2D transition metal carbides and nitrides known as MXenes are gaining increasing attention. About 20 of them have been synthesized (more predicted) and their applications in fields ranging from energy storage and electromagnetic shielding to medicine are being explored. To facilitate the search for double-transition-metal MXenes, we explore the structure-stability relationship for 8 MXene alloy systems, namely, (VMo)C, (NbMo)C, (TaMo)C, (TiMo)C, (TiNb)C, (TiTa)C, (TiV)C, and (NbV)C with 0 ≤ x ≤ 1, using high-throughput computations. Starting from density-functional theory calculated formation energies, we used the cluster expansion method to build quick-to-compute interactions, enabling us to scan through the formation energies of millions of alloying configurations. For the Mo-rich MXenes, (M1Mo)C (where M1: Ti, V, Nb, Ta) Mo atoms prefer to occupy the surface layers, and ordering persists to high temperatures, based on our Monte Carlo simulations. When Ti is alloyed with Nb or Ta, in the Ti-rich MXenes, Ti atoms prefer the surface layers (e.g., Ti-C-Nb-C-Ti sequence), and in the Nb- or Ta-rich MXenes, Ti occupies only one surface layer and the other two layers are Nb or Ta (e.g., Ti-C-Nb-C-Nb), exhibiting asymmetric ordering. However, alloying Ti with V results in solid solutions across all compositions. (NbV)C phase separates at lower temperatures but forms solid solutions at synthesis temperatures. Postsynthesis annealing at moderate temperatures (800 to 1000 K) increases the ordering for all the compositions. Lastly, by investigating the stability of their precursor MAX phases and surface-terminated MXenes, we discuss the synthesis possibilities of highly ordered MXenes.

Doping of traditional semiconductors has enabled technological applications in modern electronics by tailoring their chemical, optical and electronic properties. However, substitutional doping in two-dimensional semiconductors is at a comparatively early stage, and the resultant effects are less explored. In this work, we report unusual effects of degenerate doping with Nb on structural, electronic and optical characteristics of MoS2 crystals. The doping readily induces a structural transformation from naturally occurring 2H stacking to 3R stacking. Electronically, a strong interaction of the Nb impurity states with the host valence bands drastically and nonlinearly modifies the electronic band structure with the valence band maximum of multilayer MoS2 at the Γ point pushed upward by hybridization with the Nb states. When thinned down to monolayers, in stark contrast, such significant nonlinear effect vanishes, instead resulting in strong and broadband photoluminescence via the formation of exciton complexes tightly bound to neutral acceptors.

Tremendous research efforts are devoted to developing wide potential window aqueous supercapacitors to resolve their low energy density concern. While the operational potential window is dictated by the intrinsic electrochemical stability of water (1.23 V), such a bottleneck may be surpassed by leveraging the additional overpotential of the oxygen evolution reaction and the hydrogen evolution reaction (HER). Herein, by employing an electroreduction technique, Na+ is adsorbed onto the carbon negative electrode which effectively acts as a physical barrier to hinder intermediate HER product formation, thereby reducing HER activity. To complement the wide potential carbon electrode, Na0.25MnO2 is employed as the positive electrode to take advantage of the extra energy (i.e., increased overpotential) required for Na+ insertion process into the structure. The asymmetric supercapacitor exhibits high energy density of 61.1 W h kg−1 at a power density of 982 W kg−1, and even at an ultrahigh power density of 42.9 kW kg−1, a respectable energy density of 16.3 W h kg−1 is attained. In addition, 93.7% capacitance retention is recorded after cycling for 10 000 cycles which further demonstrates its suitability as supercapacitor. The present success in fabricating a 2.7 V asymmetric supercapacitor will open a promising research route toward achieving high energy density and high power density.