D. K. Nelson

Fractals of graphene quantum dots in photoluminescence of shungite

Viewing shungite as loosely packed fractal nets of graphene-based (reduced graphene oxide, rGO) quantum dots (GQDs), we consider photoluminescence of the latter as a convincing proof of the structural concept as well as of the GQD attribution to individual rGO fragments. We study emission from shungite GQDs for colloidal dispersions in water, carbon tetrachloride, and toluene at both room and low temperatures. As expected, the photoluminescence of the GQD aqueous dispersions is quite similar to that of synthetic GQDs of the rGO origin. The morphological study of shungite dispersions shows a steady trend of GQDs to form fractals and to drastically change the colloid fractal structure caused by the solvent exchange. Spectral study reveals a dual character of the emitting centers: individual GQDs are responsible for the spectra position while the fractal structure of GQD colloids ensures high broadening of the spectra due to structural inhomogeneity, thus causing a peculiar dependence of the photoluminescence spectra on the excitation wavelength. For the first time, photoluminescence spectra of individual GQDs were observed in frozen toluene dispersions, which paves the way for a theoretical treatment of the GQD photonics.

Enhanced Raman scattering provided by fullerene nanoclusters

Two new nonlinear optic effects are observed in crystalline solutions of fullerenes in toluene and carbon tetrachloride. Both are provided by a self-clustering of the solute molecules and are referred to fullerene-enhanced Raman scattering and solvent-enhanced luminescence. The effects are induced by the excitation of charge-transfer states of the fullerene nanoclusters, which makes the latter act as amplifiers of local electric fields.

Fullerene nanoclusters as enhancers in linear spectroscopy and nonlinear optics

The enhancement of nonlinear optical phenomena in fullerene solutions is treated in terms of the model of amplification of local electric field due to resonance excitation of clusters, which are extended chargetransfer complexes. Excitation light is transformed in the clusters into local charge-transfer excitons, inducing polarization of the medium sufficient for amplification of the local electric field of incident and exiting light. The intensity of the observed linear optical phenomena has been directly correlated with the magnitude of the nonlinear characteristics of the medium on one hand and the efficiency of clustering on the other hand. The gain coefficients have been determined. Experimental and computational testing procedures that make it possible to determine suitability of the fullerene-doped matrix for nonlinear-optics applications are proposed.

Radiative and nonradiative recombination kinetics of indirect bound excitons studied by time-resolved level anticrossing experiments

Time-resolved investigation of the level anticrossing effect in exciton emission is first used to determine separately the radiative and nonradiative indirect bound exciton lifetimes in a semiconductor crystal using GaSe as an example. A theoretical treatment of the experimental level anticrossing signal is carried out for different instances of the indirect bound exciton lifetime. The indirect bound exciton radiative lifetime τr = 4.8 µs is found to be about five times shorter than the nonradiative lifetime τ0 = 19 µs and about two orders of magnitude longer than the radiative lifetime of direct bound excitons in the same compound. Within the two-level scheme, the value of the indirect bound exciton spin relaxation time T1 is assessed to be 100 µs.

Evolution of the level anticrossing signal in magnetoluminescence of localized excitons in the GaSe–GaTe solid solution

The time dependence of the Zeeman-sublevel anticrossing signal in triplet localized exciton emission in the GaSe0.87Te0.13 semiconductor solid solution has been studied by the time-resolved spectroscopy method under conditions of unpolarized pumping. It has been shown that the anticrossing signal shape changes significantly for the lifetime t of localized excitons. At the time point t = 0, the anticrossing signal is not detected; as t increases, a maximum is formed in the dependence of the exciton emission intensity on the magnetic field (at this stage, the anticrossing signal shape is identical to that observed under conditions of steady-state excitation), which is split into a doublet as t further increases. A theoretical interpretation of the observed time dependence of the Zeeman-sublevel anticrossing signal in localized exciton emission has been proposed. The fine structure parameters and lifetimes of the triplet localized excitons have been determined in different spin states by comparing theory and experiment.

