Formation of silicon nanocrystal chains induced via Rayleigh instability in ultrathin Si/SiO₂ core/shell nanowires synthesized by an inductively coupled plasma torch process

Abstract

A new class of silicon (Si) nanohybrids is formed exploiting the Rayleigh instability experienced by Si nanowires (SiNWs) synthesized via inductively coupled plasma. These nanohybrids consist of three main categories of Si nanostructures: (i) core/shell silicon/silicon oxide nanowires (where the inner Si cores have diameters as small as 2-3 nm), (ii) sequences of almond-shaped silicon nanocrystals (SiNCs), having diameters in the range of 4-5 nm, connected by an ultrathin silicon nanowire (diameter of 1-2 nm) and embedded in a silica nanowire, and (iii) sequences of isolated spherical SiNCs (having diameters in the range of 4-7 nm) embedded in an otherwise continuous silica nanowire. The predominance of one morphology over the others can be tuned via post-synthesis thermal treatments, with a clear predominance of the spherical SiNC chain configuration after hightemperature annealing (1200°C). It is demonstrated that the Rayleigh model describes very well the morphological transformations undergone by the Si core of the nanostructures when subject to the capillarity-related instability induced by the thermal annealing. More interestingly, we have been able, for the first time to our knowledge, to follow in situ the occurrence of Rayleigh instability in SiNWs leading to the breakage of their inner core and formation of smaller SiNCs. Finally, the optoelectronic properties of the Si nanohybrids, studied via photoluminescence measurements, have confirmed the occurrence of quantum confinement effects in the ultra-small SiNCs present in these new nanohybrids.

Restructuring and breakup of nanowires with the diamond cubic crystal structure into nanoparticles

“…In this paper, we study the break-up patterns of nanowires with the diamond cubic crystal structure (i.e., Ge and Si). In contrast to the conventional understanding of the break-up process, anomalously short-wave sausage-like nanowire configurations and abnormally narrow gaps, Δ, between adjacent nanodroplets were observed in a series of experiments 12,13 . As can be seen in Fig.1a, the ratio / ~4.2 is much smaller than the critical value 2 at the stage of substantial modulation of the nanowire radius.…”

Formation of Hybrid Silicon Nanostructures via Capillary Instability Triggered in Inductively‐Coupled‐Plasma Torch Synthesized Ultra‐Thin Silicon Nanowires

“…Indeed, the observed hexagonal Si polytypes in ultra‐thin SiNWs may result from the presence of a wetting layer around their SiNWs grown via VLS, which may play a role in stabilizing the hexagonal structure. In another work, we also explained the presence of the three Si core configurations in terms of the Plateau–Rayleigh instability, which is an instability phenomenon occurring in fluid thread dynamics and later on demonstrated also for cylindrical nanostructures . As main finding of the theoretical studies carried out by Plateau, an infinite cylinder of non‐viscous fluid free of contact with any surface is unstable to any perturbation which reduces the surface area, according to the principle of minimization of surface energy (or surface tension) for jet breakup.…”

“…The other perturbations that do not respect the Plateau's condition are expected to be stable and do not induce the fluid thread breakup. Since such perturbations may be always described by the convolution of sinusoidal modes, each one having a characteristic growth rate, Rayleigh found that a specific perturbation with wavelength λ 0 grows exponentially with respect to the others, and thus induces the breakup of the cylinder into spherical droplets at periodic points . Analogously to the break‐up phenomenon of fluid threads, 1D nanostructures may undergo thermal instabilities .…”

“…Novel nanostructured materials continue to attract considerable interest as they exhibit enhanced properties and advanced functionalities not present in their bulk counterpart . A new frontier in this field concerns the formation of the so‐called hybrid nanomaterials which result from the appropriate combination of zero dimensional (0D), 1D and 2D nanomaterials . Since the optical and electronic properties at the nanoscale are dependent on the size and morphology of their nano‐constituents, hybrid nanostructures may offer unprecedented functionalities which exploit the synergetic phenomena between their single components.…”

“…All these Si nanostructures were synthesized by means of an inductively coupled plasma (ICP) torch process . The growth of these ICP‐SiNWs was mainly found to occur via the oxide assisted growth mechanism (OAG) combined with the Plateau–Rayleigh instability phenomenon, which leads to the formation of chain‐like and “chaplet‐like” configurations . Similar Si/SiO 2 nanohybrids have been also synthesized via laser ablation and thermal evaporation, whereas the presence of SiO nuclei with different crystalline orientations promotes the growth of SiNC chain in SiO 2 NWs.…”