Fluorescence Lifetime and Blinking of Individual Semiconductor

Article pubs.acs.org/JPCC

Fluorescence Lifetime and Blinking of Individual Semiconductor Nanocrystals on Graphene Benoît Rogez,*,† Heejun Yang,‡,§ Eric Le Moal,† Sandrine Lévêque-Fort,† Elizabeth Boer-Duchemin,† Fei Yao,‡,§ Young-Hee Lee,‡,§ Yang Zhang,†,∥ K. David Wegner,⊥ Niko Hildebrandt,⊥ Andrew Mayne,† and Gérald Dujardin† †

Institut des Sciences Moléculaires d’Orsay, CNRS Université Paris-Sud (UMR 8214), Orsay, France Center for Integrated Nanostructure Physics (CINAP), Institute for Basic Science (IBS), and §Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Korea ∥ Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui China ⊥ Institut d’Electronique Fondamentale, CNRS Université Paris-Sud (UMR 8622), Orsay, France ‡

ABSTRACT: A new class of optoelectronic nanodevices consisting of 0D semiconductor nanocrystals and 2D single graphene layers is attracting much attention. In particular, such a system may be used to investigate and control the transfer of energy and charge in low-dimensional systems. To this end, the fluorescence dynamics of individual colloidal quantum dots (QDs) on graphene are investigated on both the 10−9−10−8 s time scale (fluorescence lifetime) and the 100−102 s time scale (blinking statistics) in this paper. We find that (i) a nonradiative energy transfer rate of ≈5 × 108 s−1 is obtained from the reduced lifetimes of QDs on graphene as opposed to those on insulating substrates such as glass; (ii) QDs still exhibit fluorescence intermittency (“blinking”) on graphene; (iii) the cumulative distribution functions of the “off” times may be described by power-law statistics; (iv) QD coupling to graphene increases the time spent in the “on” state while the time spent in the “off” state remains relatively unchanged; and (v) the fluorescence emission spectrum of the QDs is practically unaltered by the QD−graphene coupling.

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INTRODUCTION The optoelectronic properties of semiconductor nanocrystals (or quantum dots, QDs) and graphene nanostructures are of both fundamental and practical interest.1−7 Indeed graphene, an ideal two-dimensional (2D) electron gas material, has exceptional electronic properties and offers an appealing alternative to indium−tin oxide as a transparent electrode for optoelectronic devices.8−11 On the other hand, as compared to organic dyes, QDs exhibit much higher chemical and photophysical stability, significantly larger absorption cross sections over a broad wavelength range, tunable (by size and material composition) emission spectra, and superior visible fluorescence quantum yields (virtually 100%).12 As well, dramatic advances in QD synthesis methods allow their emission spectra to be tuned within a broad spectral range. Therefore, QDs have become the material of choice for many applications such as photovoltaics,13,14 light-emitting devices,13 and fluorescent labeling.15 QDs are also the prototypical system for the fundamental study of fluorescence intermittency (blinking), a general feature of many types of quantum emitters. Despite 2 decades of research in this area, a precise description of the origin of this blinking phenomenon remains elusive and it continues to be a very topical subject of study.16−25 © 2014 American Chemical Society

The combination of the outstanding optoelectronic properties of both QDs and graphene opens unique perspectives for the design of hybrid devices. For example, with the aim of increasing solar cell efficiencies, QD-sensitized graphene photoelectrodes have been shown to enhance the photocurrent generated upon visible light exposure.2,3 QD−graphene heterostructures have been used to engineer tunable photosensors and photoswitch devices of high on/off ratio.26,27 QD light-emitting diodes using graphene as the anode interfacial layer have also been recently designed.6 In most of these studies, the authors have focused their research on the charge transfer between QDs and graphene, and they have conducted averaged measurements on an ensemble of QDs. The energy transfer between QDs and graphene and its effect on QD fluorescence dynamics have been comparatively ignored, in particular on the scale of single emitters. The study of the fluorescence properties of QDs on graphene is a powerful method for the investigation of energy and charge transfer in such hybrid systems.28−31 In particular, it has been shown that graphene strongly quenches the fluorescence of Received: June 20, 2014 Revised: July 21, 2014 Published: July 22, 2014 18445

