Science如hgmn th and Eshelby Twist NAAAS DO|:10.1126/ scIence.1157131 This copy is for your personal, non-commercial use only If you wish to distribute this article to others, you can order high-quality copies for your colleagues, clients, or customers by clicking here Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here The following resources related to this article are available online at www.sciencemag.org(thisinformationiscurrentasofSeptember17,2014): Updated information and services, including high-resolution figures, can be found in the online version of this article at http://www.sciencemag.org/content/320/5879/1060.fullhtml Supporting Online Material can be found at http://www.sciencemag.org/content/suppl/2008/05/01/1157131.dc1.html This article cites 25 articles, 5 of which can be accessed free http://ww.sciencemag.org/content/320/5879/1060.full.html#ref-list-1 This article has been cited by 58 article(s)on the iSI Web of Science This article has been cited by 6 articles hosted by HighWire Press;see http://www.sciencemag.org/content/320/5879/1060.fullhtml#related-urls This article appears in the following subject collections Materials Science http://www.sciencemag.org/cgi/collection/matsci Science(print ISSN 0036-8075: online ISSN 1095-9203)is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2008 by the American Association for the Advancement of Science; all rights reserved. The title Science is a egistered trademark of AAAs
DOI: 10.1126/science.1157131 Science 320, 1060 (2008); Matthew J. Bierman et al. Dislocation-Driven Nanowire Growth and Eshelby Twist This copy is for your personal, non-commercial use only. colleagues, clients, or customers by clicking here. If you wish to distribute this article to others, you can order high-quality copies for your following the guidelines here. Permission to republish or repurpose articles or portions of articles can be obtained by www.sciencemag.org (this information is current as of September 17, 2014 ): The following resources related to this article are available online at http://www.sciencemag.org/content/320/5879/1060.full.html version of this article at: Updated information and services, including high-resolution figures, can be found in the online http://www.sciencemag.org/content/suppl/2008/05/01/1157131.DC1.html Supporting Online Material can be found at: http://www.sciencemag.org/content/320/5879/1060.full.html#ref-list-1 This article cites 25 articles, 5 of which can be accessed free: This article has been cited by 58 article(s) on the ISI Web of Science http://www.sciencemag.org/content/320/5879/1060.full.html#related-urls This article has been cited by 6 articles hosted by HighWire Press; see: http://www.sciencemag.org/cgi/collection/mat_sci Materials Science This article appears in the following subject collections: registered trademark of AAAS. 2008 by the American Association for the Advancement of Science; all rights reserved. The title Science is a American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the on September 17, 2014 www.sciencemag.org Downloaded from
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 A. Sakurai References E Motoki, and l. Sakamaki for their experimental Supporting Online Material assistance in the microstructural observations This work .sciencemag. org/cgwcontent/fulL/320/5879/1057/DC1 5 February 2008; 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 syntheti procedure is similar to that for the hyperbranched Matthew ]. Bierman,*Y. K. Albert Lau, Alexander V.Kvit, Andrew L. Schmitt, Song Jint (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 ously 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(13, 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 (n)reproducibly yield many ntricate treelike Pbs nanowire structures o estimated magnitude of 6= 2 angstroms for the screw component. The results confirm the Eshelby theory large areas(I to 2 cm)on the growth substrate, of dislocations, and the proposed nanowire growth mechanism could be general to many materials. 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 Sl; also see fig. S4 for ambition is to synthesize nanoscale building chemical vapor deposition with PbCl, 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 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 (1). long. Individual wires grow consistently along lar, have already found many applications in Typical synthesis conditions involve reactions at the (100) crystallographic directions and their di- 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- 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 101 University Avenue, Madison, Wi 53706, USA. Materials growth 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.org
31. 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. Onedimensional (1D) nanowire materials, in particular, have already found many applications in nanoelectronics, nanophotonics, and biotechnology (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 variants 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 multiple applications of metal catalysts with subsequent 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 geometrical features of the resulting structures can be quantitatively understood with the simple elasticity theory of dislocations. The nanostructures of PbS are synthesized via chemical vapor deposition with PbCl2 and elemental 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 centimeters 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 between the growth of pine trees and the growth of hyperbranched nanowires (12) and other previously 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 diameters 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
