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光热传感对于智能穿戴设备的开发至关重要. 然而, 设计合成具有合适多波长发射的发光材料, 并在单一材料体系中利用多组探针构建宽温度范围的高灵敏温度传感器是一个巨大挑战. 本研究采用高温固相法成功制备了Li0.9K0.1NbO3:Pr3+/Er3+单掺及双掺荧光粉. 通过X射线衍射仪、扫描电子显微镜、荧光光谱仪以及自制的加热装置对其结构、形貌及激发波长和温度依赖的荧光特性进行了表征. 详细研究了Er3+单掺与Pr3+, Er3+共掺样品的上/下转换荧光及Er3+的双模荧光温度传感特性. 结果表明: Pr3+掺杂优化了Li0.9K0.1NbO3:Er3+荧光粉中源自于Er3+离子热耦合能级的双模光学测温性能. 本研究为温度探测提供了材料基础和光学测温技术.Photothermal sensing is crucial in developing smart wearable devices. However, designing and synthesizing luminescent materials with suitable multi-wavelength emission and constructing multiple sets of probes in a single material system is a huge challenge for constructing sensitive temperature sensors with a wide temperature range. In this paper, Pr3+, Er3+ single-doped and double-doped Li0.9K0.1NbO3 phosphors are successfully prepared by high temperature solid phase method, and their structures, morphologies, excitation wavelengths and temperature-dependent fluorescence properties are characterized by XRD, SEM, fluorescence spectrometer and self-made heating device. Firstly, the photoluminescences of the synthesized series of samples are investigated. The results show that comparing with the single-doped Li0.9K0.1NbO3: Er3+ sample, the up/down-conversion spectra of Pr3+, Er3+ co-doped phosphors under 808 nm/380 nm excitation show that the green fluorescence emission of Er3+ is enhanced. In addition, under 980 nm excitation, Pr3+ can effectively regulate the fluorescence energy level population pathway, so that the electrons are more effectively arranged in the 2H11/2 and 4S3/2 energy levels in the excitation process. The red emission is weakened and the green emission is enhanced, which improves the signal resolution of the fluorescent material and has a significant influence on the optical temperature measurement. Secondly, the up-conversion fluorescence property of Er3+ under 808 nm/980 nm laser excitation in Li0.9K0.1NbO3:Er3+ and Li0.9K0.1NbO3:Pr3+,Er3+ phosphors are investigated. The results show that the red and green fluorescence emissions of Er3+ are two-photon processes. Finally, the up/down-conversion dual-mode temperature sensing properties of Er3+ in Li0.9K0.1NbO3:Er3+ and Li0.9K0.1NbO3:Pr3+, Er3+ phosphors are investigated. It is found that both materials have good optical temperature measurement performances. The Pr3+ doping optimizes the dual-mode optical temperature measurement performances of Li0.9K0.1NbO3:Er3+ phosphors derived from the thermal coupling energy level of Er3+ ions. In addition, the up/down-conversion fluorescence mechanism of Li0.9K0.1NbO3:Er3+ and Li0.9K0.1NbO3:Er3+, Pr3+ phosphors are proposed, and the enhanced green fluorescence by Pr3+ co-doping is attributed to the energy transfer from Pr3+ ions to Er3+ ions, leading to the increase of green fluorescence level population and the decrease of red fluorescence level population of the Er3+ ions. This new dual-mode optical temperature measurement material provides a material basis and optical temperature measurement technology for exploring other temperature measurement materials.
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Keywords:
- niobate /
- up/down-conversion /
- multi-wavelength /
- temperature detection
1. 引 言
近年来, 稀土发光材料荧光强度比(FIR)技术因其响应时间短、灵敏度和可靠性高等优势作为光学测温技术受到极大关注. 该技术是基于荧光材料的两个发射峰的荧光强度比来实现的一种无损非接触式温度测量技术, 有效地避免了外界环境干扰和实验条件对测温性能的影响, 具有高的测温精确度, 从而受到科学家和工程师的青睐[1–3]. 其中, 荧光材料的测温灵敏度和发射波长是评价测温技术的关键参数. 一方面, 灵敏度是测温精准程度的量度, 它受到晶体形貌、尺寸、激活中心以及敏化剂的浓度等因素影响, 但具体的作用机理尚不明确, 只能通过构建合适的对照组来探究各种因素对灵敏度的影响规律[4–7]. 另一方面, 目前针对FIR技术的探究主要集中在可见光波段, 限制了实际环境温度场探测的应用[8–10]. 最近, Yu等[11]在紫外B波段开发出多个热耦合激发态的单离子(Gd3+)温度计. 该温度计将玻尔兹曼测温的概念扩展到两个以上的激发能级, 使测温范围选择性拓宽至低温、室温以及高温, 并保持了高的相对灵敏度和精确度. 这项工作不仅解决了在可见光波段FIR技术的瓶颈问题, 也为宽温度范围测温提供了解决策略. 除此之外, 采用多种激发波长的多重FIR测温技术进行多重校准也是解决实际温度场探测的一种简单有效策略.
