Morgan David, Kelley David F
Chemistry and Chemical Biology , University of California Merced , 5200 North Lake Road , Merced , California 95343 , United States.
J Phys Chem Lett. 2018 Aug 2;9(15):4160-4165. doi: 10.1021/acs.jpclett.8b01648. Epub 2018 Jul 12.
Transient absorption (TA) and time-resolved photoluminescence (PL) spectroscopies have been used to provide direct spectroscopic evidence for the recently reported phenomenon of thermal "surface charging" in II-VI quantum dots (QDs). In these studies, zincblende CdSe cores are synthesized by standard methods, and a thin CdS shell deposited by the decomposition of Cd(DDTC), resulting in core/shell QDs with chalcogenide-rich surfaces. Following ligand exchange with oleylamine, these QDs have empty low-lying surface states that can be thermally populated from the valence band. At room temperature, the surface charging equilibrium results in some fraction of the particles having a hole in the valence band, i.e., the surface acceptor states make the particle p-type. Photoexcitation of the surface charged state results in what is essentially a positive trion, which can undergo a fast Auger recombination. Both PL and TA (bleach recovery) kinetics of the CdSe/CdS QDs show a 70 ps decay component, which is assigned to Auger recombination. The empty nonbonding surface orbitals are passivated by ligation with a trialkylphosphine, and the fast decay component is absent when tributylphosphine is present. The comparison of the TA and PL kinetics shows that the relative amplitude of the 70 ps component is a factor of about 1.5 greater in the TA than in the PL. They also show that the fast component in the PL spectrum is shifted about 6 nm to the blue of the exciton luminescence. The above observations can be understood in terms of the trion versus exciton spectroscopy and strongly support the assignment of the 70 ps transient to the decay of a trion formed from the surface charged state.
瞬态吸收（TA）光谱和时间分辨光致发光（PL）光谱已被用于为最近报道的II-VI族量子点（QD）中的热“表面充电”现象提供直接的光谱证据。在这些研究中，通过标准方法合成了闪锌矿结构的CdSe核，并通过Cd(DDTC)的分解沉积了一层薄的CdS壳，从而得到了表面富含硫族化物的核/壳量子点。在用油胺进行配体交换后，这些量子点具有空的低能表面态，这些表面态可以从价带热填充。在室温下，表面充电平衡导致一部分粒子在价带中有一个空穴，即表面受体态使粒子呈p型。表面带电态的光激发产生了本质上是一个正三重态激子，它可以经历快速俄歇复合。CdSe/CdS量子点的PL和TA（漂白恢复）动力学都显示出一个70 ps的衰减分量，这被归因于俄歇复合。空的非键表面轨道通过与三烷基膦配位而被钝化，当存在三丁基膦时，快速衰减分量不存在。TA和PL动力学的比较表明，70 ps分量的相对幅度在TA中比在PL中大约大1.5倍。它们还表明，PL光谱中的快速分量相对于激子发光向蓝光方向移动了约6 nm。上述观察结果可以通过三重态激子与激子光谱来理解，并有力地支持了将70 ps瞬态归因于由表面带电态形成的三重态激子的衰减这一论断。
