Colloidal Crystal Structures Research Articles

Experimental investigation of photonic band gap influence on enhancement of Raman-scattering in metal-dielectric colloidal crystals

A simple chemical technique is implemented to fabricate a metal-dielectric colloidal crystal structure (MDCS) in order to enhance the otherwise weak Raman signals by combining the effects of localized surface plasmon resonance (LSPR) enhancement due to gold nanoparticles, precise field confinement of dielectric and air bands in the periodic dielectric structure and field enhancements at the photonic band gap (PBG) edges. The higher density of electromagnetic modes (DOS) near these band edges is explained as due to the reduced group velocity at the photonic band gap edges. Intense electric field strength due to the excitation of high DOS at the edges of PBG of MDCS and the LSPR excitation through field confinement in the dielectric medium of MDCS are employed to study the Raman-scattering signals of adsorbed benzenethiol (BT) molecule on the MDCS. Large enhancement for the Raman signal in MDCS in comparison to the Raman spectra observed for BT molecule dispersed on sputtered gold film shows the effectiveness of MDCS for the micro-level detection of the analyte.

Enhanced photoluminescence of ordered macroporous germanium electrochemically prepared from ionic liquids

Recently we reported our results on the successful synthesis of 3-D highly ordered macroporous (3DOM) structure of germanium via the template-assisted electrochemical deposition from air- and water stable ionic liquids. Herein we report our new results on the photoluminescence (PL) properties of the obtained ordered macroporous Ge and the Ge/polystyrene composite opal structure. The latter showed a strong green emission compared to a Ge film and a Ge inverse opal. The enhancement of PL intensity was ascribed to the disorder multiple scattering in polystyrene colloidal crystal structure which increased both the excitation light absorption efficiency and the light extraction efficiency. The X-ray photoelectron spectroscopy (XPS) results suggested that the ordered macroporous Ge was capped with an oxide layer including a considerable amount of GeO(2). The observed green emission (539 nm) was related to GeO(2), likely resulting from the Ge-O bond related intrinsic defects.

Structure and physical properties of colloidal crystals made of silica particles

The relationship between the particles size distribution of colloidal crystals and the structure and physical properties of the resulting 2D colloidal crystals is presented in this work. The colloids were constituted of silica sub-micron spheres with different size distributions comprised between 150 and 520nm, synthesized by the Stöber method and assembled in one monolayer through the Langmuir–Blodgett technique. Optical and wetting properties of the resulting crystals were studied by different techniques – UV–visible spectrometry, spectroscopic ellipsometry and contact angle measurements. They were related to the crystal structure which was analyzed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The conservation of orientational and translational orders over a large area is found for crystals with narrow particle size distribution. Optical interference bands in the transmittance spectra are linked to the crystal periodicity and the presence of optical anisotropy is attributed to distortions in the film generated by the deposition process. Moreover, large particle size distribution results in the most hydrophobic crystals. Thus the physical properties of the colloidal crystals can be tailored or tuned by controlling the particle size distribution and the deposition parameters.

Direct visualization of three-dimensional crystallization behavior in microgels

We use confocal microscopy to study the three-dimensional (3D) structure of colloidal crystals formed by poly(N-isopropylacrylamide)-co-(acrylic acid) microgels of diameter 1.0-1.5 μm. The confocal images are tracked to locate particle positions in 3D, which are used to compute pair-correlation functions g(r), bond order parameters, and structure factors s(q). We find that the structure remains fcc for a range of charge, size, and concentration of the particles. When the particles are weakly attractive and are at low concentrations, polycrystalline solids result. In addition, owing to the compressibility of the colloids, the crystals display remarkable structural stability when subjected to external stress.

Three-Dimensional Structure and Defects in Colloidal Photonic Crystals Revealed by Tomographic Scanning Transmission X-ray Microscopy

Self-assembled colloidal crystals have attracted major attention because of their potential as low-cost three-dimensional (3D) photonic crystals. Although a high degree of perfection is crucial for the properties of these materials, little is known about their exact structure and internal defects. In this study, we use tomographic scanning transmission X-ray microscopy (STXM) to access the internal structure of self-assembled colloidal photonic crystals with high spatial resolution in three dimensions for the first time. The positions of individual particles of 236 nm in diameter are identified in three dimensions, and the local crystal structure is revealed. Through image analysis, structural defects, such as vacancies and stacking faults, are identified. Tomographic STXM is shown to be an attractive and complementary imaging tool for photonic materials and other strongly absorbing or scattering materials that cannot be characterized by either transmission or scanning electron microscopy or optical nanoscopy.

