External Electric Field Effects on State Energy and Photoexcitation

ARTICLE pubs.acs.org/JPCC

External Electric Field Effects on State Energy and Photoexcitation Dynamics of Water-Soluble CdTe Nanoparticles Ruriko Ohshima,† Takakazu Nakabayashi,† Yoichi Kobayashi,‡ Naoto Tamai,‡ and Nobuhiro Ohta*,† † ‡

Research Institute for Electronic Science (RIES), Hokkaido University, Sapporo 001-0020, Japan School of Science and Technology, Kwansei Gakuin University, 2-1 Gakuen, Sanda, Hyogo 669-1337, Japan ABSTRACT: External electric field effects on absorption and photoluminescence (PL) spectra of colloidal CdTe nanoparticles have been measured in a poly(vinyl alcohol) (PVA) film. The electroabsorption spectra across the first exciton band are similar in shape to the second derivative of the absorption spectra, indicating the enhancement of the electric dipole moment following the optical transition to the first exciton state. The magnitude of the enhancement has been evaluated as a function of the size of the CdTe nanoparticles. The electrophotoluminescence (E-PL) spectra show a significant quenching of PL in the presence of electric fields. The direct measurements of the field-induced change in PL decay show that the field-induced quenching of PL arises from the field-induced decreases both in lifetime and in initial population of the exciton-emitting state. The E-PL spectra also show that the application of electric fields induces a red-shift or blue-shift of the PL spectra, depending on the size of the nanoparticles. It is also shown that the PL intensity of CdTe nanoparticles in PVA increases under photoirradiation at an atmospheric condition and decreases in a vacuum condition. The present results show that the emission properties of CdTe nanoparticles can be controlled by application of external perturbation such as electric field or photoirradiation.

1. INTRODUCTION The optical properties of nanometer-sized materials have been extensively studied as a subject of great importance in physical chemistry.1
6 Nanoparticles in the size regime of 2
6 nm exhibit quantum-confined effects in optical properties such as a shift of absorption to higher energy with the reduction of particle size, and II
VI semiconductor nanoparticles have received much attention in recent years for optical applications because of their high emission quantum yields.7
9 Developments of preparation methods providing high-quality nanoparticles allow one to optimize optical properties of nanoparticles for specific applications. The wavelength at the peak of the emission spectrum can be controlled by tuning the particle size. However, the factor controlling the emission quantum yield is still an unresolved issue for the design of nanoparticles, and preparation of high-luminescent nanoparticles in various conditions is still a challenging topic in material sciences. External electric field effects on absorption and photoluminescence spectra have been used to examine electronic structures in the electronically excited state and photoexcitation dynamics of molecules and molecular systems.10
12 The so-called electroabsorption and electrophotoluminescence spectra (plots of the electric-field-induced change in absorption intensity and photoluminescence intensity, respectively, as a function of wavelength or wavenumber) provide unique information about the differences in electric dipole moment (μ) and molecular polarizability (R) between the ground state and the excited state. Measurements of these spectra are also very useful to clarify the mechanism of molecular dynamics following photoexcitation, especially r 2011 American Chemical Society

for charge transfer (CT) dynamics because of its high sensitivity to an electric field.12 In a previous study, we have measured electroabsorption and electrophotoluminescence spectra of colloidal CdS nanoparticles in a poly(methyl methacrylate) (PMMA) film and observed a significant decrease in emission intensity in the presence of external electric fields.13,14 Furthermore, we found that the emission intensity of CdS-doped PMMA films increases under irradiation of UV light in air and decreases in a vacuum condition.13,14 These intensity changes are almost reversible, indicating that the emission intensity of CdS-doped PMMA films can be tuned with photoirradiation or electric fields. Benzyl mercaptan-capped CdS nanoparticles also show the remarkable field-induced decrease in emission intensity.15 In the present study, we have measured electroabsorption and electrophotoluminescence spectra and field-induced change in photoluminescence decay profiles of water-soluble thiol-capped CdTe nanoparticles dispersed in a poly(vinyl alcohol) (PVA) solid film. On the basis of the results, electronic structure and excitation dynamics following optical transition in the presence of an external electric field have been examined for CdTe nanoparticles. Effects of the photoluminescence intensity of CdTe nanoparticles on photoirradiation have also been examined. Because of high emission quantum yields of more than 80%,16 CdTe nanoparticles have been proposed for potential applications of light generation or luminescent Received: May 19, 2011 Revised: June 24, 2011 Published: July 05, 2011 15274

