Photophysical Action Spectra of Emission from Semiconductor Nanocrystals Reveal Violations to the Vavilov Rule Behavior from Hot Carrier Effects

Abstract

Semiconductor nanocrystals are known to have properties of bulk semiconductors as well as molecules. Two rules that govern molecules are that there is no dual emission (Kasha) and there is no spectrum to the emission quantum yield (Vavilov). We show that the latter rule of molecular spectroscopy is generally violated in semiconductor nanocrystals. Through experiments and theory on CdSe and perovskite nanocrystals, these violations are shown to arise via hot carrier effects. Experiments and simple phenomenology reveal that quantum yield spectra arise because of enhanced hot carrier trapping rates. A semiclassical electron-transfer theory rationalizes a microscopic picture of the carrier kinetics. These effects are especially significant when quantifying syntheses of bright emitters such as perovskite nanocrystals. These effects are also a general approach to simple steady-state measurements of the action of hot carrier kinetics.