稀土离子(Er3+, Pr3+, Tm3+, Eu3+和Ho3+)因具有丰富的能级和优异的发光性能被广泛应用于FIR光学测温, 其中, 基于合适的热耦合能级差(∆E约为800 cm–1), Er3+在不同基质中均表现出高效的绿色双峰发射和敏感的FIR效应[1,12–15]. Er3+激活的上转换发光材料, 已取得一系列进展. 但上转换发光效率依然是制约各种应用的关键因素. 基于光学荧光强度比值的FIR测温灵敏度也与荧光强度的绝对值相关, 荧光绝对值越大, 信噪比越高, 测温灵敏度就越高. 目前, 许多增强Er3+上转换发光策略被报道, 例如, 引入敏化剂Yb3+离子[4,16–19]、改变晶格结构或者掺杂碱金属离子[20–24]. 除此之外, 掺杂Ln3+来改变Er3+荧光能级电子布居路径以减少无辐射弛豫也被报道[25–27]. 总之, 提高Er3+的热耦合能级(2H11/2和4S3/2)的发射强度是改善Er3+掺杂体系测温灵敏度的关键.
基于Er3+的2H11/2和4S3/2热耦合能级, 在808或980 nm激光激发下, Er3+掺杂荧光材料的上转换FIR测温应用已被广泛研究[4,13,28–30]. 然而, 依然不能实现FIR在宽温度范围的准确测温. 为了校准测温材料的精确度, 多模测温如通过紫外和近红外激发的FIR测温是实现宽范围温度测量的一种可行解决方案, 但目前在紫外激发下的下转换光学测温研究甚少[31–33]. 对于Er3+掺杂的铌酸盐基质中实现上/下转换双模光学测温研究几乎没有报道.
本文通过高温固相法合成了一系列光学测温材料. 研究了Pr3+掺杂对Li0.9K0.1NbO3:Er3+荧光粉上/下转换双模荧光性能的影响规律. 并详细研究了Pr3+掺杂和未掺杂对Li0.9K0.1NbO3:Er3+荧光的温度依赖特性. 研究结果表明, Li0.9K0.1NbO3:Er3+和Li0.9K0.1NbO3:Pr3+, Er3+两种荧光粉都表现出源自于Er3+离子热耦合能级的上/下转换双模式荧光的温度依赖关系, 基于此探究了其双模荧光的FIR测温性能. 相比于未掺杂Pr3+的Li0.9K0.1NbO3:Er3+荧光粉, Pr3+掺杂Li0.9K0.1NbO3:Er3+荧光粉的FIR测温性能进一步得到改善, 表明其在光学测温领域的应用潜力.
2. 实 验
2.1 样品制备
采用传统高温固态反应法制备了系列Pr3+和Er3+掺杂的Li0.9K0.1NbO3微纳晶体荧光材料, 包括Li0.9K0.1NbO3:0.5%Pr3+, Li0.9K0.1NbO3:0.5%Er3+和Li0.9K0.1NbO3:0.5%Pr3+, 0.5%Er3+荧光粉. 所用原材料为K2CO3 (99%), LiCO3 (99.99%), Nb2O5 (99.9%), Pr6O11 (99.9%)以及Er2O3 (99.9%). 首先, 将原材料严格按照化学计量比称取并混合均匀, 在玛瑙研钵中研磨1 h. 然后将研磨好的粉末放置在箱式电阻炉中1000 ℃下煅烧8 h. 待自然冷却后研磨煅烧后的粉末, 以待后续表征.
2.2 性能表征
采用D/Max 2400 X射线衍射仪(XRD)对合成样品的晶体结构进行表征. 采用扫描电子显微镜(SEM, ZEISS Gemini 500)对合成样品的形貌和尺寸进行表征. 使用75 W氙灯和R928P光电倍增管的Horiba PTI荧光光谱仪测量光致发光发射(PL)谱和激发(PLE)谱. 采用功率可调的808 nm (0—2 W)和980 nm (0—5 W)近红外激光作为泵浦源, 研究上转换发光的泵浦过程. 利用自搭建的变温光谱测试系统测量23—200 ℃范围内不同激发波长的变温发射谱.
3. 实验结果与分析
3.1 物相分析
图1(a)给出了Li0.9K0.1NbO3:Ln3+系列样品的XRD衍射图谱, 可以看出, 对于不同稀土离子掺杂的Li0.9K0.1NbO3晶体, 所有衍射峰位置与标准卡PDF# 85-2456衍射峰位置匹配良好. 微量稀土离子及少量K+掺杂未引起晶体结构的明显改变. 上述结果表明: Ln3+和K+均成功掺杂至LiNbO3晶格结构中. 如图1(b)所示, 在Li0.9K0.1NbO3晶体结构中, Nb5+和Li+/K+离子分别被6个氧原子配位形成[NbO6]和[LiO6]/[KO6]八面体六配位结构, 这些八面体通过共享氧原子形成稳定的层状立体结构, 其中[KO6]八面体的引入为发光中心Er3+离子提供丰富的晶体场环境.
