Synthesis of ordered small pore mesoporous silicates with tailorable pore structures and sizes by polyoxyethylene alkyl amine surfactant

Ordered mesoporous silicates with tailorable pore structures and small pore sizes have been synthesized by using polyoxyethylene alkyl amine surfactant PN-430 [CH₃(CH₂)₁₇N(EO)_x(EO)_y, x + y = 5] as a structure-directing agent under acidic condition. Two-dimensional (2-D) hexagonal (p6mm) mesoporous silicates have been prepared via an evaporation-induced self-assembly (EISA) process. The N₂ sorption isotherms show that the product has a small uniform pore size distribution of 1.8 nm by BJH model, a BET surface area of 730 m²/g and a pore volume of 0.36 cm³/g. 3-D cubic (Pm-3n) mesoporous silicate with small uniform pore size (1.76 nm) can also be prepared at high concentration of PN-430 by EISA method in tetrahydrofuran solvent. The solvothermal post-treatment by n-hexane at 70 °C for 3 d to the above material results in the phase transition of the mesostructure from Pm-3n to P6₃/mmc based on XRD and TEM analyses. In comparison, by using nonionic oligometric alkyl-ethylene oxide surfactant such as Brij 78 (C₁₈H₃₇EO₂₀) or Triton X-100 (CH₃C(CH₃)₂CH₂C(CH₃)₂C₆H₄ EO₁₀) as co-templates, high-quality hexagonal (p6mm) small pore mesoporous silicates have also been prepared in ethanol media. Our results show that the blend templates composed of PN-430 and a small amount of nonionic surfactant can increase the efficiency of organic and inorganic hybrid species assembly, improve the quality of the structural regularity, and decrease the pore size to about 1.65 nm.     

Effect of substrate temperature on microstructure and optical properties of nanocrystalline alumina thin films

Aluminum oxide (Al₂O₃) thin films were deposited on silicon (100) and quartz substrates by pulsed laser deposition (PLD) at an optimized oxygen partial pressure of 3.0×10^?3 mbar in the substrate temperatures range 300–973 K. The films were characterized by X-ray diffraction, transmission electron microscopy, atomic force microscopy, spectroscopic ellipsometry, UV–visible spectroscopy and nanoindentation. The X-ray diffraction studies showed that the films deposited at low substrate temperatures (300–673 K) were amorphous Al₂O₃, whereas those deposited at higher temperatures (≥773 K) were polycrystalline cubic γ-Al₂O₃. The transmission electron microscopy studies of the film prepared at 673 K, showed diffuse ring pattern indicating the amorphous nature of Al₂O₃. The surface morphology of the films was examined by atomic force microscopy showing dense and uniform nanostructures with increased surface roughness from 0.3 to 2.3 nm with increasing substrate temperature. The optical studies were carried out by ellipsometry in the energy range 1.5–5.5 eV and revealed that the refractive index increased from 1.69 to 1.75 (λ=632.8 nm) with increasing substrate temperature. The UV–visible spectroscopy analysis indicated higher transmittance (>80%) for all the films. Nanoindentation studies revealed the hardness values of 20.8 and 24.7 GPa for the films prepared at 300 K and 973 K respectively.     

Influence of oxygen pressure on microstructure and dielectric properties of lead-free BaTi0.85Sn0.15O3 thin films prepared by pulsed laser deposition

Lead-free barium tin titanate BaTi_0.85Sn_0.15O₃ (BTS) ferroelectric thin films have been deposited on Pt/Ti/SiO₂/Si substrates by pulsed laser deposition. The structure and dielectric properties of thin films deposited at various oxygen pressures are investigated systematically. By optimizing the oxygen pressure during the deposition, the structure and dielectric properties are improved. The thin films grown at 15 Pa have the best crystal quality and the largest grain size, which result in the enhancement of the dielectric properties. The dielectric constant and loss tangent show the similar trend in the entire oxygen pressure range. The influence mechanisms of the oxygen pressure on the structure and dielectric properties are proposed. The BTS thin films deposited at 15 Pa with large figure of merit (FOM) of 81.1, high tunability of 72.1%, moderate dielectric constant of 341, low loss tangent of 0.009 are considered to be appropriate as a field tunable ferroelectric material for electrically tunable devices.     

