Modification of Photon Emission Statistics from Single Colloidal CdSe

Jul 18, 2014 - Hiroki Takata , Hiroyuki Naiki , Li Wang , Hideki Fujiwara , Keiji Sasaki , Naoto Tamai , and Sadahiro Masuo. Nano Letters 2016 16 (9),...
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Modification of Photon Emission Statistics from Single Colloidal CdSe Quantum Dots by Conductive Materials Hui-Wen Cheng,† Chi-Tsu Yuan,*,‡ Jyh-Shyang Wang,‡ Tzu-Neng Lin,‡ Ji-Lin Shen,‡ Yu-Ju Hung,§ Jau Tang,∥ and Fan-Gang Tseng*,†,∥ †

Department of Engineering and System Science, National Tsing Hua University, Hsinchu 30013, Taiwan Department of Physics, Chung Yuan Christian University, Chungli 32023, Taiwan § Department of Photonics, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan ∥ Research Center for Applied Sciences, Academia Sinica, Nankang, Taipei 11529, Taiwan ‡

S Supporting Information *

ABSTRACT: Colloidal CdSe/ZnS core/shell quantum dots (QDs) can be used to generate single photons at room temperature for promising applications in quantum information sciences. Despite the existence of biexciton (BX) states, they would undergo fast nonradiative Auger relaxation (AR), leading to pure single-exciton (SX) emission, even under highpower excitation. In this work, the fluorescence properties of single colloidal QDs deposited on the conductive ITO surface have been investigated by combining photon correlation spectroscopy and time-tagged, time-correlated single-photon counting (TT-TCSPC) measurement. The conductive materials can introduce extra nonradiative (NR) processes, and thus can significantly alter pristine emission properties, including emission brightness, fluorescence lifetime, and photon emission statistics. We found that such a NR process can strongly quench SX emission but has less effect on BX emission, leading to the increase of the quantum-yield (QY) ratio between BX and SX emission. As a result, the purity of single-photon emission would be degraded despite the fact that fluorescence blinking can be suppressed. This experimental understanding is significant for further designing high-performance single-photon sources based on low-cost colloidal QDs operating at room temperature.

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INTRODUCTION Colloidal CdSe/ZnS core/shell QDs can be used to generate single photons operating at room temperature even under highpower excitation for promising applications in quantum information sciences.1 The BX states within the colloidal QDs would undergo fast nonradiative Auger relaxation (NRAR), leading to pure single-photon emission arising from SX states.2 It is well-known that the fluorescence properties of single colloidal QDs can be influenced by their local environment, by which, some near-field interaction, such as energy transfer or charge transfer, can be introduced, thus altereing their pristine fluorescence properties.3−5 So far, most previous works have focused on the modification of fluorescence blinking behavior when colloidal QDs were placed in the proximity of conductive materials, such as metallic nanoparticles, metallic film, and the graphene sheet.6−9 In fact, metallic materials can significantly modify the single-QD emission properties in a number of ways, including enhancing excitation rates by the excitation of surface plasmons, the acceleration of radiative decay rates by Purcell effect, and the increase of nonradiative decay rates via energy transfer.10,11 For example, L. Novotny et al. has proposed that the blinking behavior was suppressed when single colloidal QDs are put © 2014 American Chemical Society

close to gold nanoparticles, owing to the introduction of an efficient NR energy transfer process.7 On the other hand, the fluorescence properties have also been investigated for colloidal QDs deposited on conductive ITO surface, which cannot support surface plasmons but still allow NR energy/charge transfer to occur. Previous work has been proposed that when colloidal QDs were placed on the ITO surface or n-type semiconductor, the electrons can flow from ITO to QDs.12 Such extra electrons can play two roles in modifying single-QD emission. The first one is to charge neutral QDs to form negatively charged QDs, in which, the AR can be activated, leading to lifetime shortening. The second one is to neutralize positively charged QDs, thus suppressing fluorescence blinking by shortening the duration of off-state. In contrast, some groups also pointed out that energy transfer can be introduced from excited QDs to the ITO surface, leading to the modification of single-QD emission behavior.13,14 It is known that for practical applications in light-emitting devices, colloidal QDs need to be placed nearby the conductive Received: April 7, 2014 Revised: July 18, 2014 Published: July 18, 2014 18126

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Figure 1. Fluorescence time trajectories of a single colloidal QD deposited on (a) glass and the (b) corresponding intensity distribution, as well as the (c) intensity correlation function.

