Research On Wide Bandgap Semiconductor Nanomaterials Prepared Through Femtosecond Laser Ablation In Liquid

The preparation of ultra-small??5 nm?nanoparticles through femtosecond laser ablation just takes advantage of cold processing.As the unique application of inorganic nanoparticles in optics,biology,energy and other fields are more and more prominent,and the requirement of their functional properties is more and more urgent.Moreover,in terms of evaluating,optimizing and modifying their properties,ligand-free nanoparticles with bare surface are the ideal reference materials.Therefore,the preparation of the inorganic nanoparticles without ligands has become a research interest.After nearly 30 years of research,the synthesis of nanoparticles via femtosecond pulsed laser ablation in liquid?fs-PLAL?has become a scalable and easy method to synthesis ligand-free nanoparticles.It not only solves the shortcomings of traditional methods such as a mass of by-products,complicated purification,and complex ligands,but also is a green and convenient method which can realize high output and flexible regulation for functional nanoparticles.Wide-gap semiconductor nanoparticles not only inherit the advantages of large bandgap width,high thermal conductivity,high saturation rate of electron migration and high resistance to penetrating electric field,but also possess unique properties that are not available in bulk materials.First of all,quantum dots themselves are a zero-dimensional material with a large specific surface area and a large absorption of light energy,which can greatly improve the output of electrical energy when used in photoconductors.Secondly,the gap width of quantum dots is dimensionally dependent,and the optical properties can be adjusted by changing their size.Besides,the preparation cost of quantum dots is relatively low.Therefore,It has very important applications in the new generation of flexible luminescent electronic devices and quantum dot devices with high performance.Therefore,it is very important to study the optical properties of wide-gap semiconductor materials.Nevertheless,the conventional methods for synthesizing NDs and GaN-NPs are extremely challenging.Therefore,in this paper,we fabricated nano-diamonds and GaN-NPs with ultra-small particle size through fs-PLAL.And the properties of both kinds of nanoparticles were investigated.The specific work of this paper is listed as following:?1?Investigation on the prepare parameters.Through fs-PLAL in deionized water,the laser scanning points'spacing dependent particle size distribution of NDs was investigated.It is found that when the scanning point spacing?and the laser focus diameter d satisfy the formulation d?2?,the results of the optimal uniformity of size distribution and the particle yield can be obtained simultaneously.As a result,the optimal scanning point spacing is determined to be 1?m.In addition,the effects of laser pulse energy on the yield and size distribution of NDs were investigated.It is found that the energy of femtosecond pulse laser with high repetition frequency?90KHz?has a significant effect on particles'yield,showing a positive correlation.Oppositely,the effect on size distribution is scarce.When the laser power is 828 mW,the yield of NDs can reach to 8.2 mg/h.The results of this study lay a foundation for the following experiments.?2?Preparation of NDs and investigation of their optical properties.Based on the preparation parameters determined above,the ultra-small?average size is about 3.0nm?NDs with ligand-free surface were synthesized and dominated by sp³ phase,which provides a simple green method for the synthesis of NDs.After surface passivation by PEG_200N,the fluorescent properties of NDs are changed.Before passivation,NDs exhibits dual emission when excited by 260 nm and 280 nm.After passivation,this phenomenon disappeared.While NDs are excited by 300 nm-420 nm,their emission wavelengths maintain unchanged,but the maximum emission intensity is increased by 3 times.In conclusion,this study reveals that ultra-small NDs can be obtained by fs-PLAL in water,and provides a good experimental basis for further improving the luminescence properties of NDs.?3?Through ablating bulk diamond in ethanol with femtosecond pulsed laser,the well dispersed NDs with average size of 3.8 nm can be obtained and have stable blue luminescence.Moreover,after passivation by PEG_400N,its blue emission'intensity was increased by 10 times.Their luminescence dynamics was studied by time-resolved luminescence spectra.It is found that the blue emission of NDs mainly originated from the defect state existing in the surface of NDs.After passivation,the surface is oxidized to a large extent,forming a mass of defect states and providing more capture energy levels for electron transition,which not only enhances the luminescence intensity,but also prolongs the luminescence life.The results show that the NDs with stable luminescence can be synthesized by one-step in ethanol by fs-PLAL.At the same time,it provides a theoretical basis for the surface modification of fluorescent NDs.?4?GaN-NPs preparation in various environments.GaN-NPs with different properties were obtained by using femtosecond laser ablation GaN thin films in air,water and ethanol respectively.Compared with GaN film,the change of luminescence wavelength showed different trends for three kinds of GaN-NPs,which are unchanged in air,red-shift in water and blue-shift in ethanol.XPS results show that the variable photoluminescence is caused by the different structures and compositions inside GaN-NPs,which not only affects the luminescence wavelength,but also greatly affects the luminescence intensity.It means that the structure and luminescence properties of GaN-NPs can be regulated by choosing different liquid environments,which will open up more potential applications for GaN-based photoelectric devices.In summary,based on the advantages of fs-PLAL,NDs and GaN-NPs with ultra-small size,good dispersion and free-ligand surface were obtained,which just meet the requirements of current applications.Importantly,we figured out that how the laser power and scanning point spacing affect on the output efficiency of particles and the size distribution,and put forward the formula between laser focus and scanning point spacing.Furthermore,the optical properties of NDs and GaN-NPs were studied.Firstly,it is proposed that blue emission of NDs is mainly devoted to the surface defects and these surface defect states are caused by surface oxidation,which have guiding significance for the research of fluorescent NDs.Secondly,it is proposed that the adjustable luminescence properties of GaN-NPs are caused by their different internal structures and components.The results not only demonstrate the potential of fs-PLAL in the synthesis of NDs and GaN-NPs,but also provide a reference for further optimizing the optical properties of them.