Jansen Maximilian, Juranyi Fanni, Yarema Olesya, Seydel Tilo, Wood Vanessa
Department of Information Technology and Electrical Engineering, ETH Zurich, Gloriastrasse 35, CH-8092 Zurich, Switzerland.
Laboratory for Neutron Scattering and Imaging, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland.
ACS Nano. 2021 Dec 28;15(12):20517-20526. doi: 10.1021/acsnano.1c09073. Epub 2021 Dec 8.
Nanocrystal surfaces are commonly populated by organic ligands, which play a determining role in the optical, electronic, thermal, and catalytic properties of the individual nanocrystals and their assemblies. Understanding the bonding of ligands to nanocrystal surfaces and their dynamics is therefore important for the optimization of nanocrystals for different applications. In this study, we use temperature-dependent, quasi-elastic neutron scattering (QENS) to investigate the dynamics of different surface bound alkanethiols in lead sulfide nanocrystal solids. We select alkanethiols with mono- and dithiol terminations, as well as different backbone types and lengths. QENS spectra are collected both on a time-of-flight spectrometer and on a backscattering spectrometer, allowing us to investigate ligand dynamics in a time range from a few picoseconds to nanoseconds. Through model-based analysis of the QENS data, we find that ligands can either (1) precess around a central axis, while simultaneously rotating around their own molecular axis, or (2) only undergo uniaxial rotation with no precession. We establish the percentage of ligands undergoing each type of motion, the average relaxation times, and activation energies for these motions. We determine, for example, that dithiols which link facets of neighboring nanocrystals only exhibit uniaxial rotation and that longer ligands have higher activation energies and show smaller opening angles of precession due to stronger ligand-ligand interactions. Generally, this work provides insight into the arrangement and dynamics of ligands in nanocrystal solids, which is key to understanding their mechanical and thermal properties, and, more generally, highlights the potential of QENS for studying ligand behavior.
纳米晶体表面通常覆盖有有机配体，这些配体对单个纳米晶体及其聚集体的光学、电子、热学和催化性能起着决定性作用。因此，了解配体与纳米晶体表面的键合及其动力学对于优化纳米晶体以用于不同应用非常重要。在本研究中，我们使用温度相关的准弹性中子散射（QENS）来研究硫化铅纳米晶体固体中不同表面结合的链烷硫醇的动力学。我们选择了具有单硫醇和二硫醇端基、以及不同主链类型和长度的链烷硫醇。在飞行时间光谱仪和背散射光谱仪上都收集了QENS光谱，这使我们能够研究从几皮秒到纳秒时间范围内的配体动力学。通过对QENS数据进行基于模型的分析，我们发现配体可以（1）围绕中心轴进动，同时围绕其自身分子轴旋转，或者（2）仅进行无进动的单轴旋转。我们确定了进行每种运动类型的配体百分比、平均弛豫时间以及这些运动的活化能。例如，我们确定连接相邻纳米晶体晶面的二硫醇仅表现出单轴旋转，并且较长的配体具有较高的活化能，并且由于更强的配体 - 配体相互作用而显示出较小的进动张角。总的来说，这项工作深入了解了纳米晶体固体中配体的排列和动力学，这是理解其机械和热性能的关键，更普遍地说，突出了QENS在研究配体行为方面的潜力。