dx.doi.org/10.1021/jp5061446 | J. Phys. Chem. C 2014, 118, 18445−18452

The Journal of Physical Chemistry C

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(Nilaco, lot no. 113321, purity 99.96%, thickness 100 μm). A thin layer of poly(methyl methacrylate) (PMMA A4, molecular weight 495K, diluted in 4% anisole) is then spin-coated onto the graphene layer to ensure its mechanical resistance, and the copper substrate is subsequently dissolved in an ammonium persulfate bath, leaving the thin graphene/PMMA film floating on the surface of the solution. This film is deposited on a standard microscope glass coverslip (thickness 170 μm) and the PMMA layer is then removed by dissolving in acetone and by heating the graphene/glass in an H2/N2 mixed gas atmosphere at 300 °C for 2 h. From this widely used method,9 we obtain a single layer of graphene on glass, which has monocrystalline domains of several tens of micrometers in size and no PMMA residue on the surface. The lack of a residual PMMA layer is emphasized in our experiments, since such a layer could influence QD−graphene coupling.33 If the graphene domains are noncontiguous, the resulting bare areas of the glass substrate may be easily distinguished by either optical microscopy (monolayer graphene yields a 3% decrease in the optical transmission) or atomic force microscopy (AFM). QD Sample Characterization (TEM and AFM). The lateral dimensions of the QDs are determined from transmission electron microscopy (TEM) images obtained using an Akashi Topcon EM-002B microscope operated at 100 keV. The atomic force microscopy (AFM) images of QDs on graphene are acquired using a commercial setup (JPK NanoWizard III) in intermittent contact (“tapping”) mode. Emission Spectrum Measurements. Fluorescence spectra of QDs in solution are recorded using a FluoTime 300 “EasyTau” time-resolved fluorescence spectrometer (PicoQuant, Germany) in decane at 20 °C. The sample is excited with a 300 W coaxial xenon arc lamp at 500 ± 2 nm. The spectrum is recorded from 650 to 900 nm with a bandwidth of ±2 nm. The measurement is performed with a resolution of 1 nm and an integration time per data point of 0.5 s. On graphene and on glass, the fluorescence spectra of the QDs are obtained using an inverted optical microscope in an epifluorescence configuration (Zeiss Axiovert 200; laser excitation at 632.8 nm; ×100 oil-immersion objective lens, numerical aperture NA = 1.45). The emitted light is coupled by an optical fiber to a diffraction grating spectrometer (Triax 190, Horiba Jobin Yvon) which is equipped with a liquid nitrogen cooled charge coupled device (CCD). Emission from asdeposited graphene films is similarly recorded. The transmission of the complete system (microscope + spectrometer) is ∼25% for the wavelength range 600−800 nm and then decays to ∼2% at 1000 nm. Fluorescence Lifetime Imaging Microscopy (FLIM). Fluorescence lifetimes of individual QDs are measured using a wide-field time-resolved fluorescence microscope.42 The setup is based on an inverted optical microscope (Olympus IX71) in an epifluorescence configuration equipped with an oilimmersion objective (×60, NA = 1.45). The sample is excited using a pulsed supercontinuum laser source (pulse length < 15 ps) which is spectrally filtered with a 525 ± 23 nm band-pass filter. The laser repetition rate is 20 MHz. Time-gated detection is performed thanks to a high trigger rate gated intensifier (HRI, Kentech Instrument, Ltd.) optically relayed to a CCD camera (Orca-AG Hamamatsu). This intensifier is synchronized with the laser pulse by a delay line. Thus, a temporal gate with a 1 ns width is opened at different delay times after the pulse. The resulting image series acquired for decreasing decay times is then fitted with an exponential model in order to

QDs and other fluorescent probes.32−41 Most often, this quenching is attributed to the nonradiative energy transfer from QDs to graphene.32 Until now, no clear blinking behavior has been seen for QDs in such experiments.32,33 As well, the nonradiative transfer rate of energy from QDs to graphene appears to be dependent on the different parameters of the experimental system: the type of QDs (core−shell or core only), the QD size, the type and length of the surface ligands, and the quality and synthesis method of the graphene surface. This dependence on the various sample parameters is an important point that requires further exploration. In this paper, we investigate the fluorescence lifetime and blinking of individual laser-excited CdSeTe/ZnS core/shell QDs on single-layer CVD graphene. We measure the fluorescence decay time of individual QDs using time-resolved fluorescence microscopy (10−9−10−8 s time scale) and we analyze the temporal statistics of their bright (on) and dark (off) states on the 100−102 s time scale. In order to reveal the effects of QD−graphene coupling, these measurements are conducted for both QDs on single-layer graphene deposited on glass and for QDs on bare glass substrates (control sample). Fluorescence spectra measurements show clear evidence that the emitted light may be safely ascribed to QD fluorescence (as opposed to Raman emission from graphene defects). Here we experimentally show that (i) nanocrystal fluorescence lifetimes are shorter on graphene than on insulating substrates such as glass; (ii) QDs still blink on graphene and the cumulative distribution functions of the off times may be described by power-law statistics; (iii) the QD−graphene coupling modifies the on-state statistics, which are related to the fluorescence lifetime, whereas the off-state statistics are not affected by QD− graphene coupling; (iv) the QDs remain statistically longer in the on-state when on graphene than when deposited on glass, for the time scale of our measurements; and (v) the emission spectrum of the QDs on graphene is similar to that which is found for QDs on glass, which is as expected given the optical quasi-transparency and flat dispersion curve of single-layer graphene in the visible range.

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EXPERIMENTAL MATERIALS AND METHODS Quantum Dot Deposition. For this study, we use commercially available colloidal CdSeTe/ZnS core/shell nanocrystals (Invitrogen Qdot 800, Q21771MP) stabilized in decane by long alkyl ligands (trioctylphosphine oxide, TOPO). Prior to deposition, the nanocrystal solution is diluted in pentane (purity >99%) to a concentration of 1 pM. We deposit a droplet (20 μL) of the diluted QD solution directly on graphene or glass and rinse the sample with ethanol (96% ethanol, 4% water, anhydrous purity 99.8% ethanol) in order to remove the excess TOPO ligands. We do not use spin-coating as a deposition method because the resulting concentration of QDs on graphene is found to be very low. Moreover, the droplet deposition method using a very dilute solution yields relatively homogeneous samples with optically isolated single quantum dots, an essential point for our optical experiments on individual nanocrystals. The fact that this method results in optically isolated single quantum dots is determined from blinking measurements (see Fluorescence Imaging and Blinking Measurements below). For single quantum dots, only two intensity levels (i.e., “on” and “off”) are observed in a series of measurements. Graphene Synthesis. Monolayer graphene is synthesized by chemical vapor deposition (CVD) on a copper substrate 18446



