Lanthanide doping could aid in new imaging techniques

a) The main luminescent transitions of lanthanide doping in the electromagnetic spectrum, which extend from ultraviolet to visible and then extend to the second near infrared. b) XEOL, XEPL, DS and UC processes in the perfluoride groups of the lanthanides. c) Schematic illustration of the multimodal color evolution based on the nucleofluoride @shell@shell NSs. P represents the excitation force. When different lanthanide dopants generate diverse emission wavelengths of XEOL, UC, and DS in design @shell@shell NSs, the abundant polychromatic colors can be modulated on demand by controlling the excitation wavelength and/or energy. Credit: Li Li, Yubin Wang, Andrei Kuzmin, Yuji Hua, Jingtao Zhao, Xixing Shu, Prasad in Paras

X-rays are electromagnetic waves of short wavelengths and strong penetrability into physical matter, including living things. Flash devices capable of converting X-rays into ultraviolet, visible, or near-infrared (NIR) photons are widely used to achieve indirect X-ray detection and XEOL imaging in many fields. They include medical diagnostics, computed tomography (CT), space exploration, non-destructive industrial materials, and security inspections.

Commercial bulk flash devices have high light throughput (LY) and high energy resolution. However, they suffer from many drawbacks, such as complex manufacturing procedures, expensive experimental equipment, untreatable XEOL wavelength, and poor device processability. They all produce emissions in the visible spectral range, but the presence of XEOL in the NIR range may find more interesting applications in biomedicine. The thick crystals also generate light scattering followed by clear signal interference in a photodiode array.

Recently, the metal halide perovskite has been examined for X-ray detection. Unfortunately, these materials also showed some intrinsic limitations, such as poor photo/environment stability, heavy metal toxicity and low LY. Thus, the search for the development of a new generation of scintillation devices is still a great focus of scientific research.

In a new paper published in i lite, a team of scientists led by Professor Prasad N. Barras of the University at Buffalo investigated the use of perfluoride compounds with lanthanides. Their paper investigated design strategies and nanostructures that allow the manipulation of excitation dynamics in the architecture of the nucleus.

Perovskite fluorides avoid the limitations of bulk luminescent devices and metal halide perovskite. It also displays many useful features. Shell core structures in perfluorinated lanthanides can be tuned and engineered on demand by utilizing a cheap and convenient wet chemical method. The emission wavelengths can be tuned and extended to the second NIR window, taking advantage of the abundant energy levels of lanthanide doping.

These NSs show superior photostability, low toxicity and convenient device processability. It makes them promising candidates for imaging the next generation of NS and XEOL. Furthermore, they exhibit the property of XEPL, which shows promising applications in biomedicine and optical information coding. The combination of XEOL and XEPL makes it suitable for expanding its range of applications.

In recent years, great progress has been made in the development of NS. The research team discussed the design and nanostructure strategies that allow manipulating the excitation dynamics of the core architecture. It also produces XEOL, XEPL, photon conversion (UC) and descending shift (DS). It enables emission at multiple wavelengths and at different time scales.

The basic working principle of XEOL imaging is to record the attenuation of X-rays after penetration of the subject by a flash and imaging with a camera. A flash screen is placed under the objective to absorb the transmitted X-ray photons. Low-dose x-rays that penetrate living organisms allow computed tomography to be applied. The penetrating non-living material enables product quality and safety inspection. The X-ray radiation dose should be low enough to ensure safety, while high resolution and distinct contrast are important for image analysis.

X-rays, ionizing radiation with a depth of penetration deep into the human body, have been extensively studied for radiotherapy and bioimaging applications. Powerful XEOL can activate photosensitizers to generate reactive oxygen species. They directly slow or stop tumor growth with phototherapy, which causes inflammation and affects the tiny blood vessels.

XEPL can be used in the UVC range for sterilization and in vivo killing of pathogens and cancer cells. Fluorides with large band gap and easy defects of anionic defects are suitable for continuous UV luminescence generation. Experimental characterizations along with first-principles calculations suggested that the fluorine functions induced by the introduction of oxygen acted as electronic traps.

Optical sensors have various applications in biomedical sensing, camera imaging, optical communications, and night vision. In commercial photodetectors, inorganic crystalline semiconductors are used as photodiodes and phototransistors. They do not respond effectively to a wide range of photon energy that covers X-ray, UV-vis, and NIR light.

Under NIR excitation, the lanthanide-doped fluoride layer emits UV light through UC processes for energy transfer. Subsequent radiation reabsorption occurs from the lanthanide doping to the perovskite layer. The visible emission from the perovskite layer is produced by the recombination of electrons in the CB and holes in the VB.

This nanotransducer exhibited a broad linear response to X-rays at different dose rates and UV and IR photons at different energy densities. As discussed in Section 4.4, without incorporation of the perovskite layer, perovskite fluorides can be used to generate XEOL, UC and DS as well, which may be possible to achieve broadband detection in theory and need further study in the future.

Fluoride nanoparticles impregnated with lanthanides are suitable candidates for next-generation NSs due to their low biotoxicity, high image/environment stability, easy device processing capability, tunable XEOL and XEPL properties, and other useful features.

To further the development of high-performance fluoride scopes and their practical applications, the team discussed current challenges and future interdisciplinary opportunities in this area below. Understanding the mechanism of XEOL is useful for the design and exploration of new fluoride compounds. At present, how the generated low kinetic energy charge carriers are transferred to or captured by the luminescent centers by defects and the corresponding impact factors is unclear.

The first inhabited non-radioactive excited levels and the radioactive levels of lanthanide doping are optimal when calculating or characterizing the energy differences between these charge carriers. These calculations will guide the design of power transmissions to match power differences followed by improved light production. High LY is a prerequisite for achieving extremely low dose rate applications.

Effective near-infrared luminescence in lanthanide-doped inorganic double halide perovskites

more information:
Lei Lei et al, The next generation of lanthanide-doped nano-oscillators and photon transducers, i lite (2022). DOI: 10.1186 / s43593-022-00024-0

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