Article pubs.acs.org/JPCC
Long-Range Correlated Fluorescence Blinking in CdSe/ZnS Quantum Dots Ryan Hefti,† Marcus Jones,‡ and Patrick J. Moyer*,§ †
Nanoscale Science Ph.D. Program, Department of Chemistry, The University of North Carolina at Charlotte, Charlotte, North Carolina 28223, United States ‡ Department of Chemistry, The University of North Carolina at Charlotte, Charlotte, North Carolina 28223, United States § Department of Physics and Optical Science, The University of North Carolina at Charlotte, Charlotte, North Carolina 28223, United States S Supporting Information *
ABSTRACT: We studied fluorescence intermittency (blinking) in pairs of colloidally grown nanocrystal quantum dots (NQDs) and found that the fluorescence trajectories of dots separated by up to ∼1 μm are correlated. Blinking rate enhancement was observed when nearby NQDs were in opposite emitting states. Models of fluorescence blinking in colloidal quantum dots typically invoke particle charging to explain bright and dark periods in the fluorescence trajectory. Likewise, the phenomenon of fluorescence blinking correlation observed in this study is explained by an interdot Coulomb interaction established by ejection of one or more photoinduced charges. Our results suggest that blinking can be controlled, ultimately leading to switchable nanoscale emitters.
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not enough to quench all fluorescence and multiple photoionization events are required to turn fluorescence off completely.20,21 In this work, we report on the use of roomtemperature single-molecule fluorescence measurements to demonstrate that the blinking traces of two NQDs separated by up to 1 μm are correlated. A similar result was reported by Fu et al., in which NQDs separated by up to 1.1 μm exhibited fluorescence bunching behavior.22 We propose that our remarkable results can be explained by a Coulomb effect whose interaction potential is of the order of 5−10 meV. Further, we suggest that the these results support the fact that individual NQDs can remain in an emissive state despite becoming multiply charged.
INTRODUCTION It is well-established that nanocrystal quantum dots (NQDs) are excellent fluorescent probes due to their robustness, tunable emission, and high level of brightness.1−5 Unfortunately, their application in fields such as biological labeling is often negatively impacted by the phenomenon of fluorescence blinking: a random switching between brightly fluorescing and nonemitting states.6 This phenomenon is ubiquitously observed in molecular chromophores, conjugated polymers, biomolecules, and nanoparticles;7 however, NQDs are especially well-known for their anomalous blinking behavior that follows power law statistics.6,8,9 Although some research groups have successfully reduced or eliminated blinking, either by surface functionalization with organic ligands10,11 or by synthesizing NQDs with extremely thick or alloyed shells,12−14 a comprehensive understanding of the effect remains a challenging problem. Several models have been formulated to explain NQD blinking, but none fully capture the breadth of experimental results, and questions regarding the mechanism of fluorescence intermittency remain unanswered.15 For example, fluorescence is thought to turn off after a photoexcited NQD undergoes a charge-transfer reaction, leaving an extra electron or hole in the core,16 but the fate of the remaining charge is not clear. Most predict that this charge remains trapped in surface states or ligand orbitals;17 however, some of the earliest blinking models considered the possibility that NQDs gain a net charge as the ejected electron diffuses into the surrounding matrix.18,19 In addition, recent results indicate that a single excess charge is © 2012 American Chemical Society
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EXPERIMENTAL SECTION For these experiments, a home-built laser scanning confocal microscope with a 470 nm diode, 300 ps pulse laser was used.23 Using a 10 MHz pulse rate, the average irradiance for these experiments was 2.5 kW cm−2. The excitation beam was coupled into a single-mode optical fiber and subsequently passed through a 470/20 nm band-pass filter (BPF). The excitation beam was subsequently reflected from a 500 nm dichroic beam splitter (DBS) into a 1.25 numerical aperture 100× oil-immersion objective lens, which also served as the collection lens for the fluorescent light. The DBS passed all Received: August 24, 2012 Revised: November 9, 2012 Published: November 10, 2012 25617
dx.doi.org/10.1021/jp3084343 | J. Phys. Chem. C 2012, 116, 25617−25622
The Journal of Physical Chemistry C
Article
Figure 1. Illustrating the collection of NQD fluorescence trajectories. (a) A 70 × 70 μm image. (b) Image of two NQDs. (c) FSCM image recorded along the highlighted region in (b). (d) A fluorescence intensity histogram (green dots) calculated from the FSCM data in (c), which has been fitted (thick black line) by two Gaussian curves (dashed lines) labeled “Peak 1” and “Peak 2″, corresponding to the contributions from each NQD. The long horizontal boxes that start in (c) and continue into (d) indicate the regions from which fluorescence intensities were determined for each NQD. The overlap of the fluorescence from the top (bottom) NQD onto the collection region of the bottom (top) NQD is labeled Ov2 (Ov1). Inset in (d) is the histogram of photon counting rates (gray dots) extracted from the fluorescence trajectory for the top NQD. The vertical dashed line represents the actual threshold used to separate “on” and “off” events.
