Ultrafast electron hole relaxation dynamics in CdS nanocrystals

Surface traps significantly influence the charge carrier dynamics within semiconductor nanocrystals, introducing non-radiative exciton recombination channels which are detrimental for their applications. Understanding the nature of these trap states and modulating them synthetically bears immense potential in designing defect-free colloidal semiconductor nanocrystals for efficient optoelectronic devices. Thus, systems devoid of surface traps can be used to study the relaxation pathways of excitons generated within these nanocrystals. In this work, we study the ultrafast charge carrier relaxation dynamics upon near-edge resonance excitation and above-resonance excitation in CdS nanocrystals using ultrafast transient absorption spectroscopy, in order to understand intraband cooling and mid-gap trap states. The time-resolved studies reveal that the above bandgap excitation results in a three-step process, including instantaneous growth followed by a fast sub-picosecond decay and a long-lived (>1 ns) excited state or a trap state recombination. The large percentage of long-lived excitons in CdS nanocrystals elucidates the defect-free nature of the system arising from the absence of surface states.