Effectiveness of Ultrathin Al2O3 Capping by Atomic Layer Deposition on the Stability of Single Quantum Dots

Abstract

Quantum dots (QDs) possess the capability to function as single photon sources, and their size-dependent optical properties offer a broad range of applications in nanophotonics. However, photo-bleaching and oxidation lead to the degradation of QDs at room temperature, presenting one of the main obstacles to applications. Here, we enhance the stability of single core-shell QDs at room temperature and shield them from the air by using 0.5, 0.7, and 1nm thick capping layers of Al2O3 deposited via atomic layer deposition (ALD). This also enables tip-enhanced strong coupling (TESC) spectroscopy by allowing the formation of a subnanometer plasmonic cavity between a nano-optical antenna and a metal film below the QD. After being exposed to a continuous-wave laser beam of 2.01 mW power for 3.5 hours, photoluminescence (PL) microscopy of QDs under the 0.5nm capping layer demonstrated no reduction in fluorescent QD number compared to a 51% reduction in the number of fluorescent QDs in a sample without such protection. This protection method opens up new possibilities for increasing the fluorescent lifetime of other types of QDs, such as less stable perovskite quantum dots and studying the effect of new capping materials on emitter-plasmon coupling.