Embedding Ag or Au nanoparticles within the nano-sized wall of YbPO4:Er3+ inverse opals and resulting enhanced upconversion luminescence

Metal nanoparticles preparation in the interior of nanoscale skeleton of inverse opals made up of crystallized matrix is more difficult than the preparation of pure inverse opals. In the present work, the Ag or Au nanoparticles embedded YbPO₄:Er³⁺ inverse opals were prepared by a simple approach, which involved the infiltration of opal template by using the transparent YbPO₄:Er³⁺ sol including silver nitrate or chloroauric acid and the sintering at high temperature. The 20–30?nm Au or 5–10?nm Ag nanoparticles were formed in the interior of nanoscale skeleton in the YbPO₄:Er³⁺ inverse opals, and the Ag or Au nanoparticles embedded YbPO₄:Er³⁺ inverse opals were prepared. The influence of Ag or Au nanoparticles on the upconversion photoluminescence of YbPO₄:Er³⁺ inverse opal was studied, and the upconversion luminescence enhancement induced by the Ag or Au nanoparticles was observed. The mechanisms of upconversion luminescence enhancement of YbPO₄:Er³⁺ inverse opals induced by Ag or Au nanoparticles were discussed. The enhancement of upconversion luminescence induced by Ag nanoparticles was attributed to the enhancement of the excitation field, and the enhancement of upconversion emission induced by Au nanoparticles was related to the increasing of the radiation decay rate of Er³⁺.     

Compositional and optical properties of SiO x films and (SiO x /SiO y ) junctions deposited by HFCVD

In this work, non-stoichiometric silicon oxide (SiO _x ) films and (SiO _x /SiO _y ) junctions, as-grown and after further annealing, are characterized by different techniques. The SiO _x films and (SiO _x /SiO _y ) junctions are obtained by hot filament chemical vapor deposition technique in the range of temperatures from 900°C to 1,150°C. Transmittance spectra of the SiO _x films showed a wavelength shift of the absorption edge thus indicating an increase in the optical energy band gap, when the growth temperature decreases; a similar behavior is observed in the (SiO _x /SiO _y ) structures, which in turn indicates a decrease in the Si excess, as Fourier transform infrared spectroscopy (FTIR) reveals, so that, the film and junction composition changes with the growth temperature. The analysis of the photoluminescence (PL) results using the quantum confinement model suggests the presence of silicon nanocrystal (Si-nc) embedded in a SiO _x matrix. For the case of the as-grown SiO _x films, the absorption and emission properties are correlated with quantum effects in Si-nc and defects. For the case of the as-grown (SiO _x /SiO _y ) junctions, only the emission mechanism related to some kinds of defects was considered, but silicon nanocrystal embedded in a SiO _x matrix is present. After thermal annealing, a phase separation into Si and SiO₂ occurs, as the FTIR spectra illustrates, which has repercussions in the absorption and emission properties of the films and junctions, as shown by the change in the A and B band positions on the PL spectra. These results lead to good possibilities for proposed novel applications in optoelectronic devices.

PACS

61.05.-a; 68.37.Og; 61.05.cp; 78.55.-m; 68.37.Ps; 81.15.Gh     

Enhanced visible-light catalytic activity of Au nanoparticles loaded c-axis oriented Bi2VO5.5 porous thin films

Au nanoparticles loaded c-axis oriented Bi₂VO_5.5 (BVO) porous thin films were prepared by using a simple spin-coating technique. The porous structures were formed through the hydrolysis of bismuth nitrate, Bi(NO₃)₃, one of the raw materials for synthesizing the BVO precursor solutions. The optimal photocatalytic rate of the porous thin film is three times more than that of pure BVO thin film. The enhancement of photocatalytic activity can be attributed to the Schottky barrier in the intimate interface between Au nanoparticles and BVO grains and the increase of absorption of light caused by surface plasmon resonance effect of gold nanoparticles. The possible degradation mechanism of Au–BVO-Methylene Blue system has been discussed based on the energy band structure and further trapping experiments. This study provides a simple method to prepare bismuth-containing oxide porous thin films without any pore-forming reagents, and the results suggest that the Au nanoparticles loaded BVO thin film is a promising candidate material for water or air treatment.     

