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A torchbearer for the future of applied materials and optoelectronics — Professor Chen Hsiang

Posted By Graphene Council, Wednesday, September 9, 2020
For those unfamiliar with the terms “applied materials” and “optoelectronic engineering,” a few keywords such as “semiconductors” and “sensors” should jolt one’s memory. The importance of this cutting-edge field can be illustrated by examining recent Nobel Prize winners and their research.

First, three Japanese researchers were jointly awarded the 2014 Nobel Prize for Physics "for the invention of efficient blue light-emitting diodes which has enabled bright and energy-saving white light sources," for they held the key to the elusive blue LED — Gallium nitride (GaN).

Another pair of laureates, both of Russian heritage, were awarded in 2010 "for groundbreaking experiments regarding the two-dimensional material graphene." Graphene is a newly discovered form of carbon that is prized by manufacturers of touchscreens, light panels, and solar cells for its superior transparency and heat-conducting properties.

Chen Hsiang, chair of National Chi Nan University’s Department of Applied Materials and Optoelectronic Engineering, began his academic journey in the field of electrical engineering before delving into photonics and nanomaterials. His alma mater, National Taiwan University’s fiercely competitive Department of Electrical Engineering, has been the top choice for Taiwanese students taking the university entrance exam for the past decades.

After rigorous training through NTU's undergraduate and graduate programs from 1991 to 1997, Chen left Taipei to pursue a doctoral degree at the University of California, Irvine from 2005 to 2008. It was here, in a dimly lighted campus laboratory, that he first caught a glimpse of the imperfections within the GaN transistors of that era. He proceeded to dedicate his thesis to this discovery, and graduated with both a Ph. D. degree and a book offer from a German publisher.

“At that time, researching GaN transistors was a new field,” the distinguished professor explains. “These high-power transistors are used in cellular towers, satellites, and even in outer space, but [the design then] lacked stability and contained structural flaws that could be rectified by optoelectronics.”

The materials used in his doctoral studies were procured from an American arms manufacturer that crafted F51 fighter jets and is now known as Northrop Grumman, a global aerospace and defense technology company. Unable to secure a source for such transistors upon returning to Taiwan, Chen turned his attention to the more readily available zinc oxide (ZnO) nanoparticles.

Described by the professor as “structurally identical” to the hexagonal columnar basalt found on Taiwan's Penghu Islands, crystalized ZnO particles are actually a million times larger in terms of mass. This stretch of surface is extremely advantageous in making light, portable nano-sensors that can be used to reliably measure carbon monoxide levels or ultraviolent rays.

Chen compares the process — that of introducing nanomaterials to zinc oxide to create completely new ZoN nanostructures — to “changing the toppings on a subway sandwich” to refine the properties of the end product differently each time.

Respected among his peers as a well-trained engineer who has never ceased his research efforts, Chen maintains a steady publishing average of 8 articles per year in international science master journals listed on the Science Citation Index (SCI).

This track record is matched by only a handful of NCNU faculty members, however Chen humbly redirects the compliment instead to acknowledge the collective hard work of the optoelectronics department’s instructors and student researchers.

He interjects: “There is a student who is working on those fresh perovskite [solar] cells, heard it was similar to Intel’s research.”

Chen took up the post of departmental chair last year upon completing a sabbatical and visiting at the research lab of Yale's acclaimed Professor of Technological Innovation Jung Han (韓仲). Apart from livening up his department’s recruitment and teaching process, he is also leading the way for more case studies, hands-on experiments, and industry knowledge such as the latest breakthroughs in technology and applications.

One of his recent lectures was on optical tweezers invented by the 2018 Nobel Prize in Physics team that grab atoms, molecules, and DNA with laser beam fingers; the lasers push small particles towards the center of the beam and hold them there.

The professor dutifully recites the tremendous employment opportunities that come with a bachelor's degree in the field: Taiwan Semiconductor Manufacturing Company (TSMC), United Microelectronics Corporation (UMC), Micron Technology, and Epistar. Other graduates opt for further studies at institutions such as Carnegie Mellon and Duke.

Two recent graduates are now serving as research engineers at TSMC, he says, drawing attention to the importance of deep familiarity with both the compositional and modular properties of semiconductors. “Having a background in manufacturing and sensor-testing semiconductors, as well as knowledge of the physics and materials used, will open up a lot of doors in both the electronics industry and the optoelectronics field.”

