Efficient utilization of light energy based on MoS<sub>2</sub>-XS (X=Al, B, Ga) heterostructures

Abstract

<p indent=0mm>In recent years, two-dimensional transition metal sulfide materials have received widespread attention due to their unique physical and chemical properties, and have been successfully applied in many fields such as microelectronic devices, photovoltaic technology and hydrogen evolution catalysis. However, high recombination rate, low mobility, and low cycle stability all limit the further improvement of the related properties of this kind of material to varying degrees. In our work, two-dimensional materials are composited to optimize the band structure and physical/chemical properties. This kind of approach has been proved to be feasible in many experiments. Not long ago, Demirci et al. studied the III-VI two-dimensional single-layer hexagonal structure MX (M = B, Al, Ga, In; X = O, S, Se, Te). A series of analyses show that all the above binary compounds are structurally stable. They believe that the III-VI group of binary monolayer materials can be widely used for high-performance optoelectronic devices. Based on density functional theory, this work proposes a new heterostructure form of MoS₂-XS (X=Al, B, Ga). Both MoS₂ and XS (M = B, Al, Ga) have the same hexagonal crystal structure (P-6m2), and there are reasons to believe that the MoS₂-XS heterogeneous structures will be fabricated under certain experimental condition. Therefore, it is meaningful to understand how the single-layer MoS₂ and XS form a stable two-dimensional heterostructure. At present, MoS₂-XS based structural characteristics and related physical/chemical properties of the heterostructures have not been reported. Therefore, the purpose of this research is to grasp the basic characteristics and potential application fields of the MoS₂-XS as far as possible. Based on the periodic boundary conditions of the two-dimensional structure, the establishment of a heterostructure requires us to strictly control the mismatch between MoS₂ and XS. We have obtained three highly reliable heterostructures, which maintain the lowest binding energies as well as the smallest mismatches after a series of preliminary calculations. The VBM and CBM of four pristine materials are composed of S atomic states to varying degrees, especially MoS₂ and AlS. Almost all of their band edges are derived from S atoms, which will greatly enhance the recombination of carriers. Therefore, the electrons and holes that can be actually used in the reaction are significantly reduced. How to suppress the recombination of carriers will be one of main goals in this work. The electronic properties of MoS₂-XS will be significantly affected by external strain. We conducted a detailed analysis and discussion on how the strain affects the band gap and band edges. From variable interlayer spacing, electric field to strain, MoS₂-AlS exhibits good catalytic activity, and has an ideal type-II band alignment. Therefore, we believe that MoS₂-AlS is the most ideal choice in terms of photocatalytic water splitting. It is proved that MoS₂-BS is unsuitable for the application of water splitting, but it has excellent efficiency in solar cell applications and its PCE can be as high as ~20.4%. It is worth noting that this value is very close to or even better than that of the reported heterostructure solar cells. In short, we believe that this work provides a meaningful reference for how to efficiently use light energy in two-dimensional composite materials.