Multilayer Colloidal Crystals Research Articles

Band edge induced plasmonic excitation in heterostructures consisting of rod hyperbolic metamaterials and colloidal photonic crystals

Heterostructures consisting of metal coated rod hyperbolic metamaterials (HMMs) and colloidal photonic crystals (PhCs) are proposed and fabricated. There are two different mechanisms for the reflection valleys induced from the excitation of plasmons in HMMs. Besides the common wave vector matching effects from colloidal gratings, band edge effects provide additional excitations in heterostructures. Slow light induced excitation is verified by separately modifying the photonic bandgap and grating parameters on HMMs using multilayer or monolayer colloidal crystals, 1D PhCs, or ellipsoid arrays, and by modifying the interval or metal thickness in heterostructures. Index-dependent sensitivity of the valleys is enhanced by the bandgap effect.

Formation of Free-Standing Inverse Opals with Gradient Pores.

We demonstrate the fabrication of free-standing inverse opals with gradient pores via a combination of electrophoresis and electroplating techniques. Our processing scheme starts with the preparation of multilayer colloidal crystals by conducting sequential electrophoresis with polystyrene (PS) microspheres in different sizes (300, 600, and 1000 nm). The critical factors affecting the stacking of individual colloidal crystals are discussed and relevant electrophoresis parameters are identified so the larger PS microspheres are assembled successively atop of smaller ones in an orderly manner. In total, we construct multilayer colloidal crystals with vertical stacking of microspheres in 300/600, 300/1000, and 300/600/1000 nm sequences. The inverse opals with gradient pores are produced by galvanostatic plating of Ni, followed by the selective removal of colloidal template. Images from scanning electron microscopy exhibit ideal multilayer close-packed structures with well-defined boundaries among different layers. Results from porometer analysis reveal the size of bottlenecks consistent with those of interconnected pore channels from inverse opals of smallest PS microspheres. Mechanical properties determined by nanoindentation tests indicate significant improvements for multilayer inverse opals as compared to those of conventional single-layer inverse opals.

Rapid electrostatics-assisted layer-by-layer assembly of near-infrared-active colloidal photonic crystals

Here we report a rapid and scalable bottom-up technique for layer-by-layer (LBL) assembling near-infrared-active colloidal photonic crystals consisting of large (⩾1μm) silica microspheres. By combining a new electrostatics-assisted colloidal transferring approach with spontaneous colloidal crystallization at an air/water interface, we have demonstrated that the crystal transfer speed of traditional Langmuir-Blodgett-based colloidal assembly technologies can be enhanced by nearly 2 orders of magnitude. Importantly, the crystalline quality of the resultant photonic crystals is not compromised by this rapid colloidal assembly approach. They exhibit thickness-dependent near-infrared stop bands and well-defined Fabry-Perot fringes in the specular transmission and reflection spectra, which match well with the theoretical calculations using a scalar-wave approximation model and Fabry-Perot analysis. This simple yet scalable bottom-up technology can significantly improve the throughput in assembling large-area, multilayer colloidal crystals, which are of great technological importance in a variety of optical and non-optical applications ranging from all-optical integrated circuits to tissue engineering.

Statistical characterization of self-assembled colloidal crystals by single-step vertical deposition

We have statistically characterized the self-assembly of multi-layer polystyrene colloidal crystals, using the technique of vertical deposition, with parameters chosen to produce thick layers of self-assembled crystals in one deposition step. The size distribution of domains produced with this technique was seen to follow a log-normal distribution, hinting that aggregation or fragmentation phenomena play a role. In addition, using a lithographically directed self-assembly method, we have shown that the size of multi-layer, continuous crack-free domains in lithographically defined areas can be many times larger than in the surrounding areas. In a single deposition step, we have produced continuous colloidal crystal films of 260nm diameter polystyrene spheres approximately 30–40 layers thick, with a controllable lateral size of 80–100μm without lithography, and as high as 250μm with the lithographic template. This method allows us to suppress the domain size fluctuations and produces mesoscopically thick colloidal crystals of selected size at a selected location.

