MXenes are a family of 2D transition metal carbide or nitride layers obtained from the exfoliation of nanolaminated ceramics called MAX phases. Due to the chemical diversity of the MAX phase precursors, MXenes are one of the richest family of 2D materials in terms of chemical variety and physical properties. Their chemical formula is $M_{n+1}X_nT_z$ (n = 1, 2 or 3), with M a transition metal, X being C and/or N, and T are surface terminations (e.g. O(H), F, Cl) inherited from the exfoliation process. Thanks to their hydrophilicity, MXene 2D-layers can be easily dispersed as stable colloidal suspensions which enables their easy processing as thin films by different approaches (spin-coating, spin-casting, spray-casting, etc). Combined with their very good electrical conductivity, this makes these materials very promising for various applications, including transparent conductive electrodes or energy storage devices. In this context, the functional groups $T_z$ have a very strong influence on MXene physical properties and among the several (physico)chemical approaches proposed to act on this lever, ion irradiation is a very promising route.
In this work, we demonstrate the efficiency of He+ irradiation in tuning the optical properties of $Ti_3C_2T_z$ spin-coated thin films. The introduction of a controlled amount of structural modifications both on the surface and into the MXene layers, by tuning the irradiation fluence, allows for the increase of the thin films transparency from IR to UV, while preserving the electrical conductivity. The modification of the properties is rationalized using a combination of structural, electronic and optical characterization methods including (S)TEM-EELS, thereby giving insights on the correlation between structural defects, and the induced modifications of the Ti-d bands which are responsible for the optical/electrical properties in this system. This very innovative method can be generalized to other chemistries as well as upscaled and can thus be expected to become one of the standard approaches to tailor MXenes properties.