Langmuir 2000, 16, 4049-4051
Reverse Micelle Synthesis and Characterization of ZnSe Nanoparticles Forest T. Quinlan,† Joerg Kuther,‡ Wolfgang Tremel,‡ Wolfgang Knoll,§ Subhash Risbud,† and Pieter Stroeve*,† Center on Polymer Interfaces and Macromolecular Assemblies (CPIMA), Department of Chemical Engineering and Materials Science, University of California, Davis, California 95616, Institut fur Anorganische Chemie und Analytische Chemie, Johannes Gutenberg-Universitat, Mainz D-55099, Germany, and Max-Planck-Institut fur Polymerforschung, Mainz D-55128, Germany Received July 13, 1999. In Final Form: January 18, 2000
Introduction Over the past decade, many studies have been published on the synthesis of nanoparticles of semiconducting materials.1,2 The interest in nanoparticles is largely due to potential applications in devices such as blue lasers, flat panel displays, and light-emitting diodes. A variety of chemical methods have been used for the synthesis of nanoparticles. Molecular beam epitaxy3-5 and metal organic chemical vapor deposition4 are well-known techniques to create nanoscale semiconducting materials. A limitation of these techniques is that the materials are usually nanoscale in only one dimension. Although solgel methods are also useful for creating three-dimensional nanoparticles,6-20 the silicate glass matrix is subject to shrinking and cracking upon drying. Also, since the nanoparticles are permanently trapped within the matrix, they are difficult to remove for subsequent use in other media. A procedure has been given for the production of highly luminescent ZnSe nanocrystals from organometallic pre* To whom correspondence should be addressed. E-mail:
[email protected]. Telephone: (530) 752-8778. Fax: (530) 7521031. † University of California. ‡ Johannes Gutenberg-Universitat. § Max-Planck-Institut fur Polymerforschung. (1) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226. (2) Alivisatos, A. P. Science 1996, 271, 933. (3) Akimoto, K.; Miyajima, T.; Mori, Y. Jpn. J. Appl. Phys. 1989, 28, 528. (4) Danek, M.; Jensen, K. F.; Murray, C. B.; Bawendi, M. G. J. Cryst. Growth 1994, 145, 714. (5) Buckley, A. M.; Greenbelt, M. J. Chem. Educ. 1994, 71, 599. (6) Leppert, V. J.; Mahamuni, S.; Kumbhojkar, N. R.; Risbud, S. H. Mater. Sci. Eng. B 1998, 52, 89; Nanostruct. Mater. 1998, 10, 117. (7) Leppert, L. J.; Risbud, S. Philos. Mag. Lett. 1997, 75, 29. (8) Li, G.; Nogami, M. J. Appl. Phys. 1994, 75, 4276. (9) Minti, H.; Eyal, M.; Reisfeld, R. Chem. Phys. Lett. 1991, 183, 277. (10) Nogami, M.; Shinji, S.; Nagasaka, K. J. Non-Cryst. Solids 1991, 135, 182. (11) Nogami, M.; Nagasaka, K. J. Non-Cryst. Solids 1990, 122, 101. (12) Nogami, M.; Nagasaka, K. J. Non-Cryst. Solids 1992, 147-148, 331. (13) Nogami, M.; Nagasaka, K.; Kato, E. J. Am. Ceram. Soc. 1990, 73, 2097. (14) Nogami, M.; Nagasaka, K.; Suzuki, T. J. Am. Ceram. Soc. 1992, 75, 220. (15) Nogami, M.; Zhu, Y.; Tohyama, Y.; Nagasaka, K.; Tokizaki, T.; Nakamura, A. J. Am. Ceram. Soc. 1991, 74, 238. (16) Rajh, T.; Vucemilovic, M. I.; Dimitrijevic, N. M.; Micic, O. I. Chem. Phys. Lett. 1988, 143, 305. (17) Spanhel, L.; Arpae, E.; Schmidt, H. J. Non-Cryst. Solids 1992, 147, 657. (18) Smith, C. A.; Risbud, S. H.; Cooke, J. D.; Lee, H. W. H. Opt. Photon. News 1997, 8, 55. (19) Tohge, N.; Minami, T. SPIE: Sol-Gel Opt. II 1992, 1758, 587. (20) Yamane, M.; Takada, T.; Mackenzie, J. D.; Li, C. SPIE: Sol-Gel Opt. II 1992, 1758, 577.
