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Synthesis of Amorphous Silicon Colloids by Trisilane Thermolysis in High Temperature Supercritical Solvents Lindsay E. Pell, April D. Schricker, Frederic V. Mikulec, and Brian A. Korgel* Department of Chemical Engineering, Texas Materials Institute, Center for Nano- and Molecular Science and Technology, The University of Texas, Austin, Texas 78712-1062 Received May 28, 2004 Colloidal submicrometer-diameter amorphous silicon (a-Si) particles are synthesized with >90% yield by thermal decomposition of trisilane (Si3H8) in supercritical hexane at temperatures ranging from 400 to 500 °C and pressures up to 345 bar. A range of synthetic conditions was explored to optimize the quality of the product. Under the appropriate synthetic conditions, the colloids are spherical and unagglomerated. The colloids can be produced with average diameters ranging from 50 to 500 nm by manipulating the precursor concentration, temperature, and pressure. Relatively narrow particle size distributions, as measured by transmission electron microscopy (TEM), with standard deviations about the mean as low as ∼ (10% could be obtained in some cases. We explored the thermal annealing of the amorphous silicon particles after isolation from the reactor and found that crystallization to diamond structure silicon occurred at temperatures as low as 650 °C. The amorphous and crystalline materials were characterized by X-ray diffraction and high resolution scanning and transmission electron microscopy.
Colloidal particles of high refractive index materials have a variety of potential uses in coatings and optoelectronic applications. Photonic band gap structures, for example, require materials with n > 2.8.1 Embedding high refractive index colloids in polymers could yield composites with enhanced mechanical, optical, and electrical properties.2 The synthesis of high refractive index colloids has proven to be extremely challenging, and high quality colloidal materials with established synthetic routes such as polystyrene (PS),3 SiO2,4 and TiO25 (nPS ) 1.59; nSiO2 ) 1.4; nTiO2 ) 2.5) have relatively low refractive indices compared to other potential materials, such as amorphous silicon (a-Si) (na-Si > 3.7). In fact, TiO2 is generally referred to as a high refractive index colloid in the literature, even though its refractive index is only ∼1/3 as high as that of a-Si. Here, we report the first synthesis of submicrometersize a-Si colloids, by trisilane pyrolysis in supercritical hexane at 400-500 °C. Like silane (SiH4) and disilane (Si2H6), trisilane (Si3H8) is chemically very stable in inert atmosphere, up to temperatures as high as ∼350 °C. Therefore, synthetic reactions using trisilane as a Si precursor must be carried out either in the gas phase, in very high boiling point solvents under ambient pressure (i.e., squalene (Tb ∼ 400 °C)), or in solvents at elevated pressure. We recently demonstrated the synthesis of crystalline Si nanocrystals6 and nanowires7 in supercritical solvents at temperatures above ∼400 °C using organosilane precursors. Although these reactions produce high quality Si nanocrystals and nanowires, the product yields are often low, being limited by the coproduction of organosilane byproducts. To al* Corresponding author. Phone: (512) 471-5633. Fax: (512) 4717060. E-mail:
[email protected]. (1) Norris, D. J.; Vlasov, Y. A. Adv. Mater. 2001, 13, 371-376. (2) Papadimitrakopoulos, F.; Wisniecki, P.; Bhagwagar, D. E. Chem. Mater. 1997, 9, 2928-2933. (3) Wang, L. Y.; Lin, Y.-J.; Chiu, W.-Y. Synth. Met. 2001, 119, 155156. (4) Sto¨ber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62-68. (5) See, for example: Elden-Assmann, S.; Widoniak, J.; Maret, G. Langmuir 2004, 16, 6-11 and references therein. (6) Holmes, J. D.; Ziegler, K. J.; Doty, R. C.; Pell, L. E.; Johnston, K. P.; Korgel, B. A. J. Am. Chem. Soc. 2001, 123, 3743-3748. (7) Holmes, J. D.; Doty, R. C.; Johnston, K. P.; Korgel, B. A. Science 2000, 287, 1471-1473.
