Tunable Visible-Light Emission from CdS Nanocrystallites Prepared

Oct 12, 2007 - Raúl García-Rodríguez , Mark P. Hendricks , Brandi M. Cossairt , Haitao Liu , and .... Manisree Majumder , Santanu Karan , Biswanath...
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16734

J. Phys. Chem. C 2007, 111, 16734-16741

Tunable Visible-Light Emission from CdS Nanocrystallites Prepared under Microwave Irradiation Santanu Karan and Biswanath Mallik* Department of Spectroscopy, Indian Association for the CultiVation of Science, 2A and 2B, Raja S. C. Mullick Road, JadaVpur, Kolkata-700 032, India ReceiVed: June 21, 2007; In Final Form: August 20, 2007

The microwave (MW)-assisted reaction of cadmium acetate with thiourea in N,N-dimethylformamide (DMF) was controlled by controlling the MW irradiation time in the presence of 1-thioglycerol as a capping agent. The peak position of the absorption band of the CdS nanocrystallites, dispersed in chloroform, shifts toward longer wavelength with increasing MW irradiation time, indicating growth of particle size under prolonged MW irradiation. However, the peak position of absorption band remains at the same wavelength, and only the intensity of the absorption band is increased when the MW irradiation of the colloidal solution of CdS nanocrystallites in DMF is periodically interrupted, keeping the solution at room temperature (27 °C) before each irradiation. This suggests that the particle growth occurs only during the continuous irradiation of MW and stops when the system is cooled down. From the spectral absorption edge, the diameter of CdS nanoparticles has been estimated. Photoluminescence of the CdS nanocrystallites, dispersed in chloroform, observed in the visible range shifts toward higher wavelength with increased duration of MW irradiation. When the MW irradiation was repeated for a fixed duration in colloidal solution of CdS nanocrystallites in DMF, enhancement in PL intensity was noticed without any change in the emission peak position. The relative PL quantum yield of the CdS nanocrystallites was estimated under various experimental conditions. Time-correlated singlephoton counting experiments were performed to study the time-resolved photoluminescence of CdS nanocrystallites. The observed emission decay profiles have been simulated by using the multiexponential model.

1. Introduction In the recent past there has been a rapid increase in research on nanometer-sized particles (nanoparticles) which exhibit unique properties that are greatly different from those of bulk solids.1-8 Among the numerous materials studied, CdS, one of the most important IV-VI group semiconductors, has attracted much attention.9-26 Quite recent publications include the synthesis of branched CdS nanowires and nanotrees,22 shapecontrol synthesis of CdS nanocrystals,23 monolayer behavior of CdS quantum dots,24 studies on photophysical properties,15,20 resonant Stokes shift in CdS nanocrystals,25 etc. In fact, the main interest in studying CdS nanoparticles is related to their preparation and photophysical properties which make them useful in applications such as optoelectronic,26 solar energy conversion,27 photocatalysis,28 photodegradation of water pollutants,28 etc. One aspect of photophysical properties is the photoluminescence (PL) of CdS nanocrystallites which has been studied extensively by various groups of researchers10-20 by using samples prepared by different methods. For the preparation of CdS nanoparticles, usually the chemical bath technique was employed9 by using a conventional method of heating but, in some cases, microwave (MW) was also used.10-13 In general, the reported emission spectra consist of two broad bands11,13,15,16,20 (in the range 400-520 nm and 520-800 nm) peaked at around 480 and 650 nm, respectively. Relatively strong emission peaked around 650 nm and is attributed to radiative recombination at deep trap sites originating from lattice imperfections at the * Corresponding author. E-mail: [email protected]. Fax: + 91-33-2473-2805.

