ARTICLE pubs.acs.org/JPCC
Relaxation Dynamics of Anisotropic Shaped CdS Nanoparticles Suparna Sadhu and Amitava Patra* Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata-700 032, India ABSTRACT: Here, we report the influence of shape of nanocrystals (NCs) on the carrier relaxation dynamics of photoexcited CdS NCs using time-resolved spectroscopy. The decay curves have been analyzed using stretched-exponential function and the average decay time and the stretching exponent β are found to be changed with varying the shape of CdS NCs. A stochastic model of carrier relaxation dynamics of CdS NCs has been proposed to estimate the values of the radiative recombination rate, the average number of surface trap states, the luminescence quenching rate due to surface trap states and the rate due to nonradiative recombination from trap state to ground state. Analysis suggests that surface curvature plays an important role in the relaxation dynamics, and it is found that the number of surface trap states in CdS nanospheres is lower than that of CdS nanorods/triangles.
’ INTRODUCTION A detailed understanding of the carrier relaxation dynamics of photoexcited semiconductor nanocrystals (NCs) is of major importance for both fundamental research and technological applications. The photophysical properties of semiconductor NCs are significantly different from those of bulk materials because of quantum confinement effect and an enhanced surface-to-volume ratio.1 Numerous studies have been devoted to understand the carrier relaxation dynamics in semiconductor nanomaterials.217 Significant progress has been made to understand the relaxation dynamics of semiconductor NCs based on the role of the surface properties,10,1416 temperature,8,12 NC size,13 composition,17 and so forth. Recent research has established that the time-resolved photoluminescence (PL) spectroscopy is a quantitative tool for the analysis of photoexcitation dynamics in colloidal semiconductor NCs. Very recently, Jones et al. described the recent developments in the analysis of timeresolved PL data in the context of NC dynamics.2 The role of the intrinsic bright states and surface states in the emission process both in colloidal CdSe quantum dots (QDs) and in CdSe/ZnS core/shell QDs have been studied.10 Wang et al.14 have reported the surface-related emission of CdSe QDs and their influence on the charge carrier dynamics. de Mello Donega et al.12 reported the temperature-dependence of the band-edge PL decay of efficiently luminescent organically capped CdSe QDs with different diameter over a broad temperature range. Jones et al.8 have investigated the perturbations induced by surface-localized carrier traps on the exciton dynamics of the NCs using temperature-dependent time-resolved PL measurements. They have presented a model of carrier trapping that is based on Marcus’s electron-transfer theory to get the information about the energy and distribution of surface-localized trap states. Size dependent time-resolved PL decay of colloidal CdSe NCs and their correlation with fluorescence lifetime with the quantum yield of NCs have been discussed by Zhang et al.13 The effect of shape on the r 2011 American Chemical Society
band gap and the luminescence properties of semiconductor NCs has been studied extensively.1820 Cantele et al.21 calculated the confined state energies in ellipsoidal QDs. Their study shows that the confined state energies split with respect to those of the spherical QDs due to both a volume-induced deformation effect and a geometry-induced one. Decreasing the degree of symmetry and the anisotropy effect will change the degeneracy and splitting of the excited states of semiconductor NCs. El-Sayed et al.22 have reported the shape-dependent ultrafast relaxation dynamics of CdSe nanocrstals (nanorods and nanodots) and found that the carrier relaxation dynamics of the higher energy states in nanorods are faster than nanodots due to the lowering of symmetry in the rods. In the present study, we have synthesized three different shapes of CdS NCs using a simple solution route and studied their photophysical properties by steady state and time-resolved spectroscopy. The time-resolved decay curves are analyzed by using the Kohlrausch stretched-exponential function. We are also trying to understand how the shape of the NCs influences the carrier relaxation dynamics in colloidal CdS NCs. Finally, we propose a stochastic model of carrier relaxation dynamics in semiconductor NCs and correlate it with our experimental results.
’ EXPERIMENTAL SECTION Materials. Oleylamine (70%, Aldrich), cadmium acetate (Loba Chemie, extra pure), and sulfur powder (Merck) were used as received. The spectroscopic-grade solvents (n-hexane, toluene, methanol, and ethanol) were used for optical study. Synthesis Procedure. Preparation of CdS Nanosphere. A colloidal solution of CdS NCs was prepared by a slightly Received: May 24, 2011 Revised: July 19, 2011 Published: July 27, 2011 16867
dx.doi.org/10.1021/jp2048037 | J. Phys. Chem. C 2011, 115, 16867–16872
The Journal of Physical Chemistry C
ARTICLE
Figure 1. TEM images of CdS nanosphere (a), nanorod (b), and nanotriangle (c).
