External Electric Field Effects on Optical Property and Excitation

Aug 30, 2010 - External Electric Field Effects on State Energy and Photoexcitation Dynamics of Water-Soluble CdTe Nanoparticles. Ruriko Ohshima , Taka...
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J. Phys. Chem. C 2010, 114, 15594–15601

External Electric Field Effects on Optical Property and Excitation Dynamics of Capped CdS Quantum Dots Embedded in a Polymer Film Mohan Singh Mehata,§,† Manisree Majumder,‡ Biswanath Mallik,*,‡ and Nobuhiro Ohta*,† Research Institute for Electronic Science (RIES), Hokkaido UniVersity, Sapporo 001 0020, Japan and Department of Spectroscopy, Indian Association for the CultiVation of Science (IACS), 2A and 2B, Raja S.C. Mullick Road, JadaVpur, Kolkata 700 032, India ReceiVed: May 6, 2010; ReVised Manuscript ReceiVed: August 8, 2010

Nanocomposites of benzyl mercaptan (BM)-capped cadmium sulfide (CdS) quantum dots (QDs) have been prepared and investigated by using electric field modulation spectroscopy. Applied electric fields on absorption and photoluminescence (PL) spectra as well as PL decays of BM-capped CdS QDs embedded in a poly(methyl methacrylate) (PMMA) film have revealed a field-induced alteration. Electro-photoluminescence (E-PL) exhibits field-induced quenching of photoluminescence in the presence of electric fields. The observed E-PL spectra and field-induced change in decay profile suggest that the PL intensity alternation originates from the electric field effect on emitting state population rather than PL quantum yield, probably as a result of the presence of field-assisted dissociation into hole and electron at the photoexcited state having a charge-separated character. Field-induced change in absorption spectra showed the spectral broadening caused by the stark shifts, indicating a large electric dipole moment in the exciton state. E-PL spectra also show that the emission state of BMcapped CdS QDs has a large charge transfer character. The field-assisted dissociation from the photoexcited state has been confirmed to depend on the excitation wavelength. 1. Introduction Semiconductor nanocrystals (NCs) have become the materials of great interest for both fundamental research and technical application.1-6 Nanocrystals/nanoparticles in the size regime of 2-6 nm are well-known as quantum dots (QDs), and QD of cadmium sulfide (CdS), which is a II-VI semiconductor with a band gap of 2.42 eV at 300 K, is one of the most interesting semiconductors due to their size-dependent photoluminescence (PL) tunable across the visible region, dimensional matching with biological molecules, and advances of their preparation methods.2-4 It is known that QDs of CdS exhibit quantum size effects in optical properties as evidenced by the shifts in the absorption and luminescence bands to higher energy with the reduction of grain size3 and also by observation of a discrete absorption spectrum with enhanced oscillator strength.7 In fact, the main interest in studying CdS nanoparticles is related to their preparation and photophysical properties, which make them suitable in application such as biomedical labeling, optoelectronic devices, solar energy conversion, and so on,8-11 and photoluminescence (PL) of CdS has been studied by various groups.3,4,12 Usually the reported emission spectra consist of two broad bands (in the range 400-520 and 520-800 nm) peaked at around 480 and 650 nm, respectively.4,12 Relatively strong emission peaked at around 650 nm is attributed to a radiative recombination at deep trap sites originating from lattice imperfections at the surface, that is, surface states, and the emission that peaked around 480 nm is attributed to a direct recombination of electron and hole pair at the band gap. In the recent study on 1-thioglycerol-capped CdS nanocrystallites,3 * To whom correspondence should be sent: E-mail: (N.O.) nohta@ es.hokudai.ac.jp; (B.M.) [email protected]. † Hokkaido University. ‡ Indian Association for the Cultivation of Science (IACS). § Present Address: Department of Applied Physics (Engineering Physics), Delhi Technological University, Bawana Road, Delhi-110042, India.

only one emission peak was observed, and the emission from the surface defects (surface vacancy) was absent. Thus, the study of PL of thiol-capped CdS nanocrystallites has become an interesting area of research. In the recent past, electric field modulation spectroscopy has been extensively used in small and large molecular systems to examine their excited-state electronic structure and dynamics including charge transfer, electron transfer, and electrostatic interaction in organized systems such as protein, because of their efficient sensitivity to the electric field.13-18 It has also been used to probe molecular motion and to examine stability and reliability of organic light-emitting diode (OLED) materials.16-18 The field-induced alterations in optical transitions provide unique information about the change both in the permanent electric dipole moment and in the induced dipole moment, that is, the change in molecular polarizability between the ground and excited states. On the basis of the field-induced change in optical transitions of cadmium selenide (CdSe) nanoparticles having size dependence, it has been proposed that these particles have a dipolar character in the excited state.19 On the other hand, the quantum-confinement Stark effect study of single CdSe nanocrystallite QDs and the modulation spectroscopy of CdS nanoparticles show a polar as well as a polarizable character in the lowest excited state.1,4 In the present study, we have prepared benzyl mercaptan (BM)-capped CdS quantum dots using microwave (MW) irradiation methods and have noticed that profile and peak position of the PL spectra are different from those of uncapped CdS nanocrystals. It was thought worthwhile to study the electric field-induced change in absorption and PL spectra of BM-capped CdS QDs to study the properties of the excited state under application of electric fields. Therefore, we have examined the effects of external electric fields up to ∼1 MV cm-1 on absorption, PL transitions and PL decay profiles of BM-capped CdS QDs embedded in a thin film of poly(methyl methacrylate),

