ZnS Quantum Dots

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Analysis of Trap State Dynamics of Single CdSe/ZnS Quantum Dots on an Indium Tin Oxide Thin Film with Applying External Electric Field Takashi Chiba, Jun Qi, Hideki Fujiwara,* and Keiji Sasaki* Research Institute for Electronic Science, Hokkaido University, N20, W10, Kitaku, Sapporo 001-0020, Japan ABSTRACT: To clarify the influence of trap states on the emission dynamics of single CdSe/ZnS quantum dots on an indium tin oxide (ITO) thin film, we measured the trap state lifetimes of single quantum dots by a photon interdetection time analysis method. As the applied electric field between two separated ITO substrates was changed, we found that blinking behaviors were clearly modified, and the recovery rates from trap states were also changed; on the other hand, the excited state lifetime remained almost unchanged. From the results, we concluded that not only the excited state dynamics but also the trap state dynamics would play an important role for the blinking behavior.

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INTRODUCTION Fluorescence intermittence (blinking) of single colloidal quantum dots (QDs) is one of the fundamental phenomena in single nanoemitters and is sensitive to surrounding environmental conditions such as host materials, solutions, and substrates.1−5 Because metal and semiconductor substrates are utilized in various applications such as single-photon sources,6 solar cells,7 photocatalysts,8 light-emitting diodes,9 and photoelectric devices,10 the analysis of the dynamics of single QDs on these substrates should provide important knowledge for their applications. In the past, many studies of blinking behavior on various substrates such as gold thin films,11−13 TiO2 nanoparticle films,14 rough silver nanostructures,15 and indium tin oxide (ITO) films9,16−21 have been reported. Among these substrates, ITO films are one of the most important substrates for industrial applications because of their high electric conductivity and transparency. Therefore, it is important to investigate the interactions between single QDs and ITO substrates. Recently, Jin et al. observed the blinking suppression and shortening of the fluorescence lifetime of single QDs on an ITO substrate and attributed their results to the charge transfer from QDs to the ITO substrate.18 On the other hand, Jha et al. reported that fast energy transfer from QDs to an ITO substrate quenched emission of QDs and applied negative potential suppressed oxidation of QDs, resulting in blinking suppression.19 Moreover, in 2007, Park et al. demonstrated electric field induced photoluminescence modulation in QDs placed between two planar electrodes (a gold thin film and ITO electrode with a thin SiO2 spacer layer).16 They concluded that surface charge trapping sites were modulated by the applied electric field, resulting in the modulation of exciton quenching rates. Because it has recently been considered that blinking occurs owing to the capture of photocarriers from QDs to surface or outside trap sites,5 not © 2012 American Chemical Society

only emission dynamics but also trap state dynamics should be strongly affected by the substrate. However, although the influence of substrates on the emission dynamics of the excited state of single QDs has been well studied, little emphasis has been placed on the influence of substrates on the trap state itself. In this study, we investigated the influence of trap state dynamics on the emission dynamics of single QDs on an ITO substrate using histograms of photon interdetection times (PITs)13,22,23 to directly measure trap state lifetimes. In particular, we focused on the influence of applied electric field on trap state dynamics and blinking behavior. From the results, we found that blinking behavior was strongly modified according to the applied electric field and the applied field induced modulation of trap states. Therefore, we inferred that the observed phenomena originated from the change in the surface condition (especially electron density) of an ITO substrate, which would modify the trap state dynamics in addition to the modification of the excited state dynamics.

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EXPERIMENTAL SECTION

Sample preparation included careful cleaning of glassware and ITO substrates (15−30 Ω, ∼10% SnO2 doping, SPI Supplies) by sonication in acetone, which is an alkaline detergent, and ultrapure water. After drying in air, CdSe/ZnS (QSP630, Ocean NanoTech) toluene solution was spin coated onto an ITO substrate. Observations with a confocal microscope confirmed that approximately five single QDs were dispersed Special Issue: Nanostructured-Enhanced Photoenergy Conversion Received: June 29, 2012 Revised: December 12, 2012 Published: December 12, 2012 2507

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in a 5 × 5 μm2 area. Then, polymethylmethacrylate (PMMA, Mn = 48 300, 1.0 wt %, Aldrich) toluene solution was spin coated onto another ITO substrate as an insulated spacer layer (film thickness ∼100 nm) to avoid electric shorts. The sample used in this study was composed of QDs sandwiched by these two ITO substrates (Figure 1(b)). After aluminum electrodes

with Ion = Φφf kex

where d(t) is the histogram of interdetection times; A0 and A1 are arbitrary constants; Φ is the detection efficiency; φf = kf/(kf + knr + kET) is the fluorescence quantum yield; kex is the excitation rate; and Ib is the background count. The first decay rate Ion denotes the photon count rate when a QD is not in the OFF state, whereas the second decay rate kt directly corresponds to the recovery rate from the trap state. Therefore, by fitting the histogram, Ion and kt can be obtained.