Revealing the high sensitivity in the metal toinsulator transition properties of the pulsed laser deposited VO2 thin films

Vanadium dioxide (VO₂) is known as a typical 3d-orbital transition metal oxide exhibiting the metal-to-insulator-transition (MIT) property near room temperature. However, their electronic applications have been challenged by the quality and uniformity of VO₂ thin films. In this work, we demonstrate the high sensitivity in the valence charge of vanadium and the MIT properties of the VO₂ thin films to the deposition temperature. This observation indicates the necessity to eliminate the inhomogeneity in the temperature distribution of substrate during the vacuum-deposition process of VO₂. In addition, a high thermoelectric power factor (PF, e.g., exceeding 1 μWcm⁻¹K⁻²) was achieved in the metallic phase of the VO₂ thin films and this value is comparable to typical organic or oxide thermoelectric materials. We believe this high PF enriches the potential functionality in thermoelectric energy conversions beyond the existing electronic applications of the current vacuum-grown VO₂ thin films.     

Dispersive optical constants and electrical properties of nanocrystalline CuInS2 thin films prepared by chemical spray pyrolysis

Nanocrystalline CuInS₂ thin films were deposited on borosilicate glass substrates via chemical spray pyrolysis method. The structural, morphological, optical, and electrical properties were studied as a function of increasing annealing temperature from 250 to 350 ̊C. XRD analysis showed mixed phases at lower temperatures with the preferred orientation shifting towards the (112) chalcopyrite CuInS₂ plane at higher substrate temperature. The crystallite size increased slightly between 13 and 18 nm with increase in annealing temperature. The optical band gap was determined on basis of Tauc extrapolation method and the Wemple–Di-Domenico single oscillator model. Possible structural and quantum confinement effect may have resulted in relatively larger band gaps of 1.67–2.04 eV, relative to the bulk value of 1.5 eV. The presence of Cu_xS in the as-deposited and wurtzite peaks after annealing at 350 ^̊C play a role in influencing the optical and electrical properties of CuInS₂ thin films.     

Atmospheric flame vapor deposition of WO3 thin films for hydrogen detection with enhanced sensing characteristics

Nowadays, with the increasing demand for hydrogen, sensors that can detect low concentrations of this gas are essential for its safe use. In this paper, Pd/WO₃ film hydrogen sensors are developed using a solid-feed flame vapor deposition (SF-FVD), as an atmospheric, economical, and fast film fabrication method. The crystal structure and morphology of the samples were characterized by different means. The performance of the obtained sensors was investigated for different hydrogen concentrations (1–2500 ppm) and at different operating temperatures (100–250 °C). We attempted to determine the optimum deposition conditions, including feed and substrate to flame nozzle distances. In most of the sensing conditions, the response and recovery times were measured in the order of 20 to 30 s. The layer with a more open morphology showed sensitivity at ppb hydrogen level, good stability, and selectivity. The response behavior of the samples was explained according to the power-law in the metal oxide semiconductor (MOS) gas sensors.     

New insights into synthesis of nanocrystalline hexagonal BN

Uncovering the mechanism behind nanocrystalline hexagonal boron nitride (h-BN) formation at relatively low temperatures is of great scientific and practical interest. Herein, the sequence of phase transformations occurring during the interaction of boric acid with ammonia in a temperature range of 25–1000 °C has been studied in detail by means of thermo-gravimetric analysis, X-ray diffraction, infrared spectroscopy, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy. The results indicate that at room temperature boric acid reacts with ammonia to form an ammonium borate hydrate (NH₄)₂B₄O₇x4H₂O. Its interaction with ammonia upon further heating at 550 °C for 1 h leads to the formation of turbostratic BN. Nanocrystalline h-BN is obtained either during heating in ammonia at 550 °C for 24 h or at 1000 °C for 1 h. This result is important for the development of novel cost-effective and scalable syntheses of h-BN nanostructures, such as nanosheets, nanoparticles, nanofibers, and nanofilms, as well as for sintering h-BN ceramic materials.     

