Introduction Optical temperature sensor is based on the change of optical properties of materials at different temperatures. It has the advantages of non-contact, high precision, and fast response, and is widely used in industrial and medical fields. However, optical temperature sensors still face some challenges at this stage, such as limited temperature response range of some materials, limited measurement accuracy in complex environments, and high cost. Therefore, improving the temperature measuring range of the sensor,enhancing the anti-interference ability, and reducing the cost, are the important directions of the current research and development of optical temperature sensors. Rare earth ion has a special electronic structure, 5s and 5p orbitals can shield the electron transition in 4f level, so the electronic transition in 4f level is less affected by the external environment, when the external temperature changes, the fluorescence intensity of rare earth ion does not change significantly. The luminescence intensity of CsPbI3 nanocrystalline glass varies greatly by external temperature, and the fluorescence quenching phenomenon is easy to occur at high temperature. In this paper, a fluorescence temperature sensor based on rare earth ion and cesium lead halide perovskite nanocrystalline is used to detect the ambient temperature by the ratio of fluorescence intensity of different luminous centers, which has better sensitivity and does not require complex testing instruments.
Methods The glass with the nominal compositions of 41GeO2-25B2O3-8ZnO-3.6PbI2-5.4Cs2O-11NaI-5SrO-1NaF-1.2Tb4O7-yYb2O3 (where y=0, 0.3%, 0.6%, 0.9%, 1.2%, in mole fraction denoted as Y-1, Y-2, Y-3, Y-4, Y-5 respectively), was prepared by melt-quenching method. The Tb3+–Yb3+: CsPbI3 NCs glass was synthesized by subsequent heat treatments. The raw materials were well-mixed and then put into the alumina crucible. The glass batch were melted at 1 200 ℃ for 30 minutes, and the molten glass were poured and pressed into sheets, subsequently annealed to release thermal stresses. These glass sheets were heat treated at various conditions and then optical polished for structural and performance characters.
Results and discussion XRD and TEM analysis reveal that the lattice structure is belonged to CsPbI3 NCs, with the incorporation of Tb3+ and Yb3+. Upon excitation with 365 nm, Tb3+ undergoes transitions from 7F6 to 5D3, followed by the emission of distinct wavelengths at 5D4-7F6 (485 nm), 5D4-7F5 (545 nm), and 5D4-7F4 (585 nm), indicating a non-radiation relaxation to the 5D4 level. The energy difference between the 5D4 and 7F6 states in Tb3+ is precisely two times of that in the transition from the state 2F5/2 to the ground state 2F7/2 for excited Yb3+, allowing Tb3+ to absorb a single high-energy photon and to transfer the energy from 5D4 to two Yb3+. Thus two low-energy NIR photons are emitted: Tb3+(5D4)→Yb3+(2F5/2)+ Yb3+(2F5/2). The energy transfer between Tb3+ and Yb3+ is confirmed by the NIR fluorescence spectra. When the glass without heat treatment excited by 365 nm laser, the Yb3+ luminescence peak appears at 950–1 100 nm, confirming that there is an energy transfer process from Tb3+ to Yb3+ since the sole Yb3+ ion cannot be excited by 365 nm laser. Moreover, the infrared luminescence intensity increases with the increase of Yb3+ concentration, indicating that the probability of Tb3+ transferring energy to Yb3+ increases with the increase of Yb3+ concentrations. As the concentration of Yb3+ increased, the fluorescence intensity of Tb3+ also increase in the un-heat-treated glass. Since there are no nanocrystals in the glass matrix, it was speculated that there was a back energy transfer process from Yb3+ to Tb3+. In the presence of CsPbI3 NCs, it is found that the lifetime of CsPbI3 NCs is almost the same before and after adding rare earth ions, indicating that there is no energy transfer between nanocrystals and rare earth ions. The fluorescence lifetime of Tb3+ was found to be enhanced with the increase of Yb3+ concentration, indicating the existence of back energy transfer process from Yb3+ to Tb3+. Because the luminous intensities of Tb3+ and CsPbI3 NCs have different sensitivity to temperature, good self-calibration temperature measurement performance can be obtained by using their luminous intensity ratio with temperature variation. In this work, when Tb3+ is 1.2% (in mole) and Yb3+is 0.3% (in mole), the Tb3+–Yb3+: CsPbI3 NCs glass exhibit the best temperature sensitivity.
Conclusions The optical temperature sensor was prepared by using Tb3+ and CsPbI3 NCs with different luminous colors and different sensitivity to temperature. Under the excitation of 365 nm light, Yb3+ emits light, and with the increase of Yb3+ content, the luminous intensity of Tb3+ increases. Since Yb3+ cannot absorb the energy at 365 nm light, there is an energy transfer from Tb3+ to Yb3+. The energy transfer mechanism is as follows: Firstly, Tb3+(5D4)→Yb3+(2F5/2)+ Yb3+(2F5/2) energy transfer process occurs, leading to the emission of 1 020 nm near-infrared light from Yb3+; Secondly, Yb3+(2F5/2)+ Yb3+(2F5/2) →Tb3+(5D4) occurs back energy transfer process, and Tb3+ luminescence is enhanced. Using the fluorescence intensity ratio between the 5D4–7F4 (545 nm) emission of Tb3+ and the emission of CsPbI3 NCs at 680 nm, the temperature sensor with high sensitivity (SA=0.086 K–1, SR=8.63%·K–1, δTmin=0.058 K, 298?403 K) has been prepared, which is promising to be applied in optical temperature sensor.