Academic-industry cooperation on a community level is another passion of Chen's, in which he seeks to deepen exchanges and partnerships with local LED firms and solar cell makers such as those based at Nantou's science park. “Local businesses are in need of highly skilled labor, graduates are in need of employment; we are here to create networks,” he explains.

In recent years, NCNU has been an avid participant in several programs supported by the Ministry of Education's Center for University Social Responsibility. These include cross-fertilizing Taiwan's agricultural powerhouse with optoelectronics, and now Nantou’s water bamboo and passion fruits are grown with the aid of LED lights.

Moreover, NCNU researchers are currently identifying the best wavelength, intensity, and duration for specific cultivars based on their innate growth cycle and biological characteristics.

How do a new generation of Taiwanese scholars prepare themselves for this field? To this, Chen replies with the 3 keystones of optoelectronics — light, display, and energy source.

NCNU's curriculum prides itself on providing in-depth understanding of the characteristics of the materials used, as well as the parameters for reading photonic and gaseous levels. This field is a gateway to electrical engineering, chemistry, physics, optoelectronics, and many more fascinating areas of study, so why not take the chance to learn more about semiconductors to broaden one's scientific knowledge and employability?

Professor Chen's rich scientific sensibilities have further cemented the credibility of NCNU's Department of Applied Materials and Optoelectronic Engineering. The reward for developing engaging research projects and experiment-based training? Exceeding recruitment expectations during the time of the coronavirus — full classrooms that the devoted Chen sees as a divine deliverance of grace.

Tags:  Chen Hsiang  Graphene  LED  nanomaterials  National Chi Nan University  optoelectronics  Semiconductors 

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Dr. Lutz Waldecker starts at RWTH with a fellowship of the Alexander von Humboldt Foundation

Posted By Graphene Council, Wednesday, August 26, 2020
This August, the Aachen Graphene & 2D Materials Center has gained new valuable expertise with the arrival of Dr. Lutz Waldecker as senior researcher in the Stampfer Group (2nd Institute of Physics A).

Waldecker is an expert on exciton physics and ultrafast dynamics in solids, in particular two-dimensional (2D) semiconductors. On his background, he has a Ph.D. at the Fritz Haber Institute of the Max Planck Society in Berlin, under the supervision of Ralph Ernstorfer, and a post-doc at Stanford University, in the group of Tony Heinz, where he worked on the optical and electronic properties of 2D semiconductors.

Among Waldecker’s recent results is the demonstration that dielectric screening causes a rigid shift of the single particle bands of 2D semiconductors, with little changes of the electronic dispersion [1]. This observation suggests a new, noninvasive way of inducing nanoscale functionality in 2D semiconductors by acting on the substrate dielectrics – an approach that Waldecker is planning to explore as part of his research in Aachen.

Waldecker’s position is funded by a Feodor-Lynen return fellowship of the Alexander von Humboldt Foundation. “It is great to see that the Aachen Graphene & 2D Materials Center is getting increasingly competitive in attracting top-class researchers”, says Prof. Christoph Stampfer, “I’m looking very much forward to working together with Lutz!”

Tags:  2D materials  Aachen Graphene & 2D Materials Center  Graphene  Lutz Waldecker  Semiconductors  Stampfer Group 

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Graphene sensors find subtleties in magnetic fields

Posted By Graphene Council, Friday, August 21, 2020
As with actors and opera singers, when measuring magnetic fields it helps to have range. Cornell researchers used an ultrathin graphene “sandwich” to create a tiny magnetic field sensor that can operate over a greater temperature range than previous sensors, while also detecting miniscule changes in magnetic fields that might otherwise get lost within a larger magnetic background.

The group’s paper, “Magnetic Field Detection Limits for Ultraclean Graphene Hall Sensors,” published Aug. 20 in Nature Communications.

The team was led by Katja Nowack, assistant professor of physics in the College of Arts and Sciences and the paper’s senior author.

Nowack’s lab specializes in using scanning probes to conduct magnetic imaging. One of their go-to probes is the superconducting quantum interference device, or SQUID, which works well at low temperatures and in small magnetic fields.

“We wanted to expand the range of parameters that we can explore by using this other type of sensor, which is the Hall-effect sensor,” said doctoral student Brian Schaefer, the paper’s lead author. “It can work at any temperature, and we’ve shown it can work up to high magnetic fields as well. Hall sensors have been used at high magnetic fields before, but they’re usually not able to detect small magnetic field changes on top of that magnetic field.”

