Effects of Core Size and Shell Thickness on Luminescence Dynamics

Jul 3, 2012 - ... Aswin Vijai Asaithambi , Saptarshi Mandal , and Prasun K. Mandal ... Amardeep M Jagtap , Abhijit Chatterjee , Arup Banerjee , Naresh...
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Effects of Core Size and Shell Thickness on Luminescence Dynamics of Wurtzite CdSe/CdS Core/Shell Nanocrystals Huichao Zhang,† Yonghong Ye,‡ Jiayu Zhang,*,† Yiping Cui,† Boping Yang,† and Li Shen† †

Department of Electronic Engineering, Southeast University, Nanjing 210096, China Department of Physics, Nanjing Normal University, Nanjing 210097, China

‡

S Supporting Information *

ABSTRACT: Colloidal CdSe nanocrystals (NCs), whose size is 2.8, 3.8, and 4.9 nm, respectively, were successively overcoated with CdS monolayers (MLs). The X-ray diffraction patterns indicated that the stress in the wurtzite CdSe core was increased with the epitaxial growth of CdS shell, and the CdSe lattice contraction, which was sensitive to core size, did not release with the CdS shell toward 5 MLs. The effects of the CdSe core size and the CdS shell thickness on the temperature-dependent photoluminescence (PL) lifetime were investigated. The PL lifetime undulation with temperature is indicative of the spatial distribution of trap states, and the strong interplay between intrinsic excitons and surface traps can be activated even in the case of the NCs with 5 ML CdS shell.

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INTRODUCTION In recent years, colloidal semiconductor nanocrystals (NCs) as biological probes have been attracting considerable interest due to their potential to meet the demand for better photostability over long time scales and the size-dependent photoluminescence (PL) tunable across the visible spectrum.1−3 There are surface-localized traps on the NC’s surface that are induced by the presence of surface atoms with reduced coordination number. Because of the high surface-to-volume ratio, NCs’ optical properties are significantly affected by these inhomogeneous surface traps.4−6 Steady-state PL spectra and emission decay profiles of colloidal NCs in multiple situations have been studied in detail, such as the size- and temperature-dependent optical properties,7,8 but comparatively little is understood about the carrier dynamics involved with the surface traps.9−11 On the basis of three or more states models, several kinetic schemes have been proposed to announce the interaction between intrinsic excitons and surface states.12,13 Recently, Chergui et al.9 proposed a scheme to understand the multiexponential PL decay kinetics of CdSe NCs by using stochastic ground-state dipole moments that dominantly stemmed from the surface effect. On the basis of Marcus electron transfer theory, Scholes et al.10,11 explained the trapping/detrapping process that happened between excitons in NCs and their surface traps. It has been well known that the PL of as-prepared NCs is extremely sensitive to the local environment, and the surface modification of colloidal NCs can improve the PL efficiency and chemical stability.1−3 It is a successful route in the surface modification of nanostructure particles to grow some layers of higher band gap inorganic material on the core to form a heterostructure.14−18 The growth of shell could eliminate © 2012 American Chemical Society

nonradiative defects on the NCs’ surface and thus improve the NC’s PL quantum yield (QY).18 Some interesting physical properties have been observed also, such as the suppression of the Auger recombination rate in spherical CdSe/CdS core/shell NCs with thick CdS shell.19−21 The spatial distribution of defects in the core/shell NCs can be controlled by the shell growth, accompanied by the interface strain accumulation and formation of dislocations.22 Therefore, the series of PL spectra from NCs with successive shell growths could reveal the nature of carrier recombination process via the surface defects. There is a changing delocalization of the electron wave function in the shallow potential well, which is formed by the CdSe/CdS heterostructure, with the changing of core size.15,23 It is possible that the variation of core size can alter the exciton’s trap/detrap processes between the NC and its surrounding also. Therefore, the investigation for a set of samples with different core diameters may be narrowed down on the relevant process and expose the essentialness of traps in NCs. In this work, CdSe cores with different diameters were, respectively, successively overcoated with CdS monolayers. The XRD measurement was done to study the evolution of the lattice contraction with the shell thickness and core size. The temperature-dependent emission decay profiles were taken out to explore the carrier recombination process. These experimental results clearly indicate a strong link between the PL decay dynamics and the shell’s thickness, and the core’s size influences the PL decay dynamics also. The PL average lifetime Received: February 12, 2012 Revised: July 2, 2012 Published: July 3, 2012 15660

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undulates with temperature, and the interaction among intrinsic exciton and trap state is discussed.

