Layer-by-layer self-assembly of dye-polyoxometalate multilayer composite films and their fluorescent properties

The layer-by-layer (LbL) self-assembly technique was successfully applied to the fabrication of dye-polyoxometalate multilayer composite films consisting of two dye molecules Rhodamine B (RB) and Rhodamine 6G (R6G) and a Keggin-type polyoxometalate [α-SiW₁₂O₄₀]⁴⁻ (α-SiW₁₂). The composite films were characterized by UV-vis spectroscopy, scanning electron microscopy (SEM), and fluorescence spectroscopy. UV-vis spectra show that the characteristic absorbance values of the multilayer films increase almost linearly with the number of dye/α-SiW₁₂ bilayers, suggesting that the deposition process is regular and highly reproducible from layer to layer. SEM micrographs indicate that the film surface is a little rough with some individual granular domains. In addition, the fluorescent properties of these composite films were also investigated by fluorescence spectroscopy.     

Fabrication of a 12-tungstophosphate and cadmium oxide composite film and its properties

A multilayer composite film of the 12-tungstophosphate H₃[PW₁₂O₄₀]³⁻ (PW₁₂) and cadmium oxide nanoparticles (CdO) was fabricated on quartz and silicon by the layer-by-layer (LBL) self-assembly method. The film was characterized by UV–vis spectroscopy, atomic force microscopy (AFM) and luminescence spectra. The proposed composite film exhibits higher photocatalytic activity toward methyl orange (MO) solution at pH 3.5, compared to single PW₁₂ and CdO films. The degradation rate was affected by initial concentration of PW₁₂, pH value of MO solution, inorganic ions concentration and type in MO solution. In addition, the composite film displays luminescent property and reversible electrochromic property with fast response time.     

Biopolymer-manganese oxide nanoflake nanocomposite films fabricated by electrostatic layer-by-layer assembly

Bio-nanocomposite films based on chitosan and manganese oxide nanoflake have been fabricated via the layer-by-layer (LBL) self-assembly technique. UV–vis absorption spectra showed that the subsequent growth of the nanocomposite film was regular and highly reproducible from layer to layer. X-ray photoelectron spectroscopy (XPS) spectra confirmed the incorporation of chitosan and manganese oxide nanoflake into the films. Scanning electron microscopy (SEM) images revealed that the nanocomposite film had a continuous surface and a layered structure. A sensitive hydrogen peroxide (H₂O₂) amperometric sensor was fabricated with the chitosan–manganese oxide nanoflake nanocompoite film. The sensor showed a rapid and linear response to H₂O₂ over the range from 2.5 × 10^? 6 to 1.05 × 10^? 3 M, with a sensitivity of 0.038 A M^? 1 cm^? 2.     

Growth of ZnO thin films at low temperature by plasma-enhanced atomic layer deposition using H2O and O2 plasma oxidants

Zinc oxide (ZnO) thin films were grown at 70 °C by plasma-enhanced atomic layer deposition using H₂O and O₂ plasmas. Plasma oxidants were used in order to improve the ZnO crystallinity and optoelectronic properties, avoiding high-temperature synthesis. The deposition parameters were optimized to achieve saturation in each reaction step. X-ray photoelectron spectroscopy (XPS) reveals high purity of the obtained ZnO films. X-ray diffraction (XRD) measurements indicate that the grown layers are polycrystalline and that the H₂O plasma synthesis leads to better crystallinity than the O₂ plasma as inferred from the intensity of the (100) and (002) peaks. The films are with high optical transmission, ~90%, as inferred from UV–visible (UV–Vis) transmittance measurements, and optical band gaps of 3.22 and 3.23 eV for H₂O and O₂ plasma, respectively. Atomic force microscopy (AFM) indicates that the films are smooth, with an average roughness of ~?0.22 nm. The growth rate was found to be in the range of 1.2–1.4 Å/cycle. The XPS, XRD, UV–Vis, and AFM results prove the possibility to obtain high-quality ZnO films by O₂ and H₂O plasma processes at 70 °C with chemical, structural, and optical properties promising for flexible electronics. ZnO films were successfully deposited on polyethylene terephthalate substrates using the optimal conditions for H₂O plasma process. No damage of the film surface or substrate was observed.

