Chemistry in one dimension offers surprising result

"Chemistry on the nanometer scale often appears to be different compared to chemistry in the normal scale and carbon nanotubes provide ideal conditions for studies of reactions in nanospace," says Alexandr Talyzin, docent at the Department of Physics, Umeå University.

The standard approch to make chemical recations inside of single walled carbon nanotubes, SWNTs, is to fill the inner space with molecules (e.g. fullerenes, thus forming so called peapods) and make them react with each other.

The nanotube walls will then protect the encapsulated molecules from outer space and make reactions with molecules and atoms outside the tube impossible. Once the SWNTs are filled with C60 molecules there is not enough space for hydrogen molecules to go in. That was the common opinion when the research groups started their experiments a few years ago.

But their experiments leave no doubt, hydrogen does actually penetrate into peapods and react with fullerenes. The evidence is rather direct, when the temperature and pressure of hydrogenation is taken to extreme values the fullerene cage collapses completely and large hydrogen molecules are formed. This was confirmed both by Raman spectroscopy and high resolution TEM.

The study provides one more example that chemical reactions in nanoreactors are not always the same as in “normal” conditions. In three-dimensional structure molecules can react with their neighbours in all possible directions, up, down, right, left etc.

"Inside of carbon nanotubes fullerene molecule have only two neighbours, lets say to the right and to the left. Similarly, the reaction with hydrogen is also limited to one-dimension," says Alexandr Talyzin.

A great advantage is that even single molecules inside of SWNTs can be observed using high resolution electron microscopy, something extremely difficult for bulk powders, he adds. High quality images collected at Aalto University allowed the scientists to observe not only hydrogen induced collapse of C60, but also hydrogen-driven coalescence of molecules into chain polymers and tubules.

"What we learned is a rather general result for nano-chemistry. Now we have direct evidence that molecules inside of SWNts can be reacted with gases. It opens enormous possibilities for synthesis of novel hybrid materials and chemical modification of encapsulated molecules and materials," says Alexandr Talyzin.

More information: “Hydrogen driven collapse of C60 inside of SWNTs” is published on line in Angewandte Chemie, http://onlinelibra … 946/abstract

Provided by Umea University

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New method offers control of strain on graphene membranes

A group of physicists from the University of Arkansas and other institutions have developed a technique that allows them to control the mechanical property, or strain, on freestanding graphene, sheets of carbon one-atom thick suspended over the tops of tiny squares of copper. By controlling the strain on freestanding graphene, they also can control other properties of this important material.

“If you subject graphene to strain, you change its electronic properties,” said physics professor Salvador Barraza-Lopez. Strain on freestanding graphene causes the material to behave as if it is in a magnetic field, even though no magnets are present, a property that scientists will want to exploit -- if they can control the mechanical strain.

To control the mechanical strain, University of Arkansas researchers developed a new experimental approach. Physicists Peng Xu, Paul Thibado and students in Thibado’s group examined freestanding graphene membranes stretched over thin square “crucibles,” or meshes, of copper. They performed scanning tunneling microscopy with a constant current to study the surface of the graphene membranes. This type of microscopy uses a small electron beam to create a contour map of the surface. To keep the current constant, researchers change the voltage as the scanning tunneling microscope tip moves up and down, and the researchers found that this causes the freestanding graphene membrane to change shape.

“The membrane is trying to touch the tip,” Barraza-Lopez said. They discovered that the electric charge between the tip and the membrane influences the position and shape of the membrane. So by changing the tip voltage, the scientists controlled the strain on the membrane. This control becomes important for controlling the pseudo-magnetic properties of graphene.

In conjunction with the experiments, Barraza-Lopez, Yurong Yang of the University of Arkansas and Nanjing University, and Laurent Bellaiche of the University of Arkansas examined theoretical systems involving graphene membranes to better understand this new-found ability to control the strain created by the new technique. They verified the amount of strain on these theoretical systems and simulated the location of the scanning tunneling microscopy tip in relation to the membrane. While doing so, they discovered that the interaction of the membrane and tip depends upon the tip’s location on the freestanding graphene. This allows scientists to calculate the pseudo-magnetic field for a given voltage and strain.

“If you know the strain, you can use theory and compute how big the pseudo-magnetic field may be,” said Barraza-Lopez. They found that because of the boundaries created by the square copper crucible, the pseudo-magnetic field swings back and forth between positive and negative values, so scientists are reporting the maximum value for the field instead of a constant value.

“If you were able to make the crucibles triangular, you would be closer to having non-oscillating fields,” Barraza-Lopez said. “This would bring us closer to using this pseudo-magnetic property of graphene membranes in a controlled way.”

The researchers report their findings in Physical Review B Rapid Communications.

More information: PRB 85, 121406(R) (2012)

Provided by University of Arkansas (news : web)

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