DOI: 10.1002/chem.201303605

Full Paper

& Nanomaterials

Controllable Blinking-to-Nonblinking Behavior of Aqueous CdTeS Alloyed Quantum Dots Chaoqing Dong,* Heng Liu, Aidi Zhang, and Jicun Ren*[a]

Abstract: Semiconductor quantum dots (QDs) are very important optical nanomaterials with a wide range of potential applications. However, the blinking of single QDs is an intrinsic drawback for some biological and photoelectric applications based on single-dot emission. In this work, we systematically investigated the effects of certain synthetic conditions on the blinking behavior of aqueous CdTeS alloyed QDs, and observed that blinking behaviors of QDs were able to be controlled by the structure and concentration of the thiol compounds that were used as surface ligands. In opti-

Introduction Semiconductor quantum dots (QDs) are very important optical nanomaterials with a wide range of potential applications due to their excellent physical and chemical properties such as about ten times higher brightness, enhanced stability against photobleaching, and size-tunable optical properties compared with organic dyes.[1] However, QDs, at the single-particle level, show severe fluorescence intermittency (or blinking) on a wide time scale from milliseconds to minutes, switching randomly between bright (on) and dark (off) states, which is an intrinsic drawback for certain biological and photoelectric applications that rely on single-dot emission, such as single nanoparticle tracking and single-photon light sources.[2] Earlier discoveries have shown that the “dark” state could be attributed to Auger ionization in multiply excited QDs.[3] However, more recent experimental evidences[4] have revealed that blinking is a complex phenomenon, and multiple physical mechanisms seem to be related with the blinking of QDs, such as the carrier trapping model.[5] More details about blinking of QDs can be found in the review by Schwartz and Oron.[6] With the aim of understanding the mechanism of QDs blinking, significant efforts were made towards the fabrication of

[a] Dr. C. Dong, H. Liu, A. Zhang, Prof. Dr. J. Ren School of Chemistry and Chemical Engineering State Key Laboratory of Metal Matrix Composites Shanghai Jiao Tong University 800 Dongchuan Road, Shanghai 200240 (China) Fax: (+ 86) 21-5474-1297 E-mail: [email protected] [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201303605. Chem. Eur. J. 2014, 20, 1940 – 1946

mal conditions, completely nonblinking QDs were prepared using certain thiol ligands as stabilizers in aqueous phase. The suppressed blinking mechanism was mainly attributed to elimination of QDs surface traps by coordination of thiol ligands with vacant Cd atoms, formation of appropriate CdS coating on QDs, and controlling the growth dynamics of QDs. Nonblinking QDs show high quantum yield, small size, and good solubility, and will be applied to some fields that were previously limited by blinking of traditional QDs.

nonblinking QDs since the QDs blinking was reported for the first time in 1996. So far, certain methods are developed to efficiently suppress the blinking behaviors of QDs, which mainly includes the modification of QDs with small molecular ligands[7] and inorganic shell (CdS and ZnS),[8] coupling QDs to metal nanoparticles or nanostructure,[9] and a given potential applied.[4g] Mahler et al. synthesized novel CdSe–CdS core-shell QDs with a 5 nm-thick crystalline shells and their blinking can be greatly suppressed, and found that the QD blinking probability decreased monotonically with increasing shell thickness.[8a] Chen et al. also prepared “giant” nonblinking QDs by growing very thick inorganic shells onto CdSe cores.[8b] Wang et al. found that fluorescence blinking was completely suppressed by forming alloyed Cd1xZnxSe/ZnSe QDs. The suppression was suggested to be due to a soft confinement potential originating from the alloy structure that successfully slowed down the Auger process.[8c] Recently, Bawendi et al. observed that by using Cd-octanethiol as shell precursors blinking of CdSe–CdS QDs was dramatically suppressed with a relatively thin shell.[10] Ha et al. found that the blinking of QDs was nearcompletely suppressed in presence of certain thiol compounds such as beta-mercaptoethanol (BME) although in this case the tested QDs were prepared with organometallic synthetic routes.[7] However, there are very few reports on blinking behaviors and suppression of QDs prepared in aqueous solutions.[11] In the organometallic synthetic routes of QDs, few thiol compounds were directly used as the surface ligands due to the strong quenching on fluorescence of QDs. Different to the organometallic synthetic routes, in the aqueous synthetic routes of QDs sufficient short thiol compounds such as thioglycolic acid (TGA) are directly added in the precursor as surface stabilizers to prepare aqueous QDs including CdSe, CdTe, and ZnSe