图2为Li0.9K0.1NbO3:Er3+和Li0.9K0.1NbO3:Pr3+, Er3+荧光粉的SEM图像和Li0.9K0.1NbO3:Er3+荧光粉的能量色散X射线(EDX)元素谱. 从图2(a)和图2(b)可以观察到两个样品形貌近似为立方晶粒, 晶粒大小平均为2 μm. 表明Pr3+的掺杂对荧光粉的形貌及尺寸没有显著影响. 同时, 图2(c)的Li0.9K0.1NbO3:Er3+荧光粉的EDX元素谱确认了Nb和O元素的均匀分布. Li/K和Er分别由于较小的原子量和微小的掺杂量在元素分布图上没有清晰显示.
图 2 Li0.9K0.1NbO3:Ln3+样品的SEM图片及EDX元素谱 (a) Li0.9K0.1NbO3:Er3+的SEM图片; (b) Li0.9K0.1NbO3:Pr3+, Er3+的SEM图片; (c) Li0.9K0.1NbO3:Er3+荧光粉的EDX元素分布图谱Fig. 2. SEM images and element mappings of Li0.9K0.1NbO3:Ln3+ phosphors: (a) SEM image of Li0.9K0.1NbO3:Er3+; (b) SEM image of Li0.9K0.1NbO3:Pr3+, Er3+; (c) EDX elemental distribution spectra of Li0.9K0.1NbO3:Er3+ phosphors.3.2 Pr3+掺杂对Li0.9K0.1NbO3:Er3+荧光性能的影响
图3展示了Pr3+和Er3+单掺样品及Pr3+, Er3+共掺样品的上、下转换荧光特性. 当激发波长为380 nm时, Li0.9K0.1NbO3:Er3+荧光粉呈现出Er3+的强绿色荧光发射, 其发射可归因于Er3+的2H11/2→4I15/2 (529 nm)和4S3/2→4I15/2 (554 nm)能级跃迁, 并伴随有较弱的源自于Er3+的4F9/2→4I15/2 (675 nm)能级跃迁的红色荧光发射. 在Pr3+, Er3+共掺的Li0.9K0.1NbO3样品中, 在紫外380 nm激发下, 除了展现出和Er3+单掺样品相同的源自于Er3+的绿光和红光发射外, 还展现出与Li0.9K0.1NbO3:Pr3+单掺样品相一致的发射峰(约620 nm红色发射), 由此可见其弱红色发射主要源自于Pr3+的1D2→3H4能级跃迁. 当用280 nm紫外光直接把电子从价带激发到导带时, 则可以观察到源自于Er3+的绿色和Pr3+的红色荧光发射, 没有明显的Er3+红色荧光发射, 见图3(a)所示. 图3(b)为在Pr3+, Er3+共掺杂荧光粉中分别监控554 nm和620 nm荧光发射的激发谱, 结果表明红色荧光发射和绿色荧光发射分别源自于Pr3+和Er3+的特征跃迁, Pr3+和Er3+之间没有发生明显的能量传递.
图 3 不同波长激发下, Li0.9K0.1NbO3:Ln3+的上/下转换发射谱比较 (a) Pr3+和Er3+单掺样品及Pr3+, Er3+共掺样品的发射谱 (λex = 280 nm/380 nm); (b) Pr3+, Er3+共掺样品的激发谱(λmoni = 554 nm/620 nm); Er3+单掺及Pr3+, Er3+共掺样品在(c) λex= 808 nm, (d) 980 nm激光激发下的上转换发射谱Fig. 3. Comparison of up-conversion and down-conversion emission spectra of rare earth doped Li0.9K0.1NbO3:Ln3+ under different excitation wavelengths: (a) Emission spectra of Pr3+ and Er3+ single-doped samples and Pr3+, Er3+ co-doped samples (λex = 280 nm/380 nm); (b) excitation spectra of Pr3+, Er3+ co-doped samples (λmoni = 554 nm/620 nm); (c), (d) up-conversion emission spectra of Er3+ single doped and Pr3+, Er3+ co-doped samples under 808 and 980 nm excitations.图3(c)和图3(d)展示了Li0.9K0.1NbO3:Er3+和Li0.9K0.1NbO3:Pr3+, Er3+两种荧光粉在808 nm/980 nm 激光激发下的上转换发射谱. 两个样品均表现出Er3+的特征跃迁, 得到源自于Er3+绿色荧光发射(529 nm和554 nm)和红色荧光发射(675 nm), 没有观察到明显的源自于Pr3+的发射. 与单掺杂Li0.9K0.1NbO3:Er3+样品相比, Pr3+, Er3+共掺的铌酸盐荧光粉分别在380 nm和808 nm激发下的下、上转换光谱中, Er3+的双峰绿色荧光发射增强(图3(a)和图3(c)). 有趣的是, 在980 nm激发下, Li0.9K0.1NbO3:Er3+样品展示了比源自于Er3+:2H11/2→4I15/2 (529 nm)和4S3/2→4I15/2 (554 nm)跃迁更强的红色双峰荧光发射(4F9/2→4I15/2 (675 nm)) (图3(d)). 在上转换过程中, Pr3+共掺杂的样品未观察到明显的Pr3+发射, 但Pr3+的掺杂调控了Er3+局域晶格环境的对称性和无辐射弛豫概率, 从而调控了绿色荧光强度和红绿荧光比率 (图3(c)和图3(d)). 结合Pr3+掺杂效应和激发波长, 可以有效地调控荧光能级布居途径使得在激发过程中电子更有效布居在2H11/2和4S3/2能级上, 导致红色荧光发射减弱和获得高的绿红荧光强度比率(图3(c)和图3(d)). 考虑到FIR测温应用, Pr3+共掺增强了Er3+热耦合能级2H11/2和4S3/2的荧光信号(图3(c)), 这将有利于改善Er3+热耦合能级的测温性能. 以上结果表明Pr3+掺杂可以有效地增强Er3+上/下转换热耦合能级2H11/2和4S3/2的电子布居数, 从而增加荧光材料的信号分辨率, 对光学测温有着重大的影响.