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EXPERIMENTAL SECTION The synthesis of these CdSe/CdS core/shell NCs was similar to that described with detail,14,22 but there were two factors different from ref 22. (where the CdSe cores were of zincblende structure) during the growth of the CdSe Cores: (1) TOPO (trioctylphosphine oxide) was not used and (2) the injection temperature of Se precursor was increased from 280 to 310 °C and the core’s growth temperature was increased from 250 to 270 °C. As determined by the following XRD measurement, the obtained CdSe cores were of wurtzite structure. Compared with CdSe NCs with zinc-blende structure, the wurtzite CdSe NCs could be overcoated with thicker CdS shell without release of lattice contraction. In this experiment, CdSe cores with three sizes (2.8, 3.8 and 4.9 nm, respectively) were used, which were synthesized by using different ratios of cadmium and selenium precursor and applying different amounts of octadecylamine, and they were successively overcoated with CdS shell toward five monolayers. The definition of a monolayer is a CdS shell of 0.35 nm along the major axis of a single dot. Hereafter, the CdSe core, for example, 4.9 nm, and core/shell NCs with 1, 2, 3, 4, and 5 ML (s) CdS shell are labeled as 4.9 nm core, 1 ML, 2 ML, 3 ML, 4 ML, 5 ML, respectively. A Tecnai G2 transmission electron microscope (TEM) was used to analyze the size distribution. X-ray diffraction (XRD) patterns were obtained using a Rigaku D/max 2500 VL/PC diffractometer. Absorption spectra were measured with a UV3600 spectrophotometer. PL spectra and emission decay profiles were measured with a Edinburgh F900 fluorescence spectrophotometer, and the emission was monitored at the PL peak wavelength for each sample during the time-resolved PL measurement.

Figure 1. TEM images of the CdSe cores and their corresponding core/shell (5 MLs) NCs with the core size of 2.8 (A,B), 3.8 (C,D), and 4.9 nm (E, F), respectively. The insets are the corresponding HRTEM images.

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RESULTS AND DISCUSSION Figure 1 shows the typical TEM images of CdSe cores and the CdSe/CdS core/shell NCs, which indicates a nearly spherical shape and good monodispersity. To obtain the average size and the size distribution of these NCs, we analyzed over 150 particles statistically for each sample. The average sizes yielded by the TEM images confirm that the CdS growth on the core is monolayer-by-monolayer, and the size distribution is ∼9% for the CdSe cores and is slightly increased after the growth of CdS shell, which is 10, 10, 11, 12, and 12% for the 4.9 nm series’ five core/shell NCs; 9, 10, 10, 11, and 11% for the 3.8 nm series’ five core/shell NCs, and 10, 10, 11, 11, and 12% for the 2.8 nm series’ five core/shell NCs, respectively. The high-resolution TEM (HRTEM) images, as shown in the inset of Figure 1, show well-resolved lattice fringes, and the lattice spacing of the CdSe cores is similar to that of the bulk CdSe. For the five core/shell NCs, these lattice fringes are continuous throughout the entire particles, and the coating of the CdS shell appears by the way of epitaxial growth on the CdSe NCs surface.24 A welldefined interface between CdSe core and CdS shell was not observed in any of these samples. Figure 2 shows the typical absorption and PL spectra of the CdSe core and its corresponding core/shell NCs. With the growth of the CdS shell, there is a red shift in the first exciton absorption peak, indicating an increased leakage of the exciton into the shell as well as a slight broadening of the absorption

Figure 2. Absorption (solid line) and PL spectra (dash line) of the 4.9 nm CdSe core and its corresponding CdSe/CdS core/shell NCs. The inset is the quantum yield versus the shell thickness.

