Roman Kubrin

Flame aerosol deposition of Y2O3:Eu nanophosphor screens and their photoluminescent performance

Screens of Y(2)O(3):Eu(3+)-nanophosphor (d(BET) = 24 nm) with coating densities in the range 0.23-3.8 mg cm(-2) were obtained by flame aerosol deposition (FAD) from nitrate-based precursors. The average deposition rate was 0.22 mg cm(-2) min(-1). Porosity of the obtained deposits was 0.973 +/- 0.004. Light scattering of the coatings in the visible range showed a Rayleigh-like dependence on wavelength and, in comparison to the screens made of the commercial micrometer-sized phosphor powder (d(SEM) = 4 microm), was reduced by up to two orders of magnitude. As a result, the nanophosphor coatings maintained nearly constant brightness in a very wide range of coating densities. Furthermore, it should be expected that a substantially improved screen resolution can be achieved with such screens. For excitation at a wavelength of 254 nm, the maximum brightness of the FAD-deposited (Y(0.92)Eu(0.08))(2)O(3) phosphor screens in the transmission mode was nearly one third of that of the screens made of the commercial phosphor. It was demonstrated that light reflection from the supporting substrate and porosity of the coating significantly influence its photoluminescent performance.

Thermal radiation transmission and reflection properties of ceramic 3D photonic crystals

The infrared (IR) transmission and reflection properties of the ceramic thermal barrier coatings have great implications on the overall performance of a component operated at high temperatures, where a significant amount of heat from external IR radiation will propagate through the coating toward the underlying substrate. A high-temperature photonic structure can be used to limit this radiation transport while operating at temperatures above 1000 °C. Herein, we present the concept of a broadband and angle-insensitive IR reflector, based on 3D photonic crystals (PhCs) that consists of a ceramic material with high thermal stability and low thermal conductivity. We numerically demonstrate that the multistack ceramic 3D PhCs can provide >80% of bi-hemispherical reflectance in the wavelength region of 1–5 μm.

Vertical convective coassembly of refractory YSZ inverse opals from crystalline nanoparticles.

A facile deposition method of 3D photonic crystals made of yttrium-stabilized zirconia (YSZ) was developed. YSZ nanoparticles with primary particle size below 10 nm and cubic crystalline phase were synthesized by hydrothermal treatment of solutions of zirconyl nitrate, yttrium nitrate and acetylacetone. Before coassembly with polystyrene (PS) microspheres, a dispersant Dolapix CE64 was added to the dialyzed sol of YSZ nanoparticles to render their surface negatively charged. Vertical convective coassembly resulted in 3D ordered YSZ/PS hybrid films, which were inverted at 500 °C in air to produce inverse opals. The linear shrinkage of the coatings was in the range 15-20%, below previously reported values for YSZ. The obtained coatings demonstrated pronounced photonic properties and retained their ordered structure after annealing at 1000 °C for 2 h. Increasing the filling fraction of crystalline nanoparticles in the templates should enable production of fully functional 3D photonic crystals for applications in high-temperature photonics.

Photonic properties of titania inverse opal heterostructures

Titania inverse opal heterostructures demonstrating two distinctive photonic stopgaps were fabricated by repetitive vertical self-assembly and atomic layer deposition (ALD). Angle resolved reflectance measurements of the inverse opal heterostructure are reported for the first time. The comparison with the spectra of constituents show that the ΓL stopgaps of the heterostructure obey the superposition principle and the angular dispersion of their stopgaps is well-fitted with the modified Bragg’s law at low incidence angles. Numerical simulations were used to predict the dominant features in the reflectance spectra. The total (specular and diffuse) transmission and reflectance measurements of the single inverse opals and the heterostructure reveal that the diffuse scattering could severely impair the photonic properties of the buried layers in the multi-stack photonic crystal (PhC) configurations. Ascending stacking is proposed as a means to improve the performance of the multi-layer coatings.

Facile Deposition of YSZ-Inverse Photonic Glass Films

An alternative all-colloidal and single-step deposition method of yttrium-stabilized zirconia (YSZ)-infiltrated polymeric photonic glass films is presented. Heterocoagulation of oppositely charged polystyrene (PS) microspheres and YSZ nanocrystals in aqueous dispersions created PS/YSZ core-shell spheres. These composite particles were deposited on glass substrates by a simple drop-coating process. Heterocoagulation impaired self-assembly of the particles, resulting in a disordered structure. Burn-out of the polymer yielded a random array of YSZ shells. The effect of the filling fraction of YSZ between these shells was explored. YSZ-inverse photonic glass films with a thickness below 40 μm achieved 70% reflectance of the incident radiation over a broad wavelength range between 0.4 and 2.2 μm. The YSZ structures demonstrated structural stability up to 1000 °C and maintained high reflectance up to 1200 °C for several hours, thus enabling applications as broadband reflectors at elevated temperatures.

