Solvent-Polarity Dependence of Electron-Transfer Kinetics in a CdSe

Nov 10, 2009 - The PL intermittency or blinking kinetics of single QDs were analyzed by adapting a diffusion-controlled electron transfer (DCET) theor...
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J. Phys. Chem. C 2010, 114, 1217–1225

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Solvent-Polarity Dependence of Electron-Transfer Kinetics in a CdSe/ZnS Quantum Dot-Pyromellitimide Conjugate Shi-Cong Cui, Takashi Tachikawa, Mamoru Fujitsuka, and Tetsuro Majima* The Institute of Scientific and Industrial Research (SANKEN), Osaka UniVersity, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan ReceiVed: October 6, 2009

Photoinduced electron transfer (ET) to and from semiconductor quantum dots (QDs) is of intense interest because of its important roles in QD-based devices, such as detectors, light-emitting diodes, and solar cells. In this study, we have investigated the solvent dependence of the interfacial ET between ZnS-capped CdSe (CdSe/ZnS) QDs and pyromellitimide (PI) at the ensemble and single-particle levels. The steady-state and time-resolved photoluminescence (PL) and absorption measurements revealed that the rates of ET from the excited CdSe/ZnS to the adsorbed PI-CA molecules in nonpolar solvents, such as octane and toluene, are higher than aprotic polar solvents, such as benzonitrile. Furthermore, it was found that two pathways are included in the ET processes between CdSe/ZnS and PI, that is, ET from the excited CdSe core to PI-CA molecules adsorbed on the surface of the CdSe or ZnS. The former process proceeds on the picosecond time scale, while the latter proceeds on the nanosecond time scale. The interfacial ET dynamics in individual CdSe/ZnS-PI conjugates in different solvents were next examined. The PL intermittency or blinking kinetics of single QDs were analyzed by adapting a diffusion-controlled electron transfer (DCET) theory for the probability distribution of the “on” events. The results, which are qualitatively consistent with that obtained from the bulk experiments, clearly showed that the probability of the ET events increases with the decreasing solvent polarity. Consequently, our findings provide new insight into the mechanism of the interfacial ET reactions on the semiconductor nanoparticles and potentially contribute to the development of the design of QD-based devices. Introduction 1

Semiconductor nanocrystals, also called quantum dots (QDs), have received much attention because of their device applications, such as sensors, detectors, light-emitting diodes, and solar cells.2-4 In particular, QD-based solar cells are now involved in the creation of third generation solar cells with increased power efficiencies.5 Recent reports of multiple exciton generation (MEG) by one absorbed photon in some QDs also offer the exciting possibility to dramatically improve the efficiency of QD-based solar cells.6-8 To facilitate the charge transport across the heterogeneous interface between QDs and metal oxide anodes, typically a nanocrystalline TiO2 or ZnO film, the possible rate-limiting factors must be addressed and suitably optimized. Therefore, understanding the dynamics of charge carriers generated in QDs is an important matter for using them to design those devices. The interfacial electron transfer (ET) reactions in the composite systems of semiconductor QDs and inorganic or organic substrates have been investigated in recent years.9-19 For instance, Kamat and co-workers directly observed the interparticle ET processes in the CdSe-TiO2 systems using femtosecond transient absorption spectroscopy and found sizedependent ET dynamics using different sized QDs.11 Lian et al. also demonstrated an ultrafast ET from the photoexcited CdS QDs to adsorbed rhodamine B molecules on the 10 ps time scale.15 As is well-known, the solvent polarity has strong influences on the conventional molecular ET systems. From the Marcus * Corresponding author. Phone: +81-6-6879-8495. Fax: +81-6-68798499. E-mail: [email protected].

