DOI: https://doi.org/10.29363/nanoge.incnc.2021.010
Publication date: 8th June 2021
Quantum mechanics teaches that electrons exhibit a dual behavior of particles and waves. This feat has well-known implications for the optical properties of low-dimensional semiconductors: confinement of the electronic excitation (exciton wave) in the limited space of the crystallite increases the energy of the excitation. The effects of quantum confinement on the exciton energetics is well established, and has resulted in numerous optical applications of colloidal nanocrystals. However, the optical absorption strength of excitons with different degrees of confinement has not been considered quantitatively. Here, we report generality in the band-edge light absorptance of semiconductors, independent of their dimensions. First, we provide atomistic tight binding calculations that show that the absorptance of semiconductor quantum wells is equal to m πα (m=1 or 2 with α the fine structure constant) per allowed transition, in agreement with reported experimental results. Then, we show experimentally that a monolayer superlattice of quantum dots has very similar absorptance, suggesting an absorptance quantum of m πα per (confined) exciton diameter. Extending this idea to bulk semiconductors, we experimentally demonstrate that an absorptance quantum equal to m πα per exciton Bohr diameter explains the widely varying absorption coefficient of bulk semiconductors. We thus provided compelling evidence that the absorptance quantum πα per exciton diameter rules the band-edge absorption of all direct semiconductors, regardless of their dimension.
The author wishes to thank Tim Prins, Zeger Hens, and Christophe Delerue for their strong contribution to this work that will be submitted soon.
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