Optical Properties of CdS Quantum Dots: The Key Role of the Spin-Orbit and Coulomb Interactions
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*GPS Universitds Paris VI-VII and Universit6 d'Evry val d'Essonne, France ** LPMC, Ecole Polytechnique, Palaiseau, France *** IEMN, Dept ISEN, Villeneuve d'Ascq, France ABSTRACT
We study theoretically and experimentally the effects of the Coulomb and the spin-orbit coupling on the electronic structure of small quantum dots. A tight-binding calculation with restricted configuration interaction is developed in a typical case: very small cubic quantum dots for which the electron-hole exchange interaction is of the order of magnitude of the spin-
orbit interaction. Experimentally, resonant photoluminescence and photoluminescence excitation are used to obtain information on a single size of CdS quantum dot obtained by a chemical growth method. INTRODUCTION The discrete electronic structure of quantum dots (QDs) is determined by the relative importance of different terms: the confinement-induced level splitting, the electron-hole Coulomb and exchange interactions and the spin-orbit coupling. These terms increase substantially, but in a different way, when the size of the nanocrystal is decreased into the nm range. Recently, the enhancement of exchange interaction splitting was observed in small QDs: Si [1], CdSe [2,3], InP [4] and InAs [5]. The aim of our work is to perform a detailed comparison between experiment and theory in the interesting situation where there is a crossover in the values of the parameters. The case of CdS QDs is well suited for such a study since the spin-orbit coupling and the dielectric constant are smaller than in other materials, for instance in CdSe. In order to study all these effects as a function of quantum confinement, i.e. as a function of the QD size, we have used experimental selective spectroscopic techniques like resonant photoluminescence (PL) and photoluminescence excitation (PLE). Both are based on chosing a narrow energy window, either in excitation or in detection of the PL, which selects in energy, QDs with a given size in the whole distribution and thus isolates their contribution. The paper is organized as follows: in the first part we give some details of the synthesis of the samples. This is very important since we need to control the parameters which mostly influence the optical properties, like the size distribution or the surface state of the dots. The second part is devoted to theoretical calculations in the framework of a tight-binding approach. In the third part, we present absorption and luminescence spectra of the studied nanocrystals which are compared to the calculated optical spectra. The last part presents the size-selective spectroscopy results and the excitonic fine structure as a function of QD size, is discussed. Finally, we conclude. THEORETICAL AND EXPERIMENTAL RESULTS 1.Samples and characterization The chemical synthesis of II-VI QDs has been the subject of intensive research in order to control the different parameters which determine their optical properties [6]. The size, size distribution and the surface states of the particles are among the most imp
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