C.C. Wong

Layer transfer approach to opaline hetero photonic crystals.

By taking advantage of the hydrophobicity of dry polystyrene colloidal crystal (opal) films and the large surface tension of water, a convectively self-assembled polystyrene opal film on a hydrophilic glass substrate can be peeled off from the substrate and floated on the water surface. A layer transfer technique was developed to sequentially stack floating opal films of different sphere sizes, resulting in opaline hetero photonic crystals. The feasibility of this technique to planar defect engineering in a self-assembled colloidal photonic crystal was also demonstrated. Both structural observation and optical characterization confirmed the crystalline integrity of the resultant opaline heterostructures.

Particle and substrate charge effects on colloidal self-assembly in a sessile drop.

By direct video monitoring of dynamic colloidal self-assembly during solvent evaporation in a sessile drop, we investigated the effect of surface charge on the ordering of colloidal spheres. The in situ observations revealed that the interaction between charged colloidal spheres and substrates affects the mobility of colloidal spheres during convective self-assembly, playing an important role in the colloidal crystal growth process. Both ordered and disordered growth was observed depending on different chemical conditions mediated by surface charge and surfactant additions to the sessile drop system. These different self-assembly behaviors were explained by the Coulombic and hydrophobic interactions between surface-charged colloidal spheres and substrates.

Kinetic stages of single-component colloidal crystallization.

To harness the full potential of colloidal self-assembly, the dynamics of the transition between colloids in suspension to a colloidal crystalline film should be better understood. In this report, the structural changes during the self-assembly process in a vertical configuration for colloids in the size range 200-400 nm are monitored in situ, using the transmission spectrum of the colloidal assembly treated as an emergent photonic crystal. It is found that there are several sequential stages of colloidal ordering: in suspension, with a larger lattice parameter than the solid state, in a close-packed wet state with solvent in the interstices, and, finally, in a close-packed dry state with air in the interstices. Assuming that these stages lead continuously from one to another, we can interpret colloidal crystallization as being initiated by interparticle forces in suspension first, followed by capillary forces. This result has implications for identifying the optimum conditions to obtain high-quality nanostructures of submicrometer-sized colloidal particles.

Particulate mobility in vertical deposition of attractive monolayer colloidal crystals.

In the colloidal self-assembly of charged particles on surfaces with opposite polarity, disorder often dominates. In this report, we show that ionic strength, volume fraction, and solvent evaporation temperature can be optimized in the vertical deposition method to yield hexagonal close-packed monolayer arrays with positively charged colloids on negatively charged bare glass. We further extend our study to form well-defined binary two-dimensional superlattices with oppositely charged monolayers grown layer-by-layer. Our results suggest that the lack of particulate mobility in oppositely charged systems is the main cause of disorder, and maximum mobility is attained when all three growth parameters are finely adjusted to increase the time scale for the particles to stabilize and order during crystal growth in these attractive systems. A clear understanding and control of the collective behavior of highly mobile colloids could lead to the creation of greater diversity of nanoarchitectures.

Layer-by-Layer Growth of Attractive Binary Colloidal Particles

We investigate the two-dimensional (2D) colloidal structures formed by oppositely charged polystyrene monolayers grown layer-by-layer, where the electrostatic forces are recruited to assist in the packing of the layers. Our results show a transition through several 2D-superlattices to more close-packed structures with increasing ionic strength. The observed geometrical packing constraints of the 2D-superlattice structures agree well with the estimated Debye screening length of the electric double layer. By tuning interaction forces between charged colloids, electrostatic interactions could enhance the template-directed self-assembly process to achieve more complex and diverse structures.

Controlled and rapid ordering of oppositely charged colloidal particles

Oppositely charged colloidal particles in suspension undergo rapid coagulation in the absence of any repulsive component in the interaction potential. With an added steric component serving as the repulsive force it is possible to order oppositely charged particles, which are also weakly charged, in solution. However given the novel features obtainable for an ordered structure from strong oppositely charged particles it becomes imperative to gain a full understanding of methods that can order these particles. Here we report a simple and rapid layer-by-layer method to order strongly and oppositely charged particles. Although this method is in principle scalable to order multiple layers of oppositely charged particles, herein we report ordering of one layer of positively charged particles onto a substrate made of negatively charged particles. This method utilizes a non-ionic surfactant to induce a steric repulsive force between particles and involves spin-coating to disperse and order particles on a very short time scale. The ordered structure obtained through this process is verified as the structure with one of the lowest interaction energies.

Optical absorption in a finite three-dimensional photonic crystal thin film solar cell

We report the increase in optical absorption in a finite three-dimensional photonic crystal through the dependence of the crystal film’s thickness. Varying thicknesses of TiO2 inverse opal electrodes were assembled in dye-sensitized solar cells and their optical properties were characterized. Through measuring the enhancement in the incident photon-to-electricity conversion efficiency over the reference cell, we show that optical absorption of a photosensitive dye coupled to a finite ordered structure can be influenced by the spatial properties of the structure. This report provides an insight into the changes in optical absorption of an atom when the space surrounding that atom is modified.

In Situ Visualization of Layer Transitions during Convective Self-Assembly in a Sessile Drop

Convective self-assembly of colloidal spheres provides a simple method for fabricating two and three dimensional colloidal crystals. In this work, we investigated the layer transitions phenomena during colloidal self-assembly in a sessile drop by using an in-situ videoscopic set-up. The effects of surface charge, colloidal concentration, and surfactant additions were examined. The results show that the chemical environment plays an important role in colloidal self-assembly. In the case of ordered growth, different layer transition phenomena were observed when the colloidal concentration is different.