Effect of the thiol-thiolate equilibrium on the photophysical properties of aqueous CdSe/ZnS nanocrystal quantum dots

We study the effect of thiols on the emission efficiency of aqueous CdSe/ZnS core/shell nanocrystal quantum dots (NQDs). We observe that the impact of thiol addition on emission quantum yields (QYs) is time-, concentration-, and pH-dependent. Further, we use a combination of time-resolved spectroscopic methods to determine the mechanism by which thiol addition can cause either increases or decreases in QYs. Specifically, through transient absorption measurements, we show that thiol addition can improve passivation of electron traps, increasing QYs at low thiol concentrations. Further, using nanosecond photoluminescence (PL), we observe that at higher thiol concentrations, hole traps are introduced that reduce PL QYs. Last, through a combination of pH-dependence and control studies (e.g., addition of 2-methyl thioethanol to an aqueous NQD solution and addition of dodecanethiol to a hexane NQD solution), we demonstrate that it is the ability of thiols in aqueous solution to generate thiolate that is the source of both favorable and adverse QY changes. Our results contribute to the understanding of the role of surface ligands, which is critical to the design of stable, high-quantum-yield, nonblinking NQDs.