Shifts in Valence States in Bimetallic MXenes Revealed by Electron Energy-Loss Spectroscopy (EELS)

MXenes are an emergent class of two-dimensional materials with a very wide spectrum of promising applications. The synthesis of multiple MXenes, specifically solid-solution MXenes, allows fine tuning of their properties, expands their range of applications, and leads to enhanced performance. The functionality of solid-solution MXenes is closely related to the valence state of their constituents: transition metals, oxygen, carbon, and nitrogen. However, the impact of changes in the oxidation state of elements in MXenes is not well understood. In this work, three interrelated solid-solution MXene systems (Ti2-yNbyCTx, Nb2-yVyCTx, and Ti2-yVyCTx) were investigated with scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS) to determine the localized valence states of metals at the nanoscale. The analysis demonstrates changes in the electronic configuration of V upon modification of the overall composition and within individual MXene flakes. These shifts of oxidation state can explain the nonlinear optical and electronic features of solid-solution MXenes. Vanadium appears to be particularly sensitive to modification of the valence state, while titanium maintains the same oxidation state in Ti-Nb and Ti-V MXenes, regardless of stoichiometry. The study also explains Nb's influential role in the previously observed electronic properties in the Nb-V and Nb-Ti systems.

Electron-energy-loss spectroscopy and cathodoluminescence for particles inside substrate

To simulate the interaction of a nanoparticle with an electron beam, we previously developed a theoretical description for the general case of a particle fully embedded in an infinite arbitrary host medium. The theory is based on the volume-integral variant of frequency-domain Maxwell’s equations and, therefore, is naturally applicable in the discrete-dipole approximation. The fully-embedded approximation allows fast numerical simulations of the experiments for particles inside a substrate since the host medium discretization is not needed. In this work, we study how applicable the fully-embedded approach is for realistic scenarios with relatively thin substrates. In particular, we performed test simulations for a silver sphere both inside an infinite host medium and inside a finite box or sphere. For the host medium, we considered two non-absorbing cases (the denser one causes Cherenkov radiation), as well as an absorbing case. The peak positions in the obtained spectra approximately agree between substrates a few times thicker than the sphere and the infinite one. However, a much thicker substrate (of the order of μm) would be required to have a qualitative agreement for absolute peak amplitudes. The developed algorithm is implemented in the open-source code ADDA, allowing one to rigorously and efficiently simulate electron-energy-loss spectroscopy and cathodoluminescence by particles of arbitrary shape and internal structure embedded into any homogeneous host medium.