Effective optical properties of highly ordered mesoporous thin films

This paper expands our previous numerical studies predicting the optical properties of highly ordered mesoporous thin films from two-dimensional (2D) nanostructures with cylindrical pores to three-dimensional (3D) structures with spherical pores. Simple, face centered, and body centered cubic lattices of spherical pores and hexagonal lattice of cylindrical pores were considered along with various pore diameters and porosities. The transmittance and reflectance were numerically computed by solving 3D Maxwell's equations for transverse electric and transverse magnetic polarized waves normally incident on the mesoporous thin films. The effective optical properties of the films were determined by an inverse method. Reflectance of 3D cubic mesoporous thin films was found to be independent of polarization, pore diameter, and film morphology and depended only on film thickness and porosity. By contrast, reflectance of 2D hexagonal mesoporous films with cylindrical pores depended on pore shape and polarization. The unpolarized reflectance of 2D hexagonal mesoporous films with cylindrical pores was identical to that of 3D cubic mesoporous films with the same porosity and thickness. The effective refractive and absorption indices of 3D films show good agreement with predictions by the 3D Maxwell-Garnett and nonsymmetric Bruggeman effective medium approximations, respectively.     

A combined top-down and bottom-up approach to fabricate silica films with bimodal porosity

We report a method to fabricate silica films with bimodal porosity based on the surfactant-directed self-assembly process followed by post-treatment with reactive ion etching (RIE). By RIE of a surfactant-templated mesoporous silica film with a 3D hexagonal structure, vertically-etched pores with the size of several tens of nanometers and the depth of ca. 60 nm are generated, while the original caged mesopores (ca. 5 nm in size) are still retained in the unetched parts of the film. Pre-treatment of the mesoporous silica film by wet-etching to expose the pores on the surface, followed by sputter deposition of a Pt layer for partial masking, is crucial for the anisotropic etching of the film. Such a combined top-down and bottom up approach offers an opportunity to fabricate silica films with hierarchical pore architectures.     

Highly-ordered mesoporous titania thin films prepared via surfactant assembly on conductive indium-tin-oxide/glass substrate and its optical properties

Highly ordered mesoporous titanium dioxide (titania, TiO₂) thin films on indium-tin-oxide (ITO) coated glass were prepared via a Pluronic (P123) block copolymer template and a hydrophilic TiO₂ buffer layer. The contraction of the 3D hexagonal array of P123 micelles upon calcination merges the titania domains on the TiO₂ buffer layer to form mesoporous films with a mesochannel diameter of approximately 10 nm and a pore-to-pore distance of 10 nm. The mesoporous titania films on TiO₂-buffered ITO/glass featured an inverse mesospace with a hexagonally-ordered structure, whereas the films formed without a TiO₂ buffer layer had a disordered microstructure with submicron cracks because of non-uniform water condensation on the hydrophobic ITO/glass surface. The density of the mesoporous film was 83% that of a bulk TiO₂ film. The optical band gap of the mesoporous titania thin film was approximately 3.4 eV, larger than that for nonporous anatase TiO₂ (~ 3.2 eV), suggesting that the nanoscopic grain size leads to an increase in the band gap due to weak quantum confinement effects. The ability to form highly-ordered mesoporous titania films on electrically conductive and transparent substrates offers the potential for facile fabrication of high surface area semiconductive films with small diffusion lengths for optoelectronics applications.     

Synthesis and characterization of nanostructured nickel oxide thin films prepared with chemical spray pyrolysis

Nickel oxide thin films have been deposited in an open atmosphere onto glass substrates by chemical spray pyrolysis using aqueous nickel acetate solutions and air as driving gas. The films show a strong variation in the surface morphology depending on the substrate temperature and the precursor solution flux. At 350 °C substrate temperature, a reticular tissue-like film morphology is obtained, becoming the reticular nickel oxide fibres of the film thicker with increasing precursor solution flux. At 450 °C substrate temperature, the film growth rate is 4 times slower and a highly symmetric self-ordering of the material at nanometer length scale occurs. These films consist of interconnected grains separated by pores, both of about 100 nm in size. XRD and TEM revealed that the films are cubic NiO, being the crystallite size around 10 nm. The optical band gap of the films decreases strongly for increasing film thickness from 4.3 eV to 3.65 eV.     