Spectral reflectance switching of colloidal photonic crystal structure composed of positively charged TiO2 nanoparticles

We report the tunable photonic crystal behavior of highly concentrated, mildly charged TiO2 nanoparticle colloids under external electric field. Reflectance intensity change at the certain photonic bandgap and its irreversible switching characteristics are observed and discussed. Through computer aided numerical analysis, gradient spacing between charged particles is predicted under electric field, which is well coincidence with experimental results. Also, we report the experimental evidence that the charged nanoparticle can play the role as a major charge carrier in non-aqueous liquid. Its short range moving and stacking on electrode cause the capacitive current flow and charge accumulation like ions in liquid state.

Structure of a large colloidal crystal – controlling orientation and three-dimensional order

The three-dimensional crystal structure of charge stabilised polystyrene latex in deionised water was investigated by small-angle neutron diffraction. Crystallisation with a grain size of approximately 1 × 1 cm2 was observed when the sample was flowed gently in to a 2 mm path cell. Bragg scattering peaks arising from the structure were observed under rotation about three perpendicular axes of the sample. The diffraction patterns indicate clearly that there is a cubic close packed structure with a 110 axis perpendicular to the cell wall. Rotations in small steps show large changes and indicate that the crystal is well oriented and has three-dimensional order. The crystal orientation was controlled by the meniscus and direction of flow when filling the cell.

Anisotropic photonic crystal building blocks: colloids tuned from mushroom-caps to dimers

Nonspherical polystyrene particles tuned from collapsed spheres (mushroom cap and hemisphere shapes) to two-lobed dimers are synthesized by seeded emulsion polymerization. The presence of a swelling solvent and additions of divinylbenzene crosslinker facilitate shape control through directing phase separation of the polymer network and the monomer in the swollen particles as well as the solvent release behavior. Confocal microscopy of the particles, dyed in the initial swelling and polymerization stage followed by a second stage, differentiates the locations of effective seeds and second stage polymerized material which dictate the external particle morphology. The anisotropic particles are prepared in sufficient quantity and uniformity to self-organize into colloidal crystal and rotator structures, as is demonstrated with a mushroom cap-shaped particle system condensed under confinement. Structural transitions follow the drive to maximize packing efficiency at systematically varied confinement heights. Confocal microscopy confirms monolayer, buckled and bilayer thin films with unconventional orientational ordering.

Hollow Polypyrrole Films: Applications for Energy Storage Devices

Hexagonally arrayed structures of colloidal crystals with uniform particle size are good candidates for templates for synthesizing porous nanostructures. Herein, we present the first reported use of three-dimensionally (3D) ordered porous structures to increase the energy storage abilities of conducting polymer based energy storage devices. A robust 3D colloidal crystal template was prepared using a colloidal suspension. Polypyrrole (PPy) was deposited into the interstices between the colloidal spheres by electrochemical infiltration. PPy inverse nanoporous structures were obtained by dissolving the colloidal template in a solvent (99.5% Benzene). The morphologies of nanoporous PPy structures and their electrochemical characteristics were investigated.

Fabrication of TiO<SUB>2</SUB> Colloidal Crystal Films and Characterization of Their Photocatalytic Properties

We have studied hydrolysis of organic alkyltitanate compounds and optimized reaction condition for synthesis of monodisperse titania (TiO2 colloidal particles with controlled size from nanometer to submicron. The synthesized TiO2 colloidal particles were further surface-modified with hydrophobic silane coupling agent. With the monodisperse hydrophobic particles, we fabricated TiO2 colloidal crystal thin films through transferring self-assembled colloidal crystal monolayer from water surface onto solid substrates. The TiO2 colloidal crystal films exhibit enhanced interaction with visible light. Consequently, in comparison with plain TiO2 particle thin film, the thin film with colloidal crystal structure shows enhanced photocatalytic activity, as evaluated through photodegradation of organic dye methyl orange in solution under simulated solar light.