1940

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper QDs.[12] Abundant experiments confirmed that the optical properties of these QDs such as PL QYs and photostability highly depended on these thiol ligands.[12b] In our previous work, it has been preliminarily observed that aqueous individual CdTe QDs directly synthesized in mercaptopropinic acid (MPA) solution showed nonblinking properties.[11] It is natural to question what roles these thiol ligands play in the blinking suppression of aqueous QDs. Herein, we systematically investigated the effects of certain synthetic conditions on blinking behaviors of CdTe QDs, and observed that blinking to nonblinking of aqueous QDs could be manipulated by adopting various thiol ligands or by changing the reaction parameters including molar ratio of thiol ligands to Cd and the precursor concentration. In optimum conditions, completely nonblinking CdTeS alloyed QDs were prepared using thiol ligands as stabilizers in aqueous phase. Furthermore, we investigated the mechanism of blinking suppression. The suppressed blinking of QDs was mainly attributed to elimination of QDs surface traps by coordination of thiol ligands with vacant Cd atoms, formation of appropriate CdS coating on QDs, and controlling the growth dynamics of QDs.

Results and Discussion Effect of the concentration of the thiol ligand and its molecular structure In the experiments, four thiol ligands (MPA, TGA, MSA, and NAC) were used as surface stabilizer to prepare the aqueous QDs. The effects of ligand concentration and molecular structure on blinking behaviors were investigated. Figure 1 a–c shows the typical PL intensity trajectories of individual QDs with TIRF imaging microscopy and its absorption and fluorescent spectra. Herein, the concentrations of MPA were controlled by changing the ratios of MPA-to-Cd (from 1.2 to 2.5) in the precursor while Cd concentration, precursor pH, and ratios of Te-to-Cd were fixed. The obtained fluorescence intensity trajectories of individual dot was recorded and analyzed according to the method described in the Experimental Section. The movies S1 and S2 in the Supporting Information are fluorescence image videos of 2.5MPA-capped QDs and 1.2MPA-capped QDs, respectively. The temporal resolution is 200 ms for 1.8MPA and 1.2MPAChem. Eur. J. 2014, 20, 1940 – 1946

capped QDs. As introduced in the Experimental Section, the threshold (blue horizontal line) was set to 3 times the standard deviation above the average dark count (red line). As shown in Figure 1 c, when the molar ratio of MPA-to-Cd is 1.2, the sharp decrease of fluorescence intensity from the maximum (“on”) to the threshold (“off”) was observed. And the duration time at “on” state was very short and these QDs were waiting at the long “off” state, which indicates that they are in the serious blinking. When the ratio of MPA-to-Cd increased to 1.8, “on” duration time for these QDs turned longer compared with 1.2MPA-capped QDs. Further, when the ratio of MPA-to-Cd increased to 2.5, the prepared QDs showed an “always” fluorescence emission (“on”) within the observation time judging from the trajectories and imaging video (Movie S1 in the Supporting Information). Herein, the temporal resolution of image video was set as 50 ms for 2.5MPA-capped QDs in order to discriminate the possible blinking behavior. And the intensity fluctuations may be further reduced in higher quality CCD camera with the shorter temporal resolution. This result demonstrates that the blinking of aqueous MPA-capped QDs can be suppressed with the increased ligand concentration, and nonblinking QDs can be obtained with 2.5 of MPA-to-Cd ratio. The power-law exponents (mon/off) from on/off time probability distributions also verified the blinking suppression process of MPA-capped QDs with the increased ligand concentration.

