Sc^(3+)共掺杂对单个NaGdF_4∶Yb^(3+),Er^(3+)微晶的上转换荧光调控

Upconversion Fluorescence Regulation of Single NaGdF_4∶Yb^(3+),Er^(3+) Microcrystal Induced by Sc^(3+) Co-Doping

在高温、长时的水热氛围中,通过Sc^(3+)的共掺杂,制得了一类特殊的稀土掺杂氟化物微晶。采用X射线衍射(XRD)、扫描电子显微镜(SEM)、能量色散X射线探测器(EDX)和元素分布像(element mapping)对其晶相和形貌进行表征。结果表明,无Sc^(3+)共掺时,样品为六方相NaGdF_4基六角微米盘;有Sc^(3+)共掺时,六方相NaGdF_4基微晶与单斜相Na3ScF6基微晶共存于样品之中,并且随着Sc^(3+)掺杂浓度的升高,NaGdF_4基微晶发生了显著的形貌演变。值得一提的是,实际进入基质晶格的Sc^(3+)浓度远小于前驱物中Sc^(3+)的浓度。采用激光共聚焦显微技术测试了980nm激光激发下的单颗粒上转换光谱。结果表明,晶格内少量Sc^(3+)的有效介入,可诱导单个NaGdF_4∶Yb^(3+),Er^(3+)微晶内绿色荧光(~2H_(11/2)/~4 S_(3/2)→4 I15/2)对红色荧光(4 F9/2→4 I15/2)相对强度的显著调控,且随着Sc^(3+)掺杂浓度的增加逐渐升高。结合建议的上转换机制,对红色和绿色上转换荧光衰减曲线和对应发射带的积分强度对激发功率的依赖关系展开了详细分析。上转换荧光衰减曲线的上升时间随着Sc^(3+)掺杂浓度的升高逐渐缩短,红绿发射带积分强度对激发功率的依赖也随着Sc^(3+)掺杂浓度的升高变得更为明显,暗示了半径最小的稀土离子Sc^(3+)进入基质晶格后,从Yb^(3+)到Er^(3+)能量转移速率显著提升,使得绿光的~2H_(11/2)/~4 S_(3/2)激发态较红光的4 F9/2激发态更容易布居,从而产生可观的上转换荧光调控。此类微晶有望在荧光防伪、光学波导等领域获得重要应用。

In a hydrothermal atmosphere with high temperature and long reaction time,a kind of special rare earth doped fluoride microcrystal was obtained with Sc3 ＋ co-doping.The crystal phase and morphology of as-prepared samples were characterized with X-ray diffraction （XRD）,scanning electron microscopy （SEM）,energy dispersive X-ray detector （EDX）and element map-ping.It is found that the Sc3 ＋-free sample can be ascribed to the pure hexagonal NaGdF4 ,and the Sc3 ＋-doped samples are com-posed of hexagonal NaGdF4-and monoclinic Na3 ScF6-based microcrystals.With increasing of the doped concentration of Sc3 ＋ , the morphology of NaGdF4-based microcrystal changes from lantern with cracked ends to octadecahedron with smooth surfaces, accomplished with a decreased size.It is worth mentioning that the concentration of Sc3 ＋ that really entered into the host lattice is much less than the Sc3 ＋ concentration in the precursor.Excited by 980 nm laser,the upconversion emission spectrum of single microcrystal was obtained by laser scanning confocal microscopy technology.For a single NaGdF4 ：Yb3 ＋ ,Er3 ＋ microcrystal, the significant upconversion fluorescence regulation was obtained by doping a few Sc3 ＋ into the host lattice,and the relative in-tensity of the green upconversion emission （2 H 11/2/4 S 3/2 →4 I 15/2 ）to the red counterpart （4 F 9/2 →4 I 15/2 ）increases gradually with the increasing of Sc3 ＋ doped concentration.Combining with the suggested upconversion mechanisms,the influence of Sc3 ＋ doped concentration on the rise time of upconverion fluorescence decay curve and the dependence of integrated intensity of upconverion emission band vs excitation power have been explored.It is found that the rise time of the upconversion fluorescence decay curve decreases gradually with the increase of the doped concentration of Sc3 ＋ ,which confirms that the energy transfer rate （from Yb3 ＋ to Er3 ＋ ）obtains a significant increase by the introduction of Sc3 ＋ .It makes the population of 2 H 11/2/4 S 3/2 excited state easier than the 4 F 9/2 counterpart,then induces the considerable upconversion fluorescence regulation.As the emission color of the NaGdF4-based microcrystal is easy to adjust,it is excepted to show a great potential in the fields of fluorescence anti-counter-feiting,optical waveguide,and so on.The upconversion fluorescence regulation of single rare earth doped microcrystal induced by codoping with small radius ion can be applied to other luminous hosts.