Nanowires are known to be thermally unstable and may break up into chains of nanoparticles at temperatures below their melting point. This phenomenon is reminiscent of the well-known Rayleigh-Plateau instability of liquid jets, when periodic sausage-like perturbations of the jet surface increase with time. The corresponding nanowire/jet shape modifications must be followed by a decrease in total surface energy. However, unlike isotropic liquid jet surfaces, the realization of this abatement in the system with a strong lattice structure and physical dissimilarity of bounding facets, leads to specific features of nanowire break-up, namely, significant variations in the size of nanodrops and the average distance between them, the formation of long-living dumbbells, and zigzag-like structures. Here, the physical mechanisms of such diversity are analyzed. Apart from scientific insights, the yielded results can be potentially useful in applications such as the development of optical waveguides based on ordered nanoparticles chains.

A kinetic Monte Carlo approach is applied to study physical mechanisms responsible for the breakup of nanowires with the diamond cubic crystal structure into a chain of nanoparticles discovered in preceding experiments on Silicon nanowires. We show that this process is based on the well-known mechanism of roughening transition, which specifically manifests itself in quasi-one-dimensional systems/nanowires with a pronounced anisotropy of the surface energy density. Depending on the temperature and orientation of the nanowire relative to its internal crystal structure, the wavelengths of substantial cross-sectional modulations exceed its initial radius by 4 to 18 times. For certain orientations, a straight nanowire at the initial stage of evolution forms a serpentine/helical structure. The scenarios of the stage of nanowire ruptures into single nanoclusters are also diverse: either each spindle-shaped region of the nanowire transforms into a separate drop (by long-wave surface perturbations), or the adjacent short-scale beads absorb each other due to the Ostwald ripening effect, which can be accompanied by the formation of long-lived many-body dumbbells. The discovered features of the dynamics of quasi-onedimensional systems expand our conceptions of the physical mechanisms involved in the breakup of nanowires (presented by Nichols and Mullins as a classical model for such instabilities) which could be useful in applications based on chains of ordered nanoparticles. Manuscript & Supplementary FileWe have also found out the possibility of the formation of helicoidal fragments, extended snakelike configurations, and many-body long-lived structures (see 'Movie S1' and 'Movie S2' in the Supplementary Information). In conclusion, we note that the disintegration processes of nanowires with the diamond cubic crystal structure are strikingly different from the break-up processes of nanowires with the FCC (face centered cubic) lattice structure 30-33 , even though the Wulff configurations 48,49 for these types of crystal lattices are visually very similar.

We have analyzed the expressed manifestation of the anisotropy of surface energy density in the dynamics of ultrathin nanowires, which break up into disjointed clusters when annealed below their melting temperature. The breakup process is studied for different temperatures and orientations of the nanowire axis relative to its internal crystal structure using the Monte Carlo kinetic method. We have also presented an approximate analytical model of the instability of nanowires. Generally, the interpretation of experimental results refers to the theoretical model developed by Nichols and Mullins, which is based on conceptions about the Rayleigh instability of liquid jets. In both cases, the theories -which do not take into account the anisotropy of surface energy density -predict the breakup of a nanowire/liquid jet with radius into fragments with an average length Λ = 9 . However, the observed value, Λ/ , often deviates from 9 either to lower values or to substantially greater ones (up to 24-30). Our results explain various observed features of the breakup and the significant variations in the values of its parameter Λ/ depending on experimental conditions. In particular, the ambiguous role of exchange by atoms of the surface of a nanowire with the surrounding layer of free atoms formed as a result of their rather intensive sublimation, which occurs in a number of cases, has been investigated. We have shown that this exchange can lead both to a decrease, and to a significant increase, in the parameter Λ/ . The obtained results could be potentially useful in applications such as the development of optical waveguides based on ordered nanoparticles chains.

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Peculiar scenarios in the dynamics of BCC and FCC 1D-nanostructures leading to the formation of ultra-short, and sometimes stable, high-amplitude surface modulations are analysed and the means of achieving the desired periodicity are discussed.