(Invited) Ultrafast Exciton Dynamics in Semiconductor Nanocrystals: Effects on Single Vs Dual Emission and on Optical Gain
Authors: Camellini Andrea , Zhao Haiguang , Brovelli Sergio , Vomiero Alberto , Zavelani-Rossi Margherita 
Published: May 2020

A wide variety of materials with nanometre dimensions are increasingly explored as light emitters. Among them, semiconductor nanocrystals (NCs) are very promising for a variety of applications, including light emission devices (LEDs), lasers, detectors, photovoltaic cells, biological labelling and sensing [1]. Key advantage of NCs is the possibility to tailor their optical response by controlling the electronic structure (“wave function engineering”) through the choice of composition, size and shape. Significant results have been mostly obtained with CdSe/CdS and PbS/CdS heterostructured NCs. Beyond single wavelength tuneable emission, other regimes have been demonstrated such as simultaneous emission on two different wavelengths, amplified spontaneous and laser emission.

The luminescent properties arise from exciton decay, which can proceed through radiative or nonradiative pathways, following different routes. The study of exciton dynamics can thus allow elucidating the processes connected to single or dual emission and to optical gain. This, in turn, can lead to the identification of the functional and structural characteristics that are responsible for these behaviors. Exciton relaxation occurs on picosecond timescales, so ultrafast optical techniques are required to perform these studies.

In this talk, we present studies carried out by ultrafast pump-probe spectroscopy techniques, with 100-fs time resolution, on CdSe/CdS and PbS/CdS NCs, with different structures (core/shell, dot-in-rod, dot-in-bulk, with sharp or graded interface) [2-6]. These NCs are optically active in the visible and near-infrared spectral region, show single and dual colour photoluminescence emission, optical gain and laser emission [2-7]. The analysis of the experimental data allowed us to unravel the decay processes. Exciton relaxation between core and shell states takes place typically in a few ps, but the ultimate band-edge lifetime can extend to hundreds of ps to few ns, allowing for efficient luminescence and optical gain.

Moreover, our data allowed us to clarify the role of the volume and of the shape of the outer component and the effect of the interface in the heterostructure [2-4]. We found that dual emission is possible for both thick and thin quantum-confined shells, and for different interfaces. We studied the decoupling of excitons lying in the two different component of the NC (core exciton and shell exciton) and we revealed the evolution of the exciton barrier known as dynamic hole-blockade effect. We showed that these phenomena are strictly connected to dual emission and optical gain and we identified the condition for their maximum efficiency, in term of band alignment and band transitions. Our results provide a comprehensive understanding of the physical phenomena governing dual-emission mechanisms, suppression of Auger recombination, optical gain and laser emission in heterostructured NCs.

In conclusion, the study of the exciton dynamics in different, but similar, heterostructured NCs allowed us to elucidate the relation between structural-morphological characteristics (shape, volume, and interface) and unconventional emission capabilities (dual emission and optical gain). This knowledge is very important to control NC functionalities in applications such as lasers [7], photoelectrochemical (PEC) cells [8], white light emission [9], ratiometric sensing [10].

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