Photoluminescence Enhancement of Nanogold Decorated CdS

Article pubs.acs.org/JPCC

Photoluminescence Enhancement of Nanogold Decorated CdS Quantum Dots Tamilmani Shanmugapriya and Perumal Ramamurthy* National Centre for Ultrafast Processes, University of Madras, Taramani Campus, Chennai 600 113, India S Supporting Information *

ABSTRACT: Nanoheterostructures consisting of CdS nanorods (NRs) and nanospheres (NSs) with gold nanoparticles (NPs) were synthesized by a phase transfer method. A considerable enhancement in the luminescence intensity around 9-fold and 5-fold was observed for the CdS (NR)/ Au and CdS (NS)/Au nanocomposite, respectively, with respect to the bare CdS. When compared with the singlecomponent bare CdS counterpart a substantial blue shift in the emission peak maximum was observed along with a decrease in the excited state lifetime. The plasmons of the Au NPs collectively interact with the excitons of the CdS NRs and NSs which induces the enhancement of the luminescence by reducing the trap states. Time-resolved emission studies (TRESs) of the CdS/ Au nanocomposite clearly resolved the band edge and trap state emission and also clearly revealed that the former has considerably larger contribution to the emission than the latter.

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INTRODUCTION Nanocomposites provide the possibility for enhancement of functionality and multifunctional properties in contrast with their more limited single-component counterpart.1 A wide variety of nanocomposite materials have been investigated earlier by various research groups, among which core−shell composites form an important group.2−11 These core−shell structures have enhanced luminescence,11 improved stability,12 which is exploited in designing light-harvesting devices,13 and engineered band structures.14 Considerable effort has also been made in recent years to fabricate nanocomposites with core− shell15−17 architecture, one- and two-dimensional heterostructures.18−20 Among these nanocomposites, the hybrid selfassembled heterostructures have also acquired equal importance as that of the core−shell structures owing to their enhanced multifunctionality and novel properties.21−23 The semiconductor/metal nanocomposite is a typical example of these heterostructures. Self-assembled building blocks leading to various structures like metal-tipped semiconductor nanorods24−28 and metal-shelled semiconductor nanowires29 were discussed in the literature earlier. The photophysical properties of the excitons in hybrid nanocomposites containing semiconductor and metal nanoparticles have been studied elaborately,29−35 and the observations are mainly explained as energy transfer and electron transfer between the semiconductor and metal nanoparticles in the heterostructures. These interactions generally result in either enhancement36−45 or suppression46,47 of quantum dot emission. Hence, the understanding of the variation in luminescence properties, especially of the luminescence enhancement of the semiconductor quantum dots by the other component in the nanocomposite system, becomes a very important factor in the © XXXX American Chemical Society

construction of new materials with novel applications like optoelectronic devices48 In this report, the synthesized CdS/Au nanoheterostructures display a highly enhanced and blue-shifted emission with a decrease in their luminescence lifetime. The nanocomposite synthesized in the present study is a metal-doped semiconductor nanorod and nanosphere without forming a complete core−shell type of heterostructure. The surface plasmons of Au nanoparticles that are doped on the surface of the CdS semiconductor nanoparticles show a collective effect on CdS nanoparticles which considerably decreases the trap states and brings in an enhancement in the emission intensity.

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RESULTS AND DISCUSSION The synthesis of the CdS/Au nanocomposite was carried out at room temperature where CdS quantum dots in toluene are taken and Au3+ stabilized by TOAB is added and stirred for 2− 3 h in an inert atmosphere. To this mixture, aqueous NaBH4 is added dropwise to reduce the Au3+ to Au0, and then the nanocomposite formed between CdS and Au is separated and characterized by various methods. A detailed procedure for the synthesis of the nanocomposite with CdS NRs and NSs is given in the Supporting Information. The CdS quantum dots (NRs and NSs) were synthesized by a procedure reported in our earlier work.49 Absorption spectra of the samples were recorded using an Agilent 8453 diode array UV−vis spectrophotometer. The absorption spectrum of the nanoheterostructure is shown in Received: September 21, 2012 Revised: May 17, 2013

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dx.doi.org/10.1021/jp309375v | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Figure 1a along with the absorption spectrum of bare CdS nanorods (NRs). The concentrations of CdS in the bare sample

Figure 2. Emission spectrum of (a) CdS nanorods and (b) CdS@Au nanocomposite prepared from CdS nanorods in toluene (λexc = 370 nm).

