Quantum-sized effects in oxidized silicon structures with surface II-VI nanocrystals

The Si-SiO₂ interface in oxidized macroporous silicon structures with surface CdS and ZnO nanocrystals was investigated using the methods of electroreflectance and photoconductivity. The Franz-Keldysh effect, built-in electric field and surface quantization of charge carriers in the Si-SiO₂ region were revealed. The splitting of photoconductivity peaks was detected in the area of indirect band-to-band transition due to quantization of charge carriers in the surface silicon region, too. The latter data correlate with the results of the electroreflectance spectra measurements in the area of direct interband transition of oxidized macroporous silicon structures with surface CdS and ZnO nanocrystals

Possibility of obtaining TiO2 material by plasma dynamic method into an air atmosphere

This paper shows the possibility to synthesize directly the titanium dioxide in a supersonic jet of an electric discharge erosive plasma. Using the X-ray diffractometry it is shown that the obtained product contains two main crystalline phases: anatase and rutile with tetragonal syngony. The size of the coherent scattering region is less 100 nm

Band-gap and sub-band-gap photoelectrochemical processes at nanocrystalline CdS grown on ZnO by successive ionic layer adsorption and reaction method

Cadmium sulfide nanoparticle (NP) deposition by the successive ionic layer adsorption and reaction (SILAR) method on the surface of mesoporous ZnO micro-platelets with a large specific surface area (110 ± 10 m2g− 1) results in the formation of ZnO/CdS heterostructures exhibiting a high incident photon-to-current conversion efficiency (Y) not only within the region of CdS fundamental absorption (Ymax = 90%; 0.1 M Na2S + 0.1 M Na2SO3), but also in the sub-band-gap (SBG) range (Ymax = 25%). The onset potentials of SBG photoelectrochemical processes are more positive than the band-gap (BG) onset potential by up to 100 mV. A maximum incident photon-to-current conversion efficiency value for SBG processes is observed at larger amount of deposited CdS in comparison with the case of BG ones. The Urbach energy (EU) of CdS NPs determined from the photocurrent spectra reaches a maximal value on an early deposition stage (EU = 93 mV at SILAR cycle number N = 5), then lowers somewhat (EU = 73 mV at N = 10) and remains steady in the range of N from 20 to 300 (EU = 67 ± 1 mV). High efficiency of the photoelectrochemical SBG processes are interpreted in terms of light scattering in the ZnO/CdS heterostructures

Effect of [OH-] linkages on luminescent properties of ZnO nanoparticles

Optical properties of ZnO nanoparticles prepared from a simple chemical method using sodium zincate bath show strong white light emission. X-ray absorption fine structure studies reveal a completely different local environment around Zn in these ZnO nanoparticles. The observed luminescence properties and local structural changes have been explained on the basis of a linkage between Zn and OH- ions in the surface layers of ZnO nanoparticles.Comment: J. Phys. Chem. C. (2011) (in print

Composition-Dependent Optical Band Bowing, Vibrational and Photochemical Behavior of Aqueous Glutathione-Capped (Cu, Ag)InS Quantum Dots

A mild aqueous synthesis of colloidal 2-4 nm (Cu,Ag)-In-S (CAIS) quantum dots (QDs) stabilized by surface metal complexes with glutathione was introduced. Linear variations of the interplanar distances as well as of the characteristic Ag(Cu)-S-related Raman vibrational frequencies of CAIS QDs with increasing copper content show such QDs to be solid solutions rather than a mixture of AIS and CIS phases. At the same time, the band gaps and the energies of photoluminescence (PL) band maxima of CAIS QDs show non-monotonous changes decreasing from AIS to CAIS QDs (50 molar% Cu) and then increasing back for Cu-richer CAIS compositions and pure CIS. This behavior was interpreted as a result of the band bowing phenomenon. The bowing parameters of CAIS QDs determined from both absorption spectra (1.10 eV) and PL spectra (0.38 eV) are close to the range typically reported for ternary bulk MI-MIII-S compounds with the MI sites occupied by a mixture of copper and silver cations. The PL intensity of CAIS QDs was found to decrease during PL registration due to the photochemical decomposition of QDs, the efficiency of this process increasing with increasing copper content. A similar trend was found in the photocatalytic reduction of methylviologen cations by hydrosulfide anions in the presence of CAIS. The initial rate of this reaction increased monotonously from AIS to CAIS to CIS QDs, with the activity of the CAIS QDs (50 molar% Cu) and pure CIS QDs being respectively 1.5 and 2.7 times higher than the photoactivity of pure AIS QDs. This trend is compliant with a strong decrease in the PL emission efficiency observed from AIS to CAIS to CIS QDs. Similar optical and photochemical properties were revealed for core/shell CAIS/ZnS QDs. The band-bowing effect and photochemical activity of mixed CAIS (CAIS/ZnS) QDs open good perspectives for light-conversion applications in the photon energy range down to 1.8 eV

Photocatalytic properties of rutile nanoparticles obtained via low temperature route from titanate nanotubes

Acid treatment of titanate nanotubes with 0.1 mol dm-3 H2SO4 results in their transformation into various forms of nanostructured TiO2, the shape and crystal structure of which depend on the temperature and time of treatment. Such a transformation at 25 ºC results in the formation of rutile spheroidal nanoparticles, which are characterised by good crystallinity, high specific surface area (ca 250 m2 g-1 BET) and small average diameter of particles (ca. 3 nm). These particles are also characterised by increased activity in reactions of both photocatalytic generation of hydrogen from alcohol and advanced oxidation of organic species (ethanol). Acid assisted transformation of nanotubes at higher temperatures results in formation of nanostructured anatase/titanate composites with lower photocatalytic activity. The mechanism of transformation and the reasons for increased activity of rutile nanoparticles are discusse

Mercury-indium-sulfide nanocrystals: A new member of the family of ternary in based chalcogenides

A general synthesis approach of aqueous glutathione-capped ternary Ag-In-S, Cu-In-S, and Hg-In-Snanocrystals (NCs) is introduced allowing the NC composition to be varied in a broad range. TernaryHg-In-S (HIS) NCs are reported for the first time and found to have the same tetragonal chalcopyritemotif as Cu-In-S and Ag-In-S NCs, corroborated by phonon spectra, while X-ray photoelectronspectroscopic data indicate mercury to be present as Hg+ in the Hg-In-S NCs. Colloidal HIS and Hg-InS/ZnS NCs showed little or no variations of the spectral width of the photoluminescence band uponNC size selection, temperature variation in a broad range of 10-350 K, deposition of a ZnS shell, or apost-synthesis annealing. All these observations are similar to those reported earlier for Ag-In-S andAg-In-S/ZnS NCs and allowed us to assume a general photoluminescence mechanism for all threeternary compounds, based on the model of radiative self-trapped exciton recombination