Xu et al. Nanoscale Research Letters 2011, 6:113 http://www.nanoscalereslett.com/content/6/1/113 Page 3 of 7 Figure 3 Top view SEM images of Ge NWs grown on a Si(111) that show the different growth direction. The scale bars correspond to 100 nm. growth of <110>-oriented Ge NWs obtained on the Ge (111) surface by MBE [7]. Group IV semiconductors NWs have been found to grow with different orientations, depending on the tem- perature of the surface during the growth and the eva- poration rate of elemental Si or Ge. While a high surface temperature favors the growth of <111>-oriented NWs, contradictory results have been obtained regarding the effect of the evaporation rate [7,8]. However, consistent with these previous studies, we note that the comparison of the length between the <111>-oriented Ge NW and those oriented along the <110> directions shows a strong difference. In our study, the first types of NWs are rarely higher than the overgrown Ge layer, while for the second type of NWs, the part of the NWs that surpasses the overgrown Ge layer can reach a length of 2 μm when the growth time is 60 min and the deposition rate is about 1.7 Å/s. The growth direction of the NWs thus seems to play an important role on the maximum length that group IV semiconductor NWs can reach. Although the deposition rate of Si was 0.5 Å/s in Figure 2a, a two-hour growth does not allow to build NWs higher than 400 nm, consistent with previous results that reported the difficul- ties to grow <111>-oriented Si NWs longer than 500 nm by MBE [9]. In order to understand the physical origin of the growth direction dependence on the NW length, we examined the sidewalls of both types of NWs, as shown in Figure 4. The <111>-oriented Si NWs consist of six sidewalls that exhibit small facets. It has been shown that these sidewalls correspond to {112} planes [9]. For <111>-oriented Si NWs grown by CVD at low silane partial pressure, that show similar sidewall orientations [10], gold is known to diffuse from the seed particle and to wet the sidewalls [11,12]. Adsorption of gold on Si (112) planes is also known to causes the faceting of these planes [13]. Similarly, Au diffusion from the seed particle is at the origin of the facet formation on the {112} planes, for which the crystallographic orientations alternate between {111} planes and high index planes [14]. HRTEM images of the sidewalls for the MBE grown Si NWs are consistent with the observations per- formed on <111>-oriented Si NWs grown by CVD. For example, Figure 4a2 reveals the rough morphology of one of the {112} sidewalls. Although the facets are rather rounded, probably due to the oxide layer that covered the sidewall, a corrugation of up to 2 nm is found when the height profile of the sidewall is measured. As for the <110>-oriented Ge NWs, they also exhibit an irregular hexagonal cross-section, but the orienta- tions of the sidewalls are different. They consist of {111} and {100} planes, where two {100} planes are opposite to each other and are separated on each side by two adjacent {111} planes [15,16]. In addition, the {100} planes are usually narrower than the {111} planes and their width decreases toward the base of the NWs, which undergoes the longest exposure time to the Ge deposition and diffusion (Figure 4b1). As the {111} planes are the dominant sidewalls, these sidewalls were investigated by HRTEM. Figure 4b2 shows that the {111} sidewalls are atomically flat. Scattered bright clus- ters are also seen superimposed to the atomic lattice and indicate the presence of Au-rich clusters. Similarly to the <111>-oriented Si NWs, gold diffuses from the seed particle to wet the sidewalls of the <110>- oriented Ge NWs. However, in contrast to the adsorp- tion of gold on the {112} planes, the adsorption of gold on the Si and Ge (111) surfaces does not produce a roughening of the surface. Instead, atomically flat (111) surfaces are generally observed with a √3 × √3 recon- struction, when the Au coverage is higher than one monolayer [17-19]. Our observation of a flat sidewall is thus consistent with previous surface studies about the adsorption of gold on group IV semiconductor (111) surfaces. In addition, as the {111} sidewalls contain Au- rich clusters, we might expect to have more than one monolayer of gold adsorbed on the NW sidewalls. Such result suggests that the formation of a √3 × √3 recon- struction between the clusters occurs during the NW growth in UHV. As already described in the introduction, the growth of NWs by MBE predominantly proceeds through the diffusion of adatoms that adsorb on the substrate in between the NWs. Indeed, if we consider the surface of the seed particle exposed to the flux of elemental com- pounds and the area surrounding the NWs that serves as a reservoir to collect adatoms for the NW growth,