Figure 1. Representative fluorescence trajectories (black solid line, left column) of individual 2.5MPA-capped QD629 (a), 1.8MPA-capped QD630 (b) and 1.2MPA-capped QD632 (c) from TIFR imaging movies, and the corresponding PL and UV spectra of QDs samples (right column). The temporal resolution is 50 ms for 2.5MPA-capped QD629 and 200 ms for 1.8MPA and 1.2MPA-capped QDs. All QDs were excited by continuous 488 nm Argon ion laser with intensity of 0.9 mW that was measured in front of the microscopy objective. The saturation intensity of QD629, QD630, and QD632 are about 170 mW, 114 mW, and 86 mW, respectively. They were determined with a method described in the Supporting Information. The horizontal blue line (pointed by arrow) is employed as the threshold to define fluorescent “on” and “off” intervals that calculated based on the background (red solid line) as described in the Experimental Section.

www.chemeurj.org

1941

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper The statistics calculated from fluorescence trajectories are plotted on a log–log scale as shown in Figure 2, and the fitting results (from the statistics of many QDs) of the power-law exponents mon/off express the statistics of on and off events. As evidence in Figure 2, both the on and off events of 1.8MPA-

QDs, 2.5MSA-capped QDs and 1.2TGA-capped QDs were concerned, the “on” time fraction of more than 99 % are 100 %, which implies that these QDs are nonblinking. However, in contrast to MPA, TGA and MSA-capped QDs, QDs capped with NAC show different blinking behaviors with NAC concentration. The typical PL intensity trajectories (Figure S4 in the Supporting Information) of individual NACcapped QDs demonstrate that none of the NACcapped QDs appeared nonblinking behavior even if the ratio of NAC-to-Cd increased to 4.0 (Movies S5 and S6 in the Supporting Information). As shown in Figure 3 d, for 2.5NAC-capped QDs, the percentage of dots whose “on” time fraction less than 10 % still dominate in the whole dots and the percentage of dots whose “on” time fraction greater than 90 % is zero. Relatively, blinking suppression seemed to be enhanced in the 4.0NAC-capped QDs in that the percentage of dots whose “on” time fraction larger than 90 % has been larger than 20 % and a small subpopulation of QDs appeared nonblinking characters. Interestingly, we can not obtain nonblinking QDs using NAC ligands even if further heating reflux was applied on the 4.0NAC-capped QDs (see Figure S4 d in the Supporting Information). When the PL emission wavelength of 4.0NAC-capped QD615 increase to 678 nm with the continuing reflux, these bigger size Figure 2. Log versus log plots of “on” and “off” time length histograms compiled from 1.8MPA-capped QD630 (a) and 1.2MPA-capped QD632 (b). The lines are best-fit to QD678 (diameter: 6.5 nm) displayed less frequent [Eq. (1)]. fluorescence intermittency behavior and longer “on” time fraction relative to those smaller size QD615 (diameter: 4.2 nm) but the blinking cannot be comcapped and 1.2MPA-capped QDs follow the power-law distripletely eliminated on QDs (Figure 3 C and Figure S4 d in the butions [Eq. (1)].[3a] An obvious decrease in the exponents of Supporting Information). In the power-law exponents (Figure S5 in the Supporting Information), an obvious decrease of mon (from 2.07 (0.08) to 1.39 (0.06)) and an increase in the exmon (from 1.51 (0.05) to 1.35 (0.06)) and an increase of moff ponents of moff (from 0.96 (0.08) to 1.75(0.07)) were observed as the ratio increase of MPA-to-Cd from 1.2 to 1.8. The de(from 1.55(0.06) to 1.77(0.05)) with the increasing size. The crease of mon and increase of moff demonstrated the slower decay of the “on” time distribution but faster decay of the “off” time distribution. It is in coincidence with the observed phenomena in Figure 1 b–c, that 1.8MPA-capped QDs are usually at longer “on” and shorter “off” state. These results document that the blinking behaviors can be controlled by the concentration of the thiol ligands. Also, the similar phenomena of blinking suppression with ligand concentration were observed in the TGA and MSA-capped QDs. Under the optimum conditions, the completely nonblinking TGA-capped QDs (Movie S3 in the Supporting Information) and MSA-capped QDs (Movie S4 in the Supporting Information) can also successfully be prepared with 1.2 of TGA-to-Cd ratio and 2.5 of MSA-to-Cd ratio similar to MPA-capped QDs (Figures S1 and S2 in the Supporting Information). Blinking statistics results also verified the similar blinking suppression process (Figure S3 in the Supporting Information). It implies that the influence of these ligands on the blinking behaviors (mon/off) is identical. Figure 3. Distribution of “on” time fractions of QDs constructed from analysis on PL intensity trajectories of 120  10 single dots. The compared QDs Figure 3 summarizes the effects of four thiol ligands on the were prepared using different thiol ligands: MPA (a), TGA (b), MSA (c), and blinking behaviors. For MPA, MSA, and TGA-capped QDs, the NAC (d) with different molar ratios of thiol-to-Cd. The numeric character in “on” time fraction of more than 90 % greatly increased with the inset of d (“QD608”) represents the PL emission peak position of NACthe increased ratio of thiol-to-Cd, and as far as 2.5MPA-capped capped QDs. Chem. Eur. J. 2014, 20, 1940 – 1946

