LETTERS nature PUBLISHED ONLINE: 18 OCTOBER 2009 I DOE: 10.1038/NNANO2009304 nanotechnology Single-crystalline kinked semiconductor nanowire superstructures Bozhi Tian', Ping Xie, Thomas J. Kempa', David C Bell2 and Charles M. Lieber 3 The ability to control and modulate the composition-, visible gold catalyst at the nanowire tip(Fig. Ic,d)and uniform doping 3-, crystal structure6- and morphology, 10 of semicon- diameter indicate that growth proceeds by means of the nanocluster ductor nanowires during the synthesis process has allowed catalysed VLS process- throughout the whole synthesis process. researchers to explore various applications of nanowires-15. Third, the joint angle is a constant 120 and all SBUs are confined However, despite advances in nanowire synthesis, progress in a single two-dimensional plane consistent with our model towards the ab initio design and growth of hierarchical nano-(Fig la). Finally, the yield of such a kinked two-dimensional chain structures has been limited. Here, we demonstrate a"nano- structure is greater than 40% for these 80-nm-diameter nanowires tectonic' approach that provides iterative control over the with purge times(step(2)of 15s( Supplementary Fig. $3),the nucleation and growth of nanowires, and use it to grow remaining nanowires having a one-dimensional morphology. The kinked or zigzag nanowires in which the straight sections are kinked nanowire structures could be purified to further enhance separated by triangular joints. Moreover, the lengths of the yields, and we note that the simple dispersion and deposition straight sections can be controlled and the growth direction process used to prepare samples for analysis leads to a preferential remains coherent along the nanowire. We also grow dopant- enhancement of the yield on substrates. modulated structures in which specific device functions, includ- To address the potential of ab initio design and synthesi ing p-n diodes and field-effect transistors, can be precisely prepared kinked silicon nanowires in which the arm length ocalized at the kinked junctions in the nanowires tionally varied. A representative SEM image of a structure with six d We have focused on the rational design and synthesis of two- tinct segment lengths(Fig. ld)reveals that the formation of well- dimensional multiply kinked nanowires(Fig. la), in which kinks defined SBU kinks is independent of the constituent f e introduced at defined positions during growth and are confined lengths within a range of at least 180-2, 500 nm as inv to a single plane. These hierarchical nanowires are constructed using Analysis of the segment lengths in uniformly kinked a nanotectonic approach analogous to metal-organic framework samples yields a linear dependence of segment length on axial materials 6, where we define a secondary building unit(SBU)6 con- growth time(Fig. le), further supporting our demonstration of well- sisting of two straight single-crystalline arms(blue, Fig. la)con- controlled VLS growth. The slope of the linear fit yields a nanowire nected by one fixed 1200 angle joint(green, Fig. la). Note that axial growth rate of 870 nm min- at constant pressure and uniform (11-20 or(1-100, vectors in a hexagonal structure can form data extracted from Fig. ld are also plotted (magenta squares); thes the desired 120 joint when rotating about the(111) and(0001) agree very well with the data acquired from the kinked nanowires zone axes, respectively(Fig. la; Supplementary Fig S1). SBU for- with uniform segments, demonstrating a high level of control for inde mation involves three main steps during nanocluster-catalysed pendent syntheses and, correspondingly, the capability for ab initio growth(Fig. 1b);(1)axial growth of a one-dimensional nanowire design and synthesis. Note also that these results, which show that arm segment, (2)purging of gaseous reactants to suspend nano- segment length is fully determined by growth time, are distinct from wire elongation, and (3)supersaturation and nucleation of nanowire the self-organized growth models used to explain oscillatory saw- growth following re-introduction of reactants. As illustrated for the tooth faceting in nonpolar silicon nanowires or twinning superlattices case of silicon, the concentration of silicon-reactant dissolved in the in(111)B-oriented m-v nanowires6-8 nanocluster catalyst drops during purging and then reaches a We have also determined the atomic-level structure of the two maximum upon supersaturation Steps()to(3)can be iterated dimensional kinked nanowires using transmission electron to link a number of SBUs, generating a wo-dimensional microscopy (TEM). A repres esentative TEM image(Fig 2a)of a mul chain structure tiply kinked silicon nanowire, and selected area electron diffraction We first illustrate this approach with the synthesis of two- (SAED) patterns recorded from non-adjacent joints( Fig. 2b), show imensional silicon nanowire chains. We synthesized 80-nm- that the entire nanostructure is single crystalline and that the arms diameter silicon nanowires with dominant (112)axial orientation d joints are free of bulk dislocations and defects. The SAED using a gold nanocluster-catalysed vapour-liquid-solid (VLS) terns from kink positions I and Il, which are separated by 3 um method 7-19(see Methods and Supplementary Fig S2). Scanning elec- and two intervening kinks, can be indexed for the(111)zone axis tron microscopy(SEM) images of a typical kinked silicon nanowire and show that the two-dimensional chain structure extends in the structure(Fig. Ic), produced by iterating the (1)to(3)cycle several 1ll] plane and that the segments grow along the(112)direction time so as to yield equal-length segments, highlight several notable fea- in a coherent manner. These observations are consistent with our tures. First, well-defined two-dimensional kinked nanowire structures orientation-controlled nanowire growth and SBU model( Fig. la are observed that have equal arm lengths, consistent with the constant TEM images of a single kink(Fig. 2c, d) further illustrate key SBU segment growth times, and uniform diameters. Second, the clearly features. The images demonstrate that there are no atomic-scale Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, USA, Center for Nanoscale Systems, Harvard University, Cambridge, Massachusetts 02138, USA, School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA.· e-mail cmac NatureNanotEchnOlogYIVol4IDecemBer2009Iwww.nature.com/naturenanotechnology