3.3 上转换荧光机理
图4对比展示了Li0.9K0.1NbO3:Er3+和Li0.9K0.1NbO3:Pr3+, Er3+荧光粉在808 nm/980 nm激光激发下的上转换发射谱. 所有的上转换发射谱均展示了源自于Er3+离子的绿色荧光发射(2H11/2, 4S3/2→4I15/2)以及红色荧光发射(4F9/2→4I15/2), 且这些发射峰强度均随着激发功率的增加而增加. 在上转换发射谱中未观察到明显的源自于Pr3+的荧光发射, 间接说明没有Er3+到Pr3+的能量传递. 众所周知, 上转换荧光强度与激发光功率之间的关系遵循I∝Pn[34]. 其中I为荧光发射强度, P为激发光功率, n表示每一个上转换过程中所吸收的光子数. 图4内插图中Er3+的绿色和红色发射峰积分强度与激发光功率的双对数拟合斜率值均约等于2, 证明了Er3+的红色和绿色荧光能级布居均为双光子过程. 然而, 当Pr3+共掺杂到Li0.9K0.1NbO3:Er3+中时, 实现了对荧光能级布居途径的调控. 对比图4(a)—(d), 不难发现, 当980 nm激光激发Li0.9K0.1NbO3:Er3+样品时, 图4(b)红绿荧光强度比率明显不同于图4(a)、图4(c)和图4(d). 考虑到Pr3+共掺后, 红色荧光能级布居数目减小, 而绿色荧光能级布局数目相比红色荧光能级布居数目明显增多, 这说明红色荧光能级布居不是由绿色荧光能级无辐射弛豫至红色荧光能级的. 考虑以上实验结果, 可推测出红色和绿色荧光能级电子布居途径不同.
图 4 激发功率依赖的Li0.9K0.1NbO3:Er3+和Li0.9K0.1NbO3:Pr3+, Er3+荧光粉的发射谱, 其中内插图为对应发光强度与入射激光的功率关系 (a), (b) Li0.9K0.1NbO3:Er3+荧光粉的发射谱(λex = 808 nm和λex = 980 nm); (c), (d) Li0.9K0.1NbO3:Pr3+, Er3+荧光粉的发射谱(λex = 808 nm和λex = 980 nm)Fig. 4. Excitation power-dependent emission spectra of Li0.9K0.1NbO3:Er3+ and Li0.9K0.1NbO3:Pr3+, Er3+ phosphors, where the insets are the relationships between luminescence intensity and incident laser power: (a), (b) Emission spectra of Li0.9K0.1NbO3:Er3+ phosphors (λex = 808 nm and λex = 980 nm); (c), (d) emission spectra of Li0.9K0.1NbO3:Pr3+, Er3+ phosphors (λex = 808 nm and λex = 980 nm).3.4 光学测温性能
图5(a)—(c)展示了Li0.9K0.1NbO3:Er3+样品的上/下转换荧光的温度依赖关系. 显然, 在不同波长(380, 808, 980 nm)激发下光谱轮廓独立于温度. 但随着温度的升高, 源自于Er3+的热耦合能级(2H11/2→4I15/2和4S3/2→4I15/2)的绿色双峰荧光(峰在529和554 nm)显示了明显不同的热猝灭趋势. 具体情况表现为: 双峰荧光强度均随着温度升高而减小, 但源自于较高能级的529 nm处荧光在温度梯度下猝灭效应较弱, 而源自于较低能级的554 nm处荧光温度猝灭效应较强, 这种现象源于热耦合能级的玻尔兹曼统计分布效应. 根据玻尔兹曼分布, 两个热耦合能级的荧光强度比RFIR遵循以下公式[35]:
图 5 Li0.9K0.1NbO3:Er3+荧光粉的上、下转换测温性能 (a)—(c) 分别在380, 808, 980 nm激发下的发射谱; (d)—(f) 相应于图(a)—(c)中的上/下转换发射谱的双峰绿色FIR与温度的关系; (g)—(i) 相应于图(d)—(f)中双峰荧光强度比率测温的灵敏度曲线Fig. 5. Up/down-conversion temperature measurement performance of Li0.9K0.1NbO3:Er3+ phosphor: (a)–(c) The emission spectra excited at 380, 808 and 980 nm, respectively; (d)–(f) the relationship between the bimodal green FIR and temperature corresponding to the up/down-conversion emission spectra in panel (a)–(c); (g)–(i) the sensitivity curves of temperature measurement of bimodal FIR corresponding to panel (d)–(f).RFIR=Aexp(−ΔEKBT), (1) 其中A为系数, KB为玻尔兹曼常数, ΔE为两个能级的之间的能量差. 