Nanotechnology research leads to molecular-scale batteries and motors

Paul Boughton

Since October 2004, when European Design Engineer last covered the subject of nanotechnology, this field has made tremendous progress. Vast sums of money are being invested in nanotechnology developments here in Europe, and researchers in the USA are also extremely active.

While a great deal of work is being undertaken in universities, private companies are also running substantial research programmes. With so many different projects underway and the scope for potential applications being so wide today, an article such as this can only present a small cross-section of the types of development that are bearing fruit.

Back in February 2004, researchers at semiconductor manufacturer Infineon Technologies, in Munich, Germany, claimed they had succeeded, for the first time, in using carbon nanotubes to manufacture power semiconductors (Fig.1). The first nanotube switch could control light emitting diodes (LEDs) or electric motors, which was considered to be a breakthrough for nanotechnology, as scientists previously thought nanotubes were not suitable for the high voltages and currents used in power applications.

Nanotubes are tubular structures consisting of carbon atoms and have a diameter of one millionth of a millimetre. As with almost everything else on a nano scale, the electrical properties of carbon are very different from the properties associated with the material in the bulk state. An interesting potential application that takes advantage of the unusual electrical properties of carbon nanotubes has been the subject of research by a team at the Massachusetts Institute of Technology (MIT) Laboratory for Electromagnetic and Electronic Systems (LEES) in the USA. This work was presented at the 15th International Seminar on Double Layer Capacitors and Hybrid Energy Storage Devices in Deerfield Beach, USA, in December 2005.

Joel E Schindall, the Bernard Gordon Professor of Electrical Engineering and Computer Science (EECS) and associate director of the Laboratory for Electromagnetic and Electronic Systems is working with John G Kassakian, EECS professor and director of LEES and PhD candidate Riccardo Signorelli to use carbon nanotubes to improve the energy storage characteristics of ultracapacitors.

Energy storage

Capacitors store energy as an electrical field, making them more efficient than standard batteries that get their energy from chemical reactions. Ultracapacitors are capacitor-based storage cells that provide quick, large bursts of instant energy. Today they are utilised in a range of electronic devices, from computers to cars.

Ultracapacitors have been known about since the 1960s, but they are relatively expensive and have only recently been manufactured in sufficient quantities to become cost-competitive. Their main advantages are a lifetime of 10 years or more, indifference to temperature change, high immunity to shock and vibration and high charging and discharging efficiency.

However, traditional ultracapacitors need to be much larger than batteries to hold the same charge, so the LEES team has been seeking to increase the storage capacity of existing commercial ultracapacitors by storing electrical fields at the atomic level.

Physical constraints on the electrode surface area and spacing have previously limited ultracapacitors to an energy storage capacity around 25 times less than a similarly sized lithium-ion battery. The LEES ultracapacitor, however, is reported to have the ability to overcome this energy limitation by using vertically aligned, single-wall carbon nanotubes.

Storage capacity in an ultracapacitor is proportional to the surface area of the electrodes. Conventional ultracapacitors use electrodes made of activated carbon, which is extremely porous and therefore has a very large surface area. But the pores in the carbon are irregular in size and shape, which reduces efficiency. In contrast, the nanotubes in the LEES ultracapacitor have a regular shape and a size that is only several atomic diameters in width. This results in a greater effective surface area, which equates to improved storage capacity.

While this team has been investigating the use of nanotechnology to enhance the performance of conventional products, mPhase Technologies and Lucent Technologies Bell Labs have been working on a nano-sized ‘smart’ battery prototype to store and convert energy on demand. In a test conducted at Bell Labs’ facilities, the development team demonstrated that the first fully assembled prototype device could generate enough power on demand to light a light-emitting diode (LED). The prototype is based on a novel nanostructured architecture pioneered at Bell Labs.

The new generation of reserve power cells is based on a Bell Labs discovery that electrolyte will stay on top of a nano-textured surfaces until stimulated to flow, thereby triggering a reaction producing electricity. The ‘electrowetting’ process can therefore permit activation of the batteries when required, yielding a much longer shelf life than existing battery technologies.

We have already seen carbon nanotubes can transform the performance of ultracapacitors, but carbon nanotubes also have potential applications that make no direct use of their electrical properties. For example, a new study in the USA suggests that integrating nanotubes into traditional materials dramatically improves their ability to reduce vibration, especially at elevated temperatures. The findings could pave the way for a new class of materials with a multitude of applications, from high-performance parts for spacecraft and vehicle engines, to golf clubs that do not ‘sting’ on hitting a bad shot, and stereo speakers that do not buzz when bass levels are high (Fig.2).