CO Oxidation of Au–Pt Nanostructures: Enhancement of Catalytic Activity of Pt Nanoparticles by Au

The CO oxidation activity of Pt deposited on Ta₂O₅/Ta was studied with various amounts of Au post-deposited on Pt/Ta₂O₅/Ta. For Pt nanoparticles with a mean size of 2–4 nm, an enhancement in the CO oxidation activity with increasing amount of post-deposited Au was found. The mixed Au–Pt nanoparticles with sizes in the range of 2–4 nm exhibited higher stability than the bare Au nanoparticles with a similar size range. In contrast to the results obtained with the Pt nanoparticles, the catalytic activity of a thicker Pt film gradually decreased with increasing amount of Au deposited. Based on the CO desorption experiments, it is suggested that the surface of the catalytically active Au–Pt bimetallic structures consists of both Au and Pt sites.     

Effect of Mg doping on photoluminescence of ZnO/MCM-41 nanocomposite

In this paper, photoluminescence (PL) behavior of Mg_xZn_1?xO/MCM-41 nanocomposite (where x = 0.05, 0.15, 0.25 and 0.30) is reported. Samples were characterized with small angle X-ray diffraction (SAXRD), wide angle XRD, BET (Brunauer–Emmet–Teller) surface area and pore size analyzer, field emission scanning electron microscope (FE-SEM), high resolution transmission electron microscope (HR-TEM) and PL spectrometer. The structure of MCM-41 was confirmed from both SAXRD and BET results. A broad PL band positioned at around 393 nm has been exhibited by ZnO/MCM-41 nanocomposite. With Mg doping, intensity of this PL band decreased for x = 0.05 and 0.15 and above this there was gradual enhancement in intensity. It was found that the intensity of the PL band, strongly depends on the particle size of ZnO. The increase in particle size along with MgO phase separation for x = 0.30 was proved by HR-TEM analysis. Interestingly, the differences in particle sizes at different concentrations of Mg did not account for shift in the PL band. A twofold enhancement in the intensity of PL band when x = 0.30 compared to bare ZnO/MCM-41 nanocomposite was observed. It is attributed for the increase in particle size which preserves the energy saved by passivation of ZnO nanoparticles and the other one is formation of heterojunction structures between ZnO and MgO. It was also evident from these results that there is increase in oxygen vacancies of ZnO crystallites with increase in particle size.     

ZnO-doped Y2O3 ceramic: A prospective Warm White Light Fluorescent Material

Y₂O₃ ceramics containing up to 4 at.%Zn²⁺ were prepared by infiltrating a zinc acetate water solution into a pre-sintered matrix of Y₂O₃. After pressureless sintering for 2 h at 1400 °C, ZnO grains dispersed in an Y₂O₃ matrix were identified. The photoluminescence (PL) spectra of the Y₂O₃:ZnO ceramics exhibit a very broad emission band over the entire visible spectral range with an intensity maximum at about 570?590 nm. The intensity of the PL emission increases with an increasing concentration of additive Zn²⁺, reaching a maximum between 3 and 4 at.%. The annealing temperature also affects the emission intensity. The Y₂O₃-ZnO ceramic emits yellowish (warm) white light with the CIE 1931 colour coordinates (0.39,0.43) and a correlated colour temperature in the range of 4000?4600 K. The luminescence decay kinetics at RT exhibit multi-exponential behaviour with faster (～180 ns) and slower (1.3μs and 8.0μs) decay components and the average lifetime spanning into the range of 5.3–6.5μs.     

