YVO4：Eu3+纳米微粒发射光谱与温度的关系

测量了非选择激发下YVO₄:Eu³⁺纳米微粒中Eu³⁺的⁵D₀→⁷F₂发射光谱随温度的变化.低温下的光谱是很多谱线叠加在一起形成的宽带;而室温下主要是与体材料相似的两条分离谱线.根据选择激发下YVO₄:Eu³⁺的发射光谱研究得到的结论,把发射光谱分解为两部分之和,一部分来自占据靠近纳米微粒中心的格位的Eu³⁺离子发射谱线具有与体材料相近峰值;另一部分来自占据靠近表面位置的Eu离子,发射分布在更大的光谱范围内.这两部分发光的相对强度随温度变化的分析表明,YVO₄:Eu³⁺纳米微粒发射光谱随温度的变化是由于靠近表面的Eu通过表面猝灭中心猝灭而引起的.

Temperature Dependence of the Emission Spectra in YVO4:Eu3+ Nanocrystals

Surface effect is probably the most important factor that makes the spectral properties of rear earth doped nanosized materials different from that of bulk.In many cases,surface acts as a quenching center,that increases nonradiative transition rates of the dopant.Besides,the surface diversifies the local environments of the dopantions,thus the emission spectra of nanosized materials may shift,split or get broadening.In Eu³⁺ doped nanoparticles,such a site inequivalency was studied widely with selective excitation.In this report,we show that the temperature dependent emission spectra are also a result of this effect.⁵D₀→⁷F₂ emission spectra of Eu³⁺ in nanocrystalline YVO₄ at different temperatures were measured with non-selective excitation.At low temperature,the spectrum is a broad band,superposed by a number of spectral lines,while at room temperature,it consists chiefly of two lines similar to that observed in bulk.Referring to the results of selective excitation in YVO₄:Eu³⁺ nanoparticles,we decomposed the emission spectra into two parts:that(I_C)from the Eu³⁺ ions located near the particle center with the emission similar to that in bulk;and that(I_S)from Eu³⁺ ions close to the surface with the emission wavelengths extended wider.As the temperature decreases,the relative intensity of I_S/I_C approaches to a constant 2.3 in our sample with average radius of 27 nm.That gives an estimation of the surface layer thickness,4.5 nm,in which the spectral properties of the Eu³⁺ ions are disturbed by the surface.The temperature dependence of I_S/I_C can be fitted by either an energy transfer process with energy mismatch of 900 cm^-1 or an electron transfer process from the Eu ions to the surface traps across a 900 cm^-1 barrier.The variation of the emission spectra originates from the temperature dependent quenching via surface quenching centers.