HTI5205 Chap.7 Band Structure and Luminescence
Band Structure and Luminescence
Discrete atomic states broaden into bands when many atoms form a solid. The valence band contains outer occupied states, the conduction band supports mobile carriers, and the forbidden gap controls how readily electrons enter conduction. Luminescence follows excitation and a later lower-energy transition that emits a photon.
Prompt fluorescence, delayed phosphorescence and thermally stimulated release are separated by the pathway and lifetime of trapped charge.
The chapter converts emitted wavelength to energy while warning that an optical photon may represent a trap-mediated transition rather than the entire band gap.
Move from isolated atoms to a solid
When atoms approach one another, neighbouring potentials perturb their electron states. A single discrete level splits into many closely spaced states and becomes a band for a large crystal.
The outer occupied band is called the valence band. Electrons promoted into the conduction band can move more freely through the material. The forbidden gap between them helps determine whether the solid behaves as a conductor, semiconductor or insulator.
Draw the entire detector pathway
Radiation or another stimulus creates excited carriers.
Some return promptly and emit light, some lose energy without light, and some become trapped in intermediate states. A later optical or thermal stimulus can release trapped carriers and permit recombination. The measured light therefore depends on creation, trapping, release, recombination and collection.
Treating the detector as a single upward and downward jump hides the processes needed for calibration.
Distinguish prompt and delayed emission
Fluorescence is prompt because the excited population returns quickly. Phosphorescence persists because carriers remain in longer-lived or trapped states.
The emitted photon still travels at the ordinary electromagnetic-wave speed in the medium; the delay occurs before emission.
Thermoluminescence uses later heating to release stored carriers, separating the radiation-exposure stage from the readout stage.
Interpret emitted wavelength cautiously
Converting wavelength with E = hc divided by wavelength gives the energy of the emitted photon and therefore the energy difference of the radiative transition.
That difference need not equal the full band gap when trap or impurity levels intervene. Inferring deposited radiation energy requires material calibration, light-collection efficiency and knowledge of non-radiative losses in addition to the photon-energy calculation.
What this chapter covers
- 01
From atomic levels to bands
- 02
Valence band
- 03
Conduction band
- 04
Forbidden energy gap
- 05
Fluorescence and phosphorescence
- 06
Trapping and thermal release
Worked example · free
Find optical transition energy
- 2Divide the energy-wavelength constant by wavelength.
- 2Interpret the result as a transition difference.
Key terms
- Valence Band
- The band containing the outer occupied electron states in a solid.
- Conduction Band
- The band whose electrons can move through the material and carry current.
- Band Gap
- The forbidden energy interval separating valence and conduction bands.
- Fluorescence
- Prompt light emission after optical excitation and rapid return.
- Phosphorescence
- Delayed light emission caused by longer-lived or trapped excited states.
Band Structure and Luminescence FAQ
Why do atomic levels become bands?
Neighbouring atoms perturb one another when a solid forms, splitting formerly discrete states into many closely spaced levels. With many atoms those levels appear as quasi-continuous energy bands.
What does a larger band gap imply?
More energy is needed to promote an electron into the conduction band. At the same temperature, thermal excitation becomes less likely, which changes conductivity and detector behaviour.
Why does phosphorescence last longer?
The electronic population remains in a trapped or less readily returning state before emission. The delay belongs to the state lifetime, not to a slower speed of the emitted light.
Can emitted-photon energy be treated as deposited dose?
No. Wavelength gives one transition energy. Dose inference also needs photon count, collection efficiency, trap population, non-radiative loss, detector mass and calibration. Optical emission is one link in the pathway.
Exam move
Sketch valence band, conduction band and a trap level. Draw separate arrows for excitation, trapping, release and emission, then label the energy source for each arrow. Practise converting visible wavelengths to electronvolts and explain why the answer may be smaller than the full band gap.
Make parallel sketches for fluorescence, phosphorescence and thermoluminescence, changing only the lifetime and release route that distinguishes them. Under each drawing, state whether exposure and readout occur together or at different times. Add a calibration box listing light collection, non-radiative loss and trap population so an emitted-photon calculation is never mistaken for a complete dose measurement.
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