Hierarchical Structure And Crystallite Of Nano-ZnO

Two preparation methods and their products are investigated in this project: one is synthesizing hierarchical nano-ZnO structure through liquid phase precipitation involving self-assembling process;another is preparing microcrystal ZnO thin-film by sol-gel preparation method.For nano-hierarchy of ZnO in preparation,the idea is synthesizing various morphological structures by simple method in an orderly way and step by step,and thus complicated nano-structures are obtained by their self-assembling,so various nano-morphologies can be investigated in the same sample.Nano-morphologies of flakes,noodles,nodes and globular flowers self-assembled by the formers are investigated by scanning electron microscopy(SEM).Their sizes are acquired as follows: That of flower structures is 2-3?m,that of flakes is 300 nm with 50 nm thickness in average,and the diameter is 50 and 80 nm at the tip and bottom of noodles respectively with 200 nm length.These nano-structures aggregate in cluster morphologies without enough distribution.There exist two bands around 395 and 530 nm obviously,which correspond to the exciton recombination from conduction to valence band in near UV region and the green emission origin of defect levels within band gap respectively.The emission attributed to defects is stronger than that of exciton recombination.This may be due to the attached ?-OH? at the surface of nano-structures in alkali preparation condition.In ZnO thin-film preparation,zinc acetate is used as the precursor,cholamine as the stabilizing agent,ethylene glycol methyl ether as the organic solvent for sol solution synthesis.Through cyclical operations of rotary spreading on a silicon flake and thermal treatment procedures,thin films with various thickness and crystalline are formed.The photoluminescence spectra show that the centers of UV bands owing to band to band recombination of excitons are located at 370,384 and 397 nm for thin-film of 73,110 and 197 nm thickness,which show obvious blue shift due to quantum confinement.Primary inference for this should be origin of the microcrystal structure whose size determined by the thickness.In this condition,the thin-film comprises of microcrystals rather than a uniform film.Weak violet and blue emission peaks also present at 456 and 475 nm,452 and 469 nm,and 452 and 471 nm in three groups correspond to thin-film in 73,110 and 197 nm thickness.There is not blue emission in samples.The possible explanation of its quenching could be the enlarging of electron depletion region in grain boundaries owing to the nitrogen atoms of organics bonding to zinc atoms,whose bonds are not broken totally by the thermal treatment which may be not enough.The next investigation should be the heating treatment which may determine the film structure and its photoelectric properties,and thus clear and unambiguous result should be obtained.