CdS nanoparticles is observed around 450 nm,23 whereas the trap state emission shows a peak around 600 nm. The significant observation here is that the emission maximum of the nanocomposite is significantly blue-shifted, and the intensity is highly enhanced with a considerably reduced full width half-maximum (fwhm) of ∼100 nm when compared to the luminescence spectrum of bare CdS NRs. This observation is attributed to the reduction of trap states which in turn reduces the trap state emission, and a major contribution is hence from the band edge state resulting in an enhanced band edge emission and a decrease in the fwhm. The enhancement in the band edge emission of the CdS/Au nanocomposite is quantified by comparing the quantum yield of bare CdS nanoparticles with that of the CdS/Au nanoheterostructure. The quantum yield of bare CdS nanoparticles is 0.0085 ± 0.001, and that of the CdS/Au nanoheterostructure is 0.046 ± 0.01 (quinine sulfate is used as the standard). A 5-fold increase is observed in the case of the CdS/Au nanoheterostructure when compared to that of the bare CdS nanoparticles. To confirm that the enhancement in the emission is only due to the formation of a heterostructure between CdS and Au nanoparticles, a control experiment was done by reducing the bare CdS nanoparticles using NaBH4. No significant change in the emission spectrum of bare CdS was observed. This clearly indicates that the emission enhancement is only due to the formation of the CdS/Au nanoheterostructure. The emission spectrum of CdS before and after the addition of NaBH4 is shown in the Supporting Information (Figure S1). It is also observed that the emission maximum is further blue-shifted as the concentration of Au3+ is increased (6 μM), evidencing the fact that the higher concentration of Au3+ further facilitates the exciton−plasmon interaction further on the CdS surface by increasing the density of the Au nanoparticles deposited (Figure 3). The absorption spectrum of the CdS/Au nanocomposite obtained from CdS nanospheres is given in Figure 4a. The concentration of CdS in the CdS NS/Au nanoheterostructure and that of the bare sample is 125 μM, and the concentration of Au nanoparticles is 3 μM. The CdS/Au concentration is maintained in the ratio 40:1. The absorption spectrum of the as-prepared CdS NS/Au nanocomposite showed an exciton peak at 350 nm, and the surface plasmon peak was observed at around 500 nm. The emission spectrum (Figure 4b) also showed a blue shift with an enhancement in the luminescence intensity when compared to its single-component counterpart. These observations were similar to that of the nanocomposites obtained from CdS NRs.

Figure 1. (a) Absorption spectrum of CdS quantum dots and CdS/Au nanocomposite. (b) Normalized emission spectrum of CdS quantum dots and CdS@Au nanocomposite in toluene (λexc = 370 nm).

