Study on growth kinetics of CdSe nanocrystals in oleic acid/dodecylamine

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1 Journal of Crystal Growth 286 (2006) Study on growth kinetics of CdSe nanocrystals in oleic acid/dodecylamine Bifeng Pan, Rong He, Feng Gao, Daxiang Cui, Yafei Zhang Bio-X DNA Computer Consortium, National Key Laboratory of Nano/Micro Fabrication Technology, Key Laboratory for Thin Film and Microfabrication of Ministry of Education, Department of Bio-Nano Science and Engineering, Institute of Micro-Nano Science and Technology, Shanghai JiaoTong University, 1954 Huashan Road, Shanghai , PR China Received 27 March 2005; received in revised form 20 September 2005; accepted 3 October 2005 Available online 28 November 2005 Communicated by J.M. Redwing Abstract A novel synthesis of CdSe nanocrystals is presented, which is based on the lower temperature reaction between TOPSe and cadmium acetate. An oleic acid/dodecylamine solvent was employed for studying the growth kinetics of CdSe nanoparticles at 200 1C. The concentration of growing particles remained constant following the initial nucleation step. The diameter of the CdSe nanocrystals varied from nm up to nm. About 90% of the available Cd was consumed during the growth. The rate constant for steady-state CdSe deposition was found to be ms 1 at 200 1C. r 2005 Elsevier B.V. All rights reserved. PACS: h; w Keywords: A1. Crystal growth; A1. Kinetics; B1. Nanomaterials; B2. CdSe 1. Introduction Different kinds of safe, common, and low-cost compounds were proven to be good solvents/precursors for the synthesis of high-quality CdSe nanocrystals [1 5]. The size, shape, and crystal structure of CdSe nanocrystals synthesized by these alternative routes can be varied in a controllable manner in a very broad size range, from about 1.5 nm to above 25 nm [2,3]. Cd(Ac) 2 and fatty acids were found to be the most versatile cadmium precursor and solvent/ligand, respectively [4]. Low injection/growth temperature and highly crystalline CdSe can be obtained in amines [4,5]. Spectroscopic monitoring of the growth of semiconductor nanocrystals provides a useful method for exploring the process of particle growth in solution [6,7]. Provided that there is a concrete relationship between the optical spectrum and the particle size, initial conditions during nucleation and growth can be inferred [8 10]. In the case of CdSe, several groups have carried out extensive analyses of the position of the first excited state (1S e 1S 3/2h ) as a function of the mean crystallite size, determined by electron microscopy [10 13]. The data by Banin and co-workers show an r 1 dependence of the exciton energy on particle radius [9,13,14], and this provided the first extensive correlation between size and optical band edge. Peng and colleagues have recently carried out a more systematic investigation of the extinction coefficient of CdSe, CdS, and CdTe nanocrystals over a wide range of sizes [15]. Their correlations also include the data from numerous former studies, including those of Banin et al. [14]. As reported by Mulvaney [16], it is possible to measure key parameters associated with particle nucleation, such as the number of nucleation centers and the rate constants for monomer addition to the nascent crystallites. In this report, we examine the actual nucleation conditions and the growth of CdSe in oleic acid/ Corresponding authors. Tel./fax: addresses: panbifeng@sjtu.edu.cn (B. Pan), rhe@sjtu.edu.cn (R. He) /$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi: /j.jcrysgro

2 dodecylamine using cadmium acetate as cadmium procursor, a system that provides a comparatively simple, coordinating environment for particle nucleation and growth and an economical route to highly luminescent semiconductor particles. 2. Materials and methods All chemicals were used without further purification. Selenium shot (99.999%) and trioctylphosphine (TOP, 90%) were obtained from Aldrich. Oleic acid, cadmium acetate, dodecylamine, and chloroform were sourced from Sigma. To prepare the stock selenium injection solution, g selenium shot (0.25 mmol) was reacted with 2.5 g TOP (6.73 mmol). A cadmium stock solution was prepared by heating a mixture of 33.8 g dodecylamine (183.2 mmol), 51.6 g oleic acid (183.2 mmol) and g cadmium acetate (0.25 mmol) to 220 1C until a clear, gold brown solution was obtained. The final oleic acid/dodecylamine ratio is 1:1 (mol:mol). To prepare CdSe nanocrystals, the cadmium stock solution was transferred to a 250 ml three neck round-bottom flask fitted with water-cooled condenser and thermocouple and the synthesis was then conducted under a nitrogen atmosphere. The mixture was heated to 220 1C, and the TOPSe solution swiftly injected. The growth temperature was held at 200 1C. The final cadmium/selenium ratio is 1:1 (mol/mol) and the concentration of Cd and Se is 2.5 mm. Typically color changes from yellow to orange to red to dark red were observed in min after injection. Samples for absorbance and photoluminescence (PL) spectroscopy were taken at different time intervals up to 180 min after nucleation. Addition of anhydrous methanol to the aliquot results in the reversible flocculation of the nanocrystallites. The flocculate is separated from the supernatant by centrifugation. Dispersion of the flocculate in chloroform results in an optically clear solution of nanocrystallites. 