Non-blinking (Zn)CuInS/ZnS Quantum Dots Prepared by In Situ Interfacial Alloying Approach

Semiconductor quantum dots (QDs) are very important optical nanomaterials with a wide range of potential applications. However, blinking behavior of single QD is an intrinsic drawback for some biological and photoelectric applications based on single-particle emission. Herein we present a rational strategy for fabrication of non-blinking (Zn)CuInS/ZnS QDs in organic phase through in situ interfacial alloying approach. This new strategy includes three steps: synthesis of CuInS QDs, eliminating the interior traps of QDs by forming graded (Zn)CuInS alloyed QDs, modifying the surface traps of QDs by introducing ZnS shells onto (Zn)CuInS QDs using alkylthiols as sulfur source and surface ligands. The suppressed blinking mechanism was mainly attributed to modifying QDs traps from interior to exterior via a step-by-step modification. Non-blinking QDs show high quantum yield, symmetric emission spectra and excellent crystallinity, and will enable applications from biology to optoelectronics that were previously hindered by blinking behavior of traditional QDs.


Supplementary Experimental Section
The measurement of quantum yields The PL quantum yields (QY) of various samples were measured by using Rhodamine 6G (R6G) as a reference fluorescent dye with the known QY (95%) and comparing the integrated fluorescence intensity of the solutions, both record exciting samples having the same absorbance (<0.1 au in order to minimize possible reabsorption effects). The PL QYs of the as-prepared QDs were calculated using the following equations: QY sample =QY R6G × (grad sample /grad R6G ) × (n sample /n R6G ) 2 Where grad stands for the gradient (slope) of the plot of the integrated fluorescence intensity vs absorbance, and n stands for the refractive index of the solvent (1.36 for ethanol, 1.37 for hexane, and 1.50 for toluene). The excitation wavelength was set at 450 nm. PL QYs of the QDs were calculated by comparing their integrated emissions with that of Rhodamine 6G (QY of ～95%) ethanol solution with an identical optical density of ～0.05 at 450 nm 1-3 .

The determination of single-particle emissions
In this study, we design two feasibility scenarios to identify the detected light spot came from a single dot. Firstly, the dispersion procedure of single particles is feasible.
In the individual nanoparticle detection experiment, the good dispersibility of the QDs is the necessary prerequisite for the total internal reflection fluorescence imaging. We try to prepared nanocrystal samples by spin-casting a dilute solution of CuInS based QDs diluted in a chloroform/poly(methylmethacrylate) solution (PMMA, the concentration range from 0.5%-6.0% gmL -1 ). Unfortunately, the QDs were seriously aggregated and we cannot get uniform light spot imaging ( Supplementary Fig. 30).
Pure organic solvent (toluene) or mixed organic solvent (90% hexane-10% octane) was used in single particle disperse system for blinking behavior studies 4 . In an open environment without isolating material, the luminescent process of a single QD was detected under a TIRFM. This process can rule out the possibility of PMMA spheres during the spray of polymer solution over the pretreated glass substrate 5 . Thankfully, individual QDs were shown to be homogeneously dispersed in the fluorescence images, and no aggregation was observed in this cases. We observed that the (Zn)CuInS alloyed QDs synthesized with different stoichiometric ratios of cations exhibited blinking or non-blinking behavior. It should be noted that the blinking behavior is an important feature of single individual nanoparticle 6 . This result also indicated that our dispersion procedure has good generality for single particle detection of thiol-modified QDs.
At the same time, we explored thiol-modified CdSe/CdS QDs with similar size and surface ligands composition to assess the reliability of our dispersion solutions.
Thiol-modified CdSe/CdS QDs were prepared by using our previous method 3 . These QDs at least were covered with 1-dodecanethiol as surface ligand layer. The QDs were well separated and no aggregation is observed in this case ( Supplementary Fig.   31a). These QDs exhibit severe blinking and long periods of off time ( Supplementary   Fig. 31b). This result further indicates that our dispersion procedure has good generality for single particle detection of thiol-modified QDs. Additionally, we tried to test the second-order photon correlation (or antibunching) experiments performed with a time-correlated single-photon counting system to confirm that all measurements were associated with a single dot 7 .
Unfortunately, the signal to noise ratio of (Zn)CuInS/ZnS QDs was not high enough to obtain antibunching curves even though the QDs exhibited good photostability. This is because that the size of (Zn)CuInS/ZnS QDs is smaller and their BPP