surface, i.e., the surface states, and the emission that peaked around 480 nm is attributed to a direct recombination of electron and hole pairs at the band gap. Yang et al.10 reported a PL spectrum (in the range of ∼590-615 nm) with a single strong peak at 602 nm for CdS nanoparticles synthesized via microwave irradiation. This PL band was attributed to the recombination of an electron trapped in a sulfur vacancy with a hole in the valence band of CdS. He et al.12 also reported a single PL band that peaked at 529 nm for poly(N-vinyl-2-pyrrolidone) (PVP)-capped CdS nanoparticles prepared under microwave irradiation. The reported results indicated that the PL of CdS nanocrystallites depends on the particle size and the method of their preparation. In our laboratory, a program was taken for preparing 1-thioglycerol-capped CdS nanocrystallites and to study their PL behavior while dispersed in a separate medium. We have prepared 1-thioglycerol-capped nanosized CdS using microwave (MW) heating in a periodical and intermittent mode (a kind of pulse irradiation) and observed that such nanosized CdS exhibit interesting photoluminescence properties. The timecorrelated single-photon counting (TCSPC) experiments were performed to study the time-resolved photoluminescence. Multiexponential decay profiles29 have been observed for the emission. The results are discussed in this article. 2. Experimental Section Cadmium acetate (Cd(CH3COO)2‚2H2O, 99.9%), thiourea (C(NH2)2S, GR grade), and 1-thioglycerol (HSCH2CH(OH)CH2OH) were purchased from Merck, India. N,N-dimethylformamide (DMF) from Spectrochem, India, was used as received.

10.1021/jp074849e CCC: $37.00 © 2007 American Chemical Society Published on Web 10/12/2007

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Figure 1. (a) UV-vis absorption spectra of CdS nanocrystallites dispersed in chloroform. The representative curves in the figure give the pattern corresponding to different durations of microwave (MW) irradiation during preparation. (b) Position of absorption peak with the microwave irradiation time, (c) variation of calculated nanocrystallites diameter with microwave irradiation time during preparation, and (d) UV-vis absorption spectra of CdS nanocrystallites in DMF with repeated exposure of microwave for 45 s. The representative curves in the figure give the pattern for different repeated exposures.

A MW oven (Samsung, 2.45 GHz, max power 900 W) was used to prepare the nanocrystallites. A solution of cadmium acetate (50 mM) and 1-thioglycerol (30 mM) in DMF was prepared in a glass beaker, then thiourea (30 mM) was added to the solution, and after mixing, the beaker was placed inside the MW oven. Then, the solution was irradiated with MW continuously or periodically for different durations (such as 25, 30, 35 s, etc.) after cooling the irradiated solution down to ambient temperature (room temperature, 27 °C) before each irradiation. The MW irradiation process was repeated for six times. Finally, the nanocrystallites were separated by centrifuging at 6000 rpm and by washing for several times with absolute ethanol and were dried under vacuum. The powder was then dispersed in chloroform, and a droplet of the resulting dispersion was placed on a carbon-coated copper mesh and dried in vacuum prior to analysis by transmission electron microscopy (TEM). A transmission electron microscope (JEOL, JEM-2010, Japan) was employed for the measurements. Absorption and photoluminescence emission spectra were recorded with a UV-vis absorption spectrophotometer (Shimadzu 2401PC, Japan) and a fluorescence spectrophotometer (Hitachi 4500, Japan), respectively. The Fourier transform infrared (FTIR) spectra have been recorded with a Nicolet, MAGNA-IR 750 spectrometer. Time-correlated single-photon counting (TCSPC), Fluoro Cube

(HORIBA JOBIN YVON), was used with an excitation wavelength of 375 nm from Diode Laser. 3. Results and Discussion 3.1. 1-Thioglycerol-Capped CdS Nanocrystallites. Nucleation and particle growth are two main processes in the synthesis of nanoparticles from their precursors in solutions. Termination of the particle growth is also an important factor. The preparation conditions such as starting materials, solvent, additives such as stabilizers, and reaction time and temperature influence each step of reaction significantly. The above two processes, nucleation and particle growth, depend on the local temperature distribution of the solution. However, MW heating is known to lead homogeneous heating, i.e., molecular level, of the polar solution system, achieving automatic control of the abovementioned two processes which is indispensable for the selective preparation of nanoparticles.30 CdS nanoparticles showing size distribution (shown in Figure 2c) and with a well-defined crystallinity were prepared by reacting with Cd2+ and S2- in DMF. Thiourea was selected as a source of S2- because it is known to generate S2- through the thermal decomposition, with the assistance of a base catalyst.31-34 Water included in the cadmium salt as water of crystallization and in DMF leads to hydroxide anion formation

