Elementary Physics in a Single Molecule

The molecule of about 2 nm in size is kept stable between two metal electrodes for several days. (Figure: Christian Grupe/KIT)

The molecule of about 2 nm in size is kept stable between two metal electrodes for several days. (Figure: Christian Grupe/KIT)

A team of physicists has succeeded in performing an extraordinary experiment: They demonstrated how magnetism that generally manifests itself by a force between two magnetized objects acts within a single molecule. This discovery is of high significance to fundamental research and provides scientists with a new tool to better understand magnetism as an elementary phenomenon of physics. The researchers published their results in the latest issue of Nature Nanotechnology (doi: 10.1038/nnano.2013.133).

The smallest unit of a magnet is the magnetic moment of a single atom or ion. If two of these magnetic moments are coupled, two options result: Either the magnetic moments add up to a stronger moment or they compensate each other and magnetism disappears. From the quantum physics point of view, this is referred to as a triplet or singlet. A team of researchers around Professor Mario Ruben from Karlsruhe Institute of Technology and Professor Heiko B. Weber from the Friedrich-Alexander-Universität Erlangen-Nürnberg now wanted to find out whether the magnetism of a pair of magnetic moments can be measured electrically in a single molecule.

For this purpose, the team headed by Mario Ruben used a customized molecule of two cobalt ions for the experiment. At Erlangen, Heiko B. Weber and his team studied the molecule in a so-called single-molecule junction. This means that two metal electrodes are arranged very closely to each other, such that the molecule of about 2 nm in length is kept stable between these electrodes for many days, while current through the junction can be measured. This experimental setup was then exposed to various, down to very deep, temperatures.

The scientists found that magnetism can be measured in this way. The magnetic state in the molecule became visible as Kondo anomaly. This is an effect that makes electric resistance shrink towards deep temperatures. It occurs only when magnetism is active and, hence, may be used as evidence. At the same time, the researchers succeeded in switching this Kondo effect on and off via the applied voltage. A precise theoretical analysis by the group of Assistant Professor Karin Fink from Karlsruhe Institute of Technology determines the various complex quantum states of the cobalt ion pair in more detail. Hence, the researchers succeeded in reproducing elementary physics in a single molecule…..

Read more at http://www.kit.edu/visit/pi_2013_13701.php

Redefining the ampere with the help of graphene?

Electron pumps made from graphene work 10 times faster than similar pumps made from conventional 3D materials and can be used to generate larger currents. (Courtesy: M Connolly)

Electron pumps made from graphene work 10 times faster than similar pumps made from conventional 3D materials and can be used to generate larger currents. (Courtesy: M Connolly)

The world’s first single-electron graphene pump has been built by researchers at the UK National Physical Laboratory and the Cavendish Laboratory in Cambridge. The device could be used to redefine the standard unit of current, the ampere, in terms of the electron charge – a fundamental constant of nature.
The international system of units (SI) is made up of seven base units, which are the metre, kilogram, second, kelvin, ampere, mole and candela. The ampere, volt and ohm are the three fundamental units of electricity.
Although physicists have already come up with modern ways to represent the volt and ohm (through measurements of the Josephson voltage and quantum Hall resistance, respectively), there is no equivalent for the ampere. Indeed, today, the ampere is defined as the current which, when flowing through two parallel conductors one metre apart, exerts a certain force between the conductors. Directly realizing such a macroscopic definition of current is experimentally difficult, and the accuracy of the result also depends on other base units, such as the kilogram, which drifts with time.

Enter SEPs

Ideally, a new definition of the ampere would be based on an extremely accurate source of electric current, capable of delivering one electron at a time. A single-electron pump (SEP) could be ideal in this respect because it produces a flow of individual electrons by shuttling them into a quantum dot and emitting them precisely one at a time. A good SEP also pumps the electrons quickly, so a sufficiently large current is generated.
Until recently, two types of SEP were promising contenders: tunable barrier pumps made from semiconductors, which are fast, and so-called hybrid turnstiles made from superconductors, which can be mounted in parallel to make the output current larger. Although the most accurate, a third type of pump usually made from metallic islands is too slow for making a practical current standard, but the UK researchers have now improved its performance by making it from graphene, which is a semi-metal. Graphene is a sheet of carbon just one atom thick that has a honeycomb lattice structure….
…Read more at http://physicsworld.com/cws/article/news/2013/may/28/redefining-the-ampere-with-the-help-of-graphene

Nanogenerator’s output triples previous record

(Top) The nanogenerator produces a voltage under a periodic mechanical deformation. In the deformed nanogenerator, the red and blue regions indicate a positive and negative piezoelectric potential, respectively. (Bottom) Optical photographs of the nanowire array showing its flexibility and robustness. Credit: Long Gu, et al. ©2012 American Chemical Society

(Top) The nanogenerator produces a voltage under a periodic mechanical deformation. In the deformed nanogenerator, the red and blue regions indicate a positive and negative piezoelectric potential, respectively. (Bottom) Optical photographs of the nanowire array showing its flexibility and robustness. Credit: Long Gu, et al. ©2012 American Chemical Society

aking an important step forward for self-powered systems, researchers have built a nanogenerator with an ultrahigh output voltage of 209 V, which is 3.6 times higher than the previous record of 58 V. The nanogenerator, which has an area of less than 1 cm2, can instantly power a commercial LED and could have a wide variety of applications, such as providing a way to power objects in the “Internet of Things.”
Read more at: phys.org and http://www.nanoscience.gatech.edu/paper/2012/12_NL_15.pdf

Carbon nanotubes fit by the thousands onto a chip

Carbon nanotubes’ electronic properties have long been lauded but still have not made it into chips

Scientists have demonstrated methods that could see higher-performance computer chips made from tiny straws of carbon called nanotubes.

