Observation of Fano asymmetry in Raman spectra of SrTiO3 and CaxSr1−xTiO3 perovskite nanocubes

Abstract

Bulk SrTiO3 is cubic and not expected to exhibit any first-order Raman scattering. However, nanocubes of SrTiO3 with an edge length of 80±10nm show strong first-order Raman scattering originating from the breaking of symmetry caused by frozen surface dipoles (local tetragonality) and the presence of nanoscopic polar domains (arising from incorporated impurities). Rapid polarization fluctuations within these nanoscopic ferroelectric regions interfere with a polar phonon, resulting in a Fano-like asymmetric line shape in these SrTiO3 nanocubes, as well as in Ca0.3Sr0.7TiO3 nanocubes.

Structural characterisation of slightly Fe-doped SrTiO3 grown via a sol–gel hydrothermal synthesis

“…Positive values of q indicate that the scattering intensity is greater on the high-energy side of the peak. In contrast, negative values of q indicate the interference with the electronic continuum [35]. The q parameter did not exhibit significant temperature dependence, confirming the results reported by Rabuffetti et al [36] for STO nanocubes.…”

“…This Fano-like behaviour has been observed in other perovskite nanoparticles [35,36] and thin films [34,[40][41][42].…”

“…The pronounced Fano-like asymmetric line shape of the TO 2 polar mode suggests that the activation of this firstorder process is most likely due to the presence of defectinduced ferroelectric polar regions in the nanoparticles [35]. This Fano-like behaviour has been observed in other perovskite nanoparticles [35,36] and thin films [34,[40][41][42].…”

“…However, Raman scattering studies of STO have shown that first-order Raman scattering can actually be observed when the central symmetry is broken due to various factors, such as strain effects, the presence of impurities and defects and oxygen vacancies [34]. For the STO nanoparticles, the origin of symmetry breaking comes from frozen surface dipoles or from local tetragonal arrangement as well as the presence of nanoscopic polar domains due to the incorporated impurities [35].…”

“…4a. By comparing these Raman spectra with the published data [24,[33][34][35][36][37][38][39], three firstorder phonon modes were identified at approximately 171, 540 and 800 cm -1 corresponding to the TO 2 , TO 4 and LO 4 modes, respectively. The frequencies of the TO 2 and TO 4 / LO 4 modes have been determined by fitting with Fano and Lorentzian line shapes, respectively.…”