YVO_4∶Eu^(3+)纳米微粒发射光谱与温度的关系

Temperature Dependence of the Emission Spectra in YVO_4∶Eu^(3+) Nanocrystals

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。

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 dopant ions, thus the emission spectra of nanosized materials may shift, split or get broadening. In Eu^3＋ 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 Eu^3＋ 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： Eu^3＋ nanoparticles, we decomposed the emission spectra into two parts： that （Ic） from the Eu^3＋ ions located near the particle center with the emission similar to that in bulk; and that （Is） from Eu^3＋ ions close to the surface with the emission wavelengths extended wider. As the temperature decreases, the relative intensity of Is/Ic 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 Eu^3＋ ions are disturbed by the surface. The temperature dependence of Is/Ic 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 900cm^-1 barrier. The variation of the emission spectra originates from the temperature dependent quenching via surface quenching centers.