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Nanowires and nanorods are normally fabricated using nanoscale templates or catalysts [1]. We demonstrate the organized self-assembly of Sb and Bi straight and branched nanorods on highly-oriented pyrolytic graphite and MoS2 in vapor deposition, without using the template or catalyst. Low-dimensional semimetals (e.g. Bi, Sb) are excellent thermoelectric materials with on-chip cooling device application potential. STM images reveal that the Sb and Bi nanorods start as highly compressed isotropic nuclei [2]. Later, the nuclei undergo a symmetry-breaking phase transition as strain relaxation occurs. The compressive states of the nuclei and nanorods are attributed to an enormous Laplace pressure induced by surface tension in a nanostructure. This provides an example to show the important roles of surface energetic/mechanical properties played in the synthesis and the properties of nanostructures. The stochastic growth and strain relaxation processes lead to either straight or branched nanorods. But certain control of the topology of Sb and Bi nanorods has been accomplished. At a high substrate temperature with a low deposition flux, straight nanorods grow exclusively, whereas at a low temperature and a high flux, branched nanorods can be dominant. These mechanisms can also be used to explain the self-assembly of other branched nanorods (such as II-VI semiconductor nanorods and tetrapods in solution [3]). In addition, the self-assembly of aligned nanorods has been accomplished by taking advantage of the interaction between existing nanorods and the nucleus of the nanorod on an inert substrate. Such organized self-assembly of semimetal nanorods is possible on more attractive substrates including silicon after proper surface passivation.
References
[1] Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim and H. Yan, Adv. Mater. 15 (2003) 353.
[2] X.-S. Wang, S.S. Kushvaha, Z. Yan and W. Xiao, Appl. Phys. Lett. 88 (2006) 233105.
[3] L. Manna, D.J. Milliron, A. Meisel, E.C. Scher and A.P. Alivisatos, Nature Mater. 2 (2003) 382.
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