Power/Performance Bits: Feb. 2

Single electron transistors

A group coordinated by the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) is setting out on a four year program to develop single electron transistors fully compatible with CMOS technology and capable of room temperature operation.

The single electron transistor (SET) switches electricity by means of a single electron. The SET is based on a quantum dot (consisting of just several hundred silicon atoms) embedded in an isolating layer that is sandwiched between two conducting layers. In order for a SET to function at room temperature, the silicon quantum dot needs to be smaller than five nanometers. Yet the electrons would not be able to pass through the transistor without another requirement being fulfilled: the distance between the quantum dot and the conducting layers must not be larger than two to three nanometers. As yet, these requirements could not be realized in nanoelectronics.

“Our transistor is based on a nanopillar. We have discovered a mechanism that ensures that the silicon quantum dot virtually form on their own”, says Dr. Karl-Heinz Heinig, initiator of the new EU project. “We construct slim silicon pillars of about 20 nanometers into which we embed a six nanometer thin layer consisting of the isolator silicon dioxide. Silicon atoms are pushed into the isolator by irradiating the nanopillar with fast, charged particles. When the structures are subsequently subjected to strong heat, the atoms cluster at the center of the isolating layer to form a single silicon quantum dot.”

Left: Schematic structure of a novel single-electron “gate-all-around” transistor. In a nanocolumn an insulating layer is surrounding the central quantum dot. Right: Scheme of the novel device. The SET single quantum dot consists of just several hundred silicon atoms. (Source: HDZR)

However, the task doesn’t stop at manufacturing the device.

The demonstrator cannot consist only of SET components that carry out the logical operations at room temperature. Since the energy-saving single electron transistors have too little power available to interact with the world outside their own chip, classical FET components, also in the form of nanopillars, are an additional requirement.

Participating in the project are GlobalFoundries, X-FAB, and STMicroelectronics. Research institutions aiding the effort are CEA-Leti, the Spanish National Centre for Microelectronics in Barcelona, Fraunhofer, the Institute for Microelectronics and Microsystems at the CNR in Italy and the University of Helsinki in Finland.

Graphene cage for silicon anodes

A lithium-ion battery anode made out of silicon could store 10 times more energy per charge than today’s commercial anodes and make high-performance batteries a lot smaller and lighter. But two major problems have stood in the way: Silicon particles swell to three times normal size, crack and shatter during battery charging, and they react with the battery electrolyte to form a coating that saps their performance.

To tackle the problem, a team from Stanford University and the Department of Energy’s SLAC National Accelerator Laboratory set out to wrap each and every silicon anode particle in a custom-fit cage made of graphene.

“In testing, the graphene cages actually enhanced the electrical conductivity of the particles and provided high charge capacity, chemical stability and efficiency,” said Yi Cui, an associate professor at SLAC and Stanford. “The method can be applied to other electrode materials, too, making energy-dense, low-cost battery materials a realistic possibility.”

Time-lapse images from an electron microscope show a silicon microparticle expanding and cracking within its graphene cage as lithium ions rush in during battery charging. The cage is outlined in black, and the particle in red. (Source: Y. Li et al., Nature Energy)

For the graphene cages to work, they had to fit the silicon particles exactly. To accomplish this, the team coated silicon particles with nickel, which can be applied in just the right thickness. Layers of graphene were grown on top of the nickel, which also acts as a catalyst to promote graphene growth. Finally they etched the nickel away, leaving just enough space within the graphene cage for the silicon particle to expand.

“This new method allows us to use much larger silicon particles that are one to three microns, or millionths of a meter, in diameter, which are cheap and widely available,” Cui said. “In fact, the particles we used are very similar to the waste created by milling silicon ingots to make semiconductor chips; they’re like bits of sawdust of all shapes and sizes. Particles this big have never performed well in battery anodes before, so this is a very exciting new achievement, and we think it offers a practical solution.”

Power with cardboard, tape, and a pencil

A team from EPFL, working with researchers from the University of Tokyo, used cardboard, Teflon tape and a pencil to make an 8 cm² device that can generate enough power to operate a remote micro- sensor or system.

The system is made up of two small cards, where one side of each card is covered with a graphite carbon layer by penciling. The carbon layers serve as the electrodes. Teflon is then applied to the opposite side of one of the cards. When brought together, they make a sandwich: two layers of carbon on the outside, then two layers of paper, and one layer of Teflon attached to one of paper cards. They are then taped together to form the arch-shaped geometry, enlarging the output performance.

By pressing down with your finger on the system, the two insulators come into contact. This creates charges at the surfaces: positive for the paper, negative for the Teflon. When you release your finger and the cards separate, the net charges are generated on the carbon layers due to the electrostatic induction. Pressing at a rate of 1.5 times per second, the fabricated device can generate enough power to drive micro- or nano- sensors.

That the system can be constructed with everyday items lends towards applications such as portable electronics and biomedical microsystems. Ultra low-cost sensors made of paper for various diagnostic purposes, which would be especially practical for developing countries, are already being tested.