Judith L. MacManus-Driscoll

ZnO - nanostructures, defects, and devices

ZnO has received much attention over the past few years because it has a wide range of properties that depend on doping, including a range of conductivity from metallic to insulating (including n-type and p-type conductivity), high transparency, piezoelectricity, wide-bandgap semiconductivity, room-temperature ferromagnetism, and huge magneto-optic and chemical-sensing effects. Without much effort, it can be grown in many different nanoscale forms, thus allowing various novel devices to be achieved. We review recent studies of ZnO nanostructures, fabrication, novel device applications, and its potential as an electron-acceptor material in hybrid solar cells. Control of its rich defect chemistry, which is critical for controlling properties but has not been widely addressed in the context of novel applications, is also discussed.

Greatly reduced leakage current and conduction mechanism in aliovalent-ion-doped BiFeO3

Transport properties of aliovalent-ion-doped BiFeO3 (BFO) thin films have been studied in order to identify the cause of high leakage currents. Doping of 2at.% Ti4+ ions increased the dc resistivity by more than three orders of magnitude. In contrast, doping of 2+ ions such as Ni2+ reduced the dc resistivity by two orders of magnitude. Current–voltage (I–V) characteristics indicated that the main conduction mechanism for pure and Ni2+ doped BFO was space charge limited, which was associated with the free-carriers trapped by the oxygen vacancies, whereas in the Ti4+ doped BFO, field-assisted ionic conduction was dominant.

Strongly enhanced current densities in superconducting coated conductors of YBa2Cu3O7-x + BaZrO3.

There are numerous potential applications for superconducting tapes based on YBa2Cu3O7–x (YBCO) films coated onto metallic substrates1. A long-established goal of more than 15 years has been to understand the magnetic-flux pinning mechanisms that allow films to maintain high current densities out to high magnetic fields2. In fact, films carry one to two orders of magnitude higher current densities than any other form of the material3. For this reason, the idea of further improving pinning has received little attention. Now that commercialization of YBCO-tape conductors is much closer, an important goal for both better performance and lower fabrication costs is to achieve enhanced pinning in a practical way. In this work, we demonstrate a simple and industrially scaleable route that yields a 1.5–5-fold improvement in the in-magnetic-field current densities of conductors that are already of high quality.

Strain control and spontaneous phase ordering in vertical nanocomposite heteroepitaxial thin films

Two-phase, vertical nanocomposite heteroepitaxial films hold great promise for (multi)functional device applications. In order to achieve practical devices, a number of hurdles need to be overcome, including the creation of ordered structures (and their formation on a large scale), achieving different combinations of materials and control of strain coupling between the phases. Here we demonstrate major advances on all these fronts: remarkable spontaneously ordered structures were produced in newly predicted compositions, vertical strain was proven to dominate the strain state in films above 20 nm thickness and strain manipulation was demonstrated by selection of phases with the appropriate elastic moduli. The work opens up a new avenue for strain control in relatively thick films and also promises new forms of ordered nanostructures for multifunctional applications.

Enhanced Performance in Fluorene-Free Organometal Halide Perovskite Light-Emitting Diodes using Tunable, Low Electron Affinity Oxide Electron Injectors

Fluorene-free perovskite light-emitting diodes (LEDs) with low turn-on voltages, higher luminance and sharp, color-pure electroluminescence are obtained by replacing the F8 electron injector with ZnO, which is directly deposited onto the CH3NH3PbBr3 perovskite using spatial atmospheric atomic layer deposition. The electron injection barrier can also be reduced by decreasing the ZnO electron affinity through Mg incorporation, leading to lower turn-on voltages.

