NANO LETTERS
In Situ Observation of the Nucleation and Growth of CdSe Nanocrystals
2004 Vol. 4, No. 3 465-469
Lianhua Qu, W. William Yu, and Xiaogang Peng* Department of Chemistry and Biochemistry, UniVersity of Arkansas, FayetteVille, Arkansas 72701 Received December 18, 2003; Revised Manuscript Received January 31, 2004
ABSTRACT An in situ method is introduced for the study of nucleation and growth of crystals using the size-dependent properties of a given system in the transition size regime from molecular species to bulk sized crystals. The real-time measurements for the first system studied, a CdSe one, were carried out by recording the absorption spectra during the reaction with a millisecond resolution.
Introduction. Colloidal semiconductor nanocrystals are nanometer sized fragments of the corresponding bulk crystals synthesized in solutions. Because of their interesting sizedependent properties and flexible processing chemistry, colloidal semiconductor nanocrystals have been widely studied for fundamental purposes and industrial applications, such as LEDs,1 lasers,2 solar cells,3 and biomedical labeling.4,5 The synthesis of colloidal semiconductor nanocrystals with the desired quality is the first step for realizing all these applications. Quantitative understanding of nanocrystals will certainly benefit the development of synthetic chemistry for colloidal nanocrystals.6 In addition, it will also promote the understanding of general crystallization processes that is lacking in the literature, despite the human interests in crystallization over the past thousands of years.7 In the nanometer regime, the transition size regime from single molecules to bulk sized crystals, the optical properties of semiconductor nanocrystals are strongly size dependent due to quantum confinement.8 In principle, the size and concentration of the crystals or clusters at a given time can be respectively determined by the peak position and the absorbance of the absorption spectrum of the crystals/clusters. In fact, the size dependent optical properties of semiconductor nanocrystals have already been broadly exploited for monitoring the growth reactions and studying the growth kinetics,9-12 although those measurements have been performed ex situ. Those ex situ studies of such systems have revealed some unusual phenomena10-14 and promoted the development of those greener synthetic schemes.15-17 However, those ex situ studies are based on taking aliquots from high- temperature reactions and quenching the reaction for room-temperature measurements. This limits the time resolution to tens of seconds at most. Quantitative and full * Corresponding author. Phone: 479-575-4612. Fax: 479-575-4049. E-mail:
[email protected]. 10.1021/nl035211r CCC: $27.50 Published on Web 02/14/2004
© 2004 American Chemical Society
evaluation of a system is almost impossible because it requires too many tedious, precise, and difficult purification and characterization steps. For an accurate determination of the particle and monomer concentrations in the reaction flask, a significant amount of the reaction mixture must be taken for each aliquot.11,17 Such excessive sampling undoubtedly disturbs the reaction system. Some of the above-mentioned issues may be addressable by applying the recently introduced microfluidic methods.18 In addition to these experimental difficulties, there are always some doubts regarding the difference between the events that occurred at high temperatures and the measurements performed at room temperature. This report demonstrates a nondisturbing and relatively general strategy to study in situ the crystallization systems using the size dependent optical properties of crystals in the transition regime from molecular precursors to regular nanocrystals. A greener approach for the growth of highquality CdSe nanocrystals was employed for the current study, because this specific greener scheme is a relatively easy one to handle and reproducibly yields highly emitting CdSe nanocrystals with very narrow size distributions.13 Consequently, the optical data can be converted to the desired chemical kinetic information relatively easily. A typical reaction studied is as follows: CdO, 0.0127 g (0.1 mmol), and 0.1140 g (0.4 mmol) of stearic acid were loaded into a 25 mL four-neck flask and heated to 150 °C under Ar flow. After CdO was completely dissolved, the mixture was allowed to cool to room temperature. TOPO and hexadecylamine, 3.44 g for each, were added to the flask, and the mixture was heated to 280 °C under Ar flow to form an optically clear solution. At this temperature, the Se solution containing 0.079 g (1 mmol) of Se dissolved in 2.920 g of TOP prepared in a drybox was swiftly injected into the reaction flask. After the injection, the temperature was set
Figure 2. Positions of the first exciton absorption peak of CdSe nanocrystals at 250 °C vs those at 25 °C.
