Donghwa Lee

A zero-thermal-quenching phosphor

Phosphor-converted white light-emitting diodes (pc-WLEDs) are efficient light sources used in lighting, high-tech displays, and electronic devices. One of the most significant challenges of pc-WLEDs is the thermal quenching, in which the phosphor suffers from emission loss with increasing temperature during high-power LED operation. Here, we report a blue-emitting Na3-2xSc2(PO4)3:xEu2+ phosphor (λem = 453 nm) that does not exhibit thermal quenching even up to 200 °C. This phenomenon of zero thermal quenching originates from the ability of the phosphor to compensate the emission losses and therefore sustain the luminescence with increasing temperature. The findings are explained by polymorphic modification and possible energy transfer from electron-hole pairs at the thermally activated defect levels to the Eu2+ 5d-band with increasing temperature. Our results could initiate the exploration of phosphors with zero thermal quenching for high-power LED applications.

Porous-layered stack of functionalized AuNP–rGO (gold nanoparticles–reduced graphene oxide) nanosheets as a sensing material for the micro-gravimetric detection of chemical vapor

With oleylamine as a solvent and reducing agent, Au nanoparticles (AuNPs) are grown in situ on graphene oxide (GO) sheets. Thereafter, a porous-layered stack of AuNPs–GO nanosheets is formed by a gas-phase chemical reduction step. After the AuNPs are modified with 11-mercaptoundecanoic acid (11-MUA) to graft –COOH sensing groups to the amine, the functionalized AuNPs–rGO porous-layer stacked nanosheets are loaded onto a gravimetric resonant microcantilever for use as a mass sensing material. The rGO sheets serve as multi-layer nano-shelves to support and carry the functionalized AuNPs for gas adsorbing/sensing, while the AuNPs serve as nano-spacers between the rGO sheets to provide a high specific surface area for gas molecules accessing the material. The cantilever sensor experimentally exhibits a very rapid response to ppm-level trimethylamine (TMA) vapor, which is attributed to the novel sensing material. Compared with the hydrophilic AuNP–GO, the highly hydrophobic AuNP–rGO shows a much improved suppression to the noise from changes in environmental humidity. Featuring a rapid response, high sensitivity and good resistance to interference from environmental moisture, the novel sensing nanomaterial is promising in various chemical vapor detection applications.

X‐ray Transient Absorption and Picosecond IR Spectroscopy of Fulvalene(tetracarbonyl)diruthenium on Photoexcitation

Caught in the light: The fulvalene diruthenium complex shown on the left side of the picture captures sun light, causing initial Ru-Ru bond rupture to furnish a long-lived triplet biradical of syn configuration. This species requires thermal activation to reach a crossing point (middle) into the singlet manifold on route to its thermal storage isomer on the right through the anti biradical.

Role of Four-Fold Coordinated Titanium and Quantum Confinement in CO2 Reduction at Titania Surface

Photocatalytic reduction of carbon dioxide (CO(2)) into hydrocarbons is an attractive approach for mitigating CO(2) emission and generating useful fuels at the same time. Titania (TiO(2)) is one of the most promising photocatalysts for this purpose, and nanostructured TiO(2) materials often lead to an increased efficiency for the photocatalytic reactions. However, what aspects of and how such nanomaterials play the important role in the improved efficiency are yet to be understood. Using first-principles calculations, reaction mechanisms on the surface of bulk anatase TiO(2)(101) and of a small TiO(2) nanocluster were investigated to elucidate the role of four-fold coordinated titanium atoms and quantum confinement (QC) in the CO(2) reduction. Significant barrier reduction observed on the nanocluster surface is discussed in terms of how the under-coordinated titanium atoms and QC influence CO(2) reduction kinetics at surface. It is shown that the reduction to CO can be greatly facilitated by the under-coordinated titanium atoms, and they also make CO(2) anion formation favorable at surfaces.

Interfacial Effects on the Band Edges of Functionalized Si Surfaces in Liquid Water

By combining ab initio molecular dynamics simulations and many-body perturbation theory calculations of electronic energy levels, we determined the band edge positions of functionalized Si(111) surfaces in the presence of liquid water, with respect to vacuum and to water redox potentials. We considered surface terminations commonly used for Si photoelectrodes in water splitting experiments. We found that, when exposed to water, the semiconductor band edges were shifted by approximately 0.5 eV in the case of hydrophobic surfaces, irrespective of the termination. The effect of the liquid on band edge positions of hydrophilic surfaces was much more significant and determined by a complex combination of structural and electronic effects. These include structural rearrangements of the semiconductor surfaces in the presence of water, changes in the orientation of interfacial water molecules with respect to the bulk liquid, and charge transfer at the interfaces, between the solid and the liquid. Our results showed that the use of many-body perturbation theory is key to obtain results in agreement with experiments; they also showed that the use of simple computational schemes that neglect the detailed microscopic structure of the solid-liquid interface may lead to substantial errors in predicting the alignment between the solid band edges and water redox potentials.

