Synthesis of single-crystal La0.67Sr0.33MnO3 freestanding films with different crystal-orientation

We report the synthesis of single-crystal La0.67Sr0.33MnO3 (LSMO) freestanding films with different crystal orientations. By using pulsed laser deposition, water soluble perovskite-like sacrificial layers Sr3Al2O6 (SAO) followed by LSMO films are grown on differently oriented SrTiO3 substrates. Freestanding LSMO films with different orientations are obtained by etching the SAO in pure water. All the freestanding films show room-temperature ferromagnetism and metallicity, independent of the crystal orientation. Intriguingly, the Curie temperature (TC) of the freestanding films is increased due to strain relaxation after releasing from the substrates. Our results provide an additional degree of freedom to tailor the properties of freestanding perovskite oxide heterostructures by crystal orientation and an opportunity to further integrate different oriented films together.

Over the last decades, conventional two-dimensional (2D) materials, such as graphene and MoS 2 , have been widely investigated. [1][2][3][4] The 2D material is typically one of the freestanding materials that have an isolated structure resulting in remarkable electronic properties and showing great potential for practical applications. 2,3 Apart from the conventional 2D materials, transition-metal oxide perovskites have also attracted tremendous interest owing to their abundant properties, including multiferroic, superconductivity and colossal magnetoresistance, and have been regarded as one promising candidate for emerging functional materials. [5][6][7][8] Therefore, the perovskites are expected to be prepared with freestanding form such as the 2D materials and further enrich the investigation in physics and functional devices. Indeed, single-crystal freestanding oxide films have been successfully synthesized. [9][10][11][12] In 2016, Lu and coworkers 12 developed a method to fabricate high-quality freestanding oxide films and superlattices using water-soluble perovskite-like oxide, Sr 3 Al 2 O 6 (SAO), as a sacrificial buffer layer. Then, they further explored the two-dimensional limit of the freestanding films. 13 Recently, Ji et al. 14 further prepared ultrathin isolated BiFeO 3 films even down to one-unit-cell scale. Super-elastic BaTiO 3 15 and superconductive YBa 2 Cu 3 O7−x 16 freestanding membranes were also prepared with the same method. At the same time, several prototype ARTICLE scitation.org/journal/apm devices based on the freestanding oxide films are prepared and show remarkable functionality, such as the ferroelectric capacitor 17 and resistance switching random memory. 18 These efforts demonstrate that freestanding perovskite films just like the 2D materials can be well fabricated and utilized, playing an important role in both fundamental and device research. Besides, compared with the 2D materials, perovskite oxides can be synthesized with different crystal orientations. Therefore, the different oriented oxides with freestanding form could be prepared using the sacrificial layer methods, which will provide a new degree to obtain novel properties in freestanding materials. Prior works are focused on fabricating high-quality single-crystal freestanding films on single (001) oriented substrates. [12][13][14][15][16][17][18] Little attention is paid to understand whether this technique can be appropriate for fabricating different oriented freestanding films and investigating their corresponding properties. The physical properties are closely related to the crystalline orientation in perovskites. For example, orientationdependent ferroelectric and magnetic anisotropy were identified in bulk BiFeO 3 19 and SrRuO 3 , 20,21 respectively. More importantly, at the 2D oxide interface where symmetry, surface polarity, and oxygen octahedral coupling differ from the bulk counterpart, the crystalline orientations will have a great impact on the properties of oxide heterostructures. [22][23][24] For instance, Gibert et al. 25 found a large exchange bias in (111)-oriented LaNiO 3 -LaMnO 3 superlattices, which was absent in the (001) sample, while Catalano et al. 26 observed an orientation-dependent metal-insulator transition in NdNiO 3 thin films. Therefore, it is important to investigate synthesis of TMO freestanding film with different crystal-orientations and explore their novel properties.
In this Letter, we report the synthesis of single-crystal La 0.67 Sr 0.33 MnO 3 (LSMO) freestanding films with (001), (110), and (111) orientations. The freestanding film was fabricated by the method of dissolving the water-soluble SAO buffer layer. Then, the LSMO freestanding films were achieved with the assistance of polyimide (PI) tape. By measuring their magnetic and electrical behaviors, we find that all of the different oriented films show the roomtemperature ferromagnetic and metallic behaviors before and after being released, independent of the crystal orientation.
As shown in Fig. 1(a), the hydrogarnet SAO is a perovskite-like oxide. It has a cubic structure with a lattice constant a = 15.844 Å. 12 Its constant is about four times compared with the popular perovskites, including substrate SrTiO 3 (STO, a STO = 3.905 Å, 4 × a STO = 15.620 Å). 12 Viewed on three typical planes, i.e., (001), (110) and (111) 27 Such a lattice matching relationship is the precondition of preparing single-crystal freestanding films with various orientations.

