Example of phosphorescence

Monochrome monitor

Aperture grille CRT phosphors

A phosphor, most generally, is a substance that exhibits the phenomenon of luminescence. Somewhat confusingly, this includes both phosphorescent materials, which show a slow decay in brightness (>1ms), and fluorescent materials, where the emission decay takes place over tens of nanoseconds. Phosphorescent materials are known for their use in radar screens and glow-in-the-dark toys, whereas fluorescent materials are common in CRT and plasma video display screens, sensors, and white LEDs.

Phosphors are often transition metal compounds or rare earth compounds of various types. The most common uses of phosphors are in CRT displays and fluorescent lights. CRT phosphors were standardized beginning around World War II and designated by the letter "P" followed by a number.

Phosphorus, the chemical element named for its light-emitting behavior, emits light due to chemiluminescence, not phosphorescence^[1], hence it is not a phosphor.

Principles[link]

A material can emit light either through incandescence, where all atoms radiate, or by luminescence, where only a small fraction of atoms, called emission centers or luminescence centers, emit light. In inorganic phosphors, these inhomogeneities in the crystal structure are created usually by addition of a trace amount of dopants, impurities called activators. (In rare cases dislocations or other crystal defects can play the role of the impurity.) The wavelength emitted by the emission center is dependent on the atom itself, and on the surrounding crystal structure.

The scintillation process in inorganic materials is due to the electronic band structure found in the crystals. An incoming particle can excite an electron from the valence band to either the conduction band or the exciton band (located just below the conduction band and separated from the valence band by an energy gap). This leaves an associated hole behind, in the valence band. Impurities create electronic levels in the forbidden gap. The excitons are loosely bound electron-hole pairs that wander through the crystal lattice until they are captured as a whole by impurity centers. The latter then rapidly de-excite by emitting scintillation light (fast component). In case of inorganic scintillators, the activator impurities are typically chosen so that the emitted light is in the visible range or near-UV where photomultipliers are effective. The holes associated with electrons in the conduction band are independent from the latter. Those holes and electrons are captured successively by impurity centers exciting certain metastable states not accessible to the excitons. The delayed de-excitation of those metastable impurity states, slowed down by reliance on the low-probability forbidden mechanism, again results in light emission (slow component).

Phosphor degradation[link]

Many phosphors tend to lose efficiency gradually by several mechanisms. The activators can undergo change of valence (usually oxidation), the crystal lattice degrades, atoms – often the activators – diffuse through the material, the surface undergoes chemical reactions with the environment with consequent loss of efficiency or buildup of a layer absorbing either the exciting or the radiated energy, etc.

The degradation of electroluminescent devices depends on frequency of driving current, the luminance level, and temperature; moisture impairs phosphor lifetime very noticeably as well.

Harder, high-melting, water-insoluble materials display lower tendency to lose luminescence under operation.^[2]

Examples:

• BaMgAl₁₀O₁₇:Eu²⁺ (BAM), a plasma display phosphor, undergoes oxidation of the dopant during baking. Three mechanisms are involved; absorption of oxygen atoms into oxygen vacancies on the crystal surface, diffusion of Eu(II) along the conductive layer, and electron transfer from Eu(II) to adsorbed oxygen atoms, leading to formation of Eu(III) with corresponding loss of emissivity.^[3] Thin coating of aluminium phosphate or lanthanum(III) phosphate is effective in creation a barrier layer blocking access of oxygen to the BAM phosphor, for the cost of reduction of phosphor efficiency.^[4] Addition of hydrogen, acting as a reducing agent, to argon in the plasma displays significantly extends the lifetime of BAM:Eu²⁺ phosphor, by reducing the Eu(III) atoms back to Eu(II).^[5]
• Y₂O₃:Eu phosphors under electron bombardment in presence of oxygen form a non-phosphorescent layer on the surface, where electron-hole pairs recombine nonradiatively via surface states.^[6]
• ZnS:Mn, used in AC thin film electroluminescent (ACTFEL) devices degrades mainly due to formation of deep-level traps, by reaction of water molecules with the dopant; the traps act as centers for nonradiative recombination. The traps also damage the crystal lattice. Phosphor aging leads to decreased brightness and elevated threshold voltage.^[7]
• ZnS-based phosphors in CRTs and FEDs degrade by surface excitation, coulombic damage, build-up of electric charge, and thermal quenching. Electron-stimulated reactions of the surface are directly correlated to loss of brightness. The electrons dissociate impurities in the environment, the reactive oxygen species then attack the surface and form carbon monoxide and carbon dioxide with traces of carbon, and nonradiative zinc oxide and zinc sulfate on the surface; the reactive hydrogen removes sulfur from the surface as hydrogen sulfide, forming nonradiative layer of metallic zinc. Sulfur can be also removed as sulfur oxides.^[8]
• ZnS and CdS phosphors degrade by reduction of the metal ions by captured electrons. The Me²⁺ ions are reduced to Me⁺; two Me⁺ then exchange an electron and become one Me²⁺ and one neutral Me atom. The reduced metal can be observed as a visible darkening of the phosphor layer. The darkening (and the brightness loss) is proportional to the phosphor's exposition to electrons, and can be observed on some CRT screens that displayed the same image (e.g. a terminal login screen) for prolonged periods.^[9]
• Europium(II)-doped alkaline earth aluminates degrade by formation of color centers.^[2]
• Y₂SiO₅:Ce³⁺ degrades by loss of luminescent Ce³⁺ ions.^[2]
• Zn₂SiO₄:Mn (P1) degrades by desorption of oxygen under electron bombardment.^[2]
• Oxide phosphors can degrade rapidly in presence of fluoride ions, remaining from incomplete removal of flux from phosphor synthesis.^[2]
• Loosely packed phosphors, e.g. when an excess of silica gel (formed from the potassium silicate binder) is present, have tendency to locally overheat due to poor thermal conductivity. E.g. InBO₃:Tb³⁺ is subject to accelerated degradation at higher temperatures.^[2]

Materials[link]

Phosphors are usually made from a suitable host material with an added activator. The best known type is a copper-activated zinc sulfide and the silver-activated zinc sulfide (zinc sulfide silver).

The host materials are typically oxides, nitrides and oxynitrides,^[10] sulfides, selenides, halides or silicates of zinc, cadmium, manganese, aluminium, silicon, or various rare earth metals. The activators prolong the emission time (afterglow). In turn, other materials (such as nickel) can be used to quench the afterglow and shorten the decay part of the phosphor emission characteristics.

Many phosphor powders are produced in low-temperature processes, such as sol-gel and usually require post-annealing at temperatures of ~1000 °C, which is undesirable for many applications. However, proper optimization of the growth process allows to avoid the annealing.^[11]

Phosphors used for fluorescent lamps require a multi-step production process, with details that vary depending on the particular phosphor. Bulk material must be milled to obtain a desired particle size range, since large particles produce a poor quality lamp coating and small particles produce less light and degrade more quickly. During the firing of the phosphor, process conditions must be controlled to prevent oxidation of the phosphor activators or contamination from the process vessels. After milling the phosphor may be washed to remove minor excess of activator elements. Volatile elements must not be allowed to escape during processing. Lamp manufacturers have changed composition of phosphors to eliminate some toxic elements, such as beryllium, cadmium, or thallium, formerly used.^[12]

The commonly quoted parameters for phosphors are the wavelength of emission maximum (in nanometers, or alternatively color temperature in kelvins for white blends), the peak width (in nanometers at 50% of intensity), and decay time (in seconds).

Applications[link]

Lighting[link]

Phosphor layers provide most of the light produced by fluorescent lamps, and are also used to improve the balance of light produced by metal halide lamps. Various neon signs use phosphor layers to produce different colors of light. Electroluminescent displays found, for example, in aircraft instrument panels, use a phosphor layer to produce glare-free illumination or as numeric and graphic display devices.

Phosphor thermometry[link]

Phosphor thermometry is a temperature measurement approach that uses the temperature dependence of certain phosphors for this purpose. For this, a phosphor coating is applied to a surface of interest and, usually, the decay time is the emission parameter that indicates temperature. Because the illumination and detection optics can be situated remotely, the method may be used for moving surfaces such as high speed motor surfaces. Also, phosphor may be applied to the end of an optical fiber as an optical analog of a thermocouple.