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materials for charge injection. As a result, a better understanding of photon statistics of single-QD emission is important for colloidal QDs being served as room-temperature quantum light sources. So far, most effort has been made on the understanding of the influence of conductive materials on fluorescence blinking but leaving photon statistics of single-QD emission unexplored. Such investigations are significant for colloidal QDs serving as single-photon emitting devices, in which colloidal QDs need to be placed nearby conductive materials (e.g., conductive charge transporting layer) for charge injection. Indeed, some interesting results regarding these issues have been studied for colloidal QDs deposited on metallic materials.15,16 For example, upon coupling to metallic nanostructures, superPoissonian statistics of photon emission from giant CdSe/CdS core/shell QDs was observed.15 In addition, the multiphoton emission can be enhanced from standard CdSe/ZnS core/shell QDs with coupling to roughed metallic film.16 In both aforementioned cases, complex plasmonic interactions were introduced, leading to the modification of both excitation and emission rates (including the radiative and nonradiative processes), thus complicating further analyses and the role of externally introduced NR process on photon statistics of singleQD emission cannot be disentangled. In this work, the photon statistics of single-QD emission for colloidal CdSe/ZnS core/shell QDs deposited on conductive ITO surface are investigated using photon correlation spectroscopy and time-correlated single-photon counting (TCSPC) measurements. Such conductive ITO materials can introduce extra NR pathways, thus modifying original single-QD emission behavior, altering photon statistics, in particular. We found that the purity of single-photon emission, which is characterized by the feature of photon antibunching, would be degraded accompanied by blinking suppression. This understanding is significant for colloidal QDs serving as low-cost single-photon light sources operating at room temperature.

EXPERIMENTAL SECTION

I. Sample Preparation. The thin ITO-coated cover-glass were purchased from NANOCS (∼10 ohm, ∼70 nm thickness) and flat Au coated coverslips were fabricated using an electron beam evaporator (∼20 nm thickness, the topography imaging can be seen in section 1 of the Supporting Information). Colloidal CdSe/ZnS core/shell QDs with TOP/TOPO ligands suspended in decane and emitting at ∼655 nm were purchased from Invitrogen. II. Single-QD Measurement. For single-QD experiments, a dilute solution was spin-cast onto the aforementioned substrates. In this case, the mean separation among QDs is larger than the laser spots (∼300 nm), thus an individual QD can be monitored by far-field laser scanning confocal microscope (section 2 of the Supporting Information). In addition, we also confirmed our single-QD measurement using time-gated photon correlation spectroscopy (see the main text for more detail). The pulsed laser with an emission wavelength at ∼467 nm was focused onto the sample using an oilimmersion microscope objective (Olympus, 1.4 N.A.) in an inverted geometry. The repetition rate of our pulse laser is set to be 5 MHz. The fluorescence emission was collected by the same objective and then guided into a confocal pinhole so as to reject out-of-focus light. After a pinhole, the fluorescence is split by a 50/50 beam splitter cube into two beams and then filtered by suitable band-pass filters and finally detected by a pair of single-photon avalanche photon diodes (SPADs) based on the so-called Hanbury-Brown and Twiss configuration. All signals including the synchronization of pulse laser and two SPADs are fed into the photon counting module (HydraHarp 400, PicoQuant), by which all time information can be recorded simultaneously (so-called time-tagged, time-correlated singlephoton counting, TT-TCSPC), including microtime between excitation pulse and photon emission and macro-time between experimental start and photon emission. In this way, fluorescence time trajectories, fluorescence decay profiles and intensity correlation function can be obtained simultaneously 18127

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Figure 2. Fluorescence decay profiles of a single colloidal QD deposited on (a) glass and the intensity correlation function with a time gate to remove (b) BX emission.

