The First Principle Study On The Wurtzite And Sphalerite Nanofilms

With the rapid development of science and technology,semiconductor devices are becoming more and more miniaturized and highly efficient.Wurtzite and sphalerite,as the third generation of semiconductor materials,have been widely studied and applied,and their two-dimensional structures have attracted much attention due to their unique and novel physical,chemical and electronic properties.In this thesis,the geometric and electronic structures of wurtzite and sphalerite nanofilms have been studied theoretically.The tuning effects of electric field on them have also been discussed.The purpose is to provide some theoretical insight for the application of these materials in related fields.The main contents of the thesis are as follows:(1)The geometric structure of some ultrathin wurtzite(001)and sphalerite(111)surface nanofilms with different layers were studied.The results show that all the nanofilms studied in this these have a critical number of layers.When the number of layers of nanofilm is less than or equal to the critical number,the structure will spontaneously transform into graphite like structure,while the structure is still similar to the block structure when the number of layers of nanofilm is larger than the critical number.In addition,the study also found that the two kinds of structures of the studied wurtzite nanofilms correspond to two different natures.The structure which has layers less than the critical number performs like semiconductor and the band gap decreases as the number of layers increases.The structure with layers larger than the critical number performs like metal,which is due to the surface charge caused by atom enrichment.However,the studied sphalerite nanofilms can divided into two different types.The first one is similar to wurtzite,and the second one has a special intermediate state.The intermediate state nanofilms are similar to sphalerite bulk structure but exhibit semiconductor properties.(2)The tuning effects of applying external electric field on the geometric and electronic structures of optimized sphalerite and wurtzite single layer films have been discussed.On the geometric structure,when the electric field is applied in a positive direction along the z axis,the single layer plane structure film will become buckled structure with the increase of electric field intensity.The larger the electric field is,the greater the buckling degree will be.When the intensity of the electric field is the same but with opposite direction,the direction of their buckling is opposite,and the relationship is symmetrical.When considering the electronic structure,the band gap starts to decrease when the electric field intensity reaches a threshold value,and the effect of positive electric field on the band gap is slightly larger than the negative electric field.Further analysis showed that the electric field changes the occupation state of cationic s or p orbital,moving down the bottom of the conduction band of the ? point,and the bottom of the conduction band of the semiconductor tends to ? point finally.(3)The effects of electric field on the number of critical layers and the electronic structure of multilayer wurtzite nanofilms have been studied.The results show that when the electric field applied along the negative direction of z axis reaches a certain magnitude,it can significantly increase the original critical number of layers of wurtzite nanofilms,and transform the original block structure into graphite like structure.When transforming to graphite like structure,the electronic structure of the system transforms from metal state to semiconductor state with the surface charge vanished due to structural transformation.