Exciton Dynamics in Perovskite Quantum Dots and 2D Nanoplatelets

钙钛矿型量子点和二维纳米平板中的激子动力学在这个由化学系高分子、超分子和纳米化学项目资助的项目中，连天全教授和埃默里大学的一组研究生和本科生研究了两类新兴的纳米材料。一种类型的材料(一种称为钙钛矿的无机化合物)由极小的球形粒子(称为量子点)组成，粒子的大小决定了它如何吸收和发射光线。另一种材料是非常薄的平板，它可以被视为只有两个维度的结构(称为纳米平板)，由各种半导体化合物制成。这些材料中的能量流动，以离散的数量(激子)可以被跟踪和控制。这些新材料在太阳能转换和纳米激光方面具有潜在的应用前景。开发高效、低成本的太阳能转换材料是21世纪最重要的科学挑战之一。这项工作将使人们从根本上了解这些材料的性质，促进它们的进一步发展和许多潜在的应用。块状钙钛矿材料已经在多种应用中取得了令人印象深刻的性能，包括高效太阳能电池(认证的功率转换效率为20%)和低阈值激光。胶体钙钛矿量子点通过量子限制效应为进一步调整、改善和优化这些材料的性能提供了可能。胶体二维硫族镉纳米片是一种新型的具有原子精确厚度的半导体量子阱材料。纳米板在10s到100 nm的横向尺寸上的均匀能量表明了新的激子性质的可能性，例如巨振子强度跃迁效应。这些独特的性质，与零维量子点和一维纳米棒有很大的不同，在光电子器件和太阳能转换方面有许多新的应用。这些研究的目的是为了从根本上了解这些新材料中的激子和载流子动力学，这对于它们的开发、合理设计和改进是必不可少的。该研究计划的具体目标是：1)研究钙钛矿型量子点到分子受体的电子和空穴转移速率是如何依赖于量子点尺寸和阴离子一致性的，验证量子点电荷转移的理论模型；2)研究从钙钛矿型量子点到氧化物纳米晶薄膜的界面电子转移速率如何依赖于量子点(尺寸和阴离子)和氧化物(二氧化钛、二氧化锡和氧化锌)，检验当前量子点/氧化物界面电子转移的理论模型；3)测量了二维激子玻尔半径和激子质心相干离域长度随薄膜厚度、材料、溶剂介电常数和温度的变化关系，测试了二维类氢模型对描述激子的适用性和巨谐振子强度跃迁效应的程度；4)利用电子受体(TiO2AFM)修饰的AFM探针直接成像了纯CdSe芯、I型CdSe/CDS芯/冠和II型CdTe/CdSe芯/冠NPL中激子的空间分布，考察了NPL异质结中激子的能带排列对激子空间分布的影响。除了对研究生和本科生进行培训外，拟议的工作还将导致在本科生一级编写关于清洁能源的新教材，并扩大未得到充分代表的少数民族学生的参与。

Exciton Dynamics in Perovskite Quantum Dots and 2D NanoplateletsIn this project, funded by the Macromolecular, Supramolecular and Nanochemistry program of the Chemistry Division, Prof. Tianquan Lian and a team of graduate and undergraduate students at Emory University investigate two emerging classes of nanomaterials. One type of material (an inorganic class of compounds called perovskites) consists of extremely small spherical particles (called quantum dots), where the size of the particle determines how it absorbs and emits light. The other material is a very thin plate, which can be viewed as a structure with only two dimensions (called nanoplatelets), made from various semiconductor compounds. The flow of energy in these materials, in discrete amounts (excitons) can be followed and controlled. These new materials have potential applications in solar energy conversion and nanoscale lasers. Developing efficient and cost-effective solar energy conversion materials is one of the most important scientific challenges of the 21st century. The proposed work will lead to a fundamental understanding of the properties of these materials, facilitating their further development and many potential applications.Bulk perovskite materials have achieved impressive performance in multiple applications, including highly efficient solar cells (with certified power conversion efficiencies of 20%) and low threshold lasing. Colloidal perovskite quantum dots offer the possibility to further tune, improve and optimize the properties of these materials through the quantum confinement effect. Colloidal two-dimensional cadmium chalcogenide nanoplatelets, are an emerging novel class of semiconductor quantum well materials with atomically precise thickness. The uniform energy over lateral dimensions of 10s to 100 nm in nanoplateletes suggests the possibility of novel exciton properties, such as the giant oscillator strength transitions effect. These unique properties, differing significantly from zero-dimensional quantum dots and one-dimenisonal nanorods, suggest many novel applications in optoelectronic devices and in solar energy conversion. The proposed studies are aimed at a fundamental understanding of exciton and charge carrier dynamics in these novel materials, which is essential to their development, rational design and improvement. The specific aims of the research plan are: 1) To study how the electron and hole transfer rates from perovskite QDs to molecular acceptors depend on the QD size and anion identity, testing theoretical models for charge transfer from QDs; 2) To examine how the interfacial electron transfer rates from perovskite QDs to oxide nanocrystalline thin films depend on the QD (size and anion) and oxide (TiO2, SnO2 and ZnO), testing current theoretical models of interfacial ET at QD/oxide interfaces; 3) To measure 2D exciton Bohr's radius and exciton center of mass coherent delocalization length of CdX(X=S, Se, Te) NPLs as a function of layer thickness, materials, solvent dielectric constant and temperature, testing the applicability of 2D hydrogenic model for describing excitons in NPLs and the extent of the giant oscillator strength transition effect; 4) To directly image the spatial distribution of excitons in CdSe core-only, type I CdSe/CdS core/crown and type II CdTe/CdSe core/crown NPLs using electron acceptor (TiO2) modified AFM tips, examining how band alignment in NPL heterostructures affect exciton spatial distribution. In addition to the training of graduate students and undergraduate students, the proposed work will lead to the development of new course materials on clean energy at the undergraduate level and broadened participation of under-represented minority students.