A micron-scale optical microgram showing a characteristically triangular molybdenum disulphide ultrathin film grown in York. Following the discovery of graphene, an ultra-thin wonder material made of a carbon sheet of only one atom thickness, a number of other ultra-thin membranes have become the focus of study by nanotechnologists. These ultra-thin materials can be used not only to study physics in ‘flat land’ but also can be used as building blocks to produce ultra-thin or artificially stacked and flexible electronic devices.
Using sophisticated high-resolution electron microscopy, the researchers, who included scientists from Zhejiang University in Hangzhou, Beijing University, Reming University and Chinese Academy of Science in Beijing, China and King Abdullah University of Science and Technology in Saudi Arabia, have scanned these two-dimensional sheets for defects with resolution down to the atomic scale.
They have discovered that atomically thin molybdenum disulfide (MoS2) sheets have different ‘personalities’ or dominant defects depending how they are produced. If the atomically thin sheet is cleaved from minerals or grown by chemical reaction, then the dominant defects are loss of sulphur atoms from the crystalline structure. On the other hand, if the atomically thin sheet is grown by direct evaporation of bulk MoS2, then the dominant defect is the so-called anti-site type with molybdenum atoms replacing sulphur atoms in the crystal.
An electron interferometer: Pairing of electrons takes place in the path denoted by the broken red line. So it came as a great surprise to discover electrons pairing up under certain conditions in their quantum Hall system – forming pairs that were remarkably similar to Cooper pairs. This is, indeed, the first time that this phenomenon has been observed outside of superconductivity, and the scientists are still not quite sure what to make of it. Once the electrons are pulled from their path by the magnetic field and forced to flow near the edges of the quantum Hall system, they travel in “parallel lanes” at varying distances from the edge. The scientists are now wondering if the close proximity of electrons moving in those parallel lanes could somehow cause electrons to “feel” one another more strongly and, consequently, interact in a different manner than the ubiquitous repulsion. The phenomenon was observed at the exit to the system. Electrons leaving the outer lane were measured; the surprise came when the exiting charges were found to be twice that of a normal, single electron. In other words, the current was carried by paired electrons, similar to that of Cooper pairs that flow so freely in the superconducting state. Although this phenomenon was completely unexpected and is still not understood, the question asked by the prophet Amos, with his insistence on rational cause and effect, resonates with the scientists: Why do these pairs of electrons “walk together,” apparently in total “agreement”? What causes the electrons in this system to form pairs? Or conversely, what is the effect of electron pairing on the functioning of the system? The Weizmann Institute scientists are already conducting new experiments to help sort out the riddle of the quantum Hall electron pairs.
One-pot selective monofunctionalization of CPP via a chromium complex. The study, published online on January 12, 2015 in the ("η
Obtaining substituted CPPs – traditional method and one-pot sequence (this work). (click on image to enlarge) Upon finding that a monometallic CPP complex could be obtained, Itami’s team explored the possibility of obtaining monofunctionalized CPPs from this complex. Itami and Segawa describe the steps in achieving this. “This was not an easy task as chromium arene complexes are usually air and light sensitive, and CPP chromium complexes were no exception. But Natsumi worked persistently to obtain a pure crystal of the first CPP chromium complex,” says Itami. “We then performed the subsequent reactions in one-pot, to synthesize monofunctionalized CPPs after addition of base/electrophiles and removal of the metal from the CPP chromium complex,” says Segawa. Selective monofunctionalizations of CPPs i.e. installation of one functional group at a single position on the arene ring, are difficult to achieve as all carbon-hydrogen bonds on the arene rings are chemically equivalent. Direct functionalization of metal-free CPPs usually leads to multiple substitutions on the arene rings in an uncontrolled manner. Despite CPPs being desirable components for carbon nanotubes, there has been no efficient method to obtain directly functionalized CPPs up to now. “We were pleased to see that a functional group could be selectively installed on one arene ring via chromium coordination of CPPs,” says Segawa. “As electrophiles, we utilized silyl, boryl and ester groups, which act as handles that can be easily transformed to other useful functionalities,” he continues. Itami says, “We hope that this new approach evolves to become a valuable method to construct carbon nanotubes with unique structures and properties.”