: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

日本物理学会誌及びJournal of the Physical Society of Japanの月刊について

Quantum dot (QD) light-emitting diodes (LEDs) are ideal for large-panel displays because of their excellent efficiency, colour purity, reliability and cost-effective fabrication1-4. Intensive efforts have produced red-, green- and blue-emitting QD-LEDs with efficiencies of 20.5 per cent4, 21.0 per cent5 and 19.8 per cent6, respectively, but it is still desirable to improve the operating stability of the devices and to replace their toxic cadmium composition with a more environmentally benign alternative. The performance of indium phosphide (InP)-based materials and devices has remained far behind those of their Cd-containing counterparts. Here we present a synthetic method of preparing a uniform InP core and a highly symmetrical core/shell QD with a quantum yield of approximately 100 per cent. In particular, we add hydrofluoric acid to etch out the oxidative InP core surface during the growth of the initial ZnSe shell and then we enable high-temperature ZnSe growth at 340 degrees Celsius. The engineered shell thickness suppresses energy transfer and Auger recombination in order to maintain high luminescence efficiency, and the initial surface ligand is replaced with a shorter one for better charge injection. The optimized InP/ZnSe/ZnS QD-LEDs showed a theoretical maximum external quantum efficiency of 21.4 per cent, a maximum brightness of 100,000 candelas per square metre and an extremely long lifetime of a million hours at 100 candelas per square metre, representing a performance comparable to that of state-of-the-art Cd-containing QD-LEDs. These as-prepared InP-based QD-LEDs could soon be usable in commercial displays.

Highly efficient and stable InP/ZnSe/ZnS quantum dot light-emitting diodes

In quantum-confined semiconductor nanostructures, electrons exhibit distinctive behavior compared with that in bulk solids. This enables the design of materials with tunable chemical, physical, electrical, and optical properties. Zero-dimensional semiconductor quantum dots (QDs) offer strong light absorption and bright narrowband emission across the visible and infrared wavelengths and have been engineered to exhibit optical gain and lasing. These properties are of interest for imaging, solar energy harvesting, displays, and communications. Here, we offer an overview of advances in the synthesis and understanding of QD nanomaterials, with a focus on colloidal QDs, and discuss their prospects in technologies such as displays and lighting, lasers, sensing, electronics, solar energy conversion, photocatalysis, and quantum information.

Semiconductor quantum dots: Technological progress and future challenges

Colloidal quantum dot (QD) solids are emerging semiconductors that have been actively explored in fundamental studies of charge transport1 and for applications in optoelectronics2. Forming high-quality QD solids—necessary for device fabrication—requires substitution of the long organic ligands used for synthesis with short ligands that provide increased QD coupling and improved charge transport3. However, in perovskite QDs, the polar solvents used to carry out the ligand exchange decompose the highly ionic perovskites4. Here we report perovskite QD resurfacing to achieve a bipolar shell consisting of an inner anion shell, and an outer shell comprised of cations and polar solvent molecules. The outer shell is electrostatically adsorbed to the negatively charged inner shell. This approach produces strongly confined perovskite QD solids that feature improved carrier mobility (≥0.01 cm2 V−1 s−1) and reduced trap density relative to previously reported low-dimensional perovskites. Blue-emitting QD films exhibit photoluminescence quantum yields exceeding 90%. By exploiting the improved mobility, we have been able to fabricate CsPbBr3 QD-based efficient blue and green light-emitting diodes. Blue devices with reduced trap density have an external quantum efficiency of 12.3%; the green devices achieve an external quantum efficiency of 22%. A solution-based ligand-exchange strategy enables the realization of close-packed quantum dot solid films with near-unity photoluminescence quantum yield and high charge carrier mobility.

Bipolar-shell resurfacing for blue LEDs based on strongly confined perovskite quantum dots

Hydrostatic pressure effect on the donor impurity states in asymmetric multiple quantum wells

The introduction of a high-K/metal gate stack in metal-oxide-Semiconductor field-effect transistors can cause a significant degradation of the mobility, especially at weak inversion densities. This degradation is commonly ascribed to remote Coulomb scattering (RCS, i.e., charges trapped at the SiO 2 /HfO 2 interface). However, very large densities of RCS charges are usually needed to reproduce the experimental data. In this work, we explore alternative scattering mechanisms by quantum calculations of the carrier mobilities. We consider, in particular, metal grain workfunction fluctuations and local dipoles at the SiO 2 /HfO 2 interface. Similarly to RCS, both scattering mechanisms are found to reduce the carrier mobility significantly at low carrier densities. However, the mobility exhibits a different dependence on the thickness of high-κ layer, which provides a way to identify the dominant mechanism.

/pdf/carrier-scattering-by-workfunction-fluctuations-and-hqh78vbbb2.pdf

Carrier scattering by workfunction fluctuations and interface dipoles in high-K/metal gate stacks

Based on the effective-mass approximation, the binding energy of a donor impurity in zinc-blende (ZB) InGaN/GaN asymmetric multiple quantum wells (AMQWs) is investigated variationally. Numerical results show that the ground-state donor binding energy is highly dependent on the impurity positions and the AMQWs structure parameters. The donor binding energy has a maximum value and the impurity position of the maximum value is localized inside the wide well of the AMQWs. Moreover, the variation of any well width of the AMQWs has a remarkable influence on the donor binding energy. In particular, for the impurity localized inside the wide well, the donor binding energy is insensitive to the increment of the inter-well barrier width when the inter-well barrier width is large.

Donor impurity in InGaN/GaN asymmetric multiple quantum wells

Based on the effective-mass approximation, considering the built-in electric field effect due to the spontaneous and piezoelectric polarizations, the ground-state donor binding energy of a hygrogenic impurity in a cylindrical wurzite (WZ) ZnO/MgZnO strained quantum dot (QD) is investigated variationally. Numerical results show that the ground-state donor binding energy is highly dependent on the Mg composition, the impurity positions and the QD size. The built-in electric field also induces an asymmetric distribution of the ground-state donor binding energy with respect to the center of the QD. In particular, it is found that the ground-state donor binding energy is insensible to the dot height when the impurity is located at the right boundary of the WZ ZnO/MgZnO strained QD if the dot height is large.

HYDROGENIC IMPURITY STATES IN WURTZITE ZnO/MgZnO QUANTUM DOT

Based on the effective-mass approximation, the acceptor binding energy in a cylindrical zinc-blende (ZB) InGaN/GaN single quantum dot (QD) is investigated variationally in the presence of the applied electric field. Numerical results show that the acceptor binding energy is highly dependent on the applied electric field, impurity positions and QD size. The applied electric field also induces an asymmetric distribution of the acceptor binding energy with respect to the center of the QD. Moreover, in the presence of the applied electric field, the acceptor binding energy is insensitive to dot height when the impurity is located at the left boundary of the ZB In0.1Ga0.9N/GaN QD with large dot height (H≥6 nm). In particular, the acceptor binding energy of the impurity located at the left boundary of the ZB In0.1Ga0.9N/GaN QD is identical for different dot height when the applied electric field F≥350 KV/cm. This result can be of interest for the technological purpose, as it could involve a source of control so...

EFFECT OF APPLIED ELECTRIC FIELD ON ACCEPTOR STATES IN ZINC-BLENDE InGaN/GaN QUANTUM DOT

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