J. Phys. Chem. C 2007, 111, 1239-1242
1239
Ripening Kinetics of CdSe/ZnSe Core/Shell Nanocrystals Yun-Mo Sung,* Kyung-Soo Park, and Yong-Ji Lee Department of Materials Science & Engineering, Korea UniVersity, Seoul 136-713, South Korea
Tae-Geun Kim Department of Electronic Engineering, Korea UniVersity, Seoul 136-713, South Korea ReceiVed: September 22, 2006; In Final Form: NoVember 2, 2006
CdSe and CdSe/ZnSe core shell nanocrystals were prepared via the inverse micelle technology with TOP/ TOPO/HDA surfactants, and their high crystallinity was confirmed by using X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) analyses. Ostwald ripening behavior of the nanocrystals was monitored by using the red-shift in UV-visible absorbance peaks, and their size variation was estimated by employing a quantum confinement effect equation. Lifshitz-Slyozov-Wagner (LSW) kinetics analyses were performed by using the size variation according to ripening temperature and time period. Arrhenius-type plots were created by using the slopes of the LSW curves for the CdSe and CdSe/ZnSe nanocrystals, respectively, and the activation energy values for the ripening were evaluated for the nanocrystals. At a low-temperature region, the CdSe and CdSe/ZnSe samples seem to show dissociation of Cd-Se and Zn-Se surface atomic bonds, respectively, while at a high-temperature region above 266 °C, both samples seem to show active dissociation of both Cd-Se and Zn-Se lattice atomic bonds. The CdSe-ZnSe shows relatively low activation energy for the ripening, compared to the bare CdSe possibly due to weak Zn-Se atomic bonds. One can complete the Ostwald ripening kinetics equation by using two kinetics variables, derived in this study, for the estimation of the size of CdSe/ZnSe core/shell nanocrystals. Also, this approach can be applied to ripening kinetics of other core/shell nanocrystal systems.
Introduction Colloidal semiconductor nanocrystals (quantum dots) have attracted a great deal of interest due to their size-tunable photoemission characteristics, which originate from the quantum confinement effect of both the electrons and holes in all three dimensions, leading to an increase in the effective energy band gaps of the nanocrystals.1 Among the colloidal semiconductor nanocrystals, CdSe has shown almost full range visible light emission within a reasonable size range, and thus it has been intensively studied for its distinguished role in technical applications such as light-emitting diodes, lasers, and biological labels.2 To improve quantum efficiency, the CdSe nanocrystals are often covered by another semiconductor shell with a highenergy band gap such as ZnS, ZnSe, CdS, and CdTe.3 It has been known that the size of nanocrystals is primarily determined by the precursor concentration, capping ligand and precursor anion types, and temperature at the initial nucleation and growth stage.4 However, once the precursor solution is stabilized, becoming an equilibrium with the solid-phase nanocrystals after the nucleation and growth, the Ostwald ripening dominates between the nanoparticles. The final size of nanocrystals is thus determined by temperature and time period of Ostwald ripening, and so is the color of the emitting light. Thus, the precise control of Ostwald ripening is a critical issue for the applications of not only bare CdSe but core/shell CdSe nanocrystals. Searson et al. reported the detail ripening kinetics of nanocrystals such as ZnO, TiO2, etc.5 and they used the so* Address correspondence to this author. E-mail: ymsung@korea. ac.kr. Phone: +82-2-3290-3286. Fax: +82-2-928-3584.
called, Lifshitz-Slyozov-Wagner (LSW) theory6 for ripening kinetics of the nanocrystals. They also investigated the roles of precursor anions and organic capping agents in the ripening kinetics of the nanocrystals.4 Although the ripening mechanism of core/shell nanocrystals must be different from that of bare nanocrystals, there has been no report for the ripening mechanism of core/shell nanocrystals so far. In this study, we report the ripening mechanism of CdSe/ ZnSe core/shell nanocrystals based upon the ripening kinetics study, and the result was compared with that of the bare CdSe nanocrystals. Experimental Section Cadmium oxide (CdO, 99.99%), selenium shot (Se, 99.999%), diethyl zinc, ((C2H5)2Zn in 1 mol of heptane), tri-n-octylphosphine (TOP, 90%), tri-n-octylphosphine oxide (TOPO, 90%), hexadecylamine (HDA, 90%), n-butanol, toluene, and chloroform were purchased from Aldrich (Aldrich Chemical, Milwaukee, WI). Most details of the synthetic and characterizing methods were similar to those reported in the literature.7 The concentration of ZnSe precursor solution was 1.0 mmol in the shell synthesis. The ripening experiments were performed by immersing the cleaned nanoparticles into a toluene solution to exclude the effects of unreacted chemicals in the precursor solution. Thus, the possibility of unwanted further growth of nanocrystals by the precursor solution was completely excluded in this study. The ripening experiments were performed at 240, 250, 260, 270, 280, and 290 °C for 5 to 480 min, respectively. The ripening at a higher temperature region was avoided to prevent the possible solid-solution formation in CdSe/ZnSe.
