Gold anchoring on Si sawtooth faceted nanowires

Abstract

PACS 81.07.Gf -Nanowires PACS 61.46.Km -Structure of nanowires and nanorods (long, free or loosely attached, quantum wires and quantum rods, but not gate-isolated embedded quantum wires) PACS 81.16.Hc -Catalytic methods Abstract -This paper reports on the sawtooth faceting and the related gold coverage of silicon nanowires (NWs). 111 -oriented Si NWs were grown on Si(111) substrates by ultra high vacuum chemical vapor deposition using the vapor-liquid-solid mechanism with a gold catalyst. We observed that gold nanoclusters are unequally distributed on the NW surface. They are mainly distributed on only three non-consecutive sidewalls corresponding to the (11 2), (1 21) and ( 211) planes among the six available crystallographic {112} surfaces. In addition they are anchored on upward {111} facets. This original observation brings enhanced knowledge on the faceting mechanisms. The threefold symmetry of the facet formation and gold anchoring is supported by criteria of minimal Au/Si interfacial energy. Moreover results evidence that the selective presence of gold on the NW sidewalls affects the overall morphology due to an increased radial growth on the covered sidewalls.

Revising morphology of 〈111〉-oriented silicon and germanium nanowires

“…Thus, the {112} facets characterize the shape of these nanostructures as shown in Figure 2 in the case of SiNWs. Similar morphology has been observed in experimentally grown SiNWs [12,15,16]. After structural optimization faceting is preserved while the {011} facets are mostly converted to the {112} ones except for the NW with diameter of 1.9 nm.…”

“…However, the difference in the total energy for SiNWs with different morphologies and diameters larger than 3 nm is not significant and it is hard to unambiguously state that morphology with {112} facets (Figure 2) is the most stable. Surprisingly enough, SiNWs in the triangle-like shape ( Figure 3) according to our data can be also quite thermodynamically competitive and appear during growth even for nanostructures with smaller diameters than experimentally observed (about 100 nm) [11][12][13][14][15][16]. Since our calculations have been performed at zero temperature, the entropic contribution to the total energy was neglected.…”

“…Instead, the atom in the middle of each {112} facet with two dangling bonds is found to be overcoordinated because two atoms at edges are shifted toward it. Another experimentally observed morphology of SiNWs displays alternate small and large in size {112} facets bounded in the triangle-like shape [11][12][13][14][15][16] as presented in Figure 3. We have also introduced here the {011} facets between the adjacent {112} facets with the 2×1 reconstruction which act as edges in order to avoid appearing of surface atoms with two dangling bonds.…”

“…For the 100 -oriented SiNWs with different morphologies energy differences at finite and zero temperatures are estimated to be negligible [24]. In the case of SiNWs with 111 orientations experimental observations have clearly indicated {112} facets to dominate [11][12][13][14][15][16] pointing out that the entropic contribution could even promote the appearance of latter facets. It should be noted here that the results obtained in this study do not alter the order of curves representing stability of SiNWs with 001 , 011 , 111 and 112 axes via the dependence of the total energy with respect to the number of atoms in the unit cell as indicated in Figure seven of Ref.…”

“…It is also possible to use {112} facets which play the role of edges between adjacent {011} facets in order to avoid an appearance of surface atoms with two dangling bonds [5]. However, there is some experimental evidence that mainly {112} facets define morphology of SiNWs with 111 axes and with diameters in the range of 50 -100 nm [11][12][13][14][15][16]. The latter fact obviously contradicts the common assumption that the Si(112) and Ge(112) surfaces are less stable (or unstable at all) than any of the (111), (001), (113) and (011) ones.…”