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.

表面陷阱显着影响半导体纳米晶体内的电荷载流子动力学，引入不利于其应用的非辐射激子复合通道。了解这些陷阱态的性质并对其进行综合调制，在设计用于高效光电器件的无缺陷胶体半导体纳米晶体方面具有巨大的潜力。因此，没有表面陷阱的系统可用于研究这些纳米晶体内产生的激子的弛豫路径。在这项工作中，我们使用超快瞬态吸收光谱研究了 CdS 纳米晶体中近边共振激发和高于共振激发的超快电荷载流子弛豫动力学，以了解带内冷却和中隙陷阱状态。时间分辨研究表明，上述带隙激发导致一个三步过程，包括瞬时增长，然后是快速亚皮秒衰减和长寿命（> 1 ns）激发态或陷阱态复合。CdS 纳米晶体中长寿命激子的大部分阐明了由于缺乏表面态而引起的系统的无缺陷性质。