Xu et al. Nanoscale Research Letters 2011, 6:113 http://www.nanoscalereslett.com/content/6/1/113 when the surface is atomically flat [22]. The longest diffu- sion length found for the case of <110>-oriented Ge NWs is, therefore, consistent with atomically flat {111} sidewalls in comparison with the rough-facetted {112} sidewalls of the <111>-oriented NWs. When L becomes larger than lf, then the length growth rate depends mainly on the diffusion of ada- toms that adsorb directly onto the NW sidewalls or on the seed particle. Therefore, the NWs with the smallest diameters grew slower while the NWs with the biggest diameters keep on growing with the same length growth rate. The effect of a limited diffusion length is readily visible in Figure 6. While the <110>-oriented Ge NWs appear cylindrical at the beginning of the growth (Figure 6a), some of the {111} sidewalls may show a change of their morphology, as the growth pro- ceeds. Such an example is seen in Figure 6b, where a strong overgrowth occurs at the base of the NW indi- cating that the adatoms from the substrate are rather incorporated onto the sidewalls than diffusing up to the top of the NWs. Finally, when the growth duration approaches tens of minutes, Au may have completely diffused away from the original seed particle. As a result, the NW cannot grow in length any more, but overgrowth on the sidewalls occurs. The {100} side- walls, for which the surface tension is higher [15,23], disappear through the lateral growth of the {111} Page 6 of 7 sidewalls, giving rise to a typical rhombohedral cross- section (Figure 6c). In summary, by combining SEM and TEM analysis of group IV semiconductor NWs grown by MBE on a Si (111) surface, the structural properties of Si and Ge NWs with different growth axes have been investigated. As gold diffuses from the seed particle during the growth and wets the NW sidewalls, a significant change of the sidewall roughness can occur depending on the sidewall orientations. The roughness strongly affects the diffusion length of the diffusing Si or Ge adatoms toward the interface between the seed particle and the NW, and prevents the growth of NWs with micro- meter-scale lengths. A good control of the NW growth axis is, therefore, important to obtain sidewalls with the lowest surface tension. Abbreviations CVD: chemical vapor deposition; HRTEM: high-resolution transmission electron microscope; MBE: molecular beam epitaxy; NWs: nanowires; SEM: scanning electron microscope; UHV: ultra high vacuum. Acknowledgements We thank D. Troadec for the manipulation of the Ge NWs onto the TEM grid. The authors acknowledge financial support from the DGA (Direction Générale de l’Armement) under contract REI-N02008.34.0031. Author details 1Département ISEN, Institut d’Electronique, de Microélectronique et de Nanotechnologie, IEMN (CNRS, UMR 8520), 41 bd Vauban, 59046 Lille Cedex, Figure 6 Evolution of the <110>-oriented Ge NW morphology. The NWs initially show (a) an irregular hexagonal cross-section, then (b) a reduction of the gold seed particle and a lateral overgrowth on the {111} sidewall that is exposed to the Ge flux, and finally (c) the disappearance of both the gold seed particle and the {100} sidewalls, giving rise to Ge NW with a rhombohedral cross-section.