Exciton-Delocalizing Ligands Can Speed Up Energy Migration in Nanocrystal Solids
Exciton-Delocalizing Ligands Can Speed Up Energy Migration in Nanocrystal Solids
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DOI：
10.1021/acs.nanolett.8b01079
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发表时间：
2018-05-01
期刊：
影响因子：
10.8
通讯作者：
Roberts, Sean T.
中科院分区：
文献类型：
--
作者：
Azzaro, Michael S.;Dodin, Amro;Roberts, Sean T.
长期以来，研究人员一直在寻求使用表面配体，通过减少纳米晶体之间的物理分离和加强它们的电子耦合来增强纳米固体中的能量迁移。激子-离域配体，其具有与能带边缘态强烈混合的前沿分子轨道，非常适合这种作用，因为它们可以促进载波功能扩展到能带核心之外，减少能量转移的障碍。本报告详细介绍了使用激子离域配体苯基二硫代氨基甲酸酯（PDTC）来调整激子在CdSe固体中的传输速率和扩散长度。由油酸酯封端的CdSe纳米晶体组成的膜经受固态配体交换以用PDTC替换油酸酯。激子在薄膜中的迁移随后研究飞秒瞬态吸收。我们的实验表明，PDTC处理的dielectric膜导致快速（类似于400 fs）下坡能量迁移（类似于80毫电子伏），而没有这样的迁移发生在油酸帽膜。动力学蒙特卡罗模拟使我们能够提取率和长度尺度的激子扩散在PDTC处理的薄膜。这些模拟再现了在一定温度范围内的瞬态吸收测量中观察到的动态，并确认激子通过米勒-亚伯拉罕机制跳跃。重要的是，我们的实验和模拟表明，PDTC处理的激子跳跃率增加到200 fs，5个数量级的改善相对于油酸帽膜。该激子跳跃速率是对CdSe固体测定的最快的速率之一。激子离域配体的固态交换所提供的简便，室温加工和改善的输运性质表明，它们为强耦合双金属阵列的构建提供了希望。
Researchers have long sought to use surface ligands to enhance energy migration in nanocrystal solids by decreasing the physical separation between nanocrystals and strengthening their electronic coupling. Exciton-delocalizing ligands, which possess frontier molecular orbitals that strongly mix with nanocrystal band-edge states, are well-suited for this role because they can facilitate carrier-wave function extension beyond the nanocrystal core, reducing barriers for energy transfer. This report details the use of the exciton-delocalizing ligand phenyldithiocarbamate (PDTC) to tune the transport rate and diffusion length of excitons in CdSe nanocrystal solids. A film composed of oleate-terminated CdSe nanocrystals is subjected to a solid-state ligand exchange to replace oleate with PDTC. Exciton migration in the films is subsequently investigated by femtosecond transient absorption. Our experiments indicate that the treatment of nanocrystal films with PDTC leads to rapid (similar to 400 fs) downhill energy migration (similar to 80 meV), while no such migration occurs in oleate-capped films. Kinetic Monte Carlo simulations allow us to extract both rates and length scales for exciton diffusion in PDTC-treated films. These simulations reproduce dynamics observed in transient absorption measurements over a range of temperatures and confirm excitons hop via a Miller-Abrahams mechanism. Importantly, our experiments and simulations show PDTC treatment increases the exciton hopping rate to 200 fs, an improvement of 5 orders of magnitude relative to oleate-capped films. This exciton hopping rate stands as one of the fastest determined for CdSe solids. The facile, room-temperature processing and improved transport properties offered by the solid-state exchange of exciton-delocalizing ligands show they offer promise for the construction of strongly coupled nanocrystal arrays.