Jun Hyuk Moon

Chemical Aspects of Three-Dimensional Photonic Crystals

Three-dimensional (3D) photonic crystals (PCs) are crystalline materials where the refractive index is periodically modulated on a length scale comparable to the light wavelength of interest. Interference of the light waves scattered from the dielectric lattice (i.e., Bragg scattering) leads to omnidirectional stop bands or photonic band gaps (PBGs), which are analogous to the electronic energy band gaps in a semiconductor.1,2 The bandwidth and the frequency of the PBG are determined by the refractive index contrast between the high and low (typically air) dielectric materials, the structural symmetry and periodicity, and the filling fraction and morphology of high-refractive-index materials. Photonic crystals with a large, complete PBG are highly desired, which act like an optical trap to reflect incident light from any direction at a certain frequency range of light. Thus, controlled functional structures can be engineered into the 3D structures to confine or guide photons of specific wavelengths. Therefore, PCs potentially offer revolutionary advances in the next-generation microphotonic devices and the integration of existing optoelectronic devices, including integrated optical circuits, lasers, sensing, spectroscopy, and pulse shaping. In the last two decades, there has been much interest in exploring new PC structures and studying the related new phenomenon. Existing techniques for the largescale fabrication of microstructures with submicrometer features mainly rely on the use of optical projection lithography developed for silicon IC manufacturing. This method is inherent 2D patterning and requires laborious layer-by-layer photolithography and etching processes or stacking and fold-up of the 2D layers to generate the continuous 3D structures.2-4 A promising 3D fabrication technique should be able to do the following: •produce submicrometer periodicity for PBGs in the visible to infrared (IR) spectral range •access a large number of structures with tailored shapes, functionalities, and sizes of motifs •allow for mass-production over a large area •provide fine control of defects for photonic device applications •construct structures from materials with high refractive indices for complete PBGs.

Monodispersed N-Doped Carbon Nanospheres for Supercapacitor Application

Highly monodispersed nitrogen-doped carbon nanospheres are prepared by the pyrolytic carbonization of emulsion-polymerized polystyrene-based colloidal spheres in the presence of a nitrogen-enriched molecule, melamine (1,3,5-triazine-2,4,6-triamine). The nitrogen-doped carbon spheres are successfully tested for use as electrode materials in supercapacitors. The nitrogen content incorporated into the carbon sphere is controlled by changing the weight ratio of melamine to the polymer spheres. The nitrogen doping concentration is proportional to the mixing weight ratio. The nitrogen doping produces relatively abundant pyridinic and pyrrolic configurations, and these configurations are observed to be more abundant for carbon spheres with high nitrogen doping. The nitrogen doping enhances the pseudocapacitance and the electrical conductivity of carbon, thereby enhancing the specific capacitance. We obtain a specific capacitance of up to 191.9 F g(-1) with 20% nitrogen doped carbon nanospheres, which is 14 times higher than that of the undoped carbon nanospheres. Moreover, the capacitance retention remains up to 10,000 cycles, which clearly displays a good cycling stability the nitrogen-doped carbon nanospheres as the supercapacitve electrode materials.

Hierarchically porous TiO2 electrodes fabricated by dual templating methods for dye-sensitized solar cells.

IO N Dye-sensitized solar cells (DSSCs) are of great interest due to their projected cost-effectiveness, their relatively high photoelectric conversion effi ciency, and the unique advantages of transparent cells over conventional silicon photovoltaics. [ 1 , 2 ] In DSSCs, light is harvested by a TiO 2 layer. This layer performs three functions: it acts as a substrate on which the dye molecules adsorb, it transfers the photogenerated electrons, and it serves as a diffusion pathway for ions in the electrolyte solution. The engineering of TiO 2 electrodes with regard to characteristics, such as nanostructure, crystalline morphology, and surface properties, is therefore a crucial aspect in efforts to enhance the photoconversion effi ciency. There have been many efforts to engineer the microstructure of TiO 2 electrodes. Engineered mesopores on the order of 10 nm, such as ordered pores and nanotubes, show faster transport of charge carriers than conventional nanocrystalline TiO 2 (nc-TiO 2 ) electrodes, in which random diffusion is the dominant mechanism. [ 3 , 4 ] In nanotube TiO 2 electrodes, the suppression of the recombination of photogenerated electrons was observed to be ten times higher than in conventional nanocrystalline TiO 2 (nc-TiO 2 ) electrodes, which resulted in an enhancement of the charge collection effi ciency. [ 4 ] The incorporation of macropores or particles that are several hundred nanometers in diameter induces Mie scattering of incident photons, thereby enhancing the light absorption, particularly at infrared wavelengths where the extinction coeffi cients of dyes are low. The introduction of these pores also improves the electrolyte diffusion effi ciency in TiO 2 electrodes. [ 5 , 6 ] Moreover, control of the macropore morphology (i.e., the size of macroscale particles or pores, as well as their organization) can be advantageous for the application of solidstate electrolytes with high viscosities and larger molecular volumes. [ 7 ]

Enhanced Photovoltaic Properties of Nb2O5-Coated TiO2 3D Ordered Porous Electrodes in Dye-Sensitized Solar Cells

This paper describes the use of Nb₂O₅-coated TiO₂ 3D ordered porous electrodes in dye-sensitized solar cells. We employed bilayer inverse opal structures as a backbone of 3D porous structures, and the number of Nb₂O₅ coatings was controlled, determining the concentration of Nb₂O₅ coating. XPS measurements confirmed the formation of Nb₂O₅. The uniformity of the Nb₂O₅ coating was characterized by elemental mapping using SEM and TEM measurements. Photovoltaic measurement on dye-sensitized solar cells (DSSCs) that incorporated Nb₂O₅/TiO₂ inverse opal electrodes yielded a maximum efficiency of 7.23% for a 3.3 wt % Nb₂O₅ coating on a TiO₂ IO structure. The Nb₂O₅ significantly increased the short-circuit current density (J(SC)). Electrochemical impedance spectroscopy was used to measure the J(SC), revealing an enhanced electron injection upon deposition of the Nb₂O₅ coating.

