Fiber-optic luminescent materials made from rare earth elements play a crucial role in our daily lives and work, not only providing us with light but also enriching our lives. Energy-saving light sources, televisions, mobile phones, computer monitors, and medical imaging devices we use daily all rely on luminescent materials. So, what exactly are luminescent materials?
By adding appropriate amounts of rare earth elements to fluorescent materials, these materials can emit light of different colors. Luminescent materials are substances that convert absorbed energy into light radiation. The light emitted by luminescent materials is cold light, which differs from the thermal radiation of high-temperature objects. When an object is excited by external energy and returns to its equilibrium state (ground state), it releases a portion of the absorbed energy as light radiation. Therefore, unlike the wavelength of thermal radiation, which depends on temperature, the wavelength of light emitted by luminescent materials primarily depends on the energy difference between the excited state energy level and the ground state energy level.
With the exception of La3+ and Lu3+, the divalent or trivalent ions of the other lanthanide elements are particularly suitable as luminescent materials due to their unfilled 4f shells. The 4f electrons in the f-f configuration and between the 4f-5d energy levels exhibit a rich energy structure, and the 5d orbitals are located on the surface of the ions, making them easily affected by surrounding ligands, leading to changes in the energy structure. Consequently, luminescent materials exhibit a wide and rich spectrum of absorption, excitation, and emission. Taking gadolinium ions (Gd3+) as an example, it has 3,432 electronic energy levels. The spectrum of rare earth ions can cover the vacuum ultraviolet, visible, and near-infrared spectral regions, demonstrating luminescent characteristics that cannot be achieved or replaced by other elements.
Since there are 12 kinds of rare earth elements commonly used in luminescent materials, which coincide with the number of vitamins needed by the human body, and the amount of rare earth doping in luminescent materials is trace, similar to the trace amounts of vitamins required by the human body, some people metaphorically refer to rare earths as the "vitamins" of luminescent materials.
Rare earth ions can be combined with organic materials to form rare earth organic luminescent materials, or incorporated into inorganic materials to form inorganic luminescent materials. Rare earth luminescent materials can be crystalline materials (such as phosphors), or amorphous or non-crystalline materials (such as luminescent glass). Rare earth ions are typically used as luminescent center activators or sensitizers in luminescent materials, and some rare earths are also used as matrix materials for luminescent materials due to their excellent stability.
Rare earth luminescent materials require absorbing energy and being excited to emit light, but the form of the absorbed energy can vary widely, including light energy, electrical energy, X-rays and high-energy particles, as well as chemical energy, biological energy, or mechanical energy. Therefore, in addition to being used in lighting and display applications, rare earth luminescent materials also have extensive applications in anti-counterfeiting, healthcare, agricultural production, information storage, and aerospace exploration.