Silicon rival MoS2 promises small, low-energy chips

The first computer chip made out of a substance described as a "promising" alternative to silicon has been tested by researchers. The Switzerland-based team used molybdenite (MoS2) - a dark-coloured, naturally occurring mineral. The group said the substance could be used in thinner layers than silicon, which is currently the most commonly used component in electronics. It said MoS2 could make smaller, more flexible chips that used less energy. The substance is currently used as an ingredient in engine lubricants, ski waxes and as a strengthening agent for plastics. Prof Andras Kis, the director of the Laboratory of Nanoscale Electronics and Structures (LANES) in Lausanne, published details of the research in the latest edition of the ACS Nano journal. He said the team chose to experiment with this semiconductor, rather than another material, in part because it was easily available. "There is something like 19 million metric tonnes around," Prof Kis told the BBC. "You can just go on some websites on the internet and buy a 1cm by 1cm crystal for around $100 [£64]." Surfaces oxidise To obtain a thin layer of the material to work with, Prof Kis's team put a strip of sticky plastic over the crystal, peeled it off and then attached the sliver to a support. The plastic was then peeled off to leave the very thin layer of MoS2 exposed. Using this, the team built a prototype microchip circuit to which they attached up to six serial transistors allowing them to carry out simple logic operations. Molybdenite in crystal form The researchers say molybdenite crystals are relatively abundant in the natural environment Although the integrated circuit was basic, Prof Kis said it proved that more complex designs would be possible on thinner chips than could be produced with silicon. "The problem with silicon is that you cannot make very thin things from it because it is very reactive," he said. "The surface likes to oxidise - to bind with oxygen and hydrogen - and that makes its electrical properties degrade when you want to make a very thin film." As a result the thinnest usable layers of silicon used in computer chips have been around two nanometres thick. MoS2, by contrast, can be used in layers just three atoms thick, allowing chips to be made at least three times smaller. Stiff as steel A key advantage of having a thinner material is that the transistors can also be shrunk in size. "If you have a transistor that is very thin it will also automatically dissipate less power - so it spends less power. So in a nutshell it allows you to make electronics that spend less electrical energy," Prof Kis said. MoS2 also has the advantage that it is as stiff as stainless steel, but is also capable of being flexible. Andras Kis Prof Kis says the material could help create flexible electronics that can be attached to skin "It can be bent to large angles and can be stretched a lot," said Prof Kis. "If you take a sheet of molybdenite you can stretch it so that it increases its length by 10% - that is a lot in this context. "If you did the same with silicon it would break like glass." The team said the material might be suitable for flexible electronics that could be rolled into tubes, attached to the skin or used to make mobile phones that curved themselves to fit the owner's face. Low temperatures MoS2 faces a challenge from graphene, another flexible semiconductor, as a potential replacement for silicon. But the Swiss team believe their material has a key advantage - it can amplify electronic signals at room temperature, while graphene must be cooled to 70 Kelvin - cold enough for nitrogen to turn into liquid. "If you look at the circuits in computers, for example, you have millions of transistors connected in series doing some kind of calculation," said Prof Kis. "The important thing is that the signal that goes into the processor doesn't get reduced as a consequence of the operation, because then you'd lose your electrical signal in the chip. "So it has to be constantly amplified. Silicon can do this and so can molybdenite, but graphene can only do it at very low temperatures." Despite MoS2's potential, the researchers said it would be at least 10 to 20 years before it would be likely to enter commercial use. In the meantime the group said it planned to explore whether it could make the mineral more conductive and would also try to find a less labour-intensive way of producing thin layers of the substance.