A Hierarchically Structured Ni(OH)2 Monolayer Hollow‐Sphere Array and Its Tunable Optical Properties over a Large Region

The fabrication of a hierarchically structured Ni(OH)₂ monolayer hollow‐sphere array with the shell composed of building blocks of nanoflakelets is demonstrated based on a colloidal monolayer and electrochemical deposition. The morphology can be easily controlled by the colloidal monolayer and deposition parameters. Importantly, such monolayer hollow‐sphere array shows a morphology‐ and size‐dependent tunable optical transmission stop band. This stop band can be easily tuned from 455–1855 nm by changing the size of the hollow spheres between 1000 and 4500 nm, and also fine‐adjusted by changing the deposition time. The array exhibits a nearly incident‐angle‐independent position of the stop band that 3D photonic crystals do not possess. This structure may have potential applications in optical devices, photonic crystals, and sensors for gas detection.     

Some New Developments in the Synthesis,Functionalization, and Utilization of Monodisperse Colloidal Spheres

This article provides an overview of some recent developments related to the synthesis and functionalization of monodisperse colloidal spheres, a class of colloidal materials that has found widespread use in applications such as the fabrication of photonic crystals, optical sensing, and drug delivery. Traditionally, the choice of materials has been limited to polystyrene and silica. We and other groups have recently expanded the scope of materials by developing a number of methods for producing monodisperse colloidal spheres from various semiconductors and metals. This article is confined to our own work; it covers three different synthetic strategies: the bottom–up approach, the top–down approach, and template‐directed synthesis. The colloidal spheres may have a solid, hollow, or core–shell structure, and the chemical compositions can include Se, Bi, Pb, In, Sn, Cd, Pt, Ag₂Se, CdSe, PbS, or TiO₂. As an example to illustrate the attractive features of these colloidal spheres, we demonstrate the fabrication of Ag₂Se‐based photonic crystals whose stop bands can be thermally switched between two spectral positions.     

3维函数光子晶体的特性研究

为了研究3维函数光子晶体的光子禁带特性,采用平面波展开法计算得到色散曲线,推导了平面波展开法的相关计算公式以及介质球介电常数的函数关系式,探讨了可调参量函数系数I和介质球半径R₁对光子禁带特性的影响。结果表明,3维函数光子晶体呈立方体晶格分布,由介质球填充空气背景;与常规3维介质光子晶体相比,3维函数光子晶体不仅能得到可调谐的光子禁带,而且可以拓展禁带带宽,并增加光子禁带的数量;改变函数系数I的大小可以实现对光子禁带数量、位置和带宽的调谐;改变介质球半径R₁可以对光子禁带带宽实现展宽,并改变光子禁带的位置。该研究对设计新型可调谐器件是有帮助的。     

High-quality SiO2 Colloidal Crystal Fabricated by Controllable Vertical Deposition Method

Monodispersed silica microspheres with diameter of 353 nm were assembled into photonic crystal in ethanol colloidal suspensions of varied silica volume fraction at different temperature and humidity by means of controllable vertical deposition method. The surface morphology and optical properties were studied by SEM and UV-Vis-NIR. It was found that the high quality silica colloidal photonic crystals were obtained from ethanol solutions with environment temperature between 45℃ and 55℃, humidity between 66% and 76%, the volume fraction of microspheres is between 0.8% and 1.5%, The ordered close-packed photonic crystal fabricated by controllable vertical deposition method had the two photonic bandgaps in the visible light band and near infrared band,     

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.     

High Degree of Optical Tunability of Self‐Assembled Photonic‐Plasmonic Crystals by Filling Fraction Modification

The optical properties of two‐dimensional hybrid photonic‐plasmonic crystals are fine‐tuned by modifying the dielectric component of the system. The filling fraction of the dielectric component in monolayers of spheres deposited on gold substrates is controlled by means of oxygen‐plasma etching. Doing so enables spectral tuning of the optical modes of the system. Experiments are performed on both optically passive and active samples showing the possibility for strong modification of the emission properties of samples containing emitters distributed within the spheres. The change in sphere diameter needed to substantially modify the sample's optical response points to a potential use of these samples as sensors or tunable emitting devices if appropriate polymeric components are employed.     