under debate, the NR-AR might play an important role in determining single-QD fluorescence properties.19 To gain more insight into the emission dynamics, the timeresolved fluorescence decay profiles were also measured and are shown in Figure 2a for single colloidal QDs on pure glass (only selecting the photons within the on-time duration). For most of the QDs we measured here (>90%), their decay curves can be fitted well using a biexponential function with a longer lifetime component of ∼27 ns and a shorter one of ∼0.6 ns (the statistical distribution can be found in section 4 of the Supporting Information). The longer lifetime component is commonly observed for single QDs on pure glass and can be assigned to the emission from the pure radiative decay process of SX state.20 For the shorter lifetime component, it can be attributed to the contribution from the BX state, which was further confirmed by a photon correlation measurement with and without applying a suitable time gate (will be shown and discussed in more detail later).21,22 As a result, we can extract the relaxation lifetimes for both BX and SX states by fitting decay curves with this equation, I(t) = aSX exp(−t/τSX) + aBX exp(−t/τBX), where τSX and τBX are fluorescence lifetimes of SX and BX states, respectively. For the SX state within the colloidal QDs, the radiative decay is a dominant process owing to its near-unity quantum efficiency,23 thus the SX radiative lifetime can be approximately estimated from the measured fluorescence lifetime according to this equation, η = kr/(kr + knr) = kr × τfl = τfl/τr, where η, kr, knr, τfl, and τr, stand for emission QY, radiative decay rate, nonradiative decay rate, fluorescence lifetime, and radiative lifetime, respectively. For the referenced sample, the SX radiative decay rate is estimated to be kSX r ∼ 3.7 ± 0.6 × 107 Hz. Unlike the SX state with near-unity QYs, the BX state can undergo either radiative decay or NR-AR pathways. In general, the rate of NR-AR is much larger than that of BX radiative

by postprocessing photon streams. In addition, we can also postselect specific photons based on their TCSPC channels to obtain intensity correlation function by setting a suitable time gate.

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RESULTS AND DISCUSSION In order to unravel the influence of conductive materials on single-QD emission properties, in particular, focusing on photon emission statistics, here we investigate the fluorescence properties for single colloidal CdSe/ZnS QDs deposited on a pure glass coverslip (serving as a reference sample), conductive ITO surface (the main sample studied here) and gold thin-film coated substrates. As shown in Figure 1, pristine fluorescence properties were obtained for single QDs deposited on pure glass substrates (without introducing extra NR processes), thus can serve as a reference sample for further comparison. Clearly, such single QDs exhibit typical fluorescence blinking in the fluorescence time trajectories (Figure 1a) and a reduced peak at zero time delay in the intensity correlation function (Figure 1c) under low-power excitation conditions (section 3 of the Supporting Information). The intensity correlation function is defined as g(2) (τ) = ⟨I(t) × I(t + τ)⟩)/(⟨I(t)⟩2. It implies that for referenced single QDs without intentionally introducing extra NR pathways, they can mostly emit single photons but their emission suffers from fluorescence blinking.17 These two typical behavior of single-QD emission (single-photon emission and fluorescence blinking) can be usually explained by involving NR-AR into the exciton decay processes.18 In this case, the multiple-exciton states would undergo a rapid NR-AR process, leading to dominant SX emission. Similarly, the emission of a charged-exciton state can be strongly quenched by the same process, leading to fluorescence blinking. Even though the exact mechanism for fluorescence blinking is still 18128

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Figure 3. Fluorescence decay profiles of (a) a single colloidal QD deposited on ITO coated glass and the (b) fluorescence time trajectories together with the (c) corresponding intensity distribution.

