Invited

Structure and electronic properties of single-layer MoS₂ on Au(111)

Jeppe V. Lauritsen¹

¹Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Denmark

Single-layer MoS₂ is one of the most studied of the two-dimensional transition metal dichalcogenides (TMDCs) as it was the first example of how a modified electronic structure reflected by a direct band gap and correspondingly different optical properties may develop when the TMDC becomes a single layer. Single-layer MoS₂ can furthermore give rise to interesting new physics through the possibility of exploiting spin and valley degrees of freedom. We have developed a synthesis method that allows us to selectively form single-layer epitaxial MoS₂ sheets grown with the (0001) basal plane in parallel to Au(111). This has the advantage of a well-defined and well-oriented system that enables us to systematically address the structural and electronic features pertaining to such single MoS₂ layers on a metal support using scanning tunneling microscope (STM) and angle-resolved photoemission spectroscopy (ARPES). Coherent sheets of MoS₂ at varying surface coverage can be formed by sequences of reactive physical vapor deposition of Mo in a H2S atmosphere followed by post annealing in the sulfiding atmosphere. Structurally, the single-layer MoS₂ adopts a large characteristic superstructure due to the lattice mismatch, reflected by a moiré pattern in STM images [1,2]. Our DFT studies furthermore reveal that this modulation is influenced by the preferential bonding of the bottom S in the MoS₂ sheet to on-top positions on the Au(111). The ARPES results [3] show that the Au-supported single-layer MoS₂ retains the important direct band gap at the K point seen in free-standing single-MoS₂, whereas the out-of-plane S bonding to Au leads to some distortions of the band near the Γ point. By varying the synthesis conditions we can furthermore study isolated MoS₂ islands as well as stacking faults at the interface between MoS₂ patches. This approach in particular allows us to address the edges terminating the MoS₂ sheets, which are found in photoemission and STM studies to be metallic due to the presence of edge states in contrast to the semiconducting basal plane

Figure 1: (a) Atom-resolved STM image showing the atomic lattice and moiré pattern of MoS₂/Au(111) (b) Photoemission results along the Γ-K direction for MoS₂/Au(111).

[1] S.G. Sørensen, H.G. Füchtbauer, A.K. Tuxen et al., ACS Nano 8, 6788 (2014).

[2] S.S. Grønborg, S. Ulstrup, M. Bianchi et al., Langmuir 31, 9700 (2015).

[3] J.A. Miwa, S. Ulstrup, S.G. Sørensen et al., Phys. Rev. Lett. 114, 046802 (2015).