Barium Titanate Inverted Opals—Synthesis,Characterization, and Optical Properties

Barium titanate inverted opals with powder and film morphologies were synthesized from barium ethoxide and titanium isopropoxide in the interstitial spaces of a polystyrene opal. This procedure involves infiltration of precursors into the interstices of the polystyrene opal template followed by hydrolytic polycondensation of the precursors to amorphous barium titanate and removal of the polystyrene opal by solvent extraction or calcination. In‐situ variable temperature powder X‐ray diffraction and micro‐Raman spectroscopy allow one to observe the thermally induced transformation of the as‐synthesized amorphous barium titanate inverted opal to the nanocrystalline form. In this way, a nanocrystalline barium titanate inverted opal can be engineered as either the cubic or tetragonal polymorph. Control of this process is key to the practical realization of a room‐temperature stable ferroelectric barium titanate inverted opal that can be thermally tuned through the ferroelectric–paraelectric transition around the Curie temperature. Optical characterization demonstrated photonic crystal behavior of the inverted barium titanate opals and results were in good agreement with photonic band structure calculations. The synthesis of optical quality ferroelectric barium titanate inverted opals provides an opportunity to electrically and optically engineer the photonic band structure and the possibility of developing tunable three‐dimensional photonic crystal devices.     

Controlling the Optical,Electrical and Chemical Properties of Carbon Inverse Opal by Nitrogen Doping

Nitrogen‐doped carbon inverse opal (CIO‐N) is synthesized by a two‐step process involving the infiltration of carbon‐nitrogen precursors within opals followed by the thermolysis and removal of the opal structure in hydrofluoric acid (HF). Undoped samples exhibit a reflection peak in the red region of the spectrum whereas N‐doped samples display shifts to the blue region of the spectrum as the nitrogen content is increased. The degree of crystallinity of CIO‐N strongly depends upon the nitrogen content and on the size of the precursor silica particles used to prepare the inverted opals. In addition, the introduction of nitrogen into the samples is able to increase the electrical conductivity by one order of magnitude from 2 to 30 S cm^‐1 (at room temperature). All samples are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X‐ray diffraction (XRD), Raman spectroscopy, X‐ray photoelectron spectroscopy (XPS), ultraviolet‐visible (UV‐Vis) spectroscopy, and electrical conductivity measurements. It is envisaged that CIO‐N could have important applications in the fabrication of photonic crystals, photoconducting materials, molecular sensors, field emission devices, capacitors, batteries, among many others.     

Er^3＋-activated silica inverse opals synthesized by the solgel method

We present the details of the sol-gel processing used to realize inverse silica opal,where the silica was activated with 0.3 mol% of Er3 ions. The template(direct opal) was obtained assembling polystyrene spheres of the dimensions of 260 nm by means of a vertical deposition technique. The Er3 -activated silica inverse opal was obtained infiltrating,into the void of the template,the silica sol doped with Er3 ions and subsequently removing the polystyrene spheres by means of calcinations. Scanning electron microscope showed that the inverse opals possess a fcc structure with a air hollows of about 210 nm and a photonic band gap,in the visible range,was observed from reflectance measurements. Spectroscopic properties of Er3 -activated silica inverse opal were investigated by luminescence spectroscopy,where,upon excitation at 514.5 nm,an emission of 4I13/2 → 4I15/2 of Er3 ions transition with a 21 nm bandwidth was observed. Moreover the 4I13/2 level decay curve presents a single-exponential profile,with a measured lifetime of 18 ms,corresponding a high quantum efficiency of the system.     

Luminescence of CdSe/ZnS quantum dots infiltrated into an opal matrix

The effect of the photonic band gap in the photonic crystal, the synthesized SiO₂ opal with embedded CdSe/ZnS quantum dots, on its luminescence in the visible spectral region is studied. It is shown that the position of the photonic band gap in the luminescence and reflectance spectra for the infiltrated opal depends on the diameter of the constituent nanospheres and on the angle of recording the signal. The optimal conditions for embedding the CdSe/ZnS quantum dots from the solution into the opal matrix are determined. It is found that, for the opal-CdSe/ZnS nanocomposites, the emission intensity decreases and the luminescence decay time increases in the spatial directions, in which the spectral positions of the photonic band gap and the luminescence peak of the quantum dots coincide.     

