We report on nanometer-scale cathodoluminescence (nanoCL) experiments in a scanning transmission electron microscope on individual core−shell CdSe/ CdS quantum dots (QDs). By performing combined photoluminescence (PL) and nanoCL experiments of the same individual QDs, we first show that both spectroscopies can be used equally well to probe the spectral properties of QDs. We then demonstrate that the spatial resolution of the nanoCL is only limited by the size of the QDs themselves by performing nanoCL experiments on QDs lying side by side. Finally, we show how nanoCL can be advantageous with respect to PL as it can rapidly and efficiently characterize the optical properties of a large set of individual QDs. These results contrast with pioneering CL works on II−VI QDs and pave the way to the characterization of any II−VI quantum-confined structure at the relevant scale.
Accurate energy‐size dependence of excitonic transitions in semiconductor nanocrystals in the strong confinement regime using classical theoretical approaches such as effective mass (EM) approximation, tight binding (TB), or empirical pseudo‐potential is difficult. We propose a simple empirical expression with three fitting parameters that accurately relates the size dependence of most known excitonic transitions in CdSe and in InAs nanocrystals. We show that this empirical expression can be deduced from a phase jump approach if the charge carriers are considered to travel on the atomic lattice of the nanocrystal and gain energy upon bouncing at the nanoparticle boundaries. This empirical expression is also tested on the atomically flat CdSe nanoplatelets (NPLs) without any adjustment of the parameters obtained with the CdSe spherical nanocrystals, and provides an estimation of the CdSe NPLs thickness that matches exactly the experimental observations. These results suggest that a phase shift approach could be useful to describe the electronic transitions in semiconductor nanocrystals.
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