Construction of Mono- and Multilayer Colloidal Crystals with Self-Assembled Nanospheres

A new self-assembly method for the fabrication of periodic structures using monodispersed polystyrene nanoparticles matrix was developed. The self-assembly could be formed into polystyrene nanoparticles matrix constructed by the face centered cubic (FCC) structure and hexagonally close-packed (HCP) monolayer. The polystyrene nanoparticles have been prepared by emulsion polymerization. Several aspects were investigated by using different techniques: Particle sizer, TEM and DSC etc. In this study, the feasibility of synthesizing nanoparticles of 550 nm polystyrene with a perfect spherical shape and a narrow size distribution was demonstrated. Subsequently, an investigation of the self-assembly of polystyrene nanospheres to built up an opal structure was performed. This arrangement was achieved by gravitational sedimentation under vacuum. The face centered cubic structure was identified by using SEM, thus that the different facet type {100}, {110} and {111} were composed. The self-assembly of monodispersed polystyrene nanoparticles in 2D structure was fabricated in the structure of hexagonally close-packed monolayer.

Melting of multilayer colloidal crystals confined between two walls

Video microscopy is employed to study the melting behaviors of multilayer colloidal crystals composed of diameter-tunable microgel spheres confined between two walls. We systematically explore film thickness effects on the melting process and on the phase behaviors of single crystal and polycrystalline films. Thick films (>4 layers) are observed to melt heterogeneously, while thin films (≤4 layers) melt homogeneously, even for polycrystalline films. Grain-boundary melting dominates other types of melting processes in polycrystalline films thicker than 12 layers. The heterogeneous melting from dislocations is found to coexist with grain-boundary melting in films between 5- and 12-layers. In dislocation melting, liquid nucleates at dislocations and forms lakelike domains embedded in the larger crystalline matrix; the "lakes" are observed to diffuse, interact, merge with each other, and eventually merge with large strips of liquid melted from grain boundaries. Thin film melting is qualitatively different: thin films homogeneously melt by generating many small defects which need not nucleate at grain boundaries or dislocations. For three- and four-layer thin films, different layers are observed to have the same melting point, but surface layers melt faster than bulk layers. Within our resolution, two- to four-layer films appear to melt in one step, while monolayers melt in two steps with an intermediate hexatic phase.

Colloidal Crystals as Templates for Light Harvesting Structures in Solar Cells

Abstract Monolayer colloidal crystals are formed using silica- and polystyrene beads and used as etch masks for the formation of regular, _m period hexagonal arrays of indentations in a silicon wafer. Such patterns can be used as di_raction gratings or as seeds for further processing, for example by pit-catalysed electrochemical etching. In another experiment, multilayer colloidal crystals are infiltrated with titania before subsequent removal of the beads, forming inverse opals displaying tuneable reflectivities which are interesting for use as selective reflectors.

Controlled Directionality of Ellipsoids in Monolayer and Multilayer Colloidal Crystals

We present a self-assembly approach for monolayer and multilayer deposition of ellipsoids with a controllable direction. The direction of the ellipsoids in the assembly can be conveniently tuned by external applied magnetic field. This level of control on positional and directional order suggests a way to build monolayer templates for lithography with two-dimensional patterns and three-dimensional anisotropic photonic crystals, which may open the way toward the complete photonic band gap in the visible.

Colloidal Woodpile Structure: Three-Dimensional Photonic Crystal with a Dual Periodicity

With planar photolithography and self-assembly techniques, multilayer colloidal crystals with a woodpile structure were fabricated. They represent a new kind of photonic crystals, that is, three-dimensional (3D) photonic crystals with a dual periodicity; one comes from the face-centered cubic (fcc) structure within the colloidal crystal strips and the other one results from the periodic arrangement of the colloidal crystal strips.

Engineered Multilayer Colloidal Crystals with Tunable Optical Properties

Multilayer colloidal crystals with a perfectly defined architecture have been fabricated by the successive depositions of layers of silica particles with various diameters onto a substrate. The composite materials have been characterized by scanning electron microscopy (SEM) and near-infrared (NIR) spectroscopy. Our results establish that the optical properties of these multilayer systems can be tailored by adjusting either the thickness or the stacking order of each component crystal. The versatility of our approach has been further exploited to fabricate a multilayer colloidal crystal consisting of many alternating layers of two different spheres sizes.

