Proceedings of nanoGe Fall Meeting 2018 (NFM18)
DOI: https://doi.org/10.29363/nanoge.nfm.2018.114
Publication date: 6th July 2018
Colloidally prepared, quantum-confined, semiconductor nanocrystals offer tunable energy gaps, strong photoluminescence, and, in some cases, optical gain and lasing [1]. We report ultrafast optical pump, X-ray diffraction probe experiments performed at Argonne National Lab’s Advanced Photon Source with CdSe nanocrystal (NC) colloidal dispersions as functions of particle size, polytype, and pump intensity. Shifts of diffraction peaks relate lattice heating and peak amplitude reduction conveyed transient lattice disordering (or melting). For smaller NCs, melting was observed upon absorption of as few as ∼15 electron–hole pair excitations per NC on average (0.89 excitations/nm3 for a 1.5 nm radius) with a similar electron-hole pair density inducing disordering for all examined NCs. Diffraction intensity recovery kinetics, attributable to recrystallization, occur over hundreds of picoseconds with slower recoveries for larger particles. Zincblende and wurtzite NCs revert to initial structures following intense photoexcitation suggesting melting occurs primarily at the surface, as supported by simulations. These findings suggest a need to take into account nanomaterial physical stability and transient electronic structure for high intensity excitation applications such as lasing and solid-state lighting.
[1] Klimov et al. Science, 290, 314 (2000).; Kazes et al. Adv. Mater. 14, 317 (2002).
[2] Kirschner et al. Nano Lett. 17, 5314 (2017).
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