Stokes shift

Abstract: We present a theoretical analysis of the band-edge exciton structure in nanometer-size crystallites of direct semiconductors with a cubic lattice structure or a hexagonal lattice structure which can be described within the framework of a quasicubic model. The lowest energy exciton, eightfold degenerate in spherically symmetric dots, is split into five levels by the crystal shape asymmetry, the intrinsic crystal field (in hexagonal lattice structures), and the electron-hole exchange interaction. Transition oscillator strengths and the size dependence of the splittings have been calculated. Two of the five states, including the ground state, are optically passive (dark excitons). The oscillator strengths of the other three levels (bright excitons) depend strongly on crystal size, shape, and energy band parameters. The relative ordering of the energy levels is also heavily influenced by these parameters. The distance between the first optically active state and the optically forbidden ground exciton state increases with decreasing size, leading to an increase of the Stokes shift in the luminescence. Our results are in good agreement with the size dependence of Stokes shifts obtained in fluorescence line narrowing and photoluminescence experiments in CdSe nanocrystals. Mixing of the dark and bright excitons in an external magnetic field allows the direct optical recombination of the dark exciton ground state. The observed shortening of the luminescence decay time in CdSe nanoncrystals in a magnetic field is also in excellent agreement with the theory, giving further support to the validity of our model. \textcopyright{} 1996 The American Physical Society.

Abstract: fd-excitation, absorption, reflection, and df-emission spectra presented in the literature on Eu2+ in inorganic compounds have been gathered and re-analyzed. Emission wavelength, width of the emission band, absorption wavelength, Stokes shift, and redshift pertaining to Eu2+ in more than 300 different compounds (fluorides, chlorides, bromide, iodides, oxides, sulfides, selenides, and nitrides) are presented. From the data, it is possible to predict for each of the 13 lanthanide ions (La2+, Ce2+, Pr2+, until Yb2+), doped in any of the compounds compiled, the energy of the transition from the 4fn ground state to the first 4fn−15d level and also the energy of df-emission. A brief overview on the relationships between redshift, Stokes shift, and the width of the emission with the type of compound is given.

Abstract: The literature on 4f n ↔4f n −1 5d transitions of the trivalent lanthanides in inorganic compounds has been collected. From critically analyzing fd-excitation, absorption, reflection and df-emission spectra, values for the spectroscopic red shift of 5d levels and the stokes shift were determined. Data pertaining to 368 different lanthanide sites in over 300 different compounds (fluorides, chlorides, bromide, iodides, oxides, sulfides, selenides and oxy-nitrides) has been compiled. From the data, it is possible to predict for each of the 12 lanthanide ions ( Ce 3+ , Pr 3+ until Yb 3+ ) doped in any of the compounds compiled, the position of the lowest 5d levels with typically ±600 cm −1 accuracy.

Abstract: We demonstrate that efficient shape control may be achieved in the shell of colloidally grown semiconductor nanocrystals (independent of the core), allowing the combination of a 0-D spherical CdSe core with a 1-D rodlike CdS shell. Besides exhibiting linearly polarized emission with a room-temperature quantum efficiency above 70%, these mixed-dimensionality colloidal heterostructures display large, length-dependent Stokes shifts as well as giant extinction coefficients approaching 107cm-1 M-1.

Abstract: PbSe nanocrystal colloids exhibit a well-defined excitonic structure with the lowest energy exciton tuning from 0.5 to 1 eV, as a function of size. Band-edge fluorescence is observed from 1.2 to 2 μm with a small Stokes shift, sub-μs lifetime, and near-unity quantum yield. Upon pumping at 1.064 μm, the first exciton decay is consistent with radiative relaxation at low pump intensity and with Auger recombination at higher pump intensities. Optically induced absorption is observed at approximately midgap. These transitions exhibit strengths similar to the interband exciton and are size-tunable. They are assigned to the 1Se,h−1Pe,h and 1Pe,h−1De,h intraband excitations. Intraband pump−probe measurements of the 1Se,h−1Pe,h transition reveal a short lifetime and the absence of the phonon bottleneck.