REPORTS ination of these nanostructures, particularly those handed rotating trees have roughly equal proba- formed by the branch tips, which are further ith less dense branching( Fig. 1, D to F), reveals bility of occurrence(measured ratio: 107: 126) accented when the tips are decorated with Pbs that each tree has four sets of epitaxial branches (11). The rotating branches become progressively cubes in some syntheses(see an example in the that are perpendicular to the trunk and the neigh- shorter from the base to the tip of the trees, inset of Fig. IG). These trees can grow both boring branches and rotate around the trunk in a in conelike envelopes that enforce the upward from the growth substrate and horizon- staircase fashion. The pitch of the rota- phology. The combination of the helical tally to the substrate from some nucleation clus- ranges from 16 to 220 um and can vary and the regular length progression of the ters, creating a dense copse of freestanding the length of a single tree. Right- or left- evenly spaced branches leads to beautiful curves nanowire trees, which often resembles a forest when viewed from the side of the growth sub- strates(Fig. IC). Within a common cluster, the Fig. 2. Distinctive difference in the A B trees occasionally grow epitaxially as evidenced growth rates of trunk and branch nano- by the perpendicular or parallel orientations vires and the proposed dislocation- between the trees(Fig. IB). Tree structures some- driven nanowire growth in the trunk of tree structures. (A) Approximate times coexist with or grow off from hyper- branched nanowire clusters( Fig. IE and fig S2). relative ratios of growth rates be- tween trunk and branch nanowires Occasionally, multiple levels of"tree-on-tree that are calculated as cotangents of a morphology can be observed(Fig. IG) the cone angles(0 as illustrated in Further scrutiny of the distinct morphology the inset) of the outer envelopes of and large length difference between the trunks 80 individual trees. B) Dramatized and branches of these pine tree nanowires sug scheme of the magnified tip of a tree gests that the trunks grow faster than the branches. structure highlighting the combined It was previously suggested that hyperbranched faster dislocation-driven trunk nano- Pbs nanowires are grown via a self-catalyzed wire growth and slower VLS-driven VLS mechanism: Lead itself serves as the eu- branched nanowire growth. (C) A simplified scheme illustrating that the self-perpetuating steps of a screw tectic catalyst and is consumed or evaporated dislocation spiral at the tip of a trunk can enable ld crystal growth of nanowires. ire growth(12, 13). Although no lead catalyst aps were observed at the tips of either the trunk 21]zA [111]ZAP or branch nanowires after growth, a similar length D● ●●go 220)spo limit was sometimes also observed for the tree branches(fig. S5), suggesting that the branch growth is likely VLs driven. Successive gen- -- ons of hyperbranched nanowires(12, 13) grow at a similar rate and lead to more isotropic cubic"morphology(figs. S2 and S3). In con- trast, the pine trees have steep cone angles that de not change over the duration of growth for a given tree, suggesting that the trunk nanowires grow much faster than the branches in the tree structures. The cone angles of the outer enve. lopes of the trees are dictated by the relative ratios Area of between the fastest growth rates of the trunks and branches and do not depend on the actual length and possible delays in nucleation events To quan- titatively represent the growth rate difference g-b=0 between the trunks and branches, the cotangent of the cone angle(0)of 80 tree envelopes is mea- nges from 4 to about 10(Fig. 2A). This distinct morphology and growth rate differ ence contrasts with the hyperbranched nanowires under similar reaction conditions, suggesting a different growth process for the trunks of pine tree nanowires We propose that the growth of the trunk nano- Fig 3. Diffraction contrast TEM imaging of the dislocation in the tree trunk. (A to C) TEM images wires is driven by the screw dislocation com- along the (221 zone axis under the strong two-beam conditions. (A) represents strong diffraction ponent of an axial dislocation along the length of ontrast, and(C)represents invisibility conditions as highlighted in the zone axis pattern (ZAP)(B). the nanowire, providing a continuous growth (D) Schematic superposition of real and reciprocal space of a dislocation-containing nanowire front for ID crystal growth. The self-perpetuating vector u is [1001, directions of screw and edge character Burgers vectors, bscrew and bedge, are growth fronts when the supersaturation is lower known As the Burgers vector is determined to be b11o, the high-contrast results at g1 are due to than what is required for crystal growth on per- the gillb relationship and the perpendicular go-beam results in the invisibility criterion. (E) Low fect crystal facets, and this is known as Frank's agnification TEM image showing the tree and area analyzed. (F)to( H)display [ 111] zone axis TEM mechanism for crystal growth(16, 17). When the nder the strong two-beam conditions. (F)represents glib conditions, (H)represents invisibility supersaturation is low, only fast crystal growth at conditions as highlighted in the zone axis the self-perpetuating steps of a screw dislocation www.sciencemag.orgScieNceVol32023May2008 1061
