Porous Thin Films Research Articles

Synthesis of Silicon Nano-fibrous (SiNf) thin film with controlled thickness and electrical resistivity

Abstract Considering abundance of silicon on earth with its known acceptance in semiconductor industries and increasing applications of nano porous thin films in various sensors, electrical circuits, new generation batteries, and capacitors, it is essential to comprehend the resistive behavior offered by Silicon Nano-fibrous (SiNf) thin films. We present a novel approach of laser plasma processing of silicon surface to fabricate SiNf thin films with diverse range of resistivity. Parallel plate electrode configuration was employed to measure the resistance offered by SiNf thin film fabricated using various laser parameters. Pulsation width of laser was varied to affect the plasma temperature causing variation in suitable ablation conditions. In addition, scanning parameter of leading(i.e.distance between two consecutive scan with laser beam) was modified to change the extent of silicon surface covered under direct laser gaussian beam causing variation in process overlap. Along with the resistive nature of the thin films, topological changes were also studied for the sound understanding and validation of respective resistive behavior. Significant difference was noticed in terms thickness, structure and resistivity of SiNf thin film when such variation was performed. Also, successful correlation was attained with sound support of various analysis and surface characterization tests.

Block Copolymer Self-Assembly Directed Hierarchically Structured Materials from Nonequilibrium Transient Laser Heating

We highlight two recent approaches operating far from equilibrium for the synthesis of hierarchical porous thin film materials by coupling block copolymer-directed self-assembly with transient laser heating. In first block copolymer-induced writing by transient heating experiments, or B-WRITE, an all-organic block copolymer–resols hybrid film is heated by submillisecond carbon dioxide laser irradiation, directly generating 3D mesoporous continuous resin structures and shapes. Harnessing the highly unique resin materials properties under laser heating conditions, in the second approach block copolymer-directed resin templating is coupled with nanosecond pulsed excimer laser annealing to generate complementary crystalline silicon nanostructures. The underlying structure formation mechanisms for such laser-induced organic and inorganic nanostructured materials are discussed, emphasizing that the nonequilibrium nature of these transient laser annealing approaches opens up vast and new processing windows beyon...

Fabrication and Refractive Index Control of Transparent and Luminescent HfO2:Ln3+ (Ln3+ = Eu3+, Tb3+) Thin Films for Enhanced Surface Emissions

Dense and porous thin films of luminescent Ln3+-doped HfO2 materials (Ln3+ = Eu3+ and Tb3+) were fabricated on silica glass substrates by a sol–gel dip-coating method using different kinds of coating solutions. The refractive index of the HfO2:Ln3+ thin films was controlled by changing their porosity based on the Lorentz–Lorenz relationship. Pores could be introduced systematically into the thin films with the addition of different amounts of a poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) triblock copolymer to the coating solutions and the subsequent heat-treatment at 700°C. While photoluminescence (PL) of the dense HfO2:Ln3+ thin films with higher refractive index was inefficient mainly due to the reflection problem, the porous HfO2:Ln3+ thin films with lower refractive index exhibited much improved PL intensity by suppressing total internal reflection of the emitted light. The existence of the pores was also beneficial to an increase in the absorption of the excitation light due to its multiple scattering inside the thin films. Our results then demonstrate that the porous luminescent thin films are suitable for applications where the transparency and the strong surface emission are both necessary.

Amphiphilic diblock copolymer-mediated structure control in nanoporous germanium-based thin films.

Fabrication of porous, foam-like germanium-based (Ge-based) nanostructures is achieved with the use of the amphiphilic diblock copolymer polystyrene-b-polyethylene oxide as structure directing agent. Basic concepts of block copolymer assisted sol-gel synthesis are successfully realized based on the [Ge9]4- Zintl clusters as a precursor for Ge-based thin films. Material/elemental composition and crystalline Ge-based phases are investigated via X-ray photoelectron spectroscopy and X-ray diffraction measurements, respectively. Poor-good solvent pair induced phase separation leads to pore sizes in the Ge-based films up to 40 nm, which can be tuned through a change of the molar mixing ratio between polymer template and precursor as proven by grazing incidence small angle X-ray scattering and scanning electron microscopy.

Preparation and Characterization of Porous Carbon Thin Film by Soft Template Method Using 1,4-dihydroxyanthraquinone as Carbon Source

Preparation and Characterization of Porous Carbon Thin Film by Soft Template Method Using 1,4-dihydroxyanthraquinone as Carbon Source

Diffusion controlled porous WO3 thin film photoanodes for efficient solar-driven photoelectrochemical permanganic acid production

Photoelectrochemical (PEC) oxidation of the divalent manganese ion to the permanganate ion was achieved by using porous WO3 thin film photoanodes in sulfuric acid electrolytes under simulated sunlight.

