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The transformation of a metal into a dielectric

03.05.2017

Researchers from the Higher School of Economics and the Jyväskul Nanocenter (Finland) have experimentally demonstrated for the first time the quantum size effect in a metal nanowire. The discovered effect is universal and should be taken into account when designing ultra-small nanoelectronic systems.

The study shows how the electrical resistance of a metal nanowire made of bismuth changes nonmonotonically in the process of decreasing its diameter and then sharply increases, transferring the object to a dielectric (insulating) state. According to scientists, quantum-size effects are associated with a fundamental phenomenon - quantization of the energy spectrum of electrons. And it can be observed only in objects of extremely small sizes.

"The charge carrier is usually an electron and it is impossible to determine its precise position, but we know the probability of its being in a certain area. This probability is described by a quantum mechanical wave function, and each wave has its own characteristic scale - the wavelength," explains one of the co-authors research. - So, if we make a conductor, the dimensions of which are comparable with the wavelength of this wave, there will be a qualitative change in the properties of the system. In this case, we speak of quantization of energy levels, that is, the splitting of the continuous spectrum into well-defined levels. In addition, there is called the Fermi level, which separates filled energy states from unfilled ones.So, as the size of the conductor decreases, the energy levels begin to shift relative to this threshold value, and at the moment when the last filled level crosses the Fermi level, the sample passes from the metallic to the insulating state. and that is the essence of the quantum size effect in our case."

Nanowires were chosen as "test subjects", since the wire is the basis of any electrical circuit, and a thin film was also studied as an example. There are several methods for studying quantum size effects (QSE). In the first, the size of the same sample is consistently reduced; in the second, several samples of different sizes are used.

The researchers chose the first one because they thought it matched the "cleaner" experiment. The main problem in this case was the task of reducing the structure so as not to damage it. The sizes of nanostructures were reduced by etching with a directed beam of ions of an inert gas (argon), thereby "grinding" the surface. The optimal processing mode was found, in which the surface roughness was approximately 1 nanometer (about two atomic layers). In this case, the smallest wire diameter was about 40 nm, and the starting diameter was about 300 nm. The samples themselves were fabricated using a fairly standard process of electron beam lithography and directional vacuum deposition. After making samples and carefully checking them, the best ones were selected for measurements. Then, the cycle of ion etching and measurements was repeated many times until the moment when the structure became thinner to such a limit that it simply failed (ie, torn).

"The main result of the work was that this cherished metal-insulator transition exists not only in the calculations of theorists, but can also be observed experimentally. In earlier works, this result was achieved on thin films, there were also attempts to do this in nanowires, but not too successful. So we can say that our work is one of the first to experimentally demonstrate the quantum size effect in a metal conductor. The quantum size effect is a universal phenomenon that should be present in absolutely any system of sufficiently small sizes. Thus, our research make it possible to determine the fundamental limitations on the miniaturization of elements of (nano)electronic circuits," the scientists note.

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