ination of these nanostructures, particularly those with less dense branching (Fig. 1, D to F), reveals that each tree has four sets of epitaxial branches that are perpendicular to the trunk and the neighboring branches and rotate around the trunk in a helical staircase fashion. The pitch of the rotations ranges from 16 to 220 mm and can vary down the length of a single tree. Right- or lefthanded rotating trees have roughly equal probability of occurrence (measured ratio: 107:126) (11). The rotating branches become progressively shorter from the base to the tip of the trees, resulting in conelike envelopes that enforce the tree morphology. The combination of the helical rotation and the regular length progression of the evenly spaced branches leads to beautiful curves formed by the branch tips, which are further accented when the tips are decorated with PbS cubes in some syntheses (see an example in the inset of Fig. 1G). These trees can grow both upward from the growth substrate and horizontally to the substrate from some nucleation clusters, creating a dense copse of freestanding nanowire trees, which often resembles a forest when viewed from the side of the growth substrates (Fig. 1C). Within a common cluster, the trees occasionally grow epitaxially as evidenced by the perpendicular or parallel orientations between the trees (Fig. 1B). Tree structures sometimes coexist with or grow off from hyperbranched nanowire clusters (Fig. 1E and fig. S2). Occasionally, multiple levels of “tree-on-tree” morphology can be observed (Fig. 1G). Further scrutiny of the distinct morphology and large length difference between the trunks and branches of these pine tree nanowires suggests that the trunks grow faster than the branches. It was previously suggested that hyperbranched PbS nanowires are grown via a self-catalyzed VLS mechanism: Lead itself serves as the eutectic catalyst and is consumed or evaporated during growth (15), thereby limiting the length of wire growth (12, 13). Although no lead catalyst caps were observed at the tips of either the trunk or branch nanowires after growth, a similar length limit was sometimes also observed for the tree branches (fig. S5), suggesting that the branch growth is likely VLS driven. Successive generations of hyperbranched nanowires (12, 13) grow at a similar rate and lead to more isotropic “cubic” morphology (figs. S2 and S3). In contrast, the pine trees have steep cone angles that do not change over the duration of growth for a given tree, suggesting that the trunk nanowires grow much faster than the branches in the tree structures. The cone angles of the outer envelopes of the trees are dictated by the relative ratios between the fastest growth rates of the trunks and branches and do not depend on the actual length and possible delays in nucleation events. To quantitatively represent the growth rate difference between the trunks and branches, the cotangent of the cone angle (q) of 80 tree envelopes is measured and ranges from 4 to about 10 (Fig. 2A). This distinct morphology and growth rate difference contrasts with the hyperbranched nanowires under similar reaction conditions, suggesting a different growth process for the trunks of pine tree nanowires. We propose that the growth of the trunk nanowires is driven by the screw dislocation component of an axial dislocation along the length of the nanowire, providing a continuous growth front for 1D crystal growth. The self-perpetuating steps of a screw dislocation provide facile spiral growth fronts when the supersaturation is lower than what is required for crystal growth on perfect crystal facets, and this is known as Frank’s mechanism for crystal growth (16, 17). When the supersaturation is low, only fast crystal growth at the self-perpetuating steps of a screw dislocation Fig. 2. Distinctive difference in the growth rates oftrunk and branch nanowires and the proposed dislocationdriven nanowire growth in the trunk of tree structures. (A) Approximate relative ratios of growth rates between trunk and branch nanowires that are calculated as cotangents of the cone angles (q as illustrated in the inset) of the outer envelopes of 80 individual trees. (B) Dramatized scheme of the magnified tip of a tree structure highlighting the combined faster dislocation-driven trunk nanowire growth and slower VLS-driven branched nanowire growth. (C) A simplified scheme illustrating that the self-perpetuating steps of a screw dislocation spiral at the tip of a trunk can enable 1D crystal growth of nanowires. 12 10 8 6 4 2 0 2 4 6 8 10 12 14 A C B Relative growth rate (trunk vs. branch) Number of trees (228) spot g·b = 0 C (220) spot g || b (220) spot (224) spot g·b = 0 [221] ZAP [111] ZAP Area of analysis u g1 g0 bscrew bedge b110 A 228 0 220 024 204 B D E F H 0 220 022 224 202 G 100 nm 1 µm g || b Fig. 3. Diffraction contrast TEM imaging of the dislocation in the tree trunk. (A to C) TEM images along the ½221 zone axis under the strong two-beam conditions. (A) represents strong diffraction contrast, and (C) represents invisibility conditions as highlighted in the zone axis pattern (ZAP) (B). (D) Schematic superposition of real and reciprocal space of a dislocation-containing nanowire along the [001] zone axis illustrating the Burgers vector relationship. Because the line dislocation vector u is [100], directions of screw and edge character Burgers vectors, bscrew and bedge, are known. As the Burgers vector is determined to be b110, the high-contrast results at g1 are due to the g1||b relationship and the perpendicular g0-beam results in the invisibility criterion. (E) Lowmagnification TEM image showing the tree and area analyzed. (F) to (H) display ½111 zone axis TEM under the strong two-beam conditions. (F) represents g||b conditions, (H) represents invisibility conditions as highlighted in the zone axis diagram (G). www.sciencemag.org SCIENCE VOL 320 23 MAY 2008 1061 REPORTS
