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Synthesis and Size Control of Luminescent ZnSe Nanocrystals by a Microemulsion-Gas Contacting Technique Georgios N. Karanikolos,† Paschalis Alexandridis,† Grigorios Itskos,‡ Athos Petrou,‡ and T. J. Mountziaris*,†,§ Department of Chemical and Biological Engineering and Department of Physics, University at Buffalo, The State University of New York, Buffalo, New York 14260, and National Science Foundation, Arlington, Virginia 22230 Received July 30, 2003. In Final Form: December 16, 2003 A scalable method for controlled synthesis of luminescent compound semiconductor nanocrystals (quantum dots) using microemulsion-gas contacting at room temperature is reported. The technique exploits the dispersed phase of a microemulsion to form numerous identical nanoreactors. ZnSe quantum dots were synthesized by reacting hydrogen selenide gas with diethylzinc dissolved in the heptane nanodroplets of a microemulsion formed by self-assembly of a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) amphiphilic block copolymer in formamide. A single nanocrystal is grown in each nanodroplet, thus allowing good control of particle size by manipulation of the initial diethylzinc concentration in the heptane. The ZnSe nanocrystals exhibit size-dependent luminescence and excellent photostability.
Introduction Semiconductor nanocrystals (quantum dots) are exciting materials whose optical and electronic properties can be manipulated by changing their size or composition.1,2 When the size of the nanocrystals becomes smaller than the corresponding de Broglie wavelength or Bohr radius (mean separation of an optically excited electron-hole pair), quantum confinement phenomena take place and change the nanocrystal properties dramatically. II-VI quantum dots (CdSe, CdS, ZnS, or ZnSe) with sizes of a few nanometers exhibit size-dependent luminescence, broad excitation by all wavelengths smaller than the emission wavelength, high brightness, narrow and symmetric emission, and excellent photostability.1,2 In addition to playing an important role in fundamental studies on solid-state physics,3 quantum dots can be also used in photovoltaic devices,4 in photodetectors,5 and as fluorescent biological labels.6 The most common synthesis route for II-VI nanocrystals involves reactions between organometallic compounds in a trioctylphosphine (TOP)/trioctylphosphine oxide (TOPO) and/or hexadecylamine (HDA) coordination solvent carried out in small batch reactors operating at ∼300 °C. CdSe and CdS quantum dots have been the most common materials grown by this technique.7 Luminescent ZnSe nanocrystals exhibiting high quantum yield8,9 and * Corresponding author. E-mail:
[email protected] or
[email protected]. † Department of Chemical and Biological Engineering, University at Buffalo. ‡ Department of Physics, University at Buffalo. § National Science Foundation. (1) Alivisatos, A. P. Science 1996, 271, 933. (2) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. Rev. Mater. Sci. 2000, 30, 545. (3) Empedocles, S. A.; Neuhauser, R.; Shimizu, K.; Bawendi, M. G. Adv. Mater. 1999, 11, 1243. (4) Huynh, W. U.; Peng, X.; Alivisatos, A. P. Adv. Mater. 1999, 11, 923. (5) Towe, E.; Pan, D. IEEE J. Sel. Top. Quantum Electron. 2000, 6, 408. (6) Michalet, X.; Pinaud, F.; Lacoste, T. D.; Dahan, M.; Bruchez, M. P.; Alivisatos, A. P.; Weiss, S. Single Mol. 2001, 2, 261. (7) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706.