Evidence for excitonic behavior of photoluminescence in polymer-like a-C:H films

To understand the dynamics of energy transfer and randomization of photoluminescence polarization in hydrogen-rich polymer-like amorphous carbon a-C:H films, time-resolved investigations of intensity and anisotropy decays have been performed recently. The intensity decay rates increase exponentially as a function of emission energy with a behavior very similar to that observed in wide band-gap C-rich a-Si_1–xC_x:H. In addition, in polymer-like carbon, the observation of a plateau of PL anisotropy in the 100–1000 ps range, is taken as strong evidence for the existence of a finite density of excitonic species in radiative recombination phenomena; it does not fit the phonon-assisted depolarization models proposed earlier. Polarization anisotropy decays and steady-state values are consistently interpreted using a dipole–dipole non-radiative energy transfer mechanism (Förster mechanism) with a characteristic depolarization time of 50 ps rather independent of the emission energy. The latter value is likely to be related to the density of radiative centers distribution estimated independently in the constant exciton radius approximation, rather than the result of hopping in an exponential distribution of tail states.     

Plasmonic enhancement of photocatalytic decomposition of methyl orange under visible light

By integrating strongly plasmonic Au nanoparticles with strongly catalytic TiO₂, we observe enhanced photocatalytic decomposition of methyl orange under visible illumination. Irradiating Au nanoparticles at their plasmon resonance frequency creates intense electric fields, which can be used to increase electron–hole pair generation rate in semiconductors. As a result, the photocatalytic activity of large bandgap semiconductors, like TiO₂, can be extended into the visible region of the electromagnetic spectrum. Here, we report a 9-fold improvement in the photocatalytic decomposition rate of methyl orange driven by a photocatalyst consisting of strongly plasmonic Au nanoparticles deposited on top of strongly catalytic TiO₂. Finite-difference time-domain (FDTD) simulations indicate that the improvement in photocatalytic activity in the visible range can be attributed to the electric field enhancement near the metal nanoparticles. The intense local fields produced by the surface plasmons couple light efficiently to the surface of the TiO₂. This enhancement mechanism is particularly effective because of TiO₂’s short exciton diffusion length, which would otherwise limit its photocatalytic efficiency. Our electromagnetic simulations of this process suggest that enhancement factors many times larger than this are possible if this mechanism can be optimized.     

A comparative study of the oxidation of CO and CH4 over Au/MOx/Al2O3 catalysts

The activity of Au/Al₂O₃ and Au/MO_x/Al₂O₃ (M=Cr, Mn, Fe, Co, Ni, Cu, and Zn) in low temperature CO oxidation and the oxidation of CH₄ was studied. Generally, addition of MO_x to Au/Al₂O₃ stabilizes small Au particles initially present on the support in heat treatments up to 700°C. All multi-component catalysts show a remarkable enhancement in low temperature CO oxidation compared to the mono-component catalysts. The observed activities are directly related to the average Au particle size, whereas the identity of MO_x is less important. The CH₄ oxidation activity of Au/Al₂O₃ is improved upon addition of MnO_x, FeO_x, CoO_x, and to a lesser extent NiO_x. Measured activities in CH₄ oxidation over Au/MO_x/Al₂O₃ decrease in the following order: CuO_x>MnO_x>CrO_x>FeO_x>CoO_x>NiO_x>ZnO_x. The high activity observed for CuO_x and CrO_x containing catalysts is assigned to the intrinsically high CH₄ oxidation capability of these oxides themselves. For Au/MnO_x/Al₂O₃ a lower apparent activation energy was found than for Au/Al₂O₃ and MnO_x/Al₂O₃, which might point to a promoting effect of MnO_x on Au in the oxidation of CH₄. The results presented support a similar model for both CO and CH₄ oxidation. In this model the reaction takes place at the Au/MO_x perimeter, which is defined as the boundary between Au, MO_x and the gas phase. The reductant, CO or CH₄, is adsorbed on Au, and MO_x is the supplier of O.     

Morphological,compositional, structural,and optical properties of Si-nc embedded in SiOx films

Structural, compositional, morphological, and optical properties of silicon nanocrystal (Si-nc) embedded in a matrix of non-stoichiometric silicon oxide (SiO_x) films were studied. SiO_x films were prepared by hot filament chemical vapor deposition technique in the 900 to 1,400°C range. Different microscopic and spectroscopic characterization techniques were used. The film composition changes with the growth temperature as Fourier transform infrared spectroscopy, energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy reveal. High-resolution transmission electron microscopy supports the existence of Si-ncs with a diameter from 1 to 6.5 nm in the matrix of SiO_x films. The films emit in a wide photoluminescent spectrum, and the maximum peak emission shows a blueshift as the growth temperature decreases. On the other hand, transmittance spectra showed a wavelength shift of the absorption border, indicating an increase in the energy optical bandgap, when the growth temperature decreases. A relationship between composition, Si-nc size, energy bandgap, PL, and surface morphology was obtained. According to these results, we have analyzed the dependence of PL on the composition, structure, and morphology of the Si-ncs embedded in a matrix of non-stoichiometric SiO_x films.     