Direct observation of the optical anisotropy of C70 fullerene molecules in the Shpol’skii spectra

Linear optical absorption and emission spectra of C70 fullerene molecules in single-crystal toluene are investigated. It is established that the lines of purely electronic S0-S1 transitions are significantly polarized. The degree of linear polarization of the spectral lines depends on the position of the fullerene molecule in the toluene matrix and can be as high as 100%. The polarization characteristics of the lines can be understood in the context of a model in which the S0 → S1 electronic transition is represented by the excitation of a planar oscillator whose axis is oriented along the principal axis of the C70 molecule. The relationship between the polarization of the spectral lines and the position of the fullerene molecule in the matrix is consistent with the conclusions drawn from a theoretical analysis of different configurations possible upon the embedding of C70 molecules into crystalline toluene.

Time dependence of the Hanle effect in the emission of triplet bound excitons in gallium selenide

The dynamics of the magnetic-field-induced depolarization of the circularly polarized radiation of spin-oriented triplet bound excitons in uniaxial crystals has been studied by time-resolved spectroscopy on an example of GaSe. The depolarization of radiation (the Hanle effect) is due to the difference between the behaviors of its σ+ and σ− components in a magnetic field. It has been found that the magnetic-field dependences of the σ+ and σ− component intensities vary considerably within the lifetime t of the excited states. The form of the magnetic-field dependence of the degree of polarization of the exciton radiation remains generally the same for various t values. A theoretical description of these effects has been proposed.

Time dependence of a level anticrossing signal in exciton emission from a GaSe crystal under resonant excitation conditions

The time dependence of the Zeeman-sublevel anticrossing signal in triplet-bound exciton emission from gallium selenide has been investigated under resonant excitation conditions. It has been found that the shape of the anticrossing signal varies during the lifetime of the excited state. The time dependence of the anticrossing signal in the case of the resonant excitation of exciton states differs significantly from the corresponding dependence observed in the case of the band-to-band excitation of luminescence. A theoretical description is proposed for the observed effects.

Thermally Induced Depolarization of the Photoluminescence of Carbon Nanodots in a Colloidal Matrix

The effect of temperature on fluorescence polarization in a colloidal system of carbon nanodots in glycerol under linearly polarized excitation is investigated for the first time. It is found that the experimentally obtained temperature dependence of the degree of linear polarization of fluorescence can be described by the Levshin–Perrin equation, taking into account the rotational diffusion of luminescent particles (fluorophores) in the liquid matrix. The fluorophore size determined in the context of the Levshin–Perrin model is significantly smaller than the size of carbon nanodots. This discrepancy gives evidence that small atomic groups responsible for nanodot luminescence are characterized by high segmental mobility with a large amplitude of motion with respect to the nanodot core.

Spectroscopy of resonant excitation of exciton luminescence of GaSe–GaTe solid solutions

The luminescence excitation spectra of localized excitons in GaSe0.85Te0.15 solid solutions have been investigated at the temperature T = 2 K. It has been shown that the excitation spectra of excitons with the localization energy ε > 10 mV exhibit an additional maximum ME located on the low-energy side of the maximum corresponding to the free exciton absorption band with n = 1. It has been found that the shift in the position of the maximum ME in the excitation spectrum with respect to the energy of detected photons increases as the energy of detected photons decreases, i.e., with an increase in the localization energy of excitons. Under the resonant excitation of localized excitons by a monochromatic light from the region of the exciton emission band, in the exciton luminescence spectrum on the low-energy side from the excitation line, there is also a maximum of the luminescence (ML). The energy distance between the position of the excitation line and the position of the maximum in the luminescence spectrum increases with a decrease in the frequency of the excitation light. The possible mechanisms of the formation of the described structure of the luminescence excitation and exciton luminescence spectra of GaSe0.85Te0.15 have been considered. It has been concluded that the maximum ME in the excitation spectrum and the maximum ML in the luminescence spectrum are attributed to electronic–vibrational transitions with the creation and annihilation of localized excitons, respectively.