10.1021/jp104124z  2010 American Chemical Society Published on Web 08/30/2010

Capped CdS Quantum Dots Embedded in a Polymer Film PMMA, by using a method reported previously.20 We have measured electroabsorption (E-A) and electrophotoluminescence (E-PL) spectra, that is, plots of the field-induced change in absorption intensity and in PL intensity, respectively, as a function of wavelength at the second harmonic of modulation frequency of applied electric field. Polarization measurements of the E-A spectra have been carried out, and precise analyses of the E-A spectra could be made. Plots of the field-induced change in PL intensity as a function of excitation wavelength have been also measured to examine the excitation wavelength dependence of the electric field effects on excitation dynamics. Hereafter, applied electric field is denoted by F throughout this article. On the basis of the results of the E-A and E-PL spectra as well as the electric field-induced change in decay profile, the Stark shifts and the mechanism of the field-induced modifications in PL have been discussed. 2. Experimental Section Thiol (benzyl mercaptan), BM-capped CdS quantum dots (QDs) were prepared with MW irradiation method. In the preparation of thiol-capped CdS nanocrystals, cadmium acetate [Cd(CH3COO)2 · 2H2O, 99.9%, Loba chemie, India], thiourea [C(NH2)2S, GR grade, Merck, India], benzyl mercaptan [C6H5CH2SH, Aldrich, U.S.A.] and DMF [Spectrochem, India] were used without further purification. A microwave oven (Samsung, 2.45 GHz; max. powers 900 W) was used to prepare the nanocrystallites. In a beaker, a solution of cadmium acetate (50 mM) in DMF was prepared with an appropriate amount of the capping agent (30 mM), and then thiourea (50 mM) was added to the solution. After mixing the solutions properly, the beaker was placed inside the MW oven and was irradiated with MW continuously for 40 s. The irradiation process was repeated nine times. Before each irradiation, the solution was cooled down to ambient temperature. The nanocrystallites were separated by centrifuging at 15 000 rpm and washing several times with Milli-Q water and were dried under vacuum. To measure absorption and PL spectra, a certain amount of CdS QDs was embedded in a transparent PMMA film using chloroform as a solvent. The sample film was deposited on an ITO-coated quartz substrate with a spin-coating technique. The thickness of the BMcapped CdS-doped PMMA film, measured with an interferometric microscope, was typically 0.5 µm. A semitransparent aluminum (Al) film was further deposited on the PMMA film with a vacuum vapor deposition technique. ITO and Al films were used as electrodes. A sinusoidal ac voltage with a modulation frequency of 40 Hz was used for the measurements of electroabsorption (E-A) and electrophotoluminescence (E-PL) responses.14,16,21 Absorption and PL measurements were performed with a Hitachi U-3500 spectrophotometer and JASCO FP-777 spectrofluorometer, respectively. In the E-A measurements, the conversing light beam from JASCO FP-777 spectrofluorometer was collimated with a pinhole with a lens and directed through R-barium borate polarization prism and through the sample slide on an external photomultiplier.16 A rotatory stage was used for varying the angle χ between the polarization direction of the excitation light and the direction of the applied electric field. A signal from the photomultiplier is then sent to lock-in-amplifier and then to a personal computer. E-A signals were recorded by the lock-in-amplifier at the second harmonic of the modulation frequency of the applied electric field at various angle of χ. E-PL spectra were measured at room temperature with a nonpolarized light under vacuum conditions. To measure the decay profiles of CdS QDs, we used timecorrelated single-photon-counting (TCSPC) system, and the

