By Tauc's plot you can obtain the direct and indirect band gap for a material. In your case it seems that you have a direct gap but with a contribution that could become from an impurity or secondary phase.

Best regards,

Pedro

Dear Petru,

You might have some kind of sub-bandgap absorption due to defect or different crystallinity phase (like monoclinic phase of HfO2). We tried to model such behavior in our recent paper: 

http://www.sciencedirect.com/science/article/pii/S0040609015007014

Hello Petru,

I'm also facing the similar problem. Can you help me with the same if you have able to solve the same? It would be a great help.

I think two linear segments is due to contribution of the phonons in case of indirect transitions.

You can calculate values of each linear portion (in two segments) which is intersecting the x axis

say E1(first line) and E2 (for last line) for two linears then

phonon energy = (E2-E1)/2

Energy gap = E1+ Ephonon or = E2- Ephonon

Dear Dr. Petru Lunca-Popa 

Have a nice day

Generally, the absorption coefficient (alpha) is related to photon energy (h*neo) by the known equation as:

       α = β/(hυ) (hυ - Eg)1/2        Or          (αhυ)2 =β(hυ - Eg)                 

where β is a constant called the band tailing parameter, Eg is the energy of the optical band gap and 2 is the power factor of the transition mode, which is dependent upon the nature of the material, whether it is crystalline or amorphous. According to Tauc's relation:

The plotting of  (αhυ)2 Versus the photon energy (hυ) gives a straight line in a certain region. The extrapolation of this straight line will intercept the (hυ)-axis to give the value of the indirect optical energy gap (Eg).

However, in your case and according to the attached plot, the answer is the tangents to the curve, as you do, so you have two linear segments. This is may be due to contribution of the phonons. You must take the linear part of the curve (between 2.5 and 3.25 eV). Therefore, the intersection value (in eV) is your direct band gap energy according to Tauc's Equation.

I hope this help you.

Good luck 

Hello Dear,

First of all, if you are dealing with single pure phase material (such as classical inorganic semiconductor) it is expected to be single linear segments at the low energy position, whereas presence of impurity or any defects can generates new sub level which can be the reason for getting two liner segments (one of which would be at low energy relative to the former) as mentioned by Dmitriy Likhachev , however recent days MOF/CPs are explored as new class of semiconductor where two different transitions are responsible in one single phase system, one is ligand centered (mainly bonding pi to anti bonding pi)and one is LMCT/MLCT based charge transfer, in that case you might get two different linear segments, but due to their different absorption coefficient variation, the visibility of both linear segments varies, when you will go for close look in that particular region in the tauc plot, you will get another, however I do not know for which system you have plotted.

Hi, colleagues! We recently published a new method of spectrophotometric data analysis very similar to the Tauc method but with many advantages over it. In the Tauc method, the main problem is to determine the energy gap without knowing the nature of the optical transition (described with the parameter m) and with fitting range. Our method does not require knowledge of the parameter m. The value of energy gap and parameter m can be obtained simultaneously thanks to one linear fit. Other advantages and description of how to use this method are described in the article. We encourage you to read the paper!

Article Inverse logarithmic derivative method for determining the en...

You can easily draw Tauc Plot Online on this link -

https://www.instanano.com/2020/05/draw-tauc-plot-from-uv-vis-absorbance.html

In addition to all helpful answers, I would like to add this reference

Article High Throughput Light Absorber Discovery, Part 1: An Algorit...

Dear Petru,

The emerging of a secondary hump at lower energy with low absorption is often observed in Tau’s plot in crystalline phosphor materials. I have mentioned below some of my observations:

Q. Is the secondary hump seen in both the pristine and activated samples?

(I) If it is seen in a pristine (undoped) sample, it could be due to some intrinsic defects or secondary phase present within the band gap region.

(II) If it is only observed in the doped sample, it may be attributed to some localized states of the activator. The activator’s concentration, usually being low in a host, its absorption energy may appear at a lower value with a lower energy region in Tau’s plot compared to that of the valence band to conduction band transition.

(III) The band gap obtained by extrapolation at the sharp fall of the Tau’s plot with high absorption corresponds to the VB to the CB energy gap. So, the second band gap at lower energy may be attributed to the localized defects state originated from the activator, intrinsic defect, or secondary phase.

(IV) One cannot tell the optical band gap obtained from Tau’s plot belongs to a direct or indirect band gap. In this context, the theoretical (DFT) calculation is useful to ascertain the band gap’s nature.

Hope the above points may help you

With regards

Sitakanta

Petru Lunca-Popa Tauc used an equation in 1968 to calculate the absorption edges (bandgaps) of amorphous Ge and Si from their absorption data.

(αhν)γ=A(hν-Eg)

In this equation, α is the absorption coefficient, h is Planck’s constant, υ is the frequency of the incident photon, A is a proportionality constant (which is determined by the index of refraction, electron, and hole effective masses; however, it is usually taken as 1 for amorphous materials), and Eg is the bandgap energy. The important term is the exponent γ, which denotes the nature of the electronic transition, that is, when γ=2 it is a direct allowed transition, and when it is equal to 1/2, it is an indirect allowed transition. For γ=2/3 it is a direct forbidden transition, and for γ=1/3 an indirect forbidden transition. Typically, the allowed transitions dominate the basic absorption processes, giving either direct or indirect transitions.

Thus, the basic procedure for a Tauc analysis is to acquire optical absorbance data for a sample in question that spans a range of energies from below the bandgap transition to above it.

Plotting the (αhν)γ versus (hν) is a matter of testing γ=2 or γ=1/2 to compare which provides the better fit and thus identifies the correct transition type.

I have provided the Template file (Origin file) in the video description. Thanks

https://youtu.be/LLESVSsZmlg

please see the following link:

https://www.youtube.com/watch?v=JepEAJWUh_c