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Photoluminescence (PL) is a process in which a substance absorbs photons (electromagnetic radiation) and then re-radiates photons. Quantum mechanically, this can be described as an excitation to a higher energy state and then a return to a lower energy state accompanied by the emission of a photon. - Wikipedia
The bandgap of a semiconductor can be determined from the emission wavelength. If light is shone on the material with photons having energy above its bandgap, electrons will be excited from the valence band to the conduction band. They will soon decay back down into the valence band emitting a photon with an energy equal to the bandgap. In a PL apparatus this emission is absorbed by a detector and the bandgap energy can be found.
The lab owns an Ocean Optics USB4000 spectrometer with a 300nm to 1000nm range.
A variety of excitation sources can be used. Dr. Chen's lab has a pulsed nitrogen laser emitting light with a 337 nm wavelength. They also have available a 75W high pressure mercury short arc lamp.
Talk to Dr. Chen in the physics department for permission to use the PL system. The system is located in the chemistry lab in the basement of Stirling Hall.
A lamp or a nitrogen laser (337 nm) can be used as the photon (light) source. This protocol applies for the lamp. It was used because its excitation energy is adjustable. However, the intensity is lower than that of the laser.
The system is designed for liquid samples in 1cm x 1cm cuvettes. A sample holder must be made for hard substrates. A stiff strip of paper or plastic (~1.4cm in width and 2-3 inches long) with tape to hold samples on works. Ensure the sample is taped at the right height for the beam. The laser and detector paths are 90 degrees apart so the sample must be angled at roughly 45 degrees to each.
- Turn on lamp power supply and motor driver
- Wait 20-30 min for lamp to worm up
- Open Felix program
- Define test settings:
- Excitation wavelength (set well above band gap)
- Emission range to be detected (ie. few hundred nm on either side of band gap)
- Step size (default: 1nm)
- Integration time (default is 1s)
- Averages (number of identical runs, default is 1)
- Click Acquire Prep, then Start
- Data Pointer button gives peak info
- Hide Toggle hides/shows previous runs overlayed
- Double click on run name to change name
- Save data after all runs are completed to personal folder on desktop
- Transfer data to adjacent computer using floppy, then transfer to USB
- Data will be in a text file
- Turn off lamp power supply and motor driver if not used afterwrds