theory based on the continuum dielectric model, the solvent polarity is closely related to the solvent reorganization energy, which plays a crucial role in determining the rate of the ET reaction.20 However, the solvent-polarity effects on the ET processes in semiconductor nanoparticle systems have not been adequately addressed.21-25 Lian’s group recently reported the results from an investigation of solvent-dependent (H2O, MeOH, EtOH, and DMF) ET dynamics by the photoexcited Re-bipyridyl complexes to nanocrystalline TiO2.21 In their study, the different ET rates did not depend on the conduction band (CB) positions of the TiO2 in these solvents, but the amount of water in the solvents. Although some other groups showed the solventinduced infection of the QDs luminescence character,23,25 the influence of the solvent polarity on the kinetics of the ET remains unclear. Therefore, the spectroscopic investigation of the solvent dependence of the ET processes in the conjugates of QDs and adsorbates will provide us the basic knowledge of QD-based ET systems. Our primary goal in this study is to explore the solvent dependence of the ET dynamics between a ZnS-capped CdSe (CdSe/ZnS) QD and pyromellitimide (PI) compound that was used as an electron acceptor (Figure 1).19 The ZnS capping passivates and protects the surface of the nanosized CdSe core, and hence provides an enhanced and stable band-edge luminescence and an order of magnitude increase in the quantum yield.26 Moreover, the ZnS passivation treatment on the nanostructured TiO2 sensitized with QDs has been demonstrated to increase the solar cell performance.27-30 It was recently found that the recombination in CdSe QD-sensitized solar cells was significantly reduced by the interposition of an intermediate ZnS

10.1021/jp909579j  2010 American Chemical Society Published on Web 11/10/2009

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Figure 1. (A) Schematic illustration of the ET reactions between CdSe/ ZnS QD and PI-CA. The minus and plus signs denote the photogenerated electrons (e-) and holes (h+), respectively. FET and BET denote the forward and back ET reactions. (B) Energy diagram for the ET processes.

coating layer.30 CdSe/ZnS QDs are therefore a useful luminescent substance and an attractive sensitizer for optical and electronic devices. We have employed several steady-state and time-resolved absorption and emission spectroscopies to elucidate the photoinduced ET processes in the CdSe/ZnS-PI conjugates at the ensemble and single-particle levels. Bulk absorption and PL measurements clearly revealed a strong solvent-polarity dependence of the ET kinetics. The experimental data from the ET reactions were rationalized on the basis of the Marcus theory. Single-molecule (single-particle) fluorescence spectroscopy has also yielded many insights into the photophysical and photochemical processes.31-34 In our previous study, PL from individual CdTe QD-PI conjugates modified on the glass surface in chloroform were successfully detected, and an adequate model of the intermittent ET process was proposed by analyzing the so-called PL intermittency or blinking phenomenon.19 For the CdSe/ZnS-PI systems, a similar tendency was observed for the blinking behavior. An appropriate interpretation for the solvent-polarity dependence of the ET dynamics in single CdSe/ ZnS-PI conjugates is given in terms of a diffusion-controlled ET (DCET) theory for the probability distribution of the “on” events. Experimental Methods Materials. A toluene solution of CdSe/ZnS QD capped by octadecylamine ligands was obtained from Ocean NanoTech, LLC, and used without further treatment. Chloroform (Wako, 99%), octane (Oct) (Nacalai Tesque, 98%), toluene (Tol) (Wako, 99%), chlorobenzene (ClB) (Aldrich, 99.8%), 1,2-dichlorobenzene (DCB) (Aldrich, 99%), tetrahydrofuran (THF) (Aldrich, 99.9%, inhibitor free), and benzonitrile (PhCN) (Aldrich, 99.9%) were used as solvents without further purification. The PI derivative with a carboxylic acid (PI-CA) was synthesized in our laboratory according to published procedures.19 Sample Preparation for Single-Particle PL Measurements. CdSe/ZnS QDs were modified on a glass surface by a thiol linker according to previously published methods.19 As a typical method, clean cover glasses (22 × 22 mm, Matsunami Glass) were soaked in a solution of H2SO4 (1 wt %) for 1 h, and then washed three times with Milli-Q water. The washed cover glasses were soaked in an aqueous solution of 3-aminopropyl triethoxy silane (Nacalai Tesque, 97%) (10 wt %) for 1 h. The glasses were then washed three times with Milli-Q water and heated under an Ar atmosphere for 1 h at 100 °C. The aminemodified cover glasses were next soaked in a chloroform solution containing 11-mercapto undecanoic acid (Aldrich, 95%) (50 mM), N,N-diisopropyl ethyl amine (Nacalai Tesque, 98%)

