PARTLY DIGITAL - ONLY FOR INVITEES (1,5 m) : PhD Defence Tran Phong Phu Le | Oxide nanosheets tailoring thin film orientation

Oxide nanosheets tailoring thin film orientation

Due to the COVID-19 crisis measures the PhD defence of Tran Phong Phu Le will take place (partly) online in the presence of an invited audience.

The PhD defence can be followed by a live stream.

Tran Phong Phu Le is a PhD student in the research group Inorganic Materials Science (IMS). His supervisors are prof.dr.ir. J.E. ten Elshof and prof.dr.ir. G. Koster from the Faculty of Science and Technology (TNW).

Realizing high-quality epitaxial transition metal oxide thin films on Si, glass or arbitrary substrates is promising to offer novel features and high performance for devices in electronics, optoelectronics, sensors and so on. In this thesis, the manipulation of transition metal oxide thin film orientations, both out-of-plane and in-plane directions, on arbitrary substrates is studied by two methods: oxide nanosheets and single crystal substrates. Regarding the first approach using oxide nanosheets, they can serve as templates for the thin film growth of transition metal oxides in one preferred out-of-plane orientation, enabling the free choice of substrates for both fundamental studies and practical applications. The reasons are that atomic-flat oxide nanosheets have a wide range of lattice constants and various 2D lattice symmetries, and they are easily deposited on any substrate surfaces through solution-based Langmuir-Blodgett technique. Regarding the second approach using single crystal substrates, the epitaxial lift-off of single crystalline oxide perovskite thin films with different orientations is realized using sacrificial layers deposited on single crystal substrates.

The VO₂ epitaxy on oxide nanosheets that are deposited on either amorphous SiO₂ of Si substrates or X-ray transparent Si₃N₄ membranes was studied in chapter 2. Considering lattice symmetry and lattice/domain mismatch, the epitaxial growth of VO₂ films was realized in two distinct (011)_M1/(110)_R and (-402)_M1/(002)_R orientations on Ti_0.87O₂ (TO) and NbWO₆ (NWO) nanosheets, respectively. The transport and soft X-ray absorption in transmission mode results showed the high-quality of epitaxial VO₂ films on oxide nanosheets, equal to those on bulk single crystal substrates. Furthermore, the micro-level and deterministic control over two VO₂ film orientations on a single substrate was achieved using TO and NWO nanosheets in combination with lithography. This study demonstrates the ability to obtain high-quality transition metal oxide films with desirable orientations for specific device applications and fundamental research by oxide nanosheets and free choice of substrates.

Based on the work in chapter 2, the effects of VO₂ film thickness and the underlying substrates on the properties of epitaxial VO₂ film on NWO nanosheets were investigated in chapter 3. Regarding the characteristic parameters of VO₂ metal-insulator transition (MIT), the transport measurements showed that the sharpness of MIT, DT, was unchanged and the thermal hysteresis, DH, slightly increased as the film became thicker on NWO nanosheets. Simultaneously, the MIT temperature, T_MIT, of VO₂ films decreased as the film thickness was reduced because of the shortening of the c_R axis, which was determined by X-ray diffraction. The strain effect accompanying the film thickness variation on NWO nanosheets contributed to the shortening of the c_R axis. Moreover, the underlying substrates with different thermal expansion coefficients had negligible influence on the MIT behavior of the VO₂ film on NWO nanosheets. The study shows that tuning T_MIT of VO₂ on NWO nanosheets is practicable for applications regardless of the choice of substrates.

Even though oxide nanosheets can promote the epitaxial growth of the transition metal oxide films in a single out-of-plane orientation, the in-plane orientation of the epitaxial films is totally random. Effort was made to align the in-plane orientation of oxide nanosheets (chapter 4). The synthesis of the parent layered perovskite KCa₂Nb₃O₁₀ (KCNO) in molten K₂SO₄ salt resulted in high-quality crystals with square-like platelet shape. The protonation process did not affect the shape of KCNO. Under optimal exfoliation conditions, square-like Ca₂Nb₃O₁₀ (CNO) nanosheets were obtained. The external shape of the square-like CNO nanosheets enabled the self-alignment of the in-plane orientation among nanosheets during Langmuir-Blodgett deposition on substrates because their external edges were coincident with the crystal a and b axes. The epitaxial SrRuO₃ (SRO) grains on individual square-like CNO nanosheets formed larger domains, in which each domain consisted of many nanosheets instead of one nanosheet. Although full in-plane alignment was not achieved, the improvement of in-plane orientation of square-like CNO nanosheets over longer distances led to an enhancement of the electrical properties of SRO film in terms of resistivity and residual resistivity ratio ρ_300K/ρ_2K. The study exhibits the ability to improve the in-plane orientation of oxide nanosheets, and hence of transition metal oxide films grown on them by controlling their external shape.

An alternative approach to obtain high-quality transition metal oxide films is to use sacrificial layers that are epitaxially grown on single crystal substrates. In chapter 5, the ability to obtain freestanding single crystalline oxide perovskite films with (011) and (111) orientations was investigated. Water-soluble epitaxial Sr₃Al₂O₆ (SAO) layers were successfully grown on single crystal SrTiO₃ (STO) substrates with (011) and (111) orientations. The epitaxial lift-off time of oxide perovskite films from SAO layers in water was dependent on the orientation of SAO. The freestanding single crystalline (011)_pc- and (111)_pc-oriented SRO films were successfully transferred on Si substrates. Single crystalline SRO films before the epitaxial lift-off showed the enhanced magnetism, while the transferred ones on Si exhibited bulk-like magnetism because of the difference in the strain state of SRO films. This study opens the opportunities to obtain single crystalline oxide perovskite films with desirable orientation on arbitrary substrates.