Single-crystalline kinked semiconductor nanowire superstructures Bozhi Tian1 , Ping Xie1 , Thomas J. Kempa1 , David C. Bell2 and Charles M. Lieber1,3* The ability to control and modulate the composition1–4, doping1,3–5, crystal structure6–8 and morphology9,10 of semiconductor nanowires during the synthesis process has allowed researchers to explore various applications of nanowires11–15. However, despite advances in nanowire synthesis, progress towards the ab initio design and growth of hierarchical nanostructures has been limited. Here, we demonstrate a ‘nanotectonic’ approach that provides iterative control over the nucleation and growth of nanowires, and use it to grow kinked or zigzag nanowires in which the straight sections are separated by triangular joints. Moreover, the lengths of the straight sections can be controlled and the growth direction remains coherent along the nanowire. We also grow dopantmodulated structures in which specific device functions, including p–n diodes and field-effect transistors, can be precisely localized at the kinked junctions in the nanowires. We have focused on the rational design and synthesis of twodimensional multiply kinked nanowires (Fig. 1a), in which kinks are introduced at defined positions during growth and are confined to a single plane. These hierarchical nanowires are constructed using a nanotectonic approach analogous to metal–organic framework materials16, where we define a secondary building unit (SBU)16 consisting of two straight single-crystalline arms (blue, Fig. 1a) connected by one fixed 1208 angle joint (green, Fig. 1a). Note that two k112lc or k110lc vectors in a cubic crystal structure and two k11–20lh or k1–100lh vectors in a hexagonal structure can form the desired 1208 joint when rotating about the k111lc and k0001lh zone axes, respectively (Fig. 1a; Supplementary Fig. S1). SBU formation involves three main steps during nanocluster-catalysed growth (Fig. 1b); (1) axial growth of a one-dimensional nanowire arm segment, (2) purging of gaseous reactants to suspend nanowire elongation, and (3) supersaturation and nucleation of nanowire growth following re-introduction of reactants. As illustrated for the case of silicon, the concentration of silicon-reactant dissolved in the nanocluster catalyst drops during purging and then reaches a maximum upon supersaturation. Steps (1) to (3) can be iterated to link a number of SBUs, generating a two-dimensional chain structure. We first illustrate this approach with the synthesis of twodimensional silicon nanowire chains. We synthesized 80-nmdiameter silicon nanowires with dominant k112l axial orientation using a gold nanocluster-catalysed vapour–liquid–solid (VLS) method17–19 (see Methods and Supplementary Fig. S2). Scanning electron microscopy (SEM) images of a typical kinked silicon nanowire structure (Fig. 1c), produced by iterating the (1) to (3) cycle several time so as to yield equal-length segments, highlight several notable features. First, well-defined two-dimensional kinked nanowire structures are observed that have equal arm lengths, consistent with the constant segment growth times, and uniform diameters. Second, the clearly visible gold catalyst at the nanowire tip (Fig. 1c,d) and uniform diameter indicate that growth proceeds by means of the nanoclustercatalysed VLS process17–19 throughout the whole synthesis process. Third, the joint angle is a constant 1208 and all SBUs are confined in a single two-dimensional plane consistent with our model (Fig. 1a). Finally, the yield of such a kinked two-dimensional chain structure is greater than 40% for these 80-nm-diameter nanowires with purge times (step (2)) of 15 s (Supplementary Fig. S3), the remaining nanowires having a one-dimensional morphology. The kinked nanowire structures could be purified to further enhance yields, and we note that the simple dispersion and deposition process used to prepare samples for analysis leads to a preferential enhancement of the yield on substrates. To address the potential of ab initio design and synthesis we have prepared kinked silicon nanowires in which the arm length is intentionally varied. A representative SEM image of a structure with six distinct segment lengths (Fig. 1d) reveals that the formation of welldefined SBU kinks is independent of the constituent segment lengths within a range of at least 180–2,500 nm as investigated. Analysis of the segment lengths in uniformly kinked nanowire samples yields a linear dependence of segment length on axial growth time (Fig. 1e), further supporting our demonstration of wellcontrolled VLS growth. The slope of the linear fit yields a nanowire axial growth rate of 870 nm min21 at constant pressure and uniform flow (Fig. 1e inset and Methods). The differential segment length data extracted from Fig. 1d are also plotted (magenta squares); these agree very well with the data acquired from the kinked nanowires with uniform segments, demonstrating a high level of control for independent syntheses and, correspondingly, the capability for ab initio design and synthesis. Note also that these results, which show that segment length is fully determined by growth time, are distinct from the self-organized growth models used to explain oscillatory sawtooth faceting in nonpolar silicon nanowires9 or twinning superlattices in k111l B-oriented III–V nanowires6–8. We have also determined the atomic-level structure of the twodimensional kinked nanowires using transmission electron microscopy (TEM). A representative TEM image (Fig. 2a) of a multiply kinked silicon nanowire, and selected area electron diffraction (SAED) patterns recorded from non-adjacent joints (Fig. 2b), show that the entire nanostructure is single crystalline and that the arms and joints are free of bulk dislocations and defects. The SAED patterns from kink positions I and II, which are separated by 3 mm and two intervening kinks, can be indexed for the k111l zone axis and show that the two-dimensional chain structure extends in the f111g plane and that the segments grow along the k112l direction in a coherent manner. These observations are consistent with our orientation-controlled nanowire growth and SBU model (Fig. 1a). TEM images of a single kink (Fig. 2c,d) further illustrate key SBU features. The images demonstrate that there are no atomic-scale 1 Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, USA, 2 Center for Nanoscale Systems, Harvard University, Cambridge, Massachusetts 02138, USA, 3 School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA. *e-mail: cml@cmliris.harvard.edu LETTERS PUBLISHED ONLINE: 18 OCTOBER 2009 | DOI: 10.1038/NNANO.2009.304 824 NATURE NANOTECHNOLOGY | VOL 4 | DECEMBER 2009 | www.nature.com/naturenanotechnology
NATURE NANOTECHNOLOGY DOl: 10.1038/NNANO2009304 LETTERS Axial elongation Add reactants 2000 01020304050 Time(s) 150180 Figure 1 I Design and controlled synthesis of multiply kinked nanowires. a, Schematic of a coherently kinked nanowire and the secondary building unit BU), which contains two arms(blue)and one joint (green). The multiply kinked nanowires(middle panel) are derived from the corresponding one- dimensional nanowire by introducing the joints at the points indicated by the dashed lines in the upper panel. Subscripts c and h denote cubic and hexagonal structures, respectively. b, Cycle for the introduction of a sBu by stepwise synthesis. The colour gradient accompanying the innermost blue arrows indicates the change of silicon concentration in nanocluster catalyst during synthesis of a kinked silicon nanowire. c, SEM image of a multiply kinked two-dimensional silicon nanowire with equal arm segment lengths. Scale bar, 1 um. The yellow arrow highlights the position of the nanoduster catalyst. d, SEM image of a multiply kinked silicon nanowire with decreasing arm segment lengths. Scale bar, 1 um. The growth durations are 30, 60, 90, 120, 150 and 180 s for egments 1 to 6, respectively. The yellow arrow highlights the position of the nanocluster catalyst. SEM images shown in c and d were acquired without substrate tilting, and the electron beam perpendicular to the two-dimensional plane of the multiply kinked nanowires. e, Plot of segment length versus growth time. Each blue diamond represents average segment length data(error bars: +1 s.d. )from a sample containing nanowires with uniform segment length between kinks. The green line is a linear fit to these data. Magenta solid squares are data points taken from the nanowire shown in d. Inset, growth pressure variation during kink synthesis. The black solid sphere and square denote the start of purging and re-introduction of reactants, respectively. across the complete arm-joint-arm