Glow-in-the-dark toys[link]

• Calcium sulfide with strontium sulfide with bismuth as activator, (Ca,Sr)S:Bi, yields blue light with glow times up to 12 hours, red and orange are modifications of the zinc sulfide formula. Red color can be obtained from strontium sulfide.

• Zinc sulfide with about 5 ppm of a copper activator is the most common phosphor for the glow-in-the-dark toys and items. It is also called GS phosphor.

• Mix of zinc sulfide and cadmium sulfide emit color depending on their ratio; increasing of the CdS content shifts the output color towards longer wavelengths; its persistence ranges between 1–10 hours.

• Strontium aluminate activated by europium, SrAl₂O₄:Eu(II):Dy(III), is a newer material with higher brightness and significantly longer glow persistence; it produces green and aqua hues, where green gives the highest brightness and aqua the longest glow time. SrAl₂O₄:Eu:Dy is about 10 times brighter, 10 times longer glowing, and 10 times more expensive than ZnS:Cu. The excitation wavelengths for strontium aluminate range from 200 to 450 nm. The wavelength for its green formulation is 520 nm, its blue-green version emits at 505 nm, and the blue one emits at 490 nm. Colors with longer wavelengths can be obtained from the strontium aluminate as well, though for the price of some loss of brightness.

In these applications, the phosphor is directly added to the plastic used to mold the toys, or mixed with a binder for use as paints.

ZnS:Cu phosphor is used in glow-in-the-dark cosmetic creams frequently used for Halloween make-ups. Generally, the persistence of the phosphor increases as the wavelength increases. See also lightstick for chemiluminescence-based glowing items.

Radioluminescence[link]

Zinc sulfide phosphors are used with radioactive materials, where the phosphor was excited by the alpha- and beta-decaying isotopes, to create luminescent paint for dials of watches and instruments (radium dials). Between 1913 and 1950 a radium-228 and radium-226 were used to activate a phosphor made of silver doped zinc sulfide (ZnS:Ag), which gave a greenish glow. The phosphor is not suitable to be used in layers thicker than 25 mg/cm², as the self-absorption of the light then becomes a problem. Furthermore, zinc sulfide undergoes degradation of its crystal lattice structure, leading to gradual loss of brightness significantly faster than the depletion of radium. ZnS:Ag coated spinthariscope screens were used by Ernest Rutherford in his experiments discovering atomic nucleus.

Copper doped zinc sulfide (ZnS:Cu) is the most common phosphor used and yields blue-green light. Copper and magnesium doped zinc sulfide (ZnS:Cu,Mg) yields yellow-orange light.

Tritium is also used as a source of radiation in various products utilizing tritium illumination.

Electroluminescence[link]

Electroluminescence can be exploited in light sources. Such sources typically emit from a large area, which makes them suitable for backlights of LCD displays. The excitation of the phosphor is usually achieved by application of high-intensity electric field, usually with suitable frequency. Current electroluminescent light sources tend to degrade with use, resulting in their relatively short operation lifetimes.

ZnS:Cu was the first formulation successfully displaying electroluminescence, tested at 1936 by Georges Destriau in Madame Marie Curie laboratories in Paris.

Indium tin oxide (ITO, also known under trade name IndiGlo) composite is used in some Timex watches, though as the electrode material, not as a phosphor itself. "Lighttape" is another trade name of an electroluminescent material, used in electroluminescent light strips.

White LEDs[link]

White light-emitting diodes are usually blue InGaN LEDs with a coating of a suitable material. Cerium(III)-doped YAG (YAG:Ce³⁺, or Y₃Al₅O₁₂:Ce³⁺) is often used; it absorbs the light from the blue LED and emits in a broad range from greenish to reddish, with most of output in yellow. The pale yellow emission of the Ce³⁺:YAG can be tuned by substituting the cerium with other rare earth elements such as terbium and gadolinium and can even be further adjusted by substituting some or all of the aluminium in the YAG with gallium. However, this process is not one of phosphorescence. The yellow light is produced by a process known as scintillation, the complete absence of an afterglow being one of the characteristics of the process.

