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.     

Oriented mesoporous organosilicate thin films

Coassemblies of block copolymers and inorganic precursors offer a path to ordered inorganic nanostructures. In thin films, these materials combined with domain alignment provide highly robust nanoscopic templates. We report a simple path to control the morphology, scaling, and orientation of ordered mesopores in organosilicate thin films through the coassembly of a diblock copolymer, poly(styrene-b-ethylene oxide) (PS-b-PEO), and an oligomeric organosilicate precursor that is selectively miscible with PEO. Continuous films containing cylindrical or spherical pores are generated by varying the mixing composition of symmetric PS-b-PEO and an organosilicate precursor. Tuning interfacial energy at both air/film and film/substrate interfaces allows the control of cylindrical pore orientation normal to the supported film surfaces. Our method provides well-ordered mesoporous structures within organosilicate thin films that find broad applications as highly stable nanotemplates.     

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.     

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.     

Large photocurrent-response observed at Pt/InP Schottky interface formed on anodic porous structure

A photoelectric-conversion device—based on an InP porous structure utilizing the large surface area inside pores and the low reflectance on the porous surface—is proposed. The InP walls inside the pores are covered with thin platinum films that form a Schottky barrier yielding an electric field that separates photo carriers generated under illumination. The coverage of the platinum film and its optical reflectance depended largely on the surface morphology of the porous structure. Removal of the irregular top layer formed at the initial stage of the pore formation effectively improved the coverage of the platinum film, which showed a very low optical reflectance (i.e., below 3.2%). According to current-voltage measurements under illumination, the platinum/porous InP showed larger photocurrents and higher responsivity than those of a reference planar sample.     

利用表面活性剂量改变孔洞率制备的纳米多孔SiO2膜

采用表面活性剂十六烷基三甲基溴化氨(CTAB)为模板剂,在酸性条件下产生多孔结构,再经热处理去除CTAB。实验中使用溶胶?凝胶技术,正硅酸乙酯(TEOS)为硅源,以及二次去离子水,盐酸为催化剂等原料,利用表面活性剂与硅源水解后形成的聚集体相互作用,在溶液中形成分子自组装体,制备前驱体溶胶。通过简单提拉迅速蒸发溶剂制备纳米多孔或纳米介孔SiO2薄膜,分析和研究了表面活性剂浓度对纳米多孔SiO2薄膜的结构和孔洞率的影响,通过操纵表面活性剂的含量,能控制薄膜的纳米结构、孔洞率、孔大小和孔的形态以及膜的形貌。小角度射线衍射、场发射透射电子显微镜、原子力显微镜显示可以制得具有六方、立方和由三维六方和简单立方组成的新相结构以及比介孔大的纳米多孔结构的薄膜。椭偏仪测量得到所制备薄膜的孔洞率为51.8%-65.6%,借助此孔洞率能计算薄膜的折射率和介电常数。     

Architectural optimization of porous Ultra Low K dielectric material via Finite Element simulations.

Porous thin films are the main candidates to achieve Ultra Low K materials for interlayer dielectric (ILD) applications, but their mechanical properties remain low. Indeed, the introduction of porosity significantly decreases the dielectric constant κ, but also affects the mechanical properties. However, both mechanical and dielectric performances are influenced by the pore arrangement and geometry that can thus be optimized for these two antagonist properties. A Finite Element study is conducted to quantify the influence of the arrangement and the geometry of the pores via a homogenization approach. The best architectures for ILD applications are deduced. Random arrangements as well as 3D cubic arrangements (simple, body-centered, face-centered) of monodisperse spherical pores have been considered. These arrangements are obtainable by sol-gel processing, a method that often induces a marked shrinkage of the films. This shrinkage modifies the original cubic arrangements which become orthorhombic and the pore shape which becomes ellipsoidal. The effect on the dielectric and elastic properties of these architectural changes due to the shrinkage has also been studied.     