junction. This is distinct from at the positions expected for kinks based on elongation time and other recent reports of modulated nanowires such as twinning growth rate. Higher resolution SEM or TEM images define the superlattices- that comprised twin planes and /or stacking faults. nodes as regions of slightly larger diameter with lengths of 50 Furthermore, the SBU reported in our work is unique in that it pre- nm. A summary of results for 80-and 150-nm diameters obtained h ultiple kinks, in contrast to single kinks observed previously oz, shows that this reduced kink frequency with decreasing purge ere the arms had either different growth directions 20 or compo- times is more pronounced in nanowire samples with larger diar sitions. Second, the joint has a quasi-triangular structure with eters. These results are consistent with the reactant concentration 1 top/bottom facets and two 112) side facets joining the adja- drop from the nanocluster catalyst being critical for kink formation ent arms. Finally, the nanowire growth direction changes during because the relative concentration drop will be smaller at a fixed growth of the kink, following(112)arm to (110)oint to(112) purge time in larger versus smaller diameter nanowires To shed light on the mechanism and limits of the single-crystal Overall, the above studies suggest kink formation can be qualitat line kinked junction formation, we characterized the kink frequency ively explained by the proposed stepwise model shown in Fig. 3d. In as a function of key parameters, including nanowire diameter and step 1, the reactant concentration drops in the supersaturated cata- purge time. The kink frequency is defined as Kink N/N,= lyst during the purge, and if the concentration is reduced suffi N /(Nk No), where N,, N and N, denote the number of total ciently, elongation will cease. When reactant is re-introduced in designed junctions, observed kink junctions, and observed straight step 2, the catalyst can become supersaturated again and undergo and node-like junctions, respectively. Under optimal growth con- heterogeneous nucleation,4 and growth. For short purge times ditions(see Methods), both 80-and 150-nm silicon nanowires and nanowires with larger diameter, the reactant concentration is Fig. 3a)show a high probability of kinks with a regular zigzag geo- sufficient for elongation to continue; however, as shown inin situ metry. When the purge time of step B(Fig. 1b)is reduced from TEM studies2, this situation can lead to a flattening of the catalyst NaturENanotEchNologYIVol4iDecemBer2009Iwww.naturecom/naturenanotechnology
twin defects or stacking faults, confirming a single-crystal structure across the complete arm–joint–arm junction. This is distinct from other recent reports of modulated nanowires such as twinning superlattices6–8 that comprised twin planes and/or stacking faults. Furthermore, the SBU reported in our work is unique in that it preserves crystallographic orientation and composition in arms over multiple kinks, in contrast to single kinks observed previously20,21, where the arms had either different growth directions20 or compositions21. Second, the joint has a quasi-triangular structure with f111g top/bottom facets and two f112g side facets joining the adjacent arms. Finally, the nanowire growth direction changes during growth of the kink, following k112larm to k110ljoint to k112larm. To shed light on the mechanism and limits of the single-crystalline kinked junction formation, we characterized the kink frequency as a function of key parameters, including nanowire diameter and purge time. The kink frequency is defined as Pkink ¼ Nk/Nt ¼ Nk/(Nk þ Ns ), where Nt, Nk and Ns denote the number of total designed junctions, observed kink junctions, and observed straight and node-like junctions, respectively. Under optimal growth conditions (see Methods), both 80- and 150-nm silicon nanowires (Fig. 3a) show a high probability of kinks with a regular zigzag geometry. When the purge time of step B (Fig. 1b) is reduced from optimal to 3 or 1 s, nodes or incipient kinks (Fig. 3b) are observed at the positions expected for kinks based on elongation time and growth rate. Higher resolution SEM or TEM images define the nodes as regions of slightly larger diameter with lengths of 50 nm. A summary of results for 80- and 150-nm diameters obtained for 1, 3 and 15 s purges (Fig. 3c) quantifies these observations and shows that this reduced kink frequency with decreasing purge times is more pronounced in nanowire samples with larger diameters. These results are consistent with the reactant concentration drop from the nanocluster catalyst being critical for kink formation because the relative concentration drop will be smaller at a fixed purge time in larger versus smaller diameter nanowires22. Overall, the above studies suggest kink formation can be qualitatively explained by the proposed stepwise model shown in Fig. 3d. In step 1, the reactant concentration drops in the supersaturated catalyst during the purge, and if the concentration is reduced suffi- ciently, elongation will cease. When reactant is re-introduced in step 2, the catalyst can become supersaturated again and undergo heterogeneous nucleation23,24 and growth. For short purge times and nanowires with larger diameter, the reactant concentration is sufficient for elongation to continue; however, as shown in in situ TEM studies22, this situation can lead to a flattening of the catalyst b c e a d Segment length (nm) 0 30 60 90 120 150 180 0 1,000 2,000 Growth time (s) Pressure (torr) Time (s) 0 10 20 30 40 50 0 20 40 1 6 5 4 3 2 Secondary building unit 120° 1 2 3 Nucleation Purge reactants Axial elongation Si % Add reactants Perturbation Supersaturation − − Figure 1 | Design and controlled synthesis of multiply kinked nanowires. a, Schematic of a coherently kinked nanowire and the secondary building unit (SBU), which contains two arms (blue) and one joint (green). The multiply kinked nanowires (middle panel) are derived from the corresponding onedimensional nanowire by introducing the joints at the points indicated by the dashed lines in the upper panel. Subscripts c and h denote cubic and hexagonal structures, respectively. b, Cycle for the introduction of a SBU by stepwise synthesis. The colour gradient accompanying the innermost blue arrows indicates the change of silicon concentration in nanocluster catalyst during synthesis of a kinked silicon nanowire. c, SEM image of a multiply kinked two-dimensional silicon nanowire with equal arm segment lengths. Scale bar, 1 mm. The yellow arrow highlights the position of the nanocluster catalyst. d, SEM image of a multiply kinked silicon nanowire with decreasing arm segment lengths. Scale bar, 1 mm. The growth durations are 30, 60, 90, 120, 150 and 180 s for segments 1 to 6, respectively. The yellow arrow highlights the position of the nanocluster catalyst. SEM images shown in c and d were acquired without substrate tilting, and the electron beam perpendicular to the two-dimensional plane of the multiply kinked nanowires. e, Plot of segment length versus growth time. Each blue diamond represents average segment length data (error bars:+1 s.d.) from a sample containing nanowires with uniform segment lengths between kinks. The green line is a linear fit to these data. Magenta solid squares are data points taken from the nanowire shown in d. Inset, growth pressure variation during kink synthesis. The black solid sphere and square denote the start of purging and re-introduction of reactants, respectively. NATURE NANOTECHNOLOGY DOI: 10.1038/NNANO.2009.304 LETTERS NATURE NANOTECHNOLOGY | VOL 4 | DECEMBER 2009 | www.nature.com/naturenanotechnology 825