REPORTS iral is possible, whereas growth on the cry ws the effective transfer of individual trees onto the dislocation meets the invisibility criterion lline side walls is suppressed(Fig. 2C). This grids while preserving their complex and under the perpendicular(228)spot(Fig. I breaks down the symmetry and drives the ID ile morphology (fig. S6 and movie SI)(In). Similarly, along the [111 zone axis, the(220) anisotropic crystal growth without catalysts. This Great care was taken to avoid excessive me- diffraction spot shows high contrast(Fig. 3F) dislocation-driven growth was proposed in the chanical force, which can result in the dislocation while the perpendicular(224) spot(Fig. 3H) 1950s by Sears to explain the formation of being worked out along the slip planes In trees meets the invisibility criterion. Therefore, taking micrometer-diameter metal"whiskers"(18, 19), that clearly preserve the twisting structures under the cross product of the(228)and(224)vectors which predates the VLS whisker growth. How- microscope observation(such as in Fig. 3E), shows that the Burgers vector is along the [1 ever, starting from the original Wagner and Ellis dark lines running the entire length of tree trunks direction. Electron diffraction patterns of the area VLS work(4, 20), much effort has been under- representing high dislocation contrast were ob. analyzed are shown in fig. S8. It is known that taken to rule out crystal dislocations as the served under TEM. This is shown for g=(220) the Burgers vector of the most stable dis- driving force for the ID anisotropic growth. in Fig 3A along the 221] zone axis, in Fig. 3F locations in rock salt( face-centered cubic)crys- Since then, little has been mentioned about the along the [111 zone axis, and in fig. S7 with a tals is along(110), and this has been previously ole of dislocation defects in whiskers (1, 21) more complete mapping. However, no disloca- observed in bulk Pbs crystals(23, 24). Because (and now nanowires ons were observed in the branches of any tree the dislocation line direction (u) is along the We confirm the presence of screw disloc- investigated. No dislocations were observed in [100 nanowire growth direction, the [1101 tions in the trunks of these tree structures using hyperbranched nanowires (fig. S9), with more Burgers vector represents a mixed dislocation diffraction contrast transmission electron micros- than 20 samples having been examined. These a screw dislocation component along the [100] copy(TEM) under the strong two-beam condi- observations are consistent with the suggestion (or 100) direction mostly responsible for driv- tions(22). Diffraction contrast TEM is a powerful that the nanowire trunks in the trees are driven ing the nanowire growth, and an edge dis- technique to image dislocations in crystals that by dislocation, whereas the branches of the trees location component along the [o10](or [010) relies on additional electron diffraction due to (and the hyperbranched nanowires) grow via a direction, whose role in promoting crystal growth the bending of atomic planes near the disloc- slower Vls process(Fig 2B) (25) is not clear but cannot be ruled out com- We have determined the dislocation Burgers pletely at presen ific reciprocal space diffraction spots(g) that vector(b) to be along the [110] direction. This What, then, is the reason for the helical rota are selected by a physical aperture, these addi- detailed diffraction contrast TEM analysis re- tion of branches on the screw dislocation-driven tional diffracted electrons create a visible con- quires finding two noncollinear diffraction spots nanowire trunks? All dislocations create strain trast around the dislocation. However, certain (g beams) in reciprocal space that satisfy the (and hence stress) within the otherwise perfect diffraction spots(g) with specific orientations invisibility criterion. The dislocation Burgers vec- crystalline lattice. Using elasticity theory, Eshelby to the Burgers vector of the dislocation (b) tor is along the direction of the cross product of has shown that in a finite cylindrical rod con- produce no dislocation contrast--the"invisi- these two g vectors. The tree structure shown in taining an axial screw dislocation at the center, bility criterion"(11). TEM sample preparation Fig. 3E has been analyzed under the strong two. the stress field created by the dislocation exert proved to be difficult due to the need to pre- beam conditions, as illustrated schematically in torque at the free ends of the rod, resulting in a serve the tree morphology during transfer, while Fig. 3D. The same segment of this tree was twist of the rod along the axial direction(Fig 4A) also avoiding trees with too many branches that tilted to the[] zone axis(Fig. 3, A to C)and (23, 26). This"Eshelby twist " is mathematically would obstruct the view and prevent the obser- the [111] zone axis(Fig 3, F to H), respectively pressed as vation of a dislocation. After experimenting with The image with the (220)diffraction spot many different transfer methods, we found micro(Fig. 3A)shows high dislocation contrast(cor anipulation to be the only technique that al- responding to the g b contrast maximum), while Fig. 4. Analysis of the where a is the twist of the lattice in radians pe Eshelby twists in tree unit length, R is the radius of the cylinder, and b nanostructures. (A)Sche- matic representation of aS the magnitude of the screw component of the Burgers vector(27). Attempts to observe the the forces and resulting Eshelby twist in micrometer-scale whiskers were crystal displacement due made in the late 1950s. but the results were often a screw dislocation. (B) inconclusive(28, 29). The Eshelby twist is read SEM images of a tree il- xI ily observed in the tree nanowires because the lustrating the measure- 1/R dependence makes the twist much more ment of twist (a quarte pronounced at the nanoscale compared to the of the pitch measured and the measurement of c micrometer-sized whiskers. and because the over- diameter (inset), which growth of epitaxial branching na was converted to radius =o easy visualization of the twist. This allows a di for calculation. (C) Scat- I rect measurement of Eshelby twists and a simple plot of twists measured estimate of the magnitude of the burgers vector from 247 spots on 90 individual trees agains As illustrated in Fig 4B, SEM imag be their inversed cross xamined to determine both the radius of a trunk ctional areas [GR-1 nanowire and also its twist by tracking the The red line is a least- 0408 wares fit through the A(rR)(um) periodic repeat of the branches(a Burgers vector(nm) pitch is actually measured because of the four se slope(6 A) is the magnitude of the screw component of the Burgers vector. (D)Histogram of the orthogonal epitaxial branches). The Eshelby ed Burgers vectors for each data point shown in(O with a Gaussian fit to the data. The Gaussian peak is twists (a)as a function of the inverse cross at 6 A with a standard deviation of 2a sectional areas [(R] of the nanowires wer 1062 23mAy2008Vol320ScieNcewww.sciencemag.org
