Open Submitted: 28 September 2019 Accepted: 30 January 2020 Published Online: 14 February 2020
AIP Advances 10, 025125 (2020); https://doi.org/10.1063/1.5129422
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  • Yuki Sasahara
  • Koki Kanatani
  • Hiroaki Asoma
  • Masayuki Matsuhisa
  • Kazunori Nishio
  • Ryota Shimizu
  • Norimasa Nishiyama
  • Taro Hitosugi

Materials that are thermodynamically stable at ultrahigh pressures (>10 GPa) often exhibit unique physical properties. However, few studies have addressed the fabrication of epitaxial thin films of ultrahigh-pressure phases. Herein, we combine epitaxial thin film growth techniques with ultrahigh-pressure synthetic methods. We demonstrate the synthesis of single-phase epitaxial thin films of an ultrahigh-pressure polymorph of TiO2, α-PbO2-type TiO2. A rutile TiO2(100) epitaxial thin film is used as a precursor, and a structural phase transition is induced at 8 GPa and 800–1000 °C. This study demonstrates a new synthetic route to obtain ultrahigh-pressure-phase materials. The fabrication of epitaxial thin film ultrahigh-pressure phases paves the way for investigating the physical properties that arise at surfaces and interfaces of materials.
Materials under ultrahigh pressure exhibit a variety of interesting properties ranging from magnetism to superconductivity.1–51. L. Zhang, Y. Wang, J. Lv, and Y. Ma, Nat. Rev. Mater. 2, 17005 (2017). https://doi.org/10.1038/natrevmats.2017.132. M. Azuma, K. Takata, T. Saito, S. Ishiwata, Y. Shimakawa, and M. Takano, J. Am. Chem. Soc. 127, 8889 (2005). https://doi.org/10.1021/ja05125763. A. P. Drozdov, M. I. Eremets, I. A. Troyan, V. Ksenofontov, and S. I. Shylin, Nature 569, 528 (2019). https://doi.org/10.1038/s41586-019-1201-84. M. Tsuji, H. Hiramatsu, and H. Hosono, Inorg. Chem. 58, 12311 (2019). https://doi.org/10.1021/acs.inorgchem.9b018115. J. Buckeridge, K. T. Butler, C. R. A. Catlow, A. J. Logsdail, D. O. Scanlon, S. A. Shevlin, S. M. Woodley, A. A. Sokol, and A. Walsh, Chem. Mater. 27, 3844 (2015). https://doi.org/10.1021/acs.chemmater.5b00230 To utilize these properties in electronic applications, it is crucial to be able to fabricate epitaxial thin films of these ultrahigh-pressure-phase materials. To date, thin films of Bi2NiMnO6,66. M. Sakai, A. Masuno, D. Kan, M. Hashisaka, K. Takata, M. Azuma, M. Takano, and Y. Shimakawa, Appl. Phys. Lett. 90, 072903 (2007). https://doi.org/10.1063/1.2539575 CaZn2N2,77. M. Tsuji, K. Hanzawa, H. Kinjo, H. Hiramatsu, and H. Hosono, ACS Appl. Electron. Mater. 1, 1433 (2019). https://doi.org/10.1021/acsaelm.9b00248 rock-salt-type ZnO,88. C.-Y. James Lu, Y.-T. Tu, T. Yan, A. Trampert, L. Chang, and K. H. Ploogm, J. Chem. Phys. 144, 214704 (2016). https://doi.org/10.1063/1.4950885 and diamond,9,109. H. Watanabe, K. Hayashi, D. Takeuchi, S. Yamanaka, H. Okushi, K. Kajimura, and T. Sekiguchi, Appl. Phys. Lett. 73, 981 (1998). https://doi.org/10.1063/1.12205910. S. Koizumi, T. Murakami, T. Inuzuka, and K. Suzuki, Appl. Phys. Lett. 57, 563 (1990). https://doi.org/10.1063/1.103647 which are stable under ultrahigh pressure,2,4,112. M. Azuma, K. Takata, T. Saito, S. Ishiwata, Y. Shimakawa, and M. Takano, J. Am. Chem. Soc. 127, 8889 (2005). https://doi.org/10.1021/ja05125764. M. Tsuji, H. Hiramatsu, and H. Hosono, Inorg. Chem. 58, 12311 (2019). https://doi.org/10.1021/acs.inorgchem.9b0181111. S. Desgreniers, Phys. Rev. B 58, 14102 (1998). https://doi.org/10.1103/physrevb.58.14102 have been grown epitaxially on single crystal substrates. However, only a few ultrahigh-pressure-phase materials have been successfully fabricated via the direct growth of thin films.6–10,126. M. Sakai, A. Masuno, D. Kan, M. Hashisaka, K. Takata, M. Azuma, M. Takano, and Y. Shimakawa, Appl. Phys. Lett. 90, 072903 (2007). https://doi.org/10.1063/1.25395757. M. Tsuji, K. Hanzawa, H. Kinjo, H. Hiramatsu, and H. Hosono, ACS Appl. Electron. Mater. 