattern peaks at the same 2θ as the precursor film, with a lower peak intensity, possibly due to the effect of heat treatment on the film without inducing a fluorination reaction

(Figure 4). To induce fluorination, the temperature was increased to 250ºC and 300ºC, neither of which produced identifiable peaks. To verify fluorine doping, an analysis of chemical compositions was conducted with energy-dispersive X-ray spectroscopy. Figure 5 depicts the EDS spectra near the O Kα and F Kα peaks of the LSMO4 precursor and the films fluorinated at 200ºC, 250ºC, and 300ºC. Notably, only a peak for manganese is observed in the precursor; as the temperature increases, the curve shifts to the right, possibly due to the combined influence of both manganese and fluorine. The peak also increases in intensity with increasing temperature, suggesting the

Table 3: Conditions for Topotactic Fluorination of LSMO4 Thin Films

Time Temperature Result

3 hr 200ºC Peak intensity of precursor film weakened

3 hr 250ºC No peaks

3 hr 300ºC No peaks

1 hr 250ºC No peaks

1 hr 300ºC Weak peak at 2θ = 30.325º

Log

Inte

nsity

(arb

. uni

t)

656055504540353025202θ (deg.)

004 00

6

* *

Before

300 oC

200 oC

250 oC

Figure 4: XRD patterns for initial fluorination reactions conducted for 3 hours. The 200ºC experiment resulted in the same peaks as the precursor, with lower intensity; the 250ºC and 300ºC experiments resulted in no observable peaks.

Inte

nsity

(arb

. uni

t)

1.00.50.0E (keV)

Mn

Kα

C K

α F Kα

O K

α

La K

α

Precursor 200oC 250oC 300oC

Figure 5: Energy-dispersive X-ray spectra (EDS) near the O Kα and F Kα peaks for films fluorinated for 3 hours for 200ºC, 250ºC, and 300ºC, as well as the precursor film of LSMO4. Spectra are not normalized.

sample incorporated more fluorine at higher temperatures, confirming its presence in the thin film. Because no peaks were observed in the XRD pattern, this suggests that while fluorine was successfully inserted into the structure, the film may have decomposed. To prevent film decomposition, the reaction time of the fluorination was decreased from 3 hours to one hour. XRD patterns reveal no peaks at 250ºC; however, for the fluorination experiment conducted at 300ºC, a weak, ambiguous peak located at around 30.325º was observed, suggesting that the fluorine may have been successfully incorporated in the thin film structure.

IV. CONCLUSION

Conditions to fabricate high-quality LSMO4 thin films were successfully optimized, expediting future experiments using these thin films. In addition, probable chemical reactions between the precursor LSMO4 films and PVDF were observed. Future work may investigate the effects of a further reduced fluorination reaction time, as the 1 hour reaction only produced 1 weak peak. In addition, nuclear reaction analysis (NRA) should be conducted to quantitatively determine the fluorine content of the films, as well as physical property measurement system (PPMS) measurements to constrain the electrical properties for applicability in rechargeable fluoride ion batteries.

V. ACKNOWLEDGEMENTS

Thank you to Professor Hasegawa and his lab group for providing this incredible opportunity for me to work on such cutting edge research, providing me with exposure and experience to solid-state chemistry research, as well as a welcoming, encouraging working environment for my Tokyo research internship. I would also like to thank my advisor Dr. Chikamatsu, for his guidance and support. Thank you to my supporter, Mr. Maruyama, for his constant assistance and hard work in helping me conduct all my experiments and teaching me important concepts related to my research. I would also like to express my gratitude to Mr. Yamada and Mr. Ishigami for their assistance with my XRD experiments. My deepest thanks to the UTRIP program and the International Liaison Office, for organizing an incredible program and making my stay in Tokyo enjoyable, comfortable, and incredibly

educational. Finally, I would like to extend my thanks to the FUTI scholarship, for the financial support in making this experience possible.

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