spiral is possible, whereas growth on the crystalline side walls is suppressed (Fig. 2C). This breaks down the symmetry and drives the 1D anisotropic crystal growth without catalysts. This dislocation-driven growth was proposed in the 1950s by Sears to explain the formation of micrometer-diameter metal “whiskers” (18, 19), which predates the VLS whisker growth. However, starting from the original Wagner and Ellis VLS work (4, 20), much effort has been undertaken to rule out crystal dislocations as the driving force for the 1D anisotropic growth. Since then, little has been mentioned about the role of dislocation defects in whiskers (1, 21) (and now nanowires). We confirm the presence of screw dislocations in the trunks of these tree structures using diffraction contrast transmission electron microscopy (TEM) under the strong two-beam conditions (22). Diffraction contrast TEM is a powerful technique to image dislocations in crystals that relies on additional electron diffraction due to the bending of atomic planes near the dislocation core. If an image is reconstructed from specific reciprocal space diffraction spots (g) that are selected by a physical aperture, these additional diffracted electrons create a visible contrast around the dislocation. However, certain diffraction spots (g) with specific orientations to the Burgers vector of the dislocation (b) produce no dislocation contrast—the “invisibility criterion” (11). TEM sample preparation proved to be difficult due to the need to preserve the tree morphology during transfer, while also avoiding trees with too many branches that would obstruct the view and prevent the observation of a dislocation. After experimenting with many different transfer methods, we found micromanipulation to be the only technique that allows the effective transfer of individual trees onto TEM grids while preserving their complex and fragile morphology (fig. S6 and movie S1) (11). Great care was taken to avoid excessive mechanical force, which can result in the dislocation being worked out along the slip planes. In trees that clearly preserve the twisting structures under microscope observation (such as in Fig. 3E), dark lines running the entire length of tree trunks representing high dislocation contrast were observed under TEM. This is shown for g = (220) in Fig. 3A along the ½221 zone axis, in Fig. 3F along the ½111 zone axis, and in fig. S7 with a more complete mapping. However, no dislocations were observed in the branches of any tree investigated. No dislocations were observed in hyperbranched nanowires (fig. S9), with more than 20 samples having been examined. These observations are consistent with the suggestion that the nanowire trunks in the trees are driven by dislocation, whereas the branches of the trees (and the hyperbranched nanowires) grow via a slower VLS process (Fig. 2B). We have determined the dislocation Burgers vector (b) to be along the [110] direction. This detailed diffraction contrast TEM analysis requires finding two noncollinear diffraction spots (g beams) in reciprocal space that satisfy the invisibility criterion. The dislocation Burgers vector is along the direction of the cross product of these two g vectors. The tree structure shown in Fig. 3E has been analyzed under the strong twobeam conditions, as illustrated schematically in Fig. 3D. The same segment of this tree was tilted to the ½221 zone axis (Fig. 3, A to C) and the ½111 zone axis (Fig. 3, F to H), respectively. The image with the (220) diffraction spot (Fig. 3A) shows high dislocation contrast (corresponding to the g||b contrast maximum), while the dislocation meets the invisibility criterion under the perpendicular ð228Þ spot (Fig. 1C). Similarly, along the ½111 zone axis, the (220) diffraction spot shows high contrast (Fig. 3F) while the perpendicular ð224Þ spot (Fig. 3H) meets the invisibility criterion. Therefore, taking the cross product of the ð228Þ and ð224Þ vectors shows that the Burgers vector is along the [110] direction. Electron diffraction patterns of the area analyzed are shown in fig. S8. It is known that the Burgers vector of the most stable dislocations in rock salt (face-centered cubic) crystals is along 〈110〉, and this has been previously observed in bulk PbS crystals (23, 24). Because the dislocation line direction (u) is along the [100] nanowire growth direction, the [110] Burgers vector represents a mixed dislocation: a screw dislocation component along the [100] (or ½100) direction mostly responsible for driving the nanowire growth, and an edge dislocation component along the [010] (or ½010) direction, whose role in promoting crystal growth (25) is not clear but cannot be ruled out completely at present. What, then, is the reason for the helical rotation of branches on the screw dislocation-driven nanowire trunks? All dislocations create strain (and hence stress) within the otherwise perfect crystalline lattice. Using elasticity theory, Eshelby has shown that in a finite cylindrical rod containing an axial screw