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obtain the fluorescence lifetime image (FLIM). In this way, the fluorescence decay times of each bright pixel in the imageand thus of any individual QD in the samplemay be determined. Fluorescence Imaging and Blinking Measurements. Fluorescence imaging and blinking measurements are conducted using an inverted optical microscope in an epifluorescence configuration as for the emission spectrum measurements of QDs on graphene and glass above (Zeiss Axiovert 200; laser excitation at 632.8 nm; ×100 oil-immersion objective lens, NA = 1.45). However, in the present case the emitted light is focused on a liquid-cooled CCD camera (PIXIS 1024B, Princeton Instruments), yielding a fluorescence image of the sample. Blinking statistics of individual QDs are retrieved from the analysis of a series of CCD images of the same area recorded at a rate of 1 or 10 Hz (for QDs on graphene or glass, respectively). The total acquisition time is 2500 s. Optically isolated QDs are identified and their intensity as a function of time is obtained from image series analysis (using the freeware ImageJ43). In order to eliminate the light from the Raman peaks of graphene (see the section on QD and graphene characterization below) we use a band-pass interference filter (Semrock FF02−809/81−25) with >96% transmission in the 767−853 nm range and 90 QDs give mean QD sizes of 8 ± 1 and 11 ± 2 nm along their short and long axes, respectively. A TEM image of higher magnification of a single QD in Figure 1c reveals the atomic structure of the CdSeTe core. The graphene layer is synthesized by chemical vapor deposition (CVD) on a copper substrate and then transferred to glass (see Experimental Materials and Methods). Figure 1d,e shows an atomic force microscopy (AFM) tapping mode topography image of the deposited graphene. As previously observed,9 the AFM topography of the as-deposited graphene exhibits ripples and localized defects whose height, around 6 nm, is similar to that of the QDs. Thus, after QD deposition, QDs and localized defects cannot be simply distinguished using AFM measurements. In order to unambiguously distinguish QDs from localized graphene defectsa key point for the study of individual QDs on graphenefluorescence spectroscopy may be used. Figure 2a shows the emission spectrum from two different areas on an as-deposited, monolayer graphene sample on glass (no QD deposition). Both areas are located in the center of a large domain, far from the domain boundary. Fluorescence images

Figure 2. Emission spectra of graphene and QDs. (a) Raman spectra of monolayer graphene on glass, for a defect-free region (dark blue curve, top) and a region including carbon defects (magenta curve, bottom). 2D, G, and D refer to the Raman lines expected for graphene.46 The appearance of the D band in the spectrum denotes the presence of edges or defects in the analyzed region of the graphene layer. (b) Fluorescence spectra of QDs on glass (blue curve, top), of QDs on monolayer graphene (green curve, center), and of asdeposited monolayer graphene (dark blue curve, bottom). Normalized intensity is plotted versus emission wavelength. Note the sharp peaks in the spectra from the Raman lines of graphene.

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Figure 3. Fluorescence dynamics of QDs on glass and on monolayer graphene. (a) Fluorescence decay curves of an individual QD on glass and on graphene, fitted with exponential decay functions. Note the significant decrease in the fluorescence lifetime in the case of QD−graphene coupling. (b) Fluorescence intensity as a function of time for individual QDs on glass and on graphene (longer time scale). Note the occurrence of longer time periods in the on-state for QDs on graphene. The dashed line represents the intensity threshold used to discriminate on- and off-states for statistical analysis. The histograms to the right of part b show the frequency of the measured intensities. The fact that the QDs are much more often in the onstate on graphene than on glass is clearly seen.

(not shown) of these two graphene regions show that luminescent localized defects are present in region 1 (magenta curve, bottom spectrum in Figure 2a) but not in region 2 (dark blue curve, top spectrum in Figure 2a). The emission intensity is plotted versus the shift of the emission frequency (expressed in wavenumbers) relative to the excitation He−Ne laser line (wavelength 632.8 nm). The spectrum from the defect-free region strongly resembles that which is found for large single crystal graphite samples,44,45 with peaks at ∼1580 (G) and ∼2700 cm−1 (2D or G′).46 The D band seen in the bottom spectrum (∼1350 cm−1) corresponds to a carbon ring breathing mode and is only seen when the analyzed region includes edges or defects.47,48 In particular, its intensity has been seen to vary inversely with crystallite size.47 Localized defects thus have a well-identified Raman signature in the emission spectrum (D band) and may be easily identified via spectral measurements. Figure 2b shows the fluorescence spectra of QDs on bare glass (top spectrum in Figure 2b). When QDs are deposited on a defect-free graphene area (as determined by fluorescence imaging), the fluorescence spectrum of the QDs (bottom spectrum in Figure 2b) and the Raman lines of graphene are simply superimposed. Thus, both the emissions from graphene and from carbon defects consist of sharp Raman lines which may be easily filtered. Consequently, in the following, all fluorescence measurements are performed using a band-pass interference filter that discards the contribution from the graphene and carbon defects. Thus, we can spectrally separate the light from graphene and its defects from that originating from QDs, and in this way, we can restrict ourselves in this study to QD fluorescence. Figure 2b shows that QD−graphene coupling has very little effect on the QD fluorescence spectrum. A mean increase of the full width at half-maximum (fwhm) of less than 15% is observed. Remarkably, the central wavelength of the emission spectrum, as determined from a Gaussian fit of the collected

intensity versus energy, is shifted by less than 1 meV (0.5 nm). This is in contrast to the larger spectral shifts and broadening previously reported for QD fluorescence on metallic surfaces and nanostructures.49−53 The effect of the graphene substrate on the QD spectrum is comparatively weak because of the high transparency (97%) of monolayer graphene on glass, its flat dispersion curve, and the absence of any surface plasmon resonances in the visible range.54 The preservation of the native emission spectrum of the QDs may be of importance in the context of applications of QD/graphene hybrid systems in light-harvesting or light-emitting devices. QD Fluorescence Lifetime Measurements and the Effect of QD−Graphene Coupling. Figure 3a shows the fluorescence decay of a single QD on glass (blue curve, top) and of a single QD on graphene (green curve, bottom), as measured by fluorescence lifetime imaging microscopy (FLIM; see Experimental Materials and Methods). It should be noted that the collected intensity from QDs on graphene is found to be about 10 times lower than that which is found for QDs on glass, making the experiment slightly more difficult on graphene. The experimental data is fitted with either a monoor biexponential decay function. For the QD on glass of Figure 3a, a biexponential decay with approximately equal amplitudes and with lifetimes of τ1 = 1.3 ns and τ2 = 17 ns is observed. For the QD on graphene, a monoexponential decay with τ2 = 2.5 ns is found. These measurements are limited by the temporal resolution of our FLIM setup (≈1 ns), and it is assumed that τ1 ≪ 1 ns for the graphene sample. These lifetime measurements are repeated for over 100 (50) individual and optically isolated QDs on glass (graphene) in order to obtain average values for the QD lifetimes. In this way, we find ⟨τ1⟩ = 1.2 ± 0.6 ns and ⟨τ2⟩ = 15 ± 7 ns on glass and ⟨τ1⟩ ≪ 1 ns and ⟨τ2⟩ = 1.7 ± 0.8 ns on graphene. The variation in the QD lifetimes is due to the variation of the QD shapes and sizes (see Figure 1b). These short and long fluorescence lifetimes of core−shell QDs have been assigned two different physical origins. The shorter 18448



