Researchers solve riddle of what has been holding two unlikely materials together

Mar 11, 2013
Researchers have found that the gallium arsenide and bismuth telluride are actually separated by a thin layer of a hitherto unsuspected third material -- gallium telluride. The materials are held together by the weak forces of van der Waals bonds. Credit: James LeBeau, North Carolina State University

For years, researchers have developed thin films of bismuth telluride (Bi2Te3) – which converts heat into electricity or electricity to cooling – on top of gallium arsenide (GaAs) to create cooling devices for electronics. But while they knew it could be done, it was not clear how – because the atomic structures of those unlikely pair of materials do not appear to be compatible. Now researchers from North Carolina State University and RTI International have solved the mystery, opening the door to new research in the field.

"We've used state-of-the-art technology to solve a mystery that has been around for years," says Dr. James LeBeau, an assistant professor of at NC State and co-author of a paper on the research. "And now that we know what is going on, we can pursue research to fine-tune the interface of these materials to develop more efficient mechanisms for converting electricity to cooling or heat into electricity. Ultimately, this could have applications in a wide range of electronic devices."

To study the phenomenon, the researchers had to create the nanometer-scale thin films on a GaAs substrate, or foundation. The GaAs is first placed in a chamber. Molecules containing bismuth and tellurium are then introduced into the chamber, where they react with each other and "grow" into a crystalline Bi2Te3 structure on the surface of the GaAs.

For years, researchers have developed thin films of bismuth telluride on top of gallium arsenide. But while they knew it could be done, it was not clear how -- because the atomic structures of those unlikely pair of materials do not appear to be compatible. They now know that the two materials are separated by a layer of gallium telluride -- visible here as a row of bright dates at the interface of the materials. Credit: James LeBeau, North Carolina State University

Using advanced "Super-X" X-ray spectroscopy technology in conjunction with an aberration-corrected scanning , the researchers were able to determine what was binding the Bi2Te3 to the GaAs – and it was not what they were expecting.

They found that when the tellurium molecules were introduced to the vapor deposition chamber, the tellurium reacted with the GaAs substrate to create a new surface layer of gallium telluride, which was only one molecule thick. The Bi2Te3 then formed a thin film on top of that new surface layer.

Because gallium telluride does not react with Bi2Te3, the research team knew chemical bonding could not be holding them together. Instead, the two layers are held together by the weaker force of van der Waals bonds – meaning the materials are held together by weak electrical forces.

"While these materials have been investigated previously by RTI and NC State, the state-of-the-art techniques applied by LeBeau and his team have revealed significant new insights into how the film grows," notes Dr. Rama Venkatasubramanian of RTI International, who is also a co-author of the paper.

Explore further: Researchers develop scalable methods for manufacturing metamaterials

More information: The paper, "Atomic scale structure and chemistry of Bi2Te3/GaAs interfaces grown by metallorganic van der Waals epitaxy," is published online in Applied Physics Letters.

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VendicarE
1 / 5 (3) Mar 11, 2013
van der Waals bonds = Casimer force = Zero point energy.
antialias_physorg
3.4 / 5 (5) Mar 11, 2013
van der Waals bonds = Casimer force = Zero point energy.

Van der Waals forces are electrical forces.
Casimir forces are between UNCHARGED plates.

baudrunner
3.7 / 5 (3) Mar 11, 2013
I'm puzzled that the researchers couldn't predict this. The study of molecular bonding in chemistry, whether ionic or covalent or electrostatic, is not new. Gallium telluride is a compound made up of ionic-covalent bonded layers bonded together by van der Waals forces. The solution to the riddle seems to be pretty straightforward.
ValeriaT
1 / 5 (2) Mar 11, 2013
The solution to the riddle seems to be pretty straightforward.
Actually they "knew" about it. Such a complex experiments cannot be organized, if you have absolutely no clue, what you should expect. They postulated the working hypothesis and tried to probe it with experiment.

The question rather is, what would happen, if we would crush the powder of Bi2Te3 with GaAs in the mortar with pestle. Due the large surface area the mixture should contain huge amount of Bi2Te3, despite it's formation from these precursors is thermodynamically unfavorable. In chemistry we know about many cases of endothermic reactions initiated just with thorough milling of components. The increasing of physical surface increases the free energy of the reactants, which shifts the equilibrium toward endothermic direction in accordance to Le-Chatelier's principle.
ValeriaT
1 / 5 (2) Mar 11, 2013
errata: "should contain huge amount of Bi2Te3" should be " should contain huge amount of Ga2Te3" indeed
vacuum-mechanics
1 / 5 (2) Mar 11, 2013
Because gallium telluride does not react with Bi2Te3, the research team knew chemical bonding could not be holding them together. Instead, the two layers are held together by the weaker force of van der Waals bonds – meaning the materials are held together by weak electrical forces.

It seems that we are familiar with the explanation that electrical forces between electric charge i.e. different types of charge attract, while same type repel! But the problem is that no explanation how and why they (charges) could do that? Maybe this charge interaction mechanism could help us to understand it.
http://www.vacuum...21〈=en
Tausch
1 / 5 (1) Mar 12, 2013
The formation of thin intermediate layers between adjacent crystals is not so rare and it's called the Berry phase. - V


The Berry phase has classical physics applications too. The vibrating string. Instead of measuring the frequency you measure the phase of vibration. Making the measurement independent of mass and energy. Your hearing is phase sensitive.

I have difficulty making the jump to the quantum world where the Berry phase energetically aids substrate adhesion.

Still, I am able to answer vacumm-mechanics question:
How and why charges can do that is accomplished with Berry phase - which 'aligns' charges so they attract.

Quantum physics is not my 'platform'.

Tausch
1 / 5 (1) Mar 12, 2013
Here Michael Berry's original research:
http://www.phy.br...y178.pdf

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