Fabrication of three-dimensional photonic crystals for use in thespectral region from ultraviolet to near-infrared

This paper describes a simple and convenient method that allows self-assembly of colloidal particles (50 nm-50 μm in diameter) into cubic-close-packed (c.c.p.) lattices over areas larger than 1 cm² . These three-dimensional (3D) lattices have a highly ordered structure similar to that of a natural opal, with a packing density of approximately 74%. They strongly diffract light, and each of them exhibits a stop band whose position is mainly determined by the size of the particles. These crystalline assemblies of particles have also been used as templates to fabricate inverse opals, that is, three-dimensionally porous membranes consisting of a c.c.p. lattice of interconnected air balls. Both types of periodic structures are potentially useful as 3D photonic bandgap (PBG) crystals that can be used to control the emission and propagation of light in the spectral region ranging from ultraviolet (UV) to near infrared     

Photoluminescence of ZnO infiltrated into a three-dimensional photonic crystal

The effect of the photonic band gap (stopband) of the photonic crystal, the synthesized SiO₂ opal with embedded zinc oxide, on its luminescence in the violet spectral region is studied. It is shown that the position of the photonic band gap in the luminescence and reflectance spectra of the infiltrated opal depends on the diameter of the constituent nanoglobules, the volume fraction of zinc oxide, and on the signal’s acceptance angle. It is found that, for the ZnO-opal nanocomposites, the emission intensity is decreased and the luminescence decay time is increased in the spatial directions, in which the photonic band gap coincides in spectral position with the luminescence peak of zinc oxide. The change in the decay time can be attributed to the change in the local density of photonic states in the photonic band gap.     

Electric-field-induced modification of the optical properties of CuI crystals

The study is concerned with the effect of a dc electric field applied to copper iodide crystals on the resistivity of the crystals and their transparency and luminescence in the visible and ultraviolet spectral regions. It is shown that the conductivity, transparency, and edge luminescence intensity of the crystals decrease upon application of an electric field at room temperature with characteristic times of about 10 min. The reversibility of these processes, associated with the formation and diffusion of donor- and acceptor-type intrinsic defects (Cu _i and V _Cu) in the crystal lattice of the CuI compound, is established.     

Luminescence from ZnO quantum dots deposited with synthetic opal

Photoluminescence from ZnO layers of varied thickness deposited onto the surface of synthetic opal has been studied. Narrow peaks of luminescence in the excitonic spectral range, related to quantum confinement of the electron wave functions, have been observed. The formation of ZnO quantum dots (QD) within the opal voids in the second subsurface layer has been confirmed by atomic force microscopy and by studying the angular dependence of the luminescence spectra.     

Fabrication of Photonic Microbricks via Crack Engineering of Colloidal Crystals

Evaporation‐induced self‐assembly of colloidal particles is one of the most versatile fabrication routes to obtain large‐area colloidal crystals; however, the formation of uncontrolled “drying cracks” due to gradual solvent evaporation represents a significant challenge of this process. While several methods are reported to minimize crack formation during evaporation‐induced colloidal assembly, here an approach is reported to take advantage of the crack formation as a patterning tool to fabricate microscopic photonic structures with controlled sizes and geometries. This is achieved through a mechanistic understanding of the fracture behavior of three different types of opal structures, namely, direct opals (colloidal crystals with no matrix material), compound opals (colloidal crystals with matrix material), and inverse opals (matrix material templated by a sacrificial colloidal crystal). This work explains why, while direct and inverse opals tend to fracture along the expected {111} planes, the compound opals exhibit a different cracking behavior along the nonclose‐packed {110} planes, which is facilitated by the formation of cleavage‐like fracture surfaces. The discovered principles are utilized to fabricate photonic microbricks by programming the crack initiation at specific locations and by guiding propagation along predefined orientations during the self‐assembly process, resulting in photonic microbricks with controlled sizes and geometries.     

