blue shift (rus. голубой сдвиг) — shift of the optical absorption edge to the high-frequency range observed in semiconductors with particle size decrease.

Description

The effect is due to the fact that size reduction of a semiconductor crystal is accompanied with band gap widening. Therefore, higher absorbed quantum energy is required to excite electrons from the valence band into the conduction band, and this results in absorption edge shift.

There are several models describing the blue shift effect. Historically, the first model was proposed in [1] to describe the blue shift observed in CuCl2 nanoparticles synthesised on a porous glass surface. The model describes the charge carriers (electrons and holes) generated by photon absorption as particles in a quantum well. The size of such a quantum well corresponds to the size of a nanoparticle.

The effect of nanoparticle size is also observed in the reverse process, i.e. in electron-hole recombination with emission of quant (luminescence), whose wavelength is also dependent on the particle size. The blue shift effect is most clearly observed when the crystal size decreases values are close to the exciton radius in bulk semiconductor (2-30 nm); such particles are called “quantum dots”.

Nanoparticles can experience transition from the formed band structure into discrete electronic levels (see Fig.), which leads to the transformation of the absorption edge into the absorption peak with particle size decrease.

Illustrations

a — Energy diagram of band gap expansion during conversion of a massive semiconductor into solid-

a — Energy diagram of band gap expansion during conversion of a massive semiconductor into solid-state nanoparticles. VB — valence band; BG — band gap; CB — conduction band; UFMO — upper filled molecular orbitals; LVMO — lower valence molecular orbitals. b — CdS nanoparticles absorption spectra in UV and visible bands. The blue shift of absorption band edge occurs as the colloid nanoparticles become smaller. Built using [2].


Authors

  • Veresov Alexander G.
  • Tolkachev Nikolay N.

Sources

  1. Ehfros Al.L., Ehfros A.L. Interband absorption in semiconductor sphere (in Russian). FTP. 1982. V. 16.
  2. Weller H. Colloidal semiconductor Q-particles: chemistry in the transition region between solid state // Angew. Chem. Int. Ed. Engl. 1993. V 32. P. 41–53.

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