Kinds Of Luminescence Centers Research Articles

Novel highly efficient Bi3+-activated phosphors for warm WLEDs

A novel highly efficient Bi3+-activated Ca2LuZrScAl2GeO12 phosphor has two kinds of luminescent centers, and exhibits a blue emission and a rare near-ultraviolet emission with a narrow band.

Structural Impurities and Intrinsic Defects Role in High Quality Raw Quartz Selection by Use of Luminescence Analyses

X-ray induced luminescence (XRL) spectra and thermostimulated luminescence (TSL) curves for natural quartz from a number of main quartz occurrences of Ural have been obtained. They are represented as a number of radiation bands with peaks related to different kinds of luminescence centers, such as admixtures of Al, Ge, Na, Li, or intrinsic defects of quartz’s structure. Influence of preheating and X-raying duration time increase on XRL spectra of quartz has been shown, as well as influence of preheating and irradiation of samples on TSL curves. All these arrangements give an opportunity to estimate degree of purity of natural quartz. Thus, while high-tech industries are developing fast we have to find effective way to select that raw quartz, which is right for high-pure concentrate obtaining, and luminescent study can help with that.

Luminescent properties and energy transfer of Sm3+ doped Sr2CaMo1-xWxO6 as a potential phosphor for white LEDs

A series of Sm3+ doped Sr2CaMo1-xWxO6 phosphors were synthesized through solid state reaction process and their luminescent properties have been investigated. They presented excitation bands in UV, near UV and blue regions, originating from charge transfer transitions within Mo(W)O6 or Sm3+ f–f transitions. Concentration quenching still occurs among MoO6 groups even when Mo concentration is very low, however, if there is no Mo content in Sr2CaMo1-xWxO6 phosphors, absorption in near UV region cannot happen. It was found there is a big difference between the calculated optical band gap of Sr2CaMo0.02W0.98O6: 2% Sm3+ and that of Sr2CaMo0.01W0.99O6: 2% Sm3+. Band structure and density of state of Sr2CaMo0.5W0.5O6 were examined to explore the origination of charge transfer transition. Site occupation of Sm3+ in host lattice was discussed based on Raman spectra and group theory analysis. There are two kinds of luminescence centers in Sm3+ doped Sr2CaMo1-xWxO6 (x > 0) phosphors. When 98% Mo were substituted by W, the excitation band at 350 nm by monitoring 608 nm emission in Sr2CaMo0.02W0.98O6: 2%Sm3+ is about 8.5 times stronger than that in Sr2CaMoO6: 2%Sm3+. Energy transfer process was also discussed considering the overlap between the emission spectrum of Mo(W)O6 groups and the absorption spectrum of Sm3+, as well as the atom arrays of Mo6+(W6+), O2− and Sm3+.

Photoluminescence of colloidal YVO 4:Eu/SiO 2 core/shell nanocrystals

YVO 4:Eu, and YVO 4:Eu/SiO 2 nanocrystals (NCs) were prepared by hydrothermal method with citrate as capping ligands. Their morphologies, structures, components, and photoluminescence properties were investigated and presented in this paper. A remarkable fluorescence enhancement up to 2.17 times was observed in colloidal YVO 4:Eu/SiO 2 NCs, compared to that of colloidal YVO 4:Eu NCs. This is mainly attributed to the formation of the outer protecting layers of biocompatible SiO 2 shells; which shield the Eu 3+ ions effectively from water and thus reduces the deleterious effects of water on the luminescence. Meanwhile, on the basis of laser selective excitation, two kinds of luminescent centers were confirmed in the NCs, namely, inner Eu 3+ ions and surface Eu 3+ ions. The surface modifications for YVO 4:Eu NCs effectively reduced the surface defects and accordingly enhanced the luminescence. The core/shell NCs exhibited long fluorescence lifetime and high photostability under ultraviolet radiation.

