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Accueil du site > Vie du laboratoire > Congrès / Colloques / Conférences > Optical Properties of Individual Nanowires and Quantum Dots in High Magnetic Field, Septembre 2014 > Programme du workshop > Colloidal semiconductor nanocrystals : new materials for tailoring nanoscale excitons

English

Colloidal semiconductor nanocrystals : new materials for tailoring nanoscale excitons

C. de Mello Donegá

Debye Institute for Nanomaterials Science, Utrecht University

 

Colloidal semiconductor nanocrystals (NCs) are a versatile class of nanomaterials, whose properties are determined by their size, shape, and composition. Moreover, colloidal NCs are coated with a layer of organic ligand molecules, which further extends their functionality, since it allows for easy surface manipulation and solution processing. Colloidal semiconductor NCs comprising two (or more) different materials joined together by heterointerfaces, i.e., heteronanocrystals (HNCs), offer even more exciting possibilities regarding property control [1]. The spatial localization of charge carriers in HNCs can be manipulated by controlling the offsets between the energy levels of the materials that are combined at the heterointerface. In Type-I HNCs both carriers are confined in the same material. In contrast, in Type-II HNCs a spatially indirect exciton is formed. In Type-I1/2 (or quasi-Type-II) HNCs one carrier is delocalized over the HNC, while the other is localized in one of the segments. This allows the electron-hole spatial overlap to be tailored, and gives semiconductor HNCs unique properties, which are determined by the size, shape, and composition of each segment.

 

Understanding the properties of excitons in colloidal HNCs is of great scientific interest, from both fundamental and applied viewpoints. In our group, we have applied a multistage preparation strategy that allows the combination of a variety of synthesis techniques (hot-injection, heteroepitaxial seeded growth, cation exchange, oriented attachment) in a sequential manner in order to achieve the targeted synthesis of complex colloidal HNCs. This has allowed us to systematically investigate the optical properties of a number of colloidal HNC compositions : PbSe/CdSe, CdSe/(Cd,Zn)S/ZnS, CdTe/CdSe, ZnSe/CdSe concentric core/shell quantum dots, CdTe/CdSe core/shell multipod HNCs, CdSe/CdS dot core/rod shell nanorods and ultranarrow (2 nm diameter) (Zn,Cd)Te/CdSe heteronanowires. In recent years, we have also developed methods to synthesize Cd- and Pb-free HNCs, such as CuInS2/ZnS core/shell QDs.

 

In this talk, I will focus on CdTe/CdSe HNCs, which provides an ideal model system to probe the evolution of nanoscale spatially indirect excitons as a function of the size, shape, and composition of the HNC. To this end, the optical properties of a series of efficient CdTe/CdSe core/shell HNCS [2] were investigated using a number of spectroscopic techniques (viz., absorption, photoluminescence, excitation, and time-resolved photoluminescence spectroscopies) [3,4]. Recently, we investigated the dependence of the exciton lifetimes in CdTe/CdSe HNCs on the temperature (1.7–298 K) and magnetic field strength (0-30 T), using samples representative of the Type-I1/2 and Type-II regimes. The results of our studies provide novel fundamental insights into nanoscale spatially indirect excitons and show that compositional control of HNCs can be used to tailor the exciton fine-structure and radiative lifetimes, exciton-phonon coupling strength, and spectral characteristics of colloidal HNCs.

 

References

 

[1] C. De Mello Donega, Chem. Soc. Rev. 40 (2011) 1512-1546.

[2] P. T. K. Chin, C. de Mello Donega, S. S. Bavel, S. C. J. Meskers, N. A. J. M. Sommerdijk, R. A. J. Janssen, J. Am. Chem. Soc. 129 (2007) 14880-14886.

[3] C. De Mello Donega, Phys. Rev. B 81 (2010) 165303.

[4] E. Groeneveld, C. de Mello Donega, J. Phys. Chem. C116 (2012) 16240-16250.