and the nanoheterostructure are the same (125 μM), and the concentration of Au nanoparticles in the heterostructure is 3 μM. The absorption spectrum of the CdS NR/Au nanocomposite shows a distinct peak at 356 nm which is a characteristic of the CdS exciton peak and a prominent shoulder at 500 nm characteristic of the surface plasmons of the Au nanoparticles. The absorption spectrum of its singlecomponent counterpart is also given in the figure for comparison. A blue shift of 17 nm was observed in the exciton peak (356 nm) when compared with the single-component counterpart (373 nm). The peak observed at 500 nm is typical of the surface plasmons of Au nanoparticles. An increase in the absorbance near the surface plasmon region is a clear indication of the formation of a nanocomposite between CdS QDs and Au nanoparticles, and a similar observation was reported earlier in the literature.23 Interestingly, these CdS/Au nanocomposites in the present report show an increase in the absorption at the surface plasmon region (510 nm), clearly indicating the formation of a composite between Au nanoparticles and CdS QDs. The blue shift in the exciton peak of CdS QDs in the composite when compared to the bare QDs suggests that the trap states are minimized by the Au nanoparticles in the nanocomposite. Emission spectra were recorded using a Perkin-Elmer MPF44B fluorescence spectrophotometer. The normalized emission spectrum of the CdS/Au nanoheterostructure and CdS nanorods is shown in Figure 1b. The CdS/Au nanocomposite showed a peak maximum at 490 nm with an enormous enhancement in the luminescence intensity when excited at 370 nm. The enhancement is about 9-fold when compared to that of the bare CdS nanorods which is evidenced clearly from Figure 2. The observed larger blue shift in the emission spectrum of the nanocomposite is attributed to the passivation of a larger number of trap states. The band edge emission of B



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nanoheterostructure where the presence of Au nanoparticles of diameter around 5 nm that are deposited along the length of the CdS nanorods with a length of approximately 200−300 nm is seen. It is also evident from the figure that the Au NPs do not form a complete shell over the CdS nanorod which is well in accordance with the photophysical results. The EDAX spectrum of the CdS NR/Au nanocomposite is given in the Supporting Information (Figure S2). The EDAX spectrum is very much in line with the TEM image. The spectrum showed peaks of Au, Cd, and S between 1.5 and 4 keV. The peaks at this energy region indicate the coexistence of Au, Cd, and S in the nanocomposite which is in accordance to the literature reports.11 The Au peak is also observed at 8 keV which is attributed to the free Au nanoparticles present in the sample. These free Au nanoparticles were also observed in the HRTEM image (Figure 5). The above observations reaffirm our hypothesis on the formation of the heterostructure and rule out the formation of core−shell-type structures. Figure 6 shows the HRTEM image of the hybrid nanoheterostructure obtained from CdS NSs and Au NPs. Here again, the NPs of Au are deposited on the surface of the CdS NSs excluding the probability of formation of a complete shell on the NSs. There are approximately four to five Au NPs of size ∼3−5 nm deposited on the CdS nanospheres of size 15 nm in diameter. The EDAX spectrum of the CdS NS/Au nanocomposite (Figure S3, Supporting Information) also showed the coexistence of Cd, S, and Au atoms which was in accordance with the EDAX spectrum of the CdS NR/Au nanocomposite. To completely eliminate the possibility of the formation of the core−shell structure, the CdS/Au nanocomposite was prepared with excess loading of the Au precursor. The TEM image (Supporting Information Figure S4) of the nanocomposite of CdS/Au with excess loading still shows a nanoheterostructure where Au nanoparticles are deposited on the CdS nanoparticles, and it is also clearly observed that there is a larger number of free Au nanoparticles. This clearly confirms that the excess loading of the Au precursor does not facilitate the formation of the core−shell structure between CdS and Au nanoparticles. A scheme is put forth to explain the probable mechanism for the formation of the nanoheterostructure by the current method of synthesis (Scheme 1). Time-Resolved Emission Studies (TRES). Photoluminescence (PL) lifetime decay of the CdS NR/Au nanocomposite was recorded using the time-correlated single-photon counting

Figure 3. Normalized emission spectrum of the CdS/Au nanocomposite in toluene prepared with two different concentrations, (a) 3 μM and (b) 6 μM, of Au3+@TOAB.

Figure 4. (a) Absorption spectrum of CdS nanospheres and CdS/Au nanocomposites in toluene and (b) emission spectrum of CdS nanospheres and CdS/Au nanocomposites in toluene.