Crystal Growth of SrS:Sb3+,Te from SrS-Sb2Te3 Solutions and Its Optical Properties

SrS crystals were grown from SrS-Sb2Te3 solutions at 1050°C. Its structural and compositional analysis showed that solution-grown SrS has a cubic structure and contains 0.06 at% of Sb and 0.03 at% of Te, respectively. Comparison of room-temperature (RT) cathodoluminescence (CL), selectively excited 77 K and RT photoluminescence (PL), and 77 K PL excitation (PLE) spectra of the grown SrS with those of SrS:Te and SrS:Sb showed the existence of two kinds of luminescent centers, Te isoelectronic-trap centers and a Tl-type Sb 3+ center.

The Role of Silicon Oxide Layers in Luminescence of Ensembles of Silicon Quantum Dots**The project supported by National Natural Science Foundation of China, the NMS-6 of China, National Science Foundation(DMR9803005) and INT 0001313 of USA

Based on the quantum confinement-luminescence center model, to ensembles of spherical silicon nanocrystals (nc-Si) containing two kinds of luminescence centers (LCs) in the layers surrounding the nc-Si, the relationship between the photoluminescence (PL) and the thickness of the layer is studied with the excitation energy flux density as a parameter. When there is no layer surrounding the nc-Si, the electron-heavy hole pair can only recombine inside the nc-Si, then the PL blueshift with reducing particle sizes roughly accords with the rule predicted by the quantum confinement model of Canham. When there presences a layer, some of the carriers may tunnel into it and recombine outside the nc-Si at the LCs to emit visible light. The thicker the layer is, the higher the radiative recombination rate occurred outside the nc-Si will be. When the central scale of the nc-Si is much smaller than the critical scale, the radiative recombination rate outside the nc-Si dominates, and visible PL will be possible for some nc-Si samples with big average radius, greater than 4 nm, for example. When there is only one kind of LC in the layer, the PL peak position does not shift with reducing particle sizes. All these conclusions are in accord with the experimental results. When there are two or more kinds of LCs in the layer, the PL peak position energy and intensity swing with reducing particle sizes.

Luminescent Centers of Eu 2+ in BaMgAl 10O 17 Phosphor

Two kinds of luminescent centers are observed in BaMgAl 10O 17:Eu 2+ phosphor. Influence of flux on luminescence of Eu 2+ in the phosphor is discussed in detail. There exists Eu 2+ (F −) center (a luminescent center with 277 nm excitation band and 386 nm emission band) due to the substitution of F − ions for O 2− ions. Effective energy transfer from Eu 2+ (F −) to Eu 2+ (O 2−) (a luminescent center with 334 nm excitation band and 450 nm emission band) is observed. The quenching concentration of Eu 2+ in BaMgAl 10O 17 is raised by 0.20 mole per mole host due to forming of new luminescent center Eu 2+ (F −).

Mechanism for the Correlated Swinging of Photoluminescence Intensity and Peak Energy Shift with Sizes of Nanoscale Si Particles**The project supported by National Natural Science Foundation of China

Based on the quantum confinement-luminescence center model, we focus on the relationship between the photoluminescence (PL) spectra and sizes of nanoscale silicon particles.We find that when there are two kinds of luminescence centers (LC) in the oxide layer surrounding the silicon particles, both the integrated PL intensity and the spectral peak position swing with reducing the sizes of silicon particles, which is different from the monotonous blueshift of peak position predicted by quantum confinement model. By changing the concentrations of LC we find the correlation between the spectral peak position and the integrated PL intensity when the size of silicon particles is reduced. The effects of other parameters such as the half width of size distribution and the position of photon emission levels of LC are also found to be important.

The galvanoluminescence spectra of porous oxide layers formed by aluminum anodization in oxalic acid

This paper presents the result of the most recent investigations of the galvanoluminescence (GL) spectra of porous oxide layers formed by aluminum anodization in oxalic acid. A new experimental procedure for GL spectra recording was established. This method can be easily applied to GL spectral measurements on porous anodic films. The spectra were recorded for different values of temperature and anodization current density. Two spectral peaks at 446 and 485 nm, respectively, were observed, indicating the probable existence of two kinds of luminescence centers incorporated in the oxide. In addition, it was discovered that the intensity ratio for the two peaks changes with the current density. Those GL measurements, with further photoluminescent (PL) measurements should lead to a definitive explanation of the GL phenomena, as well as of the nature and distribution of luminescent centers in both porous and barrier oxide films on aluminum.

