Quantitative evaluation of various NIR-to-red upconversion mechanisms in NaYF_(4):20%Yb^(3+),2%Er^(3+)nanoparticles

NaYF_(4):20%Yb^(3+),2%Er^(3+)纳米颗粒中多种近红外到红光上转换机制的定量研究

In the most popular NaYF_(4):Yb/Er upconversion nanoparticles(UCNPs),the red emission is attributed to four potential excitation routes encompassing two-and threephoton excitation processes.Consequently,this red emission typically exhibits a super-quadratic dependency on near-infrared(NIR)excitation intensity,with the nonlinear order n being dependent on the individual contributions(Cis)of these four excitation routes.Notably,the Cis values are not constant but significantly impacted by the surface quenching of the UCNPs,leading to a decrease in the n value.However,a quantitative assessment of these variable Cis has not been undertaken,hindering a comprehensive understanding of the quenching effect on the UC mechanisms.In this work,we prepare four NaYF_(4):Yb/Er nanocrystal samples with varying degrees of surface quenching,achieving through the modulation of particle size and core-shell structure.We quantitatively evaluate the Cis values and identify the primary excitation route responsible for the red emission.Our results reveal that the contribution of three-photon excitation increases from 7%in the 30 nm bare core to 74%in 90 nm core with shell at an excitation intensity of 200 mW cm^(−2).This observation highlights the impact of surface quenching suppression.Furthermore,we discover that the quenching effect operates by reducing the lifetimes of the Yb^(3+)2F_(5/2)and Er^(3+)4S3/2 levels,while enhancing the NIR emission intensity ratio of the Er^(3+)4I_(13/2)→4I_(15/2)transition to the Yb^(3+)2F_(5/2)→2F_(7/2)transition.Our findings provide physical insights into the excitation mechanisms underlying the red UC emission in NaYF_(4):Yb/Er UCNPs.

在NaYF_(4):Yb/Er上转换纳米颗粒中,红光发射经由四种激发路径产生,其中包括双光子和三光子激发过程.因此,红光发射对近红外激发强度通常表现出超二次方依赖关系,其非线性系数n依赖于四种激发路径的贡献占比.实际上,贡献占比受到上转换纳米颗粒猝灭效应的显著影响.然而四种路径贡献占比的定量研究还未见报道,这限制了猝灭效应对上转换发光影响的深入理解.在本工作中,我们通过调节颗粒尺寸和核壳结构制备了四种具有不同猝灭程度的纳米晶样品.定量计算了四种路径的各自贡献.结果表明,在200 mW cm^(−2)的激发强度下,三光子路径的贡献从7%增加到74%,这体现了抑制表面猝灭的重要性.还发现猝灭效应是通过降低Yb^(3+)2F_(5/2)和Er^(3+)4S_(3/2)能级寿命,以及增加Er^(3+)4I_(13/2)→4I_(15/2)与Yb^(3+)2F_(5/2)→2F_(7/2)发射强度比例来起作用的.我们的发现将为NaYF_(4):Yb/Er上转换纳米颗粒中红光上转换能级的激发机理提供理论指导.