Ann F. Marshall

In situ grown YBa2Cu3O7−d thin films from single‐target magnetron sputtering

Using single‐target off‐axis sputter deposition, high quality superconducting films of YBa2Cu3O7−δ were made in situ. These films have properties which are distinctly different from those of bulk ceramics and of post‐deposition annealed films. Their superconducting resistive transitions remain sharp regardless of the value of Tc between 75 and 86 K. Normal‐state conductivities are as high or higher than the best single crystals. Critical current densities as high as 6×107 A/cm2 at 4.2 K. Tc (R=0) falls off with film thickness down to 10 K for 35–40 A films. All of the above properties are relatively insensitive to compositional variation. The results can be explained if the in situ growth results in well‐formed CuO2 planes with defects occurring elsewhere.

Synthesis and properties of YBa2Cu3O7 thin films grown in situ by 90° off-axis single magnetron sputtering

Abstract High quality superconducting films of YBa 2 Cu 3 O 7− x were deposited in situ using single target 90° off-axis sputtering. We have investigated their superconducting DC and RF properties, their normal state properties, and their microstructures. These films are distinctly different from bulk crystals and post-deposition annealed films. Sharp superconducting transition temperatures can be reproducibly obtained by control of deposition parameters. The T c can be varied from 75 to 89 K. The optimization of properties other than T c and the control of film texture occur under conditions different from those for which the highest T c is obtained. Normal state conductivities are as high as or higher than those of single crystals. Critical current densities reach 6 × 10 7 A/cm 2 at 4.2 K. All the above properties are relatively insensitive to compositional variations. The T c s have a much weaker dependence on the c -axis lattice parameters than do those of bulk samples. The measured low-temperature penetration depth is 1400 A and surface resistance at 4.2 K and 10 GHz is as low as 16 μΩ. Microstructural studies show sharp interfaces between films and their substrates and a variety of defect structures. Many of the properties of in situ films can be explained by clean grain boundaries and the characteristics of the surface growth occuring during in situ deposition.

Epitaxial and Smooth Films of a-Axis YBa2Cu3O7

YBa2Cu3O7 films have been grown epitaxially on SrTiO3 (100) and LaAlO3 (100) substrates with nearly pure a-axis orientation and with transition temperature Tc (R = 0) of 85 K. A unique feature of these films is their smooth surface. These smooth surfaces enable the growth of short-period superlattices with well-defined modulations. The films are untwinned and the grains grow with their c-axis along one of two perpendicular directions on the substrate ([100] or [010]). The fabrication of sandwich-type Josephson junctions with good characteristics may now be possible because unlike c-axis—oriented films, the superconducting coherence length of these smooth films is appreciably large perpendicular to their surfaces.

Thin-film synthesis of the high- T c oxide superconductor YBa 2 Cu 3 O 7 by electron-beam codeposition

The successful synthesis of high- T c YBa 2 Cu 3 O 7 films by means of electron-beam codeposition are reported. Several important growth parameters have been surveyed in a preliminary way. The substrates investigated include Al 2 O 3 , ZrO 2 , MgO, and SrTiO 3 , The films were characterized by resistivity measurements, x-ray diffraction, microprobe, and Rutherford backscattering analysis. Some TEM and critical current density studies were also carried out. The best results to date have been obtained on SrTiO 3 substrates with which polycrystalline epitaxial growth has been achieved. Resistive superconducting transitions with zero resistance at 89.5 K and a 2 K width have been observed in these films.

Formation of chiral branched nanowires by the Eshelby Twist

Manipulating the morphology of inorganic nanostructures, such as their chirality and branching structure, has been actively pursued as a means of controlling their electrical, optical and mechanical properties. Notable examples of chiral inorganic nanostructures include carbon nanotubes, gold multishell nanowires, mesoporous nanowires and helical nanowires. Branched nanostructures have also been studied and been shown to have interesting properties for energy harvesting and nanoelectronics. Combining both chiral and branching motifs into nanostructures might provide new materials properties. Here we show a chiral branched PbSe nanowire structure, which is formed by a vapour-liquid-solid branching from a central nanowire with an axial screw dislocation. The chirality is caused by the elastic strain of the axial screw dislocation, which produces a corresponding Eshelby Twist in the nanowires. In addition to opening up new opportunities for tailoring the properties of nanomaterials, these chiral branched nanowires also provide a direct visualization of the Eshelby Twist.