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20 Emission intensities of semiconductor nanoparticles are sensitive to their surface states and surrounding environments, and detailed knowledge on the information on electronic structures of nanoparticles as well as the factor controlling the emission quantum yield of CdTe nanoparticles is essential for preparing the nanoparticles for specific optical applications.

2. EXPERIMENTS The CdTe nanoparticles were synthesized according to the literature methods.21
23 Briefly, 5.7 mmol of thioglycolic acid (TGA) was added to an aqueous solution (125 mL) containing 2.35 mmol of Cd(ClO4)2 3 6H2O under vigorous stirring, followed by adjustment to pH of 11.2
11.8. Under stirring, H2Te gas generated by the dropwise addition of 20 mL of 0.5 M sulfuric acid to 0.2 g of Al2Te3 chunks under N2 atmosphere was passed through the solution with a slow N2 flow for 20 min. The generated precursors were converted to CdTe nanoparticles by refluxing the solution at 100 °C under open-air conditions. The size of CdTe nanoparticles was controlled by the duration of the reflux time, and the different sizes of CdTe nanoparticles were prepared in 20
60 min after the beginning of the reflux. The evolution of the particle size was estimated from the absorption and photoluminescence spectra.24 The resulting solution was concentrated with a centrifugal concentrator (Sartorius stedim, Vivaspin 500), and the obtained CdTe nanoparticles were repeatedly washed with methanol. The precipitate nanoparticles were dissolved in ca. 0.4 mL of Milli-Q water. Then 1 mL of Milli-Q water solution containing PVA (ca. 30 mg) was mixed with the above aqueous solution containing the nanoparticles. The resultant mixture was transparent, showing a homogeneous distribution of the nanoparticles in solution. No white turbidity due to insoluble materials was observed. TGA and Cd(ClO4)2 3 6H2O were obtained from Aldrich and were used as received. Al2Te3 chunks were obtained from Mitsuwa Chemicals. PVA (Aldrich, molecular weight 124 000
186 000) was purified by precipitation with water and methanol. A certain amount of an aqueous solution of a mixture of CdTe nanoparticles and PVA was cast on an indium-tin-oxide (ITO) coated quartz substrate by a spin-coating method. Then, a semitransparent aluminum (Al) film was deposited on the polymer film by a vacuum vapor deposition technique. The ITO and Al films were used as electrodes. When the nanoparticles were treated with a centrifugal concentrator, the spin-coated films were transparent, indicating a homogeneous distribution of the nanoparticles in a polymer film. The thickness of the prepared polymer film was typically 1.2 μm. All the measurements were performed at room temperature. Electroabsorption and electrophotoluminescence spectra were measured using electric field modulation spectroscopy with the same apparatus as described elsewhere.12,25 A modulation in absorption intensity or emission intensity was induced by a sinusoidal ac voltage with a modulation frequency of 40 Hz. A field-induced change in absorption intensity or emission intensity was detected with a lock-in amplifier at the second harmonic of the modulation frequency. A dc component of the transmitted light intensity or the photoluminescence intensity was simultaneously observed. Electroabsorption and electrophotoluminescence spectra were obtained by plotting the change in absorption intensity and in photoluminescence intensity as a function of

