Analysis of Trap-State Dynamics of Single CdSe ... - ACS Publications

Article pubs.acs.org/JPCC

Analysis of Trap-State Dynamics of Single CdSe/ZnS Quantum Dots on a TiO2 Substrate with Different Nb Concentrations Yuki Nagao, Hideki Fujiwara,* and Keiji Sasaki* Research Institute for Electronic Science, Hokkaido University, N20, W10, Kitaku, Sapporo, Hokkaido 001-0020, Japan ABSTRACT: Using the photon interdetection time analysis method, we investigated the influence of Nb doping in a TiO2 substrate on the excited- and trap-state dynamics of single quantum dots (QDs) on the TiO2 substrate. From the results, we observed that although the lifetime of the excited state remained almost unchanged, the blinking behavior and the recovery rate from the trap state were modified depending on the Nb concentration. Therefore, we conjectured that these observed phenomena could be ascribed to the change in the electron density in the TiO2 substrate, which would cause the single QDs to change from the positive charged state back to the neutral state.

■

INTRODUCTION Single semiconductor quantum dots (QDs), which have the advantages of high quantum yield and wide emission wavelength selectivity, are valuable materials for composing small optoelectronic devices such as light-emitting devices,1,2 solar cells,3 photocatalysts,4 and transistors.5 To realize efficient optoelectronic devices, in which QDs are generally dispersed on a conductive substrate, it is necessary to understand the interactions between single QDs and a substrate (i.e., energy transfer and charge transfer). On the other hand, single QDs exhibit fluorescence intermittence (blinking), which is an intrinsic characteristic of single nanoemitters. Because the blinking behavior is very sensitive to the local environment (host materials, solutions, and substrates),6−13 the analysis of the emission dynamics of single QDs on various substrates should give important knowledge for improving the performance of the optoelectronic devices. Many studies on the influence of various substrates on the blinking behavior, such as gold thin films,14−17 rough silver nanoparticles,18,19 and indium tin oxide (ITO)4,20−26 and titanium dioxide (TiO2)27−30 nanoparticle films, have been reported. Among the various substrates, we paid the most attention to TiO2 substrates because of their importance for industrial applications especially in the fields of solar cells and photocatalysts. Jin et al. recently reported that, on a TiO2 nanoparticle film, blinking was accelerated and the excited-state lifetime was decreased due to the electron transfer from single QDs to TiO2 nanoparticles.27−29 On the other hand, Hamada et al. observed blinking suppression of single QDs with a TiO2 nanoparticle dispersed solution, which was attributed to isolated TiO2 nanoparticles preventing the trapped charge carrier hopping among TiO2 nanoparticles and accelerating the recovery from the trap (ionized) state to the neutral state by back electron transfer.30 In these studies, the influence of © 2014 American Chemical Society

substrates on the excited-state dynamics of single QDs has been well investigated, but there is little information from the viewpoint of the trap state. In addition to the excited-state dynamics, the substrate should also affect the trap-state dynamics because the capture of photocarriers by the QD surface or outside trap sites (positively charged state) has been considered as the origin of the blinking.10 For the trap-state analysis of single QDs, we have used the photon interdetection time (PIT) analysis method16,31,32 and recently reported the electric field modulation of blinking behavior in single QDs between two ITO substrates with a thin PMMA spacer layer.26 From the PIT analysis, we suggested that the voltage applied modulated not only the excited-state dynamics but also the trap-state dynamics by directly measuring the recovery rate from the trap state. In this study, as is the case in our previous investigation, we considered that because Nb doping in a TiO2 substrate can change the electron density in the TiO2 substrate, it could also possibly modify the trap-state dynamics as well as the excitedstate dynamics of single QDs on a Nb-doped TiO2 substrate. Therefore, using the PIT analysis method, we investigated how Nb doping modified the emission dynamics of single QDs on a TiO2 substrate with different Nb concentrations. From the results, we confirmed that, depending on the Nb concentration, the blinking behavior and trap-state dynamics changed, whereas the excited-state lifetime remained unchanged. Thus, we concluded that the modification of the trap-state dynamics performs an important role in the blinking behavior, which could arise from the change in the electron density of TiO2 substrates due to the Nb doping. Received: January 30, 2014 Revised: August 19, 2014 Published: August 19, 2014 20571

dx.doi.org/10.1021/jp501096q | J. Phys. Chem. C 2014, 118, 20571−20575

The Journal of Physical Chemistry C

Article

Figure 1. (A) Photon correlation measurements, (B) fluorescence intensity fluctuations, and (C) fluorescence intensity histograms of single CdSe/ ZnS QDs on different substrates: (a) glass, (b) nondoped TiO2, (c) 0.05 wt % Nb-doped TiO2, and (d) 0.5 wt % Nb-doped TiO2. Red and blue lines in (A) indicate the experimental data and exponential functions fitted to each histogram, respectively.

