Researchers at the Higher University of Economics and the Institute of Petrochemical Synthesis of the Russian Academy of Sciences have discovered a way to control the color and brightness of the light emitted by rare earth elements. Typically, the glow of these elements is highly predictable. For example, cerium ions typically emit light in the ultraviolet range.
However, scientists have shown that this emission can be changed. By creating a specific chemical environment, they caused the cerium ions to emit yellow light instead of the usual ultraviolet light. These findings may help develop advanced light sources, display technologies and laser systems. The research was published in the journal Optical Materials.
Rare earth elements play an important role in microelectronic devices, LEDs and fluorescent materials because they emit light with precise colors. This ability stems from the behavior of rare earth element electrons as they absorb and release energy.
When an atom absorbs energy from a source, such as light or an electric current, one of its electrons jumps to a higher energy level. This excited state is unstable. After a short time, the electron returns to its original energy level and releases the excess energy in the form of light. This process is called luminescence.
Orbital transitions and their typical stabilities
In rare earth elements, the glow is produced by electronic transitions between 4f orbitals (the region around the nucleus where electrons can reside). Typically, the energy of these transitions is fixed, which means the color of the glow remains the same: cerium emits invisible ultraviolet light, while terbium emits green light.
The 4f orbital is located deep within the atom and has minimal interaction with the surrounding environment. In contrast, the 5d orbital is sensitive to external influences, but because its energy is too high, it usually does not contribute to the luminescence of lanthanide elements.
Photographs of the synthesized cerium complexes and their molecular structures. Photo credit: Dmitrii Roitershtein
However, scientists from the Higher University of Economics and the Institute of Petrochemical Synthesis of the Russian Academy of Sciences have demonstrated that it is possible to change the color of the radiation by adjusting the chemical environment of the metal. They synthesized complexes of cerium, praseodymium and terbium using organic ligands (molecules that surround metal ions). These ligands shape the geometry of the complex and influence its properties. In all cases, the three cyclopentadienyl anions are arranged symmetrically around the metal.
These anions consist of regular pentagonal carbon atoms to which larger organic fragments are attached, giving the complex the desired structure. This environment creates a specific electrostatic field around the ions, which changes the energy of the 5d orbital, thereby affecting the luminescence spectrum.
Uncovering the mechanisms behind change
"Previously, changes in glow color were observed, but the underlying mechanism was not clear. Now, in collaboration with physicist colleagues, we are finally able to understand the mechanism behind this effect. We deliberately designed compounds with atypical electronic structures among the lanthanides. Instead of focusing on a single example, we synthesized a series of compounds from cerium to terbium to see how their properties change and identify common patterns." Daniil Bardonov, master's student at the Department of Chemistry at the Higher School of Economics Bardonov) commented.
In traditional compounds, cerium emits UV light with wavelengths between 300 and 400 nanometers. In the new complex, the emission wavelength is shifted into the red range, up to 655 nanometers. This indicates that the energy gap between the 4f energy level and the 5d energy level is reduced. Similar rearrangements of electronic energy levels were observed in other lanthanides studied, which also led to changes in their luminescence properties.
"To understand how this process works, you first have to understand the mechanism of energy transfer. Typically, a ligand molecule absorbs UV light, enters an excited state, and then transfers the energy to the metal atom, causing it to emit light," explains Dmitrii Roitershtein, academic director of the Molecular Systems and Materials Chemistry Program and co-author of the paper. "In the new compound, however, the process is different: energy is not transferred directly to the 4f electron, but through an intermediate 5d state."
The researchers believe that being able to predict the luminescence spectrum will allow us to more efficiently design materials with desired properties without the need for time-consuming trial and error. This could lead to the creation of new advanced light sources.
"We were able to demonstrate precisely how the atomic environment affects its electronic transitions and the luminescence of lanthanides," says Fyodor Chernenkiy, an undergraduate student at the Department of Chemistry at the Higher School of Economics. "We can now intentionally choose the structure of compounds to control light emission and produce materials with specific optical properties."
Compiled from /ScitechDaily