3. Results and discussion B. Pan et al. / Journal of Crystal Growth 286 (2006) In Fig. 1, we present absorption and PL spectra of CdSe nanocrystals as a function of the growth time after injection. Reaction was slow and the solution developed to a dark-red color after the 40 min injection of TOPSe. The UV vis absorption spectrum of each composite displays an absorption edge that is dependent upon the size of the CdSe nanocrystals (Fig. 1a). With increasing particle size, this feature moves to longer wavelength and we attribute this to an interband CdSe transition [17]. This shift in band edge is well established for CdSe nanoparticles [18]. In contrast to the absorption spectra, we observe that the PL intensity of the nanoparticles displays an unusual dependence on particle size. Large particles have narrow PL spectra (Figs. 1b and 2a) and the emission band shifts to progressively lower energies as the particle size increases. This behavior is typical of CdSe nanoparticles and is attributed to band-edge PL [15]. The control over luminescence quantum yield was as reproducible as was the control of the nanocrystal sizes. The PL quantum efficiency showed a tendency to drop with the increase of the particle size (Fig. 1b). At the beginning of CdSe crystal growth (from 0 to 10 min of growth time), [Cd 2+ ] and [Se 2 ] decreased, more and more CdSe nanocrytals were formed, so, PL intensity of CdSe solution increased with increasing the growth time from 0 to 10 min. The concentration of the nanocrystals in the solution did not change after 10 min growth, the peak intensity decreased with time as shown in Fig. 1b, directly reflecting the relative extinction coefficient per mole of particles. Therefore, the significant intensity drop of the spectra shown in Fig. 1b from 10 to 180 min indicates that the extinction coefficient per mole of particles decreased dramatically. The overall features and the narrow spectral band widths indicate that the size distributions of the nanocrystals are successfully narrow. These spectral changes may be due to surface traps of the excitation, FWHM can be reduced by increasing particle size or overcoating wider gap materials such as ZnS or ZnSe as is also indicated in Refs. [7,8]. The decreased width of the emission spectrum (Fig. 2a) can also be attributed to the inhomogeneous environment across the nanoparticle surface or between particles and/or their environments. In Fig. 2a, we show a plot of the particle diameters as a function of time after injection at 200 1C. The diameters have been calculated from the exciton peak energy using Eq. (1) from Yu s calibration data [15]. Similar results were obtained from Banin s calibration curve [9,14]. It is immediately apparent that the average particle size at 60 s is smallest, but this leads to the fastest particle growth. D ¼ð1: Þl 4 ð2: Þl 3 þð1: Þl 2 0:4277l þ 41:57. (1) In the above equation, D (nm) is the size of a given nanocrystal sample, and l (nm) is the wavelength of the first excitonic absorption peak of the corresponding sample. A ¼ ecl. (2) In Eq. (2), A is the absorbance at the peak position of the first exciton absorption peak for a given sample. C is the molar concentration (M) of the nanocrystals of the same sample. L is the path length (cm) of the radiation beam used for recording the absorption spectrum. In our experiments, L was fixed at 1 cm. e is the extinction coefficient per mole of

3 320 B. Pan et al. / Journal of Crystal Growth 286 (2006) Fig. 1. (a) UV vis absorption and (b) PL spectra of CdSe nanocrystals as a function of time (a) 1 min, (b) 2 min, (c) 5 min, (d) 10 min, (e) 20 min, (f) 40 min, (g) 60 min, (h) 90 min, (i) 120 min, and (j) 150 min. The CdSe concentrations for UV vis and photoluminescence are 1.0 mm and 25 nm, respectively. nanocrystals (cm 1 M 1 ) as calculated by Eq. (3) [15] e ¼ 1600DEðDÞ 3. (3) Here, DE is the transition energy corresponding to the first absorption peak and the unit is in ev. D is the diameter of the nanocrystals (nm). The format of the fitting functions was chosen according to an existing equation [15]. The smallest particle sizes we could reliably obtain correspond to samples about 60 s old; this had a diameter of about nm. The experiments were generally terminated after about 180 min growth. In Fig. 2b, we present data on the particle concentration vs. time. These numbers are calculated using the exciton energy to determine the particle radius and then the extinction coefficient for each size to determine the particle concentration using Eqs. (2) and (3) [12 15]. Within experimental error, we find that the particle concentration in these experiments is constant from 10 min growth until the reactions are terminated at around 180 min growth. This is the verification of this assumption for nanocrystal growth in a coordinating solvent. Particularly striking is the fact that the concentration of nuclei is consistently decreased within 1 10 min growth. The growth of CdSe in hot oleic acid/dodecylamine provides an ideal model system to verify kinetic models of particle growth in solution. Our aim in this study was to measure the growth rate constant (k) of CdSe nanoparticles formed during the nanocrystal synthesis and to estimate the correlation between crystal growth and time. No such measurements have appeared for this system, despite its technological relevance to the controlled synthesis of tunable, luminescent materials.