Figure 1. In situ recorded UV-vis absorption spectra of CdSe nanocrystals at 250 °C. Inset: A comparison of absorption spectra at different temperatures.
at 250 °C. For all reactions, the total mass of the reaction mixture was fixed as 10 g, and 10 times the amount of the selenium precursor was used in comparison to the cadmium precursor. In situ absorption measurements were performed using an Ocean Optics CCD absorption spectrometer (USB 2000), with a high temperature transmission-reflection dip probe immersed into the reaction solution. A similar setup was recently reported for monitoring the growth of CdSe nanocrystals although no details were discussed.19 The integration time used in this work was set as 2-4 milliseconds. Ten spectra were recorded for every second in the first five to ten minutes of the reaction, and after this initial stage, one spectrum was recorded for each minute. For the collection of the standard spectra of CdSe nanocrystals at high temperatures, a needle-tip amount of the reaction mixture was removed and diluted with chloroform at various time intervals. Absorption and photoluminescence (PL) spectra were recorded for those aliquots at room temperature. The correlation of the absorption spectra at 250 °C and those at room temperature was documented prior to the kinetics studies. Figure 1 illustrates a series of absorption spectra of CdSe particles recorded in situ at different reaction moments for a given reaction with an initial cadmium atomic concentration as of 0.02 mol/kg. As shown in the inset of Figure 1, the high- temperature absorption spectra of CdSe nanocrystals shift to red and are broad in comparison to the corresponding spectra taken at room temperature. Figure 2 demonstrates the relationship between the positions of the first exciton absorption peak of CdSe nanocrystals at 250 °C and those 466
at room temperature, which is almost a perfect linear function. With this calibration curve and the sizing curve at room temperature,20 the average size at any given moment can be readily determined. The concentration of CdSe particles at a given moment was determined by the absorbance at the first exciton absorption peak of the corresponding in situ absorption spectrum and the molar extinction coefficient of CdSe nanocrystals at 250 °C using the Beer-Lambert’s law.20 Except for the data points in the very late stage (deep ripening stage), a correction with the peak width of the absorption spectrum was found working well, using the average half width at half-maximum (hwhm) value of the standard spectra (22 nm) as the calibration standard suggested previously.20 For those spectra recorded in the deep ripening stage, a correction with the peak width of the corresponding PL spectrum was employed because of the dramatic asymmetric feature of the size distribution (Figure 3, right).20 The monomer concentration at a given moment was calculated with the size and concentration of the particles at the given time and the initial monomer concentration. The calculated particle size, particle concentration, and monomer concentration of the reaction shown in Figure 1 are illustrated in Figure 3, left panel. For this specific reaction, the formation of a small amount of CdSe particles with a size of 1.75 nm was observed at 4 milliseconds after the injection. Apparently, the initial monomer concentration in this system was not sufficiently high for the formation of the magic-sized clusters as nuclei.14 In the following 2 s, the initially formed particles grew larger and some more particles were formed. After that, the total particle concentration started to decrease until the reaction proceeded for about 130 s. In all of the above stages, the monomer concentration in the solution was in a monotonic decreasing mode, and the average size of the particles increased steadily. However, at between 130 and 670 s, the system was in a relatively stable stage with the size of the particles, the concentration of the particles and the concentration of the monomers being constants. The only noticeable change in this stage was the size distribution (see below). After 670 s, the size of the particles started to increase again, and the concentration of the particles decreased although the monomer concentration remained as a constant. The size distribution of the particles formed in the solution can be obtained from the peak width of the absorption Nano Lett., Vol. 4, No. 3, 2004
Figure 3. (Left) Temporal evolution of the size and concentration of the CdSe nanocrystals, and the concentration of cadmium monomers in the solution for the reaction shown in Figure 1. (Right) Temporal evolution of the PL spectrum of the nanocrystals for the same reaction.
