Exciton Nonlinearities and Optical Gain in Colloidal CdSe/CdS Dot/rod Nanocrystals
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1207-N09-03
Exciton Nonlinearities and Optical Gain in Colloidal CdSe/CdS Dot/rod Nanocrystals By Michele Saba*1, Marco Marceddu1, Francesco Quochi1, Stefan Minniberger2, Juergen Roither2, Agnieszka Gocalinska1, Maksym V. Kovalenko3, Dmitri V. Talapin3, Wolfgang Heiss2, Andrea Mura1 and Giovanni Bongiovanni1. [*] corresponding author, email: [email protected] 1 Dipartimento di Fisica and SLACS-CNR/INFM, Università di Cagliari, I-09042 Monserrato, Italy 2 Institute of Semiconductor and Solid State Physics, Johannes Kepler University, Austria 3 Department of Chemistry, The University of Chicago, Chicago, IL 60637 ABSTRACT We characterized the optical nonlinearities of CdSe nanocrystals surrounded by rod-like CdS shells with ultrafast measurements of time-resolved photoluminescence. We measured the exciton-exciton interaction to be, depending on structure details, attractive or repulsive, by as much as 31 meV, due to the unique band alignment in the CdSe/CdS. This feature makes CdSe/CdS dot/rods promising gain media for solution-processable lasers, as it appears combined with 80% photoluminescence quantum yield, narrow size and shape distributions and the antenna effect of the CdS rod shell enhancing optical absorption by more than a factor 50 with respect to bare dots. INTRODUCTION Semiconductor colloidal nanocrystals have been proposed as optically-active media for solution-processable optoelectronic devices, because they combine inexpensive, wet-chemistry synthesis with high photoluminescence quantum yield, large oscillator strength and size tuneability of optical transitions.i,ii,iii,iv Key to the success of nanocrystal-based devices is the possibility to design and consistently synthesize nanocrystals with desired properties. Size uniformity can be usually controlled within less than 5% uncertainty; surface capping, passivation and core/shell structures can lead to photoluminescence quantum yields exceeding 50%, optical gain and lasing.v A new frontier in nanocrystal design has appeared with heterostructures allowing spatial separation of electron and hole wavefunctions, like in type-II CdSe/CdTe core/shell nanocrystals, through staggered conduction and valence band offsets.vi,vii Charge separation inside nanocrystals is useful in photodetector and photovoltaic devices, quantum optics and lowthreshold lasers. Exciton nonlinearities also depend on the degree of separation of electron and hole wavefunction. In type II heterostructures, it has been demonstrated that charge separation can lead to a large repulsive exciton-exciton interaction. The resulting blueshift of the exciton-tobiexciton transition suppresses to a large extent resonant re-absorption of stimulated emission from single-exciton states, allowing net optical gain and lasing at excitations corresponding to less than one electron-hole pair per nanocrystal. In this regime, losses inherent to multiexciton recombinations are avoided, resulting in optical gain with a much longer lifetime, an essential step towards the demonstration of lasing under
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