1, 1433 (2019). https://doi.org/10.1021/acsaelm.9b002488. C.-Y. James Lu, Y.-T. Tu, T. Yan, A. Trampert, L. Chang, and K. H. Ploogm, J. Chem. Phys. 144, 214704 (2016). https://doi.org/10.1063/1.49508859. H. Watanabe, K. Hayashi, D. Takeuchi, S. Yamanaka, H. Okushi, K. Kajimura, and T. Sekiguchi, Appl. Phys. Lett. 73, 981 (1998). https://doi.org/10.1063/1.12205910. S. Koizumi, T. Murakami, T. Inuzuka, and K. Suzuki, Appl. Phys. Lett. 57, 563 (1990). https://doi.org/10.1063/1.10364712. Y. Jia, M. A. Zurbuchen, S. Wozniak, A. H. Carim, D. G. Schlom, L.-N. Zou, S. Briczinski, and Y. Liu, Appl. Phys. Lett. 74, 3830 (1999). https://doi.org/10.1063/1.124194
One of the possible approaches for controlling the phase of materials is to apply pressure directly to thin films. For example, it has been reported that applying a uniaxial pressure of ∼30 MPa to vanadium oxide epitaxial thin films could control its structure.1313. A. Matsuda, Y. Nozawa, S. Kaneko, and M. Yoshimoto, Appl. Surf. Sci. 480, 956 (2019). https://doi.org/10.1016/j.apsusc.2019.01.189 The combination of ultrahigh-pressure and epitaxial thin film techniques is promising for the exploration of novel synthetic routes. However, there have been no reports on the application of ultrahigh-pressure (>3 GPa) to thin films for phase control.
In this study, we developed a technique for applying ultrahigh pressure (8 GPa) to thin film samples on 5-mm single-crystal substrates, and, using a rutile TiO2(100) epitaxial thin film as a precursor, we fabricated epitaxial thin films of single-phase α-PbO2-type TiO2(100) (orthorhombic, a = 0.454 nm, b = 0.549 nm, and c = 0.491 nm)1414. L. Gerward and J. S. Olsen, J. Appl. Crystallogr. 30, 259 (1997). https://doi.org/10.1107/s0021889896011454 by inducing a structural phase transition at ultrahigh pressure. Despite being formed at ultrahigh pressure (>6 GPa), this TiO2 phase was preserved at ambient pressure.1515. J. S. Olsen, L. Gerward, and J. Z. Jiang, J. Phys. Chem. Solids 60, 229 (1999). https://doi.org/10.1016/s0022-3697(98)00274-1 α-PbO2-type TiO2 has been reported to exhibit high photocatalytic properties5,165. J. Buckeridge, K. T. Butler, C. R. A. Catlow, A. J. Logsdail, D. O. Scanlon, S. A. Shevlin, S. M. Woodley, A. A. Sokol, and A. Walsh, Chem. Mater. 27, 3844 (2015). https://doi.org/10.1021/acs.chemmater.5b0023016. H. Murata, Y. Kataoka, T. Kawamoto, I. Tanaka, and T. Taniguchi, Phys. Status Solidi RRL 8, 822 (2014). https://doi.org/10.1002/pssr.201409343 and high electrical conductivity.1717. X. Lü, W. Yang, Z. Quan, T. Lin, L. Bai, L. Wang, F. Huang, and Y. Zhao, J. Am. Chem. Soc. 136, 419 (2014). https://doi.org/10.1021/ja410810w To evaluate the catalytic properties and electrical conductivity in detail, an epitaxial thin film with a defined surface orientation and area is required because it is difficult to synthesize large single crystals.1818. T. I. Dyuzheva, L. M. Lityagina, and N. A. Bendeliani, J. Alloys Compd. 377, 17 (2004). https://doi.org/10.1016/j.jallcom.2004.01.033 Although the atomic layer deposition of an α-PbO2-type TiO2 epitaxial thin film has been reported previously,1919. A. Tarre, K. Möldre, A. Niilisk, H. Mändar, J. Aarik, and A. Rosental, J. Vac. Sci. Technol., A 31, 01A118 (2013). https://doi.org/10.1116/1.4764892 the film contained secondary phases. Therefore, there have been no reports on single-phase α-PbO2-type TiO2 epitaxial thin films. This study demonstrates that high-quality ultrahigh-pressure-phase epitaxial thin films can be fabricated by ultrahigh-pressure treatment of thin film samples.
