Role of Nitrogen in Defect Evolution in Zinc Oxide: STEM-EELS Nanoscale Investigations

Insights into ZnO-based doped porous nanocrystal frameworks

Colloidal nanocrystals play a vital role in several applications. The doping of cations in the nanocrystal matrix enhances the optical, electrical, and magnetic properties. The number and well-defined distribution of the dopant are crucial to protect the nanocrystal from clustering. The XRD, XPS, and XAS instruments reveal the change in the lattice parameters, chemical states, and local coordination environment information. In addition of detecting the position and distribution of the dopant, the 4D-STEM detector mode gathers all types of real-space atomic-resolution images by collecting all diffraction datasets from each electron probe with high-speed and efficient detection. Dopant-host ligand type, reactions conditions, and reaction time optimization during synthesis are critical for the host and dopant reactivity balance. Pearson's hard/soft acids/bases theory would be a base for balancing the solubility of the dopant-host in the given solvents/surfactant. In addition, tuning the colloidal nanocrystals to secondary structures, which enhances the mass-/ions transport, can contribute a combination of properties that do not exist in the original constituents.

Zinc Oxide Defect Microstructure and Surface Chemistry Derived from Oxidation of Metallic Zinc: Thin-Film Transistor and Sensor Behavior of ZnO Films and Rods

Zinc oxide thin films are fabricated by controlled oxidation of sputtered zinc metal films on a hotplate in air at temperatures between 250 and 450 °C. The nanocrystalline films possess high relative densities and show preferential growth in (100) orientation. Integration in thin-film transistors reveals moderate charge carrier mobilities as high as 0.2 cm2  V-1 s-1 . The semiconducting properties depend on the calcination temperature, whereby the best performance is achieved at 450 °C. The defect structure of the thin ZnO film can be tracked by Doppler-broadening positron annihilation spectroscopy as well as positron lifetime studies. Comparably long positron lifetimes suggest interaction of zinc vacancies (VZn ) with one or more oxygen vacancies (VO ) in larger structural entities. Such VO -VZn defect clusters act as shallow acceptors, and thus, reduce the overall electron conductivity of the film. The concentration of these defect clusters decreases at higher calcination temperatures as indicated by changes in the S and W parameters. Such zinc oxide films obtained by conversion of metallic zinc can also be used as seed layers for solution deposition of zinc oxide nanowires employing a mild microwave-assisted process. The functionality of the obtained nanowire arrays is tested in a UV sensor device. The best results with respect to sensor sensitivity are achieved with thinner seed layers for device construction.

A foundation for complex oxide electronics -low temperature perovskite epitaxy

As traditional silicon technology is moving fast towards its fundamental limits, all-oxide electronics is emerging as a challenger offering principally different electronic behavior and switching mechanisms. This technology can be utilized to fabricate devices with enhanced and exotic functionality. One of the challenges for integration of complex oxides in electronics is the availability of appreciable low-temperature synthesis routes. Herein we provide a fundamental extension of the materials toolbox for oxide electronics by reporting a facile route for deposition of highly electrically conductive thin films of LaNiO₃ by atomic layer deposition at low temperatures. The films grow epitaxial on SrTiO₃ and LaAlO₃ as deposited at 225 °C, with no annealing required to obtain the attractive electronic properties. The films exhibit resistivity below 100 µΩ cm with carrier densities as high as 3.6 · 10²² cm⁻³. This marks an important step in the realization of all-oxide electronics for emerging technological devices.

Designing complex oxides with appreciable low-temperature synthesis remains a challenge. Here, the authors demonstrate an atomic layer deposition process for LaNiO₃ to enable epitaxial thin films on LaAlO₃ and SrTiO₃ substrates deposited at 225 °C, with no annealing required to achieve good electronic properties.

Formation of N2 bubbles along grain boundaries in (ZnO)1-x(GaN)x: nanoscale STEM-EELS studies

Direct evidence of N₂ formation after annealing of (ZnO)_1-x(GaN)_x alloys was revealed. N₂ was trapped by V_Zn+Ga-clusters, forming faceted voids along grain boundaries. This study shows that N-N bonding is a competitive path for nitrogen after annealing, in addition to the increasing Ga-N bonds, indicating that N in O substitution sites (N_O) is not a stable configuration.