J. Phys. Chem. C, Vol. 114, No. 37, 2010 15595 second harmonic of a mode-locked Ti:sapphire laser (Spectra Physics, Tsunami, with a fundamental pulse width ∼200 fs and repetition rate of 80 MHz) was used as an excitation light. Actually, a repletion rate of 2.2 MHz was used for the decay measurements. The PL from the sample was detected with a microchannel plate photomultiplier (Hamamatsu, R3809U-52) and fed to a time-to-amplitude converter of the TCSPC system. PL decays were obtained with a multichannel pulse height analyzer (MCA). To measure the effect of applied electric field on the decay profile, we applied a modulated voltage to the sample;20 this voltage was a rectangular wave of positive, zero, negative, and zero bias in turn. The duration of each bias was 30 ms, of which the first 3 ms was a dead time to exclude an overshooting effect immediately after the alteration of the applied voltage. The memory channel of the MCA was divided into four segments. The switching of the MCA memory channel was synchronized with the modulated applied voltage. Four decays were collected, corresponding to positive, zero, negative, and zero sample biases, respectively. These decays were stored in each of the separate memory segments of the MCA. 3. Results and Discussion Figure 1a1 shows the transmission electron microscopy (TEM) image of BM-capped CdS QDs, and Figure 1a2 shows the histogram of the size distribution of the CdS QDs in the TEM image. The nanoparticles have almost spherical shape with an average size (diameter) of ca. 2.4 nm. Figure 1b shows the absorption and photoluminescence (PL) spectra of CdS QDs doped in a PMMA film. The absorption spectrum shows two broad bands at around 372 and 427 nm. Plots of the first absorption peak as a function of size of CdS QDs are shown by Peng and co-workers.22 From the plots, the position of the first absorption peak of 2.4 nm CdS QDs is estimated to be at around 365 nm. In the present experiments, the peak at 427 nm is too far, and the peak at 372 nm probably corresponds to this peak. Then, the 372 nm band is assigned to the strongly quantized CdS QDs with a size of 2.4 nm. The difference of the peak position by 7 nm may be due to both the measurement in polymer matrix instead of solution in which the measurements were done by Peng et al.22 and the BM-capping effect. The peak position of the first absorption band of CdS QDs having ∼5 nm is estimated to be 440 nm from the above-mentioned reference. So, the coagulated/aggregated CdS nanoparticles, that is, formation of bigger particles from the smaller particles may show a peak at ∼427 nm for the coagulated CdS nanoparticles having the size of ∼4.5 nm. Thus, the 372 nm band is assigned to the strongly quantized CdS QDs, and the 427 nm band may correspond to the aggregation/coagulation. Absorption spectra in solid phase are usually broader than the corresponding spectra in solution. It has been reported that even particles with a relatively narrow size distribution can exhibit broad absorption and emission bands, once they are in the strong confinement regime.23 In fact, the absorption spectra of CdS nanoparticles observed in a polymer film are very broad (see Figure 1b). The PL spectrum shows a broad band in the range of 425-750 nm with a maximum at around 564 nm. The small humps at the shorter wavelength region of PL spectrum appear to be assigned to the exciton emission caused by the direct recombination of electron-hole at the band gap of nanoparticles of different sizes. The intensity, shape, and band position of the PL spectrum observed in vacuum and in ambient air conditions remain the same, indicating that the prepared BMcapped CdS QDs are stable. In contrast to this, the spectral intensity of uncapped CdS nanoparticles, which show two

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Figure 1. (a1) TEM image (inset shows a single quantum dot; particle size ∼2.4 nm) of BM-capped CdS QDs. (a2) Size distribution (histogram) of the benzyl mercaptan-capped CdS nanocrystallites. (b) Absorption spectrum (shaded blue line) and photoluminescence (PL) spectrum (shaded green line) of BM-capped CdS QDs embedded in a PMMA film. The PL spectrum was obtained at 383 nm excitation.

emission bands, strongly depends on the atmospheric conditions;4 the observed PL spectra are different from each other in vacuum condition and in ambient air. Both the absorption and PL spectra observed for CdS QDs doped in a PMMA film are nearly the same as those observed in N,N-dimethylformamide (DMF) solvent. Generally, the PL spectra of CdS nanoparticles consist of two bands in the range of 400-520 and 520-800 nm.4,12 The longer wavelength band is attributed to radiative recombination at deep sites originating from lattice imperfections at the surface, that is, the trapped emission originating from the surface states, and the other emission located at shorter wavelength region is attributed to a direct recombination of electron-hole at the band gap. In contrast, a single strong band at 602 nm or at 529 nm has also been observed while prepared via MW irradiation method or with capping agents.24,25 In the present case, the PL spectrum shows a strong broad band, attributed to the transition from trap sites arising from surface states, that is, recombination of an electron trapped in a sulfur vacancy with a hole in the valence band of CdS QDs. The large spectral width of the PL band is caused by inhomogeneous broadening due to variation in dot’s size and broadening due to electron-phonon coupling.26

Mehata et al.

