Spectroscopic study of micro-crystals of thallium halides

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The extrema of the valence band and the conduction band of thallium halides are not at the GAMMA point but at the X point. As the result, the direct exciton has a unique and complex structure due to intervallay exchange and Coulomb interaction. In the present research we intended to study quantum size effects on an exciton state and on exchange interaction of the exciton in a micro-crystal of thallium halides.Quantum size effects on electron states have been extensively studied theoretically in these years. Kayanuma studied an exciton state in a spherical potential well to the full extent both analytically and numerically.We measured optical absorption spectra of the direct exciton of TlCI and TlBr in evaporated thin films at 5K and determined the size R of micro-crystals of TICI and TIBr by an electron microscope. The energy of the exciton was found to increase with decreasing micro-crystal size. The energy shift of the exciton was followed very well to Kayamuna's theoretical value for rather wide range of micro-crystal size R from the regime of individual particle confinement R/alpha<less than or equal>2 in the regime of exciton confinement R/alpha<greater than or equal>4, where a is the effective Bohr radiusIn TICI the absorption spectrum of the lowest exciton state has doublet peaks labeled 1A and 1B.We found that the relative intensity of the absorption bands 1A to 1B decreased with decreasing the micro-crystal size and disappeared in the film where the micro-crystal size was less than lOnm. The experimental results suggest that the exchange interaction play more important role in smaller micro-crystals. It is still open what is the confinement effect on these exchange interactions, and futher experiment and theoretical investigations will be required.