Synthesis and characterization of Mo- and W-based atomic laminates

Abstract: Mn+1AXn (MAX) phases are inherently nanolaminated compounds based on transition metals (M), A group elements (A), and carbon or/and nitrogen (X), which exhibit a unique combination of ceramic and metallic properties. My thesis work has focused on exploring novel MAX phase chemistries, including elemental combinations beyond those traditionally used for MAX phases, and their graphene-analogous 2D counterpart, MXenes.  The first part of the thesis investigates Mo-based MAX phases, which are among the least studied, despite indication of superconducting properties and potential for derivation of Mo-based MXenes. Initially, I performed theoretical calculations focused on evaluation of phase stability of the Mon+1GaCn MAX phases, and synthesized the predicted stable Mo2GaC in thin film form using DC magnetron sputtering. Close to phase pure epitaxial films were grown at ~590 °C, and electrical resistivity measurements using a four-point probe technique suggest a superconducting behavior with a critical temperature of ~7 K. The follow-up of this work, was synthesis of a new MAX related material, Mo2Ga2C, also by means of DC magnetron sputtering. The theoretical predictions as well as the materials characterization by X-ray diffraction and neutron powder diffraction, suggested a Ga bilayer interleaved between a set of Mo2C blocks, arranged in a simple hexagonal structure.   It is known that selectively etching of the A-layer in a MAX phase, shown for A=Al, can lead to realization of a MXene. Hence, the next step in my research was to explore the possibility of etching of A=Ga in Mo2GaC as well as in Mo2Ga2C, targeting a Mo2C MXene, as motivated by theoretically proposed superior thermoelectric properties of this 2D material. While Mo2GaC did not allow removal of the A-layer, I showed that Mo2C MXene could be realized from etching Mo2Ga2C thin films, removing the Ga bilayer, in 50% hydrofluoric acid at a temperature of ~50 °C for a duration of ~3 h. Hence, the results did not only produce the first Mo-based MXene, it also showed that MXenes can be obtained for etching A-elements other than Al. This, in turn, increase the pathways for expanding the family of MXenes.    I thereafter set out to explore the magnetic properties resulting from Mn-alloying of the non-magnetic Mo2GaC MAX phase. For that purpose, (Mo,Mn)2GaC was synthesized using a  DC magnetron sputtering system with Ga and C as elemental targets and a 1:1 atomic ratio  Mo:Mn compound target. Heteroepitaxial films on MgO(111) substrates were grown at  ~530 °C, as confirmed by X-ray diffraction. Compositional analysis using energy dispersive X-ray spectroscopy showed a 2:1 ratio of the M- and A-elements and a 1:1 ratio for the Mo and Mn atoms in the film. Vibrating sample magnetometry was utilized to measure the magnetic behavior of the films, showing a magnetic response up to at least 300 K, and with a coercive field of 0.06 T, which is the highest reported for any MAX phase to date.  The second part of my research has been dedicated to realizing new MAX phase related, chemically ordered compounds and their MXene derivatives, and to initiate exploration of their properties. Materials synthesis was performed by pressureless bulk sintering, and inspired by theoretical calculations we showed evidence for a new so called o-MAX phase, Mo2ScAlC2, with an out-of-plane chemically ordered structure. It is the first experimentally verified Sc-containing MAX phase, which makes its corresponding MXene, Mo2ScC2, also presented in this work, the first MXene including Sc. Moreover, I discovered two so called i-MAX phases including W, (W2/3Sc1/3)2AlC and (W2/3Y1/3)2AlC, which display in-plane chemical ordering in the M-layer. Furthermore, both was shown to allow synthesis of their corresponding 2D counterpart; W1.33C MXene, with ordered vacancies.  Initial test on these novel MXenes showed a high potential for hydrogen evolution reaction.  Altogether, I have in my thesis work realized 6 novel MAX phases and related materials, and have shown evidence for 4 new MXenes. These materials inspire a wide range of future studies, with respect to fundamental properties as well as potential for future applications.   

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