REPORTS LkHo 198 an opening angle of 45 without limb 18. T. Henning A Burkert, R. Launhardt, C Leinert, B Stecklum, Astron. Astrophys. 336, 565 (1998) port the presence of evacuated cavities in the Whitney for providing us with electronic versions of envelope of LkHo 198. The observed morphol- 20. C. Aspin, M. J McCaughrean, I.S. McLean, Astron an instead be explained by the illumination 21. M. Corcoran,TP.Ray, Ast of a cavity-free, rotationally flattened envelope 22. F. Shu et al. Astrophys. /.429, 781(1994) niversity of California at Santa Cruz tive agreem AST-9876783 and also under th 23. F. H. Shu, F. C. Adams, 5. Lizano, Annu. Rev. Astron then arise from light escaping along the path of Astrophys. 25, 23(1987) ast optical depth. However, these cavity-free 4. F. Palla, S. W. Stahler, Astron. 1. 418, 414(1993) V Mannings, A L Sargent, Astrophys. J. 490, 792(1997) nder contract w-7405-Eng-48. P K received at falling envelope models have opening angles 26. V Mannings. D w Koerner, A.L. Sargent, Nature 388 support from the NASA Origins Program (grant that increase with wavelength, whereas we ob- 55501997 serve a constant opening angle, suggesting 27. J di Francesco, N. J. Evans, P. M. Harvey, L G. Mundy. geometric rather than optical depth origin for the 28. A Natta et al, Astron. Astrophys. 371, 186(2001) Propulsion Laboratory (PL). ]PL is managed for NASA by the California Institute of Technology observed morphology. This discrepancy may be 29. B. A. Whitney, K. Wood, J.E Bjorkman, M J. wolff resolvable by varying the dust particle properties. 30. B. A Whitney, K. Wood, J E.Bjorkman, M. Cohen, upporting Online Material On the basis of these observations. LkHa Astrophys.J598.1079(2003) 233 is the more evolved of the two systems, Materials and Methods with well-defined cavities swept out by bipo- A. Bonnell, M. R Bate, H. Zinnecker, Mon. Not Figs. S1 and S2 lar outflow and bisected by a very dark lane. 33. We thank the Lick Observatory staff who assisted in LkHo 198 is a less evolved system, which is these observations, including T. Misch, K Chloros, and 11 December 2003: accepted 23 January 2004 lly in the early stages of developing bipola cavities and possesses lower extinction in the parent disk midplane The observed circumstellar environments ar Single-Crystal Nanorings Formed consistent with the rotationally flattened infall envelopes models developed for T Tauri stars by Epitaxial Self-Coiling of indicating that the process of envelope collaps as similar phases, despite the large disparities in Polar Nanobelts mass and luminosity between these two classes of young stars. This morphological similarity Xiang Yang Kong, Yong Ding, Rusen Yang, Zhong Lin Wang leads us to infer that the conservation and trans- port of angular momentum is the dominant phys- Freestanding single-crystal complete nanorings of zinc oxide were formed via a ical process for both classes of stars. Altemate spontaneous self-coiling process during the growth of polar nanobelts. The na- formation pathways have been suggested for OB noring appeared to be initiated by circular folding of a nanobelt, caused by long- stars that invoke new physical mechanisms, such range electrostatic interaction. Coaxial and uniradial loop-by-loop winding of the as magnetohydrodynamic turbulence (31)or nanobelt formed a complete ring. Short-range chemical bonding among the loop stellar mergers(32). The Herbig Ae stars studied resulted in a single-crystal structure. The self-coiling is likely to be driven by here appear to be below the mass threshhold at minimizing the energy contributed by polar charges, surface area, and elastic which such effects become important deformation. Zinc oxide nanorings formed by self-coiling of nanobelts may be 0≥日9o useful for investigating polar surface-induced growth processes, fundamental hysics phenomena, and nanoscale devices. 2. oot apper. C J. MacDonald, C E Max, F.J. Dyson, J. Self-assembly of nanocrystals can be driv- appears to be initiated by circular folding of S Hillenbrand, S. E. Strom, F. J Vrba, J. Keene, en by van der Waals forces and hydrogen a nanobelt driven by long-range electrostat Astrophys. J. 397. 