Synthesis of hollow silica spheres SBA-16 with large-pore diameter

Hollow silica SBA-16 spheres with cubic ordered mesoporous shells were synthesized by an emulsion-templating method, using Pluronic F127 as a structure-directing agent, tetraethyl orthosilicateas as a silica source and heptane as a cosolvent in the presence of NH₄F. The size of these spheres is in the range of 10 to 30 μm. The shell is about 700 nm thick and consists of large pores, ~ 9 nm in diameter, arranged in a cubic order. After calcination, the spheres maintain their mesoporosity and show a high surface area of 822 m²/g. The formation mechanism of the silica hollow spheres is discussed.     

Ordered mesoporous silicon through magnesium reduction of polymer templated silica thin films

This paper describes the process of making ordered mesoporous silicon (Si) thin films. The process begins with mesoporous silica (SiO 2) thin films that are produced via evaporation induced self-assembly (EISA) using sol-gel silica precursors with a diblock copolymer template. This results in a film with a cubic lattice of 15 nm diameter pores and 10 nm thick walls. The silicon is produced through reduction of the silica thin films in a magnesium (Mg) vapor at 675 degrees C. Magnesium reduction preserves the ordered pore-solid architecture but replaces the dense silica walls with 10-17 nm silicon crystallites. The resulting porous silicon films are characterized by a combination of low and high angle X-ray diffraction, combined with direct SEM imaging. The result is a straightforward route to the production of ordered nanoporous silicon.     

Thickness dependence of structural and optical properties of indium tin oxide nanofiber thin films prepared by electron beam evaporation onto quartz substrates

Indium tin oxide (ITO) thin films, produced by electron beam evaporation technique onto quartz substrates maintained at room temperature, are grown as nanofibers. The dependence of structural and optical properties of ITO thin films on the film thickness (99-662 nm) has been reported. The crystal structure and morphology of the films are investigated by X-ray diffraction and scanning electron microscope techniques, respectively. The particle size is found to increase with increasing film thickness without changing the preferred orientation along (2 2 2) direction. The optical properties of the films are investigated in terms of the measurements of the transmittance and reflectance determined at the normal incidence of the light in the wavelength range (250-2500 nm). The absorption coefficient and refractive index are calculated and the related optical parameters are evaluated. The optical band gap is found to decrease with the increase of the film thickness, whereas the refractive index is found to increase. The optical dielectric constant and the ratio of the free carrier concentration to its effective mass are estimated for the films.     

Gold-titania nanocomposite films with a periodic 3D nanostructure

In this study, we synthesized gold-titania nanocomposite thin films by using mesoporous titania thin films formed on indium tin oxide substrates as templates. The pore structure of our mesoporous titania thin films can be described as a periodic 3D pore network by interconnecting 7 nm sized cages. Electrochemical deposition of gold into the pores led to gold-titania nanocomposite films. Both gold and titania form continuous 3D network structures with internal periodicity. Because of the low conductivity of indium tin oxide substrate, the deposited gold formed isotropic islands. The absorption spectrum of the resultant gold-titania nanocomposite thin films showed two peaks, one at 640 nm and the other over a broad range of wavelengths longer than 1500 nm. These peaks grow with the increase of the deposition time but do not change the positions. The optical properties were explained in terms of the unique nanostructure of our gold-titania nanocomposite film.     

Compact and vertically-aligned ZnO nanorod thin films by the low-temperature solution method

Highly c-axis-oriented ZnO nanorod thin films were obtained on silica glass substrates by a simple solution-growth technique. The most compact and vertically-aligned ZnO nanorod thin film with the thickness of ∼ 800 nm and average hexagonal grain size of ∼ 200 nm exhibits the average visible transmittance 85%, refractive index 1.74, packing density 0.84, and energy band gap 3.31 eV, and it was fabricated under the optimum parameters: 0.05 M, 75 °C, 6 h, multiple-stepwise, and ZnO seed layer with an average grain size of ∼ 20 nm. The photoluminescence spectrum indicates that the densest ZnO nanorod thin film possesses lots of oxygen vacancies and interstitials. As we demonstrate here, the solution-growth technique was used to produce high-quality and dense ZnO nanorod thin films, and is an easily controlled, low-temperature, low-cost, and large-scale process for the fabrication of optical-grade thin films.     

Novel synthesis of aligned Zinc oxide nanorods on a glass substrate by sonicated sol-gel immersion

For the first time, aligned ZnO nanorod structured thin films have been synthesized on a glass substrate, which had been coated with an Al-doped ZnO thin film, using the sonicated sol-gel immersion method. These nanorods were found to have an average diameter of 100 nm and an average length of 500 nm, with hexagonal wurtzite phase grew preferentially along the c-axis direction. A sharp ultra-violet (UV) emission centred at 383 nm corresponding to the free exciton recombination was observed in a room temperature photoluminescence (PL) spectrum. The prepared ZnO nanorod structured thin film is transparent in the visible region with an average transmittance of 78% in the 400-800 nm wavelength range and high absorbance properties in the UV region (< 400 nm). The results indicate that the prepared ZnO nanorods are suitable for ultra-violet photoconductive sensor applications.     