Some rare-earth doped Sialons are photoluminescent and can serve as phosphors. Europium(II)-doped β-SiAlON absorbs in ultraviolet and visible light spectrum and emits intense broadband visible emission. Its luminance and color does not change significantly with temperature, due to the temperature-stable crystal structure. It has a great potential as a green down-conversion phosphor for white LEDs; a yellow variant also exists. For white LEDs, a blue LED is used with a yellow phosphor, or with a green and yellow SiAlON phosphor and a red CaAlSiN₃-based (CASN) phosphor.^[13][14][15]

White LEDs can also be made by coating near ultraviolet (NUV) emitting LEDs with a mixture of high efficiency europium based red and blue emitting phosphors plus green emitting copper and aluminium doped zinc sulfide (ZnS:Cu,Al). This is a method analogous to the way fluorescent lamps work.

Cathode ray tubes[link]

Spectra of constituent blue, green and red phosphors in a common cathode ray tube.

Cathode ray tubes produce signal-generated light patterns in a (typically) round or rectangular format. Bulky CRTs were used in the black-and-white household television ("TV") sets that became popular in the 1950s, as well as first-generation, tube-based color TVs, and most earlier computer monitors. CRTs have also been widely used in scientific and engineering instrumentation, such as oscilloscopes, usually with a single phosphor color, typically green.

White (in black-and-white): The mix of zinc cadmium sulfide and zinc sulfide silver, the ZnS:Ag+(Zn,Cd)S:Ag is the white P4 phosphor used in black and white television CRTs.

Red: Yttrium oxide-sulfide activated with europium is used as the red phosphor in color CRTs. The development of color TVs took a long time due to the long search for a red phosphor. The first red emitting rare earth phosphor, YVO4,Eu3, was introduced by Levine and Palilla as a primary color in television in 1964.^[16] In single crystal form, it was used as an excellent polarizer and laser material.^[17]

Yellow: When mixed with cadmium sulfide, the resulting zinc cadmium sulfide (Zn,Cd)S:Ag, provides strong yellow light.

Green: Combination of zinc sulfide with copper, the P31 phosphor or ZnS:Cu, provides green light peaking at 531 nm, with long glow.

Blue: Combination of zinc sulfide with few ppm of silver, the ZnS:Ag, when excited by electrons, provides strong blue glow with maximum at 450 nm, with short afterglow with 200 nanosecond duration. It is known as the P22B phosphor. This material, zinc sulfide silver, is still one of the most efficient phosphors in cathode ray tubes. It is used as a blue phosphor in color CRTs.

The phosphors are usually poor electrical conductors. This may lead to deposition of residual charge on the screen, effectively decreasing the energy of the impacting electrons due to electrostatic repulsion (an effect known as "sticking"). To eliminate this, a thin layer of aluminium is deposited over the phosphors and connected to the conductive layer inside the tube. This layer also reflects the phosphor light to the desired direction, and protects the phosphor from ion bombardment resulting from an imperfect vacuum.

To reduce the image degradation by reflection of ambient light, contrast can be increased by several methods. In addition to black masking of unused areas of screen, the phosphor particles in color screens are coated with pigments of matching color. For example the red phosphors are coated with ferric oxide (replacing earlier Cd(S,Se) due to cadmium toxicity), blue phosphors can be coated with marine blue (CoO·nAl₂O₃) or ultramarine (Na₈Al₆Si₆O₂₄S₂). Green phosphors based on ZnS:Cu do not have to be coated due to their own yellowish color.^[2]

Standard phosphor types[link]

Various[link]

Some other phosphors commercially available, for use as X-ray screens, neutron detectors, alpha particle scintillators, etc., are:

• Gd₂O₂S:Tb (P43), green (peak at 545 nm), 1.5 ms decay to 10%, low afterglow, high X-ray absorption, for X-ray, neutrons and gamma
• Gd₂O₂S:Eu, red (627 nm), 850 µs decay, afterglow, high X-ray absorption, for X-ray, neutrons and gamma
• Gd₂O₂S:Pr, green (513 nm), 7 µs decay, no afterglow, high X-ray absorption, for X-ray, neutrons and gamma
• Gd₂O₂S:Pr,Ce,F, green (513 nm), 4 µs decay, no afterglow, high X-ray absorption, for X-ray, neutrons and gamma

• Y₂O₂S:Tb (P45), white (545 nm), 1.5 ms decay, low afterglow, for low-energy X-ray
• Y₂O₂S:Eu (P22R), red (627 nm), 850 µs decay, afterglow, for low-energy X-ray
• Y₂O₂S:Pr, white (513 nm), 7 µs decay, no afterglow, for low-energy X-ray

• Zn(0.5)Cd(0.4)S:Ag (HS), green (560 nm), 80 µs decay, afterglow, efficient but low-res X-ray
• Zn(0.4)Cd(0.6)S:Ag (HSr), red (630 nm), 80 µs decay, afterglow, efficient but low-res X-ray

• CdWO₄, blue (475 nm), 28 µs decay, no afterglow, intensifying phosphor for X-ray and gamma
• CaWO₄, blue (410 nm), 20 µs decay, no afterglow, intensifying phosphor for X-ray
• MgWO₄, white (500 nm), 80 µs decay, no afterglow, intensifying phosphor

• Y₂SiO₅:Ce (P47), blue (400 nm), 120 ns decay, no afterglow, for electrons, suitable for photomultipliers
• YAlO₃:Ce (YAP), blue (370 nm), 25 ns decay, no afterglow, for electrons, suitable for photomultipliers
• Y₃Al₅O₁₂:Ce (YAG), green (550 nm), 70 ns decay, no afterglow, for electrons, suitable for photomultipliers
• Y₃(Al,Ga)₅O₁₂:Ce (YGG), green (530 nm), 250 ns decay, low afterglow, for electrons, suitable for photomultipliers

• CdS:In, green (525 nm), <1 ns decay, no afterglow, ultrafast, for electrons
• ZnO:Ga, blue (390 nm), <5 ns decay, no afterglow, ultrafast, for electrons
• ZnO:Zn (P15), blue (495 nm), 8 µs decay, no afterglow, for low-energy electrons

• (Zn,Cd)S:Cu,Al (P22G), green (565 nm), 35 µs decay, low afterglow, for electrons
• ZnS:Cu,Al,Au (P22G), green (540 nm), 35 µs decay, low afterglow, for electrons
• ZnCdS:Ag,Cu (P20), green (530 nm), 80 µs decay, low afterglow, for electrons

• ZnS:Ag (P11), blue (455 nm), 80 µs decay, low afterglow, for alpha particles and electrons
• anthracene, blue (447 nm), 32 ns decay, no afterglow, for alpha particles and electrons
• plastic (EJ-212), blue (400 nm), 2.4 ns decay, no afterglow, for alpha particles and electrons

• Zn₂SiO₄:Mn (P1), green (530 nm), 11 ms decay, low afterglow, for electrons

• ZnS:Cu (GS), green (520 nm), decay in minutes, long afterglow, for X-rays

• NaI:Tl, for X-ray, alpha, and electrons
• CsI:Tl, green (545 nm), 5 µs decay, afterglow, for X-ray, alpha, and electrons

• ⁶LiF/ZnS:Ag (ND), blue (455 nm), 80 µs decay, for thermal neutrons
• ⁶LiF/ZnS:Cu,Al,Au (NDg), green (565 nm), 35 µs decay, for neutrons

See also[link]

• photoluminescence
• cathodoluminescence
• laser

References[link]

Bibliography[link]

External links[link]

Look up phosphor in Wiktionary, the free dictionary.

• a history of electroluminescent displays.
• Fluorescence, Phosphorescence
• CRT Phosphor Characteristics (P numbers)
• Composition of CRT phosphors
• Safe Phosphors
• Silicon-based oxynitride and nitride phosphors for white LEDs—A review
• [1] & [2] – RCA Manual, Fluorescent screens (P1 to P24)