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What are Luminescent materials?
Luminescent materials are substances that convert an incident energy to the emission of electromagnetic waves such as UV, Visible, IR regions due to back body emission. Luminescent materials are stimulated by wide range of energy sources their diversity provides a classification for the luminescence phenomenon.
What are the types of Luminescent materials?
Luminescent materials are classified into 5 parts
Photoluminescence, Radioluminescence, Cathodoluminescence, Electroluminescence and Photoluminescent.
Photoluminescence :
Photoluminescence are those luminescence materials which are stimulated by the ultraviolet or visible waves. They are used for characterizing, dopant, impurities. They are used as application in lighting technologies for. E.g., Fluorescent lamps.
Radioluminescence :
Radioluminescence are those luminescence materials which are stimulated by the involvation of excitation which is done by ionizing radiation. They are used for the scintillation for nuclear particle detection.
Cathodoluminescence :
Cathodoluminescence from the name itself we can understand its related to gas discharge. Cathodoluminescence are those luminescence materials which are stimulated by the energetic electrons.
Electroluminescence :
Electroluminescence are those luminescence materials which are stimulated by the collisional excitation by internal electron accelerated by an applied electric field. In comparison to cathodoluminescence, Electroluminescence has low energy. they are use as application in panel lightening, LCD, LED, Back-planes.
Photoluminescent system :
Suppose transition probabilities are P12, P21>> P31. If optical pumping is stopped population of luminescing state 3.
Luminescence lifetime :
Luminescence lifetime is governed by radiative and non-radiative intensity of luminescence. It certainly is dependent on the relative magnitude of P31R.
Luminescence Centers :
In semiconductor or in insulating materials with wide variety of centers give rise to luminescence for e.g., rare earth ions, transition metal ions, excitons, donor-acceptor pair, ions with d10 & S2 electronic configuration ground state.
Rare Earth Atoms :
Rare earth Ions have n electrons (1-14). They are mostly situated in the 4f shell. The various atomic interaction resulting to eigen states is labelled by total spin ‘S’ & orbital angular momentum. In this the spin orbital coupling breaks where the L,S which are the multiplet degeneracy and the gives the total angular momentum (j) which is the sub multiplet.4fn orbitals are partly shield from effects of crystalline environment.
Excitation and luminescence transition within which the various levels are governed by rule of quantum mechanics. This can even be said like the probability of transition between 2 states i & j proportional to square of matrix element <i|H|j
Laporte’s Rule :
It governs the luminescence transition between states of same parity (zero transition probability) and so are forbidden.
Rare earth – all states of 4fn configuration which can be said as same parity for them all optical transition is forbidden.
Relaxation to Laporte’s rule :
If crystalline environment lacks inversion symmetry such transition are weakly allowed, these are mainly done on the presence of oppositely parity into ground configuration.
Admixture effect to static lattice produced by odd parity vibration and electron- phonon interaction is important for 3d ions than for 4f ones.
Transition of metal ions :
Transition of metal ionsfrom 3d series have strong interaction than 4f (No equivalent of screening by 5s, 5p). since spin orbit coupling is weaker, the order of perturbation is reversed. In this the L, S are split by the crystal field.
The ` \triangle S=0` it is the strongest selection rule. Transitions with odd parity vibration have high transition probability which is said as configuration admixing.
Laporte Allowed Transitions – 5S2 & 6S2 where the 5S2 has Sn2+ & Sb3+ elements and 6S2 has Ti+ & Pb2+ elements. Interaction between p state and crystalline environment are very strong. From the observation we have seen it has a broad spectrum.
Semiconductor :
luminescence is dominated by near bandgap which is arise from recombination of electron & holes. Most efficient for direct bandgap (ZnS). In indirect gap materials, creation or destruction of phonon is required for band-to-band luminescence which has less probable.
Interaction with lattice :
Rare earth ions: For Rare earth ions, the interaction with vibration of crystal lattice are been ignored and it is observed that the luminescence spectrum is set of sharp electron transition while for others the incorporation of latter is critical.
Ion lattice interaction: These ion lattice interactions are only for nearest ions and their displacements in cartesian co-ordinates.
Crystal field depends on the ion positions as there is a presence of electron lattice coupling, which gives a difference in harmonic vibrational potential from one electronic state to another.
This from configuration co-ordinate diagram, potential energies from ground state and excitaed state being offset parabolas of different curvature.
For Strong electron lattice coupling :
Maximum transition probability is for finite number of phonons but here it is not for zero-phonon transition.
In semiconductor Analysis,
The allowed range of transition for finite line widths is gaussian shaped band. The difference between maximum of two curves is called as stokes shift. From the following graph we observe that we have a very little intensity in zero phonon line, it has large stokes shift between energies for maximum where absorption and emission. The reverse is true for luminescence from 4f states of rare earths.
Thermal luminescence (TSL) :
Thermal luminescence (TSL) is thermally stimulated recombination of trapped electrons and holes in material which have been subjected to prior irradiation. Where irradiation is a form of Ultraviolent, x-ray, gamma rays. In this there is a free electron and hole pair created where some of them will recombine which each other, while the additional generated electron and hole get trapped as impurities in defect centers. If the trapped binding energy is sufficiently large, then by the thermal promotion of electron or hole they jump to conduction band or valence band. At irradiation temperature, it is improbable where the charge carriers remain trapped after irradiation.
The Curve of light intensity v/s temperature is called as glow curve. From the analyzed result we can extract trap depths and concentration.
Mechanism Thermal luminescence (TSL) :
The sample is heated at a fast and linear rate while light emission is monitored by sensitive filter or photomultiplier. Here , the final result of glow curve gives a fit to theoretical curve, we can extract the trap parameter. The trapped electron is attracted to T and The trapped holes is attracted to R. The trapped hole has a binding energy greater than trapped electron so the latter is depopulated first.