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cursors nucleated and grown in hot alkylamine coordinating solvent.21 The nanoparticles were relatively monodisperse with a band gap between 2.8 and 3.4 eV. Several groups have used the reverse micelle technique to make nanoparticles of CdS, CdSe, CdTe, and ZnS.22-33 The reverse micelle technique often requires specifically generated surfactants or expensive reagents. To our knowledge, no prior works on ZnSe synthesis by the reverse micelle technique have been reported. In this paper we have utilized the reverse micelle technique using an ion exchange reaction that produces cubic ZnSe nanoparticles. The chemicals used are standard, commercially available, and relatively inexpensive. The synthesis reproducibly yields nanoparticles that are nearly monodisperse and of a size less than the Bohr exciton radius for ZnSe (5.7 nm). The nanoparticles were analyzed using X-ray diffraction, UV-visible absorption spectroscopy, light scattering, transmission electron microscopy, and photoluminescence. Experimental Section Heptane (99%, spectrophotometric grade), dioctyl sulfosuccinate sodium salt (AOT, 96%), and zinc perchlorate hexahydrate Zn(ClO4)2‚6H2O were obtained from Aldrich Chemical Co., Inc. Sodium selenide (Na2Se) was purchased from Alfa Aesar. Ethyl alcohol (200 proof) was obtained from Quantum Chemical Company. Hellmanex (used as a cleaning solution) was purchased from Hellma Worldwide. The reagents were used as received. The group of W.T. at the Johannes Gutenberg-Universitat, Mainz, Germany, using standard techniques, synthesized the mercaptododecanol. The specific procedure for synthesizing nanoparticles of ZnSe we used starts with the addition of 2.65 g of AOT to 100 mL of heptane in a reaction vessel to form reverse micelles. The reverse micelle solution was constantly stirred using a magnetic stirrer. A 1 M solution of Zn(ClO4)2, was prepared by dissolving 9.31 g of Zn(ClO4)2‚6H2O in 25 mL of pure ethyl alcohol. A selenide solution was prepared by dissolving 9 mg of Na2Se in 0.33 mL of EtOH. The selenide solution was sonicated for approximately 2 min to facilitate mixing. All solutions were degassed with a rigorous nitrogen purge for more than 20 min, and the reaction flask was continually flushed with nitrogen gas. Degassing was necessary in order to avoid a side reaction of the Na2Se with dissolved hydrogen or oxygen to form H2Se and various oxides of selenium, respectively. Once the reverse micelles were formed, an 86 µL aliquot of Zn(ClO4)2 was injected through the septum into the reaction vessel. Next, the Na2Se solution was added to the mixture. The ZnSe reaction took place instantaneously upon addition of the selenide solution to the reaction vessel. The ratio of Zn:Se ions in the reaction vessel was 1:0.9 in order to ensure (21) Hines, M. A.; Guyot-Sionnest, P. J. Phys. Chem. B 1998, 102, 3655. (22) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354. (23) Dabbousi, B. O.; Bawendi, M. G.; Onitsuka, O.; Rubner, M. F. Appl. Phys. Lett. 1995, 66, 1316. (24) De, G. C.; Roy, A. M.; Saha, S. J. Photochem. Photobiol. 1995, 92, 189-192. (25) Dona, J. M.; Herrero, J. J. Electrochem. Soc. 1995, 142, 764. (26) Greenham, N. C.; Peng, X.; Alivisatos, A. P. Phys. Rev. 2 1996, 54, 17628. (27) Hoheisel, W.; Colvin, V. L.; Johnson, C. S.; Alivisatos, A. P. J. Phys. Chem. 1994, 101, 8455. (28) Lianos, P.; Thomas, J. K. Chem. Phys. Lett. 1986, 125, 299. (29) Motte, L.; Petit, C.; Boulanger, L.; Lixon, P.; Pileni, M. P. Langmuir 1992, 8, 1049. (30) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270, 1335. (31) Petit, C.; Lixon, P.; Pileni, M. P. J. Phys. Chem. 1990, 94, 1598. (32) Steigerwald, M. L.; Alivisatos, A. P.; Gibson, J. M.; Harris, T. D.; Kortan, R.; Muller, A. J.; Thayer, A. M.; Duncan, T. M.; Douglass, D. C.; Brus, L. E. J. Am. Chem. Soc. 1988, 110, 3046. (33) Smith, C. Ph.D. Thesis, University of California at Davis, 1998.