Simple and rapid synthesis of NiO/PPy thin films with improved electrochromic performance

Nickel oxide/polypyrrole (NiO/PPy) thin films were deposited by a two step process in which the NiO layer was electrodeposited potentiostatically from an aqueous solution of NiCl₂·6H₂O at pH 7.5 on fluorine doped tin oxide (FTO) coated conducting glass substrates, followed by the deposition of polypyrrole (PPy) thin films by chemical bath deposition (CBD) from pyrrole mixed with ammonium persulfate (APS). The NiO/PPy films were further characterized for their structural, optical, morphological and electrochromic properties. X-ray diffraction study indicates that the films composed of polycrystalline NiO and amorphous PPy. Infrared transmission spectrum reveals chemical bonding between NiO and PPy. Rectangular faceted grains were observed from scanning electron microscopy results. The electrochromic (EC) property of the film was studied using cyclic voltammogram (CV), chronoamperometry (CA) and optical modulation. The NiO/PPy presents superior EC properties than their individual counterparts. The coloration/bleaching kinetics (response time of few ms) and coloration efficiency (358 cm²/C) were found to be improved appreciably. The dramatic improvement in electrochemical stability (from about 500 c/b cycles for PPy to 10,000 c/b cycles for NiO/PPy) was observed. This work therefore demonstrates a cost-effective and simple way of depositing highly efficient, faster and stable NiO/PPy electrodes for EC devices.     

Preparation and characterization of ordered thin films based on aromatic poly(1,3,4-oxadiazole)s

Ultrathin layers of aromatic polyoxadiazoles by using the Langmuir-Blodgett technique are prepared for the first time. The syntheses and characterization of new soluble aromatic poly(1,3,4-oxadiazole)s are described. The polyoxadiazoles contain tetraphenyl silane units in the main chain or pendent alkylamido groups. Both the precursor route via polyhydrazide followed by thermal cyclization in bulk, and the direct spreading of poly(1,3,4-oxadiazole)s are used for film forming. The supramolecular structures of all ordered poly(arylene-1,3,4-oxadiazole) LB films are characterized by FTIR spectroscopy, X-ray scattering and atomic force microscopy, respectively.     

Chemical solution deposition of epitaxial indium- and aluminum-doped Ga2O3 thin films on sapphire with tunable bandgaps

Compared to the vacuum-required deposition techniques, the chemical solution deposition (CSD) technique is superior in terms of low cost and ease of cation adjustment and upscaling. In this work, highly epitaxial indium- and aluminum-doped Ga₂O₃ thin films are deposited using a novel CSD technique. The 2θ, rocking curve, and φ-scan modes of x-ray diffraction (XRD) measurements and high-resolution transmission electron microscopy suggest that these thin films have a pure beta phase with good in- and out-of-plane crystallization qualities. The effect of incorporating indium and aluminum into the crystallization process is studied using high-temperature in situ XRD measurements. The results indicate that indium and aluminum doping can shift the crystallization of the thin films to lower and higher temperatures, respectively. Additionally, ultraviolet-visible spectroscopy measurements indicate that the bandgap of the sintered thin films can be tuned from 4.05 to 5.03 eV using a mixed precursor solution of In:Ga = 3:7 and Al:Ga = 3:7. The photodetectors based on the (InGa)₂O₃, pure Ga₂O₃, and (AlGa)₂O₃ samples exhibit the maximum photocurrents at 280, 255, and 230 nm, respectively. The results suggest that the described CSD technique is promising for producing high-quality bandgap tunable deep-ultraviolet photoelectrical and high-power devices.     

Improving optical and electrical performances of aluminum-doped zinc oxide thin films with laser-etched grating structures

A laser etching method was performed to achieve the dual purpose of fabricating grating structures and laser annealing on aluminum-doped zinc oxide (AZO) thin films, and thus improve the film photoelectric performances. Different line spacings and laser fluences were adopted to systematically explore the optimal laser etching condition. Too narrow line spacings or too high laser fluences led to light reflections at the grating external surface to cause more light dissipation, and too wide line spacings or too low laser fluences resulted in relatively small total grating lateral areas being detrimental to multiple internal light reflections. Moreover, too narrow line spacings brought about laser-caused lattice disorder and too high laser fluences produced laser-ablated spots or overburned traces. Therefore, using the medium line spacing and laser fluence, e.g. 40 μm and 0.6 J/cm² in the present work, was more suitable for synchronously realizing grating structure fabrication and laser annealing. The corresponding AZO film exhibited the maximum figure of merit of 2.89 × 10^?2 Ω^?1, which was 1.6 times that of the untreated AZO film. This study is expected to expand performance improvement methods of TCO films and promote the application of laser-etched grating structures.     

