Study On The Optoelectronic Properties And Quantum Hall Effect Of New Type Two-dimensional Materials

Because of its unique lattice structure,two-dimensional crystal materials have excellent electronic properties,and have become a hot research topic in recent years.A new type of two-dimensional semiconductor material,black phosphorus,has been found recently,which has good electron mobility(?1000cm2V-1s-1).The excellent electrical properties show that black phosphorus related materials have great potential in the application of nanoscale electronic devices,and their optical properties are also worth exploring,and it is also of great significance to explore the blue phosphorus.In this paper,the photoelectric and quantum Hall effects of monolayer and bilayer black phosphorus and blue phosphorus are studied in detail by using the first principle and a tight binding model.Our main work is:(1)We studied the electronic band structure and optical properties of monolayer and bilayer of black phosphorus,and the dependence of various optical coefficients on the lattice structure of anisotropy was explored.The calculation results show that the monolayer and bilayer of black phosphorus are direct band gap semiconductors.The energy of bilayer black phosphorus system is smaller than that of monolayer,so the structure is more stable.As a biaxial crystal,the optical properties of black phosphorus can be divided into three light polarization directions:X,Y and Z direction.In the frequency domain of visible light,the reflectivity of monolayer black phosphorus in the X and Y direction is low for monolayer and bilayer black phosphorus,the absorption coefficient is also very small.Therefore,there is high transparency in visible light of monolayer and bilayer black phosphorus,and has strong reflection and absorption to ultraviolet(UV)light.In the infrared band,the extinction spectrum of monolayer and bilayer of black phosphorus is greater than that of X and Z direction due to the absorption of excitons in the Y direction.The photoconductivity of monolayer black phosphorus is most sensitive in the Z direction.The photoconductivity of bilayer black phosphorus is most sensitive in the X direction and the most sensitive to the monolayer and bilayer black phosphorus photoconductivity is UV light.The plasma frequency of bilayer black phosphorus is larger than that of monolayer.(2)We studied the electronic band structure and optical properties of monolayer and bilayer of blue phosphorus,and compared with the calculated results of black phosphorus.The caculation results show that there are many similarities between the optoelectronic properties of blue phosphorus and black phosphorus,but the difference is that the monolayer and bilayer blue phosphorus are indirect band gap semiconductors.As a uniaxial crystal,the optical properties of blue phosphorus can be divided into two light polarization directions:X and Z axis.In the Z direction,the absorption coefficient of monolayer blue phosphorus is always smaller than that of bilayer,while black phosphorus has a monolayer absorption coefficient larger than that of bilayer in a local range.The peak value of energy loss of bilayer blue phosphorus is larger than that of monolayer,and the peak value is blue shift compare with monolayer,and the degree of blue shift is greater than that of black phosphorus.It shows that the plasma frequency of bilayer blue phosphorus is significantly larger than that of monolayer blue phosphorus.In high frequency region,the reflectance of monolayer and bilayer is quite different.(3)We studied the effect of an external bias and disorder on the quantum Hall effects(QHE)of monolayer and bilayer black phosphorus in a strong magnetic field.In the monolayer case,the quantized Hall conductivity is similar to that of two-dimensional electron gas(2DEG),but the positions of all the Hall plateaus shift to the left due to the spectral asymmetry.By applying a bias voltage,the width of v = 0 plateaus is increased.In the presence of disorder,the higher Hall plateaus disappear first.The central zero plateaus are the most robust against disorder scattering.The longitudinal conductivity exhibits the anisotropic property that the value along the armchair direction is larger.In the bilayer case,the Hall conductivity remains the same quantization rule as in the case of the monolayer.Under a certain bias voltage,the v = 0 plateau is markedly absent,and the Landau levels(LLs)spectrum shows the appearance of doubly degenerate branches with a zero-energy level,and eventually the system undergoes a phase transition from 2DEG to Dirac semimetal.The central v = ±1 plateaus around the band center are the most robust against disorder scattering.The longitudinal conductivity around zero energy is shown to be nearly a constant about e2/h.