Dae-Ryong Jung

Optical and electronic properties of post-annealed ZnO:Al thin films

This study examined the optical and electronic properties of post-annealed Al-doped ZnO (ZnO:Al) thin films. The lowest resistivity was observed after annealing a sputter-deposited ZnO:Al film at 350 °C. X-ray photoelectron spectroscopy revealed a ∼0.4 eV shift in the Fermi level when the carrier concentration was increased to 1.6×1020 cm−3 by Al doping and annealing. The optical band gap increased from 3.2 eV for insulating ZnO to 3.4 eV for conducting ZnO:Al, and was associated with conduction-band filling up to ∼0.4 eV in a renormalized band gap. Schematic band diagrams are shown for the ZnO and ZnO:Al films.

The effects of 100 nm-diameter Au nanoparticles on dye-sensitized solar cells

Gold nanoparticles of ∼100 nm in diameter were incorporated into TiO2 nanoparticles for dye-sensitized solar cells (DSSCs). At the optimum Au/TiO2 mass ratio of 0.05, the power-conversion efficiency of the DSSC improved to 3.3% from a value of 2.7% without Au, and this improvement was mainly attributed to the photocurrent density. The Au nanoparticles embedded in the nanoparticulate-TiO2 film strongly absorbed light due to the localized surface-plasmon resonance, and thereby promoted light absorption of the dye. In the DSSCs, the 100 nm-diameter Au nanoparticles generate field enhancement by surface-plasmon resonance rather than prolonged optical paths by light scattering.

Synthesis and photoluminescence of Mn-doped zinc sulfide nanoparticles

Mn-doped zinc sulfide nanoparticles were synthesized using a liquid-solid-solution method, as a simple synthetic route for preparing nearly monodispersed nanocrystals with a diameter of 7.3±0.7nm. The influence of doping concentration for optimum luminescence properties was studied with the nonuniform distribution of local strain and the capping effect. The improved photoluminescence properties of the 450°C-annealed samples with 1.0at.% Mn doping are attributed to both the removal of water/organics and the enhanced crystallinity (reduced local strain).

Highly luminescent surface-passivated ZnS:Mn nanoparticles by a simple one-step synthesis

Highly luminescent surface-passivated ZnS:Mn nanoparticles were synthesized straightforwardly by a simple liquid-solid-solution method. Compared to the pristine Mn-doped zinc sulfide nanocrystals (quantum efficiency: ∼19%), the Li-added ZnS:Mn exhibited significantly enhanced luminescence properties (quantum efficiency: ∼43%). The surface passivation was investigated by x-ray photoelectron spectroscopy, transmission electron microscopy, and by the change in the radiative/nonradiative recombination rates. The photoluminescence enhancement is due to the formation of an effective passivation layer induced by lithium, and consequent suppression of the nonradiative recombination transitions.

Photoluminescence enhancement in CdS nanoparticles by surface-plasmon resonance

To examine the influence of metal nanoparticles on the photoluminescence of semiconductors, colloidal mixtures of CdS and Au nanoparticles were prepared with different CdS/Au fractions. Compared to the cadmium-sulfide nanocrystals (quantum efficiency ≅ 7%), the CdS/Au mixtures showed enhanced luminescence properties (quantum efficiency ≅ 14%). The existence of an optimum ratio of metal to semiconductor nanoparticles for the photoluminescence intensity indicates that interactions between the metal and semiconductor nanoparticles induced by surface-plasmon resonance occur constructively at appropriate distances.

Photoluminescence enhancement in CdS quantum dots by thermal annealing

The photoluminescence behavior of CdS quantum dots in initial growth stage was studied in connection with an annealing process. Compared to the as-synthesized CdS quantum dots (quantum efficiency ≅ 1%), the heat-treated sample showed enhanced luminescence properties (quantum efficiency ≅ 29%) with a narrow band-edge emission. The simple annealing process diminished the accumulated defect states within the nanoparticles and thereby reduced the nonradiative recombination, which was confirmed by diffraction, absorption, and time-resolved photoluminescence. Consequently, the highly luminescent and defect-free nanoparticles were obtained by a facile and straightforward process.

Surface-passivation effects on the photoluminescence enhancement in ZnS:Mn nanoparticles by ultraviolet irradiation with oxygen bubbling

This study examined the effects of surface-passivation on the photoluminescence (PL) properties of ZnS:Mn nanoparticles treated by ultraviolet (UV) irradiation with oxygen bubbling. Compared to the pristine Mn-doped zinc-sulfide nanocrystals (quantum efficiency: ∼16%), the UV-irradiated ZnS:Mn showed significantly enhanced luminescence properties (quantum efficiency: ∼35%). The photoinduced surface passivation was characterized by x-ray diffraction, x-ray photoelectron spectroscopy, and time-resolved PL. The optimum thickness of the passivation layer for the quantum efficiency was examined considering the nanocrystal size, local strain, and radiative/nonradiative recombination rates.

An effective oxidation approach for luminescence enhancement in CdS quantum dots by H2O2

The effects of surface passivation on the photoluminescence (PL) properties of CdS nanoparticles oxidized by straightforward H2O2 injection were examined. Compared to pristine cadmium sulfide nanocrystals (quantum efficiency ≅ 0.1%), the surface-passivated CdS nanoparticles showed significantly enhanced luminescence properties (quantum efficiency ≅ 20%). The surface passivation by H2O2 injection was characterized using X-ray photoelectron spectroscopy, X-ray diffraction, and time-resolved PL. The photoluminescence enhancement is due to the two-order increase in the radiative recombination rate by the sulfate passivation layer.

Ballistocardiogram of baby during sleep.

Ballistocardiogram (BCG), which displays the mechanical activity of heart, has been a subject of interest for several years due to its advantages in taking unobtrusive physiological measurements. In the field of sleep science, researchers actively study sleep architecture and clinically apply various sleep-related conditions through BCG-derived biological information such as the heartbeat, respiration and body movements of subjects. However, most of these studies have involved only adults. This area of research may be even more important with babies to monitor their biological signals without confinement. For this reason, we developed a physiological signal monitoring bed for baby by using a load cell. Heartbeat and respiration information was assessed with average respective performance errors of 1.53% and 2.53% compared to commercial equipment. The results showed the possibility of applying BCG technology to baby. Therefore, we expect that BCG-derived signals can be extensively applied to analyze sleep architecture and clinical applications in baby as they are with adults.