www.chemeurj.org

1942

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper size-dependent exponents are also observed in the CdSe and InP QDs.[13] Ito et al. believed that the blinking suppression induced by size growth could be attributed to the electron tunneling effect between a QD and the matrix.[13a] Unfortunately, the size growth of NAC-capped QDs cannot completely block the tunneling of charge carrier into a trap state on the surface of the QD. The above results illustrate that the blinking behaviors are dependent on the structure and concentration of thiol ligands. Effect of the concentration of the precursor To understand the possible effects of the concentration of the precursor on blinking, different QDs prepared with different concentrations of precursor were investigated including nonblinking QDs (1.2TGA-capped QDs) and blinking QDs (2.5NAC-capped QDs). The statistical analysis (Figure 4) revealed that for 2.5NAC-capped QDs blinking behavior still existed in the whole samples and they show weak dependence on precursor concentrations. In the 1.2TGAcapped QDs, few dots (7 %) appear blinking when the precursor concentration is reduced

Scheme 1. Schematic illustration of the blinking suppression mechanism of aqueous QDs. The blinking to nonblinking QDs can be manipulated by controlling the reaction conditions.

Figure 4. Distribution of “on” time fractions of 2.5NAC-capped QDs (a) and 1.2TGA-capped QDs (b) constructed from analysis on PL intensity trajectories of 120  10 single dots. QDs were prepared with different precursor concentration (2.5 mm, 5.0 mm and 10.0 mm).

to 2.5 mm. This result suggests that the low precursor concentration is not favorable for the preparation of nonblinking QDs. It can be attributed to ineffective passivation of nanocrystal surface and the increased surface defects. So far, certain models, such as the classical Auger recombination model (AR),[3a] the modified AR model,[14] and the multiple recombination center model[15] were proposed to explain the blinking mechanism of QDs. Although a precise blinking mechanism is not fully understood, blinking is generally considered to be from a QD charging process in which an electron (or a hole) is temporarily lost to the surrounding matrix through Auger recombination or captured to surface-related Chem. Eur. J. 2014, 20, 1940 – 1946

trap states. Regardless of the differences of these blinking mechanisms, they emphasize the important role of charge-carrier traps in regulating the photoluminescence of individual QDs. The surface traps of QDs mainly include stacking faults, vacancies, dangling bonds, and so on.[16] Therefore, eliminating the surface traps of QDs is a key step to suppress the QDs blinking. The above studies documented that the blinking behaviors are dependent on the structure and concentrations of thiol ligands. The mechanism for blinking suppression was probably attributed to the elimination of QDs surface traps by thiol ligands, which was realized in the following ways (Scheme 1):

www.chemeurj.org

1) The decomposition of thiol compounds could slowly release active “S”, and then “S” bound to surface trap sites of QDs.[12b, 17] 2) The thiol compounds might directly bind the surface traps of QDs by coordination of Cd and S in the thiol compounds. 3) The thiol compounds were used as surface ligands to control the growth dynamics of QDs and stabilize QDs in water, which led to reduction of the QDs surface traps. In order to investigate the possible function of thiol ligand and released “S” on the suppression, XPS was used to quantitatively measure the surface composition of blinking and nonblinking QDs. As shown in Figure 5, the binding energies of 572.8, 405.6, and 162.8 eV correspond to Te 3d, Cd 3d, and S 2p levels, respectively. In the measurements, the atomic sensitivity factors of S, Cd, and Te have been taken into consideration. The determined ratios among the S, Cd, and Te can be found in Table 1. As shown in Table 1, the ratios of S-to-Cd in QDs change with the concentration of thiol ligands. The different ratios of S-to-Cd reflect the difference of bound thiol ligands and S content in the QDs. In the experiment, the sizes of QDs samples capped with same ligands ensure to be identical in order to avoid possible effect of QDs sizes on compositions. As shown in Figure S6, their autocorrelation curves almost completely overlap, and their hydrodynamic diameters should be regarded to be identical (less than 6 nm) as according to the Einstein–Stokes equation they are positively proportional to

1943

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper

Figure 5. XPS data of 2.5MPA-capped QD629 (a), 1.2MPA-capped QD632 (b), and 1.2TGA- capped QD599 (c), 0.3TGA-capped QD598 (d), 2.5NAC-capped QD614 (e), 1.2NAC-capped QD608 (f), 2.5MSA-capped QD596 (g), and 1.2MSA-capped QD601 (h).

