Determination of band gap using UV-Vis spectra

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STEP1: Open the absorption graph of the material, which is obtained from the UV Vis spectroscopy.

Theory Behind Calculations: UV Vis Spectroscopy absorption peak means the Electrons are absorbing the Energy at some specific wavelength. Electrons are absorbing Energy means the Electrons are going to excited state from its ground state. Electrons are going to excited state from its ground state means the material is having band gap, thus which can be determine by absorption wavelength.

Energy Equation of Quantum Mechanics:

Energy (E) = Planks Constant (h) * Speed of Light (C) / Wavelength (λ)

Where, Energy (E) = Band gap, Planks constant (h) = 6.626×10^-34 Joules sec, Velocity of Light (C) = 2.99×10^8 meter/sec and Wavelength (λ) = Absorption peak value. Also 1eV = 1.6×10^-19 Joules (Conversion factor)

By this formula band gap can be calculated easily, from UV Vis spectroscopy absorption peak.

The basis of the spectrophotometer
In general, the amount of light absorbed by a substance in a liquid state is directly related to the concentration of that substance in the liquid. If the sample is solid, it must first be dissolved in a clear solvent to be measurable. The sample solvent (known as the control) is usually considered without adsorption or in practice its partial adsorption is less than the total adsorption (sample with solvent). The sample with the solvent is usually poured into a clear glass container or a quartz container and placed in front of the light passing through the spectrophotometer. This dish is called Cell or Quvette. Of course, using add-ons on the spectrometer device, solid or gas samples can also be analyzed, which will be discussed in detail in the articles of this article.

The spectrophotometer uses a tungsten lamp to produce visible light and a deuterium lamp to produce ultraviolet or UV light. The normally measured wavelength range in this device is from 1100 nm to 190 nm. More equipped devices are usually used to measure areas outside this range. Given that a particular molecule may absorb light in a well-defined region of the wavelength range, the light produced must be separated and adjustable to the component wavelengths in a given region. Grating Mirror or prism mirror is used to uniformize the light in the spectrophotometer.

Parts of the ultraviolet and visible spectrometer
the source of light
Prism or grating mirror
Detector, detector or photodiode
The following figure shows an overview of how this device works.

Visible ultraviolet spectrophotometer
Spectrophotometer device diagram

In the visible and ultraviolet spectrometer, after the light passes through the solution, the remaining light sample is inside a detector of Photomultiplier or Photodiode type and after computer processing as a number of one hundred as the percentage of light transmission or its logarithm with The title of the light absorption number appears on the display. Calculations of light absorption or transmission follow Lambert Beer’s law. Mathematically, the amount of light I0 passes through an environment with length X and concentration C, the intensity of the residual light I after passing through the environment is:
I = I0e-KCX
In this relation, K will be a relative constant (absorption coefficient). Therefore, the absorption of the environment or A is obtained as follows:
A = log (I0 / I) = KCX

Spectrophotometer is available in two types of single beam single beam and double beam double beam. The single beam system compares the light absorbed after placing the sample in the device with the main light before placing the sample in the device. One of the advantages of this system is its simplicity, smallness and cheapness, and one of its disadvantages is a small error due to the instability of the measurement environment.

But the two-beam system has two beams, one of which goes to the detector at the same time and the other passes through the sample and the difference between the two is calculated. One of the advantages of this system is more accuracy compared to the single-beam system, and its disadvantages are its complexity and more expensive price. The image below is a schematic of a 2-ray spectrophotometer.

Depending on the spectral region in which the spectral region is performed and which radiation properties (absorption, emission, transmission, scattering, reflection, etc.) are examined, the type of electronic transmissions and consequently the type of spectroscopy and device will be different.
In nasal spectroscopy, absorption is a process in which a chemical species in a transparent medium selectively attenuates (reduces its intensity) certain frequencies of electromagnetic radiation. In the ultraviolet / visible region, the energy of electromagnetic radiation is such that it causes electron transitions in valence electrons. For atoms and ions in the elemental state, the energy of each level is due to the movement of electrons around the nucleus. These states of energy are called electronic states. In addition to having electron energy levels, molecules also have vibrational energy levels and rotational energy levels. These alignments result from the vibration between the atoms in the molecule and from the rotation of the molecules around their own center of mass in space, respectively. In the energy level level diagram, several rotational levels are placed between the two vibrational levels and several vibrational levels are placed between the electronic level levels. Accordingly, each electronic level has vibrating levels and each vibrating level in turn has its own rotational levels. Each of these energy states is about ten times smaller than each other