Self-Assembly and Reconfigurability of Shape-Shifting Particles

Reconfigurability of two-dimensional colloidal crystal structures assembled by anisometric particles capable of changing their shape were studied by molecular dynamics computer simulation. We show that when particles change shape on cue, the assembled structures reconfigure into different ordered structures, structures with improved order, or more densely packed disordered structures, on faster time scales than can be achieved via self-assembly from an initially disordered arrangement. These results suggest that reconfigurable building blocks can be used to assemble reconfigurable materials, as well as to assemble structures not possible otherwise, and that shape shifting could be a promising mechanism to engineer assembly pathways to ordered and disordered structures.

Fabrication of colloidal crystals on hydrophilic/hydrophobic surface by spin-coating

Herein, we demonstrate the structure of the PS colloidal crystals which were fabricated on the hydrophilic/hydrophobic Si wafers by a spin-coating technique. Monodisperse PS colloids are spin-coated onto self-assembled monolayers of 3-(aminopropyl)triethoxysilane and propyltrimethoxysilane coated Si wafers. PS spheres organized as ordered close-packed face-centered cubic structure with (1 1 1) planes on the hydrophilic surface while they gathered without the crystal structure on the hydrophobic surface. This paper also reports a simple and rapid method to fabricate the close-packed structure of hollow TiO 2 spheres. The colloidal crystal of TiO 2 hollow spheres was prepared using the PS sphere template on the hydrophobic surface. The mechanism for the growing multilayers of self-assembled PS particles from a suspension onto a hydrophilic and hydrophobic Si wafer substrates using the spin-coating method at various rotating speeds is also discussed in this paper.

Electrostatic and capillary force directed tunable 3D binary micro- and nanoparticleassemblies on surfaces

We report a simple, rapid and cost-effective method based on evaporation induced assembly togrow 3D binary colloidal assemblies on a hydrophobic/hydrophilic substrate by simple dropcasting. The evaporation of a mixed colloidal drop results in ring-like or uniform areadeposition depending on the concentration of particles, and thus assembly occurs at theperiphery of a ring or uniformly all over the drop area. Binary colloidal assemblies ofdifferent crystal structure are successfully prepared over a wide range of size ratios (γ = small/large) from 0.06 to 0.30by tuning the γ of the micro- and nanoparticles used during assembly. The growth mechanism of 3D binarycolloidal assemblies is investigated and it is found that electrostatic forces facilitateassembly formation until the end of the evaporation process, with capillary forces alsoplaying a role. In addition, the effects of solvent type, humidity, and salt concentrationon crystal formation and ordering behaviour are also examined. Furthermore,long range, highly ordered binary colloidal assemblies can be fabricated by thechoice of a low conducting solvent combined with evaporation induced assembly.

Facile Synthesis of TiO2 Inverse Opal Electrodes for Dye-Sensitized Solar Cells

Engineering of TiO(2) electrode layers is critical to guaranteeing the photoconversion efficiency of dye-sensitized solar cells (DSSCs). Recently, a novel approach has been introduced for producing TiO(2) electrodes using the inverted structures of colloidal crystals. This paper describes a facile route to producing ordered macroporous electrodes from colloidal crystal templates for DSSCs. Using concentrated colloids dispersed in a volatile medium, the colloidal crystal templates were obtained within a few minutes, and the thickness of the template was easily controlled by changing the quantity of colloidal solution deposited. Here, the effects of the structural properties of the inverse opal TiO(2) electrodes on the photovoltaic parameters of DSSCs were investigated. The photovoltaic parameters were measured as a function of pore ordering and electrode film thickness. Moreover, DSSC applications that used either liquid or viscous polymer electrolyte solutions were investigated to reveal the effects of pore size on performance of an inverse opal TiO(2) electrode.

Rapid determination of colloidal crystal's structure by reflection spectrum

We prove that the reflection spectrum (RS), which was thought to be not able to determine the crystal structure, can be used to determine the colloidal crystal's structure with a rather fast sample rate. This method shows its superiority by comparing with conventional static light scattering (SLS) method. The result greatly extended the applicability of the reflection spectrum method, which has already been used in measuring lattice spacing and crystal size, etc. in the study of colloidal crystals. Therefore, it becomes possible for the reflection spectrum method to simultaneously and rapidly determine the crystal structure, lattice constant and crystal size, etc. of colloidal crystals, which is essential for deep understanding of phase behavior and crystallization kinetics.

