A new 2-D material Molybdenum Disulfide Outperforms Graphene in electronics, photonics and Water Desalination

Rajesh Uppal July 23, 2019 Material Comments Off on A new 2-D material Molybdenum Disulfide Outperforms Graphene in electronics, photonics and Water Desalination 1,795 Views

Many alternatives to silicon are being developed, as it is believed that silicon transistors will reach their technological limits after 2020. Two-dimensional (2-D) nanomaterials have shown promise for a new generation of nanoelectronic devices. The most widely studied 2-D nanomaterial is graphene, a single layer of carbon atoms and packed in a hexagonal lattice, has been found to be potentially useful as a light, low-power electronic component due to its many properties like strength, thermal and electrical conductivity. However Graphene lacks a bandgap, the key property required to create transistors, logic and memory circuits. In recent years, new materials such as molybdenum disulfide, has been researched as a substitute for graphene and silicon. The monolayer molybdenum disulfide (MoS2), one of the promising 2D materials with a direct bandgap has high potential for applications in nano electronic devices, energy storage, photocatalysts, and chemical sensors.

Molybdenum disulfide is a new 2-D material , consisting of a single-atomic layer of molybdenum sandwiched between two adjacent atomic layers of sulfide. This compound exists abundantly in nature as the mineral molybdenite, a crystal material found in rocks around the world, frequently taking the characteristic form of silver-colored hexagonal plates. The material has been used for many years as an industrial lubricant for aircraft and motorcycle engines.

Using Molybdenum disulfide , a research team led by faculty scientist Ali Javey at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) has created a smallest transistor with a working 1-nanometer gate. “We made the smallest transistor reported to date,” said Javey, lead principal investigator of the Electronic Materials program in Berkeley Lab’s Materials Science Division. “The semiconductor industry has long assumed that any gate below 5 nanometers wouldn’t work, so anything below that was not even considered,” said study lead author Sujay Desai, a graduate student in Javey’s lab. By changing the material from silicon to MoS₂, we can make a transistor with a gate that is just 1 nanometer in length, and operate it like a switch.”

The development could be key to keeping alive Intel co-founder Gordon Moore’s prediction that the density of transistors on integrated circuits would double every two years, enabling the increased performance of our laptops, mobile phones, televisions, and other electronics. MoS2 also has capability of manipulating the flow of light in atomic scale that opens an exciting avenue towards unprecedented miniaturisation of optical components and the integration of advanced optical functionalities.”

The material MoS2 being just one molecule thick and completely transparent, thus can be deposited on glass, to build large display of television sets and computer monitors thereby reducing the cost and weight and improve energy efficiency.Such thin materials would be transparent and flexible as well, in ways that would enable electronic devices that wouldn’t be possible to make with silicon. “What if your window was also a television, or you could have a heads-up display on the windshield of your car?” asked Kirby Smithe, a graduate student on Pop’s team, suggesting electronic applications that the new materials might make possible.

In the future the material is predicted to be utilized to produce a variety of new products, from whole walls that glow to clothing with embedded antennas and sensors to glasses with built-in display screens.

Electronic Properties

The two-dimensional molybdenum disulfide (MoS2) exhibits properties as good as or even better than graphene. This material, one of transition-metal dichalcogenides has a large energy gap that allows it to have semiconductor capabilities. By applying an electric field, the sheets can be switched between a state that conducts electricity and one that behaves like an insulator. Since then, researchers have already succeeded in making many electronic components from MoS2. Scientists at the Swiss university EPFL utilized the 2-D form of the material to produce a transistor.

Researchers from the University of Calfornia at Riverside and Rensselaer Polytechnic Institute have found molybdenum disulfide (MoS2) is a promising candidate for high-temperature transistors. They have made MoS2 thin-film transistors that work at temperatures exceeding 220 °C and remain stable over two months of operation. This material is also more cost effective than other alternatives like silicon carbide and gallium nitride.

1-nanometer-gate transistor

Desai et al. have constructed a transistor with a 1-nm physical gate with a MoS₂ bilayer gate and a single-walled carbon nanotube gate electrode. These ultrashort devices exhibit excellent switching characteristics with near ideal subthreshold swing of ~65 millivolts per decade and an On/Off current ratio of ~10⁶. Simulations show an effective channel length of ~3.9 nm in the Off state and ~1 nm in the On state.