10.1021/jp066203c CCC: $37.00 © 2007 American Chemical Society Published on Web 12/19/2006
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Sung et al.
Figure 1. X-ray diffraction (XRD) patterns of as-prepared CdSe and CdSe/ZnSe nanocrystal quantum dots.
Figure 3. UV-visible absorption spectra for (a) CdSe and (b) CdSe/ ZnSe nanocrystal quantum dots ripened at 290 °C. The spectra of each sample were recorded after the ripening heat treatments for 0, 5, 10, 30, 60, 120, 240, and 480 min, respectively, from bottom to top.
Figure 2. High-resolution transmission electron microscopy (HRTEM) images of as-prepared (a) CdSe and (b) CdSe/ZnSe nanocrystal quantum dots and (c, d) their corresponding nanocrystals ripened at 290 °C for 480 min.
Powder X-ray Diffraction (Rigaku XRD: Ultima 2000, Tokyo, Japan) was performed to identify the crystallinity of asprepared CdSe and CdSe/ZnSe nanocrystals, using Cu KR radiation at 40 kV and 40 mA. High-resolution transmission electron microscopy (HRTEM: JEOL JEM 4010, Tokyo, Japan) operated at 400 kV was used to identify crystallinity and morphology of the nanocrystals before and after the ripening experiments. UV-visible spectrometry (JASCO UV-Visible Spectrophotometer: V530, Tokyo, Japan) was employed to analyze photoabsorption properties of nanocrystals and to estimate their average size with the quantum confinement theory. Results and Discussion The XRD results in Figure 1 show formation of CdSe nanocrystyals with the wurtzite structure both for the CdSe and CdSe/ZnSe nanocrystals. Their crystallinity was very high with negligible content of the amorphous phase, and the broad diffraction peaks originate from the overlapping of multiple peaks and their fine size. Those from CdSe/ZnSe nanocrystals show peak shifts to high 2θ angles due to the compressive strain, originating from the small lattice parameter of the ZnSe shell. Our inductively coupled plasma (ICP) atomic emission spectroscopy analyses gave the ZnSe shell thickness of 2-3 layers
for the CdSe/ZnSe nanocrystals. The high-resolution transmission electron microscopy (HRTEM) analyses also reveal that the as-prepared CdSe and CdSe/ZnSe nanocrystals were highly crystalline, clearly showing (001) atomic planes, as shown in Figure 2 a,b. The CdSe nanocrystals were almost spherical, while some of the CdSe/ZnSe nanocrystals were slightly ellipsoidal, which would come from the preferential growth of ZnSe shell at the (001) planes of the core CdSe nanocrystals to effectively reduce their surface energy. The CdSe nanocrystals showed rapid coarsening compared to CdSe/ZnSe core/shell nanocrystals, as shown in Figure 2, panels c and d, respectivley. Both the coarsened CdSe and CdSe/ZnSe nanocrystals still showed the high crystallinity except for a few nanocrystals having a low degree of stacking faults. Both nanocrystals were heat treated at 240, 250, 260, 270, 280, and 290 °C, respectively, for different time periods. The UV-visible absorbance peak shifts, shown in Figure 3, were used to estimate the variation in the energy band gap of CdSe and CdSe/ZnSe nanocrystals, respectively (see the Supporting Information for details), which is finally used for the estimation of the size variation, based upon the following quantum confinement equation.8
Eg(dot) ) Eg(bulk) +
h2 1.8e2 2m*d2 2πod
1 1 1 + ) m* me mh
(1) (2)
Ripening Kinetics of CdSe/ZnSe Core/Shell Nanocrystals
J. Phys. Chem. C, Vol. 111, No. 3, 2007 1241
Here, Eg(dot) is the energy band gap of a nanocrystal quantum dot, Eg(bulk) is the energy band gap of a bulk semiconductor, h is the Planck constant, m* is the reduced mass of exciton, me is the effective mass of an electron, mh is the effective mass of a hole, me is the effective mass of an electron, d is the diameter of a nanocrystal, e is the electron charge, is the relative dielectric constant, and o is the space dielectric constant. The sizes of nanoparticles (d) can be determined by plugging the energy band gap, Eg(dot), of nanocrysal quantum dots and all other constant values into eq 1. The size values were in good agreement with the experimentally determined ones by HRTEM investigation within a reasonable range. The size variation was applied to the following LSW equation4-6 and the LSW plots were created as shown in Figure 5.