     

Electrostatic immobilization of polyoxometallates on silicon: X-ray Photoelectron Spectroscopy and electrochemical studies

Keggin-type dodecatungstosilicates polyoxometallates (POMs) ([SiW₁₂O₄₀]⁴⁻) were immobilized in a straightforward manner by electrostatic interactions on ammonium layers covalently grafted on silicon. This method does not require any POM modification synthetical steps. The presence of [SiW₁₂O₄₀]⁴⁻ on the surface is demonstrated by X-ray Photoelectron Spectroscopy from a specific modification of the tungsten 4f_7/2 signal. Moreover the surface coverage of [SiW₁₂O₄₀]⁴⁻ has been improved by 35% upon changing the nature of the anchoring ammonium groups from protonated to methylated amino groups. The organic-inorganic composite films have also been characterized by cyclic voltammetry showing that POMs have a specific behavior on silicon surfaces. In addition the use of a polyallylamine capping layer proved to stabilize efficiently the POM electrochemical response.     

Comparative study of photoinduced wettability conversion between [PW12O40]/brookite and [SiW12O40]/brookite hybrid films

Two tungsten-based Keggin-type heteropolyacids (PW₁₂: ([PW₁₂O₄₀]³⁻) and SiW₁₂: ([SiW₁₂O₄₀]⁴⁻)) were hybridized with brookite-type TiO₂. Then photocatalytic decomposition activity, photoinduced hydrophilicity, and sustainability of the hydrophilicity in the dark were evaluated using gaseous 2-propanol (IPA) decomposition and sessile drop method. The obtained films were transparent in the visible wavelength range. Both hybrid films exhibited higher photocatalytic decomposition activity and had higher photoinduced hydrophilicizing rates than pure brookite films under UV illumination. The PW₁₂/TiO₂ film exhibited better photocatalytic performance than the SiW₁₂/TiO₂ film did. Atmosphere dependence, XPS analysis, and electrochemical experiments indicated the cause of these two films' different levels of sustainability of hydrophilicity to be differences in their electron storage capability. Results show that the electron scavenger capability and reoxidation efficiency of the heteropolyacid are key factors affecting the overall performance of wettability conversion of this hybrid film system before and after UV illumination.     

Fabrication,characterization and electrochemistry of organic–inorganic multilayer films containing polyoxometalate and polyviologen via layer-by-layer self-assembly

The novel organic–inorganic multilayer films containing poly(butanyl viologen) (abbreviated as PBV) and phosphomolybdic acid (H₃PMo₁₂O₄₀, abbreviated as PMo₁₂) have been fabricated on quartz slides, silicon wafers and glassy carbon electrode by the layer-by-layer self-assembly technique. The highly ordered multilayer films were characterized by the UV–visible spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray reflectometry (XRR), Atomic force microscopy (AFM) and cyclic voltammetry. UV–visible spectra revealed that the growth of the films for each deposition cycle was reproducible. FT-IR and XPS spectra confirmed the incorporation of PBV and PMo₁₂ into the multilayer films. XRR revealed the film thickness at nanoscale, and AFM showed film surface with uniform and smooth morphology. In addition, the electrochemical behavior of the multilayer films at room temperature was investigated. As a result, the films presented good electrocatalytic activity toward ₃BrO⁻${{\text{BrO}}_3}^{-}$, H₂O₂ and ₂NO⁻${{\text{NO}}_2}^{-}$, providing valuable information for exploring the potential applications in ₃BrO⁻${{\text{BrO}}_3}^{-}$ sensors.     

