Engineering the Electronic Properties of Two‐Dimensional Transition Metal Dichalcogenides by Introducing Mirror Twin Boundaries

Abstract

Grain boundaries in 2D materials can have marked influence on the material properties. The effects can be not only detrimental, but also beneficial in transition metal dichalcogenides (TMDs), so that controlling the density and type of the boundaries in these systems should be important for engineering their properties. However, this is often possibly only during the growth stage. Molybdenum and tungsten dichalcogenides feature a particular set of 60° mirror twin boundaries, which are reported to occur upon merging of the growing flakes, to appear during growth to accommodate for the nonstoichiometry of the sample, or to be produced a posteriori by electron irradiation or thermal annealing. Furthermore, different preparation conditions lead to different atomic structure of the boundary, which consequently exhibit different electronic properties. This has obviously garnered interest for the ability to control grain boundary types and densities. In this progress report, the recent experimental and theoretical work related to the characterization of mirror twin boundaries is reviewed. A consistent set of formation energies for the mirror twin boundaries is provided, which then allows a coherent picture on the formation mechanisms under different conditions to be drawn. Finally, the electronic structure of these boundaries is analyzed and their potential applications are discussed.

Metallic Twin Boundaries Boost the Hydrogen Evolution Reaction on the Basal Plane of Molybdenum Selenotellurides

“…[56] It has to be noted, however, that the amount of edges and defects is quite low in our model ultrathin films and their electrochemistry is ruled mainly by the intrinsic activity of the basal plane. Therefore, the relatively high chemical activity observed in our MoTe 2 samples is likely associated with the presence of the high density of the metallic twin boundaries in our MBE-grown films, which, on the one hand, also improves the electronic transfer capability of the films, [10,45,58] and as we discuss below may also possess special chemical properties. Moreover, these boundaries are 6-7 times denser in MoTe 2 than MoSe 2 (see Figure S2, Supporting Information).…”

“…[58] These defects can be observed in several types of chalcogenides and can be obtained using several approaches (direct synthesis by MBE or electron beam bombardment or vacuum annealing). [44,58] Their special electronic structure and versatile geometry made them quite interesting to impart TMCs at the nano-or macroscale, [60] with special functional properties such as metallic conductions and unprecedented chemical activity. For the first time, we demonstrated their potential for boosting the HER activity in tellurides, however their presence in several chalcogenides [61][62][63] suggests that the same effect can be reproduced on several other materials.…”

“…[29] Theoretical calculations suggest that tellurides should have a lower activity than sulfides and selenides, in fact the Gibbs free energy of adsorption of hydrogen on MoTe 2 metallic edges is calculated to be 0.57 [29] or 0.77 eV, [27] whereas is almost thermoneutral for MoSe 2 edges and defects. Therefore, the relatively high chemical activity observed in our MoTe 2 samples is likely associated with the presence of the high density of the metallic twin boundaries in our MBE-grown films, which, on the one hand, also improves the electronic transfer capability of the films, [10,45,58] and as we discuss below may also possess special chemical properties. Therefore, the relatively high chemical activity observed in our MoTe 2 samples is likely associated with the presence of the high density of the metallic twin boundaries in our MBE-grown films, which, on the one hand, also improves the electronic transfer capability of the films, [10,45,58] and as we discuss below may also possess special chemical properties.…”

“…[58] These defects can be observed in several types of chalcogenides and can be obtained using several approaches (direct synthesis by MBE or electron beam bombardment or vacuum annealing). This novel strategy is similar to the exploitation of 1T polymorphs in WS 2 and MoS 2 that have metallic properties and therefore show HER activity not only at edges but also in the basal plane.…”

Tomonaga-Luttinger Liquid in a Box: Electrons Confined within MoS2 Mirror-Twin Boundaries

“…Their structures were unambiguously determined by transmission electron microscopy [21][22][23], and density functional theory invariably predicted the MTBs to host one-dimensional, metallic states [24][25][26][27] that are protected through the large band gap of approximately 2 eV in the surrounding 2D-layer. Intense research yielded partly conflicting results regarding the electronic structure of a specific MTB in a monolayer of MoSe 2 resting on a van der Waals substrate [19,28,29], namely the 4|4P MTB consisting of 4-fold rings sharing a point at the chalcogene site [24,30]. By using room temperature as well as low-temperature (4K) STM and STS, Liu et al [28] found a quantum well state emerging from the finite length of the interpenetrating MTBs.…”