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component τ1 has been related to radiative recombinations between QD core levels, whereas the radiative recombinations responsible for the longer τ2 have been attributed to QD localized surface states.55−59 A significant decrease in the mean fluorescence lifetime is observed for QDs on graphene as compared to QDs on glass. Due to its monolayer thickness and the fact that there are no plasmon resonances in the visible for graphene, no new radiative channels are expected when QDs are coupled to graphene. The reduction in fluorescence lifetime has thus been attributed to a nonradiative process where the energy from the excited QDs is transferred to the graphene layer.32 A quenching factor ρ may then be estimated by considering ρ ∼ ⟨τ2⟩glass/ ⟨τ2⟩graphene and may also be described by ρ = (γrad + γET)/γrad, where γrad ∼ 1/⟨τ2⟩glass is the radiative decay rate and γET is the nonradiative energy transfer rate.32 Reduction of ⟨τ2⟩ from 15 to 1.7 ns yields then a quenching factor of about 9 and a nonradiative energy transfer rate of γET ≈ 5 × 108 s−1. This value is about 10 times smaller than that which has been estimated previously for CdSe/ZnS core/shell nanocrystals on exfoliated graphene32 and about a factor 2 larger than that found for chloride-terminated CdSe core-only QDs on CVD graphene.33 The reduced nonradiative energy transfer rate in comparison with the first case may be due to the slightly larger size of the QDs used, which in turn leads to an increase in the mean distance between the QDs and graphene and a reduction in the energy transfer. In comparison with the latter case, a higher transfer rate is expected due to the fact there is no residual PMMA layer in our samples (see Experimental Materials and Methods). Blinking Statistics of QDs on Graphene. Until now, there has been no clear observation and study of the fluorescence intermittency of QDs on graphene and its relationship to fluorescence lifetime. Figure 3b shows the fluorescence intensity as a function of time for a single QD on glass (blue curve, top) and for a single QD on graphene (green curve, bottom). Here we see for the first time, unlike in previous reports,32,33 that QDs on graphene exhibit characteristic fluorescence intermittency, i.e., the intensity oscillates between a bright state and a dark (zero intensity) state. In previous reports such behavior was either completely absent32 or uncharacteristic, with no zero-intensity state observed on graphene.33 We propose that we are able to study this fluorescence intermittency as our nanocrystals are larger and thus less coupled to the surface,32 and because there are fewer surface states in our core/shell nanocrystals as opposed to chloride-terminated core-only QDs.33 As we spectrally filter the emitted light (see above), we can unambiguously assign the observed emission to QDs and not carbon defects. Qualitatively, we see immediately from Figure 3b the effect of graphene on the QD blinking behavior. QD fluorescence intermittency on glass is characterized by comparatively short time periods in the bright state, whereas these bright-state periods are significantly longer for QDs on graphene. A quantitative analysis of this effect is given below. Figure 4 shows the average cumulative distribution functions of the residence time in the on- and off-states of individual semiconductor QDs. As QDs are nonergodic systems and thus time and ensemble averages are not equivalent,60 an average value of toff and ton may not be determined. Instead, the fluorescence intensity traces of >40 (15) individual QDs on glass (graphene) are first analyzed, in order to obtain a series of probability distribution functions P(t), where P(t) = ∑t(events

Figure 4. Statistical analysis of the fluorescence intermittency of QDs on glass and on monolayer graphene. (a) The average cumulative distribution functions of the on- and off-states for QDs on glass, acquired at a rate of 10 Hz (temporal resolution 100 ms). Pc(ton|off > t) is the probability of measuring ton|off > t, i.e., the time spent in the on(off-)state before switching to the off-(on-)state. The parameter μOFF = 0.45 ± 0.05 is determined by fitting the function ⟨Pc(ton|off > t)⟩ ∝ (1/t)μOFF to the data for t ≤ 1 s. The deviation of the off-state data from a pure power law for longer times is due to the finite temporal extent of the measurement. The faster decay of the on-times has been attributed to diverse factors such as temperature, excitation intensity, QD surface morphology,17 multiexcitons, and Auger ionization.61 Uncertainties in the parameter μOFF are determined from the fits. (b) The average cumulative distribution function of the on- and off-states of QDs on glass and graphene, acquired at a rate of 1 Hz (temporal resolution 1 s). Here we see clearly that the off-times for the QDs are unchanged by graphene coupling, and that the on-times are significantly longer for QDs on graphene than for those on glass. On these log−log plots, the solid squares are the on-state data and the empty squares show the results for the off-state.

of length t).16,17 The cumulative distribution function Pc(ton|off > t) is then obtained by summing the number of events with a time longer than a time t. These functions are then averaged for all the analyzed QDs in a sample, leading to the curves in Figure 4 [⟨Pc(ton|off > t)⟩ as a function of t]. As previously mentioned, the collected intensity from QDs on glass is found to be about 10 times higher than that which is found for QDs on graphene. Thus, for the QDs on glass a high data acquisition rate may be used (see Experimental Materials and Methods), leading to time probability distribution functions with a temporal resolution of 100 ms. Figure 4a shows the average cumulative distribution functions of the onand off-states for QDs on glass. The off-state cumulative distribution may be fitted, for short times (≤1 s), by a power law Pc(toff > t) ∝ (1/t)μOFF, with μOFF = 0.45 ± 0.05. This result is consistent with previously observed off-state statistics60 and corresponds to a simple probability distribution function power law with μ + 1 = 1.5.16,17 The deviation of the off-state data from a pure power law is attributed to the finite temporal extent 18449



