Qinyuan Zhang

A novel route to control refractive index of sol-gel derived nano-porous silica films used as broadband antireflective coatings

Abstract A relatively new method to adjust refractive index of nano-porous silica films rapidly and continuously is reported. The thin films were prepared with the base/acid two-step catalytic sol-gel process and the dip-coating method. The structure, reflective spectra and refractive index of the films were analyzed with TEM, AFM, SEM, spectrophotometer, and ellipsometery, respectively. The experimental results have shown that the base/acid two-step catalysis can adjust the refractive index of the nano-porous silica films from 1.18 to 1.42 rapidly and continuously. And a broadband antireflective coating made with the method has very low reflectance. The average reflectance of the coated glass in the solar energy spectrum decreases to 1.5% from ∼7% of the glass.

ZrO2 thin films and ZrO2/SiO2 optical reflection filters deposited by sol–gel method

Abstract ZrO2 thin films were deposited by a wet chemical sol–gel method from forced hydrolysis of aqueous ZrOCl2 solution. Transmission electron microscopic observations confirmed the existence of little nanosized monoclinic ZrO2 particles in clear stable ethanolic ZrO2 colloidal suspensions. Twenty layers of ZrO2/SiO2 high reflectance filters using a quarterwave stack were fabricated by sol–gel spin-coating on K9 glass substrate. This filter exhibits a wide reflectance band near 1064 nm and a high transmittance region.

Sol–gel derived ZrO2–SiO2 highly reflective coatings

Abstract In this paper, colloid-based highly reflective coatings consisting of alternating layers of quarterwave thick high- and low-refractive index components were deposited on glass substrates by a sol–gel spin-coating method. SiO 2 was used as the low-refractive index component, and ZrO 2 as the high-refractive index material. The colloidal suspension of ZrO 2 , prepared by hydrothermal hydrolysis of ZrOCl 2 , contained monoclinic nanocrystalline ZrO 2 , with an average particle size of 15 nm. A minimum transmittance of 1% near 1064 nm was obtained from a 20-layer SiO 2 –ZrO 2 /PVP multilayer film. The ZrO 2 particles were deposited in an ethanolic suspension containing polyvinylpyroldione (PVP) as a binder. The maximum transmittance in the visible range was about 85%. A 1-on-1 laser-induced damage threshold of 16 J/cm 2 for the 20-layer SiO 2 –ZrO 2 /PVP film was observed using a Q-switched Nd:YAG high power laser at a wavelength of 1.06 μm and with a pulse width of 2.5 ns.

Properties of sol-gel derived scratch-resistant nano-porous silica films by a mixed atmosphere treatment

Abstract A strengthening mechanism for sol–gel derived nano-porous silica films was studied. The silica films were prepared with the dip method from two-step catalyzed silica sols and treated in a mixed atmosphere of ammonia and water vapor afterwards. Properties of the films were analyzed by using atomic force microscopy (AFM), Fourier-transform infrared spectroscopy (FTIR), ellipsometry and an abrasion test, respectively. The experimental results have shown that the two-step catalysis strengthens the films and the scratch resistance of the silica films is further improved after treatment in the mixed gas. The increase in strength is attributed to the cross-linking of silica particles by randomly branched or/and entangled linear chains in the sols, more Si–O–Si bonds formed by the mixed gas treatment and structural changes in the silica films.

Scratch-Resistant Improvement of Sol-Gel Derived Nano-Porous Silica Films

Scratch-resistance of sol-gel derived nano porous silica films were studied. The thin films were prepared with a dip-coating method from both one-step and two-step catalyzed silica sols, and treated in a mixture gas of ammonia and water vapour afterwards. The thin films were characterized by using Atomic Force Microscope (AFM), ellipsometer, Fourier-transform Infrared Spectroscope (FTIR), respectively. Experimental results have shown that the two-step catalysis remarkably improves strength of the films, and abrasion-resistance and adhesion of the silica films were further increased after the mixture gas treatment. It is attributed to the cross-linking of silica particles in the sols by randomly branched or/and entangled linear chains and more Si–O–Si bonds formed by the mixture gas treatment.

A new method to control nano-porous structure of sol-gel-derived silica films and their properties

Abstract To control structure of nano-porous silica films effectively, a base/acid two-step catalytic sol-gel process was investigated. The nano-porous structure and IR absorption of the thin films were characterized by means of FE-SEM, AFM, SEM, FTIR, and ellipsometry, respectively. The experimental results have shown that the base/acid two-step catalysis can adjust the refractive index of the films from 1.18 to 1.41 rapidly and continuously. With a change of the experimental conditions, the frequency shift of the ω 4 (TO 3 ) peak of FTIR absorption to a lower wave number indicates a decrease in the average Si-O-Si bridging angle. A reduction of FWHM of the ω 4 peak reveals a narrow distribution of the Si-O-Si bridging angle. These results suggest that influence of the experimental conditions on the FTIR absorption of the nano-porous silica films is attributed to a change of silica particle shape and state of aggregation of the particles in the films.