High-performance silicon transistors can have gate channel lengths as short as 5 nm before source-drain tunneling and loss of electrostatic control lead to unacceptable leakage current when the device is off. Both silicon and MoS₂ have a crystalline lattice structure, but electrons flowing through silicon are lighter and encounter less resistance compared with MoS₂. That is a boon when the gate is 5 nanometers or longer. But below that length, a quantum mechanical phenomenon called tunneling kicks in, and the gate barrier is no longer able to keep the electrons from barging through from the source to the drain terminals. “This means we can’t turn off the transistors,” said Desai. “The electrons are out of control.”

Because electrons flowing through MoS₂ are heavier, their flow can be controlled with smaller gate lengths. MoS₂ can also be scaled down to atomically thin sheets, about 0.65 nanometers thick, with a lower dielectric constant, a measure reflecting the ability of a material to store energy in an electric field. Both of these properties, in addition to the mass of the electron, help improve the control of the flow of current inside the transistor when the gate length is reduced to 1 nanometer.

Making a 1-nanometer structure, it turns out, is no small feat. Conventional lithography techniques don’t work well at that scale, so the researchers turned to carbon nanotubes, hollow cylindrical tubes with diameters as small as 1 nanometer. They then measured the electrical properties of the devices to show that the MoS₂ transistor with the carbon-nanotube gate effectively controlled the flow of electrons.

“This work demonstrated the shortest transistor ever,” said Javey, who is also a UC Berkeley professor of electrical engineering and computer sciences. “However, it’s a proof of concept. We have not yet packed these transistors onto a chip, and we haven’t done this billions of times over. We also have not developed self-aligned fabrication schemes for reducing parasitic resistances in the device. But this work is important to show that we are no longer limited to a 5-nanometer gate for our transistors. Moore’s Law can continue a while longer by proper engineering of the semiconductor material and device architecture.”

Breakthrough reported in fabricating nanochips

An international team of researchers has reported a breakthrough in fabricating atom-thin processors — a discovery that could have far-reaching impacts on nanoscale chip production and in labs across the globe where scientists are exploring 2D materials for ever-smaller and -faster semiconductors. The team, headed by New York University Tandon School of Engineering Professor of Chemical and Biomolecular Engineering Elisa Riedo, outlined the research results in the latest issue of Nature Electronics.

They demonstrated that lithography using a probe heated above 100 degrees Celsius outperformed standard methods for fabricating metal electrodes on 2D semiconductors such as molybdenum disulfide (MoS2). Such transitional metals are among the materials that scientists believe may supplant silicon for atomically small chips. The team’s new fabrication method — called thermal scanning probe lithography (t-SPL) — offers a number of advantages over today’s electron beam lithography (EBL).

First, thermal lithography significantly improves the quality of the 2D transistors, offsetting the Schottky barrier, which hampers the flow of electrons at the intersection of metal and the 2D substrate. Also, unlike EBL, the thermal lithography allows chip designers to easily image the 2D semiconductor and then pattern the electrodes where desired. Also, t-SPL fabrication systems promise significant initial savings as well as operational costs: They dramatically reduce power consumption by operating in ambient conditions, eliminating the need to produce high-energy electrons and to generate an ultra-high vacuum. Finally, this thermal fabrication method can be easily scaled up for industrial production by using parallel thermal probes.

Riedo expressed hope that t-SPL will take most fabrication out of scarce clean rooms — where researchers must compete for time with the expensive equipment — and into individual laboratories, where they might rapidly advance materials science and chip design. The precedent of 3D printers is an apt analogy: Someday these t-SPL tools with sub-10 nanometer resolution, running on standard 120-volt power in ambient conditions, could become similarly ubiquitous in research labs like hers.

