Department of Physics and Astronomy, University of Utah, 115 South 1400 East, Salt Lake City, Utah 84112, USA.
Nano Lett. 2013 Jan 9;13(1):65-71. doi: 10.1021/nl303459a. Epub 2012 Dec 7.
Large surface-to-volume ratios of semiconductor nanocrystals cause susceptibility to charge trapping, which can modify luminescence yields and induce single-particle blinking. Optical spectroscopies cannot differentiate between bulk and surface traps in contrast to spin-resonance techniques, which in principle avail chemical information on such trap sites. Magnetic resonance detection via spin-controlled photoluminescence enables the direct observation of interactions between emissive excitons and trapped charges. This approach allows the discrimination of three radical species located in two functionally different trap states in CdSe/CdS nanocrystals, underlying the fluorescence quenching and thus blinking mechanisms: a spin-dependent Auger process in charged particles; and a charge-separated state pair process, which leaves the particle neutral. The paramagnetic trap centers offer control of the energy transfer yield from the wide-gap CdS to the narrow-gap CdSe, that is, light harvesting within the heterostructure. Coherent spin motion within the trap states of the CdS arms of nanocrystal tetrapods is reflected by spatially remote luminescence from CdSe cores with surprisingly long coherence times of >300 ns at 3.5 K, illustrating coherent control of light harvesting.
半导体纳米晶体的大的表面积与体积比导致其容易发生电荷捕获，这可能会改变发光产率并引起单粒子闪烁。与自旋共振技术相比，光学光谱技术无法区分体相和表面陷阱，而自旋共振技术原则上可以提供有关这些陷阱位置的化学信息。通过自旋控制的光致发光进行磁共振检测，可以直接观察发射激子与捕获电荷之间的相互作用。这种方法可以区分位于 CdSe/CdS 纳米晶体中两个功能不同的陷阱态中的三种自由基，阐明荧光猝灭和闪烁机制：带电粒子中的自旋相关的俄歇过程；以及电荷分离态对过程，该过程使粒子保持中性。顺磁陷阱中心可以控制从宽能隙的 CdS 到窄能隙的 CdSe 的能量转移产率，即在异质结构内进行光捕获。纳米四角体的 CdS 臂的陷阱态中的相干自旋运动反映在 CdSe 核的空间上远程发光中，在 3.5 K 时，相干时间超过 300 ns，令人惊讶，说明了光捕获的相干控制。
