Syntheses And Physical Properties Of Several Two-Dimensional Heterostructures

The family of two-dimensional（2D）materials grows fast,since the first exfoliation of graphite down to monolayer graphene by Geim et al.in 2004.Novel 2D materials emerge frequently,such as hexagonal boron nitride（h BN）with the similar structure as graphene and transition metal dichalcogenides（TMDs）with diverse physical properties.Furthermore,the stacking of different 2D materials with atomically sharp van der Waals interface generates 2D heterostructures.Such artificial structures offer further degrees of freedom,e.g.,stacking order,that could be utilized to manipulate the 2D layers.Also,2D heterostructures serve as a novel platform,where synergistic and proximity effects might give rise to promising phenomena.To realize device applications and integrations,large-scale 2D heterostructures have to be controllably synthesized through bottom-up approaches,which is currently a challenge to the community.Therefore,the studies of the physical properties of 2D heterostructures have fundamental significances,and the studies regarding their syntheses are beneficial to practical applications.This paper focuses on the syntheses and physical properties of 2D heterostructures,including the graphene/Nb Se₂ heterostructure,graphene/Janus Mo SSe heterostructure,graphene/Nb Se₂ lateral heterostructure,and Nb S₂/Mo S₂ heterostructure.We study their growth mechanisms as well as their optical and electrical properties.The main results and significances of this paper are summarized below.Ⅰ.We propose an intercalation method to synthesize graphene/Nb Se₂ heterostructure.First-principle calculations demonstrate that the intercalation growth of Nb Se₂ at the graphene/Si O₂ interface is energetically favored,compared with the growth of Nb Se₂on graphene surface.Phase-field simulations further reveal that the edges of graphene serve as the intercalation channels of precursor.The asymmetric domain morphology is a direct evidence of the edge-enabled intercalation process.This intercalation method can be further extended to the synthesis of graphene/Mo S₂ heterostructure,and the intercalation process of Mo S₂ is similar to that of Nb Se₂.This work provides a novel technique for the construction of 2D heterostructures,which also realizes the in situ encapsulation of ambient-sensitive 2D materials.Ⅱ.We propose a novel method to synthesize graphene/Janus Mo SSe heterostructure,and study the second-harmonic generation（SHG）of Janus Mo SSe monolayer in details.The asymmetry between van der Waals and quasi van der Waals interfaces enables the selective intercalation of precursor with a distinct chalcogen species,thus facilitates the Janus conversion with the atomic-scale precision.The as-synthesized Janus Mo SSe monolayer has superior crystallinity and uniformity,and the interface between graphene and Janus Mo SSe is clean and sharp.In addition,the Janus Mo SSe monolayer exhibits pronounced SHG response.Its in-plane crystal orientation can be determined by the polarization-resolved SHG spectroscopy.With excitation wavelength in resonance with its C exciton state,the yyy component of second-order nonlinear susceptibility can be enhanced by a factor of～8.This work offers a novel method for the synthesis of Janus TMDs and their heterostructures without the need of hydrogen plasma treatment.This work may also promote the on-chip nonlinear optical applications of Janus TMDs.Ⅲ.We propose a novel method to synthesize graphene/Nb Se₂ lateral heterostructure.The lateral interface is characterized using scanning transmission electron microscopy（STEM）,which reveals the overlap region between graphene and Nb Se₂ as well as the effects of graphene on the vertical growth of Nb Se₂.We further investigate the transport properties of the lateral heterostructure.A superconducting gap with magnitude around0.12 me V is induced in graphene by Nb Se₂.This proximity-induced gap contributes to Andreev reflection,leading to a zero-bias enhancement of differential conductance.In addition,this proximity-induced gap is extremely sensitive to external magnetic field.This work provides a bottom-up method to synthesize 2D normal metal/superconductor heterostructures,and also offers a useful technique to manipulate the intrinsic physical properties of graphene.Ⅳ.We synthesize large-scale Nb S₂/Mo S₂ heterostructure through epitaxial growth,and the semiconductor devices based on the heterostructure have been studied in details.With the metallic Nb S₂ as the van der Waals contacts of Mo S₂,the mobility and on/off current ratio of a field-effect transistor are significantly improved,compared with the device with the conventional Ti/Au contacts.Besides,the van der Waals contacts also improve the response strength and speed of a photodetector.This work offers a bottom-up method to realize contact engineering,and might also promote the construction of TMD-based 2D electronic devices.