Three-energy-level-cascaded strategy for a more sensitive luminescence ratiometric thermometry

Comparison of Three Ratiometric Temperature Readings from the Er3+ Upconversion Emission

“…The intensity of emission from 4 F 7/2 showed a slight monotonic increase with temperature, Figure 2b, exactly like in the CaWO 4 :Yb 3+ /Er 3+ , as previously described by Li et al [6]. The same phenomenon of the increase in the intensity of downshifting emission from the higher excited state has been described for Dy 3+ activated CaWO4, where populations in Dy 3+ 4 G 11/2 and 4 I 15/2 states enlarge with the temperature at the expense of the depopulation of the 4 F 9/2 state [4]. The enlarged population of 4 F 7/2 state occurs via thermalization from 4 S 3/2 level via 2 H 11/2 level, as described by Li et al [6] showing that the 4 F 7/2 and 4 S 3/2 states are thermally linked with each other and that their populations follow Boltzmann's law, Equation (1).…”

“…1750 cm −1 ). Thus, the research has come to a conundrum: the lower the ∆E the better the thermalization, but at the cost of the loss of sensitivity [4]. Additionally, when choosing LIRs of the higher ∆E to achieve the higher S R , the population of the H level is low at low temperatures, with the consequence of low measurement resolution due to the high values of uncertainty in the measurements.…”

“…It was only until recently that this three-level strategy for increasing the sensitivity was investigated for Er 3+ ion, by including the next higher level, 4 F 7/2 . 4 Numerous Er 3+ doped materials, codoped with Yb 3+ , were investigated for luminescent thermometry [9][10][11] by the upconversion pumping mechanism. These upconverting materials have drawn significant attention due to the excitation in the NIR region of a biologically transparent window [12], allowing for numerous biological and medical applications (for example, in photothermal therapies of tumors) [13].…”

“…Upconverting Yb 3+ /Er 3+ codoped materials do possess all the desired properties: (1) a trio of thermally coupled emissive levels ( 4 S 3/2 , 2 H 11/2 and 4 F 7/2 ), (2) NIR excitation by widely available, cheap and powerful 980 nm laser, (3) emissions in the first biologically transparent window from thermally coupled levels 4 with the goal of obtaining temperature readout sensitivities and resolutions, and investigating measurement repeatability. The choice of YF 3 as the host matrix was due to its low phonon energy of~500 cm −1 [15], high chemical stability [16], and high iconicity [17], providing an excellent ground for highly efficient luminescence.…”

High temperature (nano)thermometers based on LiLuF4:Er3+,Yb3+ nano- and microcrystals. Confounded results for core–shell nanocrystals

“…It has already been shown that such an approach with more than two excited levels allows to retain a high relative sensitivity at higher temperatures based on the larger energy gap. [38,39] However, it is also important to realize that this strategy is only useful if there is appreciable luminescence intensity stemming from the radiative transition from the higher excited level ( 4 F7/2). Otherwise, the temperature uncertainty increases despite high relative sensitivity due to the connected low luminescence from the energetically highest of the three excited levels.…”