spectrum at any given moment with sophisticated computer simulations. It is more convenient to obtain this information from the band-edge photoluminescence (PL) spectrum taken at room temperature because only a single peak is involved.10,13 As demonstrated previously, the peak width and the contour of the PL spectrum are approximately determined by the standard deviation of the particles and the size distribution curve of the particles, respectively.13,14 Shown in the right panel of Figure 3, the continuous nucleation converted the nearly symmetric PL spectrum recorded for the very first aliquots of the reaction to an asymmetric one with an obvious tail on the small size side, which was caused by the formation of some small particles in this stage. After the reaction went through the maximum particle concentration point, the PL spectrum gradually came back to a symmetric shape and the peak width became narrowed when the reaction reached the stable stage. During the stable stage, the only detectable change was that both emission and absorption spectra were gradually broadened, although the peak position did not change much. Finally, the PL spectrum became significantly asymmetric and broad when the system passed the stable stage. Because the total particle concentration in the solution decreased in the last stage (Figure 3, left), it is safe to conclude that some of the particles became smaller and dissolved completely, although the average particle size in the distribution continued growing larger. The above observations suggest that the entire crystallization process can be divided into four stages. In the initial stage, the total number of particles increased as a result of a prolonged formation of relatively small particles in the solution. This stage is equivalent to the nucleation stage considered by classic theories. In the second stage, the concentration of the particles dropped significantly, the size distribution focused down, and the distribution changed from Nano Lett., Vol. 4, No. 3, 2004
an asymmetric one back to a symmetric one. This stage is probably the focusing of size distribution stage reported previously,10,13 although the decrease of the particle concentration has not been noticed with the low resolution ex situ measurements. The stable stage was likely a result of the equilibrium between the monomers and the particles in the solution, which means that, by classic theories, the monomer concentration in the solution was close to the solubility of the particles in the solution.7 The broadening of the particle size distribution in this stage was most likely due to the concentration fluctuation. The fourth stage should be the main course of the Ostwald ripening or defocusing of size distribution,10 because the relatively big particles in the distribution grow even bigger with the shrinkage and disappearance of the relatively small ones in the solution (Figure 3). Reactions with different initial monomer concentrations were performed under the same reaction conditions. The particle size, remaining monomer concentration, and particle concentration at the maximum particle concentration position and the initial point of the stable stage are plotted in Figure 4. The results indicate that at those two critical points, the remaining monomer concentration and the particle concentration in the solution, were both roughly linearly dependent on the initial monomer concentration. This means that the remaining monomer concentrations in the solution were also approximately linearly related to the particle concentration in the solution at the two critical points. In contrast, the particle size did not show much influence on either the remaining monomer concentration or the particle concentration in the solution at both points (Figure 4). We noticed that the influence of the density and the nearest neighbor of the islands on solid substrates played a determining role in the process of Ostwald ripening on the surface of solids as observed by Rosenfeld et al.,21 which similarly 467
Figure 4. (Left) The cadmium monomer concentration, particle concentration, and particle size at the maximum particle concentration point vs the initial cadmium monomer concentration in the solution. (Right) Cadmium monomer concentration, particle concentration, and particle size at the stable stage vs the initial cadmium monomer concentration in the solution.
implies that the islands can communicate with each other in the process. Rosenfeld et al. further demonstrated that such communication is built up by the influence of the chemical potential of the monomers on the substrate. In principle, the chemical potential and the motion of the free monomers on the surface of substrates are both strongly dependent on the number of the nearby islands on the substrate. The density of islands on solid substrate is equalvent to the particle concentration in solution. Therefore, the results observed by Rosenfeld et al.21 on solid substrates can be considered somewhat similar to the results shown in Figure 4, right panel. Classical crystallization theories assumed that the concentration of crystals should not be a function of the monomer concentration in the solution.7 Evidently, this is contradictory to the results shown in Figure 4. However, it should be pointed out that the results shown in Figures 3 and 4 should only be considered as semiquantitative at present. This is so because the size distribution of nanocrystals at a given moment was not quantitatively extracted from the computer simulation of the corresponding absorption spectrum. Instead, the fwhm of the PL spectrum or hwhm of the absorption spectrum was used for the correction of the size distribution of the particles. The hwhm of an absorption spectrum does not reveal the information for the sizes smaller than the peak size, and fwhm of a PL spectrum may be distorted by the emission quantum efficiency distribution of differently sized nanocrystals in the sample.12 The error of the average sizes reported here is caused by the error of the sizing curve,20 which was ultimately determined by transmission electron microscope (TEM) measurements. The error of the size of the nanocrystals determined by TEM is in the order of one monolayer of 468
Figure 5. Temporal evolution of the reaction temperature for the typical reaction. The sharp drop of the reaction temperature was caused by the swift injection of the Se solution.