(Zn,Mn)Se diluted magnetic nanocrystals10 have also been grown. The growth of monodisperse nanocrystal populations requires instantaneous injection and mixing of the reactants, followed by uniform nucleation over the entire mass of the solvent and perfect mixing in the entire reactor during processing. Selective precipitation techniques are typically used after synthesis to narrow down the size distribution of the nanocrystals.2 Other reported techniques for growing ZnSe nanocrystals include arrested precipitation,11 sol-gel processing,12 sonochemical processing,13 growth in reverse micelles,14 and vapor-phase synthesis.15 The use of a template is desirable because it creates favorable conditions for growing monodisperse particle populations. For example, precise control of particle size and shape has been recently demonstrated during growth of colloidal crystals in aqueous droplets suspended in fluorinated oil.16 Furthermore, monodisperse populations of Si quantum dots with surfaces passivated by an organic monolayer have been grown by thermally degrading diphenylsilane in supercritical octanol.17 ZnSe nanocrystals have been grown in bis-2-ethylhexylsulfosuccinate sodium salt (AOT) reverse micelles by reacting zinc perchlorate hexahydrate and sodium selenide.14 Under ideal conditions, reverse micelles can provide a template for precise control of particle size. In practice, the fast dynamics of droplet-droplet coalescence in water-in-oil (8) Hines, M. A.; Guyot-Sionnest, P. J. Phys. Chem. B 1998, 102, 3655. (9) Revaprasadu, N.; Malik, M. A.; O’Brien, P.; Zulu, M. M.; Wakefield, G. J. Mater. Chem. 1998, 8, 1885. (10) Norris, D. J.; Yao, N.; Charnock, F. T.; Kennedy, T. A. Nano Lett. 2001, 1, 3. (11) Chestnoy, N.; Hull, R.; Brus, L. E. J. Chem. Phys. 1986, 85, 2237. (12) Li, G.; Nogami, M. J. Appl. Phys. 1994, 75, 4276. (13) Zhu, J.; Koltypin, Y.; Gedanken, A. Chem. Mater. 2000, 12, 73. (14) Quinlan, F. T.; Kuther, J.; Tremel, W.; Knoll, W.; Risbud, S.; Stroeve, P. Langmuir 2000, 16, 4049. (15) Sarigiannis, D.; Peck, J. D.; Kioseoglou, G.; Petrou, A.; Mountziaris, T. J. Appl. Phys. Lett. 2002, 80, 4024. (16) Velev, O. D.; Lenhoff, A. M.; Kaler, E. W. Science 2000, 287, 2240. (17) Holmes, J. D.; Ziegler, K. J.; Doty, R. C.; Pell, L. E.; Johnston, K. P.; Korgel, B. A. J. Am. Chem. Soc. 2001, 123, 3743.
10.1021/la035397+ CCC: $27.50 © 2004 American Chemical Society Published on Web 01/08/2004
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Figure 1. Schematic representation of the microemulsiongas contacting technique for ZnSe nanocrystal synthesis. Hydrogen selenide gas is bubbled through the microemulsion, diffuses into the heptane nanodroplets, and reacts with diethylzinc. The ZnSe nuclei (clusters) formed in each droplet grow by surface reactions and particle-particle coalescence, leading to the formation of a single nanocrystal per nanodroplet.
microemulsions typically leads to the formation of droplet clusters and polydisperse particle populations.18 The technique presented in this paper utilizes heptanein-formamide microemulsions and reactions between a group-II metal-alkyl liquid, dissolved in the dispersed phase, and a group-VI hydride gas, bubbled through the microemulsion. It eliminates problems related to dropletdroplet coalescence, provides good control of particle size, and produces luminescent nanocrystals with size-dependent properties. Experimental Section A schematic representation of the technique is shown in Figure 1. A microemulsion was formed by self-assembly using a solution of diethylzinc ((C2H5)2Zn) in n-heptane (n-C7H16) as the dispersed oil phase, formamide (CH3NO) as the polar continuous phase, and an amphiphilic block copolymer, poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) or PEO-PPO-PEO, as the surfactant. PEO is the formamide-soluble block, and PPO the formamide-incompatible block.19 PEO-PPO block copolymers represent an exciting class of amphiphilic molecules with high versatility, in terms of self-assembly and corresponding practical applications.20 Imhof et al.21 tested a variety of different combinations of amphiphiles with nonaqueous polar solvents and reported that PEO-PPO-PEO block copolymers form very stable emulsions