Wooje Han

Impact of nanostructured thin ZnO film in ultraviolet protection

Nanoscale ZnO is one of the best choices for ultraviolet (UV) protection, not only because of its antimicrobial properties but also due to its potential application for UV preservation. However, the behavior of nanostructured thin ZnO films and long-term effects of UV-radiation exposure have not been studied yet. In this study, we investigated the UV-protection ability of sol gel-derived thin ZnO films after different exposure times. Scanning electron microscopy, atomic force microscopy, and UV-visible optical spectroscopy were carried out to study the structure and optical properties of the ZnO films as a function of the UV-irradiation time. The results obtained showed that the prepared thin ZnO films were somewhat transparent under the visible wavelength region and protective against UV radiation. The UV-protection factor was 50+ for the prepared samples, indicating that they were excellent UV protectors. The deposited thin ZnO films demonstrated promising antibacterial potential and significant light absorbance in the UV range. The experimental results suggest that the synthesized samples have potential for applications in the health care field.

Wavelength-tunable visible to near-infrared photoluminescence of carbon dots: The role of quantum confinement and surface states

Abstract. Carbon quantum dots (C-QDs) with different size distributions and surface characteristics can exhibit good emission properties in the visible and near-infrared (NIR) regions, which can be applicable in optoelectronic devices as well as biomedical applications. Optical properties of colloidal C-QDs in distilled water at different concentrations produced using a method of alkali-assisted surfactant-free oxidation of cellulose acetate is presented. The structural and optical properties of colloidal C-QDs at different concentrations were investigated, with the aim of clarifying the main mechanisms of photoluminescence emissions. We observed a wide range of tunable visible to NIR emissions with good stability from the C-QD colloids at different applied excitation wavelengths. The colloids show dual emissions with maxima at ∼420 and 775 nm (blue and NIR emissions) when excited at the wavelength range near the energy gaps of the C-QDs. Moreover, by increasing the excitation wavelength, tunable visible emissions at the spectral range of 475 to 550 nm are observed. A detailed analysis of the results shows that the blue and NIR luminescence of colloidal C-QDs originate from the oxide-related surface effects whereas quantum confinement is the responsible mechanism for tunable visible emissions of the C-QD colloid.

Fluorous-inorganic hybrid dielectric materials for solution-processed electronic devices

We report the synthesis and characterisation of fluorous-inorganic hybrid dielectric (FIHD) materials processable in highly fluorinated orthogonal solvents for printed electronic devices. FIHD materials were prepared successfully via ligand exchange reactions between organic ligands on the surfaces of nanomaterials and highly fluorinated carboxylic acids. When hafnium oxide (HfO2) or zirconium oxide (ZrO2) nanoparticles stabilized with trioctylphosphine oxide (TOPO) were treated with perfluoro-3,6,9-trioxatridecanoic acid in HFE-7500 at 130 °C, the modified surface characteristics of the nanoparticles resulted in excellent solubilities in the fluorous solvent. The dielectric constant of HfO2 and ZrO2 nanoparticles modified with fluorous acid was ca. 4.4 and 4.3 at 1 KHz, respectively, which is significantly higher than that of fluoropolymers. Top-gate organic thin film transistors (OTFTs) were fabricated using solution-processed organic semiconductors and HfO2-based FIHD materials. The hole mobilities of the OTFTs produced were as high as 0.08 cm2 V−1 s−1 (Vds = −40 V) and the on/off ratio reached 3.3 × 106 when 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene (diF-TESADT) was employed as the semiconductor layer. These device performances demonstrate that FIHD materials can be useful components for general printed electronic devices processed with soluble organic electronic materials.

