Fabrication Of Colloidal Crystals Research Articles

Synthesis of Polydispersed P(St-MMA-AA) Microspheres and Fabrication of Colloidal Crystals with Non-Compact Morphology

Great attention has been given to the synthesis of monodispersed and uniform colloidal polymer particles with controllable particle sizes. However, investigations on the preparation, characterization and fabrication of polydisperse polymeric particles are very few. Polydisperse polymers are also needed to meet the new demands of the modern markets due to the ease of their synthesis. This study, therefore, synthesized polydispersed poly(styrene-methyl-methacrylate acrylic-acid) (P(St-MMA-AA)) via emulsion polymerization synthetic approach under unstable reaction conditions. The synthesized P(St-MMA-AA) microparticles were characterized using Fourier transform infrared spectroscopy (FTIR), dynamic light scattering (DLS), a thermo-gravimetric analyzer (TGA) and scanning electron microscope (SEM). The DLS results revealed a highly polydispersed P(St-MMA-AA) microparticles with an average particle diameter of 803.4 nm. SEM, TEM (transmission electron microscopy) and AFM (atomic force microscopy) analyses showed that the particles were spherical and polydispersed. The unequal diameters and non-compact nature of the fabricated colloidal crystals may have affected its possible uses in some specific applications such as decorative coatings, biomedical and sensing applications. However, other materials can be incorporated into the polydispersed prepared P(St-MMA-AA) colloidal microspheres to improve their properties and possible applications.
 Keywords: Emulsion polymerization; Polydispersed; Polymer; P(St-MMA-AA).

From colloidal particles to photonic crystals: advances in self-assembly and their emerging applications.

Over the last three decades, photonic crystals (PhCs) have attracted intense interests thanks to their broad potential applications in optics and photonics. Generally, these structures can be fabricated via either "top-down" lithographic or "bottom-up" self-assembly approaches. The self-assembly approaches have attracted particular attention due to their low cost, simple fabrication processes, relative convenience of scaling up, and the ease of creating complex structures with nanometer precision. The self-assembled colloidal crystals (CCs), which are good candidates for PhCs, have offered unprecedented opportunities for photonics, optics, optoelectronics, sensing, energy harvesting, environmental remediation, pigments, and many other applications. The creation of high-quality CCs and their mass fabrication over large areas are the critical limiting factors for real-world applications. This paper reviews the state-of-the-art techniques in the self-assembly of colloidal particles for the fabrication of large-area high-quality CCs and CCs with unique symmetries. The first part of this review summarizes the types of defects commonly encountered in the fabrication process and their effects on the optical properties of the resultant CCs. Next, the mechanisms of the formation of cracks/defects are discussed, and a range of versatile fabrication methods to create large-area crack/defect-free two-dimensional and three-dimensional CCs are described. Meanwhile, we also shed light on both the advantages and limitations of these advanced approaches developed to fabricate high-quality CCs. The self-assembly routes and achievements in the fabrication of CCs with the ability to open a complete photonic bandgap, such as cubic diamond and pyrochlore structure CCs, are discussed as well. Then emerging applications of large-area high-quality CCs and unique photonic structures enabled by the advanced self-assembly methods are illustrated. At the end of this review, we outlook the future approaches in the fabrication of perfect CCs and highlight their novel real-world applications.

Photonics of Template-Mediated Lattices of Colloidal Clusters.

The recent progress in microfluidic microfabrication enables mass production of "colloidal molecules" with a preprogrammed geometry (e.g., dumbbells, tetrahedrons, etc.). Such colloids can be used as elementary building blocks in the fabrication of colloidal crystals with unique optical properties. Anisotropic clusters, however, cannot be readily assembled into regular lattices. In this paper, we study photonic properties of compact cubic templates of microdrops encapsulating complex "colloidal molecules". Because monodisperse droplets can be easily packed into dense cubic lattices and encapsulation techniques (e.g., using microfluidics) are well developed, such a material is experimentally feasible. The rationale behind such a methodology is that for a particular alignment of the encapsulated "colloidal molecules" (e.g., by applying an external magnetic or electric field), the resulting structures resemble a diamond lattice, which is known to exhibit a wide complete photonic band gap. The photonic properties of two cubic templates encapsulating dumbbells (symmetric and asymmetric) and tetrahedrons are investigated numerically. In particular, we show the emergence of the complete 3D band gap (∼8% wide for the dielectric contrast ε = 14) for symmetric dumbbells embedded within a face-centered cubic template and oriented along the space diagonal of the elementary cubic cell.

Ethanol vapor-induced fabrication of colloidal crystals with controllable layers and photonic properties.

A novel fabrication method for colloidal crystals has been proposed for the first time in this research. In this method, a suspension droplet containing colloidal particles was first spread onto a glass substrate placed in an ethanol vapor environment, and then the droplet was extracted from its center. In that case, the contact angle of the droplet reduced and the contact line receded toward the center, during which the colloidal particles self-assembled and immobilized forming a 2D colloidal crystal film on the substrate upon drying the liquid film. Alternately spreading and drying of suspension films could construct fine multi-layers of colloidal crystals, while the ethanol fraction in the suspension would be used to control roughly but rapidly the layer numbers of colloidal crystals. It was also found that the photonic properties of resultant colloidal crystal films were elevated by increasing their thickness.

Fabrication of Colloidal Crystals on Different Patterned Silicon Substrates by Self-Assembly Method

Silica and polystyrene (PS) microspheres assembled on two quite different patterned silicon substrates, cross-like pillar pattern and eye-like pattern, respectively. The results indicated that the surface pattern imposes a predetermined lattice orientation in colloidal crystals (CCs). Other influent factors, such as microsphere size, the altitude of pattern and the concentration of colloidal suspension, may also play an important role on the self-assembly process.

