Determination of near-infrared downconversion emission of Yb3+ and optical temperature sensing performances in Nd3+-sensitized SrF2 nanocrystals
Introduction
Recently, lanthanide-doped nanomaterials have become a research hotspot due to their outstanding optical and chemical properties, which have been widely reported in biological therapy [1], temperature sensors [[2], [3], [4]], anti-counterfeiting labels [5], lasers [6,7] and other aspects [[8], [9], [10], [11]]. Most present studies concentrate on upconversion luminescence (UCL) and facilitate promising applications. In these studies, some efforts have been made in the investigation of Yb3+/RE3+ (RE = Er, Ho, or Tm) co-doped nanomaterials excited by commercial 980 nm lasers [[12], [13], [14], [15]]. However, owing to the low absorption cross-section and high water absorption coefficient at 980 nm, it greatly restricts its further applications. One of the proposed solutions is to utilize 940 nm (or 915 nm) as the excitation source for Yb3+ ions due to the quite low water absorption coefficient at this wavelength [16,17]. Compared to Yb3+ ions, Nd3+ ions have a larger absorption cross-section at 808 nm, in contrast, its water absorption coefficient at 808 nm (0.02 cm−1) is much lower than Yb3+ ions at 980 nm (0.48 cm−1) [18]. This would greatly avoid the overheating effects and damage to the biological tissues when tuning the excitation wavelength from 980 nm to 808 nm.
Compared to the UCL, the DCL in the NIR range has low absorption and scattering in biological tissues and can penetrate deeper tissues, thus greatly expanding its application in the field of temperature detection and optical therapy [19,20]. More importantly, under the excitation of 808 nm laser, the efficient NIR DCL of Yb3+ ions can be realized through the ET from Nd3+ to Yb3+. Actually, the SrF2 is a promising host matrix with cubic fluorspar structure and has attracted great attention because of its low phonon energy, high thermal conductivity and quantum yield [[21], [22], [23], [24]]. However, to our knowledge, there are no such DCL studies of Yb3+ ions reported in the SrF2 hosts. Its DCL performances also remain unknown. Therefore, it is valuable to investigate the influence of the doping concentration on the morphology and size of the SrF2 NCs and DCL properties.
In addition, the accurate measurement of the temperature is of great significance. Given the most thermometers are heavily dependent on the environment and cannot be used in extreme environments. Since the fast-responding, non-contact, and accurate temperature sensing ability, the optical ratiometric nanothermometry of lanthanide-doped NCs based on FIR or LIR (luminescence intensity ratio) technology has been extensively studied [[25], [26], [27], [28], [29]]. This technology utilizes the relatively separated and tightly coupling levels inducing emission intensity ratio to measure the temperature. Currently, the study of temperature sensors mainly focuses on the homogeneous rare-earth-ion and are located in the visible UCL light region, which has been demonstrated in NaYF4, LiLuF4, CaF2, and Y2O3 matrixes. Importantly, the heterogeneously coupling states between Yb3+ and Nd3+ ions in the NIR radiate transitions can provide a relatively high emission intensity ratio dependent on the temperature, which can be utilized to detect temperature. However, up to now, there is still a lack of experimental data to investigate its mechanism.
Thus, as summarized above, in this work, we have synthesized a series of SrF2:Nd3+/Yb3+ NCs by a facile hydrothermal method. Subsequently, the structure of SrF2 NCs and its NIR DCL properties are systematically characterized. Furthermore, the dynamic properties and mechanism of the DCL are also demonstrated. Finally, based on the FIR technology, the optical temperature sensing properties are investigated and a relatively large temperature sensing sensitivity in the NIR region is obtained.
Section snippets
Synthesis of nanocrystals
The raw chemicals of SrCl2·6H2O (99.99%), YbCl3·6H2O (99.9%), NdCl3·6H2O (99.9%), Na3C6H5O7 (98%) and NH4F (98%) were purchased from Aladdin (China). The procedure of synthesis of NCs is similar to our previous literature [30]. Taking the SrF2:Nd3+/Yb3+ (15/4 mol%) NCs for example, 10 mL solution of chloride salts (1.62 mmol SrCl2, 0.3 mmol NdCl3 and 0.08 mmol YbCl3) and 10 mL aqueous solution of the sodium citrate (1 M) were mixed under stirring for 1 h. Sequentially, 20 mL aqueous solution of
Structural
Fig. 1 shows the crystal structure of the SrF2 matrix, which has a cubic phase structure with a bandgap of 6.776 eV and space group FM-3m (225) [31]. The corresponding numbers of cations and anions are 4 and 8, respectively. When the lanthanides (Nd3+ or Yb3+ ions) are doped into the lattice, parts of the Sr2+ are replaced by the corresponding Nd3+ or Yb3+ ions. Fig. 1(b) further gives the cations (e.g. Sr2+/Nd3+/Yb3+) are cubic packed in SrF2:Nd3+/Yb3+ lattice, and the anions (F−) are filled
Conclusions
In conclusion, a series of SrF2:Nd3+/Yb3+ NCs were successfully synthesized and their DCLs were also investigated in detail. The results confirm that, under the excitation of 808 nm, the NIR DCL centered at 976 nm from Yb3+ ions can be efficiently achieved by codoping with Nd3+ ions. It is also found that the SrF2:Nd3+/Yb3+ NCs emits the strongest DCL when the doping Nd3+ and Yb3+ ions are fixed at 15 mol% and 4 mol%, respectively. On this basis, the mechanism of the NIR DCLs of SrF2:Nd3+/Yb3+
CRediT authorship contribution statement
Linxuan Wang: Conceptualization, Methodology, Writing – original draft. Xu Yang: Data curation. Maohui Yuan: Conceptualization, Methodology, Writing – original draft, Writing – review & editing. Zining Yang: Visualization, Investigation. Kai Han: Visualization, Investigation. Hongyan Wang: Supervision, Writing – review & editing. Xiaojun Xu: Validation.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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These authors contributed equally to this work.