The materials, developed by researchers at Rensselaer Polytechnic Institute, are described in the 8th February 2006 issue of the journal Nano Letters. According to Nikhil Koratkar, associate professor of mechanical, aerospace, and nuclear engineering at Rensselaer and lead author of the paper, “Traditional damping polymers perform poorly at elevated temperatures; our new materials provide excellent damping at high temperatures, suggesting that these nanocomposites show great potential for a variety of applications in aircraft, spacecraft, satellites, automobiles, and even sensors for missile systems – basically any structure that is exposed to vibration” (Fig.2).

Applications such as these can be thought of as the bulk utilisation of nanoscale materials. Another major area for nanotechnology research, however, is the manipulation of individual atoms.

Take, for example, the project being undertaken by a research team led by scientists at Penn State, Rice University, and the University of Oregon, all in the USA. They have developed a means for controlling single-molecule switches by engineering their design and surrounding environment. This is said to demonstrate that single-molecule switches can be tailored to respond in predictable and stable ways, depending on the direction of the electric field applied to them. The discovery, which some people see as an essential step in the emerging field of molecular electronics, could further the development of nanocomponents for use in future generations of computers and other electronic devices.

A paper describing the research results, titled Molecular Engineering of the Polarity and Interactions of Molecular Electronic Switches, was published in the Journal of the American Chemical Society on 21st December 2005.

Stiff, stringy molecules

The research is described as the latest achievement in the team’s ongoing studies of a family of stiff, stringy molecules known as OPEs (oligo phenylene-ethynylenes), which the scientists have tailored in a number of ways to have a variety of physical, chemical, and electronic characteristics. Previously the potential for using these OPE molecules as switches had been limited by their tendency to turn on and off at random, but Weiss and his colleagues have discovered a way to reduce this random switching. In their current research, the scientists demonstrated how and why it is possible to control these molecular switches (Fig.3).

To study the properties of individual OPE molecules, the scientists first inserted them into a hairbrush-like matrix of similarly shaped molecules, which Weiss describes as a “self-assembled amide-containing alkanethiol monolayer.” One end of each molecular ‘bristle’ is attached to the thin gold base of the microscopic hairbrush. With the individual OPE molecules surrounded by the matrix of alkanethiol molecules, all anchored in gold, Weiss and his team were able to study the properties of the OPE molecules with a powerful scanning tunnelling microscope (STM). The molecules were synthesised in a laboratory at Rice University and the matrix was synthesised in a laboratory at the University of Oregon.

The team synthesised a variety of OPE molecules, some with a large dipole and others with a weaker dipole. A proportion of the OPE molecules were designed to have a positive charge on the end facing away from the gold base while others were designed to have a negative charge at that end. Weiss’s laboratory found that the tip of the microscope pulled an OPE molecule up higher than the surrounding matrix – or ‘on’ – if the OPE molecule had a sufficiently strong dipole and if the charge of its exposed end was opposite that of the STM tip, making the two electrically attractive.

Moreover, the researchers also found that if the charge of the STM tip was the same as that on the end of an OPE, and therefore electrically repulsive, the molecule was pushed down – or ‘off’ – causing it to lean sideways into the matrix. They discovered that this position alters the molecule’s interaction with the system’s gold base, changing the system’s electrical conductance.”

Surface conductivity

Silicon or, to be more precise, doped silicon, has some unusual electrical properties, and this is what makes semiconductor materials so useful. But it transpires that silicon still has some surprises for researchers. When graduate student Pengpeng Zhang successfully imaged a piece of silicon just 10 nm in thickness, she and her University of Wisconsin-Madison (USA) co-researchers were puzzled. According to established thinking, the feat should have been impossible because her microscopy method required samples that conduct electricity.

“After she did it, we realised, ‘This silicon layer is really thin – it is much thinner than what people normally use,’” says UW-Madison physicist Mark Eriksson. “In fact, it is thin enough that it should be very hard to run a current through it. So we began asking, 'Why is this working?’”

A team led by College of Engineering professors Paul Evans, Irena Knezevic and Max Lagally and physics professor Eriksson has now answered that question. Writing in the 9th February 2006 issue of the journal Nature, they have shown that when the surface of nanoscale silicon is specially cleaned, the surface itself facilitates current flow in thin layers that ordinarily will not conduct. In fact, conductivity at the nanoscale is completely independent of the added impurities, or dopants, that usually control silicon’s electrical properties.

“What this tells us is that if you are building nanostructures, the surface is really important,” says Evans. “If you make silicon half as thick, you would expect it to conduct half as well. But it turns out that silicon conducts much worse than that if the surface is poorly prepared and much better than that if the surface is well prepared.”

