Imagine that scientists assemble a material as a constructor, where properties can be "ordered": to make particles smaller or larger. Of course, this doesn't work so easily at the nanoscale: sometimes a difference of just a few nanometers completely changes the behavior of matter. Scientists at Southern Federal University have developed a method for the controlled synthesis of luminescent lanthanum fluoride nanoparticles doped with rare earth elements. Simply put, researchers have learned how to grow luminous nanoparticles with predefined characteristics and control their properties right during the synthesis process. As a result, it was possible to obtain homogeneous hexagonal nanoprisms ranging in size from 3 to 100 nanometers and learn how to change their parameters through the conditions of obtaining the material.

According to the researchers, such materials are particularly promising for biomedicine, sensors, scintillation detectors and photonic devices. For example, nanoparticles 10-30 nanometers in size can be used as luminescent markers or nanoscintillators for X-ray detection.

But getting nanoparticles of a given size is a difficult task. They are extremely sensitive to even the smallest changes in conditions.

"Even small changes in temperature, concentration of reagents, or the rate of their addition significantly affect the growth rate and can cause explosive nucleation of new particles instead of the uniform growth of existing ones," Oleg Polozhentsev said.

It is especially difficult to control particles of 3-30 nanometers in size. It is in this range that the electronic structure of the material can change, the chemical activity increases, and the properties are determined not so much by the volume of the substance as by the surface of the particles.

Scientists call the range of 3-100 nanometers the "golden zone" of nanomaterials. Up to 10 nm, quantum effects are particularly pronounced, and in the range of 10-100 nanometers, an optimal balance between luminescence, magnetic properties, and stability occurs.

To learn how to control particle growth, the researchers changed several synthesis parameters at once: reaction temperature, pH of the solution, type of solvent, concentration of reagents and surfactants.

"Temperature conditions, time, concentrations of reagents and surfactants, as well as feed and mixing speeds are the main "knobs" that can accurately select the size of nanoparticles," Oleg Polozhentsev notes.

In the course of their work, scientists have found that increasing the temperature does increase the particle size, but the growth does not continue indefinitely. After a while, the system stabilizes, and for further growth it is necessary to change the synthesis conditions again, for example, to increase the temperature or adjust the acidity of the medium.

The study showed that the particle size, as well as the type and concentration of doping, directly affect the properties of the material: with optimal doping, larger nanoparticles exhibit brighter luminescence, while reducing the size increases catalytic and biological activity.

"It is possible to purposefully change the optical, magnetic and catalytic properties by simply selecting the right particle size," the researcher emphasizes.

To enhance the glow of ultra–small particles, scientists have proposed a core-shell structure. There is an active luminescent core inside such a nanoparticle, and a protective shell on top.

"The shell helps to suppress nonradiative channels, reduces energy transfer to the surface, and increases the quantum yield of luminescence," Oleg Polozhentsev noted.

In addition, the scientists investigated the effect of size on the catalytic properties of particles. Although the work is not directly related to hydrogen energy, the researchers note that an approach to controlling the size and structure of nanoparticles can be useful in creating new photo- and electrocatalytic systems.

The method was developed and tested on lanthanum compounds, one of the most promising materials for photonics due to its optical properties and ability to interact effectively with rare earth ions. At the same time, the proposed approach can be adapted for other rare earth materials, such as fluorides, oxides, and oxyfluorides used in modern photonics and nanotechnology.

The results of the study are published in the journal Materials Today Nano.