ZnO thin films prepared by pulsed laser deposition

Zinc oxide (ZnO) thin films were deposited on soda lime glass substrates by pulsed laser deposition (PLD) in an oxygen-reactive atmosphere. The structural, optical, and electrical properties of the as-prepared thin films were studied in dependence of substrate temperature and oxygen pressure. High quality polycrystalline ZnO films with hexagonal wurtzite structure were deposited at substrate temperatures of 100 and 300 °C. The RMS roughness of the deposited oxide films was found to be in the range 2-9 nm and was only slightly dependent on substrate temperature and oxygen pressure. Electrical measurements indicated a decrease of film resistivity with the increase of substrate temperature and the decrease of oxygen pressure. The ZnO films exhibited high transmittance of 90% and their energy band gap and thickness were in the range 3.26-3.30 eV and 256-627 nm, respectively.     

Preparation and quality assessment of CuS thin films encapsulated in glass

Copper sulfide (CuS)-based thin films with different thickness have been prepared by thermal co-evaporation of the elemental constituents. Morphological and microstructural properties were shown to vary with film thickness. Optical properties of films encapsulated in a double-glazed configuration and containing an air gap have been measured. Encapsulated CuS films of thickness 100 and 150 nm showed high transmittance peak values of 48% and 36%, respectively, and a low reflectance below 20% and 15%, respectively, in the visible region. A low transmittance of ~ 10% (~ 15%) and a high reflectance of ~ 45% (~ 40%) in the near-infrared region were obtained for the film with 150 nm (100 nm). Thermal stability of the films has been proved by annealing of the films in air and Argon at 150 °C.     

Optical properties of thermochromic VO2 thin films on stainless steel: Experimental and theoretical studies

Thermochromic films of VO₂ were deposited by DC reactive magnetron sputtering on stainless steel substrate. Complex refractive indexes of VO₂ were determined by ellipsometric spectroscopy (0.35-16.5 μm) for different film thicknesses. Optical simulations were performed to model the spectral reflectance of the film/substrate system for a film thickness of 100 nm and 200 nm and to monitor the optical contrast of the thermochromic layers by comparing the spectral reflectance at 25 °C and 100 °C. The good agreement observed between experimental and theoretical spectra demonstrates the adequacy of the model for predicting the optical properties of the samples.     

Hydrothermal synthesis of β-nickel hydroxide nanocrystalline thin film and growth of oriented carbon nanofibers

Novel well-crystallized β-nickel hydroxide nanocrystalline thin films were successfully synthesized at low temperature on the quartz substrates by hydrothermal method, and the oriented carbon nanofibers (CNFs) were prepared by acetylene cracking at 750 °C on thin film as the catalyst precursor. High resolution transmission electron microscopy (HR-TEM) measurement shows that thin films were constructed mainly with hexagonal β-nickel hydroxide nanosheets. The average diameter of the nanosheets was about 80 nm and thickness about 15 nm. Hydrothermal temperature played an important role in the film growth process, influencing the morphologies and catalytic activity of the Ni catalysts. Ni thin films with high catalytic activity were obtained by reduction of these Ni(OH)₂ nanocrystalline thin films synthesized at 170 °C for 2 h in hydrothermal condition. The highest carbon yield was 1182%, and was significantly higher than the value of the catalyst precursor which was previously reported as the carbon yield (398%) for Ni catalysts. The morphology and growth mechanism of oriented CNFs were also studied finally.     

Thermally stable transparent conducting and highly infrared reflective Ga-doped ZnO thin films by metal organic chemical vapor deposition

Highly transparent conductive Ga-doped ZnO (GZO) thin films have been prepared on glass substrates by metal organic chemical vapor deposition. The effect of Ga doping on the structural, electrical and optical properties of GZO films has been systematically investigated. Under the optimum Ga doping concentration (∼4.9 at.%), c-axis textured GZO film with the lowest resistivity of 3.6 × 10⁻⁴ Ω cm and high visible transmittance of 90% has been achieved. The film also exhibits low transmittance (<1% at 2500 nm) and high reflectance (>70% at 2500 nm) to the infrared radiation. Furthermore, our developed GZO thin film can well retain the highly transparent conductive performance in oxidation ambient at elevated temperature (up to 500 °C).     