Strong Efficiency Improvements in Ultra-low-Cost Inorganic Nanowire Solar Cells

The need for sustainable power generation has encouraged research into a variety of photovoltaic materials and structures, with a greater emphasis being placed on a balance between performance and cost. The stability of many semiconducting oxides relative to other inexpensive solar cell technologies, such as organic [ 1 ] and dye-sensitized [ 2 ] cells, makes them an attractive alternative. Yet low-cost, non-toxic, inorganic solar cell technologies have received comparatively little attention. In a recent report, nine inorganic semiconductors were identifi ed as having both the potential for annual electricity production in excess of worldwide demand and material extraction costs less than that of crystalline silicon. [ 3 ] Further to materials costs, a recent study examined the high cost of modern vacuum deposition methods and highlighted the need for low-temperature, atmospheric, solution-based synthesis. [ 4 ] Solution-based synthesis of several of the nine, promising inorganic materials has been demonstrated previously. [ 5–7 ] Copper (I) oxide (Cu 2 O), in particular, has been synthesized extensively in polycrystalline form by electrodeposition from solutions near room temperature. [ 5 , 8 , 9 ]

Angular-dependent vortex pinning mechanisms in YBa2Cu3O7 coated conductors and thin films

We compare the angular-dependent critical current density (Jc) in YBa2Cu3O7 films deposited on MgO templates grown by ion-beam-assisted deposition (IBAD), and on single-crystal substrates. We identify three angular regimes in which pinning is dominated by different types of correlated and uncorrelated defects. Those regimes are present in all cases, but their extension and characteristics are sample dependent, reflecting differences in texture and defect density. The more defective nature of the films on IBAD turns into an advantage as it results in higher Jc, demonstrating that the performance of the films on single crystals is not an upper limit for the IBAD coated conductors.

Thick lead-free ferroelectric films with high Curie temperatures through nanocomposite-induced strain

Ferroelectric materials are used in applications ranging from energy harvesting to high-power electronic transducers. However, industry-standard ferroelectric materials contain lead, which is toxic and environmentally unfriendly. The preferred alternative, BaTiO(3), is non-toxic and has excellent ferroelectric properties, but its Curie temperature of ∼130 °C is too low to be practical. Strain has been used to enhance the Curie temperature of BaTiO(3) (ref. 4) and SrTiO(3) (ref. 5) films, but only for thicknesses of tens of nanometres, which is not thick enough for many device applications. Here, we increase the Curie temperature of micrometre-thick films of BaTiO(3) to at least 330 °C, and the tetragonal-to-cubic structural transition temperature to beyond 800 °C, by interspersing stiff, self-assembled vertical columns of Sm(2)O(3) throughout the film thickness. The columns, which are 10 nm in diameter, strain the BaTiO(3) matrix by 2.35%, forcing it to maintain its tetragonal structure and resulting in the highest BaTiO(3) transition temperatures so far.

Overcoming the barrier to 1000A∕cm width superconducting coatings

Remarkable progress has been made in the development of YBa2Cu3O7−δ (YBCO)-based coated conductors, and the problems of continuous processing of commercially viable tape lengths are being rapidly solved by companies around the world. However, the current carried by these tapes is presently limited to about 100A for a 1-cm-wide tape, and this is due to a rapid decrease of critical current density (Jc) as the coating thickness is increased. We have now overcome this problem by separating relatively thin YBCO layers with very thin layers of CeO2. Using this multilayer technology, we have achieved Jc values on metal substrates of up to 4.0MA∕cm2 (75K, self-field) in films as thick as 3.5μm, for an extrapolated current of 1400A∕cm width.

High-resolution x-ray diffraction and transmission electron microscopy of multiferroic BiFeO3 films

High-resolution x-ray diffraction and transmission electron microscopy (TEM) have been used to study BiFeO3 thin films grown on the bare and SrRuO3 buffered (001) SrTiO3 substrates. Reciprocal space mapping (RSM) around (002) and (103) reflections revealed that BFO films with a thickness of about 200 nm were almost fully relaxed and had a rhombohedral structure. Cross-sectional, high-resolution TEM showed that the films started to relax at a very early stage of growth, which was consistent with the RSM results. A thin intermediate layer of about 2 nm was observed at the interface, which had a smaller lattice than the overgrown film. Twist distortions about the c axis to release the shear strain introduced by the growth of rhombic (001) BiFeO3 on cubic (001) SrTiO3 were also observed. The results indicate that a strained, coherent BiFeO3 film on (001) SrTiO3 is very difficult to maintain and (111) STO substrates are preferable.