J. Phys. Chem. C 2008, 112, 1345-1350
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Photoluminescence Quenching and Intensity Fluctuations of CdSe-ZnS Quantum Dots on an Ag Nanoparticle Film Yuusuke Matsumoto,†,‡ Ryodai Kanemoto,†,‡ Tamitake Itoh,† Shunsuke Nakanishi,‡ Mitsuru Ishikawa,†,§,# and Vasudevanpillai Biju*,†,§ Nano-bioanalysis Team, Health Technology Research Center, National Institute of AdVanced Industrial Science and Technology (AIST), 2217-14 Hayashi-cho, Takamatsu, Kagawa 761-0395, Japan, and Department of AdVanced Materials Science, Faculty of Engineering, Kagawa UniVersity, 2217-20 Hayashi-cho, Takamatsu, Kagawa 761-0396, Japan ReceiVed: August 18, 2007; In Final Form: October 13, 2007
Intermittent “on” and “off” (blinking) photoluminescence (PL) of single-CdSe/ZnS quantum dots (QDs) is modified when placed on an Ag nanoparticle (NP) film into stochastic fluctuations with nonzero intensity “off” (pseudo off) periods. Also, the PL quantum efficiency (from 0.42 to 0.22) and lifetime (from 5.2 to 1.5 ns) of QDs are considerably decreased at ensemble level in the presence of Ag NPs, and a histogram of the PL lifetime of single-QDs is shifted (from 4.2 to 1.7 ns) and tapered (full width at half-maximum from 3.3 to 1.1 ns) when placed on an Ag NP film. The quenching of the PL quantum efficiency and decrease of the PL lifetime are attributed to ultrafast energy transfer from photoexcited QDs to Ag NPs. The energytransfer process competes with exciton relaxations and influences carrier trapping in surface defect-states (band gap defects) and Auger relaxation, which are considered to be the origins of blinking. The contribution of surface-states on the modified PL was identified from decreased contribution of a slow component to the PL decays of ensemble- and single-QDs in the presence of Ag NPs. On the basis of ensemble averaged PL intensity and lifetime and single-QD lifetime and intensity trajectory analyses, we propose that the energytransfer process from photoexcited QDs to Ag NPs result a redistribution of relaxation processes and provide fluctuating trajectories with nonzero intensities to single-QDs. Apart from the observations of modified blinking and narrow lifetime distribution of QDs, the current work partially supports a model proposed by Markus and co-workers that blinking is related to localization of charge carriers in defects.
Introduction Structural modifications via chemical conjugation and modified photoluminescence (PL) of semiconductor quantum dots (QDs) are of great interest in recent years for various applications including photovoltaic and optoelectronic devices,1-3 sensors,4-9 and biolabeling.10-13 Unique optical properties14,15 including broad absorption and narrow emission bands, bright and stable PL, and size-tunable electronic band gap and emission color make QDs promising for these applications. Also, the unique optical properties make QDs powerful substitutes for organic dye molecules, in particular, when photostability and brightness are required and relatively large size of QD is not a limitation. Over the last 5 years, QDs attracted much attention in bioanalytical detection and visualizing structures and functions of biomolecules and biological systems due to the above optical properties and enormous possibilities of bioconjugation with least chemical modifications. However, intrinsic intermittent “on” and “off” (blinking) PL16-43 on a wide time scale from milliseconds to minutes, with long-living “off” states, continues to be a property which limits the time averaged PL intensity and applications of QDs as single-molecule biophysical probes. Furthermore, compared to the blinking of fluorescent single* Corresponding author. E-mail:
[email protected]. Phone: (+81) 87869-3558. Fax: (+81) 87869-4113. † AIST. ‡ Kagawa University. # E-mail:
[email protected]. § Also at the Center for Arthropod Bioresources and Biotechnology, Kerala University, Kariavattom, Trivandrum 695 583, India.
molecules44-46 that of single-QDs is less sensitive to environmental changes and less useful in single-molecule analyses. Therefore, QDs with reduced blinking and non-blinking characteristics would be valuable to utilize the superior luminescence properties of QDs efficiently. Escape of a charge carrier from a photoactivated QD followed by ultrafast (,100 ps) Auger relaxation which prevails relatively slow (>100 ps) radiative relaxations is one of the origins of the PL intermittency of QDs.33 Transient trapping of carriers in surface defect-states (band gap defects) is also included in the discussion of blinking.16-18,35,37,39,40 Despite these possibilities, there are discrepancies between theoretical models of blinking and experimental results; power-law dependence of “off” time distributions over five decades of time and linear relationship between excitation intensity and “on/off” time is an example.23,27-29,31,32,34-36 Indeed, exponential and quadratic relations were theoretically predicted for the above two cases, and alternative mechanisms were conferred to explain the anomalies. In order to figure out the discrepancies of the blinking behavior, from power-law over different decades of time to single-exponential “off” time distributions, in the theoretical and experimental milieus and to rationalize the blinking behavior, researchers are involved in modification of surface chemistry and chemical environment of QDs.34,37-40,47 Mechanisms on debate are Auger ionization and involvement of surface defectstates which transiently trap and detrap electrons for short intervals and holes for an extended period of time leading to different “off” periods.41,42 Efforts based on the preparation of
10.1021/jp076659+ CCC: $40.75 © 2008 American Chemical Society Published on Web 12/01/2007
1346 J. Phys. Chem. C, Vol. 112, No. 5, 2008
Matsumoto et al.