On the etching of silica and mesoporous silica films determined by X-ray reflectivity and atomic force microscopy

X-ray reflectometry and atomic force microscopy were used to characterize the etching effect of 0.1 mol dm^− 3 NaOH solution on mesoporous silica films < 100 nm thick produced by the evaporation induced self assembly route using a nonionic triblock co-polymer as the template. The effect of this treatment on films that had been partly condensed at 80 °C or fired at 400 °C in air are compared to non-porous films produced using conventional sol-gel technique. The evolution of film structure was monitored by atomic force microscopy and X-ray reflectometry. Thicknesses obtained from these measurements were used as an order parameter to determine the etch rate. For the mesoporous films, distinct stages corresponding to (a) film compression; (b) removal of the weakly organised caplayer; (c) progressive removal of bilayers of pores/silicated layres; and finally film collapse were revealed.     

Z-scan measurement for the nonlinear absorption of Bi2.55La0.45TiNbO9 thin films

Bi_2.55La_0.45TiNbO₉ (BLTN-0.45) thin films with layered aurivillius structure were fabricated on fused silica substrates by pulsed laser deposition technique. Their structure, fundamental optical constants, and nonlinear absorption characteristics have been studied. The film exhibits a high transmittance (> 60%) in visible-infrared region. The optical band gap energy was found to be 3.44 eV. The optical constant and thickness of the films were characterized using spectroscopic ellipsometric (SE) method. The nonlinear optical absorption properties of the films were investigated by the single-beam Z-scan method at a wavelength of 800 nm laser with a duration of 80 fs. We obtained the nonlinear absorption coefficient β = 4.64 × 10^− 8 m/W. The results show that the BLTN-0.45 thin film is a promising material for applications in absorbing-type optical device.     

Preparation of transparent conductive indium tin oxide thin films from nanocrystalline indium tin hydroxide by dip-coating method

Indium tin oxide (ITO) thin films with well-controlled layer thickness were produced by dip-coating method. The ITO was synthesized by a sol-gel technique involving the use of aqueous InCl₃, SnCl₄ and NH₃ solutions. To obtain stable sols for thin film preparation, as-prepared Sn-doped indium hydroxide was dialyzed, aged, and dispersed in ethanol. Polyvinylpyrrolidone (PVP) was applied to enhance the stability of the resulting ethanolic sols. The transparent, conductive ITO films on glass substrates were characterized by X-ray diffraction, scanning electron microscopy and UV-Vis spectroscopy. The ITO layer thickness increased linearly during the dipping cycles, which permits excellent controllability of the film thickness in the range ~ 40-1160 nm. After calcination at 550 °C, the initial indium tin hydroxide films were transformed completely to nanocrystalline ITO with cubic and rhombohedral structure. The effects of PVP on the optical, morphological and electrical properties of ITO are discussed.     

Synthesis of large-pore periodic mesoporous organosilica

Periodic mesoporous organosilica (PMO) materials with large pores have been successfully synthesized using a combinational strategy by decreasing both the synthesis temperature and acidity. Herein, we use a tri-block copolymer EO106PO70EO106 [Pluronic F127, where EO is poly(ethylene oxide) and PO is poly(propylene oxide)] as the template, bis(trimethoxysilyl)ethane (BTME) as a silica source and 1,3,5-trimethylbenzene (TMB) as a pore expander. The PMO material synthesized in this approach has a face-centered cubic (fcc) structure. When the synthesis temperature is 0 °C and the acidity is 0.1 M HCl, the pore diameter of the PMO material reaches 33.6 nm, which is the largest among cubic PMO materials to our knowledge.     

Deposition of transparent conductive mesoporous indium tin oxide thin films by a dip coating process

Cetyltrimethyl ammonium bromide (CTAB) templated mesoporous indium tin oxide (ITO) thin films were deposited on quartz plates by an evaporation-induced self-assembly (EISA) process using a dip coating method. The starting solution was prepared by mixing indium chloride, tin chloride, and CTAB dissolved in ethanol. Five to fifty mole percent Sn-doped ITO films were prepared by heat-treatment at 400 °C for 5 h. The structural, adsorptive, electrical, and optical properties of mesoporous ITO thin films were investigated. Results indicate that the mesoporous ITO thin films have an ordered two-dimensional hexagonal (p6mm) structure, with nanocrystalline domains in the inorganic oxide framework. The continuous thin films have highly ordered pore sizes (>20 Å), high Brunauer-Emmett-Teller (BET) surface area up to 340 m²/g, large pore volume (>0.21 cm³/g), outstanding transparency in the visible range (>80%), and show a minimum resistivity of ρ = 1.2 × 10⁻² Ω cm.