Theoretical Studies On Surface-plasmon Resonance Enhanced Luminescence Of Single Upconversion Nanocrystals

Compared to conventional fluorescent emitters,rare earth doped upconversion nanocrystals?UCNCs?have superior photostability,longer fluorescence lifetime,and lower biotoxicity.With near-infrared excitation,which can emit visible light via anti–Stokes process with higher signal-to-noise ratio,narrow linewidth and strong tissue penetrability.UCNCs have attracted much attention due to their enormous application potential in life sciences such as bioimaging,fluorescent labeling and biosensing.However,UCNCs usually suffer from low emission efficiency,which limits their application prospects.Many methods have been proposed to solve this problem.One of the most effective solutions is to enhance the upconversion luminescence?UCL?by coupling with localized surface plasmon resonance?LSPR?.LSPR is the collective oscillation of free electrons,which usually exists at the interface between metal nanoparticles and the medium.If the LSPR resonance is near the emission or excitation wavelengths of emitters,the absorption or radiation process will be increased.In this thesis,gold nanorod?GNR?dimer was adopted as plasmonic nanostructure to enhance the upconversion luminescence from UCNCs-NaYF₄:Yb³⁺/Tm³⁺.The main contents and results are as follows:?1?The rate equation was established according to the spectrum of UCNCs-NaYF₄:Yb³⁺/Tm³⁺,which was employed to theoretically study the upconversion process in UCNCs.The results showed that with 980 nm excitation,UCNCs displayed UCL with a peak at 800 nm.In the weak excitation,UCL was adominated by 2-photon process.While in saturated excitation,the UCL intensity in 2-photon and 3-photon process were comparable.Besides,both decreasing Yb³⁺doping level and increasing Tm³⁺doping level would reduce the efficiency of UCL in UCNCs.?2?Numerical simulation was adopted to investgate the properties of LSPR in GNR dimer.The simulation results showed that GNR dimer could support two different LSPR modes corresponding to the oscillation of the free electrons along?longitudinal mode?and perpendicular?transverse mode?to its long axis.The transverse mode showed resonance at523 nm.While the longitudinal mode had a longer resonant wavelength,wich strongly depended on the diameter and length of GNR and would exhibit significant red shifts as its diameter increase or length decrease slightly.What's more,the local field and local density of photonic state could be increased by longitudinal LSPR mode with hundreds of time.?3?Enhancement effect of UCL from-NaYF₄:Yb³⁺/Tm³⁺was analysised in the presence of GNR dimer.The results showed that the enhancement of UCL had saturation effect.And GNR dimer with longitudinal LSPR resonance peak between 800 nm and 980nm could enhance UCL quite well,and the luminous intensity would be increased by hundreds of thousands of times when the concentration of doped ions was relatively high.