Spin-dependent exciton quenching and intrinsic spin coherence in CdSe/CdS nanocrystals

TL;DR

This study uses spin-resonance techniques to differentiate trap states in CdSe/CdS nanocrystals, revealing spin-dependent exciton quenching mechanisms and demonstrating long spin coherence times at low temperatures.

Contribution

It introduces a spin-resonance approach to identify and control trap states in nanocrystals, advancing understanding of their luminescence and blinking behaviors.

Findings

Identification of two trap states: spin-dependent Auger process and charge-separated pair.

Demonstration of long spin coherence times (>300 ns) at 3.5 K.

Control of energy transfer within heterostructures via paramagnetic trap centers.

Abstract

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 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 energy transfer from the wide-gap CdS to the narrow-gap CdSe, i.e. 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.