Double‐Inverse‐Opal Photonic Crystals: The Route to Photonic Bandgap Switching

Photonic crystals with a complete bandgap can stop the propagation of light of a certain frequency in all directions. We introduce double‐inverse‐opal photonic crystals (DIOPCs) as a new kind of optical switch. In the DIOPC, a movable, weakly scattering sphere is embedded within each pore of the inverse‐opal photonic crystal lattice. Switching between a diffusive reflector and a photonic crystal environment is experimentally demonstrated. Theory shows that a complete bandgap can be realized that can be opened or closed by moving the spheres. This functionality opens up new possibilities for the control of light emission and propagation. The close link and interaction between the chemical synthesis and the computational design and analysis underlines the interdisciplinary focus of this report.     

Extreme Optical Properties Tuned Through Phase Substitution in a Structurally Optimized Biological Photonic Polycrystal

Biological photonic structures evolved by insects provide inspiring examples for the design and fabrication of synthetic photonic crystals. The small scales covering the beetle Entimus imperialis are subdivided into irregularly shaped domains that mostly show striking colors, yet some appear colorless. The colors originate from photonic crystals consisting of cuticular material and air, which are geometrically separated by a triply periodic D‐surface (diamond). The structure and orientation of the photonic crystals are charactized and it is shown that in colorless domains SiO₂ substitutes the air. The experimental results are incorporated into a precise D‐surface structure model used to simulate the photonic band structure. The study shows that the structural parameters in colored domains are optimized for maximum reflectivity by maximizing the stop gap width. The colorless domains provide a biological example of how the optical appearance changes through alteration of the refractive index contrast between the constituting phases.     

Wafer‐Scale Fabrication of Ordered Binary Colloidal Monolayers with Adjustable Stoichiometries

Colloidal monolayers with high order and increased complexity beyond plain hexagonal packing geometries are useful for 2D templating of surface nanostructures and lithographic applications. Here, binary colloidal monolayers featuring a close‐packed monolayer of large spheres (L) with a superlattice of small particles (S) are prepared in a single step using a Langmuir trough. Adjustment of the stoichiometry of the two particle types at the air–water interface leads to a high degree of control over the occupation of the interstitial sites in the close‐packed layer of large spheres by the small colloids. Thus, large areas of binary 2D crystals with LS₂, LS₆, and LS₉ structures are fabricated in a controlled way. The process allows the formation of binary crystals over a wide range of particle size ratios from 0.19 to 0.40. The pH value of the subphase can be used to enhance the crystallization process by changing the contact angle of the particles at the interface. An interfacial polymerization of butyl cyanoacrylate is used to directly image the contact angle of the colloids at the interface. Transfer to solid substrates is achieved by a surface lowering technique. A variety of substrates with arbitrary topographies can thus be decorated with colloidal monolayers. Applied to a lithographic process, such monolayer architectures allow the generation of complex patterns, not accessible with conventional close‐packed monolayers.     

Morphology‐Controlled Growth of Large‐Area Two‐Dimensional Ordered Pore Arrays

A solution‐dipping template strategy for large‐area synthesis of morphology‐controlled, ordered pore arrays is reported. The morphology of the pore array can easily be controlled by concentration of the precursor solution and treatment conditions. With decrease of the concentration from a high level to a very low level nanostructured complex (pore–hole, and pore–particle) arrays, through‐pore arrays, and even ring arrays can, in turn, be obtained. The pore size is adjustable over a large range by changing the diameter of the template's latex spheres. This synthesis route is universal and can be used for various metals, semiconductors and compounds on any substrate. Such structures may be useful in applications such as energy storage or conversion, especially in integrated next‐generation nanophotonics devices, and biomolecular labeling and identification.     

对胶体球光刻中单层胶体晶体曝光特性的研究

对胶体球光刻中单层胶体晶体(MCC)的曝光特性进行了研究。利用磁控溅射的方法在蓝宝石衬底上生长SiO2薄膜并旋涂光刻胶,通过固液界面自组装的方法在光刻胶上制备了单层胶体晶体。胶体球光刻利用单层胶体晶体作为微透镜阵列,通过紫外曝光的方法在光刻胶上制备微孔阵列。光刻胶上图形的周期性与胶体球的直径有关,并且大直径的胶体球的聚光性能要强于小直径胶体球,在曝光过程中随着曝光时间的增加,由于曝光量的增加以及光刻胶的漂白现象,光刻胶上微孔的尺寸也在增加。最后以曝光后的光刻胶为掩膜,将感应耦合等离子体刻蚀技术(ICP)以及湿法腐蚀相结合,制备出了图形化蓝宝石(PSS)衬底。     

Direct Transcription of Two‐Dimensional Colloidal Crystal Arrays into Three‐Dimensional Photonic Crystals