Molecular beam epitaxial growth of layered Bi-Sr-Ca-Cu-O compounds

Abstract The in situ epitaxial growth of Bi-Sr-Ca-Cu-O films by molecular beam epitaxy (MBE) is reported. The suitability of various oxidants for the MBE growth of cuprate superconductors is discussed and the use of ozone described. Molecular beams of the constituents were periodically shuttered to grow various Bi 2 Sr 2 Ca n −1 Cu n O x phases, including 2201, 2212, 2223, 2245, and layered 2212/2223 and 2223/2234 mixtures. Some of the films grown in this way were superconducting as grown. The ability of MBE to grow layered, probably metastable Bi 2 Sr 2 Ca n −1 Cu n O x films is demonstrated. This is a major step in the development of growth method capable of producing custom layered combinations of perovskite-related compounds.

Synthesis and strain relaxation of Ge-core/Si-shell nanowire arrays.

Analogous to planar heteroepitaxy, misfit dislocation formation and stress-driven surface roughening can relax coherency strains in misfitting core-shell nanowires. The effects of coaxial dimensions on strain relaxation in aligned arrays of Ge-core/Si-shell nanowires are analyzed quantitatively by transmission electron microscopy and synchrotron X-ray diffraction. Relating these results to reported continuum elasticity models for coaxial nanowire heterostructures provides valuable insights into the observed interplay of roughening and dislocation-mediated strain relaxation.

In situ growth of superconducting MgB2 thin films with preferential orientation by molecular-beam epitaxy

We report the synthesis of superconducting MgB2 thin films grown in situ by molecular-beam epitaxy. Mg-rich fluxes are deposited with B flux by electron-beam evaporation onto c-plane sapphire substrates. The films exhibit c-axis oriented peaks of MgB2, and a full width at half maximum of 3° in their rocking curves. In-plane alignment of MgB2 shows 12-fold symmetry, which is observed by the selected area diffraction pattern in transmission electron microscopy. The MgB2 films show a superconducting transition at 34.5 K with ΔTc<1 K. Even though the residual resistivity of the films is quite high (∼60 μΩ cm), the normal-state resistivity has a very similar temperature dependence but is five times larger than that of a single crystal, implying that conduction through the whole sample is imperfectly connected or inhomogeneous. Upper critical fields are obtained from measurement of the field dependence of the resistivity. It is estimated that the upper critical field at 20 K is more than 15 T, which is one of t...

Near-Infrared Light Emitting Luciferase Via Biomineralization

A new strategy based on biomineralization is presented to rationally tune the emission wavelength of luciferase. In this study luciferase (Luc8) was used as a template to direct the synthesis of near-infrared (NIR) light emitting PbS quantum dots (QDs) at ambient conditions to form a Luc8-PbS nanocomplex. The as-synthesized PbS QDs exhibited photoluminescence in the range of 800-1050 nm, and the Luc8 enzyme remained active within the Luc8-PbS complex. Upon the addition of the substrate coelenterazine, the energy produced by Luc8 was nonradiatively transferred to PbS QDs via bioluminescence resonance energy transfer (BRET) and enabled the complex to emit NIR light. This is the first study to form dual functional bioinorganic hybrid nanostructures via active enzyme-templated synthesis of inorganic nanomaterials.

Inhibiting Strain-Induced Surface Roughening: Dislocation-Free Ge/Si and Ge/SiGe Core-Shell Nanowires

Elastic strain is a critical factor in engineering the electronic behavior of core-shell semiconductor nanowires and provides the driving force for undesirable surface roughening and defect formation. We demonstrate two independent strategies, chlorine surface passivation and growth of nanowires with low-energy sidewall facets, to avoid strain-induced surface roughening that promotes dislocation nucleation in group IV core-shell nanowires. Metastably strained, dislocation-free, core-shell nanowires are obtained, and axial strains are measured and compared to elasticity model predictions.