I am working on CdS nanoparticles. I got a band gap value of 5.89 eV whereas the bandgap value of my bulk is 2.42 eV. I am using double distilled water as my solvent in UV studies. Why did I get such a huge difference?
Jasper, what is the size of your your nanoparticles? Have you performed XRD on them?
The band gap decreases as you increase the size of the particles, say . from molecular level to macroscopic bulk size. This is due to the inter molecular interaction of the neighboring molecules or particles in the bulk medium.
The key difference to note is the difference in between discrete atomic or molecular orbitals and the bands in a solid. The organization of energy levels is quite different between nanomaterials and bulk materials in that nano materials have larger band gaps and consist of more discrete energy levels. This observation may be attributed to the various quantum confinement effects within nano particles whose sizes can be as small as only a few dozen atoms wide (couple of nanometers). Quantum confinement is defined as a confining of the movement of the particles in one or more dimensions [1]. When these dimensions are comparable to the de Broglie wavelength of the particle, quantum confinement effects take place and cause the band gap of the particle to increase. Bulk solids, on the other hand, are a composition of several thousands of atoms. All these atoms contribute to highly overlapping orbitals leading to less pronounced energy bands and consequently a smaller band gap.
The key difference to note is the difference in between discrete atomic or molecular orbitals and the bands in a solid. The organization of energy levels is quite different between nanomaterials and bulk materials in that nano materials have larger band gaps and consist of more discrete energy levels. This observation may be attributed to the various quantum confinement effects within nano particles whose sizes can be as small as only a few dozen atoms wide (couple of nanometers). Quantum confinement is defined as a confining of the movement of the particles in one or more dimensions [1]. When these dimensions are comparable to the de Broglie wavelength of the particle, quantum confinement effects take place and cause the band gap of the particle to increase. Bulk solids, on the other hand, are a composition of several thousands of atoms. All these atoms contribute to highly overlapping orbitals leading to less pronounced energy bands and consequently a smaller band gap.
How sure are you about the material quality of your nanoparticles? If you had defects in it, a lower energy transition could be oberved. Compare i.e. with yellow band in GaN.
What you have got is right. The gap between the valence and conduction band increases with the decreasing particle size. Since the bulk matter forms by the merger of numerous atoms and molecules, the freedom for movement of electrons (band) becomes lesser as matter grows from atoms and molecules to nanoparticles to bulk.
You can look at this figure for reference: http://www.coatingsys.com/yahoo_site_admin/assets/docs/How_does_nanosize_influence_the_electron_band_gap.24174621.pdf
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