decay, leading to weak or negligible BX emission (depending on QD size, QD shape, and core/shell interfaces). As a result, a single colloidal QD can mostly emit single photons even under high-power excitation. The previous report has demonstrated that the NR-AR lifetime follows a universal cubic law, that is it scales linearly with the volume of colloidal QDs.24 Therefore, the NR-AR lifetime can be calculated to be around 660 ps for colloidal QDs with emission at ∼655 nm. Combing the measured BX lifetime with the deduced NR-AR lifetime, the radiative decay rates for BX states can be deduced to be kBX r ∼ BX BX = 1/τ + 1/τ ). 108 Hz (1/τBX FL AR rad On the other hand, we can experimentally determine the BX QYs from photon correlation measurement under a low-power excitation condition (Figure 1c).25 In such a way, the BX QYs can be directly estimated by comparing the peak area ratio between a central peak and side peaks in the intensity correlation function (ηBX/ηSX = Acentral/Aside) and the value of BX QYs is ηBX = 0.1 ± 0.04. We found there exists large spreads of BX QYs among different QDs, even within the same batch, which is consistent with a previous report and might be due to tiny variations in sizes, shapes, and interfaces.25 In order to confirm the central peak in the intensity correlation function is indeed arising from BX emission of a single QD instead of SX emission from two or more independent QDs (for example, QD clusters), we reconstruct the intensity correlation function by applying a suitable time gate according to the TCSPC decay profile. For example, the photons emitted during the interval of 10−200 ns upon pulse-laser excitation (green shaded area in Figure 2a) can be selected to construct the intensity correlation function. In this case, most of the BX photons can be removed because the BX emission has decayed significantly prior to this time gating interval, leaving only pure SX emission. As shown in Figure 2b, indeed, a pure photon antibunching feature can be recovered. Such a comparison (photon correlation measurement with and without applying a suitable time gate) can provide two important pieces of evidence: (1) the QDs we

probed is an isolated single QD and (2) the shorter lifetime component is a result of the emission of the BX state.22 This confirmation is crucial for further analyses of our experimental data. Subsequently, we want to probe the fluorescence properties of single QDs upon intentionally introducing extra NR processes by depositing QDs on conductive materials, namely the ITO surface (section 5 of the Supporting Information). Figure 3a shows the fluorescence decay profile for a representative QD on the conductive ITO surface (statistical distribution can also be found in section 4 of the Supporting Information). This curve can be fitted well using an exponential decay plus a stretched exponential decay function, s(t) = aSX exp(−t/τSX)β + aBX exp(−t/τBX). Similarly, the shorter lifetime component of ∼0.5 ns is attributed to BX relaxation (again this will be confirmed later), and the longer one of ∼6 ns is assigned to a SX lifetime upon interacting with the ITO surface. By comparing the measured lifetime of single QDs on the ITO surface with that on pure glass, we found that both fluorescence lifetimes of BX and SX states can be reduced but with a distinct shortening factor. It should be plausible because the BX state has a larger BX decay rate with respect to that of the SX state and thus can be more competitive with an extra NR process, leading to a lower shortening factor. The main NR process introduced by ITO is assigned to be nonradiative energy transfer (NR-ET) from excited QDs to ITO materials (section 5 of the Supporting Information). In general, externally introduced NR processes exhibit time-varying characteristics, that is, the NR rates fluctuate with time, leading to the spread of the measured fluorescence lifetime (section 6 of the Supporting Information). In this case, the decay curve can be fitted with a stretched-exponential function.26 It is known that the NR-ET rate depends on the separation distance (d) between donors and acceptors (d−4 for dipole-surface NR-ET and d−6 for dipole−dipole NR-ET), relative dipole orientation, and spectral overlap between donor emission and acceptor absorption 18129

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Figure 4. Intensity correlation function for a single QD (a) deposited on ITO surface and with (b) a time gate to remove BX emission.

spectra.11 Single colloidal QDs exhibit large spectral diffusion and thus might be responsible for the fluctuation of NR-ET rates.27 From fluorescence lifetime variation between single QDs on ITO and pure glass, we can experimentally estimate the NR-ET rate, kET ∼ 1.23 × 108 Hz. Figure 3 (panels b and c) shows the typical fluorescence time trajectories and the corresponding intensity distribution for a representative single QD on a conductive ITO surface under a low-power excitation condition. As expected, the fluorescence intensity of on-state is quenched due to extra NR-ET accompanied by the increase of on-time events as compared with that of single QDs on pure glass. This phenomenon can also be explained by the introduction of the NR-ET process, which was also observed by other groups, for example, for single colloidal QDs deposited on conductive graphene sheet and metallic nanostructures.9,10 In addition to blinking suppression, the most interesting finding in our work is the modification of photon statistics of single-QD emission, as shown in Figure 4a. Clearly, the central peak in the intensity correlation function is raised significantly, indicating the degradation of the purity of single-photon emission. Similarly, in order to confirm this central peak indeed arising from the contribution of BX emission from the same single QD instead of SX emission from independent QDs, the time-gating method was applied to reconstruct the intensity correlation function by removing BX emission, as shown in Figure 4b. Upon selecting only the SX emission, the single-photon emission signature, that is, pure photon antibunching dip can be recovered, definitely proving that the photons detected are indeed coming from a single QD, and the contribution to residual peak at zero time delay is owing to BX emission. Such a comparison proves our observation is valid and is not due to experimental artifacts. The average value of peak-area ratio between a central peak and side peaks by compiling more than ∼70 individual QDs is ∼0.7 for single QDs deposited on the ITO surface, indicating

