β-NaGdF_4纳米球中Tb^(3+)-Er^(3+)下转换对局域态密度的尺寸效应

Size Effect on The Local Density of States of Tb^(3+)-Er^(3+) Down-conversion Pairs in β-NaGdF_4 Nanospheres

以Span 80为模板,采用水热法合成了不同尺寸(4.7～115.5 nm)的β-NaGdF4∶1%Tb3+,1%Er3+纳米晶(NCs)。在Rayleigh限(粒子尺寸小于跃迁波长)下,研究了纳米晶尺寸对局域态密度(Local density of states,LDOS)的影响以及镶嵌在β-NaGdF4纳米球中的Tb3+-Er3+的辐射和无辐射特性,进一步揭示下转换过程的物理机制。基于Tb3+-Er3+处在β-NaGdF4纳米球中的模型,用Green函数方法计算了Tb3+-Er3+发射体的自发发射速率。在介电纳米球内,Tb3+-Er3+发射体的LDOS没有显著的变化。在小尺寸(Rλ)介电纳米球外,按照Chew的理论,发现LDOS有一个类-Gauss分布。如果R>35 nm(在本实验条件下),介电纳米球外则只能观测到LDOS的下降边,LDOS与局域场强的平方E2成正比,因而LDOS的类高斯分布出现的原因应归于小尺寸发射体与局域场相互作用的增强。通过计算纳米晶尺寸与体材料自发辐射速率的比值可直接确定纳米材料中的填充因子。

The varisized samples( 4. 7 ～ 115. 5 nm) of β-NaGdF 4 ∶ 1% Tb3 +,1% Er3 +nanocrystals( NCs) were synthesized by hydrothermal method using Span 80 as template. In Rayleigh limit( the size of the particle is comparability with the wavelength of the transition),the influences of the NCs sizes on local density of states( LDOS),radiation and nonradiation of Tb3 +-Er3 +spectral pairs embedded in β-NaGdF 4 nanospheres were investigated,to reveal the physical mechanics of DC process further. Based on the Tb3 +Er3 +spectral pair located at the center of a dielectric β-NaGdF 4 nanosphere model,the spontaneous decay rate of Tb3 +-Er3 +emitter,as a probe of the LDOS,was calculated with the Green tensor approach. There is no significant variation of LDOS inside the nanospheres. For the small size( Rλ) of NCs,a new Gauss’ s distribution-like of the LDOS was obtained by Chew’s theory,for R > 35 nm( in our case) the descent side of the Gauss-like distribution of the LDOS was obtained. The LDOS is directly proportional to local field E2,thus the Gauss-like distribution of the LDOS can be attributed to the strong interaction between the small emitter and the local field. By calculating the ratio of spontaneous radiative rate of bulk medium and the size of NCs,the filling factors can be determined directly.