Photoluminescence mechanism in hydrogenated amorphous silicon studied by frequency-resolved spectroscopy.

In order to reconsider the photoluminescence mechanism in hydrogenated amorphous silicon (a-Si:H), the lifetime distribution of photoluminescence, G(\ensuremath{\tau}), was evaluated over a wide lifetime range between 5.3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}7}$ s and 8.0\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}2}$ s using frequency-resolved spectroscopy. The most remarkable finding is that G(\ensuremath{\tau}) is dominated by two kinds of lifetime components characterized by specific peak lifetimes at low temperatures. At 12 K, only the lifetime component peaked at about 1 ms is dominant, whereas another component appears distinctively at around 10 \ensuremath{\mu}s and grows gradually with increasing temperature. Such a discontinuous change of the lifetime from 1 ms to 10 \ensuremath{\mu}s takes place at temperatures lower than 60 K where the photoluminescence intensity is almost constant. Interestingly, the peak lifetimes for both lifetime components are quite insensitive to the Urbach energy or the emission energy as long as the excitation intensity is held low enough to satisfy the condition for geminate-pair recombination. These characteristic features observed in G(\ensuremath{\tau}) do not reconcile with the generally accepted model of tunneling recombination between carriers trapped at the tail states after thermalization. In particular, dominance of the 1-ms lifetime component at 12 K is interpreted following the model as the electron-hole separation enlarges up to a value corresponding to the 1-ms recombination after photogeneration.However, it is hard to understand the absence of the 10-\ensuremath{\mu}s lifetime component at 12 K, since the 10-\ensuremath{\mu}s recombination is expected to take place at much shorter electron-hole separation and is actually observed at elevated temperatures. Because of the several arguments against the generally accepted model and of the coexistence of two lifetime components having specific peak lifetimes, it is more appropriate to consider that the photoluminescence in a-Si:H comes from special localized luminescent centers corresponding to the respective lifetime components. Since the lifetime changes discontinuously from 1 ms to 10 \ensuremath{\mu}s with increasing temperature while the luminescence intensity remains constant at the low temperatures, some correlation is expected to exist between the two kinds of luminescent centers.

Two-band structure in photoluminescence spectra from porous silicon and its dependence on excitation wavelength

We have systematically measured the room-temperature photoluminescence spectra with a two-band structure from porous silicon (PS), as a function of excitation wavelengths in a wavelength range from 260 to 460 nm. Each spectrum can be fitted by two Gaussian functions centered at about 610 and 700 nm, the intensities of the two bands change with excitation wavelength and the intensity maxima occur when the excitation wavelength is about 340 and 400 nm, respectively. When the excitation wavelength exceeds 420 nm, the band at 610 nm is very weak. The above phenomena can be accounted for in the quantum confinement/luminescence centers model [G. G. Qin and Y. Q. Jia, Solid State Commun. 86, 559 (1993)], where it is supposed that there are two kinds of luminescence centers in SiOx layers covering the nanoscale silicon units in PS.

N-lines in the luminescence spectra of Cr 3+ -doped spinels (I) identification of N-lines

Low-temperature luminescence spectra ( T=75 K) which have been measured at various excitation wavelengths in the range 450 nm⩽λ exc ⩽650 nm are reported for the chromium-doped spinels MgAl 2O 4, ZnAl 2O 4, ZnGa 2O 4 and for chromium-doped samples with the general composition AB 2O 4 · xB 2O 3 (A=Mg,Zn; B=Al,Ga). It is shown that the measurement of luminescence spectra at various excitation wavelengths provides a very powerful and highly sensitive method for the detection and identification of lines which arise from different kinds of luminescence centers. By their differing intensity dependences on the excitation wavelength, a number of N-lines which must be due to different kinds of Cr 3+ ions with slightly different short-range order can be identified in the spectra of the three stoichiometric spinels at frequencies v(R 1)− v<250 cm −1 .