LETTERS NATURE NANOTECHNOLOGY DOL: 10.1038/NNANO2009304 Figure 2 Crystallographic structure of kinked silicon nanowires. a, Bright-field TEM image of a multiply kinked silicon nanowire. Scale bar, 1 um. The green (region D)and red (region ii) dashed circles highlight non-adjacent kinks where diffraction data were recorded. The yellow arrow highlights the position of the anocluster catalyst. b, SAED patterns recorded from regions I and ll in a. The SaEd patterns were recorded along the( 111) zone axis. c TEM image of a single kink with crystallographic directions and facets indicated by arrows and dashed lines, respectively. Scale bar, 50 nm. The green (region D) and blue I and ll in c. Scale bars, 5nm. Dashed lines and arrows denote crystallographic planes and growth directions, respectively. All TEM images and SAEo'regions (region II)open squares highlight regions of the joint and one arm where high-resolution images were recorded. d, Lattice-resolved TEM images from patterns were acquired with the electron beam perpendicular to the two-dimensional plane of the kinked nanowires nanodroplet and an increase in nanowire diameter consistent with we believe that these results already highlight an emerging potential the formation of nodes(Fig. 3b, marked with blue stars). In step 3, of our nanotectonic approach to generate, in a predictable manner, growth proceeds with preservation of the most stable (111 complex two-dimensional nanowire structures facets, thus implying that the heterogeneous nucleation should Furthermore, we have used our model for the designed synthesis of occur preferentially at the active (110) edges of the three-phase two-dimensional kinked nanowire structures in other materials. For direction about the(111)axis. This growth along(110)is transient using the iterative approach of Fig. la(see Methods)show o S uoo boundary This model yields a transition from the (112)to(110) example, SEM images(Fig. 4a) of german because this direction is not thermodynamically favourable in this with well-defined kinks, where the kink angle, 1200, is consistent with diameter regime 9,20(Supplementary Fig. S2), and in step 4, the that for the SBU. TEM images(Fig 4b) further demonstrate that(i)the kink is completed with a transition to another (112)direction, growth direction of the arms of the two-dimensional kinked germa- thus completing a single SBU with coherent arm growth directions. nium nanowires are along the(112) direction and (i)the joint is We did not observe(112)to(111)growth switching2 in our kinked single-crystalline. These structural details are consistent with the structures,most probably because the growth of a(111) segment general features observed in kinked silicon nanowires(Figs 1 and 2) requires the formation of six new 112) facets and the disappear- Our model also predicts that the arm-joint-arm kink SBU could be ance of two stable 111 facets of the initial(112)segment. realized in very different materials such as the wurtzite phase of the Although additional experiments will be necessary to clarify group I-VI semiconductor CdS. Notably, designed iterative modulation ails of the kink formation hypothesis in our proposed growth of the growth of(11-20 direction CdS nanowires yields a regular two- model, we note that this model now enables the design and synthesis dimensional kinked structure with 120 kink angle as shown in Fig. 4c. of specific structures in silicon nanowires and, more generally, TEM images(Fig. 4d)demonstrate that the Cds two-dimensional nanowire systems with distinct compositions. To illustrate this kinked nanowire structure is single-crystalline, with arms all along point, we first designed and synthesized (kink-node)m and(kink- the (11-20) direction of the wurtzite phase. Finally, we suggest node),(kink), modulated silicon nanowire structures, where our approach could also be used for the designed synthesis of two- and n are indices denoting the number of times the growth of the dimensional kinked group I-v nanowire materials such as gan nano. structural unit is repeated. We chose 150-nm gold as the catalysts, wires, which have been reported with almost pure(11-20)orientation!. d 15 and I s as the purge durations( Fig. 3c) for the growth of These results highlight an emerging potential for our bottom-up kinks and nodes, respectively. Notably, SEM images of the(kink- nanotectonic approach to generate more complex nanowires with node)m structure(Fig. 3e, I and II) show that the nodes(highlighted potentially unique function integrated at the nanoscale in the topo with yellow stars)are reproducibly inserted between kinks over mul- logically defined points of the kinks. We illustrate this capability by tiple modulations. These results also show that the formation of combining our iterative growth approach with additional modu individual kinks or nodes is independent of adjacent elements lation of the dopant to vary the electronic characteristics in a and is controlled by growth conditions. This latter point and poss- well-defined manner with respect to the kinks. A kinked silicon ible control is further demonstrated by the synthesis of coherent nanowire SBU with integrated n-and p-type arms was synthesized (kink) SBUs following modulated(kink-node) units(Fig. 3e, III). by switching phosphine and diborane dopants during the kink Interestingly, the observation of coherent zigzag chain structures growth sequence (see Methods). Current-voltage (I-v)data suggests that 'steering of kinks is not random and might be due, recorded on a representative single kink device( Fig. 5a)reveal a for example, to a minimization of stress or maintenance of the clear current rectification in reverse bias with an onset at centre-of-mass of the whole structure. Although further studies will a forward bias voltage of 0. V, consistent with the synthesis of be required to understand coherence in multiply kinked structures, a well-defined p-n diode within the kinked structure. Moreover, 826 NatureNanotEchnOlogYIVol4IDecemBer2009Iwww.nature.com/naturenanotechnology
nanodroplet and an increase in nanowire diameter consistent with the formation of nodes (Fig. 3b, marked with blue stars). In step 3, growth proceeds with preservation of the most stable f111g facets25, thus implying that the heterogeneous nucleation should occur preferentially at the active f110g edges26 of the three-phase boundary24. This model yields a transition from the k112l to k110l direction about the k111l axis. This growth along k110l is transient because this direction is not thermodynamically favourable in this diameter regime19,20 (Supplementary Fig. S2), and in step 4, the kink is completed with a transition to another k112l direction, thus completing a single SBU with coherent arm growth directions. We did not observe k112l to k111l growth switching20 in our kinked structures, most probably because the growth of a k111l segment requires the formation of six new f112g facets9 and the disappearance of two stable f111g facets26 of the initial k112l segment. Although additional experiments will be necessary to clarify details of the kink formation hypothesis in our proposed growth model, we note that this model now enables the design and synthesis of specific structures in silicon nanowires and, more generally, nanowire systems with distinct compositions. To illustrate this point, we first designed and synthesized (kink-node)m and (kinknode)m(kink)n modulated silicon nanowire structures, where m and n are