Cui et al. (10 wt %), and N,N′-diisopropyl carbodiimide (Aldrich, 99%) (10 wt %) for 1 h, and again washed three times with chloroform. The thiol groups on the surface were activated in a chloroform solution of dithiothreitol (Nacalai Tesque, 99%) (10 mM) for 30 min. The thiol linker-modified cover glasses were soaked in a chloroform solution of CdSe/ZnS QD (5 nM) for 5 min. The glasses were then soaked in a chloroform solution of PI-CA (1 µM) for 5 min. The glasses were then washed three times with chloroform. The modified glasses were kept in chloroform in the dark before use. A homemade cell, which was composed of the QDs-modified cover glass and a clean glass slide with a spacer, was used for the single-particle PL experiments. For the solvent dependence experiments, the chloroform in the cell chamber was replaced by the chosen solvent and washed three times with the solvent. Ensemble Spectroscopies. The steady-state UV-visible absorption and fluorescence spectra were measured using a Shimadzu UV-3100 and a Hitachi 850, respectively. Time-Resolved Emission Measurements. The time-resolved emission spectra and decays were measured by the time correlated single photon counting (TCSPC) using a streak scope (Hamamatsu Photonics, C4334-01) equipped with a polychromator (Acton Research, SpectraPro150). The second harmonic oscillation (400 nm) of the output of the femtosecond laser (Spectra-Physics, Tsunami 3941-M1BB; full width at halfmaximum (fwhm), 80 fs; 800 nm) pumped by a diode-pumped solid-state laser (Spectra-Physics, Millennia VIIIs) was used to excite the sample in a quartz cell. The multiexponential decay curves were fitted using a nonlinear least-squares method with a multicomponent decay law given by I(t) ) a1 exp(-t/τ1) + a2 exp(-t/τ2) + · · · + an exp(-t/τn). The average lifetime, 〈τ〉, was then evaluated using i)n i)n aiτ2i /∑i)1 aiτi.35 The instrument response the equation: 〈τ〉 ) ∑i)1 function (IRF) was also obtained by measuring the scattered laser light to analyze the temporal profile. By using this method, a time resolution of about 50 ps was obtained after the deconvolution procedure. The observed temporal emission profiles were well fitted by three exponential functions. All measurements were carried out at room temperature. Picosecond Transient Absorption Measurements. The picosecond transient absorption spectra were measured by the pump and probe method using a regeneratively amplified titanium sapphire laser (Spectra Physics, Spitfire Pro F, 1 kHz) pumped by a Nd:YLF laser (Spectra Physics, Empower 15). The seed pulse was generated by a titanium sapphire laser (Spectra-Physics, Tsunami 3941m1BB, 80 fs fwhm) pumped with a diode-pumped solid-state laser (Spectra-Physics, Millennia VIIIs). A second harmonic generation of the fundamental (400 nm, 10 µJ pulse-1) was used as the excitation pulse. The excitation light was depolarized. A white light continuum pulse, which was generated by focusing the residual of the fundamental light on the flowing water cell after the computer-controlled optical delay, was divided into two parts and used as the probe and the reference lights, of which the latter was used to compensate the laser fluctuation. Both the probe and the reference lights were directed to the rotating sample cell with a 1.5 mm optical path and detected by the CCD detector equipped with the polychromator (Solar, MS3504). The pump pulse was chopped by the mechanical chopper synchronized to one-half of the laser repetition rate, resulting in a pair of spectra with and without the pump, from which the absorption change induced by the pump pulse was estimated. All measurements were carried out at room temperature.

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Figure 2. (A) Steady-state absorption spectra of CdSe/ZnS QDs (0.1 µM) in different solvents. The absorption at 505 nm is assigned to the 1S3/21Se absorption band of the CdSe. (B) Steady-state PL spectra of CdSe/ZnS QDs (0.1 µM) in different solvents. The excitation wavelength is 400 nm. Note that the φPL values were determined using solutions with the same absorbance at 400 nm.