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Karan and Mallik reaction of cadmium salt with S2- under MW irradiation. It will be shown later in this article that 1-thioglycerol bonds with the CdS nanoparticles through the sulfur end and prevents the coalescence of nanoparticles, as observed for mercaptopropanoic-acid-capped CdS nanoparticles.35 When the DMF solution containing cadmium acetate, thioglycerol, and thiourea was irradiated by MW, the reaction mixture became clearer and its yellow color gradually deepened with further irradiation, indicating the formation of colloidal CdS in the system. The formation of CdS nanoparticles was monitored by the change in the UV-vis absorption spectra. A TEM image of the solution after the irradiation for a minimum of 25 s revealed the formation of CdS nanocrystallites. CdS nanocrystallites were not observable for the sample solution irradiated for less than 25 s, suggesting the presence of a threshold for the release of S2- from thiourea. 3.2. Absorption Spectra of the CdS Nanocrystallites. Figure 1a shows the absorption spectra of the CdS powder samples (collected from the colloidal CdS solution prepared by MW irradiation for different durations) dispersed in chloroform. The absorption peak is moved to a longer wavelength in its onsets with increasing duration of MW irradiation (Figure 1a). In Figure 1b, the plot of absorption peak position with MW irradiation time/duration is shown. The trend line obtained from this plot indicates the nonuniform growth of the size of CdS nanocrystallites with duration of MW irradiation. This shift in the absorption peaks indicated the size-growth11,20 of the CdS nanocrystallites induced by the continued irradiation. For the increase in MW irradiation time higher than 45 s, the absorption peak position shifts rapidly toward red indicating rapid enhancement in CdS nanoparticle size. The integrated absorption intensities (Figure 1a) also increased with increasing irradiation time, showing that the concentration of CdS nanocrystallites was also increased with increasing size of nanocrystallites. From Figure 1a, the presence of tail in the visible spectral region in the absorption spectra is noticed, i.e., the absorption spectra appear to broaden out as well as grow with longer MW irradiation times. We believe that the presence of tail in the visible spectral region in the absorption spectra (i.e., broadness in the absorption spectra) is related to the size distribution of CdS nanocrystallites. The absorption edge (λe, being obtained from the intersection of the sharply decreasing region of the spectrum with the baseline) could be used to assess the size of semiconductor crystallites.12,36 From the spectral absorption edge (Figure 1a), the diameter of CdS nanoparticles was calculated using Henglein’s empirical curve that relates the wavelength of the absorption threshold/edge to the diameter (2RCdS) of the CdS clusters12,37 as

2RCdS ) 0.1/(0.1338 - 0.0002345λe) nm

Figure 2. HRTEM image of CdS nanocrystallites on a carbon-coated copper grid prepared for microwave irradiation of (a) 25 s and (b) 50 s. Insets show the corresponding selected area electron diffraction pattern and (c) size distribution (hystogram) of the CdS nanocrystallites for the samples prepared for microwave irradiation of 50 s.

in the presence of DMF. The MW-assisted hydrolysis of thiourea catalyzed by hydroxide leads to the release of sulfide anion (S2-) in the system.11 We successfully prepared a colloidal dispersion of 1-thioglycerol-capped CdS nanocrystallites through the

(1)

The absorption edge and the estimated size (diameter) of CdS nanoparticles/crystallites prepared under MW irradiation for different amounts are shown in Table 1. Plot of the size of CdS nanoparticles as a function of irradiation time is shown in Figure 1c. Panels b and c of Figure 1 demonstrate clearly that the shift in the absorption peak corresponds to the change in particle size of the CdS nanocrystallites with duration of MW irradiation. With increasing duration of MW irradiation, a characteristic red shift occurred with increasing size of the CdS nanocrystallites. The increase in particle size with the MW irradiation time indicated that Ostwald ripening appeared to determine the final size of CdS nanocrystallite in the present preparation procedure.12

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TABLE 1: Absorption Edge (λe) and the Calculated (from Equation 1) Nanocrystallite Diameter (2RCdS) Corresponding to Samples Prepared under Different Times of MW Irradiation microwave irradiation time (s)

λe (nm)

2RCdS (nm)