■

EXPERIMENTAL SECTION

trap state to the ground state (kt). Under the weak excitation conditions, the histogram of PIT (d(t)) can be expressed as a double-exponential function:16,31,32

TiO2 substrates with different Nb concentrations were commercially available (Shinkosha, rutile TiO2(110) single crystals, size 10 × 10 × 0.5 mm, Nb concentration 0, 0.05, and 0.5 wt %). Before sample preparation, the glassware and TiO2 substrates were cleaned by sonication in acetone, detergent, and ultrapure water. Then CdSe/ZnS quantum dots (QSP630, Ocean NanoTech, octadecylamine surface group, powder form) were dispersed in a toluene solution to dilute the concentration and spin coated onto each substrate. To adjust the QD concentration, we diluted the QD dispersed toluene solution until we confirmed approximately 5 single QDs in a 10 × 10 μm2 area on each substrate by observation with a confocal microscope. The prepared TiO2 substrates were adhered on a glass substrate (size 24 × 50 × 0.15 mm) by an optical adhesive, and then the glass substrate was mounted on the piezo stage that was set on the confocal microscope stage (IX70, Olympus). The QD dispersed surface of the TiO 2 substrates was faced toward an objective lens, and the excitation laser light (wavelength 488 nm, Showa Optronics) was focused on the sample surface by an objective lens (100×, NA = 0.9, excitation intensity 140 W/cm2).16 Emitted photons from single QDs were divided by a 50% beam splitter and detected by two single-photon-counting modules (SPCMs; SPCMAQR-14, EG&G). To make a histogram of PIT and fluorescence intensity time traces with a desired time resolution, we recorded the time intervals between adjacent detection pulses from the two SPCMs using a time interval analyzer (PCI-6602, National Instruments, time resolution 12.5 ns).16,31,32 At the same time, to estimate the excited-state lifetime (τ) of single QDs, we also measured the photon correlation (g(2)(t)) using a time-correlated single-photon counting module (SPC-430, Becker and Hickl).7 For PIT analysis, a three-level system of single QDs (ground, excited, and trap states) was assumed with the rates of fluorescence (kf), nonradiative deactivation (knr), transfer from the excited state to the trap state (kET), and recovery from the

d(t ) = A 0 exp[−(Ion + Ib)t ] + A1 exp[−(k t + Ib)t ]

(1)

where A0 and A1 are arbitrary constants and Ib is the background count. Ion = Φφfkex represents the photon count rate when a QD is in the on state, where Φ, kex, and φf are the detection efficiency, the excitation rate, and the quantum fluorescence yield (= kf/(kf + knr + kET)). Thus, we can obtain Ion and kt by fitting the PIT histogram.

■

RESULTS AND DISCUSSION Figure 1A shows the photon correlation measurements (g(2)(t)) of single QDs on (a) glass and (b−d) TiO2 substrates with Nb concentrations of 0, 0.05, and 0.5 wt %. Red and blue lines represent the experimental data (binning time 235 ps) and fitting results with exponential functions. From the results, because g(2)(0) ≈ 0 for all cases, single QDs were observed on each substrate and the excited-state lifetimes (τ) were estimated to be (a) 41, (b) 18, (c) 16, and (d) 18 ns. We found that, on each TiO2 substrate, the excited-state lifetimes did not show an obvious change regardless of the different Nb concentrations, whereas the lifetimes on the TiO2 substrates became shorter than that on the glass substrate. To investigate the blinking behavior on each substrate, the fluorescence intensity time traces of the same single CdSe/ZnS QDs are simultaneously shown in Figure 1B, in which the binning time for each data point was 100 ms. Also, Figure 1C shows fluorescence intensity histograms calculated from the data in Figure 1B (binning count 50). The intensities in the on state were about (a) 4.0 × 104, (b) 1.8 × 104, (c) 1.8 × 104, and (d) 2.5 × 104 counts/s for each substrate. We found that, compared with the intensity on the glass substrate, the intensity on the nondoped TiO2 substrate became smaller and slightly recovered on the Nb-doped TiO2 substrates. In addition, on the glass and nondoped TiO2 substrates, blinking behavior was 20572

