REPORTS 31. We thank 5. Kuroda, M a. Y. Ueno, T. Hibaru and was completed in March 2006 at the National SOM Text and S. Iwasaki for the materials processing; K. Nakazato stitute for Materials Science in Japan Figs. S1 to 53 pact test; and Y. Hirota, A. Sakurai, References Supporting Online Material assistance in the microstructural observations This work .sciencemag. org/cgwcontent/fulL/320/5879/1057/DC1 accepted 14 April 2008 is a fruit of the Ultra-Steel Project, which began in 1997 Materials and Methods 10.1126/ science.1156084 Dislocation-Driven nanowire timeters per minute(sccm) of argon flow and 900 torr pressure with the hydrogen flow at 1.8 sccm for the first 1 min and 1.0 sccm for the Growth and eshelby twist ning 14 min. Even though the synth procedure is similar to that for the hyperbranched Matthew ] Bierman, 'Y.K. Albert Lau, I*Alexander V Kvit, 2 Andrew L Schmitt, Song Jin't PbS nanowires(see examples in figs. S2 and s3) (12), the nanowire growth appears to be driven Hierarchical nanostructures of lead sulfide nanowires resembling pine trees were synthesized by chemical by different mechanisms. The key difference be- vapor deposition. Structural characterization revealed a screwlike dislocation in the nanowire trunks with tween the growth of pine trees and the growth of helically rotating epitaxial branch nanowires. It is suggested that the screw component of an axial hyperbranched nanowires(12) and other previ- dislocation provides the self-perpetuating steps to enable one-dimensional crystal growth, in contrast to sly reported PbQ(Q is S, Se, or Te)nanowire mechanisms that require metal catalysts. The rotating trunks and branches are the consequence of the growth(3, 14)is the hydrogen flow profile.The Eshelby twist of screw dislocations with a dislocation Burgers vector along the(110) directions having an optimized reactions (ln) reproducibly yield many estimated magnitude of 6+2 angstroms for the screw component. The results confirm the Eshelby theory intricate treelike Pbs nanowire structures over of dislocations, and the proposed nanowire growth mechanism could be general to many materials. large areas(l to 2 cm )on the growth substrate, a as revealed by scanning electron microscopy n the burgeoning field of nanoscience, a major The nanostructures of PbS are synthesized via (SEM)(Fig. I and fig. SI, also see fig. 4 for 2 ambition is to synthesize nanoscale building chemical vapor deposition with PbCl2 and ele- phase identification). These trees have trunks that blocks of arbitrary dimensions, morphologies, mental sulfur as precursors under argon flow with are up to hundreds of micrometers in length and o nd materials of increasing complexity. One- a co-flow of H2 at atmospheric pressure and with branches that are commonly tens of micrometers dimensional(ID) nanowire materials, in partic mperatures between 600 and 650C (II). long. Individual wires grow consistently along llar, have already found many applications in Typical synthesis conditions involve reactions at the (100) crystallographic directions and their di- y nanoelectronics, nanophotonics, and biotech- 600C for 15 min under 150 standard cubic cen- ameters range from 40 to 350 nm. Closer exa ology(L, 2). To break the symmetry of bulk crystals and enable the anisotropic ID crystal growth of inorganic nanowires, the well-known A B apor-liquid-solid (VLS) growth method uses metal nanoparticles that form low-melting point eutectic alloys with the targeted materials to serve as the catalytic seeds for ID anisotropic growth (3, 4). Except for direct vapor-solid growth (5), most nanowire-formation mechanisms including solution-liquid-solid growth (SLS)(6) and var- c00≥sEo=vooEso ants of VLS such as vapor-solid-solid growth (7), require the use of catalytic nanoparticles, either added intentionally or generated in situ, o enable the ID anisotropic crystal growth. "Treelike"or hyperbranched nanostructures 10 have also been reported, but they all rely on mul- tiple applications of metal catalysts with subse- quent VLs (8, 9)or SLS (10) growth steps We suggest a nanowire growth mechanism that does not depend on catalysts but instead is driven by an axial screwlike dislocation along the length of the nanowire. It results in hierarchical lead sulfide(PbS) nanostructures of pine tree morphology when combined with a slower in situ VLs branching nanowire growth. The geo- metrical features of the resulting structures can be 10m quantitatively understood with the simple elas- ticity theory of dislocations. Fig. 1. SEM micrographs of Pbs pine tree nanowires. (A) Overview of dense forest of many nanowire trees. B)Tree clusters showing epitaxial growth along(100)directions. (C) Side view of owth substrate showing forest growth. (D to F)High-magnification views of trees highlighting the Science Center, University of Wisconsin-Madison, 1509 twisting (Eshelby twist)of the central trunk and helical rotating branches, with(E) further University Avenue, Madison, WI 53706, USA lustrating branch epitaxy on the tree trunk and (F)showing a tree with fewer branches (G)An xample of tree-on-tree" morphology that can be occasionally observed. (Inset) A magnified view tTo whom correspondence should be addressed. E-mail: of the tips of nanowires after synthesis highlighting the cubes that sometimes decorate the tips. @chem. wisc. edu The inset scale bar is 200 nm. The images are false colored. 