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of the measurement (longer measurement times would provide data that would follow a line more closely on this log−log scale). The truncation of the on-state cumulative distribution function (Figure 4a) occurs earlier and has been shown to depend on several experimental features, such as temperature, excitation intensity, and QD surface morphology,17 and has also been linked to multiexciton excitation and Auger ionization.61 As the emission intensity is low for QDs on graphene, the maximum recording rate is 1 Hz (acquisition time 1 s). Figure 4b shows the cumulative distribution functions of the on- and off-states for QDs on both graphene and glass, as determined with a binning time of 1 s. The off-state cumulative distribution functions for QDs on glass and on graphene are very similar. This confirms the universality of the statistical behavior of the off-state and its insensitivity to different environments.17 On the other hand, the on-state statistics for the QDs on glass and on graphene are markedly different. In particular, longer onstate times are observed for QDs on graphene than on glass, as determined qualitatively from Figure 3b. It should also be noted that the localized luminescing defects of the as-deposited (no QDs) CVD graphene layer have been investigated. As expected, they do not exhibit any blinking. Thus, fluorescence intermittency is a second method (complimenting emission spectrum measurements) that may be used to distinguish deposited QDs from localized defects on graphene. The Relationship between Blinking Statistics and Fluorescence Lifetime for QDs on Graphene. While the exact mechanism for blinking is still under much debate,20,24,25,62,63 charge carrier trapping is consistently evoked as an essential ingredient for blinking.64 Such charge trapping may occur when the QD is in an excited state. Therefore, the longer the lifetime of the excited state, the longer the QD will be in an excited state (provided that the excitation power is below the saturation threshold) and thus the higher the probability that the QD will switch to an off-state. This is precisely what we observe experimentally, since the QDs on glass exhibit a longer fluorescence lifetime and reside a statistically shorter time in the on-state. Conversely, the transition from the off-state back to the on-state is governed by the return of the charge, transiently trapped in the periphery, to the QD core where the excited electron−hole pair can recombine. Therefore, the mean residence time in the off-state is not expected to depend on the fluorescence lifetime of the QD, unlike that of the on-state, which is again in good agreement with our experimental observations reported above. A very simplified version of the Monte Carlo simulation of Efros and Rosen65 may be used to demonstrate that a shorter fluorescence lifetime leads to a shorter time spent in the onstate (see Figure 5).

Figure 5. A very simple Monte Carlo simulation based on the work of Efros and Rosen65 showing the effect of fluorescence lifetime on the occurrence of on- and off-events. Simulated intensity versus time traces for the case of (a) long (10 ns) and (c) short (1 ns) fluorescence lifetimes. Frequency of on- and off-events for the same (b) long and (d) short fluorescence lifetimes. A shorter lifetime clearly leads to more time spent in the on-state. Please note that this simulation is very simplified and does not take into account all the complicated dynamics of blinking in semiconductor QDs. In particular, it does not reproduce the power-law behavior of blinking. Only a short time scale is simulated due to computer memory limitations (time step 5 × 10−12 s, binning time 1 μs).

on a number of parameters: (i) the type of QDs (core−shell or core only), (ii) the QD size, (iii) the type and length of the surface ligands, (iv) the quality and synthesis method of the graphene, and (v) the existence of impurities (e.g., PMMA) or defects on the graphene surface. An important result of the present study is that fluorescence intermittency is still observed for QDs on graphene, unlike the case of QDs on metals for which blinking is suppressed.66−69 In particular, a graphene substrate affects the on-time statistics but leaves the off-time statistics relatively unchanged. The longer times in the on-state on graphene may be related to a reduction in the fluorescence lifetimes. CVD graphene often includes localized luminescent defects which must not be confused with deposited QDs. In order to unambiguously identify QDs on graphene, either blinking imaging microscopy or emission spectrum measurements may be used. From such emission spectrum measurements it is shown that the QD emission spectrum is virtually unchanged by coupling to graphene. These results, which show how graphene affects QD fluorescence on the 10−9−10−8 and 100− 102 s time scales, are important for future solar cell applications, where it is hoped that QDs may be used to enhance the efficiencies and graphene may be used as a transparent electrode.

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CONCLUSION Monolayer graphenea 2D transparent conductor―uniquely influences the optoelectronic properties of semiconductor nanocrystals. A combination of techniques such as AFM, emission spectroscopy, fluorescence lifetime imaging microscopy, and blinking imaging microscopy is necessary to study these properties in detail. Fluorescence lifetimes of QDs decrease on graphene (as opposed to on an insulator such as glass), and these shorter lifetimes are attributed to the transfer of energy between QDs and graphene. This energy transfer rate is estimated at γET ≈ 5 × 108 s−1. From comparisons with the literature,32,33 it appears that the energy transfer rate depends