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RESULTS AND DISCUSSIONS Figure 2(A) shows the typical results of photon correlation measurements (g(2)(t)) of QDs on each substrate: (a) glass, (b)

Figure 1. Schematic diagrams of prepared sample configurations ((a) glass and (b) ITO substrates with applied voltage).

were glued with Ag paste onto both ITO substrates, the electrodes were connected to a constant voltage source (3 V). Hereafter, the case that the negative (positive) electrode was connected to a QD-dispersed ITO substrate is referred to as ITO(−) (ITO(+)), and the case that zero voltage is applied is referred to as ITO(0). For comparison, we also prepared QDdispersed glass substrates (Figure 1(a)). The experimental setup was essentially the same as that reported elsewhere.13 The sample was mounted on a threedimensional piezo stage set on a confocal microscope stage (IX70, Olympus). A CW Ar+ laser (wavelength = 488 nm, Showa Optronics) was used as excitation light. This light was introduced into the confocal microscope and focused on the sample by an oil-immersion objective lens (100×, N.A. = 1.4, focal spot size ∼500 nm, excitation intensity ∼300 W/cm2). Emission from the single QDs was collected by the same objective lens and passed through a notch filter to eliminate excitation light. This emission was divided into two beams by a 50% half mirror and detected by two avalanche photodiodes (single-photon counting modules (SPCMs), SPCM-AQR-14, EG&G). Time intervals between two adjacent detection pulses from the two SPCMs were recorded using a time interval analyzer (PCI-6602, National Instruments; time resolution, 12.5 ns). The locations of single QDs were confirmed by the fluorescence intensity images produced by two-dimensionally scanning the stage. The focal spot was then moved to specific single QDs, and the intervals of detection pulses from the SPCMs were continuously recorded. Next, histograms of PIT and fluorescence intensity time traces with a desired time resolution were constructed by a computer.13,22,23 In addition, photon correlation measurements (g(2)(t)) were simultaneously performed using a time-correlated single-photon counting module (SPC-430, Becker and Hickl) to estimate the fluorescence lifetime (τ) of single QDs.2 By analyzing these measurements, we could understand the influence of each substrate on the emission and trap state dynamics of single QDs. The PIT analysis method assumes a three-level system of single QDs, in which transitions occur among ground |G⟩, excited |E⟩, and trap states |T⟩ with rates of the fluorescence kf, nonradiative deactivation knr, transfer from |E⟩ to |T⟩ kET, and recovery from |T⟩ to |G⟩ kt.13 If the experiment is conducted under the weak excitation condition (that is, the excitation rate is considerably smaller than the decay rate of the excited state), the histogram of PIT can be expressed as d(t ) = A 0 exp[−(Ion + Ib)t ] + A1 exp[−(k t + Ib)t ]

(2)

Figure 2. (A) Photon correlation measurements, (B) fluorescence intensity fluctuations, and (C) fluorescence intensity histograms of single CdSe/ZnS QDs on different substrates: (a) glass, (b) ITO(0), (c) ITO(−), (d) ITO(+). Excitation intensity was 300 W/cm2. Red and blue lines in (A) indicate the experimental data and exponential functions fitted to each histogram, respectively.

