No abstract

We were able to systematically control crystallographic characteristics and electrical properties of nickel oxide epitaxial thin films integrated with cubic yttria-stabilized zirconia (c-YSZ)-buffered silicon(001) substrates. The NiO epilayers were grown under several oxygen partial pressures by pulsed laser deposition. The out-of-plane orientation of the NiO layers showed an interesting behavior where it changed from ⟨111⟩ at lower pressures (7 × 10–6 Torr) to ⟨100⟩ at higher pressures (5 × 10–2 Torr). This observation was attributed to the nature of surface termination and templating effect of the c-YSZ{100} platform at different pressures. With the use of θ–2θ and φ scans of X-ray diffraction, the epitaxial alignment across the NiO/c-YSZ interface was determined to be {111}NiO|||{100}c‑YSZ and ⟨110⟩NiO||⟨100⟩c‑YSZ for the heterostructure grown under a low pressure and {100}NiO||{100}c‑YSZ and ⟨100⟩NiO||⟨100⟩c‑YSZ for the heterostructure grown under a high oxygen pressure. Our high-resolution TEM studies revealed formation of atomically sharp interfaces with no evidence of interfacial reaction and confirmed the established epitaxial relationships. n-Type electrical conductivity was observed in the NiO epilayers grown under lower pressures, which was turned into p-type in the films deposited under higher oxygen partial pressures. Besides, the electrical resistivity increased with the growth pressure. These observations were correlated to the nature of point defects in the NiO crystal. The formation of oxygen vacancies and metallic nickel at lower pressures and excess oxygen and trivalent nickel at higher pressures was revealed by XPS. We established a structure–property correlation in NiO/c-YSZ{100}/Si{100} thin film epitaxial heterostructures with special emphasis on the stoichiometry and crystallographic characteristics.

We report the control of semiconductor to metal transition in VO2(010) epilayers integrated with Si{100} substrates buffered with an NiO[111]/YSZ[100] intermediate layer. VO2 epitaxial thin films were grown at different thicknesses varying from 10 to 200 nm using pulsed laser deposition technique. An epitaxial relationship of VO2(010)‖NiO(111)‖ YSZ(001)‖Si(001) and VO2[100]‖NiO[110]‖ YSZ[100]‖Si[100] was established at room temperature. The crystallographic alignment across the VO2/NiO interface changes to VO2(100)‖NiO(111) and VO2[001]‖NiO[110] at the temperature of growth giving rise to a misfit strain of about 33.5% and 3.0% along two orthogonal in-plane orientations. The transition temperature was observed to vary from about 353 to 341 K, the transition amplitude increased by about five orders of magnitude, and the hysteresis decreased to about 3 K, as the thickness of VO2 layers increased from about 10 to 200 nm. These observations were explained based on strain characteristics, overall defect content and grain boundaries, and phenomenological thermodynamic models.

We have demonstrated dark hydrophilicity of single crystalline rutile TiO2(100) thin films, in which rapid switching from a hydrophobic to a hydrophilic surface was achieved using nanosecond excimer laser irradiation. The TiO2/YSZ/Si(001) single crystalline heterostructures were grown by pulsed laser deposition and were subsequently irradiated by a single pulse of a KrF excimer laser at several energies. The wettability of water on the surfaces of the samples was evaluated. The samples were hydrophobic prior to laser annealing and turned hydrophilic after laser annealing. Superhydrophilic surfaces were obtained at higher laser energy densities (e.g., 0.32 J.cm−2). The stoichiometries of the surface regions of the samples before and after laser annealing were examined using XPS. The results revealed the formation of oxygen vacancies on the surface, which are surmised to be responsible for the observed superhydrophilic behavior. According to the AFM images, surface smoothening was greater in films that were annealed at higher laser energy densities. The samples exhibited hydrophobic behavior after being placed in ambient atmosphere. The origin of laser induced wetting behavior was qualitatively understood to stem from an increase of point defects near the surface, which lowered the film/water interfacial energy. This type of rapid hydrophobic/hydrophilic switching may be used to facilitate fabrication of electronic and photonic devices with novel properties.