(Ge,Sn)Se基范德瓦尔斯异质结的构筑及其输运性质研究

Two-dimensional (2D) nanomaterials have been extensively studied ever since a monolayer graphene was isolated from graphite by mechanical exfoliation in 2004. Thereafter, the interest is promptly extended to other nove 2D nanomaterials, such as hexagonal boron nitride, silicene, phosphorene, transition metal dichalcogenides, group-IV monochalcogenides, and so on. Furthermore, it is found that if one kind of 2D nanomaterials is placed upon other 2D nanomaterials to construct van der Waals (vdW) heterostructures, the systems will show some novel structures, physical properties and the pontential applies in the next-generation electronics. Recently, a new kind of novel 2D nanomaterials (group-IV monochalcogenides, (Ge,Sn)Se) has attracted great attentions. (Ge,Sn)Se monolayers are structurely similar and isoelectronic to monolayer phosphorene. In contrast with graphene, (Ge,Sn)Se and phosphorene are direct semiconductors and have the intrinsic ferroelasticity. However, the electron and hole effective mass in (Ge,Sn)Se monolayers are less than those in phosphorene. That is to say, (Ge,Sn)Se maybe have a higher mobility than phosphorene and are the promising candidates in electronic devices. Due to the lack of inversion symmetry in (Ge,Sn)Se monolayers, they show ferroelectricity and piezoelectricity, with piezoelectric constants higher than those in boronitrene and MoS2. Density functional theory predicts that the thermoelectric figure of merit reaches 3.27 or 2.76 along the armchair or zigzag directions with an optimal n-type carrier concentration in SnSe monolayer because of the exceptionally low lattice thermal conductivity. However, the thermoelectric figure of merits of SnSe would drastically reduce with the temperature decreasing. Moreover, density functional theory in combination with Boltzmann transport calculations show that the thermoelectric figure of merits of phosphorene reaches to 1 at room temperature. Thus, as expected, (Ge,Sn)Se/phosphorene vdW heterostructures will show better thermoelectric performance in a wider temperature region. Owing to the spontaneous electron-hole charge separation in GeSe/phosphorene vdW heterostructures, they are the promising candidates for high-performance optoelectronic devices. However, few papers on (Ge,Sn)Se/phosphorene vdW heterostructures have been reported. Therefore, we firstly construct (Ge,Sn)Se/phosphorene vdW heterostructures by taking into account the different stacking ways between (Ge,Sn)Se and phosphorene. Secondly, the formation energies are calculated to investigate the thermodynamic stability with density functional theory and the phonon spectra are caluclated to discuss the dynamical stability within density functional perturbation theory. Thirdly, the comparative calculations including van der Waals corrections and spin-orbital coupling interactions are carried out. Fourthly, the electric or thermoelectric transport properties are discussed within density functional theory in combination with non-equilibrium Green function. Furthermore, the effects of strains and vertical electric fields on electronic structures and transport properties are investigated. To understand the effect of electric fields, the redistribution of electron density as well as the transferred electrons between intra- and inter-layers are systemically calculated by the integration of electron density in real-space grids. At last, all the above-mentioned investigations will be expand to the studies on (Ge,Sn)Se/ transition metal dichalcogenides vdW heterostructures.

把不同晶体结构和能带结构的二维纳米材料叠加并利用层间范德瓦耳斯力来构筑异质结，极有可能产生新的材料结构和物理性质及在未来信息器件方面的潜在应用价值。(Ge,Sn)Se与黑磷具有相似结构和外层价电子数并都是半导体且具有本征铁弹性。但(Ge,Sn)Se中载流子有效质量要小于黑磷并且分别在高温和室温表现出优异热电特性。(Ge,Sn)Se因不具有反演对称性表现出压电和铁电特性。GeSe/黑磷异质结中因存在自发电子-空穴分离是优异的光电材料，但是(Ge,Sn)Se/黑磷异质结方面的研究还较少。我们首先构筑(Ge,Sn)Se/黑磷异质结，研究自旋-轨道耦合效应、范德瓦耳斯力对不同层间叠加方式稳定性的影响及相应的能带结构特征，接着利用密度泛函理论结合非平衡格林函数的方法来研究异质结的量子输运特性及应力和电场的影响并设计一两种器件模型。最后，将研究内容扩展到(Ge,Sn)Se/过渡金属二硫化物异质结中去。

自从2004年石墨烯被剥离以来，二维纳米材料就成为举世瞩目的研究热点。把不同晶体结构和能带结构的二维纳米材料叠加并利用层间范德瓦耳斯作用来构筑异质结，极有可能产生新的材料结构和物理性质以及在未来信息器件等方面的潜在应用。本项目我们以具有铁电、压电及热电特性的二维SnSe纳米材料为出发点，来构筑其双层结构以及与黑磷构筑范德瓦尔斯异质结并研究其对应的电子结构、输运性质等。本项目重点关注其电子输运特性，因此，依托本项目我们首先测试了两种第一性原理的计算方法：“密度泛函理论+半经典玻尔兹曼输运理论”和“密度泛函理论+非平衡格林函数方法”。我们利用密度泛函理论结合半经典玻尔兹曼输运理论计算了一类1T相结构的二维过渡金属二硫属化物（ZrS2，ZrSe2）的电导率、塞贝克系数以及电子热导率等输运性质，并进而发现这类材料具有非常高热功的P型热电材料。这主要来自于其具有较高的空穴迁移率。把单个B80富勒烯分子耦合在两个金纳米线电极之间构筑成分子结，并利用密度泛函理论结合非平衡格林函数的方法计算其透射谱，进而得到电导、塞贝克系数、电子热导等输运性质，发现分子结的输运性质尤其是塞贝克系数可以由应变、化学势等调制。接着，在探讨层间范德瓦尔斯异质结之前，我们构筑了由过渡金属二硫属化物的1T相和2H相组成的层内异质结的结构并计算了其弹性杨氏模量跟泊松比，发现界面对其力学性质是具有破坏性的。这与范德瓦尔斯异质结形成了对比。然后，我们考虑了四种不同堆叠形式的双层SnSe等纳米结构，发现AB堆叠形式的双层结构能量最低并且为直接带隙半导体而其它三种堆叠形式的都表现为间接带隙半导体。无论何种形式的堆叠方式双层结构的带隙都比单层的小，这是由于层间相互作用导致价带和导带能带宽度增加。输运性质计算，发现AA堆叠形式的电导率和塞贝克系数相同条件下更好，这主要是其较高对称性导致能带的简并性更高。最后，我们考虑四种不同堆叠形式构筑了由单层GeS，GeSe，SnS，SnSe等与黑磷组成的范德瓦尔斯异质结，发现AB堆叠形式仍然具有最低的能量。然而，电子结构计算发现无论何种堆叠形式SnSe/黑磷构筑成的范德瓦尔斯异质结都表现为金属性，具有良好的导电性。依托该项目，我们利用密度泛函理论结合半经典玻尔兹曼理论/非平衡格林函数方法研究了不同堆叠形式双层SnSe及SnSe/黑磷异质结的电子结构和输运性质并为其信息器件设计奠定基础

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