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 18450



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(17) Shimizu, K. T.; Neuhauser, R. G.; Leatherdale, C. A.; Empedocles, S. A.; Woo, W. K.; Bawendi, M. G. Blinking Statistics in Single Semiconductor Nanocrystal Quantum Dots. Phys. Rev. B 2001, 63, 205316. (18) Stefani, F. D.; Knoll, W.; Kreiter, M.; Zhong, X.; Han, M. Y. Quantification of Photoinduced and Spontaneous Quantum-Dot Luminescence Blinking. Phys. Rev. B 2005, 72, 125304. (19) Margolin, G.; Protasenko, V.; Kuno, M.; Barkai, E. Photon Counting Statistics for Blinking CdSe−ZnS Quantum Dots: A Lévy Walk Process. J. Phys. Chem. B 2006, 110, 19053−19060. (20) Frantsuzov, P.; Kuno, M.; Janko, B.; Marcus, R. A. Universal Emission Intermittency in Quantum Dots, Nanorods and Nanowires. Nat. Phys. 2008, 4, 519−522. (21) Orrit, M.; Basché, T. Steady Light from Quantum Dots, at Last. But How? ChemPhysChem 2009, 10, 2383−2385. (22) Spinicelli, P.; Mahler, B.; Buil, S.; Quélin, X.; Dubertret, B.; Hermier, J.-P. Non-Blinking Semiconductor Colloidal Quantum Dots for Biology, Optoelectronics and Quantum Optics. ChemPhysChem 2009, 10, 879−882. (23) Qin, W.; Liu, H.; Guyot-Sionnest, P. Small Bright Charged Colloidal Quantum Dots. ACS Nano 2014, 8, 283−291. (24) Voznyy, O.; Sargent, H. E. Atomistic Model of Fluorescence Intermittency of Colloidal Quantum Dots. Phys. Rev. Lett. 2014, 112, 157401. (25) Schmidt, R.; Krasselt, C.; Göhler, C.; von Borczyskowski, C. The Fluorescence Intermittency for Quantum Dots Is Not Power-Law Distributed: A Luminescence Intensity Resolved Approach. ACS Nano 2014, 8, 3506−3521. (26) Peng, L.; Feng, Y.; Lv, P.; Lei, D.; Shen, Y.; Li, Y.; Feng, W. Transparent, Conductive, and Flexible Multiwalled Carbon Nanotube/ Graphene Hybrid Electrodes with Two Three-Dimensional Microstructures. J. Phys. Chem. C 2012, 116, 4970−4978. (27) Zhang, D.; Gan, L.; Cao, Y.; Wang, Q.; Qi, L.; Guo, X. Understanding Charge Transfer at PbS-Decorated Graphene Surfaces toward a Tunable Photosensor. Adv. Mater. 2012, 24, 2715−2720. (28) Klekachev, A. V.; Asselberghs, I.; Kuznetsov, S. N.; Cantoro, M.; Mun, J. H.; Cho, B. J.; Hotta, J.-i.; Hofkens, J.; van der Veen, M.; Stesmans, A. L.; et al. Charge Transfer Effects in Graphene−CdSe/ ZnS Quantum Dots Composites. Proc. SPIE 2012, 8462, 84620L. (29) Zhang, C.; Xu, J.; Zhang, S.; Ji, X.; He, Z. One-Pot Synthesized DNA−CdTe Quantum Dots Applied in a Biosensor for the Detection of Sequence-Specific Oligonucleotides. Chem.Eur. J. 2012, 18, 8296−8300. (30) Kim, Y.-T.; Shin, H.-W.; Ko, Y.-S.; Ahn, T. K.; Kwon, Y.-U. Synthesis of a CdSe-Graphene Hybrid Composed of CdSe Quantum Dot Arrays Directly Grown on CVD-Graphene and its Ultrafast Carrier Dynamics. Nanoscale 2013, 5, 1483−1488. (31) Debgupta, J.; Mandal, S.; Kalita, H.; Aslam, M.; Patra, A.; Pillai, V. Photophysical and Photoconductivity Properties of Thiol-Functionalized Graphene−CdSe QD Composites. RSC Adv. 2014, 4, 13788− 13795. (32) Chen, Z.; Berciaud, S.; Nuckolls, C.; Heinz, T. F.; Brus, L. E. Energy Transfer from Individual Semiconductor Nanocrystals to Graphene. ACS Nano 2010, 4, 2964−2968. (33) Ajayi, O. A.; Anderson, N. C.; Cotlet, M.; Petrone, N.; Gu, T.; Wolcott, A.; Gesuele, F.; Hone, J.; Owen, J. S.; Wong, C. W. TimeResolved Energy Transfer from Single Chloride-Terminated Nanocrystals to Graphene. Appl. Phys. Lett. 2014, 104, 171101. (34) Kasry, A.; Ardakani, A. A.; Tulevski, G. S.; Menges, B.; Copel, M.; Vyklicky, L. Highly Efficient Fluorescence Quenching with Graphene. J. Phys. Chem. C 2012, 116, 2858−2862. (35) Liu, X.; Wang, G.; Song, X.; Feng, F.; Zhu, W.; Lou, L.; Wang, J.; Wang, H.; Bao, P. Energy Transfer from a Single Nitrogen-Vacancy Center in Nanodiamond to a Graphene Monolayer. Appl. Phys. Lett. 2012, 101, 233112. (36) Tisler, J.; Oeckinghaus, T.; Stoehr, R. J.; Kolesov, R.; Reuter, R.; Reinhard, F.; Wrachtrup, J. Single Defect Center Scanning Near-Field Optical Microscopy on Graphene. Nano Lett. 2013, 13, 3152−3156.

ACKNOWLEDGMENTS This work is supported by the ANR project NAPHO (contract ANR-08-NANO-054) and the European STREP ARTIST (contract FP7 243421). This work was partly supported by the French RENATECH network. We acknowledge technical support from the Centrale de Technologie Universitaire IEFMinerve in Orsay. Thanks to D. Canneson for discussions and a critical reading of the manuscript and to M. Kociak for TEM measurements.

