Self-activated phosphors

ZnO Zn is a typical example of a self-activated phosphor. In the case of zinc oxide, it is an excess of zinc which enables the phosphor to luminesce. The production is carried out by thermal oxidation of crystallized zinc sulfide in air at ca. 400 °C. The green luminescence, with a broad maximum at 505 nm, has a very short decay time of 10-6 s. As a phosphor for cathode-ray tubes, ZnO.Zn is classified in the TEPAC list as P 24 and in the WTDS system as GE. 

The Srs(P04)3Cl crystals are hexagonal needles with lattice parameters ah = 9.953 A. and ch = 7.194 A. The needle axis corresponds to the crystallographic c axis. The europium(II) doped sample is a phosphor, readily excitable with electrons, x-rays, and both short and long ultraviolet light. It emits in the blue with a peak at 445 nm. Crystals of strontium chloride vanadate(V) are orthorhombic platelets with lattice constants a = 7.43 A., b = 11.36 A., and c = 6.54 A., with the b axis corresponding to the thin dimension of the flakes. Strontium chloride vanadate(V) is a self-activated phosphor giving broadband emission with a peak at 423 nm. when excited with 2537-A. radiation. All compounds are insulators, with resistivities >1012 ft-cm. 

The only activators which can be used with self-activated phosphors are the rare earths. The reason for this is that the rare earths have the general electronic configuration 4P> 5s 5p6. These outer shielding electron shells isolate the Ln3+ center from the influence of the tungstate center, and vice-versa. We will discuss this type of phosphor in more detail later in this chapter. 

YNb04 has been known to be a self-activated phosphor with a band-gap value of 4.1 eV [84]. Under vacuum ultraviolet (UV) and UV excitation, the four excitation bands—labeled as A, B, C and D—are observed in the PL excitation spectrum. Based on the LDA -i- U approach, the calculated band gap is 4.28 eV, which agrees well with the experimental measurement value of 4.1 eV. Comparing the excitation spectra and DOS plots, bands B, C, and D are ascribed to the charge-transfer transition from O to Nb or Y. A and B originate firom transition between the VB and defect state in the forbidden band. 

As a famous and typical host emission or self-activated phosphor, Sr2Ce04 is novel blue luminescent material that has attracted much attention because it possesses some desirable qualities such as (i) ligands-to-metal charge transfer (CT) transition of Ce" " at approximately 340 nm, (ii) efficient energy transfer that can occur from the CT state to the trivalent rare-earth in Sr2Ce04 RE, ... 

Phosphors usually contain activator ions in addition to the host material. These ions are dehberately added in the proper proportion during the synthesis. The activators and their surrounding ions form the active optical centers. Table 1 Hsts some commonly used activator ions. Some soflds, made up of complexes such as calcium tungstate [7790-75-2] CaWO, are self-activated. Also in many photolurninescence phosphors, the primary activator does not efficiently absorb the exciting radiation and a second impurity ion is introduced known as the sensitizer. The sensitizer, which is an activator ion itself, absorbs the exciting radiation and transfers this energy to the primary activator. 

Copper-activated zinc and cadmium sulphides exhibit a rather long afterglow when their irradiation has ceased, which is favourable for application in radar screens and self-luminous phosphors. 

Use Bombarding particle in cyclotrons, activator in self-luminous phosphors, in cold cathode tubes, tracer in biochemical research and various special problems in chemical analysis, luminous instrument... 

There is one other class of anions which are important. These are the so-called "self-activated" anions. In this case, if we combine certain of the optically-inert cations of 6.3.1. with those of the following diagram, we obtain a phosphor which does not require an activator cation ... 

It is easily seen that while the CaW04 W phosphor has its maximum efficiency of emission close to room temperature, the BaW04 W is most efficient close to liquid nitrogen temperature. Therefore, the former is nearly inert at the temperature of maximum emission of the latter and vice-versa. This phenomenon is quite common for many phosphor systems, particularly the self-activated ones. 

Phosphors. Infrared radiation can stimulate the emission of visible radiation from certain types of phosphors which have been previously excited by ultraviolet radiation. Becker and Risgin [2.71] have reported that self-activated ZnS crystals have exhibited both visible luminescence and photoconductivity when stimulated by radiation in the 1-3 pm interval. Geusic et al. [2.72] have shown that rare earth phosphors can convert very near infrared (0.93 pm) radiation into the visible with an efficiency as high as 1 %. Although phosphors can be employed to convert images directly, their lack of sensitivity has limited their usefulness. 

Chen X, Xia ZG, Yi M, Wu XC, Xin H (2013) Rare-earth free self-activated and rare-earth activated Ca2NaZn2V30i2 vanadate phosphors and their color-tunable luminescence properties. J Phys Chem Solids 74 1439... 

Sulfates. The basic lattice of sulfate phosphors absorbs very short wavelength UV radiation. On excitation with X rays or radiation from radioactive elements, a large proportion of the energy is stored in deep traps. For this reason, CaS04 Mn is used in solid-state dosimeters. Of the glowpeaks which can be selected by thermoluminescence, more than 50 % fail to appear at room temperature because of a self selection of the shallow traps. Other activators, such as lead or rare-earth ions (Dy3 +, Tm3 +, Sm3+), stabilize the trapped electrons [5.399]—[5.401]. 

Use Activation of phosphors for self-luminous markers, detecting leaks, medicine to trace blood flow, and in measurement of standard meter. 

Krypton, Kr, is an elemental, colorless, odorless, inert gas. It is noncombustible, nontoxic, and nonreactive however, it is an asphyxiant gas and will displace oxygen in the air. Krypton 85 is radioactive and has a half-life of 10.3 years. The four-digit UN identification number for krypton is 1056 as a compressed gas and 1970 as a cryogenic liquid. These forms of krypton are not radioactive. Radioactive isotopes of krypton are shipped under radioactive labels and placards as required. Its primary uses are in the activation of phosphors for self-luminous markers, detecting leaks, and in medicine to trace blood flow. 

In this paper we describe, for the first time, a self-assembly technique that employs alkyl phosphoric acid esters to produce dense, highly ordered monolayers in a tails-up configuration, on a TazOs surface. Tantalum oxide was chosen because of its high refractive index, which renders it ideal for application in a planar-waveguide-based bioaffinity sensor. Upon appropriate m-function-alization, alkanephosphate-based SAMs have the potential to be used as the interface that anchors active sensing elements or as the basis of passive, biomolecule-resistant regions on the sensor surface. 

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