Results are presented for the second-harmonic generation of light in K, Li Ta03 for x =1. 6%, 2.6%, 3.4~o, and 6%%uo. From the second-harmonic eKciency and from birefringence results, it is concluded that below the freezing temperature Tf, KTa03.Li polar displacements are correlated over distances g", which increase continuously with dopant concentration x from 6 nm at x = 1.6% to 23 nm at x = 6%. Below Tf, gd is nearly independent of T. Quadrupolar distortions are correlated over much larger distances, comparable to the wavelength of light.

Photoconductivity (PC), thermally stimulated conductivity (TSC), photoluminescence (PL), thermoluminescence (TL), and electron spin resonance (ESR) measurements have been made on single crystals of potassium tantalate over the temperature range 4.2–290 K. We revealed two sorts of O− shallow hole centers which are responsible for the two temperature regions of PL and PC enhancement: T<70 K and 100–150 K. Both O− centers were identified by their ESR spectra. While at low temperatures PL and PC have a rather intrinsic origin, i.e., they do not depend essentially on the sort or quality of crystals, at 100–150 K both quantities strongly depend on the defect content and vanish in well-oxidized crystals. We show that O− centers serve as radiative electron–hole recombination centers. Their energy levels are situated at 0.08 and 0.16 eV above the top of the valence band. Measurements of TSC and TL after UV irradiation revealed several glow peaks at temperatures 18–30 K and 65–70 K. There is a good correlation between TSC and TL intensity in different samples as well as after annealing in O2 and H2 atmospheres. Because electrons are mobile species in KTaO3, we attribute both TSC and TL to the thermal ionization of the same shallow donor centers related with isolated oxygen vacancies. The experimental data were treated in a one-trap/one-recombination center model, which takes into account the presence of “thermally disconnected” deep electron traps.

A comparative study of first-order Raman scattering in K 0.984 Li 0.016 TaO 3 and KTa 0.976 Nb 0.024 O 3 single crystals has been performed. The soft-phonon line shows marked differences in the two systems, revealing a distinctive behavior with regard to the scale of polar order. Analysis of the low-frequency spectrum allows for an estimate of the polar correlation length in the Li-doped sample; this is limited to the nanometric scale, in agreement with the view of a dipole-glass transition. No evidence of a similar situation is found in the Nb-doped sample, where the general trend of soft-mode frequencies and intensities suggests a transition to a state of long-range order, for the quoted concentration. The intensities of the hard-phonon spectra and the TO 4 linewidth are shown to confirm the different evolution of the polar transition in the two systems.

The last decade has come across an increasing demand for theranostic biocompatible nanodevices possessing the double ability of diagnosis and therapy. In this work, we report the design, synthesis and step-by-step characterization of rationally coated gold nanostars (GNSs) for the SERS imaging and photothermal therapy of HeLa cancer cells. The nanodevices were realized by synthesizing GNSs with a seed growth approach, coating them with a controlled mixture of thiols composed of a Raman reporter and a polyethylene glycol with a terminal amino group, and then reacting these amino groups with folic acid (FA), in order to impart selectivity towards cancer cells which overexpress folate receptors on their membranes. After a complete characterization, we demonstrate that these FA-functionalized GNSs (FA-GNSs) are able to bind selectively to the membranes of HeLa cells, acting as SERS tags and allowing SERS imaging. Moreover, we demonstrate that once bound to HeLa cell membranes, FA-GNSs exhibit photothermal effect which can be exploited to kill the same cells in vitro using laser irradiation in the NIR at a very low and safe irradiance. We thus demonstrate that the FA-GNSs designed following the described approach are an efficient prototype of theranostic nanodevices.