dislocation at the center, the stress field created by the dislocation exerts a torque at the free ends of the rod, resulting in a twist of the rod along the axial direction (Fig. 4A) (23, 26). This “Eshelby twist” is mathematically expressed as: a ¼ b pR2 ð1Þ where a is the twist of the lattice in radians per unit length, R is the radius of the cylinder, and b is the magnitude of the screw component of the Burgers vector (27). Attempts to observe the Eshelby twist in micrometer-scale whiskers were made in the late 1950s, but the results were often inconclusive (28, 29). The Eshelby twist is readily observed in the tree nanowires because the 1/R2 dependence makes the twist much more pronounced at the nanoscale compared to the micrometer-sized whiskers, and because the overgrowth of epitaxial branching nanowires allows easy visualization of the twist. This allows a direct measurement of Eshelby twists and a simple estimate of the magnitude of the Burgers vector screw component. As illustrated in Fig. 4B, SEM images can be examined to determine both the radius of a trunk nanowire and also its twist by tracking the periodic repeat of the branches (a quarter of the pitch is actually measured because of the four orthogonal epitaxial branches). The Eshelby twists (a) as a function of the inverse crosssectional areas [(pR2 ) −1 ] of the nanowires were Fig. 4. Analysis of the Eshelby twists in tree nanostructures. (A) Schematic representation of the forces and resulting crystal displacement due to a screw dislocation. (B) SEM images of a tree illustrating the measurement of twist (a quarter of the pitch measured) and the measurement of diameter (inset), which was converted to radius for calculation. (C) Scatterplot of twists measured from 247 spots on 90 individual trees against their inversed crosssectional areas [(pR2 ) −1 ]. The red line is a leastsquares fit through the data whose slope (6 Å) is the magnitude of the screw component of the Burgers vector. (D) Histogram of the calculated Burgers vectors for each data point shown in (C) with a Gaussian fit to the data. The Gaussian peak is centered at 6 Å with a standard deviation of 2 Å. 10 µm 500 nm A B 25 20 15 10 5 Count 0.4 0.8 1.2 1.6 2.0 2.4 2.8 Burgers vector (nm) R b unit length 0.4 D 0.3 0.2 0.1 0.0 Twist (rad/µm) 100 200 300 400 500 600 1/(πR2 ) (µm-2) C α 1062 23 MAY 2008 VOL 320 SCIENCE www.sciencemag.org REPORTS
measured at 247 points on 90 individual trees (23, 30), an experimentally observed mechanistic 16. w.K. Burton, N. Cabrera, F. C. Frank, Nature 163, 398 from 16 synthetic batches(Fig. 4C). To extract the understanding is currently lacking. We suggest magnitude of the Burgers vector, a line can be fit that this dislocation-driven nanowire growth mech- 17. w.K.Burton, N. Cabrera, F. C. Frank, Philos. Trans. R. Soc London A 243. 299(1951) ing b from Eq. 1 above. This can be more and is underappreciated in the synthesis of lD 19. G. w. Sears, Acta Metall. 3, 361(1955) directly seen in a histogram of the calculated nanostructures, particularly in cases where the 20. R. 5. Wagner, W.C. Ellis, KA Jackson, S M. Arnold, Burgers vectors(Fig. 4D). A Gaussian fit to these growth mechanism is inconclusively explained 21. D.R. veblen, 1. E. Post, Am. Mineral. 68. 79 data yields the average magnitude of the screw and especially when free of catalysts. Besides the (1983) component of the Burgers vector b=6+2 A alogous PbSe for which we have found pre- 22. D. B. Williams, C. B. Carter, Transmission Electron Because the Burgers vector direction is con- liminary evidence of similar growth phenomena, Microscopy: A Textbook for Materials Science(Plenum, firmed to be [110] by TEM, a 6 A screw the dislocation-driven nanowire growth mecha- New York, 1996), chaps. 22 and 25. component of the Burgers vector(the projection nism is likely to occur in materials that are prone 23.ke, Hit J. tothe, Theory of Dislocations (MCGraw-Hill, of b onto the dislocation line u [100) is ap- to have screw dislocations, such as SiC, GaN, 24. A Foitzik, w. Skrotzki, P Haasen, Physica Status SolidiA proximately equal to the lattice constant of ZnO, and Cas, both in vapor-phase growth and 121.81(1990 Pbs,a=5.94 A. It is known that smallest b in solution-phase synthesis. However, we caution 25. E Bauser, H. trunk, L. Cyst Growth 51. 362(1981) allowable is the shortest lattice translation vector that postgrowth mechanical perturbation could 26. 5,de dielbltin o h ys. 4c. 6 d crystals is (110), whose screw component is one might not be able to observe dislocations in mponents. when the cross section of the nanowire is no v100)(half the lattice constant a). Given various the final nanowire products if samples are not sources of errors in this estimate (27), it is sat- handled properly. can be used together with a sma dculation. when the dislocation line is not at the center isfying to see that no data were observed sub of the cylinder, a small correction factor is applied tantially below the theoretical minimal vector References and Notes 28. R. D. Dragsdorf, W. W. Webb, J. Appl. Phys. 29, 817 and the average estimated b value of twice the 1. Y Xia et al. Adv Mater. 15. 353(2003 minimal theoretical length is reasonable. Addi- 2. C M Lieber, Z L Wang MRS Bull. 32, 99(2006) 29. G. W. Sears, I. Chem. Phys. 31, 53(1959). tionally, theory predicts that left-handed disloca- 3. A M. Morales, C M Lieber, Science 279, 208(1998) tion spirals lead to right-handed Eshelby twists 4. R.S. Wagner, W C. Ellis, Appl. Phys. Lett.4, 89 (Oxford Univ. Press, London, 1967) 31. 5.] thanks NSF(CAREER DMR-0548232), Research and vice versa(23, 26); therefore, the equal prob- 5. Z w. Pan, Z R. Dai, Z L Wang, Science 291, 1947 ability of twist handedness implies equal prob- ofessor Grant, and 3M Nontenured Faculty Award for bility of Burgers vector sense(sign). 