Theoretical Investigation On Group-? Nanocrystals

Recently,semiconductor nanocrystals have attract much attention and interest due to numerous potential application,such as nanoelectric and optical devices,fluorescent biological detection and quantum computation et al.Semiconducting group-?nanocrystals play vital role in the development of nanoscience.The hydrogenated group-?(C,Si,Ge)nanocrystals maintain sp~3 hybridizations,such as nanodiamonds and silicon nanocrystals,providing an idea model to verify quantum confinement effect by theory and experiment.Till now,numerous experiment and theory investigation indicate that the electronic properties of nanocrystals are controlled by the size,shape and symmetry.Moreover,modifying the nanodiamonds with different element or group can tune the electronic properties of system effectively.Generally,nanodiamonds(C_nH_m)can be regard as the superposition of adamantane and the centers form a fragment of diamond.For the isomers which cannot be distinguished by the bond energy model,we propose a new nomenclature to enumerate the nanodiamonds with the carbon number varying from 10 to 41.In addition,our method greatly enhances the efficiency to screen candidates and predict the nanodiamonds which are not pre-determined in the experiment.Combined density functional based tight binding(DFTB)method and the first-principle calculation,we study the structural stability and electronic properties of system,finding that the energy gaps obtained by these two methods are in agreement with each other indicating that DFTB method is reliable.Furthermore,the shape of stable structures will be influenced by the hydrogen chemical potential and the energy gap will be determined by its shape and structural stability.For the surface absorption of nanodiamonds,we study the energy barrier of sulfur absorption to understand the complexity of absorption.Meanwhile,based on the nanodiamonds,we study the influence of sulfur number and position on the electronic properties of system.Besides the number and position of C=S,the size and symmetry of nanodiamonds will affect the energy gap distribution of system,where the increase of C=S the energy gap will decrease accordingly.The investigation on the sulfur absorption will provide theoretical guidance for the design of electron device.Cubic silicon carbide nanocrystals(SiCNCs)is reported to be potential UV and blue light emitter.Combined bond energy model with Wang-Landau method,we systematically investigate the hydrogenated silicon carbide nanocrystal(H-SiCNCs),including the relationship between structural stability and size/shape of nanocrystals.Moreover,how the shape of nanocrystals influences the energy gap distribution will also be investigated.With the Wang-Landau method we first search the candidates with the total number of silicon and carbon are 10,14,18,22 and 26.From the phase diagram of hydrogen and carbon chemical potential,we notice that the configuration of stable structure varies with the chemical potential and the nanocrystals with typical shape such as tetrahedron,hexahedron and octahedron will produce structural evolution and hexahedral configuration is metastable.For the tetrahedral and octahedral H-SiCNCs with same size,the energy gap of latter is larger,besides,the highest occupied orbitals(HOMOs)and lowest unoccupied orbitals(LUMOs)of hexahedral structure will generate charge separation.Therefore,the carbon and hydrogen chemical potential can adjust the configuration of stable structures and then design the electron devices.Since the lattice constant of silicon and germanium nearly same with each other,they can be mixed with arbitrary ratio.Based on the nanodiamond,we enumerate the SiGe nanocrystals with all the possible mixing ratio for given the total number of silicon and germanium.Combined bond energy model and first principle calculation,we study the stability of hydrogenated SiGe nanocrystals and find that the results obtained by these two methods are consistent.Furthermore,the ratio of Si/Ge and configuration will influence the electronic properties of system,thus,the energy gap can be tune within certain limits.