Phosphor hosts

Phosphor host materials for EL applications must have the following properties 11... 

The UV photon may be assumed to have an energy less than the band gap of the phosphor host so the absorption by A, S, and P takes place at the centers. Many activators, particularly Mn2+, have a low absorption cross-section so that direct pumping of the activator is inefficient. Sensitizers, such as Pb2+, are selected to have large absorption cross-sections at UV wavelengths. They then transfer the excitation to the activator which re-emits it as luminescent radiation. Most co-activators have a luminescent emission of their own so that the efficiency of the system depends on the kinetics of energy transfer compared with direct emission of the co-activator. In many mineral systems, one observes both and thus the activator radiation Ra and the sensitizer radiation Rg are... 

Excitation by impinging electrons (or ions) takes place by a somewhat different mechanism. Energetic electrons penetrate the phosphor grains and the lose energy by multiple collision processes. The energy is sufficient to pump the conduction band of the phosphor host and the excitation can move anywhere in the crystal to pump the localized luminescence centers. Cathodoluminescence thus appears at lower concentration thresholds than photoluminescence but is more susceptible to the influence of defects and other luminescence poisons. 

The hosts that we have discussed to date have been ionic in nature, and dielectric as well (they are non-conductive). There is also another class of phosphor hosts which are covalent and semi-conductive in nature, namely the zinc and cadmium sulfides and/or selenides. The criterion for selection of a semi-conducting host for use as a phosphor includes choice of a composition with an energy band gap of at least 3.00 ev. This mandates the use of an optically inactive cation, combined with sulfide, selenide and possibly telluride. The ojq gen-dominated groupings such as phosphate, or silicate or arsenate, etc. are not semi-conductive in nature. And, none of the other transition metal sulfides have band gaps sufficiently... 

The rare-earth-containing p-alumina type of phases can also be directly synthesized at elevated temperatures (Veistegen et al. 1973, Mizuno et al. 1974, Kumar and Kay 1985, Warner et al. 1996) they are of considerable interest not only as optical materials for laser and phosphor hosts (Abraham et al. 1987) but also in electrochemical cells at elevated... 

Srs(P04)3X (X = F, Cl, Br) acts as another important apatite-type halo-phosphate phosphor host. As shown in Fig. 10.11a, the photoluminescence emission spectra of Srio(P04)6X Eu (X = F2, CI2, Br2 and O) under 365-nm excitation are demonstrated. AU of the samples can show violet-blue (when X = O) or blue... 

Na2M(P04)F (M = Sr, Ca) belongs to another typical non-apatite family halo-phosphate phosphor host [63, 64]. The novel green-emitting phosphor Na2CaP04F Eu was synthesized, which has broad excitation bands between 250 and 450 nm with the green emission centered at 506 nm. The optimum concentration of Eu " in Na2CaP04F Eu is determined to be 0.02 mol. Moreover, Dy " ... 

Borate compounds have a versatile crystal stmcmre. Boron-containing polyhedra can be formed in different forms, and the connections of the polyhedra will in mm lead to many possibilities to obtain new phases [79]. Furthermore, the introduction of halogen atoms will produce more chances to discover many new halo-borate compounds, of which many will act as the excellent phosphor hosts [80, 81]. Herein, we first summarizes the reported halo-borate phosphors including their host, activators, and the corresponding excitation and emission peaks as shown in Table 10.2. Then we will select some important halo-borate phosphors for detailed introduction. 

Suehiro T, Onuma H, Hirosaki N, Xie R-J, Sato T, Miyamoto A (2009) Powder synthesis of Y-ot-SiAlON and its potential as a phosphor host J Phys Chem C 114(2) 1337-1342... 

XEOL of rare earths 445 5.2. Preparation of phosphor hosts 452... 

Genesis of ultrasensitive non-rare earth phosphor hosts... 

A generalized table of oxygen dominated hosts, which is an extension of the classification proposed by Johnson (1966) and DeKalb ( 970), is shown in table 37E.1. The emphasis in our studies has been to evolve analytically useful phosphors with a high degree of reproducibility in their preparation, freedom from interelement effects (see 5.1) and with the best possible limits of detection. In the periodic table shown in fig. 37E.6 the 57 elements enclosed by boxes when converted to an appropriate phosphor host support the luminescence of one or more of the lanthanides down to at least the 100 ppm level. 

Generalized composition of oxidic phosphor hosts in which XEOL of rare earths has been observed. 

The wavelengths of the analytical lines and the estimated limits of detection for different phosphor hosts are summarized in table 37E.2. The table is not exhaustive as several investigators who have utilized the XEOL technique for the determination of trace level rare earths have not tabulated the limits of detection. Typical analytical curves that have been obtained in our studies utilizing the internal reference element principle are shown in fig. 37E.12. 

This brief report summarizes the analytical potential of the XEOL technique for the determination of trace level rare earths in a variety of phosphor hosts. It has not been designed as an exhaustive review but should serve as a guide to those who wish to evaluate the XEOL technique for the determination of trace level rare earths in a wide variety of matrices. 

The anhydrous sulfates are not very satisfactory luminescent materials but oxosulfates R2O2SO4 prepared from them have been studied as possible phosphor host materials (Haynes and Brown, 1968 Porcher et al., 1983). See also section 4.6. 

La, Lu, and Y do not show any UV absorption when used as host cations in rare earth orthovanadate phosphors. Of the usual host cations only gadolinium has absorption and emission bands in the UV region. The rare earth orthovana-dates are exceptional phosphor host materials in that the vanadate anion can be excited by both long and short UV radiation (fig. 121). The excitation of YVO4 in the 320 nm region occurs via the Aj T2 allowed transition of VO4 (Burrus and Paulusz, 1969). The excitation at shorter wavelengths (250 nm) involves... 

Semiconductors in nano-crystallized form exhibit markedly different electrical, optical and structural properties as compared to those in the bulk form [1-10]. Out of these, the ones suited as phosphor host material show considerable size dependent luminescence properties when an impurity is doped in a quantum-confined structure. The impurity incorporation transfers the dominant recombination route from the surface states to impurity states. If the impurity-induced transition can be localized as in the case of the transition metals or the rare earth elements, the radiative efficiency of the impurity- induced emission increases significantly. The emission and decay characteristics of the phosphors are, therefore, modified in nanocrystallized form. Also, the continuous shift of the absorption edge to higher energy due to quantum confinement effect, imparts these materials a degree of tailorability. Obviously, all these attributes of a doped nanocrystalline phosphor material are very attractive for optoelectronic device applications. 

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