Mastering Electrons in LOw Dimensional Innovative Carbides (MELODIC) – MELODIC

Crystal growth.
Mechanical exfoliation.
e-beam lithography.
Electrical characterization.
Various Microscopy techniques (STM, EFM, KPFM).

1/ we show, for the first time, that mechanical exfoliation using the adhesive tape method can be used on nanolamellar phases where the bonding is not weak. It thus opens the door for others to try and exfoliate layered solids that, to date, have been deemed impossible to mechanically exfoliate because of the interlayer strengths.
2/ One can now produce MAXenes, the MXenes counterparts of which have not been possible by chemical etching such as Cr2AlC.
3/ One can apply the exfoliation process independently of the nature of the A element. For example, to date Ti2SnC has been unetchable, but as shown herein can be mechanically exfoliated.
4/ It is now possible to produce few layers-thick MX layers wherein the A layers remain. This is of a considerable interest in the case of more “exotic” MAX phases, such as the Mo4Ce4Al7C3 ferromagnetic phase.
5/ Like in graphene, it is reasonable to assume that the mechanically exfoliated flakes would be significantly less defective than those that were obtained by etching and thus will allow for a more in-depth understanding of intrinsic electronic properties of these new materials.

We will investigate the predicted semiconducting nature of some MXenes. The hydrophilicity and optimal surface-to-volume ratio of MXenes opens the door to biological sensing applications. Our study will allow us to quantify the extent to which such 2D systems can be made sensitive to charge variation or polarization close to their surface

1.D. Pinek, T. Ito, K. Furuta, Y. Kim, M. Ikemoto, S.I. Ideta, K. Tanaka, M. Nakatake, P. Le Fèvre F. Bertran and T. Ouisse, “Near Fermi level Ti3SiC2 electronic structure revealed by angle-resolved photoemission spectroscopy”, Phys. Rev. B 102, 075111 (2020)
2.Y. Kim, A. Gkountaras, O. Chaix-Pluchery, I. Gélard, J. Coraux , C. Chapelier, M.W. Barsoum and T. Ouisse, “Elementary processes governing V2AlC chemical etching in HF”, RCS Advances 10, 25266–25274 (2020)
3.A. Gkountaras, Y. Kim, J. Coraux, V. Bouchiat, S. Lisi, M.W. Barsoum and T. Ouisse, “Mechanical Exfoliation of Select MAX Phases and Mo4Ce4Al7C3 Single Crystals to Produce MAXenes”, Small 16, 1905784 (2020)
4.A. Champagne, F. Ricci, M. Barbier, T. Ouisse, D. Magnin, S. Ryelandt, T. Pardoen, G. Hautier, M.W. Barsoum and J.C. Charlier, “Insights into the elastic properties of RE-i-MAX phases and their potential exfoliation into two-dimensional RE-i-MXenes”, Phys. Rev. Mat. 4, 013604 (2020)
5.T. Ouisse, D. Pinek and M.W. Barsoum, “Modelling in-plane magneto-transport in Cr2AlC”, Ceramics Int. 45, 22956-22960 (2019)
6.A. Champagne, O. Chaix-Pluchery, T. Ouisse, D. Pinek, I. Gélard, L. Jouffret, M. Barbier, F. Wilhelm, Q. Tao, J. Lu, J. Rosen, M. W. Barsoum, and J.-C. Charlier, “First-order Raman scattering of rare-earth containing i-MAX single crystals (Mo2/3RE1/3)2AlC (RE = Nd, Gd, Dy, Ho, Er)”, Phys. Rev. Mat. 5, 053609 (2019)

Our project is motivated by the recent discovery of a large (> 20) family of 2D materials labelled MXenes. They are derived from the nanolayered Mn+1AXn or MAX phases, where M is a transition metal, A is an A-group element, X is C or N. The 3D to 2D transformation is brought about by selectively etching the A layers, resulting in 2D flakes with various surface terminations Ts. All MXenes synthesized to date have been hydrophilic 2D metals characterized by a huge density of states at the Fermi energy, even if theory predicts that it is possible to open a bandgap by an appropriate choice of T. Little is currently known about MXene monolayers and their heterojunctions with other 2D materials. MXenes are the functional nanomaterials forming the building blocks of an impressive number of potentially useful industrial applications: they have demonstrated extreme capacitance values (1000 Fcm-3), high electrical conductivities, excellent electromagnetic shielding ability, electrode materials for Li and multivalent ion batteries, electrochemical catalytic surfaces, hydrogen storage and sensors, among many other unique and exciting properties and characteristics with high technological potential. In all previously published data, there is a clear lack of quantitative information about the basic electron properties. Yet the latter ultimately control the performance of any of the applications cited above. Also lacking is a precise appreciation of the role of surface functionalization on performance. Our project, albeit standing aside from direct applications, will fill this gap by producing a wealth of data permitting in turn a thorough knowledge of the fundamental properties of MXene monolayers.

Our goal is to develop appropriate technological processes to isolate large area single MXene flakes with tunable surface functionalizations and to characterize their properties – mostly electronic – as well as the properties of vertically stacked hetero-junctions with other 2D materials, such as graphene or BN. The end goal is to obtain a quantitative and qualitative overview of the intrinsic properties of functionalized MXenes, and to fabricate heterojunctions or gated devices with unique and exceptional characteristics. This will allow us to identify and circumscribe the application domains where those new 2D materials offer sustainable competitive advantages over alternate competing materials.
MXene flakes are obtained from MAX powders, which limits their size to about 1µm2. LMGP is the only lab growing single crystals of macroscopic size. The latter will allow us to produce better controlled MXene layers. We will most particularly focus on two aspects:

1/ We will synthesize and deposit large area MXene monoflakes of various chemistries; control their Ts; measure the resulting magnetotransport properties as a function of gating voltages and electrolytes; create vertical heterojunctions with other 2D materials At this time it is fairly widely appreciated that 2D heterojunctions can result in new phenomena and physics. We thus wish to understand electron transport in MXenes and create functional materials with unique characteristics. Connecting high conductivity 2D metals to existing 2D semiconductors and insulators is indeed urgently needed in most innovative 2D-based applications. MXenes could fill this gap and operating MXene-based heterojunctions forms one of our ultimate goals.
2/ We will investigate the predicted semiconducting nature of some MXenes. The hydrophilicity and optimal surface-to-volume ratio of MXenes opens the door to biological sensing applications. Our study will allow us to quantify the extent to which such 2D systems can be made sensitive to charge variation or polarization close to their surface. We will also engineer M-vacancies in metallic MXenes so as to reduce the conductivity down to values acceptable for bio-sensing. Fulfilment of a demonstrator involving DNA hybridization is our second main objective.