Nonspherical ZnS colloidal building blocks for three-dimensional photonic crystals

The asymmetry introduced by a complex or nonspherical basis promotes photonic band gap formation in three-dimensional photonic crystals. However, relatively few techniques have been demonstrated to produce uniform nonspherical colloids for use as photonic crystal bases. Here we expand the menu of basis types with high refractive index by preparing nonspherical zinc sulfide colloids of uniform size and shape. Dimers, trimers, and planar tetramers were precipitated from aqueous solution by the thermal decomposition of thioacetamide in the presence of zinc nitrate, manganese nitrate, and nitric acid. The well-defined morphological types were obtained from suspensions aged for 4-6 h at 26-32 degrees C and then for 20-35 min at 85 degrees C. Stereological techniques were used to analyze SEM images and determine the percentage of each particle class. For example, the quantitative characterization of a particle population prepared at 29 degrees C for 6 h and 85 degrees C for 22 min had the composition 59+/-3% spheres, 31+/-2% dimers, 7+/-1% trimers, 0.4+/-0.2% tetramers, and 2.5+/-0.8% complex clusters (encompasses all other varieties of shape). X-ray diffraction and X-ray photoelectron spectroscopy confirmed the zinc blend crystal structure and the stoichiometric composition of the particles. The refractive index was estimated as 2.25 (413 nm) -2.09 (709 nm) by fitting experimental absorption spectra to curves derived from Mie scattering calculations. This indicated an average porosity approximately 24%. Such colloids offer the potential to form diamond-like lattices with large, stable photonic band gaps.     

Fabrication and characterization of core-shell photonic crystals via a dipping process

High-quality polystyrene (PS) colloidal photonic crystals in large area were fabricated in 24 h via a capillary-enhanced process. Then, the photonic crystals with core-shell structure were obtained by incorporating silica nanoparticles into the interstitial space of opal template via a dipping process. The filling ratio (V_silica) of interstitial space could be manipulated by dipping colloidal crystals into suspensions with different concentrations of silica nanoparticles, which in turn renders the obtained core-shell photonic crystals. The absorptive peak of opal without dipping process is at 445 nm as measured by UV–vis spectrometry. The filling ratios of 0.130, 0.167 and 0.253 can be calculated according to the modified Bragg's Law, which corresponds to the absorptive peaks for core-shell opals at 453, 463 and 469 nm obtained from suspensions with silica nanoparticles of 0.017, 0.122, and 0.244 wt%, respectively. Therefore, by using this dipping process, the characteristic absorption wavelength for photonic crystal will be varied easily, efficiently and cost effectively than that by traditional methods for constructing opal from monodispersed colloids of different diameters.     

Luminescent core-shell photonic crystals from poly(phenylene ethynylene) coated silica spheres

We describe the preparation and characterization of a photonic crystal filled with a luminescent conjugated polyelectrolyte, sulfonated poly(phenylene ethynylene). The conjugated polymer was coated onto the nanospheres by the layer-by-layer method and assembled directly into a fluorescent opal structure avoiding the defects associated with post-filling schemes. These structures exhibit strong angle-dependent luminescent properties. By using multiple layers, we further demonstrate control over the emissive bands of the opal.     

All-clay photonic crystals

Hierarchically structured "all-clay" 1D and 3D inverse photonic crystals were fabricated, featuring the crystalline layered microstructure of the phyllosilicate Laponite combined with crystalline macroporosity on the submicron scale. By using simple hard-templating protocols based on monodisperse polystyrene spheres, periodic layered silicate and inverse opal architectures with photonic crystal properties are accessible, which exhibit reflectivities of the stop band in excess of 50%. The combination of intrinsic chemical functionality with structural color makes such hierarchical "all-clay" architectures potential scaffolds in color-tunable chemo-optical devices.     

Responsive photonic crystals

This Review summarizes recent developments in the field of responsive photonic crystal structures, including principles for design and fabrication and many strategies for applications, for example as optical switches or chemical and biological sensors. A number of fabrication methods are now available to realize responsive photonic structures, the majority of which rely on self-assembly processes to achieve ordering. Compared with microfabrication techniques, self-assembly approaches have lower processing costs and higher production efficiency, however, major efforts are still needed to further develop such approaches. In fact, some emerging techniques such as spin coating, magnetic assembly, and flow-induced self-assembly have already shown great promise in overcoming current challenges. When designing new systems with improved performance, it is always helpful to bear in mind the lessons learnt from natural photonic structures.     

Three-dimensional quantum photonic crystals and quantum photonic glasses

The structure and optical properties of quantum-dot photonic crystals (quantytes) were studied. Such crystals have the so-called band gaps, which leads to a strong reflection of electromagnetic waves from the crystal surface. It was shown that in certain spectral regions the refractive index has a negative sign, and the group velocity of electromagnetic waves essentially decreases. The applications of quantytes as optical information recording and rewriting devices, negative refractive optical elements, selective mirrors, narrowband filters, and others were discussed.     

Nanoparticle-based one-dimensional photonic crystals

Herein we present a fast, reliable method for building nanoparticle-based 1D photonic crystals in which a periodic modulation of the refractive index is built by alternating different types of nanoparticles and by controlling the level of porosity of each layer. The versatility of the method is further confirmed by building up optically doped photonic crystals in which the opening of transmission windows due to the creation of defect states in the gap is demonstrated. The potential of this new type of structure as a sensing material is illustrated by analyzing the specific color changes induced by the infiltration of solvents of different refractive indexes.     

