Carbon dots are promising luminescent nanoparticles which possess unique optical properties together with the simplicity of their synthesis. The revealing of their energy level structure is a crucial for further implementation in various applications from biology to photonics. This work is devoted to the investigation of optical responses of citric acid-based carbon dots with respect to the chemical environment, as solvent polarity. Small spectral shifts of the luminescent and absorption bands for CDs are observed. Assuming the PL is originated from the emission of various luminescent centers in CDs we explain these shifts by their intensity redistribution within PL band in different solvents. The redistribution leads to blue shift of PL band in non-polar solvents, while in polar solvents strong dependence of PL band on polarity value is absent.
We report on the structural investigation of self-organized assemblies of PbS nanocrystals (NCs) of different sizes, which were deposited on a glass substrate or embedded in a porous matrix. Regardless of the NC size and the type of the substrate and matrix, the assemblies were ordered in two-dimensional superlattices with densely packed NCs.
Concentration-dependent optical properties of lead sulfide (PbS) quantum dots (QDs) embedded into a porous matrix are investigated. A red shift of luminescence peak position depends on QD concentration and evidences for F¨orster resonant energy transfer (FRET) inside the quasi-monodispersed assemblies. PL decay curves show significant influence of FRET process on QD optical properties. PL lifetime size-dependencies are found to be dependent on the QD concentration. The growing impact of FRET process with increasing QD concentration leads to transformation of anomalous size-dependence of PL lifetimes to the inverse. FRET efficiency gradually increases with QD concentration and saturates at 35% that corresponds to the formation of an ordered QD superstructure. Formation of PbS QD superstructure is confirmed by X-ray analysis, while calculated interdot distance in the superstructure coincides with the one obtained by FRET technique.
X-ray structural analysis is used for investigation of structures, obtained by self-organization from nanoparticles - lead
sulfide (PbS) quantum dots (QDs) of different sizes - deposited on a glass substrate or embedded in a porous matrix.
These nanostructures obtained by both methods represent ordered close-packed structures of the nanoparticles. The
configuration of obtained structures does not depend on the nanoparticle size and type of substrate and matrix.
We investigate quantum dots (QDs) of ternary compounds CdXHg1-XSe with 0<X<1 using a Raman scattering technique
at room temperature. We obtained the correlation between QDs chemical composition and the frequencies of CdSe-like
LO and the HgSe-like TO and LO modes. It is shown that the crystalline structure of original CdSe QDs used for Cd/Hg
substitution, either zinc blende or wurtzite affects strongly on structural properties of resultant CdXHg1-XSequantum dots.
KEYWORDS: Lead, Luminescence, Quantum dots, Energy transfer, Fluorescence resonance energy transfer, Data analysis, Energy efficiency, Absorption, Optical filters, Nanocrystals
Nonradiative fluorescence resonance energy transfer (FRET) between lead sulfide quantum dots (QDs) of two different
sizes embedded in porous matrix is observed by a fluorescence spectroscopy. Analysis of decays of photoluminescence
from QD mixture shows that energy transfer in studied systems is determined by static quenching, specific for direct
contact between QD-donor and QD-acceptor in the QDs close-packed ensembles. From steady-state spectral analysis it
was found that efficiency of energy transfer depends on the molar ratio QD-donor/QD-acceptor and energy transfer from
the donor to the acceptor passes by several channels.
The resonant and off-resonant Raman spectra of optical phonons in two-dimensional CdSe nanocrystals of 5, 6, and 7
monolayers are analysed. The spectra are dominated by SO and LO phonon bands of CdSe, whose frequencies are
thickness-independent in the off-resonant Raman scattering but demonstrate an evident thickness dependence in the
case of the resonant Raman scattering.
We study size dependence of kinetic and spectral properties of near-infrared luminescence from PbS quantum
dots in colloidal solution. Luminescence lifetimes are found to lie between 250 ns for the largest quantum
dots and 2:5 μs for the smallest ones, while the Stoke's shift is found to increase from 4-5 to 300 meV. These
results are explained by the presence of the long-living in-gap state, with the size-dependent energy. Analytical
modeling shows that the relaxation from this state is dominant in small quantum dots and negligible in large ones.
Biexponential luminescence decay with the size-dependent recombination rates is predicted for quantum dots
of all sizes.
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