Spectroscopic study of micro-crystals of thallium halides

卤化铊的价带和导带的极值不在伽马点，而在X点。结果表明，由于层间交换和库仑相互作用，直接激子具有独特而复杂的结构。在本研究中，我们打算研究卤化铊微晶中激子态的量子尺寸效应和激子的交换相互作用。近年来，量子尺寸效应对电子态的理论研究已经非常广泛。Kayanuma对球形势垒中的激子态进行了全面的解析和数值研究。在5K下测量了蒸发薄膜中TLCI和TlBr的直接激子的光学吸收光谱，并用电子显微镜测定了TICI和TIBr微晶的尺寸R。发现激子能量随着微晶尺寸的减小而增大。在激子约束R/α和lt；大于或等于&gt；4的情况下，在相当宽的微晶尺寸范围内，激子的能量漂移很好地符合Kayamura的理论值。在TICI中，最低激子态的吸收光谱有双峰，标记为1A和1B。我们发现，在微晶尺寸小于10 nm的薄膜中，吸收带1A到1B的相对强度随着微晶尺寸的减小而减小，并消失在微晶尺寸小于10 nm的薄膜中。实验结果表明，在较小的微晶中，交换作用起着更为重要的作用。这些交换相互作用的限制效应是什么，还有待于进一步的实验和理论研究。

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.