基于(1)式可知测温材料的FIR值与温度具有确定的函数关系, 在一定温度范围内FIR值变化越明显, 测温性能越优异, 因此可以利用FIR技术来准确地测量温度. 图5(d)—(f)给出了(2H11/2→4I15/2)和(4S3/2→4I15/2)热耦合能级FIR的温度依赖关系, 通过指数函数拟合得出相应的函数关系, 随着温度的升高, FIR值逐渐增大. 图5(d)—(f)证实了不同激发波长激发同一材料时表现出不同的光学测温特性, 这种多波长激发的设计为高灵敏度的多重校准提供了一种有效策略. 此外, 本文研究发现, 与下转换380 nm激发相比, 在相同的温度梯度下, 上转换模式的FIR值变化较大, 测温性能更优异, 这主要是因为碱金属离子K+掺杂减少了稀土离子的无辐射跃迁, 增强了Er3+的上转换发射强度, 提高了信噪比, 更有利于提高温度探测的性能[16].
灵敏度作为光学测温性能的重要参数, 它直接反映光学测温性能. 相对灵敏度Sr是指将温度变化1 K时荧光强度比值相对其自身的变化率, 而绝对灵敏度Sa为在变温过程中荧光强度比值的绝对变化率, 其公式定义如下[35]:
Sr=|1RFIR∂RFIR∂T|, (2) Sa=|∂RFIR∂T|=RFIR×Sr. (3) 根据(2)式和(3)式, 绝对灵敏度和相对灵敏度与温度T的拟合曲线如图5(g)—(i)所示, 显然, Sa和Sr在紫外和两种NIR激光激发下具有极高的拟合度, 其中Sr值随温度升高呈下降趋势, Sr最大值分别为0.97 × 10–2 K–1 (380 nm), 1.286 × 10–2 K–1 (808 nm)和1.221 × 10–2 K–1 (980 nm); 同时, Sa值随温度升高而升高, 且Sa最大值分别为0.44 × 10–2 K–1 (380 nm), 0.89 × 10–2 K–1 (808 nm)和0.81 × 10–2 K–1 (980 nm). 通过本文的光学测温材料与先前报道关于Er3+光学测温材料(表1)的Sa和Sr值[36–38]进行比较, Li0.9K0.1NbO3:Er3+荧光粉的多波长光学测温具有优异的相对灵敏度和绝对灵敏度. 基于多波长条件的光学测温表现出具有超灵敏度和多重校准性, 表明该荧光粉在光学测温方面有很大的应用前景.
表 1 基于FIR技术下不同基质中掺杂Er3+的温度传感材料光学参数Table 1. Optical parameters of temperature sensing materials doped with Er3+ in different substrates based on FIR technologyMaterials Wavelength/nm Sr-Max/(10–2 K–1) Sa-Max/(10–2 K–1) References SrSnO3:Er 975 nm 0.997(294 K) 0.791(368 K) [30] BaBiNb2O9:Er 980 nm 0.959(300 K) 0.996(483 K) [36] La2CaZnO5:Er 378 nm 1.454(300 K) — [31] Sr2Gd8(SiO4)6O2:Er 379 nm — 0.34(463 K) [32] Ca3Bi(PO4)3:Er 376 nm 1.21(300 K) 0.312(473 K) [33] La2Mo2O9:Er 980 nm 1.16(293 K) 0.527(493 K) [37] (K, Na)NbO3:Er 980 nm375 nm 0.96(303 K)16.17(80 K) 0.28(433 K)0.37(280 K) [38] Cs3Bi2Cl9:Er 808 nm980 nm 1.4(303 K)1.38(303 K) 0.62(573 K)0.61(573 K) [13] Li0.9K0.1NbO3:Er 380 nm808 nm980 nm 0.97(303 K)1.286(297 K)1.221(297 K) 0.44(463 K)0.89(443 K)0.81(443 K) This work 灵敏度作为光学测温的重要指标, 如何提高其灵敏度成为目前的主要问题. 其中, 离子共掺杂是一种常见的提高非接触式光学温度计灵敏度的一种有效途径. 为了进一步优化Li0.9K0.1NbO3:Er3+荧光粉的光学测温性能, 进一步考察了Pr3+和Er3+共掺Li0.9K0.1NbO3荧光粉的测温性能. 图6(a)—(c)展示了Li0.9K0.1NbO3:Pr3+, Er3+荧光粉的温度依赖特性. 结果表明: 随着温度的升高, 不同波长激发下的Er3+热耦合能级2H11/2→4I15/2处的发射强度温度猝灭效应较弱, 而4S3/2→4I15/2处的发射强度温度猝灭效应较明显, 其主要原因与Li0.9K0.1NbO3:Er3+测温材料相似. 图6(d)—(f)为优化后的Li0.9K0.1NbO3:Pr3+, Er3+荧光粉的2H11/2/4S3/2热耦合能级FIR的温度依赖关系. 与Li0.9K0.1NbO3:Er3+相比, Li0.9K0.1NbO3:Pr3+, Er3+的FIR数据在测量温度范围区域内仍能保持良好的指数函数关系, 并且FIR值变化明显, 可以用来作为光学测温材料.
