2D materials beyond MXenes: recent progress, exotic properties, and future perspectives of alkaline earth metal based structures and their isoelectronic analogues

The advent of computational and experimental approaches of novel two-dimensional (2D) metal carbides and nitrides, encompassing a variety of metal species of group IIA, IIB, IIIA and various transition metals (collectively known as MXenes) has unveiled significant advances in materials science and technology. Among them, alkaline-earth metal nitrides and carbides (AEXenes) have attracted enormous attention particularly following the experimental realization of few AEXenes as a 2D electride. Herein we have systematically reviewed the structural, electronic, thermal, mechanical, magnetic and optical properties of the 2D AEXenes, reported in recent literature. Leveraging these intriguing features, we have assigned their prospective applications across diverse domains including energy storage, energy harvesting, catalytic and spintronic devices. For instance, BeN monolayers possess significantly high storage capacity (3489 mAh gm−1) with low diffusion barrier and placing them on par with functionalized MXenes. However, Mg2C and Mg3C2 display remarkably low lattice thermal conductivity of 20.26 Wm−1K−1 and 1.5 Wm−1K−1, attributed to distinctive structural complexity, elevated scattering rate and ultimately result better energy conversion efficiency. We have further outlined viable approaches i.e. external carrier doping, adsorption, various defect engineering to modulate their electronic and magnetic properties to facilitate themselves for different applications. In this topical review, we aim to explore recent advancement of these functionalized materials and their isoelectronic analogues to harness their exceptional properties from both theoretical and experimental perspectives.