单层二硫化钼带的量子输运

Since the discovery of graphene, it has received widespread attention because it exhibits rich physics such as anomalous quantum Hall effect due to its massless Dirac charged carriers and its high mobility that is a must for the next generation of nano-electronics. However, pristine graphene does not have an intrinsic band gap, a property that is essential for many applications including transistors. Layered transition metal dichalcogenides such as MoS2 represent another class of materials which give rise to new physical phenomena. In its bulk form MoS2 is an indirect band gap semiconductor which turns into a direct band gap semiconductor for monolayer structure.This intrinsic semiconducting nature of MoS2 is a major advantage over graphene as a channel material. Recent experiments show that single layer MoS2 has direct band gap with luminescence quantum efficiency by more than a factor of 104 compared with the bulk material. Because of its direct band gap, MoS2 can be used in light-emitting diodes, photodetectors or solar cells. Because of the thinness of these materials, the devices could be made transparent, light and flexible, and could provide a viable alternative to devices based on organic semiconductors, which degrade relatively quickly in ambient conditions. Monolayer MoS2 high performance transistor was also produced experimentally at room temperature showing the highest on-current, highest transconductance and highest estimated mobility reported to date for a 2D transition metal dichalcogenide, while keeping the extremely high room-temperature current on/off ratio of 108. In addition, due to the broken inversion symmetry and strong spin orbit coupling, monolayer MoS2 exhibits interesting interplay between valley Hall and spin Hall effects. This strong valley-spin coupling can be used to integrate valleytronics and spintronics in monolayer MoS2. Most recently, experimental evidence on the optical selection rules for interband transitions at K points was reported in monolayer MoS2. Due to the importance of monolayer MoS2 from both scientific and application point of view, it is timely to exploit various quantum transport properties through monolayer MoS2. It is the purpose of this project to provide such a study. In this project, we will study the valley Hall effect and spin Hall effect of monolayer MoS2 nanoribbon systems. We will study statistical properties of valley Hall effect and spin Hall effect in the presence of disorders. We will also study ac transport properties of monolayer MoS2 nanoribbon systems. This includes dynamic conductance, electrochemical capacitance, charge relaxation resistance, as well as frequency dependent shot noise that are essential to describe ac characteristics of monolayer MoS2 nanoribbon systems.

石墨烯展现了丰富的物理，但石墨烯没有本征带隙。体材料过渡金属硫化物MoS2是间接带隙半导体，单层MoS2却是直接带隙，这点优于石墨烯。实验证明单层MoS2发光效率比体材料高1万倍，可用来做发光二极管、光探测器和太阳能电池，而且，由于很薄，可做得透明、轻便和可塑，是有机半导体的可行替代品。单层MoS2高效晶体管可在室温下制备，具有大电流、高互导、高迁移率，室温下电流开/断比高达10的8次方。由于反演对称性的破坏和强自旋轨道耦合，也展现了强Valley-Spin耦合。且单层MoS2中K点处光的带间跃迁规则最近也被实验证实。由于单层MoS2的科学价值和应用方面的重要性，其量子输运特性更值得深入研究。本项目将在非平衡格林函数框架下，研究单层MoS2层带的Valley Hall效应和自旋Hall效应，及有杂质下的统计特性，研究其交流输运特性，包括：动力学电导、电化学电容、电荷弛豫电阻及交流散粒噪声。

单层MoS2高效晶体管可在室温下制备，具有大电流、高互导、高迁移率，室温下电流开/断比高、强自旋轨道耦合引起的强Vallye-Spin耦合等性质，因此，其量子输运特性值得深入研究。本项目利用非平衡格林函数理论，建立量子输运的交流输运理论和全计数统计理论，进一步研究单层MoS2的自旋霍尔效应和谷霍尔效应，研究单层MoS2带的直流、交流输运行为。首先，我们建立了包含声子的满足电流守恒和规范不变性的交流输运理论框架，建立了纳米器件的电荷、自旋、能量的瞬态响应行为的全计数统计理论，为研究纳米器件性质的深入研究打下了一定的理论基础；其次，对单层二硫化钼形成的场效应晶体管随门电压变化的特性进行了研究，发现其电导随门电压的大小以及门电压区域的宽度成周期性变化，并从自旋轨道耦合的Rashba效应以及电子进动角度给予了定性解释；第三，对MoS2在左旋光和右旋光照射下的电子极化电流进行了理论计算，发现两种不同旋光可以分别激发K点或者K'点处的电子跃迁，形成自旋极化电流，并对单层和双层下的极化电流进行了理论解释；对光照射下，谷电流的泵浦特性进行了研究；第四，发展了一套随时间变化的满足规范不变性的电子热输运理论，并将其应用于MoS2纳米带的交流电子热流的研究；第五，对MoS2带的半导体带隙进行了理论计算，并通过在带边缘掺入各种不同原子来调整带隙，从而为MoS2光器件的研制提供理论指导；由于WSe2和WTe2与MoS2属于同类过渡金属硫化物，我们也对WSe2纳米管的输运性质（包括：电流电压曲线、散粒噪声、热电势和动力学行为等）进行了详细研究；此外，也对单层WTe2的负微分电阻行为进行了研究；根据研究发展，我们也对单层黑磷和石墨烯的各种性质也进行了研究，并与MoS2等的场效应管进行了比较。