Tunable Visual Detection of Dew by Bare Artificial Opals

The photonic properties of artificial opals directly depend on the atmosphere (air, usually) filling the voids in the constitutive dielectric material. Here, a novel use of bare artificial silica opals (direct and inverse) is presented for straightforward water dew detection, in which the stop‐band exhibits a drastic change upon soaking of the voids. In particular, the opal reflectance drops to one‐fifth within 100 ms scale, so the fading of the opal structural color upon dew formation is evident for the naked eye. Moreover, the stop‐band redshifts up to 60 nm, leading, by convenient selection of the stop‐band wavelength, to perceptible color change. Due to the porous nature of the opal, the dew formation in the opal voids is shifted to lower saturation pressures compared to an open system. This is experimentally demonstrated by controlled reduction of the pores size, so that the advancement of the dew point temperature is tuned between 2.5° and 6.3°. The outstanding color change together with the easy fabrication (without postprocessing) and selectable size (from miniature to large‐area) make artificial opals cost‐effective and versatile visual dew detectors. The tunable advance of the opal response over the actual dew point is practical to adequately anticipate and prevent dew formation in neighboring surfaces. Furthermore, opals might serve as useful probe systems for the study of fundamental phenomena such as condensation of vapors in porous and particulate media.     

Photoinduced Tuning of Optical Stop Bands in Azopolymer Based Inverse Opal Photonic Crystals

Photo‐tunable photonic crystals were prepared from three dimensional (3D) colloidal crystal templates using a photoresponsive azopolymer. For the preparation of azopolymer infiltrated photonic crystals, silica colloidal crystals were fabricated by gravity sedimentation, a self‐assembly technique. The interstitial voids between colloidal particles were filled with azopolymer and azopolymer inverse opals were produced by treatment with aqueous hydrofluoric acid. These photonic crystals exhibited stop bands in their transmission spectra measured in the normal incidence to the (111) plane of face centered cubic (fcc). The photonic bandgap of the azopolymer infiltrated opal and inverse opal could be controlled by the refractive index change due to the photoinduced orientation of azobenzene chromophores. When the azopolymer photonic crystals were irradiated with linearly polarized light, their bandgap positions were shifted to shorter wavelength regions with increasing irradiation time. This behavior experimentally produced a photoinduced orientation of the azobenzene groups in parallel with the incidence of the excitation light. Through such an out‐of‐plane orientation of azo chromophores, parallel to the [111] fcc crystallographic axis, the effective refractive index of the photonic crystal medium was decreased. Therefore, a blue‐shift in bandgap positions was consequently induced with 20–40 nm tuning ranges. The out‐of‐plane orientation was confirmed by angular resolved absorption spectral measurements.     

Silicon Inverse‐Opal‐Based Macroporous Materials as Negative Electrodes for Lithium Ion Batteries

Several types of silicon‐based inverse‐opal films are synthesized, characterized by a range of experimental techniques, and studied in terms of electrochemical performance. Amorphous silicon inverse opals are fabricated via chemical vapor deposition. Galvanostatic cycling demonstrates that these materials possess high capacities and reasonable capacity retentions. Amorphous silicon inverse opals perform unsatisfactorily at high rates due to the low conductivity of silicon. The conductivity of silicon inverse opals can be improved by their crystallization. Nanocrystalline silicon inverse opals demonstrate much better rate capabilities but the capacities fade to zero after several cycles. Silicon–carbon composite inverse‐opal materials are synthesized by depositing a thin layer of carbon via pyrolysis of a sucrose‐based precursor onto the silicon inverse opals. The amount of carbon deposited proves to be insufficient to stabilize the structures and silicon–carbon composites demonstrate unsatisfactory electrochemical behavior. Carbon inverse opals are coated with amorphous silicon producing another type of macroporous composite. These electrodes demonstrate significant improvement both in capacity retentions and in rate capabilities. The inner carbon matrix not only increases the material conductivity but also results in lower silicon pulverization during cycling.     