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wavenumber, respectively.√The rms output of the lock-in amplifier was multiplied by 2 2 to convert it to what would be obtained if an equivalent dc field was applied.10 The angle between the direction of the applied electric field and the electric vector of the excitation light was set to be 90° or 54.7° (magic angle) in the electroabsorption measurements, and depolarized emission was detected in the electrophotoluminescence measurements. Applied field strength was evaluated from the applied voltage divided by the thickness. Hereafter, electroabsorption and electrophotoluminescence spectra are abbreviated as E-A and E-PL spectra, respectively. Measurements of the field-induced change in emission decay profile were carried out with a single-photon counting system combined with a pulse generator supplying a bipolar square wave.26 The second harmonic of the output from a mode-locked femtoseocnd Ti:sapphire laser (Spectra Physics, Tsunami) was used for excitation. The repetition rate was reduced to be ∼6 MHz with a pulse picker (model 350-160, Conoptics) from the original 81 MHz. Emission from the sample was dispersed with a monochromator (Nikon, G-250) and detected with a microchannel-plate photomultiplier (Hamamatsu, R3809U-52). The emission signal was discriminated and then led to a time-toamplitude converter system. Emission decays were obtained with a multichannel pulse height analyzer (SEIKO EG&G, model 7700). Applied voltage was a repetition of rectangular waves of positive, zero, negative, and zero bias in turn. The time duration of each bias was 30 ms, but the first 2 ms was a deadtime to exclude an overshooting effect of applied field just after the change in applied voltage. Four different decays were collected, corresponding to positive, zero, negative, and zero sample bias. The instrumental response function had a pulse width of ∼60 ps (fwhm). The steady-state absorption and emission spectra were recorded with an absorption spectrometer (Hitachi, U-3500) and a fluorescence spectrometer (FP-777, JASCO), respectively. Continuous irradiation was carried out with a 150 W Xe lamp in combination with a monochromator and neutral density filters.

3. RESULTS AND DISCUSSION 3.1. Electric Field Effect on Absorption Spectra (E-A Measurements). When an electric field is applied to molecules,

each energy level is shifted, depending on the electric dipole moment (μ) and the molecular polarizability (R) of the state concerned. When the magnitude of μ or R in the excited state is different from the one in the ground state, the absorption spectra as well as the emission spectra are shifted since the magnitudes of the level shift in both states are different from each other. For an isotropic distribution of chromophores in rigid matrices such as PVA, the E-A spectrum (ΔA(ν)) is given by the following equation10
12,27 "

ΔAðνÞ ¼ ðf FÞ

2

# 2 dfAðνÞ=νg 0 d fAðνÞ=νg þ Cν A AðνÞ þ B ν dν dν2 0

0

ð1Þ where f is the internal field factor and F represents applied electric field. The coefficient A0 depends on the change in transition dipole moment following absorption. B0 and C0 correspond to the spectral shift and spectral broadening resulting from the differences in molecular polarizability (ΔR) and in electric dipole moment (Δμ), respectively, between the ground state and the 15275

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Figure 1. E-A spectra (shaded line) and absorption spectra (solid line) of CdTe nanoparticles with the average diameter of 2.2 (a), 2.6 (b), 3.2 (c), 3.5 (d), and 4.1 nm (e) in a PVA film. The simulated E-A spectrum is also shown by a dotted line in every case. Applied field strength was 0.4 MV cm
1. The discontinuity of the absorption spectrum in the high wavenumber region is the experimental artifact in every case.

excited state, which are given as follows B0 ¼

ΔR=2 þ ðΔRm
ΔRÞð3 cos2 χ
1Þ=10 hc

ð2Þ

½5 þ ð3 cos2 ξ
1Þð3 cos2 χ
1Þ 30h2 c2

ð3Þ

C0 ¼ jΔμj2

ΔR denotes the trace of ΔR; ΔRm is the diagonal component of ΔR with respect to the direction of the transition dipole moment; χ is the angle between the direction of F and the electric vector of the light; and ξ is the angle between the direction of Δμ and the transition dipole moment. It is worth mentioning that the first-order term in F, i.e., the term which is proportional to F (not shown in eq 1), becomes zero when this term is integrated over the full space in a randomly distributed system; however, the second-order term in F, i.e., the term proportional to the square of F, can give the nonzero value even when this term is integrated over the full space. At a magic angle condition of χ = 54.7°, the coefficients B0 and C0 are reduced, respectively, as B0 ¼

ΔR 2hc

ð4Þ

C0 ¼

jΔμj2 6h2 c2

ð5Þ

From eqs 4 and 5, the values of |Δμ| and ΔR can be obtained from the first and second derivative components of the E-A spectrum, respectively.

