Tailoring Band Edge Photoluminescence in CdSe Nanoplatelets

The photoluminescence of quasi-2D CdSe nanoplatelets (NPLs) shows unique signatures like up to three distinct emission lines below 100 K [1]. While one emission line is usually attributed to the neutral exciton, the other two are still under debate. There seems to be consensus that negative trions contribute to the emission [2], but their prominent role is rather puzzling and there are hints that emission lines originating from higher excitonic p-states are also observable under specific conditions [3]. The understanding of the necessary conditions for specific luminescence mechanisms is crucial to intentionally manipulate band edge emission in CdSe NPLs.

In this work [4], we demonstrate by time integrated and time resolved photoluminescence spectroscopy that Förster resonant energy transfer (FRET) in stacks of NPLs combined with hole trap states in specific NPLs within the stack triggers trion formation, while single NPL spectra are dominated by neutral excitonic emission originating from both, s- and p-states. This interpretation is verified by implementing copper (Cu⁺) dopants into the lattice as intentional hole traps. Trion emission gets strongly enhanced, and due to the large amount of hole trapping Cu⁺ states in each single NPL, trion formation does not necessarily require stacking of NPLs. Consequently, the ratio between trion and neutral exciton emission can be controlled by either changing the amount of stacked NPLs during sample preparation or by implementing copper dopants into the lattice which act as additional hole traps.

With these findings we can thus consistently explain the controversial findings in literature for the low temperature NPL emission spectra. Moreover, the ability to control trion emission opens attractive opportunities for single-photon emitters, which play an important role in quantum information technology. This utility is due to a lack of a dark state in trions that broadens or dephases the neutral exciton emission [5]. Thus, our approach to intentionally increase trion emission in CdSe nanoplatelets might pave an attractive path for single-photon emitters.

References:
[1] F. V. Antolinez et al., Nano Lett., 20, 5814 (2020)
[2] E. V. Shornikova et al., Nano Lett., 20, 1370 (2020)
[3] A. W. Achtstein et al., Phys. Rev. Lett. 116, 116802 (2016)
[4] M. Riesner et al., ACS Nano, 18, 24523 (2024)
[5] M. Califano et al., Phys. Rev. B 75, 115401 (2007)