Layer-dependent anisotropic electronic structure of freestanding quasi-two-dimensional Mo S 2
KAUST DepartmentAdvanced Nanofabrication and Thin Film Core Lab
Imaging and Characterization Core Lab
Physical Sciences and Engineering (PSE) Division
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AbstractThe anisotropy of the electronic transition is a well-known characteristic of low-dimensional transition-metal dichalcogenides, but their layer-thickness dependence has not been properly investigated experimentally until now. Yet, it not only determines the optical properties of these low-dimensional materials, but also holds the key in revealing the underlying character of the electronic states involved. Here we used both angle-resolved electron energy-loss spectroscopy and spectral analysis of angle-integrated spectra to study the evolution of the anisotropic electronic transition involving the low-energy valence electrons in the freestanding MoS2 layers with different thicknesses. We are able to demonstrate that the well-known direct gap at 1.8 eV is only excited by the in-plane polarized field while the out-of-plane polarized optical gap is 2.4 ± 0.2 eV in monolayer MoS2. This contrasts with the much smaller anisotropic response found for the indirect gap in the few-layer MoS2 systems. In addition, we determined that the joint density of states associated with the indirect gap transition in the multilayer systems and the corresponding indirect transition in the monolayer case has a characteristic three-dimensional-like character. We attribute this to the soft-edge behavior of the confining potential and it is an important factor when considering the dynamical screening of the electric field at the relevant excitation energies. Our result provides a logical explanation for the large sensitivity of the indirect transition to thickness variation compared with that for the direct transition, in terms of quantum confinement effect.
CitationLayer-dependent anisotropic electronic structure of freestanding quasi-two-dimensional Mo S 2 2016, 93 (7) Physical Review B
SponsorsThis work is financially supported by the National Basic Research Program of China (Grants No. 2014CB932500 and No. 2015CB921004) and National Science Foundation of China (GrantsNo. 51222202 and No. 51472215). The research reported in this paper was partially supported by King Abdullah University of Science and Technology (KAUST). J.Y. acknowledges Pao Yu-Kong International Foundation for a visiting Chair Professorship in ZJU and EPSRC and Royal Society for partial support. Dr. Ray Egerton and Dr. He Tian are kindly acknowledged for critical reading, comments, and revisions. J.H. and K.L. contributed equally to this work.
PublisherAmerican Physical Society (APS)
JournalPhysical Review B