M. Stefanski

Influence of grain size on optical properties of Sr2CeO4 nanocrystals

The absorption, excitation, and emission spectra of the Sr2CeO4 nanocrystals prepared by the modified sol-gel method were investigated. The impact of the average grain size of Sr2CeO4 nanocrystals on their optical properties was investigated. It was observed that with increasing the average grain size of Sr2CeO4 nanocrystals, the emission decay times decreased significantly. A similar behavior was observed for the emission quantum efficiencies and the Huang-Rhys factors. The grain size dependence of optical parameters of Sr2CeO4 nanocrystals was found well fitted by functions of the reciprocal of the grain diameter. It was shown that this dependence may be rationalized assuming that the correction for electric local field associated with effective refractive index affecting the spherical nanoparticle is governed by its shell.

Synthesis and up-conversion luminescence of Er3+ and Y b3+ codoped nanocrystalline tetra- (KLaP4O12) and pentaphosphates (LaP5O14)

The up-converting nanocrystals of KLa0.95Er0.05Y bxP4O12 and La0.95-xEr0.05Y bxP5O14 were prepared using co-precipitation method. The spectroscopic properties of these materials were investigated in a function of Y b(3+) concentration. The up-conversion emission, power dependence of emission intensities, and the luminescence decay times were investigated. It was found that the green to red and (2)H11/2 → (4)I15/2 to (4)S3/2 → (4)I15/2 emission intensity ratio were strongly affected by the Y b(3+) concentration. Moreover, the order of up-conversion emission and threshold power rises up with Y b(3+) concentration for (4)S3/2 → (4)I15/2 transition. The luminescence decay time of the (4)S3/2 → (4)I15/2 emission increases with Y b(3+) concentration while the (4)F9/2 → (4)I15/2 emission is independent of dopant concentration. The influence of the Y b(3+) concentration on the up-conversion emission intensities was discussed in terms of concentration dependent hetero looped photon avalanche process. A comparison of the up-conversion properties of KLa0.95Er0.05Y bxP4O12 and La0.95-xEr0.05Y bxP5O14 nanocrystals was presented.

Broadband anti-Stokes white emission of Sr2CeO4 nanocrystals induced by laser irradiation

The laser induced white emission (LIWE) from Sr2CeO4 nanocrystals upon irradiation with a focused IR laser beam was investigated. It was observed to be a threshold phenomenon with its intensity increasing exponentially with the excitation power density. This process was investigated under double laser beam simultaneous excitation in the UV range leading to Stokes emission in the visible range and in the IR range leading to anti-Stokes LIWE. With increasing LIWE intensity, the Stokes emission intensity strongly decreased. The LIWE is accompanied by efficient photocurrent generation depending on laser excitation density followed by multiphoton absorption and ionization processes. Photoimpedance measurements showed a sharp increase of the dielectric constant by several orders of magnitude in the Sr2CeO4 nanocrystals during the LIWE process demonstrating a metallic-like behaviour. The mechanisms of LIWE include multiphoton absorption and ionization that lead to the creation of a coupled pair of Ce3+ and Ce4+ ions that allow for the intervalence charge transfer (IVCT) emission transitions in the white light range. A strong decrease of absorption band intensity of Sr2CeO4 with increasing LIWE intensity confirms the creation of (Ce3+, Ce4+) pairs.

Modulation of thulium upconversion in potassium tetraphosphate (KLaP4O12) nanocrystals by co-doping with Yb3+ ions

The influence of sensitizer concentration on the up-conversion properties of KLa0.99−xTm0.01YbxP4O12 nanocrystals was investigated in a wide range of Yb3+ concentrations (x = 0.05–0.5). This impact was manifested by Yb3+ concentration dependent variation of up-conversion emission spectra, blue to NIR emission ratio variation, luminescence decay times as well as on the order of upconversion processes. The observed processes of energy transfer between Yb3+ and Tm3+ were discussed in terms of reduction of the average distance between dopants, Tm3+ → Yb3+ back energy transfer and energy migration to surface quenchers over a Yb3+ sublattice.

Laser induced white emission generated by infrared excitation from Eu3+:Sr2CeO4 nanocrystals

The laser induced white emission (LIWE) was observed from Eu3+:Sr2CeO4 nanocrystals. The samples were obtained in form of powders by the modified sol-gel route. The structure and morphology of the phosphors were investigated by powder X-ray diffraction and transmission electron microscope techniques. The intense LIWE occurred under reduced pressure and focused beam of near infrared laser excitation. The power and pressure dependencies exhibit evident threshold character typical for the avalanche effect. The photoconductivity of the Eu3+:Sr2CeO4 nanocrystals measured as a function of different powers of excitation source was analyzed.

Laser induced broad band anti-Stokes white emission from LiYbF4 nanocrystals

Abstract Spectroscopic properties of tetragonal LiYbF 4 nanocrystals under high dense NIR excitation at vacuum condition were investigated. White, broad band emission covering whole visible part of the spectrum from LiYbF 4 nanocrystals was observed. Its intensity strongly depended on the excitation power, excitation wavelength and ambient pressure. Temperature of the nanocrystals under 975 nm excitation was determined as a function of excitation power. Strong photo-induced current was observed from LiYbF 4 pallet. The emission kinetic was analyzed. The mechanism of the anti-Stokes white emission was discussed in terms of the laser-induced charge transfer emission from Yb 2+ states.

Synthesis, structure and NIR luminescence properties of Yb3+ and Bi3+-activated vanadate GdVO4

Abstract Near-infrared emission of Yb3+ and Bi3+-co-doped GdVO4 vanadate phosphors obtained via the modified Pechinis method was reported. The phase purity and structure of samples were characterized by X-ray powder diffraction (XRD), X-ray energy dispersion spectroscopy (EDS), scanning electron microscopy (SEM), Raman and infrared (IR) spectroscopy. Photoluminescence emission (PL) and excitation (PLE) spectra were recorded and investigated in details. Results indicated that upon near-UV excitation (340 nm) in the (Bi3+-V5+) charge transfer state this phosphor had a strong broad band yellow-green emission (3P1→1S0) of Bi3+ ions and also NIR emission (2F5/2→2F7/2) of Yb3+ ions in the range of 950–1050 nm, which matched well with the spectral response of the silicon-based solar cells. The decreasing Bi3+ emission with increasing Yb3+ concentration proved the occurrence of energy transfer from Bi3+ to Yb3+ ions. Results demonstrated that Yb3+ and Bi3+-co-doped GdVO4 vanadate phosphors might act as a NIR down-conversion (DC) solar spectral converter to enhance the efficiency of the silicon-based solar cells.