25 30 35 40 45 50

400.2 417.3 440.6 448.5 449.6 463.9

2.5 2.7 3.2 3.4 3.5 3.9

The irradiation of the colloidal CdS solution in DMF was periodically interrupted several times, keeping the irradiation time constant (45 s). After each irradiation the solution was cooled down to ambient temperature. Under such periodical MW heating the absorption onset was pinned at the same wavelength and only the optical density of the absorption band of the CdS solutions was increased (Figure 1d). Similar to Figure 1a, in the case of Figure 1d the presence of tail in the visible spectral region in the absorption spectra is also noticed, i.e., the absorption spectra appear to broaden out as well as grow with longer MW irradiation times. In this case also, we believe that the presence of tail in the visible spectral region in absorption spectra (i.e., broadness in the absorption spectra) is related to the size distribution of CdS nanocrystallites. The enhancement in absorption intensity almost at the same peak position indicated that the periodic and intermittent MW irradiation (for same duration) increased the number of CdS nanocrystallites but not the particle size. From this experiment it is indicated that the particle growth occurs only during the continuous MW irradiation and stops when the system is cooled down. Thus, MW irradiation influences the nucleation and growing rates of CdS nanocrystallites. 3.3. HRTEM Image of the CdS Nanocrystallites. HRTEM images of the CdS nanocrystallites prepared for 25 and 50 s of MW irradiation are shown in panels a and b of Figure 2, respectively. Insets of Figure 2a,b show the corresponding selected area electron diffraction (SAED) patterns for CdS nanocrystallites. Figure 2a,b indicates that the particle size of CdS nanocrystallites depends on the duration of MW irradiation. Figure 2c shows the particle size versus frequency (hystogram) curve based on the analysis of few HRTEM images corresponding to CdS nanocrystallites prepared under MW irradiation for 50 s. The average particle size is found to be ∼2.75 nm, and this value is slightly lower than the value calculated from eq 1. 3.4. FTIR Spectroscopy and X-ray Diffraction. FTIR spectra recorded from the CdS nanocrystallites prepared via microwave irradiation for 50 s (curve 1) and for pure 1-thioglycerol (curve 2) are shown in Figure 3a. The FTIR spectrum of the nanocrystallites is similar to that of 1-thioglycerol except for the absence of the S-H vibration peak38 at 2557 cm-1. The thiolates are connected to the Cd++ sites on the CdS nanocrystallites surface via sulfur atoms, and act as the ‘skin’ of the CdS particles.38 The new band at near 1558 cm-1 for the CdS nanocrystallites could be due to the water bending of the adsorbed water molecules on the surface of the nanocrystallites.13 Thus the capping of CdS nanocrystallites by 1-thioglycerol has been confirmed by FTIR spectroscopy. EDX of CdS nanocrystallites prepared via microwave irradiation for 25 s, as shown in Figure 3b, supports the presence of S (44.32%) and Cd (47.79%) with a little amount of oxygen (7.89%) in the sample used for taking the HRTEM image. The XRD pattern of CdS nanocrystallites (Figure 3c) showed the presence of

Figure 3. (a) FTIR spectra of CdS nanocrystallites prepared for microwave irradiation of 50 s (curve 1) and for pure 1-thioglycerol (curve 2), (b) EDX spectrum for the CdS nanocrystallites prepared under microwave irradiation of 25 s, and (c) X-ray diffraction pattern for the powder samples prepared for microwave irradiation of 25, 40, and 50 s, respectively.

reflections characteristic of the hexagonal phase of the nanocrystallites prepared under microwave irradiation for different

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Figure 4. Steady-state PL spectra of CdS nanocrystallites for an excitation wavelength of 350 nm: (a) dispersed in chloroform (the representative curves in the figure give the pattern for different times of microwave irradiation during preparation), (b) plot of PL peak position with the microwave irradiation time, (c) for different excitation wavelengths for the sample prepared with 25 s microwave irradiation, and (d) in DMF with repeated exposures (1-6) of microwave for 45 s for an excitation wavelength of 350 nm.

times/durations. The intense and wide peaks are positioned at 2θ ) 26.6°, 43.8°, and 51.2°, which oriented along the (002), (110), and (112) directions and are in agreement with the JCPDS file 41-1049. With the increase in MW irradiation time the (110) and (112) peaks become prominent and sharp due to the increase in nanocrystalline size. 3.5. Steady-State Photoluminescence. Bulk CdS is reported to have a broad emission in the 500-700 nm range of the luminescence spectrum.39 Photoluminescence of CdS nanocrystallites prepared under MW heating can give us much information on their size and surface structures. Figure 4a shows the PL emission spectra of CdS in chloroform collected from the colloidal CdS solutions in DMF under MW irradiation for different durations. In Figure 4b, the plot of PL peak position versus MW irradiation time is shown. It is noticed from Figure 4b that the PL peak position varies with MW irradiation time in a similar way/fashion as the absorption peak position (Figure 1b) and CdS nanoparticle diameter (Figure 1c) vary with MW irradiation time. The trend line in Figure 4b indicates the variation of PL peak position with the size of CdS nanocrystallites grown depending on the MW irradiation time. Figure 4c shows the variation in PL peak intensity for the change in photoexcitation wavelength. For the photoexcitation from 300 to 350 nm, an increase in PL intensity was noticed, but for further increase in photoexcitation wavelength a decrease in PL intensity was noted. However, the PL peak position remained the same. Observation of emission almost at the same position suggests that the CdS nanoparticle size (average) representing the PL peak position was independent of the photoexcitation

