Thermoelectric Transport Properties Of Germanene And Black Phosphorene

In the past 20 years,the total energy consumption in China has been increasing,but the energy utilization rate is only about 30%.Thermoelectric conversion technology can directly convert waste heat into electric energy,which can not only improve energy efficiency,but also alleviate environmental pollution.The technical index to measure the efficiency of thermoelectric conversion is the value of thermoelectric coefficient.The thermoelectric coefficient of the traditional bulk structure is low,while the low dimensional nanostructure has high thermoelectric coefficient due to the quantum size effect,so it is likely to be integrated into the next generation of micro-nano electronic and energy conversion equipment.Due to the high integration of nanoelectronic devices,the thermal power potency ineluctably becomes larger and larger,which leads to the deterioration of reliability and performance of the devices.On the other hand,the energy lack and environmental problem are increasingly austere.The majority of energy is wasted in the form of "waste heat".The study of thermal transport and thermoelectric transport properties in low-dimensional nanostructures is becoming a very promising subject In the field of Engineering Thermophysics and energy utilization,which has great significance in both theory and application.It not only can solve the problem of heat dissipation that currently puzzling the development of nanodevices,but also provide theoretical guidance for the efficient recycling of energy and the design and construction of phonon devices that can realize the regulation of thermal transport.In this thesis,according to the non-equilibrium Green’s function(NEGF)theory,we simulate the thermoelectric transport and thermodynamic properties of low-dimensional nanosystems(nanowires,nanosheet),which can provides theoretical guide for the design and manufacture of thermoelectric devices.The cardinal substance of this thesis are as follows:1.Based on the tight-binding approximation model and NEGF,the thermoelectric transport properties and electric field modulation of germanene nanoribbons(GeNRs)with edge defects are investigated.For the perfect GeNRs,we find the maximal thermoelectric figure of merit(ZT)decreases monotonously and the phonon thermal conductivity increases linearly with the increase of the bandwidth of nanobelts.For those containing defects,the maximal ZT increases monotonously with the increase of the number of longitudinal defects,but the phonon thermal conductivity decreases monotonously.Comparing with ZGeNRs,the seebeck coefficient of AGeNRs is larger and the phonon thermal conductivity of AGeNRs is lower,so AGeNRs has greater thermoelectric performance.In addition,the thermoelectric performance of perfect GeNRs is improved and GeNRs with edge defects is reduced under external electric field.Based on this theoretical simulation,the thermoelectric performance of GeNRs can be greatly improved by modulate the number and geometry of defects and electric fields,and it provides theoretical guidance for the thermoelectric application of GeNRs.2.Using the first principle simulation and NEGF,we study the ballistic thermal transport properties and strain modulation of thermal properties for phosphorene nanosheet.For perfect phosphorene nanosheets,the anisotropy of phonon thermal conductivity is significantly high.When the temperature is larger than the Debye temperature,the phonon thermal conductivity is straightly proportional to the negative first power of temperature.By applying uniaxial strain in zigzag and armchair direction of phosphorene nanosheets,there are imaginary frequency happening and photonic band gap changing in the phonon spectra,and it demonstrates that the phonon heat transport can be tuned by apply strain.According to the phonon spectrum under the qualification of strain,we calculate free energy,entropy,specific heat capacity of phosphorene nanosheets,and it shows that the free energy,entropy,specific heat capacity can be modulated by applying strain.Our study can provides theoretical guidance for the design and manufacture of phosphorene-based thermal nano-devices.