Structure and photoluminescence of ultrathin films of SnO_2 nanoparticles synthesized by means of pulsed laser deposition

摘要

Tin oxide (SnO_2) ultrathin films were deposited by pulsed laser deposition (PLD) onto SiO_2/Si and quartz substrates, at various nominal thicknesses ranging from isolated nanoparticles (NPs) to ~300 nm-thick films, under an oxygen background pressure of 10 mlbrr. The microstructural and surface morphologies of the NP-based SnO_2 films were characterized by x-ray diffraction and atomic force microscopy, as a function of their nominal film thickness. The PLD-SnO_2 films were found to be composed of NPs (in the 1-6 nm range), whose size increases with the film thickness. The energy band gap, as determined from the absorption edge, was found to shift to higher values with decreasing the film thickness (i.e., decreasing the NPs size). It was found that an annealing at 700℃ under O_2 ambient is a prerequisite to get a photoluminescence (PL) emission from the PLD-SnO_2 films. The PL of the annealed SnO_2 films was found to consist of two broad emission bands, regardless of the SnO_2 film thickness. The first band is composed of 3 PL subbands peaking at 3.20, 3.01, and 2.90 eV, while the second one is centered on 2.48 eV. In spite of the observed band-gap widening (as confirmed by theoretical calculation), we show that surface state (e.g., oxygen vacancies) dominate completely the PL emission of SnO_2 NPs, which becomes more luminescent as the NPs size decreases while the PL energy remains unchanged. The PL properties of the PLD-SnO_2 NPs are discussed in terms of defects and/or oxygen vacancies related transitions.