Vlastnosti vertikálně uspořádaných struktur kvantových teček

We have studied the overlap of electron wave functions of stacked QDs. We have found that for sample with the spacer thickness 15 nm the electron wave functions of QDs in stack are fully separated. We have found that for spacer thickness around 7 nm the electron wave functions of smaller QDs from deeper layers still overlap while the electron wave function of bigger QDs at top layers remain separated

InAs/GaAs quantum-size structures grown by MOVPE

Structures with quantum size objects as self-organised quantum dots, quantum rings, wires and quasi-quantum wells could be reproducibly grown by MOVPE. The advantage of MOVPE technology is its material variability, operating costs and relation to the industry

Dependence of photocatalytic activity of ZnxCd1-xS quantum dot composition

Aqueous colloidal dispersions containing ZnxCd1-xS quantum dots (QDs) of different x compositions were prepared by precipitating zinc and cadmium acetates with sodium sulphide, in the presence of a cetyltrimethylammonium bromide stabilizer. Ultraviolet-visible absorption spectroscopy was used to determine the transition energies of the QDs, which in turn were used to calculate their sizes, which depended on their composition. The QD size decreased with increasing Zn content. The photocatalytic activity of the ZnxCd1-xS QDs was studied by the decomposition of methylene blue under ultraviolet irradiation, at a maximum intensity at 365 nm (3.4 eV). Three different photocatalytic activity regions were observed, which depended on the Zn content. The quantum levels of the QDs could be excited by incident irradiation, and influenced the resulting photocatalytic activity. Maximum photocatalytic activity was achieved at x = 0.6, where the QD transition energy was equal to the irradiation photon energy. The photocatalytic efficiency of the QDs depended on their surface area and arrangement of quantum levels, because of the quantum size effect.Web of Science36333532

Core/shell CdS/ZnS nanoparticles: Molecular modelling and characterization by photocatalytic decomposition of Methylene Blue

Core/shell CdS/ZnS nanoparticles were modelled in the Material Studio environment and synthesized by one-pot procedure. The core CdS radius size and thickness of the ZnS shell composed of 1–3 ZnS monolayers were predicted from the molecular models. From UV–vis absorption spectra of the CdS/ZnS colloid dispersions transition energies of CdS and ZnS nanostructures were calculated. They indicated penetration of electrons and holes from the CdS core into the ZnS shell and relaxation strain in the ZnS shell structure. The transitions energies were used for calculation of the CdS core radius by the Schrödinger equation. Both the relaxation strain in ZnS shells and the size of the CdS core radius were predicted by the molecular modelling. The ZnS shell thickness and a degree of the CdS core coverage were characterized by the photocatalytic decomposition of Methylene Blue (MB) using CdS/ZnS nanoparticles as photocatalysts. The observed kinetic constants of the MB photodecomposition (kobs) were evaluated and a relationship between kobs and the ZnS shell thickness was derived. Regression results revealed that 86% of the CdS core surface was covered with ZnS and the average thickness of ZnS shell was about 12% higher than that predicted by molecular modelling.Web of Science29282281

InGaN/GaN Structures: Effect of the Quantum Well Number on the Cathodoluminescent Properties

International audienc

Effect of silver doping on the TiO2 for photocatalytic reduction of CO2

Pure TiO2 and various silver-enriched TiO2 powders were prepared by the sol–gel process controlled in the reverse micellar environment. The catalysts were tested in CO2 photocatalytic reduction and characterized by X-ray diffraction (XRD), nitrogen adsorption measurement and UV–vis. Methane and methanol were the main reduction products. The yield of methane and methanol increases when modifying the TiO2 by silver incorporation is caused by two mechanisms: up to 5% of Ag in TiO2 the Ag impurity band inside the TiO2 bandgap decreases the absorption edge and increases so the electron–hole pair generation, above 5% of Ag in TiO2 Ag metallic clusters are formed in TiO2 crystals with Shottky barrier at the metal–semiconductor interface, which spatially separates electron and holes and increases their lifetime (decreases probability of their recombination)

Optimization of cerium doping of TiO2 for photocatalytic reduction of CO2 and photocatalytic decomposition of N2O

The cerium-doped TiO2 photocatalysts (0–0.8 mol% Ce) were prepared by using a sol–gel method. Textural, structural, optical and electronic properties of Ce/TiO2 photocatalysts were characterized in detail by using nitrogen physisorption, powder X-ray diffraction, diffuse reflectance UV–Vis spectroscopy and contact potential difference measurements. It was proved that increasing amount of cerium ions in TiO2 (1) decreased the anatase crystallite size, which corresponded to the increase in specific surface area of the photocatalysts, and (2) decreased the absorption edge (shifting the spectral response toward the visible light region). The prepared photocatalysts were tested for CO2 photocatalytic reduction in a stirred batch annular reactor, and methane was a main product. The photocatalytic decomposition of nitrous oxide was carried out in batch reactor with circulation, and only oxygen and nitrogen were detected as reaction products. It was found out that the energies of electrons and holes played the key role in both photocatalytic reactions and can be markedly affected by doping of TiO2 by cerium.Web of Science78355855

Preparation, characterization and photocatalytic properties of cerium doped TiO2: On the effect of Ce loading on the photocatalytic reduction of carbon dioxide

The parent TiO2 and cerium doped TiO2 photocatalysts with Ce loadings 0.28–10 mol.% were prepared by the sol-gel method controlled within reverse micelles of nonionic surfactant Triton X-114. Photocatalysts were comprehensively characterized using nitrogen physisorption, XRD, XPS, contact potential difference measurements, Raman spectroscopy, and DR UV–vis spectroscopy and their performance was explored in the CO2 photocatalytic reduction for the first time. Concerning photocatalysts properties, it was revealed that the inhibiting effect of cerium on the TiO2 crystallites growth occurred only up to 3 mol.% of Ce when the incorporation of Ce4+ into the anatase lattice took place. This phenomenon was correlated with the expansion of anatase cell volume. At higher Ce loadings (≥5 mol.%) the anatase lattice was saturated and the formation/separation of amorphous ceria and/or ceria (∼1 nm) nucleation occurred, accompanied by the increase of TiO2 anatase crystallite-size and the limitation of value of anatase cell volume. Further, it was found out that the mesoporosity of photocatalysts may be preferentially attributed to voids existing between the individual crystallites and thus can be influenced by changes in the crystallite size. The modification of TiO2 with cerium affected also the spectral response of photocatalysts, shifting it to the visible light region. However, this property itself was not crucial in the CO2 photocatalytic reduction. The key role in the CO2 photocatalytic reduction played the energies of electrons and holes within the electronic structure of photocatalysts, which were markedly affected by the Ce atoms addition. For 0.28 mol.%Ce/TiO2, both electrons and holes have required potentials for the photocatalytic reduction of CO2, while for 3 mol.% and higher Ce loadings the energy of electrons was already below H+ reduction potential and thus the photocatalytic performance of these catalysts was decreasing.Web of Science152-15318317