26.03.2014 change 26.03.2014

Rival of graphene - molybdenum compound present in rocks - better understood

The layers of molybdenum disulfide have better prospects for use in electronics than graphene. In nature, molybdenum disulfide occurs as molybdenite, crystalline mineral often taking the form of characteristic hexagonal flakes with silvery color. Source: Faculty of Physics UW The layers of molybdenum disulfide have better prospects for use in electronics than graphene. In nature, molybdenum disulfide occurs as molybdenite, crystalline mineral often taking the form of characteristic hexagonal flakes with silvery color. Source: Faculty of Physics UW

Monatomic layers of molybdenum disulfide - compound present in many rocks - can declass graphene in electronic applications. These structures have been closely examined at the Faculty of Physics, University of Warsaw.

Graphene has already been hailed the future of electronics. Coal forming monatomic layers has different properties than when the layers are thicker. But there are more similar materials with monolayer structures. Importantly, some of them, such as molybdenum disulfide, have properties as interesting as that of graphene.

Researchers at the Faculty of Physics, University of Warsaw have shown that the phenomena occurring in the crystal lattice of molybdenum disulfide layers have a slightly different nature than previously thought. The study allowed the researchers to propose a more precise model of the phenomena occurring in the crystal lattice of molybdenum disulfide, reported the Faculty of Physics UW in a release sent to PAP.

Work describing the discovery, prepared in collaboration with the Laboratoire National des Champs Magnétiques Intenses in Grenoble, has been published in the journal "Applied Physics Letters".

"Complex electronic circuits made of sngle atomic layers can be built only when we have sufficient understanding of the physics of the phenomena occurring in the crystal lattice of the material. Our research shows that science still has much to do in this area" - said Dr. Adam Babinski from the University of Warsaw.

According to the physicists from the University of Warsaw, a few years ago scientists noticed that just like graphene is obtained from graphite, many other crystals can be used to obtain single atom layers. These have already been produced, inter alia, for chalcogenides of transition metals, namely sulfides, selenides, and tellurides. Particularly interesting material are layers of molybdenum disulfide (MoS2). It occurs in nature as molybdenite, crystalline mineral often taking the form of characteristic hexagonal flakes with silvery color. Molybdenite - which resembles graphite and has often been confused with it - is found in the rocks all over the world. For years it has been used in the manufacture of lubricants and metal alloys. As in the case of graphite, properties of MoS2 single layers have long remained unnoticed.

From the viewpoint of applications in electronics, layered molybdenum disulfide has an important advantage over graphene: it is characterized by the presence of so-called energy gap. Its existence means that electrons can not take any energy and by applying an electric field the material can be switched between the state in which it conducts electricity, and the state in which it behaves as an insulator. According to current estimates, a switched off molybdenum disulfide transistor would actually consume even hundreds of thousands of times less energy than a silicon transistor. Graphene, on the other hand, does not have an energy gap and transistors built with it can not be completely turned off.

Analysis of the light scattered in the material provides valuable information on the crystalline structure and phenomena taking place therein. Researchers from the Faculty of Physics UW looked at known Raman spectra of molybdenum disulfide, they also conducted their own microscopic measurements at low temperature. Increased equipment sensitivity and a detailed analysis of the results allowed the researchers to propose a more precise model of the phenomena occurring in the crystal lattice of molybdenum disulfide.

"In the case of layered materials, the shape of the Raman lines has been explained with the phenomena associated with certain vibration characteristic of the crystal lattice. We have shown that in a layered molybdenum disulfide, the effects attributed to vibration must actually be caused, at least in part, by other, hitherto disregarded lattice vibrations" - explained PhD student Katarzyna Gołasa (Faculty of Physics UW).

The presence of a new type of vibration in layered materials affects the behavior of electrons. Consequently, these materials must exhibit slightly different electronic properties than previously anticipated.

"Graphene was the first. Its unique properties arouse considerable and still growing interest among scientists and the industry. However, we should not forget about other layered materials. If we study them, they may prove to be better than graphene in many applications" - concluded Adam Babiński.

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