Novel Semiconductive Ternary Hybrid Heterostructures for Artificial Optoelectronic Synapses

Synaptic devices that mimic biological synapses are considered as promising candidates for brain-inspired devices, offering the functionalities in neuromorphic computing. However, modulation of emerging optoelectronic synaptic devices has rarely been reported. Herein, a semiconductive ternary hybrid heterostructure is prepared with a D-D’-A configuration by introducing polyoxometalate (POM) as an additional electroactive donor (D’) into a metalloviologen-based D-A framework. The obtained material features an unprecedented porous 8-connected bcu -net that accommodates nanoscale [α-SiW₁₂O₄₀]⁴⁻ counterions, displaying uncommon optoelectronic responses. Besides, the fabricated synaptic device based on this material can achieve dual-modulation of synaptic plasticity due to the synergetic effect of electron reservoir POM and photoinduced electron transfer. And it can successfully simulate learning and memory processes similar to those in biological systems. The result provides a facile and effective strategy to customize multi-modality artificial synapses in the field of crystal engineering, which opens a new direction for developing high-performance neuromorphic devices.     

Self-assembly of ultrathin composite TiO2/polymer films

A wet layer-by-layer self-assembly of composite TiO₂/polymer films on Si and Al₂O₃/Al, substrates has been studied by AFM, STM, and ellipsometry techniques. The quality of the first adsorbed TiO₂ layer has been found to be the governing factor in multilayer film growth. The first layer consists of single particles and particle agglomerates 30–120 nm wide The surface coverage in the layer is determined by the chemical composition of the substrate surface and water pH in post-adsorption rinsing procedure. Well-packed TiO₂/polymer film completely covering the surface has been prepared in five adsorption cycle on Al₂O₃/Al substrate. The film remained crack-free after heat treatment at 300°C. I–V curves measurement reveals high resistivity (R∼10¹⁰ O in the voltage range from −2 to +3 V) of TiO₂/polymer films prepared in ten adsorption cycles.     

Electrochemical behaviour of self-assembly multilayer films based on iron-substituted α-Keggin polyoxotungstates

Ultrathin multilayer films containing metal-substituted polyoxometalates, [PW₁₁Fe^III(H₂O)O₃₉]⁴⁻ (PW₁₁Fe) or [SiW₁₁Fe^III(H₂O)O₃₉]⁵⁻ (SiW₁₁Fe), and poly(ethylenimine) (PEI) were prepared by the electrostatic layer-by-layer self-assembly method on a glassy carbon electrode. The multilayer films were characterized by cyclic voltammetry and scanning electron microscopy and UV-Vis absorption spectroscopy on a quartz slide was used to monitor film growth. Cyclic voltammetry indicates that the electrochemical properties of the polyoxometalates are completely maintained in the multilayer films, and the influence of scan rate on the voltammetric features showed that the first tungsten reduction process for immobilized PW₁₁Fe and SiW₁₁Fe is a surface-confined process. Studies with [Fe(CN₆)]^3−/4− as electrochemical probe showed that their permeability depends on the thickness of the multilayer films, if the outermost layer is negatively charged. Additionally, the (PEI/SiW₁₁Fe)_n multilayer films showed electrocatalytic properties towards nitrite reduction.     

Effect of annealing time on the structural,optical and electrical characteristics of DC sputtered ITO thin films

Using an Indium tin oxide (ITO) ceramic target (In₂O₃:SnO₂, 90:10 wt%), ITO thin films were deposited by conventional direct current magnetron sputtering technique onto glass substrates at room temperature. The obtained ITO films were annealed at 400 °C for different annealing times (1, 2, 5, 7, and 9 h). The effect of annealing time on their structural, optical and electrical properties was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microcopy (AFM), ultra violet–visible (UV–Vis) spectrometer, and temperature dependence Hall measurements. XRD data of obtained ITO films reveal that the films were polycrystalline with cubic structure and exhibit (222), (400) and (440) crystallographic planes of In₂O₃. AFM and Scanning Electron Microscopy SEM have been used to probe the surface roughness and the morphology of the films. The refractive index (n), thickness and porosity (%) of the films were evaluated from transmittance spectra obtained in the range 350–700 nm by UV–Vis. The optical band gap of ITO film was found to be varying from 3.35 to 3.47 eV with the annealing time. The annealing time dependence of resistivity, carrier concentration, carrier mobility, sheet resistance, and figure of merit values of the films at room temperature were discussed. The carrier concentration of the films increased from 1.21 × 10²⁰ to 1.90 × 10²⁰ cm^?3, the Hall mobility increased from 11.38 to 18 cm² V^?1 s^?1 and electrical resistivity decreased from 3.97 × 10^?3 to 2.13 × 10^?3 Ω cm with the increase of annealing time from 1 to 9 h. Additionally, the temperature dependence of the carrier concentration, and carrier mobility for the as-deposited and 400 °C annealed ITO films for 2 and 9 h were analysed in the temperature range of 80–350 K.     

