The Synthesis And Tribological Peoperties Of ?-NaYF₄:Yb/Er Upconversion Film Codoped With Cr³⁺ Ions

?-NaYF₄:Yb/Er crystal,possessing low phonon energy,wide band-gap and small expansion of electron cloud,has become a new focus of research.?-NaYF₄:Yb/Er upconversion?UC?film has more advantages than powder and bulk materials in terms of structural uniformity,higher thermal conductivity,and better thermal resistance,therefore,it has great potential for solar cell application.There are two key issues to be resolved for the application of?-NaYF₄:Yb/Er UC film in solar cells.Firstly,the UC luminescence efficiency of the?-NaYF₄:Yb/Er crystals is still too low to obtain the target performance.Secondly,the UC film synthesized via current methods has low interfacial bonding strength to the substrate leading to short lifespan of the UC film in the application.In this work,Cr³⁺ions are doped into the lattice of NaYF₄ to enhance the UC efficiency of?-NaYF₄:Yb/Er crystals.The influence of Cr³⁺ion doping concentration on crystalline structure and stability of the intermediate phonon level is investigated.?-NaYF₄:Yb/Er UC film is synthesized on silicon?Si?substrate using self-assembly process,and the interfacial bonding strength between UC film and Si substrate has been evaluated by tribological tests.The major research work and achievements are summarized as follows:1.?-NaYF₄:Yb/Er crystals have been synthesized by hydrothermal method.The influences of hydrothermal temperature,time,and content of NaF on crystal phase and morphology have been investigated.The optimum technological parameters for synthesis of?-NaYF₄:Yb/Er crystals have been obtained.The structure and morphology of crystals in different growth stage have been investigated,and the growth mechanism has been clarified.Furthermore,the size control of?-NaYF₄:Yb/Er crystals are achieved by tuning pH values and concentration of NaNO₃.The emission colors of?-NaYF₄:Yb/Er crystals are shifted from green to orange yellow.2.The luminescence intensity of?-NaYF₄:Yb/Er crystals have been enhanced by doping Cr³⁺ions into the crystal.It was found that Cr³⁺ions could substitute Y³⁺ions in the lattice sites,changing the lattice structure and local symmetry around Er³⁺ions,and making more Er³⁺and Yb³⁺ions into luminescence band leading to increased stability of the Er³⁺intermediate phonon level.The influence of doping concentration of Cr³⁺ions on the luminescence properties of?-NaYF₄:Yb/Er crystals has been systematically investigated,thereby the optimal doping concentration is defined.3.?-NaYF₄:Yb/Er UC film has been assembled on Si substrate via self-assembly process.The surface morphology and elemental composition of the UC film are characterized by Scanning Electron Microscope?SEM?,X-ray Photoelectron Spectroscopy?XPS?,and Fourier Transform Infrared Spectroscopy?FT-IR?.The mechanism of the interfacial reactions between UC film and Si substrate is studied,and the formation mechanism of the UC film has been clarified.4.Macro-tribological properties of?-NaYF₄:Yb/Er UC film are systematically investigated.The influences of assembly time,sliding speed and loads on the tribological performance of the UC film are discussed.The UC film synthesized via self-assembly method is found being covalently bonded to the Si substrate,therefore,improved the interfacial bonding strength.Its tribological lifespan is five times longer than that of the film prepared by physical deposition.The UC luminescence efficiency of the?-NaYF₄:Yb/Er crystals has been enhanced and the interfacial bonding strength between UC film and Si substrate has been increased.This work suggests a new process for increased luminescence intensity of UC materials and provides original data and research methodology for the application of UC film in the field of solar cells.