Effect of annealing on the development of fully transparent ternary V-O-Na-Si mixed metal oxide thin films from polymer-assisted dip-coated V2O5

Both transparent oxides and transparent conductive oxides are of particular research interest for future applications in flexible, optically transparent thin film transistors and luminescent devices. We report the formation of a transparent oxide material based on the interdiffusion of Na-O and Si-O species with dip-coated V2O5 thin films on a borosilicate glass substrate. The deposition process used a facile solution processed dip-coating technique in the high-rate draining regime. Liquid precursors of vanadium alkoxide and alkoxide-polyethylene glycol mixtures were used for thin film deposition. We examine the effect of annealing condition on the phase conversion process, morphology and optical transmittance due to the conversion of the V2O5 films to completely transparent ternary mixed metal oxide thin films. The work also examines the role of polymer-assisted deposition on the development of the V-O-Na-Si transparent thin films during different annealing conditions. Polymer-assisted V2O5 thin films on glass are shown to convert to optically clear thin films during annealing, with a transparency >95% across the visible spectrum, and a blue-shift of the absorption edge to maintain >90% transparency at 380 nm

Transparent antireflective layers of oxide nanowires grown from thin films by pressurized contact interdiffusion processes

Oxide phase nanowires are important for applications ranging from optoelectronics to water splitting, but prove difficult to grow in high density with good crystalline quality and phase purity. Heterogeneous catalysts are typically required to nucleate growth. This work demonstrates that dispersions of oxide nanowires can be formed directly from solution processed oxide thin films. We also examine the effect of changes in applied pressure between a solution processed vanadium oxide thin film and a surface-contacted glass coupon on the catalyst-free formation of interconnected sodium vanadate nanowire structures by interdiffusion. Under different applied pressures, meshes of high quality crystalline oxide nanowires formed on the surface, and we examine the nature of phase conversion and nanostructure growth including larger shards composed of multiple conjoined nanowires are also examined. The optical properties of the oxides NWs formed by interdiffusion from oxide thin films show promising properties for application as antireflective coatings across a broadband spectral range. This interdiffusion technique is effective for high quality oxide nanowire growth without catalysts directly from insulating or conducting thin films by direct contact with a source of diffusing species

Solution processable metal oxide thin film deposition and material growth for electronic and photonic devices

A comprehensive review of recent advances in solution processing and growth of metal-oxide thin films for electronic and photonic devices is presented, with specific focus on precise solution-based technological coatings for electronics and optics, and new concepts for oxide material growth for electrochemical, catalytic, energy storage and conversion systems, information technology, semiconductor device processing and related devices. Throughout, the nature of the soluble precursors solutions and their relationship to film formation process by various solution coating techniques are collated and compared, highlighting advantages in precursor design for creating complex oxides for devices. Because of the versatility of solution-processable oxides and functional material coating, it is important to capture the advances made in oxide deposition for plastic electronics, see-through and wearable devices, and high-fidelity thin film transistors on curved or flexible displays. Solution processing, even for oxides, allows control over composition, thickness, optical constants, porosity, doping, tunable optical absorbance/transmission, band structure engineering, 3D-substrate coating, complex composite oxide formation and multi-layered oxide systems that are more difficult to achieve using chemical vapor deposition (CVD) or atomic layer deposition (ALD) processes. We also discuss limitations of solution processing for some technologies and comment on the future of solution-based processing of metal-oxide materials for electronics, photonics and other technologies

Development and characterisation of solution processed vanadium oxide and transparent metal oxide thin films

Metal oxide thin films are important for modern electronic devices ranging from thin film transistors to photovoltaics and functional optical coatings. Solution processed techniques allow for thin films to be rapidly deposited over a range of surfaces without the extensive processing of comparative vapour or physical deposition methods. The production of thin films of vanadium oxide prepared through dip-coating was developed enabling a greater understanding of the thin film formation. Mechanisms of depositing improved large area uniform coverage on a number of technologically relevant substrates were examined. The fundamental mechanism for polymer-assisted deposition in improving thin film surface smoothness and long range order has been delivered. Different methods were employed for adapting the alkoxide based dip-coating technique to produce a variety of amorphous and crystalline vanadium oxide based thin films. Using a wide range of material, spectroscopic and optical measurement techniques the morphology, structure and optoelectronic properties of the thin films were studied. The formation of pinholes on the surface of the thin films, due to dewetting and spinodal effects, was inhibited using the polymer assisted deposition technique. Uniform thin films with sub 50 nm thicknesses were deposited on a variety of substrates controlled through alterations to the solvent-alkoxide dilution ratios and employing polymer assisted deposition techniques. The effects of polymer assisted deposition altered the crystallized VO thin films from a granular surface structure to a polycrystalline structure composed of high density small in-plane grains. The formation of transparent VO based thin film through Si and Na substrate mediated diffusion highlighted new methods for material formation and doping

