Quantum State Resolved Spectroscopy of Excitonic and Multi-Excitonic Dynamics in Quantum Confined Nanostructures and Heterojunctions

技术综述多共振相干多维光谱(CMDS)是表征复杂纳米结构的一种新的、具有潜在变革意义的方法。它的基础是利用多束可调谐激光激发不同的量子态，形成多个量子相干(MQCs)，在MQCs保持其量子力学相位相干性的同时重新发射输出光束。该计划由NSF固态和材料化学计划支持，使用与量子态的共振来创建复杂纳米结构中单个子结构的多维签名。它分离了影响输出强度的单个相干路径，并使用这些路径来获得具有量子态分辨率的相干和非相干动力学。相干动力学包括退相相互作用和相干转移，非相干动力学包括电荷转移和布居驰豫。该计划特别感兴趣的是开发更大纳米结构中施主-受主亚结构之间电荷转移动力学的量子态解析方法。感兴趣的量子态包括子结构的量子受限激子态和多激子态以及表面态。多激子态的多维谱以及相干和非相干动力学特性确定了多激子产生的机制。高阶波混频探测不同高能激子的势能面。替代的CMDS方法包括三色通路和多路传输检测。该计划中使用的纳米结构是简单的、表征良好的模型系统，代表了在开发新的纳米技术方面感兴趣的不同量子受限亚结构和形态。提供未来所需的能源是一项巨大的挑战，需要高效地收集太阳能并将其转化为增长经济所需的电力和太阳能燃料的新技术。纳米技术是提供这种能力的一个有前途的方向，因为发生在小尺寸上的量子效应为设计复杂的纳米结构提供了机会，这些结构是高效和坚固的太阳能转换器。这些纳米结构的小尺寸决定了新技术的创造，这些技术可以访问各个子结构中的各个量子态，并跟踪从最初吸收光到最终转换为电能或太阳能燃料的能量流动。在这个项目中开发的激光方法将提供这些能力。这一方法不仅将确定控制能量如何获取和使用的基本科学原则，而且还将通过网站教程、科学会议和公开讲座、在线课程材料以及本科生和研究生的培训向更广泛的科学界传播。这种传播将是变革性的，因为这种新的方法提供了对材料如何工作的更深层次的见解，并可以解决当前技术无法回答的问题。

TECHNICAL SUMMARYMultiresonant Coherent Multidimensional Spectroscopy (CMDS) is a new and potentially transformative approach for characterizing complex nanostructures. It is based on using multiple tunable laser beams to excite different quantum states to form multiple quantum coherences (MQCs) that re-emit output beams during the time the MQCs retain their quantum mechanical phase coherence. This program, supported by the NSF Solid State and Materials Chemistry Program, uses the resonances with the quantum states to create multidimensional signatures of the individual substructures within complex nanostructures. It isolates the individual coherence pathways that contribute to the output intensity and uses these pathways to obtain the coherent and incoherent dynamics with quantum state resolution. The coherent dynamics includes both dephasing interactions and coherence transfer and the incoherent dynamics includes charge transfer and population relaxation. The program is particularly interested in developing methods for quantum state resolution of the charge transfer dynamics between donor-acceptor substructures within larger nanostructures. The quantum states of interest include the quantum confined excitonic and multiexcitonic states of the substructures as well as surface states. The multidimensional spectra and the coherent and incoherent dynamics of the multiexcitonic states identify the mechanisms responsible for multiexciton generation (MEG). Higher order wave mixing probes the potential energy surface of different energetic excitons. Alternative CMDS methodologies include 3-color pathways and multiplex detection. The nanostructures used in this program are simple, well-characterized model systems that represent the different quantum confined substructures and morphologies that are of interest in developing new nanotechnologies. NON-TECHNICAL SUMMARYProviding the energy required for the future is an enormous challenge that requires new technologies that efficiently harvest solar energy and turn it into the electrical power and solar fuels needed for growing economies. Nanotechnology is a promising direction for providing this capability because the quantum effects that occur at small dimensions provide opportunities for engineering complex nanostructures that are efficient and robust solar converters. The small size of these nanostructures dictates the creation of new technologies that can access the individual quantum states within the individual substructures and follow the flow of energy from the initial absorption of light to the final conversion into electricity or solar fuels. The laser methods developed in this program will provide these capabilities. Not only will this methodology define the fundamental scientific principles controlling how the energy is harvested and used but it will be disseminated to the wider scientific community through web site tutorials, scientific conferences and public lectures, on-line course materials, and the training of undergraduate and graduate students. The dissemination will be transformative because this new methodology provides deeper insights into how materials function and can address questions that cannot be answered by current technologies.