To protect monolayer semiconductor transition metal dichalcogenides (S-TMDs) from oxidation, they must be entirely shielded from light, with even short exposure causing oxidation severe enough to damage electrical contacts and completely destroy optical characteristics.
A new, Monash University-led collaboration has shown that the oxidation of monolayers of tungsten disulphide (WS2) in ambient conditions is due to the absorption of visible wavelengths of light.
The new work, in collaboration with researchers from the US. Naval Research Laboratory and the University of Autonomous University of Madrid, informs researchers working in the field to the heretofore unappreciated photo-sensitive nature of these materials, and more importantly, acts as a guide for completely avoiding oxidation in samples exposed to ambient conditions.
“This work should guide researchers in best practices for S-TMD device manufacture,” says lead author, Mr. Jimmy Kotsakidis.
Scientists have used precisely tuned pulses of laser light to film the ultrafast rotation of a molecule. The resulting “molecular movie” tracks one and a half revolutions of carbonyl sulphide (OCS)—a rod-shaped molecule consisting of one oxygen, one carbon and one sulphur atom—taking place within 125 trillionths of a second, at a high temporal and spatial resolution. The team headed by DESY’s Jochen Küpper from the Center for Free-Electron Laser Science (CFEL) and Arnaud Rouzée from the Max Born Institute in Berlin are presenting their findings in the journal Nature Communications. CFEL is a cooperation of DESY, the Max Planck Society and Universität Hamburg.
“Molecular physics has long dreamed of capturing the ultrafast motion of atoms during dynamic processes on film,” explains Küpper, who is also a professor at the University of Hamburg. This is by no means simple, however, as the realm of molecules normally requires high-energy radiation with a wavelength of the order of the size of an atom in order to be able to see details. So Küpper’s team took a different approach: They used two pulses of infrared laser light precisely tuned to each other and separated by 38 trillionths of a second (picoseconds) to set the carbonyl sulphide molecules spinning rapidly in unison, i.e., coherently. They then used another laser pulse with a longer wavelength to determine the position of the molecules at intervals of around 0.2 trillionths of a second each. “Since this diagnostic laser pulse destroys the molecules, the experiment had to be restarted again for each snapshot,” reports Evangelos Karamatskos, the principal author of the study from CFEL.
Investigating the remarkable excitonic effects in two-dimensional (2-D) semiconductors and controlling their exciton binding energies can unlock the full potential of 2-D materials for future applications in photonic and optoelectronic devices. In a recent study, Zhizhan Qiu and colleagues at the interdisciplinary departments of chemistry, engineering, advanced 2-D materials, physics and materials science in Singapore, Japan and the U.S. demonstrated large excitonic effects and gate-tunable exciton binding energies in single-layer rhenium diselenide(ReSe2) on a back-gated graphene device. They used scanning tunneling spectroscopy (STS) and differential reflectance spectroscopy to measure the quasiparticle (QP) electronic and optical bandgap (Eopt) of single-layer ReSe2 to yield a large exciton binding energy of 520 meV.
The scientists achieved continuous tuning of the electronic bandgap and exciton binding energy of monolayer ReSe2 by hundreds of milli-electron volts via electrostatic gating. Qiu et al. credited the phenomenon to tunable Coulomb interactions arising from the gate-controlled free carriers in graphene. The new findings are now published on Science Advances and will open a new avenue to control bandgap renormalization and exciton binding energies in 2-D semiconductors for a variety of technical applications.
Researchers at the Center for Theoretical Physics of Complex Systems (PCS), within the Institute for Basic Science (IBS, South Korea), and colleagues have reported a novel phenomenon, called Valley Acoustoelectric Effect, which takes place in 2D materials, similar to graphene. This research is published in Physical Review Letters and brings new insights to the study of valleytronics.
In acoustoelectronics, surface acoustic waves (SAWs) are employed to generate electric currents. In this study, the team of theoretical physicists modelled the propagation of SAWs in emerging 2D materials, such as single-layer molybdenum disulfide (MoS2). SAWs drag MoS2 electrons (and holes), creating an electric current with conventional and unconventional components. The latter consists of two contributions: a warping-based current and a Hall current. The first is direction-dependent, is related to the so-called valleys – electrons’ local energy minima – and resembles one of the mechanisms that explains photovoltaic effects of 2D materials exposed to light. The second is due to a specific effect (Berry phase) that affects the velocity of these electrons travelling as a group and resulting in intriguing phenomena, such as anomalous and quantum Hall effects.
The fabrication of electronic devices from exfoliated 2-D materials can be tricky. The group of Daniel Granados at IMDEA Nanociencia has engineered a solution that consists of the after-fabrication tailoring of MoS2-FET transistors using pulsed-focused electron beam induced etching.
Transition metal dichalcogenides are 2-D, atomically thin layers bound together by Van der Waals forces. These materials exhibit thickness-dependent variations in their physical properties that can be exploited in distinct optoelectronic applications. For example, the band structure of molybdenum disulphide (MoS2) has a direct bandgap of 1.8 eV in a single layer that narrows down with thickness of 1.2 eV indirect bandgap in bulk.
The atomically thin layers of MoS2 can be separated by micromechanical exfoliation, nonetheless the fabrication of optoelectronic devices from mechanically exfoliated MoS2 is an intricate process. The geometry of the device is limited in all cases by the shape of the exfoliated flake, even when a deterministic stamping method is employed. Even when using CVD (chemical vapour deposition) techniques the device fabrication is hindered by the material growing in islands with reduced sizes and different physical properties.