Preparation and characterization of polyoxometalate-Ag nanoparticles composite multilayer films

The multifunctional thin films (BW₁₂/Ag NPs)_n (BW₁₂ = BW₁₂O₄₀, NPs = nanoparticles) were prepared by layer-by-layer self-assembly method. The (BW₁₂/PEI-Ag⁺)_n (PEI = polyethylenimine) composite films were achieved through alternately depositing anionic BW₁₂ and cationic PEI-Ag⁺ complex. The deposition process of (BW₁₂/PEI-Ag⁺)₁₀ multilayer is linear layer-by-layer self-assembly. Under UV irradiation, Ag ions in (BW₁₂/PEI-Ag⁺)_n multilayer films were reduced photochemically into Ag NPs and (BW₁₂/Ag NPs)₁₀ films were obtained. Through UV-vis measurements, the presence of surface plasma absorption peak at 445 nm demonstrated the formation of silver NPs. The electrochemical and antibacterial activities of (BW₁₂/Ag NPs)_n films were investigated. The electrochemical results indicate that the glassy carbon electrode modified with (BW₁₂/Ag NP)_n film exhibits the electroreduction toward O₂. Moreover, the (BW₁₂/Ag NP)₁₀ multilayer films exhibit long-lasting antibacterial properties toward Escherichia coli (E. coli).     

PPy/α-Fe2O3 hybrid nanocomposites: effect of CSA doping on structural, morphological, optical and electrical transport properties

Hybrid nanocomposites of camphor sulfonic acid (CSA) doped organic polypyrrole and inorganic alpha-ferric oxide (PPy/α-Fe₂O₃) have been successfully prepared using different weight percentages of CSA (10–50 %) dispersed in PPy/α-Fe₂O₃ hybrid nanocomposite by solid state synthesis method. These hybrid nanocomposites are characterized by using various techniques such as X-ray diffraction (XRD), fourier transform infra red (FTIR), scanning electron microscopy (SEM), atomic force microscopy (AFM), UV–visible spectroscopy and two probe techniques respectively. The XRD spectra revealed that the addition of CSA has no effect on crystallinity of PPy/α-Fe₂O₃ hybrid nanocomposite. The FTIR results show that, the characteristic peaks of PPy/α-Fe₂O₃ hybrid nanocomposite shift to higher wave number after addition of CSA in the PPy/α-Fe₂O₃ nanocomposites indicates some chemical interactions and better conjugation between CSA and PPy/α-Fe₂O₃ hybrid nanocomposites. SEM studies revealed that strong effect on morphology of PPy/α-Fe₂O₃ nanocomposite. The AFM analysis show uniform nano porous granular morphology. UV–visible spectroscopy studies have provided insight into the electronic interaction between the PPy, α-Fe₂O₃ and CSA. DC electrical conductivity showed a steeply increase in electrical conductivity of PPy/α-Fe₂O₃ nanocomposites with increase in amount of CSA from 10 to 50 %.     