图 6 Li0.9K0.1NbO3:Pr3+, Er3+荧光粉的上/下转换双模式光学测温性能 (a)—(c) 分别在380, 808, 980 nm激发下的发射谱; (d)—(f) 相应于图(a)—(c)中的上/下转换发射谱的双峰绿色FIR与温度的关系; (g)—(i) 相应于图(d)—(f)中双峰FIR测温的灵敏度曲线Fig. 6. Up/down-conversion temperature measurement performance of Li0.9K0.1NbO3:Pr3+, Er3+ phosphor: (a)–(c) The emission spectra excited at 380, 808, and 980 nm, respectively; (d)–(f) the relationship between the bimodal green FIR and temperature corresponding to the up/down-conversion emission spectra in panel (a)–(c); (g)–(i) the sensitivity curves of temperature measurement of bimodal FIR corresponding to panel (d)–(f).通常Sr具有普适性, 可以用来衡量不同发光材料的温度探测能力; 但考虑到光信号相对发射强度的影响, Sa更适用于衡量相同基质发光材料下FIR技术的测温材料, 尤其是基于相同热耦合能级的测温材料[25]. 因此对于优化后的Li0.9K0.1NbO3:Pr3+, Er3+材料与Li0.9K0.1NbO3:Er3+材料测温性能的优劣主要参考Sa值.
图6(g)—(i)为不同波长激发下的绝对灵敏度和相对灵敏度与温度T的关系, 显然, 在不同波长激发下的Sa和Sr具有极高的拟合度, 随温度变化的趋势与Li0.9K0.1NbO3:Er3+材料相似, 其中Sr最大值分别为1.12 × 10–2 K–1 (380 nm), 1.284 × 10–2 K–1 (808 nm)和1.106 × 10–2 K–1 (980 nm), 具有较大的相对灵敏度; Sa最大值分别为0.54 × 10–2 K–1 (380 nm), 1.12 × 10–2 K–1 (808 nm)和0.83 × 10–2 K–1 (980 nm). 与Li0.9K0.1NbO3:Er3+相比, Li0.9K0.1NbO3:Pr3+, Er3+的绝对灵敏度都有所提升, 说明Pr3+掺杂提高了Er3+的热耦合能级的测温性能, 表明该荧光材料具有作为温度传感材料的潜力. 表2列举了其他稀土离子作为辅助剂调控Er3+掺杂测温材料的Sr与Sa[39–42], 对比表中数据, 不难发现我们优化后的测温材料性能优异并且可以通过上/下转换双模式进行测温和校准.
表 2 基于FIR技术下不同基质中掺杂Er3+-Ln3+的温度传感材料光学参数Table 2. Optical parameters of temperature sensing materials doped with Er3+-Ln3+ in different substrates based on FIR technology.Materials Wavelength/nm Sr-Max/(10–2 K–1) Sa-Max/(10–2 K–1) References La2MgGeO6:Bi, Er 980 nm 1.23(293 K) 0.94(473 K) [39] K3Gd(PO4)2:Yb, Er, Tm 980 nm 1.35(300 K) 0.456(608 K) [40] NaLuF4:Er, Tm 1532 nm 1.265(293 K) 0.4(519 K) [41] BiVO4:Er, Tm 980 nm 1.1(293 K) 0.7(473 K) [42] 1550 nm 1.1(293 K) 0.56(453 K) Y2SiO5:Er, Tm 808 nm 0.395(298 K) — [29] KYb(MoO4)2:Er, Gd 980 nm 1.1(303 K) 0.97(513 K) [25] KYb(MoO4)2:Er, La 1.1(303 K) 0.95(513 K) KYb(MoO4)2:Er, Y 1.11(303 K) 0.91(513 K) Li0.9K0.1NbO3:Pr, Er 380 nm 1.12(296 K) 0.54(434 K) Thiswork 808 nm 1.284(296 K) 1.12(443 K) 980 nm 1.106(296 K) 0.83(443 K) 以上结果表明, 在Li0.9K0.1NbO3:Er3+测温材料中掺杂Pr3+离子可以提高Er3+热耦合能级的上/下转换双模式的光学测温性能, 且灵敏度优于其他荧光测温材料, 因此有望成为多重校准光学测温的候选材料.
