【作者】 王婷；

【导师】 王伟；

【作者基本信息】 西安理工大学 ， 物理学， 2021， 硕士

【摘要】 自2004年石墨烯首次从石墨中机械剥离出来,一个新的材料领域—二维材料引起了人们的广泛关注。近年来关于二维材料的合成、表征及应用的相关研究呈指数增加。二维多层材料是以层内强共价键及层间弱范德瓦尔斯力结合在一起。由于维度降低所带来的量子限制效应和弱屏蔽效应赋予二维半导体许多新奇物理效应,展示出不同于块体材料的独特物理性质。其中二维半导体材料在多个领域都具备重要的应用前景,其优异的电学性质既可以降低功耗,又可大幅度提升元件的集成度,在新型电子器件应用中具有天然优势。正因为如此,未来电子元件的核心材料将有可能由传统的硅材料转变为二维材料。二维材料中最有代表性的半导体材料是石墨烯,该材料具有非常优越的电子、光学性质以及高机械强度。但是本征石墨烯没有带隙,目前也无法通过有效手段获得较大的带隙,这大大限制了该材料在光电领域中的应用,因此越来越多的学者开始探索新型的二维材料。近年来以二维过渡金属硫化物材料最备受关注。理论和实验结果均表明过渡金属硫化物的电子能带结构与材料的结构体系有关,当材料由体相结构转变为二维结构时,其带隙特性由间接变为直接,光学性质和电学特性等也发生显著变化。二维过渡金属硫化物材料具有多种优于石墨烯的物理和化学性能,主要应用于电子元件领域,另外其在新能源电池、能量存储和传感器等方面的应用前景也非常广泛。最近一种新型的二维过渡金属硫化物材料—CrS₂通过化学气相沉积法成功合成,该材料具有优异的谷极化、压电性、光催化和相变本征磁性等性质。本论文采用基于密度泛函理论的第一性原理计算（First-principles calculations）方法对二维半导体材料CrS₂的稳定性、电子结构与输运性质、光学性质以及带隙调控进行了计算分析,结果如下:首先计算了二维单层CrS₂材料的形成能、弹性常数、声子谱以及从头算分子动力学,计算结果表明该材料满足四个稳定性条件,即该材料在常温下能够稳定存在;接下来又对该材料的电子结构进行了分析,主要研究了能带结构、态密度。计算结果表明二维单层CrS₂是直接带隙半导体材料,带隙值为1.32eV,其导带底和价带顶均位于高对称点K。此外,也对CrS₂材料的输运性质进行了研究,计算结果表明电子和空穴的有效质量呈各向异性,其最大（最小）有效质量分别为1.10m₀（0.88m₀）和1.05m₀（0.87 m₀）,基于形变势理论计算得到的电子和空穴载流子迁移率分别为199cm²V^-1s^-1和231cm²V^-1s^-1;最后计算了该材料的吸收率、反射率、透射率,结果表明二维单层CrS₂材料在可见光波段红色光区域透射率达到峰值,吸收率则为谷值,与反射率一致;而在紫色光区域,吸收率达到峰值,反射率也达到最大值,透射率则相反。接下来又对二维单层CrS₂材料电子结构和光学性质的调制进行了研究。首先分析了应变对CrS₂材料能带结构的影响。在压应变到拉伸应变的过程中,单轴应变和双轴应变均会改变单层CrS₂的带隙宽度,且带隙值均呈逐渐减小的趋势。通过分别施加单（双）轴应变,该材料的带隙值可在2.57eV（3.00eV）到0.96eV（0.44eV）的范围内进行调控。此外,带隙特性也发生相应改变,在单轴应变状态下,当施加应变超过+6.4%时,单层CrS₂的带隙特性由直接变为间接。双轴应变状态下,施加应变超过+3.2%后,单层CrS₂变为间接带隙。施加应变也会使CrS₂材料的吸收率、反射率和透射率的波峰（谷）发生偏移,单轴应变下,该材料的峰（谷）值位于可见光波段的蓝光处,双轴应变下则位于绿光处。最后分析了厚度对CrS₂材料能带结构演化的影响。结果表明:当CrS₂厚度从一层增加到四层时,带隙值从1.32eV减小到0.74eV,导带底位于K点,价带顶则由K点移向Γ点,导致三层以上的CrS₂带隙特性由直接变为间接。随着层数的增加,CrS₂材料的在可见光区域的吸收率和反射率增大,透射率减小。以上计算结果表明,通过施加应变和改变层数手段可使CrS₂材料的带隙值和带隙特性以及光学性质发生变化,扩展了该材料在多个不同领域的应用潜力。更多还原