jr3 - jro3 ) Kt
(3)
Here, jro is the average initial size of original nanocrystals before ripening occurs, jr is the average size of nanocrystals after ripening occurs, K is the ripening rate coefficient, and t is the ripening time. The ripening rate constant, K, was derived as follows:
K)
8γDVm2C∞ 9RT
(4)
Here, γ is the interfacial energy, D is the diffusion coefficient, Vm is the molar volume, C∞ is the equilibrium concentration at a flat surface, R is the gas constant, and T is the temperature. The bare CdSe nanocrystals show higher ripening kinetics than the CdSe/ZnSe nanocrystals. From the slopes of each curve, the ripening coefficient, K, was determined according to temperature and used for kinetics analysis. As the ripening coefficient contains the diffusion coefficient term (D), eq 4 can be written as the following Arrhenius-type equation.
(
K ) K0 exp -
Q RT
)
Figure 4. Lifshitz-Slyozov-Wagner (LSW) plots showing ripening of (a) CdSe and (b) CdSe/ZnSe nanocrystal quantum dots.
(5)
Here, K0 is a constant and Q is the activation energy for ripening. By plugging the K values, obtained from LSW plots in Figure 4, into eq 5, Arrhenius plots were obtained as shown in Figure 5. From the slope and y-intercept of each curve, the activation energy for ripening (Q) and the constant values (K0) were determined, respectively, and listed in Table 1. Bare CdSe nanocrystals show higher activation energy values than CdSe/ ZnSe shows. The dissociation of Cd-Se bonds requires ∼310 kJ/mol for the lattice diffusion of Cd2+ and Se2-, while that of Zn-Se requires ∼136 kJ/mol.9 The reason for the very low activation energy of ripening both for the bare CdSe and CdSe/ ZnSe, especially at a low-temperature range, compared to that of lattice diffusion lies in that the surface diffusion would be a dominating mechanism for the ripening reaction. Surface atoms are weakly bonded to the inner atoms and their outward bonds are missing, so the diffusion of surface atoms is rather easier than that of the inner atoms. The bare CdSe shows ∼42% of activation energy required for the dissociation of Cd-Se lattice bonds, and the CdSe/ZnSe shows ∼51% of that of Zn-Se. Due to the low dissociation energy of Zn-Se, the activation energy for the CdSe/ZnSe nanocrystals is lower than that of the bare CdSe. Both samples show change in the activation energy for the ripening at ∼266 °C, which is indicative of change in the ripening mechanism. The bare CdSe and CdSe/ZnSe show 302
Figure 5. Arrhenius plots of CdSe and CdSe/ZnSe nanocrystal quantum dots, showing the activation energy for Ostwald ripening.
and 287 kJ/mol, respectively, for the ripening at a high reaction temperature region. At this high-temperature range, the activation energy is comparable to the dissociation energy of Cd-Se bonds, and thus it can be safely stated that the dissociation of lattice atoms dominates in the bare CdSe. The activation energy for ripening of 287 kJ/mol for the CdSe/ZnSe implies that its atomic bonding is relatively weak, compared to that of CdSe in this temperature region. For the CdSe/ZnSe, the red-shift in UV-visible absorption spectra would come from the increase
1242 J. Phys. Chem. C, Vol. 111, No. 3, 2007
Sung et al. Acknowledgment. This study was supported by a Korea Research Foundation (KRF) grant funded by the Korean Government (MOEHRD) (KRF-2005-041-D00391).
