Joohyun Lim

Rapid Imaging of Latent Fingerprints Using Biocompatible Fluorescent Silica Nanoparticles

Fluorescent silica nanoparticles (FSNPs) are synthesized through the Stöber method by incorporating silane-modified organic dye molecules. The modified fluorescent organic dye molecule is able to be prepared by allylation and hydrosilylation reactions. The optical properties of as-prepared FSNPs are shown the similar optical properties of PR254A (allylated Pigment Red 254) and have outstanding photostability. The polyvinylpyrrolidone (PVP) is introduced onto the surface of FSNP to enhance the binding affinity of PVP-coated FSNP for latent fingerprints (LFPs) detection. The simple preparation and easy control of surface properties of FSNPs show potential as a fluorescent labeling material for enhanced latent fingerprint detection on hydrophilic and hydrophobic substrates in forensic science for individual identification.

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.

Correlations of Optical Absorption, Charge Trapping, and Surface Roughness of TiO2 Photoanode Layer Loaded with Neat Ag-NPs for Efficient Perovskite Solar Cells

We systematically investigated the effect of silver nanoparticles (Ag-NPs) on the power conversion efficiency (PCE) of perovskite solar cells (PSCs). Neat, spherical Ag-NPs at loading levels of 0.0, 0.5, 1.0, and 2.0 wt % were embedded into the titanium dioxide (TiO2) photoanode layer. The plasmonic effect of the Ag-NPs strongly enhanced the incident light absorption over a wide range of the visible wavelength region in addition to the inherent absorbance of the perovskite sensitizer. The low conduction energy level of the Ag-NPs compared to that of TiO2 provides trap sites for free charge carriers. Thus, the correlation between the enhancement of the optical absorption and the number of charge traps provided by the Ag-NPs is critical to determine the device performance, especially current density (Jsc) and PCE. This is confirmed by the quantitative comparison of the incident light absorption and the time-resolved photoluminescence decay according to the loading levels of the Ag-NPs in the TiO2 layer. The absorption enhancement from 380 to 750 nm in the UV-visible spectrum is proportional to the increase in the loading levels of the Ag-NPs. However, the Jsc increases with the device with 0.5 wt % Ag-NPs and gradually decreases with increases in the loading level above 0.5 wt % because of the different contributions to the absorbance and the charge trapping by different Ag-NP loading levels. In addition, the suppression of the surface roughness with dense packing by the Ag-NPs helps to improve the Jsc and the following PCE. Consequently, the PCE of the PSC with 0.5 wt % Ag-NPs is increased to 11.96%. These results are attributed to the balance between increased absorbance by the localized surface plasmon resonance and the decreased charge trapping as well as the decreased surface roughness of the TiO2 layer with the Ag-NPs.

Stability enhancement of an electrically tunable colloidal photonic crystal using modified electrodes with a large electrochemical potential window

The color tuning behavior and switching stability of an electrically tunable colloidal photonic crystal system were studied with particular focus on the electrochemical aspects. Photonic color tuning of the colloidal arrays composed of monodisperse particles dispersed in water was achieved using external electric field through lattice constant manipulation. However, the number of effective color tuning cycle was limited due to generation of unwanted ions by electrolysis of the water medium during electrical switching. By introducing larger electrochemical potential window electrodes, such as conductive diamond-like carbon or boron-doped diamond, the switching stability was appreciably enhanced through reducing the number of ions generated.

Oligothiophene-modified silver/silica core–shell nanoparticles for inhibiting open-circuit voltage drop and aggregation in polymer solar cells

Metal nanoparticles (NPs) have attracted much attention owing to their particular characteristics such as localized surface plasmon resonance (LSPR) and scattering properties, which can improve the light-harvesting ability of photovoltaic cells. However, modification of the metal NP surface is needed to prevent particle aggregation and photoinduced charge trapping. Surface modification of silica-coated Ag NPs with oligothiophene (OT) provides silver/silica core–shell (Ag@SiO2–OT) NPs, which are well dispersed in nonpolar organic solvents and miscible with the bulk-heterojunction (BHJ) layer of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) (P3HT:PCBM). Incorporation of the Ag@SiO2–OT NPs into the P3HT:PCBM layers as the active layer of photovoltaic devices improves the light-harvesting ability and enhances the photo-conversion efficiency (PCE) by about 18%. There is no significant change in the open-circuit voltage (Voc) value even when up to 30 wt% of Ag@SiO2–OT NPs are incorporated, confirming that the OT-modified silica layer on silver NPs contributes to improving light absorption and photo-current without causing aggregation and photo-induced charge trapping. Moreover, the deliberately designed transmission electron microscopy (TEM) investigation of the composite films of P3HT:PCBM and Ag@SiO2–OT NPs reveals that Ag@SiO2–OT NPs are mainly located in the P3HT domain owing to the favorable interaction between the similar molecular structures of OT and P3HT.