leviate organosilane byproduct formation, we began to explore trisilane as an alternative Si precursor. We found that trisilane reactions produce only amorphous Si (a-Si) colloids with average diameters ranging between 60 and 450 nm, depending on the reaction conditions, with very high yields of >90% conversion of trisilane to a-Si colloidal product. The colloidal product is of high quality: the particles are not aggregated, they exhibit spherical shape, and they have relatively narrow size distributions with standard deviations about the mean diameter as low as ∼(10%. The a-Si colloids were produced from solutions of anhydrous hexane (Aldrich) and trisilane (Voltaix) prepared in a nitrogen-filled glovebox. The reaction solutions were loaded into a 10 mL high pressure titanium reactor, which was sealed and removed from the glovebox to be placed into a brass block that had been preheated by cartridge heaters. The heating block was typically heated ∼50-75 °C higher than the desired reaction temperature. Two thermocouples were used to measure the temperature: one was placed into the reactor, and the other was in the heating block. The reactor required ∼5 min to reach the reaction temperatures. The reactor pressure was determined by the reaction temperature and the solution volume loaded in the reactor. The highest quality colloidal product was produced at pressures between 200 and 400 bar. At pressures above and below this range, a polymeric product was obtained instead of colloidal particles. All reactions were carried out well above the critical point of hexane (Tc ) 235 °C; Pc ) 30 bar). (Note: The titanium reactors are not structurally reliable above 690 bar; therefore, care was taken to keep the reactor pressure below this point!) The reactor was placed in the heating block for a total of 10-20 min, which was found to be sufficient for the reaction to come to completion. The reaction was quenched by removing the reactor from the brass block and submerging it into an ice bath. The product was extracted from the reactor with hexane and centrifuged (9000 rpm, 10 min). The precipitate could be redispersed in hexane or chloroform with mild sonication to give an optically opaque suspension. Figure 1 shows transmission and high resolution scanning electron microscopy [TEM (Figure 1A,C) and
10.1021/la048671o CCC: $27.50 © 2004 American Chemical Society Published on Web 07/08/2004
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Figure 1. a-Si colloids obtained from trisilane pyrolysis in hexane at 500 °C and 276 bar (10 min residence time). (A and C) TEM images of particles obtained from 10 and 1 mM trisilane. (B) HRSEM image of a-Si colloids formed from 10 mM trisilane in hexane at 500 °C and 414 bar (10 min residence time). (D) XRD pattern of the a-Si colloids imaged in part B. TEM images were obtained using either a Philips 208 transmission electron microscope with an 80 kV accelerating voltage or a JEOL 2010F transmission electron microscope operated at 200 kV. XRD measurements were taken using a Bruker-Nonius D8 Advance Theta-2 Theta powder diffractometer with KR radiation and a scintillation detector. HRSEM images were obtained by drop casting a hexane dispersion of a-Si colloids onto a carbon substrate using a Leo 1530 scanning electron microscope operated at a 4 kV accelerating voltage.
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Figure 2. (A and B) HRSEM images and (E) XRD data of annealed colloids using molybdenum boats as the substrate, while (C and D) HRTEM was performed on annealed particles that had been redispersed and deposited on a TEM grid. The unlabeled peaks in part E correspond to the molybdenum substrate and a molybdenum silicon alloy that forms during annealing at the point of contact between the a-Si colloids and the Mo substrate. The DTA scan of the original sample, shown in part F, exhibits a crystallization temperature of 885 °C.
HRSEM (Figure 1B)] images and an X-ray diffraction [XRD (Figure 1D)] pattern of the a-Si colloids synthesized in hexane at 500 °C and 276 bar from 10 mM (Figure 1A) and 1 mM (Figure 1C) trisilane solutions. The colloids are spherical and reasonably size-monodisperse. The particle diameter, dp, is sensitive to the precursor concentration, with larger particles (dp ) 257 ( 46 nm) being produced at higher concentrations. The lower concentration reactions yielded nanocrystals 59 ( 14 nm in diameter. The XRD pattern exhibits broad diffraction peaks at 2θ ) 28 and 52°, characteristic of amorphous Si.8 High resolution transmission electron microscopy (HRTEM) also confirmed their amorphous core. Fourier transform infrared (FTIR) spectra exhibit a very small Si-H stretch at 2100 cm-1 indicating some residual hydrogen in the core (see the Supporting Information).9 The product yield is very high, ranging from 93.7 to 91.9%, assuming 0 and 20% hydrogenation, respectively. Silane pyrolysis in the gas phase at ∼550 °C produces partially hydrogenated a-Si nanoparticles, which are highly aggregated with broad log-normal size distributions.10 In contrast to the aerosol particles, which are up to tens of nanometers in diameter, the a-Si colloids produced in solution are much larger, relatively sizemonodisperse, and not aggregated. The solution-phase synthesis provides much higher reactant concentrations and residence times than synthesis in the gas phase, which promotes the formation of larger primary particles. At 500 °C, the most likely mechanism for trisilane pyrolysis is through heterogeneous insertion at a hydrogenterminated Si surface site11 and not homogeneous decomposition to lower silanes. This diffusion-limited growth
mechanism would explain the relatively narrow size distribution.12 Furthermore, the insertion reaction expels hydrogen from the trisilane and Si particle surface; however, complete loss of secondary hydrogen does not necessarily occur,13 which is consistent with the presence of residual hydrogen in the particles. We found that the reaction temperature required to produce crystalline Si colloids from trisilane is significantly higher than 500 °C. Growth temperatures in the gas phase must exceed ∼750 °C to obtain crystalline Si particles.10 These extreme temperatures are not experimentally accessible in supercritical hexane due to the thermal instability of the solvent. However, using toluene, we could obtain polycrystalline nanometer-size Si particles at ∼675 °C. At these temperatures, however, significant amounts of toluene also pyrolyze, making these reactions impractical for particle production. To better determine the crystallization temperature of the a-Si colloids, samples were annealed as a dry powder either in flat-bottom molybdenum (Mo) boats (0.005 in. thick) (purchased from Midwest Tungsten Service) heated by applying 50-100 A of current in a Denton vacuum chamber evaporator (DV-502A, 0.002-0.004 mTorr) or in an alumina crucible using a Perkin-Elmer Series 7 differential thermal analyzer at a scan rate of 10 deg/min, under a nitrogen atmosphere (20 mL/min nitrogen purge). Figure 2 shows microscopy images and XRD data of annealed a-Si colloids. Table 1 summarizes the XRD data obtained from ∼285 nm diameter a-Si colloids annealed in Mo boats under
(8) Mamiya, M.; Takei, H.; Kikuchi, M.; Uyeda, C. J. Cryst. Growth 2001, 229, 457-461. (9) Sailor, M. J.; Lee, E. J. Adv. Mater. 1997, 9, 783-793. (10) Onischuk, A. A.; Strunin, V. P.; Ushakova, M. A.; Panfilov, V. N. J. Aerosol Sci. 1997, 28, 207-222.
