Free standing luminescent silicon quantum dots: evidence of quantum confinement and defect related transitions

Abstract

We report the synthesis of luminescent, free standing silicon quantum dots by dry and wet etching of silicon and silicon oxide core/shell nanostructures, which are synthesized by controlled oxidation of mechanically milled silicon. Dry and wet etching performed with CF(4) plasma and aqueous HF, respectively, result in the removal of the thick oxide shell of the core/shell nanostructures and affect an additional step of size reduction. HF etch is capable of producing isolated, spherical quantum dots of silicon with dimensions ∼ 2 nm. However, the etching processes introduce unsaturated bonds at the surface of the nanocrystals which are subsequently passivated by oxygen on exposure to ambient atmosphere. The photoluminescence spectra of the colloidal suspensions of these nanocrystals are characterized by double peaks and excitation dependent shift of emission energy. Comparison of the structural, absorption and luminescence characteristics of the samples provides evidence for two competing transition processes--quantum confinement induced widened band gap related transitions and oxide associated interface state mediated transitions. The results enable us to experimentally distinguish between the contributions of the two different transition mechanisms, which has hitherto been a challenging problem.

On the role of the atomic bond types in light emission from Si nanoparticles

“…8,13 Therefore, despite XS Si being a practical labeling figure at this point, we must go further in the search for a better parameter that can be used to succinctly but precisely define all SRO types, regardless of fabrication technique and conditions.…”

“…One of the most useful features of the SRO is the possibility of tuning the characteristics of its emission spectrum, which can comprehend nearly all the visible range, and is susceptible of modification trough the use of different fabrication techniques and conditions. 6,7 Whereas it has been reached a consensus in that the luminescence in these materials is caused by a combination of defect-related emission and quantum confinement phenomena, 6,8,9 it still exists a lack of a precise knowledge regarding how each contributes to the overall spectrum, and how this is related to the atomic characteristics of the material. Furthermore, the parameters used to define its characteristics are not common for all fabrication techniques.…”

Luminescent Properties of Silicon Nanocrystals:Spin on Glass Hybrid Materials

“…It is noticeable that both before and after annealing, the IR spectra of the pure SOG film are similar to the IR spectra obtained from the coated and embedded films with Si-NCs (Figure 5b,c). Figure 6 shows the IR peak related to the Si-O-Si asymmetric stretching mode deconvoluted in their longitudinal (LO) and transversal (TO) modes of the different samples, in order to qualitatively assess the structural changes produced by the thermal annealing [7,[33][34][35] In all of the spectra in Figure 6, it is possible to see the stretching peak (at ~1078 cm −1 ) and the asymmetric stretching peak (from 1100 cm −1 to 1200 cm −1 ). Notice that these spectra show a shift to lower or higher wavenumbers depending on its structure or composition after annealing.…”

“…A deconvolution of the IR stretching peak (symmetric and asymmetric) in the LO and TO modes shows a change in the full width at a half maximum (FWHM) associated with the formation of amorphous phases of SiO 2 (peak at~1025 cm −1 ) in the Si-NCs/SOG interface after thermal annealing. This is an indication of a possible combination of non-stoichiometric and stoichiometric SiO 2 [7,36]. The Si-NCs could react with the oxygen atoms of the SOG due to the silicon atoms presenting a higher probability of having oxygen neighboring atoms, forming Si=O related bonds [14].…”

“…One of the proposed approaches is to use dielectric matrices with silicon nanocrystals (Si-NCs) or nano-agglomerates as active media. Among several other techniques, chemical methods have been used to fabricate such materials [2][3][4][5][6][7][8][9][10]. The origin of the photoluminescence (PL) of Si-NCs embedded into oxide films is still controversial [11], but there are two principal models commonly used to explain the emission mechanism.…”