Ferroelectric Tunnel Junction for Dense Cross-Point Arrays

Cross-point array (CPA) structure memories using a memristor are attracting a great deal of attention due to their high density integration with a 4F2 cell. However, a common significant drawback of the CPA configuration is crosstalk between cells. To date, the CPA structure using a redox-based memristor has restrictions to minimize the operating current level due to their resistive switching mechanism. This study demonstrates suitable characteristics of a ferroelectric tunnel junction (FTJ) for the memristor of the CPA structure using an electrostatic model. From the FTJ of the Au/p-type Pr0.98Ca0.02MnO3 (4 nm)/BaTiO3 (4.3 nm)/n-type Ca0.98Pr0.02MnO3 (3 nm)/Pt(111) structure, which has a higher and thicker potential barrier, a good memristive effect for the CPA structure with a high nonlinear current-voltage curve and low current operation, was obtained by Δ Fowler-Nordheim tunneling with effectively blocked direct tunneling and thermionic emission. The FTJ demonstrated reduced sneak current and the possible for high nonlinearity.

Tunable Dielectric Properties of Poly(vinylidenefluoride-co-hexafluoropropylene) Films with Embedded Fluorinated Barium Strontium Titanate Nanoparticles

Fluoropolymer nanocomposites of poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) were prepared using fluorinated barium strontium titanate (Ba1−xSrxTiO3, BSTO) nanoparticles (NPs) by low-temperature synthesis using a modified liquid–solid solution process. The exact stoichiometry of as-synthesized BSTO NPs was confirmed by X-ray diffraction analysis along with lattice parameter calculations. The synthesized BSTO NPs were fluorinated using 2,2,2-trifluoroacetic acid as a fluorous ligand. The BSTO NPs showed high solubility in the fluorous system (polymer and solvent) on account of their modified surface. The root-mean-square roughness of the fluorinated BSTO/PVdF-HFP nanocomposite was 76 times lower than that of the nonfluorinated BSTO/PVdF-HFP nanocomposite. The dielectric constant of the fluorinated BSTO/PVdF-HFP nanocomposite exhibited Curie temperature behavior. The dielectric constant of the nanocomposite predicted using the modified Kerner model at room temperature agreed well with the experimental values.

Highly stable and efficient green luminescent CdS colloidal nanocrystals

Abstract. A chemical route was used to synthesize highly stable CdS colloidal nanocrystals. Different characterization techniques, such as UV-Vis, photoluminescence, and time-resolved photoluminescence spectroscopy, were used to study the optical properties of synthesized sample. A strong and narrow green emission peak centered at 532 nm with 40-nm full widths at half maximum and 57.2% quantum efficiency was recorded. The absorption cross section at the first absorption peak is about 11.69×10−16  cm2 that has not been reported before. Variation in optical properties, such as emission peak intensity and full widths at half maximum of emission peak, was evaluated with various long-term measurements (after 1 year). Obtained results confirm that the prepared CdS nanocrystals can be employed as green source material.

Barium Titanate Nanoparticles Formed by Chlorine-Free Ambient Condition Sol Process Using Tetrabutylammonium Hydroxide

Barium titanate (BaTiO3: BTO) nanoparticles (NPs) were synthesized by chlorine-free ambient condition sol (ACS) process using heat reflux at low temperature of 90°C. The size distribution and morphology of BTO NPs were investigated by varying the concentration of tetrabutylammonium hydroxide (TBAH). The crystalline size of BTO NPs was decreased with increasing the amount of TBAH capping agent (average size changes from 54.3 to 38.7 nm for 0 to 0.5 M TBAH in X-ray diffraction measurement). The particle size of BTO NPs was principally controlled by a synthetic control of butyl chain of TBAH and also a steric effect of excess amount of TBAH. The dielectric constant of BTO NPs was decreased from 152 to 144 at 1 MHz after an adoption of TBAH capping agent with almost uniform dielectric loss (<0.027). But the dielectric constant of BTO NPs synthesized with various molar ratio of TBAH (0.1, 0.3, and 0.5) did not show a distinguished decrease. At the particle size range in this experiment, the dielectric behavior of BTO NPs was found to be mainly dependent on the TBAH ligands at BTO NPs formed during capping process, not on the size of BTO NPs.