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peak due to a distribution of shell thickness.24 The growth of the CdS shell results in a considerable shift of the emission peak and a slight broadening at higher coverage also. The red shift of the first excited state (1S(h)-1S(e)) between the core and its corresponding 5 ML sample is 206.6, 134.4, and 80.45 meV for the cases of 2.8, 3.8, and 4.9 nm CdSe core, respectively. As determinate by the D value between the initial energy of the core and the final energy of the core/shell NCs when the energy loss upon continued growth of the shell can be negligible, the total energy shift (ΔEtot) is 233.9, 137.8, and 80.8 meV for the above three cases,15 which means that the shell of 5 ML CdS can effectively confine the leakage of the exciton in NCs with the 3.8 and 4.9 nm cores. As the coverage of CdS shell on the CdSe surface is increased, a dramatic increase in the fluorescence QY can be observed, as shown in the inset of Figure 2. Figure 3 shows the typical XRD patterns of the CdSe core and the CdSe/CdS core/shell NCs, indicating all of the NCs of

one important parameter for the formation of zinc-blende CdSe NCs.29 Some impurities such as alkyl phosphonic and phosphinic acids usually appear in the technical-grade TOPO, which was used in our previous work, in which zinc-blende CdSe cores were yielded. The shift to larger angle of XRD peaks results from the presence of the lattice contraction in the particles22,30 rather than the prolate shape of NCs (which could be eliminated by TEM images).25 The lattice constants of wurtzite NCs, which are calculated from the positions of the XRD peaks, are shown in Figure 3. When the CdSe core is overcoated with CdS shell, its lattice constants (a and c) become smaller, and such lattice contraction is increased with the increase in the CdS shell thickness. With the same thickness of CdS shell, the smaller CdSe core shows a larger lattice contraction than the bigger core. The lattice contraction with 5 ML CdS shell is 3.1, 2.8, and 2.4% for the 2.8, 3.8, and 4.9 nm CdSe core, respectively. It should be noted that in the case of zinc-blende CdSe core, the overcoating of CdS shell results in lattice contraction also. The contraction reaches its maximum (2.2%) with the 3 ML CdS shell, but the compression strain relaxes with further growth of CdS shell. The release of the lattice contraction, which induces some nonradiative defects in zinc-blende NCs, results in the decrease in the PL QY with >3 ML CdS shell. Although the wurtzite NCs exhibit larger lattice contraction with thick shell, such contraction’s release does not appear in this lowersymmetrical crystal, so overcoating with thicker shell does not lead to a drop in PL QY. Attenuant NC solution was spincoated onto a quartz substrate so the distance between neighboring NCs was greater than 1 μm, and the sample was put in an Oxford cryostat for the temperature-dependent optical measurement. The emission decay profiles of these as-prepared NCs are not singleexponential but multiexponential. Figure 4 shows the typical PL decay. Although it has been suggested that the PL decay of colloidal II−VI group semiconductor NCs can be extremely well-analyzed by five- or six-component exponential functions with the reduced chi-square (χ(2)) close to the ideal value (1.0),31 we use triexponential decay to fit all of the emission decay profiles obtained in this experiment. The χ(2) value is below 1.2 for all of the numerical fits, which is suggested to be acceptable for the PL decay obtained with a time-correlated single photon counting method whose noise is of Poissonian statistics. The triexponential PL decay means that there are three kinds of exciton recombination processes: a fast decay (τ1) in the level of several nanoseconds (ns), a middle one (τ2) in the level of 10 ns, and a slow one (τ3) close to 100 ns. The fast component is attributed to the recombination of intrinsic excitons,12,17,22 and the middle/slow ones are related to the interplay between the exciton state and the surface traps. These traps exhibit complex energy distribution and are differentiated into two groups.10,11,31 If a purification process is done to remove some surface ligands on the colloidal NCs, then some nonradiative traps will be induced. The effect of the nonradiative traps on the longest component is larger than that on the other two components, so the longest component will be significantly weakened and the emission decay profiles can be fitted well with a biexponential function in the case of the purification.22 Figure 5 shows the temperature dependence of the three lifetimes and the average lifetime (τav) for the series of NCs with 4.9 nm core. The τ1 value is slightly decreased with the

Figure 3. Left: XRD patterns of the 4.9 nm CdSe core and its core/ shell NCs. Vertical lines indicate wurtzite CdSe and CdS bulk reflections. Right: The wurtzite lattice constants a and c of the CdSe cores with diameters of 2.8 nm (square), 3.8 nm (circle), 4.9 nm (triangle) and their corresponding CdSe/CdS core/shell NCs.

