Xu et al. Nanoscale Research Letters 2011, 6:113 http://www.nanoscalereslett.com/content/6/1/113 Page 4 of 7 Figure 4 SEM observation of NW sidewalls for (a1) a <111>-oriented Si NWs and (b1) a <110>-oriented Ge NWs grown on a Si(111) surface by MBE. Some sidewall orientations are indicated for both types of NWs. Lattice-resolved TEM images showing the roughness of (a2) a {112} sidewall on a <111>-oriented Si NWs and (b2) a {111} sidewall on a <110>-oriented Ge NWs. their ratio is usually quite small. When the NW is still short, typically at the beginning of the growth, the con- tribution of the adatoms diffusing from the substrate up along the NW is thus the strongest to the growth rate. This mechanism implies that the crystallization rate at the interface between the seed particle and the NW is related to the flow of diffusing adatoms that become incorporated when they reach the circumference of the interface [4]. It yields a characteristic signature: the NW length varies like the inverse of the diameter. Such a behavior appears in Figure 5a for the case of the <111>- oriented Si NWs grown by MBE. It is also visible after a 15 min for the growth of <110>-oriented Ge NWs (Figure 5b). Although the