Figure 2. (a) Absorption spectrum with G1 and G2 Gaussian bands, (b) second derivative spectra of G1 and G2 bands, (c) electroabsorption (E-A) spectra of BM-capped CdS QDs embedded in a PMMA film observed at two different angles of χ () 90 and 54.7°) at a field strength of 0.7 MV cm-1 at room temperature. (d) E-A spectrum and the simulated spectrum.

E-A spectra were measured at different angle χ between the direction of the applied electric field and the electric vector of the excitation light. Figure 2 shows the polarized E-A spectra of BM-capped CdS QDs doped in a PMMA film in the range of 17 500-34 000 cm-1 recorded with a field strength of 0.7 MVcm-1, together with the absorption and its derivative spectra. The absorption spectrum in the low-energy region is reproduced by a superposition of two broad Gaussian bands, that is, G1 (with a peak at 427 nm) and G2 (with a peak at 372 nm) (Figure 2a). The second derivative spectra of G1 and G2 bands are shown in Figure 2b. The polarized E-A spectra obtained using lockin-amplifier at the second harmonic of the modulation frequency of the applied electric field at normal incidence of χ () 90°) and the magic angle of χ () 54.7°) are identical to each other in shape as well as in peak position (Figure 2c). The positive and negative lobs of the E-A signals are crossing to the zero intensity line at the same point (see Figure 2c), indicating that CdS QDs are distributed isotropically in a PMMA film, even when the applied electric field strength is as strong as 0.7 MV cm-1. Each of the negative lobs of the E-A signals is probably assigned to the transition from the ground state to the excited states; the first two lower energy lobs are corresponding to the G1 and G2 absorption transitions. The E-A band maxima (negative lobs) are approximately the same in position to the absorption peaks, confirming the predominance of the spectral broadening, as the basis for the observed effects. Thus, the E-A

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Figure 3. Plots of ∆A as a function of the square of the applied electric field. Dotted line is just a guide for eye.

spectra follow closely the second derivative of the absorption spectrum or the sum of the second derivative of Gaussian bands G1 and G2 (cf. Figure 2b,c). The dependence of the amplitude of the measured E-A signals on applied electric field revealed a quadratic dependence on the electric field strength (Figure 3). With the assumption that the original isotropic distribution in a PMMA film is maintained in the presence of F, the E-A spectrum may be given by a sum of the zeroth-, first-, and second-derivatives of the absorption spectrum.27,28 Actually, the E-A spectrum obtained at the magic angle of χ () 54.7°) is well fitted with the second derivative spectra of the G1 and G2 bands, as shown in Figure 2d, where the second derivative coefficient is as large as 19 000 MV-2 for G1 and 9000 MV-2 for G2 at a field strength of 0.7 MV cm-1. The zeroth- and firstderivatives of the absorption spectrum were not necessary to reproduce the observed E-A spectra. The second derivative coefficient (Cχ) can be expressed as13,27,28

Cχ ) (|∆µ|)2[5 + (3 cos2 χ - 1)(3 cos2 η - 1)]/30h2c2 (1) in which |∆µ| ) |µe - µg| with subscripts e and g representing the excited and the ground states, respectively, h and c are Planck’s constant and the speed of light, respectively, and η is the angle between the direction of ∆µ and the transition dipole moment. At χ ) 54.7°, the angle dependent term is zero in eq 1, which results in Cχ ) (∆µ)2/6h2c2. Then, a large change in permanent electric dipole moment (∆µ) associated with the optical transition from g to e state is evaluated to be 29 D/f for G1 and 20 D/f for G2, where f is the internal field factor. The magnitude of ∆µ obtained for the G2 band is close to the one reported for the 2.9 nm diameter of CdS nanoparticles.4 For the G1 band, the magnitude of ∆µ is relatively large, which may result in the higher number of charge carrier on or near the nanocrystallite surface. Figure 4 shows the PL and E-PL spectra of BM-capped CdS QDs doped in a PMMA film with excitation at 383 nm under vacuum condition near 298 K with a different field strength of 0.2-0.8 MV cm-1 from each other. PL as well as E-PL spectra obtained at 353 and 411 nm excitation are essentially the same in shape to those at 383 nm excitation. PL and E-PL spectra were recorded simultaneously, and the excitation was performed at the wavelengths at which the field-induced change in absorption intensity was negligibly small (Figure 2c), which is an essential requirement to examine the exact magnitude of the field-induced change in PL spectra. The PL spectrum with a maximum near 564 nm has a large Stokes shift (9150 cm-1) relative to the absorption band, which implies that the emitting state is different from the optically excited state. The E-PL spectrum obtained at 0.2 MV cm-1 is similar in shape to the

Figure 4. Electrophotoluminescence (E-PL) spectra (shaded line) and photoluminescence (PL) spectra (dotted line) of BM-capped CdS QDs embedded in a PMMA film obtained with field strengths of 0.2-0.8 MV cm-1 in vacuum at room temperature (298 K).

