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Even Better Than Graphene?
July 4, 2014   
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Molybdenum disulfide, a compound naturally occurring in rocks, may soon outclass graphene in electronic applications. Like graphene, the structure of molybdenum disulfide features single-atomic layers.

Scientists from the Faculty of Physics at the University of Warsaw carrying out research into molybdenum disulfide say it could potentially be used as an extremely thin, “two-dimensional” semiconductor.

Graphene is a new wonder material that is ultralight, flexible, more than 100 times harder than steel and an excellent conductor of electricity. It could have many hi-tech applications and may even replace silicon in the electronic devices of the future. Transparent, flexible and durable, graphene offers a huge range of potential applications. Graphene consists of a single layer of carbon atoms that form a flat, practically two-dimensional grid (length and width) with hexagonal meshes and a honeycomb structure.

Scientists call graphene a material of the future—ideal for the production of thin, flexible touchscreens and new-generation semiconductors and supercapacitors. But the problem with graphene is that while the carbon atoms in each layer are tied to their neighbors with very strong bonds, layers are bonded to one another with significantly weaker forces. An ordinary adhesive tape is enough to separate single layers of graphene from graphite crystal.

However, scientists know of more materials with a similar monolayer structure. Molybdenum disulfide has proved to be a particularly interesting material. This compound occurs in nature as molybdenite, a crystalline mineral that often takes the form of silvery hexagonal flakes. For years it has been used in the manufacture of lubricants and metal alloys. But the properties of single-atomic layers of molybdenum disulfide remained unnoticed for a long time.

“Complex electronic circuits built from single atomic layers can be constructed only when we sufficiently understand the physics of the phenomena occurring in the crystal lattice of these materials,” said Adam Babiński, a professor at the University of Warsaw. “Our research shows that a lot still remains to be done in terms of scientific research in this area.”

Physicists at the university say that, in terms of applications in electronics, layered molybdenum disulfide has an important advantage over graphene: the presence of the so-called energy gap. This means that after applying an electric field, the material can be “switched” from a state in which it conducts electricity to a state in which it behaves like an insulator.

Researchers estimate that a turned-off transistor from molybdenum disulfide would consume hundreds of thousands times less energy than a silicon transistor. In contrast, graphene does not have such an energy gap and transistors built from it cannot be completely turned off.

The Warsaw physicists have examined the known Raman spectra of molybdenum disulfide, analyzing the scattered light in the material. They have also carried out microscopic measurements at low temperature. The increased sensitivity of the apparatus and a detailed analysis of the results allowed them to propose a new, more accurate model of the phenomena occurring in the crystal lattice of molybdenum disulfide.

“In the case of layered materials, the shape of the Raman lines was until now attributed to phenomena associated with certain characteristic vibrations of the crystal lattice. We have demonstrated that, in layered molybdenum disulfide, the effects attributed to these vibrations in reality come, at least in part, from other, hitherto disregarded, lattice vibrations,” said Ph.D. student Katarzyna Gołasa, a member of the project research team at the University of Warsaw.

Babiński said, “Graphene was first. Its unique features still arouse considerable and growing interest among both scientists and industry. However, we should not forget other layered materials. If we get to know them well, they may prove to be better than graphene in many applications.”
Olga Majewska
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