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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 > Abstract : Observation of the full exciton and phonon fine-structure in CdSe/CdS dot-in-rod hetero-nanocrystals

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Abstract : Observation of the full exciton and phonon fine-structure in CdSe/CdS dot-in-rod hetero-nanocrystals

A. Granados del Águila,1 B. Jha,1 F. Pietra,2 C.de Mello Donegá,2 J.C.Maan1, D. Vanmaekelbergh2 and P.C.M. Christianen*1

1High Field Magnet Laboratory, Radboud University Nijmegen, the Netherlands

2Condensed Matter and Interfaces, Debye Institute, Utrecht University, the Netherlands

 

E-mail : P.Christianen@science.ru.nl 

 

Semiconductor colloidal nanocrystals (NCs) exhibit very efficient light emission that can be tuned in wavelength by varying their size, composition and shape. The use of core-shell hetero-nanostructures (HNCs) that combine two semiconductor materials with different bandgap energies values, gives rise to an extra degree of freedom to tailor the NC properties by separate engineering of the spatial extensions of the electron- and hole wavefunctions [1,2]

 

Figure(a) Schematic representation of CdSe/CdS dot-in-rods and the corresponding band alignment diagram. (b) Fluorescence line-narrowing spectrum of CdSe/CdS dot-in-rods showing ultra narrow emission lines that reveal the full exciton and phonon fine-structure.

We have investigated the optical properties of dot-in-rod HNCs ; consisting of a spherical CdSe core embedded in a rod-like CdS shell (Fig. (a)). Their overall rod-like shape permits to self-assemble them in well-defined arrays with their long axis perpendicular to the deposition substrate [3].

We have used fluorescence line-narrowing (FLN) spectroscopy with resonant laser excitation to select a subsection of HNCs out of the inhomogeneously broadened ensemble. The FLN spectrum shows an unprecedented number of peaks (Fig. (b)), some of whichwith very narrow linewidths (< 1 meV) approaching those of a single-dot experiment. The origins of all peaks have been established by measuring their spectral position as a function of NC size and magnetic field strength up to 30 T, allowing us to characterize the full exciton- and phonon fine-structure of the dot-in-rods. We identify a clear signature of an acoustic-phonon assisted transition (P4 in Fig (b)), separated from the zero-phonon emission (P3) and optical-phonon replicas, evidencing the importance of vibrational modes for exciton recombination in NCs. The overall symmetry of the exciton wavefunction has been found to be spherical, and not rod-like, although the electron-hole exchange energy is reduced, relative to a CdSe NC, due to the extension of the electron wavefunction into the CdS shell, leading to a smaller electron-hole overlap.

Our results demonstrate that self-assembled arrays of ultrahigh quality CdSe/CdS dot-in-rods provide an ideal model system to investigate the fundamental optical properties of semiconductor nanocrystals.

 

References

 

[1] Donega, C d M 2011Chem. Soc. Rev. 40, 1512

[2] Brovelli S, Schaller R D, Crooker S A, García-Santamaría F, Chen Y, Viswanatha R, Hollingsworth J A, Htoon H and Klimov V I 2011 Nat. Commun. 2, 280

[3] Pietra F, Rabouw F T, Evers W H, Byelov D V, Petukhov A V, Donega, C. d. M. and Vanmaekelbergh D 2012 Nano Lett. 12, 5515