Phase engineering of 2D X2C monolayers: insights into metal–semiconductor transitions and multifunctional applications

Abstract

In pursuit of advanced materials with potential to invigorate next-generation multifunctional electronic devices, we focus our investigation on a novel class of two-dimensional (2D) binary compounds derived from group IV elements. Specifically, we examined the structural, thermodynamic, and electronic characteristics of X₂C (X = Si, Ge, Sn) monolayers through first-principles calculations. Our findings indicate that these monolayers can stabilize in no fewer than six distinct space groups. Notably, the triclinic P1 configuration emerges as the most stable for Si₂C and Ge₂C, while for Sn₂C, the trigonal Pm1 phase is preferred. The existence of alternative configurations, such as the polar P3m1 phase, points to possible polymorphic coexistence within these materials. Phonon dispersion analyses confirm the dynamic stability of the monolayers, and ab initio molecular dynamics simulations further verify their thermal robustness at ambient temperature. Particularly intriguing is the observation of a metal–semiconductor transition between the P3m1 and P1 structures, underscoring the potential to tune electronic properties via structural modification. This transition likely stems from a Peierls-type distortion in the P3m1 phase, which relaxes into lower symmetry forms like P1; an effect especially pronounced in Sn₂C. Such phase transitions are especially relevant for multifunctional device applications, where controlled phase manipulation can be harnessed for electronics, memory technologies, and even emerging quantum devices. While enthalpy of formation calculations suggest that X₂C synthesis is endothermic (requiring external heat or pressure) alternative fabrication strategies, such as substrate-assisted growth, may mitigate these challenges. Further, our heterostructure analyses reveal that Si₂C in the P3m1 phase interacts only weakly with insulating h-BN, preserving its metallic character. Similarly, Sn₂C maintains both structure and metallicity when supported on WSe₂. Altogether, these results position X₂C monolayers as promising candidates for 2D electronic and optoelectronic platforms, where judicious substrate selection and phase engineering can be leveraged to tailor material properties for specific technological applications.