Failla Michele, García Flórez Fransisco, Salzmann Bastiaan B V, Vanmaekelbergh Daniel, Stoof Henk T C, Siebbeles Laurens D A
Chemical Engineering Department, Delft University of Technology, Van der Maasweg 9, 2629 HZDelft, The Netherlands.
Institute for Theoretical Physics and Center for Extreme Matter and Emergent Phenomena, Utrecht University, Princetonplein 5, 3584 CCUtrecht, The Netherlands.
J Phys Chem C Nanomater Interfaces. 2023 Jan 19;127(4):1899-1907. doi: 10.1021/acs.jpcc.2c07292. eCollection 2023 Feb 2.
We studied the initial nature and relaxation of photoexcited electronic states in CdSe nanoplatelets (NPLs). Ultrafast transient optical absorption (TA) measurements were combined with the theoretical analysis of the formation and decay of excitons, biexcitons, free charge carriers, and trions. In the latter, photons and excitons were treated as bosons and free charge carriers as fermions. The initial quantum yields of heavy-hole (HH) excitons, light-hole (LH) excitons, and charge carriers vary strongly with photon energy, while thermal relaxation occurs always within 1 ps. After that, the population of LH excitons is negligible due to relaxation to HH excitons or decay into free electrons and holes. Up to the highest average number of about four absorbed photons per NPL in our experiments, we found no signatures of the presence of biexcitons or larger complexes. Biexcitons were only observed due to the interaction of a probe-generated exciton with an exciton produced previously by the pump pulse. For higher pump photon energies, the initial presence of more free charge carriers leads to formation of trions by probe photons. On increasing the number of absorbed pump photons in an NPL, the yield of excitons becomes higher as compared to free charge carriers, since electron-hole recombination becomes more likely. In addition to a TA absorption feature at energy below the HH exciton peak, we also observed a TA signal at the high-energy side of this peak, which we attribute to formation of LH-HH biexcitons or trions consisting of a charge and LH exciton.
我们研究了CdSe纳米片（NPLs）中光激发电子态的初始性质和弛豫过程。超快瞬态光吸收（TA）测量与激子、双激子、自由电荷载流子和三重态的形成与衰减的理论分析相结合。在理论分析中，光子和激子被视为玻色子，自由电荷载流子被视为费米子。重空穴（HH）激子、轻空穴（LH）激子和电荷载流子的初始量子产率随光子能量强烈变化，而热弛豫总是在1皮秒内发生。在此之后，由于弛豫到HH激子或衰变成自由电子和空穴，LH激子的数量可以忽略不计。在我们的实验中，每个NPL吸收的光子平均数量最高约为四个，我们没有发现双激子或更大复合物存在的迹象。双激子仅在探测产生的激子与泵浦脉冲先前产生的激子相互作用时被观测到。对于更高的泵浦光子能量，更多自由电荷载流子的初始存在会导致探测光子形成三重态。随着NPL中吸收的泵浦光子数量增加，与自由电荷载流子相比，激子的产率变得更高，因为电子 - 空穴复合更有可能发生。除了在HH激子峰能量以下的TA吸收特征外，我们还在该峰的高能侧观测到一个TA信号，我们将其归因于LH - HH双激子或由一个电荷和LH激子组成的三重态的形成。
