Podder Chinmoy, Gong Xiangtao, Pan Heng
J. Mike Walker '66 Department of Mechanical Engineering, Texas A&M University, College Station, TX, 77843, USA.
Department of Mechanical and Aerospace Engineering, Missouri University of Science and Technology, Rolla, MO, 65401, USA.
Small. 2021 Dec;17(50):e2103436. doi: 10.1002/smll.202103436. Epub 2021 Oct 7.
The carrier excitation, relaxation, energy transport, and conversion processes during light-nanocrystal (NC) interactions have been intensively investigated for applications in optoelectronics, photocatalysis, and photovoltaics. However, there are limited studies on the non-equilibrium heating under relatively high laser excitation that leads to NCs sintering. Here, the authors use femtosecond laser two-pulse correlation and in-situ optical transmission probing to investigate the non-equilibrium heating of NCs and transient sintering dynamics. First, a two-pulse correlation study reveals that the sintering rate strongly increases when the two heating laser pulses are temporally separated by <10 ps. Second, the sintering rate is found to increase nonlinearly with laser fluence when heating with ≈700 fs laser pulses. By three-temperature modeling, the NC sintering mechanism mediated by electron induced ligand transformation is suggested. The ultrafast and non-equilibrium process facilitates sintering in dry (spin-coated) and wet (solvent suspended) environments. The nonlinear dependence of sintering rate on laser fluence is exploited to print sub-diffraction-limited features in NC suspension. The smallest feature printed is ≈200 nm, which is ≈¼ of the laser wavelength. These findings provide a new perspective toward nanomanufacturing development based on probing and engineering ultrafast transport phenomena in functional NCs.
为了实现光电子学、光催化和光伏领域的应用，人们对光与纳米晶体（NC）相互作用过程中的载流子激发、弛豫、能量传输和转换过程进行了深入研究。然而，关于在相对较高激光激发下导致NC烧结的非平衡加热的研究却很有限。在此，作者利用飞秒激光双脉冲关联和原位光透射探测来研究NC的非平衡加热和瞬态烧结动力学。首先，双脉冲关联研究表明，当两个加热激光脉冲在时间上间隔小于10皮秒时，烧结速率会大幅增加。其次，在用约700飞秒激光脉冲加热时，发现烧结速率随激光能量密度呈非线性增加。通过三温模型，提出了由电子诱导配体转变介导的NC烧结机制。这种超快和非平衡过程在干燥（旋涂）和潮湿（溶剂悬浮）环境中都有利于烧结。利用烧结速率对激光能量密度的非线性依赖关系，在NC悬浮液中打印出亚衍射极限特征。打印出的最小特征约为200纳米，约为激光波长的四分之一。这些发现为基于探测和调控功能化NC中超快传输现象的纳米制造发展提供了新的视角。
