Bandgap of Nanomaterials

BANDGAP OF NANOMATERIALS

The electronic properties of metals and semiconductors are determined by their electronic band structure. The band structure changes with particle size. Molecular orbitals get converted into delocalized band states as shown in fig. 5.8.

The band structure of nanocrystals lies between the discrete density of states as in atoms and molecules and es as continuous band as in crystals.

As the size of the material decreases, the energy separation between the adjacent levels increases. This size quantization effect is responsible for the transition of electronic states from a bulk metal or semiconductor to nanoparticles.

The particles that show this size quantization effect are called Q - particles or quantum dots.

In case of the particle size being less than the de Broglie wavelength, charge carriers can be quantum-mechanically understood as particles in a box and the size of the box can provide the dimensions of the particle.

With a decrease in particle size of metals, the quasi-continuous density of states splits into discrete electronic levels with an increase in the spacing between these levels.

Quantum size effect is most significant for semiconductor nanoparticles. In semiconductor, a bandgap already exists in the bulk state. It also increases and the energy bands gradually convert into discrete molecular electronic levels with a decrease in particle size.

As the size of metal nanoparticles decreases, they tend to the lose their metallic character and become semiconductors.

In metals, the quantum size effect exists but it can be seen only in particles smaller than 2 nm where localization of energy levels can be observed when the spacing between the levels exceeds thermal energy (about 26 meV).

Quantum Size Effect

When the size of a nanocrystal becomes smaller than the de Broglie wavelength, electrons and holes get spatially confined, electrical dipoles get generated, the discrete energy levels are formed.

As the size of the material decreases, the energy separation between adjacent levels increases. The density of states of nanocrystals is positioned in between discrete (as that of atoms and molecules) and continuous (as in crystals).

Quantum size effect is most significant for semiconductor nanoparticles. In semiconductors, the bandgap energy is of the order of a few electron volts. It increases with a decrease in particle size.

When photons of light fall on a semiconductor, the photons are absorbed. A sudden rise in absorption is observed when the photon energy is equal to the bandgap.

As the size of the particle decreases, absorption shifts towards the shorter wavelength (blue shifts). This indicates an increase in the bandgap energy (Fig.5.9).

A change in absorption causes a change in the colour of the semiconductor nanoparticle.

For example, bulk cadmium sulfide is orange in colour and has a bandgap of 2.42 eV. It becomes yellow and then ultimately white as its particle size decreases and the bandgap increases.

TUNNELING

The phenomenon of penetration of charge carriers directly through the potential barrier, instead of climbing over it, is called tunneling.