Table 1. The calculated atomic ratio of Te/Cd, S/Cd and (Te + S)/Cd by XPS data. Samples [a]

2.5MPA 1.2MPA[b] 1.2TGA[c] 0.3TGA[d] 2.5NAC[e] 1.2NAC[f] 2.5MSA[g] 1.2MSA[h]

PL QYs [%]

Blinking state

Te/Cd

S/Cd

(Te+S)/Cd

63.2 56.3 46.6 40.8 60.7 43.5 55.1 64.0

nonblinking blinking nonblinking blinking blinking blinking nonblinking nonblinking

0.36 0.24 0.44 0.45 0.18 0.31 0.31 0.27

0.87 0.70 0.61 0.50 0.90 0.61 0.98 0.79

1.23 0.94 1.05 0.95 1.08 0.92 1.29 1.06

[a] 2.5MPA-capped QD629. [b] 1.2MPA-capped QD632. [c] 1.2TGAcapped QD599. [d] 0.3TGA-capped QD598. [e] 2.5NAC-capped QD614. [f] 1.2NAC-capped QD608. [g] 2.5MSA-capped QD596. [h] 1.2MSA-capped QD601.

the diffusion times.[18, 19] XPS results indicated that the samples prepared under higher concentration thiols displayed higher ratio of S-to-Cd in the surface atomic composition of QDs and the structure of QDs was actually an S-rich CdTeS alloys. As mentioned above, the S in QDs can be explained in two ways: 1) The decomposition of thiol compounds could slowly release active “S”, and then “S” bound to surface trap sites of QDs as CdS coating. 2) The thiol compounds directly bound to surface traps of QDs by coordination of Cd and S in the thiol compounds. In the aqueous synthesis of QDs, the thiol ligands are Chem. Eur. J. 2014, 20, 1940 – 1946

www.chemeurj.org

strongly coordinated with Cd atoms via the CdS bond, which is much different to the weak coordination such as that of myristic acid with metal atoms in the organometallic synthetic routes.[13b] The strong coordination effect was regarded to more effectively quench the surface traps of the QDs by electron donation.[8d, 9a] As shown in Table 1, when the (Te + S)/Cd ratios of samples were less than 1.0, QDs showed blinking. In this case, some Cd atoms in QDs remain to be vacant due to lack coordination of sufficient Te or S, and the remaining Cd vacancies are served as channels for efficient charge tunneling from the interior of the core to the surrounding matrix as reported by Mulvaney et al.[20] When large amounts of thiols were bound to the vacant sites, thiols can donate an electron to the trap state and render inaccessible to electrons ejected from the core of QDs and the blinking process is suppressed. The above results suggest that the increase in the amount of thiol ligands can saturate Cd vacancies and remove surface traps, and effectively block these channels and thereby suppress the blinking. Meanwhile, the structures of 2.5NAC-capped QD655 (blinking) and 2.5MSA-capped QD651 (nonblinking) were characterized by TEM method. TEM images (Figure 6) reveal that both of them belong to a predominant cubic (zinc blende) structure, which is accordance with the powder XRD characterization (Figure S7 in the Supporting Information). And the XRD peaks and the lattice plane distances ([111], ~ 3.60 ) derived from the TEM images confirmed that the structure was the inter-

1944

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper

Figure 6. Representative TEM overviews of 2.5MSA-capped QD651 (a) and 2.5NAC-capped QD655 (b). The scale bar is 5 nm.