The first stage of this study was the development of an ultrahigh-pressure treatment cell. An important consideration for this design was that thin films (several tens of nanometers thick) are extremely vulnerable to physical impact and that the substrate used for thin film fabrication is only ∼0.5 mm thick. Therefore, it was necessary to develop a Kawai-type multi-anvil apparatus2020. N. Kawai and S. Endo, Rev. Sci. Instrum. 41, 1178 (1970). https://doi.org/10.1063/1.1684753 cell assembly that could be used to apply ultrahigh pressure to a millimeter-sized thin film sample without causing damage.
A schematic of the cell assembly is shown in Fig. 1(a). Most of the components are commonly used in Kawai-type multi-anvil apparatus; Cr2O3-doped MgO was used as the pressure medium, LaCrO3, as the resistance heater, and Cu, as the electrode. In addition, to prevent damage, we used NaCl powder—which is soft and stable at high temperature and ultrahigh pressure—as the pressure medium in direct contact with the thin film sample [Fig. 1(b)]. Furthermore, the sample periphery was covered with a Cu cylinder to prevent sintering of the sample and the MgO pressure medium. To collect the samples, we simply dissolved the NaCl in water. Hence, we could collect the sample without applying a strong force that could break the substrate.
To assess the possible damage to a sample subjected to ultrahigh pressure using our apparatus, we applied 8 GPa pressure at room temperature to a precursor rutile TiO2(100) epitaxial thin film on a 5-mm-diameter Al2O3(0001) substrate. The precursor epitaxial thin films were deposited by pulsed laser deposition (PLD).21–2421. T. Hitosugi, N. Yamada, S. Nakao, Y. Hirose, and T. Hasegawa, Phys. Status Solidi A 207, 1529 (2010). https://doi.org/10.1002/pssa.20098377422. T. Hitosugi, A. Ueda, S. Nakao, N. Yamada, Y. Furubayashi, Y. Hirose, T. Shimada, and T. Hasegawa, Appl. Phys. Lett. 90, 212106 (2007). https://doi.org/10.1063/1.274231023. Y. Furubayashi, T. Hitosugi, Y. Yamamoto, K. Inaba, G. Kinoda, Y. Hirose, T. Shimada, and T. Hasegawa, Appl. Phys. Lett. 86, 252101 (2005). https://doi.org/10.1063/1.194972824. R. Shimizu, I. Sugiyama, N. Nakamura, S. Kobayashi, and T. Hitosugi, AIP Adv. 8, 095101 (2018). https://doi.org/10.1063/1.5048441 According to the phase diagram of rutile TiO2, the sample should not undergo a phase transition when a pressure of 8 GPa is applied at room temperature.1515. J. S. Olsen, L. Gerward, and J. Z. Jiang, J. Phys. Chem. Solids 60, 229 (1999). https://doi.org/10.1016/s0022-3697(98)00274-1 Figures 1(c) and 1(d) show the out-of-plane x-ray diffraction (XRD) patterns of the film before and after ultrahigh-pressure treatment at room temperature. A very small peak shift of the 400 peak [from 2θ = 84.78° to 84.84°; Fig. 1(d)] and a slight decrease in the rocking curve full-width-at-half-maximum (FWHM) of the 200 peak [from 0.055° to 0.039°; Fig. 1(e)] were observed after the treatment. The average surface roughness (Ra) of a 2 µm × 2 µm section of the sample exhibited negligible change upon the application of pressure (1.93 nm before and 1.92 nm after), and there was no significant change in the sample surfaces before and after treatment, as observed in atomic force microscopy (AFM) images (Fig. S2). These results confirm that applying ultrahigh pressure at room temperature using our apparatus does not damage the samples.