Universal Approach toward Hysteresis-Free Perovskite Solar Cell via Defect Engineering

Organic-inorganic halide perovskite is believed to be a potential candidate for high efficiency solar cells because power conversion efficiency (PCE) was certified to be more than 22%. Nevertheless, mismatch of PCE due to current density (J)-voltage (V) hysteresis in perovskite solar cells is an obstacle to overcome. There has been much lively debate on the origin of J-V hysteresis; however, effective methodology to solve the hysteric problem has not been developed. Here we report a universal approach for hysteresis-free perovskite solar cells via defect engineering. A severe hysteresis observed from the normal mesoscopic structure employing TiO2 and spiro-MeOTAD is almost removed or does not exist upon doping the pure perovskites, CH3NH3PbI3 and HC(NH2)2PbI3, and the mixed cation/anion perovskites, FA0.85MA0.15PbI2.55Br0.45 and FA0.85MA0.1Cs0.05PbI2.7Br0.3, with potassium iodide. Substantial reductions in low-frequency capacitance and bulk trap density are measured from the KI-doped perovskite, which is indicative of trap-hysteresis correlation. A series of experiments with alkali metal iodides of LiI, NaI, KI, RbI and CsI reveals that potassium ion is the right element for hysteresis-free perovskite. Theoretical studies suggest that the atomistic origin of the hysteresis of perovskite solar cells is not the migration of iodide vacancy but results from the formation of iodide Frenkel defect. Potassium ion is able to prevent the formation of Frenkel defect since K+ energetically prefers the interstitial site. A complete removal of hysteresis is more pronounced at mixed perovskite system as compared to pure perovskites, which is explained by lower formation energy of K interstitial (-0.65 V for CH3NH3PbI3 vs -1.17 V for mixed perovskite). The developed KI doping methodology is universally adapted for hysteresis-free perovskite regardless of perovskite composition and device structure.

Structure and diffusion of intrinsic defect complexes in LiNbO3 from density functional theory calculations

Organized defect clusters in non-stoichiometric LiNbO₃ are known to dominate macroscale ferroelectric properties; yet the detailed nature of these defects is currently unknown. Here, the relative stabilities of various defect cluster arrangements of lithium vacancies around a niobium antisite in LiNbO₃ are determined using density functional theory combined with thermodynamic calculations. Their effects on the ferroelectricity of the system are also discussed. It is found that at room temperature the non-uniaxial dipole moments associated with the defect clusters could affect the properties of the system locally. The diffusion mechanism is predicted to be through first nearest neighbor jumps on the Li sublattice. The diffusivity of the lithium vacancy is found to be extremely low at room temperature, which indicates that the defect complexes should be rather stable.

Hyper-branched sensing polymer directly constructed on a resonant micro-cantilever for the detection of trace chemical vapor

A hyper-branched polymer is layer-by-layer self-assembled on a resonant micro-cantilever and, then, functionalized with sensing-terminals for the specific detection of the trace chemical vapor of dimethyl methylphosphonate (DMMP, a typical simulant for nerve agents). The hyper-branched polymer is directly constructed on the SiO2 surface of the cantilever via an A2 + B4 layer-by-layer route, where A2 and B4 are complementary interacting groups which undergo coupled linking. After modification with 4-(2-(4-(allyloxy)phenyl)-1,1,1,3,3,3-hexafluoropropan-2-yl)phenol (APHFPP) groups specific to DMMP, the high specific-surface-area hyper-branched polymer provides very dense sensing sites to adsorb a great number of DMMP molecules for micro-gravimetric detection. Moreover, the sensing polymer possesses a “more branches but fewer roots” configuration on the cantilever surface to depress the cross-talk effect caused by adsorption induced cantilever spring-stiffening. Experimental results indicate that, self-assembled with the hyper-branched sensing polymer, the resonant cantilevers exhibit rapid and reproducible detection of trace DMMP (with the detection limit lower than 7.2 ppb) and effectively depressed parasitic frequency-shift from the cantilever spring stiffening effect. In addition, the sensor features satisfactory selectivity in the presence of water and organic solvents. When an alternative sensing-group of 2-allylhexafluoroisopropanol (AHFIP) is modified on the hyper-branched architecture, the cantilever becomes specifically sensitive to trace explosive vapor. Therefore, the developed technique for the functionalization of hyper-branched polymer directly grown on a cantilever provides a widely usable micro/nano sensing-platform for the detection of trace chemical vapors.

Structure and energetics of 180° domain walls in PbTiO3 by density functional theory

Density functional theory at the level of the local density approximation with the projector augmented wave method is used to determine the structure of 180° domain walls in tetragonal ferroelectric PbTiO(3). In agreement with previous studies, it is found that PbO-centered {100} walls have lower energies than TiO(2)-centered {100} walls, leading to a Peierls potential barrier for wall motion along <010> of ∼36 mJ m(-2). In addition to the Ising-like polarization along the tetragonal axis, it is found that near the domain wall, there is a small polarization in the wall-normal direction away from the domain wall. These Néel-like contributions to the domain wall are analyzed in terms of the Landau-Ginzburg-Devonshire phenomenological theory for ferroelectrics. Similar characteristics are found for {110} domain walls, where OO-centered walls are energetically more favorable than the PbTiO-centered walls.

Local probing of the interaction between intrinsic defects and ferroelectric domain walls in lithium niobate

We demonstrate the capability of confocal Raman spectroscopy to characterize nanoscale interactions of defects with ferroelectric domain walls by identifying defect-related frequency shifts in congruent lithium niobate. These shifts resemble those observed for an external field applied anti-parallel to the ferroelectric axis, suggesting a small reduction of the electric polarization. Density functional theory calculations suggest that this reduction results from a change in the intrinsic defect cluster structure and polarization at the domain wall.