Wrinkled silica/titania nanoparticles with tunable interwrinkle distances for efficient utilization of photons in dye-sensitized solar cells.

Efficient light harvesting is essential for the realization of high energy conversion efficiency in dye-sensitized solar cells (DSCs). State-of-the-art mesoporous TiO2 photoanodes fall short for collection of long-wavelength visible light photons, and thus there have been efforts on introduction of scattering nanoparticles. Herein, we report the synthesis of wrinkled silica/titania nanoparticles with tunable interwrinkle distances as scattering materials for enhanced light harvesting in DSCs. These particles with more than 20 times larger specific surface area (>400 m2/g) compared to the spherical scattering particles (<20 m2/g) of the similar sizes gave rise to the dye-loading amounts, causing significant improvements in photocurrent density and efficiency. Moreover, dependence of spectral scattering properties of wrinkled particles on interwrinkle distances, which was originated from difference in overall refractive indices, was observed.

Soft Template Strategy to Synthesize Iron Oxide–Titania Yolk–Shell Nanoparticles as High‐Performance Anode Materials for Lithium‐Ion Battery Applications

Yolk-shell-structured nanoparticles with iron oxide core, void, and a titania shell configuration are prepared by a simple soft template method and used as the anode material for lithium ion batteries. The iron oxide-titania yolk-shell nanoparticles (IO@void@TNPs) exhibit a higher and more stable capacity than simply mixed nanoparticles of iron oxide and hollow titania because of the unique structure obtained by the perfect separation between iron oxide nanoparticles, in combination with the adequate internal void space provided by stable titania shells. Moreover, the structural effect of IO@void@TNPs clearly demonstrates that the capacity retention value after 50 cycles is approximately 4 times that for IONPs under harsh operating conditions, that is, when the temperature is increased to 80 °C.

Interface electronic structure of the organic light-emitting devices: Photoemission and x-ray absorption studies of Al∕KF∕Alq3 interface

The chemistry of organic light-emitting diode interface composed of KF sandwiched between the Al and Alq3 was investigated using near-edge x-ray absorption fine structure (NEXAFS) as well as x-ray and ultraviolet photoelectron spectroscopy techniques. At the earliest stages of KF deposition on Alq3, changes in F K-edge NEXAFS spectra indicated a strong chemical reaction, which is responsible for the dipole layer formation seen in valence-band spectra. For Al deposition on KF∕Alq3, the reactions inferred from observed spectral changes are not consistent with the commonly believed KF dissociation and AlF3 formation scenarios.

Synthesis of hierarchical iron oxide nanostructures from primary nanoparticles and their morphology control via hydrolysis

Flower-like hierarchical iron oxide structures have attracted much research attention due to their large surface area, easy separation and re-use, and ability to adsorb toxic ions. However, harsh synthetic conditions such as the use of surfactants and autoclave reactors limit their broader application. Moreover, control over the porosity and morphology of such structures has not yet been investigated. Facile preparation of hierarchical Fe-glycolate particles is possible using pre-prepared (Fe, Ti) oxide nanoparticles without the use of surfactants or autoclave reactors as they act as both precursors and intermediate seeds for self-assembly. The porosity and morphology of the particles are controlled via variable rates of hydrolysis. Replacement of bulky glycolates with smaller water molecules induces surface area and porosity changes in the hierarchical structures. Hierarchical magnetite particles are obtained from the Fe-glycolate and hydrolyzed particles after reductive annealing, and exhibit different surface areas, porosities and magnetic properties. Porosity-controlled magnetites from hydrolyzed particles also show greater adsorption for removing toxic arsenate ions in water.