Size effect in self-trapped exciton photoluminescence fromSiO2-based nanoscale materials

Abstract

Direct evidence for a size effect in self-trapped exciton ͑STE͒ photoluminescence ͑PL͒ from silica-based nanoscale materials as compared with bulk type-III fused silica is obtained. Two kinds of mesostructures were tested: ͑1͒ silica nanoparticle composites with primary particle size of 7 and 15 nm, ͑2͒ ordered and disordered mesoporous silicas with pore size ranging from ϳ2 to ϳ6 nm and wall thickness ϳ1 nm. The PL was induced by the two-photon absorption of focused 6.4 eV ArF laser light with intensity ϳ10 6 W cm Ϫ2 and measured in a time-resolved detection mode. Two models are applied to examine the blue shift of the STE PL ͑STEPL͒ band with decreasing size of nanometer-sized silica fragments. The first model is based on the quantum confinement effect on Mott-Wannier-type excitons developed for semiconductor nanoscale materials. However, the use of this model leads to a contradiction showing that the model is completely unusable in the case of wide-band-gap nanoscale materials ͑the band-gap of bulk silica E g Х11 eV͒. In order to explain the experimental data, we propose a model that takes into account the laser heating of Frenkel-type free excitons ͑FE's͒. The heating effect is assumed to be due to the FE collisions with the boundary of nanometer-sized silica fragments in the presence of an intense laser field. According to the model, laser heating of FE's up to the temperature in excess of the activation energy required for the self-trapping give rise to the extremely hot STE's. Because the resulting temperature of the STE's is much higher than the lattice temperature, the cooling of STE's is dominated by the emission of lattice phonons. However, if the STE temperature comes into equilibrium with the lattice temperature, the absorption of lattice phonons becomes possible. As a result, the blue shift of the STEPL band is suggested to originate from the activation of hot ͑phonon-assisted͒ electronic transitions. Good agreement between experimental data and our FE laser heating model has been obtained.

Size dependence of nanostructures: Impact of bond order deficiency

“…The energy gained from laser heating of the exciton increases with decreasing nanosolid diameter in an exp(1/R) form. Based on the analysis, Glinka et al [73] indicated that the size-dependent PL blueshift of a nanosolid in general does not need to be related always to the QC effect.…”

“…A free-exciton collision model [73] proposed for the PL blueshift suggests that the E G expansion arises from the contribution of thermally activated phonons in the grain boundaries rather than the QC effect. During PL measurement, the excitation laser heats the free-excitons that then collide with the boundaries of the nanometer-sized fragments.…”

“…(ii) The free-exciton collision model [73] proposes that, during the PL measurement, the excitation laser heats the free-excitons which then collide with the boundaries of the nanometer-sized fragments. The PL blueshift is suggested to originate from the activation of hotphonon-assisted electronic transitions rather than from the effect of quantum confinement.…”

“…(i) The band gap expands with reducing particle size, which gives rise to the blueshift in the PL and photo-absorbance (PA) spectra of nanometric semiconductors such as Si oxides [73,478,479,480], IIIeV [481] (GaN [482,483], InAs [484], GaP, and InP [485,486]) and IIeVI (CdS [487e489], ZnS [490], CdSe [491,492], ZnTe [493], CdTe/CdZnTe [494]) compounds. (ii) The energies of PL and PA involve the contribution from electronephonon coupling that shifts the optical band gap E G from the true E G by the well-known value of Stokes shift arising from electronephonon interaction that also changes with solid size [47].…”

Techniques for the Study of the Electronic Properties.

“…The most widespread, simple theoretical framework to study the influence of confinement effects and, particularly, the primary particle size influence in exciton energy, is the so-called effective-mass approximation (EMA). 4,48 Other theories as the free-exciton collision model (FECM) 49 have been implemented and although are much less utilized may become alternatives in the near future for massive use. The EMA theory assumes parabolic energy bands, infinite confining potential at the interface of the spherical semiconductor particle, and limits main energy terms to electron-hole interaction energy This indicates a blue shift of E(R) as a function of R -2 .…”