The Unusual Optical Properties of Semiconducting Nanoparticles

While I have indicated “semiconducting nanoparticles” in the title, nanoparticles made of silicon, germanium or cadmium are not themselves semiconductors. A nanoparticle of Si_n can be made by laser evaporation of a Si substrate in the region of a helium gas pulse. The beam of neutral clusters is photolyzed by UV laser producing ionized clusters whose mass to charge ratio is then measured in a mass spectrometer.

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The most unusual property of nanoparticles made of semiconducting elements is the pronounced changes in their optical properties compared to those of the bulk material. There is a significant shift in the optical absorption spectra toward the blue (shorter wavelength) as the particle size is reduced. The figure below shows the optical absorption spectra of a CdSe nanoparticle at two different sizes measured at 10K. You can see that the lowest energy absorption region, referred to as the absorption edge is shifted to higher energy as the particle size decreases. Because the absorption edge is due to the band gap, this means that the band gap increases as particle size decreases. Also note that, the intensity of the absorption increases as the particle size is reduced.

The higher energy peaks are associated with the exciton, and they shift to higher energies with the decrease in particle size. These effects are a result of the confinement of the exciton. Basically, as the particle size is reduced, the hole and the electron are forced closer together, and the separation between the energy levels changes.

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