Figure 1 shows the E-A spectra of CdTe nanoparticles in a PVA film with different particle sizes, together with the corresponding absorption spectra. All the absorption spectra exhibit a shoulder in the region of 15 000
25 000 cm
1, which is assigned to the transition to the first exciton state. The peak position of the absorption to the first exciton state as well as the absorption onset shifts to a longer wavelength with increasing particle size, due to quantum-confined effects. Enlarged views of the absorption and E-A spectra of CdTe with a size of 2.2 and 4.1 nm, respectively, are shown in Figure 2, together with the first and second derivatives of the separated exciton absorption band. Absorption and E-A spectra shown in Figures 1 and 2 were obtained with the magic angle of χ, but it is noted that the spectral shape of both spectra obtained with 90° of χ was essentially the same, indicating that the field-induced reorientation is negligible.28 The magnitude of the field-induced change in absorption intensity is proportional to the square of the applied field strength, as expected from eq 1, as shown in Figure 3. All the E-A spectra around the first exciton band are similar in shape to the second derivative of the first exciton band, indicating that the field-induced change in absorption intensity mainly comes from the change in μ, i.e., Δμ, following the transition to the first exciton state. Actually, all the E-A spectra around the first exciton band region could be fitted quite well by the second derivative of the corresponding absorption spectrum. The coefficient A0 in eq 1 is small in all the E-A spectra, indicating that the transition dipole moment to the first exciton state is hardly influenced by F. The values of |Δμ| between the first exciton state and the ground state were obtained from the second derivative part in the E-A spectra using eq 5. The results are shown in Figure 4, where the local field was assumed to be the same as the applied electric field, i.e., f = 1 in eq 1. In the determination of |Δμ|, the absorption profile of the first exciton band was assumed to be a Gaussian, as shown in Figure 2. A mismatch between the observed E-A spectrum and the simulated one in that the simulation is underestimating the low-energy peak and both underestimating and overestimating the high-energy feature may come from the residuals of the absorption. Actually, the observed E-A spectra could be reproduced fairly well even when the second derivative of the observed absorption spectrum was used, and the estimated values of |Δμ| were not so different from the ones determined with the Gaussian profile. All the CdTe nanoparticles exhibit a large value of |Δμ| following the transition to the first exciton state. It is likely that the magnitude of μ in the first exciton state is larger than that in the ground state, indicating that the dipole moment of the CdTe nanoparticles is fairly large in the first exciton state. We can therefore conclude that the CdTe nanoparticles have large CT character in the first exciton state. As shown in Figure 4, the |Δμ| value remains unchanged with the particle size in the region of 2
4 nm diameter, i.e., roughly 16 D, and it appears that the value of |Δμ| slightly increases as the size increases with a diameter larger than 4 nm. This behavior is different from the ones of CdS and CdSe nanoparticles in a film.13,27 It has been reported that both CdS and CdSe nanoparticles exhibit a significant increase in |Δμ| with increasing particle size, indicating that positive and negative charges in the first exciton state are further separated as the particle size increases. Such charge asymmetry in the first exciton state may result from interactions between surface and interior states.29 3.2. Electric Field Effect on Spectrum and Decay of Photoluminescence (E-PL Measurements). The E-PL spectrum 15276

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Figure 2. Absorption spectrum (top), the derivatives of the absorption spectrum (middle), and E-A spectrum (bottom) of CdTe nanoparticles with a diameter of 2.2 nm (left) and 4.1 nm (right). In (a) and (d), the exciton absorption band (thick solid line) was extracted by assuming a Gaussian shape, and the subtracted spectrum is shown by a dotted line. In (b) and (e), the first and second derivative spectra of the extracted absorption spectrum are shown by a solid line and dotted line, respectively. In (c) and (f), the observed E-A spectrum and the simulated one are shown by a shaded line and dotted line, respectively. The applied field strength was 0.4 MV cm
1.

