Excited state localisation cascades in inorganic semiconductor nanoparticles

Excited state relaxation in zinc sulfide (ZnS) nanoparticles is studied as a model for the fate of the excited state in inorganic nanoparticles in general. A series of time-dependent density functional theory optimisations on the S1 and T1 excited states predict the existence of not merely isolated minima, as found before, but rather a connected cascade of excited state minima ending up in a conical intersection between the excited state energy surface and the ground state. The localisation of the excited state in the different minima increases down the cascade, while the barriers separating these minima, studied here for the first time for nanoparticles, are predicted to be in some cases electronic (strongly avoided crossing) in origin. The cartoon picture of excited state relaxation in inorganic nanoparticles that involves relaxation to the bottom of only one approximately harmonic well followed by photoluminescence appears for the ZnS nanoparticles studied here to be at best rather simplistic. The localisation cascade is finally found to strongly affect the excited state properties of nanoparticles and predicted to lead to the formation of defected nanoparticles after de-excitation in selected cases.