For the next stage of the study, experiments were performed under ultrahigh pressure and elevated temperature. According to the phase diagram of TiO2 at 8 GPa,1414. L. Gerward and J. S. Olsen, J. Appl. Crystallogr. 30, 259 (1997). https://doi.org/10.1107/s0021889896011454 α-PbO2-type TiO2 has maximum stability at approximately 550–1050 °C. Additionally, since the Al2O3 substrate reacts with water at temperatures below 900 °C,2525. E. Ohtani, K. Litasov, A. Suzuki, and T. Kondo, Geophys. Res. Lett. 28, 3991, https://doi.org/10.1029/2001gl013397 (2001). https://doi.org/10.1029/2001gl013397 the ultrahigh-pressure treatment was performed at 1000 °C. Figures 2(a) and 2(b) show out-of-plane XRD patterns for the thin film before and after ultrahigh-pressure treatment (8 GPa) at 1000 °C. Remarkably, the 400 peak shifted to a higher angle after treatment, indicative of a phase transition from rutile to α-PbO2-type TiO2. Raman spectroscopy also confirmed the conversion to α-PbO2-type TiO2 [Fig. 3(d)]. In addition, the FWHM of the rocking curve for the α-PbO2-type TiO2 200 peak was 0.11° [Fig. 2(c)], which indicates the fabrication of a high crystallinity film. Note that the obtained α-PbO2-type TiO2 film was relaxed (Fig. S3).
The ϕ-scan measurements revealed that the film was epitaxially grown on the Al2O3(0001) substrate. The peaks of Al2O3 30-30 (2θ = 68.2°, χ = 90°) and α-PbO2-type TiO2 020 (2θ = 32.4°, χ = 90°) appear at the same ϕ-angles [Fig. 2(d)], which demonstrates the in-plane epitaxial relationship of [10-10]Al2O3//[010]TiO2 [Fig. 2(e)]. Note that the six diffraction peaks for α-PbO2-type TiO2 020 indicate the existence of three rotational domains. The in-plane epitaxial relationship is consistent with a previous report that utilized atomic layer deposition1919. A. Tarre, K. Möldre, A. Niilisk, H. Mändar, J. Aarik, and A. Rosental, J. Vac. Sci. Technol., A 31, 01A118 (2013). https://doi.org/10.1116/1.4764892 and with those found in nature.2626. S.-L. Hwang, P. Shen, H.-T. Chu, and T.-F. Yui, Science 288, 321 (2000). https://doi.org/10.1126/science.288.5464.321 We stress that this is the first report of a single-phase epitaxial film of α-PbO2-type TiO2. The films became single phase because the thermodynamically stable conditions for α-PbO2-type TiO2 were realized in the fabrication process.
Although epitaxial thin films were obtained, the flat precursor surface (Fig. S2) developed a wrinkled structure after ultrahigh-pressure treatment at 1000 °C, as shown by AFM measurements [Fig. 3(a)]. The height profile indicates that the wrinkles are over 200 nm high, which are larger than the thickness of the thin film precursor (100 nm). These wrinkled structures did not develop on the surface of the Al2O3 substrate; the Al2O3 surface without the TiO2 precursor maintained a surface roughness (Ra = 0.11 nm) that was comparable to that of pristine Al2O3 substrates (Ra = 0.09 nm).
Subsequently, we used scanning electron microscopy–energy-dispersive x-ray spectroscopy (SEM-EDX) to confirm the composition of the wrinkled structures [Fig. 3(b)]. The EDX mappings of Ti and Al [Fig. 3(c)] show that Ti existed only in the wrinkled structure. These results indicate that the TiO2 thin film aggregated during the ultrahigh-pressure treatment (8 GPa at 1000 °C). Note that Raman spectroscopy also indicated aggregation of α-PbO2-type TiO2 on the surface [Fig. 3(d)].