Figure 3. Plots of the E-A intensity relative to the absorption intensity of CdTe nanoparticles having a size of 2.5 nm as a function of the square of the applied electric field. The intensity was monitored at 501 nm. The broken straight line is just a guide for the eye.

(ΔIPL(ν)) is also expected to be given by a combination of the zeroth, first, and second derivatives of the PL spectrum, as in the case of E-A spectrum.12 The first and second derivative components arise from ΔR and Δμ, respectively. The coefficient of the zeroth derivative component depends on the field-induced change in emission quantum yield, arising from the field-induced change in radiative decay rate and/or in nonradiative decay rate at the emitting state. Then, electric field effects on photoexcitation dynamics can be evaluated from the zeroth derivative component of the E-PL spectrum. Figure 5 shows the E-PL spectra of the CdTe nanoparticles in PVA with different particle sizes, together with the PL spectra simultaneously observed. Excitation was done at the wavelength where the field-induced change in absorption intensity is negligible. All the PL spectra are considered to arise from the excitonemitting state because the peak of the PL spectra is close to that of the exciton absorption band, but the emitting sate is regarded as different from the Franck
Condon state of the exciton band reached by the optical absorption, as will be mentioned later. In contrast with the E-A spectra, field-induced quenching of emission is predominantly observed in all the E-PL spectra. Actually, the E-PL spectra could be reproduced by a superposition of

Figure 4. Plots of the magnitude of Δμ as a function of size following absorption of the first exciton band of CdTe nanoparticles. The broken line is just a guide for the eye.

the PL spectrum, its first and second derivative spectra (see Figure 6). The magnitude of the field-induced quenching of photoluminescence is shown in Figure 7. When the emission quantum yield at zero field and the field-induced change in yield are denoted by Φ and ΔΦ, respectively, the field-induced change in emission quantum yield relative to the total emission quantum yield is given by ΔΦ/Φ. In the analysis of the E-PL spectrum, the magnitude of ΔΦ was evaluated from the integration of the intensity of the E-PL spectrum across the whole spectrum region since the derivative components do not give a change in total intensity. As shown in Figure 7,
ΔΦ/Φ is roughly 0.1 with a field strength of 0.4 MV cm
1; i.e., about 10% of PL is quenched by application of the field of 0.4 MV cm
1. The large fluctuation of the field-induced quenching observed in Figure 7 may come from the variety in surface structure of the sample. It is worth mentioning that the signal intensity of E-PL is not proportional to the square of the applied electric field, in contrast with the E-A intensity (cf. Figures 3 and 8). The spectral bandwidth of the E-PL spectra is smaller than that of the corresponding PL spectra in each size, as shown in Figure 5. The narrowness of the E-PL spectra is ascribed to the contribution of the second derivative of the PL spectra. It is also 15277

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Figure 5. E-PL spectra (shaded line) and PL spectra (thick solid line) of CdTe nanoparticles with the average diameter of 2.2, 2.5, 3.0, 3.2, 3.9, and 4.1 nm, respectively, in a PVA film (from top to bottom). Excitation wavelength was 362, 392, 381, 385, 392, and 402 nm, respectively (from top to bottom). Applied field strength was 0.4 MV cm
1 in every case.

known that the peak position of the E-PL spectra is shifted in comparison with the corresponding PL spectra except for the nanoparticles having a diameter of 3 nm; the peak of the |ΔIPL| spectrum is a little red-shifted in a small size of CdTe nanoparticles, e.g., 2.2 nm, whereas the peak of the |ΔIPL| spectra is a little blue-shifted in a large size of nanoparticles, e.g., 4.1 nm, in comparison with the peak of the corresponding PL spectrum (see Figures 5 and 6). This shift is ascribed to the contribution of the first derivative of the PL spectrum, indicating the change in polarizability following the optical transition from the excitonemitting state to the ground state. In fact, the observd E-PL spectra could be simulated by a linear combination of zeroth, first, and second derivatives of the PL spectra, as already mentioned above. From the second derivative component in the E-PL spectra, combined with eq 5, the value of |Δμ| between the emitting state and the ground state is estimated to be 40 ( 10 D irrespective of the size. This value is much larger than that determined from the E-A spectra, i.e., 16 D, indicating that the exciton-emitting state is very different from the first exciton band reached by optical transition. The first derivative component in the E-PL spectra is negligible in the size of 3 nm, where the peak position is the same in both PL and E-PL spectra, indicating that the polarizability is nearly the same both in the emitting state and