Sol-Gel Processing of Zirconia Coating for HR Mirrors with High Laser Damage Threshold

High laser-damage resistant coatings are very important in high power laser systems. In this study ZrO2 thin films are prepared by sol-gel spin-coating technology from suitable zirconia aqueous colloidal suspensions containing nano-crystalline ZrO2 at room temperature synthesized by a hydrothermal process from an inorganic precursor (ZrOCl2·8H2O). By adding a soluble organic binder PVP to the suspension prior to application, it is possible to substantially increase the coating refractive index and the abrasion-resistance as well as the laser damage threshold. The features of the coatings and the colloidal suspensions are investigated. Multilayer highly reflective dielectric coatings are also elaborated by applying quarterwave-thick alternating coatings of the binder-aided zirconia and silica, which is prepared with the sol-gel process from TEOS. To achieve 99% reflectivity, 19–21 layers are required. Single shot laser damage tests are carried out using a high power laser at 1064 nm wavelength with a pulse duration of 2.5 ns. The laser damage thresholds of 18 and 15 J/cm2 are achieved for single ZrO2-PVP coating and ZrO2-PVP/SiO2 multilayers respectively.

Nanoporous TiO2 Coatings for Infrared Detectors

In GaAs based infrared detectors, a considerable part of the incident light will be reflected at the surface, thus it decreases the detectors sensitivity considerably. In this paper, a TiO2 nanoporous coating was prepared successfully on the GaAs substrate by sol-gel method. The optical parameters of the coating were also controlled successfully. It was proved that the coating could greatly improve the transmittance of the incident light, which agrees with the theoretical results quite well. In the 2.5–6.0 μm waveband, the maximum transmittance of GaAs substrate is 56%, while the transmittance of the GaAs substrate coated with a nanostructured TiO2 coating is about 94%.

Strengthening mechanism of porous silica films derived by two-step catalysis

A scratch-resistant improvement of sol-gel derived nano-porous silica films by a base/acid two-step catalysis is reported. The silica films were prepared with the two-step catalytic sol-gel process and a dip-coating method, and then post annealed in air and a mixed gas atmosphere. The films were characterized with FE-SEM, AFM, TEM, XPS, FTIR, an ellipsometer, and an abrasion test. With the post annealing, the frequency shift of the FTIR absorption ω4(TO3) peak of the films to a lower wavenumber results from a decrease in the average Si-O-Si bridging angle. An increase in the full-width at half-maximum of the ω4 peak is ascribed to a wide distribution of the bridging angle because of formation of strained Si-O-Si bonds caused by the post annealing. XPS analysis shows that the effect of the mixed gas annealing on the binding energy and full-width at half-maximum (FWHM) for Si (2p) and O (1s) peaks is attributed to the further condensation reaction of the films resulting from the disappearance of the OH groups. The abrasion and adhesion tests indicate that the two-step catalysis obviously improves the scratch resistance and adhesion of the films, and that the mixed gas treatment further strengthens the silica network. The increase in strength is attributed to more Si-O-Si bond cross-linkages between silica particles formed by the two-step catalysis and the mixed gas treatment.

Influence of some physical parameters on sol-gel-derived TiO2 thin films

We present some physical parameters that affect the deposition of TiO2 films by the sol-gel method. A set of solutions with different TiO2 concentrations has been prepared. After synthesis of a set of TiO2 films by spin coating at different spinning speeds, the films were annealed at different temperatures. The viscosities of the start solutions were measured by a viscometer. The thickness of the films and their refractive index were measured by a monochromatic ellipsometer. The results showed that the thickness is decreasing exponentially with increasing spinning speed and decreasing linearly with temperature according to the relation d(nm) equals 104 - 0.125 T( degree(s)C) + 322e-S/900. The thickness of the films ranges from 56 to 129 nm. The refractive index is decreasing with the spinning speed and increasing with the temperature according to the relation n equals 1.791 + 6.107X10-4T( degree(s)C) + 1.733e-S/600. The refractive index of the prepared films ranges from 1.89 to 2.125 depending on annealing temperature and spinning speed. The viscosity of the start solutions increased with increasing the TiO2 concentration and the aging time. One week, after preparing the solutions the viscosity was found to be in the range of 1.4 to 4.4 mPa.s depending on the concentrations.