With a simple twist of the fingers, one can create a beautiful spiral from a deck of cards. In the same way, scientists at the University of California, Berkeley, and Lawrence Berkeley National Laboratory (Berkeley Lab) have created new inorganic crystals made of stacks of atomically thin sheets that unexpectedly spiral like a nanoscale card deck.
Their surprising structures, reported in a new study appearing online Wednesday, June 20, in the journal Nature, may yield unique optical, electronic and thermal properties, including superconductivity, the researchers say.
These helical crystals are made of stacked layers of germanium sulfide, a semiconductor material that, like graphene, readily forms sheets that are only a few atoms or even a single atom thick. Such “nanosheets” are usually referred to as “2-D materials.”
“No one expected 2-D materials to grow in such a way. It’s like a surprise gift,” said Jie Yao, an assistant professor of materials science and engineering at UC Berkeley. “We believe that it may bring great opportunities for materials research.”
I finally got to try my hand at sample preparation these past 2 weeks. Materials can exhibit striking electronic and mechanical properties when stripped to few layers / monolayer. Graphene (single layer graphite) is one popular example, first to be discovered among the family of 2D materials in 2004. But the search for conducting 2D materials has now extended beyond graphene.
I tried to exfoliate a single layer of molybdenum disulfide (MoS2), one type of transition metal dichalcogenides (TMD), under the guidance of my lab colleague. The properties of these TMD in the form of 2D sheets have application in more efficient electronics, semiconductors, solar cells, and touch screen display panels, just to name a few.
Exfoliation in this context is actually applying the material of interest onto a scotch tape, then folding and opening the tape several times until the flakes on it are as thin as possible. Judging with my naked eyes, this usually just means for the flakes to not appear as shiny (for metal compounds) anymore. Then, I will have to transfer the flakes on the Scotch tape to a tiny piece of transparent gel (PDMS gel). The procedure is pretty simple, but the flakes produced are not guaranteed to be thin enough. Through my many trials, I find random thickness of flakes distributed all over the gel under optical microscope. And among them, we need to locate the thinnest of all that could potentially be a monolayer.
Below is the flake that I suspect to be a single layer of MoS2 at different magnification level. Can you find it in the last picture?
Here is the relative size of the PDMS gel on which all these flakes lie.
This also happened when I managed to transfer plenty of flakes onto the PDMS gel, while most of the time I do not get enough to choose from when observed under the microscope. After identifying this flake, I had to transfer it onto another substrate (such as a silicon nitride grid), before I can put it into an electron microscope that offer higher resolution through electrons scattering method. The transfer patience takes some patience, as pressing or lifting the gel too quickly may break the hard-found flake. So it is just a lot of going back and forth between the sample preparation table and microscope to get one nice specimen. But it is important to get through this process for us to get to the characterization part, which is where all the fun stuff about science comes in!
Physicists have discovered a novel kind of nanotube that generates current in the presence of light. Devices such as optical sensors and infrared imaging chips are likely applications, which could be useful in fields such as automated transport and astronomy. In future, if the effect can be magnified and the technology scaled up, it could lead to high-efficiency solar power devices.
Working with an international team of physicists, University of Tokyo Professor Yoshihiro Iwasa was exploring possible functions of a special semiconductor nanotube when he had a lightbulb moment. He took this proverbial lightbulb (which was in reality a laser) and shone it on the nanotube to discover something enlightening. Certain wavelengths and intensities of light induced a current in the sample—this is called the photovoltaic effect. There are several photovoltaic materials, but the nature and behavior of this nanotube is cause for excitement.
“Essentially our research material generates electricity like solar panels, but in a different way,” said Iwasa. “Together with Dr. Yijin Zhang from the Max Planck Institute for Solid State Research in Germany, we demonstrated for the first time nanomaterials could overcome an obstacle that will soon limit current solar technology. For now solar panels are as good as they can be, but our technology could improve upon that.”
Using optical and electrical measurements, a two-dimensional anisotropic crystal of rhenium disulfide was found to show opposite piezoresistant effects along two principle axes, i.e. positive along one axis and negative along another. Piezoresistance was also reversible; it appeared upon application of a strain, but the relative resistance returned to its original value on strain removal. This novel finding is expected to lead to wide application of rhenium disulfide.
Upon application of mechanical stress such as pressure on crystals and some kinds of ceramics, a surface charge proportional to the applied strain is induced; this phenomenon is called the piezoelectric effect. The piezoelectric effect has been known since the mid-18th century and has found use, for example, in the ignition device of cigarette lighters. Today it is widely applied in sensors, actuators, etc. On the other hand, when mechanical strain is applied to semiconducting materials, some of them show a change in electrical resistance, called the piezoresistive effect. Materials showing the piezoresistive effect are used in pressure sensors, strain sensors etc.
A technique that introduces carbon-hydrogen molecules into a single atomic layer of the semiconducting material tungsten disulfide dramatically changes the electronic properties of the material, according to Penn State researchers at Penn State who say they can create new types of components for energy-efficient photoelectric devices and electronic circuits with this material.
“We have successfully introduced the carbon species into the monolayer of the semiconducting material,” said Fu Zhang, doctoral student in materials science and engineering lead author of a paper published online today in Science Advances.
Prior to doping—adding carbon—the semiconductor, a transition metal dichalcogenide (TMD), was n-type—electron conducting. After substituting carbon atoms for sulfur atoms, the one-atom-thick material developed a bipolar effect, a p-type—hole—branch, and an n-type branch. This resulted in an ambipolar semiconductor.