MOSCOW, Oct. 4This year, the Nobel Committee decided to award a prize in chemistry for work in the field of nanotechnology. Eleven million crowns will be shared by three scientists from the United States who discovered quantum dots and developed methods for their stabilization and production — Alexey Ekimov, Louis Bruce and Mungi Bawendi. These studies paved the way for the creation of LED devices for widespread use. The director of the A. F. Ioffe Institute of Physics and Technology of the Russian Academy of Sciences, Doctor of Physical and Mathematical Sciences, Corresponding Member of the Russian Academy of Sciences Sergei Ivanov spoke about the history and practical significance of quantum dots. Interviewed by Tatyana Pichugina.
— Sergei Viktorovich, your institute traditionally deals with the topic that was awarded the Nobel Prize today. At one time, the laureate was Zhores Ivanovich Alferov, who worked at the Ioffe Physicotechnical Institute. Please tell us about the discovery of quantum dots by Alexey Ekimov.
— Alexey Ivanovich Ekimov graduated from Leningrad University, he was a student of Evgeniy Fedorovich Gross, the founder of exciton physics, who experimentally proved that excitons—excited particles—exist in a semiconductor. Gross invited him to his laboratory at Phystech (Leningrad Institute of Physics and Technology named after Ioffe — Ed.), where they studied spin phenomena. Together with Gross, he was awarded the USSR State Prize. We can safely say that the optical school to which Ekimov belongs is the Gross School of Physics and Technology.
In the late 1970s, A.I. Ekimov went to the Vavilov State Optical Institute, where he worked on the synthesis of optical filters capable of changing color depending on annealing conditions. This is quartz glass with alloying impurities. If they gather in clusters, the color also changes. Then the idea arose that the optical properties are determined by the sizes of nanoclusters, which consist of semiconductor compounds, for example, copper chloride, cadmium selenide, cadmium sulfide. In their pure form, these materials emit and absorb light in the visible region — yellow, green, and so on. And if the emitting object is so small that quantum effects appear — these are several tens of nanometers, ten to fifteen atoms, then such crystals formed in the glass radically change the optical properties — color, absorption spectrum of the glass.
About In 1981, Alexey Ekimov, in collaboration with his GOI colleague Alexey Onushchenko, published an articlein «JETP Letters» («Quantum size effect in three-dimensional semiconductor microcrystals.» — Ed.). And a year later, together with Alexander Efros, a Physics and Technology theorist, Ekimov published another work, which theoretically explained the optical properties of semiconductor quantum dots in glass. Until the end of the 1990s, he continued his research at the Ioffe Physicotechnical Institute, then went abroad and ended up in the USA — in a private company dealing with nanostructured objects.
< br />Around the same years, Louis Bruce (then working at the American Bell Laboratories — editor's note) discovered that nanoobjects with quantum properties can be formed in a liquid — under certain conditions of heating, concentration, and doping impurities. This is how colloidal quantum dots were discovered. About ten years later, Mungi Bawendi (a native of France, at that time an employee of the Massachusetts Institute of Technology in the USA, a former colleague of Bruce at Bell Laboratories — Ed.) developed the industrial chemical technology of colloidal quantum dots.
— How are quantum dots obtained?
— They were discovered in glass, where they arise spontaneously, because at high temperatures it is energetically more favorable for impurities (cadmium, selenium, sulfur) to form nanoclusters than to remain in a freely distributed state.
It is difficult to work with such objects in glass, for example, to apply electric current to them. And the synthesis of colloidal quantum dots in liquid made it possible to use them in medicine. They are small, composed of group VI atoms and easily combine with organic medicinal radicals. These balls serve as carriers, among other things, of chemical therapeutic drugs — molecules for delivery through the blood to any point in the body. It is important to ensure that they gather in the right place. To do this, they are doped with magnetic impurities, such as manganese. It turns out to be a semiconductor ball, hung with medicines, inside which sits a particle of manganese. Then, using a magnet, you can collect particles in a diseased organ or tumor. There they are activated, the drugs are separated and interact with human tissue.
< br />If quantum dots from a colloidal solution are smeared on the surface and illuminated with LEDs, various emissions arise in the optical range. This gives color television — such displays are already on sale.
The third way to obtain quantum dots is in semiconductor technology. During the growth of multilayer heterostructures, very strained thin monolayers can be grown. For example, in gallium arsenide there is a layer of indium arsenide. Due to the enormous stresses accumulated in the thin layer, it breaks and clusters of indium arsenide are formed. If, during sputtering, it is overgrown with gallium arsenide, then clusters of indium arsenide are formed in the gallium arsenide matrix. This also happens spontaneously, but under certain conditions (temperature, deposition rate). Quantum dots, artificially formed inside a semiconductor heterostructure, actively emit light when an electric current is applied. Such semiconductor lasers with arsenic quantum dots were synthesized for the first time in the world at our institute in 1994. In 1999, we developed a technology for cadmium selenide quantum dots and grew them inside a semiconductor heterostructure with zinc selenide. This research led to the creation of the world's first green light laser diodes. We worked then together with German scientists, but the idea and technology were created at Phystech.
— Surely this is not the last Nobel Prize for your institute.
“I think they don’t award you twice for the same thing.” But it is pleasant that the beginning of such optical research and the explanation of this effect, the development of the discovery took place within the framework of Gross’s laboratory, then in the laboratory of Alferov, then his student Pyotr Sergeevich Kopyev. I replaced him in 2014.
Alexey Ivanovich Ekimov and I maintained contacts until recently, meeting at scientific conferences.
— The story is amazing. In those years, the world actually competed to see who would be the first to make a discovery.
— It must be said that semiconductor quantum dots using molecular beam epitaxy were first demonstrated in America in Petroff’s laboratory (P. M. Petroff. — Ed. .). And it was possible to synthesize them and insert them into a laser diode, thereby finding practical application for them, at the Physicotechnical Institute in 1993-1994.
It is important that Alexey Ekimov received the Nobel Prize for research related to the Physicotechnical Institute, although at that moment he worked at GOI. But optics, spectroscopy, theory were created here.