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in the ground state at 3 nm. In other sizes, the peak position of the PL spectrum is different from the one of the E-PL spectrum, as mentioned above. The present results indicate that the negative peak of the E-PL spectrum shifts to the red with a diameter smaller than 3 nm and shifts to the blue with a diameter lager than 3 nm, as the second-order Stark shift caused by the difference in polarizability between the emitting state and the ground state. Then the polarizability in the exciton-emitting state is regarded as larger than that in the ground state in large sizes of CdTe nanoparticles, whereas the polarizability in small sizes of CdTe nanoparticles is smaller in the excited state than that in the ground state. The magnitude of the polarizability in the emitting state of CdTe nanoparticles with a size of 4.1 and 2.2 nm is estimated to be larger and smaller than that in the ground state by ∼5 and ∼6 nm3, respectively. From the steady state measurements of the E-PL spectra, it is difficult to confirm whether the field-induced decrease in emission intensity is due to the increase of the nonradiative decay rate at the emitting state or the decrease in initial population of the emitting state. In the present study, therefore, we have carried out the measurements of the field-induced change in PL decay profile. Figure 9a shows a typical PL decay of the 3.5 nm CdTe nanoparticles in PVA observed in the absence of F. Excitation and monitoring wavelengths were 400 and 610 nm, respectively. The difference between the decays observed at zero field (I0PL(t)) and at 0.5 MV cm
1 (IFPL(t)), i.e., IFPL(t)
I0PL(t), referred to as ΔIPL(t), and the ratio IFPL(t)/I0PL(t) are shown in Figures 9b and 9c, respectively. The ΔIPL(t) value is negative during the full decay, showing that the PL is quenched by F in the whole time region. The time profile of ΔIPL(t) is different from that of I0PL(t), indicating that the emission lifetime of the CdTe nanoparticle is influenced by F. It is important to note in Figure 9c that the magnitude of IFPL(t)/I0PL(t) is lower than unity just after photoexcitation. This result indicates that the initial population of the exciton-emitting state of the CdTe nanoparticle is reduced in the presence of F. All the time profiles of I0PL(t), IFPL(t), ΔIPL(t), and IFPL(t)/I0PL(t) are reproduced by assuming a tetraexponential decay, i.e., ∑iAi exp(
t/τi), where Ai and τi denote preexponential factor and lifetime of component i, respectively. The lifetime and the preexponential factor of each decay component at zero field were evaluated as follows: τ1 = 2.05 ns, τ2 = 5.80 ns, τ3 = 11.37 ns, τ4 = 60.83 ns, A1 = 0.738, A2 = 0.160, A3 = 0.094, A4 = 0.008. The corresponding values in the presence of 0.5 MV cm
1 were determined as follows: τ1 = 1.78 ns, τ2 = 5.12 ns, τ3 = 10.09 ns, τ4 = 61.75 ns, A1 = 0.704, A2 = 0.149, A3 = 0.085, A4 = 0.006. The average emission lifetime, defined by ∑iAiτi/∑iAi, is determined to be 4.00 ns at zero field and 3.44 ns at 0.5 MV cm
1. The magnitude of Ai corresponds to the population of each emitting state, and the ratio of ∑iAi at 0.5 MV cm
1 to that at zero field is 0.944. These results lead us to a conclusion that the field-induced quenching of PL of the CdTe nanoparticles is due to the fieldinduced decreases both in lifetime and in initial population of the exciton-emitting state. The exciton state is regarded as a charge-separated state in nature. The observed field-induced decrease in initial population of the exciton-emitting state suggests that the field-assisted charge separation into the hole and electron within each nanoparticle occurs from the excited state in competition with the relaxation to the exciton-emitting state; i.e., the formation of the exciton-emitting state is decelerated in the presence of F, as in the case of π-conjugated polymers.30 The shortening of the emission 15278