modified method developed by Joo et al.23 Cadmium acetate (0.133 g; 0.5 mM) in 5 mL of oleylamine was heated to 150 C temperature under Ar flow for 20 min to form a clear solution. Separately, an oleylaminesulfur solution was prepared by dissolving 0.016 g (0.5 mM) of sulfur powder in 2.5 mL of oleylamine and quickly injected into the above hot reaction mixture under gentle stirring. The reaction mixture was kept at the desired growth temperature (175 C). After 2 h, the reaction was quenched by the addition of a large volume of anhydrous toluene into the reaction mixture. The NCs were separated from the toluene solution by the addition of ethanol and centrifuged. The yellow precipitate was then redispersed in n-hexane. To remove unused excess precursors, we carried out reprecipitation and centrifugation steps three times. Finally, these were redispersed in n-hexane for further measurements. Three individual syntheses of these NCs were prepared to check the reproducibility. Preparation of CdS Nanorod. For the preparation of CdS nanorod, the ratio of cadmium and sulfur precursor was taken as 1:3. Cadmium acetate (0.133 g; 0.5 mM) in 5 mL of oleylamine was heated to 150 C under Ar flow for 20 min to form a clear solution. An oleylaminesulfur solution (dissolving 0.048 g (1.5 mM) of sulfur powder in 2.5 mL of oleylamine) was quickly injected into the above hot reaction mixture under gentle stirring. The reaction mixture was kept for 12 h at the desired growth temperature (175 C). The growth of the CdS NCs was terminated as before, and the same washing procedure was used to remove unused excess precursors. Three individual syntheses of these NCs were prepared by a similar method. Preparation of CdS Nanotriangle. For the preparation of the CdS nanotriangle, the ratio of cadmium and sulfur precursor was taken as 2:1. Cadmium acetate (0.266 g; 1 mM) in 5 mL of oleylamine was heated to 150 C under Ar flow for 20 min to form a clear solution. Separately, an oleylaminesulfur solution was prepared by dissolving 0.016 g (0.5 mM) of sulfur powder in 2.5 mL of oleylamine and quickly injecting it into the above hot reaction mixture under gentle stirring. The reaction mixture was kept for 8 h at the desired growth temperature (175 C). The growth of the CdS NCs was terminated as before, and the same washing procedure was used to remove unused excess precursors. Three individual syntheses of these NCs were prepared by a similar method. Characterization. Physical Characterization. The transmission electron microscopy (TEM) images were taken using a JEOLTEM-2010 transmission electron microscope with an operating
voltage of 200 kV to analyze the shape, size, size distribution, and structure of the resulting QDs. Samples for TEM were prepared by making a clear solution of samples in choloroform and placing a drop of the solution on a carbon-coated copper grid. Optical Measurements. Absorption and fluorescence spectra of CdS NCs in n-hexane (spectroscopic grade) solution were taken at room temperature with a Shimadzu UV-2450 UVvis spectrometer and a Horiba Jobin Yvon Fluoro Max-P fluorescence spectrometer, respectively. PL quantum yields (QYs) were obtained by comparison with standard dye (Coumarine 500 in methanol), using the following equation:24 QY s ¼ ðF s Ar ns 2 QY r Þ=ðF r As nr 2 Þ
ð1Þ
where Fs and Fr are the integrated fluorescence emission of the sample and the standard, respectively. As and Ar are the absorbance at the excitation wavelength of the sample and the reference, respectively, and QYs and QYr are the quantum yields of the sample and the reference (QYr = 90%),25 respectively. The refractive indices of the solvents in which the sample and reference are prepared are given by ns (1.426) and nr (1.375), respectively. The values of Fs and Fr are determined from the PL spectra corrected for the instrumental response, by integrating the emission intensity over the desired spectral range. Only the band-edge luminescence peak was integrated (any other luminescence bands, such as defect-associated luminescence or solvent fluorescence were discarded as background). The measured average quantum yield values are 58% (63%, 55%, and 57% for three sets of CdS nanospheres), 38% (38%, 42%, and 35% for three sets of CdS nanorods) and 37% (41%, 38%, and 34% for three sets of CdS nanotriangles) for CdS nanospheres, nanorods, and nanotriangles, respectively. We used average quantum yield values to reduce instrumental error and experimental error. The fluorescence decay traces of three different shapes of CdS NCs were recorded in time-correlated single-photon counting (TCSPC) mode using a IBH Fluorocube apparatus. The samples were excited at 375 nm by a picoseconds diode laser (IBH Nanoled-07) with 1 MHz repetition rate. The pulse duration is
