Irène Cucchi (group of F. Baumberger) receives the Flatlands 2019 poster prize for her work on two-dimensional (2D) materials entitled “Microfocus laser-ARPES on encapsulated mono-, bi- and few-layer 1T’-WTe2”. In this study, researchers at the University of Geneva measured for the first time the electronic structure of exfoliated air-sensitive flakes of van der Waals materials. The unique facility at the department of quantum matter physics allows to perform such measurements with unprecedented resolution.
Just like graphene, transition metal dichalcogenides can be exfoliated down to a few crystalline sheets and hold much potential for the realization of novel electronic phases linked to their two-dimensional (2D) nature. Among them, the semimetallic 1T’-WTe2 (tungsten ditelluride) lies at the frontier between 2D materials and topological physics. For the monolayer, transport measurements report edge conduction consistent with a quantum spin Hall insulating state up to 100 K, a newly discovered state of matter where the bulk is insulating while the edges can still carry a current. In bilayer form and above 20 K, WTe2 belongs to the very small group of polar metals.
Understanding these phases is difficult, because little is known from experiment about the momentum space electronic structure of ultrathin crystals. Angle-resolved photoelectron spectroscopy (ARPES), the most direct probe of electronic band structures, proved challenging on exfoliated WTe2 flakes because of their high chemical reactivity, small lateral size, and because of the small relevant energy scales in this compound. At the University of Geneva, these difficulties could be overcome by combining two advances. First, researchers in the group of Prof A. Morpurgo encapsulated such a WTe2 flake with monolayer graphene, preventing the dichalcogenide from reacting with oxygen. Second, the group of Prof F. Baumberger, developed a laser-based micro-ARPES system that combines a spatial resolution of 2 µm with sub-meV energy resolution and permits for the first time lab-based ARPES measurements of exfoliated flakes.
These data, published in Nano Letters, support the recent identification of the quantum spin Hall state in monolayer WTe2 and reveal strong signature of the broken inversion symmetry in the bilayer. More generally, this measurement opens the way for electronic structure measurements with ARPES on previously inaccessible layered materials.