Tag Archives: lattice

In square ice (left) water molecules are locked at a right angle. This looks nothing like the familiar hexagonal ice (right).

Sandwiching water between graphene makes square ice crystals at room temperature

In a most unexpected find, the same  University of Manchester team that isolated graphene for the first time in 2003 found that water flattens into square crystals – a never encountered lattice configuration – when squeezed between two layers of graphene. The square ice qualifies as a new crystalline phase of ice, joining 17 others previously discovered. The finding could potentially improve  filtration, distillation and desalination processes.

Water, don’t be square

In square ice (left) water molecules are locked at a right angle. This looks nothing like the familiar hexagonal ice (right).

In square ice (left) water molecules are locked at a right angle. This looks nothing like the familiar hexagonal ice (right).

Previously, Andre Geim of the University of Manchester, UK – who shared a Nobel Prize in physics in 2010 for his groundbreaking graphene research – was left scratching his head after he found water vapours could pass through laminated sheets of graphene oxide. This was peculiar since helium couldn’t do this, a molecule that’s a lot smaller than water. To complicate the puzzle, liquid water – which is more tightly bonded than vapor – could also pass through the graphene oxide.

Then, simulations showed that water was forming square ice crystals between the graphene sheets. “But you never trust molecular-dynamics simulations,” says Geim. The team thus proceeded with a simple experiment. They dropped just one milliliter of water on a sheet of graphene (an one atom thick layer carbon arranged in a hexagon lattice), then placed a second one on top. As the water slowly evaporated, it was reduced to an one atom thick layer (just like the graphene!), all arranged in a square lattice at room temperature.

This electron scan microscope image clearly shows how the square ice looks like. Image: NATURE

This electron scan microscope image clearly shows how the square ice looks like. Image: NATURE

In normal conditions (temperature and pressure), the water molecule has a V shape, with the two hydrogen atoms bonded to the oxygen atom at a 105° angle. Imagine Mickey Mouse, that’s water! In ice form,  four bonds are usually arranged in a tetrahedral (pyramid) shape. In the square ice, however, all the atoms line up with a right angle between each oxygen–hydrogen bond.

After several iterations of the experiment, Geim’s team ended up with one, two or three atom thick layers of square ice crystals, all aligned one atop another. Remember, I mentioned the water molecules were squeezed by the graphene. In fact, the pressure exerted by the two layers could be more than 10,000 times that of  atmospheric pressure, according to the paper published in Nature. This happens because as the graphene sheets get closer, they distort each others’ electron cloud. The sheets are attracted to one another by a huge intermolecular force known as the van der Waals force, like “having millions of little springs holding them together,” according to Alan Soper, a physicist at the Rutherford Appleton Laboratory in Harwell, UK.

This might not be some queer finding confined to a laboratory setting. Square ice might be encountered in nature where enormous pressure is exerted over tight quarters. It just may be that we haven’t found it yet. On a practical level, the square ice method might  improve desalination filters based on graphene.

“Finding out how the water behaves in a capillary is a big part of what we need to do to make a good filter,” says Geim. “This is a very important step.”

 

In semiconductors like silicon, electrons attached to atoms in the crystal lattice can be mobilized into the conduction band by light or voltage. Berkeley scientists have taken snapshots of this very brief band-gap jump and timed it at 450 attoseconds. Stephen Leone image.

For the first time, physicists measure electron as it jumps from semiconductor. Yes, it’s a big deal!

All our modern electronics are based on a class of wonder materials called semiconductors. What makes these so valuable is their ability to free electrons when subjected to an electrical current or when hit by light, becoming mobile and eventually routed and switch through a transistor. It’s the very basis of our digital age, be it solar cells or computers. Now, researchers at UC Berkeley have taken a real-time snapshot of electrons being stripped from silicon’s valence shell for the very first time.

A brief jump

In semiconductors like silicon, electrons attached to atoms in the crystal lattice can be mobilized into the conduction band by light or voltage. Berkeley scientists have taken snapshots of this very brief band-gap jump and timed it at 450 attoseconds. Stephen Leone image.

In semiconductors like silicon, electrons attached to atoms in the crystal lattice can be mobilized into the conduction band by light or voltage. Berkeley scientists have taken snapshots of this very brief band-gap jump and timed it at 450 attoseconds. Image: Stephen Leone.