Influence of annealing on the structure and stoichiometry of europium-doped titanium dioxide thin films

This work presents the influence of annealing on the structure and stoichiometry of europium (Eu)-doped titanium dioxide (TiO₂). Thin films were fabricated by magnetron sputtering from a metallic Ti-Eu target in oxygen atmosphere and deposited on silicon and SiO₂ substrates. After deposition the selected samples were additionally annealed in air up to 1070 K.Film properties were examined by means of X-ray diffraction (XRD), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) and the results were analyzed together with the undoped TiO₂ thin films prepared under similar technological conditions.XRD results showed that depending on the Eu content, as-deposited thin films consisted of the TiO₂-anatase or TiO₂-rutile.An additional annealing will result in the growth of anatase crystals up to 35 nm, but anatase to rutile phase transformation has not been recorded. AFM images display high quality and a dense nanocrystalline structure. From the XPS Ti2p spectra the 4+oxidation state of Ti was confirmed. The O1s XPS spectra displayed the presence of an O²⁻ photoelectron peak accompanied by an additional broader peak that originates from hydroxyl species chemisorbed at the sample surface. It has been found that Eu dopant increases the OH⁻ content on the surface of prepared TiO₂:Eu thin films. The calculated O/Ti ratio was in the range of 1.85-2.04 depending on the sample.     

Photocatalytic degradation of gaseous toluene over hollow “spindle-like” α-Fe2O3 loaded with Ag

In this work, hollow “spindle-like” α-Fe₂O₃ nanoparticles were synthesized by a hydrothermal route. The Ag/α-Fe₂O₃ catalyst was prepared based on the spindle-shaped α-Fe₂O₃ with CTAB as the surfactant, which showed excellent photoelectric property and photocatalytic activity. The structural properties of these samples were systematically investigated by X-ray powder diffraction, scanning electronic microscopy, transmission electronic microscopy, energy-dispersive X-ray spectra, and UV–Vis diffuse reflectance spectroscopy techniques. The photo-induced charge separation in the samples was demonstrated by surface photovoltage measurement. The photocatalytic performances of the Ag/α-Fe₂O₃ and α-Fe₂O₃ samples were comparatively studied in the degradation of toluene under xenon lamp irradiation by in situ FTIR spectroscopy. Benzaldehyde and benzoic acid species could be observed on the α-Fe₂O₃ surface rather than Ag/α-Fe₂O₃ surface. The results indicate that the Ag/α-Fe₂O₃ sample exhibited higher photocatalytic efficiency.     

Modulation of the structural and optical properties of sputtering-derived HfO2 films by deposition power

High-k gate dielectric HfO₂ thin films have been deposited on Si and quartz substrate by radio frequency magnetron sputtering. The structural and optical properties of HfO₂ thin films related to deposition power are investigated by X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM), ultraviolet–visible spectroscopy (UV–Vis), and spectroscopic ellipsometry (SE). Results confirmed by XRD have shown that the as-deposited HfO₂ thin films are not amorphous state but in monoclinic phase, regardless of deposition power. Analysis from FTIR indicates that an interfacial layer has been formed between the Si substrate and the HfO₂ thin film during deposition. AFM measurements illustrate that the root mean square (RMS) of the as-deposited HfO₂ thin films’ surface demonstrates an apparent reduction with the increase of deposition. Combined with UV–Vis and SE measurements, it can be noted reduction in band gap with an increase in power has been observed. Additionally, increase in refractive index (n) has been confirmed by SE.     

Synthesis and characterization of porous platelet-shaped α-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> with enhanced photocatalytic activity for 17α-ethynylestradiol

The porous platelet-shaped α-Bi₂O₃ photocatalyst was successfully synthesized by a novel hydrothermal–calcination method assisted with ethylenediamine and polyvinylpyrrolidone. The physical and chemical properties of α-Bi₂O₃ photocatalyst were characterized based on XRD, XPS, SEM, TEM, EDS, UV–Vis DRS, and PL techniques. The influence of preparation conditions on the formation of α-Bi₂O₃ photocatalyst was investigated, and the effect of catalyst dosage and pH value on the EE2 removal rate was also investigated. The synthesized porous platelet-shaped α-Bi₂O₃ photocatalyst exhibited excellent photocatalytic activity for 17α-ethynylestradiol (EE2), and 97.8% of EE2 was removed after 75 min of visible light irradiation using α-Bi₂O₃ as photocatalyst. The reaction rate constant over the porous platelet-shaped α-Bi₂O₃ photocatalyst was 11.6 and 11.4 times of that of traditional α-Bi₂O₃ and N-TiO₂, respectively. The possible photocatalytic mechanism has been discussed on the basis of the theoretical calculation and the experimental results. The porous platelet-shaped α-Bi₂O₃ was a stable and efficient photocatalyst, proving that it is a promising photocatalyst.     

