Heterointerface And Optical Property Modulation Of Two-Dimensional Semiconductors

Since the invention of the first transistor,integrated circuits have been rapidly developed in accordance with Moore’s Law.Integrated circuit technology will also lead the trend of science and technology in the new era.In the next twenty-five years,science and technology will continue to develop in high-tech directions,such as artificial intelligence,big data analytics,the Internet of Things and quantum computing.There is no doubt that the new generation of integrated circuits will be an important part of these emerging technologies.However,when the size of transistors is reduced to the nanometer scale,the components of integrated circuits have encountered physical bottlenecks,such as heat generation and power consumption caused by quantum effects.The slowdown of the semiconductor integrated circuit industry will affect the entire technology industry progress.Therefore,how to obtain high-performance components has become the key to promote the next generation of scientific and technological progress,which is also a concern of the majority of academic workers and entrepreneurs.Graphene has super hardness,ultra-thin thickness,high carrier mobility,and good ductility.Scientists believe that it can continue to write the magic material of Moore’s Law.Two-dimensional metal dichalcogenides,such as transition metal dichalcogenides,are a class of semiconductors that also have a layered structure.They have novel physical properties,such as ultra-thin physical dimensions,novel electronic structures,continuously tunable band gaps,and flexible properties.As well as their huge application potential,they have received extensive attention from the scientific community.More importantly,combine of different two-dimensional semiconductors to form lateral or vertically stacked heterojunctions can not only remain the excellent nature of each component,but also introduce unique physical properties related to the junction regions,such as interlayer energy transfer,interlayer carrier transfer,interlayer band gap,etc.,as well as unique device applications,such as dual-channel transistors,rectifiers,logic optoelectronic devices,and memories.The band alignment determines the application direction of two-dimensional heterojunction.Therefore,directional manipulation and regulation the band alignments of two-dimensional heterojunctions are very important for their future applications.At the same time,due to the defects and many body effects,the vapor-grown transition metal chalcogenides monolayers have relatively poor photoluminesce emission and single conductivity type,which also limit their further application in photoelectric devices.In view of this,in this dissertation,using a dual heating zone furnace with direction-switchable gas flow,a series of two-dimensional lateral heterostructures with tunable band alignments were synthesized successfully.A traditional two-step chemical vapor deposition method was used to synthesize two-dimensional vertical heterostructures with tunable band alignment,conduction type and stacked behaviours.Meanwhile,ultrahigh photoluminescence quantum yield was demonstrated in WO₃-WS₂ bilayer heterostructures.And foreign V atoms substitution doped MoS₂ was achieved,with enhanced b-exciton emission and p-type conduction.The main achievements are summarized as follows:1.We report a one-step chemical vapor deposition method for the growth of band alignment continuously modulated WS₂-WS_2（1-x）Se_2x（0<x≤1）monolayer lateral heterostructures,with atomically sharp interfaces at the junction area.Local photoluminescence and Raman measurements demonstrate the position-dependent composition and band gap information on the as-grown nanosheets.Kelvin probe force microscopy investigations further confirm the tunable band alignments in the heterostructures,where a continuously decreased Fermi level difference between the core and the shell regions is observed with the x value varied from 1 to 0.The direct growth of high-quality atomic-level junctions with controllable band alignment marks an important step toward the potential applications of two-dimensional semiconductors in integrated electronic and optoelectronic devices.2.We show the controlled preparation of WS₂-WS_2（1-x）Se_2x-WSe₂ lateral multilayer nip heterostructure with atomic sharp interfaces,tunable band alignments.By using micro-Raman photoluminescence characterizations,the sharp one-dimensional interfaces between two single layers were been observed,which were further confirmed by scanning transmission electron microscopy.Additionally,using nano-angle-resolved photoemission spectroscopy,we demonstrate that the energy band gap together with the spin splitting of the valence band maximum can be gradually tuned across the lateral heterostructure,which presents a type II band alignment.These findings confirm the possibility of controlling the band alignment in two-dimensional lateral heterostructures,which represents a key feature for designing multifunctional electronic and optoelectronic devices.3.We report for the first time a two-step chemical vapor deposition approach for a series of WS_2（1-x）Se_2x/Sn S₂ vertical heterostructures with high quality and large areas.The steady-state photoluminescence results exhibit an obvious composition-related quenching ratio,revealing a strong coherence between the band offset and the charge transfer efficiency at the junction interface.Based on the achieved heterostructures,dual-channel back-gate field-effect transistors were successfully designed and exhibited typical composition-dependent transport behaviors,and pure n-type unipolar transistors to ambipolar transistors were realized in such systems.The direct vapor growth of these novel vertical WS_2（1-x）Se_2x/Sn S₂heterostructures could offer an interesting system for probing new physical properties and provide a series of layered heterostructures for high-quality devices.4.We used a modified two-step chemical vapor deposition method to prepare WSe₂/WS₂ heterojunction,which shows controlled stack behavior.Optical characterization shows that the heterojunction has two stack behavior of A-A and A-B,which was further confirmed by scanning transmission electron microscope characterizations.More importantly,in the obtained heterojunctions,we observed periodic Moirésuperlattices.More importantly,our collaborators have used the heterojunction realized the study of anomalous interlayer exciton diffusion in twist-angle-dependent Moirépotentials.5.We present a direct physical vapor growth of WO₃-WS₂ bilayer heterostructures,with WO₃ monolayer domains attached on the surface of large-size WS₂ monolayers.Optical characterizations revealed that the photoluminescence quantum yield of the as-grown WO₃-WS₂ heterostructures can reach up to 11.6%,which is 2 orders of magnitude higher than that of WS₂ monolayers by the physical vapor deposition growth method and about 13-times higher than that of mechanical exfoliated WS₂ monolayers,representing the highest photoluminescence quantum yield reported for direct growth transition metal dichalcogenides materials so far.The photoluminescence enhancement mechanism has been well investigated by time-resolved optical measurements.The fabrication of WO₃-WS₂ heterostructures with ultrahigh photoluminescence quantum yield provides an efficient approach for the development of highly efficient two-dimensional integrated photonic applications.6.We systematically demonstrate the controllable growth of V-doped MoS₂monolayer via alkali metal-assisted vapor growth method.Scanning transmission electron microscopy demonstrates that V atoms successfully substituted the Mo atoms and uniformly distribute in the MoS₂ monolayers,which was also confirmed by Raman and X-ray photoelectron spectroscopy spectra.Power-dependent photoluminescence spectra clearly indicates that,distinct B excitons emission characteristics in observed in V-doped MoS₂ monolayers（low doping concentration）,indicating that V atoms can effectively induce the valley splitting in MoS₂.Carrier transport behaviors of the V-doped MoS₂ monolayers were further probed,where typical p-type conduction gradually arises and enhances with increasing the V mole fractions in the designed devices.The achieved V-doped MoS₂ monolayers will provide new platforms for probing new physics and provide novel materials for high-quality optoelectronic devices.