Effective Ratiometric Luminescent Thermal Sensor by Cr³⁺‐Doped Mullite Bi₂Al₄O₉ with Robust and Reliable Performances

Abstract

The design of effective optical systems featuring high thermal sensitivity able to discriminate ever smaller variations of temperature in noncontact mode is of critical importance to face the challenges brought by the modern‐day technological revolution. If from one hand, the ratiometric optical thermometers based on Boltzmann distribution are demonstrated to be characterized by a unique reliability, on the other hand, robust performances in different environments are highly desired for new applications such as in situ thermal sensing of catalytic reactions. Here, the crystal field experienced by Cr3+ in Bi2Al4O9 is investigated demonstrating the potential of this system as ratiometric self‐referencing thermal sensor being characterized by high relative sensitivity (1.24% K−1 at 290 K). The remarkable absolute sensitivity results in an exceptional low thermal resolution of ≈0.2 K, 15‐fold lower than for the conventional Nd3+‐based thermometers used in biological applications. The comparison of the performances among different systems evidences the potential of Cr3+‐based thermometers with thermal resolution even lower than the state of the art diamond. In addition, the pH dependence of the photoluminescence emission confirms a high stability also at extreme conditions of basic and acid environments and the aging effects are tested for one week.

Boltzmann Thermometry in Cr3+‐Doped Ga2O3 Polymorphs: The Structure Matters!

“…It is interesting to note the large difference between the R ‐line splitting energies between the two polymorphs: β‐Ga 2 O 3 :Cr 3+ system is characterized by a large energy split Δ E R = 164 cm −1 , similar to the value estimated in Bi 2 Ga 4 O 9 (168 cm −1 at 80 K [ 20 ] ), while α‐Ga 2 O 3 :Cr 3+ shows a small Δ E R = 34 cm −1 , similar to the values reported for the garnets GGG (36.2 cm −1 [ 40 ] ) and GSGG (31.5 cm −1 [ 41 ] ), ruby (29 cm −1 [ 42 ] ), and ZnGa 2 O 4 (38 cm −1 [ 43 ] ). The relatively higher value for the α‐Ga 2 O 3 :Cr 3+ with respect to the isostructural α‐Al 2 O 3 :Cr 3+ (ruby) confirms the trend when we move from gallates to aluminates as for the perovskites LaGaO 3 (24.1 cm −1 [ 44 ] ) and LaAlO 3 (5.4 cm −1 [ 45 ] ), the garnets YGG (22 cm −1 [ 46 ] ) and YAG (19 cm −1 [ 47 ] ), the mullites Bi 2 Ga 4 O 9 (168 cm −1 [ 20 ] ) and Bi 2 Al 4 O 9 (131 cm −1 [ 21 ] ), and the spinels ZnGa 2 O 4 (38 cm −1 [ 43 ] ) and ZnAl 2 O 4 (6 cm −1 [ 48 ] ).…”

“…These values are much better than that one for the conventional Er 3+ ‐ and Nd 3+ ‐based Boltzmann thermometers reflecting the different nature of the transitions involved, as recently demonstrated. [ 21 ]…”

“…Because of the mutual configurational offset between the potentials of the 4 T 2 and 2 E states, this class of thermometers are thermal crossover thermometers. The comparison recently reported between the thermometric properties of Cr 3+ into the isostructural mullites Bi 2 Ga 4 O 9 [ 20 ] and Bi 2 Al 4 O 9 [ 21 ] gives information about the effect of different substituting GaO 6 and AlO 6 octahedral sites. The understanding of the role of a single parameter such as the CF in the thermometric properties of Cr 3+ thermometers is critical to construct a set of guiding principles to explain and predict the performance of this class of materials.…”

A Theoretical Framework for Ratiometric Single Ion Luminescent Thermometers—Thermodynamic and Kinetic Guidelines for Optimized Performance

“…If those states couple more strongly to local odd‐parity vibrational modes or phonons, the transition can become vibronically allowed. A very prominent example for that is the excited 2 E state (in cubically symmetric fields) of the 3d 3 configuration like in Mn 4+ or Cr 3+ , [ 64–77 ] but also the lowest excited (dominantly) triplet states 3 T 2u and 3 E u of the 4f 13 5d 1 configuration of Yb 2+ . [ 189 ]…”

A Ho3+‐Based Luminescent Thermometer for Sensitive Sensing over a Wide Temperature Range

“…[ 1,2 ] Remote temperature sensing based on luminescence thermometry offers an alternative that is capable of measuring heat generation and diffusion on the microscopic scale. [ 3 ] Among the various choices of luminescent systems, [ 4–9 ] crystals doped with lanthanide (Ln 3+ ) ions represent a particularly promising class of luminescent thermometers, because their dimensions can be tuned from a few nanometers to several micrometers and their photoluminescence spectrum is sensitive to temperature. A characteristic feature of Ln 3+ ions is their rich energy level structure, which results in emission spectra with well‐separated lines.…”