Single-Particle PL Measurements. The experimental setup for the single-particle PL measurements is based on an Olympus IX71 inverted fluorescence microscope.19,33 A CW light emitted from a 488 nm argon ion laser (Melles Griot, 35LAS450, 50 mW, ∼2 kW cm-2 at the cover glass surface) that passed through an objective lens (Olympus, UPlanSApo, 1.40 NA, 100×) after the reflection at a dichroic mirror (Olympus, DM505) was used to excite the QDs. The emission from single QDs on the cover glass was collected using an oil-immersion microscope objective, magnified by the built-in 1.6× magnification changer, passed through an emission filter (Olympus, BA510-550) to remove the undesired scattered light, and imaged by an electron-multiplying charge-coupled device (EM-CCD) camera (Roper Scientific, Cascade II:512). The images were recorded at 50 frames s-1 and processed using Image-Pro Plus software (Roper Scientific, ver. 6.2), ImageJ software (http:// rsb.info.nih.gov/ij/). A general approach was used to define the intensity threshold to distinguish between the on and off states.36 To determine the threshold that separates these states, the PL intensity distribution was fitted by a sum of two or three Gaussian functions. The threshold was chosen to be 100 counts per 20 ms, which is greater than the background noise and low PL intensity levels. Counts above the threshold level were assigned to the “on state”. Results and Discussion Steady-State UV-Visible Absorption and PL Spectra. First, the steady-state absorption and PL spectra of the CdSe/ ZnS QDs in different solvents were measured. The results are shown in Figure 2. The absorption spectra showed a peak at around 510 nm for all solvents used in this study, indicating that the QDs can easily dissolve in these solvents without any aggregation or structural change. The possibility of aggregation of the QDs is also ruled out by the fact that a linear relationship between the absorbance and concentration for the solutions was confirmed under the present experimental conditions ( 15 µM. When we assume that the PL quenching occurs only in the associated complex between CdSe/ZnS and PI-CA, the apparent association constant (Ka) is given by19,40

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I0 I0 I0 ) + I0 - I I0 - I′ Ka(I0 - I′)[PI-CA]

Cui et al.

(1)

where I0 and I are the PL intensities observed in the absence and presence of PI-CA, respectively, and I′ is the initial emission intensity of the associated complex. The validity of the equilibrium model was confirmed by a straight-line plot of I0/ (I0 - I) and 1/[PI-CA], and the determined Ka values are summarized in Table 1. The Ka value in THF is lower than those in other solvents, suggesting that the surface adsorption of the PI-CA molecules is inhibited by the strong interaction between the QDs and THF molecules.41 When considering that there is no overlap between the emission from the QD and the absorption of PI-CA ( λ. In fact, as shown in Figure 5, the curve of the calculated kET reproduced the inverse solvent-polarity dependence of 〈kET〉 at a λs lower than 0.5 eV. This result might infer the existence of the Marcus inverted region in the present systems. Picosecond Transient Absorption Measurements. To directly observe the ET process between the excited CdSe/ZnS and PI, the transient absorption of the CdSe/ZnS-PI conjugate in toluene was measured on the picosecond time scale. Figure 6A presents the time profiles obtained at 720 nm during the 400 nm laser flash photolysis of the CdSe/ZnS QDs in the absence and presence of PI-CA in toluene. The transient absorption spectrum at 800 ps is also presented in the inset of the figure. An absorption band with a peak at around 720 nm, which is assigned to the PI•-,57,58 was clearly observed. No absorption peak was observed for the sample in the absence of the PI-CA or QDs. The bleaching curves for both samples at 520 nm are given in Figure 6B. This bleaching is assigned to the 1S3/2(h)1S1/2(e) exciton interband transition of CdSe/ZnS, and its recovery was measured to assess the rate of ET.11,16 The time profile observed for the unmodified CdSe/ZnS QDs was fitted by a biexponential curve with lifetimes of τ1 ) 11.0 ps (79.6%) and τ2 ) 43.3 ps (20.4%). In comparison, for the CdSe/ZnS-PI system, the time profile exhibits lifetimes of τ1 ) 1.7 ps (72.2%) and τ2 ) 43.3 ps (27.8%). Using eq 2, 〈kET〉 was calculated to be 5.0 × 1011 s-1, which is about 3 orders greater than that determined from PL lifetime measurements (〈kET〉 ) 1.2 × 108 s-1, see Table 3). This large difference strongly suggests that there is an ultrafast ET process within the CdSe/ZnS-PI conjugate in toluene. Similar ultrafast ET rates (about 1010-1011 s-1) were recently reported for various conjugations consisting of QDs and electron acceptors.11,15,16

