Preparation And Optical Properties Of Various Crystal Structure Carbon Nanoparticles

Owing to their special characteristics,carbon nanoparticles have wide application potential in optoelectronic devices,biological sensors,biological detection,etc.Carbon has an electronic configuration of ls’2s2p2 and it can possess sp-,sp2-,and sp3-hybridized atomic orbitals.As a result,carbon has various allotropes and carbon nanoparticles exist in different crystal structures.The carbon nanopartilces exhibit good luminescence properties;however,their luminescence mechanisms are still under debate.The luminescence mechanism of the material determines its luminescence properties,which then determine its applications.Therefore,the study of the luminescence mechanism of carbon nanoparticles is necessary.Here,we report the preparation of the carbon nanoparticles of different crystal structures by using chemical synthesis methods and study their luminescence properties.The crystal structures of the synthesized carbon nanoparticles include graphite,diamond,and C8 structures.The reversible phase transformation between diamond and Cg-structured nanoparticles is investigated.The carbon nanopartlcles with diameters of 1-9 nm are synthesized by using cellulose acetate as the carbon source material.It is found that the luminescence properties of a given collection of carbon nanoparticles change remarkably as the concentration of the nanoparticles in the solution varies.The photoluminescence shifts to blue(as large as 100 nm)with decreasing concentration.In addition,the luminescence intensity increases gradually with decreasing concentration.The pH of the solution has little influence on the luminescence properties.A model based on photon reabsorption is proposed to explain this phenomenon.Our result indicates that photon reabsorption can strongly affect the luminescence properties of the colloidal nanoparticles.By using glucose as the carbon precursor,we synthesize the oxygen-terminated diamond nanoparticles.They exhibit multiple emission bands centered at around 390,433,470,527,and 614 nm,respectively.These bands cover nearly the full visible region.The quantum yield is about 2.5-5%.By using ammonium citrate as the carbon precursor,we synthesize the amide-terminated diamond nanoparticles.They exhibit photoluminescence with two emission bands centered at around 435 and 447 nm.The quantum yield is 50.4%.The investigations of the surface structure and the luminescence properties in conjunction with the density functional theory calculation reveal fruitful oxygen-related surface defects in these diamond nanoparticles.Such defects create respective energy levels in the band gap leading to observed luminescence.The calculation shows that significant spatial overlap between the highest occupied and lowest unoccupied molecular orbitals of amide-terminated diamond nanoparticles causes large radiative decay rate as well as unprecedented high quantum yield.These surface defects are popular and they play important roles in luninescence of the large family of various(graphite,diamond,and C8)carbon as well as SiC and C3N4 nanoparticles.By using EDTA as the carbon source material,we synthesize the C8 structured carbon quantum dots.They have fruitful morphologies.The synthesis process yields well-crystallized diamond nanopartilces when the alkali solvent is substituted by an acid solvent.There is reversible phase transformation between these two types of carbon allotropes when the solvent varies between alkali and acid.A model based on surface structure is proposed to explain this phase transition process.It is found that the variation of the surface chemical structure triggers such a phase transition.This finding will help us better understand the structural phase transition of carbon allotropes.