Synthesis Of New Type Two-dimensional Van Der Waals Heterostructure And Superlattices

Two-dimensional（2D）materials have drawn rapidly growing scientific interest in the past several years for their rich and tunable electronic,optical and mechanic properties and their exciting potential applications in various fields such as photodetectors,transistors,and light-emitting diodes.The associated van der Waals（vd W）heterostructures and vd W superlattice have provided great flexibility for integrating distinct atomic layers beyond the traditional limits of lattice-matching requirements,which defines a rich material platform for both fundamental studies and technological applications.So far,2D vd W heterostructures and vd W superlattice have been mostly obtained through a mechanical exfoliation and arduous layer-by-layer restacking process.This approach is generally applied to create diverse heterostructures from a wide range of layered crystals,but typically with limited yield and reproducibility,and becomes exponentially more challenging for high-order superlattice.Here,we present a straightforward approach to synthesize these atomic-layer thick materials and construct complex heterostructure and superlattice.The main contents as follows:（1）2D materials exhibit fascinating properties that are tunable by the number of atomi c layers.It remains a significant challenge to produce high quality single crystals of 2D materials with precisely controlled layer numbers.Here,we report a one-step chemical vapor deposition（CVD）approach to NiTe₂ nanosheets with precisely controlled layer numbers.Typical optical microscopy（OM）images show the resulting NiTe₂ nanosheets mostly exhibit a hexagonal or triangular shape with the lateral domain size varying from～5 to～440μm.With the increase of the growth temperature,the NiTe₂ nanosheet undergo a transition from kinetics to thermodynamics,and the number of NiTe₂ nanosheets increace from monolayer to 2,3,4,5...2D NiTe₂ nanosheets show excellent layer dependent electrical properties.The excellent conductivity of NiTe₂ nanosheets is 7.8×10⁵ S m^-1 and the extraordinary breakdown current density of NiTe₂ nanosheets is 4.7×10⁷ A cm^-2.These studies define a reliable way to2D NiTe₂ nanosheets with precisely controlled layer number,which is essential for fundamental investigation of the thickness-dependent properties of 2D NiTe₂,such as electronic and optoelectronic properties,superconductivity,charge-density-wave order,and their potential applications.（2）The junctions formed at the contact between metal and two-dimensional semiconductors are crucial for two-dimensional electronic and optoelectronic devices.We report the in situ growth of ultrathin metallic NiSe single crystals on WSe₂,forming NiSe/WSe₂ vd W heterostructure,in which the metallic NiSe nanosheets can function as the contact electrodes to WSe₂,creating an interface that is essentially free from chemical disorder and improve the performance of related 2D semiconductor.Typical OM images show the resulting NiSe nanosheets mostly exhibit a triangular or hexagonal shape with thickness down to～0.8 nm.The structure and composition of these NiSe nanoplates were characterized by the X-ray photoelectron spectroscopy（XPS）and TEM.The excellent conductivity of NiSe nanoplates is 1.6×10⁶ S m^-1,which has enabled a variety of applications such as electrodes contact for 2D semiconductors devices.The field-effect mobilities of WSe₂ transistors with NiSe electrodes contact is much higher than that with vacu um deposited Cr/Au electrodes contact.Our work provides accessible strategy to improve the performance of 2D electronic devices,opening up new avenues for engineering future 2D electronic and optoelectronic devices.（3）So far,2D vd W heterostructures and vd W superlattices have been mostly obtained through a mechanical exfoliation and arduous layer-by-layer restacking process.This approach is generally applied to create diverse heterostructures from a variety of 2D materials,but typically with limited yield and reproducibility,and becomes exponentially more challenging for high-order superlattices.Herein we report a straightforward approach to create high-order vd W superlattices by rolling up 2D vd W heterostructures.By exposing CVD-grown 2D/2D vd W heterostructures（for example,Sn S₂/WSe₂）to an ethanol-water-ammonia solution,we show that the capillary force can drive a spontaneous delamination and rolling-up process to produce vd W heterostructure roll-ups containing high-order 2D/2D vd W superlattices,without going through multiple transfer and restacking processes.Electrical transport studies reveal an evolution of the transport characteristics from 2D to one-dimensional（1D）with greatly increased conductance,as well as an angle-dependent linear magnetoresistance,in the vd W superlattices.Beyond the material compositions,roll-up vd W superlattices with different chiralities（or Eshelby twists）and the associated moiréstructures may be obtained,depending on the roll-up angles.Indeed,the electron diffraction patterns show that a series of the roll-up structures with variable chiral angles are obtained in both WSe₂ and Sn S₂/WSe₂ roll-ups.Furthermore,we show that this rolling-up strategy can be extended to create diverse 2D/2D vd W superlattices and complex three-component2D/2D/2D vd W superlattices,as well as beyond-2D materials,including three-dimensional（3D）or 1D materials,generating a wide range of multi-dimensional vd W superlattices,such as 3D/2D,3D/2D/2D,1D/2D and 1D/3D/2D vd W superlattices.This study demonstrates a general approach to producing high-order vd W superlattices with widely variable material compositions,dimensions,chirality and topology,and defines a rich material platform for both fundamental studies and technological applications.