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High-Resolution Remote Thermometry and Thermography Using Luminescent Low-Dimensional Tin-Halide Perovskites
Remote thermal imaging, or thermography, lends itself to numerous applications ranging from medicine1 and defence, to biological research or the diagnosis of technical failures. In all these applications, remote thermal detection falls into two main categories: infrared (IR) or visible. The alternative and less expensive bolometric detectors, following advances in MEMS technologies (micro-electro-mechanical systems), have already entered the consumer electronics market, and are able to record thermal images with both high speed and high resolution. However, their thermographic performance, based on measurements of IR radiation intensity, is inherently limited by the transparency and emissivity/reflectivity of an observed object and, more importantly, by any material and medium (window, coating, matrix, solvent and so on). As one of the major consequences, IR thermography cannot be easily combined with conventional optical microscopy or other enclosed optical systems such as cryostats or microfluidic cells.
Figure 1. Visible-light and IR thermography comparison.
An alternative method for remote thermography, which is unhindered by enclosures or IR-absorptive media, utilizes temperature-sensitive luminophores (that is, fluorophores or phosphors) with photoluminescence (PL) in the visible spectral range that are deposited onto, the object of interest as temperature probes. To probe an object’s temperature, the luminophore is excited by an ultraviolet or visible (UV–vis) pulsed source (for example, a laser or light-emitting diode, LED), and the temperature-dependent PL lifetime decay is then analysed by time resolving detectors.
This PL lifetime approach has several benefits: the excitation power, and consequently the PL intensity, can be adjusted to a value appropriate for the dynamic range of the detector. Additionally, the use of UV–vis light rather than mid- to long wavelength IR radiation allows for the direct integration of this method with conventional optical spectroscopy and microscopy applied in biological studies and materials research. Furthermore, higher spatial resolutions can be obtained with visible light (400–700 nm) as the diffraction limit is ~20 times sharper than for LWIR (7–14µm); this potentially extends the utility of remote thermography to intracellular, in vitro and in vivo studies.
To promote the advancement and widespread use of remote thermography, a much broader portfolio of luminescent, thermally sensitive and temperature-range-tunable materials is required. These emitters must exhibit a fully reproducible radiative lifetime versus temperature dependence, and demonstrate invariant behaviour towards excitation-light intensity. We present (1) a family of low-dimensional tin-halide luminophores well-suited for remote thermography due to their strongly temperature-dependent, compound-specific PL lifetimes, (2) thermometric precision down to 0.013 °C with operation in a broad temperature range from −100 to 110 °C and (3) a thermographic method utilizing time-of-flight (ToF) cameras19,20 for costeffective, high-resolution, fast thermal imaging.
Figure 2. Thermographic image of a sample that consists of encapsulated Cs4SnBr6 powder.
In summary, we discovered that the de-trapping process of STEs in low-dimensional tin-halides exhibits extreme thermal sensitivity over a compositionally tunable range of temperatures. In particular, such emission is characterized by monoexponential decays with a steep dependence of PL lifetime on temperature. We then applied these features to high-precision thermometric measurements over a wide temperature range (−100 to 110 °C) and demonstrated an approach to remote optical thermography by combining these low-dimensional tin-halide luminophores with ToF-FLI. By doing so, we succeeded in achieving low-cost, precise and high-speed PL-lifetime thermographic imaging.
Sergii Yakunin , Bogdan M. Benin , Yevhen Shynkarenko, et al. High-resolution remote thermometry and thermography using luminescent low-dimensional tin-halide perovskites. Nature Materials. 18:846-852, 2019.