Synthesis And Modulation Of Chemical Composition And Space Structural Distribution Of Two-dimensional Sensing Materials

The discovery of graphene has ignited intense interest in a new generation of atomically thintwo-dimensional（2D）material,including 2D semiconductors that may exhibit superior immunity to short channel effect.In particular,2D transition metal dichalcogenides（2D-TMDs）（e.g.,Mo S2 or WSe₂）are of great interest for their tunable band gap and the associated electronic and optical properties.Many unique attributes,such as single atomic thickness,extraorindary flexiblity,affluent excition dynamics,valley pol arization,make 2D-TMDs aexciting class of materials for a new generation electronics and optoelectronics and sensors based on electronics and optoelectronics.The application of 2D-TMDsoffer solutions to many standing problems in morden electronic industry and sensor technology,and open up totally new opportunities for future electronics and optoelectronics and sensors based onelectronics and optoelectronics.Thefeature s of nearly total surface,flexible,new chemical modification possibility,ultrathin th ichness and low cost make the 2D semiconductors,such as 2D-TMDs,have important application in morden sensor technology and industry.To fully explore the potential of such 2D semiconductor,it is necessary to modulate their chemical composition,structur e,electronic properties and their space distribution,much like the traditional semiconductor heterostructures that have essentially defined the foundation of all modern electronic and optoelectronic devices.This thesis aims to address these challenges,develop rational strategies to synthesis and analysize 2D-TMDs with tunable the chemical composition,structure,electronic properties and their space distribution,conduct the fundamental research on application in electronics and optoelectronics and sensors based on electronics and optoelectronics.The main research results are summarized as the following:（1）We have first developed rataionl strategies for the preparation 2D-TMDs（e.g.,WS2,WSe₂）using a thermal chemical vapor deposition（CVD）process.We have further designed a CVD process that can allow in situ switch of the vapor phase reactants to allow lateral epitaxial growth of single or few-layered TMD lateral heterostructures.Under the guide of morden analysis technology,we have demonstrated,for the first time in the world,2D WS2–WSe₂ and 2D MoS₂-Mo Se₂ lateral heterojunctionscan be grown on SiO₂/Si substrate.Optical microscopy studies show that the heterojuctions nanosheets typically exh ibit a triangle or truncated triangle morphology with well-definedsharp angles and straight lines.Atomic force microscopy（AFM）studies show that the samples had uniform thickness（monolayer,bilayer,or few-layer）and smooth surfaces.Raman and Photoluminece nce（PL）microscopy studies demonstrate the resulting heterostructure nanosheets exhibit clear structural and optical modulation.Transmission electron microscopy and electron diffraction studies reveal a single crystalline structure,and the energy dispers ive x-ray elemental mapping confirms spatial modulation of chemical compositions in the heterostructure nanosheets.Electrical transport studies demonstrate that the WSe2-WS₂ heterojunctions can form lateral p-n diodes with excellent current rectification behaviour and photocurrent generation characteristics,and can be used to create CMOS inverters with high voltage gain up to 24.We have point out the possibility in sensor design and fabrication appling the two-dimensioal semiconductors heterojunctions.On the basis of preparation and analysis of atomic-thin lateral heterojunctions,the concept “interline”in two-dimentional heterojuctionwas proposed,in analogy to the concept “interface”in three-dimentioal heterojunction.This study represents an important advance in developing 2D semiconductor heterostructures,marking an essential step towards 2D electronics and optoelectronics and sensor technology based on2 D electronics and optoelectronics.（2）To explore the full potential of these 2D semiconductors in 2D electronics and optoelectronics and sensor technology based on2 D electronics and optoelectronics requires a precise control of their band gap and electronic properties,which represents a significant challenge in the 2D material systems.Here we demonstr ate a systematic control of the electronic properties of 2D-TMDs by creating a mixing alloy of the intrinsically p-type WSe₂ and intrinsically n-type WS2 with variable alloy compositions.Under the guide of morden analysis thechnology,2D WS 2x Se₂-2x alloy nanosheets were prepared using a home-built CVD equipment with precisely controlled chemical compositionby adjusting the WS 2 and WSe₂ vapor ratio in the CVD equipment.We show a series of WS 2x Se₂-2x alloy nanosheets can be synthesized with fully tunable chemical compositions and optical properties.Micro-Raman studies indicate the Raman resonant frequency show a systematic change with the chemical composition,suggesting the modulation of the structure of the alloy nanosheets.Micro-photoluminecence research indicates the photoluminescence peaks position change with the chemical composition,suggesting the realization bandgap modulation inthe alloy nanosheets.Transmission electron microscopy study confirms the nearly perfect single crystalline structur e,and the lattice parameters change with the chemical composition systematically.Electrical transport studies using back-gated field-effect transistors demonstrate that charge carrier types and threshold voltages of the alloy nanosheet transistors can be systematically tuned by the alloy composition.A highly p-type behavior is observed in selenium rich alloy,which gradually shifts to lightly p-type,and then switches to lightly n-type characteristics with the increasing sulfur atomic ratio,and eventuall y evolves into highly n-doped semiconductors in sulfur rich alloys.This study represents a first systematic control of the electronic properties of 2D-TMDs.The synthesis of WS2 x Se₂-2xnanosheets with tunable optical and electronic propertiesrepresents a critical step towards rational design of 2D electronics and optoelectronics and sensor technology based on 2D electronics and optoelectronics with tailored spectral responses and device characteristics.（3）The use impurity atoms to control the charge carri ers represents a key strategy to control the electronic properties of traditional semiconductors.Here we demsontrate that the electronic properties properties of 2D-TMDs（e.g.,WS2 nanosheets）can be trationally tailored bythe treatment of phousporous（P）vapour in a CVD equipment realized.Optical microscopy observation indicates the treatment of P vapour treatment did not alter the morphology of these nanosheets.AFM studiesdetermined these nanosheets in the work are monolayer,bilayer or few-layer material.Micro-Raman studiesindicate the treatment with P vapour did not obviously alter the phonon structure（i.e.the main crystalline structure）.Micro-PL studies indicates the treatment of P vapour alter the PL peaks position and spectral characteristics,sug gesting the alteration of the electron structure of the nanosheets.Electronic properties studies show that the treatment of P vapour change the semiconducting characters of these nanosheets,showinga gradual evolution from n-type semiconductor behavior,to bipolar semiconducting behavior,and then p-type semiconducting behavior with the increasing treatment tempreture.This work lay a foundation for two-dimensional integrated circuits and sensor miniaturization,flexibility,intelligentizing,integrating,an d networking,etc.The research of 2D heterojunctions,modulation the electronic properties by alloying,doping of the 2D semiconductorby impurity vapor treatment to modulate the charge carries represent important progress in 2D semiconductors.The successf ul growth of 2D semicodnuctors with precisely controlled chemical,structural and electronic properties defines an exciting and robust material platform for a new generation of atomically thin electronics and optoelectonics and sensors based on electronics and optoelectronics,and could enable many exciting opportunites in ultrasensitive sensors,integrated circuits and integrated sensors,flexible electronics and flexible sensors,etc.