Chunjoong Kim

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).

Charge trap dynamics in a SiO2 layer on Si by scanning capacitance microscopy

Trapped electrons and holes, and their dynamics, were visualized from spatially resolved capacitance–voltage (C–V) curves and dC/dV images using scanning capacitance microscopy. A trapped charge of 10−16–10−18 C, localized within 2 μm diam circular test structures, was imaged. The detrapping process of the trapped electrons can be explained with a quantum-mechanical tunneling model.

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.

Imaging of a silicon pn junction under applied bias with scanning capacitance microscopy and Kelvin probe force microscopy

Scanning capacitance microscopy (SCM) and Kelvin probe force microscopy (KPFM) are used to image the electrical structure of a silicon pn junction under applied bias. With SCM, the carrier density inside a diode is imaged directly. With KPFM, the surface potential distribution of an operating diode is measured, revealing different behavior from that in bulk. The surface potential drop is extended deep into the lightly p-doped region at reverse bias, reflecting the existence of the surface space-charge region as confirmed by the numerical simulation.

Depth dependent carrier density profile by scanning capacitance microscopy

The depth dependent carrier density was measured on an arsenic implanted silicon sample using scanning capacitance microscopy (SCM). The capacitance versus voltage scan was performed by applying dc biases with a dither ac signal. A strong dc bias dependence was observed at the interface of an abrupt junction between n+ and p. The bias dependent SCM images show good agreement with quasi-three dimensional simulations, suggesting that they can be used to map a device structure.

The Effect of AlPO[sub 4]-Coating Layer on the Electrochemical Properties in LiCoO[sub 2] Thin Films

The electrochemical properties of nanoscale Al2O3-coated LiCoO2 thin films were examined as a function of the coating coverage. Al2O3-coated LiCoO2 films showed enhanced cycle-life performance with increasing degree of coating coverage, which was attributed to the suppression of Co dissolution and F− concentration in the electrolyte. Moreover, an Al2O3-coating layer with partial coverage clearly improved the electrochemical properties, even at 60 °C or with a water-contaminated electrolyte. Even though metal-oxide coating on LiCoO2 has been actively investigated, the mechanisms of nanoscale coating have yet to be clearly identified. In this article, surface analysis suggested that the Al2O3-coating layer had transformed to an AlF3∙3H2O layer during cycling, which inhibited the generation of HF by scavenging H2O molecules present in the electrolyte.

Iron-phosphate∕platinum∕carbon nanocomposites for enhanced electrocatalytic stability

The effects of FePO4–Pt∕C nanocomposites on the electrocatalytic properties were examined after 1000 accelerated cycles (up to 1.48V vs normal hydrogen electrode). The FePO4–Pt∕C nanocomposites exhibited similar electrocatalytic properties to Pt∕C without any initial degradation, and also showed enhanced long-term stabilities. By optimizing the synthetic conditions of FePO4–Pt∕C nanocomposites, the well-dispersed FePO4 nanoparticles played important roles in preserving the Pt surface activities, by preventing both the dissolution and agglomeration of Pt nanoparticles.

Nanostructured Platinum/Iron Phosphate Thin-Film Electrodes for Methanol Oxidation

Nanostructured Pt-FePO 4 thin-film electrodes consisting of Pt nanocrystals and amorphous FePO 4 were fabricated by cosputtering, and nanocomposites containing ∼3 or ∼5 nm Pt crystalline phases were obtained. Compared with the pure Pt electrodes, the Pt-FePO 4 electrodes containing ∼3 nm Pt nanophases showed a significantly high efficiency, stability, and low charge-transfer resistance for methanol oxidation. The improved catalytic activities of the Pt-FePO 4 thin-film electrodes can be attributed to both an increase in the active surface area, and the possible ability of FePO 4 for the effective transfer of protons and the CO oxidation during methanol oxidation.

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.