Fabrication and characterization of single-crystal metal-assisted chemically etched rough Si nanowires for lithium-ion battery anodes

Silicon nanowires were fabricated by a metal assisted chemical (MAC) etching process and routes toward ohmic contacting of substrates for Li-ion battery anode application were developed. Si nanowire layers are comprised of wires that are single crystal with rough outer surfaces. The nanowires are epitaxial with the underlying Si(100) substrate, maintain equivalent doping density and crystal orientation, and are coated with a stoichiometric SiO2. Electrical backside contacting using an In-Ga eutectic allows low-resistance ohmic contacts to low-doped nanowire electrodes for electrochemical testing

The nature of silicon nanowire roughness and thermal conductivity suppression by phonon scattering mechanisms

The nature of the surface roughness of electrolessly etched p-type Si nanowires (NWs) is examined using high resolution transmission electron microscopy and shown to comprise individual silicon nanocrystallites throughout the waviness of the roughness features. As the frequency of roughness features are believed to be sources of surface and boundary scattering, the thermal conductivity below the Casimir limit is still not fully explained. The frequency shift and development of asymmetry in the optical phonon mode in silicon was monitored by Raman scattering measurements as a function of temperature (>1000 K). We assessed the influence of Si NW roughness features on phonon scattering mechanisms including quantum confinement of phonons from roughness nanocrystals, boundary scattering, and optical phonon decay to interacting 3- and 4-phonon processes that may contribute to the cause of significant thermal conductivity suppression in rough Si nanowires. High temperature studies and detailed examination of the substrate of roughness revealed high frequency optical phonon contributions to thermal conductivity suppression

Patterning optically clear films: co-planar transparent and color-contrasted thin films from interdiffused electrodeposited and solution-processed metal oxides

Transparent thin films can now be site-selectively patterned and positioned on surface using mask-defined electrodeposition of one oxide and overcoating with a different solution-processed oxide, followed by thermal annealing. Annealing allows an interdiffusion process to create a new oxide that is entirely transparent. A primary electrodeposited oxide can be patterned and the secondary oxide coated over the entire substrate to form high color contrast coplanar thin film tertiary oxide. The authors also detail the phase formation and chemical state of the oxide and how the nature of the electrodeposited layer and the overlayer influence the optical clearing of the patterned oxide film

Mesoporosity in doped silicon nanowires from metal assisted chemical etching monitored by phonon scattering

Si nanowires (NWs) are shown to develop internal mesoporosity during metal assisted chemical etching from Si wafers. The onset of internal porosity in n+-Si(100) compared to p-Si(100) is examined through a systematic investigation of etching parameters (etching time, AgNO3 concentration, HF % and temperature). Electron microscopy and Raman scattering show that specific etching conditions reduce the size of the internal Si nanocrystallites in the internal mesoporous structure to 3–5 nm. Mesoporous NWs are found to have diameters as large as 500 nm, compared to ~100 nm for p-NWs that develop surface roughness. Etching of Si (100) wafers results in (100)-oriented NWs forming a three-fold symmetrical surface texture, without internal NW mesoporosity. The vertical etching rate is shown to depend on carrier concentration and degree of internal mesoporosity formation. Raman scattering of the transverse optical phonon and photoluminescence measurements confirm quantum size effects, phonon scattering and visible intense red light emission between 685 and 720 nm in internally mesoporous NWs associated with the etching conditions. Laser power heating of NWs confirms phonon confinement and scattering, which is demonstrated to be a function of the internal mesoporosity development. We also demonstrate the limitation of mesoporosity formation in n+-Si NWs and development of porosity within p-Si NWs by controlling the etching conditions. Lastly, the data confirm that phonon confinement and scattering often reported for Si NWs is due to surface-bound and internal nanostructure, rather than simply a diameter reduction in NW materials

Pseudocapacitance of α-CoMoO4 nanoflakes in non-aqueous electrolyte and its bi-functional electro catalytic activity for methanol oxidation

Nanocrystalline cobalt molybdate (CoMoO4) nanoflakes were grown directly on carbon fibre cloth (CFC) via a simple hydrothermal method without any template or surfactant. A symmetric supercapacitor was fabricated using CoMoO4 nanoflakes/CFC as both negative and positive electrodes. The device has delivered the maximum specific capacitance of 8.3 F g−1 at a constant current density of 1 A g−1 in organic electrolyte. It offers the reasonable energy (2.6 Wh kg−1) and power density (748.8 W kg−1) as comparable to the carbon based symmetric supercapacitors. As a catalyst for methanol oxidation, the CoMoO4 nanoflakes showed high current density (25 mA cm−2) and low onset potential (0.38 V). The impressive bi-functional electrochemical activity of CoMoO4 on CFC is mainly attributed to its porous microstructure, where reasonable electrical conductivity resulted from binder-free and intimate metal oxide/substrate integration