613(199 bonding among the passivating organic ic interactions. Short-range chemical bond- 4. D. A Weintraub, A A Goodman.R L Akeson. in molecules on the particle surfaces(1-3). ing among the loops leads to the final 2000),pp.247-271 Arizona Press, Tucson, Az, charge-polarized surfaces, such as nano- ing is driven by minimizing the energy Science 277 belts of oxides like Zno(4), electrostatic contributed by polar charges, surface area, P. Lloyd et al, Proc. SPIE 4008, 814(2000) forces can drive self-assembly, especially and elastic deformation Single-crystal nanorings of ZnO were grown by a solid-vapor process. The raw material was a 9. J.R. Kuhn, D. Potter, B. Parise, Astrophys. J. 553, L18: mixture of Zno(melting point 1975C), indium vapor environment, one type of polar oxide, and lithium carbonate powders at a weight 10. W Li, N J Evans, P. M. Harvey, C Colome, Astrophys. ge-induced helical and spiral Zno structure ratio of 20: 1: 1, and it was placed at the highest 12. D. Copys 1282, 631 N Calvet, R.M. Levreault. was previously reported(5). We now report temperature zone of a horizontal tube fumace a distinct nanoring structure that is formed Before heating to a desired temperature of by spontaneous self-coiling polar 1400oC, the tube furnace was evacuated to 13. R Hajar, P. Bastien, Astrophys. J. 531, 494 nanobelt during growth. Nanoring growth "10- torr to remove the residual oxygen. The 14. G. Sandell, D. A. Weintraub, Astron. Astroph source materials were then heated to 1400 c at a eating rate of 20C/min. Zno decomposes into 15.C D. Koresko, P. M. Harvey, J C. Christou, R. Q. Institute of Technology, Atlanta, GA 30332-0245 Zn- and O--at high temperature(1400C)and low pressure(-10-3torr), and this decomposi 17. M. Fukagawa et al. ]. Psychiatry Neurosci. 54. 969 "To whom correspondence should be addressed. E- tion process is the key step for controlling the 2002 mail: zhong wang @mse gatech. edu th of the nanobelts. After a few 27FebRuaRy2004Vol303ScieNcewww.sciencemag.orgLkH
198 an opening angle of 45° without limb brightening. Thus, our observations do not sup￾port the presence of evacuated cavities in the envelope of LkH
198. The observed morphol￾ogy can instead be explained by the illumination of a cavity-free, rotationally flattened envelope by the central star; the bipolar appearance would then arise from light escaping along the path of least optical depth. However, these cavity-free infalling envelope models have opening angles that increase with wavelength, whereas we ob￾serve a constant opening angle, suggesting a geometric rather than optical depth origin for the observed morphology. This discrepancy may be resolvable by varying the dust particle properties. On the basis of these observations, LkH
233 is the more evolved of the two systems, with well-defined cavities swept out by bipo￾lar outflow and bisected by a very dark lane. LkH
198 is a less evolved system, which is only in the early stages of developing bipolar cavities and possesses lower extinction in the apparent disk midplane. The observed circumstellar environments are consistent with the rotationally flattened infall envelopes models developed for T Tauri stars, indicating that the process of envelope collapse has similar phases, despite the large disparities in mass and luminosity between these two classes of young stars. This morphological similarity leads us to infer that the conservation and trans￾port of angular momentum is the dominant phys￾ical process for both classes of stars. Alternate formation pathways have been suggested for OB stars that invoke new physical mechanisms, such as magnetohydrodynamic turbulence (31) or stellar mergers (32). The Herbig Ae stars studied here appear to be below the mass threshhold at which such effects become important. References and Notes 1. W. Happer, G. J. MacDonald, C. E. Max, F. J. Dyson, J. Opt. Soc. Am. 11, 263 (1994). 2. L. A. Hillenbrand, S. E. Strom, F. J. Vrba, J. Keene, Astrophys. J. 397, 613 (1992). 3. P. Bastien, Astrophys. J. 317, 231 (1987). 4. D. A. Weintraub, A. A. Goodman, R. L. Akeson, in Protostars and Planets IV, V. Mannings, A. P. Boss, S. S. Russell, Eds. (Univ. of Arizona Press, Tucson, AZ, 2000), pp. 247–271. 5. C. Max et al., Science 277, 1649 (1997). 6. J. P. Lloyd et al., Proc. SPIE 4008, 814 (2000). 7. Additional information on materials and methods is available as supporting material on Science Online. 8. D. E. Potter et al., Astrophys. J. 540, 422 (2000). 9. J. R. Kuhn, D. Potter, B. Parise, Astrophys. J. 553, L189 (2001). 10. W. Li, N. J. Evans, P. M. Harvey, C. Colome, Astrophys. J. 433, 199 (1994). 11. J. Canto, L. F. Rodriguez, N. Calvet, R. M. Levreault, Astrophys. J. 282, 631 (1984). 12. D. Corcoran, T. P. Ray, P. Bastien, Astron. Astrophys. 293, 550 (1995). 13. R. Hajjar, P. Bastien, Astrophys. J. 531, 494 (2000). 14. G. Sandell, D. A. Weintraub, Astron. Astrophys. 292, L1 (1994). 15. C. D. Koresko, P. M. Harvey, J. C. Christou, R. Q. Fugate, W. Li, Astrophys. J. 485, 213 (1997). 16. P. O. Lagage et al., Astrophys. J. Lett. 417, L 79 (1993). 17. M. Fukagawa et al., J. Psychiatry Neurosci. 54, 969 (2002). 18. T. Henning, A. Burkert, R. Launhardt, C. Leinert, B. Stecklum, Astron. Astrophys. 336, 565 (1998). 19. S. Terebey, F. H. Shu, P. Cassen, Astrophys. J. 286, 529 (1984). 20. C. Aspin, M. J. McCaughrean, I. S. McLean, Astron. Astrophys. 144, 220 (1985). 