Carbon nanotubes have long been known to have electronic properties superior to current silicon-based devices.

But difficulties in manipulating them have hampered nanotube-based chips.

The experiments, reported in Nature Nanotechnology, show a kind of two-part epoxy approach to individually place the nanotubes at high density.

The race is on in the semiconductor chip industry to replace current silicon technology – methods to make smaller and therefore faster devices will soon come up against physical limits on just how small a silicon device can be.

Study co-author James Hannon, a materials scientist at IBM, said that there are few realistic successors to silicon’s throne.

“The problem is you have to put it in to production on a 10- or 15-year time scale, so the kinks have to be worked out in the next few years,” he said.

“If you look at all the possibilities out there, there are very few that have actually produced an electronic device that would outperform silicon – there are exotic things out there but they’re all still at the ‘PowerPoint stage’.”

Though single nanotubes have shown vastly superior speed and energy characteristics in lab demonstrations, the challenge has been in so-called integration – getting billions of them placed onto a chip with the precision the industry now demands…..

Read more at: www.bbc.co.uk

Single molecule nanocar takes its first spin

THE tiniest car in the world has gone for a drive. Made of a single molecule, the “vehicle” has four wheel-like paddles that rotate in the same direction when zapped with a beam of electrons.

“The molecule is autonomous,” says Syuzanna Harutyunyan, a chemist at the University of Groningen in the Netherlands who worked on the mini motor vehicle. “You don’t need to touch it. Just give it energy and it’s capable of converting that energy into movement.”

The nanocar could be used to transport miniature loads of cargo and to help unravel why tiny motors in nature tend to be so much more efficient than large-scale ones.

To create the nanocar, Harutyunyan and her team designed a molecule with a long central body and one pivoted paddle at each of four corners. The paddles are free to swing around in circles, not unlike wheels.

Ordinarily they arrange themselves so as to minimise crowding with the central body, as this costs the molecule the least amount of energy. But when the team applies a pulse of electrons to the “wheels”, some gain energy and move a quarter turn.

In this new position, the wheels experience overcrowding against the body of the nanocar and will move to a more spacious position as soon as possible. They get this opportunity when the bonds holding the wheels to the body stretch, prompting the wheels to move another quarter turn in the same direction, to a more “comfortable” position. A further pulse of electrons repeats the process (see diagram).

Frigid temperatures of 7 kelvin (-266 °C) help this clunky forward motion by effectively freezing the wheels in place except when excited by the electron pulse and during their subsequent self-adjustment. This keeps them from rolling backwards.

nanocar had been built before but its wheels only spun in place, equivalent to placing a car on blocks to test it. By contrast, the new vehicle moves in a straight line (NatureDOI: 10.1038/nature10587).

It’s a slow road trip: it takes 10 pulses of electric fuel to move the vehicle 6 nanometres. The head of a pin is about a million nanometres wide.

Nonetheless, nanocar team member Karl-Heinz Ernst at the University of Zurich, Switzerland, is anxious to put it to work. “We have a locomotive,” he says, “but it’s time to put some cars at the back and pull them along.”

Paul Weiss at the University of California, Los Angeles, says the car can help us unravel why tiny motors in nature, such as the motor proteins that move material around in cells, are so highly efficient. “The reason we work at these small scales is so that we can really understand the motion and efficient energy conversion.” He hopes this will lead to more efficient large-scale motors.


http://youtu.be/lASdcW-BtiU

http://www.newscientist.com

World’s smallest electric motor made from a single molecule

Chemists at Tufts University have developed the world’s first single molecule electric motor, which may potentially create a new class of devices that could be used in applications ranging from medicine to engineering. The molecular motor was powered by electricity from a state of the art, low-temperature scanning tunneling microscope. This microscope sent an electrical current through the molecule, directing the molecule to rotate in one direction or another. The molecule had a sulfur base (yellow); when placed on a conductive slab of copper (orange), it became anchored to the surface. The sulfur-containing molecule had carbon and hydrogen atoms radiating off to form what looks like two arms (gray); these carbon chains were free to rotate around the central sulfur-copper bond. The researchers found that reducing the temperature of the molecule to five Kelvin (K), or about minus 450 degrees Fahrenheit (ºF), enabled them to precisely impact the direction and rotational speed of the molecular motor The Tufts team plans to submit this miniature electric motor to the Guinness World Records. The research was published online Sept. 4 in Nature Nanotechnology. Credit: Heather L. Tierney, Colin J. Murphy, April D. Jewell, Ashleigh E. Baber, Erin V. Iski, Harout Y. Khodaverdian, Allister F. McGuire, Nikolai Klebanov and E. Charles H. Sykes.

Chemists at Tufts University’s School of Arts and Sciences have developed the world’s first single molecule electric motor, a development that may potentially create a new class of devices that could be used in applications ranging from medicine to engineering.
In research published online September 4 in Nature Nanotechnology, the Tufts team reports an electric motor that measures a mere 1 nanometer across, groundbreaking work considering that the current world record is a 200 nanometer motor. A single strand of human hair is about 60,000 nanometers wide…… Continue reading World’s smallest electric motor made from a single molecule