**9. Open questions**

Ideally, for an exact explanation of nanowire crystal phases at different conditions discussed above, one could theoretically model the system and compare the contact angles where the structure is predicted to switch phases and compare it with experimental values. Such calculations, and also models for other phenomena in nanowires, involve different interface energy terms. However, there are hardly any direct experimental measurements of interface energies at the different growth conditions even for common material systems. The solid surface energies (the solid-vapor interface energy to be more precise) would depend on the surface relaxations/reconstructions adapted by the system, which again depends on the growth condition [119]. There exists post-growth surface energy measurements on bulk materials, [120] but is inadequate for knowing nanowire surface energies during growth conditions. Some roundabout estimates have been made by comparing experimental observations with approximate models for finding surface energies during growth; [99, 108] having these values are certainly better than having nothing, but we need better measurements. The reason for not having more direct measurements is simple – they are challenging to perform and observe at nanowire growth conditions. The surface tension of Au-Si liquid catalyst has been beautifully measured by studying electric field-induced deformation [85]. This method can be used to study other material systems as well. We have to come up with smart strategies for measuring solid–liquid and solid-vapor interface energies.

A seemingly basic, but still ambiguous topic is what are the key parameters deciding the nanowire growth direction. For example, unless at very peculiar growth conditions, most III-V and II-VI nanowires grow in the <111>/<0001> B direction, [3, 4] even on amorphous substrates, [89, 113, 121] in fact even without a substrate [122]. (The <0001>B direction in wurtzite structure is equivalent to the <111>B of zincblende polytype.) A complete and accurate description in the VLS case would involve catalyst chemical potential, solid–liquid interface energy for different possible crystal planes in contact with the liquid catalyst, solid-vapor surface energy of nanowire sides, liquid–vapor interface energy, edge energies of the top facet, edge energies of the growing island, [49] edge energies at nanowire side corners, if or not a new layer has well-defined low-index facets or is the surface rather rounded, [123] effect of liquid ordering, [45] etc. These individual terms are a function of the growth parameters and catalyst composition. As mentioned in the previous paragraph, most of these values have not been measured yet. But all these factors present, perhaps there is some key factor(s) which overpowers at typical growth conditions?

A very interesting but unresolved question is the diffusion pathway of the reactants. If the group V or group VI species is expected to hardly dissolve in the catalyst during compound nanowire growth, is it necessary that it should diffuse through the volume of the catalyst? Could these species be diffusing through the catalyst-nanowire surface instead? With the current technology it is not possible to watch the trajectory of each individual atom. However, perhaps there could be

**107**

**Author details**

Carina B. Maliakkal

carinab.maliakkal@gmail.com

provided the original work is properly cited.

Centre for Analysis and Synthesis, Lund University, Lund, Sweden

\*Address all correspondence to: carina\_babu.maliakkal@chem.lu.se,

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

*In Situ TEM Studies of III-V Nanowire Growth Mechanism*

some strategic in situ experiments, which in combination with appropriate rigorous theoretical simulations, can solve this puzzle. The radius dependence of the ledge-flow time might distinguish if the diffusion is through bulk or interface. The dynamics of the ledge-flow and the shape of the growing layer might also serve as a tool. That said, it is not necessary that there be a unique answer to this puzzle even for a given catalyst-nanowire system and catalyst phase (i.e. VLS or VSS), perhaps it could be dependent on the growth conditions. Another approach to this puzzle could be – diffusion need not even be the rate limiting process; in such a case why care about it. But this is nonetheless an interesting unanswered riddle, where in situ

Several in situ techniques, including in situ TEM, has been used to study nanowire growth. In situ TEM studies revealed that the growth dynamics of compound nanowires (e.g. III-V nanowires like GaAs) is fundamentally different from elemental nanowires. This can be understood by the difference in solubility of the nanowire species in the catalyst, which was also investigated by in situ TEM. Due to this concentration difference the layer nucleation and layer completion processes could be independently controlled. The growth dynamics has been studied in relation

with the crystal structure and nanowire-catalyst interface morphology.

*DOI: http://dx.doi.org/10.5772/intechopen.95690*

TEM can be extremely valuable.

