Exciton Dynamics in Colloidal Lead Sulfide (PbS) Nanosheets

半导体在许多现代技术中发挥着核心作用，从计算机芯片到发光二极管（LED）。半导体的一个关键特性是它的带隙，这是晶体中电子可以吸收的最小能量。虽然带隙取决于半导体的类型，但它也取决于晶体的大小。 随着晶体的收缩，电子被限制在越来越小的体积中。 当它的尺寸只有几纳米，或者比一张纸薄10万倍时，它的带隙可能会非常不同。 人们可以使用这种限制效应来制造不同颜色的LED，只需改变颗粒的大小，而不是材料本身。然而，虽然发射可见光的半导体纳米结构很常见，但红外光新材料的发展仍然是一个挑战。 在化学系大分子，超分子和纳米化学项目的支持下，Bowling绿色州立大学的Liangfeng Sun教授正在研究硫化铅（PbS）纳米片中的电子。与他的学生合作，孙教授正在开发方法来生长PbS纳米片，并精确控制其尺寸。 他们使用复杂的实验方法来研究他们的受限电子。 他们的发现可能会导致更高效的太阳能电池和更好的半导体激光器。该项目还为未来的科学家提供先进实验技术方面的培训机会。此外，该项目还吸引了中央州立大学（位于俄亥俄州威尔伯福斯的一所历史悠久的黑人大学）的本科生，并向高中生介绍了纳米材料研究。研究小组正在研究辐射复合率、电荷转移、激子-激子相互作用和共振能量转移过程作为纳米片结构的函数（片面积、厚度、共面接触以及与量子点的比较）。稳态光谱和电化学方法被用来阐明静态性能，如能级，发射和吸收光谱的过渡线宽度作为尺寸和温度的函数。时间分辨光谱技术（荧光和皮秒和飞秒时间尺度上的瞬态吸收，分别）被用来调查激子动力学在这些新的结构。研究活动包括：合成具有可调厚度和横向尺寸的高质量PbS纳米片，研究孤立纳米片中单个激子的动力学，研究纳米片中相互作用的多个激子的动力学，以及研究激子解离和从纳米片到电荷受体的电荷转移。 该研究小组还研究了纳米片之间的激子共振能量转移。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Semiconductors play a central role in many modern technologies, from computer chips to light emitting diodes (LEDs). A key property of a semiconductor is its band gap, which is the smallest energy an electron in the crystal can absorb. While the band gap depends on the type of semiconductor, it also depends on the size of the crystal. As the crystal shrinks, the electrons become confined in smaller and smaller volumes. When it reaches only a few nanometers in size, or about 100,000 times thinner than a sheet of paper, its band gap can be very different. One can use this confinement effect to make LEDs of different colors simply by changing the size of the particle, rather than the material itself. However, while semiconductor nanostructures that emit visible light are common, the advancement of new materials for infrared light has remained a challenge. With support from the Macromolecular, Supramolecular and Nanochemistry Program in the Division of Chemistry, Professor Liangfeng Sun at Bowling Green State University is studying electrons confined in lead sulfide (PbS) nanosheets. Working with his students, Professor Sun is developing methods to grow PbS nanosheets with precise control over their dimensions. They use sophisticated experimental methods to study their confined electrons. Their discoveries could lead to more efficient solar cells and better semiconductor lasers. The project also provides training opportunities for future scientists in advanced experimental techniques. In addition, the project is engaging undergraduates from Central State University, an historically black university in Wilberforce, Ohio, as well as introducing high-school students to nanomaterial research.The research team is studying the radiative recombination rates, charge transfer, exciton-exciton interactions, and resonance energy transfer processes as a function of the nanosheet structure (sheet area, thickness, co-facial contact, and comparison to quantum dots). Steady-state spectroscopy and electrochemical methods are used to elucidate static properties such as energy levels, transition line widths of emission and absorption spectra as a function of size and temperature. Time-resolved spectroscopic techniques (fluorescence and transient absorption on the picosecond and femtosecond time scales, respectively) are used to investigate exciton dynamics in these novel structures. The research activities include: synthesizing high-quality PbS nanosheets with tunable thickness and lateral size, studying the dynamics of a single exciton in isolated nanosheets, studying the dynamics of the interacting multiple excitons in a nanosheet, and investigating the exciton dissociation and charge transfer from nanosheets to charge acceptors. The research group also investigates the exciton resonance energy transfer between the nanosheets.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.