dx.doi.org/10.1021/jp501096q | J. Phys. Chem. C 2014, 118, 20571−20575

The Journal of Physical Chemistry C

Article

decay components (kt), which exhibit the recovery rates of single QDs on different substrates, were estimated to be (a) 1.4 × 103, (b) 1.1 × 103, (c) 1.9 × 103, and (d) 2.4 × 103 s−1. From the results, we found that kt on the nondoped TiO2 substrate was not evidently changed compared with that on the glass substrate, whereas kt values on the Nb-doped TiO2 substrates increased depending on the Nb concentration. To clarify the influence of the Nb doping, we measured about 30 single QDs for each substrate. In Figure 3, the scatter

clearly observed. These observed results on the nondoped TiO2 substrate (shorter excited lifetime, blinking behavior, decrease in intensity) were similar to the results reported in previous papers,27−29 in which the electron transfer from QDs to TiO2 was responsible for the shorter lifetime and decrease in the intensity. On the other hand, on the Nb-doped TiO2 substrates, the blinking seemed to be suppressed because the probability of a lower count level evidently decreased in the histograms of the Nb-doped TiO2 substrates. Furthermore, the blinking behavior was further suppressed on the 0.5 wt % Nb-doped TiO2 substrate than on the 0.05 wt % Nb-doped TiO2 substrate because the peak at higher level on the 0.5 wt % Nb-doped TiO2 substrate was narrower than that on the 0.05 wt % Nbdoped TiO2 substrate. Thus, from these results, because the blinking behavior strongly depended on the Nb concentration, we assumed that both the excited- and trap-state dynamics would be affected. To examine the recovery rate (kt) of single QDs on each substrate, PIT analysis for the same QDs in Figure 1 were performed, and the results are shown in Figure 2. In the figure,

Figure 3. Scatter plots of (a) τ vs kt, (b) Ion vs kt, and (c) τ vs Ion for different Nb concentrations. Blue circles, red squares, and green triangles indicate the results for TiO 2 substrates with Nb concentrations of 0, 0.05, and 0.5 wt %, respectively. The number of single QDs that we measured was about 30 for each substrate.

plots of (a) τ vs kt, (b) Ion vs kt, and (c) τ vs Ion are represented (blue circles, red squares, and green triangles indicate the results for the TiO2 substrates with Nb concentrations of 0, 0.05, and 0.5 wt %, respectively). Vertical and horizontal bars in the figure indicate the typical variations in each measurement. In addition, we also summarize the averaged values of τ, Ion, and kt in Table 1. From the results in Table 1, the average excitedstate lifetime and fluorescence intensity decreased on each TiO2

Figure 2. Histograms of photon interdetection times emitted from single CdSe/ZnS QDs on each substrate: (a) glass, (b) nondoped TiO2, (c) 0.05 wt % Nb-doped TiO2, (d) 0.5 wt % Nb-doped TiO2. Red and blue lines indicate the experimental data and fitting results using eq 1.

red and blue lines indicate the experimental data (binning time 12.5 μs, accumulation time 300 s) and the fitting function calculated from eq 1. Either the fast (Ion) or slow (kt) decay component depended on the conditions of each substrate. The fast decay rates (Ion), which show the fluorescence intensity in the on state of single QDs, were estimated to be (a) 4.1 × 104, (b) 1.8 × 104, (c) 2.1 × 104, and (d) 2.5 × 104 counts/s. We found that these values were consistent with the counting rates of the time traces in Figure 1B. Moreover, the second slow

Table 1. Averaged Values of Ion, kt, and τ Evaluated from about 30 Single CdSe/ZnS QDs for Each Substrate Ion/104 (s−1) glass nondoped TiO2 0.05 wt % Nb-doped TiO2 0.5 wt % Nb-doped TiO2 20573

