Reflectance of surfactant-templated mesoporous silica thin films: Simulations versus experiments

In this study, cubic and hexagonal mesoporous amorphous silica thin films were synthesized using evaporation-induced self-assembly process followed by calcination leaving highly ordered spherical or cylindrical pores in a silica matrix. The films featured pores with diameter between 4 and 11 nm, lattice parameter from 7.8 to 24 nm, and porosity between 22% and 45%. All films were dehydrated prior to reflectance measurements except for one film which was fully hydrated. The present study compares the spectral reflectance measured experimentally between 400 and 900 nm with that computed numerically by solving three-dimensional Maxwell's equations in mesoporous silica thin films with the same morphology as those synthesized. The matrix was assumed to have the same optical properties as bulk fused silica. The pore optical properties were either those of air or liquid water whether the film was dehydrated or hydrated, respectively. Excellent agreement was found between experimental and numerical reflectance for both cubic and hexagonal mesoporous silica films. This study experimentally validates our simulation tool and offers the prospect of ab-initio design of nanocomposite materials with arbitrary optical properties without using effective medium approximation or mixing rules.     

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.     

Silica@Carbon mesoporous nanorattle structures synthesised by means of a selective etching strategy

A facile route for the fabrication of nanorattles composed of tunable silica spherical nanoparticles confined inside mesoporous carbon shells is presented. The synthetic strategy involves several steps: i) the synthesis of solid core/mesoporous shell silica microspheres, ii) the infiltration of the mesoporous shell with a carbon precursor and its conversion to carbon through a carbonization process and iii) the controlled dissolution of the silica by means of a soft etching agent (NaOH 1.5 M). Following this procedure, a variety of Silica@Carbon nanorattles of diameter ∼ 430 nm is produced. The diameter of the silica core can be uniformly tuned between 330 nm and 160 nm by varying the etching time in a range of 2 h-44 h. The rate constant for the dissolution process of the silica core was estimated to be k ≈ 1.2 × 10^− 10 g cm^− 2 s^− 1. These nanorattles have a high BET surface area and a large pore volume, depending on the amount of silica in the composite.     

Preparation and electrochemical characteristics of mesoporous carbon spheres for supercapacitors

Mesoporous carbon spheres serving as electrode materials for supercapacitors were synthesized by a facile polymerization-induced colloid aggregation method using melamines as a carbon precursor and commercial colloidal silica as a silica source for hard template. After the carbonization of as-formed resins-template composites at 1000 °C and the removal of the silica template by hydrofluoric acid, the resulting mesoporous carbon spheres with a diameter size of ∼5 μm, specific surface area (up to 1280 m²/g) and uniform pore size as large as 30 nm could be obtained. Due to the enriched nitrogen content and the large pore size of the mesoporous carbon spheres affecting the surface wettability, resistance, and ion diffusion process in the pores, the mesoporous carbon spheres showed a high specific capacitance of 196 F/g in 5 mol/l H₂SO₄ electrolytes at a discharge current density of 1 A/g.     

Mesoporous titania films with adjustable pore size coated on stainless steel substrates

Mesoporous layers of titania were prepared on stainless steel substrates of defined roughness by dip coating. Ordered arrays of micelles formed from amphiphilic block copolymers served as pore templates during film drying. Coating of the precursors solution on freshly grinded steel resulted in extensively fractured films with severely distorted templated porosity. In contrast, films produced on precalcined steel showed good integrity, high substrate coverage and narrow pore size distribution with pores interconnected and ordered in a short range. This difference in film quality and morphology was ascribed to the reaction between template polymers and metal ions leached from the steel of grinded substrate surfaces. Films were ca. 700 nm thick and composed of nanocrystalline titania. The pore size of titania coatings was varied between 5 and 16 nm employing polymer templates of different structure and molecular weight.     

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.     

Combined Langmuir-Blodgett and sol-gel coatings

Optical properties and in-depth structure of double-layer coatings on glass substrates were investigated. One of the layers was prepared by dip coating either from silica or titania sol, the other layer was made from ca. 130 nm Stöber silica particles by the Langmuir-Blodgett (LB) technique. Two different types of combined coatings were prepared: (1) nanoparticulate LB films coated with sol-gel (SG) films and (2) nanoparticulate LB films drawn onto SG films.Scanning electron microscopy and optical methods, i.e. UV-vis spectroscopy and scanning angle reflectometry were applied for analyzing the structure and thickness of coatings. These measurements revealed that the precursor sols could not penetrate into the particulate LB film completely in case of coating type (1). For coating type (2) very little overlap between the SG and LB layers was found resulting in significant improvement of light transmittance of combined coatings compared to single SG films.To show some possible advantages of the combination of these techniques additional studies were carried out. Surface morphology of combined coatings (1) was studied by atomic force microscopy. Surfaces with different roughness were developed depending on the thickness of the sol-gel film (titania: ca. 70 nm; silica: ca. 210 nm). The adhesive peel off test revealed improved mechanical stability of combined coatings (2), in comparison to LB films which makes them good candidates for further applications.     