negative of the PL spectrum (shaded line in Figure 4). The total integrated intensity of the E-PL spectrum is negative, demonstrating that the quantum yield (QY) of PL decreases in the presence of F. Thus, PL of BM-capped CdS QDs is diminished by application of electric field even as low as 0.2 MV cm-1. As shown in Figure 4, the magnitude of the PL quenching monotonically increases with increasing field strength from 0.2 to 0.8 MV cm-1. As an example, the fractional field-induced quenching of PL determined by integrating the ratio of ∆IPL to IPL is ∼6% at a field strength of 0.8 MV cm-1. Here, ∆IPL and IPL represent the field-induced change in PL intensity and the PL intensity at zero field, respectively. The field strength dependence of ∆IPL was obtained by integrating the E-PL spectra over the spectral region of 421-770 nm, in which plots of ∆IPL/ IPL is shown as a function of the square of the applied electric field (Figure 5). The field-induced quenching of PL is roughly proportional to the square of the applied electric field. Actually, the peak of the E-PL spectrum shows a red shift with respect to PL at a low electric field of 0.2 MV cm-1, which indicates the Stark shift. With increasing the electric field strength, the magnitude of the red shift becomes smaller, suggesting that the field-strength dependence of the quenching in PL is more

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Figure 5. Plots of ∆IPL/IPL as a function of the square of the applied electric field. Dotted line is just a guide for eye.

Figure 6. (a) E-PL spectrum (inverted scale) and PL spectrum (solid purple line) and (b) E-PL spectrum (black solid line) of BM-capped CdS QDs embedded in a PMMA film under vacuum condition observed with excitation at 383 nm with a field strength of 0.2 MV cm-1 and the simulated spectrum (dotted red line).

efficient than the Stark shift, whose magnitude is probably proportional to the square of the applied electric field in the present case. As mentioned above, a spectral shift was noticed in the E-PL spectra with respect to the PL spectra, especially at low electric fields, for example, at 0.2 MV cm-1 (see Figure 6). The E-PL spectrum at 0.2 MV cm-1 was reproduced by a linear combination of the zeroth- and first-derivatives of the PL spectrum (Figure 6b), and the second derivative spectrum was not necessary to be considered. If the isotropic distribution of the excited species is assumed, the first derivative coefficient can be expressed as

Bχ )

{

(∆Rm - ∆R¯ )(3 cos2 χ - 1) ∆R¯ + 2hc 10hc

}

in which ∆R j ) (1/3)Τr(∆R), where ∆R ) Re - Rg, and ∆Rm represents the diagonal component of ∆R j with respect to the direction of the transition dipole moment. The magnitude of ∆R j appeared to be associated with the transitions from the emitting state to the ground state was estimated to be ∼9000 j , since the magnitude of the Å3 by assuming ∆Rm ) ∆R coefficient of the first derivative of the E-PL spectrum obtained at 0.2 MV cm-1 was 10 cm MV-2. The extremely large value of ∆R j thus obtained appears to show that the presence of the first derivative comes from the orientational polarizability probably because the emitting state has a large electric dipole

