Phosphor

A phosphor is a luminescent substance that emits light when exposed to various forms of radiant energy. Phosphors exhibit fluorescence or phosphorescence depending on whether the emitted light appears immediately or persists after excitation ceases. Their behaviour is fundamental to technologies such as cathode-ray tubes (CRTs), fluorescent lamps, plasma displays, scintillation detectors, and glow-in-the-dark materials. Phosphors function by absorbing energy that excites their electrons to higher energy states; when these electrons return to their ground state, the excess energy is released as visible or ultraviolet light.

Luminescence and Light-Emission Processes

Luminescence in phosphors arises from electronic transitions within the host crystal. When a phosphor is irradiated—by ultraviolet light, visible light, or an electron beam—electrons in the material absorb energy and transition to excited states. In fluorescent materials, this emission occurs almost immediately, ceasing as soon as excitation stops. In phosphorescent materials, the energy is trapped in metastable states, causing a delayed release as the electron gradually returns to its ground state; the material may continue glowing for milliseconds to several hours or days depending on composition.
In inorganic scintillators, a related process involves transitions within the electronic band structure. High-energy particles promote electrons from the valence band to the conduction band or excitonic states. Impurities, known as activators, introduce discrete energy levels in the band gap, capturing electrons and holes. Rapid de-excitation of excitons produces fast scintillation components, whereas metastable impurity states give rise to delayed light emission. Activator choice ensures emissions occur at wavelengths suitable for photomultiplier detection.

Materials and Composition

Phosphors are typically composed of a host lattice combined with small quantities of dopant activators. These activators determine both emission wavelength and afterglow duration. Common host materials include oxides, nitrides, oxynitrides, sulfides, selenides, halides, and silicates based on zinc, cadmium, manganese, aluminium, silicon, or rare-earth metals.
Key examples include:

• Copper-activated zinc sulfide (ZnS:Cu) and silver-activated zinc sulfide (ZnS:Ag), widely used in early displays and glow-in-the-dark products.
• Strontium aluminate doped with europium and dysprosium (SrAl₂O₄:Eu²⁺,Dy³⁺), known for its high brightness and long persistence. Developed in 1993, this phosphor emits intense green or blue-green light and provides significantly longer afterglow than zinc sulfide.
• Mixed zinc–cadmium sulfides, which exhibit colour tuning through variation in cadmium content.
• Strontium sulfide with bismuth or copper activators, yielding long-lasting blue or red emissions.

Manufacturing involves careful control of composition, particle size, firing conditions, and atmosphere to prevent oxidation or contamination. Many powders undergo milling, washing, and annealing, although optimised growth techniques may eliminate the need for high-temperature treatments. Lamp manufacturers have progressively substituted less toxic materials in response to health and environmental regulations.

Phosphor Applications

Fluorescent phosphors are integral to continuously excited systems such as:

• Cathode-ray tubes and oscilloscopes
• Plasma and early flat-panel displays
• Scintillation counters
• Fluorescent lighting and white LEDs
• Black-light pigments for artistic applications

Phosphorescent materials serve applications requiring delayed emission:

• Glow-in-the-dark toys and signage
• Watch dials and instrument panels
• Radar screens, where persistence allows targets to remain visible between beam sweeps

CRTs historically used standardised phosphors labelled with designations beginning with “P” and a number, separating types used for display, radar, oscilloscope, and television applications.

Phosphor Properties and Performance

Important parameters for characterising phosphors include:

• Emission wavelength, often expressed in nanometres
• Colour temperature, used particularly for white blends
• Peak width, specifying spectral purity
• Decay time, indicating the rate of brightness reduction after excitation

Strontium aluminate, for example, shows excitation wavelengths between 200–450 nm, with emission peaks near 520 nm (green), 505 nm (blue-green), and 490 nm (blue).
Activators influence persistence and brightness, whereas additional dopants such as nickel may reduce afterglow where rapid decay is needed. Conversely, certain high-performance phosphors rely on optimally engineered dopant combinations to achieve long duration and intense emission.

Degradation and Ageing Mechanisms

Phosphors degrade over time due to several processes:

• Oxidation of activators, changing valence states and reducing emissivity
• Crystal lattice degradation, including diffusion of dopant atoms
• Surface reactions with oxygen or moisture, creating non-emissive layers
• Charge accumulation, leading to thermal quenching or surface damage in devices such as CRTs
• Deep-level trap formation, particularly in electroluminescent materials exposed to humidity

Examples include the degradation of BaMgAl₁₀O₁₇:Eu²⁺ (BAM), a common plasma display phosphor, through oxidation of europium. Protective coatings such as aluminium phosphate or lanthanum phosphate can mitigate oxygen attack, though at the cost of reduced efficiency. In some systems, hydrogen-bearing atmospheres help restore Eu³⁺ to Eu²⁺, extending operational lifespan.
Zinc sulfide–based phosphors used in CRTs and field-emission displays experience surface damage due to electron bombardment, Coulombic effects, and heat. Similarly, Y₂O₃:Eu phosphors may form non-phosphorescent layers when exposed to oxygen under electron excitation.