Uncovering the personality of wonder ultrathin materials

micron-scale optical microgram showing a characteristically triangular molybdenum disulphide ultrathin film

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.


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Unlikely pairs

In the rational world of science, everything can be explained by cause and effect. Sometimes when the cause is known, the ensuing effects can be predicted. But more often scientists try to measure something that “should be there”, or else they discover new phenomena that have no apparent reason or cause. Indeed, most scientific research arises from these two starting points, working from opposite directions to connect cause with effect. Prof. Moty Heiblum of the Condensed Matter Physics Department and his research group recently conducted an experiment ("Robust Electron Pairing in the Integer Quantum Hall Effect Regime") looking for an effect that “should be there” and ended up with an unexplained phenomenon. Working in the Braun Center for Submicron Research, the group was experimenting with a phenomenon known as the quantum Hall effect. In this system, electrons flow in a two-dimensional plane and are exposed to a strong magnetic field perpendicular to the plane. The electrons, which “prefer” to run in straight lines, get pulled from their original paths by the magnetic field and end up traveling around the edges of the plane. But what Heiblum and his group observed in the electron flow seemed to belong to a different type of system: superconductivity. Electrons, which all carry negative charges, normally repel one another. However, under very special conditions, in some materials and at extremely low temperatures, electrons can actually “hook up” to form pairs called Cooper pairs. Cooper pairs can move through a material with no resistance whatsoever, and this state is thus known as superconductivity. An electron interferometer: Pairing of electrons takes place in the path denoted by the broken red line 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.
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Chromium-centered cycloparaphenylene rings as new tools for making functionalized nanocarbons

Professor Kenichiro Itami, Yasutomo Segawa and Natsumi Kubota of the JST-ERATO Itami Molecular Nanocarbon Project and the Institute of Transformative Bio-Molecules (ITbM), Nagoya University have synthesized novel cycloparaphenylene (CPP) chromium complexes and demonstrated their utility in obtaining monofunctionalized CPPs, which could become useful precursors for making carbon nanotubes with unprecedented structures. CPPs consist of a chain of benzene rings and are the shortest segment of carbon nanotubes. Since their first synthesis and isolation in 2008, CPPs have attracted wide attention in the fields of materials science and supramolecular chemistry. Applying the basic concepts of chromium arene chemistry, Itami and his coworkers have performed the first selective installation of a functional group on CPP, which has previously been difficult due to multiple reactive arene sites on the CPP ring. By being able to selectively install and tune the functional groups on CPPs, it is envisaged that carbon nanotubes with new properties can be constructed by this method. One-pot selective monofunctionalization of CPP via a chromium complex One-pot selective monofunctionalization of CPP via a chromium complex. The study, published online on January 12, 2015 in the ("η6-Cycloparaphenylene Transition Metal Complexes: Synthesis, Structure, Photophysical Properties, and Application to the Selective Monofunctionalization of Cycloparaphenylenes"), illustrates the first synthesis, isolation and analysis of a CPP chromium complex, which enables a one-pot access to monofunctionalized CPPs. This outcome is believed to be a significant advance in the fields of both CPP chemistry and organometallic chemistry. Arenes are known to coordinate to transition metals and the corresponding metal complexes exhibit different reactivities relative to the free arene. CPPs, which consist of a chain of arenes, also reacted with chromium carbonyl to successfully generate the first chromium complex of CPP. Interestingly, the main product was a CPP with one chromium moiety complexed to one arene on the outer side of the ring, as confirmed by 1H NMR (nuclear magnetic resonance) spectroscopy, high-resolution mass spectrometry and X-ray crystallography. “Chromium arene chemistry is a well-established area and we decided to apply this organometallic method to synthesize the first CPP chromium complex,” says Itami, the Director of the JST-ERATO project and the Institute of Transformative Bio-Molecules. “As CPPs have a number of arene rings, we initially expected that chromium would form a complex with each arene ring,” says Segawa, a group leader of the JST-ERATO project. “However, we were surprised to see that CPP reacted with chromium in a 1:1 ratio in all the conditions that we tried. Simulation of the molecular structure suggested that the first equivalent of chromium complexed to CPP lowers its reactivity, thus preventing the reaction with a second chromium moiety.” Obtaining substituted CPPs – traditional method and one-pot sequence 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.”
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