A simple protocol for the fabrication of three‐dimensional (3D) photonic crystals in silicon is presented. Surface structuring by nanosphere lithography is merged with a novel silicon etching method to fabricate ordered 3D architectures. The SPRIE method, sequential passivation reactive ion etching, is a one‐step processing protocol relying on sequential passivation and reactive ion etching reactions using C₄F₈ and SF₆ plasma chemistries. The diffusion of fresh reactants and etch product species inside the etched channels is found to play an important role affecting the structural uniformity of the designed structures and the etch rate drift is corrected by adjusting the reaction times. High quality photonic crystals are thus obtained by adding the third dimension to the two‐dimensional (2D) colloidal crystal assemblies through SPRIE. Careful adjustments of both mask design and lateral etch extent balance allow the implementation of even more complex functionalities including photonic crystal slabs and precise defect engineering. 3D photonic crystal lattices exhibiting optical stop‐bands in the infrared spectral region are demonstrated, proving the potential of SPRIE for fast, simple, and large‐scale fabrication of photonic structures.     

MS2 (M = Co and Ni) Hollow Spheres with Tunable Interiors for High‐Performance Supercapacitors and Photovoltaics

We demonstrate in this paper facile synthesis of CoS₂ and NiS₂ hollow spheres with various interiors through a solution‐based route. The obtained CoS₂ microspheres constructed by nanosheets display a three‐dimensional architecture with solid, yolk‐shell, double‐shell, and hollow interiors respectively, with continuous changes in specific surface areas and pore‐size distributions. Especially, the CoS₂ hollow spheres demonstrate excellent supercapacitive performance including high specific capacitance, good charge/discharge stability and long‐term cycling life, owing to the greatly improved faradaic redox reaction and mass transfer. Furthermore, CoS₂ hollow spheres exhibit superior electrocatalytic activity for disulfide/thiolate (T₂/T^?) redox electrolyte in dye‐sensitized solar cells (DSCs). Therefore, this work provides a promising approach for the design and synthesis of structure tunable materials with largely enhanced supercapacitor behavior, which can be potentially applied in energy storage devices.     

Significant Exciton Brightening in Monolayer Tungsten Disulfides via Fluorination: n‐Type Gas Sensing Semiconductors

Monolayer transition‐metal dichalcogenides (TMDCs) have recently emerged as promising candidates for advanced photonic and valleytronic applications due to their unique optoelectronic properties. However, the low luminescence efficiency of monolayer TMDCs has significantly hampered their use in these fields. Here it is reported that the photoluminescence efficiency of monolayer WS₂ can be remarkably enhanced up to fourfold through the fluorination, surpassing the reported performance of molecular and/or electrical doping methods. Its degree is easily controlled by changing the fluorine plasma duration time and can also be reversibly tuned via additional hydrogen plasma treatment, allowing for its versatile tailoring for interfacial band alignment and customized engineering. The striking photoluminescence improvement occurs via a substantial transition of trions to excitons as a result of the strong electron affinity of fluorine dopants, and the fluorination enables unprecedented detection of n‐type NH₃ gas in WS₂ due to changed excitonic dynamics showing excellent sensitivity (at least down to 1.25 ppm). This work provides valuable strategies and insights into exciton physics in monolayer TMDCs, opening up avenues toward highly‐efficient 2D light emitters, photovoltaics, nanosensors, and optical interconnects.     

Inside Front Cover: Fine Control of the Wettability Transition Temperature of Colloidal‐Crystal Films: From Superhydrophilic to Superhydrophobic (Adv. Funct. Mater. 2/2007)

A facile strategy for finely controlling the wettability transition temperature of colloidal‐crystal films from superhydrophilic to superhydrophobic is demonstrated by Song and co‐workers on p. 219. The films are assembled from poly(styrene‐n‐butyl acrylate–acrylic acid) latex spheres. The wettability transition temperature of the films is tuned by adjusting the n‐butyl acrylate/styrene balance. This approach offers flexibile fabrication of colloidal crystals with tunable wettability, and can be further extended to general materials. A facile strategy for finely controlling the wettability transition temperature of colloidal‐crystal films from superhydrophilic (water contact angle, CA, 0°) to superhydrophobic (water CA, 150.5°) is demonstrated. The colloidal‐crystal films are assembled from poly(styrene‐n‐butyl acrylate–acrylic acid) amphiphilic latex spheres. The wettability transition temperature of the films can be well tuned by adjusting the n‐butyl acrylate/styrene balance of the latex spheres. Superhydrophobic films are achieved when assembled at 90, 80, 70, 60, 40, or even 20 °C. This approach offers the flexibility of fabricating colloidal crystals with desired and tunable wettability, and can be further extended to general materials, opening up new perspectives in controlling the wettability behavior by chemical composition.     