The HRTEM images of the nanoheterostructures give clear insight into the morphology and structure of the hybrid nanocomposites of CdS/Au obtained from the present synthesis method. Figure 5 shows the CdS NR/Au NP hybrid

Figure 5. HRTEM images of the CdS/Au nanoheterostructure synthesized from CdS nanorods. C



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Figure 6. HRTEM images of the CdS/Au nanoheterostructure prepared from CdS nanospheres.

amplitude of 58% and another component of lifetime 4 ns with amplitude of 42% were observed. The single-component counterpart (inset Figure 9) showed triexponential decay (Table 1). It was observed that the longer lifetime of the CdS

Scheme 1. Mechanism Showing the Stepwise Formation of CdS/Au Nanoheterostructures

Table 1. Lifetime Analysis Data of the CdS/Au Nanocomposites Obtained from CdS Nanorods (NRs) and CdS Nanospheres (NSs) sample

τ1 (ns)

τ2 (ns)

τ3 (ns)

B1 (%)

B2 (%)

B3 (%)

χ2

CdS nanorods CdS/Au CdS nanospheres CdS/Au

2.04 0.99 3.38 1.08

20.4 3.83 24.3 4.78

88.4 86.9 -

18 58 21 76

25 42 47 24

57 32 -

1.16 1.13 1.12 1.29

quantum dot (88 ns) completely disappears in the CdS/Au nanocomposite. A similar observation was made in the nanocomposite prepared from CdS NSs. The lifetime decay monitored at its emission maximum is given in Figure S5, and the lifetime decay of the CdS NS is given in the inset of Figure S5 in the Supporting Information. The dependence of the luminescence intensity on the amount of attached Au nanoparticles as well as the spectral shift of CdS nanorods and nanosphere emission indicated that the observed optical effects are likely to be related to the interaction of plasmons in Au NPs and the excitons in NRs and NSs. Interestingly, as the luminescence intensity increases, the photoluminescence lifetime gradually decreases. This is in contrast to the classical observation in organic fluorophores where an emission enhancement is generally accompanied by longer lifetimes. Several factors contributing to the luminescence enhancement have been discussed in the literature both theoretically and experimentally:29 “(1) the decrease of nonradiative rate of an exciton in the presence of Au nanoparticles which reduces the enhancement coefficient, (2) the discreteness of nanoparticles increases the electromagnetic enhancement since almost any type of deviation from the simplest geometries results in local enhancement of the electromagnetic fields, and (3) the randomness of nanoparticle positions is known from photoluminescence that the random fields induced by a rough metallic surface result in a strong electric field enhancement”. One can conclude that the plasmon-induced enhancement of electric fields can originate from collective excitations in a system of interacting Au nanoparticles. Single, noninteracting Au nanoparticles cannot provide any enhancement. In contrast, single Au nanoparticles

(TCSPC) technique by exciting at 370 nm and by monitoring the decay at its emission maximum (Figure 7). The decay was fitted biexponentially. A component of lifetime 1 ns with

Figure 7. Lifetime decay of the CdS/Au nanocomposite in toluene prepared by adding 0.2 mL of Au3+@TOAB when exciting at 370 nm (inset shows the lifetime decay of bare CdS nanorods). D



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will likely lead to a suppression of fluorescence due to an increase of nonradiative losses. It is well explained in the literature30 that the reciprocal of the lifetime of an exciton of a semiconductor is composed of two terms, the radiative and nonradiative rates. The radiative rate increases with the field enhancement factor which is due to the local density of states of photons. The observed blue shift in the emission of the composite can also be explained on the basis of plasmon− exciton interaction. In the presence of the Au nanoparticles the lifetime of the excitons decreases which consequently decreases the diffusion length of the excitons. These excitons cannot diffuse into the regions of smaller band gaps, and hence the excitons emit photons in the regions of comparatively larger band gaps and hence blue shift in the emission spectrum of the superstructures. A similar observation is reported in the present investigation, which is well correlated with the plasmon− exciton interaction along the CdS nanorods and nanospheres which depends on the homogeneity of the quantum dot surface. Time-resolved emission studies (TRES) were carried out to further add strength to the above discussion. The time-resolved emission studies of bare CdS quantum dots are explained and reported elsewhere in detail.49,50 The lifetime decay of CdS/Au nanocomposites was monitored from 380 to 580 nm at an interval of 10 nm and shown in Figure 8. It is observed that the