6. I. J Trentler et al., Science 270, 1791(1995) upport. M. J.B. was partially supported by an Air Products Fellowship. We thank R. Selinsky for assistance with the The observation of Eshelby twist in these pine ee nanowires is a clear demonstration and val- K A Dick et al, Nat. Mater. 3, 380(2004) illustrations in Fig. 2. idation of Eshelby's theory on dislocations. The 9.D. Wang. FQian, CYang, Z H Zhong, CMLieber, results also provide evidence for a catalyst-free 10. A Dong, R. Tang, W E. Buhro, J Am. Chem. Soc. 129, upporting Online Material wwsciencemag. org/cgi//full/1157131/DC1 nanowire growth mechanism driven by axial screw dislocations and imply that VLS and screw laterals and methods are available as supporting erial on Science online Figs. 51 to $9 dislocation-driven nanowire growth can coexist 12. M 1. Bierman, Y. K.A. Lau, S Jin, Nano Lett. 7, 290 Because of the distinct morphology difference 13. ]. Zhu et al, Nano Lett. 7, 1095(2007) that the dislocation is a result of cool-down or other 14. M. Fardy, A.L. Hochbaum, ).Goldberger, M. M. Zhang, 2008: accepted 16 April 2008 postgrowth perturbation. Although some general 15. B. Hannon, S. Kodambaka, F. M. Ross, R. M. Tromp 08 nce.1157131 discussions on the origins of dislocations exist atue440,6902006) information when citing this paper Detection of silica-Rich sulfate-rich sedimentary rocks at Meridiani Planum (4. The rover Spirit recently investigated the Deposits on Mars Eastem Valley between Home Plate and the Mitcheltree/Low Ridge complex(Fig. 1)in Gusev crater. Here we describe the discovery of silica- S.W. Squyres, R.E. Arvidson, 2 S. Ruff R. Gellert, R.V. Morris, 'D. W. Ming, 'L. Crumpler, rich deposits in the Eastem Valley and farther east 1. D. Farmer ,D ] Des Marais, A. Yen, S. M. McLennan, W. Calvin, 0]. F. Bell lll, 1 near sulfate-rich soil depos BC.Clark,A Wang, 2 T ] McCoy, M. E. Schmidt, 2P. A de Souza ]r 3 Home plate consists of laminated- -to-cross- bedded tephra that shows evidence for a volcanic Mineral deposits on the martian surface can elucidate ancient environmental conditions on the explosive origin, including a bomb sag produced planet. Opaline silica deposits (as much as 91 weight percent Sio2) have been found in association when an ejected -4-cm clast fell into deformable and as bedrock. We interpret these materials to have forros are present both as light-toned soils ash deposits(5). Mitcheltree Ridge and Low with volcanic materials by the Mars rover Spirit. The d Ridge, located east of Home Plate, are partially and therefore to be strong indicators of a former aqueous environment. This discovery is important eroded synclinal structures that expose tephra deposits (including lapillistones) capped by a upport thriving microbial ecosystems. deposit of vesicular basalt boulders. Soils in the Inner Basin -250 m to the north(Samra)and -50 m to the east(tyrone)(Figs. I and 2)of O paline silica deposits are an indicator of coatings of amorphous silica on rocks(1, 2), Home Plate contain hydrated ferric sulfate de- aqueous activity. Some regions of although this interpretation is not unique (3). posits(6, 7). The mobility of ferric iron under lars exhibit a thermal infrared spectral Results from the Mars rover Opportunity have apparently oxidizing conditions, leading to ferric gnature that has been interpreted to result from suggested that opaline silica could be present in sulfates and oxides, is suggestive of low pH con- www.sciencemag.orgScieNceVol32023May2008 1063
measured at 247 points on 90 individual trees from 16 synthetic batches (Fig. 4C). To extract the magnitude of the Burgers vector, a line can be fit to the data as plotted, with the slope representing b from Eq. 1 above. This can be more directly seen in a histogram of the calculated Burgers vectors (Fig. 4D). A Gaussian fit to these data yields the average magnitude of the screw component of the Burgers vector b = 6 ± 2 Å. Because the Burgers vector direction is confirmed to be [110] by TEM, a 6 Å screw component of the Burgers vector (the projection of b onto the dislocation line u [100]) is approximately equal to the lattice constant of PbS, a = 5.94 Å. It is known that smallest b allowable is the shortest lattice translation vector in a material (23), which in the case of rock salt crystals is ½〈110〉, whose screw component is ½〈100〉 (half the lattice constant a). Given various sources of errors in this estimate (27), it is satisfying to see that no data were observed substantially below the theoretical minimal vector, and the average estimated b value of twice the minimal theoretical length is reasonable. Additionally, theory predicts that left-handed dislocation spirals lead to right-handed Eshelby twists and vice versa (23, 26); therefore, the equal probability of twist handedness implies equal probability of Burgers vector sense (sign). The observation of Eshelby twist in these pine tree nanowires is a clear demonstration and validation of Eshelby’s theory on dislocations. The results also provide evidence for a catalyst-free nanowire growth mechanism driven by axial screw dislocations and imply that VLS and screw dislocation-driven nanowire growth can coexist. Because of the distinct morphology difference from the hyperbranched nanowires, it is unlikely that the dislocation is a result of cool-down or other postgrowth perturbation. Although some general discussions on the origins of dislocations exist (23, 30), an experimentally observed mechanistic understanding is currently lacking. We suggest that this dislocation-driven nanowire growth mechanism proposed for PbS trees is likely general to and is underappreciated in the synthesis of 1D nanostructures, particularly in cases where the growth mechanism is inconclusively explained and especially when free of catalysts. Besides the analogous PbSe for which we have found preliminary evidence of similar growth phenomena, the dislocation-driven nanowire growth mechanism is likely to occur in materials that are prone to have screw dislocations, such as SiC, GaN, ZnO, and CdS, both in vapor-phase growth and in solution-phase synthesis. However, we caution that postgrowth mechanical perturbation could work the dislocation out of the nanowires, and one might not be able to observe dislocations in the final nanowire products if samples are not handled properly. References and Notes 1. Y. Xia et al., Adv. Mater. 