mediate between cubic CdTe and cubic CdS phases or an alloyed CdTeS structure.[12b] However, it is difficult to discriminate the difference between the inner nanocrystal structure of 2.5NAC-capped QDs and 2.5MSA-capped QDs. It suggests that the thiol coordination and CdS coating processes induced by different ligands have little influence on their inner crystal structure.[21] But these happened in the QDs interface is one of the important ways that can effectively remove the surface traps and suppress the blinking. Interestingly, although the ratios of S-to-Cd in the 2.5NACcapped QDs samples are high, these QDs still show blinking behaviors. This result suggests that another factor controls the blinking behavior of QDs besides the mechanism of blinking suppression above. The surface traps of QDs should be associated with the growth dynamics of QDs. In the aqueous synthesis, the growth dynamics of QDs are significantly controlled by the concentration and the structure of thiol ligands. We investigated the effects of the concentration and the structure of thiol ligands on the growth dynamics of QDs. As shown in Figure 7 a, the growth rate of QDs slowed down with an increase

ligands-capped QDs, and this result can explain the reason why NAC-capped QDs appeared the serious blinking characters (Figure S4 in the Supporting Information). MPA, TGA, and MSA have similar molecular structures with the same terminal groups of thiol and carboxyl, but NAC is a derivative of l-cysteine in that one of terminal group is substituted by acetyl. Herein the possible reason for different growth rates of these QDs should be attributed to the nature of thiol layer on the QDs due to their different structure. According to the classical model of La Mer, the nanocrystals grow via coalescence and fusion of smaller clusters.[24,25] When NAC was used as capping ligands, the terminal groups of NAC-capped clusters can coordinate with the Cd atoms of other clusters by forming a more stable pentagonal configuration. The coordination favored the coalescence and fusion of clusters, so remarkably accelerated the growth of QDs. As a result, more surface traps were formed in the fast growth.

Conclusion

In this work, we systematically investigated the blinking behaviors of CdTeS alloyed QDs prepared under variable reaction parameters using different thiol ligands, and observed that blinking behavior of aqueous QDs was able to be controlled by the structure and concentration of thiol ligands. In optimum conditions, completely nonblinking QDs were prepared using thiol ligands as stabilizers in aqueous phase. The mechanism for blinking suppression was probably attributed to the elimination of QDs surface traps by thiol ligands, which was realized in the following ways. 1) The decomposition of thiol compounds could slowly release active “S”, and then “S” could bind the surface trap sites of QDs. 2) The thiol compounds might directly bind the surface traps of QDs through the coordination of surface Cd with S in the thiol compounds. 3) The thiol compounds as surface ligands were used to control the growth dynamics of QDs and stabilize QDs in water, which led to reduction of the QDs surface Figure 7. The temporal growth of MPA-capped QDs prepared with different ratio of MPA-to-Cd (a) and growth dytraps. Nonblinking QDs show namics comparison of MPA, TGA, NAC and MSA- capped QDs in the 90 8C water bath (b). The PL emission peak high quantum yield, small size, positions were measured from the samples extracted at different reaction time interval excited with 380 nm. and good solubility, and will be applied to some fields that were of thiol ligands. In consideration of the results of Figure 3 a, previously limited by the blinking of traditional QDs. Our this result hinted that the growth rate had positive relationship method can also be extended to the synthesis of other nonwith blinking behavior. The fast growth rate of QDs leads to blinking QDs. the generation of surface traps, and it is consistent with the reported result that the moderate growth rate of QDs is important to cut down the generation of surface traps and improve Experimental Section the quality of QDs.[22, 23] Meanwhile, the influences of growth Fluorescence trajectories from single QDs were acquired by total dynamics on blinking can also be observed on other thiolsinternal reflection fluorescence (TIRF) imaging system based on an capped QDs. Figure 7 b shows the effects of the structure of Olympus IX 71 inverted fluorescence microscope. To extract the thiol ligands on the growth dynamics of QD. The growth rate probability distributions for both “on” and “off” duration times for of 2.5NAC-capped QDs is remarkably faster than that of other QDs, the threshold level is assigned as three times the standard deChem. Eur. J. 2014, 20, 1940 – 1946

www.chemeurj.org

1945

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper viation (3s) above the average background counts. The average background counts are obtained from five blank dots with the same pixel (6  6) to bright QDs in one TIRF imaging video. A software program is compiled with MATLAB to acquire both “on” and “off” duration times by analyzing the PL intensity trajectories of single QD gained from MetaMorph software. The probability distributions of on and off duration times and percentage of QDs with different “on” duration times are calculated based on PL intensity trajectories of about 120  10 bright dots. Then the probability distributions are fit to a power law of the form[3a] [Eq. (1)].

