O. V. Buganov

Decay of Electronic Excitations in CdS and CdS/ZnS Colloidal Quantum Dots: Spectral and Kinetic Investigations

Using the spectral methods of induced absorption, luminescence, and photostimulated luminescence flash, we have experimentally investigated processes of decay of electronic excitations in CdS colloidal quantum dots and in CdS/ZnS “core-shell” systems synthesized in gelatin by the sol-gel method. It has been shown that the decay of electronic excitations in colloidal quantum dots of this type is predominantly related to a fast localization of nonequilibrium charge carriers on surface defects and their subsequent recombination during times on the order of units and tens of picoseconds. The passage to core-shell systems eliminates, to a large extent, surface defects of the core, some of which are luminescence centers. However, upon using the sol-gel synthesis, a noticeable fraction of luminescence centers are formed in the interior of the CdS quantum dot, which, as well as in the case of CdS/ZnS systems, ensures localization of exciton, blocks its direct annihilation, and maintains recombination radiation.

Fluorescence and picosecond induced absorption from the lowest singlet excited states of quercetin in solutions and polymer films

The spectroscopic and photophysical properties of the biologically important plant antioxidant quercetin in organic solvents, polymer films of polyvinyl alcohol, and a buffer solution at pH 7.0 are studied by stationary luminescence and femtosecond laser spectroscopy at room temperature and 77 K. The large magnitude of the dipole moment of the quercetin molecule in the excited Franck–Condon state μeFC= 52.8 C m indicates the dipolar nature of quercetin in this excited state. The transient induced absorption spectra S1→Sn in all solvents are characterized by a short-wave band at λabsmax = 460 nm with exponential decay times in the range of 10.0–20.0 ps. In the entire spectral range at times of >100 ps, no residual induced absorption was observed that could be attributed to the triplet–triplet transitions Т1 → Тk in quercetin. In polar solvents, two-band fluorescence was also recorded at room temperature, which is due to the luminescence of the initial enol form of quercetin (~415 nm) and its keto form with a transferred proton (550 nm). The short-wave band is absent in nonpolar 2-methyltetrahydrofuran (2-MTHF). The spectra of fluorescence and fluorescence excitation exhibit a low dependence on the wavelength of excitation and detection, which may be related to the solvation and conformational changes in the quercetin molecule. Decreasing the temperature of a glassy-like freezing quercetin solution in ethanol and 2-MTHF to 77 K leads to a strong increase in the intensity (by a factor of ~100) of both bands. The energy circuits for the proton transfer process are proposed depending on the polarity of the medium. The main channel for the exchange of electronic excitation energy in the quercetin molecule at room temperature is the internal conversion S1 ⇝ S0, induced by the state with a proton transfer.

Decay of electronic excitations in colloidal thioglycolic acid (TGA)-capped CdS/ZnS quantum dots

Investigations of dynamics of exciton relaxation in colloidal thioglycolic acid (TGA)-capped CdS/ZnS core/shell systems with diameter of 3.6 nm by means of femtosecond transient absorption spectroscopy, thermostimulated luminescence (TSL), and decay of luminescence are presented in this paper. It was found that the intensity of trap-state luminescence increases when one and two ZnS monolayers are formed. Also, the lifetime of trap-state luminescence increases. Two types of trap states with different depths were found, using thermostimulated luminescence technique. Localized states of the first type with depth of 0.085 eV do not change their concentration during sell formation. In contrast, trap state of the second type with depth of 0.125 eV are almost completely removed. It was found that the electron lifetime, investigated femtosecond transient absorption is not changed during formation of ZnS shell. It was concluded that localized states are channels of non-radiative recombination, direct quenching the center of trap-state luminescence. The absence of exciton luminescence is caused by rapid localization of holes at luminescence center.

Time-Dependent Absorption Spectra of 1D, 2D Plasmonic Structures Obtained by the Ordering of Ag Nanoparticles in Polymer Matrix

We investigated the dynamics of electron excitations in plasmonic nanocomposite based on a polymer matrix with a periodic substructure of ordered silver nanoparticles. The nanoparticles were synthesized in the polymer matrix from a metal precursor periodically distributed by holographic method. A two-dimensional distribution of silver nanoparticles was obtained by exposing to the interference pattern formed by three (for hexagonal structures) or four (for quadratic structures) laser beams. The influence of the structure symmetry on the transient surface plasmon spectra and relaxation dynamics of induced changes near plasmon resonance was discussed.

Photodynamics of intramolecular proton transfer in polar and nonpolar biflavonoid solutions

Using methods of steady state luminescence and femtosecond spectroscopy, we have studied the mechanism of intramolecular proton transfer in synthesized 3,7-dihydroxy-2,8-di(4-methoxyphenyl)-4H,6H-pyrano[3,2-g]chromen-4,6-dion in polar and nonpolar solutions, films, and polycrystals at 293 and 77 K. In an excited singlet state, intramolecular proton transfer occurs in two stages. At the first stage, a tautomer with one transferred proton (OTP tautomer) is formed from the Franck-Condon state within τ1 = 0.6 ps. At the second stage, the second proton is transferred within τ2 = 3.1 ps and a tautomer with two transferred protons (TTP tautomer) is formed, which fluoresces in toluene at 293 K with a high quantum yield, Φf = 0.66, and the fluorescence spectrum of which is characterized by a large Stokes shift, 9900 cm−1. At 293 K, polar solvents (dimethylformamide, dimethyl sulfoxide, ethanol, etc.) solvate the BFV molecule in the ground state, while, in the excited state, an OTP tautomer is mainly formed. In polar ethanol at 77 K, a dual fluorescence spectrum is observed, which is caused by the fluorescence emission of polysolvates with λmaxf = 460 nm and TTP phototautomers at λmaxf= 610 nm.

A negative feedback controlled hybrid mode locked Nd:YAG laser using a semiconductor quantum dot saturable absorber

The parameters of a solid-state PbS quantum dot saturable absorber, optical elements, resonator and feedback circuit have been adjusted for stable and highly efficient hybrid mode-locking Nd:YAG laser operation. A highly stable microsecond pulse train (40‐50µs) containing about 5000ps pulses and 3mJ total train energy was generated with a repetition rate of 10Hz. This all solid-state hybrid mode-locked Nd:YAG laser was used for synchronous pumping of a femtosecond Ti:sapphire laser.

The effect of plasmonic nanocomposite structure on the dynamics of electron excitations by ultra-short laser pulses

We investigated the dynamics of electron excitations in plasmonic nanocomposite based on a polymer matrix with a periodic substructure of ordered silver nanoparticles. The nanoparticles were synthesized in the polymer matrix from a metal precursor periodically distributed by holographic method.

Unusual transient absorption dynamics of silver nanoparticles in solutions of carboxylated amine complexons

We present the results of research on fast relaxation dynamics in the electronic excitation of silver nanoparticles synthesized in the presence of carboxylated amine complexons (NTA, Na2EDTA, DTPA) without any reductant or polymeric stabilizer. Unusual transient absorption dynamics in these objects after femtosecond laser irradiation was found, manifesting as the appearance of an additional long-lived bleaching band. The effect may be assigned to the inhomogeneous and porous shell of silver nanoparticles synthesized by such a procedure, as the consequence of a partial fragmentation of this shell due to heating under femtosecond laser excitation of plasmonic nanoparticles and subsequent electron–phonon energy relaxation.