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Hence, in this work, we epitaxially deposit SAO/STO/LSMO heterostructures on the STO substrates by pulsed laser deposition (PLD). Before growth, the substrates are annealed at 850 ○ C in the PLD chamber for 30 min to achieve a smooth surface with welldefined step-and-terrace morphology. After that, there are extra STO with several unit cells deposited on the substrates to eliminate possible surface reconstruction. Then, the SAO layer is deposited on the STO substrates at a substrate temperature T sub = 700 ○ C with an oxygen pressure Po2 = 1 × 10 −3 mbar and a laser fluence of 1.3 J/cm 2 . During the growth, reflection high-energy electron diffraction (RHEED) was used to monitor the thicknesses and surface quality of SAO deposited on different substrates. The deposition condition for the SAO layer on (001) STO was carefully optimized to achieve layer-by-layer growth, and the thickness was controlled with 6 unit cells (Fig. S1a). The (110) and (111)  Based on the epitaxial SAO layers, 3-unit-cell STO buffer layers and 80-unit-cell LSMO films are successively grown. The STO layers are used to prevent cation diffusion between SAO and LSMO, which has been found in previous work. 28 At first, they are grown on (001) STO with the same laser fluence (1.3 J/cm 2 ) at T sub = 700 ○ C and Po2 = 0.1 mbar. Judging from the RHEED oscillating curve (Fig. S1b), the LSMO film was optimized to grow in the layer-by-layer mode too. Its thickness was controlled with 80 unit cells. Next, the (110) and (111) films were deposited with identical pulse counts. It should be noted that the T sub for (110) and (111) LSMO depositing is optimized at 600 ○ C to reduce the influence due to the surface polarity of the substrates and obtain smooth films. 23 Finally, these heterostructures are immersed in pure water and etch the SAO to separate the LSMO films from the substrates, as shown in Fig. 1(e). During the process, polyimide (PI) tape is used as a support to achieve continuous and complete films. This is beneficial to prevent fragmenting

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scitation.org/journal/apm the film in the case of directly lifting off without supporting layer or with the assistance of a cover layer such as polymethyl methacrylate or polystyrene. 9 Figure 2 illustrates the sample quality of strained LSMO films epitaxially deposited on STO substrates with different orientations including (001), (110), and (111). The atomic force microscopy (AFM) and RHEED are used to characterize the surface and crystallization quality of the three strained films. Figures 2(a)-2(c) show the surface morphology of different oriented LSMO films. Atomically flat (001) LSMO surfaces are obtained with the clear terrace-andstep, as shown in Fig. 2(a) (with a roughness of 0.183 nm), while (110) and (111) 31,32 These variations reveal that the orientation and stress release can modulate the magnetism to a degree.
To reveal the electrical properties of the oriented film before and after being transferred, the temperature dependence of resistivity was measured in the strained and freestanding LSMO films. As shown in Figs. 4(a)-4(c), a metallic behavior is observed in the strained films and kept well in the freestanding films. Therefore, these films are room-temperature ferromagnetic metals regardless of the orientation and stress. However, we find that the resistivity (ρ) is enhanced after being released for every oriented film. This is due to the cracks generated during the transfer process, as shown in Fig. S3d. Additionally, around the TC temperature, the (111) freestanding LSMO film shows a metal-insulator transition behavior, which is not observed in the other films. These results indicate that electric properties of the LSMO film can be modulated by the crystalline orientation and strain relaxation.
It has been widely recognized that the use of the water-soluble sacrificial layer presents a general approach to synthesize singlecrystal freestanding oxide films. [12][13][14][15][16] Some advantages of this freestanding technique can be predicted, such as the reusability of perovskite substrates, 12,16 the ability to grow freestanding metal films and prepare flexible oxide-based electronic devices. 15,18 Our study demonstrates that the technique can be utilized for producing the oxide membranes with different orientations. This provides an additional degree of freedom to tailor the properties of freestanding perovskite oxide heterostructures using crystal orientation. Moreover, one can further integrate differently oriented films together to explore novel physics and chemistry properties and develop emerging functional devices.
In summary, we have epitaxially grown high-quality LSMO films on SAO-coated STO substrates with various crystal orientations. By comparing the magnetic and electric characterizations of the oriented films, they are room-temperature ferromagnetic metals. With the assistance of the PI tape, we obtain single-crystal LSMO freestanding films with different orientations by dissolving the SAO layer. We find that the T C of freestanding films is increased in freestanding films due to strain relaxation after being released from the substrates. These demonstrate that inserting a sacrificial layer is a feasible way to prepare the single-crystal freestanding perovskite film with different crystal orientations. Our results provide the opportunity for further tailoring the properties of freestanding perovskite oxide heterostructures by crystal orientation and integrating different oriented films together.