Despite its prediction, observation of slowed hot-carrier cooling in strongly confined quantum dots (QDs) have been limited to date due to reasons that are not fully understood. We used nanosecond, broadband, transient absorption spectroscopy to investigate the size dependence of the high energy intraband transitions in PbS colloidal QDs. Direct measurements of the higher lying states of QDs permitted detailed analysis of the electronic overlap between the quantum confined states and how this overlap may affect relaxation processes. In smaller PbS nanocrystals, where the hot-phonon bottleneck is expected to be most pronounced, we found that relaxation of parity selection rules combined with stronger coupling to phonons led to greater spectral overlap of the intraband states. This overlap creates pathways for fast energy transfer and thermalization processes that bypass the predicted hot- phonon bottleneck. In contrast, large PbS QDs do not exhibit such relaxation of parity selection rules and possess narrow intraband transitions expected of quantum confined systems. These observations are consistent with slower relaxation dynamics measured for larger quantum confined nanocrystals. These findings indicate that, despite the predicted benefit of increasing intraband electronic state separation on the potential for slowed carrier cooling, small diameter QDs exhibit increased carrier-phonon interactions which increase relaxation rates.<br/>

High energy state relaxation pathways in PbS QDs through direct observation of the forbidden S-D intraband transition

256th National Meeting (2018)

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