J. Phys. Chem. C, Vol. 114, No. 2, 2010 1221 From the structure of the CdSe/ZnS QDs, the ZnS cover layer does not completely cover all of the surface of the CdSe core.59 Thus, it is acceptable that some PI-CA molecules were directly connected to the CdSe core in which the faster ET process (τ ) 2 ps) is expected as shown on the right side of Figure 7. The slower ET process (〈τ〉 ) 4-7 ns) measured by the PL lifetime experiments would result from the PI-CA molecules adsorbed on the ZnS layer. By taking the dependence of kET on r into account (kET ∝ exp(-βr), where β is the decay parameter) and assuming a β of 10 nm-1,20,60 kET decreases by a factor of 0.05 (0.0025 for two ZnS monolayers) because of the increase in r from 0.8 to 1.1 nm (1.4 nm for two ZnS monolayers). From this estimation, it is likely that the 2- or 3-order of magnitude increase in kET observed by the femtosecond laser flash photolysis is a consequence of a short ET distance (and additionally, a strong electronic interaction) between the CdSe and PI chromophore due to the absence of the ZnS layer. Single-Particle PL Measurements. In the single-particle experiments, a number of CdSe/ZnS-PI conjugates were modified on the glass surface by alkylthiol linkers (Figure 8).19 The thiol groups can strongly bind with Cd2+ and Zn2+ (Ka > 108 M-1).41,61 This modification method is very useful because ET dynamics in the conjugates are explored under the same conditions for both the ensemble and the single-particle measurements. To confirm whether or not the conjugation between the CdSe/ ZnS QDs and PI-CA molecules is substantially stable during the solvent exchange and washing procedures, the PL intensity measured for the CdSe/ZnS QDs-modified glasses was first examined at the ensemble level before and after the solvent exchange steps (see the Supporting Information, Figure S6). In these experiments, the CdSe/ZnS QDs were modified on the surface of the cover glasses by the same procedures adapted for the single-particle experiments (see the Experimental Methods for details). By exchanging chloroform with benzonitrile after the modification, the PL intensity significantly increased. Again, by introducing chloroform into the sample, the quenching of the PL emission was observed. All results support the fact that the PI-CA molecules can stably associate with the surface of the CdSe/ZnS QDs during the course of the single-particle experiments. Figure 9 shows the typical trajectories of the PL intensity (left panel) and intensity distributions (right panel) of single CdSe/ZnS QDs before (A) and after (B) the modification of the PI-CA molecules in octane. The threshold was chosen to be 100 counts per 20 ms, and the PL intensity above the threshold level was assigned to the “on state” (see blue lines). The “on times” with an average intensity of ∼150 counts on the trajectory were clearly shorter when PI-CA is present. We then calculated the average on time by analyzing all of the single-particle data for each sample because of the limited number of on-off blinking events for a single particle. These results also show the shortened on time by the modification of the PI-CA molecules (vide infra). Another interesting result is that the PL intensity trajectories of both systems show a lower PL intensity level (∼50 counts). For other solvent systems, this weak PL can be occasionally observed. Very recently, Lian et al. reported similar weak PL levels for single CdSe/ZnS QDs and their conjugations with electron acceptors, such as Fluorescein 27 and TiO2.17,18 Their statistical analyses of the PL intensities and lifetimes of the individual QDs revealed that both parameters coincidentally change due to the fluctuation in the nonradiative decay rates. The transition from on to off states is most probably attributed

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Figure 6. (A) Time profiles of transient absorption at 720 nm observed during the 400 nm laser flash photolysis of CdSe/ZnS toluene solutions (5 µM) in the absence (blue) and presence of PI-CA (100 µM) (red). Inset shows the transient absorption spectrum observed at 800 ps after the laser flash. (B) Time profiles of transient absorption at 520 nm observed during the 400 nm laser flash photolysis of CdSe/ZnS toluene solutions in the absence (blue) and presence of PI-CA (100 µM) (red). The inset shows the transient absorption spectra observed for a toluene solution containing CdSe/ZnS and PI-CA. The black lines indicate the biexponential curves fitted to the kinetic traces.