moment, and the dipole moment of each QD may align along the applied electric field. In such a case, the coefficient Βχ may correspond to [(µe∆µ)/(3hckT)]cos γ, where µe and ∆µ present electric dipole moment at the emitting state and difference in electric dipole moment between the emitting state and the ground state, respectively, and γ represents the angle between the vectors of µe and ∆µ.28 If the dipole moment in the ground state is not negligible, it is expected that the field-induced orientation of the dipole moment of QDs along the applied electric field induces a change in absorption intensity, and the E-A spectra obtained with the magic angle and other angles of χ would be different in shape from each other. As shown in Figure 2, however, the E-A spectrum obtained with the magic angle of χ () 54.7°) is the same in shape as that obtained with χ ) 90°. These results show that the dipole moment in the ground state of QDs is negligible. Then, ∆µ is regarded as µe, and γ is regarded as zero. By using the above-mentioned Bχ value obtained from the E-PL spectrum at 0.2 MVcm-1, the electric dipole moment in the emitting state, that is, µe, is estimated to be 24 D, which is very similar to the dipole moment obtained from the E-A spectra for the G1 or G2 band. Such a large dipole moment in the emitting state shows that the emitting state of BM-capped CdS QD is regarded as a charge separated state, as menioned for the exciton state prepared by photoabsorption, that is, for the G1 and G2 bands. Large permanent electric dipole moment in the ground state had been reported for CdSe nanospheres dissolved in nonpolar solvents by dielectric dispersion measurements, that is, 25 and 47 D for the 3.4 and 4.6 nm diameter CdSe.29 In CdS, larger permanent dipole moment has been also predicted from the model based on the small crystallographic deviations from the ideal wurtzite structure, that is, ∼20 D for 2.4 nm.30 From the present E-A spectra, there is no doubt that there is a large difference in the dipole moment between the ground state and the excited state of G1 or G2 of CdS QDs. The fact that the first derivative is dominant as the Stark shift of the PL spectra suggests that a large dipole moment at the emitting state induces the orientational polarizability, as already mentioned above. In the present experiments, however, the electric dipole moment in the ground state is regarded as negligible. The reason why the present result of the polarized E-A spectra is different from the one proposed by Nann and Schneider (about the electric dipole moment in the ground state) is not clear at the moment. To examine the excitation energy dependence of the fieldinduced change in PL, furthermore, plots of the E-PL intensity as a function of excitation wavelength, that is, electrophotoluminescence-excitation (E-PL-Ex) spectra as well as plots of the PL intensity as a function of excitation wavelength, that is, PLexcitation (PL-Ex) spectra were measured simultaneously in the excitation wavelength region of 250-500 nm, by monitoring the emission at 560 nm with a field strength of 0.8 MV cm-1. The results are shown in Figure 7. PL-excitation spectrum is essentially the same in shape as the absorption spectrum, indicating that the QY of BM-capped CdS QDs in the absence of F is nearly independent of the excitation wavelength. The ratio of E-PL-Ex relative to the PL-Ex, which gives the fieldinduced change of QY of PL at each excitation wavelength, is shown in Figure 7. The ratio between these two spectra clearly indicates that the field-induced PL quenching varies with the excitation wavelength. It is worth mentioning that the quenching is more efficient at the G1 band excitation than that at the G2 band excitation. The field-induced alteration in PL intensity, that is, the fieldinduced quenching of PL may be regarded either due to an

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Figure 7. Photoluminescence excitation (PL-Ex) spectrum (shaded line) and the ratio of the electro-photoluminescence excitation (E-PLEx) spectrum relative to the PL-Ex spectrum (red dotted line) of BMcapped CdS QDs embedded in a PMMA film. Emission was monitored at 560 nm, and the applied electric field strength was 0.8 MV cm-1.

altered rate of the nonradiative decay of the emitting state, as reported for CdSe nanoparticles,31 or an altered initial population of the emitting state. The alteration in nonradiative decay rate may decrease or increase the PL lifetime, which consequently alters the PL QY, whereas the latter produces an altered population yield of the emitting state.4,17 To examine the origin of the field-induced change in PL, we have measured the timeresolved decay profiles of BM-capped CdS QDs doped in a PMMA film both in the absence of F and in the presence of 0.8 MV cm-1 under vacuum conditions near 298 K. The results are shown in Figures 8 and 9. The PL decays were monitored at 560 nm in the absence of F and presence of 0.8 MV cm-1 upon excitation at 383 nm (Figure 8a). The difference ∆IPL(t) between the decay at zero field (I0(t)) and the decay at 0.8 MVcm-1 (IF(t)), that is, IF(t) - I0(t) ≡ ∆IPL(t), and the ratio of these two decay profiles IF(t)/I0(t) are shown in traces b and c of Figure 8. The decay profiles I0(t) and IF(t), the difference ∆IPL(t), and the ratio IF(t)/I0(t) were simulated by assuming a tetra-exponential decay, that is, ∑4i Ai exp(- t/τi), where τi is the lifetime and Ai is the pre-exponential factor of the ith component. In the simulation of the four decay profiles, the lifetimes were estimated to be τ1 ) 0.99 ns (0.903), τ2 ) 10.6 ns (0.059), τ3 ) 47.5 ns (0.021), and τ4 ) 282 ns (0.017) at zero field, while τ1 ) 0.98 ns (0.858), τ2 ) 10.7 ns (0.054), τ3 ) 48.7 ns (0.018), and τ4 ) 297 ns (0.015) at 0.8 MV cm-1, where pre-exponential factor is shown in parentheses. The average lifetime (τav) given by τav ) ∑iAiτi/∑iAi was estimated to be 7.30 ns at zero field and 7.14 ns at 0.8 MV cm-1. The nonexponential decay profiles may result from the size distribution of CdS QDs. The lifetime is roughly independent of the applied electric field. The dominant and rapid component is ∼1 ns in lifetime. In the simulation, the pre-exponential factor of the rapid component significantly decreases from 0.903 to 0.858, whereas the lifetime τ1 () 0.98 ns) remains constant in the presence of F. The pre-exponential factor also decreases for the slower components of τ2-τ4, while the lifetime slightly increases, for example, τ3 ) 47.5 f 48.7 ns and τ4 ) 282 f 297 ns. These results are illustrated in the plots of IF(t)/I0(t) shown in Figure 8c. Similar decay trend was observed at 411 nm excitation (Figure 9) in the sense that the pre-exponential factor decreases significantly by F and that the lifetime increases a little, and the distribution of lifetime varies to some extent. The simulated decays in the presence and absence of F, together with the simulated difference and ratio, are shown in Figure 9. In the direct measurements of the field-induced change in decay profile, the evaluated lifetimes may include an experimental error of 1-2%, but the obtained difference ∆IPL(t) and the ratio IF(t)/

