6190
J. Phys. Chem. B 2006, 110, 6190-6197
Surface Chemistry of Aerosolized Nanoparticles:Thermal Oxidation of Silicon Ying-Chih Liao, Amanda M. Nienow, and Jeffrey T. Roberts* UniVersity of Minnesota, Department of Chemistry, Minneapolis, Minnesota 55455 ReceiVed: July 29, 2005; In Final Form: February 6, 2006
The kinetics of reaction between silicon nanoparticles and molecular oxygen were studied by tandem differential mobility analysis . Aerosolized silicon nanoparticles were extracted from a low-pressure silane plasma into an atmospheric pressure aerosol flow tube reactor. Particles were initially passed through a differential mobility analyzer that was set to transmit only those particles having mobility diameters of approximately 10 nm. The monodisperse particle streams were mixed with oxygen/nitrogen mixtures of different oxygen volume fractions and allowed to react over a broad temperature range (600-1100 °C) for approximately one second. Particles were size-classified after reaction with a second differential mobility analyzer. The particle mobility diameters increased upon oxidation by up to 1.3 nm, depending on the oxygen volume fraction and the reaction temperature. Oxidation is described by a kinetic model that considers both oxygen diffusion and surface reaction, with diffusion becoming important after formation of a 0.5 nm thick oxide monolayer.
Introduction Nanoscale science is predicted to become a critical driver of economic growth and technological development in the early 21st century.1-3 Increasing demands for nanoscale devices will require new developments not only in nanoparticle synthesis but also in the control of interfacial reactivity. Nanoparticles, by their very nature, have high surface area-to-volume ratios,4 and their surfaces are, therefore, susceptible to reactions with ambient gases. Moreover, many critical properties of nanoparticles are wholly or partly dictated by the surface. For these and other reasons, it is important to develop ways of understanding surface reactivity and controlling surface functionality of nanoparticles. Many methods of nanoparticle production have been reported in the literature. One important synthetic approach is gas-toparticle conversion, which can occur in furnaces5-8 and flames,9,10 thermal plasmas,11-13 and nonthermal plasmas.14 Recently, a new method for synthesizing crystalline silicon nanoparticles from silane (SiH4) in a low-pressure plasma reactor was reported.14 This method is simple in design and is capable of producing crystalline silicon nanoparticles less than 5 nm in diameter at rates of up to several milligrams per hour. Because of the fast production rate and the nonagglomerated nature of products, this method has the potential of becoming a synthetically useful route to silicon nanoparticles for different applications. One potential problem with silicon nanoparticle-based systems is that the silicon surface is susceptible to oxidation by oxygen and reaction with gas-phase water. Reactions with these and other ambient gases could limit the application of nanoparticle silicon in certain types of devices, for instance, those employing silicon as quantum dot emitters. The oxidation of silicon wafers by molecular oxygen has been studied for at least forty years. In 1965, Deal and Grove proposed a kinetic model for oxidation that postulates a mechanism in which oxidation occurs via oxygen migration through an oxide layer and eventual reaction with silicon at a * To whom correspondence should be addressed. Telephone: (612) 6241880. Fax: (612) 626-8659. E-mail:
[email protected].
