Understanding the mechanism for generating electric current without energy consumption at room temperature


Hiroshima U id42209A group of researchers in Japan and China identified the requirements for the development of new types of extremely low power consumption electric devices by studying Cr-doped (Sb,Bi)2Te3 thin films. This study has been reported in Nature Communications (“Carrier-mediated ferromagnetism in the magnetic topological insulator Cr-doped (Sb,Bi)2Te3).
At extremely low temperatures, an electric current flows around the edge of the film without energy loss, and under no external magnetic field. This attractive phenomenon is due to the material’s ferromagnetic properties; however, so far, it has been unclear how the material gains this property. For the first time, researchers have revealed the mechanism by which this occurs. “Hopefully, this achievement will lead to the creation of novel materials that operate at room temperature in the future,” said Akio Kimura, a professor at Hiroshima University and a member of the research group.
Ferromagnetism mediated by Sb or Te atoms
Ferromagnetism mediated by Sb or Te atoms. (Image: Hiroshima University)
Their achievement can be traced back to the discovery of the quantum Hall effect in the 1980’s, where an electric current flows along an edge (or interface) without energy loss. However, this requires both a large external magnetic field and an extremely low temperature. This is why practical applications have not been possible. Researchers believed that this problem could be overcome with new materials called topological insulators that have ferromagnetic properties such as those found in Cr-doped (Sb,Bi)2Te3.
A topological insulator, predicted in 2005 and first observed in 2007, is neither a metal nor an insulator, and has exotic properties. For example, an electric current is generated only at the surface or the edge of the material, while no electric current is generated inside it. It looks as if only the surface or the edge of the material has metallic properties, while on the inside it is an insulator.
At extremely low temperatures, a thin film made of Cr-doped (Sb,Bi)2Te3 shows a peculiar phenomenon. As the film itself is ferromagnetic, an electric current is spontaneously generated without an external magnetic field and electric current flows only around the edge of the film without energy loss. However, it was previously unknown as to why Cr-doped (Sb,Bi)2Te3 had such ferromagnetic properties that allowed it to generate electric current.
“That’s why we selected the material as the object of our study,” said Professor Kimura.
Because Cr is a magnetic element, a Cr atom is equivalent to an atomic-sized magnet. The N-S orientations of such atomic-sized magnets tend to be aligned in parallel by the interactions between the Cr atoms. When the N-S orientations of Cr atoms in Cr-doped (Sb,Bi)2Te3 are aligned in parallel, the material exhibits ferromagnetism. However, the interatomic distances between the Cr atoms in the material are, in fact, too long to interact sufficiently to make the material ferromagnetic.
The group found that the non-magnetic element atoms, such as the Sb and Te atoms, mediate the magnetic interactions between Cr atoms and serve as the glue to fix the N-S orientations of Cr atoms that face one direction. In addition, the group expects that its finding will provide a way to increase the critical temperature for relevant device applications.
The experiments for this research were mainly conducted at SPring-8. “We would not have achieved perfect results without the facilities and the staff there. They devoted themselves to detecting the extremely subtle magnetism that the atoms of non-magnetic elements exhibit with extremely high precision. I greatly appreciate their efforts,” Kimura said.
Source: Hiroshima University

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Organic semiconductors open path for flexible electronics


News & Trends • Posted:16 October 2013

mix-id328072.jpgScientists from the Emergent Molecular Function Research Group at the RIKEN Centre for Emergent Matter Science have developed a synthetic procedure that according to them makes it easier to tailor the chemical structure of an important organic semiconductor.

 

Organic semiconductors made from small aromatic molecules can be dissolved and screen-printed onto many substrates, including plastics, opening the path for ‘flexible’ electronic devices such as low-cost polymer solar cells.

Kazuo Takimiya and colleagues, in collaboration with researchers from Hiroshima University, have now developed a synthetic procedure that makes it easier to tailor the chemical structure of an important organic semiconductor.

Kazuo Takimiya and his team, in collaboration with researchers from Hiroshima University, were studying molecules known as diacene-fused thienothiophenes when they discovered their new synthetic procedure. Diacene-fused thienothiophenes are composed of interlocking benzene and sulphur-containing aromatic rings and are more durable, and have higher charge carrier mobilities, than most other organic semiconductors. Although current schemes to make these compounds are relatively straightforward, they are also difficult to modify. Thus, chemists have a hard time producing derivatives based on this ring system with more desirable properties.

The researchers devised a creative synthesis that, instead of relying on bulky aromatic precursors, generates diacene-fused thienothiophenes from small molecules through two consecutive ring-forming reactions. First, they generated an active reagent called phenylsulfenyl chloride that joins to a benzene–acetylene molecule and transforms it into a three ring system. Then, they used selective carbon–hydrogen bond activation to set off a rare intramolecular coupling that produces a molecule with four fused rings known as benzothieno-benzothiophene (BTBT). Takimiya explained that this approach produces excellent yields and makes it possible to scrutinise numerous BTBT derivatives by making simple changes to the starting reagents.

Trials revealed that this technique was particularly useful for extending the ring structure of BTBT-type molecules. For example, by substituting double- and triple-fused benzene molecules into the synthetic procedure, the team linearly constructed the BTBT substructure to form five, six and seven aromatic rings. Intriguingly, these new derivatives have an asymmetric structure that may dramatically improve their solubility—an important processing feature for printed electronics and one that is difficult to achieve using existing synthetic techniques.

Lengthening the BTBT framework to an eight-ringed symmetric structure also yielded a potent new organic semiconductor with excellent thermal stability and a charge carrier mobility five times higher than that of BTBT. “This mobility is among the highest recorded for thin film organic field-effect transistors, meaning that this molecule could be a candidate for real flexible electronics applications in the future,” stated Takimiya.