in formamide. Formamide was used instead of water, because it is sufficiently polar to be immiscible in heptane and it does not react with diethylzinc. Hydrogen selenide (H2Se) gas diluted in hydrogen was bubbled through the microemulsion at room temperature and atmospheric pressure, diffused into the nanodroplets, and reacted with diethylzinc to yield ZnSe and ethane (C2H6). By assuming (and subsequently verifying) that a single ZnSe particle is formed in each nanodroplet, the initial concentration of diethylzinc was used to specify the particle size. All chemicals were used “as received” without any additional purification. Care was taken to avoid exposure of the hygroscopic formamide and PEO-PPO-PEO to atmospheric moisture. Standard airless techniques were used to avoid exposure of diethylzinc to oxygen and moisture. Diethylzinc (1 M solution in n-heptane), formamide (99.5+%), and n-heptane (99%) were purchased from Aldrich. Electronic-grade hydrogen selenide gas (in the form of a 5% mixture with hydrogen) was purchased from Solkatronic Chemicals. Pluronic P105 PEO-PPO-PEO block copolymer (EO37PO58EO37 with MW of 6500 and 50% PEO content) was obtained from BASF Corp. The diethylzinc-containing microemulsions were formed as follows: (a) 3.33 g of PEO-PPO-PEO was added to 20 mL of (18) Zhao, H.; Douglas, E. P.; Harrison, B. S.; Schanze, K. S. Langmuir 2001, 17, 8428. (19) Alexandridis, P.; Yang, L. Macromolecules 2000, 33, 3382. (20) Amphiphilic Block Copolymers: Self-Assembly and Applications; Alexandridis, P., Lindman, B., Eds.; Elsevier Science B.V.: Amsterdam, 2000. (21) Imhof, A.; Pine, D. J. J. Colloid Interface Sci. 1997, 192, 368.
Figure 2. X-ray diffraction pattern from ZnSe particle aggregates obtained by reacting diethylzinc (diluted in heptane) with hydrogen selenide gas (diluted in hydrogen) at room temperature. The vertical lines at 27.2°, 45.2°, and 53.6° correspond to the expected diffraction angles by the (111), (220), and (311) planes of cubic ZnSe. formamide, and the mixture was stirred for 1.5 h. (b) Diethylzincheptane solution (0.5 mL) was added to PEO-PPO-PEO/ formamide solution under nitrogen. (c) The final mixture was sonicated for 1.5 h. The resulting liquid was transparent and homogeneous, an indication that a microemulsion was formed.22 It was subsequently transferred to the reactor (located in a vented enclosure) under nitrogen. A flow of 20 sccm 5% hydrogen selenide in hydrogen was established, and the gas was allowed to bubble through the microemulsion for 15 min. This time was found to be sufficient for converting all the diethylzinc to ZnSe. The reactor was subsequently purged with nitrogen for 1 h to remove all traces of hydrogen selenide. The gases exiting the reactor were passed through a cracking furnace and a bed of adsorbents before release into a fume hood. A hydride detector was used to ensure personnel safety. Photoluminescence (PL) emission spectra were obtained by loading samples from the reactor into quartz cuvettes and analyzing them using a 0.5 m single-stage spectrometer (CVI Laser Corp.), equipped with a thermoelectrically cooled multichannel CCD detector (Camera AD-205 working in the wavelength range of 200-1100 nm). A 325 nm line of a 20 mW He-Cd UV laser (Melles Giot) was used to excite the nanocrystals. Transmission spectra from samples loaded in quartz cuvettes were obtained by using a 150 W xenon lamp (UV) and a 0.35 m scanning monochromator (McPherson) with PMT phase sensitive detection. The samples used for transmission electron microscopy were prepared by placing a drop of the processed microemulsion on a 400-mesh carbon-coated copper grid (Ernest F. Fullam, Inc.) and leaving it under a vacuum for 48 h to evaporate the solvents. The instrument used was a JEOL JEM 2010 high-resolution transmission electron microscope, operated at 200 kV, with a point-to-point resolution of 0.193 nm. Dynamic light scatterering was performed to measure the microemulsion droplet size, prior to each synthesis experiment, using a model 95 argon ion laser (Lexel Corp.) with a BI-200SM goniometer detector (Brookhaven Instruments Corp.) operating at an angle of 90°. X-ray diffraction was performed using a Siemens D500 XR diffractometer.