**10. Summary**

*In Situ TEM Studies of III-V Nanowire Growth Mechanism DOI: http://dx.doi.org/10.5772/intechopen.95690*

some strategic in situ experiments, which in combination with appropriate rigorous theoretical simulations, can solve this puzzle. The radius dependence of the ledge-flow time might distinguish if the diffusion is through bulk or interface. The dynamics of the ledge-flow and the shape of the growing layer might also serve as a tool. That said, it is not necessary that there be a unique answer to this puzzle even for a given catalyst-nanowire system and catalyst phase (i.e. VLS or VSS), perhaps it could be dependent on the growth conditions. Another approach to this puzzle could be – diffusion need not even be the rate limiting process; in such a case why care about it. But this is nonetheless an interesting unanswered riddle, where in situ TEM can be extremely valuable.

#### **10. Summary**

*Nanowires - Recent Progress*

**9. Open questions**

zincblende structure or (ii) with nucleation at the triple-phase-line only, but with the zincblende structure having lower energy at those growth conditions. Note that in the above explanation or in references [1, 49] truncation was not explicitly needed to explain crystal phase switching. At extremely high contact angles and at extremely low contact angles, there could be either truncation or large tapering; [84, 99] but truncation is not a necessity for zincblende growth. The truncation might be responsible for the observed quasi-instantaneous ledge-flow though.

Ideally, for an exact explanation of nanowire crystal phases at different conditions discussed above, one could theoretically model the system and compare the contact angles where the structure is predicted to switch phases and compare it with experimental values. Such calculations, and also models for other phenomena in nanowires, involve different interface energy terms. However, there are hardly any direct experimental measurements of interface energies at the different growth conditions even for common material systems. The solid surface energies (the solid-vapor interface energy to be more precise) would depend on the surface relaxations/reconstructions adapted by the system, which again depends on the growth condition [119]. There exists post-growth surface energy measurements on bulk materials, [120] but is inadequate for knowing nanowire surface energies during growth conditions. Some roundabout estimates have been made by comparing experimental observations with approximate models for finding surface energies during growth; [99, 108] having these values are certainly better than having nothing, but we need better measurements. The reason for not having more direct measurements is simple – they are challenging to perform and observe at nanowire growth conditions. The surface tension of Au-Si liquid catalyst has been beautifully measured by studying electric field-induced deformation [85]. This method can be used to study other material systems as well. We have to come up with smart strate-

gies for measuring solid–liquid and solid-vapor interface energies.

A seemingly basic, but still ambiguous topic is what are the key parameters deciding the nanowire growth direction. For example, unless at very peculiar growth conditions, most III-V and II-VI nanowires grow in the <111>/<0001> B direction, [3, 4] even on amorphous substrates, [89, 113, 121] in fact even without a substrate [122]. (The <0001>B direction in wurtzite structure is equivalent to the <111>B of zincblende polytype.) A complete and accurate description in the VLS case would involve catalyst chemical potential, solid–liquid interface energy for different possible crystal planes in contact with the liquid catalyst, solid-vapor surface energy of nanowire sides, liquid–vapor interface energy, edge energies of the top facet, edge energies of the growing island, [49] edge energies at nanowire side corners, if or not a new layer has well-defined low-index facets or is the surface rather rounded, [123] effect of liquid ordering, [45] etc. These individual terms are a function of the growth parameters and catalyst composition. As mentioned in the previous paragraph, most of these values have not been measured yet. But all these factors present, perhaps there is some key factor(s) which overpowers at typical

A very interesting but unresolved question is the diffusion pathway of the reactants. If the group V or group VI species is expected to hardly dissolve in the catalyst during compound nanowire growth, is it necessary that it should diffuse through the volume of the catalyst? Could these species be diffusing through the catalyst-nanowire surface instead? With the current technology it is not possible to watch the trajectory of each individual atom. However, perhaps there could be

**106**

growth conditions?

Several in situ techniques, including in situ TEM, has been used to study nanowire growth. In situ TEM studies revealed that the growth dynamics of compound nanowires (e.g. III-V nanowires like GaAs) is fundamentally different from elemental nanowires. This can be understood by the difference in solubility of the nanowire species in the catalyst, which was also investigated by in situ TEM. Due to this concentration difference the layer nucleation and layer completion processes could be independently controlled. The growth dynamics has been studied in relation with the crystal structure and nanowire-catalyst interface morphology.
