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Project Topic
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Discovered in 2011, MXenes are by far the youngest family of 2D materials known and thus the least understood. MXenes are so called because they are derived from the Mn+1AXn (or MAX) phases where M is an early transition metal, A is an A-group element (mostly 13 and 14), X is C and/or N. To convert the MAX phases to their MXenes, the A-layers – the vast majority of which are Al - are selectively etched using HF-containing solutions. The A-layers are replaced by surface terminations, T, (-O, -OH and –F) which is why their proper designation is Mn+1XnTx. In 2017, one of the PI's (IFM) of this proposal predicted and then synthesized quaternary compounds with the formula (M'2/3,M"1/3)2AlC, where M' and M'' are two metals, one of which is an early transition metal. What distinguishes these phases from all other MAX phases is the in-plane ordering of the M' and M" atoms, which is why we are labelling them i-MAX phases. The same group established the existence of i-MAX phases with the chemical formula: (M2/3RE1/3)2AlC, where RE = Ce, Pr, Nd, Sm, Gd, Tb, Dy, Er, Ho, Tm and Lu. These phases were also predicted to be and indeed found to be magnetic. Members of a simultaneously discovered, similar layered family, with stoichiometry M4RE4Al7C3, were found to be ferromagnetic (Fig. 2b). Since all these phases are Al-containing, the Al should be readily selectively etched to create 2D RE-i-MXenes, where the RE elements are ordered in the basal planes. The goal of the proposed work is to fundamentally understand the electronic and magnetic properties of single RE-i-MXenes layers. The latter will be produced in two ways: i) mechanical exfoliation of large single crystals and, ii) chemical exfoliation/etching of powders. The ultimate objective is to deposit relatively large (100 μm2) single layers on SiO2/Si substrates and characterize them. The two approaches should result in different terminations, that in turn will elucidate the crucial role that surface terminations and etching induced defects play on the transport, and more importantly, magnetic properties. The work involves 4 labs: i) LMGP in Grenoble, is the only lab in the world that routinely produces large MAX single crystals. It will grow RE-i-MAX crystals. ii) INEEL, also in Grenoble, will mechanically exfoliate the crystals and, along with the UCL group in Louvain, will concentrate on processing devices and measuring the low temperature magnetic properties of both Re-i-MAX and RE-i-MXenes phases. iii) IFM in Sweden will be responsible for supplying high purity powders to the other partners and continuing in their trail blazing efforts to understand the magnetism of these phases by DFT calculations. iv) UCL will measure and simulate - using ab initio calculations - magnetotransport and vibrational (Raman) properties of both the Re-i-MAX and RE-i-MXenes, with a particular focus on the role of surface termination in the case of the latter. The possibility to significantly increase the palette of available of metallic/magnetic MXene 2D crystals is important in the perspective of "full 2D" electronic and spintronic devices, as they could be used both as interconnects with low electrical resistivities and low contact resistances. Most importantly if 2D single sheets are produced, such a major breakthrough can lead to spin injection/detection.
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