Calculation And Design Of Novel MoS₂/TiO₂?ZnO? Van Der Waals Heterostructures

In recent years,two-dimensional?2D?ultrathin materials with different properties from bulk materials have attracted more and more attention,but single 2D material more or less has some deficiencies.For instance,the monolayer MoS₂ is a direct band gap semiconductor,with a small band gap?1.90 eV?and the light absorption window in the visible region,so the absorption efficiency is high,but the total amount of light absorption is very small as a result of monolayer.The novel 2D lepidocrocite-type TiO₂is a broad band gap semiconductor with strong redox power,but the light harvesting window is restricted to ultraviolet range,and thus is very inefficient for solar light absorption.Therefore,if two or more different 2D materials are combined into heterostructures,it is expected to make the best of individual advantages of different layers to achieve some special structures and novel properties.Moreover,the related theories and numerical algorithms of density functional theory?DFT?have been rapidly developed,and the first principle methods based on DFT have become common research methods in condensed matter physics,quantum chemistry and material science.Therefore,in this thesis,monolayer MoS₂,novel 2D lepidocrocite-type TiO₂ and graphene-like ZnO?g-ZnO?as well as corresponding heterostructures have been studied systematically by first principle methods,including the geometric structures,electronic properties,etc.,and a novel design scheme based on MoS₂/TiO₂ heterostructure is proposed for solar cells and other photoelectric nano devices.The research results have certain reference value and guiding significance for the preparation of new types of high performance solar cells and photocatalysts.The obtained main results are as follows:?1?For the first time,the structural,electronic and optical properties of 2D MoS₂/TiO₂ van der Waals?vd W?heterostructure that consists of 2D lepidocrocite-type TiO₂ and monolayer MoS₂ are investigated.It is found that there is a type-II band alignment between the MoS₂ and TiO₂ layers,and the conduction band offset?CBO?is larger than the exciton binding energy in monolayer MoS₂,which will be beneficial for the separation of photogenerated electrons and holes and improve photoelectric conversion efficiency.In addition,it is also found that the electronic structures of monolayer MoS₂ and 2D lepidocrocite-type TiO₂ are well retained in their individual layers,namely,both monolayer MoS₂ and 2D TiO₂ in the heterostructure exhibit individually a direct band gap,and the band gaps are almost the same as those of isolated monolayer MoS₂ and 2D lepidocrocite-type TiO₂.Furthermore,a novel design scheme based on the new 2D MoS₂/TiO₂ heterostructure is proposed for a solar cell,and the power conversion efficiency?PCE?of the MoS₂/TiO₂ heterostructure solar cell is estimated to be about 8%.?2?Based on the study of 2D MoS₂/TiO₂ vd W heterostructure,MoS₂/TiO₂multilayer heterostructure and superlattice are further studied by combining monolayer MoS₂ with 2D lepidocrocite-type TiO₂ to explore the effect of the layer number on the electronic properties of MoS₂/TiO₂ heterostructure.It is found that the electronic properties of MoS₂/TiO₂ multilayer heterostructure are similar to those of MoS₂/TiO₂bilayer heterostructure.Moreover,the stacking pattern of MoS₂/TiO₂ superlattice is...ABACABAC...,and a type-II band alignment between the MoS₂ and TiO₂ layers is still discovered in the MoS₂/TiO₂ superlattice,and the electronic properties of the superlattice are almost the same as those of MoS₂/TiO₂ bilayer heterostructure.?3?The adsorption of TBP and Imidazole on MoS₂ is investigated to explore the change of electronic properties of 2D MoS₂/TiO₂ heterostructure.It is found that both TBP and Imidazole can move the band structure of MoS₂/TiO₂ heterostructure as a whole to lower energy direction,and then enhance the oxidizing ability of MoS₂/TiO₂heterostructure,especially imidazile,which will make 2D MoS₂/TiO₂ heterostructure potential applications in the photocatalytic degradation of water pollutants,etc.?4?Analogous to 2D MoS₂/TiO₂ vd W heterostructure,by combining monolayer MoS₂ with g-ZnO into heterostructures with six different stacking patterns,the most stable stacking pattern is obtained after comparing their energies.Based on the calculated electronic properties,it is found that the MoS₂/ZnO heterostructure is still of a type-II band alignment between the MoS₂ and ZnO layers,and there is a stronger interlayer coupling compared to MoS₂/TiO₂ heterostructure,which is beneficial for charge transfer between layers.Furthermore,it is found that the MoS₂/ZnO heterostructure will undergo a quantum phase transition from semiconductor to semimetal when the thickness of ZnO reaches five layers,and electrons will automatically transfer from the valence band of ZnO to the conduction band of MoS₂,leading to the automatic separation of electrons and holes in space,as well as the formation of two-dimensional electron gas?2DEG?and two-dimensional hole gas?2DHG?.