the wurzite structure. The peak broadening is due to the finite size of these particles.25 The XRD pattern of the core/shell NCs matches the core’s pattern, but the XRD peaks shift to larger angle with the increase in the shell thickness. Duplicate phases were not observed, indicating that the shell grows on the surface of NCs by the epitaxial growth manner18 and ruling out the possible of the formation of CdS NCs and obvious dislocations between interface of core/shell NCs.26 The attenuation of the (102) and (103) reflexes is often observed for CdSe NCs and their related heterostructures, which arises as a result of stacking faults in the (002) direction.15,25,26 The crystal structure of CdSe NCs could be modified by the injection temperature of the precursors, the growth temperature, and the nature of the ligands (such as the chain length of the amines).27,28 The crystal lattice prefers to be wurtzite when CdSe NCs are synthesized at high temperature with the ligand of octadecylamine. It appears that the alkyl phosphonic acid is 15662

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Figure 4. Typical PL decay (open circle) and its triexponential fitted curve (dashed line). The top shows the residual data, which are of random noise distributed around zero, indicating that the triexponential fit is quite appropriate. The numerical fit was done with the Edinburgh F900 software based on the Marquardt− Levenberg algorithm.

Figure 5. τ1 value (open square), τ2 value (open circle), τ3 value (open triangle), and the PL average lifetime (solid hexagon) versus temperature of these NCs. Note that the value of τ3 is reduced onethird.

increase in temperature for the core and the five core/shell samples, indicating that there are some thermally activated nonradiative recombination processes of intrinsic excitons, which is evidenced further with the temperature-dependent PL QY, as shown in Figure 6.17 Because the PL signal comes from the same area of the substrate coated with NCs during the temperature-variation measurement, the PL intensity is proportion to the emission QY. The PL QY is normalized with the QY value at the lowest temperature (77 K) for each sample. All six samples exhibit a decreased PL QY with the increase in temperature, and the QY’s attenuation with temperature becomes small in the case of thick CdS shell (≥4 ML), which is consistent with the temperature feature of τ1. The decrease in the τ1 value with temperature becomes slight for the samples with thick shell (≥4 ML). Both τ2 and τ3 are increased to their maxima when temperature is increased from 77 K; then, they are gradually decreased with the further increase in temperature although the temperature values corresponding to their maxima are different, so the τav fluctuates with temperature. The initial increase in τav with temperature appears for all core and core/shell NCs, and the temperature (Tmax) corresponding to the maximum of τav is increased with overcoating CdS shell. As shown in Figure 7, the amplitudes of the τ2 and τ3 decay channels are decreased first with temperature, and that of the τ2 decay channel is slightly increased at high temperature, indicating that the thermally activated nonradiative traps effect the two decay channels also, whereas the amplitude of the short component is increased and then is decreased. It should be noted that the three decay

Figure 6. Normalized PL QY as a function of temperature for 4.9 nm core and core/shell samples.

channels are corrivals, so the initial increasing of the short component results from the decreasing of the other two components. Therefore, the increase in the channels’ amplitudes does not mean that the PL absolute intensity is 15663

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PL lifetime fluctuation with temperature in an ensemble of NCs.10,11 The increase in PL lifetime is indicative of a growing trap state population, which is determined by the reorganization energy (λ), the free-energy difference (ΔG) between exciton and trap, and the electronic coupling integral (V) between the exciton and the trap state. Our samples are similar to these core/shell NCs used in Scholes group’s experiment, but our successive overcoating can be used to explore the evolution of the factors λ, ΔG, and V on the shell thickness, and the XRD patterns indicate that the nonradiate defects induced by the lattice contraction’s release do not appear concomitantly with the overcoating. The τ2 and τ3 components are related to exciton’s thermal trapping/detrapping at the localized states. The shell growth, which happens in an epitaxial manner, eliminates traps at the core/shell interface, and the intrinsic excitons can be confined well within NCs by a thick shell, and there is a lower possibility that excitons are trapped at surface states around NCs’ surroundings. Then, the activated temperature of a surface state to trap/detrap an exciton becomes higher in the case of thicker shell. A higher Tmax implies larger λ or ΔG values to activate thermally a surface state to trap/detrap an exciton.22 The distance between the surface trap and the exciton’s center is increased with overcoating CdS shell, so the reorganization energy, which enables the environment to accommodate the change due to carrier trapping, will be increased. Figure 8 show the dependence of τav with temperature for the series of 2.8 and 3.8 nm cores. Similar to the 4.9 nm case, the 2.8/3.8 nm ones exhibit the up−down undulation feature due to the trapping/detrapping effect also, and the Tmax value is decreased with the decrease in CdS shell thickness. Dissimilar to the 4.9 nm case, the 2.8/3.8 nm samples with ≥2 ML CdS shell exhibit an initial decrease in the average lifetime with the increasing of temperature in the low-temperature region because of the increase in the Boltzmann population of the “bright” exciton states. Although the energy difference between “bright” and “dark” excitons is very small for the large core (4.9 nm), the splitting of “bright” and “dark” fine structure states is large for the small cores (2.8/3.8 nm) series, so the “bright/ dark” exciton effect should play a role in low-temperature dynamics. In addition, for the 2.8/3.8 nm cores and the samples with 1 ML CdS shell, because of their low Tmax, the initial decrease due to the “dark/bright” effect will overlap with the up part of the up−down undulation in the low-temperature region, so it cannot be observed distinctly. Figures 5 and 8 indicate that the average lifetime fluctuates up and down with temperature due to the trapping/detrapping process in the high-temperature region, and the Tmax value is increased with overcoating CdS shell. With the same thick shell (≥3 ML), the 2.8 nm core series have smaller Tmax than the other two series. The large red shift of the first excited state, shown in the absorption spectra, for the 2.8 nm series implies high possibility for exciton to approach the NC’s surface, and thus there is strong interplay between the surface traps and excitons. This observation is consistent with the high sensitivity to surface-bound adsorbates at CdSe/CdS core/shell NCs with small cores.11,15 The absorption spectra, shown in Figure 2, indicates that 5 ML CdS shell can effectively confine the leakage of exciton into the NC’s surrounding for the core/shell NCs with 3.8 and 4.9 nm cores, and Figures 5 and 8 show that the Tmax values of the 5 ML-shell samples with large cores are above room temperature, implying that the interplay between intrinsic

