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Dr. Rafael Besse

Fields of interest

2D materials and heterostructures

DFT / TDDFT charge dynamics

PublicationsGoogle Scholar

 

Research

Ab initio Studies of Two-Dimensional Transition Metal Dichalcogenides: Transition metal dichalcogenides are a prominent class of compounds in the context of two-dimensional materials due to their unique electronic and optical properties. The intense research activity on these materials in the past decade has opened up several routes to understand and exploit their properties and the phenomena observed in them. In this talk I will present a summary of contributions to the understanding of the physical properties of two-dimensional transition metal dichalcogenides from theoretical investigations based on density functional theory and time-dependent density functional theory combined with molecular dynamics. (1) The Peierls transition mechanism has an important role in the stabilization of the distorted octahedral phase of MoSe2 and a change in the energetically prefered phase induced by nanoflakes sizes was demonstrated. (2) With a wide exploration of dichalcogenides of transition metals of groups 8 to 11, the contrast between weak interlayer binding and strong contribution of chemical bonds was observed. Semiconductor monolayers were identified among these compounds and the chemical trends of their band offsets were explained. (3) Although interlayer binding is dominated by weak interactions, interlayer coupling can significantly influence band gaps of van der Waals heterostructures and two crucial mechanisms were idetified, namely, the interlayer hybridization of electron states and the formation of electric dipole at the interface. (4) In the MoS2/PtSe2 heterobilayer, it was found that photoexcited electrons in MoS2 transfer to the PtSe2 layer at a faster rate than hole transfer, leading to an effective charge separation, despite the type-I band alignment. The level crossings induced by the interfacial dipole caused by the imbalance in charge transfer have an important contribution to affect the rate of charge transfer.

To improve our understanding of how the stacking order can affect the physical properties of these systems, we present a density functional theory study of the energetic, electronic, and optical properties of vdW heterostructures of graphene (Gr)−transition-metal dichalcogenides (TMDs), namely, monolayer (graphene, MoS2, and WS2), bilayer, and tetralayer systems. We found that the Gr−TMD interactions are dominated by mainly vdW interactions, while the TMD−TMD systems also display a weak hybridization contribution of out-of-plane orbitals, which enhances the binding energy. An analysis of the bilayer electronic structure reveals that a stronger interaction induces small band gap deviations from Anderson’s rule. Thus, the key effect of the stacking order in tetralayer systems is primarily to control the level of interaction between TMD layers, thereby controlling the TMD band gap. 

Vertical stacking of two-dimensional materials with weak van der Waals (vdW) interactions has laid the ground for breakthroughs in physics as well as in technological applications. Although vdW interactions dominate interlayer binding, interlayer electronic coupling may not be negligible and can lead to properties beyond the superposition of constituent monolayers. Here, studying heterobilayers of transition-metal dichalcogenides (MQ2; M = Mo, Ni, Pt; Q = S, Se) by means of density functional theory calculations, we show two mechanisms that influence the band gaps of vdW heterostructures beyond the Anderson rule: (1) interfacial hybridization (mainly involving out-of-plane states, such as chalcogen pz-states), which leads to an upshift in the valence band maxima and accordingly a decrease in the band gap. (2) Formation of an interfacial electric dipole, resulting in an effective gap increase in type-II junctions. While the former is material specific, depending on the proximity of pz-states to each other and the valence band maxima, the latter can be generally described using a model based on the charge density decay outside the monolayers and the pristine band edge positions with respect to the vacuum level, irrespective of junction type.