Search

CdS quantum dots prepared by the Langmuir–Blodgett method have been investigated using UV spectroscopy and time-resolved photoluminescence. It has turned out that at temperatures 5–300 K, most prominent features in photoluminescence spectra are an intense high-energy peak and a wide low-energy shoulder. The high-energy peak observed in the spectra is due to transitions between states in the valence and conduction bands, and the wide shoulder is associated with defects in quantum dots. The kinetics of band-edge photoluminescence is biexponential, which is interpreted as the simultaneous recombination of negative excitons and trions. It has been found that trion recombination makes a major contribution to the photoluminescence intensity. It has been shown that the recombination rate of both excitons and trions drops with increasing temperature. The slowdown of exciton recombination is attributed to the thermal occupation of higher lying optically inactive states and is well described in terms of the available theoretical models of the exciton state fine structure. The radiative lifetime of trions grows with temperature owing, first, to the occupation of optically passive states and, then, to delocalization of one electron from the quantum dot into the overbarrier continuum of states, which decreases the overlapping integral. [ABSTRACT FROM AUTHOR]

The absorption, photoluminescence, and photoluminescence excitation spectra of CdS nanocrystals formed by the Langmuir-Blodgett method are explored. Features of the absorption and photoluminescence excitation spectra defined by optical transitions in the matrix and nanocrystals are identified. The efficiency of electronic excitation transfer from an organic matrix to nanocrystals is studied. It is shown that charge carriers efficiently transfer from the matrix to electron and hole size-quantization levels in nanocrystals and to acceptor defect levels in the band gap of nanocrystals. A large Stokes shift defined by fine exciton structure (bright and dark excitons) is observed. The shift is in the range 140-220 meV for nanocrystals 2.4 and 2.0 nm in radius. [ABSTRACT FROM AUTHOR]

Cadmium sulfide (CdS) nanoparticles have been obtained by chemical precipitation onto the surface of single-crystalline silicon from an aqueous solution of ammonia, cadmium chloride (CdCl), and thiourea, as well as from water-DMSO and water-DMF mixtures with the same concentrations of the reagents. According to data of atomic force microscopy, the samples obtained from the aqueous solution consist of individual nanoparticles and agglomerates thereof with sizes of no larger than 1 µm. Materials obtained from the water-organic mixtures are distinguished by the aggregation of CdS nanoparticles into threadlike chains. The length of the formed curved chains and the size of CdS nanoparticles composing them depend on the nature and amount of an organic component of a mixture. Atomic force microscopy, transmission electron microscopy, and photoluminescence spectroscopy data have shown that the average size of CdS nanoparticles is 2-2.5 nm depending on solvent composition. [ABSTRACT FROM AUTHOR]

The photoluminescence kinetics in CdS nanocrystals produced by the Langmuir-Blodgett technique is studied at a temperature of 5 K. The photoluminescence kinetics is described by the sum of two exponential functions, with characteristic times of about 30 and 160 ns. It is found that the fast and slow decay times become longer, as the nanocrystal size increases. Analysis of the data shows that the fast decay time is controlled by trion recombination in nanocrystals with defects, whereas the slow decay time is controlled by the annihilation of optically inactive excitons in nanocrystals without defects. It is established that, as the nanocrystal size is decreased, the fraction of imperfect nanocrystals is reduced because of an increase in the energy of defect formation. [ABSTRACT FROM AUTHOR]

Stationary and transient photoluminescence of CdS quantum dots deposited on silicon substrates and carbon nanotubes is investigated. The photoluminescence spectrum of quantum dots on a silicon substrate is dominated by a band originating from electron transitions between the quantum-confinement levels in the dots. When the quantum dots are deposited on carbon nanotubes, the intensity of this band decreases significantly. Furthermore, the kinetics of the photoluminescence decay becomes faster, which brings evidence of an additional channel for the quantum-dot deexcitation. The analysis of the experimental data demonstrates that the Förster energy transfer from CdS quantum dots to carbon nanotubes is most probably responsible for this channel. The efficiency of this process exceeds 60%. [ABSTRACT FROM AUTHOR]