Characterization of Porous Carbon Thin Films Using 1,4-dihydroxyanthraquinone

Characterization of Porous Carbon Thin Films Using 1,4-dihydroxyanthraquinone

Multifunctional nanopatterned porous bismuth ferrite thin films

Nanopatterned porous thin films of bismuth ferrite (BiFeO3) with porosity perpendicular to the plane are prepared by an evaporation-induced self-assembly methodology using nitrate metal salts and a commercial block copolymer as a structure-directing agent.

Environmentally Tight TiO2–SiO2 Porous 1D‐Photonic Structures

AbstractAlthough thin film porosity is the basis of many optical sensors, it can be deleterious for a stable optical behavior of passive optical elements due to the condensation of water and other vapors in their pores. This paper proposes a new strategy for the magnetron sputtering (MS) fabrication of environmentally tight SiO2–TiO2 porous multilayers. Thin films of these two oxides deposited in an oblique angle configuration (MS‐OAD) present a nanocolumnar and highly porous nanostructure and, as a consequence, experience significant changes in their optical properties when exposed to water vapor. Similarly, the optical properties of Bragg reflectors and Bragg microcavities made of the stacking of porous and compact SiO2 and TiO2 thin films experience reversible changes when these 1D‐photonic structures are exposed to water pressure. A key finding of this work is that a very thin capping layer of SiO2 deposited on the surface of porous SiO2 films in the stack, at the interlayer between the two oxides, efficiently seals the pores making the photonic structures environmentally tight. This capping layer approach is a useful strategy to incorporate porosity as an additional parameter to design the optical behavior of planar photonic structures while preserving optical and environmental stability.

Liquid switchable radial polarization converters made of sculptured thin films

A radial polarization converter is a super-structured optical retarder that converts a conventional linearly polarized light beam into a structured beam with radial or azimuthal polarization. We present a new type of these sophisticated optical elements, which is made of porous nanostructured sculptured single thin films or multilayers prepared by physical vapor deposition at an oblique angle. They are bestowed with an axisymmetric retardation activity (with the fast axis in a radial configuration). In particular, a Bragg microcavity multilayer that exhibits a tunable transmission peak in the visible range with a retardance of up to 0.35 rad has been fabricated using this methodology. Owing to the highly porous structure of this type of thin films and multilayers, their retardance could be switched off by liquid infiltration. These results prove the possibility of developing wavelength dependent (through multilayer optical design) and switchable (through vapor condensation or liquid infiltration within the pore structure) radial polarization converters by means of oblique angle physical vapor deposition.

Hyperspectral and Color Imaging of Solvent Vapor Sorption Into Porous Silicon.

A porous silicon thin film photonic crystal (rugate) sample with both a radial gradient in the rugate reflectance band wavelength and two spatially separated pore-wall surface chemistries (methylated and oxidized) was monitored by hyperspectral and color imaging while it was dosed with vapors of acetone, ethanol, heptane, 2-propanol, and toluene at concentrations ranging from 100 to 3,000 mg m−3. The shift in the wavelength of the rugate reflectance band maximum at each position along a transect across the two surface chemistries, as derived from the hyperspectral imaging, could discriminate between the different solvents and concentrations of solvents, while the change in hue derived from the color camera data along an analogous transect did not provide discrimination. The discrimination between solvents was mainly due to the two different surface chemistries, and the gradient associated with the change in the rugate reflectance band wavelength did not affect the selectivity significantly. There was spatial variability in the spectral and color responses along the transect independent of the overall rugate reflectance band wavelength gradient and pore-wall surface chemistries, and this was attributed to factors such as the presence of striations in the silicon wafer from which the porous silicon was prepared.

Porous silicon thin films as anodes for lithium ion batteries deposited by co-evaporation of silicon and zinc

Porous silicon thin films were fabricated by an innovative method using vacuum processing for the application as high capacity anode in lithium ion batteries. The deposition procedure comprises a co-evaporation of silicon and zinc, resulting in a deposition of a compound layer with deposition rates up to 100 nm/s and a subsequent thermal annealing. Due to its high vapor pressure, the zinc fraction is expelled and hence, a porous silicon matrix is formed. Herein, we introduce a novel and potentially scalable synthesis method for porous silicon films and show first analytical investigations concerning the layer morphology and the electrochemical properties. With the novel silicon anode excellent electrochemical performance, particularly high capacities of ≥3000 mAh/g, reasonable coulombic efficiencies of ≥90% in the initial cycle and comparably high cycle life >150 cycles can be demonstrated, which reveals their great potential for battery anode applications.