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Figure 6. PL as well as E-PL spectra (top), the derivatives of the PL spectrum (middle), and E-PL spectrum (bottom) of CdTe nanoparticles with a diameter of 2.2 nm (left) and 4.1 nm (right). E-PL spectra are shown by a shaded line in any figure. In (a) and (d), the PL spectrum is shown by a thick solid line. In (b) and (e), the first and second derivative spectra of the PL spectrum are shown by a solid line and dotted line, respectively. In (c) and (f), the simulated E-PL spectra are shown by a dotted line. The applied field strength was 0.4 MV cm
1.

Figure 7. Plots of the field-induced quenching of PL relative to the total PL intensity in CdTe nanoparticles as a function of size in the presence of the external electric field of 0.4 MV cm
1. The dotted line is just a guide for the eye.

lifetime suggests that the nonradiative process at the exciton-emitting state is accelerated by F. The significant nonradiative process from the exciton-emitting state may be the formation of surface trap states undergoing further nonradiative decays.31 It has been shown that the rate of the CT process is notably influenced by F in mixtures of donor and acceptor molecules in a film.12 It is therefore conceivable that the observed field-induced acceleration of the nonradiative process arises from the large CT character of the CdTe nanoparticles in the first exciton state, confirmed from the above-mentioned analysis of the E-A spectra as well as the E-PL spectra. 3.3. Photoirradiation Effect on Photoluminescence Intensity. It has been also found that the emission intensity of the CdTe nanoparticles in PVA is influenced by photoirradiation. Figure 10a shows the PL spectrum of the 3.5 nm diameter CdTe nanoparticles in a PVA film as a function of photoirradiation time at an atmospheric condition. Continuous irradiation was carried out with 400 nm light. It is clearly shown in Figure 10a that the emission intensity of the CdTe-doped film increases with increasing photoirradiation time. Plots of the peak intensity of the emission against photoirradiation time are shown in the inset of Figure 10a. The emission intensity of the CdTe nanoparticle after continuous photoirradiation for 180 min was about 1.4 times higher than that just after the photoirradiation. It should also be

Figure 8. Plots of the E-PL intensity relative to the PL intensity of CdTe nanoparticles having a size of 2.5 nm as a function of the square of the applied electric field. The intensity was monitored at 541 nm with excitation at 392 nm.

noted that the peak of the emission shows a small blue-shift following photoirradiation. Figure 10b shows the time dependence of the emission spectrum of CdTe nanoparticles in PVA in a vacuum condition, i.e., at ∼4.5  10
6 Torr. In this experiment, the CdTe-doped film was irradiated with 400 nm light at an atmospheric condition until the exciton emission intensity reaches a saturated value. Then the sample chamber was evacuated, and the emission spectra were recorded periodically with photoirradiation. It is clearly shown that the emission intensity decreases with time in vacuo, being accompanied by a small red-shift of the peak position. Such a decrease in the emission intensity was observed, irrespective of photoirradiation. We have also observed reenhancement of the emission intensity by irradiating the same sample with 400 nm light at an atmospheric condition. This result suggests that the alternative change in emission intensity at an atmospheric condition with photoirradiation and in vacuo is repeatable and that the intensity change due to photoirradiation is not due to an irreversible degradation of the nanoparticles. Figure 10c shows the absorption spectra of the 3.5 nm diameter CdTe nanoparticle in a PVA film as a function of photoirradiation time of 400 nm light at an atmospheric condition. 15279

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Figure 9. (a) Decay profiles of PL of 3.5 nm diameter CdTe nanoparticles in a PVA film observed at 610 nm in the absence (dotted line) and in the presence of 0.5 MV cm
1 (solid line). (b) The difference between the decays observed at 0.5 MV cm
1 and at zero field (thin solid line), together with the decay at zero field (dotted line) and the simulated difference (thick solid line). (c) The ratio of the decay profile observed at 0.5 MV cm
1 relative to that at zero field (noisy solid line), together with the simulated one (clear solid line). These results were obtained at an atmospheric pressure with excitation at 400 nm.