The Hall effect is a well-known phenomenon in condensed matter physics. When a current flows through a sample, it is bent by a magnetic field, creating a voltage across both sides of the sample that is proportional to the magnetic field.

Hall-effect sensors are used in a variety of technologies, from cellphones to robotics to anti-lock brakes. The devices are generally built out of conventional semiconductors like silicon and gallium arsenide.

Nowack’s group decided to try a more novel approach. The last decade has seen a boom in uses of graphene sheets – single layers of carbon atoms, arranged in a honeycomb lattice. But graphene devices often fall short of those made from other semiconductors when the graphene sheet is placed directly on a silicon substrate; the graphene sheet “crumples” on the nanoscale, inhibiting its electrical properties.

Nowack’s group adopted a recently developed technique to unlock graphene’s full potential – sandwiching it between sheets of hexagonal boron nitride. Hexagonal boron nitride has the same crystal structure as graphene but is an electrical insulator, which allows the graphene sheet to lie flat. Graphite layers in the sandwich structure act as electrostatic gates to tune the number of electrons that can conduct electricity in the graphene.

The sandwich technique was pioneered by co-author Lei Wang, a former postdoctoral researcher with the Kavli Institute at Cornell for Nanoscale Science. Wang also worked in the lab of co-senior author Paul McEuen, the John A. Newman Professor of Physical Science and co-chair of the Nanoscale Science and Microsystems Engineering (NEXT Nano) Task Force, part of the provost’s Radical Collaboration initiative.

“The encapsulation with hexagonal boron nitride and graphite makes the electronic system ultraclean,” Nowack said. “That allows us to work at even lower electron densities than we could before, and that’s favorable for boosting the Hall-effect signal we are interested in.”

The researchers were able to create a micron-scale Hall sensor that functions as well as the best Hall sensors reported at room temperature while outperforming any other Hall sensor at temperatures as low as 4.2 kelvins (or minus 452.11 degrees Fahrenheit).

The graphene sensors are so precise they can pick out tiny fluctuations in a magnetic field against a background field that is larger by six orders of magnitude (or a million times its size). Detecting such nuances is a challenge for even high-quality sensors because in a high magnetic field, the voltage response becomes nonlinear and therefore more difficult to parse.

Nowack plans to incorporate the graphene Hall sensor into a scanning probe microscope for imaging quantum materials and exploring physical phenomena, such as how magnetic fields destroy unconventional superconductivity and the ways that current flows in special classes of materials, such as topological metals.

“Magnetic field sensors and Hall sensors are important parts of many real-world applications,” Nowack said. “This work puts ultraclean graphene really on the map for being a superior material to build Hall probes out of. It wouldn’t be really practical for some applications because it’s hard to make these devices. But there are different pathways for materials growth and automated assembly of the sandwich that people are exploring. Once you have the graphene sandwich, you can put it anywhere and integrate it with existing technology.”

Tags:  Cornell University  Graphene  Katja Nowack  Semiconductors  Sensors 

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Combining graphene and nitrides for high-power, high-frequency electronics

Posted By Graphene Council, Thursday, August 20, 2020
Researchers at Graphene Flagship partners CNR-IMM, Italy, CNRS-CRHEA, France, and STMicroelectronics, Poland, in collaboration with Graphene Flagship Associate Member TopGaN, Poland, collaborated on the Partnering Project GraNitE to produce graphene-enabled hot electron transistor (HET) devices. Thanks to nitride semiconductors, they achieved devices with current densities a million times higher than previous prototypes.

Nitride semiconductors are in the spotlight for their potential to be incorporated into HETs to improve their properties and performance. HETs are a type of vertical transistor that can operate at frequencies in the terahertz (THz) range, making them very valuable for applications in communications, medical diagnostics and security. Graphene is promising for applications in HETs, owing to its thinness and high conductivity. They are typically made from nitrides of gallium, aluminium or indium, or alloys of these metals. Aluminium and gallium nitrides are key ingredients in high-electron mobility transistors (HEMTs) – one of the technological foundations of 5G communications.