10.1021/la9909291 CCC: $19.00 © 2000 American Chemical Society Published on Web 03/15/2000
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Langmuir, Vol. 16, No. 8, 2000
an excess of zinc ions. An excess of Zn ions on the outside of the nanoparticles allows the reaction of a protective capping agent (mercaptododecanol) on the surface of the particles. The nanoparticles were capped by dissolving the mercaptododecanol in the reaction solution at a concentration of 0.1 mg/mL. Nanoparticles with the capping agent were removed from the solution and used for X-ray diffraction and TEM analysis. For UV-visible spectroscopy, light scattering, or photoluminescence, the nanoparticle suspension was used directly without introducing the capping agent. For X-ray diffraction analysis the capped nanoparticles of ZnSe were removed from solution. The solution was centrifuged and excess fluid decanted, and the pellet was rinsed with ethyl alcohol. Centrifugation and rinsing were repeated several times, and the nanoparticles were dried under flowing nitrogen. X-ray diffraction patterns were acquired with the samples held between Scotch tape on a Siemens D5000 diffractometer operating in the transmission geometry. In this instance, nanoparticle samples from the above procedure and a modified procedure were used. The modified procedure was used to produce a larger amount of sample. To this end, concentrations of Zn and Se in the synthesis were increased by a factor of 10. Hence, the modified procedure will be referred to as the factor 10 scale-up in this paper. The synthesis was as follows: 2.65 g of AOT was dissolved in 100 mL of heptane, 0.514 g of Zn(ClO4)2‚6H2O was dissolved in 1.4 mL of EtOH, and 90 mg of Na2Se was dissolved in 3.3 mL. In all other respects the preparation was the same as the previous procedure. The samples for the factor 10 scale-up procedure were also capped with mercaptododecanol. Absorption spectra of the nanoparticle solution in the range of 200-550 nm were measured using a Varian Cary 1/3 UVvisible spectrometer using a 1 cm quartz cuvette. Particle size was determined by dynamic light scattering and transmission electron microscopy (TEM). Light scattering was conducted on a Barnstead model 2030 dynamic light scattering apparatus. Data were taken at 30° in order to measure the shorter relaxation times associated with such small particles. A 100 kV Hitachi H-600 was used for electron microscopy. The 300 mesh copper grids were Formvar coated. Samples for the TEM were prepared by making a batch of ZnSe nanoparticles. However, the mercaptododecanol was added to the nanoparticle solution shortly after synthesis, thus capping the nanoparticles. The heptane was then repeatedly washed with EtOH in an attempt to remove excess AOT from the solution. The nanoparticles were resuspended in solution, and then a drop of this solution was placed onto a Formvar-coated TEM grid and allowed to evaporate. Photoluminescence experiments were performed on a Jobin Yvon-Spex Fluorolog-3 spectrophotometer using a 450 W xenon UV source. Emission spectra for the nanoparticles were collected in reflection mode with spectral resolution of 0.2 nm. Particles from both the standard synthesis and the factor 10 scale-up were measured.
Results and Discussion The initial mixture of AOT and heptane during the first step of the synthesis was clear. When the Zn(ClO4)2 solution was added to the solution, there was no change in color, but upon addition of the Na2Se solution, the reaction took place in less than ∼0.5 s. The reaction yielded a stable, translucent yellow solution. The yellow tint suggests the presence of the ZnSe nanoparticles. A larger quantity of nanoparticles for X-ray diffraction measurements was obtained by the factor 10 scale-up procedure. Figure 1 shows a typical example of a TEM micrograph of the ZnSe nanoparticles; the images were scanned and processed with the NIH Image Software program which allows a statistical analysis of the particle size and size distribution. Care was taken to adjust the contrast level such that larger artifacts in some of the micrographs were eliminated and only the nanoparticles were counted. Artifacts were due to the presence of excess surfactant not removed during washing. The average measured
Notes
Figure 1. Typical TEM micrograph of ZnSe nanoparticles. The bar is 10 nm in size.