3.5 1.6 2.2 2.4

± ± ± ±

0.8 0.2 0.3 0.3

kt/103 (s−1) 1.4 1.2 1.7 2.3

± ± ± ±

0.2 0.2 0.2 0.3

τ (ns) 32 17 16 16

± ± ± ±

6 4 3 4

dx.doi.org/10.1021/jp501096q | J. Phys. Chem. C 2014, 118, 20571−20575

The Journal of Physical Chemistry C

Article

single-molecule spectroscopy and the PIT analysis method can provide useful knowledge for the potential applications of QDs on conductive or semiconductor substrates.

substrate, compared with those on glass. The decrease in the excited-state lifetime on the TiO2 substrates suggests that the TiO2 substrates affected the excited-state dynamics, which corresponded to the results reported by Jin et al.27,28 However, the fluorescence intensities on the Nb-doped TiO2 substrates were increased depending on the Nb concentration, whereas the excited-state lifetime was almost unchanged. Since the emission intensity and lifetime would typically have a proportional relationship, the inconsistency between τ and Ion on the Nb-doped TiO2 substrates seemed to be strange, while the ratios of Ion and τ on the nondoped TiO2 against those on glass corresponded well with each other and were evaluated to be about 44% from the data in Figures 1 and 2. Although the reason for this inconsistency is not clear yet, we conjecture that, as the surface conditions (permittivity, conductivity) of the TiO2 substrates were changed by the Nb concentration, the excitation conditions of QDs in the vicinity of the surface were changed. Furthermore, the average recovery rates from the trap state (kt) did not change much compared those on the glass and nondoped TiO2 substrates, which increased with the Nb concentration. Similar tendencies were clearly observed in the scatter plots of Figure 3, in which, with increasing Nb concentration, we found that the fluorescence intensity (Ion) and the recovery rates from the trap state (kt) clearly increased, although variations of individual QDs on each substrate were observed. However, no clear dependence of the excited-state lifetime (τ) on the Nb concentration was observed. Thus, because we found that the recovery rates obviously changed depending on the Nb concentration and the change in the recovery rates corresponded to the modification of the blinking behaviors in Figure 1, we concluded that the Nb doping in the TiO2 substrate affected the trap-state dynamics as well as the excited-state dynamics. Because the excited-state lifetime did not change much and QDs in the off state are considered to be positively charged,9,10 the transfer rate from the excited state to the trap state would not change depending on the Nb concentration and a positively charged state of single QDs would occur for all TiO2 substrates with different Nb concentrations. Therefore, on the nondoped TiO2 substrate, the blinking behavior was observed. However, as the substrate conductance becomes larger (resistivity >107 Ω·cm for 0 wt %, 5 Ω·cm for 0.05 wt %, and 0.3 Ω·cm for 0.5 wt %) due to an increase in the Nb concentration, the recovery rate from the trap state increases. This change would quickly cause the single QDs to change from the positive charged state back to the neutral state, via a mechanism that is similar to that of blinking suppression by a reducing agent,33 whereas the transfer rate from the excited state to the trap state would remain unchanged regardless of the Nb concentration. Therefore, blinking with a binning time of 100 ms is suppressed when the Nb concentration increases.

■

AUTHOR INFORMATION

Corresponding Authors

*Phone: +81-11-706-9395. E-mail: [email protected]. *Phone: +81-11-706-9396. E-mail: [email protected] Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI Grants 22681011, 24651111, and 23246016.