Figure 7. A schematic illustration of two ET pathways from the photoexcited CdSe/ZnS to PI.

Figure 8. An illustration of CdSe/ZnS-PI conjugates modified on the cover glass surface by mercaptotriethoxysilane molecules. PI-CA molecules were absorbed on the surface of the CdSe/ZnS QDs by the carboxyl group. The solvent in the flow cell chamber was exchanged with various solvents.

to the photoinduced charging of the QDs by the ET to trap states in the QDs.62,63 Although the PL lifetime measurements were not performed for individual CdSe/ZnS-PI conjugates, our results infer that there are at least two ET processes with different rates, that is, faster ET (picosecond time domain) and slower ET (nanosecond time domain) processes, as illustrated in Figure 7. The high PL intensity level, that is, the “on state”, is undoubtedly understood as the emission from the excited CdSe/ZnS without the ET to the adsorbed PI. This intensity level should correspond to the intrinsic PL lifetime of the QDs (〈τ0〉 ≈ 15 ns). When the relative ratio of average intensities of two levels (160 and 40 counts) is considered, the lower PL intensity level could be attributed to the component with the

average lifetime of ∼ns observed for the bulk samples. This lifetime would correspond to the slower ET process between the CdSe and PI-CA adsorbed on the ZnS surface, while the “off state” corresponds to the faster ET process (∼1011 s-1) between the CdSe and PI-CA directly adsorbed on the core surface. Single-Particle Blinking Behaviors. The fluorescence intermittency, also called a blinking phenomenon, is one of the important characteristics of single-molecule (single-particle) spectroscopy. Numerous studies have developed a model mechanism for the blinking wherein a QD switches between the on and off states via charging events.64-66 The histograms of the on times for single CdSe/ZnS QDs modified on a glass surface with and without PI-CA molecules in toluene were analyzed and compared. As shown in Figure 10A, individual CdSe/ZnS QDs excited with a 488 nm laser show a fast blinking with an on time of a few hundred milliseconds in toluene. The characteristic decay times (τon) for the on events are tentatively fitted by a single exponential equation. As listed in Table 4, τon dramatically decreased by modification of the PI-CA molecules on the surface of the single CdSe/ZnS QDs in nonpolar solvents, while it did not change in relatively highly polar solvents, such as THF and benzonitrile. At sufficiently long times, the histograms have only one or no counts per bin time due to finite counting statistics. Therefore, we analyzed the probability density, Pon(t), by weighting each point in the on histograms by the average time (∆tav) between the nearest neighbor event bins using the following equation:36

Pon(t) )

Non(t) 1 × Ntotal ∆tav

(5)

where Non(t) and Ntotal are the number of on events of duration (t) and the total number of on or off events, respectively. As is well-known, the time distributions of the on-events follow a simple power law behavior as given by36

Pon(t) ) At-mon

(6)

where A is the scaling coefficient and mon is the power-law exponent that describes the distribution. Figure 10B shows the on-time distributions of single CdSe/ ZnS QDs with and without PI-CA molecules in toluene. The results of the other solvents are provided in the Supporting

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Figure 9. Typical trajectories of PL intensities (left panels) and intensity distributions (right panels) of single CdSe/ZnS QDs in the absence (A) and presence of PI (B) in octane. The blue lines indicate the threshold intensity that separates the on and off states.

Figure 10. Histograms of on times (A) obtained for CdSe/ZnS and CdSe/ZnS-PI in toluene. Solid lines indicate the single-exponential fits. (B) On-time probability density, Pon(t), obtained for a single QD. The Pon(t) for the CdSe/ZnS-PI system was constructed of trajectories for 20 single particles. The dashed lines indicate the simple power law fits (see eq 6). The solid lines indicate the best fits based on the equation: Pon(t) ) At-1.8exp(-Γont), where A is the scaling coefficient. All of the fitting parameters are summarized in Table 4.

