Advanced Photonics 封面|钙钛矿纳米晶：从基础研究到器件应用

半导体钙钛矿薄膜材料近年来在太阳能电池等光电器件领域取得了飞速的发展。半导体钙钛矿纳米晶是一类具有钙钛矿晶格结构、由几千个原子组成、尺寸通常在10纳米左右的新型低维半导体光电材料。

钙钛矿纳米晶的尺寸可比于其激子波尔半径，因此具有量子受限效应。得益于特殊的钙钛矿结构，钙钛矿纳米晶还表现出许多优异的光学性质。钙钛矿纳米晶不仅具有近100%荧光量子产率，能够高效发射荧光，同时还可以通过改变材料的尺寸和化学组分实现从近紫外到近红外的宽范围可调荧光光谱。

近日，南京大学王晓勇教授团队从宏观群体（系综）和微观单颗粒两个尺度回顾了全球相关研究者在钙钛矿纳米晶光学特性研究方面所取得的进展情况，相关工作以 “Optical studies of semiconductor perovskite nanocrystals for classical optoelectronic applications and quantum information technologies: a review”为题发表于Advanced Photonics 2020年第5期，并被选为封面文章。

图1. Advanced Photonics2020年第5期封面图

在宏观群体尺度下，时间分辨荧光光谱和瞬态吸收光谱技术被广泛用来表征钙钛矿纳米晶中载流子的弛豫、复合以及迁移和提取等动力学过程，进而指导基于钙钛矿纳米晶的太阳能电池和发光二极管等光电器件的性能提升。钙钛矿纳米晶不仅具有可调荧光光谱（图2a），同时得益于其振子强度强、吸收截面大、荧光量子产率高等优点，钙钛矿纳米晶可以产生从近紫外到近红外的低阈值的放大自发辐射过程。

Yakunin等在钙钛矿纳米晶中测得阈值∼5 μJ cm−2的单光子激发放大自发辐射，随后，阈值为0.9 mJcm−2（5.2 mJcm−2）的双（三）光子激发自发放大辐射也相继被报道(如图2(b)所示)。上述研究表明钙钛矿纳米晶能够应用于开发低阈值激光。

图2. (a) 紫外光激发下不同卤素组分CsPbX3纳米晶溶液的光学图像。(b) 不同卤素组分CsPbX3纳米晶的放大自发辐射光谱。(c) 单个FAPbBr3纳米晶的激子精细能级结构。(d) 单个CsPbI3纳米晶的量子干涉测量。

此外，在高度有序的钙钛矿纳米晶超晶格结构中，由于钙钛矿纳米晶之间能够相干耦合，Rainò等测量发现钙钛矿纳米晶超晶格发射的荧光不仅具有140 fs的长相干时间，同时具有光子聚束（Photon Bunching）效应，呈现出独特的超荧光发射现象，这一光学特性使得钙钛矿纳米晶有望实现多光子纠缠量子光源。

在微观单颗粒尺度，量子受限效应使得钙钛矿纳米晶得以高效发射单光子，而缺陷容忍特性有效地抑制了钙钛矿纳米晶的荧光闪烁和光谱漂移。

继Park等在钙钛矿纳米晶中观测到单光子发射现象后，Hu等进一步测量发现钙钛矿纳米晶能够无荧光闪烁、无光谱漂移地高效发射单光子，是一种高品质单光子源。

同时，如图2(c)所示，Tamarat等通过磁光实验研究揭示了钙钛矿纳米晶包含最低暗态和亮三重态的激子精细能级结构。作为一类胶体半导体纳米晶，钙钛矿纳米晶在单颗粒尺度表现出类似于原子和自组装量子点的稳定光学性质，这进一步促进了对钙钛矿纳米晶量子光学性质的探索研究和开发应用。

Utzat等测量发现单个钙钛矿纳米晶在发射态精细结构中具有约20μeV的超窄光谱线宽，表明钙钛矿纳米晶可用来实现不可区分性单光子源。Lv等在单个钙钛矿纳米晶吸收态精细结构中完成了量子干涉测量（图2(d)），获得约10ps激子相干时间，预示着相干操作激子波函数的可能性。

以上对群体钙钛矿纳米晶所进行的光学特性研究为提高相关光电器件的运行指标提供了重要的学术指导，而在单粒子层面进行的深入光学表征为其进入量子信息技术这一新兴研究方向奠定了坚实的实验基础。因此，钙钛矿纳米晶的独特性在于其兼备了经典与量子光学的杂化特性，从而在器件应用和基础科学方面会得到大量的科研关注而拥有广阔的发展空间。

Recent progress in the optical studies of semiconductor perovskite nanocrystals

As a novel type of low-dimensional semiconductor perovskite nanocrystals (NCs) can possess a fluorescent quantum yield of about 100% with size- and composition-tunable emission covering the broad spectral range from the near-UV to the near-infrared.

In a review paper published in Advanced Photonics, entitled "Optical studies of semiconductor perovskite nanocrystals for classical optoelectronic applications and quantum information technologies: a review", Prof. Xiaoyong Wang' group from Nanjing University summarize recent progress made in the optical studies of ensemble and single perovskite NCs. The mentioned review is selected as the On the Cover of Advanced Photonics (Vol.2, Iss.5).

Figure 1 On the Cover of Advanced Photonics (Vol.2, Iss.5)

At the ensemble level, the carrier relaxation and recombination dynamics, as well as the charge transport and extraction processes, can be routinely probed by the time-resolved photoluminescence (PL) and transient absorption optical techniques. Under both one- and multiple-photon excitations, low-threshold amplified spontaneous emission has been realized in perovskite NC ensembles, whose highly-ordered superlattices can additionally reveal the novel optical effect of superfluorescence.

At the single-particle level, the perovskite NCs can emit single photons with suppressed PL blinking and spectral diffusion as dictated by the quantum confinement and defect tolerance features, thus allowing the observation of stable exciton fine energy-level structures. The ultranarrow PL linewidth obtained from the fine-structured emission states of a single perovskite NC can be utilized to prepare indistinguishable photons, while the quantum interference measurement performed on the fine-structured absorption states implies the possibility of coherently manipulating the exciton wavefunctions.

Optical studies of perovskite NC ensembles are surely helpful in guiding their enhanced performance in relevant optoelectronic devices, with the single-particle characterization being equally important in advancing single perovskite NCs into the exciting regime of quantum information technologies. In this sense, the perovskite NCs are unique in that they are carrying a hybrid feature of classical and quantum optics, which will surely attract great research attention from both fundamental studies and practical applications for their wide development in future works.

Figure 2. (a) Optical images of solution CsPbX₃ NCs with varying halide compositions under the excitation of an UV lamp. (b) PL spectra of amplified spontaneous emission measured for film CsPbX₃ NCs with different halide compositions. (c) Band-edge exciton fine structures of a single FAPbBr₃ NC. (d) Quantum interference measurement for a single CsPbI₃ NC.