Figure 5. Variation of PL peak position for an excitation wavelength of 350 nm with the corresponding absorption peak position.

wavelength. When the MW irradiation was repeated, an enhancement in PL intensity was noticed as shown in Figure 4d for an excitation wavelength of 350 nm; but almost no change in the emission peak position was observed. In contrast, Wade et al.11 reported that in the MW-assisted prepared CdS nanocrystallites in DMF, when the irradiation was repeated, an appreciable shift to longer wavelength was observed for the emission peak. They also mentioned11 that by further repetition

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TABLE 2: Relative Photoluminescence (PL) Quantum Yield (ΦCdS), Component of Emission Lifetime, and the Preexponential Factora for the Decays Corresponding to Samples Prepared for Different Durations of MW Irradiation MW irradiation time (s)

quantum yield (ΦCdS)

τ1 (ns)

τ2 (ns)

τ3 (ns)

τ4 (ns)

τ5 (ns)

χ2

τave (ns)

25 30 35 40 45 50

0.0192 0.0098 0.0011 0.00043 0.00036 0.00039

2.79 (9.39) 1.53 (9.43) 1.20 (14.32) 5.32 (20.20) 1.04 (17.53) 0.21 (12.61)

20.34 (28.71) 19.30 (27.74) 7.00 (15.56) 24.70 (16.07) 5.73 (18.10) 1.16 (13.74)

90.21 (42.77) 89.60 (42.90) 39.80 (25.71) 0.96 (23.15) 27.70 (25.37) 6.55 (17.13)

0.27 (19.12) 0.16 (19.93) 93.40 (30.52) 109.00 (11.58) 139.00 (21.95) 35.20 (29.31)

0.22 (13.89) 0.14 (29.00) 0.17 (17.05) 175.00 (27.21)

0.90 0.94 1.10 1.17 1.19 1.14

81.55 80.68 77.17 82.52 114.50 146.30

a

Preexponential factors for each component are given in parentheses.

of the irradiation, the emission intensity decreased and the extent of the peak shift was diminished. The decrease in the emission intensity and the shift of the emission peak was attributed to the changes in the surface structures, which should be related to the emission from surface defects. Ultimately, they concluded that PL was attributed to the sulfur vacancy. However, as in the present experiment no change in the peak position was noticed upon repeated MW irradiation, and the possibility of occurrence of emission from the surface defects (sulfur vacancy) could be excluded. In the present case, only one emission peak has appeared which corresponds to the reported peak16,20 in the lower wavelength region, originating from the direct recombination of electron and hole pairs at the band gap. From Figures 1b and 4b it is clear that the nature of change in absorption property corresponds to the change in emission from the CdS nanocrystallites. In Figure 5, a good linear plot of the emission peak position against the absorption peak position is shown. Such emissions can be primarily attributed to the quantum confinement effect in the nanoscaled CdS crystals. The quantum confinement effect for explaining the luminescence has been proposed by Cullis and Canham40,41 in the case of porous silicon. The relative PL quantum yield of CdS nanocrystallites (ΦCdS) were estimated by integrating the area under the PL curves using the following formula (eq 2):42,43

ΦCdS )

ODstandard × ACdS × Φstandard ODCdS × Astandard

(2)

where A is the area under the respective PL curve and OD is optical density at the excitation wavelength. The standard used for the PL quantum yield measurement is anthracene (Φstandard ) 0.36 in cyclohexane).44 A systematic variation in the relative PL quantum yield of CdS nanocrystallites (ΦCdS) with MW irradiation time has been shown in Table 2. From Table 2 it is seen that relative PL quantum yield decreases rapidly with MW irradiation time up to 35s, then it attains almost a saturation/ stable value. 3.6. Time-Resolved Photoluminescence. The time-correlated single-photon counting (TCSPC) experiments were performed. The solutions were excited at 375 nm, and the PL decay was monitored at the peak of the emission. The emission decays for the CdS nanocrystallites, formed by MW irradiation of 25, 35, and 50 s and dispersed in chloroform, are plotted in Figure 6. The observed emission decay profiles were simulated by using the multiexponential model29,45 described by the following equation (eq 3) n

I(t) )

Ri exp(-t/τi) ∑ i)1

(3)

where τi represents the decay times, Ri represents the amplitudes of the components at t ) 0, and n is the number of decay times.