Self-growth of nanocrystalline structures for amorphous Sr0.925Bi0.05TiO3 thin films with high energy density

Process of self-growth nanocrystalline structure was explored to improve the dielectric properties of amorphous Sr_0.925Bi_0.05TiO₃ (SBT) thin films through controlling the annealing temperature. The crystallinity of the material was 15% when annealed at 550?°C, and the resulting nanocrystalline particles were 14?nm in size as determined by XRD analysis. Therefore, the proposed process could produce a novel film of an amorphous matrix coating nanocrystalline particles. Finite element analysis results showed that the applied electric field was focused primarily in the amorphous matrix, which could effectively decrease the probability of low voltage breakdown of the nanocrystalline particles. Simultaneously, the nanocrystalline particles supplied more polarization charges, which helped to improve the dielectric constant of the inorganic composite. Combining the merits of amorphous and crystalline phases, the ultimate energy storage density of the modified sample was as high as 21.2?J/cm³, which represents an improvement of 5.1?J/cm³ compared with that of pure amorphous SBT thin film.     

Facile synthesis of thin coating C/ZnO composites with strong electromagnetic wave absorption

C/ZnO composites with increased electromagnetic (EM) wave absorbing features have been synthesized through a simple one-pot hydrothermal process and subsequent high temperature carbonization under the protection of argon. The results depict that the maximum absorption of C/ZnO composites synthesized with the optimal molar ratio of zinc acetate to glucose is ?50.43?dB at 15.77?GHz. The 1.16-mm-thick coating shows a wide effective absorption bandwidth (3.52?GHz) of EM wave (RL≤?10?dB). The thin coating thickness of the C/ZnO composites is desirable for decreasing the absorber weight in EM wave absorption. And there are no other reagents used throughout the synthesis process except for the green glucose and zinc acetate. Thus, C/ZnO composites would be highly promising lightweight EM wave absorbing materials.     

Effect of sputtering pressure on structural and dielectric tunable properties of BaSn0.15Ti0.85O3 thin films grown by magnetron sputtering

Lead?free ferroelectric BaSn_0.15Ti_0.85O₃ (BTS) thin films are grown on Pt-coated Si substrates by magnetron sputtering at 650?°C, the effect of sputtering pressure on the microstructural, surface morphological, dielectric properties and leakage characteristic is systematically investigated. XRD analysis shows the crystallinity of BTS thin films with perovskite structure can be improved by appropriate control of the sputtering pressure. The surface morphology analyses reveal that grain size and roughness can be affected by sputtering pressure. The BTS thin films prepared at sputtering pressure of 3.0?Pa exhibit a low dispersion parameter of 0.006, a medium dielectric constant of ~357, a high dielectric tunability of 65.7%@?400?kV/cm and a low loss tangent of 0.0084?@?400?kV/cm. Calculation of figure of merit (FOM) displays a high value of 84.1, and the measurement of leak current shows a very low value of 4.39?×?10^–7 A/cm² at 400?kV/cm. The results indicate that BTS thin film deposited sputtering pressure of 3.0?Pa is an excellent candidate for electrically steerable applications     

Ferroelectric and dielectric properties of manganese-doped Na0.5Bi0.5TiO3 nanocrystalline thin films prepared by metal organic decomposition: A single-layer thickness-dependent study

2 at% Manganese-doped Na_0.5Bi_0.5TiO₃ (NBTMn) thin films with single-layer thicknesses ranging from 15 to 45 nm/l were deposited on the indium tin oxide/glass substrates by a metal organic decomposition process and spin coating technique. The influence of single-layer thickness on the crystal structure, surface morphology, insulating ability, ferroelectric and dielectric properties was mainly investigated. Compared with the other films, NBTMn film with a single-layer thickness of 30 nm/l exhibits the (110)-preferred orientation and dense structure. Also, it shows the enhanced ferroelectricity with a large remanent polarization (P_r) of 38 μC/cm² due to the preferred orientation and low leakage current density. Meanwhile, a high dielectric tunability of 39% for NBTMn with 30 nm/l can be observed by varying the measuring applied voltage and frequency. These results indicate that the suitable layer thickness is beneficial to improve the electrical performances of NBTMn thin film.     