3.5 Li0.9K0.1NbO3:Pr3+/Er3+荧光粉的上/下转换荧光机理图
基于上/下转换光谱学分析, 提出的Er3+的上/下转换发光机理如图7所示. 简述如下: 在紫外380 nm (280 nm)激发下, 电子被有效的激发至Er3+的2G11/2能级(导带), 在声子能辅助下无辐射弛豫至Er3+的2H11/2与4S3/2能级, 并跃迁至基态4I15/2能级, 实现了Er3+的下转换绿色发光. 由于低效的红色荧光能级布居(通过2H11/2/4S3/2无辐射弛豫到4F2/9), 导致了Er3+的弱红色荧光发射. 在808 nm激发下处于Er3+的基态能级4I15/2通过吸收一个光子到达4I9/2能级, 接着再吸收同样的光子能量到2H9/2或4F7/2能级, 然后, 快速无辐射弛豫至绿色荧光能级2H11/2/4S3/2导致双峰绿色发射. 在Er3+单掺样品中, 在980 nm激发下, Er3+的基态能级4I15/2吸收一个光子到达4I11/2, 接着吸收同样的光子能量从4I11/2到2H11/2或从4I13/2到4F9/2, 导致了可以相比较的红色和绿色荧光发射. 而共掺Pr3+后(图7), 一个Pr3+和一个Er3+各吸收一个980 nm光子, 接着, Pr3+将能量传递给Er3+离子, 使激发态Er3+到达绿色荧光能级, 抑制了红色荧光能级的布居, 从而增加了绿色荧光能级的布居.
我们知道, Er3+的2H11/2与4S3/2属于热耦合能级, 依据玻尔兹曼统计分布原理, 随温度上升, 室温下主要占据4S3/2低能级的电子倾向占据2H11/2, 导致2H11/2与4S3/2能级之间的荧光强度比率发生规律性变化, 为光学测温应用提供了可能.
4. 结 论
本文通过传统高温固相法合成了Li0.9K0.1NbO3:Pr3+, Li0.9K0.1NbO3:Er3+和Li0.9K0.1NbO3:Pr3+, Er3+荧光粉, 首先研究了多种波长激发下Pr3+掺杂对Er3+热耦合能级2H11/2与4S3/2的影响; 其次对Er3+的绿色上转换发光的过程进行了探究; 最后对Li0.9K0.1NbO3:Er3+, Li0.9K0.1NbO3:Pr3+, Er3+荧光粉的上下转换双模式光学测温性能进行了研究. 结果表明两种材料都具有良好的光学测温性能, 其中Pr3+掺杂可以优化Li0.9K0.1NbO3: Er3+测温材料的测温性能. 这种新型的双模光学测温材料为探索其他测温材料的应用提供了思路.
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图 2 Li0.9K0.1NbO3:Ln3+样品的SEM图片及EDX元素谱 (a) Li0.9K0.1NbO3:Er3+的SEM图片; (b) Li0.9K0.1NbO3:Pr3+, Er3+的SEM图片; (c) Li0.9K0.1NbO3:Er3+荧光粉的EDX元素分布图谱
Fig. 2. SEM images and element mappings of Li0.9K0.1NbO3:Ln3+ phosphors: (a) SEM image of Li0.9K0.1NbO3:Er3+; (b) SEM image of Li0.9K0.1NbO3:Pr3+, Er3+; (c) EDX elemental distribution spectra of Li0.9K0.1NbO3:Er3+ phosphors.
图 3 不同波长激发下, Li0.9K0.1NbO3:Ln3+的上/下转换发射谱比较 (a) Pr3+和Er3+单掺样品及Pr3+, Er3+共掺样品的发射谱 (λex = 280 nm/380 nm); (b) Pr3+, Er3+共掺样品的激发谱(λmoni = 554 nm/620 nm); Er3+单掺及Pr3+, Er3+共掺样品在(c) λex= 808 nm, (d) 980 nm激光激发下的上转换发射谱
Fig. 3. Comparison of up-conversion and down-conversion emission spectra of rare earth doped Li0.9K0.1NbO3:Ln3+ under different excitation wavelengths: (a) Emission spectra of Pr3+ and Er3+ single-doped samples and Pr3+, Er3+ co-doped samples (λex = 280 nm/380 nm); (b) excitation spectra of Pr3+, Er3+ co-doped samples (λmoni = 554 nm/620 nm); (c), (d) up-conversion emission spectra of Er3+ single doped and Pr3+, Er3+ co-doped samples under 808 and 980 nm excitations.
