Cr3+-Cr3+ aggregation of charge transfer between valence states to achieve near-infrared zone II luminescence


The near-infrared spectrum contains the characteristic vibration absorption of a large number of organic functional groups, and the near-infrared phosphor-converting light-emitting diode (pc-LED) has attracted more and more attention in the fields of non-destructive testing and night vision. In addition, near-infrared luminescence imaging can assist in in vivo diagnosis and drug delivery. Compared to the traditional first biological imaging window (750-950 nm), luminescent contrast agents operating within the second imaging window (1000-1800 nm) exhibit lower tissue absorption and scattering coefficients, enabling greater detection depths and higher imaging signal-to-noise ratios. However, according to crystal field engineering, it is difficult for Cr3+ doped inorganic luminescent materials to achieve near-infrared secondary luminescence, while broadband near-infrared second-region luminescent ions such as Ni2+ and Cr4+ show low luminous efficiency and poor luminescence thermal stability under commercial blue chips. Inorganic luminescent materials based on rare earth ion doping are usually multilinear narrowband emission, and it is difficult to perform spectral control.

Recently, the team of Professor Liu Quanlin of the University of Science and Technology Beijing and Professor Ma Chonggeng of Chongqing University of Posts and Telecommunications reported for the first time that Cr3+ ion aggregation-induced broadband luminescence in the near-infrared region II near 1200 nm through the matrix of Cr3+ ion doped magnetite structure, and the external quantum efficiency reached 18.9% at room temperature, and clarified that the luminescence originated from the charge transfer between valence states of Cr3+-Cr3+ ion pairs. The NIR II luminescence can be effectively excited in the UV-NIR region, which provides new ideas for the design of new NIR II luminescence materials, devices and spectroscopic applications.

Research background

In 2016, Osram reported the first near-infrared pc-LED (model SFH4735), noting the potential application of infrared pc-LEDs in smart agriculture, but the LED only covers the spectral region of 650-1050 nm, while the output power of the device is only 16 mW driven by 350 mA. Therefore, in order to improve the performance of packaged devices, it is necessary to improve the luminous efficiency of fluorescence conversion materials and extend the luminescence wavelength.

Cr3+ ion is considered to be an ideal NIR luminescence activator, and since the outer D orbital is affected by the coordination crystal field, adjustable NIR luminescence can be achieved by lattice tuning. However, based on the regulation of crystal field engineering, the currently reported Cr3+ doped perovskite, pyroxene, garnet and other inorganic luminescent materials are usually located in the near-infrared region, and with the increase of the luminescence wavelength, their luminous efficiency and luminescence thermal stability are significantly reduced. Therefore, how to expand the luminescence wavelength of Cr3+ and improve its luminescence thermal stability has become particularly important.

In 2001, Tang et al. first reported anomalous luminescence induced by the aggregation of luminescent molecules in 1-methyl-1,2,3,4,5-pentaphenylsilrole. Transition metals such as Mn, Fe, Cr, etc., have uncovered d-orbital electrons, which make transition metals usually have rich ion valence states as well as magnetic properties. Under the condition of high doping concentration, aggregation between ions is prone to occur, resulting in strong interactions between electrons in the D orbital of the underlayer, typical effects include magnetic interactions. Zhang et al. studied the aggregate luminescence behavior of high Mn2+ ion doped MgAl2O4, Li2ZnSiO4, etc., and found that under high doping conditions, these materials usually exhibit abnormal luminescence different from single Mn2+ centers, and interpreted this abnormal luminescence as a strong magnetic interaction between Mn2+ ions. Liu et al. also found the magnetic interaction of Cr3+-Cr3+ pairs in Sr(Al,Ga)12O19, but the magnetic interaction between Cr3+ ions did not lead to a large difference from the single Cr3+ center in the spectrum, because the magnetic interaction is the interaction of electron spin, and its exchange coupling energy is usually low.

