Towards ferromagnetic 2D MXenes: Thermal activation of Ti3C2Tx and Fe intercalation in ultra high vacuum

Achieving intrinsically ferromagnetic behavior of 2D materials at room temperature (RT) is > the challenge < of the 2D community. Success here promises a variety of applications, for example as novel spintronic devices or magnetic sensors. MXenes, a new family of 2D van der Waals (vdW) materials, are promising candidates in this respect. However, MAX phases from which MXenes are synthesized by selective etching of the A element are not compatible with Fe, Co or Ni. Therefore, Fe is intercalated into MXenes in ultra high vacuum (UHV) within the scope of this work to master ferromagnetic properties. First, Ti3C2Tx MXenes, functionalized by surface groups Tx : = O, −OH, −F and −Cl from the wet-chemical etching, are deposited on Si(100)/SiO2 substrates and annealed under UHV conditions up to 1000 K (thermal activation). This leads to a removal of Tx : −OH, −F and −Cl as well as H2O intercalated in between MXene sheets to various degrees as confirmed by in situ mass spectrometry and Auger electron spectroscopy (AES). As a result, the interplanar spacing between MXene sheets is decreased from initially d = 1.44 ± 0.06 nm to d = 1.21 ± 0.09 nm and d = 1.03 ± 0.07 nm, respectively, as shown by wide-angle X-ray scattering (WAXS). A 6 nm Fe film deposited on MXene samples is intercalated in situ by thermally driven diffusion at temperatures between 400 K and 1000 K. The diffusivity of Fe perpendicular to layered MXene sheets (D⊥~10-21 m2s-1 to ~10-20 m2s-1) and the activation energy of Fe diffusion (EA = 0.10 ± 0.01 eV) are remarkably low, as extracted from AES measurements, X-ray photoelectron spectroscopy (XPS) depth profiles and scanning transmission electron microscopy together with energy-dispersive X-ray spectroscopy (STEM-EDS) cross section analysis. Despite intercalated water in MXenes, Fe remains metallic under vacuum conditions and is stable in air after intercalation. In MXenes thermally activated at 1000 K, Fe penetrates up to 25 nm, whereby the interplanar spacing does not change, as WAXS measurements have shown. Here, near-surface Fe clusters are formed in voids of MXene samples showing a bcc structure as demonstrated by X-ray absorption near-edge structure (XANES) spectroscopy and the X-ray linear dichroism (XLD) at the Fe K-edge. For lower thermal activation temperatures Tth < 675 K, 35 times higher diffusivities of Fe into stacked MXenes as well as a higher penetration depth are observed. Furthermore, XANES and XLD spectra at the K-edge of Fe indicate a formation of Fe3C and/or Fe in a local environment similar to that of Fe3C, e.g. upon binding to MXene sheets as a termination species. Diffusion of Fe in between MXene sheets is supported here by a systematic upward deviation of the interplanar spacing (d = 1.39 ± 0.11 nm) after Fe intercalation, based on X-ray diffraction (XRD) measurements. For MXenes, thermally activated at Tth < 675 K with intercalated Fe, vibrating sample magnetometry (VSM) measurements at 300 K in in-plane geometry show hysteresis loops with a saturation magnetization of 740 ± 130 kA/m and a coercive field of 58 ± 1 mT. At T < 100 K there is a significant increase of the coercive field, which amounts to 239 ± 1 mT at 5 K. The Curie temperature equals TC = 544 ± 8 K and is 60 K higher than the one of Fe3C (TC = 485 K). Moreover, X-ray magnetic circular dichroism (XMCD) spectra at the Fe K-edge at 300 K show spectroscopic features of Fe3C, but also additional features pointing to diverse atomic configurations of intercalated Fe in MXenes thermally activated at temperatures Tth < 675 K.