Figure 9. TRES spectrum of the CdS/Au nanocomposite shown at different time intervals monitored at different wavelengths.

Figure 10. TRANES spectrum of the CdS/Au nanocomposite constructed from the TRES spectrum.

in the TRANES, which confirms the number of emitting species present in the sample. In the TRANES, the emission intensity at 435 nm decreases and shifts to longer wavelength, thus creating a clear iso-emissive point at 450 nm. Though the band edge emission at 435 nm decreases as we increase the delay time, there is no observed increase in the intensity in the longer-wavelength region, whereas in the case of bare CdS quantum dots, the longer-wavelength emission maximum attributed to the trap state increases as we increase the delay time. This observation clearly infers that the trap state emission density is considerably decreased when the Au nanoparticles are deposited on the CdS nanoparticles, and hence the band edge emission is enhanced.

Figure 8. Lifetime decay of the CdS/Au nanocomposite monitored at different emission wavelengths (λexc = 373 nm).

lifetime increases as we change the monitoring wavelength to a longer wavelength region which is confirmed by the decay analysis data given in Table S1 (Supporting Information). The decay is fitted biexponentially where the shorter lifetime increases from 0.85 to 1.44 ns with a decrease in amplitude as we vary the wavelength from 400 to 580 nm, and the longer lifetime increases from 3.61 to 5.8 ns with an increase in amplitude. The lifetime was also constructed using the decay analysis data and is given in the Supporting Information S6. A TRES spectrum was constructed using the literature methods,51,52 and the peak maximum was found to shift to the red region as the time delay was increased from 0 to 300 ps (Figure 9) without any isoemissive point. The time-resolved area-normalized emission spectrum (TRANES) (Figure 10) was constructed from the TRES spectrum. The area normalization of TRES results in the TRANES. It is generally constructed by normalizing the area of each TRES spectrum such that the area of the spectrum at time ‘t’ is equal to the area of the spectrum at the shortest time, i.e., t = 0. Area normalization of the TRES results in a clear isoemissive point

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CONCLUSIONS The CdS/Au nanoheterostructure synthesized by a solution process resulted in a highly enhanced and blue-shifted emission, and a quenching in the emission lifetime is observed. The enhanced blue-shifted emission and the lifetime quenching are nicely correlated and attributed to the collective interaction of the plasmons and the excitons in the hybrid nanoheterostructure. The HRTEM images confirmed the decoration of Au nanoparticles on the surface along the length of the nanorod and similarly on the surface of the nanospheres. The TRANES spectrum resolved the emission states in the CdS/Au nanocomposite and confirmed the decrease in the density of the trap states. E



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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information consists of synthesis procedure of CdS/Au heterostructure, EDAX spectrum of CdS NR/Au nanocomposite and CdS NS/Au nanocomposite, TEM image of CdS/Au nanocomposite, Lifetime decay of CdS NS/Au nanocomposite, TRES of CdS/Au nanocomposite, Lifetime analysis data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the DRDO (Defence Research and Development Organization) for the financial support. We also thank Dr. K. George Thomas, Photosciences and Photonics Division, National Institute for Interdisciplinary Science and Technology (CSIR), Trivandrum 695 019 for helping us in recording the TEM images. We also thank the National Centre for Nanoscience and Nanotechnology (NCNSNT) University of Madras for helping us in recording the EDAX spectrum.

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Photoluminescence Enhancement of Nanogold Decorated CdS

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