Electron Beam Evaporation Of Two-dimensional Bismuth And Its Electrical Performance Control

Due to the tunable band gap between graphene and transition metal dichalcogenides(TMDs),the elemental group IVA and VA two-dimensional(2D)material,X-ene,has attracted increasing research attention.As the last element of group VA,bismuth has been expected to yield excellent electrical properties,large magnetoresistance effect,quantum spin Hall effect and other exotic physical characteristics.As the thickness of bismuth decreases to less than 30 nm(2D bismuth),it turns from a semimetal to semiconductor.Together with the increase of specific surface area resulting in an increased sensitivity,2D bismuth is promising to realize transistor-type electronics and sensors.Nevertheless,there has been a lack of cost-effective and scalable method to prepare 2D bismuth directly on various device substrates.In this thesis,electron beam(e-beam)evaporation route was explored to produce large-area and high-quality 2D bismuth with emphasis on its electrical performance control and sensing functions.The key findings or results are as follows:(1)It is feasible to prepare large-area pristine 2D bismuth with smooth(R_a?1 nm)and uniform surface,high crystallinity in(001)preferential orientation directly on a SiO₂/Sisubstrate via e-beam evaporation at the optimized condition:substrate temperature?75?,and a deposition rate?0.6(?)/s.While having few tiny pinholes on 5-nm 2D bismuth,a pin-hole free uniform surface is guaranteed for a thickness?10 nm.(2)The crystal structure and electrical properties of e-beam evaporated 2D bismuth can be tuned via thickness and annealing treatment.As the thickness increases,the grain size of2D bismuth increases and preferred to align along(001)orientation.Interesting,this work delivered a simultaneous increase of the Hall coefficient,Seebeck coefficient value and electrical conductivity.Besides the thickness effect,annealing improves the crystallinity and conductivity(?10⁴-10⁵S/m)of 2D bismuth with an optimized condition at 100-200?for 1 h.(3)The transistor-type devices on 2D bismuth were fabricated,followed by electrical characterization and multifunctional sensing tests.2D bismuth and its transistor devices in this work yield excellent electrical performance:the sheet resistance is?1 k?/?,the Hall coefficient is?10^-2cm³C^-1,the carrier concentration is 5�10²⁰cm^-3and the field-effect mobility is 235 cm²V^-1s^-1at room temperature.The aforementioned performance is comparable to molecular beam epitaxy(MBE)counterparts in similar thickness.2D bismuth-based transistor can realize the sensing response to 532 nm laser as photosensors.As the resistance of 2D bismuth shows a rapid response(?ms)and high sensitivity?32%to the temperature.Moreover,2D bismuth transistors respond quickly to heavy metal ions As³⁺in aqueous solution and the detection limit is?130 nM.E-beam evaporated 2D bismuth has demonstrated decent electrical properties and multifunctional sensing effects,holding great potential for innovative and multifunctional micro/nano electronics and sensors.