TABLE 1: Ripening Kinetics Variables for CdSe and CdSe/ZnSe Nanocrystals CdSe kinetics variables
266 °C
266 °C
K0 (nm3/min) 2.341 × 1011 8.7031 × 1028 1.155 × 105 8.770 × 1025 Q (kJ/mol) 131 302 69 287
in the leakage of the electron wave function with the increase of the ZnSe shell thickness10 at a low-temperature region. On the other hand, the red-shift would originate mostly from the increase in the CdSe core size by lattice diffusion in a hightemperature region. The temperature of 266 °C can be considered as a transition temperature from the surface diffusion to the lattice diffusion mechanism for the ripening of both nanocrystals. By substituting eq 5 into eq 3, the ripening kinetics can be rewritten as follows:
(
jr3 - jro3 ) K0 exp -
Q t RT
)
(6)
By plugging the kinetics data listed in Table 1 into eq 6, one can complete the ripening kinetics equation for the bare CdSe and CdSe/ZnSe nanocrystals, respectively, and can estimate the size (rj) of nanocrystals at a certain temperature and time period. Conclusions To summarize, for the ripening reactions, the bare CdSe nanocrystals show dissociation of surface and lattice Cd-Se bonds in low- and high-temperature regions, respectively, while the CdSe/ZnSe nanocrystals show dissociation of Zn-Se in a low-temperature region and that of both Cd-Se and Zn-Se in a high-temperature region. By determining the kinetics variables from an Arrhenius-type analysis, the ripening kinetics equation could be completed for both the bare CdSe and CdSe/ZnSe nanocrystals. This type of analysis cannot only provide the theoretical estimation for the ripening of CdSe/ZnSe nanocrystals, but also it can be applied to the ripening kinetics of other core/shell nanocrystals.
Supporting Information Available: All UV-visible absorption spectra of CdSe and CdSe/ZnSe. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Alivisatos, A. P. Science 1996, 271, 933. (b) Shim, M.; GuyotSionnest, P. Nature 2000, 407, 981. (2) (a) Klimov, K. I.; Mikhailovsky, A. A.; Xu, S.; Malko, A.; Hollingsworth, J. A.; Leatherdale, C. A.; Eisler, H. J.; Bawendi, M. G. Science 2000, 290, 314. (b) Bruchez, M. P.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2031. (c) Han, M.; Gao, X.; Su, J. Z.; Nie, S. Nat. Biotechnol. 2001, 19, 631. (3) (a) Peng, X. G.; Schlamp, M. C.; Kadavanich, A. V.; Alivisatos, A. P. J. Am. Chem. Soc. 1997, 119, 7019. (b) Sung, Y.-M.; Lee, Y.-J.; Park, K.-S. J. Am. Chem. Soc. 2006, 128, 9002. (c) Dabbousi, B. O.; Rodriguez-Vejo, J.; Mikulec, F. V.; Heine, J. R.; Mattoussi, H.; Ober, R.; Jensen, K. F.; Bawendi, M. G. J. Phys. Chem. B 1997, 101, 9463. (d) Danek, M.; Jensen, K. F.; Murray, C. B.; Bawendi, M. G. Chem. Mater. 1996, 8 173. (e) Reiss, P.; Bleuse, J.; Pron, A. Nano Lett. 2002, 2, 781. (f) Huang, G. W.; Chen, C. Y.; Wu, K. C.; Ahmed, M. O.; Chou, P. T. J. Cryst. Growth 2004, 265, 2004. (g) Peng, P.; Milliron, D. J.; Hughes, S. M.; Johnson, J. C.; Alivisatos, A. P.; Saykally, R. J. Nano Lett. 2005, 5, 1809. (h) Lee, Y.-J.; Kim, T.-G.; Sung, Y.-M. Nanotechnology 2006, 17, 3539. (4) (a) Wong, E. M.; Hoertz, P. G.; Liang, C. J.; Shi, B. M.; Meyer, G. J.; Searson, P. C. Langmuir 2001, 17, 8362. (b) Hu, Z. S.; Oskam, G.; Penn, R. L.; Pesika, N.; Searson, P. C. J. Phys. Chem. B 2003, 107, 3124. (5) (a) Wong, E. M.; Bonevich, J. E.; Searson, P. C. J. Phys. Chem. B 1998, 102, 770. (b) Oskam, G.; Nellore, A.; Penn, R. L.; Searson, P. C. J. Phys. Chem. B 2003, 107, 1734. (6) (a) Lifshitz, I. M.; Slyozov, V. V. J. Phys. Chem. Solids 1961, 19, 35. (b) Wagner, C. Z. Elektrochem. 1961, 65, 581. (7) Reiss, P.; Bleuse, J.; Pron, A. Nano Lett. 2002, 2, 781. (8) Parak, W. J.; Manna, L.; Simmel, F. C.; Gerion, D.; Alivisatos, P. Nanoparticles; Schmid, G., Ed.; Wiley-VCH: Weinheim, Germany, 2004;pp 4-49. (9) Lange’s handbook of chemistry; Dean, J. A., Ed.; McGraw-Hill: New York, 1999; pp 4.41-4.53. (10) Prasad, P. N. Nanophotonics; Wiley-Interscience: Hoboken, NJ, 2004; pp 79-127.
