Van Der Waals Epitaxial Growth of Superconductive NbS2

Abstract

Metallic NbS2 is one of the layered transition metal dichalcogenides (TMDCs) that exhibits intrinsic physical phenomena at low temperature limits such as superconductivity and charge density waves (CDW), although it has not been studied as extensively as other compounds. The unique properties of NbS2 were sufficiently confirmed in mechanically exfoliated NbS2. Chemical synthesis of NbS2 with atomically thickness is of great importance to property investigation and device applications. However, it is of great challenge to synthesize NbS2 with uniform layer. Unlike chemical vapor deposition (CVD) which loads the sources into a process chamber with a substrate, metalorganic chemical vapor deposition (MOCVD) can regulate the flow of the sources and control the material growth by supplying the sources in separate canisters. Several TMDCs such as MoS2 and WS2 can be grown well by MOCVD, and various studies using these materials have already been conducted. However, MOCVD for NbS2 has been rarely reported due to still lacking suitable MO precursors. In this study, heteroepitaxial growth of single crystalline NbS2 monolayer over a large area of graphene is achieved uniformly by MOCVD. By transmission electron microscope studies, it is verified that single crystalline NbS2 monolayer has high crystallinity and epitaxial relationship with graphene. We also try scanning tunneling microscope (STM) on single crystalline NbS2/graphene heterostructures for studying about its intrinsic superconductivity. In order to protect the unique properties of NbS2 from oxidation, synthesized NbS2 was unloaded after in-situ passivation with S film that was thermally removed in the analysis chamber. Our synthetic process provides a new way towards uniform synthesis of NbS2 nanosheets and could be widely applied to further study.