Phosphor luminescent center

The Configuration Coordinate Model. To illustrate how the luminescent center in a phosphor works, a configurational coordinate diagram is used (2) in which the potential energy of the luminescent or activator center is plotted on the vertical axis and the value of a single parameter describing an effective displacement of the ions surrounding the activator, is plotted on the horizontal axis (Fig. 2). At low temperatures, near room... 

The luminescent centers require a range of properties that include a large cross-section for the collision excitation to occur, an ionic radius and valency to fit the lattice and be stable under the applied high electronic fields, and the capability to display high luminous efficiency when excited.11 Metal ions suitable for EL devices include Mn, Tb, Sm3+, Tm3+, Pr3+, Eu2+, and Ce3+.12-17 ZnS lattices doped with Mn2+ (yellow-orange emission at ca. 585 nm) have proved to be one of the best phosphors for EL devices. 

This luminescence center is not detected in minerals yet, but is well known in synthetic phosphors, for example in halophosphates, which are closely related to hydroxyapatite (Blasse and Grabmaier 1994). The Sb doped calcium halophosphate is a very efficient blue-emitting phosphor under short wave 254 excitation (Fig. 5.56). When the halophosphate host lattice contains not only Sb, but also Mn, part of the energy absorbed by the Sb ions is transferred to Mn ", which shows an orange emission. By carefully adjusting the... 

The most important mineral example is natural scheelite. ScheeUte emits a bright blue emission in a broad band centered at 425 nm (Fig. 4.9) with a decay time of several ps. Calcium tungstate CaW04 has long been known as a practical phosphor, and has been carefully studied. The intrinsic blue luminescence center is the complex ion in which the central W metal ion is... 

Energy Transfer. The excitation of phosphors does not always take place at the luminescence center. A recombination and the associated emission require transfer of the absorbed energy to the luminescence center. This can take place by the following mechanisms ... 

Most minerals fall into the class of insulator phosphors. The characteristics of the luminescence are usually defined by the electronic structure of an activator ion as modified by the crystal field of the host crystal structure. Although some energy transfer takes place between nearby ions, appearing as the phenomena of co-activation, luminescence poisons, and activator pair interactions, the overall luminescence process is localized in a "luminescent center" which is typically 2 to 3 nm in radius. From a perspective of band theory, luminescent centers behave as localized states within the forbidden energy gap. 

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 luminescent center in organic phosphors has been described in... 

There are two types of ener transfer from a point in a lattice to a luminescent center. For semi-conducting phosphors like ZnS, migration of electrons in the conduction band, or holes in the valence band, conve3rs the excitation energy to localized luminescent centers. Excltons (bound electron-hole pairs) is another mechanism. For insulators (which are generally oxygen dominated compositions), the excitation energy can be transferred from an excited point in the lattice, or from an excited luminescent center to other unexcited centers in the lattice by Quantum Mechanical resonance. 

As a consequence the band gap decreases, so that the emission wavelength shifts to the ted. Actually Zno.eBCdo.32S Ag is a gieen-. and Zno.i3Cdoa7S Ag+ a red-emitting cathode-ray phosphor. The emission color is not determined by the nature of the luminescent center, but by the value of the band gap. Figure 7.2 shows the chromaticity diagram with three phosphors from the (Zn,Cd)S Ag+ family. 

Another X-ray storage phosphor is RbBr TI+. The luminescent center is the TP (6s ) ion which emits by a 6s6p - 6 transition (Sect. 3.3.7). The electron is trapped at a bromine vacancy, the hole is assumed to be trapped at a Tl ion. The storage state can, therefore, be characterized by F -f Tl . The PSL center consists of these two centers optical stimulation excites the F center, and the electron recombines with the hole on thallium yielding TP in the excited state [19]. The efficiency of the photostimulated luminescence of RbBr TI+ decreases above 230 K due to a thermal instability of one of the trapped charge carriers. 

A report in the same line [380] involves suspension of ZnS.Mn nanoparticles in microemulsions, with the surface modified by polyoxyethylene(l)laurylether phosphoric acid or polyoxyethylene(4.5)laurylether acetic acid. The coated particles exhibited several times higher photoluminescence intensity and quantum efficiency compared to the uncoated ones. The phosphate group P = O or the carboxyl group C = O, with their luminescent centers, were functional in this improvement, thus opening new routes for imparting superior optical properties on microemulsion-synthesized particles. Gan etaL [376] used four routes for the synthesis of ZnS Mn (i) the relatively new route that involved hydrothermal... 

A review of the progress in the understanding and applications of the TL of R-doped (and Mn-doped) Cap2 phosphors was given by Jain (1990). It also discusses effects of the radiation doses and of the LET on the GCs. Optical absorption ESR and TL emission spectra which lead to the identification of traps and luminescence centers are also given. 

As the luminescent centers D and A are codoped in a phosphor, energy transfer may allow both D and A to emit, thus generating two color emissions on a single color excitation. Such a doubly doped system, or even triply doped system, exhibits attractive application for fiiU-color white-light generation on blue and/or UV LED (light-emitting diode) excitation. 

If a nonexponential decay of luminescence results from different radiative rates, then I t) no longer follows the same function as N t), achieving the average excited-state lifetime using Eq. (3.12) is invalid. Luminescent materials with different radiative rates may be caused by codoping different types of luminescent centers or the same center on different lattice sites or mixing different phosphors, etc. 

In this chapter, we have summarized recent accomplishments in the area of high-pressure luminescence spectroscopy of phosphor materials. It was shown that the unique insight and understanding of phosphor performance is possible from high-hydrostatic study of luminescence centers created by TM and RE ions. The eff ect of pressure on luminescence related to f-f, d-d, and d-f ionization and charge-transfer transitions has been discussed. Several recent examples from the literature have been presented to illustrate the influence of pressure on luminescence energy, intensity, hneshape, and luminescence kinetics and efficiency. 

In addition to Eu ", Pb " can also show characteristic emission when Pb-doped Sr5(P04)3Cl phosphors were prepared [55]. The red emission peaking at 725 nm was observed to be excited by the wavelength at 486 nm, which was ascribed to the Pb -phosphate charge-transfer transition. The optimum concentration for Pb " luminescence was 1 %. At > 1 %, luminescence quenching occurred. Because the radius of Pb " ion (1.19 A) is almost the same as that of Sr " ion (1.21 A), Pb " ions can easily substitute for Sr " ions and act as a luminescence center. The luminescence of Pb " ion is quite diverse and depends strongly on the host lattice due to the... 

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