Tailoring of highly porous SnO2 and SnO2-Pd thin films

Tin oxide is a material that attracts attention due to variety of technological applications. The main parameters that influence its properties are morphology, crystalline structure and stoichiometry. Researchers try to develop nanostructured thin films with tunable parameters that would conform its technological applications. Herein, we report on the preparation and characterization of highly porous SnO2 and Pd-doped SnO2 thin films. These films were deposited in the form of nanorods with controllable geometry. Such morphology was achieved by utilizing glancing angle deposition (GLAD) with assisted magnetron sputtering. This arrangement allowed preparation of slanted pillars, zig-zag structure, vertically standing posts, spiral posts and “bush”-like structures. We calculated that slanted pillars feature the highest surface area among the listed. Then, sets of slanted pillars were deposited and studied in more details. Tin oxide films were thoroughly characterized by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and synchrotron radiation photoelectron spectroscopy (SRPES). The influence of substrate annealing during the deposition and Pd doping on the morphology, crystalline structure and stoichiometry of the films are discussed. GLAD with assisted magnetron sputtering allowed us to deposit broad range of SnO2 nanostructures while annealing of the substrate during deposition affected the films crystallinity. Also, we found out that doping of the SnO2 films with Pd leads to alloy phase formation. These findings can be applied in variety of applications including gas sensing, catalysis, optics and electronics.

Effect of graphene and fabrication technique on the release kinetics of carvacrol from polylactic acid

Porous membranes and thin films containing poly-lactic acid (PLA), carvacrol (CRV) and graphene nanoplatelets (GNP) were fabricated by electrospinning and solvent casting at different formulations.The systems were characterized from a mechanical, morphological, calorimetric and spectroscopic point of view. CRV release as a function of time was studied and a mathematical model was used to fit and interpret the data in order to investigate the release mechanism.The results indicate that the incorporation of GNP generally determined a simultaneous strengthening, stiffening and toughening effect, while preserving a good ductility. Furthermore, integrating GNP allowed tuning the amount and kinetics of CRV release, and proved to reduce the initial burst release effect.

Porous SnO2–CuxO nanocomposite thin film on carbon nanotubes as electrodes for high performance supercapacitors

Metal oxides are promising materials for supercapacitors due to their high theoretical capacitance. However, their poor electrical conductivity is a major challenge. Hybridization with conductive nanostructured carbon-based materials such as carbon nanotubes (CNTs) has been proposed to improve the conductivity and increase the surface area. In this work, CNTs are used as a template for synthesizing porous thin films of SnO2–CuO–Cu2O (SnO2–CuxO) via an electroless deposition technique. Tin, with its high wettability and electrical conductivity, acts as an intermediate layer between copper and the CNTs and provides a strong interaction between them. We also observed that by controlling the interfacial characteristics of CNTs and varying the composition of the electroless bath, the SnO2–CuxO thin film morphology can be easily manipulated. Electrochemical characterizations show that CNT/SnO2–CuxO nanocomposite possesses pseudocapacitive behavior that reaches a specific capacitance of 662 F g−1 and the retention is 94% after 5000 cycles, which outperforms any known copper and tin-based supercapacitors in the literature. This excellent performance is mainly attributed to high specific surface area, small particle size, the synergistic effect of Sn, and conductivity improvement by using CNTs. The combination of CNTs and metal oxides holds promise for supercapacitors with improved performance.

Bilayer Au nanoparticle-decorated WO3 porous thin films: On-chip fabrication and enhanced NO2 gas sensing performances with high selectivity

We present the on-chip fabrication of bilayer Au NP-decorated WO3 nanoporous thin films (B-Au/WO3) via layer-by-layer stacking of a sacrificial colloidal template with periodic Au sputtering deposition. Technical analysis shows that the B-Au/WO3 film is homogeneous and consists of a hexagonally ordered bowl-like structure, whose surface is uniformly decorated with crystalline Au NPs. The B-Au/WO3 sensing film shows a sensitivity of 96.0, a response time of 9.0 s and a recovery time of 16.0 s for 1 ppm NO2 at a low operating temperature of 150 °C. This significantly exceeds performances of bare WO3 counterpart. Also, the B-Au/WO3 sensor demonstrates obviously enhanced sensing responses over operating temperature ranges of 75–225 °C and perfect selectivity to NO2 gas. We proposed a combined sensing mechanism, the surface Au NPs dominated chemical sensitization and inter-bedded Au NP-induced electronic sensitization, to explain the perfect sensing performances in sensitivity, response, and selectivity. The template-mediated approach focuses on precise single-layer control and facilitates direct integration of the sensing film onto the required sensor substrate. The addressed on-chip fabrication strategy along with Au NPs decoration technique might provide promising applications in construction of advanced chemiresistive gas sensors, photocatalysts and electrochromic smart windows.

