Electronic Transport And Optical Absorption In Semiconductor And Graphene Nanostructures

With the rapid development of science and technology, the sizes of electronic de-vices have reduced to nanoscale. Quantum effects become very obvious and important in nanodevices, and make these devices own unique properties and extensive application potential in a lot of fields, which have attracted many researchers’ interests. People find in these nanodevices that it is better to carry signal by the spin degree of freedom than the charge, due to spin has a longer decoherence time and lower energy consumption. In com-parison with spin, the valley degree of freedom in graphene, an emerging carbon-based material, can also carry information for a long distance.We study the electron transport properties in nanostructures like the conventional semiconductor quantum dots, graphene quantum dot, graphene nanoribbons and so on. Therein, we pay attention to the spin polarized transport in the semiconductors, and the electric dipole transition in graphene quantum dots and valley polarized transport in graphene strips. We get many interesting results, which maybe benefit the design of the future electric devices owing lower power consumption and quicker operation speed. The concrete arrangements of our paper are as following:In chapter one, we first introduce the basic properties of the semiconductor quantum dots, graphene and graphene quantum dots. Then we describe the theoretical methods used in our paper.In chapter two, we study the spin-polarized transport through a semiconductor quan-tum dot connected to a normal metal lead and a ferromagnetic lead, applied with different temperatures. Using the master equation approach, we find that in such a system the spin polarized thermal electric current has a rectification effect; that is, in the positive temper-ature bias regime, the current polarization has a nonzero plateau, while in the negative temperature bias regime, the current polarization vanishes. In addition, the current po-larization exhibits a spin-valve effect, which corresponds to the existence of a finite zero region controlled by the gate voltage, and the size of the zero region is determined by Coulomb interaction and temperature bias.In chapter three, we theoretically propose a unipolar spin diode consisting of two serially coupled quantum dots connected to two normal metal electrodes, where the two electrodes have different temperatures and the two dots have different Zeeman splittings. Using the master equation approach, we can observe that the current through this system can be from zero to nearly100%spin-polarized by tuning the difference of the electrode temperatures or the gate voltages applied to the quantum dots. This particular transport property is attributed to the interplay of strong intradot Coulomb interactions, the asym-metrical energy level distributions in the two dots induced by different Zeeman splittings and different Fermi distributions in the two electrodes caused by different temperatures. Also, we propose a bipolar spin diode using the two unipolar spin diodes. These configu-rations may provide the essential elements for designing logic circuits and even magnetic heads.In chapter four, we investigate the uniaxial strain modulated electronic structure and optical absorption of a triangular zigzag graphene quantum dot within the tight-binding approach. The electronic structure and optical absorption can be correctly characterized by the symmetry analysis before and after the strain is applied. We can observe the red-shift or blueshift of the absorption peaks in the optical spectrum by uniaxial tensile or compressive strain, indicating that the strained triangular zigzag graphene quantum dot can be used as a strain sensor. We also consider the influence of dot sizes on the sensor sensitivity. Furthermore, we confirm the robustness of such a function against a single vacancy defect. On the other hand, by applying a gate voltage on the strained dot, the Fermi energy is shifted away from zero, obvious far-infrared absorption peaks can appear in the optical spectrum, which means it is possible to realize far-infrared photodetectors based on strained graphene quantum dots. In chapter five, we investigate the valley-dependent electronic transport through a graphene monolayer modulated simultaneously by a uniform uniaxial strain and linearly polarized light. Within the Floquet formalism, we calculate the transmission probabilities and conductances of the two valleys. We find that valley polarization can appear only if the two modulations coexist. Under a proper stretching of the sample, the ratio of light intensity and light frequency squared is important. If this quantity is small, the electron transport is mainly contributed by the valley-symmetric central band and the conductance is valley unpolarized; but when this quantity is large, the valley-asymmetric sidebands also take part in the transport and the valley polarization of the conductance appears. Furthermore, we can tune the degree of polarization by the strain strength, light intensity and light frequency. We propose that the detection of valley polarization can be realized utilizing the valley beam splitting. Thus, a graphene monolayer can be used as a mechanically and optically controlled valley filter.In the last chapter, we make a brief summary of our paper and give an outlook about our future study.