Results and Discussion To investigate the feasibility of the proposed chemistry for room-temperature synthesis of crystalline ZnSe, a 5% hydrogen selenide/hydrogen gas mixture was first bubbled through a 0.1 M solution of diethylzinc in heptane at room temperature and atmospheric pressure. This “proof-ofprinciple” experiment did not involve a microemulsion and produced a suspension of ZnSe particle aggregates in heptane. Samples taken from the suspension were placed on clean quartz wafers, and the heptane was evaporated under a vacuum. The resulting deposits of ZnSe particles on quartz were analyzed by X-ray diffraction. The diffraction peaks obtained (Figure 2) match the standard peaks corresponding to cubic (zinc blende) ZnSe, confirm(22) Danielsson, I.; Lindman, B. Colloids Surf. 1981, 3, 391.
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ing that the reaction does yield crystalline ZnSe at room temperature. No indication of a hexagonal (wurtzite) structure was found. We subsequently conducted experiments using microemulsions in order to precisely control the particle size. The effect of surfactant to dispersed phase molar ratio on the droplet size of the heptane/PEO-PPO-PEO/ formamide microemulsions was investigated first. We measured the size and uniformity of the heptane nanodroplets using dynamic light scattering. A mixture of 12.6 wt % PEO-PPO-PEO and 1.3 wt % n-heptane in formamide was found to form stable microemulsions with an average droplet diameter of 40 nm and was used for the nanocrystal growth experiments. We estimated the diethylzinc concentration in the heptane to obtain a single ZnSe crystal of a particular size in each droplet, after full conversion. The underlying assumption is that, even if several clusters nucleate in a single droplet, cluster diffusion within that droplet will enable the formation of a single nanocrystal through particle-particle coalescence. The overall reaction forming ZnSe is H2Se(g) + Zn(C2H5)2(l) f ZnSe(s) + 2C2H6(g). It occurs spontaneously at room temperature and is exothermic with heat of reaction of -380 kJ/mol.23,24 A similar reaction has been used by our group for growing single crystalline thin films of ZnSe by metallo-organic vapor-phase epitaxy (MOVPE)25,26 and ZnSe nanoparticles by vapor-phase processing.15 In the experiments discussed here, the nucleation of ZnSe clusters most probably occurs simultaneously at different locations inside each droplet. The clusters subsequently grow by reaction with precursors (surface growth reactions) and by cluster-cluster coalescence. A single ZnSe particle eventually forms in each droplet through coalescence of smaller clusters. The energy released by the chemical reactions and by the particleparticle coalescence27 is responsible for raising the temperature of the nanoparticles and converting them to crystals through annealing. The annealing mechanism is aided by a depression of the apparent melting point of ZnSe nanoparticles with size.15 As a result, the annealing of ZnSe nanoparticles requires a smaller temperature increase than the one required for bulk crystals. The nanoparticle heating from the energy released during growth is very localized, and the macroscopically observed temperature of the mixture remains constant at room temperature. Three concentrations of diethylzinc in heptane were used to obtain nanocrystal populations for photoluminescence experiments: 0.3, 0.03, and 0.003 M. The estimated particle size assuming 40 nm droplets is 8, 3.7, and 1.7 nm, respectively. The ZnSe bulk exciton Bohr diameter, below which confinement phenomena are expected, has been reported to be 9 nm.28 The PL emission spectra of the three representative particle populations obtained from the experiments are shown in Figure 3a. The PL spectra exhibit size-dependence with peaks at 449 nm for curve 1, 444 nm for curve 2, and 439 nm for curve 3 corresponding (23) CRC Handbook of Chemistry and Physics, 3rd electronic ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 2000. (24) NIST Chemistry WebBook: NIST Standard Reference Database Number 69; July 2001 Release; National Institute of Standards and Technology: Gaithersburg, MD, 2001. (25) Jones, A. C. J. Cryst. Growth 1993, 129, 728. (26) Peck, J.; Mountziaris, T. J.; Stoltz, S.; Petrou, A.; Mattocks, P. G. J. Cryst. Growth 1997, 170, 523. (27) Lehtinen, K. E. J.; Zachariah, M. R. Phys. Rev. B 2001, 63, 205402-1. (28) Leppert, V. J.; Mahamuni, S.; Kumbhojkar, N. R.; Risbud, S. H. Mater. Sci. Eng. B 1998, 52, 89.
