Xu et al. Nanoscale Research Letters 2011, 6:113 http://www.nanoscalereslett.com/content/6/1/113 Page 5 of 7 Figure 5 Correlation between the length and the diameter of <111>-oriented Si NWs and <110>-oriented Ge NWs. For Ge NWs, the data are measured for two different growth times. The evaporation rate for both experiments was 1.5 ± 0.2 Å/s. deposition rate was almost similar, comparison of the maximum length that can be reached between the <111> and <110>-oriented NWs clearly shows that the <110>-oriented NWs are quickly much longer. Based on the mass transport model described in [20], in the case of NWs that are still short, the length growth rate dependence on the adatoms diffusing onto the sidewalls from the substrate toward the interface between the Au droplet and the NW is expressed as follows: dL dt  4 J D cosh  L / f  (1) where Ω, J, and lf are, respectively, the atomic volume of the growth species, the flow of adatoms from the sub- strate toward the NW sidewalls, and the diffusion length along the NW sidewalls. Considering that Ω and J do not significantly vary between the growth of Si and Ge NWs, for a given diameter, the length growth rate is found to vary as the inverse of a cosh function that depends on lf. A higher diffusion length on the side- walls results in an increase of the length growth rate. Surface diffusion requires overcoming an energy barrier [21]. The smaller the surface corrugation is, the lower the activation energy is. For a given time, elemental Si or Ge can thus diffuse further away from their adsorption site