TABLE 4: Fitting Parameters in Different Solvents from Single-Particle PL Measurements τon/s

a

monb Γon/s-1c

no PI with PI no PI with PI no PI with PI

Oct

Tol

ClB

THF

PhCN

0.113 0.098 2.22 2.35 6.2 8.9

0.114 0.099 2.16 2.22 6.0 8.0

0.113 0.096 2.22 2.36 6.9 8.2

0.075 0.080 2.09 2.09 6.2 6.2

0.075 0.080 2.33 2.36 11.8 11.2

a The kinetic parameters were determined by single-exponential functions. Errors within (10%. b See eq 6. Errors within (10%. c See eq 7 and ref 70 for details. Errors within (15%.

Information (Figures S7-10). Each data set is fitted to eq 6 with the different mon values in toluene. As summarized in Table 4, these values are not plausible because they are obviously higher than that reported for the single CdSe/ZnS QDs (mon ≈ 1.6).67 In addition, the distributions exhibit a downward deviation from the pure power law. A similar breakdown of the power law with a bending tail for the on-time events was reported for various QDs, such as CdSe and CdTe, and CdSe nanorods.24,67,68 Recently, Marcus and co-workers reported a nonadiabatic ET theory with a diffusion-controlled ET (DCET) model for the PL blinking of CdSe/ZnS QDs based on the ET processes

between a QD and its localized surface states (charge trapping sites).69-72 To better match the shape of the probability distribution of the on times, Pon(t), we fit them to a truncated power law predicted by the DCET theory:

Pon(t) ≈

tc -mon exp(-Γont) t 4π

(7)

where tc is the critical time that is a function of the electronic coupling strength and other quantities, and Γon is the saturation rate. The exponential drop in the probability distribution with a time scale longer than the saturation time (1/Γon) arises due to the activation barrier at the intersection of the free energy potentials of the light-emitting and dark states. From the normal DCET theory, mon is considered to be 1.5, but in the experiments, the mon of 1.4-1.7 can be frequently adapted.70 In fact, our data were best fitted with mon ≈ 1.8. As shown by the solid lines in Figure 10B, this function well matches the shape of the on-time distributions rather than the simple power law fits (dotted lines). All of the calculated Γon values in the different solvents are listed in Table 4. The Γon values obtained for the unmodified QDs increased with the increasing polarity of the solvents.73 Meanwhile, the Γon values obtained in the nonpolar

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Cui et al. examination of the dependences of the ET kinetics on a variety of factors including the particle size and composition of the QDs and the redox potential of the adsorbates will be carried out. Conclusions

Figure 11. Solvent reorganization energy (λs) dependence of relative Γon (ΓonCdSe/ZnS-PI/ΓonCdSe/ZnS) in different solvents (red squares). The broken line is a visual guide.

solvents significantly increased by the conjugation with PI, but those in the relatively highly polar solvents were almost identical. With the assumption that two parabolas along a reaction coordinate, which represent the free energy curves of the lightemitting and dark states, have the same curvature, Γon is given by69-72

Γon )

(λ + ∆GET)2 8tdiff λkBT

(8)

where tdiff is the diffusion correlation time constant for motion on a parabolic energy surface. According to eq 8, when -∆GET . λ, a more negative value for ∆GET should lead to an increase in Γon, that is, a greater bending. Our experimental results are well explained by the fact that -∆GET estimated for the CdSe/ ZnS-PI system (0.75 eV) is significantly higher than the λ values (0.28-0.37 eV) for the ET in nonpolar solvents, such as octane and toluene, and also greater than -∆GET (0.041 eV) for the ET with surface states just below the edge of the CB of the QDs.75 At the same time, the solvent-polarity dependence of Γon for the unmodified CdSe/ZnS QDs might be explained by the possibility that -∆GET (0.041 eV) for the ET with surface states75 is almost equal to or lower than λ for the ET in all of the solvents used in this study (i.e., λ g -∆GET).76 The ratios between the Γon values obtained for the unmodified and PI-modified CdSe/ZnS QDs (ΓonCdSe/ZnS-PI/ΓonCdSe/ZnS) are plotted versus the λs values calculated for each solvent. As shown in Figure 11, it seems that these values increase with the decreasing polarity of the solvents. This tendency implies an increase in kET in the nonpolar solvents18,19 and is qualitatively consistent with that obtained from the bulk experiments (Table 3). The small λs values for the ET in nonpolar solvents lead to the shortening of the “on time”, that is, the increase in Γon, which is considered as an implication of the Marcus inverted region for very exothermic heterogeneous ET reactions at the solid-liquid interface. At present, however, it is still difficult to accurately predict the ∆GET dependence of Γon in the systems because of the uncertainties of the energy levels and reorganization processes of the intrinsic surface states, etc. Furthermore, the temporal fluctuations in the local environments, the conformation of adsorbates, and the vibronic coupling between the QDs and adsorbates should result in a distribution of the diffusion correlation times due to the different energy barriers. In such cases, one might apply the extended DCET model involving a contribution of anomalous diffusion in the energy configuration space.69 To thoroughly resolve this issue, a detailed