Figure 8. (a) Photoluminescence (PL) decays observed at zero field (green solid line) and at a field strength of 0.8 MV cm-1 (red solid line) of BM-capped CdS QDs embedded in a PMMA film. Excitation and emission wavelengths were 383 and 560 nm, respectively. (b) Difference (black solid line) between PL decays observed at 0.8 MV cm-1 and at zero field, together with the simulated difference (dotted red line). (c) Ratio of the decay observed at 0.8 MV cm-1 relative to that at zero field (black solid line), together with the simulated one (broken red line).

I0(t) of Figures 8 and 9 clearly indicate the significant alteration in decay profiles. If the initial population of the emitting state of the PL is unaffected by F, the ratio of IF(t)/I0(t) would be unity at t ) 0. Moreover, if the fluorescence lifetime is independent of F, IF(t)/ I0(t) would remain constant over the period. As shown in Figures 8b and 9b, the difference between IF(t) and I0(t) and the integral of ∆IPL(t) over the observation period are both negative, indicating that field-induced quenching of PL occurs upon excitation at 383 and 411 nm in conformity with the observed E-PL spectrum. In Figures 8c and 9c, the ratio of IF(t)/I0(t) is less than unity at t ) 0 following photoexcitation, demonstrating that the population of the emitting state decreases significantly in the presence of F. The pre-exponential factor decreased by ∼8% at 383 nm excitation and ∼15% at 411 nm excitation at a field strength of 0.8 MV cm-1. Hence, the field-induced quenching of PL is attributed to the field-induced decrease in the initial population of the fluorescent state just after the photoexcitation. As discussed above, the excited state is regarded as a charge-separated state having a large dipole moment. Then, the field-induced PL quenching which depends on the excitation wavelength probably results from the field-assisted dissociation of the photoexcited state (charge-transfer state) into a carrier of hole and electron, which leads to the decrease of the initial population of the emitting state. The fact that the field-induced PL quenching at the G1 band excitation is more efficient than the G2 band excitation may be interpreted in terms of the difference in polarity of the excited state; the dipole moment at the G1 band is larger than the G2 band, and so the field-induced

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Mehata et al. the excited states. Field-induced orientational polarizability is suggested from the observed E-PL spectra; BM-capped CdS QDs have a significant charge-transfer character in the emitting state, and the dipole moment of the QDs may be aligned along the applied electric field. The intensity, shape, and the band position of the PL spectrum of BM-capped CdS QDs in vacuum and in ambient air conditions remained the same, indicating that the prepared BM-capped CdS QDs are stable, though the spectral intensity of uncapped CdS nanoparticles depends strongly on the atmospheric conditions.4 PL of CdS is quenched significantly in the presence of F. Field-induced change in PL decay profile shows that the PL quenching resulted from the field-induced decrease of the emitting state population probably as a result of field-assisted dissociation of the photoexcited state into a carrier of hole and electron. It has been confirmed that the efficiency of the field-assisted dissociation depends on the excitation wavelength. The lifetime of the PL assigned to the so-called trap emission increases in the presence of applied electric fields, which probably results of the field-induced deenhancement of the charge-recombination process.

Figure 9. (a) PL decays observed at zero field (black solid line) and at a field strength of 0.8 MV cm-1 (blue solid line) of BM-capped CdS QDs embedded in a PMMA film, together with the simulated decays. Excitation and emission wavelengths were 411 and 560 nm, respectively. (b) Difference (black solid line) between the PL decays observed at 0.8 MV cm-1 and at zero field, together with the simulated difference (dotted red line). (c) Ratio of the decay observed at 0.8 MV cm-1 relative to that at zero field (black solid line), together with the simulated one (broken red line).

