Optical Properties Of Er³⁺ Highly Doped Upconverting Nanocrystals

Lanthanide doped upconversion nanoparticles(UCNPs)are capable of converting near-infrared laser into visible light and ultraviolet emission.In stark contrast to quantum dots and organic dyes,UCNPs contain individual and variable absorption and emission centres,and it has unique advantages such as large spectral shift,long fluorescence lifetime,narrow radiation bandwidth,and relatively low cost.Based on its unique optical properties,UCNPs have a wide range of applications in the fields of fluorescence microscopy,deep tissue bioimaging,nanomedicine,optogenetics,and security.However,owing to the constraint of concentration quenching,the doping amount of the activator is limited to a lower concentrations,thus the tunable luminescence properties has been largely hindered at relatively low-doping concentrations.For lanthanide doped nanomaterials with a high ratio of surface area to volume,high-doping concentration is likely to induce both cross-relaxation energy loss and energy migration to the surface quenchers.Interestingly,recent research highlight the strong coupling of concentration and surface quenching effects in colloidal lanthanide-doped nanocrystals,and that inert epitaxial shell growth can overcome concentration quenching.However,although highly efficient UC luminescence has been reported in recent years,UC mechanisms,especially some abnormal optical properties aroused by high concentration doping,have not been explored in detail.In this paper,the new UC phenomenon and corresponding mechanism are investigated in Er³⁺heavily doped fluoride core and core-shell nanocrystals focused on a scale of around 10nm.The detailed work of this paper is as follows:(1)The effect of Er³⁺doping concentration on the optical transition probability was investigated by two independent methods.Specifically,Er³⁺doped NaGdF₄ nanocrystals with similar size and sub-10 nm were synthesized by high temperature thermal decomposition method.The transition probability of samples with various doping concentrations was calculated by J-O theory.On the other hand,after the epitaxial growth of about 3 nm of NaGdF₄on the core nanocrystals by the hot injection method,the enhanced upconversion luminescence were achieved.Based on the FIR technique,the temperature sensing is performed and the dependence of the doping concentration on the transition probability was obtained.The dependence trends are acquired from the two independent methods are highly consistent.Also,we found that a lower doping concentration is more conducive to obtaining greater sensitivity results.(2)The optical properties of Er³⁺heavily doped nanocrystals at 980 nm and 1530 nm laser excitation were compared.Especially,under the excitation of 1530 nm,the luminescence intensity of Er³⁺heavily doped core-shell nanocrystals increases with the increase of doping concentration,and the brightest upconversion luminescence is realized in Na Er F4@NaGdF₄,which is superior to the traditional well known 2Er/18Yb co-doped core-shell nanocrystals.In Na Er F₄@NaGdF₄ nanocrystals,abnormal temperature dependent lifetime and abnormal power dependent photon number were analyzed.In addition,based on FIR technology,using traditional thermal coupling and non-thermally coupled level for temperature sensing,and the higher sensitivity can be achieved through the non-thermally coupled one.The non-thermally coupled level has also shows the promising candidate for in vivo temperature sensing.(3)A novel Nd³⁺-Yb³⁺sensitized Er³⁺highly doped core-shell nanocrystal was designed to realize high-efficiency single-band red light emission,and the luminescence dynamics and mechanism were analyzed thoroughly.The brightness of the proposed nanocrystals is brighter than the one obtained with the traditional low doped cascade sensitized layer,indicating that a higher doping level is required,especially in the transition layer,to match the feeding requirements of the heavily doped core.In addition,both the excitation laser(800 nm)and the emission band(652 nm)are located in the first biological window,and the nanocrystal has a high photothermal conversion efficiency(40%),which favor for deeper biological tissue penetration characteristics.Based on the efficient single-red emission characteristics of the nanocrystals,the FIR technology was used to demonstrate the potential for in vivo temperature feedback.