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REFERENCES

(1) Cao, A.; Liu, Z.; Chu, S.; Wu, M.; Ye, Z.; Cai, Z.; Chang, Y.; Wang, S.; Gong, Q.; Liu, Y. A Facile One-step Method to Produce Graphene−CdS Quantum Dot Nanocomposites as Promising Optoelectronic Materials. Adv. Mater. 2010, 22, 103−106. (2) Chang, H.; Lv, X.; Zhang, H.; Li, J. Quantum Dots Sensitized Graphene: In situ Growth and Application in Photoelectrochemical Cells. Electrochem. Commun. 2010, 12, 483−487. (3) Geng, X.; Niu, L.; Xing, Z.; Song, R.; Liu, G.; Sun, M.; Cheng, G.; Zhong, H.; Liu, Z.; Zhang, Z.; et al. Aqueous-Processable Noncovalent Chemically Converted Graphene−Quantum Dot Composites for Flexible and Transparent Optoelectronic Films. Adv. Mater. 2010, 22, 638−642. (4) Lightcap, I. V.; Kamat, P. V. Fortification of CdSe Quantum Dots with Graphene Oxide. Excited State Interactions and Light Energy Conversion. J. Am. Chem. Soc. 2012, 134, 7109−7116. (5) Song, Y. M.; Yoon, M.; Jang, S. Y.; Jang, D. M.; Cho, Y. J.; Kim, C. H.; Park, J.; Cha, E. H. Size and Phase Controlled Synthesis of CdSe/ZnS Core/Shell Nanocrystals Using Ionic Liquid and Their Reduced Graphene Oxide Hybrids as Promising Transparent Optoelectronic Films. J. Phys. Chem. C 2011, 115, 15311−15317. (6) Wang, D.-Y.; Wang, I.-S.; Huang, I.-S.; Yeh, Y.-C.; Li, S.-S.; Tu, K.-H.; Chen, C.-C.; Chen, C.-W. Quantum Dot Light-Emitting Diode Using Solution-Processable Graphene Oxide as the Anode Interfacial Layer. J. Phys. Chem. C 2012, 116, 10181−10185. (7) Huang, X.; Qi, X.; Boey, F.; Zhang, H. Graphene-Based Composites. Chem. Soc. Rev. 2012, 41, 666−686. (8) Katsnelson, M. I. Graphene: Carbon in Two Dimensions. Mater. Today 2007, 10, 20−27. (9) Bae, S.; Kim, H.; Lee, Y.; Xu, X.; Park, J.-S.; Zheng, Y.; Balakrishnan, J.; Lei, T.; Kim, H. R.; Song, Y. I.; et al. Roll-to-Roll Production of 30-in. Graphene Films for Transparent Electrodes. Nat. Nanotechnol. 2010, 5, 574−578. (10) Vaziri, S.; Lupina, G.; Henkel, C.; Smith, A. D.; Ostling, M.; Dabrowski, J.; Lippert, G.; Mehr, W.; Lemme, M. C. A GrapheneBased Hot Electron Transistor. Nano Lett. 2013, 13, 1435−1439. (11) Ojeda-Aristizabal, C.; Bao, W.; Fuhrer, M. S. Thin-Film Barristor: A Gate-Tunable Vertical Graphene−Pentacene Device. Phys. Rev. B 2013, 88, 035435. (12) Brokmann, X.; Coolen, L.; Dahan, M.; Hermier, J. P. Measurement of the Radiative and Nonradiative Decay Rates of Single CdSe Nanocrystals through a Controlled Modification of their Spontaneous Emission. Phys. Rev. Lett. 2004, 93, 107403. (13) Talapin, D. V.; Lee, J.-S.; Kovalenko, M. V.; Shevchenko, E. V. Prospects of Colloidal Nanocrystals for Electronic and Optoelectronic Applications. Chem. Rev. 2010, 110, 389−458. (14) Wanger, D. D.; Correa, R. E.; Dauler, E. A.; Bawendi, M. G. The Dominant Role of Exciton Quenching in PbS Quantum-Dot-Based Photovoltaic Devices. Nano Lett. 2013, 13, 5907−5912. (15) Wang, Y.; Fruhwirth, G.; Cai, E.; Ng, T.; Selvin, P. R. 3D SuperResolution Imaging with Blinking Quantum Dots. Nano Lett. 2013, 13, 5233−5241. (16) 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. 18451



Article

(37) Guo, X. T.; Ni, Z. H.; Yan Liao, C.; Yan Nan, H.; Zhang, Y.; Wei Zhao, W.; Hui Wang, W. Fluorescence Quenching of CdSe Quantum Dots on Graphene. Appl. Phys. Lett. 2013, 103, 201909. (38) Liao, C. Y.; Zhu, X. H. Energy Transfer from CdSe Quantum Dots to Graphene. Proc. SPIE 2013, 9068, 90680Q. (39) Gaudreau, L.; Tielrooij, K. J.; Prawiroatmodjo, G. E. D. K.; Osmond, J.; Garcia de Abajo, F. J.; Koppens, F. H. L. Universal Distance-Scaling of Nonradiative Energy Transfer to Graphene. Nano Lett. 2013, 13, 2030−2035. (40) Li, Z.; He, M.; Xu, D.; Liu, Z. Graphene Materials-Based Energy Acceptor Systems and Sensors. J. Photochem. Photobiol. C 2014, 18, 1− 17. (41) Hirose, T.; Shigemasa, H.; Kutsuma, Y.; Kaneko, T.; Tamai, N. Energy Transfer Dynamics of CdTe Quantum Dots on Epitaxial Graphene Prepared by Si Sublimation of 4H-SiC(0001). Chem. Lett. 2014, 43, 125−127. (42) Blandin, P.; Lévêque-Fort, S.; Lécart, S.; Cossec, J. C.; Potier, M.-C.; Lenkei, Z.; Druon, F.; Georges, P. Time-Gated Total Internal Reflection Fluorescence Microscopy with a Supercontinuum Excitation Source. Appl. Opt. 2009, 48, 553−559. (43) Schneider, C. A.; Rasband, W. S.; Eliceiri, K. W. NIH Image to ImageJ: 25 Years of Image Analysis. Nat. Meth. 2012, 9, 671−675. (44) Nemanich, R. J.; Solin, S. A. First- and Second-Order Raman Scattering from Finite-Size Crystals of Graphite. Phys. Rev. B 1979, 20, 392−401. (45) Vidano, R.; Fischbach, D.; Willis, L.; Loehr, T. Observation of Raman Band Shifting with Excitation Wavelength for Carbons and Graphites. Solid State Commun. 1981, 39, 341−344. (46) Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S.; et al. Raman Spectrum of Graphene and Graphene Layers. Phys. Rev. Lett. 2006, 97, 187401. (47) Tuinstra, F.; Koenig, J. L. Raman Spectrum of Graphite. J. Chem. Phys. 1970, 53, 1126−1130. (48) Cancado, L. G.; Jorio, A.; Martins Ferreira, E. H.; Stavale, F.; Achete, C. A.; Capaz, R. B.; Moutinho, M. V. O.; Lombardo, A.; Kulmala, T. S.; Ferrari, A. C. Quantifying Defects in Graphene via Raman Spectroscopy at Different Excitation Energies. Nano Lett. 2011, 11, 3190−3196. (49) Gryczynski, I.; Malicka, J.; Jiang, W.; Fischer, H.; Chan, W. C. W.; Gryczynski, Z.; Grudzinski, W.; Lakowicz, J. R. Surface-PlasmonCoupled Emission of Quantum Dots. J. Phys. Chem. B 2005, 109, 1088−1093. (50) Biteen, J. S.; Lewis, N. S.; Atwater, H. A.; Mertens, H.; Polman, A. Spectral Tuning of Plasmon-Enhanced Silicon Quantum Dot Luminescence. Appl. Phys. Lett. 2006, 88, 131109. (51) Van Vlack, C.; Kristensen, P. T.; Hughes, S. Spontaneous Emission Spectra and Quantum Light−Matter Interactions from a Strongly Coupled Quantum Dot Metal−Nanoparticle System. Phys. Rev. B 2012, 85, 075303. (52) Ferri, C. G. L.; Inman, R. H.; Rich, B.; Gopinathan, A.; Khine, M.; Ghosh, S. Plasmon-Induced Enhancement of Intra-Ensemble FRET in Quantum Dots on Wrinkled Thin Films. Opt. Mater. Express 2013, 3, 383−389. (53) Briscoe, J. L.; Jayasundara, N.; Cho, S.-Y. Surface-PlasmonPolariton Assisted Modification of Spontaneous Emission of Colloidal Quantum Dots in Metal Nanostructures. Appl. Phys. Lett. 2013, 102, 023113. (54) Mak, K. F.; Sfeir, M. Y.; Wu, Y.; Lui, C. H.; Misewich, J. A.; Heinz, T. F. Measurement of the Optical Conductivity of Graphene. Phys. Rev. Lett. 2008, 101, 196405. (55) Nirmal, M.; Murray, C. B.; Bawendi, M. G. Fluorescence-Line Narrowing in CdSe Quantum Dots: Surface Localization of the Photogenerated Exciton. Phys. Rev. B 1994, 50, 2293−2300. (56) Wang, X.; Qu, L.; Zhang, J.; Peng, X.; Xiao, M. Surface-Related Emission in Highly Luminescent CdSe Quantum Dots. Nano Lett. 2003, 3, 1103−1106.