Riedo’s work on thermal probes dates back more than a decade, first with IBM Research — Zurich and subsequently SwissLitho, founded by former IBM researchers. A process based on a SwissLitho system was developed and used for the current research. She began exploring thermal lithography for metal nanomanufacturing at the City University of New York (CUNY) Graduate Center Advanced Science Research Center (ASRC), working alongside co-first-authors of the paper, Xiaorui Zheng and Annalisa Calò, who are now post-doctoral researchers at NYU Tandon; and Edoardo Albisetti, who worked on the Riedo team with a Marie Curie Fellowship.

Converting Wi-Fi Signals To Electricity With New 2-D Materials

Researchers from MIT and elsewhere have taken a step in that direction, with the first fully flexible device that can convert energy from Wi-Fi signals into electricity that could power electronics. Devices that convert AC electromagnetic waves into DC electricity are known as “rectennas.” All rectennas rely on a component known as a “rectifier,” which converts the AC input signal into DC power. Traditional rectennas use either silicon or gallium arsenide for the rectifier. These materials can cover the Wi-Fi band, but they are rigid. And, although using these materials to fabricate small devices is relatively inexpensive, using them to cover vast areas, such as the surfaces of buildings and walls, would be cost-prohibitive.

To build their rectifier, the researchers used a novel 2-D material called molybdenum disulfide (MoS2), which at three atoms thick is one of the thinnest semiconductors in the world. In doing so, the team leveraged a singular behavior of MoS2: When exposed to certain chemicals, the material’s atoms rearrange in a way that acts like a switch, forcing a phase transition from a semiconductor to a metallic material. The resulting structure is known as a Schottky diode, which is the junction of a semiconductor with a metal.

“By engineering MoS2 into a 2-D semiconducting-metallic phase junction, we built an atomically thin, ultrafast Schottky diode that simultaneously minimizes the series resistance and parasitic capacitance,” says first author and EECS postdoc Xu Zhang, who will soon join Carnegie Mellon University as an assistant professor.

Parasitic capacitance is an unavoidable situation in electronics where certain materials store a little electrical charge, which slows down the circuit. Lower capacitance, therefore, means increased rectifier speeds and higher operating frequencies. The parasitic capacitance of the researchers’ Schottky diode is an order of magnitude smaller than today’s state-of-the-art flexible rectifiers, so it is much faster at signal conversion and allows it to capture and convert up to 10 gigahertz of wireless signals. “Such a design has allowed a fully flexible device that is fast enough to cover most of the radio-frequency bands used by our daily electronics, including Wi-Fi, Bluetooth, cellular LTE, and many others,” Zhang says.

Optical Properties

The relatively weak absorption co-efficiency of graphene (2.3 % of incident light per layer) significantly delimit its light modulation ability for optical communication devices such as light detector, modulator and absorber. Molybdenum disulfide’s semiconducting ability, strong light-matter interaction and similarity to the carbon-based graphene make it of interest to scientists as a viable alternative to graphene in the manufacture of electronics, particularly photoelectronics.

Rice University engineering researcher Isabell Thomann are exploring the light-absorbent properties of the substance that may be used to develop future energy-efficient optoelectronic and photocatalytic devices. “By using simple strategies, we were able to absorb 35 to 37 percent of the incident light in the 400- to 700-nanometer wavelength range, in a layer that is only 0.7 nanometers thick.” “Squeezing light into these extremely thin layers and extracting the generated charge carriers is an important problem in the field of two-dimensional materials,” she said. “That’s because monolayers of 2-D materials have different electronic and catalytic properties from their bulk or multilayer counterparts.” “The goal, of course, is 100 percent absorption, and we’re not there yet.” The research has many applications, including development of efficient and inexpensive photovoltaic solar panels.

The thinness of these materials, however, becomes a limiting factor in their efficiency as photovoltaics, or light-energy conversion devices. Light absorbing devices, such as solar cells and photodetectors, require a certain amount of optical thickness in order to absorb photons, rather than allowing them to pass through.

Petoukhoff, under the supervision of Professor Keshav Dani, seeks to improve optoelectronic devices by adding a 2-D layer of MoS2 to an organic semiconductor, which has similar absorption strengths as MoS2. The researchers from the Femtosecond Spectroscopy Unit added an array of silver nanoparticles, or a plasmonic metasurface, to the organic semiconductor-MoS2 hybrid to focus and localize the light in the device. The addition of the metasurface increases the optical thickness of the material while capitalizing on the unique properties of the ultra-thin active layer, which ultimately increase the total absorption.

