Highly efficient Fe³⁺-doped A₂BB’O₆ broadband near-infrared-emitting phosphors for spectroscopic analysis

Numerous NIR-emitting phosphors have been reported for the development of NIR-emitting phosphor-converted light-emitting diodes (pc-LEDs), but exploitation of efficient and broadband NIR-emitting phosphors is one of the key obstacles encountered. Currently reported broadband NIR-emitting phosphors are mainly based on Cr³⁺ as it can usually show broadband emission in the range 650–1200 nm. However, there is a potential risk of oxidation of Cr³⁺ to Cr⁶⁺, which affects the NIR luminescence efficiency and increases the chromium toxicity of the phosphors, thereby limiting their practical applications in certain fields that requires high safety. Recently, several Bi³⁺-, Eu²⁺-, and Mn²⁺-activated NIR-emitting phosphors have also been reported. Their emission wavelengths are near the deep-red light region, which are not long enough for the NIR region. Fe³⁺ is another activator, which is non-toxic and can be considered a friendly dopant. However, the reported emission commonly occurs in the red and far-red light regions. Therefore, efforts should be further made to achieve efficient long-wavelength NIR emission of Fe³⁺ luminescence.

 

In a new paper published in Light Science & Application, a team of scientists, led by Professor Jun Lin from Changchun Institute of Applied Chemistry, Chinese Academy of Sciences and Professor Guogang Li from China University of Geosciences have reported the Fe³⁺-activated Sr_2-yCa_y(InSb)_1-zSn_2zO₆ broadband NIR-emitting phosphor materials with tunable emission from 885 to 1005 nm. The Ca₂InSbO₆:Fe³⁺ phosphor peaking at 935 nm shows an ultra-high IQE of 87%, which demonstrates a potential application in NIR spectroscopy detection. This work represents an important step towards realizing efficient and broadband NIR-emitting phosphor materials.

 

Given that the d–d transition of Fe³⁺ is influenced by the strength of crystal field, its emission energy can be modulated by tuning host composition. In this work, the authors chose Sr₂InSbO₆ with a double perovskite structure as an initial host for Fe³⁺ doping. Through a cation substitution of Ca²⁺ for Sr²⁺ and a further cosubstitution of [Sn⁴⁺–Sn⁴⁺] for [In³⁺–Sb⁵⁺], a series of Sr_2-yCa_y(InSb)_1-zSn_2zO₆:Fe³⁺ long-wavelength NIR-emitting phosphors were synthesized using a traditional high-temperature solid-state method.

Under 340 nm excitation, Sr₂InSbO₆:Fe³⁺ shows a broadband NIR emission peak at 885 nm. After Ca²⁺ substitutes Sr²⁺ and [Sn⁴⁺–Sn⁴⁺] further cosubstitutes [In³⁺–Sb⁵⁺], the emission spectra show a red shift from 885 to 935, and then to 1005 nm. In this process, the full width at half maximum is broadened from 108 to 146 nm. In addition, the emission intensity is enhanced by two times after the complete introduction of Ca²⁺. The time-resolved photoluminescence spectra reveal only one luminescent center. Considering the valence state and ionic radius, Fe³⁺ ions are believed to occupy the In³⁺ sites. Based on the analysis of crystallographic site occupation and crystal field environment, the photoluminescence tuning mechanism of emission spectra is revealed. Structural analysis shows that the introduction of Ca²⁺ and Sn⁴⁺ leads to the host lattice shrinkage, which can result in an increased crystal field strength around Fe³⁺. Thus, the emission spectra show a normal red shift, as observed. In addition, Ca²⁺ incorporation lowers the site symmetry of Fe³⁺. This contributes to breaking the forbidden transition of Fe³⁺, thereby increasing the emission intensity.

Sr₂InSbO₆:Fe³⁺ shows a IQE value of 48%, which is comparable to that of the reported Cr³⁺-doped phosphors with similar emission wavelengths. The IQE of Ca₂InSbO₆:Fe³⁺ (peaking at 935 nm) is as high as 87%, which is rare for the broadband NIR-emitting phosphors with emission wavelength over 900 nm. This indicates that Fe³⁺ can be a candidate activator for efficient NIR-emitting phosphors. These emission-tunable Fe³⁺-activated NIR-emitting phosphors synthesized in this work show a potential application in spectroscopic analysis. This study provides new insights into the luminescence of Fe³⁺ and offers a new way for developing efficient broadband NIR-emitting phosphor materials.

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