(57) Chen, X.; Lou, Y.; Samia, A. C.; Burda, C. Coherency Strain Effects on the Optical Response of Core/Shell Heteronanostructures. Nano Lett. 2003, 3, 799−803. (58) Lee, W. Z.; Shu, G. W.; Wang, J. S.; Shen, J. L.; Lin, C. A.; Chang, W. H.; Ruaan, R. C.; Chou, W. C.; Lu, C. H.; Lee, Y. C. Recombination Dynamics of Luminescence in Colloidal CdSe/ZnS Quantum Dots. Nanotechnol. 2005, 16, 1517−1521. (59) Liu, I.-S.; Lo, H.-H.; Chien, C.-T.; Lin, Y.-Y.; Chen, C.-W.; Chen, Y.-F.; Su, W.-F.; Liou, S.-C. Enhancing Photoluminescence Quenching and Photoelectric Properties of CdSe Quantum Dots with Hole-Accepting Ligands. J. Mater. Chem. 2008, 18, 675−682. (60) Brokmann, X.; Hermier, J. P.; Messin, G.; Desbiolles, P.; Bouchaud, J. P.; Dahan, M. Statistical Aging and Nonergodicity in the Fluorescence of Single Nanocrystals. Phys. Rev. Lett. 2003, 90, 120601. (61) Peterson, J. J.; Nesbitt, D. J. Modified Power Law Behavior in Quantum Dot Blinking: A Novel Role for Biexcitons and Auger Ionization. Nano Lett. 2009, 9, 338−345. (62) Cui, J.; Beyler, A. P.; Bischof, T. S.; Wilson, M. W. B.; Bawendi, M. G. Deconstructing the Photon Stream from Single Nanocrystals: From Binning to Correlation. Chem. Soc. Rev. 2014, 43, 1287−1310. (63) Fernee, M. J.; Tamarat, P.; Lounis, B. Spectroscopy of Single Nanocrystals. Chem. Soc. Rev. 2014, 43, 1311−1337. (64) Cordones, A. A.; Leone, S. R. Mechanisms for Charge Trapping in Single Semiconductor Nanocrystals Probed by Fluorescence Blinking. Chem. Soc. Rev. 2013, 42, 3209−3221. (65) Efros, A. L.; Rosen, M. Random Telegraph Signal in the Photoluminescence Intensity of a Single Quantum Dot. Phys. Rev. Lett. 1997, 78, 1110−1113. (66) Fu, Y.; Zhang, J.; Lakowicz, J. R. Suppressed Blinking in Single Quantum Dots (QDs) Immobilized Near Silver Island Films (SIFs). Chem. Phys. Lett. 2007, 447, 96−100. (67) Ito, Y.; Matsuda, K.; Kanemitsu, Y. Mechanism of Photoluminescence Enhancement in Single Semiconductor Nanocrystals on Metal Surfaces. Phys. Rev. B 2007, 75, 033309. (68) Ma, X.; Tan, H.; Kipp, T.; Mews, A. Fluorescence Enhancement, Blinking Suppression, and Gray States of Individual Semiconductor Nanocrystals Close to Gold Nanoparticles. Nano Lett. 2010, 10, 4166−4174. (69) Canneson, D.; Mallek-Zouari, I.; Buil, S.; Quélin, X.; Javaux, C.; Mahler, B.; Dubertret, B.; Hermier, J.-P. Strong Purcell Effect Observed in Single Thick-Shell CdSe/CdS Nanocrystals Coupled to Localized Surface Plasmons. Phys. Rev. B 2011, 84, 245423.

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Fluorescence Lifetime and Blinking of Individual Semiconductor

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