Abstract
Each plane of layered ReS2 and ReSe2 materials has 1D chain structure, from which intriguing properties such as 1D character of the exciton states and linearly polarized photoluminescence originate. However, systematic studies on the 1D character of charge carriers have not been done yet. Here, we report on systematic and comparative studies on the energy-momentum dispersion relationships of layered transition metal dichalcogenides ReS2 and ReSe2 by angle resolved photoemission. We found that the valence band maximum or the minimum energy for holes is located at the high symmetric Z-point for both materials. However, the out-of-plane ({k}_{z}) dispersion for ReSe2 (20 meV) is found to be much smaller than that of ReS2 (150 meV). We observe that the effective mass of the hole carriers along the direction perpendicular to the chain is about 4 times larger than that along the chain direction for both ReS2 and ReSe2. Remarkably, the experimentally measured hole effective mass is about twice heavier than that from first principles calculation for ReS2 although the in-plane anisotropy values from the experiment and calculations are comparable. These observation indicate that bulk ReS2 and ReSe2 are unique semiconducting transition metal dichalcogenides having strong one-dimensional characters.
Highlights
Very recently, layered ReS2 and ReSe2 began to draw attention as their layer-layer interactions are much weaker than other layered TMDs12
As the above-mentioned intriguing 1D properties are determined by the characteristics of hole and electron band dispersions, electronic structure studies by angle resolved photoemission spectroscopy (ARPES) are naturally desired
(1) The valence band maximum (VBM) is located at the Z-point for both systems, while it was reported in a previous ARPES study reported that VBM of ReSe2 may be located at non-high-symmetric momentum point[30,33]
Summary
Very recently, layered ReS2 and ReSe2 began to draw attention as their layer-layer interactions are much weaker than other layered TMDs12. High-resolution transmission electron microscopy and electron diffraction studies reveal distorted 1T structure with Re chain formation[12] Such structure distortion is directly related to the high anisotropy in the Raman response from in-plane lattice vibrations[14,15,16,17,18,19]. The effective mass is a fundamental parameter that can be obtained from the energy and momentum dispersion relationship and governs the electrical and optical properties such as mobility, conductivity, light absorption and photoluminescence. A couple of ARPES studies on ReS2 and ReSe2 have been reported and the results indicate an in-plane anisotropy in the hole band dispersion as expected from the chain structures in ReS227–29 and in ReSe230. Our data show a striking difference from what were reported in previous experimental and theoretical studies29,30,32. (1) The valence band maximum (VBM) is located at the Z-point for both systems, while it was reported in a previous ARPES study reported that VBM of ReSe2 may be located at non-high-symmetric momentum point30,33. (2) The kz dispersion of ReSe2 is much smaller than that of published quasiparticle band structure within the LDA + GdW approximation32. (3) The effective hole masses along and perpendicular to the chain direction are quite different from the reported experimental and theoretical values[29,30,34]
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