21. M. Corcoran, T. P. Ray, Astron. Astrophys. 336, 535 (1998). 22. F. Shu et al., Astrophys. J. 429, 781 (1994). 23. F. H. Shu, F. C. Adams, S. Lizano, Annu. Rev. Astron. Astrophys. 25, 23 (1987). 24. F. Palla, S. W. Stahler, Astron. J. 418, 414 (1993). 25. V. Mannings, A. I. Sargent, Astrophys. J. 490, 792 (1997). 26. V. Mannings, D. W. Koerner, A. I. Sargent, Nature 388, 555 (1997). 27. J. di Francesco, N. J. Evans, P. M. Harvey, L. G. Mundy, H. M. Butner, Astrophys. J. 432, 710 (1994). 28. A. Natta et al., Astron. Astrophys. 371, 186 (2001). 29. B. A. Whitney, K. Wood, J. E. Bjorkman, M. J. Wolff, Astrophys. J. 591, 1049 (2003). 30. B. A. Whitney, K. Wood, J. E. Bjorkman, M. Cohen, Astrophys. J. 598, 1079 (2003). 31. C. F. McKee, J. C. Tan, Astrophys. J. 585, 850 (2003). 32. I. A. Bonnell, M. R. Bate, H. Zinnecker, Mon. Not. R. Astron. Soc. 298, 93 (1998). 33. We thank the Lick Observatory staff who assisted in these observations, including T. Misch, K. Chloros, and J. Morey; the many individuals who have contributed to making the laser guide star system a reality; and B. Whitney for providing us with electronic versions of models. Onyx Optics fabricated our YLF Wollaston prisms. Supported in part by NSF Science and Tech￾nology Center for Adaptive Optics, managed by the University of California at Santa Cruz under cooper￾ative agreement AST-9876783 and also under the auspices of the U.S. Department of Energy, National Nuclear Security Administration, by the University of California, Lawrence Livermore National Laboratory, under contract W-7405-Eng-48. P.K. received addi￾tional support from the NASA Origins Program (grant NAG5-11769). M.D.P. is supported by a NASA Mich￾elson Graduate Fellowship, under contract to the Jet Propulsion Laboratory (JPL). JPL is managed for NASA by the California Institute of Technology. Supporting Online Material www.sciencemag.org/cgi/content/full/303/5662/1345/ DC1 Materials and Methods Figs. S1 and S2 Table S1 11 December 2003; accepted 23 January 2004 Single-Crystal Nanorings Formed by Epitaxial Self-Coiling of Polar Nanobelts Xiang Yang Kong, Yong Ding, Rusen Yang, Zhong Lin Wang* Freestanding single-crystal complete nanorings of zinc oxide were formed via a spontaneous self-coiling process during the growth of polar nanobelts. The na￾noring appeared to be initiated by circular folding of a nanobelt, caused by long￾range electrostatic interaction. Coaxial and uniradial loop-by-loop winding of the nanobelt formed a complete ring. Short-range chemical bonding among the loops resulted in a single-crystal structure. The self-coiling is likely to be driven by minimizing the energy contributed by polar charges, surface area, and elastic deformation. Zinc oxide nanorings formed by self-coiling of nanobelts may be useful for investigating polar surface–induced growth processes, fundamental physics phenomena, and nanoscale devices. Self-assembly of nanocrystals can be driv￾en by van der Waals forces and hydrogen bonding among the passivating organic molecules on the particle surfaces (1–3). For inorganic nanostructures that expose charge-polarized surfaces, such as nano￾belts of oxides like ZnO (4), electrostatic forces can drive self-assembly, especially in gas-phase environments where these forces are unscreened by solvents. For crys￾talline nanomaterials grown in a solid￾vapor environment, one type of polar char￾ge–induced helical and spiral ZnO structure was previously reported (5). We now report a distinct nanoring structure that is formed by spontaneous self-coiling of a polar nanobelt during growth. Nanoring growth appears to be initiated by circular folding of a nanobelt driven by long-range electrostat￾ic interactions. Short-range chemical bond￾ing among the loops leads to the final single-crystalline structure. The self-coil￾ing is driven by minimizing the energy contributed by polar charges, surface area, and elastic deformation. Single-crystal nanorings of ZnO were grown by a solid-vapor process. The raw material was a mixture of ZnO (melting point 1975°C), indium oxide, and lithium carbonate powders at a weight ratio of 20:1:1, and it was placed at the highest temperature zone of a horizontal tube furnace. Before heating to a desired temperature of 1400°C, the tube furnace was evacuated to 103 torr to remove the residual oxygen. The source materials were then heated to 1400°C at a heating rate of 20°C/min. ZnO decomposes into Zn2 and O2– at high temperature (1400°C) and low pressure (103 torr), and this decomposi￾tion process is the key step for controlling the anisotropic growth of the nanobelts. After a few School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332– 0245 USA. *To whom correspondence should be addressed. 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