Types Of Optical Centers Research Articles

Assignment of Core and Surface States in Multicolor-Emissive Carbon Dots.

It is important to reveal the luminescence mechanisms of carbon dots (CDs). Herein, CDs with two types of optical centers are synthesized from citric acid in formamide by a solvothermal method, and show high photoluminescence quantum yield reaching 42%. Their green/yellow emission exhibits pronounced vibrational structure and high resistance toward photobleaching, while broad red photoluminescence is sensitive to solvents, temperature, and UV-IR. Under UV-IR, the red emission is gradually bleached due to the photoinduced dehydration of the deprotonated surface of CDs in dimethyl sulfoxide, while this process is hindered in water. From the analysis of steady-state and time-resolved photoluminescence and transient absorption data together with density functional theory calculations, the green/ yellow emission is assigned to conjugated sp2 -domains (core state) similar to organic dye derivatives stacked within disk-shaped CDs; and the broad red emission-to oxygen-containing groups bound to sp2 -domains (surface state), whereas energy transfer from the core to the surface state can happen.

Spectroscopic and Laser Properties of Neodymium Doped Yttrium Titanate Crystals

The possibility of using monocrystals of yttrium titanate in quantum electronics is considered. Optical zone melting technology is developed for growing monocrystals of yttrium titanate doped with neodymium ions. Absorption and luminescence spectra in the range from helium to room temperatures and laser generation are investigated. Based on the analysis of spectroscopic data, a diagram of the energy levels of the Nd3+ ion in yttrium titanate lattice is constructed. It is shown that neodymium ions in the yttrium titanate lattice form two types of optical centers. The results of the study of laser generation in pulsed mode at room and nitrogen temperatures are presented.

Structure and transport properties of zirconia crystals co-doped by scandia, ceria and yttria

This work is a study of the effect of co-doping (ZrO2)0.9(Sc2O3)0.1 solid solution with yttria and/or ceria on the phase composition, local structure and transport properties of the crystals. The solid solution crystals were grown using directional melt crystallization in cold crucible. We show that ceria co-doping of the crystals does not stabilize the high-temperature cubic phase in the entire crystal bulk, unlike yttria co-doping. Ceria co-doping of the (ZrO2)0.9(Sc2O3)0.1 crystals increases their conductivity, whereas the addition of 1 mol.% yttria tangibly reduces the conductivity. Equimolar co-doping of the (ZrO2)0.9(Sc2O3)0.1 crystals with ceria and yttria changes the conductivity but slightly. Optical spectroscopy of the local structure of the crystals identified different types of optical centers. We found that the fraction of the trivalent cations having a vacancy in the first coordination sphere in the ceria co-doped crystals is smaller compared with that in the yttria co-doped crystals.

New amorphous nanophosphors obtained by evaporation of silicates and germanates REE

Abstract Amorphous nanophosphors have been produced by pulsed electron beam evaporation of bulk phosphors of rare earth elements (REE) silicates and germanates with oxyapatite structure activated with europium. The processes of crystallization of amorphous nanopowders (NP) have been studied by differential scanning calorimetry and thermogravimetry (40–1400°C) methods. A modification of the Raman and IR spectra of the samples was found when the particle size decreased from bulk to nanostate due to insignificant polymerization of Si(Ge)O4 tetrahedra. The spectral and luminescence characteristics of the bulk samples as well as nanoamorphous samples have been examined. It was established that Eu3+ forms two types of optical centers. Upon the transition of Sr2Gd8(SiO4)6O2:Eu3+ phosphor from bulk state to nanostate during laser excitation, the luminescence intensity of Eu3+ ions increases 10 times. Blue and white amorphous nanophosphors have been obtained for the first time by evaporation of Ca2M8(SiO4)6O2:Eu (M=Y, Gd) and Ca2La8(GeO4)O2:Eu phosphors. Radiation reduction of ions Eu3+→Eu2+ in electron beam has been presumably found.

Low-temperature spectroscopy of optical centers in cerium-yttrium (Ce1-xYxO2-x/2) and cerium-zirconium (Ce1-xZrxO2) oxides

The results of studying the impact of isovalent (Zr4+) and non-isovalent (Y3+) impurity ions on the formation of oxygen vacancies in the lattice of cerium dioxide nanocrystals. It is shown that the oxygen vacancy content increases significantly with the introduction of both the Y3+ and Zr4+ ions, and that the oxygen vacancies play a part in the formation of two types of optical centers: Ce3+-V··-Ce3+ and F+-centers. The ratio of these optical centers can be manipulated by varying the concentration of impurity ions and the atmosphere of high-temperature nanocrystal processing.

Thulium optical centers in Tm,Nb:SrMoO4 crystal

Formation of three types of thulium optical centers is observed in SrMoO4 crystal doped with Tm3+ in the form of thulium niobate. Two types of optical centers are found to have quite similar spectroscopic properties, while the third one is characterized by richer absorption and fluorescence spectra and a twice shorter 1G4 level lifetime.

Spectral characteristics of nanoamorphous phosphors obtained by evaporation of Sr2Gd8(SiO4)6O2: Eu polycrystals

Amorphous nanophosphors have been obtained for the first time by pulsed electron beam evaporation of targets from bulk polycrystalline phosphors of the composition Sr2Gd8(SiO4)6O2: Eu with oxyapatite structure. The processes of crystallization of amorphous nanopowder were studied using differential scanning calorimetry and termogravimetry (40−1400°C). It was found that the Raman spectra of the samples are modified when the particles decrease from bulk to nanodimensional state. The spectral and luminescent characteristics of the phosphors in polycrystalline and nanoamorphous states have been examined. It was established that Eu3+ forms two types of optical centers. Unlike bulk phosphors, in the nanophosphors no Eu2+ ions are formed. The ratio of red to orange luminescence intensity for nanophosphors was found to increase as compared with bulk phosphors. This effect is due to disordering of nanoparticles. A resonant transfer of excitation energy from Gd3+ to Eu3+ ions was found in bulk and nanophosphors. Upon transition from polycrystalline to nanoamorphous state and laser excitation of phosphors, the luminescence intensity of Eu3+ ions increases approximately 10 times. This effect is explained in the framework of the configuration coordinates model.

