Molecular beam epitaxy of quasi-freestanding transition metal disulphide monolayers on van der Waals substrates: a growth study

Abstract

Based on an ultra-high vacuum compatible two-step molecular beam epitaxy synthesis with elemental sulphur, we grow clean, well-oriented, and almost defect-free monolayer islands and layers of the transition metal disulphides MoS2, TaS2 and WS2. Using scanning tunneling microscopy and low energy electron diffraction we investigate systematically how to optimise the growth process, and provide insight into the growth and annealing mechanisms. A large band gap of 2.55 eV and the ability to move flakes with the scanning tunneling microscope tip both document the weak interaction of MoS2 with its substrate consisting of graphene grown on Ir(1 1 1). As the method works for the synthesis of a variety of transition metal disulphides on different substrates, we speculate that it could be of great use for providing hitherto unattainable high quality monolayers of transition metal disulphides for fundamental spectroscopic investigations.

Tomonaga-Luttinger Liquid in a Box: Electrons Confined within MoS2 Mirror-Twin Boundaries

“…The spectrum reveals a finite density of states throughout the measured energy range, except for a narrow gap E gap of the order of 100 meV located at E F (E = 0). All states visible in the spectrum lie within the 2.5 eV band gap of the surrouding MoS 2 [33] and hence are strongly confined within the MTB. Fig.…”

“…The synthesis of MoS 2 on the substrate graphene on Ir(111) is conducted in a two-step process [33]: During the first step, Mo is evaporated from a rod with a rate of 0.125 monolayers/min on the graphene surface at room temperature in a S pressure of p ≈ 5 × 10 −9 mbar.…”

Comprehensive tunneling spectroscopy of quasifreestanding MoS2 on graphene on Ir(111)

“…[29], a closed monolayer of graphene (Gr) is grown on Ir(111) via temperature programmed growth and chemical vapour deposition (CVD) at T ≈ 1370 K. ML-to few-layer MoS 2 is subsequently grown on the Gr/Ir(111) substrate by van der Waals MBE, according to the methods developed in Ref. [3]. Mo is evaporated from an e-beam evaporator and S from FeS 2 granules in a Knudsen cell.…”

“…Amongst these its large, direct bandgap is promising for the electronics communities, and is a basic quality to be characterized. Largescale flakes can be grown epitaxially [3][4][5] or exfoliated [6,7], but reliable characterization of the pristine electronic bandgap remains problematic.…”

“…Large bandgaps of E g ≈ 2.65 eV [24] and ≈ 2.7 eV [25] have been reported, but only in locations where the ML-MoS 2 is locally decoupled from an inhomogeneous substrate. On top of all this, practical difficulties due to sulfur's relatively high vapour pressure had, until recently [3], hindered molecular beam epitaxy (MBE) synthesis of MoS 2 . Thus close-to-freestanding MoS 2 flakes of sufficient size, quality, and cleanness on STS-permitting substrates have remained elusive.…”