Synthesis and electroluminescence properties of a novel tetraphenylsilane-oxadiazole-diphenyl(para-tolyl)amine polymer

A novel triarylaminooxadiazole-containing tetraphenylsilane light-emitting polymer (PTOA) has been synthesized. Excellent thermal stability was observed due to the presence of a rigid tetraphenylsilane-based polymer backbone (T_g = 218 °C, T_d = 373 °C). In solution, PTOA shows photoluminescence (PL) with an emission maximum at 426 nm, which is attributed to the light-emitting unit of the triarylaminooxadiazole group. In solid film, the emission maximum of PL is observed at 458 nm, a 32 nm red-shift from the PL in solution. The solvatochromic effect and excimer formed in the solid film are responsible for the red-shifting and broadening of the PL emission band. The PL stability and morphology of the PTOA solid film were further investigated by thermal annealing at elevated temperatures. No significant difference in the PL spectra or morphology was observed between a pristine sample and a repeatedly thermally annealed film (at 200 °C). PTOA-based PLED shows EL with a main peak at 458 nm accompanied by a shoulder at around 530 nm. The light emission from electromer or electroplex leads to a broadening of the EL spectra (400-650 nm), which corresponds to the interaction between the oxadiazole and diphenyl(4-tolyl)amine groups in different polymer segments or chains. A sky blue emission (Commission Internationale de L'Eclairage (CIE_x,y) coordinates (0.20,0.23)) was obtained for PTOA-based PLED. The brightness and efficiency of the PLED can be as high as 248 cd/m² and 0.54 cd/A, respectively. The EL of PTOA-based PLED has been further improved by blending the PTOA with poly(n-vinylcarbazole) (PVK) in different concentrations. The effects of concentration on the PL and EL were studied for the PTOA-PVK composite film-based PLEDs.     

Preparation and Enhanced Luminescence of Au Nanoparticles Including SiO2:Tb3+ Three‐Dimensional Ordered Macroporous Films

Novel photoluminescent films consisting of three‐dimensionally ordered macroporous (3DOM) SiO₂:Tb³⁺ and Au nanoparticles were prepared by template assisted method, and their luminescence properties were investigated. The results show that Au nanoparticles have significant influence on luminescence property of SiO₂:Tb³⁺ 3DOM materials. The Au nanoparticles embedded in the skeleton of SiO₂:Tb³⁺ 3DOM materials could strongly improve their luminescence properties. The enhancement mechanism was discussed and demonstrated, which is attributed to the increasing of radiative decay rates of Tb³⁺ induced by surface plasmon effects of Au nanoparticles.     

Optical properties of porous silicon coated with ultrathin gold film by RF-magnetron sputtering

Optical properties of porous silicon (PS) with ultrathin gold (Au) coatings were investigated. The gold films were deposited by using an RF-sputter-deposition technique on PS prepared by electrochemical anodization of P-type (1 0 0) Si. Photoluminescence (PL) spectroscopy and UV/VIS photospectroscopy analyses were performed to investigate the PL and optical transmittance properties of the Au-coated PS samples. Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron emission spectroscopy (XPS) analyses were also performed to investigate the origin of the PL enhancement by Au deposition. The PL intensity of PS is 6.4% increased by depositing 5.3 nm Au film using an RF-sputtering technique, but it is decreased 28.4% by postannealing. FTIR, spectrophotometry and XPS analysis results suggest that the PL enhancement by Au film deposition is attributed to the oxidation inhibiting effect of the Au film. However, it is not desirable to deposit an Au film thicker than 5.3 nm on PS as the PL intensity is decreased rather than increased owing to a significant decrease in the transmittance. Deterioration in the PL of the Au-coated PS by postannealing is ascribed to oxidation of the PS layer occurring at the high annealing temperature in spite of the Au passivation.     

