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/naturenanotechnologytwin 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 pre￾serves 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 compo￾sitions21. Second, the joint has a quasi-triangular structure with f111g top/bottom facets and two f112g side facets joining the adja￾cent 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-crystal￾line 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 con￾ditions (see Methods), both 80- and 150-nm silicon nanowires (Fig. 3a) show a high probability of kinks with a regular zigzag geo￾metry. 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 diam￾eters. 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 qualitat￾ively explained by the proposed stepwise model shown in Fig. 3d. In step 1, the reactant concentration drops in the supersaturated cata￾lyst 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 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 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