Epitaxial Film Growth of LiBH4 via Molecular Unit Evaporation

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Epitaxial Film Growth of LiBH4 via Molecular Unit Evaporation Hiroyuki Oguchi, Sangryun Kim, Shingo Maruyama, Yuhei Horisawa, Shigeyuki Takagi, Toyoto Sato, Ryota Shimizu, Yuji Matsumoto, Taro Hitosugi, and Shin-ichi Orimo ACS Appl. Electron. Mater., Just Accepted Manuscript • DOI: 10.1021/acsaelm.9b00350 • Publication Date (Web): 15 Aug 2019 Downloaded from pubs.acs.org on August 19, 2019

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ACS Applied Electronic Materials

Epitaxial Film Growth of LiBH4 via Molecular Unit Evaporation　 Hiroyuki Oguchi*,†,‡, Sangryun Kim$, Shingo Maruyama#, Yuhei Horisawa‖, Shigeyuki Takagi$, Toyoto Sato$, Ryota Shimizu+, Yuji Matsumoto#, Taro Hitosugi+, and Shin-ichi Orimo†, $.

†WPI-Advanced

Institute for Materials Research (WPI-AIMR), Tohoku University, Sendai 980-

8577, Japan ‡New

Industry Creation Hatchery Center (NICHe), Tohoku University, Sendai 980-8579, Japan

$Institute

for Materials Research (IMR), Tohoku University, Sendai 980-8577, Japan

#Department

of Applied Chemistry, School of Engineering, Tohoku University, Sendai, 980-

8579, Japan ‖Department

of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, Yokohama 226-8502, Japan +School

of Materials and Chemical Technology, Tokyo Institute of Technology, Tokyo, 1528550, Japan

ABSTRACT: Complex hydrides have attracted considerable attention in fields including fast ion conduction and hydrogen storage. To understand the physical properties and to expand the fields of application of complex hydrides, physically well-defined epitaxial films that can facilitate the investigation of intrinsic properties and interfacial effects need to be fabricated. However, epitaxial films of complex hydrides remain difficult to produce. This study reports the growth of single-phase epitaxial films of the complex hydride LiBH4, and their high Li-ion conductivity of 1×10−2 S cm−1 at 423 K. To achieve this, we used a low1 ACS Paragon Plus Environment

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power infrared laser to induce the evaporation of LiBH4 molecular units; with this technique, we were able to deposit [BH4]- complex anions while preserving their molecular integrity, producing epitaxial films with high crystallinity, flat surface, and high Li-ion conductivity. These achievements will establish a solid basis for epitaxial growth of complex hydrides, and pave the way for advanced studies of complex hydrides including surface and interfacial phenomena. KEY WARDS: complex hydride, epitaxial film, fast Li-ion conduction, molecular unit evaporation, infrared pulsed laser deposition

1. INTRODUCTION Complex hydrides are a class of materials denoted as Mx(M’yHz), where M represents a metal cation such as Li+, Na+, and Mg2+; and M’yHz represents a complex anion such as [BH4]–, [B12H12]2−, or [AlH4]–. These materials have attracted considerable attention as reducing agents and hydrogen storage materials.1–3 Since the discovery of fast Li-ion conduction in LiBH4 and Li(BH4)1-xIx solid solution systems,4,5 complex hydrides have been considered as a new class of solid-electrolytes for all-solid-state batteries.6–8 Fast ion conduction is also very important in electronic applications, such as resistive memory and neuromorphic devices.9–11 Understanding the intrinsic properties and interfacial 2 ACS Paragon Plus Environment

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phenomena of LiBH4 and Li(BH4)1-xIx is essential for their applications in devices, and therefore, establishing synthesis processes for epitaxial films is quite important to provide a platform for investigating these materials.12–14

Although growth of LiBH4 film is highly desirable, researchers have failed to fabricate high-quality epitaxial films. To date, beside one example of polycrystalline film,15 there is no report of the fabrication of LiBH4 epitaxial films. Even for other complex hydrides, only several reports on polycrystalline or amorphous films had been published.16–19 In LiBH4 film growth, one of the greatest difficulties is the formation of [BH4]– complex anions: the high energy barrier for the formation of B-H covalent bonds prohibits the direct growth of LiBH4 films from their constituent elements.20 Further, the comparable formation energies of [BH4]– and [B12H12]2– inevitably result in a mixture of both anions in the films.21 Pulsed laser deposition (PLD) has been unsuccessful (Figure S1) because irradiation by a KrF excimer laser decomposes LiBH4 targets to their constituent elements, leading to the formation of impurity phases, including unreacted boron and Li2B12H12.15 To selectively obtain [BH4]– complex anions, conventional wet chemical powder production uses

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compounds such as diborane (B2H6), which has a B-H bond structure similar to that of [BH4]– complex anions.22,23 These results suggest that preserving the complex anions during the deposition process is a crucial factor for forming high-quality LiBH4 films.