that the QY ratio between BX and SX is increased as compared with that of single QDs on pure glass. This implies that the purity of single-photon emission is degraded upon introducing extra NR processes. It is known that the fluorescence intensity can be estimated by this equation, Sfl = Ccollection × γexcitation × ηemission, where Sfl, Ccollection, γexcitation, and ηemission stand for measured fluorescence intensity, collection efficiency, excitation rates, and emission QYs, respectively. The main modification of single-QD emission on ITO as compared with that on pure glass could be mainly attributed to the increase of NR rates due to the NR-ET process, resulting in the reduction of fluorescence QYs, while leaving the collection efficiency and the excitation rates less affected. As a result, the SX QYs for single QDs deposited on ITO can be estimated to be ∼0.16 from fluorescence time trajectories (see section 7 of the Supporting Information) and then the BX QY can be estimated to be around ∼0.1 by photon correlation measurement. As a result, the purity of single-photon emission, which is determined by the QY ratio of BX and SX emission, would be degraded. In order to explain our experimental finding, that is, the purity of single-photon emission is degraded after introducing the extra NR process, we simply calculate the QY ratio between BX and SX emission using the following equation: ⎛η ⎞ ⎜⎜ BX ⎟⎟ ∼ ⎝ ηSX ⎠ET =

krBX krBX

BX + kAR + kET

krSX krSX + kET

4(krSX + kET) BX + kET 4krSX + kAR

, (krBX ∼ 4krSX)

(1)

Here, the QY of the SX state, ηSX was assumed to be near unity,23 the radiative decay rates of SX emission, kSX r , and the rate of BX Auger relaxation, kBX AR, have been determined 18130

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function of our system) than that of colloidal QDs on conductive ITO and on pure glass, as shown in Figure 6a. For such QDs, no quantitative data can be extracted for further analyses due to much faster decay for both BX and SX emission and no time-gating method can be applied for photon correlation measurement. Despite that, this measurement can still give a qualitative insight into the role of the extent of the NR-ET process on the purity of single-photon emission. As shown in Figure 6b, the area of central peak in the intensity correlation function is similar to that of side peaks, which is consistent with the trend predicted by our calculation.

previously. As shown in Figure 5, a plot of the BX/X QY ratio as a function of externally introduced NR-ET rates (red line)

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CONCLUSIONS In conclusion, the influence of extra NR-ET process on singleQD emission behavior, in particular focusing on photon statistics, has been investigated using photon correlation spectroscopy and TT-TCSPC measurement. By performing photon correlation measurement with and without applying a suitable time gate, the BX emission from a single colloidal QD can be definitely confirmed, and the QY ratio between BX and SX emission can be determined, thus the purity of singlephoton emission from a single colloidal QD can be estimated. We found that after introducing an extra NR-ET process, the SX emission can be strongly quenched but has less effect on BX emission. As a result, the QY ratio between BX and SX emission was increased, leading to the degradation of the purity of single-photon emission. This understanding can facilitate the researchers to further design high-performance single-photon sources based on low-cost colloidal QDs.

