The influence of a magnetic field on the electron and hole energy spectra of spherical uniform and multilayer semiconductor nanocrystals is investigated. The calculations are performed within the k•p method and envelope function approximation. The valence subband mixing is taken into account by considering a two-band Hamiltonian for the hole states. It is shown that the magnetic-field dependence of the energy spectrum varies strongly with the size and composition of the nanocrystals. Several interesting phenomena, like spatial polar separation of the one-electron charge density in quantum dot-quantum well structures or crossover from confinement in the external shell to the internal core in quantum dot-quantum barrier systems under the influence of a magnetic field are reported.
Optical spectra of CdS nanocrystals are interpreted by using both the atomistic tight-binding method and multiband effective mass theory. Both methods correctly describe the energy splitting between the two lowest optically active transitions and their relative strengths, providing the same labeling of the two main absorption peaks of the spectrum. Our calculations unambiguously show that these peaks correspond to the 1S 3/2 f 1s and 1P 3/2 f 1p transitions. Both zinc blende and wurtzite-type structures for CdS nanocrystals are considered. Similar optical spectra are predicted for the two lattice structures. We also study how the spectrum, and in particular, the 1S 3/2 -1S 1/2 splitting, is changed by modifying parameters, within the experimental uncertainties, including size and shape fluctuations, surface passivation and spin-orbit coupling. Our results are robust to small variations in all of these parameters.
IntroductionProgress in experimental techniques allows both the synthesis and characterization of high quality, monodisperse nanocrystals with a well-established shape, crystal structure, and a consistent surface derivatization. 1-3 Transmission electron microscopy (TEM) and X-ray powder diffraction (XRD) are used in combination with computer simulations to characterize nanocrystallite structural features. TEM allows imaging of individual nanocrystals and the development of a statistical description of the size and shape of the particles in a sample, whereas XRD determines the crystal structure. Different spectroscopies, such as fluorescence line narrowing and hole burning, are able to eliminate the effects of inhomogeneous broadening due to their size-selectivity so that the near-band-edge electronic fine structure can be observed. 2 Recently, attenuated low-energy photoelectron spectroscopy, A-LEP, has been proposed as a new technique for studying the hole states of the quantum particles (QPs), including the splitting between the light/heavy hole and split-off bands. 4 Both the effective mass method (EMM) and tight-binding (TB) theory have been successfully applied to model the optical properties of quantum dots and nanocrystals. The multiband effective mass theory has correctly predicted a large resonant Stokes shift in small CdS nanocrystals 5 consistent with recent photoluminescence excitation measurements, 6 whereas the atomistic tight-binding theory has provided a detailed description of the excitonic fine structure for multilayer nanocrystals with changes in composition on the monolayer scale. 7-9 In the abovementioned studies, the optical properties and the various observed phenomena are described well in terms of a single isolated dot.In this paper, we present both tight-binding and multiband effective-mass theory to describe the electronic states and optical response of CdS nanocrystals. We determine the effects of
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.