Surface Charge Control of Quantum Dot Blinking

A characteristic property of colloidal semiconductor nanocrystal quantum dots (QDs) is their emission intermittency. Although a unifying theory of QD photoprocesses remains elusive, the importance of charged states is clear. We now report a new approach to directly study the role of surface charge on QD emission by adding metal ions to individual, core-only QDs immobilized in aqueous solution in an agarose gel. The CdTe QDs show very stable emission in the absence of metal ions but a dramatic and reversible increase in blinking due to the presence of trivalent metal ions. Our results support a charge-separation model, in which the major blinking pathway is the surface trapping of electrons; transiently bound metal ions close to the QD surface enhance this process. ■ INTRODUCTION Photoluminescence (PL) intermittency, also termed blinking, is a universal feature of emitters ranging from small molecular fluorophores, fluorescent proteins, and conjugated polymers to nanoscale emitters such as semiconductor quantum dots (QDs) and diamond nanocrystals. Until recently, the blinking of QDs was primarily seen as a disadvantage, especially for applications such as particle tracking and solar cells where long-lived dark states are unwanted. In contrast, the emergence of super-resolution imaging techniques that utilize the stochastic switching of fluorescence between “on” and “off” states has meant that blinking QDs are also highly desirable. The resistance to photobleaching, together with other desirable optical properties, gives blinking QDs a distinct advantage in such applications. It is well established that the PL intensity for QDs randomly jumps between highly emissive “on” states and nonemissive “off’ states under continuous excitation. Rather than displaying two-state on/off dynamics, the on states display a continuous distribution of emissive states. The distribution of on and off times span up to 6 orders of magnitude, corresponding to almost 9 orders of magnitude in probability density, which are generally assigned to power law kinetics. Many studies have also reported truncated power law behavior, though the exact form of on/off distributions is sensitive to the methods employed for data analysis. There is also recent evidence that the apparent power law distributions are due to a superposition of exponential functions. Therefore, in spite of nearly two decades of experimental studies of the photophysical and charge transport properties of QDs, the mechanism of blinking in QDs is still the subject of vigorous debate. It is possible that several different mechanisms are operating in parallel, depending on the particular sample, environment, and experimental conditions. Nevertheless, there has been an attempt to find a universal blinking mechanism. The first model that gained acceptance involved the long-time charging of a single QD core, with subsequent excited-state energy lost nonradiatively via Auger recombination. However, various studies have questioned this model. The recently developed multiple recombination centers (MRC) model invokes multiple surface hole traps with fluctuating trapping rates. This has been recently extended to include additional electron trapping pathways, in part to accommodate experimental evidence that QD blinking can be altered through surface or solution modifications. More recent experiments have confirmed the existence of delayed emission, which can only be explained by longlived charge-separated states; a modified version of the Auger quenching model was proposed, with a suggested assignment of the electron as the trapped carrier. One of the reasons that it has been so difficult to develop a detailed blinking mechanism for QDs is the difficulty in correlating bulk charge distributions with specific surface processes. Important previous studies have involved adjusting the bulk solution environment with a change in pH, via electrochemical methods, by intraparticle charge transfer by altering the bulk matrix, by adding a shell or capping ligands, or by applying external electric fields. In this work, we describe a new experimental approach to directly probe the effect of surface-localized charge on the QD emission by immobilizing core-only QDs in agarose gel and adding metal ions in aqueous solution (Figure 1a). We attribute the blinking to the occupation of surface traps enabled by kinetically labile positive metal ions bound to the agarose close to the QD surface. Our data are consistent with a model in which blinking results from the surface trapping of electrons. Received: August 2, 2016 Revised: August 5, 2016 Published: August 8, 2016 Article