Figure 7. Amplitudes of the three decay channels for the 4.9 nm series.

enhanced with temperature. In fact, as shown in Figure 6, the PL absolute intensity is decreased with temperature. As indicated in Figure 6, some nonradiative traps are thermally activated, and they can quench the exciton in the NC. The middle/long components rise from the interplay between exciton and a radiative surface trap, the interaction between them will increase the residence time of exciton around the QD’s surface, which will increase the probability of exciton’s capture by the nonradiative trap, so the effect of the nonradiative trap on the long component is larger than that on the short component. The long component has a very long residence time of exciton around the QD’s surface, so the more nonradiative traps are thermally activated, the more their amplitude is decreased. For the middle component, it has a shortened residence time of exciton in the high-temperature region because the τ2 value is decreased when temperature is over Tmax, so its amplitude is increased although there are more thermally activated nonradiative traps at high temperature. Such τav undulation with temperature has been observed in a few semiconductor nanostructures. Self-assembly InAs NCs show an initial rise of τav with temperature, and it is explained by the exciton dissociation at elevated temperature and escape of carriers into the wetting layer, leading to a subsequent recapture into NCs, followed by relaxation and recombination with an increased decay time.32,33 An increased PL lifetime with temperature has been observed in InAs/GaAs quantum rings, and it is attributed to the thermal occupation of dark states and the reduced wave function overlapping between electrons and holes at a high temperature induced by the presence of piezoelectric/strain potential and a large asymmetry in ring profiles, which resulted in the suppression of radiative emission and longer exciton lifetime.34 The numerical simulation, which was based on Marcus electron transfer theory, revealed that a complex interplay between an activated trapping/detrapping process and the density of accessible trap states could induce a 15664

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ASSOCIATED CONTENT

S Supporting Information *

Information on synthesis process and determination of the emission QY. Amplitudes of the three decay channels and the emission QYs as a function of temperature. 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 Fund supports from the National Basic Research Program of China (973 Program, 2012CB921801) and the National Natural Science Foundation of China (10774023) are acknowledged.

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Figure 8. PL average lifetime versus temperature of the 2.8 nm CdSe core and its CdSe/CdS core/shell NCs (A) and the 3.8 nm CdSe core and its CdSe/CdS core/shell NCs (B).

excitons and surface states could not be sufficiently activated at room temperature. Therefore, the passivation of 5 ML CdS shell could effectively prevent the influence from surface trap in the case of large CdSe cores.

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REFERENCES

CONCLUSIONS

Overcoating thick CdS shell on wurtzite CdSe core was done with monolayer control of shell thickness, and the XRD patterns indicate that the release of lattice contraction does not occur. The PL lifetime undulation with temperature manifests the interplay between intrinsic excitons and surface states, and the dependence of the interrelation on the core size and the shell thickness is discussed. 15665

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