SUPPLEMENTARY INFORMATION

Transcription

1 SUPPLEMENTARY INFORMATION 1. Head-to-side welding mode In addition to aforementioned head-to-head and side-to-side joining geometries, cold-welding can also be realized in other geometries depending on (a) the relative positions of two nanowires; (b) the degree-of-freedom of nano-manipulation; (c) the matching orientation of two nanowires. For example, a head-to-side welding was successfully performed and form a T shape nanowire junction (Fig. s1 a-c). Combining multiple welding geometries, more complex configuration of nanowires can be bottom-up assembled, such as illustrated in Fig. s1 d. (a) (b) (c) (d) Fig. s1. A head-to-side welding for two ultrathin gold nanowires: (a) An illustration of welding geometry; (b) Before welding and (c) After cold-welding; (d) The joining of multiple (three) nanowires by cold-welding (Scale bar 10nm in all three images) 2. Cold-welding of ultrathin Ag-Ag and Au-Ag nanowires In addition to cold-welding of ultrathin Au nanowires, cold-welding also occurs between silver-silver nanowires, (Fig. s2), as long as they are single crystals with matching orientation and clean surfaces, and with small diameters (usually less than 20nm). On the other hand, for joining between dissimilar materials, so far we observed nature nanotechnology 1

2 supplementary information that cold-welding could easily occur between gold and silver (Fig. s3), which should be attributed to their similar crystalline structures and lattice spacing. (a) (b) (c) (e) (d) Fig. s2 Cold-welding of ultrathin silver nanowires/nanorods: (a) a silver nanowire (top) was approaching to a silver nanorod (bottom); (b) cold-welding occurred once their front surfaces touched each other; (c) high magnification images of the welding zone; (d) the as-welded silver nanowire after relaxation; (e) in situ pulling test was performed and the aswelded silver nanowire broke at a different location rather than the welding section. Scale bars in (a), (b), (d), (e) are 10nm, and in (c) is 5nm. 2 nature nanotechnology

3 supplementary information (a) (b) (c) (d) (e) nature nanotechnology 3

4 supplementary information (f) Fig. s3 Cold-welding process between a silver nanowire and a gold nanowire: in (a) an ultrathin gold nanowire (right, diameter ~10nm) was approaching to a silver nanowire (left, diameter ~15-20nm); (b) cold-welding didn t occur instantly as they touched each other due to the existence of surface surfactants on gold nanowires; (c) after removal of the surface layer on their front edges, cold-welding occurred quickly; (d) the as-welded Au-Ag nanowire structure after relaxation; (e) in situ pulling of the as-welding nanowire structures was performed and the breaking point was in the original silver nanowire section, rather than at the Au-Ag welding zone; (f) EDAX showing the composition of the nanowires. Scale bars in (a), (b), (d) and (e) are 10nm and in (c) is 5nm. 3. Discussions on electron beam irradiation effect First of all, it might be noted that cold-welding experiments were also successfully performed at 100KV low dose condition as well as the beam-blank condition. However, even when the TEM was operated at 300KV, according to [31] and the Supporting 4 nature nanotechnology

5 supplementary information Information of [15], temperature increase due to electron beam irradiation effect will be limited within about 10 C with low intensity electron beam condition. In addition, as estimated from the energy-loss spectrum [32], the increasing of temperature associated with electron beam irradiation can be really small (less than 7K) for gold nanoparticles with radius above 2nm on substrates with good thermal conductivity. Considering that the radii of the ultrathin nanowires used in this study were right all above 2 nm, and samples were attached to gold or tungsten probes with conductive silver paste, electron beam heating effects were thought to be not significant in current study. 4. Discussion on the increased mechanical strength of gold nanowires It has been well recognized that there exists a strong size effect in mechanical behaviours for metals and more generally, smaller is stronger ([26] and the references therein). At nanometer length scale, the structure of materials such as nanowires is normally free of defects, and the deformation mechanism is quite different from that of bulk materials. As such, the strength of nanowires can even approach the ideal strength of materials. The observed high mechanical strength for gold nanowires in this study correlates well with earlier results obtained both experimentally and computationally [26, 27]. 5. Supplementary Movies Movie s1 --- A head-to-head welding of gold nanorods. The video was recorded at 2 frames per second and played at 20x speed. Movie s2 --- A side-to-side welding of ultrathin gold nanowires followed by in situ pulling, and a second welding with a head-to-head geometry, followed by a nature nanotechnology 5

6 supplementary information second in situ pulling until breaking of the welded nanowire. The video was recorded at 2 frames per second and played at 50x speed. Movie s3 --- A side-to-side welding of ultrathin gold nanowires using TEM- AFM holder, followed by a quantitative in situ tensile test of the as-welded gold nanowire. The video was recorded at 2 frames per second and played at 50x speed. 6 nature nanotechnology