Gallium-based technologies do have their limitations, however, and GraNitE seeks to take advantage of graphene and layered materials to overcome them. The GraNitE team incorporated graphene as an active ingredient into high-powered aluminium-gallium nitride (AlGaN) and gallium nitride (GaN) based nitride transistors to better dissipate heat, by taking advantage of graphene's high thermal conductivity. The devices also operate at higher frequency thanks to the incorporation of high-quality graphene.

The team devised two approaches. Their first was to deposit graphene onto the surface of the nitride semiconductor using chemical vapour deposition (CVD). This resulted in highly homogeneous, nanocrystalline graphene films,1 ­­which could lead to uptake by industry. The second was to grow monolayer graphene using CVD on a copper surface, then to transfer and integrate it into thin layers of AlGaN and GaN. This method resulted in a graphene/AlGaN junction with excellent rectifying properties, ideal for applications in switches, with an injection mechanism tuneable by modifying the AlGaN composition and thickness.2

Graphene Flagship partnering project GraNitE used their graphene nitride junction as a key building block to fabricate prototype HET devices. Their devices had a low voltage threshold and an electric current density six orders of magnitude higher than those in previous silicon tests,2 representing an important advance in the development of hybrid graphene/nitride semiconductors, and paving the way for future exploitation of this technology.

"The integration of graphene and nitride semiconductors is one of the most viable approaches to harness the unique properties of these materials for industrial applications," says Filippo Giannazzo, GraNitE Project Leader and Senior Scientist at Graphene Flagship partner CNR-IMM, Italy.

Tags:  chemical vapour deposition  CNR-IMM  Filippo Giannazzo  Graphene  Graphene Flagship  semiconductors  transistor 

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Porous graphene ribbons doped with nitrogen for electronics and quantum computing

Posted By Graphene Council, Friday, July 10, 2020
Graphene consists of a single layer of carbon atoms arranged in a honeycomb structure. The material is of interest not only in basic research but also for various applications given to its unique properties, which include excellent electrical conductivity as well as astonishing strength and rigidity. Research teams around the world are working to further expand these characteristics by substituting carbon atoms in the crystal lattice with atoms of different elements. Moreover, the electric and magnetic properties can also be modified by the formation of pores in the lattice.

Ladder-like structure

Now, a team of researchers led by the physicist Professor Ernst Meyer of the University of Basel and the chemist Dr. Shi-Xia Liu from the University of Bern have succeeded in producing the first graphene ribbons whose crystal lattice contains both periodic pores and a regular pattern of nitrogen atoms. The structure of this new material resembles a ladder, with each rung containing two atoms of nitrogen.

In order to synthesize these porous, nitrogen-containing graphene ribbons, the researchers heated the individual building blocks step by step on a silver surface in a vacuum. The ribbons are formed at temperatures up to 220°C. Atomic force microscopy allowed the researchers not only to monitor the individual steps in the synthesis, but also to confirm the perfect ladder structure - and stability - of the molecule.

Extraordinary properties

Using scanning tunneling microscopy, the scientists from the Department of Physics and the Swiss Nanoscience Institute (SNI) at the University of Basel also demonstrated that these new graphene ribbons were no longer electrical conductors, like pure graphene, but actually behaved as semiconductors. Colleagues from the Universities of Bern and Warwick confirmed these findings by performing theoretical calculations of the electronic properties. "The semiconducting properties are essential for the potential applications in electronics, as their conductivity can be adjusted specifically," says Dr. Rémy Pawlak, first author of the study.

From the literature, it is known that a high concentration of nitrogen atoms in the crystal lattice causes graphene ribbons to magnetize when subjected to a magnetic field. "We expect these porous, nitrogen-doped graphene ribbons to display extraordinary magnetic properties," says Ernst Meyer. "In the future, the ribbons could therefore be of interest for applications in quantum computing."

Tags:  Department of Physics  Ernst Meyer  Graphene  Remy Pawlak  semiconductors  Shi-Xia Liu  Swiss Nanoscience Institute  University of Basel  University of Bern 

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Scientists Use Light to Choreograph Electronic Motion in 2D Materials

Posted By Graphene Council, Monday, June 29, 2020
A team of scientists led by the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and UC Berkeley has demonstrated a powerful new technique that uses light to measure how electrons move and interact within materials. With this technique, the researchers observed exotic states of matter in stacks of atomically thin semiconductors called transition metal dichalcogenide (TMD) moiré superlattices.