Vertically oriented hexagonal mesoporous films formed through nanometre-scale epitaxy

Polymer- and surfactant-templated mesoporous inorganic materials offer a unique combination of controllable nanoscale architecture, materials variation and low-cost solution processing. Inorganic materials can be produced with a range of periodic pore structures, with feature size ranging from 2 to 30 nm, and from a diverse set of materials. Unfortunately in thin-film form, the pores of the ubiquitous hexagonal honeycomb phase tend to lie in the plane of the substrate making these materials unsuitable for applications where diffusion into the pores is required. Here, we show that nanometre-scale epitaxy on a patterned substrate can be used to form vertically oriented pores in honeycomb-structured films. We use the surface of cubic mesoporous films to form the pattern; as such, our method does not sacrifice the simple processing advantages of a self-assembled system. A precise lattice match between the hexagonal and cubic films is needed for vertical orientation, a condition that can be achieved using mixed templates or selective pore swelling. Pore orientation is characterized by a combination of microscopy and diffraction. Here, we present alignment data on oriented nanopores in the 10-15 nm range, but the method should be applicable across the 2-30 nm pore size range of these self-organized materials.     

Chemical gas sensor application of open-pore mesoporous thin films based on integrated optical polarimetric interferometry

Chemical gas sensors that employ integrated optical polarimetric interferometry were fabricated by the sol-gel synthesis of transparent mesoporous thin films of TiO2-P2O5 nanocomposite on tapered layers of TiO2 sputtered on tin-diffused glass waveguides. Atomic force microscopy images of the mesoporous thin film clearly show the open pore mouths on the film surface that favor rapid diffusion and adsorption of gas-phase analytes within the entire film. Adsorption of gas and vapor induces changes (Deltan) in the refractive index of the mesoporous thin film that lead to shifts in the phase difference between the fundamental transverse electric and magnetic modes simultaneously excited in the glass waveguide via single-beam incidence. Upon exposure to NH3 gas at concentrations as low as 100 ppb in dry air at room temperature, the sensor exhibits a reversible change in the phase difference with the response and recovery times of less than 60 and 90 s, respectively. It is unexpected that the sensor is unresponsive to either NO2 or C6H6 vapor, leading to a somewhat selective sensitivity to NH3. Determination of Deltan was carried out with a combination of the experimental results and the theoretical calculations. The sensor design represents a novel, effective, and easily accessible approach to mesoporous thin-film-based integrated optical chemical sensors.     

WO3电致变色薄膜制备过程中光学膜厚测量监控技术的研究

采用真空电子束蒸发方法制备WO3电致变色薄膜过程中，利用极值法光学膜厚测量技术监控薄膜的光学特性，对不同光学膜厚的WO3薄膜的原始态、着色态和退色态的光谱特性进行了对比分析。测试采用二电极恒电压方法，用分光光度计实时测量透过率的变化。结果证明以ITO玻璃作为比较片，极值法监控薄膜光学膜厚，当反射率达到第一极小值，即透过率达到第一极大值时，WO3薄膜得到最好的综合电致变色特性。     

Preparation and characterisation of thickness dependent nano-structured ZnS thin films by sol–gel technique

Zinc sulfide (ZnS) thin films of different thickness were coated on glass substrates by the sol–gel dip-coating technique. Thickness dependent structural and optical properties of the films were studied in detail. X-ray diffraction (XRD) analysis indicated that the films had mixture of cubic (β) and hexagonal (α) phases with cubic (β) phase being predominant. Scanning electron microscope (SEM) showed that the film surfaces were smooth and crack free. Energy dispersive X-ray (EDX) measurement showed no impurity in the ZnS compound with elemental concentration of Zn/S being 50.38/49.62. Optical measurements showed that optical transmittance of the films were decreased in the visible range as the film thickness increased and band gap of the thin films were estimated to be about 3.61, 3.56, and 3.39?eV for the films with thickness of 100, 220, and 360?nm, respectively. Reactive indices and extinction coefficients of the films were measured by Spectroscopic Ellipsometer. Both the refractive index (n) and extinction coefficient (k) of the films were increased as the film thickness decreased. Electrical measurements showed that the resistivity of the films were decreased as the substrate temperature and film thickness increased.     

