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2005 Nuggets

For both device application and fundamental research, it is of great interest to establish techniques for modifying and welding nanomaterials at the atomic and nanometer scales, e.g., to cut an atomic gap for molecular transport studies and molecule devices, to make tailored nano-holes or nano-bridges for quantum transport effects, and to weld nanowire (NW) junctions for nano-transistors, or weld NW loops for magnetic flux sensors. However, to date such techniques are not available.

Recently we demonstrated that by using a high-intensity electron beam (HIEB), a contamination-free source, we could perform both local modification and welding down to the sub-nanometer scales. We showed that a HIEB of 10^5-6 A/cm², obtained in a field-emission transmission electron microscope (FETEM), could be applied to create holes, gaps, and other patterns of atomic and nanometer dimensions on a single NW (Fig. 1). A HIEB with slightly lower intensity can be used to weld two NWs together, forming metal-metal or metal-semiconductor junctions (Fig. 2), and, to remove the oxide shell from a crystalline NW. The results have been published in Small, Issue 12 of 2005 with a cover picture [1]. We also showed that the HIEB nano-welding technique could help reduce the contact resistance between an Au NW and the Au leads of a test circuit by 3-5 orders of magnitude [2].

We believe that in the near future, if the beam size of a scanning electron microscopy (FESEM) is further scaled down by ~ 10 times from the current level, it is very possible to combine the HIEB techniques with the well-established e-beam lithography technology for large-scale applications.

Fig. 1 TEM images of nanoscale patterns made on individual Si NWs with a HIEB. (a) Letters “PSU” written on a 24-nm Si NW. (b) & (c) Letters “Si” on a 41-nm Si NW, being written twice at the same location. The lattice distortion highlighted by a red arrow in (b) is annealed away in (c). (d) & (e) On the same NW as that in (b), a nano-bridge is further sculpted. From (d) to (e), the shapes of the letters “Si” and the crystalline structure of the NW have not been obviously changed.

Fig. 2 (a-h) HIEB-welding of Au-Sn nano-junction from t = 0s to 900 s. (i) In the welded junction, at t = 600 s, Au, AuSn, AuSn and Sn (or possibly AuSn4) and Sn nanophases are found at regions “A”-“C” and “E”, respectively. (j) Energy dispersive X-ray spectra taken at the regions “A” to “E” in (i).

References

[1] Nanometer-scale modification and welding of silicon and metallic nanowires with a high-intensity electron beam, S. Y. Xu, M. L. Tian, J. G. Wang, J. Xu, J. M. Redwing, and M. H. W. Chan, Small 1 (2005) 1221-1229, with a cover picture (see the left image).

[2] A low-cost platform for bridging transport property to structure of nanomaterials, S. Y. Xu, J. Xu and M. L. Tian, accepted by Nanotechnology.