Decay waves generated using eq 3 were found to produce a good fit to the data when four exponential components were considered (i.e., n ) 4) in the case of MW irradiation for 25 and 30 s. However, it was observed that MW irradiation for 35 s and more produced a good fit to the data for n ) 5. The resulting decay times, τi, the average decay lifetimes, τave (calculated from eq 4), and the values of the goodness-of-fit parameter,29 χ2R are listed in Table 2 for the decays corresponding to samples prepared for different durations of MW irradiation. n

τave )

∑ i)1

n

Riτ2i /

Riτi ∑ i)1

(4)

As expected for a good fit, the values of χ2R are near unity.29 It may be noted that the samples corresponding to MW irradiation of 35, 40, 45, and 50 s produced fits with χ2R in the range of ∼1.19-1.25 while four exponential components were considered; however, for five components the value of χ2R remained in the range of ∼1.14-1.19. Consequently, samples corresponding to MW irradiation of 25 and 30 s, four exponential components were considered, and for MW irradiation times longer than 30 s, five exponential components were considered. It should be mentioned here that it is difficult to distinguish between some multiexponential functions or, conversely, that it is difficult to recover the actual values of Ri and τi for a multiexponential decay. For the exciton emission of CdSe nanoparticles, time-resolved measurements have been performed in the femtosecond to microsecond time range, and several decay components have been observed.45-49 These results of CdSe nanoparticles may be useful to understand the decay profiles of CdS nanocrystallites in the present study. Underwood et al.46 applied the femtosecond time-resolved upconversion method to the exciton emission of CdSe nanoparticles and observed a fast decaying component occurring in several picoseconds, and a long-lived component with a lifetime longer than tens of picoseconds. They assigned the short-lived component to the immediate recombination of the hole and electron in the first exciton state and the long-lived component to the recombination in the triplet (spinforbidden) state. Nanosecond exciton emission decays of CdSe nanoparticles have been measured by Jones et al.45 The observed nanosecond decay of CdSe was ascribed to the emission from a mixture between the lowest spin-forbidden state and the nexthighest spin-allowed state. The significant nonradiative process from the exciton-emitting state was considered to be the formation of the surface trap state undergoing further nonradiative decay to levels producing the trap emission.45 However, in the present study as no trap emission was observed, such a possibility could be excluded. As in the present case, there is a size distribution of CdS nanocrystallites contributing to PL, and it appears that the time-

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Figure 7. Variation of the relative PL quantum yield and the average lifetime of CdS nanocrystallites prepared under microwave irradiation for different durations/times and dispersed in chloroform.

the lifetime at the band-edge emission. From Table 2, a systematic variation in the average decay lifetimes (τave) with MW irradiation time has been noticed. For increasing amounts of MW irradiation in the range from 35 to 50 s, clear enhancement in the value of τave has been noticed. Such an enhancement in the value of τave could be related to the population of surface states capturing the electrons and their thermal detrapping depending on the size and size distribution of CdS nanocrystallites. In Figure 7, plots of relative PL quantum yield versus MW irradiation time and average decay lifetime time (τave) versus MW irradiation time are shown. From Figure 7 it is seen that the values of relative quantum yield are much higher in the range of MW irradiation for 25-35 s; when in the same range of MW irradiation, the values of average decay lifetime are much lower. Interestingly, the variation in the values of relative PL quantum yield and average decay lifetime reverses in the range of MW irradiation higher than 35 s. The nature of variation in the value of relative PL quantum yield and average decay lifetime with duration of MW irradiation sounds reasonable and significant. 4. Conclusions

Figure 6. Emission decay profiles of CdS nanocrystallites dispersed in chloroform and prepared under microwave irradiation for (a) 25, (b) 35, and (c) 50 s.

correlated fluorescence measurements arise from a convolution of emitting states from different sized nanocrystallites. A fast decay component less than the order of nanoseconds can be attributed to the recombination of electrons and holes produced by photoexcitation, and a slow decay component of the order of a few nanoseconds or more can be attributed to thermal detrapping of the electrons from the surface states to the conduction band since such thermal activation could enhance