shells with different thickness and different materials, different surface ligand coverage, interfacing of QD surface with electron donor molecules and electric fields, etc. were examined to clarify these discrepancies.17,21,26,34,37-40,47 Also, correlation between noble metal environment and PL showed fluctuations, enhancements, and quenching of PL intensities both at ensemble- and single-QD levels.34,47-61 In a recent report, Kanemitsu and coworkers discussed quenching and enhancement of emission from single-QDs present on Au surfaces as a function of surface roughness in which they attributed energy transfer to the quenching and increased absorption and plasmon effect to the enhancement.47 Also, they observed considerable suppression of blinking when QDs are placed on flat/rough Au substrates which is analogous to suppressed blinking of single-QDs in the presence of thiols.37 More recently, Nienhaus and co-workers reported that blinking of QDs is independent of ZnS shell thickness, on the basis of which they ruled out Auger ionization via tunneling of electrons to an outside trap.34 On the other hand, they proposed hole trapping, which was originally proposed by Markus and Frantsuzov,41 to account for a deviated power-law exponent and cutoff time in intensity autocorrelations. Here, we observed that the PL intermittencies of singleCdSe-ZnS QDs are modified when placed on an Ag nanoparticle (NP) film. In place of two-state “on” and “off” blinking of pristine CdSe-ZnS QDs, we observed fluctuating intensities with incomplete (pseudo) “off” states for QDs present on an Ag NP film. Also, the frequency of long-living “off” states was considerably reduced on an Ag NP film. We correlated the fluctuating PL intensity of single QDs with quenching of PL quantum efficiency and lifetime at ensemble- and single-QD levels and narrowing of single-QD lifetime distributions. Energy transfer from dark exciton state and surface defect-states of photoexcited QDs to Ag NPs are invoked in the discussion. Experimental Section A colloidal CdSe-ZnS QD sample with PL intensity maximum ∼600 nm (QD605) was obtained from Evident Technologies (NY). From this, dilutions were made using chloroform thus preparing 1 nM QD solutions. This was followed by preparing 200∼50 pM solutions of QDs in ethanol. Analytical grade chloroform and ethanol were obtained from Wako, Japan and used as supplied. Ag NP solutions were prepared by the Lee-Meisel62 method in which a solution of AgNO3 was reduced by citrate ions. Samples for ensemble PL measurements were prepared by adding required amounts of an Ag NP solution to QD solutions. Aggregations of Ag NPs and QDs were minimized under the selected concentrations which we verified from AFM imaging of samples prepared by dispersing the QD and Ag NP+QD solutions on glass substrates. Samples for single-QD imaging, and PL intensity time-trajectory and lifetime measurements were prepared by dispersing a 50 pM QD605 solution on a slide glass. Ag NP + QD samples were prepared by dropping and dragging of Ag NP + QD (50 pM) solutions on slide glasses. AFM image of a thin film of Ag NPs and PL image of QDs present on an Ag NP film are shown in Figure 1, panels A and B, respectively. It may be noted that the PL and AFM images were recorded independently and are not correlated. A cartoon of an Ag NP + QD sample is given in Figure 1C which was prepared based on difference in the PL intensity and intensity trajectory characteristics of singleQDs in the sample, details of which are discussed below. PL spectra of QD solutions were recorded using a Hitachi4500 spectrofluorometer. AFM images were recorded using an Asylum-MFP3D microscope. Tapping-mode AFM images were
Figure 1. (A) Tapping mode AFM image of Ag NPs distributed on a glass substrate. (B) PL image of single-CdSe-ZnS QDs present on an Ag NP film. The thick-line, narrow-line, and dotted-line circles classify single-QDs based their PL intensities. (C) Cartoon presentation of a combination of A and B: (i) a high-intensity QD in B present at a hot-spot formed between particle plasmons of two or more adjacent Ag NPs, (ii) a medium-intensity QD in B which is not interacting with Ag NPs, and (iii) a low-intensity QD in B which is involved in energy transfer interactions with Ag NPs.
collected in air using reflective-aluminum-coated ultrasharp (radius of curvature