Pðton =toff Þ1ðton =toff Þm

ð1Þ

Acknowledgements This work was financially supported by NFSC (20905048, 21075081, 21135004 and 21327004), Innovation Program of Shanghai Municipal Education Commission (14ZZ024), Shanghai Educational Development Foundation (2008CG12) and SMC-Chenxin Young Scholar project sponsored by Shanghai Jiao Tong University. Keywords: fluorescence · nonblinking · quantum dots · suppression mechanism · thiol ligands [1] a) M. C. S. V. L. Colvin, M. C. Schlamp, A. P. Alivisatos, Nature 1994, 370, 354 – 357; b) S. Coe, W. K. Woo, M. Bawendi, V. Bulovic, Nature 2002, 420, 800 – 803; c) W. U. Huynh, J. J. Dittmer, A. P. Alivisatos, Science 2002, 295, 2425 – 2427. [2] B. O. D. M. Nirmal, M. G. Bawendi, J. J. Macklin, J. K. Trautman, T. D. Harris, L. E. Brus, Nature 1996, 383, 802 – 804. [3] a) A. L. Efros, M. Rosen, Phys. Rev. Lett. 1997, 78, 1110 – 1113; b) K. T. Shimizu, R. G. Neuhauser, C. A. Leatherdale, S. A. Empedocles, W. K. Woo, M. G. Bawendi, Phys. Rev. B 2001, 63, 205316; c) M. Kuno, D. P. Fromm, H. F. Hamann, A. Gallagher, D. J. Nesbitt, J. Chem. Phys. 2000, 112, 3117 – 3120. [4] a) E. Hendry, M. Koeberg, F. Wang, H. Zhang, C. D. Donega, D. Vanmaekelbergh, M. Bonn, Phys. Rev. Lett. 2006, 96, 057408; b) P. Spinicelli, S. Buil, X. Quelin, B. Mahler, B. Dubertret, J. P. Hermier, Phys. Rev. Lett. 2009, 102, 136801; c) P. P. Jha, P. Guyot-Sionnest, ACS Nano 2009, 3, 1011 – 1015; d) D. E. Gomz, J. van Embden, P. Mulvaney, M. J. Fernee, H. Rubinsztein-Dunlop, ACS Nano 2009, 3, 2281 – 2287; e) S. Rosen, O. Schwartz, D. Oron, Phys. Rev. Lett. 2010, 104, 157404; f) J. Zhao, G. Nair, B. R. Fisher, M. G. Bawendi, Phys. Rev. Lett. 2010, 104, 157403; g) C. Galland, Y. Ghosh, A. Steinbruck, M. Sykora, J. A. Hollingsworth, V. I. Klimov, H. Htoon, Nature 2011, 479, 203 – 208. [5] a) P. A. Frantsuzov, R. A. Marcus, Phys. Rev. B 2005, 72, 155321; b) P. A. Frantsuzov, S. Volkan-Kacso, B. Janko, Phys. Rev. Lett. 2009, 103, 207402; c) J. Tang, R. A. Marcus, Phys. Rev. Lett. 2005, 95, 107401.

