Ghosh Tufan, Dehnel Joanna, Fabian Marcel, Lifshitz Efrat, Baer Roi, Ruhman Sanford
The Institute of Chemistry, and the Fritz Haber Center for Molecular Dynamics , The Hebrew University of Jerusalem , Jerusalem 9190401 , Israel.
Schulich Faculty of Chemistry, Solid State Institute, Russell Berrie Nanotechnology Institute, Nancy and Stephen Grand Technion Energy Program , Technion Israel Institute of Technology , Haifa 3200003 , Israel.
J Phys Chem Lett. 2019 May 16;10(10):2341-2348. doi: 10.1021/acs.jpclett.9b00992. Epub 2019 Apr 26.
The conjecture that, as in bulk semiconductors, hot multiexcitons in nanocrystals cool rapidly to the lowest available energy levels is tested here by recording the effects of a single cold "spectator" exciton on the relaxation dynamics of a subsequently deposited hot counterpart. Results in CdSe/CdS nanodots show that a preexisting cold "spectator exciton" allows only half of the photoexcited electrons to relax directly to the band-edge. The rest are blocked in an excited quantum state due to conflicts in spin orientation. The latter fully relax in this sample only after ∼25 ps as the blocked electrons spins flip, prolonging the temporal window of opportunity for harvesting the retained energy more than 100 fold! Common to all quantum-confined nanocrystals, this process will delay cooling and impact the spectroscopic signatures of hot multiexcitons in all envisioned generation scenarios. How the spin-flipping rate scales with particle size and temperature remains to be determined.
如同在体相半导体中一样，纳米晶体中的热多激子会迅速冷却至最低可用能级，这一猜想在此通过记录单个冷“旁观者”激子对随后沉积的热激子弛豫动力学的影响进行了测试。CdSe/CdS纳米点的结果表明，预先存在的冷“旁观者激子”仅允许一半的光激发电子直接弛豫到带边。其余的由于自旋取向冲突而被阻挡在激发量子态中。在该样品中，只有当被阻挡的电子自旋翻转后，后者才会在约25皮秒后完全弛豫，将收集保留能量的时间窗口延长了100多倍！所有量子限域纳米晶体都存在这种情况，这一过程将延迟冷却，并影响所有设想的产生场景中热多激子的光谱特征。自旋翻转速率如何随颗粒尺寸和温度变化仍有待确定。