We have investigated the solvent dependence of the interfacial ET between CdSe/ZnS QDs and PI-CA molecules at the bulk and single-particle levels. The steady-state and time-resolved PL and absorption measurements revealed a strong solventpolarity dependence on the ET process; the rates of ET in nonpolar solvents, such as octane and toluene, are higher than in the less polar solvents, such as benzonitrile. The results were consistently interpreted in terms of the Marcus theory. Furthermore, it was found that two pathways are included in the ET process in the CdSe/ZnS-PI conjugates, that is, direct and ZnS layer-mediated ET from the CdSe core to PI-CA molecules adsorbed on the surfaces of CdSe and ZnS, respectively. The former process proceeds on the picosecond time scale, while the latter proceeds on the nanosecond time scale. Using the single-particle experiments, the blinking behaviors of individual CdSe/ZnS-PI conjugates in different solvents were examined. The Γon values, which are analyzed by adapting the DCET theory, clearly increased with the decreasing polarity of the solvents. The small λs values for the ET within the conjugates in nonpolar solvents lead to a shortening of the “on time”, that is, an increase in Γon, thus providing the possibility for the Marcus inverted region behavior in very exothermic heterogeneous ET reactions at the solid-liquid interface. This observation is qualitatively consistent with the conclusion drawn from the analysis of the results from the bulk experiments. Consequently, our findings bring new insight into the mechanism of interfacial ET reactions on semiconductor nanoparticles and potentially contribute to the development and the design of QDbased devices. Acknowledgment. This work has been partly supported by a Grant-in-Aid for Scientific Research (Projects 17105005, 21750145, and others) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government. Supporting Information Available: Figures and tables containing the results of PL lifetime measurements in Oct (S1), ClB (S2), DCB (S3), THF (S4), and PhCN (S5), solvent exchange experiments (S6), and single-particle blinking in Oct (S7), ClB (S8), THF (S9), and PhCN (S10). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Alivisatos, A. P. Science (Washington, D.C.) 1996, 271, 933–937. (2) Gill, R.; Zayats, M.; Willner, I. Angew. Chem., Int. Ed. 2008, 47, 7602–7625. (3) Sargent, E. H. AdV. Mater. 2008, 20, 3958–3964. (4) Kamat, P. V. J. Phys. Chem. C 2008, 112, 18737–18753. (5) Brown, G. F.; Wu, J. Laser Photonics ReV. 2009, 3, 394–405. (6) Schaller, R. D.; Sykora, M.; Jeong, S.; Klimov, V. I. J. Phys. Chem. B 2006, 110, 25332–25338. (7) Nozik, A. J. Chem. Phys. Lett. 2008, 457, 3–11. (8) Sukhovatkin, V.; Hinds, S.; Brzozowski, L.; Sargent, E. H. Science (Washington, D.C.) 2009, 324, 1542–1544. (9) Sykora, M.; Petruska, M. A.; Alstrum-Acevedo, J.; Bezel, I.; Meyer, T. J.; Klimov, V. I. J. Am. Chem. Soc. 2006, 128, 9984–9985. (10) Robel, I.; Subramanian, V.; Kuno, M.; Kamat, P. V. J. Am. Chem. Soc. 2006, 128, 2385–2393. (11) Robel, I.; Kuno, M.; Kamat, P. V. J. Am. Chem. Soc. 2007, 129, 4136–4137.

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