Ion Implantation Induced Structural Modifications and Functionalization of Ti3C2Tx MXenes
Authors: Pazniak Hanna , Bilyk Thomas , Chartier Patrick , Hurand Simon , Marteau Marc , Pacaud Jérôme , Liedl Andrea , Busby Yan , Noel Celine , Houssiau Laurent , Larciprete Rosanna , Lacovig Paolo , Lizzit Daniel , Tosi Ezequiel , Lizzit Silvano , Benchakar Mohamed , Célérier Stéphane , Mauchamp Vincent , David Marie-Laure 
Published: July 2021

MXenes are single or few-layered two-dimensional (2D) transition metals carbides and/or nitrides successfully synthesized in 2011, with chemical composition Mn+1XnTx, where M stands for an early transition metal, X is C and/or N, Tx are a surface terminations and n = 1, 2, or 3 [1]. These materials are considered as promising candidates in many applications such as energy storage, sensing, catalysis, and optoelectronics. For instance, MXenes combine high conductivity and hydrophilicity making them interesting candidates for applications involving aqueous media and requiring good electronic properties or easy processing. Moreover, they exhibit a wide chemical variability thanks to their parent MAX phases precursors, a 150-plus member family of layered ternary carbides or nitrides with chemical composition Mn+1AXn (where the A-element mainly belong to the columns 13 and 14 of the periodic table). This offers the unique opportunity to design the MXenes on demand, with considerably different physicochemical properties for target applications [2-4]. Tuning the MXene properties is also possible through the manipulation of their surface termination groups T (i.e., T = −F, −OH, −O, or −Cl). The crucial role of the surface groups has recently triggered many studies aiming at controlling and characterizing the MXene surface structure/chemistry mainly by using different etching protocols [5]. However, the nature of the surface groups, inherited from the A element exfoliation step of the MAX phases by chemical reactions, is hardly tunable by this mean except by changing the etching protocole which might have significant impact on the quality of the synthetized MXenes.
Here, we demonstrate that medium energy ion implantation can be used to modify the MXene surface functionalization, through the controlled formation of structural defects and the incorporation of foreign species in Ti3C2Tx thin films fabricated by spin coating (Fig.1 a-b). Defects created by Mn implantation in a wide fluence range (1x1014-5x1016 cm-2) are characterized at different depths by XPS on Ti2p core-level spectra, ToF-SIMS and EELS analyses. For the latest, we use an original approach that we have developed [6], allowing an estimation of the level of damage in the material by studying the C-K edge. Our results show that the 60 keV Mn implantation leads to successful incorporation of Mn in the Ti3C2 thin film. This is accompanied by the structural distortion of the Ti-C layers as evidenced by EELS experiments (Fig. 1c), preferential Ti sputtering as compared to C without any significant amorphization or phase transformation up to a fluence of 1x1016 cm-2. However, for the highest fluence studied (5x1016 cm-2), complete amorphization was observed. For the subthreshold amorphization fluence, the study of the fine structure of the O K-edge and of the electron diffraction pattern suggest the formation of extended defects in the Ti layers including the occupation of the defective Ti sites by oxygen, leading to local environment for the oxygen atoms like graphene oxide. Finally, our results show that the number of defects can be directly and in a reproductible way tunned by the implantation fluence.
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Figure 1