Surface And Photoluminescence Properties Of Graphene And Silicon Quantum Dots

Due to the pronounced quantum effects,quantum dots(QDs)assume numerous novel chemical/physical properties,which make them effectively and widely used in various fields.The optical property is one of the most important issues of QDs.Compared with organic light-emitting dyes,QDs have some unique advantages,such as size-dependent luminescence,high photoluminescence(PL)intensity,stable PL,long fluorescent life,biocompatibility.However,traditional II-VI(CdS,CdSe,ZnSe)and III-V(InP,GaAs,GaN)QDs usually contain heavy metals,which are harmful to the ecosystem.Furthermore,the traditional QDs may have some damage to cells,which impedes their applications in the biological field.Recently,graphene quantum dots(GQDs)and silicon quantum dots(SiQDs),as new kinds of quantum dots,have emerged and ignited tremendous research interest.Carbon and silicon are abundant on the earth,and they are completely green materials.Therefore,GQDs and SiQDs may be the perfect substitutes for traditional QDs,which would break a new path for the development and application of QDs.On the basis of many previous documents,we investigated the effect of Gamma ray irradiation on the structure and optical properties of GQDs.In addition,we also synthesized SiQDs embedded in silica wire array using a simple and low-cost high temperature decomposition method,and investigated its structure and optical properties.The main results of this dissertation are listed as follow:(1)The Gamma-ray irradiation effects on the structure of GQDs were studied with the help of UV-visible absorption,X-ray photoelectron spectroscopy,Raman spectra and Fourier transform infrared spectra.In the range of 0 to 200 kGy Gamma-ray irradiation,the GQDs are oxidized,making the GQDs to aggregate together easier.The oxygen atoms come from the residual air in the suspension will bond with the ring-opening carbon-bear free radicals caused by the high-energy Gamma-ray irradiation.When the irradiation dose increases to 300 and 400 kGy,the GQDs will be reduced by eaq The oxygenated functional groups reacted with the carbon free radicals to form C-H.After Gamma-ray irradiation,GQDs exhibit a low capacity to quench the fluorescence of R6G due to the synergistic effect of dynamic and static quenching.The positive relationship between the static quenching coefficient VqNa and the?-conjugated content of GQDs may be used to quantitative measurement.(2)We reduced non-radiative surface states and achieved PL modulation of GQDs via gamma-ray irradiation.Gamma-ray irradiation was found to reduce the amount of non-radiative surface states or traps,which results in enhanced emission.Depending on the chemical conditions of the GQD surface,emission can be in blue color with dominating n-?*transition or in green with strong contribution from lower lying surface state sub-bandgaps.As all samples show excitation dependence,emissive pathways between these energy levels are possible,resulting in broad emission.Furthermore,persistent excitation dependent emission indicates the availability of emissive high and low energy transitions.Coupled with low cytotoxicity and good biocompatibility,irradiated GQDs performed well as bioimaging probes of 4T1 cancer cells.(3)We synthesized SiQDs embedded in highly oriented,fairly long silica wire array using a simple and low-cost high temperature decomposition method.XRD pattern,HRTEM image,PL and Raman spectra all prove the existence of SiQDs in the silica wire array.The phenomenological phonon confinement model describes the asymmetrically broadened Raman peak of the SiQDs shape satisfactorily.Additionally,the products show three broad luminescence bands centered at 490,800,1550 nm respectively.The unique optical properties of the SiQDs embedded silica wire array,combined with relative ease of fabrication,would provide opportunities for developing cheap and novel types of silicon-based optical and electronic devices in the future.