4 B. Pan et al. / Journal of Crystal Growth 286 (2006) D/nm FWHM/nm (a) t/min ε/10 5 cm -1 M [CdSe]/10-6 M (b) t/min 4 Fig. 2. (a) Diameters and full-width-half-maximum (FWHM) vs. time curves for CdSe nanocrystals, the CdSe diameters were calculated from Eq. (1); (b) extinction coefficients E and calculated CdSe nanocrystal concentrations ([CdSe]) as a function of time, [CdSe] and E are calculated using Eqs. (2) and (3). The data in Fig. 2b clearly indicate that the CdSe concentrations decrease at the beginning of the reaction; furthermore nucleation is a slow process that ceases after 10 min injection. This considerably simplifies the analysis of the data. To do so requires some sort of kinetic model for the particle growth. The Cd concentration must obey an equation of the form [16] d½cdš t dt ¼ kaðtþ½cdš t NðtÞ, (4) where [Cd] t is the concentration of available Cd at time t, A(t) is the surface area of each particle at time t, N(t) is the number of particles at time t, and k is an interfacial rate constant with units (length time 1 ) that reflects the rate determining steps during deposition. The Cd concentration at time t can be calculated from Eq. (5) as follows [16]: VðtÞr ½CdŠ t ¼½CdŠ 0 ½CdSeŠN a M ½4=3 prðtþ 3 Šr ¼½CdŠ 0 ½CdSeŠN a. ð5þ M Since [Cd] 0 ¼ [Cd] t +[Cd] con, therefore, ½CdŠ con % ¼ consumed½cdš% ¼ ð½cdš 0 ½CdŠ t Þ ½CdŠ 0 100%, ð6þ

5 322 B. Pan et al. / Journal of Crystal Growth 286 (2006) [Cd] t /mm [Cd] con /% (a) t/min Q(10-5 ) (b) t/s Fig. 3. (a) Calculated [Cd] t and [Cd] con percentage at 200 1C as a function of time from Eqs. (5) and (6); (b) a linear plot of Q vs. t and the slope equals k, whereas the value of Q at t ¼ 0 equals C. where [Cd] 0 ¼ M, [CdSe] is from Fig. 2b, N a is Avogadro number ( mol 1 ), r is the density of CdSe (5800 g L 1 ), M is the relative molecular weight of CdSe ( g mol 1 ), r(t) is the radius of CdSe at time t as given in Fig. 2a. In Fig. 3a, we present plot on the [Cd] t and the [Cd] con vs. time. Within experimental error, we find that the [Cd] t decreased from the moment of initial growth until 150 min growth. This is the verification of this assumption for nanocrystal growth in a coordinating solvent. We assume [Cd] ¼ 0att¼1so that 4=3 pr 3 max r ½CdŠ 0 ¼½CdSeŠ 1 N a. (7) M The [CdSe] N is the CdSe concentration at t ¼1. [CdSe] N E M assuming that the [CdSe] remains constant after 20 min growth. Therefore, the maximal CdSe radius r max ¼ 1.86 nm is obtained by Eq. (7). The correlation between CdSe radius r and time t is provided by Mulvaney s method as follows [16]: p 2 ffiffi h p ffiffi i 3 arctan 1 þ 2A 1=3 B r. 2 1=3 3 þ Log A 2=3 þ B 1=3 A 1=3 r þ B 2=3 r 2 A 1=3 B 1=3 r t ¼ þ C ¼ Q þ C. (8) 6B 1=3 ka 2=3 k With p 2 ffiffi h. p ffiffi i. 2 3 arctan 1 þ 2A 1=3 B 1=3 r 3 þ Log A 2=3 þ B 1=3 A 1=3 r þ B 2=3 r 2 A 1=3 B 1=3 r Q ¼. (9) 6B 1=3 A 2=3

