I have a data for UV visible characterisation. Will any body share the how to find the enegy gap by plotting the graph..and if you have the excel sheet for x and y variables kindly contact me? thanks a lot...
If you have Transmittance or absorption vs. wavelength data in your hand you can plot Tauc plot . Tauc plot is the (khv)n vs. hv and n is depend on your material .
k=(1-R)2/2R where R is reflectancy.
The method is called "Tauc plot". You have (hv) on the x-axis and (f(r)*hv))n on the y-axis. n is 1/2 for direct and 2 for indirect bangap materials and f(r) is Kubelka-Munk function, a function which approximates the optical absorption of your material from the reflectance spectrum. (You should also specify if you are using absorption)
If you have Transmittance or absorption vs. wavelength data in your hand you can plot Tauc plot . Tauc plot is the (khv)n vs. hv and n is depend on your material .
k=(1-R)2/2R where R is reflectancy.
Dear Avcl and Erfan.
How to fix n-value for following material? How do we know about direct or indirect bandgap depends on our material? Please give some tips for following
1. Cu doped TiO2, 2. Cu doped ZnO 3. Cu doped SnO2.
Dear Avc
Thank you very much, f(r) is Kubelka-Munk function, how o find the same, any mathematical expression to determine? kindly suggest for uv-absorption wavelength. data on TiO2.
Erfan..sir
thank you very much for the tips given....I have uv data, absorbance versus wavelength, can I use the mentioned formula for k, if yes how to find the R? for Tio2
Dear Kumar,
More in detail, n-value can be 1/2, 2, 3/2 or 3 corresponding to allowed direct, allowed indirect, forbidden direct or forbidden indirect transitions, respectively. As for your materials -which i am not an expert on them but- you should specify their phases as well. For instance, I know that for pure TiO2, in the anatase phase, the bandgap is indirect, so n should be 2 but rutile has a direct bandgap, so n=1/2.
So, I recomend you to check the XRD patterns and identify all the phases of your materials and do a quick literature search on their bandgap type.
Dear Vankatesha,
The mathematical expression of f(r) is basically what is written for "k" in Efran's answer. But it is not suitable for your case since you are using an absorption spectra. In the absorption case, the relation becomes (αhv)n where α is the absorption coefficient of your material which has a different value for different wavelengths.
So I would recommend you to look for the corresponding α for your corresponding wavelength. And please also identify which phase of titania you are using because different phases have different α values.
There are at least two possibilities:
1. Measure the optical absorption spectrum and treated it with standard methods. From experimental frequency dependence of the absorption coefficient to determine the width of the band gap.
2. Determine the frequency dependence of the real and imaginary dielectric constants f rom the ellipsometric measurements. The position of the low-energy peak of the imaginary part determines the width of the band gap
The band gap energy can be determined from wavelength at which a steep raise is initiated in absorbance. This is due to the fact that the measured wavelength of photon is the minimum required for electron excitation across the band gap. However, the accuracy of this approach can be put to question by prevalence of non linear absorbance increase that depends on the presence of defect sites and local density of states at the valance and conduction band edges.
An ideal approach for exhaustive band gap analysis would involve the use of Tauc plots for fitting of absorption coefficients to match the expected trends of direct and indirect band gap semiconductors. The absorption coefficient α can be calculated from path length l and absorbance A using the following formula.
α(cm^(-1) )=(ln(10)×A)/(l(cm))
With the use of absorption coefficient α, base line extrapolation of Tauc plots specific to direct and indirect band gap determination can be employed for computation of band gap energies (Where hυ=E, the energy gap, so general trends are to be known to assume determine x and y axes E)
Plot co-ordinates for different transitions
(αhυ)^2 vs hυ Direct (Allowed)
(αhυ)^2/3 vs hυ Direct (Forbidden)
(αhυ)^1/2 vs hυ Indirect (Allowed)
(αhυ)^1/3 vs hυ Indirect (Forbidden)
You may refer literature to get examples of such band gap determination. One paper is linked below.
http://pubs.rsc.org/en/content/articlelanding/2014/ta/c4ta01494d#!divAbstract
Dear Arun
thank you very much for sparing your time to give the explanation..this will be useful for the people just they entered the research..again thank you..
Here you can find good explanation and band gap calculator,
http://www.instanano.com/characterization/theoretical/uv-vis-spectroscopy-band-gap-calculation/
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