We propose that this correlated blinking is most likely caused by the photogeneration of highly charged NQDs, which produce an electric field effect (i.e., a change in interaction potential energy) capable of modulating the fluorescence trajectories of neighboring NQDs. Figure 1a shows a 70 × 70 μm image of a 5 × 10−11 M concentration of CdSe/ZnS core/shell NQDs that have been spin-cast into a thin PMMA film. Zooming into the image (Figure 1b) reveals individual dots with characteristic blinking evidenced by dark vertical slices intermixed with bright fluorescent slices for each dot. NQD blinking manifests in this way because the image is collected line-by-line by scanning in the vertical (fast scan) direction. The fast scan image shown in Figure 1c was recorded at a 100 ms/line scan speed to build a 512 × 512 pixel image. Using this scan speed with this particular image size, dots separated by a distance of 1 μm are temporally separated by 12.5 ms. Additionally, as shown in Figure 1d, each image was summed horizontally to yield a plot of time-integrated intensity, and to localize the NQDs as precisely as possible, Gaussian point-spread functions (dashed lines in Figure 1d) were fit to each of the fluorescence peaks. The two NQDs in Figure 1b are ∼460 nm apart and display the expected blinking dynamics with probability densities that obey power law statistics typical of nanocrystal fluorescence.25,26 Once the centroid of each NQD was identified, then each fast scan image was analyzed on a line-by-line basis to establish an intensity threshold delineating on and off behavior for each dot. Thresholds were determined by considering a 10-pixel region close to the peak intensity position for each NQD. Horizontal boxes in Figure 1c,d highlight these regions. These 10 pixels were summed and compared to the average of the summed values for all 512 locations across an FSCM image. Any value below the average was considered indicative of an NQD “off” state, whereas values above the average reflected an NQD “on” state. This process of establishing a threshold was consistent throughout all data analysis and always corresponded to a value slightly higher than the modal intersection point of the distributions, indicating that the NQDs were “on” a little more than they were “off”. An example histogram describing the distribution of fluorescence intensities determined for each 10-pixel region across the fast scan image is shown in the inset in Figure 1d.
wavelengths above 500 nm, after which a second DBS, at 585 nm, was used to split the emission signal for two-color imaging. Finally, a 565/40 nm BPF and a 605/52 nm BPF were used in front of two separate single-photon avalanche diodes. A high-speed PZT actuated stage gives us the ability to vary the scan speed according to the system being studied. While studying various NQD labeled samples, we noticed a possible correlation between the blinking patterns of neighboring NQDs. This phenomenon was then investigated further on single NQD samples prepared by spin-coating a small sample of NQDs in 0.5% poly(methyl methacrylate) in toluene onto a glass coverslip. Three commercial CdSe/ZnS core/shell NQD samples were studied: sample I (peak emission at 617 nm), II (596 nm), and III (564 nm). Single NQD samples were prepared using concentrations ranging from 1 × 10−11 to 8 × 10−10 M. Using a method that we call fast-scanning confocal microscopy (FSCM), samples of NQDs were analyzed. FSCM takes advantage of the typically inconvenient limit of scanning confocal microscopy: serial imaging. Instead of building a complete two-dimensional image with each series of scans, we limit the horizontal (slow scan) direction to a negligibly small distance and confine the image to a narrow line, which is highlighted in Figure 1b. Since this horizontal distance is about 1 nm and is much smaller than the spot size of the focused laser, the system is scanning the same vertical line repeatedly. Two adjacent NQDs are centered along this line. By rapidly scanning along this axis, the gathered image is simply a distance versus time plot, as shown in Figure 1c. This technique allows us to monitor the fluorescence dynamics of two vertically oriented NQDs that are less than 5 μm from one another.
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RESULTS AND DISCUSSION We have observed that two NQDs in close proximity to each other (
