Photoluminescence

 
Photoluminescence in simple terms is a reverse process of absorption. However, the absorption spectrum of a material contains all possible transitions while photoluminescence involves emission of a particular wavelength that corresponds to the transition. Photoluminescence occurs when an excited electron in an excited state returns to the initial state by emission of a photon whose energy gives the difference between the excited state and the initial state energies. The process can be direct or indirect depending on the gap energy. A schematic diagram of a direct process of a band structure near the band gap of a heavily dope n-type semiconductor.

An advanced software package for the analysis of all our optical data (Raman _IR vibrational spectroscopy, Reflectivity- Kramers-Kronig, photoluminescence, etc.) is available (see Prof. Eklund).

In a direct intrinsic semiconductor, the photoluminescence energy is the optical gap.

                                                  

However, in the case of the doped,

                                                 

In the phonon assisted photoluminescence process in indirect semiconductors;

                                               

Where    is the incident photon energy, Egap is the band gap and   is the absorbed or emitted phonon energy.

 Excitonic behavior can be important in photoluminescence and it is an indicator of sample quality. If the semiconductor is very pure, the coulombic attraction between the generated electron and hole can bind them into a quasi-hydrogenic exciton. The exciton can move as a whole through the crystal, but with net charge of zero, it carries no charge. Excitons from when photon absorption occurs at the critical points given by the equation

                           

Due to excitonic effect, the photoluminescence in a direct  transitions is

                                  

In nanomaterials, the PL energy is upshifted due to diameter distribution, thus the PL energy is given by

                                 

In such case, for an indirect semiconductor

                           

In our group, PL is used to probe the band gap and to determine the mean diameter of our nanomaterials. We also use our PL data to study alloy composition (as in the case of GeSi alloys), crystallinity and stress in the nanofilaments. Apart from these, PL as a tool is used to probe impurity and defects in the materials. All results are correlated with Raman and IR results for confirmation.

Most of the PL measurements are done with a single grating monochrometer spectramax 270M Spectrometer shown below.