Self-poisoning of crystal nuclei in hard-rod liquids.

We report a Monte Carlo study of the pathway for crystal nucleation in a fluid of hard, colloidal rods. In the earliest stages of nucleation, a lamellar crystallite forms. Subsequent thickening of this lamella is hampered by the fact that the top and bottom surfaces of this crystallite are preferentially covered by rods that align parallel to the surface. As a consequence, subsequent growth of individual crystals is stunted. Experimental evidence for such stunted crystal growth has recently been reported by Maeda and Maeda in experiments on suspensions of colloidal rods [Phys. Rev. Lett. 90, 018303 (2003)]]. The simulations suggest that, in experiments, the growth of multilayer colloidal crystals can be selectively enhanced by the application of an external aligning field.

The Fabrication and Bandgap Engineering of Photonic Multilayers

This communication describes a method to fabricate multilayer colloidal crystals formed by the layer-by-layer deposition of silica beads on a glass substrate. Each layer of the crystal consists of a three-dimensionally ordered array of close-packed colloids. These multilayer samples are amenable to templating methods for tuning the dielectric contrast of the material. The resulting photonic crystal structures exhibit optical properties which resemble the superposition of the properties of each individual crystal, with additional structure that suggests the onset of superlattice-type miniband formation. These multilayer structures thus afford new opportunities for engineered photonic behavior. Traditionally colloidal crystals are three dimensional periodic structures formed from monodisperse colloids. Because of their diffractive optical properties they are a type of photonic crystal and may have applications as optical filters and switches, high density magnetic data storage devices, chemical and biochemical sensors, or as removable scaffolds for the formation of highly ordered, macroporous materials. They are also useful as model systems for fundamental studies of crystal melting and phase transition behavior. The process of colloidal crystallization has been extensively studied, leading to the development of several methods to make high quality colloidal crystals with few crystalline defects. These techniques include electrostatically induced crystallization, gravity sedimentation, electro-hydrodynamic deposition, colloidal epitaxy, physical confinement and convective self-assembly. Bimodal AB2 and AB13 colloidal crystals with complex structures have also been observed in binary mixtures of hard-sphere colloids with specific radii ratios. Here we report a method to make a new form of colloidal crystal, a multilayer crystal, using successive deposition of crystals of colloids of arbitrary sizes. The multilayer colloidal crystal is schematically represented in Figure 1A. Spheres of different colors represent submicrometer silica or polystyrene colloids of different sizes. Each layer of the crystal is a close-packed array of colloids, and the overall structure consists of successively stacked crystals, formed of colloids of arbitrary diameters. The preparation of these structures is described in the experimental section. The high uniformity of the resulting crystals can be illustrated by the transmission (Fig. 1B) and reflection (Fig. 1C and D from different angles) photographs of a threelayer crystal. This sample is formed by consecutive deposition of 13 layers of 430 nm silica spheres, followed by 16 layers of 253 nm silica spheres, followed by 10 layers of 338 nm silica spheres. We describe the multilayer colloidal crystal pattern by listing the sphere size from bottom to top. For example, the sample in Figure 1 is referred to as 430 nm/253 nm/338 nm. The reflected colors are caused by Bragg diffraction of visible light by the three-dimensionally ordered arrays of submicrometer colloids. When two overlapping layers are made from colloids with extremely different sizes, most of the reflected light from the bottom layer will transmit through the upper layer, resulting in the transparent appearance of the second layer in Figure 1C (430 nm/253 nm). Crystalline quality is among the most important parameters in determining the performance of colloidal crystals in optical applications. Figure 2 shows the typical top-view and crosssectional scanning electron microscopy (SEM) images of each astepo of the sample shown in Figure 1. In Figure 2A, the hexagonal close-packed (hcp) arrangement of the top 430 nm layer is evident. The sharp peaks in the two-dimensional fast Fourier transform (FFT, inset) of a low-magnification image confirm the presence of long-range crystalline order extending over the largest length scales (40 ” 40 lm) accessible in a single image. The stacking of close-packed layers shown in Figure 2B demonstrates the high degree of order along the (111) crystallographic axis, perpendicular to the substrate.