【作者】 刘春明；

【导师】 刘才龙；

【作者基本信息】 吉林大学 ， 原子与分子物理， 2019， 硕士

【摘要】 在石墨烯被报道其内部含有独特的层状结构以后,具有类似结构的IV-VI族二维层状半导体材料吸引了人们广泛的研究兴趣。硫化亚锡(Sn S)作为IVVI族化合物中极具代表性的半导体材料之一,引发了科学家们的研究热潮。本文选取应用直流电弧等离子体放电方法合成的少层Sn S纳米片作为研究对象,通过X射线衍射(XRD)、扫描电子显微镜(SEM)和透射电子显微镜(TEM)对样品在常压时的结构和形貌进行了表征。通过对其高压原位拉曼光谱(Raman spectroscopy)和高压同步辐射X射线衍射光谱的分析,研究了Sn S纳米片在高压下的结构性质。通过高压原位室温电阻率、高压原位变温电阻率以及高压原位Hall效应的测量,对其在高压下的电输运性质进行了系统的研究。探索压力对Sn S纳米片结构及电输运性质的调控作用。具体的研究成果如下:1、我们通过直流电弧等离子体放电方法成功合成了少层Sn S纳米片。利用SEM和TEM对Sn S纳米片的形貌进行了表征,得知Sn S纳米片样品的平均长度和宽度为400-500nm,平均厚度为20-30nm。利用X射线衍射技术对其常压结构进行了表征,结果表明Sn S纳米片为正交结构具有Pnma对称性的单晶粉末样品。通过对其高压原位拉曼光谱及高压原位同步辐射X射线衍射光谱的分析,发现Sn S纳米片在3.0GPa时发生压致正交结构(Pnma)→正交结构(Cmcm)的二阶连续等结构相变,在13.0GPa时发生由正交结构(Cmcm)→单斜结构(P21/C)的一阶结构相变。2、通过高压原位直流电阻率实验,我们发现在整个压力范围内Sn S纳米片电阻率随压力增加而减小。在3.0GPa和12.0GPa处,Sn S纳米片电阻值均发生不连续变化,我们把两次不连续变化归因于结构相变。值得注意的是,相比于体材料Sn S(相变压力点约为12.6GPa和18.2GPa),Sn S纳米片两次相变的压力点均提前了,其中第一个相变压力点提前9.6GPa,第二个相转变压力点提前6.2GPa。结构相转变的差异可以归因于Sn S纳米片微观尺寸效应以及其层状形貌所具有的特殊表面和界面结构。卸压后Sn S纳米片电阻率基本恢复到原来的数量级,说明其相变过程是可逆的。过高压原位霍尔效应测量系统研究了压力对Sn S纳米片样品电输运性质的影响。霍尔系数(RH)始终为正值,这表明在整个压力区间内Sn S纳米片中起主要传导作用的载流子是空穴。电输运参数如电阻率、霍尔系数、载流子浓度和载流子迁移率等在3.0GPa和12.4GPa左右都发生了明显的不连续变化,这些变化归因于结构相变。3、通过高压原位变温电阻率测量实验,通过数据分析得出Sn S纳米片在常压到7.0GPa电阻率随温度的升高呈下降趋势,可判断在此压力区间内Sn S纳米片表现为明显的半导体特性。到8.0GPa时,Sn S纳米片的电阻率随着温度的升高呈上升趋势,表现为金属特性。对比霍尔效应测量结果,我们发现该金属或半金属相为“坏金属”,以空穴导电为主。我们给出了半导体Sn S纳米片的激活能随压力的依赖关系来反映其杂质能级随压力变化情况。此外,我们还发现:与Sn S体材料数据对比,Sn S纳米片金属化相变点也提前2.3GPa,在高压下展示出极强的尺度效应。更多还原

【Abstract】 After graphene has been reported to contain a unique layered structure,the IV-VI two-dimensional layered semiconductor material with similar structure has attracted wide research interest.As one of the most representative semiconductor materials in its family,stannous sulfide（SnS）has aroused great interest among scientists.In this thesis,the few layer SnS nanosheets synthesized by direct current（DC）arc plasma discharge method are selected as the research object,the microstructure and morphology of the samples at ambient conditions were characterized by X-ray diffraction（XRD）,scanning electron microscopy（SEM）and transmission electron microscopy（TEM）,respectively.The structural properties of SnS nanosheets under high pressure were investigated by high pressure in situ Raman spectroscopy and synchrotron radiation XRD spectroscopy.The electrical transport properties under high pressure were systematically studied by high-pressure in situ room temperature resistivity,high-pressure in situ temperature-dependence resistivity and high-pressure in situ Hall effect measurements.Our goal is to explore how pressure tune the structure and electrical transport properties of SnS nanosheets.The specific research results are as follows:1.We successfully synthesized SnS nanosheets by DC arc plasma discharge method.The morphology of SnS nanosheets was characterized by SEM and TEM.The average length and width of SnS nanosheets were 400-500 nm,and the average thickness was 20-30 nm.The structure of SnS nanosheets under ambient conditions was characterized by XRD technique.The results show that the SnS nanosheets are single crystal powder samples with orthorhombic structure（Pnma）.By high-pressure in situ Raman spectroscopy and high-pressure in situ synchrotron radiation XRD spectroscopy measurements,we find that the SnS nanosheets undergo a second-order isostructural continuous phase transition from orthorhombic structure（Pnma）to orthogonal structure（Cmcm）at 3.0 GPa,and then another structural phase transition occurred from orthorhombic structure（Cmcm）to monoclinic structure（P2₁/C）at 13.0GPa.2.Through high-pressure in situ DC resistivity experiments,we find that the electrical resistivity of SnS nanosheets decreases with increasing pressure over the entire pressure range.The resistance values have two discontinuous changes at 3.0GPa and 12.0 GPa,which are attributed to the structural phase transitions.It is worth noting that both the two phase transition pressures of the SnS nanosheets are considerably lower than the bulk material SnS.The first phase transition pressure reduces about 9.6 GPa,and the second phase transition pressure reduces about 6.2GPa,respectively.These differences can be attributed to the microscopic size effect of the SnS nanosheets and their special surface and interface structure of the layered morphology.After the pressure release,the electrical resistivity value of the SnS nanosheets recovered to its original order of magnitude,indicating that the phase transition process is reversible.3.The effect of pressure on the electrical transport properties of SnS nanosheets was investigated by high-pressure in-situ Hall effect measurements.It is found that the value of Hall coefficient（R_H）was always positive in the whole compression process,which indicates that the holes are the dominant carrier under compression.The electrical transport parameters include electrical resistivity,Hall coefficient,carrier concentration and carrier mobility change obviously at around 3.0 GPa and12.4 GPa,mainly due to the structural phase transition.4.Through the high-pressure in situ temperature dependent electrical resistivity experiment,we find that the electrical resistivity of the sample decrease with increasing temperature from ambient pressure to 7.0 GPa,which indicates that the SnS nanosheets exhibited obvious semiconductor characteristics.At the pressure above 8.0 GPa,the electrical resistivity of SnS increased with increasing temperature,and the SnS nanosheets exhibit metallic behaviors.Compared with the hall effect measurement results,we find that the metal or semi-metal phase is a“bad metal”,which is dominated by hole conduction.The dependence of activation energy of semiconductor SnS nanosheets with pressure is given to reflect the change of impurity level with pressure.In addition,compared with the data of SnS bulk materials,we also find that the metallization phase transition pressure of SnS nanosheets was about2.3 GPa lower than that in bulk materials,showing a strong size-dependent behavior under high compression.更多还原