Stereolithography additive manufacturing of multi-ceramic triangle structures with tunable thermal expansion

Ceramics and their structures with tunable thermal expansion are deemed necessary to develop. In this study, multi-ceramic structures with tunable thermal expansion, including negative thermal expansion (NTE), zero thermal expansion (ZTE), and positive thermal expansion (PTE), were developed for the first time. Firstly, multi-ceramic triangle structures with tunable thermal expansion were designed based on ZrO₂ and Al₂O₃ ceramics. Then, multi-ceramic triangle structures were fabricated by using the stereolithography additive manufacturing technology. Functionally gradient ceramic (FGC) layers were applied for releasing the thermal mismatch stress among the structure. Finally, the thermal expansion behavior of the multi-ceramic triangle structures were characterized and analyzed by using a homemade thermal expansion testing system. The proposed design, fabrication and characterization methods of multi-ceramic structures with tunable thermal expansion are believed to promote the engineering application.     

Solution-based synthesis of dense,large grained CuIn(S,Se)2 thin films using elemental precursor

Compared with the expensive and complicated vacuum techniques, the solution-based process to deposit I-III-VI₂ chalcogenide thin films (I=Cu, III=In or Ga, VI=S or Se) has attracted great interests due to its lower cost, higher scalable production and better application in flexible substrate. Herein, a low-toxic and high-active mixture solvent comprised of 1, 2-ethanedithiol and 1,2-ethylenediamine is utilized to dissolve elemental Cu, In and S powders at 60 °C, forming the CuInS₂ (CIS) precursor solution. After spin coating and annealing in a both Ar gas and selenium atmosphere, a dense and large-grained chalcopyrite CuIn(S,Se)₂ (CISSe) thin films with a close-packed grain size of ~800 nm are prepared, eliminating a undesired fine fine-grained bottom layer. In addition, the selenization temperature of the CISSe thin films is also discussed, which influences the phase composition, crystallinity and morphology of CISSe thin films. Photovoltaic device of the CISSe-based thin films is fabricated, obtaining a power conversion efficiency of 6.2% with an active cell area of 0.5 cm² under AM 1.5 illumination.     

Correlation between chain conformations and optical anisotropy of thin films of an organo-soluble polyimide

An organo-soluble polyimide was successfully synthesized by two step polycondensation accompanied with chemical imidization. Optical anisotropy of thin films was detected by a prism-coupler technique. The results showed that the optical anisotropic properties of thin films prepared from solutions in different solvents depend on the solution properties. It is concluded that the more expanded the chain conformation in solution, the larger the negative birefringence of thin films. © 1997 Elsevier Science Ltd.     

Reduction in thermal conductivity of Bi thin films with high-density ordered nanoscopic pores

We prepared two-dimensional Bi thin films with high-density ordered nanoscopic pores by e-beam evaporation of Bi metal. For this structure, we used polystyrene beads ranging from 200 to 750 nm in diameter as an etch mask. The typical hole and neck sizes of the Bi thin films with approximately 50 nm in thickness on SiO₂/Si substrates were in the range of 135 to 490 nm and 65 to 260 nm, respectively. By measuring the thermal characteristics through a 3ω technique, we found that the thermal conductivities of nanoporous Bi thin films are greatly suppressed compared with those of corresponding bulk materials. With a decrease in pore size to approximately 135 nm, the thermal conductivity decreased significantly to approximately 0.46 W/m·K at 300 K.     

Impact of Sr-doping on structural and electrical properties of Pb(Zr0.52Ti0.48)O3 thin films on RuO2 electrodes

In this paper, we investigated the impact of Sr-doping on the structural properties and electrical characteristics of lead zirconate titanate [Pb(Zr_0.52Ti_0.48)O₃, PZT] thin films deposited on RuO₂ electrodes by a sol-gel process and spin-coating technique. We used X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy, and field-emission transmission electron microscopy to explore the structural, morphological, chemical, and microstructural features, respectively, of these films as a function of the growth condition (strontium doping concentrations varied from 1, 3, and 5 mol%). The PZT thin film processed at the 3 mol% Sr exhibited the best electrical characteristics, including a low leakage current of 2.27×10⁻⁷ A/cm² at an electric field of 50 kV/cm, a large capacitance value of 2.74 μF/cm² at a frequency of 10 kHz, and a high remanent polarization of 37.95 μC/cm² at a frequency of 5 kHz. We attribute this behavior to the optimal amount of strontium in the PZT film forming a perovskite structure and a thicker interfacial layer at the PSZT film-RuO₂ electrode interface.