Xiao James, Liu Yun, Steinmetz Violette, Çaǧlar Mustafa, Mc Hugh Jeffrey, Baikie Tomi, Gauriot Nicolas, Nguyen Malgorzata, Ruggeri Edoardo, Andaji-Garmaroudi Zahra, Stranks Samuel D, Legrand Laurent, Barisien Thierry, Friend Richard H, Greenham Neil C, Rao Akshay, Pandya Raj
Cavendish Laboratory, University of Cambridge, J.J. Thomson Avenue, CB3 0HE, Cambridge, United Kingdom.
Department of Chemical Engineering & Biotechnology, University of Cambridge, Philippa Fawcett Drive, CB3 0AS, Cambridge, United Kingdom.
ACS Nano. 2020 Nov 24;14(11):14740-14760. doi: 10.1021/acsnano.0c01752. Epub 2020 Oct 12.
Luminescent colloidal CdSe nanorings are a recently developed type of semiconductor structure that have attracted interest due to the potential for rich physics arising from their nontrivial toroidal shape. However, the exciton properties and dynamics of these materials with complex topology are not yet well understood. Here, we use a combination of femtosecond vibrational spectroscopy, temperature-resolved photoluminescence (PL), and single-particle measurements to study these materials. We find that on transformation of CdSe nanoplatelets to nanorings, by perforating the center of platelets, the emission lifetime decreases and the emission spectrum broadens due to ensemble variations in the ring size and thickness. The reduced PL quantum yield of nanorings (∼10%) compared to platelets (∼30%) is attributed to an enhanced coupling between (i) excitons and CdSe LO-phonons at 200 cm and (ii) negatively charged selenium-rich traps, which give nanorings a high surface charge (∼-50 mV). Population of these weakly emissive trap sites dominates the emission properties with an increased trap emission at low temperatures relative to excitonic emission. Our results provide a detailed picture of the nature of excitons in nanorings and the influence of phonons and surface charge in explaining the broad shape of the PL spectrum and the origin of PL quantum yield losses. Furthermore, they suggest that the excitonic properties of nanorings are not solely a consequence of the toroidal shape but also a result of traps introduced by puncturing the platelet center.
发光胶体CdSe纳米环是一种最近开发的半导体结构，由于其非平凡的环形形状可能产生丰富的物理现象而引起了人们的兴趣。然而，这些具有复杂拓扑结构的材料的激子性质和动力学尚未得到很好的理解。在这里，我们结合飞秒振动光谱、温度分辨光致发光（PL）和单粒子测量来研究这些材料。我们发现，通过在CdSe纳米片的中心打孔将其转变为纳米环时，由于环尺寸和厚度的整体变化，发射寿命缩短，发射光谱展宽。与纳米片（约30%）相比，纳米环的PL量子产率降低（约10%），这归因于（i）激子与200 cm处的CdSe LO声子之间增强的耦合以及（ii）带负电荷的富硒陷阱，这使得纳米环具有高表面电荷（约-50 mV）。这些弱发射陷阱位点的占据主导了发射特性，与激子发射相比，低温下陷阱发射增加。我们的结果详细描绘了纳米环中激子的性质以及声子和表面电荷在解释PL光谱的宽形状和PL量子产率损失起源方面的影响。此外，它们表明纳米环的激子性质不仅是环形形状的结果，也是通过刺穿纳米片中心引入陷阱的结果。
