Exciton Dynamics in Perovskite Quantum Dots and 2D Nanoplatelets

Percent量子点和2D纳米片中的激子动力学在这个由化学系的大分子，超分子和纳米化学项目资助的项目中，Tianquan Lian教授和埃默里大学的一个研究生和本科生团队研究了两种新兴的纳米材料。一种类型的材料（称为钙钛矿的无机化合物）由极小的球形颗粒（称为量子点）组成，其中颗粒的大小决定了它如何吸收和发射光。另一种材料是一种非常薄的板，它可以被看作是一种只有两个维度的结构（称为纳米片），由各种半导体化合物制成。可以跟踪和控制这些材料中离散量（激子）的能量流。这些新材料在太阳能转换和纳米激光器方面具有潜在的应用前景。开发高效、经济的太阳能转换材料是21世纪最重要的科学挑战之一。这项工作将使人们对这些材料的性质有一个基本的了解，促进它们的进一步发展和许多潜在的应用。大块钙钛矿材料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（经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提供了进一步调整，改善和优化这些材料性能的可能性。胶态二维硫族镉纳米片是一类新兴的具有原子级精确厚度的新型半导体量子阱材料。纳米片晶中横向尺寸为10 s至100 nm的均匀能量表明了新激子性质的可能性，例如巨振子强度跃迁效应。这些独特的性质，显着不同于零维量子点和一维纳米棒，提出了许多新的应用在光电器件和太阳能转换。所提出的研究旨在对这些新型材料中的激子和载流子动力学有一个基本的了解，这对它们的开发、合理设计和改进至关重要。本研究计划的具体目标是：1）研究钙钛矿量子点到分子受体的电子和空穴转移速率如何依赖于量子点的尺寸和阴离子身份，测试量子点电荷转移的理论模型; 2）研究从钙钛矿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（TiO 2、SnO 2和ZnO），测试QD/氧化物界面处的界面ET的当前理论模型; 3）测量CdX的二维激子玻尔半径和激子质心相干离域长度（X=S，Se，Te）NPL作为层厚度、材料、溶剂介电常数和温度的函数，检验二维类氢模型对描述NPL中激子的适用性和巨振子强度跃迁效应的程度; 4）使用电子受体（TiO 2）修饰的AFM针尖直接成像仅CdSe核、I型CdSe/CdS核/冠和II型CdTe/CdSe核/冠NPL中激子的空间分布，检查NPL异质结构中的能带对准如何影响激子空间分布。除了对研究生和本科生进行培训外，拟议的工作将导致在本科生一级编制关于清洁能源的新教材，并扩大代表性不足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.