In this study, we report on the PLD growth of LiBH4 epitaxial thin films by preserving the B-H bonds of the [BH4]– complex anions. We used an infrared (IR) laser to heat a LiBH4 target, producing intact [BH4]– complex anion vapor for deposition to a substrate. We found that a low-power IR laser effectively evaporated LiBH4 molecular units from the LiBH4 targets. Such IR lasers are known to be useful for maintaining molecular units of organic semiconductors and ionic liquids owing to their low photon energy and low irradiation power.24–28 The obtained films showed high crystallinity and Li-ion conductivity exceeding 1.5×10−3 S cm−1 at temperatures higher than 395 K. We also report on the deposition of epitaxial thin films of Li(BH4)0.92I0.08 (often written as LiBH4+0.09LiI)4,29 and their Li-ion conductivities, which reached as high as 1.0×10−2 S cm−1 at 423 K. To our knowledge, this value is the highest reported in epitaxial films to date.


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2. EXPERIMENTAL SECTION To avoid sample degradation, sample fabrication and characterization were performed in non-air-exposure environments. Targets were prepared in a glove box filled with purified Ar. Samples and targets were transferred in air-tight vessels.

Films were deposited in a vacuum chamber (background pressure: 6×10−9 Torr) using the molecular unit evaporation. The LiBH4 and Li(BH4)0.92I0.08 phases were successfully grown on α-Al2O3 (0001) substrates at temperatures from RT to 423 K without the assistance of H2 gas. A vacuum pressure of less than 1×10−6 Torr during the deposition ensured that the films were grown in the laser molecular beam epitaxy (L-MBE) mode. For the deposition, the targets were heated by a focused pulsed IR laser beam (wavelength of 808 nm, spot size of ~4 mm2, irradiation energy of ~200 W cm−2, repetition rates of 1–3 Hz, pulse widths of 10–30 ms) (LIMO32, LIMO GmbH). To prevent gradual hydrogen release and decomposition of LiBH4, the targets were rotated at a speed of approximately 2 rpm to change the location of heating. The distance between the target and a substrate was 3–5 cm. The typical film growth rate was 15 nm min−1. The target of LiBH4 and Li(BH4)0.75I0.25 had a diameter of 20 mm and a thickness of ~2 mm, and was 5 ACS Paragon Plus Environment


prepared by uniaxial pressing of a mixture of LiBH4 (or LiBH4 and LiI) and carbon black powders at 60 MPa. The amount of iodine in the film Li(BH4)0.92I0.08 obtained by using Li(BH4)0.75I0.25 target was approximately 27% that of the target, probably because of the different equilibrium vapor pressures of LiBH4 and LiI (Note S1).

Both the evaporation of the LiBH4 and the LiI molecular unit were monitored by a quadrupole mass spectrometer (Q-mass). Film thickness was measured by a quartz crystal thickness monitor (Q-pod, Inficon) and a stylus profiler (DektakXT, Bruker AXS). The composition of boron and iodine was determined by nuclear reactor analysis (NRA) and Rutherford backscattering spectroscopy (RBS) measurements (Figure S2), respectively. A Raman spectrometer (Nicolet Almega-HD) with a 532 nm laser was used to determine the phases. The crystallinity of the films was evaluated by X-ray diffraction (XRD) measurements (D8 Discover, Bruker AXS and SmartLab 9MTP, RIGAKU). Homogeneity and surface flatness were examined with a scanning electron microscopy (SEM) (JSM-7800F, JEOL) and atomic force microscopy (AFM) (MultiMode 8, Bruker AXS) conducted at the National Institute for Materials Science (NIMS) Battery Research


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Platform. All measurements were conducted at room temperature if not otherwise specified.

The in-plane Li-ion conductivity was measured by using electrochemical impedance measurements. The samples were placed in a vacuum chamber (back pressure: 1×10−7 Torr), and the impedance spectra were measured over a temperature range from 323 K to 423 K with an applied frequency range of 5 Hz to 0.5 MHz (SP-150, Biologic). The thicknesses of the LiBH4 and the Li(BH4)0.92I0.08 epitaxial films were approximately 2 and 3 μm, respectively. We deposited comb-shaped electrodes on films at room temperature by evaporating Au in a vacuum chamber using a patterned shadow mask (Figure S3). To determine the Li-ion conductivity of the powder samples, the powder was pressed into a disc-shaped pellet, 8 mm in diameter and 1 mm in thickness. Then electrochemical impedance measurements were implemented with Li electrodes under an Ar atmosphere over a temperature range from 323 to 423 K with an applied frequency range of 4 Hz to 1 MHz with the use of a frequency response analyzer 3532-80, HIOKI.



