Department of Chemistry and Biochemistry, University of Colorado Boulder , Boulder, Colorado 80309, United States.
Nano Lett. 2017 Jun 14;17(6):3764-3774. doi: 10.1021/acs.nanolett.7b01101. Epub 2017 Jun 1.
Type-II and quasi type-II heterostructure nanocrystals are known to exhibit extended excited-state lifetimes compared to their single material counterparts because of reduced wave function overlap between the electron and hole. However, due to fast and efficient hole trapping and nonuniform morphologies, the photophysics of dot-in-rod heterostructures are more rich and complex than this simple picture. Using transient absorption spectroscopy, we observe that the behavior of electrons in the CdS "rod" or "bulb" regions of nonuniform ZnSe/CdS and CdSe/CdS dot-in-rods is similar regardless of the "dot" material, which supports previous work demonstrating that hole trapping and particle morphology drive electron dynamics. Furthermore, we show that the longest lived state in these dot-in-rods is not generated by the type-II or quasi type-II band alignment between the dot and the rod, but rather by electron-hole dissociation that occurs due to fast hole trapping in the CdS rod and electron localization to the bulb. We propose that specific variations in particle morphology and surface chemistry determine the mechanism and efficiency of charge separation and recombination in these nanostructures, and therefore impact their excited-state dynamics to a greater extent than the heterostructure energy level alignment alone.
II 型和准 II 型异质结构纳米晶体的激子态寿命比它们的单一组分材料长得多，这是因为电子和空穴的波函数重叠减少了。然而，由于空穴的快速和有效捕获以及不均匀的形态，点-棒异质结构的光物理比这个简单的图像要丰富和复杂得多。通过瞬态吸收光谱，我们观察到在非均匀 ZnSe/CdS 和 CdSe/CdS 点-棒异质结构中，CdS“棒”或“球”区域中的电子行为与“点”材料无关，这支持了先前的工作，证明了空穴捕获和颗粒形态驱动电子动力学。此外，我们表明，这些点-棒异质结构中寿命最长的状态不是由点和棒之间的 II 型或准 II 型能带排列产生的，而是由于 CdS 棒中的空穴快速捕获和电子局域到球而发生的电子-空穴解离。我们提出，颗粒形态和表面化学的特定变化决定了这些纳米结构中电荷分离和重组的机制和效率，因此比异质结构能级排列更能影响它们的激发态动力学。