图 4 激发功率依赖的Li0.9K0.1NbO3:Er3+和Li0.9K0.1NbO3:Pr3+, Er3+荧光粉的发射谱, 其中内插图为对应发光强度与入射激光的功率关系 (a), (b) Li0.9K0.1NbO3:Er3+荧光粉的发射谱(λex = 808 nm和λex = 980 nm); (c), (d) Li0.9K0.1NbO3:Pr3+, Er3+荧光粉的发射谱(λex = 808 nm和λex = 980 nm)
Fig. 4. Excitation power-dependent emission spectra of Li0.9K0.1NbO3:Er3+ and Li0.9K0.1NbO3:Pr3+, Er3+ phosphors, where the insets are the relationships between luminescence intensity and incident laser power: (a), (b) Emission spectra of Li0.9K0.1NbO3:Er3+ phosphors (λex = 808 nm and λex = 980 nm); (c), (d) emission spectra of Li0.9K0.1NbO3:Pr3+, Er3+ phosphors (λex = 808 nm and λex = 980 nm).
图 5 Li0.9K0.1NbO3:Er3+荧光粉的上、下转换测温性能 (a)—(c) 分别在380, 808, 980 nm激发下的发射谱; (d)—(f) 相应于图(a)—(c)中的上/下转换发射谱的双峰绿色FIR与温度的关系; (g)—(i) 相应于图(d)—(f)中双峰荧光强度比率测温的灵敏度曲线
Fig. 5. Up/down-conversion temperature measurement performance of Li0.9K0.1NbO3:Er3+ phosphor: (a)–(c) The emission spectra excited at 380, 808 and 980 nm, respectively; (d)–(f) the relationship between the bimodal green FIR and temperature corresponding to the up/down-conversion emission spectra in panel (a)–(c); (g)–(i) the sensitivity curves of temperature measurement of bimodal FIR corresponding to panel (d)–(f).
图 6 Li0.9K0.1NbO3:Pr3+, Er3+荧光粉的上/下转换双模式光学测温性能 (a)—(c) 分别在380, 808, 980 nm激发下的发射谱; (d)—(f) 相应于图(a)—(c)中的上/下转换发射谱的双峰绿色FIR与温度的关系; (g)—(i) 相应于图(d)—(f)中双峰FIR测温的灵敏度曲线
Fig. 6. Up/down-conversion temperature measurement performance of Li0.9K0.1NbO3:Pr3+, Er3+ phosphor: (a)–(c) The emission spectra excited at 380, 808, and 980 nm, respectively; (d)–(f) the relationship between the bimodal green FIR and temperature corresponding to the up/down-conversion emission spectra in panel (a)–(c); (g)–(i) the sensitivity curves of temperature measurement of bimodal FIR corresponding to panel (d)–(f).
表 1 基于FIR技术下不同基质中掺杂Er3+的温度传感材料光学参数
Table 1. Optical parameters of temperature sensing materials doped with Er3+ in different substrates based on FIR technology
Materials Wavelength/nm Sr-Max/(10–2 K–1) Sa-Max/(10–2 K–1) References SrSnO3:Er 975 nm 0.997(294 K) 0.791(368 K) [30] BaBiNb2O9:Er 980 nm 0.959(300 K) 0.996(483 K) [36] La2CaZnO5:Er 378 nm 1.454(300 K) — [31] Sr2Gd8(SiO4)6O2:Er 379 nm — 0.34(463 K) [32] Ca3Bi(PO4)3:Er 376 nm 1.21(300 K) 0.312(473 K) [33] La2Mo2O9:Er 980 nm 1.16(293 K) 0.527(493 K) [37] (K, Na)NbO3:Er 980 nm375 nm 0.96(303 K)16.17(80 K) 0.28(433 K)0.37(280 K) [38] Cs3Bi2Cl9:Er 808 nm980 nm 1.4(303 K)1.38(303 K) 0.62(573 K)0.61(573 K) [13] Li0.9K0.1NbO3:Er 380 nm808 nm980 nm 0.97(303 K)1.286(297 K)1.221(297 K) 0.44(463 K)0.89(443 K)0.81(443 K) This work 表 2 基于FIR技术下不同基质中掺杂Er3+-Ln3+的温度传感材料光学参数
Table 2. Optical parameters of temperature sensing materials doped with Er3+-Ln3+ in different substrates based on FIR technology.
Materials Wavelength/nm Sr-Max/(10–2 K–1) Sa-Max/(10–2 K–1) References La2MgGeO6:Bi, Er 980 nm 1.23(293 K) 0.94(473 K) [39] K3Gd(PO4)2:Yb, Er, Tm 980 nm 1.35(300 K) 0.456(608 K) [40] NaLuF4:Er, Tm 1532 nm 1.265(293 K) 0.4(519 K) [41] BiVO4:Er, Tm 980 nm 1.1(293 K) 0.7(473 K) [42] 1550 nm 1.1(293 K) 0.56(453 K) Y2SiO5:Er, Tm 808 nm 0.395(298 K) — [29] KYb(MoO4)2:Er, Gd 980 nm 1.1(303 K) 0.97(513 K) [25] KYb(MoO4)2:Er, La 1.1(303 K) 0.95(513 K) KYb(MoO4)2:Er, Y 1.11(303 K) 0.91(513 K) Li0.9K0.1NbO3:Pr, Er 380 nm 1.12(296 K) 0.54(434 K) Thiswork 808 nm 1.284(296 K) 1.12(443 K) 980 nm 1.106(296 K) 0.83(443 K) -
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