Chem. Eur. J. 2014, 20, 1940 – 1946

www.chemeurj.org

[6] O. Schwartz, D. Oron, Isr. J. Chem. 2012, 52, 992 – 1001. [7] S. Hohng, T. Ha, J. Am. Chem. Soc. 2004, 126, 1324 – 1325. [8] a) B. Mahler, P. Spinicelli, S. Buil, X. Quelin, J. P. Hermier, B. Dubertret, Nat. Mater. 2008, 7, 659 – 664; b) Y. Chen, J. Vela, H. Htoon, J. L. Casson, D. J. Werder, D. A. Bussian, V. I. Klimov, J. A. Hollingsworth, J. Am. Chem. Soc. 2008, 130, 5026 – 5027; c) X. Y. Wang, X. F. Ren, K. Kahen, M. A. Hahn, M. Rajeswaran, S. Maccagnano-Zacher, J. Silcox, G. E. Cragg, A. L. Efros, T. D. Krauss, Nature 2009, 459, 686 – 689; d) T. D. Krauss, J. J. Peterson, J. Phys. Chem. Lett. 2010, 1, 1377 – 1382; e) B. Chon, S. J. Lim, W. Kim, J. Seo, H. Kang, T. Joo, J. Hwang, S. K. Shin, Phys. Chem. Chem. Phys. 2010, 12, 9312 – 9319. [9] a) C. T. Yuan, P. Yu, J. Tang, Appl. Phys. Lett. 2009, 94, 243108; b) N. I. Hammer, K. T. Early, K. Sill, M. Y. Odoi, T. Emrick, M. D. Barnes, J. Phys. Chem. B 2006, 110, 14167 – 14171. [10] O. Chen, J. Zhao, V. P. Chauhan, J. Cui, C. Wong, D. K. Harris, H. Wei, H. S. Han, D. Fukumura, R. K. Jain, M. G. Bawendi, Nat. Mater. 2013, 12, 445 – 451. [11] H. He, H. F. Qian, C. Q. Dong, K. L. Wang, J. C. Ren, Angew. Chem. 2006, 118, 7750 – 7753; Angew. Chem. Int. Ed. 2006, 45, 7588 – 7591. [12] a) A. L. Rogach, A. Kornowski, M. Y. Gao, A. Eychmuller, H. Weller, J. Phys. Chem. B 1999, 103, 3065 – 3069; b) N. Gaponik, D. V. Talapin, A. L. Rogach, K. Hoppe, E. V. Shevchenko, A. Kornowski, A. Eychmuller, H. Weller, J. Phys. Chem. B 2002, 106, 7177 – 7185; c) H. Zhang, L. P. Wang, H. M. Xiong, L. H. Hu, B. Yang, W. Li, Adv. Mater. 2003, 15, 1712 – 1715; d) H. F. Qian, C. Q. Dong, J. F. Weng, J. C. Ren, Small 2006, 2, 747 – 751; e) N. Ma, E. H. Sargent, S. O. Kelley, Nat. Nanotechnol. 2009, 4, 121 – 125; f) A. L. Rogach, T. Franzl, T. A. Klar, J. Feldmann, N. Gaponik, V. Lesnyak, A. Shavel, A. Eychmuller, Y. P. Rakovich, J. F. Donegan, J. Phys. Chem. C 2007, 111, 14628 – 14637. [13] a) Y. Ito, K. Matsuda, Y. Kanemitsu, Phys. Status Solidi C 2009, 6, 221 – 223; b) F. Zan, C. Q. Dong, H. Liu, J. C. Ren, J. Phys. Chem. C 2012, 116, 3944 – 3950. [14] R. Verberk, A. M. van Oijen, M. Orrit, Phys. Rev. B 2002, 66, 233202. [15] P. A. Frantsuzov, S. Volkan-Kacso, B. Janko, Nano Lett. 2013, 13, 402 – 408. [16] Z. Xu, M. Cotlet, Small 2012, 8, 253 – 258. [17] H. B. Bao, Y. J. Gong, Z. Li, M. Y. Gao, Chem. Mater. 2004, 16, 3853 – 3859. [18] C. Q. Dong, R. Bi, H. F. Qian, L. Li, J. C. Ren, Small 2006, 2, 534 – 538. [19] S. Doose, J. M. Tsay, F. Pinaud, S. Weiss, Anal. Chem. 2005, 77, 2235 – 2242. [20] D. E. Gmez, J. van Embden, J. Jasieniak, T. A. Smith, P. Mulvaney, Small 2006, 2, 204 – 208. [21] A. L. Rogach, Mater. Sci. Eng. B 2000, 69, 435 – 440. [22] C. L. Wang, H. Zhang, S. H. Xu, N. Lv, Y. Liu, M. J. Li, H. Z. Sun, J. H. Zhang, B. Yang, J. Phys. Chem. C 2009, 113, 827 – 833. [23] L. Qu, X. Peng, J. Am. Chem. Soc. 2002, 124, 2049 – 2055. [24] H. Zhang, D. Wang, B. Yang, H. Mohwald, J. Am. Chem. Soc. 2006, 128, 10171 – 10180. [25] V. K. La Mer, R. H. Dinegar, J. Am. Chem. Soc. 1950, 72, 4847 – 4854.

Received: September 11, 2013 Revised: November 21, 2013 Published online on January 15, 2014

1946

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Controllable blinking-to-nonblinking behavior of aqueous CdTeS Alloyed quantum dots.

Semiconductor quantum dots (QDs) are very important optical nanomaterials with a wide range of potential applications. However, the blinking of single...
2MB Sizes 0 Downloads 0 Views