Temperature-dependent Luminescence Properties Of Upconversion Nanocrystals

Upconversion luminescence materials can convert near-infrared ray into visible light through a multiphoton mechanism.This unique luminous mode bestows them great potential for many applications such as biomedicine,temperature sensing,three-dimensional display,solar cells and anti-counterfeiting,etc.It is generally known that luminescence is a competing process between radiative and non-radiative transitions,and the latter depends on temperature to a large extent.Therefore,investigations on temperature-dependent luminescence of upconversion nanoparticles not only contributes to clarifying the energy consumption mechanism of nanoparticles,but also provides clues to improve luminescence properties and create new applications.In this paper,we use the co-precipitation method to prepare NaGdF4:Yb,Ln(Ln=Ho,Tm)upconversion nanoparticles,investigate the growth behavior to improve their size distribution uniformity.Also,temperature-dependent upconversion luminescence properties and emission color tuning features of the nanoparticles were studied.Firstly,we prepared the NaGdF4:Yb,Ln(Ln=Ho,Tm)upconversion nanoparticles by co-precipitation method and explored the effects of preparation parameters such as temperature,reaction time and OA/ODE ratio on the nanoparticle morphology,size and crystal phase.It was found that there is no phase transition progress from cubic to hexagonal structure in the hexagonal NaGdF4 synthesis process.That is very different from the synthesis process of hexagonal NaYF4,and ultrasmall hexagonal NaGdF4 upconversion nanoparticles(<10nm)can be achieved by this method.Raising the synthesis temperature or extending the reaction time brings better crystalline nanoparticles,but also leads to worse particle uniformity due to the Ostwald ripening effect.Increasing the OA/ODE ratio will change the nanoparticle morphology from sphere to rod-like.Upconversion nanoparticles with good uniformity and ultrasmall size(?5 nm)can be obtained by optimizing the synthesis temperature,reaction time and OA/ODE ratio.Secondly,we focused on the temperature effect on luminescence properties of NaGdF4:Yb,Ln(Ln=Ho,Tm)upconversion nanoparticles.The phenomena was discovered that when we raise the temperature of nanoparticles,luminescence is enhanced greatly and the enhancement factor is different between NaGdF4:Yb,Ho and NaGdF4:Yb,Tm upconversion nanoparticles.We thought this abnormal phenomenon is closely related to surface quenching effect.Besides,we also studied the effects of activator concentration and pump power on luminescence properties.Cross relaxation and concentration quenching effect lead to the luminescent quenching.The emission intensity and pump power meet the relationship I?Pn.Finally,based on the different temperature and pump power dependences of upconversion luminescence between Tm and Ho,we designed and prepared three types of Tm/Ho co-doped upconversion nanoparticles:core-only NaGdF4:Yb,Ho,Tm,core-shell NaGdF4:Yb,Ho@NaGdF4:Yb,Tm and core-shell NaGdF4:Yb,Tm@NaGdF4:Yb,Ho.Their emission color tuning features caused by modulating temperature and pump power were investigated.The result showed that NaGdF4:Yb,Tm@NaGdF4:Yb,Ho core/shell nanoparticles exhibited better color shift properties,and the emission color can shift from white to green with the temperature increasing and from white to bule with the pump power increasing.This kind of thermochromic property can be used to construct more secure anti-counterfeiting patterns.