6 Therefore Q ¼ kt C, (10) where A ¼ V m ð½cdš 0 ½CdŠ eq Þ8: assuming that [Cd] eq -0 at growth time t ¼1; B ¼½CdSeŠ 1 N a 4p/ 3 ¼ (m 3 ). The rate constant k is obtained by plotting Q vs. t (Fig. 3b). The slope of the linear plot obtained from the experimental data is rate constant k, whereas the value of Q at t ¼ 0 equals to C. Values of k and C obtained from Fig. 3b are ms 1 and , respectively. 4. Conclusions In conclusion, safe, low-cost, and common compounds were chosen for reproducibly synthesizing high-quality CdSe nanocrystals in a size range of 1 4 nm. Oleic acid/dodecylamine provides a useful medium for studying the growth kinetics of CdSe nanocrystals. The extinction coefficient of semiconductor nanocrystals is determined for convenient and accurate measurements of the concentrations of nanocrystals. Ideal kinetics models were developed with the aim to measure the growth rate constant (k) of CdSe formed during the nanocrystal synthesis. We have found preliminary values of the radius and concentrations of CdSe nanoparticles as a function of time with rate constant k ¼ ms 1. Acknowledgements This work was supported by Shanghai Municipal Commission for Science and Technology (Grant No. 03DZ14025), National Natural Science Foundation of China (No ), National 973 project (2005CB G), Shanghai Development Foundation of Science and Technology (No. 03ZR14057) and 2003 Major Basic Research Program of Shanghai (No. 03DJ14002). References B. Pan et al. / Journal of Crystal Growth 286 (2006) [1] L.H. Qu, Z.A. Peng, X.G. Peng, Nano Lett. 1 (6) (2001) 333. [2] Z.A. Peng, X.G. Peng, J. Am. Chem. Soc. 123 (2001) 183. [3] L. Manna, E.C. Scher, A.P. Alivisatos, J. Am. Chem. Soc. 122 (2000) [4] X.G. Peng, L. Manna, W.D. Yang, J. Wickham, E. Scher, A. Kadavanich, A.P. Alivisatos, Nature 404 (2000) 59. [5] L. Manna, D.J. Milliron, A. Meisel, E.C. Scher, A.P. Alivisatos, Nat. Mater. 2 (2003) 382. [6] Z.A. Peng, X.G. Peng, J. Am. Chem. Soc. 123 (2001) [7] D.V. Talapin, A.L. Rogach, A. Kornowski, M. Haase, H. Weller, Nano Lett. 1 (4) (2001) 207. [8] D. Tonti, F. van Mourik, M. Chergui, Nano Lett. 4 (12) (2004) [9] E. Nahum, Y. Ebenstein, A. Aharoni, T. Mokari, U. Banin, N. Shimoni, O. Millo, Nano Lett. 4 (1) (2004) 103. [10] D.V. Talapin, R. Koeppe, S. Goltzinger, A. Kornowski, J.M. Lupton, A.L. Rogach, O. Benson, J. Feldmann, H. Weller, Nano Lett. 3 (12) (2003) [11] M.Y. Han, X.H. Gao, J.Z. Su, Sh.M. Nie, Nat. Biotechnol. 19 (2001) 631. [12] C.B. Murray, D.J. Noms, M.G. Bawendi, J. Am. Chem. Soc. 115 (1993) [13] C.A. Leatherdale, W.-K. Woo, F.V. Mikulec, M.G. Bawendi, J. Phys. Chem. B 106 (2002) [14] V.N. Soloviev, A. Eichho fer, D. Fenske, U. Banin, J. Am. Chem. Soc. 122 (2000) [15] W.W. Yu, L.H. Qu, W.Zh. Guo, X.G. Peng, Chem. Mater. 15 (2003) [16] C.R. Bullen, P. Mulvaney, Nano Lett. 4 (2004) [17] B.R. Fisher, H.J. Eisler, N.E. Stott, M.G. Bawendi, J. Phys. Chem. B 108 (2004) 143. [18] W.W. Yu, L. Qu, W. Guo, X. Peng. Chem. Mater. 15 (2003) 2854.