New transistors: An alternative to silicon and better than graphene

New transistors: An alternative to silicon and better than graphene

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This is a digital model showing how molybdenite can be integrated into a transistor. Credit: Credit: EPFL

Smaller and more energy-efficient electronic chips could be made using molybdenite. In an article appearing online January 30 in the journal Nature Nanotechnology, EPFL's Laboratory of Nanoscale Electronics and Structures (LANES) publishes a study showing that this material has distinct advantages over traditional silicon or graphene for use in electronics applications.

A discovery made at EPFL could play an important role in electronics, allowing us to make transistors that are smaller and more energy efficient. Research carried out in the Laboratory of Nanoscale Electronics and Structures (LANES) has revealed that molybdenite, or MoS₂, is a very effective semiconductor. This mineral, which is abundant in nature, is often used as an element in steel alloys or as an additive in lubricants. But it had not yet been extensively studied for use in electronics.

100,000 times less energy

"It's a two-dimensional material, very thin and easy to use in nanotechnology. It has real potential in the fabrication of very small transistors, light-emitting diodes (LEDs) and solar cells," says EPFL Professor Andras Kis, whose LANES colleagues M. Radisavljevic, Prof. Radenovic et M. Brivio worked with him on the study. He compares its advantages with two other materials: silicon, currently the primary component used in electronic and computer chips, and graphene, whose discovery in 2004 earned University of Manchester physicists Andre Geim and Konstantin Novoselov the 2010 Nobel Prize in Physics.

One of molybdenite's advantages is that it is less voluminous than silicon, which is a three-dimensional material. "In a 0.65-nanometer-thick sheet of MoS₂, the electrons can move around as easily as in a 2-nanometer-thick sheet of silicon," explains Kis. "But it's not currently possible to fabricate a sheet of silicon as thin as a monolayer sheet of MoS₂." Another advantage of molybdenite is that it can be used to make transistors that consume 100,000 times less energy in standby state than traditional silicon transistors. A semi-conductor with a "gap" must be used to turn a transistor on and off, and molybdenite's 1.8 electron-volt gap is ideal for this purpose.

Better than graphene

In solid-state physics, band theory is a way of representing the energy of electrons in a given material. In semi-conductors, electron-free spaces exist between these bands, the so-called "band gaps." If the gap is not too small or too large, certain electrons can hop across the gap. It thus offers a greater level of control over the electrical behavior of the material, which can be turned on and off easily.

The existence of this gap in molybdenite also gives it an advantage over graphene. Considered today by many scientists as the electronics material of the future, the "semi-metal" graphene doesn't have a gap, and it is very difficult to artificially reproduce one in the material.

Better batteries from the bottom up

Better batteries from the bottom up

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A nanostructured lithium ion battery developed at Rice University may charge faster and last longer than Li ion batteries in current use. Nanowires with a PMMA polymer coating, seen in a transmission electron microscope image at right, solve a long-standing problem of forming ultrathin electrolyte layers around nanostructured electrode materials. (Credit: Ajayan Lab/Rice University)

(PhysOrg.com) -- Rice University researchers have moved a step closer to creating robust, three-dimensional microbatteries that would charge faster and hold other advantages over conventional lithium-ion batteries. They could power new generations of remote sensors, display screens, smart cards, flexible electronics and biomedical devices.

The batteries employ vertical arrays of nickel-tin nanowires perfectly encased in PMMA, a widely used polymer best known as Plexiglas. The Rice laboratory of Pulickel Ajayan found a way to reliably coat single nanowires with a smooth layer of a PMMA-based gel electrolyte that insulates the wires from the counter electrode while allowing ions to pass through.

The work was reported this week in the online edition of the journal Nano Letters.

"In a battery, you have two electrodes separated by a thick barrier," said Ajayan, professor in mechanical engineering and materials science and of chemistry. "The challenge is to bring everything into close proximity so this electrochemistry becomes much more efficient."

Ajayan and his team feel they've done that by growing forests of coated nanowires -- millions of them on a fingernail-sized chip -- for scalable microdevices with greater surface area than conventional thin-film batteries. "You can't simply scale the thickness of a thin-film battery, because the lithium ion kinetics would become sluggish," Ajayan said.

"We wanted to figure out how the proposed 3-D designs of batteries can be built from the nanoscale up," said Sanketh Gowda, a graduate student in Ajayan's lab. "By increasing the height of the nanowires, we can increase the amount of energy stored while keeping the lithium ion diffusion distance constant."

The researchers, led by Gowda and postdoctoral researcher Arava Leela Mohana Reddy, worked for more than a year to refine the process.

"To be fair, the 3-D concept has been around for a while," Reddy said. "The breakthrough here is the ability to put a conformal coat of PMMA on a nanowire over long distances. Even a small break in the coating would destroy it." He said the same approach is being tested on nanowire systems with higher capacities.

The process builds upon the lab's previous research to build coaxial nanowire cables that was reported in Nano Letters last year. In the new work, the researchers grew 10-micron-long nanowires via electrodeposition in the pores of an anodized alumina template. They then widened the pores with a simple chemical etching technique and drop-coated PMMA onto the array to give the nanowires an even casing from top to bottom. A chemical wash removed the template.

They have built one-centimeter square microbatteries that hold more energy and that charge faster than planar batteries of the same electrode length. "By going to 3-D, we're able to deliver more energy in the same footprint," Gowda said.

They feel the PMMA coating will increase the number of times a battery can be charged by stabilizing conditions between the nanowires and liquid electrolyte, which tend to break down over time.

The team is also studying how cycling affects nanowires that, like silicon electrodes, expand and contract as lithium ions come and go. Electron microscope images of nanowires taken after many charge/discharge cycles showed no breaks in the PMMA casing -- not even pinholes. This led the researchers to believe the coating withstands the volume expansion in the electrode, which could increase the batteries' lifespans.