Electroluminescence spectra of ultraviolet light-emitting diodes based on <Emphasis Type="Italic">p</Emphasis>-<Emphasis Type="Italic">n</Emphasis>-heterostructures coated with phosphors

The electroluminescence spectra of light-emitting diodes based on p-n heterostructures of the InGaN/AlGaN/GaN type are studied in the near-ultraviolet spectral region (360–405 nm). The spectra are peaked at the wavelengths 385 and 395 nm, and the intensity of emission falls exponentially with the photon energy in the shorter-wavelength and longer-wavelength regions. The emitters in the green and yellow spectral regions based on these light-emitting diodes coated with silicate phosphors are studied. The luminescence spectra of phosphors have the Gaussian shape and maximums in the range from 525 to 560 nm. The color characteristics of emitters depend on the ratios of intensities of the ultraviolet and yellow-green bands. The possibilities of fabrication of light-emitting diodes of visible luminescence based on ultraviolet light-emitting diodes that excite colored phosphors are discussed.     

Na_5Tm（WO_4）_4发光晶体的光谱特性研究

以Ｎａ２ＷＯ４为助熔剂，采用缓冷法成功地生长出新型发光晶体Ｎａ５Ｔｍ（ＷＯ４）４，晶粒尺寸为２ｍｍ×２ｍｍ×２ｍｍ。根据四方晶系面间距公式及最小二乘法，可以确定晶体的晶胞参数ａ＝１．１３８６ｎｍ，ｃ＝１．１２８３ｎｍ，测定并分析了该晶体室温下的红外光谱、吸收光谱、发射光谱及激发光谱。根据Ｊｕｄｄ－Ｏｆｅｌｔ理论计算了Ｔｍ３＋离子的吸收振子强度和配位场作用强度参数。结果表明该晶体有可能成为一类很重要的蓝色发光材料。     

Bimodality in Arrays of In<Subscript>0.4</Subscript>Ga<Subscript>0.6</Subscript>As Hybrid Quantum-Confined Heterostructures Grown on GaAs Substrates

Hybrid quantum-confined heterostructures grown by metal-organic vapor-phase epitaxy (MOVPE) via the deposition of In_0.4Ga_0.6As layers with various nominal thicknesses onto vicinal GaAs substrates are studied by photoluminescence spectroscopy and transmission electron microscopy. The photoluminescence spectra of these structures show the superposition of two spectral lines, which is indicative of the bimodal distribution of the size and/or shape of light-emitting objects in an array. The dominant spectral line is attributed to the luminescence of hybrid “quantum well–dot” nanostructures in the form of a dense array of relatively small quantum dots (QDs) with weak electron and hole localization. The second, lower intensity line is attributed to luminescence from a less dense array of comparatively larger QDs. Analysis of the behavior of the spectral line intensities at various temperatures showed that the density of larger QDs grows with increasing thickness of the InGaAs layer.     

Designing Multicolor Micropatterns of Inverse Opals with Photonic Bandgap and Surface Plasmon Resonance

Structural coloration provides unique features over chemical coloration, such as nonfading, color tunability, and high color brightness, rendering it useful in various optical applications. To develop the structural colors, two different mechanisms of coloration–photonic bandgap (PBG) and surface plasmon resonance (SPR)–have been separately utilized. In this work, a new method is suggested to create structurally colored micropatterns by regioselectively employing SPR in a single film of inverse opal with PBG. The inverse opals are prepared by thermal embedding of opal into a negative photoresist and its subsequent removal. The inverse opals have a hexagonal array of open pores on the surface which serves as a template to make SPR‐active nanostructures through a directional deposition of gold, a perforated gold film and an array of curved gold disks are formed. With a shadow mask lithographically prepared, the gold is regioselectively deposited on the surface of the inverse opal, which results in two distinct regions of gold‐free inverse opal with PBG and gold nanostructure with SPR. As PBG and SPR develop their own structural colors respectively, the resultant micropatterns exhibit pronounced dual colors. More importantly, the micropatterns show the distinguished optical response for evaporation of volatile liquids that occupy the pores.     