Control of wall thickness in the formation of ordered mesoporous silica films

Ordered mesoporous silica thin films using block copolymer have drawn an attention for low-k application due to its ordered pore structure. From the respect of dielectric and mechanical properties of the film, there is trade-off between pore size and wall thickness. In this work, factors for increase of wall thickness were investigated. It was found that body-centered cubic structure was maintained irrespective of the concentration of catalytic acid. The catalytic acid thickens the framework wall because counterion reduces the repulsion force between silicic acids. The highly ordered mesoporous silica films were obtained although high concentration of acid was added to the silica sol. However, wormlike micelle exists more with high HCl concentration due to fast gellation rate. And excess water, which has the role similar to the humid atmosphere, also increases the thickness of silica wall. However, large amount of excess water at the micelle interface disrupts organic-inorganic electrostatic interaction. As a conclusion, optimization of HCl concentration in the silica sol and control of humidity during spin coating can simultaneously increase the framework thickness while maintaining the pore periodicity.     

Growth of highly c-axis oriented aluminum nitride thin films on β-tantalum bottom electrodes

We have investigated the influence of tantalum (Ta) bottom electrodes on the crystallinity and crystal orientation of aluminum nitride (AlN) thin films. AlN thin films and Ta electrodes were prepared by using rf magnetron sputtering method. The crystal structure of the Ta electrodes was tetragonal (β-Ta, a metastable phase) at room temperature. The crystallinity and orientation of the AlN thin films and Ta electrodes strongly depended on sputtering conditions. Especially, the crystallinity and crystal orientation of the Ta electrodes were influenced by their film thickness and the substrate temperature. When the thickness of the Ta bottom electrodes was 200 nm and the substrate temperature was 100 °C, the AlN thin films indicated high c-axis orientation (the full width at half maximum of rocking curve of 3.9°). The crystal orientation of the AlN film was comparable to that of AlN thin films deposited on face centered cubic (fcc) lattice structure metal, such as Au, Pt and Al, bottom electrodes.     

Ferromagnetic films deposited on nanochannel alumina

Interest in artificially grown nanostructures has grown enormously in the last few years because of the potential applications in the magnetic recording industry. In this work we present ferromagnetic resonance investigations performed on Fe films sputter-deposited on nanochannel alumina (NCA). These films form a network-like nanostructure on top of the walls that separate the pores. The geometry is fixed by the channel size, the porosity of the substrate and the film thickness. From the experimental results in NCA of 20 nm pore diameter we have found that thinner films (10 nm or less) are discontinuous and formed by isolated, partially oriented anisotropic particles. An average aspect ratio of ∼1.5 was estimated for the particles forming the film. As the film thickness increases the effective anisotropy (mostly shape anisotropy) tends to reach a saturation value for films thicker than 75 nm. For NCA substrates of larger pore diameter (100 nm and 200 nm pore size) the effective anisotropy is greatly reduced and even changes orientation for the thinner films. This behavior is interpreted as comming from a faster filling of the pores with the sputtered material in the substrates with smaller pore size. Received: 30 March 2000     

Interfacial Synthesis of Conjugated Crystalline 2D Fluorescent Polymer Film Containing Aggregation‐Induced Emission Unit

A fully conjugated 2D fluorescent film containing a tetraphenylethene (TPE) unit is constructed by Glaser–Hay coupling reaction on the surface of copper foil. A large‐area, freestanding fluorescent films with an average thickness 4.5 nm can be obtained through the strategy of solid–liquid interfacial synthesis. The film and the pore structure are characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), X‐ray photoelectron spectroscopy (XPS). High‐resulution TEM and selected area electron diffraction (SAED) further confirm the dual pores structure with triangular‐ and hexagonal‐shaped pores. The as‐prepared 2D films exhibit excellent solid‐state fluorescence emission arising from the confinement of TPE units.     

Fabrication and optical characterization of template-constructed thin films with chiral nanostructure

We report the fabrication of thin films perforated by high aspect ratio helical or chevron pores by an extension of the glancing angle deposition (GLAD) technique. The perforated films were created by transferring the nanostructure of a GLAD template film into target materials such as polymers and spin-on-glasses and subsequently removing the template. The pore shapes are shown to be highly controllable and films designed to suit particular applications are discussed. By a double templating technique, we replicate the structure of the original film using alternate materials, which are typically less suited to the unmodified GLAD technique. Helical films of Cu and Ni were created by this method and the process should be transferable to additional electrodeposited materials. The optical rotatory power of perforated thin films formed on glass substrates was characterized and perforated films were shown to be effective in rotating the polarization plane of linearly polarized incident light by as much as 1.4/spl deg///spl mu/m.     