(11) Swihart, M. T.; Girshick, S. L. J. Phys. Chem. B 1999, 103, 64-76. (12) Shah, P. S.; Husain, S.; Johnston, K. P.; Korgel, B. A. J. Phys. Chem. B 2002, 106, 12178-12185. (13) Onischuk, A. A.; Panfilov, V. N. Usp. Khim. 2001, 70, 368-381.
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Table 1. Crystal Domain Sizes of Si Colloids Determined by XRD after Thermal Treatment in Mo Boats under Vacuuma annealing conditions current (A)
T (°C)
0 50 60 70 80 90 100
unannealed 226 482 589 648 703 743
Si(111) fwhm (degrees) domain size (nm) ∼5 ∼4 ∼4 ∼2.5 0.516 0.435 0.212
amorphous amorphous amorphous amorphous 25.4 32.9 168.6
a The particles were synthesized from [Si H ] ) 10 mM at 276 3 8 bar and 500 °C. The colloids were annealed for 1 min after a 15 s stabilization period. The substrate temperature was determined using an IRCON Series R infrared thermometer calibrated with a pure Al standard with an emissivity of 0.35. The domain size was calculated using the Si(111) full width at half-maximum (fwhm) and the Scherrer equation.
vacuum at temperatures ranging from 220 to 740 °C. TEM and XRD indicate that the particles begin to crystallize at ∼650 °C. At 650 °C, as shown in Figure 2E, the (111) and (220) diffraction peaks for diamond cubic Si at 2θ ) 28.4 and 47.3° appear and narrow significantly with increasing annealing temperature. The crystallization temperature measured by differential thermal analysis (DTA) was higher than what was measured in the Mo boats; the exothermic event at 885 °C in the DTA scan in Figure 2F corresponds to Si crystallization. The lower crystallization temperature on Mo substrates most likely results from Mo metal-induced crystallization. It is well-known that low temperature crystallization of amorphous silicon can occur in the presence of metals with Si solubility; for example, Al14 (14) Al-Dhafiri, A. M. J. Saudi Chem. Soc. 2002, 6, 365-376.
crystallizes Si at temperatures as low as ∼350 °C. HRSEM and TEM imaging of the crystallized Si colloids also revealed a morphological difference in particles annealed on Mo and alumina substrates. The colloids crystallized on Mo substrates under vacuum break apart into smaller domains of crystalline Si, as seen in Figure 2B; whereas the Si colloids crystallized under nitrogen in alumina boats do not, remaining intact with polycrystalline Si cores. The morphological difference may be related to the underlying substrate or the different annealing atmosphere; it is possible that aggressive hydrogen evolution from the Si colloids under vacuum may “break apart” the large colloids into smaller particles, which would not be expected under nitrogen. Trisilane pyrolysis in supercritical hexane yields submicrometer a-Si colloids that are qualitatively different from a-Si particles produced by silane pyrolysis in the gas phase at similar temperatures. The solvent environment provides high precursor concentrations, long residence times, and slower interparticle collisions, with product yields over 90%. High temperature supercritical solvents provide a potentially useful temperature range for the synthesis of a variety of ceramic colloidal materials. Acknowledgment. We thank A. Saunders for the HRSEM image in Figure 1B and D. Aherne and D. Jurbergs for helpful discussions. We also thank the NSF, the Welch Foundation, and the Texas Higher Education Coordinating Board through their ATP program for partial support of this work. Supporting Information Available: FTIR spectrum and XRD patterns. This material is available free of charge via the Internet at http://pubs.acs.org. LA048671O
