Tunable and highly sensitive fluorescent thermometers from La2CaZrO6:Cr3+ with a time-resolved technology

Non-contact optical temperature measurement can effectively avoid the disadvantages of traditional contact thermometry and thus, become a hot research topic. Herein, a fluorescence intensity ratio (FIR) thermometry using a time-resolved technique based on La2CaZrO6:Cr3+ (LCZO) is proposed, with a maximum relative sensitivity (Sr - FIR) of 2.56% K-1 at 473 K and a minimum temperature resolution of 0.099 K. Moreover, the relative sensitivity and temperature resolution can be effectively controlled by adjusting the width of the time gate based on the time-resolved technique. Our work provides, to our knowledge, new viewpoints into the development of novel optical thermometers with adjustable relative sensitivity and temperature resolution on an as-needed basis.

Broadband short-wave near-infrared-emitting phosphor MgNb2O6:Cr3+ for pc-LED applications

Broadband short-wave near-infrared (NIR) phosphor-converted light-emitting diodes (pc-LEDs) have been attracting keen interest for miniature NIR spectroscopy, while still lacking sufficient novel broadband NIR-emitting phosphors. Herein, we report a novel MgNb2O6:Cr3+ polycrystalline phosphor with a broad NIR emission band centered at 970 nm and a large full-width at half-maximum of approximately 155 nm under excitation of bluish-green light at around 515 nm. The optimized phosphor MgNb2O6:1%Cr3+ features a high internal quantum efficiency (IQE) of ∼85.5% and a moderate external QE of 25.2%. The fluorescence properties determined by two distorted hexa-coordination octahedral sites (i.e. [MgO6] and [NbO6]), low crystal field strength (Dq/B ∼ 1.65), and Cr3+-doping concentration were systematically investigated for comprehensive understanding of photophysical mechanisms. Besides, this broadband NIR phosphor MgNb2O6:Cr3+ exhibits a moderate thermal quenching of 21.4%@373 K for pc-LED application. An NIR pc-LED self-built by combining the optimal phosphor with a commercial cyan of ∼515 nm exhibits an NIR output power increase from 3.19 to 11.38 mW as the drive current is varied from 40 to 220 mA. With the help of this prototype pc-LED device, multiple applications were successfully performed to clearly recognize blood vessel distributions in the human finger, penetrate a plastic cap, and distinguish multi-color text. Undoubtedly, further development of such broadband short-wave NIR-emitting phosphors will make novel pc-LED devices for significant applications in biomedical imaging, nondestructive safety detection, intelligent identification, etc.

Tb3+-based multi-mode optical ratiometric thermometry

Owing to some special superiority, luminescence ratiometric thermometry, mainly including dual excitations single emission and single excitation dual emissions, has gained popularity over the past few years. However, developing novel ratiometric thermometry that can work in multi-mode is still a challenge. Here we report a temperature measurement method based on the photoinduced luminescence of Tb³⁺ in the low-cost and easy to prepare calcium tungstate. Both the conventional luminescence intensity ratio (LIR) and recently developed single-band ratiometric (SBR) strategies have been achieved in our materials. On the one hand, upon excitation of the charge transfer state, the emissions from the excited ⁵D₄ and ⁵D₃ states present different responses to temperature. A thermometry depending on the LIR between these two emissions has thus been developed, with adjustable relative sensitivity that is sensitive to the excitation wavelength. On the other hand, both the emissions from the excited ⁵D₄ and ⁵D₃ states respond dissimilarly to the temperature variation. A SBR thermometer has thus been constructed with two excitation modes, reaching the maximum relative sensitivity of 1.83% K^-1 at 573 K. The present work is expected to inspire other researchers to exploit more multi-mode optical ratiometric thermometries.

Tetrahedrally coordinated rigid crystal structure enables partial self-reduction of mixed-valence europium for optical thermometric application

Mixed-valence europium-activated phosphors are receiving attention in many fields, such as lighting, anti-counterfeiting, optical recording, encryption, and temperature sensing. However, it remains difficult to construct mixed-valence europium-activated compounds due to the reductive and oxidative synthesis conditions required to obtain Eu²⁺ and Eu³⁺ ions, respectively. Herein, mixed-valence Eu²⁺/Eu³⁺ was realized in the CaBPO₅ built by rigidly connected BO₄ and PO₄ tetrahedrons by partial Eu³⁺ → Eu²⁺ self-reduction in the air atmosphere. Commendably, the CaBPO₅:Eu²⁺/Eu³⁺ phosphor exhibits excellent ratiometric temperature sensing performance with the maximum absolute and relative sensitivity being as high as 0.184 K^-1 and 3.444% K^-1 with good signal discriminability, due to the high and low, respectively, temperature-dependence of Eu²⁺ and Eu³⁺ emissions. The rapid dropping intensity of Eu²⁺ in CaBPO₅ with increasing temperature was due to the small energy gap (∼0.48 eV) between the Eu²⁺-5d state and the conduction band. Our work not only provides a novel thermometer candidate but also enlightens researchers to a method of effectively designing new mixed-valence metal-ion activated luminescent thermometers via selective tetrahedrally coordinated rigid crystal structure.

Near-UV excited multifunctional near-infrared phosphors for anti-counterfeiting and ratiometric thermometer applications

Considerable efforts have been devoted to the development of visible light-emitting phosphors for anti-counterfeiting application, but the design of multifunctional near-infrared materials with suitable luminescent properties was lacking. Based on the crystal field theory, a near ultraviolet excitable and near-infrared emitting Ca₂Sn_2(1-x)Al_2(1-x)O₉:2xCr³⁺,2xSi⁴⁺ phosphor was designed. On the one hand, the Ca₂Sn_2(1-x)Al_2(1-x)O₉:2xCr³⁺,2xSi⁴⁺ phosphor showed great potential for applications including anti-counterfeiting and secret signals under a 405 nm near UV laser. The integrated intensity at 150 °C was 75.9%, implying high thermal stability when irradiated with a high power light source. On the other hand, an unexpected red emission component originating from the Cr³⁺-Cr³⁺ dimer was observed and was confirmed by fast luminescence decay and electron spin resonance signals. The different thermal stabilities of Cr³⁺ near-infrared and Cr³⁺-Cr³⁺ red emissions opened up a new opportunity for luminescence ratiometric thermometers.

Ultra-sensitive luminescence thermometry based on MgNb2O6:Dy3+/Pr3+ thermochromic phosphors

A novel ultra-sensitive ratiometric luminescence thermometry strategy combining the redshift of the charge transfer band with dramatic quenching of the intervalent charge transition in MgNb2O6:Dy3+/Pr3+ thermochromic phosphors.