Taeho Moon

Electrochemical Properties of Disordered-Carbon-Coated SnO2 Nanoparticles for Li Rechargeable Batteries

The effects of carbon coating on ∼9 nm SnO 2 particles were examined. The C-coated SnO 2 nanoparticles were synthesized from SnCl 4 , glucose, and ethylene glycol through a solvothennal method. Raman spectra indicated that the coated carbon was a disordered carbon. The C-coated SnO 2 nanoparticles showed superior cycling properties to the uncoated ones. Transmission electron microscopy after cycling confirmed that the nanoparticles were well dispersed without aggregation. The enhanced cycling property is believed to be attributed to the effective hindrance of nanoparticle growth by the core/shell structure of Sn/Li 2 O and carbon layer after phase separation during the first discharge.

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).

Dielectric relaxation of atomic-layer-deposited HfO2 thin films from 1kHzto5GHz

The dielectric relaxation of HfO2 thin films grown by atomic-layer deposition (ALD) was studied as a function of frequency from 1kHzto5GHz. The dielectric relaxation of the ALD HfO2 films followed a power-law dependence known as the Curie–von Schweidler relaxation law both in the kHz and GHz ranges, and the relaxation exponents were consistent with the measured dielectric losses. The behavior of the dielectric response for the HfO2 thin films may be attributed to defect sites in the HfO2 layer and∕or interface.

Crystallinity Dependence of Microwave Dielectric Properties in (Ba,Sr)TiO3 Thin Films.

The dielectric properties of (Ba0.43Sr0.57)TiO3 (BST) thin films were investigated in the microwave-frequency range. The dielectric losses (tan δ) and dielectric constants (e) were successfully measured up to ~6 GHz using a circular-patch capacitor geometry. The deposition temperatures were varied from room temperature to 750°C to investigate the effects of crystallinity on the dielectric properties. As the film crystallinity was enhanced, the dielectric losses increased from 0.0024±0.0018 at room-temperature to 0.0102±0.0017 at 750°C deposition. The dielectric constants varied from 10.29±0.02 to 243±1 in the same deposition-temperature range. Raman spectroscopy showed that the increase in dielectric losses of the BST thin films was correlated with the growth of microscopic polar regions induced by symmetry-breaking defects. Because the sizes of these regions are proportional to the dielectric constants of host lattices (crystallinity), dielectric losses increase with the deposition temperatures.

Nanostructural Effect of AlPO4-Nanoparticle Coating on the Cycle-Life Performance in LiCoO2 Thin Films

To control the nanostructure of an AlPO 4 -coating layer, nanoparticles with three AlPO 4 phases were synthesized: amorphous, tridymite, and cristobalite phases. These colloids were layered on LiCoO 2 thin films by spin coating, and subsequently annealed at 400°C. The interdiffusion variations at the interface were eliminated by the spin-coating method, while the cycle-life performance of the coated cathode depended on the nanostructure of the AlP04-nanoparticle-coating layer. The LiCoO 2 thin-film cathode coated with amorphous nanoparticles and annealed at 400°C showed the best cycle-life performance, and effectively suppressed the degradation of Li + -diffusion kinetics during cycling.

Microwave dielectric relaxation of the polycrystalline (Ba,Sr)TiO3 thin films

The microwave dielectric properties of the (Ba,Sr)TiO3 thin films annealed at various oxygen pressures ranging from 5to500mTorr were investigated over the frequency range 0.5–5GHz using a circular-patch capacitor geometry. The dielectric constant (e) followed Curie–von Schweidler relaxation in the microwave-frequency range, and the degree of relaxation corresponded qualitatively with the measured dielectric loss (tanδ). As the oxygen pressure varied, the dielectric loss had a maximum value of ∼0.03 at 100mTorr, and its behavior was correlated with the Raman strength of the polar modes.

Recent advances in the transparent conducting ZnO for thin-film Si solar cells

The key challenge for solar-cell development lies in the improvement of power-conversion efficiency and the reduction of fabrication cost. For thin-film Si solar cells, researches have been especially focused on the light trapping for the breakthrough in the saturated efficiencies. The ZnO-based transparent conducting oxides (TCOs) have therefore received strong attention because of their excellent light-scattering capability by the texture-etched surface and cost effectiveness through in-house fabrication. Here, we have highlighted our recent studies on the transparent conducting ZnO for thin-film Si solar cells. From the electrical properties and their degradation mechanisms, bilayer deposition and organic-acid texturing approaches for enhancing the light trapping, and finally the relation between textured ZnO and electrical cell performances are sequentially introduced in this review article.

Effect of crystallinity on the dielectric loss of sputter-deposited (Ba,Sr)TiO 3 thin films in the microwave range

The crystallinity dependence of the microwave dielectric losses in (Ba,Sr)TiO 3 thin films was investigated. The sputter-deposition temperatures were altered to vary the level of thin-film crystallinity on a Pt/Si substrate. The dielectric losses (tan ) were measured up to 6 GHz without parasitic (stray) effects by using a circular-patch capacitor geometry and an equivalent-circuit model. The microwave dielectric losses increased from 0.0024 ± 0.0018 to 0.0102 ± 0.0017 with increasing crystallinity. These deteriorated dielectric losses showed a good correlation with the symmetry-breaking defects, as confirmed by Raman spectra at approximately 760 cm �1 , inducing microscopic polar regions above the Curie temperature of the bulk (Ba0.43Sr0.57)TiO3.

Rapid crystallization in ambient air for planar heterojunction perovskite solar cells

Organic-inorganic hybrid perovskite solar cells have attracted great interest because of rapid improvement of power-conversion efficiency and strong potential for low fabrication cost. The development of cost-effective routes producing high quality perovskite films has been continuously demanded. Here, it is shown that crystalline perovskite films with completely coated morphology can be formed using the precursors of MACl and PbI2 without post-annealing under atmosphere. The dense perovskite films composed of the closely packed islands are observed with the smooth surface. The planar cells with p-i-n heterojunction geometry are successfully demonstrated using PEDOT:PSS and PCBM. Significantly, the outstanding electrical properties are observed, which demonstrates the good coverage and crystallinity of the perovskite layers.

Fabrication and characterization of porous silicon nanowires

We report the synthesis of porous silicon nanowires through the metalassisted chemical etching of porous silicon in a solution of hydrofluoric acid and hydrogen peroxide. The morphology of porous silicon nanowires was characterized by scanning electron microscopy and transmission electron microscopy. The etch rate of the porous silicon nanowires was faster than that of silicon nanowires, but slower than that of porous silicon. The porous silicon nanowires distributed uniformly on the entire porous silicon layer and the tips of the porous silicon nanowires congregated together. The single crystalline and sponge-like porous structure with the pore diameters of less than 5 nm was confirmed for the porous silicon nanowires.