Combinations with 2D materials have the potential to revolutionize the marketability of optoelectronic devices. Conventional optoelectronic devices are expensive to manufacture and are often made from scarce or toxic elements, such as indium or arsenic. Organic semiconductors have low manufacturing costs, and are made of earth-abundant and non-toxic elements. This research can potentially improve the cost and efficiency of optoelectronics, leading to better products in the future. Scientists have found that the physical properties of two-dimensional (2D) MoS2 change markedly when it has nanoscale properties. A slab of MoS2 that is even a micron thick has an “indirect” bandgap while a two-dimensional sheet of molybdenum disulfide has a “direct” bandgap. It shows thickness dependent band-gap properties, allowing for the production of tunable optoelectronic devices with diversified spectral operation.

2-D materials make photodetectors ultra-efficient

The performance of devices like cameras and solar panels, depend on the efficiency of photodetectors or how efficiently the detectors can turn incident photons into electrons and corresponding positively charged species called holes. These electron-hole (e-h) pairs can then move through the material to generate electricity. One way to improve this efficiency is to shrink the materials down to the nanoscale. Scientists have used nanocrystal quantum dots, carbon nanotubes, and graphene to achieve efficiencies beyond 100%, meaning a single photon produces more than one e-h pair, an effect known as e-h multiplication

A team of researchers at the University of California, Riverside, led by Nathaniel M. Gabor, now reports greater than 300% efficiency with a class of ultrathin two-dimensional materials called transition-metal dichalcogenides. The photodetector, made of two atomic layers of WSe2 stacked on a single layer of MoSe2, is almost transparent and about the size of a camera pixel (Nat. Nanotechnol. 2017, DOI: 10.1038/nnano.2017.203).

The researchers propose that when a photon strikes the top WSe2 layer, it sets an electron in motion that can then hop to the MoSe2 layer to create another e-h pair. By applying a small voltage to the layers, the team was able to further enhance the device’s efficiency to 350%, creating between 3–4 e-h pairs per photon. Work on the 2-D metal dichalcogenides is still in the early stages, Gabor says, but the materials’ efficiencies are “on par” with those of more mature devices made with nanocrystal quantum dots. He adds that one advantage of using dichalcogenides is that they tend to be crystalline, which could make them better at electron transport compared with quantum dots.

World’s thinnest lens to revolutionize cameras

Dr Yuerui (Larry) Lu from The Australian National University (ANU) and his team has created the world’s thinnest lens, from molybdenum disulphide crystal. The 6.3-nanometre lens outshines previous ultra-thin flat lenses, made from 50-nanometre thick gold nano-bar arrays, known as a metamaterial. “This type of material is the perfect candidate for future flexible displays,” said Dr Lu, “We will also be able to use arrays of micro lenses to mimic the compound eyes of insects.” “Molybdenum disulphide is an amazing crystal,” said Dr Lu “It survives at high temperatures, is a lubricant, a good semiconductor and can emit photons too.

“The capability of manipulating the flow of light in atomic scale opens an exciting avenue towards unprecedented miniaturisation of optical components and the integration of advanced optical functionalities.” Dr Lu’s team created their lens from a crystal 6.3-nanometres thick – 9 atomic layers – which they had peeled off a larger piece of molybdenum disulphide with sticky tape. They then created a 10-micron radius lens, using a focussed ion beam to shave off the layers atom by atom, until they had the dome shape of the lens.

The team discovered that single layers of molybdenum disulphide, 0.7 nanometres thick, had remarkable optical properties, appearing to a light beam to be 50 times thicker, at 38 nanometres. This property, known as optical path length, determines the phase of the light and governs interference and diffraction of light as it propagates. Molybdenum disulphide crystal’s refractive index, the property that quantifies the strength of a material’s effect on light, has a high value of 5.5. For comparison, diamond, whose high refractive index causes its sparkle, is only 2.4, and water’s refractive index is 1.3.