silicon-silicon oxide interface.15 The model accurately describes the kinetics of thick oxide layer growth under wet (i.e., oxidation in the presence of water vapor) and dry conditions.15,16 However, the Deal-Grove model does not adequately describe the growth of ultrathin oxide layers (800 °C). Smaller ∆Dp are predicted than are measured at low furnace setting temperatures, possibly because oxidation at low temperatures results in suboxides of lower density than SiO2. In any case, the model does successfully capture the functional dependence of Dp on FO2, especially the abrupt transition from fast to slow oxidation rate at ∆Dp ∼ 0.3 nm. To understand the importance of the first oxide monolayer on oxidation kinetics, predictions from the shrinking core model with oxygen diffusion throughout the oxidation are compared with measurements in Figure 7b. The best-fit predictions do not reproduce the experimental data as well as do those in Figure 7a. Moreover, this whole-time diffusion model cannot predict the abrupt transition from fast to slow oxidation at ∆Dp ∼ 0.3 nm. Thus, the hypothesis of negligible diffusion resistance during the formation of the first oxide monolayer gives a more reasonable explanation for silicon oxidation kinetics. It is interesting to compare parameters that emerge from the fitting procedure described here to those used in the standard Deal-Grove model. For the nanoparticle system, the oxidation rate prefactor k0 is larger and the activation energy Ereact is smaller than those suggested by Deal and Grove, as shown in Table 3. It is well-known that the Deal-Grove model generally does not apply to ultrathin oxide films. Oxidation rates on silicon wafers have been found to increase exponentially as the SiO2
6196 J. Phys. Chem. B, Vol. 110, No. 12, 2006
Liao et al. ity analysis (T-DMA). Particles of initial mobility diameter 10 nm were reacted with O2 over broad range of temperature and oxygen volume fraction. Oxidation-induced increases in particle mobility diameter, which were as large as 1.3 nm depending on the oxygen volume fraction and the reaction temperature, were measured accurately online by tandem differential mobility analysis. An abrupt transition from fast to slow oxidation rate at ∆Dp ∼ 0.3 nm were observed in the experiment. This transition can be explained by a simple shrinking core model. Before the formation of a 0.5 nm thick oxide monolayer, oxygen molecules react directly with silicon surface and leads to faster reaction. Once the silicon nanoparticle surface is covered with an oxide monolayer, oxygen molecules must diffuse through the SiO2 layer, and the reaction kinetics becomes diffusion-controlled. A rate constant of 0.015 m/s and activation energy of 106 kJ/mol are reported for the thermal oxidation process. This is the first time, to our knowledge, that accurate thermal oxidation data in such thin film regime (2 nm), other effects, such as stress from density change34,35 and transient effects of a moving Si/SiO2 interface,36 may increase reaction resistance and, hence, the effective oxidation activation energy. The time scales calculated from the fitted parameters support the steady-state kinematics assumption that is implicit in the model. The diffusion time scale τD ≡ FjSiO2‚Ri2/(De0‚C*) is 16 s in these experiments, and the reaction time scale, τk ≡ FjSi‚Ri/ (k0‚C*) ) 0.29 s, is much smaller. Therefore, after the formation of the first oxide monolayer, film growth slows and diffusion becomes progressively more important than the interfacial reaction. Due to the slower supply from oxygen diffusion flux, the oxygen concentration near the Si/SiO2 interface may eventually be close to zero. Thus, the concentration profile is likely to be at steady-state, i.e., the diffusion flux is constant in the SiO2 shell, and the oxygen uptake rate at the Si/SiO2 interface might be nearly equal to the diffusion flux. Conclusions The kinetics of thermal oxidation of aerosolized monodisperse silicon nanoparticles were studied by tandem differential mobil-