■

REFERENCES

(1) Nakajima, H.; Ekuni, S.; Kumano, H.; Idutsu, Y.; Miyamura, S.; Kato, D.; Ida, S.; Sasakura, H.; Suemune, I. Strongly Suppressed MultiPhoton Generation from a Single Quantum Dot in Metal-Embedded Structure. Phys. Status Solidi C 2011, 23, 337−339. (2) Sun, Q.; Wang, Y. A.; Li, L. S.; Wang, D.; Zhu, T.; Xu, J.; Yang, C.; Li, Y. Bright, Multicoloured Light-Emitting Diodes Based on Quantum Dots. Nat. Photonics 2007, 1, 717−722. (3) Robel, I.; Subramanian, V.; Kuno, M.; Kamat, P. V. Quantum Dot Solar Cells. Harvesting Light Energy with CdSe Nanocrystals Molecularly Linked to Mesoscopic TiO2 Films. J. Am. Chem. Soc. 2006, 128, 2385−2393. (4) Dibbell, R. S.; Soja, G. R.; Hoth, R. M.; Watson, D. F. Photocatalytic Patterning of Monolayers for the Site-Selective Deposition of Quantum Dots onto TiO2 Surfaces. Langmuir 2007, 23, 3432−3439. (5) Koh, W. K.; Saudari, S. R.; Fafarman, A. T.; Kagan, C. R.; Murray, C. B. Thiocyanate-Capped PbS Nanocubes: Ambipolar Transport Enables Quantum Dot Based Circuits on a Flexible Substrate. Nano Lett. 2011, 11, 4764−4767. (6) Michler, P.; Imamoglu, A.; Mason, M. D.; Carson, P. J.; Strouse, G. F.; Buratto, S. K. Quantum Correlation among Photons from a Single Quantum Dot at Room Temperature. Nature 2000, 406, 968− 970. (7) Lounis, B.; Bechtel, H. A.; Gerion, D.; Alivisatos, P.; Moerner, W. E. Photon Antibunching in Single CdSe/ZnS Quantum Dot Fluorescence. Chem. Phys. Lett. 2000, 329, 399−404. (8) Kuno, M.; Fromm, D. P.; Hamann, H. F.; Gallagher, A.; Nesbitt, D. J. Nonexponential “Blinking” Kinetics of Single CdSe Quantum Dots: A Universal Power Law Behavior. J. Chem. Phys. 2000, 112, 3117−3120. (9) Brokmann, X.; Messin, G.; Desbiolles, P.; Giacobino, E.; Dahan, M.; Hermier, J. P. Colloidal CdSe/ZnS Quantum Dots as SinglePhoton Sources. New J. Phys. 2004, 6, 99. (10) Frantsuzov, P.; Kuno, M.; Jankó, B.; Marcus, R. A. Universal Emission Intermittency in Quantum Dots, Nanorods and Nanowires. Nat. Phys. 2008, 4, 519−522. (11) Wang, X.; Ren, X.; Kahen, K.; Hahn, M. A.; Rajeswaran, M.; Maccagnano-Zacher, S.; Silcox, J.; Cragg, G. E.; Efros, A. L.; Krauss, T. D. Non-Blinking Semiconductor Nanocrystals. Nature 2009, 459, 686−689. (12) Fomenko, V.; Nesbitt, D. J. Solution Control of Radiative and Nonradiative Lifetimes: A Novel Contribution to Quantum Dot Blinking Suppression. Nano Lett. 2008, 8, 287−293. (13) Bharadwaj, P.; Novotny, L. Robustness of Quantum Dot PowerLaw Blinking. Nano Lett. 2011, 11, 2137−2141. (14) Ito, Y.; Matsuda, K.; Kanemitsu, Y. Mechanism of Photoluminescence Enhancement in Single Semiconductor Nanocrystals on Metal Surfaces. Phys. Rev. B 2007, 75, 033309. (15) Matsuda, K.; Ito, Y.; Kanemitsu, Y. Photoluminescence Enhancement and Quenching of Single CdSe/ZnS Nanocrystals on

■

CONCLUSIONS We investigated the influence of Nb doping on the emission dynamics of single CdSe/ZnS quantum dots on a TiO2 substrate with different Nb concentrations using singlemolecule spectroscopy and the PIT analysis method. We observed that, depending on the Nb concentration, the trapstate dynamics was modified, while the excited-state lifetime did not change. Therefore, we conjectured that the Nb doping affected the trap-state dynamics and accelerated the recovery rate from the trap (ionized) state to the neutral state, which is the reason for the blinking suppression. Thus, we believe that 20574