Biosensors

The unique properties of two-dimensional molybdenum disulfide (2D MoS2) have so far led to immense research regarding this material’s fundamentals, applications, and, more recently, its potential for biosensing. 2D MoS2 has properties that make it of great interest for developing biosensors. These properties include large surface area, tunable energy band diagrams, a comparatively high electron mobility, photoluminescence, liquid media stability, relatively low toxicity, and intercalatable morphologies

Molybdenum Disulfide Outperforms Graphene in Water Desalination

Graphene has been considered one of the promising material for easing the energy demands of water desalination. Here the material acts a porous membrane that allows water through but blocks the flow of salt ions—a pressure-driven process called reverse osmosis. Researchers at the University of Illinois recently found that molybdenum disulfide (MoS2) outperforms graphene in water desalination role. In research published in the journal Nature Communications, the Illinois scientists modeled various thin-film membrane materials and found that MoS2 was the most efficient, filtering up to 70 percent more water than graphene membranes.

“Reverse osmosis is a very expensive process,” Narayana Aluru, a professor at the university and leader of the research said. “It’s very energy intensive. A lot of power is required to do this process, and it’s not very efficient. In addition, the membranes fail because of clogging. So we’d like to make it cheaper and make the membranes more efficient so they don’t fail as often. We also don’t want to have to use a lot of pressure to get a high flow rate of water.” For any new material to take its place as a reverse osmosis membrane it must have pores of a precisely-controlled size; a resistance to fouling and clogging; and significantly reduce energy costs.

“MoS2 has inherent advantages in that the molybdenum in the center attracts water, then the sulfur on the other side pushes it away, so we have much higher rate of water going through the pore,” said graduate student Mohammad Heiranian, the first author of the study. “It’s inherent in the chemistry of MoS2 and the geometry of the pore, so we don’t have to functionalize the pore, which is a very complex process with graphene.” “These materials are efficient in terms of energy usage and fouling, which are issues that have plagued desalination technology for a long time,” Aluru said.

Challenges being overcome

The key challenge has been a large electrical resistance between metal contacts and single-atomic layers of the material that limits the flow of current and hindering performance. Now, researchers have developed a technique, molecular level doping, wherein material is doped with the chemical compound 1,2 dichloroethane (DCE), resulting in a 10-fold reduction of contact resistance. This 2D materials—in which all atoms are at the surface—are by their nature extremely sensitive to their environment, and their performance often falls far short of theoretical limits due to contamination and trapped charges in surrounding insulating layers

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 molybdenum disulfide (MoS2) can be improved by boron nitride (BN) -encapsulation. They found that the room-temperature mobility was improved by a factor of about 2, approaching the intrinsic limit. Upon cooling to low temperature, the mobility increased dramatically, reaching values 5-50× that those measured previously.

Manufacturing processes

Another challenge is producing the material on an industrial scale, MIT researchers, including Yi-Hsien Lee, have found a good way to make large sheets of the material using a chemical vapor deposition process. It could also be made into solutions that serve as inks for printable electronics.

Now a team led by Stanford electrical engineering Associate Professor Eric Pop has demonstrated how it might be possible to mass-produce such atomically thin materials and electronics. “The question was whether the team could manufacture a molybdenum disulfide crystal big enough to form a chip. That requires building a crystal roughly the size of your thumbnail. This may not sound like a big deal until you consider the aspect ratio of the crystal required: a chip just three atoms thick but the size of your thumbnail is like a single sheet of paper big enough to cover the entire Stanford campus.”

The Stanford team manufactured that sheet by depositing three layers of atoms into a crystalline structure 25 million times wider than it is thick. Smithe achieved this by making ingenious refinements to a manufacturing process called chemical vapor deposition. This approach essentially incinerates small amounts of sulfur and molybdenum until the atoms vaporize like soot. The atoms then deposit as an ultra-thin crystalline layer on a “handle” substrate, which can be glass or even silicon.

During chip manufacturing, circuits must be etched into the material. To demonstrate how a large-scale, single-layer chip manufacturing process might perform this step in the future, the team used standard etching tools to cut the Stanford logo into their prototy