Si-rich Al2O3 films grown by RF magnetron sputtering: structural and photoluminescence properties versus annealing treatment

Silicon-rich Al₂O₃ films (Si_x(Al₂O₃)_1−x) were co-sputtered from two separate silicon and alumina targets onto a long silicon oxide substrate. The effects of different annealing treatments on the structure and light emission of the films versus x were investigated by means of spectroscopic ellipsometry, X-ray diffraction, micro-Raman scattering, and micro-photoluminescence (PL) methods. The formation of amorphous Si clusters upon the deposition process was found for the films with x ≥ 0.38. The annealing treatment of the films at 1,050°C to 1,150°C results in formation of Si nanocrystallites (Si-ncs). It was observed that their size depends on the type of this treatment. The conventional annealing at 1,150°C for 30 min of the samples with x = 0.5 to 0.68 leads to the formation of Si-ncs with the mean size of about 14 nm, whereas rapid thermal annealing of similar samples at 1,050°C for 1 min showed the presence of Si-ncs with sizes of about 5 nm. Two main broad PL bands were observed in the 500- to 900-nm spectral range with peak positions at 575 to 600 nm and 700 to 750 nm accompanied by near-infrared tail. The low-temperature measurement revealed that the intensity of the main PL band did not change with cooling contrary to the behavior expected for quantum confined Si-ncs. Based on the analysis of PL spectrum, it is supposed that the near-infrared PL component originates from the exciton recombination in the Si-ncs. However, the most intense emission in the visible spectral range is due to either defects in matrix or electron states at the Si-nc/matrix interface.     

Enhanced solid-state electrogenerated chemiluminescence of Au/CdS nanocomposite and its sensing to H2O2

Au nanoparticles (AuNPs) are good quenchers once they closely contact with luminophore. Here we reported a simple approach to obtain enhanced electrogenerated chemiluminescence (ECL) behavior based on Au/CdS nanocomposite films by adjusting the amount of AuNPs in the nanocomposite. The maximum enhancement factor of about 4 was obtained at an indium tin oxide (ITO) electrode in the presence of co-reactant H₂O₂. The mechanism of this enhancement was discussed in detail. The strong ECL emission from Au/CdS nanocomposites film was exploited to determine H₂O₂. The resulting ECL biosensors showed a linear response to the concentration of H₂O₂ ranging from 1.0 × 10⁻⁸ to 6.6 × 10⁻⁴ mol L⁻¹ with a detection limit of 5 nmol L⁻¹ (S/N = 3) and good stability and reproducibility.     

Effect of H2O and CO2 on NOx emission control for lean-burn engines by electrochemical-catalytic cells

Lean-burn engines can offer superior fuel efficiency but require advanced technology for NO_x emission control. Electrochemical-catalytic cell has been proposed for lean DeNO_x. This work demonstrates that the DeNO_x rate can be enhanced by the presence of H₂O and/or CO₂, and can increase with increasing H₂O and CO₂ concentrations, although the increased extent is quite small. In the low NO_x concentration range, relatively constant DeNO_x rates were observed and can result in zero NO_x emissions, where the presence of H₂O and CO₂ has important enhancement effect. Higher temperature generally results in larger N₂ selectivity in the low NO_x concentration region.     

Nanocrystallization and optical properties of CsPbBr3−xIx perovskites in chalcogenide glasses

The confinement of CsPbX₃ (X = Cl, Br, and I) perovskite nanocrystals (NCs) in a stabilized inorganic glass matrix is a new strategy for improving their long-term stability and promoting their applications in the optoelectronic field. Here, in situ nanocrystallization strategy is developed to precipitate CsPbBr_3?xI_x NCs with arbitrary I/Br ratio among an elaborately designed GeS₂–Sb₂S₃-based chalcogenide glass matrix. Spherical CsPbBr_3?xI_x NCs are homogeneously distributed in the glass matrix after thermal treatment. The photoluminescence (PL) spectra show that the emission peaks of CsPbBr_3?xI_x NCs can be tuned from 570 nm to 722 nm with the replacement of Br by I. The fs transient absorption (TA) spectra reveal that there exists some structural defects in the NCs, leading to short PL decay life. This work would shed light on confining CsPbX₃ NCs into glassy matrices, facilitating their future applications in photoelectronic fields.     