Next, we pursued the fabrication of flat films by suppressing the aggregation of TiO2. It has been reported that Fe3O4 thin films on a SrTiO3 substrate aggregate during high-temperature annealing (1100 °C under vacuum).2727. R. Takahashi, H. Misumi, T. Yamamoto, and M. Lippmaa, Cryst. Growth Des. 14, 1264 (2014). https://doi.org/10.1021/cg5000414 Indeed, we confirmed the aggregation of rutile TiO2 thin films on the Al2O3 substrate by high-temperature annealing (1000 °C under ambient pressure; Fig. S5). Accordingly, we performed an ultrahigh-pressure treatment (8 GPa) at a lower temperature (800 °C). Since the Al2O3 substrate may react with water at temperatures below 900 °C,2525. E. Ohtani, K. Litasov, A. Suzuki, and T. Kondo, Geophys. Res. Lett. 28, 3991, https://doi.org/10.1029/2001gl013397 (2001). https://doi.org/10.1029/2001gl013397 the samples were sealed under dry conditions before the ultrahigh-pressure treatment (Fig. S1). After the treatment, an epitaxial thin film was obtained (Fig. S6). The film had a flatter surface; however, the film was slightly aggregated, and holes were observed by AFM [Fig. 4(a)]. The thin film showed a flatness of Ra = 16.7 nm for 20 µm × 10 µm, with the flattest area exhibiting Ra = 2.60 nm for 2 µm × 2 µm [Fig. 4(b)]. Note that no peak originating from the α-AlO(OH) diaspore nor a secondary phase was detected by XRD.
In conclusion, we successfully developed a technique for applying ultrahigh pressure to a rutile TiO2(100) thin film sample using a Kawai-type multi-anvil apparatus to fabricate a flat α-PbO2-type TiO2(100) epitaxial thin film. The surface structure changed depending on the temperature used during the ultrahigh-pressure treatment: a flat thin film was obtained with treatment at a relatively low temperature (800 °C, 8 GPa), and a coagulated and wrinkled film was obtained with treatment at a higher temperature (1000 °C, 8 GPa). With a further reduction in the sample size to a diameter of 2.5 mm, it would be possible to apply a pressure of 20 GPa to the films using our experimental apparatus. As the α-PbO2-type TiO2 phase was preserved at ambient pressure, this successful growth method opens the pathway for fabricating a variety of high-quality ultrahigh-pressure-phase epitaxial thin films. Furthermore, practical applications can be developed to exploit the physical properties arising from surface and interface effects of ultrahigh-pressure phases.
See the supplementary material for the experimental procedures, AFM images of ultrahigh-pressure treated rutile TiO2 thin films at room temperature, lattice constant determination of α-PbO2-type TiO2, EDX spectrum after ultrahigh-pressure treatment (8 GPa, 1000 °C), AFM image of the aggregates of rutile TiO2 thin films that formed upon annealing at 1000 °C, and structural properties of flat α-PbO2-type TiO2 epitaxial thin films (PDF).
Y.S. acknowledges funding from the Hiki Foundation, Tokyo Institute of Technology. R.S. acknowledges funding from JSPS Kakenhi, Grant Nos. JP17H05216, and JST-PRESTO, Grant No. JPMJPR17N6, Japan. N.N. acknowledges funding from JSPS Kakenhi Grant No. JP18H03836. T.H. acknowledges funding from JSPS Kakenhi Grant Nos. JP18H03876 and JP18H05514 and the JST-CREST (Grant No. JPMJCR1523) program. The authors thank Ms. Emi Oshinoya for assistance with ultrahigh-pressure treatments, Professor Yuji Wada for assistance with SEM measurements, and Mr. Naoto Nakamura for assistance with Raman spectroscopy, which was performed at the Ookayama Materials Analysis Division in the Technical Department of the Tokyo Institute of Technology. The authors thank Dr. Ryo Nakayama, Professor Mamoru Yoshimoto, and Dr. Akifumi Matsuda for fruitful discussions. The crystal structures were illustrated using the computer program VESTA.2828. K. Momma and F. Izumi, J. Appl. Crystallogr. 44, 1272 (2011). https://doi.org/10.1107/s0021889811038970
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