The results also mean that the powerful concepts, methods and instruments of silicon electronics honed by scientists and the semiconductor industry over decades – many of which require conductive samples – can also be used to explore the nanoworld.

The team studied silicon-on-insulator substrates, in which a silicon wafer 0.5 mm thick is covered by a much thinner layer of insulating silicon oxide. Another silicon layer, in turn, is formed on the oxide layer. In the UW-Madison investigation, this uppermost layer was a ‘nanomembrane’ just 10 nm thick.

Silicon nanomembranes could one day become the platform for future high-speed electronics and a host of novel sensor technologies, says Lagally. But, like all silicon, they naturally develop another unwanted layer of oxide on top when exposed to air, resulting in an oxide-silicon-oxide structure. This top oxide layer is usually driven off by heating the material to more than 1200°C, but his causes nanomembranes to ball up.

What Zhang originally developed was a method to remove the top oxide without causing this damage. Under ultra-high vacuum, she slowly deposited several additional silicon or germanium layers, each just one atom thick, at 700°C. Scanning tunnelling microscopy revealed that this process somehow allowed the nanomembrane to conduct electricity.

To find out why, the team analysed the resistance of silicon layers ranging from to 200to15nm in thickness. More importantly, they compared silicon’s resistance when sandwiched between two oxide layers – the usual case – and when cleaned of the top oxide and exposed to vacuum through Zhang’s method. Knezevic then created a model predicting resistance as a function of layer thickness in both situations.

Knezevic’s model indicates that, in layers thinner than 100nm, the properties of silicon itself become irrelevant; what matters is the surface. Even in relatively thick layers of 200nm, silicon cleaned of the top oxide was at least 10 times more conductive than silicon sandwiched between oxide layers. And as layer thickness decreased, this difference eventually grew to six orders of magnitude. The team has proposed that cleaning promotes conductivity by creating new electronic states on the silicon surface where electrons can reside.

Sunlight-powered nano-motor

Electric current is usually what makes an electric motor turn, but chemists at Italy’s University of Bologna, UCLA and the California Nanosystems Institute (CNSI) have designed and constructed a molecular nano-scale motor that is powered only by sunlight. The research findings were published on 31st January 2006 in the Proceedings of the National Academy of Sciences (PNAS).

The nano motor can work continuously without any external interference, and it operates without consuming or generating chemical fuels or waste, according to Fraser Stoddart, UCLA’s Fred Kavli professor of Nanosystems Sciences and CNSI director.

Precisely how light-powered nano motors will be used in the future is not yet clear, but possible applications include nanoelectronics, molecular computers, and nano valves that perhaps could be used for the delivery of anti-cancer drugs and other medications.

The nano motor is a multi-component molecular-scale system called a rotaxane, a mechanically interlocked molecule consisting of one or more rings trapped on a rod by bulky stoppers at either end, in a manner reminiscent of an abacus. The system is built up from two separate molecular components: a dumbbell-shaped one, which is more than 6 nm long, and a ring component of a diameter of approximately 1.3nm. The ring component is trapped on the rod portion of the dumbbell by two bulky stoppers attached to the ends of the rod so that, although the ring can move along the rod, it cannot go over the stoppers at the ends. The rod portion of the dumbbell contains two ‘stations’ that can be called A and B.

“It is the attractiveness between the ring and stations A and B that assists us in making the molecules in the first place,” Stoddart explains. “The attractiveness for these two stations (A and B) lives on in the two-state or bistable rotaxane after it has been made. The final requirement in the design of the nanomotor is that the ring prefers, in the starting state of the molecule, to surround one of the two stations, let us say A. In order to induce the ring to move from A to B we have to make A temporarily a less desirable station such that the ring will spontaneously migrate to station B.

“The linear nanomotor works as follows: the absorption of sunlight by one of the two stoppers, a light-harvesting one, causes the transfer of one electron to station A, which is deactivated as far as wanting the ring to encircle it. As a consequence, the ring moves to its second port of call, station B. Station A is subsequently reactivated by the return of the transferred electron to the light-harvesting stopper, and the ring moves back to this station.

“The system operates according to a four-stroke cycle which is reminiscent of an internal combination engine in a motor car: (1) light excitation and subsequent transfers of an electron (‘combustion’), (2) displacement of the ring along the rod from A to B (‘piston displacement’), (3) removal of the electron received by station A (‘exhaust removal’, and (4) relocation of the piston (Fig. 4). The motions executed by the nano motor are quite rapid: a full cycle is carried out in less than one thousandth of a second, which means that the motor can operate at a frequency of 1000Hz – a speed that is equivalent, using the car engine analogy, to 60000rpm.”