Luminescence.manish
- 1. LUMINESCENCE BY MANEESH TIWAREE (NIT NAGALAND)
- 2. INTRODUCTION An electron absorbs a photon from the incident beam and it jumps from valance band to the conduction band. Thus ,this electron is in exited state or higher state ,but it no longer be in the higher state and eventually get back into lower energy state by emitting radiation. This phenomenon of emission of light by a material after absorbing such type of energy ,is known as luminescence. 1
- 4. PHOTOLUMINESCENCE Photoluminescence is a type of luminescence. The process in which the emitting radiation arises due to the absorption of photon from the source such as infrared,visible,ultravoilet or x-rays. Photoluminescence is divided into two parts namely,fluroscence and phosphorescence. 3
- 5. FLUORESCENCE If a material absorbs light by any one of the above intorduced source ,it radiates or emit light within 10-8 sec. after the excitation is removed. This luminescence is known as fluorescence or fast photoluminescence. It is a temperature independent phenomenon. Substance like fluorspar,fluorescein,quinine sulphate,uranium oxide,calcite,uranium glass exhibit the phenomena of fluorescence. 4
- 6. Characterstic luminescent materials Activator ion Stokes effect- The same energy levels are involved for both absorption and emission and hence,the wavelengths are identical . The only difference between the two wavelegnths is a shift in the emission spectrum towards the red end of the absorption spectrum . This phenomenon is known as stokes effect . Reason: when the activator gets exited state ,gives up heat energy to surrounding by means of vibration,this loss of energy is the reason behind stokes shift. Application : fluorescent lamps. Therefore ,in Characterstic luminescent materials,the excitation and emission ofolumi light takes place very rapidly in less than 10-8 s and hence called as fast photoluminescence. 5
- 7. 6
- 8. PHOSPHORESCENCE Certain materials emit light after the incident radiaton is cut off. This process is known as phosphorescence. The time delay between the excitation of electron and emission of light ranges from 10-8 s to several min.or Hrs. Or even days. 7
- 9. Non-Characterstic luminescent materials Activator Coactivators Hole trap(acceptor energy level) Electron trap(donor energy level) Electron -hole generation and recombination process 8
- 11. Holes trapped into acceptor sites Ec Ed Ea Ev 10
- 12. Trapping of electron occur at donor site Ec before recombination Ed Ea Ev 11
- 13. Electron recombined with hole and hence Ec release a luminescent radiation. Ea Ev 12
- 14. The time taken by the electron in a trap depend on the depth of the trap below conduction band Ec- Ed The emission of electron slow due to the electron hole recombination through impurity levels. Therefore ,this process is known as slow photoluminescence. 13
- 15. PHOSPHORS Materials are used to produce luminescence. 15 BASE MATERIALS ACTIVATORS LIGHT COLOURS Zinc sulfide Silver Blue Yttrium silicate cerium Purple blue Zinc sulfide copper green Gadolium oxysulfide terbium Yellowish green Zinc orthosilicate Manganese Yellowish green Zinc cadmium sulfide Silver Green Yttrium oxysulfide Europium Red
- 16. APPLICATIONS Fluorescent materials are used in display devices such as LED,LCD,CRT etc. A Fluorescent lamp consist of a thin coating of Fluorescent materials like cadmium borate zinc silicateetc. When uv radiation fall on it ,and they absorbs incident radiation and re-emit the light having the wavelength in visible region ,and hence, increases luminous flux. When x-rays are made to fall on glass coated with such materials produces fluorescent light. Therefore it is used as a x-ray detector. 16
- 17. REFERENCES https://en.wikipedia.org/wiki/Stokes_shift Materials science (v rajendran) 17
- 18. PHOTOLUMINESCENCE Presntation given by- MANEESH TIWAREE (2014466003) Under the supervision of – DR.DEBRUN DHAR PURKYASTHA 18