Electron mobility in molybdenum disulfide increases when encapulated in boron nitride

90696_webEncapulating molybdenum disulfide in boron nitride increases how quickly electrons can move through the material, finds new research. Room-temperature mobility was improved by a factor of about 2, and mobility increased to 5-50 times that previously measured when the material was cooled down to a low temperature.

In 2013 James Hone, Wang Fong-Jen Professor of Mechanical Engineering at Columbia Engineering, and colleagues at Columbia demonstrated that they could dramatically improve the performance of graphene – highly conducting two-dimensional (2D) carbon – by encapsulating it in boron nitride (BN), an insulating material with a similar layered structure. In work published this week in the Advance Online Publication on Nature Nanotechnology’s website, researchers at Columbia Engineering, Harvard, Cornell, University of Minnesota, Yonsei University in Korea, Danish Technical University, and the Japanese National Institute of Materials Science have shown that the performance of another 2D material – molybdenum disulfide (MoS2) – can be similarly improved by BN-encapsulation.

James Hone, leader of this new study and director of Columbia’s NSF-funded Materials Research Science and Engineering Center, said: “These findings provide a demonstration of how to study all 2D materials. Our combination of BN and graphene electrodes is like a ‘socket’ into which we can place many other materials and study them in an extremely clean environment to understand their true properties and potential. This holds great promise for a broad range of applications including high-performance electronics, detection and emission of light, and chemical/bio-sensing.” [Read more…]

New method found for creating two-dimensional materials, enables use in electronic devices

materialsResearchers have developed a new method for creating large areas of atom-thin material for use in electronic devices.

Two-dimensional materials have a whole host of exotic properties because they are just one atom thick. Graphene, a single layer of carbon atoms arranged into a honeycomb-like pattern, is the most famous example of a two-dimensional material. It is stronger than steel, has excellent electrical properties, and could be used to make two-dimensional devices that are much smaller than those currently made from bulk or thin-film silicon. However, it is not a semiconductor. Therefore, scientists are turning to other materials that have this essential property for creating transistors.

Researchers have now demonstrated a technique for creating a single atomic layer of molybdenum disulfide — a two-dimensional semiconductor.

Molybdenum disulfide belongs to a family of materials called transition-metal dichalcogenides. They have two chalcogenide atoms (such as sulfur, selenium or tellurium) for every transition-metal atom (molybdenum and tungsten are examples). These materials and their wide range of electrical properties provide an excellent platform material system for versatile electronics. However, creating high-quality material over areas large enough for industrial-scale production is difficult.

Shijie Wang, from the A*STAR Institute of Materials Research and Engineering, said: “Traditional mechanical exfoliation methods for obtaining two-dimensional materials have limited usefulness in commercial applications, and all previous chemical methods are incompatible for integration with device fabrication.”

“Our technique is a one-step process that can grow good-quality monolayer films, or few layers of molybdenum disulfide films, at wafer scale on various substrates using magnetron sputtering.”

The team fired a beam of argon ions at a molybdenum target in a vacuum chamber. This ejected molybdenum atoms from the surface where they reacted with a nearby sulfur vapor. These atoms then assembled onto a heated substrate of either sapphire or silicon. The team found that they could grow monolayer, bilayer, trilayer or thicker samples by altering the power of the argon-ion beam or the deposition time.

They confirmed the quality of their material using a number of common characterization tools including Raman spectroscopy, atomic force microscopy, X-ray photoelectron spectroscopy and transmission electron microscopy. The researchers also demonstrated the excellent electrical properties of their molybdenum disulfide films by creating a working transistor.

Wang added, “Our next step in this work will focus on the application of this technique to synthesize other two-dimensional materials and integrate them with different materials for various device applications.”

The researchers were from the A*STAR Institute of Materials Research and Engineering.

Light emission of molybdenum disulfide enhanced

solving-molybdenum-disulfides-thin-problem-headerResearchers have enhanced the light emission of molybdenum disulfide by laying a series of nanodisks on its surface, overcoming one of the major drawbacks of the material.

Monolayer MoS2’s ultra-thin structure is strong, lightweight, and flexible, making it a good candidate for many applications, such as high-performance, flexible electronics. But such a thin semiconducting material has very little interaction with light, limiting the material’s use in light emitting and absorbing applications.

NMN previously interviewed A.T Charlie Johnson, a professor at Penn, about molybdenum disulfide and its practical applications.

To overcome this problem, the research team designed and fabricated a series of silver nanodiscs and arranged them in a periodic fashion on top of a sheet of MoS2. Not only did they find that the nanodiscs enhanced light emission, but they determined the specific diameter of the most successful disc, which is 130 nanometers. [Read more…]

INTERVIEW: Is molybdenum disulfide the new graphene? – Prof. A.T.C Johnson (Part 1)

a.t.johnsonGraphene has long been regarded as a “wonder material”, but its use in a number of interesting practical applications has been held back by its poor performance as a semiconductor. This is because graphene does not have an energy band gap, unlike molybdenum disulfide (MoS2), another monolayer material. [Read more…]

Heat propagates through graphene as a wave

Grahene heat waveResearch reveals that heat travels through 2D materials, like graphene, as a wave. This phenomenon had only previously been observed in substances at temperatures close to absolute zero.

