Spatially confined excitons undergo two distinct quantization effects depending on the confinement length, the enhanced binding energy under strong confinement, and the center-of-mass quantization under weak one. However, the transition between them has not been experimentally identified in two-dimensional (2D) materials due to the lack of thin films satisfying large-scale uniformity and atomic-level flatness in a wide thickness range. Here, we reveal the crossover in high-quality epitaxial thin films of a 2D semiconductor PbI2 grown by molecular beam epitaxy. The absorption spectra exhibit oscillatory structures manifesting the exciton center-of-mass quantization, and the quantization energies show an additional blue shift associated with the strong confinement effect below five-layer thickness (35 Å). The precise control of exciton quantum states will lead to the further development of optoelectronic functionalities of 2D materials.
Crossover from strong to weak exciton confinement in thickness-controlled epitaxial PbI2 thin films
Masao Nakamura, Ryuichi Namba, Takahiro Yasunami, Naoki Ogawa, Yoshinori Tokura, Masashi Kawasaki; Crossover from strong to weak exciton confinement in thickness-controlled epitaxial PbI2 thin films. Appl. Phys. Lett. 13 February 2023; 122 (7): 073101. https://doi.org/10.1063/5.0138536
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