International Collaboration in Chemistry: Mechanism for Cation Exchange and Doping in Semiconducting Nanostructures

来自加州大学伯克利分校的Phillip Geissler获得了由大分子、超分子和纳米化学计划以及化学理论、模型和计算方法计划颁发的国际化学合作奖；他的合作者特拉维夫大学的Eran Rabani同时得到了美国-以色列双国科学基金会的支持。该项目的动机是，在过去的20年里，化学家在极小规模上制造材料的能力发生了一场革命，特别是以“纳米晶体”(晶体材料的纳米尺寸摘录)的形式。然而，要实现纳米晶体的巨大技术前景，需要比目前更多地控制它们的成分、形状、结构和相对排列。这个项目涉及最近发现的一种获得这种控制的方法：将纳米晶体中的一组离子与一组不同的化学离子交换，特别是在不改变纳米晶体结构的情况下。人们对这种“阳离子交换”的机理、能量学、动力学和物理后果知之甚少，这极大地限制了它的精确应用。该项目使用理论和计算工具来开发和探索阐明离子交换的化学基础的模型。它进一步评估了交换对与理想的技术进步相关的电子性质的影响。该项目的具体活动包括：(I)建立用于探索掺杂和离子交换反应机制的模型，(Ii)开发和应用探索这些模型的长期行为所需的最先进的计算技术，以及(Iii)评估由此产生的电子性质。通过与电子结构计算和实验测量的合理比较，建立了相互作用模型。分子动力学模拟结合跃迁路径采样等技术被用来研究杂质/阳离子如何从溶液进入纳米晶体，它们如何在主体晶格中扩散，以及它们如何与主体原子交换位置。为了计算出射电子性质，发展了适用于掺杂材料和异质结的半经验赝势模型。最后，一个重掺杂纳米晶体的理论正在构建中，该理论遵循多重杂质的类氢模型，以在基本水平上阐明带的排列、激子的局域化、电子-空穴相互作用，以及这些性质如何依赖于尺寸、形状和材料的选择。

Phillip Geissler from the University of California Berkeley is supported in an International Collaboration in Chemistry award by the Macromolecular, Supramolecular and Nanochemistry program and the Chemical Theory, Models and Computational Methods program; his collaborator, Eran Rabani at Tel Aviv University, is simultaneously supported by the U.S.-Israel Binational Science Foundation. The project is motivated by the fact that the past 20 years have seen a revolution in chemists' ability to make materials on extremely small scales, notably in the form of "nanocrystals" (nanometer-sized excerpts of crystalline materials). Realizing the immense technological promise of nanocrystals, however, requires much more control over their composition, shape, structure, and relative arrangements than is currently possible. This project concerns a recently discovered route towards gaining control of this kind: exchanging one set of ions in a nanocrystal with a chemically distinct set of ions, remarkably without altering the nanocrystal's structure. The mechanism, energetics, dynamics, and physical consequences of such "cation exchange" are very poorly understood, greatly limiting its precise application. The project uses tools of theory and computation to develop and explore models that shed light on the chemical underpinnings of ion exchange. It further evaluates the consequences of exchange on electronic properties relevant to desirable technological advances. Specific activities of this project include (i) construction of models for exploring reaction mechanisms of doping and ion exchange, (ii) development and application of state-of-the-art computational techniques needed to explore the long-time behaviors of these models, and (iii) the assessment of resulting electronic properties. Interaction models are built through judicious comparison with electronic structure calculations and experimental measurements. Molecular dynamics simulations combined with techniques such as transition path sampling are employed to examine how impurities/cations enter the nanocrystal from solution, how they diffuse through the host lattice, and how they exchange places with host atoms. In order to calculate emergent electronic properties, semiempirical pseudopotential models adequate for doped materials and heterostructures are developed. Finally, a theory for heavily doped nanocrystals in the spirit of a "hydrogenic-like" model for multiple impurities is being constructed to elucidate at a fundamental level the alignment of bands, localization of excitons, electron-hole interactions, and how these properties depend on the size, shape, and choice of materials.