Significance: The new ability to generate circularly polarized coherent (laser-like) beams of short wavelength high harmonics in a tabletop-scale setup is attracting intense interest worldwide. Although predicted in 1995, this capability was demonstrated experimentally only in 2014. However, all work to date (both theory and experiment) studied circularly polarized harmonics only in the extreme UV (EUV) region of the spectrum at wavelengths >18 nm. In this new work done in a broad international collaboration, we demonstrate the first soft X-ray high harmonics with circular polarization to wavelengths λ < 8 nm and the first tabletop soft X-ray magnetic circular dichroism (XMCD) measurements, and also uncover new X-ray light science that will inspire many more studies of circular high-harmonic generation (HHG).

Electrocatalysis has the potential to become a more sustainable approach to generate hydrogen as a clean energy source and chemical feedstock. Finding a stable, eco-friendly, low cost and highly efficient catalyst is one of the prerequisites to realize large-scale industrial electrocatalytic hydrogen production. Two-dimensional metal carbide and nitride (MXene) materials have shown characteristics of promising hydrogen evolution reaction (HER) catalysts, but challenges in terms of both hydrogen adsorption strength and reaction rate still need to be addressed. In addition, previous theoretical studies of MXenes for the HER focus mainly on the thermodynamics (e.g. hydrogen adsorption energy) rather than the kinetics of the reaction (e.g. energy barrier and reaction rate). In this work, we utilize high-throughput computational methods to study both the HER thermodynamics and kinetics of M₂XO₂ type MXenes and how their HER activity can be enhanced by the modification of different transition metal (TM) adatoms. Compared to the relatively weak HER activity observed for the majority of pristine MXenes, the addition of TM adatoms on the MXene surface is predicted to enhance their HER activity significantly. The presence of TM not only optimizes the Gibbs free energy of hydrogen adsorption (ΔG_H) but also reduces the H₂ production activation barrier. Intriguingly, we observed a HER mechanism preference switch from Volmer–Heyrovsky observed on pristine MXenes to Volmer–Tafel after modification with TM adatoms. On the basis of in-depth and systematic exploration of the electronic structure and interaction between hydrogen and MXenes, the origin of the mechanism preference switch is linked to the TM-induced electron redistribution on the surface of the MXene.

Two-dimensional (2D) materials have attracted considerable interest due to their remarkable properties and potential applications for nanoelectronics, electrodes, energy storage devices, among others. However, many well-studied 2D materials lack appreciable conductivity and tunable mechanical strength, limiting their applications in flexible devices. Newly developed MXenes open up the opportunity to design novel flexible conductive electronic materials. Here, using density functional theory (DFT), we investigate systematically the effects of several functional groups on the stabilization, mechanical properties, and electronic structures of a representative MXene. It is found that oxygen possesses the largest adsorption energy as compared to other functional groups, indicating its good thermodynamic stabilization. In comparison with bare and other functionalized titanium carbides, the oxygen functionalized one exhibits the most superior ideal strength; however, the premature softening of the long-wave phonon modes might limit the intrinsic strength for $\mathrm{T}{\mathrm{i}}_3{\mathrm{C}}_2{\mathrm{O}}_2$. Furthermore, the introduction of functional groups can induce a strong anisotropy under tensile loading. By analyzing the deformation paths and the electronic instability under various loadings, we demonstrate that the unique strengthening by oxygen functional groups is attributed to a significant charge transfer from inner bonds to outer surface ones after functionalization. Our results shed a novel view into exploring a variety of MXenes for their potential applications in flexible electronic and energy storage devices.