3. RESULTS AND DISCUSSION We used an IR laser (wavelength of 808 nm) at a low irradiation energy (~200 W cm−2) to suppress decomposition of LiBH4 molecular units. Tuning the irradiation energy was critical as the temperature of the target is sensitive to the power, and a high-power laser might decompose LiBH4. To evaporate LiBH4, we added carbon powder as an IR absorber to the target, because LiBH4 has a large band gap (~7 eV).30 The evaporation processes are schematically shown in Figure 1a.

Time-dependent Q-mass spectroscopy indicated that LiBH4 was not decomposed and existed as LiBH4 molecular units in the evaporated gas. The Q-mass intensity of the atomic mass unit (amu) signal at 22, corresponding to the LiBH4 molecular unit, increased and decreased in response to turning the IR laser on and off, respectively (Figure 1b). We did not detect Li-B-H compounds, such as Li2B10H10 (amu = 132) and Li2B12H12 (amu = 156), which are possible intermediate species in the decomposition of LiBH4.31 These results clearly indicate that the evaporated gas phase mainly consisted of LiBH4 molecular units.


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This observation was corroborated by Raman spectroscopy of the obtained films grown on α-Al2O3(0001) substrates. Even films deposited at room temperature (RT), which is considered low enough to suppress chemical reactions on the substrate, showed only peaks from LiBH4. Thus, LiBH4 molecular units, rather than atomic species, were supplied to the substrate.

The XRD patterns indicated that LiBH4 epitaxial films were grown on the α-Al2O3 (0001) substrate (Figure 2). During the evaporation, the substrate temperature was set to be 423 K to promote crystal growth while suppressing hydrogen desorption from the films (two-dimensional XRD images of the films grown at room temperature, 393 K, and 423 K are compared in the Figure S4). To clearly observe diffraction peaks of LiBH4 composed of only light elements, the thickness of the films was increased to be more than 2 μm. Because the films were extremely sensitive to moisture, all experimental processes were performed without air-exposure. Photographs and scanning electron microscope (SEM) images of the 2 μm-thick films are shown in Figure S5.



The out-of-plane XRD pattern (Figure 2a) showed peaks at 2θ = 26.20°, 53.86°, and 85.52°, assigned to the 002, 004, and 006 diffraction features of orthorhombic LiBH4, respectively, indicating that the films were in the c-axis orientation.32 The pole figure of the LiBH4 011 reflection showed periodical diffraction spots indicating epitaxial growth of LiBH4 (Figure 2b). The six-fold symmetric pattern revealed the coexistence of epitaxial domains of orthorhombic LiBH4 rotated 60° with respect to each other. The pole figures of the films and of the α-Al2O3 substrate show that the in-plane epitaxial relationship was [010]LiBH4 // [10-10]Al2O3 for one of the three 60° rotational domains (Fig. 2d). The mismatches were determined to be 7.1% and 0.5% for [010]LiBH4 and [100]LiBH4, respectively. Additional XRD studies for the film heated at 423 K showed that LiBH4 epitaxial phase was stable at that temperature (Fig. S6).

The films were of very high quality in terms of their crystallinity and surface flatness. The Raman spectra (Figure 1c) and the out-of-plane XRD pattern (Figure 2a) revealed that the films had a single phase. In addition, the full width at half maximum (FWHM) of the rocking curve of the LiBH4 002 peak was ~1.1° (Figure 2e), indicating high degree of


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orientation comparable to that of the reported metal hydride epitaxial films.33–36 The surface roughness Ra determined by atomic force microscopy (AFM) was 4 nm over an area of 2 μm × 2 μm (Figure 2f). This level of roughness has never been attainable for complex hydrides. The AFM image of the initial stage of the film growth revealed the formation of islands suggesting that the films were grown in Volmer-Weber growth mode (Figure S7).