ITO(0), (c) ITO(−), and (d) ITO(+). Gray and black lines indicate the experimental data and fitting to exponential functions for each histogram, respectively. All histograms were binned for 235 ps and accumulated for 600 s. From the results, because g(2)(0) becomes almost zero for all QDs on the different substrates, we confirmed that single QDs were observed on each substrate. From the fitting, the excited state lifetimes were estimated to be (a) 34.8, (b) 12.7, (c) 11.5, and (d) 10.5 ns. QDs on the ITO substrates have significantly lower excited state lifetimes compared with those on glass substrates. In addition, comparing the results on each ITO substrate, no obvious change in lifetime was observed regardless of the applied electric field. To investigate blinking behavior, time traces of fluorescence intensities from single CdSe/ZnS QDs on each substrate condition ((a) glass, (b) ITO(0), (c) ITO(−), and (d) ITO(+)) were simultaneously measured and are shown in Figure 2(B). Emitted photons from single QDs were binned for 100 ms and collected for 600 s. Figure 2(C) shows fluorescence intensity histograms, which were calculated from the data in Figure 2(B) on each substrate. All histograms were binned for 20 counts and accumulated for 600 s. The intensities in the ON state were estimated to be (a) 2.7 × 104, (b) 1.2 × 104, (c) 2.5 × 104, and (d) 2.5 × 104 cps for each substrate, and we found that the intensity on ITO(0) decreased compared to that on the glass substrate and recovered on ITO(−) and ITO(+). In

(1) 2508

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addition, the single QDs on a glass substrate clearly showed blinking behavior, whereas the blinking was suppressed on ITO(0) because of the appearance of a peak at higher count level and the disappearance of a peak around the background level in the histogram. Furthermore, on applying voltage, the blinking was further suppressed on ITO(−) because the histogram became narrower than that on ITO(0), while blinking clearly occurred again on ITO(+), resulting in a broader distribution of ON state and the appearance of a peak around the background level in the histogram. These results suggest that blinking behavior is clearly modified by the applied electric field. The tendencies of these observed results on ITO(0) (shorter lifetime, slight blinking suppression, and decrease in intensity) were in good agreement with the previous results reported in refs 18 and 19. They explained that the shorter lifetimes of single QDs on an ITO substrate were originated from the charge transfer between QDs and an ITO substrate in ref 18, while in ref 19 it was explained by the energy transfer from QDs to an ITO substrate. In either case, the lifetime of single QDs on ITO was evaluated to be 3−4 ns, which differed from our results (11−12 ns). However, we confirmed that the ratios of the intensity and the lifetime on ITO(0) against those on glass were evaluated to be 44% and 37%, which almost agreed with each other. We considered that our obtained lifetimes on ITO(0) would be also conceivable because the intensity would be typically proportional to the excited state lifetime. On the other hand, although there is the consistency between the changes in the lifetime and intensity on ITO(0), we found the inconsistency between them when the voltage was applied, in which the excited state lifetimes were unchanged while the intensities in the ON state increased. As this voltage dependence could not be explained from our obtained results at the present stage, further investigations must be necessary to clarify the mechanism. However, we thought that because the blinking behavior was strongly modified according to the applied electric field not only the excited state dynamics but also the trap state dynamics would be modified. To discuss the recovery rate from the trap state of single QDs on each substrate, we performed PIT analysis for the same single QDs shown in Figure 2. Figure 3 shows the PIT histograms for single CdSe/ZnS QDs on each substrate: (a) glass, (b) ITO(0), (c) ITO(−), and (d) ITO(+). In the figure, dots represent the experimental data, which were binned for 12.5 μs and measured for 600 s, and solid lines indicate the fitting functions. The results indicate that the fast (Ion) and slow (kt) decay components change depending on the substrates and applied electric fields. We confirmed that the fast decay rates (Ion) were almost equal to those from the time traces in Figure 2(B). Compared to Ion in the case of the glass substrate, Ion of ITO(0) was clearly decreased, whereas Ion of both ITO(−) and ITO(+) remained almost unchanged. Moreover, we found that the slow decay rates (kt) were strongly modified depending on each ITO substrate condition. The recovery rate on ITO(0) was slightly faster than that on the glass substrate. However, when the electric field was applied, the recovery rates on ITO(−) became faster than that on ITO(0); on the other hand, on ITO(+), the recovery rate became almost the same as that on the glass substrate. To discuss the dynamics of single QDs on the ITO substrates, we summarize the averaged values of Ion, τ, and kt by measuring 20−30 single QDs for each substrate (Table 1). Figure 4 also shows the scatter plots of Ion and kt for different

Figure 3. Histograms of photon interdetection times emitted from single CdSe/ZnS QDs on each substrate: (a) glass, (b) ITO(0), (c) ITO(−), (d) ITO(+). Red dots and blue lines indicate the experimental data and fitting results using eq 2.

