Determination of near-infrared downconversion emission of Yb³⁺ and optical temperature sensing performances in Nd³⁺-sensitized SrF₂ nanocrystals

Highlights

• Efficient near-infrared downconversion of Yb³⁺ was achieved in Nd³⁺-sensitized SrF₂ nanocrystals under 808 nm excitation.

• The thermometric performances located in near-infrared range were systematically investigated.

• A maximum thermal sensitivity up to 0.00346 K^-1 is obtained at 473 K.

Abstract

In this work, a series of Nd³⁺ and Yb³⁺ codoped SrF₂ nanocrystals (NCs) were synthesized by the hydrothermal method. The TEM results show that the average size of these NCs is approximately ∼50 nm. The structure of SrF₂ NCs and its optical properties of near-infrared (NIR) downconversion luminescence (DCL) are systematically characterized. Under the excitation of 808 nm laser, intense NIR DCL centered at 976 nm from Yb³⁺ ions is experimentally determined. The mechanism of the DCL is further interpreted based on the efficient energy-transfer (ET) processes from Nd³⁺ to Yb³⁺. In addition, utilizing the fluorescence intensity ratio (FIR) technique, the temperature sensing behaviors of SrF₂:Nd³⁺/Yb³⁺ (15/4 mol%) are also investigated through the heterogeneously coupled states between Nd³⁺ (⁴F_3/2 → ⁴I_11/2) and Yb³⁺ (²F_5/2 → ²I_7/2) in a temperature range of 298–573 K. The results confirm that the maximum thermal sensitivity is up to 0.00346 K⁻¹ at 473K. The excellent thermal sensitivity of these NCs, particularly the DCL located at the NIR region, have the potential for biological applications and temperature measurement at the nanoscale.

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.