ZnO/ZnS core-shell composites for low-temperature-processed perovskite solar cells

Electron transport layers(ETLs)in perovskite solar cells(PSCs)are a key factor to determine the photovoltaic performance.Herein,we demonstrate preparation of Zn O/ZnS core-shell composites through directly synthesizing ZnS on the ZnO nanoparticles in solution.We confirmed the formation of ZnO/ZnS core-shell composites by the uses of X-ray diffraction patterns and the Fourier transform infrared spectroscopy.ZnO/ZnS composites exhibit much homogeneous surface morphology as compared with the bare Zn O as revealed in the scanning electronic microscopy.Moreover,the upper shift of conduction band level upon composition of the Zn O/Zn S film results in a better alignment of energy level,which facilitates cascade charge extraction and thus improves the current density of perovskite solar cell.The shift of conduction band also improves the voltage of the PSCs.The photoluminescence(PL)spectroscopies measured in both steady and transient states were carried out to characterize the charge extraction at the interface between CH_3NH_3PbI_3and the electron transport layers of either ZnO or ZnO/ZnS composite.The ZnO/ZnS composite can more efficiently quench the PL signal of perovskite absorber than bare Zn O resulting in enhanced photocurrent generation in PSCs.     

Transferable pair potentials for CdS and ZnS crystals

A set of interatomic pair potentials is developed for CdS and ZnS crystals. We show that a simple energy function, which has been used to describe the properties of CdSe [E. Rabani, J. Chem. Phys. 116, 258 (2002)], can be parametrized to accurately describe the lattice and elastic constants, and phonon dispersion relations of bulk CdS and ZnS in the wurtzite and rocksalt crystal structures. The predicted coexistence pressure of the wurtzite and rocksalt structures as well as the equation of state are in good agreement with experimental observations. These new pair potentials enable the study of a wide range of processes in bulk and nanocrystalline II-VI semiconductor materials.     

均质核壳结构ZnS的一步法合成及光催化性能

以硝酸锌和硫脲为原料,十二烷基硫酸钠为保护剂,通过一步水热合成反应,制备了具有核壳结构的微米级Zn S半导体材料.该Zn S材料的核、壳成分相同,晶体构造一致,均为立方闪锌矿结构;调节反应时间,可以方便地控制壳层构造的生长与闭合程度;优化反应条件得到整体尺寸约为3μm、核壳结构特征突出的形貌新颖的Zn S微粒;将Zn S材料用于亚甲基蓝的光降解反应,其对目标降解物的降解效率与形貌特征有紧密联系,壳层闭合程度越高,降解效率越低.在最优条件下,该材料对亚甲基蓝的降解率可达97.3%.     

Fluorescent and pH-responsive diblock copolymer-coated core-shell CdSe/ZnS particles for a color-displaying, ratiometric pH sensor

A color distinctive, ratiometric pH sensor was demonstrated using pH responsive and fluorescent (PyMMP-b-P2VP) diblock copolymer coated CdSe/ZnS QDs. Due to the change in the P2VP conformations in response to pH change, the color of QDs in solution changes distinctly from blue to red.     

均匀沉淀法制备单分散纳米ZnS及发光性能

采用均匀沉淀法,乙酸锌和硫代乙酰胺(TAA)分别提供Zn源和S源,无水乙醇为分散介质,十八胺(ODA)为分散剂,成功制备了小于50nm的单分散ZnS纳米颗粒,并就反应温度、分散剂浓度、溶液浓度和陈化时间对ZnS纳米颗粒粒径的影响进行系统研究。用SEM、激光纳米粒度仪、XRD、PL、EL对样品进行了表征,结果表明:维持一定的分散剂和溶液浓度于0℃陈化24h,样品粒径最小且结晶度佳,干燥和退火后的发光样品在275nm的紫外光激发下发射出490nm左右的蓝绿光,在2kV(样品厚度1mm)激励下发射出520nm的绿光。     

Polystyrene@TiO2 core-shell microsphere colloidal crystals and nonspherical macro-porous materials

High-quality and stable PS@TiO(2) core-shell microsphere colloidal crystals were prepared by electrostatic colloid stabilization combined with two-substrate vertical deposition method. The polyelectrolyte stabilized colloids self-assembled into face-centered cubic arrays with the (111) face perpendicular to the substrate. These colloidal crystals are gifted with high mechanical stability toward the flow of solution. Structure-property correlations were made using scanning electron microscopy and UV-vis-NIR spectroscopy. Optical spectra showed the presence of an L-stopband peak in the photonic band structure. Besides, these PS@TiO(2) colloidal crystals can be used as templates to fabricate the nonspherical macro-porous materials, and complete band gaps can be more easily obtained from such structure than from their spherical counterparts due to their lower symmetries. This work will hold the promise of enhanced photonic band gap materials.     

Optical properties and photonic density of states in one-dimensional and three-dimensional liquid-crystalline photonic crystals

ABSTRACT

Complex optical investigations were performed in one-dimensional (cholesteric) and three-dimensional (Blue Phase II) liquid-crystalline photonic crystals. Spectra of optical transmission and luminescence in the range of the photonic stop band contain information about the local anisotropy, characteristics of the photonic stop band, photonic density of states. We determine the photonic density of states in one-dimensional and three-dimensional liquid-crystalline photonic crystals employing measurements of polarised luminescence. The width of the photonic band gap, density of states and the optical characteristics related to the density of states essentially change with temperature in one-dimensional cholesteric photonic crystal. Drastic transformation of the density of states was found at the transition from one-dimensional to three-dimensional (Blue Phase II) photonic crystal. The results of our investigations demonstrate the possibility to employ the applied method for various types of photonic structures.     

Self-assembly of photonic crystals from polymer colloids

This article reviews recent developments in self-assembly of polymer colloids into colloidal crystals, a good candidate material for photonic crystals. Self-assembly strategy has developed as a facile and efficient method to fabricate colloidal crystals. Much research work has been focused on controlling the morphology and improving the quality, as well as finding applications of the colloidal crystals.