indices denoting the number of times the growth of the structural unit is repeated. We chose 150-nm gold as the catalysts, and 15 and 1 s as the purge durations (Fig. 3c) for the growth of kinks and nodes, respectively. Notably, SEM images of the (kinknode)m structure (Fig. 3e, I and II) show that the nodes (highlighted with yellow stars) are reproducibly inserted between kinks over multiple modulations. These results also show that the formation of individual kinks or nodes is independent of adjacent elements and is controlled by growth conditions. This latter point and possible control is further demonstrated by the synthesis of coherent (kink)8 SBUs following modulated (kink-node)4 units (Fig. 3e, III). Interestingly, the observation of coherent zigzag chain structures suggests that ‘steering’ of kinks is not random and might be due, for example, to a minimization of stress or maintenance of the centre-of-mass of the whole structure. Although further studies will be required to understand coherence in multiply kinked structures, we believe that these results already highlight an emerging potential of our nanotectonic approach to generate, in a predictable manner, complex two-dimensional nanowire structures. Furthermore, we have used our model for the designed synthesis of two-dimensional kinked nanowire structures in other materials. For example, SEM images (Fig. 4a) of germanium nanowires grown using the iterative approach of Fig. 1a (see Methods) show nanowires with well-defined kinks, where the kink angle, 1208, is consistent with that for the SBU. TEM images (Fig. 4b) further demonstrate that (i) the growth direction of the arms of the two-dimensional kinked germanium nanowires are along the k112l direction and (ii) the joint is single-crystalline. These structural details are consistent with the general features observed in kinked silicon nanowires (Figs 1 and 2). Our model also predicts that the arm–joint–arm kink SBU could be realized in very different materials such as the wurtzite phase of the group II–VIsemiconductor CdS. Notably, designed iterative modulation of the growth of k11–20l direction CdS nanowires yields a regular twodimensional kinked structure with 1208 kink angle as shown in Fig. 4c. TEM images (Fig. 4d) demonstrate that the CdS two-dimensional kinked nanowire structure is single-crystalline, with arms all along the k11–20l direction of the wurtzite phase. Finally, we suggest our approach could also be used for the designed synthesis of twodimensional kinked group III–V nanowire materials such as GaN nanowires, which have been reported with almost pure k11–20l orientation4 . These results highlight an emerging potential for our bottom-up nanotectonic approach to generate more complex nanowires with potentially unique function integrated at the nanoscale in the topologically defined points of the kinks. We illustrate this capability by combining our iterative growth approach with additional modulation of the dopant to vary the electronic characteristics in a well-defined manner with respect to the kinks. A kinked silicon nanowire SBU with integrated n- and p-type arms was synthesized by switching phosphine and diborane dopants during the kink growth sequence (see Methods). Current–voltage (I–V) data recorded on a representative single kink device (Fig. 5a) reveal a clear current rectification in reverse bias with an onset at a forward bias voltage of 0.6 V, consistent with the synthesis of a well-defined p–n diode within the kinked structure. Moreover, a (220) (022) (202) I II (022) (202) 1/3(242) 1/3(422) c I II I II 110 112 112 I II {112} {110} b d − (220) − − − 1/3(242) 1/3(422) − − Figure 2 | Crystallographic structure of kinked silicon nanowires. a, Bright-field TEM image of a multiply kinked silicon nanowire. Scale bar, 1 mm. The green (region I) and red (region II) dashed circles highlight non-adjacent kinks where diffraction data were recorded. The yellow arrow highlights the position of the nanocluster catalyst. b, SAED patterns recorded from regions I and II in a. The SAED patterns were recorded along the k111l zone axis. c, TEM image of a single kink with crystallographic directions and facets indicated by arrows and dashed lines, respectively. Scale bar, 50 nm. The green (region I) and blue (region II) open squares highlight regions of the joint and one arm where high-resolution images were recorded. d, Lattice-resolved TEM images from regions I and II in c. Scale bars, 5 nm. Dashed lines and arrows denote crystallographic planes and growth directions, respectively. All TEM images and SAED patterns were acquired with the electron beam perpendicular to the two-dimensional plane of the kinked nanowires. LETTERS NATURE NANOTECHNOLOGY DOI: 10.1038/NNANO.2009.304 826 NATURE NANOTECHNOLOGY | VOL 4 | DECEMBER 2009 | www.nature.com/naturenanotechnology
NATURE NANOTECHNOLOGY DOl: 10.1038/NNANO2009304 LETTERS c100 20 Diameter (nm) Figure 3 I Mechanistic studies of kinked nanowire growth. a, SEM images of 150-(upper) and 80-nm(lower) diameter kinked silicon nanowires grown with periodic 155 purges. Scale bar, 1 um. b, TEM images at low (left panel)and high(right panel)magnification of one 80-nm-diameter silicon nanowire segment subjected to a l s purge. The blue stars mark incipient kinks or nodes, and the dashed square corresponds to the region where the right panel was recorded Scale bars, 500(left) and 50 nm(right). c, Kink frequency(Pkink) histogram for 80-and 150-nm diameter silicon nanowires grown with different purge durations. Green, red and blue bars denote results from 1, 3 and 15s purges, respectively, and are averaged over at least 15 multiply kinked nanowires. d, Schematic illustrating the key stages of kink formation. Arrows 1-4 denote purge, re-introduction of reactant, joint growth and subsequent arm growth, spectively. e, SEM images of two-dimensional silicon nanowires with modulated kinks and incipient kinks (starred nodes). i corresponds to a designed (kink-node) structure: Il is an enlargement of one node from the region indicated by the dashed yellow square in l; and lll corresponds to a(kink- node)(kink), structure, where m and n are integers. The angle between the norm of the kinked nanowire plane and the electron beam in Ill is 50. All ther images were acquired with the electron beam perpendicular to the two-dimensional plane of the kinked nanowires. Scale bars in l ll and ll are 1, 0.2 and 1 um, respectively. Furthermore, our concept can be extended to the design and syn- thesis of nanowires with distinct functionality at sequential kinks. A representative atomic force microscopy(AFM) image of a double kink structure synthesized with n* and n dopant profiles at the two kink joints(Fig. 5c)shows that the characteristic SBU described above is unaffected by multiple modulations of dopant concen- tration. notably scanned gat ata demonstrate enhanced(decreased) nanowire conductance as the tip with positive(negative) gate potential is scanned across the designed n-type segment immediately adjacent to the upper-left kink junction, thus confirming the integration of an n-type field effect transistor at a well-defined and recognizable point on the structure. The absence of gate response from the lower-right kink junction(Fig. 5d) further shows that the single-crystalline kink structure itself will not alter the electrical we Figure 4 I Generality of kinked nanowire synthesis. a, SEM image of one believe that these synthetic results and demonstrated topologically multiply kinked germanium nanowire Scale bar, 1 um. b, Lattice-resolved defined