AVS 64th International Symposium & Exhibition
    2D Materials Focus Topic Monday Sessions
       Session 2D+EM+MI+MN-MoM

Invited Paper 2D+EM+MI+MN-MoM10
Properties of Single Layer Transition Metal Dichalcogenides Grown by Van der Waals Epitaxy

Monday, October 30, 2017, 11:20 am, Room 15

Session: Properties of 2D Materials including Electronic, Magnetic, Mechanical, Optical, and Thermal Properties
Presenter: Matthias Batzill, University of South Florida
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It is well documented that the electronic properties of transition metal dichalcogenides (TMDs) vary as their dimensions are reduced to a single layer. Also, variations depending on the substrate have been reported. In our studies we grow single- to few- layers of TMDs by molecular beam epitaxy on van der Waals substrates (mainly HOPG or bulk-MoS2). Despite the weak interactions between the monolayer and the substrate the film grows rotational aligned so that a film exhibits a single crystal orientation. This enables for example electronic structure characterization by angle resolved photoemission spectroscopy. The versatile growth procedure allows us to characterize many materials systems. First we discuss the role of the substrate for semiconducting TMDs. We study the electronics structure variation for MoSe2 grown on another TMD (MoS2) and compare it with that grown on HOPG. While the band dispersion of MoSe2 on HOPG resembles the expectations for free-standing MoSe2 it is modified for MoSe2/MoS2 due to interlayer hybridization of the chalcogen p-orbitals. A big advantage of MBE growth in vacuum is that it enables the synthesis and study of more reactive systems – like most metallic TMDs. Thus, in the second part of this talk we investigate the properties of single layer TiSe2. TiSe2 is an unconventional charge density wave (CDW) material whose charge density wave transition has been associated with an excitonic insulator phase. Such an excitonic insulator is formed spontaneously if the excitonic binding energy exceeds the band gap and thus formation of excitons may become the ground state. By scanning tunneling spectroscopy we observe significant increase in the CDW-band gap opening at the Γ-point for the monolayer compared to few-layer materials. Furthermore, the opening of the gap varies with the substrate material, consistent with expectations for excitonic binding energies. Interestingly, we observe coherence peaks in the tunneling spectra below 50 K suggesting the formation of an excitonic condensate.