How can you measure band-gap by UV-Vis
Spectroscopy?
To measure the band-gap by UV-Vis spectroscopy, you can follow these steps:
1. Prepare a sample: Choose a solid or liquid material that you want to determine the band-gap of. Ensure that the sample is in a suitable form for UV-Vis spectroscopy, such as a thin film or a solution.
2. Set up the UV-Vis spectrometer: Turn on the UV-Vis instrument and allow it to warm up for a few minutes. Set the wavelength range to cover the UV and visible spectrum. Calibrate the instrument if required.
3. Baseline correction: Obtain a blank sample by using a solvent or a reference material that does not absorb UV or visible light. Place it into the spectrometer and measure the absorption spectrum. This will serve as a reference for background correction.
4. Measure the sample: Place your prepared sample into the spectrometer and measure its absorption spectrum. Ensure that the sample is uniformly aligned and covers an appropriate portion of the light path.
5. Analyze the absorption spectrum: The absorption spectrum will show peaks and valleys corresponding to the absorption of light by the material at different wavelengths. Look for the absorption onset, where the absorption suddenly increases. This onset indicates the energy needed to promote an electron from the valence band to the conduction band, which is the band-gap energy.
6. Calculate the band-gap: Measure the corresponding wavelength at the absorption onset and convert it to energy using the equation E = hc/λ, where E is the energy, h is Planck's constant, c is the speed of light, and λ is the wavelength. Subtract the energy value from the measured sample from the energy value obtained from the baseline correction. This difference gives you the band-gap energy of the material.
It is important to note that band-gap measurement by UV-Vis spectroscopy gives an indirect estimation and may have some limitations, particularly for materials with complex electronic structures or those that exhibit other types of absorption phenomena. For more accurate band-gap determination, other techniques such as photoluminescence spectroscopy or electrical measurements may be required.