| Janus monolayers with MXY type structure,the middle metal atom M and the chalcogen elements or halogens X and Y on both sides form a symmetry breaking energy level,which greatly improves the exciton absorption intensity.In structural phase change materials,Photocatalytic water splitting,photovoltaic devices,piezoelectric devices,etc.have broad application prospects.The rich surface structure of Janus monolayers makes it easy to form van der Waals heterojunctions with various stacked structures.By changing the external electric field or stress,the van der Waals interaction can be vertically stacked to adjust the maximum value of the valence band and the minimum value of the conduction band.Improve the electronic structure and optical properties of materials and design advanced optoelectronic devices.This paper is based on the first-principles calculation method of density functional theory（DFT）,the Janus BiXI monolayers（X=S、Se、Te）and its BiX1I-BiX2I（X1 and X2 are different Chalcogen atoms）heterojunction structure materials,the influence of different heterojunction structures on their optoelectronic properties was analyzed and discussed,and the following conclusions were obtained:1.In this article,the phonon spectrum of the Janus structure monolayers BiXI does not show any imaginary frequency,the structure is stable,and the phonon dispersion has three obvious acoustic branches at lower frequencies,including longitudinal acoustic mode（LA）,transverse acoustic mode（TA）,and acoustic mode（ZA）,which are distributed from top to bottom in order;BiSI,BiSeI,and BiTeI are indirect band gaps with band gaps of 1.75eV,1.43eV and 1.12eV respectively.The bottom of the conduction band is mainly determined by the polarization strength of the Bielement.The bottom of the conduction band is mainly contributed by the p orbitals of the Bielement,which directly affects the position of the bottom of the conduction band.The top of the valence band corresponds to a much larger contribution from the p orbitals of element X（X=S,Se,Te）than from I and Bielements.As the atomic number of chalcogen element X increases,the electronegativity of the atom decreases and the strength of chemical bonds formed between atoms within a layer weakens,resulting in a decrease in band gap values.Choosing different materials can change the band structure to meet different practical needs.2.We constructed 54 high-symmetry stacked structure models of BiX1I-BiX2I heterojunctions and calculated and analyzed the structural stability of model-I（X1-I）,model-Ⅱ（X1-X2）and model-Ⅲ（I1-I2）,resulting in 9 stable heterojunction structures;Janus van der Waals heterojunction structures changed the band gap structure of single-layer materials,with staggered band gaps.Heterojunction model-Ⅱ（X1-X2）was transformed into a direct band gap type Ⅱ heterojunction semiconductor,while model-I（X1-I）and model-Ⅲ（I1-I2）remained indirect band gaps.The bottom of their conduction bands was mainly contributed by p orbitals of Bielements.The top of their valence bands was different from that of single layers and was mainly contributed by p orbitals of highly electronegative X and I elements.In different stacking modes,s orbitals of Bialso had a small contribution.3.Different stacking modes of Janus van der Waals heterojunctions have different effects on optical properties.Compared with single-layer materials,heterojunctions have larger absorption peak values and both absorption bands and absorption peak wavelengths have redshifted.Compared with single-layer absorption peak wavelengths located in ultraviolet light region,same type heterojunctions have increased absorption rate in visible light region by up to 1.44 times compared to single layer,providing design ideas for design and application of visible light photovoltaic devices.4.After applying biaxial strain to heterojunctions,band gap values decreased under tensile strain and increased under compressive strain.Valence band top degenerated at high symmetry G point.BiSI-BiTeI and BiSeI-BiTeI model-Ⅲ（I1-I2）were converted into direct band gap semiconductors.At same time,absorption peak value was in visible light region under 5%compressive strain,with an increase in absorption rate by 12%-14.4%compared to that without applied stress.Among several heterojunctions studied in this paper,BiSI-BiTeI had highest absorption rate in visible light region.The results of this study show that through heterojunction structure design and external electric field or strain action,material’s electronic structure and energy band gap can be changed to improve material’s photoelectric properties and provide ideas for design and application of photoelectric devices with different needs. |
