Two-dimensional P3¯m1Ca3N2, Ba3P2, and Ba3As2: Promising stable narrow-gap semiconductors for infrared and broadband photodetectors

Exploring two-dimensional (2D) narrow-gap materials with exceptional stability and outstanding photoelectric performance has become a key focus in nano-optoelectronics. However, most existing 2D materials contain relatively large band gaps, and those with narrow band gaps tend to have inadequate stability. This study employed first-principles calculation to predict three alternative narrow-gap 2D binary group (${\mathrm{II}}_{3}$-${\mathrm{V}}_{2}$) materials in the $P\overline{3}m1$ space group: ${\mathrm{Ca}}_{3}{\mathrm{N}}_{2}$, ${\mathrm{Ba}}_{3}{\mathrm{P}}_{2}$, and ${\mathrm{Ba}}_{3}{\mathrm{As}}_{2}$. All these materials exhibit excellent energetic, mechanical, dynamic, and thermal stability. Their mechanical properties reveal isotropic characteristics and demonstrate excellent in-plane stiffness and flexibility. Regarding electronic properties, monolayer ${\mathrm{Ca}}_{3}{\mathrm{N}}_{2}$, ${\mathrm{Ba}}_{3}{\mathrm{P}}_{2}$, and ${\mathrm{Ba}}_{3}{\mathrm{As}}_{2}$ possess indirect narrow band gaps of $0.41$, $0.61$, and $0.68$ eV, respectively. Moreover, they exhibit high electron mobilities (about ${10}^{3}$--${10}^{4}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.2em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.2em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$) and are nearly isotropic. In terms of optical properties, they demonstrate a significantly broad absorption range, spanning from the IR to visible and UV regions, with remarkably high absorption coefficients (approximately ${10}^{4}$--${10}^{5}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$). Additionally, their exciton binding energies are higher than those observed in traditional bulk materials while lower than most other 2D materials, facilitating excellent light-driven performance. We propose that these alternative 2D $P\overline{3}m1\phantom{\rule{0.2em}{0ex}}{\mathrm{Ca}}_{3}{\mathrm{N}}_{2}$, ${\mathrm{Ba}}_{3}{\mathrm{P}}_{2}$, and ${\mathrm{Ba}}_{3}{\mathrm{As}}_{2}$ binary narrow-gap semiconductors will hold promising application prospects in nano-optoelectronic fields such as IR light detection, ambipolar transistors, medical imaging, electrodes, optical communication, and remote sensing.