A new thermographic
nanocrystalline Sr
4
Al
14
O
25
:Mn
4+
,Tb
3+
phosphor was developed,
and the concentrations of both dopants and the synthesis conditions
were optimized. The combination of the thermally quenched luminescence
from the Mn
4+
ions to the almost temperature-independent
emission from Tb
3+
provides a sensitive luminescent thermometer
(
S
R
= 2.8%/°C at 150 °C) with
strong emission color variability. In addition, a figure of merit
for this luminescence thermochromism was proposed, as the relative
sensitivities of the
x
and
y
CIE
coordinates, which for this phosphor reaches at 150 °C
S
R
(
x
) = 0.6%/°C and
S
R
(
y
) = 0.4%/°C, respectively.
Noncontact thermal imaging was demonstrated with this phosphor using
a single consumer digital camera and exploiting the ratio of red (R)
and green (G) channels of the RGB images, thereby confirming the high
application potential of Sr
4
Al
14
O
25
:Mn
4+
,Tb
3+
nanocrystals for thermal sensing
and mapping.
The near-infrared luminescence of Ca6Ba(PO4)4O:Mn5+ is demonstrated and explained. When excited into the broad and strong absorption band that spans the 500–1000 nm spectral range, this phosphor provides an ultranarrow (FWHM = 5 nm) emission centered at 1140 nm that originates from a spin-forbidden 1E → 3A2 transition with a 37.5% internal quantum efficiency and an excited-state lifetime of about 350 μs. We derived the crystal field and Racah parameters and calculated the appropriate Tanabe–Sugano diagram for this phosphor. We found that 1E emission quenches due to the thermally-assisted cross-over with the 3T2 state and that the relatively high Debye temperature of 783 K of Ca6Ba(PO4)4O facilitates efficient emission. Since Ca6Ba(PO4)4O also provides efficient yellow emission of the Eu2+ dopant, we calculated and explained its electronic band structure, the partial and total density of states, effective Mulliken charges of all ions, elastic constants, Debye temperature, and vibrational spectra. Finally, we demonstrated the application of phosphor in a luminescence intensity ratio thermometry and obtained a relative sensitivity of 1.92%K−1 and a temperature resolution of 0.2 K in the range of physiological temperatures.
The influence of the crystal field strength and host material composition on the spectral position of the emission band and thermal stability of Fe3+ luminescence was investigated in order to develop a highly sensitive luminescence thermometer.
This paper describes Mn 5+ -activated Sr 3 (PO 4 ) 2 and Ba 3 (PO 4 ) 2 phosphors as near-infrared lifetime-based luminescence thermometry probes. Materials were prepared by a solid-state method, and their rhombohedral structures were confirmed by Xray diffraction analysis. Diffuse reflectance measurements showed broad and strong absorption between 650 and 950 nm covering the first biological transparency window and having an absorption maximum at ∼660 nm. By switching Sr with Ba, the following changes in the photoluminescent properties were observed: (i) a red shift of the emission maximum (1173 nm → 1191 nm) and (ii) a decrease in the excited-state lifetime. Thermometric properties of the phosphors were assessed by measuring and analyzing the temperature dependence of the Mn 5+ excited-state lifetime. Lifetime-based luminescence thermometry revealed a relative sensitivity of 0.5% K −1 at 310 K (physiologically relevant range) and a maximal value of ∼1% K −1 at temperatures between 400 and 500 K.
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