Periodic TiO2 Nanorod Arrays with Hexagonal Nonclose‐Packed Arrangements: Excellent Field Emitters by Parameter Optimization

Periodic TiO₂ nanorod arrays with hexagonal nonclose‐packed (hncp) arrangements are synthesized by pulsed laser deposition (PLD) using polystyrene colloidal monolayers as templates and with subsequent annealing in air. The hncp‐array formation is governed by in situ volume shrinkage of amorphous TiO₂ nanorods in the crystallizing process during annealing. The array periodicity can easily be tuned by different sphere sizes of the colloidal template, whereas the distance between neighboring nanorods can be controlled by altering the background gas pressure during the PLD process, at a given periodicity for the nanorod array. Parameter‐controlled growth is helpful for investigating and optimizing the parameter‐dependent field‐emission properties. The hncp nanorod array exhibits an enhanced field‐emission (FE) performance compared to both particle films and nanorod arrays with top aggregation. With an increase in periodicity of a hncp nanorod array, the field‐enhancement factor decreases and the turn‐on FE field increases. FE characteristics can be further enhanced by increasing the distance between adjacent nanorods while maintaining the same periodicity. The parameter‐optimized results suggest that the arrays with a smaller periodicity and a larger distance display the best FE performance and could be highly valuable for designing field‐emission devices based on these periodic nanorod arrays.     

The Race for the Photonic Chip: Colloidal Crystal Assembly in Silicon Wafers

This paper surveys recent developments in engineering physics approaches and self‐assembly chemistry methodologies for creating 3D photonic crystals and how this has led to in‐wafer patterned colloidal crystals. These materials are comprised of single crystal micrometer scale features of silica colloidal crystals that have controlled thickness, area, and orientation and are embedded within a single crystal silicon wafer. Two processes for growing opal‐patterned chips are described. One is based upon microfluidic and the other spin coating driven self‐assembly of colloidal silica micro‐spheres within a lithographic patterned silicon wafer.     

Site‐Selective Self‐Assembly of Colloidal Photonic Crystals

A scalable method for site‐selective, directed self‐assembly of colloidal opals on topologically patterned substrates is presented. Here, such substrate contains optical waveguides which couple to the colloidal crystal. The site‐selectivity is achieved by a capillary network, whereas the self‐assembly process is based on controlled solvent evaporation. In the deposition process, a suspension of colloidal microspheres is dispensed on the substrate and driven into the desired crystallization sites by capillary flow. The method has been applied to realize colloidal crystals from monodisperse dielectric spheres with diameters ranging from 290 to 890 nm. The method can be implemented in an industrial wafer‐scale process.     

A Microfluidic Chip with Integrated Colloidal Crystal for Online Optical Analysis

A microfluidic chip that incorporates colloidal crystals inside a microchannel system for on‐chip integration of optical components is presented. It is demonstrated that the use of fluorescent spheres offers added advantages and functionality to the colloidal crystal array. Multifunctional optical components are demonstrated that are able to serve as a reference wavelength calibration line in measured reflectance spectra. Integrating colloidal crystals with varying filtering effects into a microfluidic chip enables selective transmission or blocking of a particular range of wavelengths locally. In addition, combinations of double‐band colloidal crystal filters provide further tunability of the working wavelength for on‐chip detection applications.     

垂直沉积法制备高质量胶体光子晶体

利用乳液聚合方法制备了粒径约为262 nm的单分散聚苯乙烯(PS)微球。通过控制溶剂蒸发温度和液体表面下降的速度,用垂直沉积法较快速地制备出了在较大范围呈现很好有序性的密排结构聚苯乙烯胶体光子晶体,其在626 nm波长处存在光子带隙。在扫描电子显微镜(SEM)下,观察到该胶体光子晶体是面心立方(fcc)密排结构。实验结果表明,对于粒径为262 nm的聚苯乙烯微球,在温度为55℃,质量分数为0.3%的情况下,当液体表面下降的速度约为每天3 mm时,可以得到高质量的胶体光子晶体。这种高质量的胶体光子晶体可以为利用模板技术制备具有完全带隙的有序孔结构提供较理想的模板。