The Mbe Synthesis Of 2D Van Der Waals Semiconductor Structures And Their Surface And Interface Electronic Behaviors

The field of two-dimensional(2D)materials and heterojunctions(HJs)has rapidly developed since the first isolation of graphene.Although graphene exhibits high carrier mobility,flexibility,toughness,and near-transparency,the zero band gap severely limits its application in logic devices.Therefore,the search for semiconductors alternative to graphene(namely,the van der Waals semiconductors)is imperative.Van der Waals semiconductors are layered materials with covalent in-plane bonding and weak out-of-plane interactions enabling the exfoliation or the direct growth of two-dimensional layers with a single atomic thickness.Due to the quantum confinement and surface effects,van de Waals semiconducting materials in monolayer and few layers exhibit very different properties from corresponding bulk.Semiconducting HJs,comprised of atomically thin transition metal dichalcogenides(TMDs),have shown great potentials in electronic and optoelectronic applications.Organic/TMD hybrid bilayers allow combining the advantages of both organic and inorganic semiconductors,which hold enhanced pumping efficiency of interfacial excitons,tunable electronic structures,and optical properties,and other superior advantages to these inorganic HJs.In this paper,we have prepared the atomic clean monolayer(ML)-organic/ML-TMD HJ using molecular beam epitaxy(MBE).We use the scanning tunneling microscope(STM)to measure the electronic structure and interface behavior directly and provide detailed information of the HJ at the 2D limit.Our findings are of fundamental importance to comprehend and design novel electronic/optoelectronic devices based on the emergent organic/TMD HJs.Strong PTCDA/WSe₂ interfacial interactions lead to appreciable hybridization of the WSe₂ conduction band with PTCDA unoccupied states,accompanying with a significant amount of PTCDA-to-WSe₂ charge transfer(by 0.06 e/PTCDA).A type-II band alignment was directly determined with a valence band offset of?1.69 e V,and an apparent conduction band offset of?1.57 e V.On the other hand,2D van der Waals semiconductors with intrinsic magnetism are also on the rise,which have great potential in sensing and data storage.Such materials present a new platform for spintronics and magnet optoelectronic devices(e.g.,spin tunnel field-effect transistors)due to the inherent long-range magnetic order which is tunable by applied electric and magnetic fields.Though bottom-up preparation of Cr I₃ and Cr Br₃ monolayers has been reported,the Cr Cl₃ is mostly obtained by mechanical exfoliation.Here,the controlled growth and electronic structure of the van der Waals magnet Cr Cl₃ films are studied.The morphology of Cr Cl₃ films is strongly dependent on the substrate temperature(T_sub).As the T_sub gradually increases,the Cr Cl₃/EG epitaxy growth will show as the fractal,the ribbon,and the compact island(with straight or smooth edges),respectively.For the compact island growth,Cr Cl₃ follows a layer-by-layer mode,and the deposition temperature of the first layer is larger than that of second layer.Moreover,we directly determined the band edges(-0.98 V?+0.955 V)and band gap of the ML-Cr Cl₃,which agree well with the first-principles calculations.Compared to the ML,the band edges of bilayer Cr Cl₃ move towards the lower energy level(i.e.,the conduction band is closer to the Fermi surface and the valence band is away from that),and the band gap becomes smaller.Finally,ML-Cr Cl₃films hold grain boundaries with different tilt angles due to the randomness of the vd W epitaxial growth.The band gap at the boundaries would be broadened with respect to the intrinsic ML-Cr Cl₃ while the metallic state appears near that.