【Abstract】 Since the successful discovery and isolation of graphene in 2004,the field of two-dimensional（2D）materials and research on the discovery,characterization,and application of novel 2D materials has grown exponentially.In few-layer 2D materials,each layer is composed of a covalently bonded lattice and is weakly coupled to its neighboring layers by van der Waals interactions.The quantum confinement effect and reduced screening effect arising from thickness reduction in 2D materials often manifest themselves in features clearly different from those of their bulk counterparts.The 2D semiconductors have many potential applications in high performance microelectronic devices,which can significantly improve device integration and reduce power consumption.It is expected that the 2D semiconductors will replace the traditional silicon material as the core material of future electronic devices.Graphene is one of the most representative 2D materials with excellent electronic and optical properties as well as high mechanical strength.However,the intrinsic graphene has zero band gap,which greatly limits the application of this material in the field of optoelectronics.Therefore,the researchers are continually exploring more new 2D semiconductors.In recent years,2D transition metal sulfide materials（TMDs）have attracted the most attention.Both theoretical and experimental results show that when the thickness of a TMD is reduced from bulk into monolayer,the electronic band structure changes from an indirect band gap to a direct band gap,which results in a significant change in its photoelectric and other properties.TMDs have many similar and even superior properties to graphene,especially in electronic devices,and have a wide range of potential applications in new energy batteries,energy storage and sensors.2D layered CrS₂ material is a novel TMD which has successfully synthesized via the chemical vapor deposition method.It has excellent valley,polarization,piezoelectric,photocatalytic and phase change intrinsic magnetic properties.In this paper,we perform first-principles calculations based on density functional theory to systematically investigate the structure stability,electronic structure,optical properties of CrS₂ monolayer.The results are as follows:First,the formation energy,elastic constants,phonon spectrum and ab-initio molecular dynamics of CrS₂ monolayer were calculated,with the aim of determining the its structure stability.Our results indicate that this material satisfies four stability conditions.This means that it might exist stably at room temperature.Next the electronic properties of CrS₂ monolayer including band structure,density of states,effective mass and carrier mobility were investigate.It is found that the CrS₂ monolayer is a direct band gap semiconductor with a band gap value of 1.32eV,with both valence band maximum and conduction-band minimum locating at the high symmetry point K.Meanwhile,the effective masses of both electron and hole display anisotropic,and the maximum（minimum）effective masses are 1.10m₀（0.88m₀）and 1.05（0.87m₀）,respectively.Based on deformation potential theory,the electron and hole carrier mobility are predicted to 199 cm²V^-1s^-1 and 231 cm²V^-1s^-1,respectively.Furthermore,the optical properties of CrS₂ monolayer including absorbance,reflectance and transmittance were also calculated.The results show that both absorption and reflectivity increase monotonically from infrared region to visible light region.Then,the modulation of electronic structure of monolayer CrS₂ material was further studied by applying strain or change the number of layer.We found that the band gap value decreases monotonically from 2.57eV for-8%strain（3.00eV）to 0.96 eV（0.44eV）for+8%strain under uniaxial（biaxial）strain.In addition,the CrS₂ monolayer can undergo an direct-to-indirect band gap transition when the uniaxial（biaxial）strain exceeds+6.4%（+3.2%）.The wave peaks（valleys）of the absorptivity,reflectivity and transmittance of the CrS₂ will be changed by strain.Under uniaxial strain,the peaks（valleys）of the CrS₂ are located in the blue part of the visible band,while under biaxial strain,the peaks（valleys）are located in the green part.Finally,the effect of thickness on the evolution of band-edges in few-layer CrS₂ was also investigated.The results show the band gap value of 2D CrS₂ semiconductor decreases from 1.32 eV for monolayer to 0.74 eV for quadrilayer.The few-layer CrS₂ thicker than trilayer exhibits an indirect band gap due to the interlayer coupling.With the increase of the number of layers,the absorptivity and reflectivity of CrS₂ in the visible light region increase,and the transmittance decreases.The above calculated results indicate that few-layer CrS₂ semiconductor with direct and tunable band gaps in the visible range make it a potential candidate for novel applications in nanoelectronics and optoelectronics.更多还原