Theoretical Studies On The Opto-electronic Properties Of Two Dimensional Materials And Van Der Waals Heterostructures

Due to the nano-size effect,the two-dimensional materials possess unique and excellent photoelectric,mechanical and magnetic properties,so it has been used in microelectronics,optoelectronics,electronic devices,sensors,solar cells,super capacitors and photocatalytic application.Moreover,two dimensional polarized materials,which possess a vertical intrinsic electric field induced by the dipole moment,show promising excellent properties in photocatalytic water splitting.On the one hand,the carriers can be separated from interior to surface efficiently under the effect of the induced vertical intrinsic electric field.On the other hand,the effective solar light harvesting can be realized.In this paper,the keys,the effective solar light harvesting and carrier separation,to enhance the efficiency of photocatalytic water splitting process are considered.We design and investigate the opto-electronic properties based on the 2D polarized material Ge X?X=S,Se?by first principles calculations.The structural,opto-electronic and photocatalytic properties were systematically investigated.The effect of biaxial strain and forming vd W heterostructure were also discussed.The results were showed as following.?1?We systematically investigated the electronic,optical and photocatalytic properties of the?phase monolayer Ge X?X=S,Se?under external biaxial strain.The band gaps of monolayer Ge S and Ge Se are 3.265 e V and 2.993 e V,respectively.The direction pointing of the dipole-induced internal electric field is from Ge atomic surface to X?X=S?Se?atomic surface.Moreover,it is effective to tune the biaxial strain to control the electronic and optical properties of the monolayer Ge X?X=S,Se?.The the band gaps of the monolayer Ge X?X=S,Se?could be modulated to 1.732 e V?1.130 e V?,and the absorption for visible light could be enhanced at the same time.A surface potential difference and larger electric field were obtained in the monolayer Ge X?X=S,Se?by a tensile strain.All the opto-electronic and photocatalytic properties support that the photocatalytic activity of the monolayer Ge X?X=S,Se?under ultraviolet and visible light could be enhanced by strain engineering,especially tensile strain.?2?We proposed a 2D polarized Ge S/Mo Se₂ vd W heterostructure to get better photocatalytic performance than isolated monolayer.The opto-electronic properties and photocatalytic activity of the Ge S/Mo Se₂ heterostructure were systematically investigated.Thanks to the advantageous properties of Ge S monolayer and Mo Se₂ monolayer were well preserved in the Ge S/Mo Se₂ vd W heterostructure,the performance of Ge S/Mo Se₂ heterostructure was better than isolated materials.Such as,the Ge S/Mo Se₂ heterostructure possess a suitable band gap?1.387 e V?that can harvest visible light effectively.There is a dipole induced internal electric field in the Ge S/Mo Se₂heterostructure,which is helpful to reduce the probability of recombination of photo-generated electron-hole pairs.Furthermore,the Ge S/Mo Se₂ heterostructure possesses strong solar absorption performance with a high absorption coefficient(about 10⁶ cm^-1).The band alignments of the Ge S/Mo Se₂ heterostructure well agree with the redox potential required for the water splitting reaction.In addition,strain engineering is a useful method to tune the opto-electronic properties and enhances photocatalytic activity of Ge S/Mo Se₂ heterostructure.?3?Forming heterostructure based on the 2D polarized materials can get better photocatalytic performance than isolated monolayer.However,the coupling effect of dipole-induced electric field and interface built-in electric field in the 2D polarized materials is not clearly.Therefore,we constructed Ge S/Mo Te₂ heterostructure and investigated the coupling effect of dipole-induced electric field and interface built-in electric field based on the 2D polarized materials.The photocatalytic activity of the Ge S/Mo Te₂heterostructure was systematically investigated.The results indicate that the dipole-induced electric field in all stacking order are almost the same.But the interface built-in electric field is different in different stacking order.Moreover,the dipole-induced electric field is much larger than the interface built-in electric field,indicating the dipole-induced electric field contributed dominatly to the total built in electric field.Moreover,our results suggest that the 2D polarized Ge S/Mo Te₂ heterostructure is a high-efficienct photocatalyst candidate for water splitting.Such as,direct semiconductor with a band gap about 1.505 e V,wide light spectrum ranging from ultraviolet to near-infrared light can be adsorbed with a high absorption coefficient(about 10⁶ cm^-1),total built in electric field and suitbable band alignments.We concluded that 2D polarized material based Ge S/Mo Te₂ van der Waals heterostructure is a high-efficiency photocatalyst candidate for water splitting under ultraviolet to near-infrared light.?4?In order to understand the distinctive advantage properties of the 2D polarized materials system,we have investigated the electronic and optical properties of the heterostructures based on the general two dimensional material which did not possess a dipole-induced internal electric field.Due to the monolayer blue?P and monolayer Ge X?X=S,Se?share a similar hexagonal crystal structure,we have constructed blue?P/Mo X?X=S,Se?₂ heterostructures based on the blue?P.The results showed the blue?P/Mo X?X=S,Se?₂ vd W heterostructures were indirect gap semiconductors,and the band gap values can be tuned by the strain.For blue?P/Mo Se₂ heterostructure,the system could be transited to be a direct semiconductor in the range of-2%to-5.5%biaxial compressive strain.The blue?P/Mo X?X=S,Se?₂ heterostructures occured the semiconductor-metal transition at critical strains of-0.085?-0.105?and+0.09?+0.135?.The blue?P/Mo X?X=S,Se?₂ heterostructures can absorb strongly the visible-UV light,which is in the range of 50 to 800 nm.Strain engineering is an effective method to tune and enhance the optical properties of blue?P/Mo X?X=S,Se?₂ vd W heterostructures.The results suggest that blue?P/Mo X?X=S,Se?₂ heterostructures are promisingly applied in visible-ultraviolet optoelectronic due to their tunable electronic and optical properties.We have designed and investigated the opto-electronic properties and photocatalytic properties of the 2D polarized materials and their heterostructures by first principles calculations.The 2D polarized materials possess a dipole induced internal electric field,which reduce the probability of recombination of electron-hole pairs and enhance the solar energy conversion efficiency.Moreover,thanks to the advantageous properties of constitiuent material can be preserved in the heterostructure,the performance heterostructure is better than isolated monolayer.Our results suggest that 2D polarized material and its heterostructures could be a promising candidate in solar cells,optoelectronic sensors and photocatalytic applications.