Transparent nanophosphor films with high quantum efficiency through cold compaction

A facile method to improve the transparency, mechanical stability and quantum yield (QY) of luminescent nanoparticle films is presented. Porous layers of the crystalline Y2O3:Eu nanophosphor with an average particle size of 24 nm were produced by flame spray pyrolysis. The coatings were sandwiched between two rigid substrates and exposed to cold isostatic pressing (CIP) at 900 MPa. The second substrate could be removed afterwards without damage. Compaction increased the particle packing density up to 60 vol% and nearly eliminated light scattering in the films, thus making them transparent throughout the visible range. At the same time, the luminescence decay time constant decreased from 3.3 to 1.4 ms confirming an increase in the internal QY of the nanophosphor from 0.31 to 0.60. A good match between the experimental data with the nanocrystal cavity model of radiative decay of photoluminescence was demonstrated. The increase of the external brightness of the coatings was limited to 28% (for thin coatings it even decreased) due to the onset of light trapping by multiple internal reflection. Deliberate introduction of scatterers on the surface of the film allowed the extracted intensity to increase by at least 70%, thus reaching 55% of the maximum brightness of a commercial micrometer-sized Y2O3:Eu phosphor powder. The CIP-processed coatings possessing a final thickness between 1 and 12 μm behaved as smooth crack-free solid films with excellent mechanical stability. The proposed method of cold compaction offers an advantage of rapid processing avoiding a high-temperature post-treatment for all applications of transparent phosphor films and other optical coatings.

Bottom-up Fabrication of Multilayer Stacks of 3D Photonic Crystals from Titanium Dioxide

A strategy for stacking multiple ceramic 3D photonic crystals is developed. Periodically structured porous films are produced by vertical convective self-assembly of polystyrene (PS) microspheres. After infiltration of the opaline templates by atomic layer deposition (ALD) of titania and thermal decomposition of the polystyrene matrix, a ceramic 3D photonic crystal is formed. Further layers with different sizes of pores are deposited subsequently by repetition of the process. The influence of process parameters on morphology and photonic properties of double and triple stacks is systematically studied. Prolonged contact of amorphous titania films with warm water during self-assembly of the successive templates is found to result in exaggerated roughness of the surfaces re-exposed to ALD. Random scattering on rough internal surfaces disrupts ballistic transport of incident photons into deeper layers of the multistacks. Substantially smoother interfaces are obtained by calcination of the structure after each infiltration, which converts amorphous titania into the crystalline anatase before resuming the ALD infiltration. High quality triple stacks consisting of anatase inverse opals with different pore sizes are demonstrated for the first time. The elaborated fabrication method shows promise for various applications demanding broadband dielectric reflectors or titania photonic crystals with a long mean free path of photons.

Photoluminescence of (YGd)2O3:Eu phosphors produced by nanoparticle-seeded flame-assisted spray pyrolysis

We report on synthesis of europium-doped (Y1?xGdx)2O3 by flame-assisted spray py-rolysis in premixed propane/air flames. Phosphor powders with an average particle size on the order of 0.5 ?m were obtained from 0.2 M aqueous solutions of the corresponding metal salts. Relative concentration of gadolinium (x) was varied in the range 0 to 1. For excitation at a wavelength of 254 nm, the highest intensity of photoluminescence was achieved for x = 0.2 (110% of brightness of the samples without gadolinia). Doping pure yttria and gadolinia with nanoparticles of Y2O3:Eu (dBET = 24 nm) was tested. Addition of nanoseeds (up to 50 mol.% in the obtained powders) noticeably decreased surface roughness of phosphor particles.

Towards ceramic 3DOM-materials as novel high-temperature reflective coatings and filters for thermophotovoltaics

Studies on evaporation-induced vertical convective self-assembly as well as horizontal self-assembly are presented including inversion of the opals and deposition of heterostruc-tures. Opaline coatings were deposited on microscope slides from monodisperse particles of polystyrene and silicon dioxide with the particle sizes in the range from 400 to 1100 nm. Well defined photonic stopgaps were observed for both direct and inverted structures. We show that the horizontal deposition method facilitates self-assembly of large silica particles, so that the limitation of the conventional vertical convective self-assembly can be overcome. Successful stacking of the self-assembled polystyrene templates with unequal particle size is demonstrated.