Scientists from the Russian Chemical Technical University named after DI. Mendeleev synthesized an airgel from silicon oxide with embedded luminescent particles of the organometallic substance Alq₃.
The authors of the study note that this approach is promising for the creation of new light-emitting devices, since the porous structure of the airgel protects luminescent substances from the destructive effects of the external environment, and also allows combining different luminophores in one matrix, which makes it possible to obtain a smoother and more uniform radiation spectrum than that of modern LEDs. The results of the work were published in the Journal of Solid of State Chemistry, and now the researchers are developing a prototype of a light-emitting device based on a new approach.
There are millions of LEDs in use in the world today, but they still have serious drawbacks. Chief among them is the irregularity and unnaturalness of radiation. Most often, LEDs themselves emit light in a narrow wavelength range, that is, only a certain color – for example, only green or only red. Therefore, in order to make truly efficient light-emitting devices with them, they resort to various tricks, which significantly increases their cost.
So, in a typical modern white LED, there are two light-emitting substances at once. One of them is a luminescent substance that emits blue and ultraviolet light under the action of an electric current, and the second is a semitransparent phosphorescent film, which, under the influence of blue radiation, also begins to emit light, but only this time yellow. A mixture of yellow and blue in the right proportions gives white, but this combination, of course, differs from natural white light: there is too much ultraviolet radiation in it, as well as other ratios between radiation intensities at different wavelengths, and as a result, eyes get tired faster from such light. Therefore, scientists are looking for new approaches to creating LEDs.
Researchers from the RCTU proposed using aerogels for this – this is the name for materials that are hard light sponges, the pores of which are filled with gas. Aerogels have a very low density, huge porosity, up to 99% of the airgel is occupied by air, as well as a huge internal surface area of up to 1500 m2 / g, that is, if you add up the total area of the inner surface of all pores of a piece of airgel weighing only five grams, you get a whole football field. Therefore, aerogels are already used to create various thermal insulation materials, supercapacitors, and other applications.
“We tried to incorporate luminescent substances into aerogels for two main reasons. Firstly, for many phosphors, the radiation spectrum deteriorates markedly with the appearance of even the smallest impurities, and they also rapidly degrade when in contact with humid air, which oxidizes them – the airgel can act in such cases as a kind of protector of the phosphor from the environment, – says one of the authors of the work, senior researcher of the Russian Chemical Technical University, Artyom Lebedev. – Secondly, the airgel can be used as a volumetric emitter, that is, to build into it not one, but several luminescent substances, the radiation of which together will give a smooth and uniform spectrum. The airgel is also well suited for the classical white LED circuit, in which the ultraviolet radiation of one substance excites the photoluminescence of another substance. The airgel absorbs ultraviolet light well and prevents it from escaping, but instead sends it on a journey through a complex maze of pores until ultraviolet light reaches the phosphor molecules. The result is a uniform spectrum, smoothed out by this intricate internal airgel architecture. ”
In the work, the scientists used the organometallic compound of tris (8-hydroxyquinoline) aluminum (Alq₃) as a luminescent substance. It is one of the best known compounds used to make organic light emitting diodes. Alq₃ is excited by ultraviolet light, and itself emits green light with a maximum intensity in the region of 500 nm. The most common silicon dioxide airgel was used as the Alq₃ matrix. The synthesis of such a hybrid material was carried out in several stages.
First, scientists obtained a hydrogel from organosilicon precursors. This material is very similar to an airgel – the same lightweight porous sponge, a framework that is made of silicon dioxide molecules stitched together, but the pores of this system are filled not with gas, but with liquid – in this case, it was isopropanol, in which the hydrogel was synthesized. Next, it was necessary to introduce into this matrix Alq₃, which is poorly soluble in isopropanol, but has a higher solubility in acetone. Therefore, isopropanol in the pores of the hydrogel was gradually replaced by acetone, and then the entire sponge was immersed in a solution of Alq₃ in acetone, as a result of which the porous structure of the gel absorbed the phosphor.
After that, the hydrogel had to be turned into an airgel. If you try to simply dry the hydrogel in air, then its internal structure will collapse, and you will not get a solid porous material. Therefore, hydrogels are dried in an environment of supercritical carbon dioxide heated inside a special apparatus at a pressure of 120 atmospheres to a temperature above 31 degrees. Under such conditions, CO₂ mixes indefinitely with the solvent in the pores of the gel. For successful drying, CO₂ is continuously fed into the apparatus for several hours, due to which the solvent is completely removed from the gel. When it is completely removed and the pressure gradually decreases, CO₂ turns into gas and finally a hybrid airgel with embedded Alq₃ is obtained. Under normal conditions, it looks like a solid translucent material, but when irradiated with ultraviolet light, it begins to actively glow green.
Scientists have shown that such a multistage synthesis does not harm the airgel itself: Alq₃ does not clog or destroy pores, but is embedded in the bulk of the material, practically without changing its basic properties. In addition, the researchers optimized the synthesis conditions, or rather the ratio between the amount of solvent (isopropanol) used and the organosilicon precursor. They showed that the most intense glow are aerogels obtained from mixtures in which isopropanol was 7 times more than the airgel precursor.
From demonstration of capabilities to the first prototype
The authors of the work emphasize that their study is only the first demonstration of the possibilities of the new approach, and for the obtained aerogels it is still incorrect to evaluate such final technical characteristics of light-emitting devices as energy efficiency. Now scientists continue to work and introduce other organometallic luminescent substances into aerogels in order to combine their emission spectra. In the near future, the researchers plan to make a prototype of a light-emitting device based on aerogels.
“In this first work, we have already shown that the approach with luminescent aerogels is promising, but this approach has another very important perspective,” says Artyom Lebedev. “The fact is that Alq₃ itself is very expensive. This is due to the need for its multiple purification, with the difficulties of synthesis. At the same time, the starting quinoline, from which it is synthesized, is much cheaper. And if we figure out how to synthesize an organometallic complex from its precursors directly inside the “protective” shell of the airgel, in an inert medium of supercritical carbon dioxide, then it would be very, very beneficial. We are actively working on this now. ”