Deformation of Colloidal Crystals for Photonic Band Gap Tuning

In this article, we demonstrate the fabrication of colloidal crystals made of polystyrene oblate particles with face centered lattice structures. Monodisperse spherical polystyrene nanospheres were self-assembled into colloidal crystal followed by the infiltration of polydimethylsiloxane (PDMS) precursors into the self-assembled structure. After curing the infiltrated PDMS precursors, the composite colloidal crystals were deformed by compressive stress acting parallel or vertical to the [111] direction of the face centered cubic lattice of the colloidal crystals. After deformation, the shape of the constituent spherical polystyrene particles was changed into oblate ellipsoidal morphology. Depending on the direction of the compressive stress, the stop bands of deformed colloidal crystals were controlled and reflectance spectra were measured to observe the blue-shift or red-shift phenomena of the stop band after deformation. Shear stress was also applied to the composite colloidal crystal to change the morphologies of the constituent polystyrene particles into oblate spheroids, and the stop band position and the band gap width of the reflectance spectrum were modified.

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.

Fabrication of Colloidal Crystals with Hierarchical Structure and Its Water Adhesion Properties

A facile fabrication method of colloidal crystals with hierarchical structure has been presented. The colloidal crystals were assembled from latex spheres with various uneven surfaces, which were simply synthesized via emulsion polymerization by varying initiator feeding times. The as-prepared colloidal crystals demonstrated the water adhesion properties, which could be modulated by designing the hierarchical structure of latex surface. The result provided a simple fabrication method for colloidal crystals with hierarchical structure and high water adhesion force. This would greatly extend the applications of colloidal crystals to many other important fields, for instance, microfluidic device.

Fabrication of Tunable Spherical Colloidal Crystals Immobilized in Soft Hydrogels

Spherical colloidal crystals canbe fabricated using droplets as templates; droplets of aqueouscolloidalsuspensionaredispensedonahydrophobicsurfaceorin oil and then dried. The evaporation of the liquid mediumleads to the assembly of particles into close-packed structureswithasphericalgeometry.Sinceupondryingtheparticlestouchoneanother,thecrystallatticeconstantisuniquelydeterminedby the diameter of the particles. Therefore, fabrication ofcolloidal crystals with different wavelengths for their opticalstop band requires colloidal particles with different size orrefractiveindex.Itisdifficulttotunethediffractionwavelengthafter the colloidal crystal spheres are formed; this lack oftunabilitylimitstheutilityofthesestructuresformanypracticalapplications.Analternativeroutetotheformationofsphericalcolloidalcrystals that may overcome the limitations in tunability isthrough the use of charged colloids. They can form crystallinestructures but with a lower packing density in a liquid mediumdue to their repulsive electrostatic interaction between theparticles.

Fabrication of Colloidal Crystals with Defined and Complex Structures via Layer-by-Layer Transfer

A new and versatile way--using poly(dimethylsiloxane) (PDMS) sheets to layer-by-layer (LbL) transfer hexagonal-close-packed particle monolayers from preformed colloidal crystals and stack them on substrates-has been demonstrated to create colloidal crystals. This approach allows LbL control of the thickness of the resulting crystals and especially of the size and the packing structure of the particles in each layer. Furthermore, it also allows fabrication of binary colloidal crystals over large areas by deformation of the PDMS sheets during LbL transfer. Two new binary crystals-one composed of identically sized particles but in different densities and the other of a nonclose-packed monolayer of large particles and a close-packed monolayer of small particles-were created, which are hard grown by other colloidal crystallization techniques developed thus far.

Formation and Drying of Colloidal Crystals Using Nanosized Silica Particles

Much interest has been generated in the fabrication of colloidal crystals from suspensions because of the promise of photonic band gap applications. However, since the case of small, nonsedimenting colloidal particles indeed remains rather rarely treated, spherical silica particles with diameters varying from 75 down to 20 nm have been used in the present work to fabricate colloidal crystals by drying the suspending liquid. Typical events that take place during the drying process of a particulate film, such as cracking, compaction and penetration of air into a porous network, have been evaluated using existing theories, and the maximum stress in the drying film could be approximated. Investigation on the dry film structure by scanning electron microscopy showed the arrangement of particles in a close-packed system. To interpret the formation of such crystals, the amplitudes of the interparticle and capillary forces have been estimated from existing models. The repulsive interparticle forces allow the particles to remain stable and thus rearrange up to fairly high particle concentration. These modeling results showed the dominance of the capillary contribution at the end of the drying process. Nitrogen adsorption/desorption measurements gave very coherent results regarding both pore volume and pore size of the dry particulate films when compared to the expected ordered packing arrangements.

The Effects of Surfactants on the Morphology of Colloidal Crystals in Self-assembly

AbstractIn this research, sodium dodecyl sulfates (SDS) and N-cetyl-n,n,n-trimethyammonium bromide (CTAB) surfactant solutions are used as solvents of polystyrene (PS) colloidal suspension during the fabrication of colloidal crystals. The effects of the surfactant on the quality and the morphology of the colloidal crystals are studied. It was found that surfactants not only change the charge of PS colloidal particles, but also significantly changed the surface tension and the 3 phase contact angle of the suspension with respect to the glass substrate, in turn they change the thickness of the formed crystal as well as the crystal structure. The derived knowledge will be potentially useful in clarifying the mechanisms involved in the formation of colloidal crystals.