Demystifying\nTrion Emission in CdSe Nanoplatelets

At\ncryogenic temperatures, the photoluminescence spectrum of CdSe\nnanoplatelets (NPLs) usually consists of multiple emission lines,\nthe origin of which is still under debate. While there seems to be\nconsensus that both neutral excitons and trions contribute to the\nNPL emission, the prominent role of trions is rather puzzling. In\nthis work, we demonstrate that Förster resonant energy transfer\nin stacks of NPLs combined with hole trap states in specific NPLs\nwithin the stack trigger trion formation, while single NPL spectra\nare dominated by neutral excitonic emission. This interpretation is\nverified by implementing copper (Cu<sup>+</sup>) dopants into the\nlattice as intentional hole traps. Trion emission gets strongly enhanced,\nand due to the large amount of hole trapping Cu<sup>+</sup> states\nin each single NPL, trion formation does not necessarily require stacking\nof NPLs. Thus, the ratio between trion and neutral exciton emission\ncan be controlled by either changing the amount of stacked NPLs during\nsample preparation or implementing copper dopants into the lattice\nwhich act as additional hole traps.