Structural and photoluminescence properties of silicon nanowires extracted by means of a centrifugation process from plasma torch synthesized silicon nanopowder

Abstract

We report on a method for the extraction of silicon nanowires (SiNWs) from the by-product of a plasma torch based spheroidization process of silicon. This by-product is a nanopowder which consists of a mixture of SiNWs and silicon particles. By optimizing a centrifugation based process, we were able to extract substantial amounts of highly pure Si nanomaterials (mainly SiNWs and Si nanospheres (SiNSs)). While the purified SiNWs were found to have typical outer diameters in the 10-15 nm range and lengths of up to several μm, the SiNSs have external diameters in the 10-100 nm range. Interestingly, the SiNWs are found to have a thinner Si core (2-5 nm diam.) and an outer silicon oxide shell (with a typical thickness of ∼5-10 nm). High resolution transmission electron microscopy (HRTEM) observations revealed that many SiNWs have a continuous cylindrical core, whereas others feature a discontinuous core consisting of a chain of Si nanocrystals forming a sort of 'chaplet-like' structures. These plasma-torch-produced SiNWs are highly pure with no trace of any metal catalyst, suggesting that they mostly form through SiO-catalyzed growth scheme rather than from metal-catalyzed path. The extracted Si nanostructures are shown to exhibit a strong photoluminescence (PL) which is found to blue-shift from 950 to 680 nm as the core size of the Si nanostructures decreases from ∼5 to ∼3 nm. This near IR-visible PL is shown to originate from quantum confinement (QC) in Si nanostructures. Consistently, the sizes of the Si nanocrystals directly determined from HRTEM images corroborate well with those expected by QC theory.

Ultrafast Carrier Relaxation Dynamics in Quantum Confined Non-Isotropic Silicon Nanostructures Synthesized by an Inductively Coupled Plasma Process

“…In particular, QC-induced PL emission has been reported to occur at 1.93 eV (640 nm) for 5 nm-diameter SiNW and to redshift with increasing diameters [ 13 ], exhibiting a few tens of μs recombination time [ 14 ]. In the recent past, we measured the PL spectra on SiNC and SiNW with diameter down to 2–3 nm, confirming the presence of PL features located between 600 and 950 nm (2.0–1.3 eV) [ 15 ]. More recently, we showed that amorphous silicon oxide presents PL in such a spectral range due to band-tail recombination [ 16 ].…”

“…From the process described in refs. [ 15 ] and [ 28 ], we obtained a condensate consisting of a blend of NW (with diameters in the few nm range and up to several microns long) and silicon spheres with diameters ranging from 10 s of nm to few µm. Aimed by the purpose of selecting the NW population from the silicon spheres, the as-grown (AG) Si nanopowder was dispersed in isopropanol (IPA, Merck KGaA, Darmstadt, Germany) at a concentration of 0.5 mg/mL and ultrasonicated for 5 min in a 200 W ultrasonic bath.…”

“…The resulting sprayed nano-Si films were very thin and semi-transparent. The 5000× g centrifuged films (hereafter named 5k) were also annealed (for 1 h at 800 °C) under H 2 (5%)/N 2 (95%) forming gas environment at atmospheric pressure [ 15 ]. To these centrifuged and annealed films, hereafter we will refer as 5k@.…”

“…The SiNS investigated here mainly consist of spherical and non-isotropic SiNC, which have been separated from the largest spherical Si microstructures by means of a centrifugation based extraction process, as detailed in Ref. [ 15 ]. By performing TT measurements with a tunable pump photon energy and a supercontinuum probe pulse (1.2–2.8 eV energy range) as a function of different parameters (i.e., the pump light energy (in the UV and NIR), the light polarization, and high fluence regime), we were able to point out, for the first time, the carrier relaxation mechanism in the ps regime for non-isotropic SiNS.…”

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

“…The formation of new Si-based nanohybrids was demonstrated via high-temperature thermal annealing of ultrathin core/shell Si/SiO 2 nanowires synthesized via an ICP-based process [5,34,35]. The formation mechanisms of these Si hybrid nanostructures was described on the basis of instability phenomena undergone by the inner Si core of the ICP-SiNWs when subjected to high-temperature annealing conditions.…”