functional devices represent a significant advance towards TEM image of the joint region of a representative germanium nanowire the realization of ab initio designed and 'self-labelledtwo-dimen- kink Scale bar, 5nm Inset highlights one secondary building unit (SBU) sional nanowire structures. Such designed and self-labelled two- with arrows corresponding to(112)growth directions and the red square dimensional nanowire structures may open up unique applications to the region where the high-resolution image was recorded. Inset scale in the bottom-up integration of active devices in nanoelectronics, ar, 50 nm. c, SEM image of one multiply kinked CdS nanowire Scale photodetector arrays, multiplexed biological sensors and the devel bar, 1 um. d, Lattice-resolved TEM image of the arm region of a representative Cds adjacent to the kink joint Scale bar, 5 nm. The inset opment of multi-terminal nanodevices in three dimensions highlights two SBUs with the arrows corresponding to the (11-20 Methods resolution image was recorded. Inset scale bar, 50 nm. All images were nano wure synthesis. ingke-crystalline kinked na growth directions and the red square to the region where the high- were synthesized using the acquired with the electron beam perpendicular to the two-dimensional connected to a gas plane of the kinked nanowire controlled tube furnace Monodisperse gold nanoparticles (Ted Pella)were dispersed an electrostatic force microscopy(EFM)image of a typical kinked paruct/ silicon or sapphire growth substrates (gold surface coverage: 0.01-0.1 P-n nanowire in reverse bias (Fig. 5b)showed that the voltage yields. The silicon nanowires were synthesized at 450-460 C using silane(Sih)a drop occurs primarily at the designed p-n junction localized and the silicon reactant source, H2 as the carrier gas, and phosphine(PH3, 1,000 ppm in labelled by the kink during growth. H,)and diborane(B, Hg, 100 ppm in H,)as the n-and p-type dopants In a typical NaturENanotEchNologYIVol4iDecemBer2009Iwww.naturecom/naturenanotechnology
an electrostatic force microscopy (EFM) image of a typical kinked p–n nanowire in reverse bias (Fig. 5b) showed that the voltage drop occurs primarily at the designed p–n junction localized and labelled by the kink during growth. Furthermore, our concept can be extended to the design and synthesis of nanowires with distinct functionality at sequential kinks. A representative atomic force microscopy (AFM) image of a double kink structure synthesized with nþ and n dopant profiles at the two kink joints (Fig. 5c) shows that the characteristic SBU described above is unaffected by multiple modulations of dopant concentration. Notably, scanned gate microscopy (SGM) data (Fig. 5d) demonstrate enhanced (decreased) nanowire conductance as the tip with positive (negative) gate potential is scanned across the designed n-type segment immediately adjacent to the upper-left kink junction, thus confirming the integration of an n-type fieldeffect transistor at a well-defined and recognizable point on the structure. The absence of gate response from the lower-right kink junction (Fig. 5d) further shows that the single-crystalline kink structure itself will not alter the electrical transport properties. We believe that these synthetic results and demonstrated topologically defined functional devices represent a significant advance towards the realization of ab initio designed and ‘self-labelled’ two-dimensional nanowire structures. Such designed and self-labelled twodimensional nanowire structures may open up unique applications in the bottom-up integration of active devices in nanoelectronics, photodetector arrays, multiplexed biological sensors and the development of multi-terminal nanodevices in three dimensions. Methods Nanowire synthesis. Single-crystalline kinked nanowires were synthesized using the nanocluster-catalysed VLS method described previously18,19 in a quartz tube connected to a gas manifold and vacuum pump and heated by a temperaturecontrolled tube furnace. Monodisperse gold nanoparticles (Ted Pella) were dispersed on SiO2/silicon or sapphire growth substrates (gold surface coverage: 0.01–0.1 particles mm22 ), which were placed within the central region of the quartz tube reactor. Nanowires grown on both substrates yielded similar kink morphologies and yields. The silicon nanowires were synthesized at 450–4608C using silane (SiH4) as the silicon reactant source, H2 as the carrier gas, and phosphine (PH3, 1,000 ppm in H2) and diborane (B2H6, 100 ppm in H2) as the n- and p-type dopants. In a typical a c 0 20 40 60 80 100 80 150 Diameter (nm) e d b I II III {110} 1 2 3 112 112 112 4 {111} 110 110 Figure 3 | Mechanistic studies of kinked nanowire growth. a, SEM images of 150- (upper) and 80-nm (lower) diameter kinked silicon nanowires grown with periodic 15 s purges. Scale bar, 1 mm. b, TEM images at low (left panel) and high (right panel) magnification of one 80-nm-diameter silicon nanowire segment subjected to a 1 s purge. The blue stars mark incipient kinks or nodes, and the dashed square corresponds to the region where the right panel was recorded. Scale bars, 500 (left) and 50 nm (right). c, Kink frequency (Pkink) histogram for 80- and 150-nm diameter silicon nanowires grown with different purge durations. Green, red and blue bars denote results from 1, 3 and 15 s purges, respectively, and are averaged over at least 15 multiply kinked nanowires. d, Schematic illustrating the key stages of kink formation. Arrows 1–4 denote purge, re-introduction of reactant, joint growth and subsequent arm growth, respectively. e, SEM images of two-dimensional silicon nanowires with modulated kinks and incipient kinks (starred nodes). I corresponds to a designed (kink-node)m structure; II is an enlargement of one node from the region indicated by the dashed yellow square in I; and III corresponds to a (kinknode)m(kink)n structure, where m and n are integers. The angle between the norm of the kinked nanowire plane and the electron beam in III is 508. All other images were acquired with the electron beam perpendicular to the two-dimensional plane of the kinked nanowires. Scale bars in I, II and III are 1, 0.2 and 1 mm, respectively. 1120 Ge CdS 112 a b c d − Figure 4 | Generality of kinked nanowire synthesis. a, SEM image of one multiply kinked germanium nanowire. Scale bar, 1 mm. b, Lattice-resolved TEM image of the joint region of a representative germanium nanowire kink. Scale bar, 5 nm. Inset highlights one secondary building unit (SBU) with arrows corresponding to k112l growth directions and the red square to the region where the high-resolution image was recorded. Inset scale bar, 50 nm. c, SEM image of one multiply kinked CdS nanowire. Scale bar, 1 mm. d, Lattice-resolved TEM image of the arm region of a representative CdS adjacent to the kink joint. Scale bar, 5 nm. The inset highlights two SBUs with the arrows corresponding to the k11–20l growth directions and the red square to the region where the highresolution image was recorded. Inset scale bar, 50 nm. All images were acquired with the electron beam perpendicular to the two-dimensional plane of the kinked nanowires. NATURE NANOTECHNOLOGY DOI: 10.1038/NNANO.2009.304 LETTERS NATURE NANOTECHNOLOGY | VOL 4 | DECEMBER 2009 | www.nature.com/naturenanotechnology 827