Innovative research

Professor Liu Quanlin’s team from University of Science and Technology Beijing and Professor Ma Chonggeng of Chongqing University of Posts and Telecommunications reported for the first time that Cr3+ ion aggregation-induced charge transfer between Cr3+-Cr3+ valence states emitting in the near-infrared second region by redoping LaMgGa11O19 with magnetite structure by Cr3+ ions. When the concentration of Cr3+ ions is low, the emission spectrum is located at 700-1000 nm, which is derived from the 4T2→4A2 spin of a single Cr3+ center. When the Cr3+ ion content exceeds 0.5, it exhibits near-infrared II luminescence at 1200 nm, and reaches a maximum at a Cr3+ content of 0.7 (see Figure 1a), and the external quantum efficiency reaches 18.9% at room temperature. Simultaneous monitoring of luminescence at 890 and 1200 nm, the materials exhibited characteristic excitation of a single Cr3+ center at 300-750 nm (Figure 1c), and the results of low-temperature diffuse reflection also showed that no characteristic absorption signal of Cr4+ was observed in the entire UV-NIR region, indicating that Cr4+ ions were not present in the material (Figure 1d). Electron paramagnetic resonance experiments showed that there was a single Cr3+ center resonance signal at low Cr3+ concentration, and then after increasing Cr3+ concentration, there was a strong Cr-Cr pair signal in the material, indicating that there was a strong Cr3+-Cr3+ interaction at high Cr3+ concentration (see Figure 1f).

Figure 1: Spectral characteristics of Cr3+ and Cr3+-Cr3+ aggregated signals

The luminescence of Cr3+-Cr3+ pairs mainly includes two forms: magnetic interaction and charge transfer between valence states, of which Andries et al. calculated that the magnitude of magnetic interaction is only a few hundred wavenumbers, and the second-zone luminescence of the material has a very large Stokes displacement, and the magnetic interaction of the isomorphic Sr(Al,Ga)12O19:Cr3+ is only a few hundred wavenumbers, so the authors believe that Cr3+ in LaMgGa11O19The anomalous near-infrared II luminescence comes from the intervalence charge transfer of Cr3+-Cr3+→ Cr2+, Cr4+. In addition, the 1200 nm luminescence lifetime at low temperatures is up to 2.3 ms (Figure 2a), which is much higher than the 3A2→3T2 radiative transition lifetime of Cr4+. The charge transfer between the valence states of Cr3+-Cr3+ ions can be understood as: one of the Cr3+ ions loses electrons and is oxidized to Cr4+ ions (Cr3+ e(VB) + Cr4+), and the other Cr3+ ions get an electron that is reduced to Cr2+ ions (Cr3+ + e(VB) ® Cr2+®); The luminescence process transitions from the 5E ground state of the Cr2+ ion to the 4A2 ground state of Cr3+ (see Figure 2c). Since charge transfer between valence states involves the migration of electrons between two ions, the luminescence process exhibits a very large Stokes shift.

Figure 2: Luminescence decay and charge transfer between Cr3+-Cr3+ valence states

The near-infrared luminescence of the second region also exhibits luminescence antithermal quenching characteristics, and its luminous intensity increases to 432% of the low temperature (80 K) at a temperature of 290 K (Figures 3a and 3b), which the authors analyzed may be caused by the increase in energy transfer efficiency between Cr3+ ions due to the increase in temperature and the increased probability of electron migration in neighboring Cr3+ ions. Although the internal efficiency of the material is only 27.2% under the excitation of commercial blue light chip, due to the high Cr3+ concentration doping, its absorption efficiency is as high as 69.7%, and its external efficiency can reach 18.9% (see Figure 3d).

Figure 3: Variable temperature luminescence characteristics and device performance

Application and outlook

In this paper, Cr3+ ion redoping was used to realize the near-infrared second-region luminescence of charge transfer between valence states. This is conducive to a deeper understanding of the luminescence behavior of Cr3+ ions, and provides guidance for the development of near-infrared secondary luminescent materials. However, whether the NIR II luminescence phenomenon can be realized in other systems needs to be explored. And the luminous efficiency needs to be further improved.

The study was published online in Light: Science & Applications under the title “Intervalence charge transfer of Cr3+-Cr3+ aggregation for NIR-II. luminescence.”

The first author is Shengqiang Liu, a doctoral student at University of Science and Technology Beijing, and the corresponding authors include Professor Liu Quanlin, Associate Professor Song Zhen of University of Science and Technology Beijing, and Professor Ma Chonggeng of Chongqing University of Posts and Telecommunications. (Source: LightScience Applications WeChat public account)

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