Transient spectral and dynamic properties of magic-size Cd 3 P 2 nanoclusters in the limit of strong confinement

Both semiconductor nanocrystals and organic molecules are important photofunctional materials for an array of applications. It is interesting to examine the intermediate regime between these two families, which can be interpreted as the strong-confinement limit of the nanocrystals or alternatively as the large-size limit of molecules. Here, we choose Cd 3 P 2 magic-size clusters (MSCs) as a unique platform and apply time-resolved spectroscopy to investigate their spectral and dynamic properties. We find that these small clusters display molecular-like vibronic progression on their absorption and emission spectra and a large Stokes shift, which leads to well-separated transient absorption bleach and stimulated emission signals distinct from typical nanocrystals. On the other hand, such small size MSCs can still accommodate biexciton states, and the strongly enhanced Coulombic interactions lead to very fast dephasing of the biexciton resonance as well as rapid biexciton Auger annihilation (1.5 ps). Further, temperature-dependent measurements provide evidence for the transformation of band-edge excitons to localized excitons, with the localization likely driven by the softened lattice in these small-size clusters. These collective results demonstrate that strongly-confined nanoclusters indeed bridge the gap between nanocrystals and molecules, and can be a unique library to search for exotic excited state properties.