Modification of electrical and optical properties of ZnO films under ultraviolet irradiation

The effect of ultraviolet radiation of polycrystalline zinc oxide films (with a thickness of 200 nm) on their resistivity, transparency, and luminescence in the visible and violet spectral regions is studied. It is shown that, under irradiation of the films in air and vacuum, the conductivity, transmittance, and edge luminescence intensity increase with characteristic times of about 100 min. It is established that the corresponding processes controlled by desorption of oxygen atoms and molecules from the surface of nanocrytals in the ZnO films are reversible.     

Composite Luminescent Material for Dual Sensing of Oxygen and Temperature

A novel kind of composite material is presented that contains two indicators incorporated into a single polymer matrix, thus allowing simultaneous determination of oxygen partial pressure and temperature. The temperature‐sensitive dye (ruthenium tris‐1,10‐phenanthroline) was chosen for its highly temperature‐dependent luminescence which is the highest among the Ru^II polypyridyl complexes. A fluorinated palladium(II) tetraphenylporphyrin served as the oxygen probe. The indicators were incorporated into either poly(styrene‐co‐acrylonitrile) microparticles (to sense oxygen) or into poly(acrylonitrile) (for temperature sensing, since this polymer is virtually impermeable to oxygen). The luminescence of both dyes can be separated either spectrally (due to different absorption and emission spectra of the indicators) or via luminescence decay time. The material is suitable for temperature‐compensated oxygen sensing, for example, in high‐resolution oxygen profiling, and for imaging temperature in the range between 0 and 60 °C. This enables one to “see” (rather than to “feel”) temperature in this important range. Simultaneous imaging of pressure and temperature also has been achieved. It enables contactless imaging of the two parameters, for example, in wind tunnels. Due to the use of a biocompatible hydrogel matrix, the material conceivably is suited for biomedical applications.     

Acoustic Feature Analysis and Discriminative Modeling of Filled Pauses for Spontaneous Speech Recognition

Most automatic speech recognizers (ASRs) concentrate on read speech, which is different from spontaneous speech with disfluencies. ASRs cannot deal with speech with a high rate of disfluencies such as filled pauses, repetitions, lengthening, repairs, false starts and silence pauses. In this paper, we focus on the feature analysis and modeling of the filled pauses “ah,” “ung,” “um,” “em,” and “hem” in spontaneous speech. Karhunen-Loéve transform (KLT) and linear discriminant analysis (LDA) were adopted to select discriminant features for filled pause detection. In order to suitably determine the number of discriminant features, Bartlett hypothesis testing was adopted. Twenty-six features were selected using Bartlett hypothesis testing. Gaussian mixture models (GMMs), trained with a gradient decent algorithm, were used to improve the filled pause detection performance. The experimental results show that the filled pause detection rates using KLT and LDA were 84.4% and 86.8%, respectively. A significant improvement was obtained in the filled pause detection rate using the discriminative GMM with KLT and LDA. In addition, the LDA features outperformed the KLT features in the detection of filled pauses.     

Designing Multicolored Photonic Micropatterns through the Regioselective Thermal Compression of Inverse Opals

Colloidal assemblies develop pronounced structural colors due to the selective diffraction of light. Micropatterns with multiple structural colors are appealing for the use in a variety of photonic applications. Here, a lithographic approach is reported, which provides a high level of control over the size, shape, and color of a micropattern using the anisotropic shrinkage of inverse opals made of a negative photoresist heated to high temperatures. Shrinkage occurs uniformly across the thickness of the film, leading to a blueshift in the structural color while maintaining a high reflectivity across the full visible spectrum. The rate of shrinkage is determined by the annealing temperature and the photoresist crosslinking density. The rate can, therefore, be spatially modulated by applying UV radiation through a photomask to create multicolor micropatterns from single‐colored inverse opals. The lateral dimensions of the micropattern features can be as small as the thickness of the inverse opal.