Design of polar XC₃ (X = P, As, Sb, Bi) monolayers with coupled bandgap, polarization, and optical responses

Bandgap engineering and polarization control in graphene-based systems are crucial for developing high-performance two-dimensional (2D) semiconductors. However, simultaneously achieving a sizable bandgap, intrinsic polarity, and strong light-matter interaction remains challenging. Here, we propose a new class of carbon-based polar semiconductors, monolayer XC₃ (X = P, As, Sb, Bi), designed by substituting group-V elements into graphene to break its sublattice symmetry. This symmetry breaking not only opens wide bandgaps (2.23-3.11 eV) but also induces spontaneous out-of-plane (OOP) electric polarization (-3.1-8.1 pC m^-1) and an internal electric field, stabilizing polar phases and facilitating photocarrier separation. The resulting electronic structures exhibit a distinctive Mexican-hat-shaped valence band and strong band nesting, leading to intense visible-to-near-ultraviolet optical absorption (>10⁵ cm^-1). Moreover, XC₃ monolayers possess large and anisotropic carrier mobilities and exhibit band-edge alignments suitable for photocatalytic water splitting across a wide pH range (0-10). These findings establish a general route to 2D polar semiconductors that integrate coupled electronic, optical, and catalytic functionalities, offering a promising platform for graphene-derived optoelectronic and energy applications.