Octahedral faceted Si nanoparticles as optical traps with enormous yield amplification

Abstract

We describe a method for the creation of an efficient optical scatter trap by using fully crystalline octahedral Silicon nanoparticles (Si-NPs) of approximately 100 nanometres in size. The light trapping, even when probing an isolated nanoparticle, is revealed by an enormous amplification of the Raman yield of up to 108 times that of a similar Si bulk volume. The mechanism conceived and optimised for obtaining such a result was related to the capability of a Si octahedron to trap the light because of its geometrical parameters. Furthermore, Si-NPs act as very efficient light scatterers not only for the direct light beam but also for the trapped light after it escapes the nanoparticle. These two effects are observed, either superimposed or separated, by means of the Raman yield and by photoluminescence enhancements. The inductively coupled plasma synthesis process performed at a temperature of only 50°C allows for the ubiquitous use of these particles on several substrates for optical and photovoltaic applications.

Strong Raman yield enhancement in large Si nanocrystals from ultraviolet to infrared: Density and shape dependence

“…Before to further quantify the observed signal amplification, two other features of the peak have to be carefully discussed, that is, the peak shift position and broadening, that permit to monitor the local temperature …”

“…A second key aspect of the synthesis process is that the shape of Si‐NP changes with gas precursors mixture. When using pure Ar/SiH 4 , one obtains crystalline octahedral Si‐NP exposing energetically favored large <111> planes . By adding H 2 gas to the mixture, the crystalline growth is not as continuous as in the previous case, and so an irregular‐shaped, polycrystalline Si‐NP can be obtained …”

“…Samples are prepared in vacuum (5 × 10 −7 Torr) in an inductively coupled plasma chemical vapor deposition (CVD) reactor exploiting the capability of this nonthermal plasma to enhance Si crystalline nanoparticles formation, rather than SiH 4 decomposition into amorphous Si (a‐Si) deposition . The reactor has a vertical geometry, the plasma is highly ionized (2–6 × 10 11 /cm 3 ), and the substrate is maintained at low temperature (50°C) . Process pressure is instead in the 5–30 mTorr range.…”

“…using laser‐induced heating of silicon nanocrystals achieved a large increase in the radiative emission rate . Studying Si‐NP, in a preliminary experiment, we detected huge Raman amplification by octahedral faceted Si‐NP, but the data were collected in a limited range, with visible light and mainly on samples with few Si‐NP per unit area …”

“…Recently, a great effort has been devoted to nanotechnology for the excellent results obtained in pure and applied research related to Si nanoparticles (Si‐NP), not only in quantum confinement regime 2–10 nm but also in the contiguous size range 10–150 nm . In fact, larger particles permit much easier and less expensive investigation of many properties, which can be applied to a variety of devices.…”