Supporting Information Available: T-DMA scans of 10.3 nm initial mobility diameter silicon particles subjected to different oxidation conditions at different furnace setting temperatures. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Mazzola, L. Nat. Biotechnol. 2003, 21, 1137. (2) Paull, R.; Wolfe, J.; He´bert, P.; Sinkula, M. Nat. Biotechnol. 2003, 21, 1144. (3) Hassan, M. H. A. Science 2005, 309, 65. (4) Poole, C. P.; Owens, F. J. Introduction to nanotechnology; J. Wiley: Hoboken, NJ, 2003. (5) Lin, S. Y.; Ferg, J.; Biswas, P.; Enzweiler, R.; Boolchand, P. J. Magn. Magn. Mater. 1996, 159, 147. (6) McMillin, B. K.; Biswas, P.; Zachariah, M. R. J. Mater. Res. 1996, 11, 1552. (7) Wang, Z. M.; Yang, G.; Biswas, P.; Bresser, W.; Boolchand, P. Powder Technol. 2001, 114, 197. (8) Axelbaum, R. Powder Metall. 2000, 43, 323. (9) Yang, S.; Yi, J. H.; Son, S.; Jang, J.; Altman, I. S.; Pikhitsa, P. V.; Choi, M. Appl. Phys. Lett. 2003, 83, 4842. (10) Lee, D.; Yang, S.; Choi, M. Appl. Phys. Lett. 2001, 79, 2459. (11) Girshick, S. L. Plasma Sources Sci. Technol. 1994, 3, 388. (12) Rao, N.; Micheel, B.; Hansen, D.; Fandrey, C.; Bench, M.; Girshick, S.; Heberlein, J.; McMurry, P. J. Mater. Res. 1995, 10, 2073. (13) Rao, N.; Girshick, S.; Heberlein, J.; McMurry, P.; Bench, M.; Jones, S.; Hansen, D.; Micheel, B. Plasma Chem. Plasma Process. 1995, 15, 581. (14) Mangolini, L. T., E.; Kortshagen, U. Nano Lett. 2005, 5, 655. (15) Deal, B. E.; Grove, A. S. J. Appl. Phys. 1965, 36, 3770. (16) Wong, H.; Cheng, Y. C. J. Appl. Phys. 1988, 64, 893. (17) Beresford, R. Semicond. Sci. Technol. 2003, 18, 973. (18) da Silva, E. F., Jr.; de Vasconcelos, E. A.; Stosic, B. D. Appl. Surf. Sci. 2002, 190, 30. (19) El-Oyoun, M. A.; Inokuma, T.; Kurata, Y.; Hasegawa, S. SolidState Electron. 2003, 47, 1669. (20) Kotani, Y.; Omura, Y. Jpn. J. Appl. Phys. 2000, 39, 4549. (21) Gayathri, N.; Banerjee, S. Appl. Phys. Lett. 2004, 84, 5192.
Surface Chemistry of Aerosolized Nanoparticles (22) Liu, B. Y. H.; Pui, D. Y. H.; Whitby, K. T.; Kittelson, D. B.; Kousaka, Y.; McKenzie, R. L. Atmos. EnViron. 1978, 12, 99. (23) Rader, D. J.; McMurry, P. H. J. Aerosol Sci. 1986, 17, 771. (24) McMurry, P. H.; Takano, H.; Anderson, G. R. EnViron. Sci. Technol. 1983, 17, 347. (25) Higgins, K. J.; Jung, H.; Kittelson, D. B.; Roberts, J. T.; Zachariah, M. R. J. Phys. Chem. 2002, 106, 96. (26) Higgins, K. J.; Jung, H.; Kittelson, D. B.; Roberts, J. T.; Zachariah, M. R. EnViron. Sci. Technol. 2003, 37, 1949. (27) Nienow, A. M.; Roberts, J. T.; Zachariah, M. R. J. Phys. Chem. B 2005, 109, 5561. (28) Kesten, J.; Reineking, A.; Porstendorfer, J. Aerosol Sci. Technol. 1991, 15, 107. (29) Chen, D.-R.; Pui, D. Y. H.; Hummes, D.; Fissan, H.; Quant, F. R.; Sem, G. J. J. Aerosol Sci. 1998, 29, 497.
J. Phys. Chem. B, Vol. 110, No. 12, 2006 6197 (30) Bates, D. M. a. W., D. G. Nonlinear Regression and Its Applications; Wiley: New York, 1988. (31) Jennings, H. M. J. Mater. Sci. 1983, 18, 951. (32) Gusev, E. P.; Lu, C. H.; Gustafsson, T.; Garfunkel, E. Phys. ReV. B: Condens. Matter Mater. Phys. 1995, 52, 1759. (33) Massoud, H. Z.; Plummer, J. D.; Irene, E. A. J. Electrochem. Soc. 1985, 132, 2685. (34) Massoud, H. Z.; Plummer, J. D.; Irene, E. A. J. Electrochem. Soc. 1985, 132, 1745. (35) Irene, E. A. J. Appl. Phys. 1983, 54, 5416. (36) de Almeida, R. M. C.; Goncalves, S.; Baumvol, I. J. R.; Stedile, F. C. Phys. ReV. B: Condens. Matter Mater. Phys. 2000, 61, 12992. (37) Norton, F. J. Nature 1961, 191, 701.
