What is Luminescence?



What is Luminescence?
The property of emission of light when a energetic particle impinges on  material (semiconductors)  leading to creation of electron hole pairs and excitation of carriers. When these carriers come to their equilibrium states they emit light.

Mechanism of Recombination

Radiative Recombination

When the excited excess carriers reach equilibrium positions by emission of photons it is said to be Radiative recombination  



Non-Radiative Recombination
When the excited excess carriers reach equilibrium positions by emission of phonons due to surface / bulk defects / other defects it is said to be non-radiative recombination.
 

What Is Activator?

Impurity atom occurring in relatively small concentrations in host material or a small stoichometric excess of one of constituents of material which exhibits the property of Luminescence. 

What Is Killer?

Presence of certain type of impurity may also inhibit Luminescence of other centers, in which case they are referred as killers.

After a particular level of concentration, the efficiency decreases as excess carrier returns to the ground state only if there is no other activator with in a sphere of
radius ‘R’ around central activator atom

Dependence of luminescence efficiency on Activator concentration




CHARACTERISTICS OF A PHOSPHOR

  1. High concentration of carrier traps.
  2. High emission efficiency.
  3. Large trap depth.
  4. Traps, Luminescence & lattice are not to be damaged by repeated irradiation & heating process.

BETA DECAY





There are 3 models of beta radioactivity

i) Negatron (β-) Emission



ii) Orbital Electron Capture


iii) Positron (β+) Emission


BETA SPECTROSCOPY



Beta ray spectra is of two types:

i) Continuous Spectrum

ii) Sharp Line Spectrum

The continuous spectrum is due to Negatrons and Positrons.

There is a possibility of the daughter nucleus being created in an excited state, which may decay by gamma emission.

Rutherford suggested that part of gamma radiation is absorbed by outer electrons of same atom and as a result the secondary beta ray electrons are ejected.  The process is called “Internal Conversion”.

The sharp Line Spectrum is due to secondary beta rays emitted due to internal conversion.

The process of Beta disintegration differs from alpha disintegration in following two respects:

i) The alpha particles are already present in the initial nucleus while beta particles are not present in the initial nucleus. Beta particles are created at the time of emission.

ii) The energy spectrum of alpha particles is discrete while energy spectrum of beta particles is continuous.

iii) Beta decay is a three body problem while alpha decay is a two body problem.

In the Neutrino, the spin and angular momentum vector are oppositely directed and in anti neutrino these vectors are aligned together.

Since a matter-antimatter pair is formed whenever energy is converted into mass, in a conversion of a nuclear Neutron to a Proton, the negative electron (matter) should accompany anti neutrino (anti matter). Similarly Neutrino is emitted with positron emission and orbital electron capture.

FERMI THEORY OF BETA DECAY

In 1934, Fermi made a successful theory of beta decay. The theory is based on following assumptions:
1) The light particles, the electron and neutrino ar created by transformation of a neutron into a proton in a nucleus or vice versa.

Note:- Neutron or Proton that are transformed to beta particles are not free particles but are bound to Nucleus by Nuclear Forces.

2) The energy remains conserved in decay process, the available energy being shared among the electron and the neutrino.

3) The beta decay process is analogous to the emission of electromagnetic radiation by an atom, with electron-neutrino field acting in place of Electromagnetic field.

4) “Electron-Neutrino” field is weak in contrast to short range strong interactions which exist between Nucleons bound in the nucleus.

5) Time   Dependent Perturbation theory is a very good approximation because of smallness of coupling constants.

6) As Nucleons move with velocities of only nearly c/10 in nuclei calculations can be made with non relativistic nuclear wave functions.

What is Covalent bonding?

The covalent bond is formed by sharing of pairs of valence electrons between like atoms rather than by electron transfer.
eg : consider the hydrogen molecule H2 ; when two isolated 'H' atoms, each with its electron in the ground state 1S orbital approach each other , the 1S clouds begin to overlap. Each electron is attracted to the other nucleus and the overlap increases( provided the electrons have opp spin) . The two atomic orbitals merge into a molecular orbital. when the repulsive forces have been balanced the attractive forces a molecule results , having stability greater than that of two isolated atoms.
The covalent bonding is also known as "Home polar" or "electron - pair bonding".

Saturation in covalent bonds
   Hydrogen molecule can be stable with only two atoms. If a third atom is brought near 'H2' it is repelled due to allowed exchange of spins is repulsive.Thus covalent bond exhibits.

Direction nature of Covalent bond
    The covalent bond is formed as a result of pairing of two electrons in the atomic orbitals of two atoms, the bond then should lie along the direction of overlapping of atomic orbitals.Hence covalent bonds will have strong preferences.

Hybrid bonding
         Covalent bonds are not only formed by pure 'S' orbitals or pure 'P' orbitals but can also be formed by the overlapping of 'S' and 'P' orbitals called hybrid bonding. eg:- H2O; The HOH bond angle is 104.5 deg  

NEUTRON DIFFRACTION

X-Ray diffraction techniques have certain limitations. In 1936, "W M Elgasser" suggested that moving Neutrons should have debroglie waves associated with them and therefore could be diffracted. The debroglie wavelength of Neutron moving with most probable speed at 20 is 1.80. this is of order of interplanar spacing in crystals. so neutrons can be diffracted by crystals and can be used to study their structure.
          A beam of thermal neutronsfrom an atomic pile possessing all wavelengths is collimated and allowed to fall upon a single crystal. The diffracted beams are photographed on a photographic plate. A Laue pattern is obtained. The Laue pattern can be used to study the crystal structure. The Laue pattern with Lead clearly shows the greater transparency of matter to Neutrons than X-Rays.

The diffraction patterns are formed in a way similar to that for X-rays. For X-rays of 1Amstrong, one requires energies of order 10^4 eV and for electrons about 10^2 eV.

Neutrons are scattered chiefly by Nuclei of atoms, and since wavelength of Neutrons is much greater than dimensions of scattering nucleus, the atomic scattering factor is nearly independent of  scattering angle. Experimentally it was observed that when a beam of Neutrons from a Radium Beryllium source was diffracted by MgO crystal, a maximum occured where predicted by Bragg's relation. The scattering crossection of Nuclei for thermal Neutrons does not depend on atomic number of element, as it does for X-rays.

The scattering of X-Rays by light elements is relatively weak because X-Ray scattering is done by electrons. The Neutrons can penetrate into the matter very easily enables us to deduce the positions of Hydrogen and Carbon atoms in a number of organic crystals.

A major role of Neutron Diffraction has been investigating the magnetic structure of solids. This is a result of fact that Neutrons possess magnetic moments and that these magnetic moments interact with magnetic moments of scattering atoms of solid. This gives an additional scattering mechanism for Neutrons which often outweighs Nuclear Scattering.

If the atomic moments are randomly oriented as in a paramagnetic solid, the magnetically scattered Neutrons are incoherent in phase leading to a diffuse background. This diffuse background of magnetic scattering is then super imposed on lines produced by scattering.