Highly ordered bilayer structures of gold(I)–alkanethiolates having two types of alkyl groups

The luminescent gold(I)–alkanethiolates containing two types of alkyl groups, Au(I)–(SR)_x(SR′)_ys, were synthesized by mixing HAuCl₄ with two n-alkanethiols in tetrahydrofuran. Au(I)–(SR)_x(SR′)_ys have highly ordered bilayer structures and the dominant conformations of the alkyl groups were found to be all-trans from X-ray diffraction and FT-IR spectroscopy studies. The gold nanoparticles stabilized by two types of alkanethiolates could be prepared by reducing the Au(I)–(SR)_x(SR′)_ys using tetrabutylammonium borohydride (TBABH) as a reducing agent. Elemental analysis results of Au(I)–(SR)_x(SR′)_ys and gold nanoparticles prepared from Au(I)–(SR)_x(SR′)_ys indicated that the longer alkanethiols are more reactive for the formation of Au(I)–(SR)_x(SR′)_ys and also the longer alkylthiolates are preferentially absorbed on the gold particles compared with the shorter ones.     

A simple synthesis of Ag2+x Se nanoparticles and their thin films for electronic device applications

A simple method to synthesize silver selenide nanoparticles has been proposed. By changing the ratio of Se-oleylamine complex and silver acetate in the reacting mixture at different temperatures, both size and stoichiometry of the silver selenide particles could be successfully controlled. The size of the nanoparticles was adjusted by changing reaction temperatures. The synthesized silver selenide nanoparticles showed size changes from 3 to 10 nm when the corresponding reaction temperatures were 40–100°C, respectively. In addition to the size change, the stoichiometry of the synthesized nanoparticles (Ag_2+x Se) could be adjusted by simply varying the ratio of Ag to Se precursors. Through XPS analyses the x value in Ag_2+x Se was determined, and it changed between 0.54 and ?0.03 by varying Ag/Se ratio from 2/0.75 to 2/4. The optical property of the nonstoichiometric Ag_2+x Se nanoparticles was different from that of stoichiometric Ag₂Se nanoparticles, but showed the plasmon absorption of Ag-Ag network. The plasmon absorption was decreased with the increased concentration of the Se precursor. Finally, the Ag_2+x Se thin film in this work showed large magnetoresistance and successfully applied to prepare high-performance Schottky diode. The Ag_2.06Se film exhibited the magnetoresistance effect up to 0.9% at only 0.8 T at room temperature. The voltage drop and breakdown voltage of the Schottky diode were 0.5 V and 9.3 V, respectively.     

Electrocatalysis by nanoparticles: Optimization of the loading level and operating pH for the oxygen evolution at crystallographically oriented manganese oxide nanorods modified electrodes

The electrocatalytic evolution of oxygen gas is investigated at manganese oxide nanorods (nano-MnO_x) modified Au, Pt and GC electrodes in a wide range of pH values, ranging from highly acidic to highly basic. Morphological investigation has been carried out by a scanning electron microscopy (SEM), which revealed the deposition of nano-MnO_x in a nanorod morphology. A significant enhancement of the electrocatalytic activity of the Au, Pt and GC electrodes towards the oxygen evolution reaction (OER) was observed upon the electrodeposition of nano-MnO_x onto the aforementioned electrodes. The effect of the surface coverage of the manganese oxide and the pH of the electrolyte was investigated to seek an optimization. The highest cathodic shift in the onset potential of the OER was obtained in 0.5 M KOH irrespective of the substrate whereas the optimum loading (surface coverage) was about ca. 52%. The origin of the enhancement of the OER is addressed with the assistance of an X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) techniques. The preferential electrodeposition of crystallographically oriented nano-MnOx (in the manganite phase, γ-MnOOH) is thought to play the primary role in the observed enhancement.