Hi
I have calculated the band gap of my visible-active photocatalyst using tauc's method. The calculated band gap was 2.86. According to the following equation, I have calculated the wavelength which is corresponding to that band gap:
Wavelength(in nm)=1240.8/Energy(in eV) = 433.57 nm
What does this WAVELENGTH mean?
I'm sure that it is not the maximum wavelength for absorption because my absorption vs. wavelength plot shows absorption until the end of visible region (800 nm). Furthermore, I'm sure that it is not highest wavelength at which absorption occurs because my absorption vs. wavelength plot has maximum @ 480-503 nm.
Thank you very much in advance.
Bandgap means that no light above the BG would enter your material. Tauc's plot is only valid for thin films and differ between direct and indirect bandgap semiconductors (you can check the classical books by Pakove). Essentially one cannot thus make a one-to-one correlation unless you take the nature of semiconductor into consideration
Here are some points to think of:
1) The equation is correct. It gives value for wavelength corresponding to the band gap value
2) Different ways of measurement may give different results. This is normal, and there is no need to panic.
3) As mentioned by Prof. Dutta, things depend on type of semiconductor (direct or indirect) type of solid (nano vs. bulk crystals vs. thin films, ... etc
4) If you need to have accurate measurement, you may need to use other additional characterizations (Photo-luminescence, Band edge position measurement, etc.)
Thank you very much.
But I still don't know what is the exact meaning of 433.57 nm (the wavelength corresponding to band gap)? according to the definition of band gap, my semiconductor could not have absorption at higher amounts of 433.57 nm but the absorption vs wavelength plot of my sample shows absorption until the end of visible region!!
Please read about what are tail states....... you will understand automatically then
It is very important to know uniformity of your solids. It is well known that BG of semicodonducters is curved near the surface layer . If you measure BG by absorption high surface area can influence on value. more over if you have width particle size distribution absorption band also can be wider. The following reason can be chemical heterogeneity of particles. I recomend you to acquainted with methods for measuring the BG
Dear Parham
without any doubt your material has an important impact on your spectrum, to put it in a nutshell, this equation gives you the energy edge in EQE-Energy which your material can absorbed photon with this amount of energy and higher or wavelength of 433.57 nm or less. this explanation is according to the EQE-E plot or EQE-wavelength plot. my supervisor says you should check your result because there is the possibility of making some thing wrong in analysing your results.
Here you can find good explanation and band gap calculator,
http://www.instanano.com/characterization/theoretical/uv-vis-spectroscopy-band-gap-calculation/
Kirithiga Ramalingam · National Institute of Technology Tiruchirappalli
what does the value 1240.8 represent?
Normally,
A photon is characterized by either a wavelength, denoted by λ or equivalently an energy, denoted by E. There is an inverse relationship between the energy of a photon (E) and the wavelength of the light (λ) given by the equation:
E=hc/λ
where h is Planck's constant and c is the speed of light. The value of these and other commonly used constants is given in the constants page.
h = 6.626 × 10 ^-34 joule·s
c = 3 × 10^-8 m/s
hc = 1.99 × 10^-25 joules-m
1 eV = 1.602 × 10^-19 J.
converting joules to eV----
and if wavelength is given as 1Angstrom= 10^-10m
so energy band gap is given by
Eg= 1240.8/λ(nm)
It's an old thread, but still relevant for its basic nature. Let me add a different perspective.
In an ideal semiconducting material (i.e. with no defects and other factors) the wavelength calculated from the bandgap corresponds, as mentioned in the previous answers, to the edge of absorption: photons with a lower energy (higher wavelenght) cannot be absorbed, photons with a higher energy are instead absorbed.
On the other hand, if we look at emission characteristics, instead, the photons being emitted by the semiconductor (either spontaneously or by stimulated emission) will have a wavelength very close to the one corresponding to the bandgap, ideally identical, if no other effects are considered.
This is the reason why, for instance, GaN ("large" bandgap) is used for short wavelength (blue/violet) LEDs and lasers, and InGaAs ("small" bandgap) for long wavelength (near IR) ones.
Tauc-plot can be applied for all samples, whether they are crystalline, partial crystalline or even the amorphous. But the sample should be thin films.
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