atoms, although this error should be somewhat suppressed by incorporating experimental results from several different research groups.20 The error of the particle and monomer concentrations (Figures 3 and 4) would be a combination of the errors from the size distribution and the average size mentioned here. In addition to the semiquantitative treatment of the size distribution of the nanocrystals discussed in the above paragraph, the temperature variation within the first few seconds after the swift injection of the Se solution (Figure 5) might also have caused some uncertainty in the data. The third error came from the accuracy of the injection rate of the Se solution. With all these standing issues in the presented experiments, the conflict between the classical crystallization theory and the experimental results discussed above should be treated with caution. Experiments and simulation methods yielding quantitatively reliable data are our next step to further explore this very stimulating and Nano Lett., Vol. 4, No. 3, 2004
new research direction, quantitatively probing the crystallization system using size dependent properties of the crystals. To our knowledge, the understanding of crystallization, especially the nucleation process, has been very much limited mainly due to a lack of quantitative experimental data. Acknowledgment. Financial support was provided by the NSF-CHE and Quantum Dot Corporation, Hayward, CA. References (1) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354-357. (2) Klimov, V. I.; Mikhailovsky, A. A.; Xu, S.; Malko, A.; Hollingsworth, J. A.; Leatherdale, C. A.; Eisler, H. J.; Bawendi, M. G. Science 2000, 290, 314-317. (3) Greenham, N. C.; Peng, X. G.; Alivisatos, A. P. Phys. ReV. B 1996, 54, 17628-17637. (4) Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013-2016. (5) Chan, W. C. W.; Nie, S. M. Science 1998, 281, 2016-2018. (6) Peng, X. Chem. Eur. J. 2002, 8, 334-339.
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(7) Mullin, J. W. Crystallization, 3rd ed.; Butterworth-Heinemann: Oxford, Boston, 1997. (8) Brus, L. J. Phys. Chem. 1986, 90, 2555-60. (9) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706-8715. (10) Peng, X.; Wickham, J.; Alivisatos, A. P. J. Am. Chem. Soc. 1998, 120, 5343-5344. (11) Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. 2001, 123, 1389-1395. (12) Talapin, D. V.; Rogach, A. L.; Shevchenko, E. V.; Kornowski, A.; Haase, M.; Weller, H. J. Am. Chem. Soc. 2002, 124, 5782-5790. (13) Qu, L.; Peng, X. J. Am. Chem. Soc. 2002, 124, 2049-2055. (14) Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2002, 124, 3343-3353. (15) Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2001, 123, 183-184. (16) Qu, L.; Peng, Z. A.; Peng, X. Nano Lett. 2001, 1, 333-336. (17) Yu, W. W.; Peng, X. Angew. Chem., Int. Ed. 2002, 41, 2368-2371. (18) Chan, E. M.; Mathies, R. A.; Alivisatos, A. P. Nano Lett. 2003, 3, 199-201. (19) Reiss, P.; Carayon, S.; Bleuse, J.; Pron, A. Synth. Met. 2003, 139, 649-652. (20) Yu, W. W.; Qu, L.; Guo, W.; Peng, X. Chem. Mater. 2003, 15, 28542860. (21) Rosenfeld, G.; Morgenstern, K.; Esser, M.; Comsa, G. Appl. Phys. A 1999, 69, 489-493.
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