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Figure 3. (a) Room-temperature photoluminescence spectra of ZnSe quantum dots obtained by microemulsion-gas contacting. The emission wavelengths are blue-shifted compared to that of bulk ZnSe (460 nm) as the particle size decreases. Curves 1-3 correspond to 0.3, 0.03, and 0.003 M diethylzinc concentration in heptane. The inset is a schematic of a ZnSe quantum dot encapsulated in a heptane nanodroplet. (b) Superposition of photoluminescence and transmission spectra for curve 3 in panel a.
to an initial concentration of 0.3, 0.03, and 0.003 M diethylzinc in heptane. There is a systematic blue shift of the emission peak from sample 1 to sample 3, which clearly indicates that the average size of the ZnSe nanocrystals decreases. Each peak is single, symmetric, and narrow, indicating good control of particle size and crystal structure, without any postprocessing step. The width of the peaks compares well to a variety of reported data from ZnSe nanocrystal populations synthesized by other techniques.8,9,13,14 Some of the results reported in the literature were obtained after postprocessing of the particles to narrow down the size distribution. The PL data reported here are from samples that did not undergo any postprocessing.
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Figure 4. TEM (a) and HR-TEM (b) images of ZnSe nanocrystals with an average diameter of 6 nm obtained by processing a 0.3 M diethylzinc solution in 40 nm heptane nanodroplets of a heptane/PEO-PPO-PEO/formamide microemulsion.
The evidence of a low-energy shoulder observed on the three samples of Figure 3a is attributed to emission from larger ZnSe particle aggregates, formed by coalescence of nanoparticles on the container (vial) walls, where the structure of the microemulsion may be disturbed. A comparison between the PL emission and transmission spectra for sample 3 is shown in Figure 3b. The limits of the absorption spectrum are indicated by the vertical arrows a (at 428 nm) and c (at 453 nm). Feature b (at 467 nm) is attributed to bulk ZnSe due to the presence of
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particle aggregates near the wall of the container, as discussed above. The peak of the emission (arrow d) (at 439 nm) lies in the middle of the absorption wavelength interval between arrows a and c. The nanocrystal-loaded microemulsions exhibit excellent stability. Photoluminescence spectra obtained from samples stored for 1 month after preparation exhibited no distinguishable variations, when compared to the original ones. Additional characterization of the ZnSe nanocrystals was performed by using transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM). The evaporation of solvents leaves a residue on the TEM grid consisting of the Pluronic P105 block copolymer with nanocrystals embedded in it. Figure 4 shows TEM and HR-TEM images of ZnSe nanocrystals corresponding to a sample prepared using a 0.3 M initial diethylzinc concentration in heptane. The resulting particles are crystalline with an average diameter of 6 ( 0.7 nm (estimated from the TEM image). This indicates that the initial hypothesis that a single nanocrystal is formed in each nanodroplet is reasonable. This technique may also offer flexibility for functionalization of the surface of the nanocrystals. For example, thiol-conjugated molecules can be dissolved in the heptane nanodroplets for in situ functionalization of the nanocrystals upon synthesis. Presently, nanocrystal functionalization requires several postgrowth processing steps. In conclusion, a new microemulsion-gas contacting technique has been developed for controlled synthesis of compound semiconductor nanocrystals. The technique utilizes the dispersed phase of a heptane/PEO-PPOPEO/formamide microemulsion to form numerous identical nanoreactors, thus enabling precise control of particle size. It employs reactions between group-II alkyls and group-VI hydrides similar to the ones used by the microelectronics industry for MOVPE of thin films and can be scaled up for commercial production. The growth of luminescent ZnSe quantum dots was demonstrated by reacting diethylzinc, dissolved in the heptane nanodroplets, with hydrogen selenide gas, bubbled through the microemulsion. Nanocrystal size and luminescence were tuned by changing the initial concentration of diethylzinc in the heptane nanodroplets. The nanocrystal-loaded microemulsions are stable, and their photoluminescence spectra remain unchanged over a period of several months. Acknowledgment. We thank C. Sarigiannidis, J. Wang, M. Koutsona, L. Guo, D. Borden, K.-T. Yong, and K.-K. Chain for assistance with the experiments and for helpful discussions. This work was partially supported by SUNY-Buffalo (IRCAF Seed Grant Program) and by the National Science Foundation. LA035397+
