Abstract
ReSe2 and ReS2 are unusual compounds amongst the layered transition metal dichalcogenides as a result of their low symmetry, with a characteristic in-plane anisotropy due to in-plane rhenium ‘chains’. They preserve inversion symmetry independent of the number of layers and, in contrast to more well-known transition metal dichalcogenides, bulk and few-monolayer Re-TMD compounds have been proposed to behave as electronically and vibrational decoupled layers. Here, we probe for the first time the electronic band structure of bulk ReSe2 by direct nanoscale angle-resolved photoemission spectroscopy. We find a highly anisotropic in- and out-of-plane electronic structure, with the valence band maxima located away from any particular high-symmetry direction. The effective mass doubles its value perpendicular to the Re chains and the interlayer van der Waals coupling generates significant electronic dispersion normal to the layers. Our density functional theory calculations, including spin-orbit effects, are in excellent agreement with these experimental findings.
Highlights
The layered transition metal dichalcogenides (TMDs) family includes a rich palette of superconductors, metals[1] and semiconductors with direct and indirect gaps, and offers fascinating possibilities for the realisation of nanoscale electronic, optoelectronic and photonic devices through the assembly of heterostructures[2]
One of the most illuminating modes of angle-resolved photoemission (ARPES) measurement nowadays is to monitor with high energy and angular resolution the photoemission signal from states in a given energy window near the Fermi level as a function of electron wavevector parallel to the crystal surface, since it is this wavevector component that is conserved in photoemission[35]
This discussion was based initially on optical studies of few-micron sized bulk samples and was extended to the monolayers as these became available; a consensus is gradually emerging that ReSe2 has an indirect band gap with a valence band maximum located away from the Brillouin zone centre, and that it remains indirect down to one monolayer, whilst ReS2 was claimed until recently to have a direct gap at all thicknesses
Summary
The layered TMD family includes a rich palette of superconductors, metals[1] and semiconductors with direct and indirect gaps, and offers fascinating possibilities for the realisation of nanoscale electronic, optoelectronic and photonic devices through the assembly of heterostructures[2]. The TMD family includes materials which do not conform to the typical expectations above[1], and this much less well-known group of TMDs expands the range of possible heterostructures One such material is ReSe2 (and the closely-related ReS2), as discussed in a recent review[6], in which the only symmetry operation is inversion[10,11,12,13]. Even if a full understanding of the momentum-resolved electronic structure of ReSe2 is complex due to its triclinic crystal structure, the two-fold theoretical and experimental approach taken here allows us to identify the electronic hallmark of this compound as well as how the bulk band structure relates to that of Re-TMD monolayers
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