Sorbonne Université, CNRS, Institut des NanoSciences de Paris, INSP, 4 place Jussieu, F-75005 Paris, France.
Sorbonne Université, CNRS, ENS-PSL Research University, Collège de France, Laboratoire Kastler Brossel, LKB, 4 place Jussieu, F-75005 Paris, France.
Nanoscale. 2018 Apr 5;10(14):6393-6401. doi: 10.1039/C7NR09334A.
All inorganic CsPbX3 (X = Cl, Br, I) nanocrystals (NCs) belong to the novel class of confined metal-halide perovskites which are currently arousing enthusiasm and stimulating huge activity across several fields of optoelectronics due to outstanding properties. A deep knowledge of the band-edge excitonic properties of these materials is thus crucial to further optimize their performances. Here, high-resolution photoluminescence (PL) spectroscopy of single bromide-based NCs reveals the exciton fine structure in the form of sharp peaks that are linearly polarized and grouped in doublets or triplets, which directly mirror the adopted crystalline structure, tetragonal (D4h symmetry) or orthorhombic (D2h symmetry). Intelligible equations are found that show how the fundamental parameters (spin-orbit coupling, ΔSO, crystal field term, T, and electron-hole exchange energy, J) rule the energy spacings in doublets and triplets. From experimental data, fine estimations of each parameter are obtained. The analysis of the absorption spectra of an ensemble of NCs with a "quasi-bulk" behavior leads to ΔSO = 1.20 ± 0.06 eV and T = -0.34 ± 0.05 eV in CsPbBr3. The study of individual luminescence responses of NCs having sizes comparable to the exciton Bohr diameter, 7 nm, allows us to estimate the value of J to be around ≈3 meV in both tetragonal and orthorhombic phases. This value is already enhanced by confinement.
所有无机 CsPbX3（X = Cl、Br、I）纳米晶体（NCs）都属于新型受限金属卤化物钙钛矿，由于其出色的性能，目前在光电等多个领域引起了人们的兴趣和极大的关注。因此，深入了解这些材料的能带边缘激子特性对于进一步优化其性能至关重要。在这里，通过对单个溴化物基 NCs 的高分辨率光致发光（PL）光谱研究，揭示了激子的精细结构，表现为尖锐的峰，这些峰具有线性偏振，并以二聚体或三聚体的形式分组，直接反映了所采用的晶体结构，四方（D4h 对称性）或正交（D2h 对称性）。发现了一些可理解的方程，这些方程表明基本参数（自旋轨道耦合、ΔSO、晶体场项、T 和电子空穴交换能、J）如何控制二聚体和三聚体的能量间隔。从实验数据中，获得了每个参数的精细估计值。对具有“准体相”行为的 NCs 集合的吸收光谱进行分析，得到 CsPbBr3 中的 ΔSO = 1.20 ± 0.06 eV 和 T = -0.34 ± 0.05 eV。对尺寸与激子玻尔直径（7nm）相当的单个 NCs 的发光响应进行研究，可使我们估计出在四方相和正交相中的 J 值约为 ≈3 meV。这个值已经通过限制得到了增强。