Structure, radiation damage and annealing effects in diamond implanted with a high fluence of a few MeV carbons

Crystallinity of the type-Ib single crystal diamond damaged with 3MeV carbon ions to a fluence of 2×1016cm−2 and subsequent post-implantation anneal at 1200°C in a hydrogen plasma atmosphere was investigated. The annealed sample had very low optical transmission, suggesting occurrence of graphitization due to an excess introduction of radiation defects and the following post-implantation annealing. In this optical transmission spectrum, an interference pattern was also observed. Combination of such an interference pattern with a TRIM simulation demonstrated that a thin highly defective layer was formed around the depth of ∼1.6μm, whereas the optical transparent diamond is preserved as an over-layer in the depth region from the surface to ∼1.6μm. In the photoluminescence (PL) spectra of such a residual thin diamond over-layer, a very broad luminescent band appeared in the spectral range 1.4–2.3eV. The data of time-resolved PL measurements allowed one to resolve this broadband to two types of bands peaking at ∼1.6eV and ∼1.9eV. The ∼1.6eV band is probably the “B-band” originated from the radiation defects introduced by the ion-implantation. It seems that the ∼1.9eV band consists of phonon sidebands of zero phonon lines (around 2.1–2.3eV) of two types of optical centers, but the further studies are required for detailed identifications of these optical centers.

Synthesis and spectral characteristics of Sr 2Y 8(SiO 4) 6O 2: Eu polycrystals

Spectral-luminescent characteristics of Sr 2Y 8(SiO 4) 6O 2: Eu powder crystal phosphor with the apatite structure and high-intensity luminescence of Eu 3+ ions have been studied. The charge state of europium in the samples has been characterized by means of X-ray L 3-adsorption spectroscopy. It was established that Eu 3+ forms two types of optical centers. Besides, luminescence of Eu 2+ions was found. Reduction Eu 3+→Eu 2+ was considered, which may be due to V Sr ∥ vacancy formation in the 4 f crystal lattice position and to negative charge transfer by this vacancy to two Eu Y 3 + ions. Thus, in the silicate lattice there exist inhomogeneously distributed oxygen-deficient centers, which are responsible for nonradiative transfer of excitation energy to Eu 3+ and Eu 2+ ions. To study electron-vibrational interactions in the crystal phosphor samples, their IR and Raman spectra were examined. In the luminescence spectrum of Eu 2+, a series of low-intensity bands caused by interaction of the 4 f 65 d state of Eu 2+ with silicate lattice vibrations was observed.

Spectroscopy of chromium centers inLiScGeO4andLiInGeO4single crystals

The detailed spectroscopy of $\mathrm{Cr}:\mathrm{Li}\mathrm{Sc}\mathrm{Ge}{\mathrm{O}}_{4}$ and $\mathrm{Cr}:\mathrm{Li}\mathrm{In}\mathrm{Ge}{\mathrm{O}}_{4}$ is presented. Low temperature absorption, emission, and excitation spectroscopies reveal two types of optical centers: trivalent chromium in the distorted octahedral sites and tetravalent chromium in the tetrahedral sites. Energies of the transitions calculated from the exact atom positions using the angular overlap model are in a satisfactory agreement with the experimental data and describe the unusually low energy position of the ${\mathrm{Cr}}^{3+}$ metastable level. The spin-forbidden $^{2}E$ and $^{2}T_{1}$ transitions of ${\mathrm{Cr}}^{3+}$ appear as a Fano antiresonance in the absorption and excitation spectra. The temperature quenching of ${\mathrm{Cr}}^{3+}$ fluorescence is described in terms of the Struck and Fonger model. ${\mathrm{Cr}}^{3+}$ ions in highly distorted octahedral positions are active centers responsible for recently discovered ${\mathrm{Cr}}^{3+}$ laser action in the near-infrared range in these crystals.

Nonradiative relaxation and inhomogeneous splitting of aggregated optical centers in the Nd 3+-doped CaF 2 and SrF 2 crystals (FLN and decay study)

Site-selective fluorescence and laser excitation spectroscopy for different types of optical centers in SrF 2 : Nd 3+ (0.3 – 1.0%) and SrF 2 : La 3+ (1.0%) : Nd 3+ (0.2%) were performed at 4.2 K. The crystal-field levels of the 4I 9/2, 4F 3/2 , and 4G 5/2 multiplets for different Nd 3+ optical centers were determined in SrF 2 : Nd 3+ (0.3%). An attempt was made to identify some of the new optical centers on the basis of their fluorescence lifetimes and crystal-field splitting. Some models of these centers have been discussed. Two–six times reduction of the 4F 3/2 multiplet lifetime was observed in the pair M-center in comparison with the single optical centers due to energy cross-relaxation in the pair. A fine splitting of the levels at the 4I 9/2(1)→ 4G 5/2(1) crystal-field transition (1.5 cm −1) similar to that in CaF 2 (2.4 cm −1) was found for the Nd–Nd pair in SrF 2. The variation of the splitting from CaF 2 to SrF 2 nicely demonstrates the R −5 dependence of coherent quadrupole–quadrupole interaction.