Excitons in 2D colloidal semiconductor nanostructures

Elsa CASSETTE

(Department of Chemistry, Princeton University, USA)

Colloidal semiconductor nanocrystals present unique optical properties which depend on their size, shape and composition. It is therefore crucial to be able to precisely control these parameters.

In my research, I focus mainly on the optical properties of two-dimensional nanostructures. First I will present my PhD work. I will show it is possible to grow a flat CdS shell around spherical CdSe cores to obtain “dot-in-plate” nanocrystals. The lattice mismatch between the two materials, together with the anisotropic shape, induces a large splitting in the fine structure of the first exciton peak. Consequently, the emission narrows and becomes 2D-polarized, in the plane of the plate. This latter property is amplified by the dielectric effect due to the anisotropic shape of the nanocrystal and is verified by polarization measurements in ensemble and at the single nanocrystal level. Moreover, the dot-in-plate nanocrystals deposit spontaneously flat on substrates providing a fine control of the emission dipole orientation. This later property is particularly interesting in application involving dipole-dipole or dipole-plasmon couplings or interaction with electric or magnetic field.

Then I will present my postdoctoral work on the electronic coherence in semiconductor nanoplatelets. Electronic coherence has attracted considerable attention for its possible role in dynamical processes in molecular systems. However, its detection is challenged by inhomogeneous line broadening and interference with vibrational coherences. In particular, reports of ‘persistent’ coherent exciton superpositions at room temperature remain controversial, as the related transitions give typically shorter optical dephasing times of about 10–20 fs. Colloidal semiconductor nanoplatelets are an ideal system where details are not obscured by vibrational coherences nor ensemble dephasing. Electronic coherence was observed up to about 100 fs in a ‘warm and wet’ (room-temperature solution) environment. Comparing the exciton and optical coherence times evidences a partial correlation of fluctuations underlying dephasing and allows us to elucidate decoherence mechanisms occurring in these samples. Finally, I will present the first results on charge transfer in in-plane heteronanostructures with type-II band alignement.

Structural Study of Transition Metal Oxide Thin Films

Prof. Yusuke Wakabayashi
Osaka University, Japan

Transition metal oxides have been studied extensively by many physicists because of their variety of electronic/magnetic properties realized in rather simple structures, such as perovskites, spinels, etc. One of the major developments in the study of transition metal oxides in this century is the establishment of the film growth technique. It makes us able to apply strong uniaxial pressure. More interestingly, it makes interfaces between two different oxides. Such interfaces sometimes show very different properties from either side of it. Taking manganese oxide thin films and LaAlO3/SrTiO3 interface for examples, structural studies on metal oxide thin films by means of synchrotron x-ray diffraction will be presented.

Apport de la Toxicologie à l’élaboration de nanomatériaux safe-by-design

Prof. Françoise PONS

Faculté de Pharmacie, Université de Strasbourg