The films showed high Li-ion conductivity (Figure 3a). The in-plane Li-ion conductivity of the LiBH4 epitaxial film was 1.7×10−7 S cm−1 at 323 K, as determined by electrochemical impedance measurements. Notably, this value was almost identical to that of commercial LiBH4 powder. The Li-ion conductivity increased sharply at 393 K in the heating and cooling process. This jump is consistent with a phase transition between low-temperature (LT, orthorhombic) and high-temperature (HT, hexagonal) phases.4,529 We measured the conductivity to be 3.4×10−3 S cm−1 at 423 K, which is the highest value reported to date for any hydride and oxide epitaxial films37–40 (we note that there are no



report of sulfide epitaxial film). A reversible temperature dependence of the conductivity indicated that the films were thermally stable.

We further fabricated epitaxial thin films of a Li(BH4)1-xIx solid solution system, which is known to show a high Li-ion conductivity. A mixture of LiBH4 and LiI with a molar ratio of 1:0.33 [Li(BH4)0.75I0.25] was used as a target (see Figure S9). The films grown at 423 K on α-Al2O3 (0001) substrates were epitaxial films of Li(BH4)0.92I0.08. Again, we succeeded in the fabrication of epitaxial films with high degree of orientation (Figure S10).

A high Li-ion conductivity was also confirmed for the Li(BH4)0.92I0.08 epitaxial film (Figure 3b). The Li-ion conductivity increased sharply between 323 and 373 K in the heating process. This transition temperature was lower than that of pure LiBH4, indicating that the iodine addition lowered the phase-transition temperature of LiBH4, as is known to occur for Li(BH4)1-xIx powders.4,29 The Li-ion conductivity of the film above 373 K was similar to that of the powder having the same composition.

The temperature dependence of the Li-ion conductivity in the cooling process indicates that the HT-phase was maintained even at 323 K. Notably, the Li-ion conductivity of the 12 ACS Paragon Plus Environment

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films at 323 K during the cooling process was 1.1×10−4 S cm−1, which is sixteen times higher than that in the initial state of the as-prepared sample (6.7×10−6 S cm−1). When the film was kept at 323 K, the Li-ion conductivity slightly decreased after 12 h, and significantly decreased after 168 h (Figure S11). This result suggests that the HT-phase was kinetically stabilized in the cooling process, likely because of epitaxial strain. The stabilization was observed only for Li(BH4)0.92I0.08, and not for LiBH4, possibly because of the better lattice matching between the HT-phases of the Li(BH4)0.92I0.08 films with the αAl2O3 substrate (the lattice parameters of HT-phase LiBH4 are smaller than those of the substrate)32 [ionic radius of I− (0.211 nm) > [BH4]− (0.205 nm)].41 The high Li-ion conductivity at 323 K was reproducible in the second heating-cooling process.

Only the films maintained the HT-phase down to 323 K. In contrast to the films, the Liion conductivity of the powder showed a small hysteresis and returned to almost the same value as that of the initial state in the cooling process. This result indicates that the powder could not maintain a HT-phase and it remained in the LT-phase at 323 K. The phase



stabilization owing to epitaxial strain might be used to increase Li-ion conductivity by choosing an appropriate substrate.

This study will not only help future syntheses of complex hydrides epitaxial films, but it will also contribute to new fundamental and applied researches. For example, the in-plane Li-ion conductivities of the LiBH4 and Li(BH4)0.92I0.08 epitaxial films in the HT-phase in this study were observed to be almost the same as that of the powder complex hydrides, indicating that there is little or no anisotropy in the Li-ion conduction of these materials (contrary to what had been suggested elsewhere4).

4. CONCLUSION In summary, we succeeded in growing high-quality complex hydride LiBH4 epitaxial films based on evaporation of LiBH4 molecular units. A low-power IR laser preserved the LiBH4 molecular units in the evaporation process. We also grew Li(BH4)0.92I0.08 epitaxial films. The films showed a high Li-ion conductivity of 1.0×10−2 S cm−1 at 423 K. Results of this study will form a solid basis to conduct advanced research into the merits of epitaxial film experiments essential to realizing innovative complex-hydride-based devices.


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Figure 1. LiBH4 molecular unit evaporation. (a) A schematic of LiBH4 infrared pulsed laser deposition processes. (b) Quadruple-mass spectra of atomic mass unit of 22 (LiBH4), 132 (Li2B10H10), and 156 (Li2B12H12). (c) Raman spectra of the films deposited at the indicated temperatures (all spectra were collected at room temperature). The spectra of a LiBH4 power and an α-Al2O3 substrate are shown as reference. Except for peaks of LiBH4, all 15 ACS Paragon Plus Environment


peaks originate from the α-Al2O3 substrates. Peaks corresponding to species such as [B10H10]2−, [B12H12]2−, and boron were not observed. Moreover, there was no indication of carbon contamination from carbon added to the target.