Table 1. Averaged Values of Ion, τ, and kt of Single CdSe/ZnS QDs on Each Substrate Ion/103 cps glass ITO(0) ITO(−) ITO(+)

36.6 16.1 25.0 27.7

± ± ± ±

13.7 6.7 6.4 10.4

kt/103 s−1

τ/ns 36.1 12.0 10.2 10.2

± ± ± ±

7.6 2.6 3.0 4.8

1.6 2.5 6.3 1.9

± ± ± ±

0.3 1.5 1.9 0.7

Figure 4. Scatter plots of Ion and kt for different ITO substrate conditions. Red squares, blue triangles, and black circles indicate the results for ITO(0), ITO(−), and ITO(+), respectively.

ITO substrate conditions (red squares, blue triangles, and black circles indicate the results for ITO(0), ITO(−), and ITO(+), respectively). From the results, it is seen that the intensity Ion and excited state lifetime τ on any ITO(0) substrate are decreased compared to that on the glass substrate. However, in the cases of ITO(−) and ITO(+), the average intensity increases compared with the case of ITO(0), whereas the average excited state lifetime does not show obvious differences against different applied electric fields. Furthermore, the average recovery rates from trap states clearly suggest that trap state dynamics is affected by the applied electric fields. Regarding recovery rates, those of ITO(−) clearly become faster than those on ITO(0) and ITO(+), suggesting blinking suppression; on the other hand, recovery rates on ITO(+) are slightly slower than those on ITO(0) and are almost the same as those on the glass substrate. These differences in the 2509

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recovery rates correspond well to the blinking behaviors in Figure 2. From these results, it is confirmed that the applied electric field affects not only emission dynamics but also trap state dynamics. In recent studies, it has been considered that QDs in the OFF state are positively charged, and blinking suppression on ITO substrates would be originated from the negatively charged QDs by charge transfer between QDs and ITO.18 Because the insulation of the sample was confirmed and the trap state lifetime clearly changed with the applied voltage, which would mean the existence of the OFF state even when QDs were dispersed on an ITO substrate, we thought that the surface conditions of an ITO substrate (especially electron density) probably affected on the blinking behavior of single QDs. Especially, when negative voltage was applied (ITO(−)), we conjectured that the blinking suppression was accelerated by the removal of long-lived holes in QDs, similar to those of QDs in a reducing solution.24 On the other hand, because the surface of ITO(+) has less electrons compared to ITO(0) and ITO(−), the hole removal would be suppressed and the recovery rate becomes slower, resulting in the appearance of blinking. Thus, the mechanism of blinking behaviors depending on the applied electric fields can be intuitively explained, as described above. However, the change in the intensity on both ITO(−) and ITO(+) cannot be simply explained by the change in the excited state lifetimes, in which the intensity increases whereas the excited state lifetime slightly decreases or remains almost unchanged. Although we considered that the inconsistency on ITO(−) and ITO(+) should come from the change in the surface conditions of an ITO substrate, the mechanism for explaining whole behaviors is still unclear. Moreover, we should note that the blinking suppression was also explained by the suppression of the surface oxidation of QDs under a negative potential.19 If it occurred on ITO(−), the fluorescence intensity should be decreased by quenching, but our obtained intensity contrarily increased and could not also be explained. For further clarification of the electric field dependence of excited and trap state dynamics, we have to perform further investigations (e.g., measurements of single QDs with continuously changing the applied voltage, introduction of a spacer layer).

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AUTHOR INFORMATION

Corresponding Author

*Keiji Sasaki. Phone: +81-11-706-9396. E-mail: sasaki@es. hokudai.ac.jp. Hideki Fujiwara. Phone: +81-11-706-9395. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Grant-in-Aids for Young Scientists (A), Exploratory Research, and Scientific Research (A) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan and the PRESTO program of the Japan Science and Technology Agency.

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CONCLUSIONS Emission dynamics of single CdSe/ZnS quantum dots on ITO substrates was studied by single molecule spectroscopy and a photon interdetection time analysis method. We observed the switching of blinking behavior depending on the applied electric field on ITO substrates. According to the change in the blinking behavior, we confirmed that not only excited state dynamics but also recovery rates from trap states were modified. We conjectured that one of the reasons for the blinking suppression was possibly due to the acceleration of the removal of long-lived holes in QDs by the change in the surface condition of an ITO substrate because we found the change in the trap state dynamics, which would mean the existence of the OFF state even when QDs were dispersed on an ITO substrate. Although further studies are required, considering the potential for various applications by and focusing on QDs, we believe that the observed phenomena can provide a novel viewpoint for technological applications of QDs on conductive thin films. 2510

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