Vertically aligned porous silica thin films functionalized by nickel chloride incorporated in walls

Abstract The article concerns a novel functional material in the form of thin film: vertically aligned mesoporous silica layer precisely functionalized by nickel chloride molecules. The functional units are placed exclusively inside silica walls, where they are chelated by cyclam (1,4,8,11-tetraazacyclotetradecane) molecules. The synthesis routes, based on a self-assembly approach, resulted in materials with the assumed molecular structure. The microscopic studies of the silica matrices structure confirmed that tetrasilylated cyclam did not perturb the formation of porous silica films. Vibrational spectroscopy showed incorporation of the cyclam inside silica structure. In turn, the EPR spectroscopy confirmed, that nickel dichloride was chelated by cyclam molecules, which resulted in a homogeneous distribution of functional nickel-containing groups.

In-plane orthotropic property characterization of a polymeric battery separator

The in-plane characterization of an orthotropic porous polymer thin film used as separators in lithium-ion batteries was investigated. The mechanical behavior of an orthotropic material is described by the stress-strain relationships in the principal material directions and shear, and the Poisson's ratios. The measurements of the shear properties and the Poisson's ratio for polymer films with a thickness of tens micrometers has not been well established. In this work, these measurements were attempted for a separator of 25 μm thickness. The mechanical testing was performed with a dynamic mechanical analyzer (DMA). Digital image correlation (DIC) was used for strain measurements. The measured major Poisson's ratio was at a constant value whereas the minor Poisson's ratio increased linearly with strain. The symmetry condition for the elastic orthotropic relation was found to be valid up to 1% strain under transverse tension. The shear properties were measured with an off-axis tensile experiment. The shear modulus determined from the DIC strain measurement agreed well with that from the off-axis modulus and elastic constants. Based on the analogy for anisotropy between the elastic and linear viscoelastic domains, the shear creep response was measured with the off-axis tensile creep experiments. The creep compliances in shear and in the principal material directions were determined. This work laid the foundation for the development of orthotropic elastic and viscoelastic models for battery separators.

Effects of PEG1000 and Sol Concentration on the Structural and Optical Properties of Sol⁻Gel ZnO Porous Thin Films.

ZnO porous thin films were synthesized as antireflection coatings via a sol–gel dip-coating method with polyethylene glycol (PEG1000) utilized as a polymeric porogen on alumina transparent ceramics. The pore formation mechanism of the ZnO porous thin films was proposed through thermal and Fourier transformation infrared spectrometer (FTIR) analyses. The effect of sol concentrations on crystal structure, microstructure, and optical properties was also discussed. The experiment results indicated that all the ZnO thin films exhibited a hexagonal wurtzite structure with their preferred orientation along a (0 0 2) plane by X-ray diffraction (XRD) patterns. The grain size of the films increased from 30.5 to 37.4 nm with the sol concentration ranging from 0.2 to 1.0 M. Furthermore, scanning electron microscopy (SEM) images show that the pores on the surface were observed to first decrease as the sol concentration increased and then to disappear as the sol concentration continued to increase. The UV spectrum presents a maximum transmittance of 93.5% at a wavelength of 600 nm at a concentration of 0.6 M, which will be helpful in the practical applications of ZnO porous film on alumina transparent ceramic substrates. The pore formation mechanism of ZnO porous thin films can be ascribed to ring-like network structures between the PEG1000 and zinc oligomers under the phase separation effect.

Nanoporous Films and Nanostructure Arrays Created by Selective Dissolution of Water-Soluble Materials.

Highly porous thin films and nanostructure arrays are created by a simple process of selective dissolution of a water‐soluble material, Sr3Al2O6. Heteroepitaxial nanocomposite films with self‐separated phases of a target material and Sr3Al2O6 are first prepared by physical vapor deposition. NiO, ZnO, and Ni1− xMgxO are used as the target materials. Only the Sr3Al2O6 phase in each nanocomposite film is selectively dissolved by dipping the film in water for 30 s at room temperature. This gentle and fast method minimizes damage to the remaining target materials and side reactions that can generate impurity phases. The morphologies and dimensions of the pores and nanostructures are controlled by the relative wettability of the separated phases on the growth substrates. The supercapacitor properties of the porous NiO films are enhanced compared to plain NiO films. The method can also be used to prepare porous films or nanostructure arrays of other oxides, metals, chalcogenides, and nitrides, as well as films or nanostructures with single‐crystalline, polycrystalline, or amorphous nature.