Figure 2. Characterization of LiBH4 epitaxial films. (a) An out-of-plane X-ray diffraction pattern of the film deposited at a substrate temperature of 423 K. The film was capped with an Au layer (thickness: ~600 nm) to protect the surface from humidity. (b) A pole figure of LiBH4 011 reflection. (c) A pole figure of Al2O3 01-12 reflection. (d) In-plane epitaxial relationship between the film and the substrate, determined from the pole figures shown in (b) and (c). The unit cells of the 60° rotational domains of LiBH4 are shown as red, green, and blue rectangles in (d). A black rhombus in (d) denotes the unit cell of Al2O3. (e) The rocking curve of the LiBH4 002 reflection. The full width at half maximum 16 ACS Paragon Plus Environment

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(FWHM) was ~1.1°. (f) Atomic force microscope (AFM) image of a LiBH4 film surface. The average roughness (Ra) values over areas of 10 μm × 10 μm and 2 μm × 2 μm (dotted square) were 37 nm and 4 nm, respectively.



Figure 3. Temperature-dependence of Li-ion conductivity of epitaxial films deposited at a substrate temperature of 423 K. (a) LiBH4. (b) Li(BH4)0.92I0.08. Samples were first heated (323 K →423 K) and then cooled (423 K →323 K). The Li-ion conductivities of powders with the same composition are shown in grey as reference. Original impedance spectra are shown in Figure S8.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected]

Notes The authors declear no competing financial interest.

ACKNOWLEDGMENT The authors gratefully acknowledge support from the Target Project 4 of WPI–AIMR, Tohoku University; JSPS KAKENHI Grant No. 17H06519, 18H01727, 18H03876, JP18H05513, JP18H05514; and The TEPCO Memorial Foundation. We thank Tomoteru Fukumura for his support for film growth, Kyosuke Matsushita and Makiko Oshida for their help for AFM and SEM measurements at National Institute for Materials Science (NIMS)



Battery Research Platform, Jumpei Yahiro and Masahiro Saito for their contribution to NRA and RBS measurements and data analysis at Toray Research Center. We also thank Kuniko Yamamoto and Kesami Saito for their assistance for XRD measurements at WPI-AIMR Common Equipment Unit. We thank Patrick Han and SayEdit.com for editing a draft of this manuscript.


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Page 30 of 34

Page 31 of 34

a

Substrate Epitaxial film

H

IR

H B H

las

er

LiBH4 molecular unit

H

Li

[BH4]-

Carbon-added LiBH4 target

Li

c

15

amu 22 (LiBH4) 132 (Li2B10H10) 156 (Li2B12H12)

Off

LiBH4

Al2O3

423 K Intensity (arb. unit)

b

Current (pA)

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


10 On

393 K RT LiBH4 powder Al2O3 substrate

0

0

500

1000 Time (sec)

1500

3000

1000 2000 Wave number (cm-1) ACS Paragon Plus Environment


a

d 105 104 103

002

004

[010]

[001] [100] LiBH4

LiBH4 Al2O3 Au

106 Intensity (cps)

006 Al O

102 101 20

b

40 60 80 2θ (degree)

100

[1010] [001] [1210] Al2O3

e

LiBH4 011

20

φ

Intensity (x103 cps)

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

Page 32 of 34

15 FWHM = 1.1°

10 5 0 8

c

10

12 14 ω (degree)

16

f

Al2O3 0112

18 75 nm

φ

2 μm

ACS Paragon Plus Environment

-150 nm

Page 33 of 34

a 423

373

323

Conductivity (S cm-1)

Cooling Heating

Film Powder

-5

10-6

LiBH4

10-7 10-8

323

373

10-2

10-3

10

423

10-1

10-2

10-4

Temperature (K)

b

Temperature (K) 10-1

Conductivity (S cm-1)

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


2.2

2.4

2.6

Cooling

10-3

Film

10-4

Powder Heating

10-5 10-6

Li(BH4)0.92I0.08

10-7 2.8

3.0

3.2

10-8

2.2

2.4

2.6

2.8

1000/T (K ) -1

1000/T (K-1)


3.0

3.2


Substrate

LiBH4 epitaxial film H

IR

las

er

Li

B H

H H

LiBH4 is evaporated as its molecular unit !! 10-2 Cond. (S cm-1)

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

Page 34 of 34

LiBH4 target


10

Li-ion cond. higher than 10-2 S cm-1 Li(BH4)0.92I0.08

-4

10-6 10-8 2.2

LiBH4 Heating Cooling

2.6 3.0 1000/T (K-1)

Epitaxial Film Growth of LiBH4 via Molecular Unit Evaporation

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