Metal nanocrystals exhibit important optoelectronic and photocatalytic functionalities in response to light. These dynamic energy conversion processes have been commonly studied by transient optical probes to date, but an understanding of the atomistic response following photoexcitation has remained elusive. Here, we use femtosecond resolution electron diffraction to investigate transient lattice responses in optically excited colloidal gold nanocrystals, revealing the effects of nanocrystal size and surface ligands on the electron-phonon coupling and thermal relaxation dynamics. First, we uncover a strong size effect on the electron-phonon coupling, which arises from reduced dielectric screening at the nanocrystal surfaces and prevails independent of the optical excitation mechanism (i.e., inter- and intraband). Second, we find that surface ligands act as a tuning parameter for hot carrier cooling. Particularly, gold nanocrystals with thiol-based ligands show significantly slower carrier cooling as compared to amine-based ligands under intraband optical excitation due to electronic coupling at the nanocrystal/ligand interfaces. Finally, we spatiotemporally resolve thermal transport and heat dissipation in photoexcited nanocrystal films by combining electron diffraction with stroboscopic elastic scattering microscopy. Taken together, we resolve the distinct thermal relaxation time scales ranging from 1 ps to 100 ns associated with the multiple interfaces through which heat flows at the nanoscale. Our findings provide insights into optimization of gold nanocrystals and their thin films for photocatalysis and thermoelectric applications.

中文翻译：

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超快电子衍射和光学散射显微镜监测胶态金纳米晶体的非平衡热力学。

金属纳米晶体响应光表现出重要的光电和光催化功能。迄今为止，瞬态光学探针已经普遍研究了这些动态能量转换过程，但是对光激发后的原子响应的理解仍然难以捉摸。在这里，我们使用飞秒分辨率电子衍射研究光学激发的胶体金纳米晶体中的瞬态晶格响应，揭示了纳米晶体尺寸和表面配体对电子-声子耦合和热弛豫动力学的影响。首先，我们发现了对电子-声子耦合的强烈尺寸效应，这是由于纳米晶表面的介电屏蔽减少所致，并且普遍存在，与光激发机制（即带内和带内）无关。第二，我们发现表面配体充当热载流子冷却的调节参数。特别地，由于在纳米晶体/配体界面处的电子耦合，与基于胺的配体相比，在带内光激发下，具有基于硫醇的配体的金纳米晶体显示出显着更慢的载流子冷却。最后，我们通过结合电子衍射和频闪弹性散射显微镜在时空上解决了光激发纳米晶体薄膜中的热传递和散热问题。综上所述，我们解决了与热在纳米尺度上流过的多个界面相关的从1 ps到100 ns的不同热弛豫时间尺度。我们的发现为光催化和热电应用中金纳米晶体及其薄膜的优化提供了见识。