Ratiometric temperature measurement using negative thermal quenching of intrinsic BiFeO3 semiconductor nanoparticles.

A strategy for optical nanothermometry using the negative thermal quenching behavior of intrinsic BiFeO3 semiconductor nanoparticles has been reported here. X-ray diffraction measurement shows polycrystalline BiFeO3 nanoparticles with a rhombohedral distorted perovskite structure. Transmission electron microscopy shows agglomerated crystalline nanoparticles around 20 nm in size. Photoluminescence measurements show that intensity of the defect level emission increases significantly with temperature, while the intensity of near band emission and other defect levels emissions show an opposite trend. The most important figures of merit for luminescence nanothermometry: the absolute (S a) and the relative sensor sensitivity (S r) and the temperature resolution (ΔT m) were effectively resolved and calculated. The relative sensitivity and temperature resolution values are found to be 2.5% K-1 and 0.2 K, respectively which are among the highest reported values observed so far for semiconductors.

Structural, optical and vibrational properties of self-assembled Pbn+1(Ti1-x Fex)nO(3n+1)-δ Ruddlesden-Popper superstructures.

Bulk crystals and thin films of PbTi(1-x)FexO3(-δ) (PTFO) are multiferroic, exhibiting ferroelectricity and ferromagnetism at room temperature. Here we report that the Ruddlesden-Popper phase Pbn+1(Ti(1-x)Fex)nO3(n+1)-δ forms spontaneously during pulsed laser deposition of PTFO on LaAlO3 substrates. High-resolution transmission electron microscopy, x-ray diffraction and x-ray photoemission spectroscopy were utilised to perform a structural and compositional analysis, demonstrating that n ≃ 8 and x ≃ 0.5. The complex dielectric function of the films was determined from far-infrared to ultraviolet energies using a combination of terahertz time-domain spectroscopy, Fourier transform spectroscopy, and spectroscopic ellipsometry. The simultaneous Raman and infrared activity of phonon modes and the observation of second harmonic generation establishes a non-centrosymmetric point group for Pbn+1(Ti0.5Fe0.5)nO3(n+1)-δ, a prerequisite for (but not proof of) ferroelectricity. No evidence of macroscopic ferromagnetism was found in SQUID magnetometry. The ultrafast optical response exhibited coherent magnon oscillations compatible with local magnetic order, and additionally was used to study photocarrier cooling on picosecond timescales. An optical gap smaller than that of BiFeO3 and long photocarrier lifetimes may make this system interesting as a ferroelectric photovoltaic.