March 13, 2014
MoS2 May Outperform Graphene For Electronics Applications, Researchers Say
redOrbit Staff & Wire Reports - Your Universe Online
Layers of molybdenum disulfide (MoS2), a naturally occurring compound in rocks, of just a single atom in thickness may be better than graphene for electronics applications, according to new research from the University of Warsaw. However, the researchers caution that the nature of the phenomena occurring in layered materials are not well understood and still require further research.
Graphene is a material built of six-atom carbon rings arranged in a honeycomb-like structure, which forms extremely resilient sheets just a single atom thick. Although many believe graphene represents the future of electronics, there are alternate materials with a layered structure and other similar properties, such as molybdenum disulfide.
In the current study, researchers at the University of Warsaw Faculty of Physics (FUW) investigated many of the properties of molybdenum disulfide, and found that the phenomena occurring in the crystal network of molybdenum disulfide sheets are of a slightly different nature than previously believed.
"It will not become possible to construct complex electronic systems consisting of individual atomic sheets until we have a sufficiently good understanding of the physics involved in the phenomena occurring within the crystal network of those materials. Our research shows, however, that research still has a long way to go in this field", said Professor Adam Babiński at the UW Faculty of Physics.
The simplest method of creating graphene is exfoliation, in which a piece of scotch tape is first stuck to a piece of graphite, then peeled off. Among the particles that remain stuck to the tape, one can find microscopic layers of graphene due to the fact that graphite consists of many graphene sheets adjacent to one another.
Although the carbon atoms within each layer are very strongly bound to one another (by covalent bonds, to which graphene owes its legendary resilience), the individual layers are held together by significantly weaker bonds (van de Walls bonds). Conventional scotch tape is strong enough to break the latter and tear individual graphene sheets away from the graphite crystal.
Just as graphene can be obtained from graphite, sheets a single atom thick can similarly be obtained from many other crystals. This has been successfully demonstrated, for instance, with transition metals chalcogenides (sulfides, selenides, and tellurides).
Layers of molybdenum disulfide (MoS2) have proven to be a particularly interesting material. The compound exists in nature as molybdenite, a crystal material found in rocks throughout the world, and frequently takes the characteristic form of silver-colored hexagonal plates.
For years molybdenite has been used in the manufacturing of lubricants and metal alloys. However, as with graphite, the properties of single-atom sheets of MoS2 have long gone unnoticed. In fact, for electronics applications, molybdenum disulfide sheets exhibit a significant advantage over graphene in that they have an energy gap – an energy range within which no electron states can exist. This means that by applying electric field, the material can be switched between a state that conducts electricity and one that behaves like an insulator. By current calculations, a switched-off molybdenum disulfide transistor would consume as little as several hundred thousand times less energy than a silicon transistor, the researchers said.
Graphene has no energy gap, and transistors made of graphene cannot be fully switched off.
Additional valuable information about a crystal's structure and phenomena occurring within it can be obtained by analyzing how light gets scattered within the material. Photons of a given energy are typically absorbed by the atoms and molecules of the material, and then reemitted at the same energy. In the spectrum of the scattered light one can then see a distinctive peak corresponding to that energy.
However, it turns out that one out of many millions of photons is able to use some of its energy otherwise, such as to alter the vibration or circulation of a molecule. The reverse situation may sometimes occur, with a photon taking away some of the energy of a molecule, slightly increasing its own energy. In this situation, known as Raman scattering, two smaller peaks are observed to either side of the main peak.
The scientists at the UW Faculty of Physics analyzed the Raman spectra of molybdenum disulfide carrying on low-temperature microscopic measurements. The higher sensitivity of the equipment and detailed analysis methods enabled the researchers to propose a more precise model of the phenomena occurring in the crystal network of molybdenum disulfide.
"In the case of single-layer materials, the shape of the Raman lines has previously been explained in terms of phenomena involving certain characteristic vibrations of the crystal network. We have shown for molybdenum disulfide sheets that the effects ascribed to those vibrations must actually, at least in part, be due to other network vibrations not previously taken into account,” said Katarzyna Gołasa, a doctorate student at the UW Faculty of Physics.
The presence of the new type of vibration in single-sheet materials has an impact on how electrons behave. As a result, these materials must have somewhat different electronic properties than previously thought, the researchers concluded.
"Graphene was the first. Its unique characteristics have triggered a considerable, still-growing interest among scientists and also from industry. However, we must not forget about other single-layer materials. If we study them well, they may prove to be better than graphene for many applications," Professor Babiński said.
The research is published this month in the journal Applied Physics Letters.