Scanning tunneling microscopic study of defects and heterointerfaces of transition metal dichalcogenides

Abstract

Since graphene, the first two-dimensional (2D) material, was exfoliated in 2004, 2D materials have attracted intensive attention for decades. Different from traditional bulk materials, 2D materials with monolayer or few-layer thicknesses exhibit many extraordinary properties. Transition-metal dichalcogenides (TMDs) are one important category of 2D materials that possesses unique electronic and optoelectronic properties as well as excellent application potentials. The electronic properties of TMDs can be tuned by various methods, among which defect engineering is one effective way. Moreover, the heterointerface formed by bringing two materials together can give rise to exciting properties and promising applications. Based on these envisions, this thesis is focused on scanning tunneling microscopic studies of defects and heterointerfaces of TMDs. To be specific, the band-bending of MoSe2 near line defects, intentional doping of MoSe2, and several heterointerfaces are studied. Mirror-twin domain boundaries (MTBs) are commonly seen in epitaxial monolayer TMDs such as MoSe2. They have attracted many research interests as ideal one-dimensional (1D) metallic systems. Band-bending of 1H-TMD in the vicinity of MTBs has been reported and ascribed to a charging effect of the defects from the substrate. The band-bending of 1H-MoSe2 normal to 4|4P-MTBs on different substrates as well as the filling levels of the corresponding MTB band is investigated by low-temperature scanning tunneling spectroscopy (LT-STS). Various band-bending magnitudes and directions and the filling levels of MTB band are observed, corresponding to different charge transfer extents between substrates and MoSe2 monolayers. The band-bending of 1H-MoSe2 across a curved defect, i.e., the vertex of an MTB triangular loop, is also investigated, where the asymmetric band-bending on two sides reveals the geometric charge distribution effect, further establishing the static charge-induced energy potential variations. Compared to the apparent bending of the valence band edge, the conduction band has bent little, resulting in bandgap variation near the MTBs. It is argued that piezoelectric effect-induced strain may be a relevant factor. This work will help to establish the origin of band bending of 1H-TMDs near MTBs. Intentional doping of Nb and Re atoms in MoSe2 is conducted by molecular beam epitaxy (MBE) and characterized by a combination of scanning tunneling microscopy (STM) and scanning transmission electron microscopy (STEM) techniques. Nb and Re atoms are proved to be shallow dopants with hole doping and electron doping, respectively. Post-deposition of Nb(Re) and Se on MoSe2 can form NbSe2/MoSe2 (ReSe2/MoSe2) heterointerface. The characteristics of NbSe2/MoSe2 heterointerface depends on deposition sequences: a sharp interface is discernable when MoSe2 is grown prior to NbSe2, while the interface is less obvious with a reversed deposition sequence. A sharp interface is formed between 1T’-ReSe2 and 2H-MoSe2. Post-deposition of In on MoSe2 is also performed, leading to the formation of In2Se¬3 with various phases. The mechanism is hypothesized as that In would combine with Se inside MoSe2, calling for more experimental and theoretical evidences.