Tristan Richard

Quantum confinement effects of CdS nanocrystals in a sodium borosilicate glass prepared by the sol‐gel process

Experimental evidences of both weak and strong confinement regimes are reported on CdSnanocrystals embedded in a sodiumborosilicate glass matrix. A method, based on the sol‐gel technique, is used for the preparation of CdS‐activated glass. This route is capable of providing nanocrystals covering a wide range of radii with small size dispersion. Low‐temperature linear‐absorption spectra have been analyzed in terms of excitons and electron‐hole confinements by fitting the results of a numerical calculation to experimental findings. The model used, in the envelope‐function formalism, involves both a Lorentzian broadening of the exciton energy states inside each nanocrystal and a Gaussian size distribution. The improvement of crystal quality and the sharpening of the size distribution by thermal annealing is also studied versus both time and temperature of treatment. It is shown that we can keep a tight control on the crystallinity, average size, and size distribution of the nanocrystals by rather simple adjustments and short treatments.

Influence of spin-orbit split-off band on optical properties of spherical semiconductor nanocrystals the case of CdTe

The multiband envelope-function formalism, including the effects of finite spin-orbit splitting energy Δ, is used to calculate valence-band confined states in spherical semiconductor nanocrystallites. A compact analytical formulation of the necessary equations is proposed. In the particular case of CdTe, despite the large value of Δ, the split-off band has a sizeable influence on the fundamental interband transition, which is found dipole-forbidden, by symmetry. This result cannot be accounted for by usual models in which the influence of the split-off band is a priori neglected, when Δ is large. The consequences of basic assumptions on this prediction, which should have important consequences on intrinsic luminescence, are discussed.

Sol-gel preparation and optical characterization of sodium borosilicate glasses doped with II-VI semiconductor nanocrystals

An original sol-gel technique is used to prepare sodium borosilicate glasses doped by II-VI semiconductor nanocrystallites. A complete study of CdS and CdSe crystallites is presented, as well as preliminary results on CdTe crystallites. The synthesis procedure involves the preparation of complex cadmium-based molecules in an aqueous solution to which other inorganic precursors are added in order to start the gelation of the vitreous matrix. In the cases of CdSe and CdTe, selenious and telluric acids, respectively, added to the starting solution induce the formation of crystallites by a redox reaction during thermal degradation of the gel. This gel is then melted and quenched to provide the glass. The effects of further thermal annealing, at temperatures around and above the Tg of the glass, are observed by optical absorption measurements. The latter are analyzed by using a theoretical calculation of excitonic energies and oscillator strengths, based on the envelope-function approximation. This model includes the confinement-induced complex mixing of valence-band states. Our results show that disorganized clusters with a rather narrow size-distribution controlled by initial conditions are present before the annealing. The latter quickly improves the crystalline quality. Meanwhile, the average size is slightly increased and the width of the size-distribution reduced down to an optimum, after a few hours (less than 10). For instance, CdS crystals of approximately 4 nm in diameter, with a half width at half maximum of the size distribution of approximately 0.23 nm are obtained after 6 hours of annealing.