LETTERS NATURE NANOTECHNOLOGY DOL: 10.1038/NNANO2009304 GND Vbias (v) Figure 5 I Topologically defined nanoelectronic devices. a, I-V data recorded from a kinked p-n silicon nanowire device Inset: SEM image of the device structure. Scale bar, 2 um. b, EFM image of a p-n diode reverse-biased at 5 V. The AFM tip voltage was modulated by 3 V at the ca frequency. The signal brightness is proportional to the nanowire device surface potential, and shows an abrupt drop around the kink position. The dashed hes mark the nanowire position. Scale bar, 2 um. c, d, AFM and SGM images of one nt-kink-nt-kink-(n-nt) dopant modulated double-kinked silicon nanowire structure Scale bar in c, 2 um. The SGM images were recorded with a Vin of 10V(i) and -10V(ID), respectively, and Vsd of 1 V. The dark and bright regions correspond to reduced and enhanced conductance, respectively. the black dashed lines mark the nanowire position. synthesis of uniform n-type, 80-nm kinked silicon nanowires, the flow rates of SiHg, meast were carried out with a Digital Instruments Nanoscope Illa respectively, and the total pressure 40 torr and purge duration 10-15 s; the minimum surface potential maps and SGM con tips(Nanosensors, PPP-NCHPt).The EFM PH, and H, were 1-2, 2-10 and 60 standard cubic centimetres per minute, MultiMode AFM and metal-coated pressure during the purge cycle was x3 x 10- torr. The dopant feed-in ratios lift heights of 40 and 20 nm, respectively. In the surface potential measurements, the (silicon: boron/phosphorus) in kinked p-n silicon nanowires were 500: 1 for both p-n diode was reverse-biased at 5 V and the tip voltage was modulated by 3 V at the p-and n-type segments. In nt-kink-nt-kink-(n-nt)dopant modulated silicon resonance frequency. In SGM measurements, the tip functions as a local gate V n*-and n-type segments, respectively, and the n-type segment was grown for 30s +10V, and the conductance versus position provides a measure of local anowires, the silicon-phosphorus feed-in ratios were 200: l and 10,000:1 for Germanium nanowires were synthesized at 270-290.. 40 torr, with (GeHa, 10% in H2)and H, as the reactant and carrier gas, respectively. Cds Received 2 June 2009; accepted 15 September 2009 with nanowire growth using gold nanocluster-catalysed VLS method at published online 18 October 2009 The purge cycle used to form kinks in the germanium and Cds Refere Gudiksen, M. S, Lauhon, LJ, Wang, J, Smith, D. C. Lieber, C M. Growth of Structure characterization. Zeiss Ultra55/Supra55VP field-emission SEMs and a nanowire superlattice structures for nanoscale photonics and electronics. Nature JEOL 2010 field-emission TEM were carry out SEM and TEN 415,617-620(2002 espectively. For sample preparation, kinked nanowires 2. Bjork, M. T et al. One-dimensional heterostructures in semiconductor cohol and dispersed onto heavily doped silicon substrate whiskers. Appl. Phys. Left. 80, 1058-1060(2002). 200-nm nitride; resistivity, 1-10 n2 cm, Nova Electronic Mater M.S. Wang, D. L. grids (Ted Pella). anowire heterostructures. Nature 420, 57-61(2002 4. Qian, F, Li, Y, Gradecak, S, Wang, D L, Barrelet, c Device n and me ent. Devices were fabricated on silicon substrates itride-based nanowire radial hete tures for nanophotonics Nano Lett. 4 (Nova Electronic Materials, n-type 0.005 Q cm)with 100-nm thermal oxide and 5. followed by titanium/palladium(1.5nm/100 nm)contact deposition in a thermal Pathrc, Zhong,, z. H.&Lieber, C.MEncoding electronic properties by waporator Current-voltage(I-V) data 1304-130702005) iconductor parameter analyser(Model 4156C)with contacts to devices made 6. Algra, R. E et al. Twinning superlattices in indium phosphide nanowires. Nafure a probe station(Desert Cryogenics, Model TTP4) EFM and SGM 456,369-372(2008 828 NatureNanotEchnOlogYIVol4IDecemBer2009Iwww.nature.com/naturenanotechnology
synthesis of uniform n-type, 80-nm kinked silicon nanowires, the flow rates of SiH4, PH3 and H2 were 1–2, 2–10 and 60 standard cubic centimetres per minute, respectively, and the total pressure 40 torr and purge duration 10–15 s; the minimum pressure during the purge cycle was 3 1023 torr. The dopant feed-in ratios (silicon:boron/phosphorus) in kinked p–n silicon nanowires were 500:1 for both p- and n-type segments. In nþ–kink–nþ–kink–(n–nþ) dopant modulated silicon nanowires, the silicon–phosphorus feed-in ratios were 200:1 and 10,000:1 for nþ- and n-type segments, respectively, and the n-type segment was grown for 30 s. Germanium nanowires were synthesized at 270–2908C, 40 torr, with germane (GeH4, 10% in H2) and H2 as the reactant and carrier gas, respectively. CdS nanowires were grown in a three-zone furnace by evaporating CdS power at 650–720 8C, with nanowire growth using gold nanocluster-catalysed VLS method at 550–500 8C. The purge cycle used to form kinks in the germanium and CdS nanowires was typically 15 s. Structure characterization. Zeiss Ultra55/Supra55VP field-emission SEMs and a JEOL 2010 field-emission TEM were used to carry out SEM and TEM analyses, respectively. For sample preparation, kinked nanowires were gently sonicated in isopropyl alcohol and dispersed onto heavily doped silicon substrates (100-nm oxide/200-nm nitride; resistivity, 1–10 V cm, Nova Electronic Materials) or lacey carbon grids (Ted Pella). Device fabrication and measurement. Devices were fabricated on silicon substrates (Nova Electronic Materials, n-type 0.005 V cm) with 100-nm thermal oxide and 200-nm SiN at the surface. Devices were defined by electron-beam lithography followed by titanium/palladium (1.5 nm/100 nm) contact deposition in a thermal evaporator. Current–voltage (I–V) data were recorded using an Agilent semiconductor parameter analyser (Model 4156C) with contacts to devices made using a probe station (Desert Cryogenics, Model TTP4). EFM and SGM measurements were carried out with a Digital Instruments Nanoscope IIIa MultiMode AFM and metal-coated tips (Nanosensors, PPP-NCHPt). The EFM surface potential maps and SGM conductance maps were acquired in lift mode with lift heights of 40 and 20 nm, respectively. In the surface potential measurements, the p–n diode was reverse-biased at 5 V and the tip voltage was modulated by 3 V at the resonance frequency. In SGM measurements, the tip functions as a local gate Vtip ¼ +10 V, and the conductance versus position provides a measure of local accumulation or depletion of carriers in the device. Received 2 June 2009; accepted 15 September 2009; published online 18 October 2009 References 1. Gudiksen, M. S., Lauhon, L. J., Wang, J., Smith, D. C. & Lieber, C. M. Growth of nanowire superlattice structures for nanoscale photonics and electronics. Nature 415, 617–620 (2002). 2. Bjork, M. T. et al. One-dimensional heterostructures in semiconductor nanowhiskers. Appl. Phys. Lett. 80, 1058–1060 (2002). 3. Lauhon, L. J., Gudiksen, M. S., Wang, D. L. & Lieber, C. M. Epitaxial core–shell and core–multishell nanowire heterostructures. Nature 420, 57–61 (2002). 4. Qian, F., Li, Y., Gradecˇak, S., Wang, D. L., Barrelet, C. J. & Lieber, C. M. Gallium nitride-based nanowire radial heterostructures for nanophotonics. Nano Lett. 4, 1975–1979 (2004). 5. Yang, C., Zhong, Z. H. & Lieber, C. M. Encoding electronic properties by synthesis of axial modulation-doped silicon nanowires. Science 310, 1304–1307 (2005). 6. Algra, R. E. et al. Twinning superlattices in indium phosphide nanowires. Nature 456, 369–372 (2008). c n+ n+ n n+ b GND 5 V p n a −4 −2 0 2 0 20 40 60 80 Vbias (V) I II d Figure 5 | Topologically defined nanoelectronic devices. a, I–V data recorded from a kinked p–n silicon nanowire device. Inset: SEM image of the device structure. Scale bar, 2 mm. b, EFM image of a p–n diode reverse-biased at 5 V. The AFM tip voltage was modulated by 3 V at the cantilever-tip resonance frequency. The signal brightness is proportional to the nanowire device surface potential, and shows an abrupt drop around the kink position. The dashed lines mark the nanowire position. Scale bar, 2 mm. c,d, AFM and SGM images of one nþ2kink2nþ2kink2(n2nþ) dopant modulated double-kinked silicon nanowire structure. Scale bar in c, 2 mm. The SGM images were recorded with a Vtip of 10 V (I) and 210 V (II), respectively, and Vsd of 1 V. The dark and bright regions correspond to reduced and enhanced conductance, respectively. The black dashed lines mark the nanowire position. LETTERS NATURE NANOTECHNOLOGY DOI: 10.1038/NNANO.2009.304 828 NATURE NANOTECHNOLOGY | VOL 4 | DECEMBER 2009 | www.nature.com/naturenanotechnology
