A Simple Route to Silicon-Based Nanostructures

Communications

tant was decanted. This peptization step was repeated 3±4 times. Finally, the combined supernatants were centrifuged at 12 500g for 5 min and the weakly opalescent colorless supernatant, containing the smallest nanoparticles, was carefully decanted. Thin fibers of LaPO4:Eu were synthesized in acidic solution as follows: A solution of La(NO3)3×6H2O (12.34 g; 28.5 mmol) and Eu(NO3)3×5H2O (0.642 g; 1.5 mmol) in water (100 mL) was adjusted to pH 4.5 and combined with stirring with (NH4)2HPO4 (3.96 g; 30 mmol) dissolved in water (100 mL). The resulting suspension having pH 1.7 was poured into a Teflonlined autoclave (Berghof, HR-500) and treated as described above. Similarly, fibers of LaPO4:Ce(5 %) and La0.4Ce0.45Tb0.15PO4 were prepared by employing a total amount of 30 mmol of the respective nitrates. Prior to heating, the suspension in the autoclave was purged with forming gas (N2:H2 = 9:1) for 60 min, in order to prevent oxidation of Ce3+ to Ce4+. Powders of the nanoparticles were obtained from the colloidal solutions by removing the water with a rotary evaporator (bath temperature 50 C). Transmission electron micrographs of the samples were taken using a Philips CM 300 UT electron microscope, working at 300 kV acceleration voltage. A Philips Xpert system was used to measure the X-ray diffraction pattern of powder samples. UV-vis absorption spectra of the colloidal solutions were recorded with a Lambda 40 spectrometer (Perkin±Elmer). Photoluminescence spectra were recorded with a Spex Fluoromax 2 spectrometer having a spectral resolution of 0.5 nm. Received: November 19, 1998 Final version: February 22, 1999 ± [1] G. Blasse, B. C. Grabmaier Luminescent Materials, Springer, Heidelberg, 1994. [2] T. Ye, G. W. Zhao, W. P. Zhang, S. D. Xia, Mater. Res. Bull. 1997, 32, 501. [3] Q. Li, L. Gao, D. Yan, Nanostruct. Mater. 1997, 8, 825. [4] D. K. Williams, B. Bihari, B. M. Tissue, J. M. McHale, J. Phys. Chem. B 1998, 102, 916. [5] B. Bihari, H. Eilers, B. M. Tissue, J. Lumin. 1997, 72±74, 190. [6] E. T. Goldburt, B. Kurkarni, R. N. Bhargava, J. Taylor, M. Liberia, J. Lumin. 1997, 75, 1. [7] Y. L. Soo, S. W. Huang, Z. H. Ming, Y. H. Kao, G. C. Smith, E. Goldburt, R. Hodel, B. Kulkarni, J. V. D. Veliadis, R. N. Bhargava, J. Appl. Phys. 1998, 83, 5404. [8] R. N. Bhargava, J. Lumin. 1996, 70, 85. [9] B. M. Tissue, Chem. Mater. 1998, 10, 2837. [10] K. Kawano, K. Arai, H. Yamada, N. Hashimoto, R. Nakata, Sol. Energy Mater. Sol. Cells 1997, 48, 35. [11] W. G. Becker, A. J. Bard., J. Phys. Chem. 1983, 87, 4888. [12] D. Gallagher, W. E. Heady, J. M. Racz, R. N. Bhargava, J. Cryst. Growth 1994, 138, 970. [13] R. N. Bhargava, D. Gallagher, X. Hong, A. Nurmikko, Phys. Rev. Lett. 1994, 72, 416. [14] Y. L. Soo, Z. H. Ming, S. W. Huang, Y. H. Kao, R. N. Bhargava, D. Gallagher, Phys. Rev. B 1994, 50, 7602. [15] K. Sooklal, B. S. Cullum, S. M. Angel, C. J. Murphy, J. Phys. Chem. 1996, 100, 4551. [16] G. Counio, S. Esnouf, T. Gacoin, J.-P. Boilot, J. Phys. Chem. 1996, 100, 20 021. [17] L. Levy, J. F. Hochepied, M. P. Pileni, J. Phys. Chem. 1996, 100, 18 322. [18] L. Levy, F. Feltin, D. Ingert, M. P. Pileni, J. Phys. Chem. B 1997, 101, 9153. [19] G. Counio, T. Gacoin, J.-P. Boilot, J. Phys. Chem. B 1998, 102, 5257. [20] K. Riwotzki, M. Haase, J. Phys. Chem. B 1998, 102, 10 129. [21] A. Henglein, Chem. Rev. 1989, 89, 1861. [22] L. E. Brus, Appl. Phys. A 1991, 53, 465. [23] Y. Wang, N. Herron, J. Phys. Chem. 1991, 95, 525. [24] H. Weller, Adv. Mater. 1993, 5, 88. [25] H. Weller, A. Eychmüller, in Advanced Photochemistry 20 (Eds: D. C. Neckers, D. H. Volman, G. V. Bünau), Wiley, New York 1995, p. 165. [26] A. P. Alivisatos, J. Phys. Chem. 1996, 100, 13 226. [27] A. P. Alivisatos, Science. 1996, 271, 933. [28] R. D. Peacock, Struct. Bond. 1975, 22, 83. [29] W. L. Wanmaker, A. Bril, J. W. ter Vrugt, J. Broos, Philips Res. Rep. 1966, 21, 270. [30] J.-C. Bourcet, F. K. Fong, J. Chem. Phys. 1974, 60, 34. [31] N. Hashimoto, Y. Takada, K. Sato, S. Ibuki, J. Lumin. 1991, 48±49, 893. [32] J. W. Anthony, Am. Mineral. 1957, 42, 904. [33] Y. Hikichi, K. Hukuo, J. Shiokawa, Bull. Chem. Soc. Jpn. 1978, 51, 3645.