Figure 5. A plot of the QY ratio between BX and SX emission as a function of energy transfer rates.

can be obtained according to eq 1, and our experimental data are also shown in the figure (blue dots, the error bar is due to QD-to-QD heterogeneities). Without introducing the NR-ET process, that is, the case of single QDs on pure glass, this ratio is small due to fast BX-AR rates. Once introducing the extra NR-ET process, this QY ratio can be modified and would increase as increasing NR-ET rates. It also implies that as NRET rates are further increased, the QY ratio might be close to unity. In order to demonstrate this trend, we put single colloidal QDs on gold thin film, by which a more efficient NR-ET process can be introduced. As expected, the fluorescence decay profile for a single colloidal QD directly deposited on a gold thin film is much faster (close to the instrument response

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ASSOCIATED CONTENT

S Supporting Information *

(1) Fabrication of gold thin film. (2) Fluorescence imaging of colloidal QDs. (3) Number of excitons generated within the QDs. (4) Statistical distribution. (5) Interaction between colloidal QDs and ITO. (6) Fluorescence lifetime analyses.

Figure 6. Fluorescence decay profiles of (a) a single colloidal QD deposited on gold thin-film and the (b) corresponding intensity correlation function. 18131

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Shell Nanocrystals Coupled to Metal Nanostructures. Phys. Rev. Lett. 2013, 110, 117401/1−117401/5. (16) LeBlanc, S. J.; McClanahan, M. R.; Jones, M.; Moyer, P. J. Enhancement of Multiphoton Emission from Single CdSe Quantum Dots Coupled to Gold Films. Nano Lett. 2013, 13, 1662−1669. (17) Frantsuzov, P.; Kuno, M.; Janko, B.; Marcus, R. A. Universal Emission Intermittency in Quantum Dots, Nanorods and Banowires. Nat. Phys. 2008, 4, 519−522. (18) Galland, C.; Ghosh, Y.; Steinbruck, 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, 203−207. (19) Rosen, S.; Schwartz, O.; Oron, D. Transient Fluorescence of the Off State in Blinking CdSe/CdS/ZnS Semiconductor Nanocrystals Is Not Governed by Auger Recombination. Phys. Rev. Lett. 2010, 104, 157404/1−157404/4. (20) Fisher, B. R.; Eisler, H. J.; Stott, N. E.; Bawendi, M. G. Emission Intensity Dependence and Single-Exponential Behavior in Single Colloidal Quantum Dot Fluorescence Lifetimes. J. Phys. Chem. B 2004, 108, 143−148. (21) Fisher, B.; Caruge, J. M.; Zehnder, D.; Bawendi, M. G. RoomTemperature Ordered Photon Emission from Multiexciton States in Single CdSe Core-Shell Nanocrystals. Phys. Rev. Lett. 2005, 94, 087403/1−087403/4. (22) Mangum, B. D.; Ghosh, Y.; Hollingsworth, J. A.; Htoon, H. Disentangling the Effects of Clustering and Multi-Exciton Emission in Second-Order Photon Correlation Experiments. Opt. Express. Opt. Express 2013, 21, 7419−7426. (23) 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, 107403/1−107403/ 4. (24) Klimov, V. I.; Mikhailovsky, A. A.; McBranch, D. W.; Leatherdale, C. A.; Bawendi, M. G. Quantization of Multiparticle Auger Rates in Semiconductor Quantum Dots. Science 2000, 287, 1011−1013. (25) Nair, G.; Zhao, J.; Bawendi, M. G. Biexciton Quantum Yield of Single Semiconductor Nanocrystals from Photon Statistics. Nano Lett. 2011, 11, 1136−1140. (26) Issac, A.; Jin, S.; Lian, T. Intermittent Electron Transfer Activity from Single CdSe/ZnS Quantum Dots. J. Am. Chem. Soc. 2008, 130, 11280−11281. (27) Neuhauser, R. G.; Shimizu, K. T.; Woo, W. K.; Empedocles, S. A.; Bawendi, M. G. Correlation between Fluorescence Intermittency and Spectral Diffusion in Single Semiconductor Quantum Dots. Phys. Rev. Lett. 2000, 85, 3301−3304.

(7) Estimation of SX QYs for single QDs on ITO surface. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support by Academia Sinica. This work was also supported by grants from the National Science Council of Taiwan under the programs: NSC 99-2221-E-001002-MY3, NSC 99-2113-M-001-023-MY3, NSC 101-2627-M007-009, and NSC 102-2112-M-033-001-MY3.

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