Preparation,Photoluminescence Properties and Phase Transformation of Sic Nanoparticles

Author:Guo Zuo Zuo

Supervisor:fan ji yang


Degree Year:2017





Silicon carbide(SiC)is one of the third-generation semiconductor materials.It has important applications in high temperature,high frequency,and high power electronic devices owing to its superior physical and mechanical properties.As a wide bandgap semiconductor,SiC has more than 250 crystalline structures.They can be used in blue,green,and ultraviolet light-emitting devices and optical detectors.In addition,SiC has good biocompatibility,which makes it be widely used in biomedical field.SiC is an indirect bandgap semiconductor with low luminescence efficiency,which limits its application in optoelectronic field.However,recent studies show that when the size of a SiC crystal diminishes to be several or tens of nanometers,its quantum efficiency can be significantly increased due to the.spatial confinement effect.As a result,it shows strong yellow to blue photoluminescence,hence it has good application potential in biological imaging and solid-state lighting.The luminescence of the SiC QDs is complex,and it may have both intrinsic and different defects-associated origins.On the other hand,the phase transformation process between different polytypes of SiC is unclear.We prepared the SiC nanoparticles through chemical etching or electrochemical etching.We studied their size-dependent luminescence Imechanisms and their phase transformation property at ambient temperature and pressure.The SiC nanoclasters with an average diameter of 7 A were fabricated through diminishing the sizes of the SiC microcrystals under high pressure and high temperature;their electronic structures and light absorption/emission properties were investigated.We observed quantum confinement luminescence of the 6H-SiC QDs through selected blue luminescence quenching by using the Ag nanoparticles.The SiC nanoparticles with different average sizes were prepared by using either chemical etching or electrochemical etching method.The microstructural and optical characterizations demonstrated there is hexagonal to cubic phase transformation in the SiC QDs at ambient temperature and pressure.The study shows that both the quantum confinement luminescence and the defect luminescence play important roles in luminescence of the SiC QDs.The quantum confinement luminescence dominates in larger SiC QDs,whereas the surface-defect luminescence dominates in ultrasmall SiC QDs.We observed the rare phenomenon of multiple-phonon-assisted light absorption process in the SiC QDs,which confirms the indirect bandgap nature of the SiC QDs.We studied the electronic structures and light absorption/emission properties of the SiC nanoclusters with an average diameter of 7 A that were fabricated by using a two-step size-reduction method.We clarified the luminescence mechanism of the SiC nanoclusters by analyzing their photoluminescence and light absorption properties.The result shows that the SiC nanoclusters behave like an indirect-energy-gap semiconductor and there are two types of oxygen-related surface defects that determine their luminescence properties.An energy gap versus size relation for the SiC nanoparticle was derived experimentally and it reveals that the increase rate of energy gap with decreasing size is smaller than the predicted rate of the density-functional-theory calculation.We studied the photoluminescence properties of the coupled colloidal system of SiC QDs and Ag nanoparticles.The experimental result in conjunction with the theoretical calculation reveals that there is strong coupling between the localized electron-hole pair in the SiC QD and the localized surface plasmon in the Ag nanoparticle.We observed the quantum confinement luminescence of the 6H-SiC QDs through Ag nanoparticle-induced selected quenching of the previous strong blue emission band.Our result shows that the high degree of overlap between the blue emission band of the SiC QDs and the surface plasmon resonance absorption spectrum of the Ag NPs causes strong exciton-plasmon coupling which leads to selected quenching of the blue emission band.This leads to observation of the otherwise hidden near-UV emission of the 6H-SiC QDs that follows the quantum confinement effect.