| In 2004,Novoselov and Geim and their collaborators successfully used tape to strip graphene from graphite.Due to its excellent electrical properties,excellent mechanical properties,and high thermal conductivity,large specific surface area,etc,graphene is called"the king of new materials"in many fields.In 2014,Zhao et al.found that the ZT value of SnSe single crystal（Pnma phase）with black phosphorous layered structure could reach 2.6 at 923 K.As an analogue of SnSe,it has been proved to be a potential thermoelectric and optoelectronic material.In order to further improve the thermoelectric and optoelectronic properties of two dimensional GeSe,the effects of strain on the electrical properties,thermoelectric properties and optical properties of single-layer GeSe are studied based on density functional theory,Boltzmann transport theory and deformation potential theory,Our main conclusions are as following:（1）The effective mass,relaxation time and mobility of single-layer GeSe under different strain states were calculated by density functional theory and deformation potential theory.Single-layer GeSe was found to be a direct-bandgap semiconductor with a band-gap of 1.12 eV.Under compressive strains,as the strain increases,the single-layer GeSe gradually undergoes a transition from semiconductor to metal.When a compressive strain 1%is applied in the Zigzag direction or Biaxial direction,the single-layer GeSe is converted from direct-bandgap to indirect-bandgap.Under tensile strains,the bandgap of the single-layer GeSe increases gradually as strain increases,and transforms to indirect-bandgap semiconductor under the tensile strain above 7%or 6%in the Zigzag or Biaxial direction,respectively,the single-layer GeSe is converted from direct-bandgap to indirect-bandgap.The carrier effective mass of single-layer GeSe has anisotropy without strain,with heavy band in the Zigzag direction,and light band in the Armchair direction.And the effective mass of the hole carriers is heavier than the electrons.We have found that the effective mass gradually decreases with increasing compressive strain regardless of the strain direction and in contrast,the effective mass gradually increases as the tensile strain increases.Its carrier mobility at room temperature is 383.84 cm2 V-11 s-1（Zigzag,n-type）,107.73 cm2 V-11 s-1（Zigzag,p-type）,155.24 cm2 V-11 s-1（Armchair,n-type）,266.11cm2 V-11 s-1（Armchair,p-type）.With strains,the carrier mobility increases as the compressive strain increases,and decreases as the tensile strain increases.The carrier lifetime of single-layer GeSe at room temperature is 59.40 fs（Zigzag,n-type）,18.28 fs（Zigzag,p-type）,24.02 fs（Armchair,n-type）and 45.15 fs（Armchair,p-type）.The effect of strains on the carrier lifetime is consistent to that of mobilities.（2）Thermoelectric properties of single-layer GeSe at room temperature,including Seebeck coefficient S,electrical conductivityσand power factor PF,were calculated using first-principles and semi-classical Boltzmann transport theory.We have found that strain has a significant modulation effect on the thermoelectric properties of single-layer GeSe.A tensile strain in the Zigzag or the Biaxial direction increases the Seebeck coefficient of the p-type GeSe monolayer.While under the strains in the Armchair or the Biaxial direction,the electrical conductivity decreases as the tensile strain increases,and increases as the compressive strain increases.The maximum power factor of single-layer GeSe is 1432.76?W/mK2（Zigzag direction,p-type doping）,7076.21?W/mK2（Zigzag direction,n-type doping）,3807.52?W/mK2（Armchair direction,p-type doping）,and 2962.45?W/mK2（Armchair direction,n-type doping）.The corresponding optimal carrier concentrations are 7.9468×1019 cm-3、1.3639×1020 cm-3、6.6590×1019 cm-3、1.1728×1020 cm-3.A tensile strain in the Zigzag direction increases the p-type doping power factor for both in-plane directions.A compressive strain in the Armchair direction increases power factors for the p-type GeSe in the Zigzag direction and n-type GeSe in the Armchair direction,effectively.Applying compressive strain in the Biaxial direction increases the power factor of the p-type doping and n-type doping in the Armchair direction.（3）The influence of strain on the optical properties of single-layer GeSe is studied by the first principle calculation method.Without strains,the peak value of the dielectric function,absorption spectrum and photoconductive conductivity of single-layer GeSe in the Zigzag directions are:2.25 eV、2.35 eV、2.25 eV.The peak values correspond to the wavelengths of light:551.11 nm、527.66 nm、551.11 nm.The values for the direction of Armchair are:2.30 eV、2.65 eV、2.50 eV.They correspond to the wavelengths:539.13 nm、467.92 nm、496.00 nm.Both are in the visible range,indicating that single-layer GeSe is a promising solar material.The influence of strain in the Zigzag direction on the optical properties is weaker than the Armchair and the Biaxial direction.Applying tensile strain will weaken dielectric function,absorption spectrum,and photoconductive intensity peak of single-layer GeSe,and the peak position will be blue-shifted as the strain increases.Applying compressive strain will enhance dielectric function,absorption spectrum and photoconductivity intensity peak of the single-layer GeSe,and the peak position will be red-shifted as the strain increases. |
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