Magnetic Properties of CoFe2O4 Thin Films Synthesized by Radical-Enhanced Atomic Layer Deposition.

A radical-enhanced atomic layer deposition (RE-ALD) process was developed for growing ferrimagnetic CoFe2O4 thin films. By utilizing bis(2,2,6,6-tetramethyl-3,5-heptanedionato) cobalt(II), tris(2,2,6,6-tetramethyl-3,5-heptanedionato) iron(III), and atomic oxygen as the metal and oxidation sources, respectively, amorphous and stoichiometric CoFe2O4 films were deposited onto SrTiO3 (001) substrates at 200 °C. The RE-ALD growth rate obtained for CoFe2O4 is ∼2.4 Å/supercycle, significantly higher than the values reported for thermally activated ALD processes. Microstructural characterization by X-ray diffraction and transmission electron microscopy indicate that the CoFe2O4 films annealed between 450 and 750 °C were textured polycrystalline with an epitaxial interfacial layer, which allows strain-mediated tuning of the magnetic properties given its highly magnetostrictive nature. The magnetic behavior was studied as a function of film thickness and annealing temperature: saturation magnetization (Ms) ranged from 260 to 550 emu/cm3 and magnetic coercivity (Hc) ranged from 0.2 to 2.2 kOe. Enhanced magnetic anisotropy was achieved in the thinner samples, whereas the overall magnetic strength improved after annealing at higher temperatures. The RE-ALD CoFe2O4 thin films exhibit magnetic properties that are comparable to both bulk crystal and films grown by other deposition methods, with thickness as low as ∼7 nm, demonstrating the potential of RE-ALD for the synthesis of high-quality magnetic oxides with large-scale processing compatibility.

Atmospheric and Aqueous Deposition of Polycrystalline Metal Oxides Using Mist-CVD for Highly Efficient Inverted Polymer Solar Cells.

Large scale, cost-effective processing of metal oxide thin films is critical for the fabrication of many novel thin film electronics. To date, however, most of the reported solution-based techniques require either extended thermal anneals or additional synthetic steps. Here we report mist chemical vapor deposition as a solution-based, readily scalable, and open-air method to produce high-quality polycrystalline metal oxide thin films. Continuous, smooth, and conformal deposition of metal oxide thin films is achieved by tuning the solvent chemistry of Leidenfrost droplets to promote finer control over the surface-local dissociation process of the atomized zinc-bearing precursors. We demonstrate the deposited ZnO as highly efficient electron transport layers for inverted polymer solar cells to show the power of the approach. A highest efficiency of 8.7% is achieved with a fill factor of 73%, comparable to that of conventional so-gel ZnO, which serves as an indication of the efficient vertical transport and electron collection achievable using this material.

Crossover from incoherent to coherent phonon scattering in epitaxial oxide superlattices.

Elementary particles such as electrons or photons are frequent subjects of wave-nature-driven investigations, unlike collective excitations such as phonons. The demonstration of wave-particle crossover, in terms of macroscopic properties, is crucial to the understanding and application of the wave behaviour of matter. We present an unambiguous demonstration of the theoretically predicted crossover from diffuse (particle-like) to specular (wave-like) phonon scattering in epitaxial oxide superlattices, manifested by a minimum in lattice thermal conductivity as a function of interface density. We do so by synthesizing superlattices of electrically insulating perovskite oxides and systematically varying the interface density, with unit-cell precision, using two different epitaxial-growth techniques. These observations open up opportunities for studies on the wave nature of phonons, particularly phonon interference effects, using oxide superlattices as model systems, with extensive applications in thermoelectrics and thermal management.

Nanodiamond landmarks for subcellular multimodal optical and electron imaging.

There is a growing need for biolabels that can be used in both optical and electron microscopies, are non-cytotoxic, and do not photobleach. Such biolabels could enable targeted nanoscale imaging of sub-cellular structures, and help to establish correlations between conjugation-delivered biomolecules and function. Here we demonstrate a sub-cellular multi-modal imaging methodology that enables localization of inert particulate probes, consisting of nanodiamonds having fluorescent nitrogen-vacancy centers. These are functionalized to target specific structures, and are observable by both optical and electron microscopies. Nanodiamonds targeted to the nuclear pore complex are rapidly localized in electron-microscopy diffraction mode to enable "zooming-in" to regions of interest for detailed structural investigations. Optical microscopies reveal nanodiamonds for in-vitro tracking or uptake-confirmation. The approach is general, works down to the single nanodiamond level, and can leverage the unique capabilities of nanodiamonds, such as biocompatibility, sensitive magnetometry, and gene and drug delivery.