Figure 2. X-ray diffraction traces for ZnSe nanoparticles and results for cubic ZnSe simulation.
particle diameters were 3.5 ( 1.7 nm. These results compare well with those obtained from our light scattering experiments which gave average diameters for the nanoparticles of 3.3 ( 0.8 nm. The average size is well below the Bohr exciton diameter of 11.4 nm for ZnSe. X-ray diffraction traces for the scaled up sample are given in Figure 2. Also shown are the peak lines for the cubic simulation of bulk ZnSe. The figures show that the major peaks fit the peak lines of the cubic ZnSe simulation. Similar results have also been obtained for ZnSe crystals obtained with the regular synthesis, although the XRD pattern was weaker due to a smaller amount of sample (not shown). The experimental peaks of the XRD in Figure 2 are characteristically broader than normally found for large crystallites, which is expected for nanoparticles. The pattern is centered on the bulk crystal lines with no other features. There is no evidence that the diffraction signature corresponds to the hexagonal ZnSe phase. Our results are similar to those reported by Hines and Guyot-Sionnest, who reported XRD data for cubic ZnSe nanoparticles of 4.3 and 6.0 nm average diameter.21 Photoluminescence emission spectra were taken with a sample from the regular synthesis, the regular synthesis with the mercaptododecanol capping agent, and the factor 10 scale-up solution. The spectra, shown in Figure 3, show remarkable consistency for both the capped and uncapped samples from the regular synthesis. Both curves show luminescence peaks at 355 nm, and there is noticeable
Notes
Langmuir, Vol. 16, No. 8, 2000 4051
Figure 4. UV-visible spectra of ZnSe nanoparticles and bulk ZnSe.
Figure 3. Photoluminescence of ZnSe nanoparticles.
broadening for the sample from the factor 10 scale-up solution. The broadening is probably due to a higher polydispersity of particle size in the factor 10 scale-up procedure than in the regular synthesis (from TEM data; not shown). Also, along with the polydispersity in size, there is an increase in larger-sized particles, which accounts for the red shift in the factor 10 scale-up. The blue and red shifts in the peak emission imply that one can tailor the emission from the nanoparticles by using the reverse micelle synthesis procedure although additional refinements in the procedure may be needed. The UV-visible spectra for bulk ZnSe, solution samples from the regular synthesis, and solution samples from the factor 10 scale-up are shown in Figure 4. From these spectra one can clearly see the blue shift due to quantum confinement. The absorption onset (λ) is obtained from the intersection of the tangent slopes as shown by the inset in Figure 4. The onset is about 480 nm (2.6 eV) for bulk ZnSe, 410 nm (3.0 eV) for the factor 10 scale-up, and 390 nm (3.2 eV) for the ZnSe obtained by the regular synthesis. The wavelength for the absorption onset can be represented in photon energy E (in eV) by the relationship E ) 1241/λ. The absorption onset for the regular synthesis, shifted by about 0.6 eV, is consistent with respect to other methods of synthesizing ZnSe.18,21 Moreover, the regular synthesis shows a distinct peak in the spectra at 280 nm and a broad shoulder at about 350 nm. These UV-visible results are similar for those found for ZnSe nanoparticles embedded in glasses.33 For such small diameters of the ZnSe nanoparticles, the blue shift
is significant and is readily shown in the UV-visible spectra. The peak observed in the curve for the sample from the regular procedure is indicative of monodispersity in the particle size. The solution contains a large amount of particles whose absorbance is around 280 nm. This is well into the blue regime. However, it should be noted that the solution sample from the factor 10 scale-up does not have the same degree of monodispersity as the sample from the regular synthesis. There is not a distinct hump in the spectra, reflecting a lack of concentration of one particle size. However, the nanoparticles prepared in this manner exhibit quantum confinement since the absorption onset is shifted 0.4 eV for the factor 10 scale-up. It should be noted that the photoluminescence (PL) data substantiate the UV-visible data. The results for the factor 10 scale-up show a shift in the PL maximum from 355 to 395 nm, as well as broadening of the peak. Conclusions Nanoparticles of ZnSe were fabricated using the reverse micelle technique. The nanoparticles exhibit quantum confinement, having radii less than that of the Bohr exciton radius (5.7 nm). X-ray diffraction shows that the nanoparticles are cubic ZnSe. Light scattering and TEM measurements give the diameters of the nanoparticles to be from 3.3 to 3.5 nm. The absorption shift in the UVvisible spectra shows an offset of 0.6 eV, and the peak emission of the nanoparticles is 355 nm, as obtained by photoluminescence. Acknowledgment. The MRSEC Program of the National Science Foundation (Grant DMR-9400354) supported this work through the Center on Polymer Interfaces and Macromolecular Assemblies (CPIMA). F.T.Q. thanks Drs. B. Zimmer and V. Bliznyuk (IBM-Almaden) and V. Leppert, M. Myer, and Y. Yeh (all from UC Davis) for assistance in the identification of ZnSe. LA9909291