NATURE NANOTECHNOLOGY DOl: 10.1038/NNANO2009304 LETTERS 7. Caroff, P et al. Controlled polytypic and twin-plane superlattices in I-v 21. Dick, K. A. et al. The morphology of axial and branched nanowire restructures Nano Lett. 7, 1817-1822(2007). 8. Davidson, F M, Lee, D. C, Fanfair, D. D. Korgel B A Lamellar twinning in 22. Kodar S, Tersoff, Reuter, M. C& Ross, F M. Germanium nanowire F.M., Tersoff, ]& Reuter, M. C. Sawtooth faceting in silicon nanowires. 23. Kim, B J et al. Kinetics of individual nucleation events observed in nanoscale 10.Gao, P.x.et al. Conversion of zinc oxide nanobelts into superlattice-structured 4daapor-liquid-solid growth. Science 322, 1070-1073(2008) Phys.Rev.Let.95,146104(2005) Wacaser, B. A. et al Preferential interface nucleation: an expansion of the VLS nanohelices. Science 309, 1700-1704(2005). growth mechanism for nanowires. Adv Mater. 21, 153-165(2009) l1.Lu,W.& Lieber, C M. Nanoelectronics from the bottom up. Nature Mater. 6, 25 Jaccodine, R J Surface energy of germanium and silicon. J. Electrochem. Soc. 110,524-527(1963) 12.Sirbuly, D J, Law, M, Yan, H Q& Yang, P. D Semiconductor nanowires for 26. Pan, L, Lew, K.K., Redwing. J M. Dickey, E C Stranski-Krastanow growth of subwavelength photonics integration. J. Phys. Chem. B 109, germanium on silicon nanowires Nano Lett. 5, 1081-1085(2005). 5190-15213(2005) 13. Patolsky, F, Timko, B. P, Zheng, G.& Lieber, C M. Nanowire-based Acknowledgement: electronic devices in the life sciences. MRS Bull. 32, 142-149(2007) The authors would like to thank Y. J. Dong, x C. Jiang and Q. Qing for help with 14. Stern, E. ef al. Label-free immunodetection with CMOS-compatib experiments. C ML acknowledges support from a National Institutes of Health Directors semiconducting nanowires. Nature 7127, 519-522(2007) Pioneer Award, a McKnight Foundation Neuroscience Award and a contract from MITRE mad. Corporation. T.J. K acknowledges support from the National Science Foundation Graduate owire arrays on plastic substrates for ultrasensitive flexible chemical sensors. Research Fellowship. Nature Mater..6,379-384(2007) 16. Yaghi,O.M et al. Reticular synthesis and the design of new materials. Nature Author contributions 17. Wagner,R.S.& Ellis, w.C. Vapor-liquid-solid mechanism of single crystal analyses. B.T. and C ML co-wrote the paper. All authors discussed the results and nd 3,705-714(2003) B.T. and C.M. L designed the experiments. B.T. PX and T J. K performed experimen growth. Appl. Phys. Lett. 4, 89-90(1964) commented on the manuscript. 18. Morales, A. M.& Lieber, C. M. A laser ablation method for the synthesis of talline semiconductor nanowires. Science 279, 208-211(1998 Additional information 19. Wu, Y et al. Controlled growth and structures of molecular-scale silicon Supplementaryinformationaccompaniesthispaperatwww.nature.com/ nanowires Nano Lett. 4, 433-436(2004) naturenanotechnology.rEprintsandpermissioninformationisavailableonlineathttp://npg. 20. Lugstein, A et al. Pressure-induced orientation control of the growth of epitaxial nature. com/reprintsandpermissions/ Correspondence and requests for materials should be silicon nanowires. Nano Lett. 8, 2310-2314(2008) ad dressed to C mL NaturENanotEchNologYIVol4iDecemBer2009Iwww.naturecom/naturenanotechnology
7. Caroff, P. et al. Controlled polytypic and twin-plane superlattices in III–V nanowires. Nature Nanotech 4, 50–55 (2009). 8. Davidson, F. M., Lee, D. C., Fanfair, D. D. & Korgel B. A. Lamellar twinning in semiconductor nanowires. J. Phys. Chem. C 111, 2929–2935 (2007). 9. Ross, F. M., Tersoff, J. & Reuter, M. C. Sawtooth faceting in silicon nanowires. Phys. Rev. Lett. 95, 146104 (2005). 10. Gao, P. X. et al. Conversion of zinc oxide nanobelts into superlattice-structured nanohelices. Science 309, 1700–1704 (2005). 11. Lu, W. & Lieber, C. M. Nanoelectronics from the bottom up. Nature Mater. 6, 841–850 (2007). 12. Sirbuly, D. J., Law, M., Yan, H. Q. & Yang, P. D. Semiconductor nanowires for subwavelength photonics integration. J. Phys. Chem. B 109, 15190–15213 (2005). 13. Patolsky, F., Timko, B. P., Zheng, G. & Lieber, C. M. Nanowire-based nanoelectronic devices in the life sciences. MRS Bull. 32, 142–149 (2007). 14. Stern, E. et al. Label-free immunodetection with CMOS-compatible semiconducting nanowires. Nature 7127, 519–522 (2007). 15. McAlpine, M. C., Ahmad, H., Wang, D. W. & Heath, J. R. Highly ordered nanowire arrays on plastic substrates for ultrasensitive flexible chemical sensors. Nature Mater. 6, 379–384 (2007). 16. Yaghi, O. M. et al. Reticular synthesis and the design of new materials. Nature 423, 705–714 (2003). 17. Wagner, R. S. & Ellis, W. C. Vapor–liquid–solid mechanism of single crystal growth. Appl. Phys. Lett. 4, 89–90 (1964). 18. Morales, A. M. & Lieber, C. M. A laser ablation method for the synthesis of crystalline semiconductor nanowires. Science 279, 208–211 (1998). 19. Wu, Y. et al. Controlled growth and structures of molecular-scale silicon nanowires. Nano Lett. 4, 433–436 (2004). 20. Lugstein, A. et al. Pressure-induced orientation control of the growth of epitaxial silicon nanowires. Nano Lett. 8, 2310–2314 (2008). 21. Dick, K. A. et al. The morphology of axial and branched nanowire heterostructures. Nano Lett. 7, 1817–1822 (2007). 22. Kodambaka, S., Tersoff, J., Reuter, M. C. & Ross, F. M. Germanium nanowire growth below the eutectic temperature. Science 316, 729–732 (2007). 23. Kim, B. J. et al. Kinetics of individual nucleation events observed in nanoscale vapor–liquid–solid growth. Science 322, 1070–1073 (2008). 24. Wacaser, B. A. et al. Preferential interface nucleation: an expansion of the VLS growth mechanism for nanowires. Adv. Mater. 21, 153–165 (2009). 25. Jaccodine, R. J. Surface energy of germanium and silicon. J. Electrochem. Soc. 110, 524–527 (1963). 26. Pan, L., Lew, K.-K., Redwing, J. M. & Dickey, E. C. Stranski–Krastanow growth of germanium on silicon nanowires. Nano Lett. 5, 1081–1085 (2005). Acknowledgements The authors would like to thank Y. J. Dong, X. C. Jiang and Q. Qing for help with experiments. C.M.L. acknowledges support from a National Institutes of Health Director’s Pioneer Award, a McKnight Foundation Neuroscience Award and a contract from MITRE Corporation. T.J.K acknowledges support from the National Science Foundation Graduate Research Fellowship. Author contributions B.T. and C.M.L. designed the experiments. B.T., P.X. and T.J.K. performed experiments and analyses. B.T. and C.M.L. co-wrote the paper. All authors discussed the results and commented on the manuscript. Additional information Supplementary information accompanies this paper at www.nature.com/ naturenanotechnology. Reprints and permission information is available online at http://npg. nature.com/reprintsandpermissions/. Correspondence and requests for materials should be addressed to C.M.L. NATURE NANOTECHNOLOGY DOI: 10.1038/NNANO.2009.304 LETTERS NATURE NANOTECHNOLOGY | VOL 4 | DECEMBER 2009 | www.nature.com/naturenanotechnology 829