The interlayer coupling interaction between few-layer g-C3N4 plays a key role in optimizing the photocatalytic hydrogen evolution performance.

Employing size effect to tailor photoabsorption and photoemission properties of quantum dot (QD) nanostructures is a widely used method for optoelectronic and biomedical applications. In this work, based on time-dependent density functional theory calculations, we systematically research the size-dependent optical absorption and emission properties of two-dimensional phosphorene, arsenene, and antimonene single-element QDs. As expected, the photoabsorption gap is monotonically decreasing with the increase of QD diameter, which is consistent with the well-accepted quantum confinement effect (QCE). Surprisingly, the emission gap increases with the increase of size within ∼3 nm diameter range. It is revealed that the anomalous phenomenon is derived from the competition between the QCE and excited-state structural rearrangement during the relaxation of the excited state. The smaller two-dimensional QDs is more prone to structural deformation at the excited state accompanied by the greater change of the electronic structure, which prevails over the QCE and reduces the emission gap. The atomic level picture of the anomalous emission phenomenon revealed in our research is valuable for the fundamental understanding of the size effect of QDs as well as for their optical utilization in optoelectronics and biomedical fields.

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