Study Of The Electronic Properties Of TH-Si_xC_y Surface Adsorption And Nanoribbons

Two-dimensional tetrahexa-ring silicon carbide(TH-SixCy)has the characteristics of low formation energy,limited band gap,and indirect or direct leapfrogging modes,giving it potential for applications in nanodevices,optoelectronic devices,gas sensor devices,etc.As surface functionalisation,gas surface adsorption and nanoribbons can all modul,ate the energy band structure of 2D materials,the indirect leapfrogging mode of TH-SixCy and the small band gap limit its application in the field of semiconductor devices.Therefore,this thesis work investigates the effects of H/F adsorption surface functionalization,surface adsorption of hazardous gases and water,different nanoribbon tailoring and passivation methods on the electronic properties of THSixCy using a first principles density flooding theory(DFT)approach,provides an in-depth analysis of the mechanism of their energy band structure changes,explores the applications of TH-SixCy in the fields of optoelectronic devices,heterojunction devices,photocatalytic hydrolysis,and serves as a theoretical guide for the practical application of TH-SixCy as a gas sensor,with the following main conclusions:1.H/F adsorption modulates the energy band structure,effective mass,work function,electron affinity energy and other properties of TH-SixCy.TH-SixCy behaves as a metal or semiconductor after H/F adsorption;TH-SiC2 achieves a direct-indirect leap mode transition;the effective mass of TH-SixCy exhibits anisotropy at the semiconductor nature;the H adsorption TH-SixCy work function as well as the electron affinity energy decrease with increasing adsorption rate,and the work function as well as the electron affinity energy increase with increasing adsorption rate at F adsorption.When the H/F adsorption rate is 1/8,1/4D,TH-SixCy exhibits a metallic nature.This is due to the adsorption of H/F atoms leading to the breaking of covalent bonds in TH-SixCy and the creation of suspension bonds,which in turn lead to the appearance of super-zero energy bands in the energy band,exhibiting a metallic nature,the number of super-zero energy bands depending on the number of suspension bonds appearing in TH-SixCy.While the TH-SixCy energy band exhibits a semiconducting nature when H/F is at 1/4S,1/2,and 1 adsorption rates,the TH-SiC band gap varies between 1.09eV-2.83eV and the TH-SiC2 band gap is regulated between 0.02eV-2.21eV.This is due to the gradual conversion of the double bond(π+σ bond)in TH-SixCy to a single bond(σ bond)for each pair of H/F atoms adsorbed and the gradual disappearance of the band edge contributed by the π bond,resulting in a band gap larger than that of the pristine TH-SixCy.2.NO,SO2 and NO2 are adsorbed on TH-SiC with a charge transfer of 0.42e,0.64e and 0.37e from the gas to the substrate respectively,which is chemisorption.Compared to the pristine TH-SiC with a forbidden band width of 1.8eV,the three gases become magnetic after surface adsorption and the band gap is reduced by 0.66eV,1.63eV and 0.43eV respectively.Of these,the NO and SO2 gas molecules contribute to the magnetic properties of the adsorption system as well as the VBM,while the magnetic properties of the NO2 adsorption system originate from the pz state of the C atom.This is due to the formation of a chemical bond between the O atom of NO2 in the NO2 adsorption system and the Si atom of the substrate sp2 hybridisation,leading to the breaking of the Si-C covalent bond and the creation of a hanging bond by the C atom,resulting in the formation of an energy band located in the conduction band and acting as a CBM.The conductivity is greater than that of pristine TH-SiC due to the reduced band gap after gas adsorption,which is more conducive to electron leap from the valence band to the conduction band at the same temperature.Therefore,TH-SiC is sensitive to NO,SO2 and NO2.When a gas is adsorbed on the surface of TH-SiC2,NO2 forms a physical adsorption with TH-SiC2 and the charge is transferred from the gas to the substrate by 0.15e,exhibiting magnetic as well as metallic properties in the energy band.This is due to the introduction of an excess zero energy band near the Fermi energy level for NO2 gas.The conductivity of the NO2 adsorption system is therefore greater than the pristine TH-SiC2 conductivity at the same temperature,and TH-SiC2 has a certain sensitivity to NO2.The research results provide a theoretical basis for the design and preparation of future TH-SixCy gas sensing materials.3.The results of the TH-SiC nanoribbons show that different cuts,edge passivation of different atoms and different widths can change the electronic properties of the nanoribbons.The three different structures of the nanoribbons behave as small band gap semiconductors(0.18eV,0.07eV)or metals due to the influence of the edge atoms.The influence of edge atoms on band edges can be eliminated by edge passivation of H/F atoms,and the leapfrogging mode achieves an indirect-direct transition,thus making nanoribbons promising for applications in optoelectronic devices.In addition,changing the position of the nanoribbon band edges by controlling the position of the edge passivated H/F atoms has led to broad applications for nanoribbons in type Ⅰ and type Ⅱ heterojunctions as well as photocatalytic hydrolysis.Finally,studies for TH-SiC nanoribbons of different widths show that the band gap decreases from 1.79eV to 1.49eV as the bandwidth increases,and that a direct to indirect transition occurs in the mode of leapfrogging.