The present experiment shows a size distribution of CdS nanocrystallites prepared by manipulating the duration/time of MW irradiation of the chemical bath containing cadmium acetate, thiourea, and 1-thioglycerol in DMF. Thioglycerol bonds with the CdS nanocrystallites through the sulfur end and prevents the coalescence of nanocrystallites. In the present study, 1-thioglycerol-capped CdS nanocrystallites in DMF as well as those separated from DMF solution and dispersed in chloroform have exhibited light emission in the visible range, having an emission peak at a wavelength depending on the duration of MW irradiation. The visible emission from the nanocrystallites is also tunable over a large wavelength scale. On the basis of the present experiment, the possibility of occurrence of emission from the surface defects (sulfur vacancy) could be excluded. Such tunable emissions are attributed to the quantum-confined effect in the nanoscale CdS crystals. As in the present case, there is a size distribution of CdS nanocrystallites contributing to PL; it appears that the time-correlated fluorescence measurements arise from a convolution of emitting states from different sized nanocrystallites. From the present experiment, a systematic

Tunable Emission from CdS Nanocrystallites variation in the average decay lifetimes (τave) with MW irradiation time has been noticed. When increasing the amount of MW irradiation in the range from 35 to 50 s, clear enhancement in the value of τave has been observed. Such an enhancement in the value of τave could be related to the population of surface states capturing the electrons and their thermal detrapping depending on the size and size distribution of CdS nanocrystallites. The estimated relative PL quantum yield decreases rapidly with MW irradiation time up to 35 s; then it attains almost a saturation/stable value. The variation in relative PL quantum yield and average decay lifetime for the CdS nanocrystallites with duration of MW irradiation for their preparation is quite reasonable and interesting. References and Notes (1) Collier, C. P.; Vossmeyer, T.; Heath, J. R. Annu. ReV. Phys. Chem. 1998, 49, 371. (2) Bawendi, M. G.; Steigerwald, M. L.; Brus, L. E. Annu. ReV. Phys. Chem. 1990, 41, 477. (3) Alivisatos, A. P. Science 1996, 271, 933. (4) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13 226. (5) Brus, L. E. J. Chem. Phys. 1984, 80, 4403. (6) Klein, D. L.; Roth, R.; Lim, A. K. L.; Alivisatos, A. P.; McEuen, P. L. Nature 1997, 389, 699. (7) Basak, D.; Karan, S.; Mallik, B. Chem. Phys. Lett. 2006, 420, 115. (8) Karan, S.; Basak, D.; Mallik, B. Chem. Phys. Lett. 2007, 434, 265. (9) Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2001, 123, 183. (10) Yang, H.; Huang, C.; Li, X.; Shi, R.; Zhang, K. Materials Chem. Phys. 2005, 90, 155. (11) Wada, Y.; Kuramoto, H.; Anand, J.; Kitamura, T.; Sakata, T.; Mori, H.; Yanagida, S. J. Mater. Chem. 2001, 11, 1936. (12) He, R.; Qian, X-F.; Yin, J.; Hong-an, Xi.; Li-juan, Bian.; Zi-kang, Zhu. Colloids Surf. A 2003, 220, 151. (13) Caponetti, E.; Martino, D. C.; Leone, M.; Pcdone, L.; Saladino, M. L.; Vetri, V. J. Colloid Interf. Sci. 2006, 304, 413. (14) Murakoshi, K.; Hosokawa, H.; Yanagida, S. Jpn. J. Appl. Phys. 1999, 38, 522. (15) Babu, K. S.; Vijayan, C.; Haridoss, P. Mater. Lett. 2006, 60, 124. (16) Fujiwara, H.; Hosokawa, H.; Murakoshi, K.; Wada, Y.; Yanagida, S.; Okada, T.; Kobayashi, H. J. Phys. Chem. B 1997, 101, 8270. (17) Nanda, K. K.; Sarangi, S. N.; Sahu, S. N. J. Phys. D: Appl. Phys. 1999, 32, 2306. (18) Deshmukh, N. V.; Bhave, T. M.; Ethiraj, A. S.; Sainkar, S. R.; Ganesan, V.; Bhoraskar, S. V.; Kulkarni, S. K. Nanotechnology 2001, 12, 290. (19) Lin, Y.; Zhang, J.; Sargent, E. H.; Kumacheve, E. Appl. Phys. Lett. 2002, 81, 3134.

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