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.     

Phase separated AlSi thin films prepared by filtered cathodic arc deposition

Phase separated AlSi films composed of Al cylinders embedded in an amorphous Si matrix were prepared on conducting Si substrates by filtered cathodic arc deposition. The compositional dependence of AlSi films on a negative substrate bias showed a different trend depending on the cathode composition because of the self-sputtering process during the deposition. The porous structure was obtained from the phase separated AlSi film after removal of Al cylinders by wet etching in an ammonia solution. Scanning electron microscope images of the etched AlSi films showed that the average diameter of pores was increased from 3 nm to 7 nm by applying a negative substrate bias voltage during the deposition. The honeycomb ordered arrangement of pores was observed at 0 V and − 25 V substrate bias. The substrate temperature during the depositions had almost the same effect on the film morphologies as the negative substrate bias.     

Fabrication of TiO2 nanotube film by well-aligned ZnO nanorod array film and sol-gel process

High density TiO₂ nanotube film with hexagonal shape and narrow size distribution was fabricated by templating ZnO nanorod array film and sol-gel process. Well-aligned ZnO nanorod array films obtained by aqueous solution method were used as template to synthesize ZnO/TiO₂ core-shell structure through sol-gel process. Subsequently, TiO₂ nanotube array films survived by removing the ZnO nanorod cores using wet-chemical etching. Polycrystalline anatase TiO₂ nanotube films were ∼ 1.5 μm long and ∼ 100 nm in inter diameter with a wall thickness of ∼ 10 nm.     

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.     

Texture optimization process of ZnO:Al thin films using NH4Cl aqueous solution for applications as antireflective coating in thin film solar cells

ZnO:Al thin films varying the thickness from 80 to 110 nm were deposited on polished float zone < 100 > Si wafers by radio frequency magnetron sputtering at 100 °C. To texturize these surfaces with the aim of being used as antireflective coating, a wet etching process based on NH₄Cl was applied. Taking into account that the layer thickness was small, the control of the etch parameters such as etchant concentration and etching time was evaluated as a function of the textured film properties. An appropriate control of the etching rate to adjust the final thickness to the 80 nm required for the application was realized. Using NH₄Cl concentrations of 10 wt.% and short times of up to 25 s, an increase of the film roughness up to a factor of 5.6 of the as-deposited films was achieved. These optimized textured films showed weighted reflectance values below 15% and considerable better electrical properties than the as-deposited 80 nm-thick ZnO:Al films.     

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.     

Fabrication and transfer of nanoporous alumina thin films for templating applications: Metal dots array deposition and porous ZnO film growth

An approach to fabricate nanoporous alumina thin films on aluminum foils followed by their transfer to desired substrates via chemical lift-off and Van der Waals bonding is presented. By employing a black wax technique, we demonstrate a crack free (within the area of 2 × 2 cm²) lift-off and bonding of a nanoporous alumina film with thickness of as small as ~ 300 nm onto a substrate. The liftoff-and-bonding process changes neither the morphology nor the structure of the nanopores in the alumina film. Templating applications of the bonded alumina thin films are demonstrated and discussed. The results reveal that the alumina nanopores, combined with the liftoff-and-bonding technique, have great potential for templating applications in both nano-sized dots array deposition and functional materials growth.     

Highly ordered hexagonally arranged nanostructures on silicon through a self-assembled silicon-integrated porous anodic alumina masking layer

A combined process of electrochemical formation of self-assembled porous anodic alumina thin films on a Si substrate and Si etching through the pores was used to fabricate ideally ordered nanostructures on the silicon surface with a long-range, two-dimensional arrangement in a hexagonal close-packed lattice. Pore arrangement in the alumina film was achieved without any pre-patterning of the film surface before anodization. Perfect pattern transfer was achieved by an initial dry etching step, followed by wet or electrochemical etching of Si at the pore bottoms. Anisotropic wet etching using tetramethyl ammonium hydroxide (TMAH) solution resulted in pits in the form of inverted pyramids, while electrochemical etching using a hydrofluoric acid (HF) solution resulted in concave nanopits in the form of semi-spheres. Nanopatterns with lateral size in the range 12-200?nm, depth in the range 50-300?nm and periodicity in the range 30-200?nm were achieved either on large Si areas or on pre-selected confined areas on the Si substrate. The pore size and periodicity were tuned by changing the electrolyte for porous anodic alumina formation and the alumina pore widening time. This parallel large-area nanopatterning technique shows significant potential for use in Si technology and devices.     