This jump happens so fast that extremely fast lasers,  femtosecond lasers, are unable to measure it. This time, scientists turned to a type of laser that sends pulses of light even faster – attosecond pulses of soft X-ray light lasting only a few billionths of a billionth of a second. Experiments show that the time it takes from an electron to transit from the atom’s valence shell, across the band-gap, and into the conduction region is 450 attoseconds or 450 quintillionths of a second.

“Though this excitation step is too fast for traditional experiments, our novel technique allowed us to record individual snapshots that can be composed into a ‘movie’ revealing the timing sequence of the process,” explains Stephen Leone, UC Berkeley professor of chemistry and physics.

In the experiment published in Science, Leone and colleagues zapped a silicon crystal with ultrashort pulses of visible light using a laser. Immediately after the laser was fired, a subsequent X-ray beam was directed which lasted only a few tens of attoseconds (10-18 seconds) to take snapshots of the evolution of the excitation process triggered by the laser pulses. The experimental data was then interpreted by a supercomputer simulation at the  at the University of Tsukuba and the Molecular Foundry. Not only did the simulation model the excitation of the electrons, but also  the subsequent interaction of X-ray pulses with the silicon crystal.

Physics has identified two distinct states that occur when a semiconducting atom is “activated”. First, the electron absorbs energy and jumps to a higher state where it’s free to roam – it gets excited. Then, the lattice made up of the individual atoms arranged in an orderly manner to form the crystal rearranges itself in response to the redistribution of electrons. In this second stage, part of the energy used to excite the electron is transformed into heat carried by vibrational waves called phonons.

The present experiment confirms this once more, while offering a more refined look of what happens inside. The experiments show that initially, only electrons react to the energy from the laser. Then, after the laser has stopped firing or  60 femtoseconds later, they observed the onset of a collective movement of the atoms, that is, phonons. The researchers estimate the lattice spacing rebounded about 6 picometers (10-12meters) as a result of the electron jump, consistent with other estimates.

“These results represent a clean example of attosecond science applied to a complex and fundamentally important system,” Neumark says.

 

Defect in graphene opens up even more possibilities

Graphene is probably the ‘substance of the century’, and it will probably be for us what plastics were in the 1900s. Now, a flower-like defect in the material that can occur during the fabrication process could have a significant effect on graphene’s already impressive mechanical, magnetic, and electrical properties.

Amazing graphene

Graphene is practically a one atom thick layer of carbon atoms, densely packed in a hexagonal (honey comb) lattice. The carbon-carbon bond length in graphene is about 0.142 nanometers. It is already known that it has unbelievable strength and conductivity, both of which are a result of its structure.

Graphene differs from conventional 3D materials in that it is a semi-metal or zero-gap semiconductor. It has a remarkable electron mobility at room temperature and it has been showed that electrical current going through it can magnetize it.

Seven deffects

A team of researchers from the National Institute of Standards and Technology (NIST) and Georgia Tech described for the first time a class of seven defects that can occur during its fabrication. Basically, these defects appear as a result of the movement of the carbon atoms at high temperatures when producing graphene by heating silicon carbide under ultrahigh vacuum. The rearrangements which require the least energy from graphene are switching from six-member carbon rings to rings containing five or seven atoms, which keeps all the carbon atoms happy with no unsatisfied bonds. However, these changes create a new type of defect or grain boundary loop in the honeycomb lattice. According to researchers, the fabrication process plays a huge role in this structural malfunction.

“As the graphene forms under high heat, sections of the lattice can come loose and rotate,” says NIST researcher Eric Cockayne . “As the graphene cools, these rotated sections link back up with the lattice, but in an irregular way. It’s almost as if patches of the graphene were cut out with scissors, turned clockwise, and made to fit back into the same place, only it really doesn’t fit, which is why we get these flowers.”

The incredibly lattice is already stronger than steel, but it is also extremely rigit; these (technically speaking) defects might do it a world of good, giving it much needed flexibility, thus making it even more resistant to fractures or tears. Further research will provide some new insight as to how these flower-like structures can be eliminated or created at will, depending on the needs. Furthermore, these seven new structures would each have different not only mechanical, but also electrical and magnetic properties. All hail graphene !