1060 23mAy2008voL320scIencEwww.sciencemag.org31. We thank S. Kuroda, M. Fujiwara, Y. Ueno, T. Hibaru, and S. Iwasaki for the materials processing; K. Nakazato for the Charpy impact test; and Y. Hirota, A. Sakurai, E. Motoki, and I. Sakamaki for their experimental assistance in the microstructural observations. This work is a fruit of the Ultra-Steel Project, which began in 1997 and was completed in March 2006 at the National Institute for Materials Science in Japan. Supporting Online Material www.sciencemag.org/cgi/content/full/320/5879/1057/DC1 Materials and Methods SOM Text Figs. S1 to S3 References 5 February 2008; accepted 14 April 2008 10.1126/science.1156084 Dislocation-Driven Nanowire Growth and Eshelby Twist Matthew J. Bierman,1* Y. K. Albert Lau,1* Alexander V. Kvit,2 Andrew L. Schmitt,1 Song Jin1 † Hierarchical nanostructures of lead sulfide nanowires resembling pine trees were synthesized by chemical vapor deposition. Structural characterization revealed a screwlike dislocation in the nanowire trunks with helically rotating epitaxial branch nanowires. It is suggested that the screw component of an axial dislocation provides the self-perpetuating steps to enable one-dimensional crystal growth, in contrast to mechanisms that require metal catalysts. The rotating trunks and branches are the consequence of the Eshelby twist of screw dislocations with a dislocation Burgers vector along the 〈110〉 directions having an estimated magnitude of 6 ± 2 angstroms for the screw component. The results confirm the Eshelby theory of dislocations, and the proposed nanowire growth mechanism could be general to many materials. I n the burgeoning field of nanoscience, a major ambition is to synthesize nanoscale building blocks of arbitrary dimensions, morphologies, and materials of increasing complexity. One￾dimensional (1D) nanowire materials, in partic￾ular, have already found many applications in nanoelectronics, nanophotonics, and biotech￾nology (1, 2). To break the symmetry of bulk crystals and enable the anisotropic 1D crystal growth of inorganic nanowires, the well-known vapor-liquid-solid (VLS) growth method uses metal nanoparticles that form low–melting point eutectic alloys with the targeted materials to serve as the catalytic seeds for 1D anisotropic growth (3, 4). Except for direct vapor-solid growth (5), most nanowire-formation mechanisms, including solution-liquid-solid growth (SLS) (6) and var￾iants of VLS such as vapor-solid-solid growth (7), require the use of catalytic nanoparticles, either added intentionally or generated in situ, to enable the 1D anisotropic crystal growth. “Treelike” or hyperbranched nanostructures have also been reported, but they all rely on mul￾tiple applications of metal catalysts with subse￾quent VLS (8, 9) or SLS (10) growth steps. We suggest a nanowire growth mechanism that does not depend on catalysts but instead is driven by an axial screwlike dislocation along the length of the nanowire. It results in hierarchical lead sulfide (PbS) nanostructures of pine tree morphology when combined with a slower in situ VLS branching nanowire growth. The geo￾metrical features of the resulting structures can be quantitatively understood with the simple elas￾ticity theory of dislocations. The nanostructures of PbS are synthesized via chemical vapor deposition with PbCl2 and ele￾mental sulfur as precursors under argon flow with a co-flow of H2 at atmospheric pressure and with temperatures between 600° and 650°C (11). Typical synthesis conditions involve reactions at 600°C for 15 min under 150 standard cubic cen￾timeters per minute (sccm) of argon flow and 900 torr pressure with the hydrogen flow at 1.8 sccm for the first 1 min and 1.0 sccm for the remaining 14 min. Even though the synthetic procedure is similar to that for the hyperbranched PbS nanowires (see examples in figs. S2 and S3) (12), the nanowire growth appears to be driven by different mechanisms. The key difference be￾tween the growth of pine trees and the growth of hyperbranched nanowires (12) and other previ￾ously reported PbQ (Q is S, Se, or Te) nanowire growth (13, 14) is the hydrogen flow profile. The optimized reactions (11) reproducibly yield many intricate treelike PbS nanowire structures over large areas (1 to 2 cm2 ) on the growth substrate, as revealed by scanning electron microscopy (SEM) (Fig. 1 and fig. S1; also see fig. S4 for phase identification). These trees have trunks that are up to hundreds of micrometers in length and branches that are commonly tens of micrometers long. Individual wires grow consistently along the 〈100〉 crystallographic directions and their di￾ameters range from 40 to 350 nm. Closer exam- 1 Department of Chemistry, University of Wisconsin–Madison, 1101 University Avenue, Madison, WI 53706, USA. 2 Materials Science Center, University of Wisconsin–Madison, 1509 University Avenue, Madison, WI 53706, USA. *These authors contributed equally to this work. †To whom correspondence should be addressed. E-mail: jin@chem.wisc.edu Fig. 1. SEM micrographs of PbS pine tree nanowires. (A) Overview of dense forest of many nanowire trees. (B) Tree clusters showing epitaxial growth along 〈100〉 directions. (C) Side view of growth substrate showing forest growth. (D to F) High-magnification views of trees highlighting the twisting (Eshelby twist) of the central trunk and helical rotating branches, with (E) further illustrating branch epitaxy on the tree trunk and (F) showing a tree with fewer branches. (G) An example of “tree-on-tree” morphology that can be occasionally observed. (Inset) A magnified view of the tips of nanowires after synthesis highlighting the cubes that sometimes decorate the tips. The inset scale bar is 200 nm. The images are false colored. 1060 23 MAY 2008 VOL 320 SCIENCE www.sciencemag.org REPORTS on May 23, 2008 www.sciencemag.org Downloaded from