Direct template synthesis of mesoporous carbon and its application to supercapacitor electrodes

A direct templating method which is facile, inexpensive and suitable for the large scale production of mesoporous carbon is reported herein. A meso-structure surfactant/silicate template was made in a solution phase and resorcinol-formaldehyde as a carbon precursor was incorporated into the template solution. After aging, carbonization and hydrofluoric acid (HF) etching, mesoporous carbon was obtained. Using X-ray diffraction, scanning and transmission electron microscopy and nitrogen sorption, the synthesis mechanism of the mesoporous carbon was elucidated. According to the small angle X-ray scattering measurements, the surface became smoother after the removal of the silica, indicating that the silica was mostly located at the pore surface of the carbon. Also, the calculation of the pore volume demonstrated that the silica was transferred into the pores of the carbon without structural collapse during HF etching. When the prepared mesoporous carbon was applied to a supercapacitor electrode, the rectangular shape of the cyclic voltammogram was less collapsed, even at a high scan rate, which is indicative of its high rate capability. This was due to the low resistance of the electrolyte in the pores (3.8 Ω cm²), which was smaller than that of conventional activated carbon electrodes and even comparable to that of ordered mesoporous carbon electrodes. This improved performance was probably due to the well developed mesoporosity and high pore connectivity of the prepared mesoporous carbon.     

EBL- and AFM-based techniques for nanowires fabrication on Si/SiGe

Two approaches for sub-100 nm patterning are applied to Si/SiGe samples.The first one combines electron beam lithography (EBL) and anisotropic wet etching to fabricate wires with triangular section whose top width is narrower than the beam size. Widths as small as 20 nm on silicon and 60 nm on Si/SiGe heterostructures are obtained.The second lithographic approach is based on the local anodization of an aluminum film induced by an atomic force scanning probe. Using atomic force microscopy (AFM) anodization and selective wet etching, aluminum and aluminum oxide nanostructures are obtained and used as masks for reactive ion etching (RIE). Sub-100 nm wide wires are fabricated on Si/SiGe substrates.     

Nickel pulse reversal plating for image reversal of ultrathin electron beam resist

The effects of various pulse reversal plating parameters on the grain size and smoothness of Ni film on silver seed layers has been studied. The duty cycle, frequency, bath temperature and agitation methods have been tested. The objective was to form a thin continuous hard etch mask (20-30 nm of thickness) of Ni films for image reversal of thin film resist using electroplating. While nickel sulfamate solution without additives or brighteners has been used to plate Ni films, reactive ion etching (RIE) has been used to test the durability of the plated Ni films in fluorine plasma. It was found that pulse reversal plating with current intensity of 12 mA/cm², duty cycle of 90%, bath temperature of 45 °C, ultrasonic agitation of power 80 W, and 400 kHz wave frequency resulted in a plating rate as low as 0.2 nm/s. This plating rate made the control of the film thickness an easy task to achieve. This yielded to a smooth plated surface free from defects or voids, with 25 nm film thickness. Combining electron beam lithography with pulse reversal plating for image reversal and RIE offers the prospect of patterning patterns with the desired aspect ratio. Holes of 100 nm diameter, 250 nm period, and 300 nm depth are achieved using this process.     

The size modulation of hollow mesoporous carbon spheres synthesized by a simplified hard template route

Hollow mesoporous carbon spheres (HMCSs) have been prepared by a simplified replication route from a solid silica core/mesoporous silica shell aluminosilicate (SCMS-Al) template, which was synthesized by directly incorporating aluminum species into the mesoporous framework during template synthesis. The size of HMCSs can be tuned between 80 and 470 nm by simply changing the diameters of SCMS-Al. The HMCSs have uniform mesopores with a narrow pore size distribution (3.4-4.1 nm), and high surface area, (890-1150 m²/g) and total pore volumes (0.75-1.15 cm³/g). The techniques of N₂ sorption isotherms, TEM, EDX and SEM were used to characterize the as-synthesized spheres.