Dong Shuo, Pal Sougata, Lian Jie, Chan Yinthai, Prezhdo Oleg V, Loh Zhi-Heng
Division of Chemistry and Biological Chemistry, and Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University , 21 Nanyang Link, Singapore 637371.
Departments of Chemistry, and Physics and Astronomy, University of Southern California , Los Angeles, California 90089, United States.
ACS Nano. 2016 Oct 25;10(10):9370-9378. doi: 10.1021/acsnano.6b04210. Epub 2016 Sep 21.
Quasi-two-dimensional colloidal nanoplatelets (NPLs) have recently emerged as a class of semiconductor nanomaterials whose atomically precise monodisperse thicknesses give rise to narrow absorption and emission spectra. However, the sub-picosecond carrier dynamics of NPLs at the band edge remain largely unknown, despite their importance in determining the optoelectronic properties of these materials. Here, we use a combination of femtosecond transient absorption spectroscopy and nonadiabatic molecular dynamics simulations to investigate the early time carrier dynamics of CdSe/CdS core/shell NPLs. Band-selective probing reveals sub-picosecond Auger-mediated trapping of holes with an effective second-order rate constant of 3.5 ± 1.0 cm/s. Concomitant spectral blue shifts that are indicative of Auger hole heating are found to occur on the same time scale as the sub-picosecond trapping dynamics, whereas spectral red shifts that emerge at low excitation densities furnish an electron-cooling time scale of 0.84 ± 0.09 ps. Finally, nonadiabatic molecular dynamics simulations relate the observed sub-picosecond Auger-mediated hole-trapping dynamics to a shallow trap state that originates from the incomplete passivation of dangling bonds on the NPL surface.
准二维胶体纳米片（NPLs）最近作为一类半导体纳米材料出现，其原子精确的单分散厚度导致窄的吸收和发射光谱。然而，尽管NPLs在带边的亚皮秒载流子动力学对确定这些材料的光电特性很重要，但仍基本未知。在这里，我们结合飞秒瞬态吸收光谱和非绝热分子动力学模拟来研究CdSe/CdS核壳NPLs的早期载流子动力学。带选择性探测揭示了亚皮秒级俄歇介导的空穴俘获，有效二级速率常数为3.5±1.0 cm/s。发现与俄歇空穴加热相关的伴随光谱蓝移与亚皮秒俘获动力学发生在同一时间尺度上，而在低激发密度下出现的光谱红移给出了0.84±0.09 ps的电子冷却时间尺度。最后，非绝热分子动力学模拟将观察到的亚皮秒级俄歇介导的空穴俘获动力学与一个浅陷阱态联系起来，该陷阱态源于NPL表面悬空键的不完全钝化。