15, 353 (2003). 2. C. M. Lieber, Z. L. Wang, MRS Bull. 32, 99 (2006). 3. A. M. Morales, C. M. Lieber, Science 279, 208 (1998). 4. R. S. Wagner, W. C. Ellis, Appl. Phys. Lett. 4, 89 (1964). 5. Z. W. Pan, Z. R. Dai, Z. L. Wang, Science 291, 1947 (2001). 6. T. J. Trentler et al., Science 270, 1791 (1995). 7. A. I. Persson et al., Nat. Mater. 3, 677 (2004). 8. K. A. Dick et al., Nat. Mater. 3, 380 (2004). 9. D. Wang, F. Qian, C. Yang, Z. H. Zhong, C. M. Lieber, Nano Lett. 4, 871 (2004). 10. A. Dong, R. Tang, W. E. Buhro, J. Am. Chem. Soc. 129, 12254 (2007). 11. Materials and methods are available as supporting material on Science Online. 12. M. J. Bierman, Y. K. A. Lau, S. Jin, Nano Lett. 7, 2907 (2007). 13. J. Zhu et al., Nano Lett. 7, 1095 (2007). 14. M. Fardy, A. L. Hochbaum, J. Goldberger, M. M. Zhang, P. Yang, Adv. Mater. 19, 3047 (2007). 15. J. B. Hannon, S. Kodambaka, F. M. Ross, R. M. Tromp, Nature 440, 69 (2006). 16. W. K. Burton, N. Cabrera, F. C. Frank, Nature 163, 398 (1949). 17. W. K. Burton, N. Cabrera, F. C. Frank, Philos. Trans. R. Soc. London A 243, 299 (1951). 18. G. W. Sears, Acta Metall. 1, 457 (1953). 19. G. W. Sears, Acta Metall. 3, 361 (1955). 20. R. S. Wagner, W. C. Ellis, K. A. Jackson, S. M. Arnold, J. Appl. Phys. 35, 2993 (1964). 21. D. R. Veblen, J. E. Post, Am. Mineral. 68, 790 (1983). 22. D. B. Williams, C. B. Carter, Transmission Electron Microscopy: A Textbook for Materials Science (Plenum, New York, 1996), chaps. 22 and 25. 23. J. P. Hirth, J. Lothe, Theory of Dislocations (McGraw-Hill, New York, 1968). 24. A. Foitzik, W. Skrotzki, P. Haasen, Physica Status Solidi A 121, 81 (1990). 25. E. Bauser, H. Strunk, J. Cryst. Growth 51, 362 (1981). 26. J. D. Eshelby, J. Appl. Phys. 24, 176 (1953). 27. Edge dislocations do not produce such distortions. Mixed dislocations can be evaluated as separate screw and edge components. When the cross section of the nanowire is not circular (which is often the case), the cross-section area can be used together with a small correction factor for this calculation. When the dislocation line is not at the center of the cylinder, a small correction factor is applied. 28. R. D. Dragsdorf, W. W. Webb, J. Appl. Phys. 29, 817 (1958). 29. G. W. Sears, J. Chem. Phys. 31, 53 (1959). 30. F. R. N. Nabarro, Theory of Crystal Dislocations (Oxford Univ. Press, London, 1967). 31. S. J. thanks NSF (CAREER DMR-0548232), Research Corporation Cottrell Scholar Award, DuPont Young Professor Grant, and 3M Nontenured Faculty Award for support. M.J.B. was partially supported by an Air Products Fellowship. We thank R. Selinsky for assistance with the illustrations in Fig. 2. Supporting Online Material www.sciencemag.org/cgi/content/full/1157131/DC1 Materials and Methods Figs. S1 to S9 References Movie S1 29 February 2008; accepted 16 April 2008 Published online 1 May 2008; 10.1126/science.1157131 Include this information when citing this paper. Detection of Silica-Rich Deposits on Mars S. W. Squyres,1 * R. E. Arvidson,2 S. Ruff,3 R. Gellert,4 R. V. Morris,5 D. W. Ming,5 L. Crumpler,6 J. D. Farmer,3 D. J. Des Marais,7 A. Yen,8 S. M. McLennan,9 W. Calvin,10 J. F. Bell III,1 B. C. Clark,11 A. Wang,2 T. J. McCoy,12 M. E. Schmidt,12 P. A. de Souza Jr.13 Mineral deposits on the martian surface can elucidate ancient environmental conditions on the planet. Opaline silica deposits (as much as 91 weight percent SiO2) have been found in association with volcanic materials by the Mars rover Spirit. The deposits are present both as light-toned soils and as bedrock. We interpret these materials to have formed under hydrothermal conditions and therefore to be strong indicators of a former aqueous environment. This discovery is important for understanding the past habitability of Mars because hydrothermal environments on Earth support thriving microbial ecosystems. Opaline silica deposits are an indicator of past aqueous activity. Some regions of Mars exhibit a thermal infrared spectral signature that has been interpreted to result from coatings of amorphous silica on rocks (1, 2), although this interpretation is not unique (3). Results from the Mars rover Opportunity have suggested that opaline silica could be present in sulfate-rich sedimentary rocks atMeridiani Planum (4). The rover Spirit recently investigated the Eastern Valley between Home Plate and the Mitcheltree/Low Ridge complex (Fig. 1) in Gusev crater. Here we describe the discovery of silicarich deposits in the Eastern Valley and farther east near sulfate-rich soil deposits. Home Plate consists of laminated–to–crossbedded tephra that shows evidence for a volcanic explosive origin, including a bomb sag produced when an ejected ~4-cm clast fell into deformable ash deposits (5). Mitcheltree Ridge and Low Ridge, located east of Home Plate, are partially eroded synclinal structures that expose tephra deposits (including lapillistones) capped by a deposit of vesicular basalt boulders. Soils in the Inner Basin ~250 m to the north (Samra) and ~50 m to the east (Tyrone) (Figs. 1 and 2) of Home Plate contain hydrated ferric sulfate deposits (6, 7). The mobility of ferric iron under apparently oxidizing conditions, leading to ferric sulfates and oxides, is suggestive of low pH conwww.sciencemag.org SCIENCE VOL 320 23 MAY 2008 1063 REPORTS