In the race to miniaturize electronic components, researchers are challenged with a major problem: the smaller or the faster your device, the more challenging it is to cool it down. One solution to improve the cooling is to use materials with very high thermal conductivity, such as graphene, to quickly dissipate heat and thereby cool down the circuits. Scientists were, however, perplexed by how heat propagated through a 2D materials.

Researchers have now demonstrated that heat propagates in these materials in the form of a wave, just like sound in air.

Generally, heat propagates in a material through the vibration of atoms. These vibrations are are called “phonons”, and as heat propagates through a three-dimensional material, these phonons keep colliding with each other, merging together, or splitting. All these processes can limit the conductivity of heat along the way. Only under extreme conditions, when temperature goes close to the absolute zero (-200 0C or lower), it is possible to observe quasi-lossless heat transfer.

The situation is very different in two-dimensional materials. The research team demonstrated that heat can propagate without significant losses in 2D even at room temperature, thanks to the phenomenon of wave-like diffusion, called “second sound”. In that case, all phonons march together in unison over very long distances.

Andrea Cepellotti, a doctoral assistant at EPFL, said:

“Our simulations, based on first-principles physics, have shown that atomically thin sheets of materials behave, even at room temperature, in the same way as three-dimensional materials at extremely low temperatures.”

“We can show that the thermal transport is described by waves, not only in graphene but also in other materials that have not been studied yet. This is an extremely valuable information for engineers, who could exploit the design of future electronic components using some of these novel two-dimensional materials properties.”

Black phosphorus is yet another competitor to graphene

Black PResearchers have used an ultrathin black phosphorus film – only 20 layers of atoms – to demonstrate high-speed data communication on nanoscale optical circuits. The devices showed vast improvement in efficiency over comparable devices using graphene.

As consumers demand electronic devices that are faster and smaller, electronics makers cram more processor cores on a single chip, but getting all those processors to communicate with each other has been a key challenge for researchers. The goal is to find materials that will allow high-speed, on-chip communication using light.

While the existence of black phosphorus has been known for more than a century, only in the past year has its potential as a semiconductor been realized. Due to its unique properties, black phosphorus can be used to detect light very effectively, making it desirable for optical applications. But now, for the first time, researchers have created intricate optical circuits in silicon and then laid thin flakes of black phosphorus over these structures.

The team explained that after the discovery of graphene, new two-dimensional materials continue to emerge with novel optoelectronic properties and because these materials are two-dimensional, it makes perfect sense to place them on chips with flat optical integrated circuits to allow maximal interaction with light and optimally utilize their novel properties.

They demonstrated that the performance of the black phosphorus photodetectors even rivals that of comparable devices made of germanium – considered the gold standard in on-chip photodetection. Germanium, however, is difficult to grow on silicon optical circuits, while black phosphorus and other two-dimensional materials can be grown separately and transferred onto any material, making them much more versatile.

They also showed that the devices could be used for real-world applications by sending high-speed optical data over fibers and recovering it using the black phosphorus photodetectors. The group demonstrated data speeds up to three billion bits per second, which is equivalent to downloading a typical HD movie in about 30 seconds.

While black phosphorus has much in common with graphene – another two-dimensional material – the materials have significant differences, the most important of which is the existence of an energy gap, often referred to as a “band gap.”

Materials with a band gap, known as “semiconductors,” are a special group of materials that only conduct electricity when the electrons in that material absorb enough energy for them to “jump” the band gap. This energy can be provided through heat, light, and other means.

While graphene has proven useful for a wide variety of applications, its main limitation is its lack of a band gap. This means that graphene always conducts a significant amount of electricity, and this “leakage” makes graphene devices inefficient. In essence, the device is “on” and leaking electricity all the time.

Black phosphorus, on the other hand, has a widely-tunable band gap that varies depending on how many layers are stacked together. This means that black phosphorus can be tuned to absorb light in the visible range but also in the infrared. This large degree of tunability makes black phosphorus a unique material that can be used for a wide range of applications – from chemical sensing to optical communication.

Additionally, black phosphorus is a so-called “direct-band” semiconductor, meaning it has the potential to efficiently convert electrical signals back into light. Combined with its high performance photodetection abilities, black phosphorus could also be used to generate light in an optical circuit, making it a one-stop solution for on-chip optical communication.

The past several years have seen a flurry of two-dimensional material discoveries, first with graphene, more recently with transition metal dichalcogenides (TMDs) such as molybdenum disulphide (MoS2), and now black phosphorus. All of the previous two-dimensional materials have serious trade offs, but black phosphorus provides the “best of both worlds,” said the team, with a tunable band gap and high-speed capability.

Nathan Youngblood, a graduate student at the University of Minnesota, said:

“It is really exciting to think of a single material that can be used to send and receive data optically and is not limited to a specific substrate or wavelength. This could have huge potential for high-speed communication between CPU cores which is a bottleneck in computing industry right now.”

Professor Steven Koester, Professor of Electrical and Computer Engineering at the University of Minnesota, added:

“Black phosphorus is an extremely versatile material. It makes great transistors and photodetectors, and has the potential for light emission and other novel devices, making it an ideal platform for a new type of adaptable electronics technology.”

The work by University of Minnesota Department of Electrical and Computer Engineering Professors Mo Li and Steven Koester and graduate students Nathan Youngblood and Che Chen was published today in Nature Photonics – a leading journal in the field of optics and photonics.