Figure 10. Time dependence of the PL spectrum of the 3.5 nm diameter CdTe nanoparticle in a PVA film with irradiation of 400 nm light at an atmospheric pressure (a) and at a low pressure of 6  10
4 Pa (b). Plots of the PL intensity as a function of time are shown in the insets of (a) and (b). Time dependence of the absorption spectrum of the same CdTe-doped film with irradiation of 400 nm light at an atmospheric pressure is shown in (c).

The absorption intensity is not changed with photoirradiation, indicating that the photoinduced enhancement of the emission is

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not due to the increase in absorption intensity. The average emission lifetime also remains constant with photoirradiation within our experimental uncertainty. These results suggest that the photoenhancement of the emission intensity at an atmospheric condition arises from the photoinduced increase in the yield of the process from the optically excited Franck
Condon state to the emitting exciton state. The photoinduced enhancement of emission has been reported in colloidal CdS and CdSe nanoparticles.13,14,31
35 We have shown in our previous papers13,14 that the exciton emission of colloidal CdS nanoparticles in a PMMA film is enhanced by irradiation of UV light at an atmospheric condition and was deenhanced at a low pressure of 10
5 Torr. Adsorption of water molecules on the surface32 or photooxidation of the surfacepassivating dangling bonds34 was suggested to explain the photoinduced enhancement of emission intensity at an atmospheric condition. In the present study, however, it may be said that photoirradiation at an atmospheric condition induces the modification of the surface environment, which decreases the rate of the nonradiative process from the emitting exciton state to the nonemissive surface state, resulting in the emission enhancement. The core particle and the surface ligands remain unchanged with photoirradiation because the shape of the absorption spectra is hardly influenced by photoirradiation. Depending on the materials, however, the photoirradiation effect may depend not only on the surface condition but also on the core/shell particles. To clarify the mechanism as well as the core/shell dependence of the photoirradiation effect, further studies are necessary with different nanomaterials and with different modifications by capped materials.

4. SUMMARY E-A and E-PL spectra of colloidal CdTe nanoparticles have been measured at room temperature in a PVA film. The analysis of the E-A spectra indicates the large enhancement of the dipole moment following the optical transition to the first exciton state. The magnitude of Δμ with excitation into the first exciton state is roughly 16 D in the size of 2
4 nm, and its value increases a little with an increase of the size above 4.0 nm. The electric-fieldinduced quenching of PL is dominant in the E-PL spectra, which arises from the field-induced decrease both in lifetime and in initial population of the exciton-emitting state. The magnitude of the field-induced quenching is roughly 10% of the total intensity in the presence of 0.4 MV cm
1. These results show that the exciton state of the CdTe nanoparticles has large CT character and that the rate of the nonradiative process from this state to the nonemissive surface trapped state is accelerated by F. The PL spectrum of the small size of CdTe nanoparticles shows a blueshift in the presence of F, while the PL spectrum of the large size of CdTe nanoparticles shows a red-shift in the presence of F, in addition to the Stark shift given by the second derivative of the PL spectrum caused by the change in electric dipole moment following the optical transition from the exciton-emitting state to the ground state. Thus, the change in polarizability of the nanoparticles following emission depends on the size of the nanoparticle. It is known that the polarizability is larger in the excited state than in the ground state for nanoparticles with a size larger than 3 nm, while the relation is opposite for nanoparticles with a size smaller than 3 nm. At around 3 nm size of CdTe, the first derivative component is negligible. It is also found that photoirradiation of CdTe-doped films at an atmospheric 15280

dx.doi.org/10.1021/jp204660m |J. Phys. Chem. C 2011, 115, 15274–15281

The Journal of Physical Chemistry C condition enhances the emission intensity, but the emission intensity is de-enhanced in a vacuum condition.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone: +81-11-706-9410.

’ ACKNOWLEDGMENT This work was in part supported by a Grant-in-Aid for Scientific Research (No. 20043005) from the MEXT in Japan.

ARTICLE

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