Controlled Deposition of Silver Nanoparticles in Mesoporous Single‐ or Multilayer Thin Films: From Tuned Pore Filling to Selective Spatial Location of Nanometric Objects

Silver nanoparticle assemblies are embedded within mesoporous oxide thin films by an in situ mild reduction leading to nanoparticle–mesoporous oxide thin‐film composites (NP@MOTF). A quantitative method based on X‐ray reflectivity is developed and validated with energy dispersive spectroscopy in order to assess pore filling. The use of dilute formaldehyde solutions leads to control over the formation of silver nanoparticles within mesoporous titania films. Inclusion of silver nanoparticles in mesoporous silica requires more drastic conditions. This difference in reactivity can be exploited to selectively synthesize nanoparticles in a predetermined layer of a multilayered mesoporous stack leading to complex 1D‐ordered multilayers with precise spatial location of nanometric objects. The metal oxide nanocomposites synthesized have potential applications in catalysis, optical devices, surface‐enhanced Raman scattering, and metal enhancement fluorescence.     

Effective optical properties of non-absorbing nanoporous thin films

Numerous effective medium models have been proposed for the effective optical properties of nanoporous media. However, validations of these models against experimental data are often contradictory and inconclusive. This issue was numerically investigated by solving the two-dimensional Maxwell's equations in non-absorbing nanoporous thin films with various morphologies. It was found that below a certain critical film thickness, the effective index of refraction depends on the porosity and on the pore size, shape and spatial distribution. For thick enough films the effective index of refraction depends solely on porosity and on the indices of refraction of the two constitutive phases. The numerical results agree very well with a recent model obtained by applying the Volume Averaging Theory to the Maxwell's equations. However, commonly used models systematically and sometimes significantly underpredict the numerical results.     

Optical properties of porous anodic aluminum oxide thin films on quartz substrates

Porous anodic aluminum oxide (AAO) thin films on quartz substrates were fabricated via evaporation of a 100-nm thick Al, followed by anodization with different durations and pore widening and Al removal by chemical etching. The transmittance and reflectance of AAO films on quartz substrates were measured by optical spectrophotometry. The microstructure and morphology were examined by scanning electron microscopy. The pore diameter of AAO films after pore widening and Al removal is 60 ± 4 nm and the interpore distance is 88 ± 5 nm. It is found that the reflectance decreases and the transmittance increases with the increase of the anodization time and pore widening. Compared to a bare substrate, the transmittance of AAO films after pore widening and Al removal is about 3.0% higher, while the reflectance is about 3.0% lower over a wide wavelength range. Additionally, after pore widening and Al removal, when AAO films are prepared on both sides of the quartz substrate, the highest transmittance is about 99.0% in the wavelength range 570-680 nm. The optical constants and thickness of AAO films after pore widening and Al removal were retrieved from normal incidence transmittance data. Results show that the refractive index is lower than 1.25 in the visible optical region and that the porosity is about 0.70.     

Tuning the mesopore structure of 3D hexagonal thin films using butanol as a co-solvent

Distorted 3D hexagonal (space group R-3m) mesoporous titania thin films with high visible-light transparency are produced by introducing a small amount of 1-butanol into sol-gel coating sols containing P123 as the primary pore template. The mesostructure of the titania thin films identified by grazing incidence small angle x-ray scattering (GISAXS), transmission electron microscopy and scanning electron microscopy has rhombohedral symmetry (space group R-3m) with the [111] direction normal to the substrate. In the absence of butanol at the ratio of P123:titania used, poorly ordered films which may contain 2D or 3D hexagonal domains form, but introducing as little as a 1:10 ratio (by mass) of 1-butanol to P123 (with ethanol as coating solvent) leads to a well-defined 3D hexagonal structure in the resulting films. Simulated GISAXS patterns obtained using the NANODIFT program confirm the assignment of the experimental patterns to this structure. The characterization results indicate that over a 1-butanol:P123 mass ratio of 0.1 to 2, the films are composed of ordered arrays of cage-like cavities, and that in contrast to the hypothesized role of butanol as a swelling agent, the pore size decreases with an increase in the ratio of 1-butanol to P123.