CdS annealing treatments in various atmospheres and effects on performances of CdTe/CdS solar cells

In this work, a systematic research on CdS annealing treatments under various atmospheres had been done to understand their effects on CdS/CdTe solar cells. CdS films were prepared by a standard CBD method and annealed under various atmospheres, including Ar, Ar+H₂, O₂, Ar+S and Ar+CdCl₂. Morphological, structural, optical and chemical properties were investigated using Atom force microscope (AFM), X-ray diffraction (XRD), UV–VIS spectroscopy and X-ray photoelectron spectroscopy (XPS). Annealing treatments enhanced modifications of morphology, structure and electrical properties of CdS films. AFM showed different surface morphologies and roughnesses of CdS films annealed under various atmospheres. XRD indicated the transition of CdS films from metastable cubic structure to stable hexagonal structure after annealing treatment, especially annealed in Ar+CdCl₂. From XPS analysis, Fermi levels of CdS films shifted closer to conduction band after annealing under O₂ and Ar+CdCl₂, while the levels shifted away from conduction band under Ar+H₂ and Ar+S. The relationships between those modifications by annealing treatments and effects on the performance of solar cells were discussed. Solar cell based on CdS annealed with Ar+CdCl₂ had the best performance due to the high n-doping of CdS layer introduced by annealing process.     

Nonaqueous preparation of nanocomposite films containing both polyoxometalate and phthalocyanine with bifunctional electrocatalytic properties

New bifunctional nanocomposite films containing water-insolvable cobalt tetraaminophthalocyanine (CoTAPc) and 2:18 tungstophosphate anions (P₂W₁₈) were successfully fabricated using the electrostatic layer-by-layer (LbL) assembled method in nonaqueous (DMF) media. UV–vis spectroscopy, IR spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy (AFM) and electrochemical method were used to characterize the assembled multilayer films. UV–vis measurements revealed regular film growth with each four-layer {P₂W₁₈/CoTAPc/PSS/PAH} adsorption. AFM analysis provides the morphology of the multilayer films. The cyclic voltammetric curves indicate that the film shows electrocatalytic activity for chlorate anion.     

Ultrahigh Detectivity Broad Spectrum UV Photodetector with Rapid Response Speed Based on p-β Ga2O3/n-GaN Heterojunction Fabricated by a Reversed Substitution Doping Method

An excellent broad-spectrum (220–380 nm) UV photodetector, covering the UVA-UVC wavelength range, with an ultrahigh detectivity of ≈10¹⁵ cm Hz^1/2 W⁻¹, is reported. It is based on a p-β Ga₂O₃/n-GaN heterojunction, in which p-β Ga₂O₃ is synthesized by thermal oxidation of GaN and a heterostructure is constructed with the bottom n-GaN. XRD shows the oxide layer is (−201) preferred oriented β-phase Ga₂O₃ films. SIMS and XPS indicate that the residual N atoms as dopants remain in β Ga₂O₃. XPS also demonstrates that the Fermi level is 0.2 eV lower than the central level of the band gap, indicating that the dominant carriers are holes and the β Ga₂O₃ is p-type conductive. Under a bias of −5 V, the photoresponsivity is 56 and 22 A W⁻¹ for 255 and 360 nm, respectively. Correspondingly, the detectivities reach an ultrahigh value of 2.7 × 10¹⁵ cm Hz^1/2 W⁻¹ (255 nm) and 1.1 × 10¹⁵ cm Hz^1/2 W⁻¹ (360 nm). The high performance of this UV photodetector is attributed mainly to the continuous conduction band of the p-β Ga₂O₃/n-GaN heterojunction without a potential energy barrier, which is more helpful for photogenerated electron transport from the space charge region to the n-type GaN layer.