Dimer‐Based Three‐Dimensional Photonic Crystals

AbstractThe self‐assembly of polystyrene dimer‐ and spherocylinder‐shaped colloids is achieved via controlled drying on glass and silicon substrates. 3D monoclinic colloidal crystal structures are determined from scanning electron microscopy images of sections prepared using focused ion‐beam (FIB) milling. Full photonic bandgaps between the eighth and ninth bands are found for a systematic range of colloidal dimer shapes explored with respect to the degree of constituent lobe fusion and radius ratio. The pseudogap between bands 2 and 3 for spherocylinder‐based monoclinic crystals is also probed using normal incidence reflection spectroscopy.

Colloidal Self‐Assembly Meets Nanofabrication: From Two‐Dimensional Colloidal Crystals to Nanostructure Arrays

Self-assembly of colloidal microspheres or nanospheres is an effective strategy for fabrication of ordered nanostructures. By combination of colloidal self-assembly with nanofabrication techniques, two-dimensional (2D) colloidal crystals have been employed as masks or templates for evaporation, deposition, etching, and imprinting, etc. These methods are defined as "colloidal lithography", which is now recognized as a facile, inexpensive, and repeatable nanofabrication technique. This paper presents an overview of 2D colloidal crystals and nanostructure arrays fabricated by colloidal lithography. First, different methods for fabricating self-assembled 2D colloidal crystals and complex 2D colloidal crystal structures are summarized. After that, according to the nanofabrication strategy employed in colloidal lithography, related works are reviewed as colloidal-crystal-assisted evaporation, deposition, etching, imprinting, and dewetting, respectively.

Fabrication of Unconventional Colloidal Self-Assembled Structures

Although colloidal self-assembled structures have many potential applications in many fields, the limited structure of colloidal crystals has hindered their widespread use. In this study, we have proposed an approach to fabricating various types of 2D colloidal self-assembled structures that cannot be achieved using a conventional colloidal self-assembly method. Several types of colloidal assembly structures including lines, zigzags, and dimers were fabricated by adjusting the lattice space of the bottom layer of binary colloidal crystals. In addition, by selectively sintering the top layer of these new types of binary structures, nanohole arrays and zigzag colloidal rods were produced. This proposed method expands the diversity of colloidal crystal structures, thus colloidal crystals could possibly replace other expensive nanofabrication methods.

Drying dissipative structures of colloidal crystals of silica spheres coated with polymer brushes of poly(carboxymethyl betaine)

Drying patterns of colloidal crystals of colloidal silica spheres coated with the brushes of zwitterionic poly(carboxymethyl betaine) (SiP-PCMB) and their parent silica spheres (SiP) were studied on a cover glass, a watch glass, and a Petri glass dish. Crystal structures kept the whole process of dryness of the suspensions of SiP-PCMB and SiP. Crystal structures of the dried films of SiP-PCMB were kept stable even when the initial suspensions contained 5 mM of sodium chloride, which is the important role of the excluded volume effects of the shells of the polymer brushes. On the other hand, crystal structures of SiP spheres in the dried films were much unstable and melted in the presence of 5 mM sodium chloride. In the suspension state, colloidal crystallization of SiP-PCMB took place stably by the contribution of the excluded volume effects besides the extended electrical double layers compared with that of SiP spheres, where only the double layer effect contributes to the crystallization. The fractal patterns of the complexation of SiP-PCMB or SiP spheres with sodium chloride were observed microscopically in the dried films. Several kinds of dissipative crystallization such as array and/or accumulation of the crystallites were observed, and the importance of the convectional and sedimentation processes during the course of dryness was demonstrated.

Coherent x-ray imaging of defects in colloidal crystals

Coherent x-ray diffractive imaging (CXDI) was applied to reveal the structure of colloidal crystals. The colloidal sample was illuminated by a coherent x-ray beam through a $7\text{ }\ensuremath{\mu}\text{m}$ pinhole aperture. The resulting diffraction patterns contain several Bragg peaks and an additional interference structure between the peaks due to the coherent illumination of a finite part of the sample. The inversion of these diffraction patterns reveals the arrangement of colloidal particles in a face-centered cubic (fcc) lattice as well as defects in the form of stacking faults in the (111) planes.