dx.doi.org/10.1021/jp501096q | J. Phys. Chem. C 2014, 118, 20571−20575

The Journal of Physical Chemistry C

Article

Metal Surfaces Dominated by Plasmon Resonant Energy Transfer. Appl. Phys. Lett. 2008, 92, 211911. (16) Fujiwara, H.; Ohta, H.; Chiba, T.; Sasaki, K. Temporal Response Analysis of Trap States of Single CdSe/ZnS Quantum Dots on a Thin Metal Substrate. J. Photochem. Photobiol., A 2011, 221, 160−163. (17) Shimizu, K. T.; Woo, W. K.; Fisher, B. R.; Eisler, H. J.; Bawendi, M. G. Surface-Enhanced Emission from Single Semiconductor Nanocrystals. Phys. Rev. Lett. 2002, 89, 117401. (18) Masuo, S.; Naiki, H.; Machida, S.; Itaya, A. Photon Statistics in Enhanced Fluorescence from a Single CdSe/ZnS Quantum Dot in the Vicinity of Silver Nanoparticles. Appl. Phys. Lett. 2009, 95, 193106. (19) Naiki, H.; Masuo, S.; Machida, S.; Itaya, A. Single-Photon Emission Behavior of Isolated CdSe/ZnS Quantum Dots Interacting with the Localized Surface Plasmon Resonance of Silver Nanoparticles. J. Phys. Chem. C 2011, 115, 23299−23304. (20) Park, S.; Link, S.; Miller, W. L.; Gesquiere, A.; Barbara, P. F. Effect of Electric Field on the Photoluminescence Intensity of Single CdSe Nanocrystals. Chem. Phys. 2007, 341, 169−174. (21) Qin, W.; Shah, R. A.; Guyot-Sionnest, P. CdSeS/ZnS Alloyed Nanocrystal Lifetime and Blinking Studies under Electrochemical Control. ACS Nano 2012, 6, 912−918. (22) Jin, S.; Song, N.; Lian, T. Suppressed Blinking Dynamics of Single QDs on ITO. ACS Nano 2010, 4, 1545−1552. (23) Jha, P. P.; Guyot-Sionnest, P. Electrochemical Switching of the Photoluminescence of Single Quantum Dots. J. Phys. Chem. C 2010, 114, 21138−21141. (24) Galland, C.; Ghosh, Y.; Steinbrü c k, A.; Sykora, M.; Hollingsworth, J. A.; Klimov, V. I.; Htoon, H. Two Types of Luminescence Blinking Revealed by Spectroelectrochemistry of Single Quantum Dots. Nature 2011, 479, 203−208. (25) Yalcin, S. E.; Labastide, J. A.; Sowle, D. L.; Barnes, M. D. Spectral Properties of Multiply Charged Semiconductor Quantum Dots. Nano Lett. 2011, 11, 4425−4430. (26) Chiba, T.; Qi, J.; Fujiwara, H.; Sasaki, K. Analysis of Trap State Dynamics of Single CdSe/ZnS Quantum Dots on an Indium Tin Oxide Thin Film with Applying External Electric Field. J. Phys. Chem. C 2013, 117, 2507−2510. (27) Jin, S.; Lian, T. Electron Transfer Dynamics from Single CdSe/ ZnS Quantum Dots to TiO2 Nanoparticles. Nano Lett. 2009, 9, 2448− 2454. (28) Jin, S.; Martinson, A. B. F.; Widerrecht, G. P. Reduced Heterogeneity of Electron Transfer into Polycrystalline TiO2 Films: Site Specific Kinetics Revealed by Single-Particle Spectroscopy. J. Phys. Chem. C 2012, 116, 3097−3104. (29) Liu, Z.; Zhu, H.; Song, N.; Lian, T. Probing Spatially Dependent Photoinduced Charge Transfer Dynamics to TiO2 Nanoparticles Using Single Quantum Dot Modified Atomic Force Microscopy Tips. Nano Lett. 2013, 13, 5563−5569. (30) Hamada, M.; Nakanishi, S.; Itoh, T.; Ishikawa, M.; Biju, V. Blinking Suppression in CdSe/ZnS Single Quantum Dots by TiO2 Nanoparticles. ACS Nano 2010, 4, 4445−4454. (31) Molski, A.; Hofkens, J.; Gensch, T.; Boens, N.; De Schryver, F. Theory of Time-Resolved Single-Molecule Fluorescence Spectroscopy. Chem. Phys. Lett. 2000, 318, 325−332. (32) Martynski, M.; Zydlewicz, J.; Boens, N.; Molski, A. Determination of Photophysical Parameters from Photon Arrival Time Trajectories in Single Molecule Fluorescence Spectroscopy. J. Chem. Phys. 2005, 122, 134507. (33) Hohng, S.; Ha, T. Near-Complete Suppression of Quantum Dot Blinking in Ambient Conditions. J. Am. Chem. Soc. 2004, 126, 1324− 1325.

20575

dx.doi.org/10.1021/jp501096q | J. Phys. Chem. C 2014, 118, 20571−20575