Excitonic Transport in Van der Waals Solids: Insights from Experiment and Predictive Calculations

非技术描述：当半导体吸收光子时，电子被激发到更高的能量状态。带负电荷的激发电子和留下的正电荷（空穴）相互吸引，形成一个被称为激子的束缚对。这些激发对能够在材料中传输能量，并在光合作用等自然过程以及光化学和发光二极管等有机器件中发挥重要作用。了解激子的传输增加了一个新的维度，不仅可以提高当前激子器件的性能，还可以为下一代光激子器件引入一个平台。目前的工作研究的能量输运和它之间的联系，形成激子的束缚电子和空穴对之间的分离。除了解决基础和技术挑战之外，光电和量子材料前沿的这项研究为各级真正的跨学科教育提供了理想的场所。除了博士培训外，研究成果还被纳入研究生和本科生课程，并用于培训本科生。从这项研究中获得的知识有可能推动研究，创新和教育下一代科学家和工程师的前沿，以创造更美好的未来。技术说明：由于实现室温稳定激子器件的前景，二维半导体中的激子输运最近受到了极大的关注。这些货车德瓦耳斯（vdW）半导体支持稳定的室温激子，由于减少介电屏蔽，导致高结合能和小玻尔半径。然而，由于这些激子在几个晶格间距上保持离域，因此难以定性和定量地理解输运行为。这反映在将激子分类为Wannier-Mott激子或Frenkel激子在这些材料中并不是微不足道的事实。了解传输对于作为设备平台的材料系统的开发至关重要，因为它决定了设备的架构。为了应对这一挑战，研究团队进行了联合实验和计算研究工作，使用扩散成像显微镜，超快泵浦-探测和非线性光学技术，光致发光光谱和基于密度泛函理论的第一性原理计算，对激子能量输运性质进行了系统的研究，可以提供对激子态的见解。该团队研究了夹在六方氮化硼之间的不同碘化铅层中的激子能量传输。该材料系统使得两个独立的旋钮（i）碘化铅的厚度（ii）氮化硼的厚度，以控制激子的介电屏蔽，从而控制离域量。这种对激子态的原子级控制以及介电屏蔽提供了研究同一材料系统中局域和离域激子能量传输之间关系的机会。在这样做的过程中，它可以潜在地实现对带状或跳跃式能量传输机制的控制。对传输行为的控制具有潜在的变革性，因为它将改变电荷和/或激发态如何被用于各种设备（如光电子学、光产生、晶体管等）的规则。通过这项工作，研究团队试图控制纳米级系统之间和内部的能量流，这将促进量子信息科学、能量收集、计量学、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。

Nontechnical description: When a semiconductor absorbs a photon, an electron is excited to a higher energy state. The negatively charged excited electron and the positive charge left behind (hole) attract each other and form a bound pair known as an exciton. These excited pairs are capable of transporting energy in materials and play an important role in natural processes such as photosynthesis as well as organic devices such as photovoltaics and light-emitting diodes. Understanding the transport of excitons adds a new dimensionality to not only bolster the performance of the current excitonic devices but also introduce a platform for next-generation optoexcitonic devices. The current work studies the energy transport and its relation to the separation between the bound electron and hole pair that form the exciton. Above and beyond addressing fundamental and technological challenges, this research at the frontiers of optoelectronic and quantum materials provides an ideal venue for truly interdisciplinary education at all levels. In addition to PhD training, the results of the research findings are being incorporated into the curriculum of graduate and undergraduate courses as well as used for training of undergraduate students. The knowledge gained from this research has a potential to push forward the frontiers of research, innovations and educating the next generation of scientists and engineers for a better future. Technical description: Exciton transport in two-dimensional semiconductors has recently received significant attention due to the prospects of achieving room-temperature-stable excitonic devices. These van der Waals (vdW) semiconductors support stable room temperature excitons due to reduced dielectric screening that results in high binding energies and small Bohr radii. However, since these excitons remain delocalized over a few lattice spacings, qualitative as well as quantitative understanding of the transport behavior has been difficult. This is reflected from the fact that classifying the excitons as Wannier-Mott or Frenkel excitons is not trivial in these materials. Understanding the transport is crucial for the development of the material system as a device platform as it determines the architecture of the devices. To meet this challenge, the research team undertake a joint experimental and computational research effort using diffusion imaging microscope, ultrafast pump-probe and nonlinear optical techniques, photoluminescence spectroscopy and first-principles calculations based on density functional theory to conduct a systematic study of excitonic energy transport properties that can provide insight on the excitonic states. The team investigates excitonic energy transport in varying Lead Iodide layers that are sandwiched between hexagonal Boron Nitride. This material system enables two independent knobs (i) thickness of Lead Iodide (ii) thickness of Boron Nitride, to control the dielectric screening of the excitons and hence the amount of delocalization. Such atomic level control over the excitonic states as well as dielectric screening provides an opportunity to study the relationship between localized and delocalized excitonic energy transport in the same material system. In doing so, it could potentially enable control over band-like or hopping-like energy-transport mechanisms. Control over transport behavior is potentially transformative as it will change the rules on how charge and/or excited states are exploited for various devices such as photovoltaics, light generation, transistors, etc. Through this work, the research team attempts to gain control over the energy flow between and within nanoscale system that will enhance progress in quantum-information science, energy harvesting, metrology, and light sources.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.