dissociation from the G1 band excitation is expected to be more efficient than the other, in agreement with the present results. With a passage of time, IF(t)/I0(t) increases slightly, suggesting that the lifetime of PL becomes longer in the presence of F (Figures 8c and 9c). The increase of PL lifetime in the presence of F is expected when the nonradiative decay rate is decreased by F at the emitting state of PL. The presence of the orientational polarizability on the PL of BM-capped CdS QDs indicates that the emitting state of PL has a polar character, as mentioned above. Then, the main nonradiative process at the emitting state of the CdS QDs may be a kind of charge recombination process. As shown in the field-induced enhancement of the exciplex fluorescence of linked compounds of carbazole and terephthalic acid methyl ester,32 which show photoinduced electron transfer, the charge recombination process can be de-enhanced by application of external electric fields. As the origin of the field-induced lengthening of the PL lifetime of QDs, therefore, field-induced de-enhancement of the charge recombination process at the PL emitting state may be considered. 4. Conclusion BM-capped CdS quantum dots were prepared by using the microwave irradiation technique and probed by recording E-A and E-PL signals as well as electric field-induced change in PL decay profile. The E-A spectra are similar in shape to the second derivative of the absorption spectrum, demonstrating that the field-induced change in absorption spectrum comes from the change in electric dipole moment following the transition to

Acknowledgment. M.S.M. thanks the Japan Society for the Promotion of Science (JSPS) for a Postdoctoral fellowship and for a Grant-in-Aid for JSPS fellows. B.M. thanks the Department of Science and Technology (DST), Govt. of India for the financial support against the project (Order No. DST/INT/JAP/ P-44/08 dt 8.05.2008) and JSPS, Japan for supporting his stay in Japan. References and Notes (1) Empedocles, S. A.; Bawendi, M. G. Science 1997, 278, 2114. (2) Thangadurai, P.; Balaji, S.; Manoharan, P. T. Nanotechnology 2008, 19, 435708. (3) Karan, S.; Mallik, B. J. Phys. Chem. C 2007, 111, 16734, and reference cited therein. (4) Ohara, Y.; Nakabayashi, T.; Iwasaki, K.; Torimoto, T.; Ohtani, B.; Hiratani, T.; Konishi, K.; Ohta, N. J. Phys. Chem. B 2006, 110, 20927– 20936. (5) Karan, S.; Mallik, B. Nanotechnology 2008, 19, 495202. (6) Karan, S.; Mallik, B. J. Phys. Chem. C 2008, 112, 2436. (7) Wang, Y.; Herron, N. J. Phys. Chem. 1991, 95, 525. (8) Chan, W. C. W.; Nie, S. Science 1998, 218, 2016–2018. (9) Chory, C. B.; Buchold, D.; Muller, G.; et al. Chem. Phys. Lett. 2003, 379, 443. (10) Warner, J. H.; Tilley, R. D. AdV. Mater. 2005, 17, 2997. (11) Guha, S.; Wu, B. J.; Cheng, H.; DePuydt, J. M. Appl. Phys. Lett. 1993, 63, 2129. (12) Wada, Y.; Kuramoto, H.; Anand, J.; Kitamura, T.; Sakata, T.; Mori, H.; Yanagida, S. J. Mater. Chem. 2001, 11, 1936. (13) Bublit, G. U.; Boxer, S. G. Annu. ReV. Phys. Chem. 1997, 48, 213. (14) Ohta, N. Bull. Chem. Soc. Jpn. 2002, 75, 1637. (15) Chowdhury, A.; Locknar, S. A.; Premvardhan, L. L.; Peteanu, L. A. J. Phys. Chem. A 1999, 103, 9614. (16) Jalviste, E.; Ohta, N. J. Chem. Phys. 2004, 121, 4730. (17) Mehata, M. S.; Iimori, T.; Yoshizawa, T.; Ohta, N. J. Phys. Chem. A 2006, 110, 10985. (18) Mehata, M. S.; Hsu, C.-S.; Lee, Y.-P.; Ohta, N. J. Phys. Chem. C 2009, 113, 11907. (19) Colvin, V. L.; Cunningham, K. L.; Alivisatos, A. P. J. Chem. Phys. 1994, 101, 7122. (20) Tsushima, M.; Ushizaka, T.; Ohta, N. ReV. Sci. Instrum. 2004, 75, 479. (21) Umeuchi, S.; Nishimura, Y.; Yamazaki, I.; Murakami, H.; Yamashita, M.; Ohta, N. Thin Solid Films 1997, 311, 239. (22) Yu, W. W.; Qu, L.; Guo, W.; Peng, X. Chem. Mater. 2003, 15, 2854. (23) van Dijken, A.; Janssen, A. H.; Smitsmans, M. H. P.; Vanmaekelbergh, D.; Meijerink, A. Chem. Mater. 1998, 10, 3513. (24) Yang, H.; Huang, C.; Li, X.; Shi, R.; Zhang, K. Mater. Chem. Phys. 2005, 90, 155. (25) He, R.; Qian, X.-F.; Yin, J.; Xi, H.-A.; Bian, L.-J.; Zhu, Z.-K. Colloids Surf., A 2003, 220, 151. (26) Wuister, S. F.; Meijerink, A. J. Lumin. 2003, 105, 35.

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