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A Simple Route to Silicon-Based Nanostructures** By Yan Qiu Zhu, Wei Bing Hu, Wen Kuang Hsu, Mauricio Terrones, Nicole Grobert, Turgay Karali, Humberto Terrones, Jonathan P. Hare, Peter D. Townsend, Harold W. Kroto, and David R. M. Walton* Following the discovery of carbon nanotubes,[1] one-dimensional nanostructures have been the subject of intensive research due to their potential in applied nanotechnology. Examples include gas storage,[2,3] field emission probes,[4,5] and reinforced composite materials.[6] As a consequence, heteroatom nanotubes (e.g., BN,[7,8] B±C±N[9±11]) and inorganic fullerene-like (e.g., WS2) nanostructures[12,13] have been prepared. Recently, silicon-containing materials (e.g., Si[14,15] and SiOx[16,17] nanowires, SiC[18] and Si3N4[19] nanorods, SiC nanowires,[20,21] and SiC coaxial nanocables[22]) have been generated using carbon nanotubes as reacting templates,[18±20] by laser ablation methods,[14,15,17,22] and by high-temperature carbothermal reduction of silica xerogels.[21] The resulting structures may be used as optical devices[17] and semiconductors.[14] In a recent paper,[16] we reported the generation of threedimensional SiOx nanoflowers using Co as the catalyst under Ar. By changing the ambient gas to CO, we have now achieved a greater degree of uniformity in terms of distribution and size of the nanoflowers and, by substituting Fe for Co, we have obtained high-quality Si and SiC nanowires, rather than nanoflowers (as expected), sheathed with SiOx. The SiOx nanoflowers and Si-cored nanowires are believed to form by chemical reactions and analogous growth mechanisms involving a vapor±liquid±solid (V±L±S) pro± [*] Dr. D. R. M. Walton, Dr. Y. Q. Zhu, W. B. Hu, Dr. W. K. Hsu, Dr. M. Terrones, N. Grobert, T. Karali, Dr. J. P. Hare, Prof. P. D. Townsend, Prof. H. W. Kroto School of Chemistry, Physics and Environmental Science University of Sussex Brighton BN1 9QJ (UK) Dr. H. Terrones Instituto de Física, UNAM Apartado Postal 20-364, MØxico, D.F. 01000 (Mexico) [**] We thank the Royal Society (YQZ, WKH, MT), the DERA (NG), Conacyt-MØxico and DGAPA-UNAM IN 107-296 (HT), and the EPSRC for financial support. We are grateful to J. Thorpe and D. Randall (Sussex) for assistance with TEM and SEM facilities. 0935-9648/99/1007-0844 $ 17.50+.50/0

Adv. Mater. 1999, 11, No. 10

Communications

cess.[23] The resulting nanoflowers exhibit luminescent emission comparable to that of bulk SiOx. A 1:1 wt.-% mixture of SiC powder (99.9 % purity, 400 mesh) and Co (325 mesh) or Fe (