The Broad Chromatic Range of Two‐Dimensional Transition Metal Carbides

Transition metal carbides and nitrides (MXenes) are a relatively new class of 2D materials, which include metallic and semiconducting examples. The chemical compositions of the vast MXene family span a broad range of the periodic table, yet the optical spectra of only a handful of MXene compositions have been reported, and in only one example are the spectroscopic features assigned. The optical properties of nine carbide MXenes, representing a systematic variation of chemical composition and atomic structure are presented here. The observed optical phenomena span the UV to IR and include interband transitions and plasma excitations. The spectral features involving excitation of the plasma provide an optical readout of the composition‐dependent carrier concentration, revealing even subtle changes due to modification of the surface chemistry. The high carrier concentration found in many MXenes differentiates them from other known 2D materials and the authors present evidence in favor of the hypothesis that MXenes host optically active plasmon resonances that naturally span the UV to near‐IR, as a function of composition. This study introduces optical properties of novel MXenes, benefits the interpretation of 2D materials spectroscopy, and suggests the tremendous potential for this class of optoelectronic building blocks. 2D transition metal carbides (MXenes) show a broad spectroscopic response to electromagnetic radiation. Characterization of colloidal solutions and corresponding thin films supported on glass reveals distinct optical features across the UV, visible, and near‐IR regimes.