Demulsification of water-in-crude oil emulsions by a continuous electrostatic dehydrator

The demulsification rates of water-in-crude oil emulsion in high AC fields were investigated under various conditions by using a model dehydrator. A continuous electrostatic dehydrator was constructed using a glass vessel of 6.5 cm diameter and 10 cm height equipped with a copper electrode and a perforated plate. The separation rate of water from the simulated crude oil increased along with the applied field, frequency, demulsifier concentration, temperature, and contact time. As the applied field increased up to 2.5 kV/cm, the separation percentage increased up to 90%, and as the concentration of the demulsifier reached 100 ppm, 80% of the water were separated at 2.5 kV/cm. Also it was observed that the separation percentage increased as the temperature, frequency of field, and contact time. It is proposed that the breakup of droplets depends on the interfacial polarization and the proper deformation of water droplets in the field induced by the electrostatic charge.

Inverse Opal Carbons for Counter Electrode of Dye-Sensitized Solar Cells

We investigated the fabrication of inverse opal carbon counter electrodes using a colloidal templating method for DSSCs. Specifically, bare inverse opal carbon, mesopore-incoporated inverse opal carbon, and graphitized inverse opal carbon were synthesized and stably dispersed in ethanol solution for spray coating on a FTO substrate. The thickness of the electrode was controlled by the number of coatings, and the average relative thickness was evaluated by measuring the transmittance spectrum. The effect of the counter electrode thickness on the photovoltaic performance of the DSSCs was investigated and analyzed by interfacial charge transfer resistance (R(CT)) under EIS measurement. The effect of the surface area and conductivity of the inverse opal was also investigated by considering the increase in surface area due to the mesopore in the inverse opal carbon and conductivity by graphitization of the carbon matrix. The results showed that the FF and thereby the efficiency of DSSCs were increased as the electrode thickness increased. Consequently, the larger FF and thereby the greater efficiency of the DSSCs were achieved for mIOC and gIOC compared to IOC, which was attributed to the lower R(CT). Finally, compared to a conventional Pt counter electrode, the inverse opal-based carbon showed a comparable efficiency upon application to DSSCs.

Carbon-Deposited TiO2 3D Inverse Opal Photocatalysts: Visible-Light Photocatalytic Activity and Enhanced Activity in a Viscous Solution

We for the first time demonstrated carbon-deposited TiO2 inverse opal (C-TiO2 IO) structures as highly efficient visible photocatalysts. The carbon deposition proceeded via high-temperature pyrolysis of phloroglucinol/formaldehyde resol, which had been coated onto the TiO2 IO structures. Carbon deposition formed a carbon layer and doped the TiO2 interface, which synergistically enhanced visible-light absorption. We directly measured the visible-light photocatalytic activity by constructing solar cells comprising the C-TiO2 IO electrode. Photocatalytic degradation of organic dyes in a solution was also evaluated. Photocatalytic dye degradation under visible light was only observed in the presence of the C-TiO2 IO sample and was increased with the content of carbon deposition. The IO structures could be readily decorated with TiO2 nanoparticles to increase the surface area and enhance the photocatalytic activity. Notably, the photocatalytic reaction was found to proceed in a viscous polymeric solution. A comparison of the mesoporous TiO2 structure and the IO TiO2 structure revealed that the latter performed better as the solution viscosity increased. This result was attributed to facile diffusion into the fully connected and low-tortuosity macropore network of the IO structure.

Holographic fabrication of three-dimensional nanostructures for microfluidic passive mixing†

In this study, we incorporated mixing units of three-dimensional (3D) interconnected pore network inside microfluidic channels by combining single prism holographic lithography and photolithography. 3D pore network structures were generated by the interference of four laser beams generated by a truncated triangular pyramidal prism. The levelling between the 3D porous structures and the channel walls was greatly improved by employing supercritical drying, which induced negligible internal capillary stresses and reduced substantially anisotropic volume shrinkage of 3D structures. Also, complete sealing of the microfluidic chips was achieved by attaching flexible PDMS cover substrates. Overall mixing performance of the systems with completely sealed mixing units was 84% greater than that obtained without such mixers. Splitting and recombination of flows in the 3D interconnected pore structures enhanced the mixing efficiency by decreasing the diffusion path and increasing the surface contact between two liquid streams. Because the flow splitting and recombination was developed through the 3D interconnected pore network, high mixing efficiency (>0.60) was achieved at low Reynolds numbers (Re < 0.05) and Péclet numbers in the regime of Pe < 1.4 x 10(3).

Core-shell diamond-like silicon photonic crystals from 3D polymer templates created by holographic lithography.

We have fabricated diamond-like silicon photonic crystals through a sequential silica/silicon chemical vapor deposition (CVD) process from the corresponding polymer templates photopatterned by holographic lithography. Core-shell morphology is revealed due to the partial backfilling of the interstitial pores. To model the shell formation and investigate its effect to the bandgap properties, we developed a two-parameter level-set approach that closely approximated the core-shell morphology, and compare the bandgap simulation with the measured optical properties of the 3D crystals at each processing step. Both experimental and calculation results suggest that a complete filling is necessary to maximize the photonic bandgap in the diamond-like structures.

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.