ZnO thin film
Optical constants (k) converted to absorption.
Tauc Plot Generator
FigranTurn a UV-Vis spectrum into a Tauc plot and read the optical band gap.
- Absorbance or reflectance
- Direct or indirect
- SVG and PNG figure
A band gap calculator for UV-Vis absorbance, transmittance and reflectance spectra.
Paste columns with a shared X column, or import CSV/TXT.
Linear fit range (eV)
Drag the handles or enter values. Review the selected region for your material.
A baseline can shift the intercept. Compare None, subtraction and intersection, and report the method used.
Methods
The optical band gap of ZnO was estimated from a Tauc plot of (αhν)² against photon energy (direct allowed transition, n = 1/2), using absorbance data. A linear fit over 3.306–3.594 eV was extrapolated to the x axis (no baseline correction), giving Eg = 3.176 eV (standard error of the fit 0.007 eV, R² = 0.9968, 7 points).
Uncertainty is the first-order standard error of the fitted intersection; it does not include model or systematic uncertainty.Band gap
ZnO: 3.176 eV · fit SE 0.007 eV
R² 0.9968 · 7 points
Tauc plot examples
Explore different spectra and compare the fitted regions.
TiO₂ anatase thin film
Optical constants (k) converted to absorption.
CdS thin film
Optical constants (k) converted to absorption; the negative-k transparent region was excluded.
Powder diffuse reflectance
Simulated powder spectrum with a tail and post-edge downturn.
Silicon indirect band gap
Optical constants (k) converted to absorption, plotted for an indirect band gap.
Compare three samples
Compare simulated spectra with tails and saturating absorption edges.
How to calculate band gap from UV-Vis
Make a Tauc plot from UV-Vis data in four steps.
1Add a spectrum
Paste the spectrum or import a CSV or TXT file.
2Choose the transition
Select the input signal and a physically justified transition.
3Fit the linear region
Inspect and adjust the fit range and baseline.
4Download the figure
Save the plot or export converted values.
Direct or indirect band gap?
Choose the transition using independent physical evidence. Straightness alone cannot establish the transition. The strict Tauc gap applies to amorphous materials; (αhν)² is a common convention for crystals (Zanatta, 2019).
| Transition | n | Ordinate |
|---|---|---|
| Direct allowed | ½ | |
| Indirect allowed | 2 | |
| Direct forbidden | 3⁄2 | |
| Indirect forbidden | 3 |
Direct allowed
Indirect allowed
References: Zanatta, Scientific Reports 9, 11225 (2019)
Diffuse reflectance and Kubelka–Munk
For a sufficiently thick, diffusely scattering sample, the Kubelka–Munk function can serve as a proxy for absorption when scattering is approximately constant. Specular reflection and wavelength-dependent scattering can bias the result.
Choosing the linear region and baseline
Fit a defensible linear absorption edge rather than a curved tail or excitonic peak. A baseline can shift the intercept; compare None, subtraction and intersection and report the method used. Automatic fits are starting points, not a substitute for judgment.
None (x-axis intercept)
Subtract flat baseline
Baseline intersection
References: Makuła, Pacia and Macyk (2018) · Klein et al. (2023) · Zhong et al., J. Phys. Chem. Lett. 14, 6702 (2023)Zhong et al. (2023) · Escobedo-Morales et al., Heliyon 5, e01505 (2019)Escobedo-Morales et al. (2019)
Tauc plot FAQ
What is a Tauc plot?
A Tauc plot graphs a transformed absorption signal against photon energy to estimate an optical band gap from a linear absorption edge.
How do I calculate a band gap from UV-Vis?
Convert wavelength to photon energy, transform the absorption signal using the chosen transition exponent, and extrapolate an appropriate linear region to the baseline.
Can the plot tell me whether the gap is direct or indirect?
No. Use band structure, literature or other independent measurements to select the transition; a straight line alone is not proof.
Which exponent n should I use?
Under the convention used here, n is 1/2 for direct allowed and 2 for indirect allowed transitions. Forbidden transitions are available under advanced options.
How do I use diffuse reflectance data?
Choose reflectance to calculate the Kubelka–Munk function, or select F(R) if you have already calculated it. The approximation depends on the sample and scattering conditions.
How should I choose a linear region?
Select an absorption-edge region that is reasonably linear and supported by your material model. Avoid curved tails and excitonic peaks, and report the range used.
How does the baseline affect the result?
A nonzero sub-bandgap signal can move the x-axis intercept. Compare None, subtraction and intersection, and report the method used.
Can I reproduce this Tauc plot in Origin?
Yes. Use the same conversion, transition exponent, fit points and baseline in Origin; different choices change the result.
Is my spectrum uploaded?
No. Calculation and figure export happen in your browser; the session draft stays in this browser tab.