Their study, which was published in the journal Nature, is the first to prove that interactions between electrons play a significant role in how charge flows in TMD moiré superlattices.

“Moiré superlattices provide a unique method for introducing exotic electronic behavior in materials where they don’t typically exist,” said lead author Emma Regan, a doctoral researcher in Berkeley Lab’s Materials Sciences Division and the UC Berkeley physics department. “Understanding and engineering electronic behavior in quantum materials may provide new approaches for electronic devices in the future.”

In most materials, electrons move fast and rarely interact. But in previous studies, other researchers have shown that a moiré superlattice – which creates an energy landscape for electrons – can slow the electrons down enough that they feel interactions between each other.

“We suspected that these electron-electron interactions in TMD moiré superlattices are very strong – even stronger than what you would find in stacks of graphene,” said Regan.

Typically, physicists investigate electron-electron interactions by attaching wires to a material and measuring how easily electrical current flows. But in stacks of TMDs, electrons don’t flow easily between the wires and the material, which makes it difficult to understand how the electrons interact.

So the researchers turned to light instead.

The research team, led by senior author Feng Wang, fabricated the TMD moiré superlattice from atomically thin layers of tungsten diselenide and tungsten disulfide – two common semiconductors known for their ability to efficiently absorb and emit light. They then formed a device just 25 nanometers (25 billionths of a meter) thick by sandwiching the tungsten diselenide/tungsten disulfide moiré superlattice between boron nitride and graphene.

In Wang’s ultrafast nano-optics lab, the researchers shone lasers on the TMD device to observe how electrons flowed in the superlattice as they varied the number of electrons injected into the material. Wang is a faculty scientist in Berkeley Lab’s Materials Sciences Division and professor of physics at UC Berkeley.

Tags:  2D materials  Emma Regan  Feng Wang  Graphene  Lawrence Berkeley National Laboratory  Semiconductors  UC Berkeley 

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Enabling room temperature ferromagnetism in monolayer MoS2 via in situ iron-doping

Posted By Graphene Council, Saturday, May 2, 2020
Two-dimensional semiconductors, including transition metal dichalcogenides, are of interest in electronics and photonics but remain nonmagnetic in their intrinsic form. Previous efforts to form two-dimensional dilute magnetic semiconductors utilized extrinsic doping techniques or bulk crystal growth, detrimentally affecting uniformity, scalability, or Curie temperature. 

Here, we demonstrate an in situ substitutional doping of Fe atoms into MoS2 monolayers in the chemical vapor deposition growth. The iron atoms substitute molybdenum sites in MoS2 crystals, as confirmed by transmission electron microscopy and Raman signatures. We uncover an Fe-related spectral transition of Fe:MoS2 monolayers that appears at 2.28 eV above the pristine bandgap and displays pronounced ferromagnetic hysteresis. 

The microscopic origin is further corroborated by density functional theory calculations of dipole-allowed transitions in Fe:MoS2. Using spatially integrating magnetization measurements and spatially resolving nitrogen-vacancy center magnetometry, we show that Fe:MoS2 monolayers remain magnetized even at ambient conditions, manifesting ferromagnetism at room temperature.

Tags:  2D materials  Graphene  Semiconductors 

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The ACS Publishes a Chicken-Sh!T Article About Graphene

Posted By Graphene Council, Friday, February 14, 2020

A joke targeted at graphene research seems driven more by envy than providing a check on its excesses 

Last month, the venerable American Chemical Society (ACS) in its journal ACS Nano decided to take a step off the path of its mandate to promote scientific discovery and thought it might be fun to ridicule it. ("Will Any Crap We Put into Graphene Increase Its Electrocatalytic Effect?" by Lu Wang, Zdenek Sofer and  Martin Pumera*

Of course, the target of the ridicule was graphene, which over the last decade has been taking much of the research funding targeted for advanced materials. Let’s say it was an easy target to malign, especially for those who are not invested in graphene’s development.

The ridicule was formulated in this way: a lot of research papers are published on how to dope graphene (add impurities to it), so what if we published a paper on someone doping graphene with chicken guano. Hysterical, right?

There’s just one problem with this joke, one of the cornerstones of semiconducting engineering for at least the last half-century has been doping. Doping is used to enhance, or just tweak, the conductivity of semiconductors by intentionally introducing impurities into them. 

To forego a long explanation of solid-state physics and band gaps, suffice it to say that digital electronics (the kind of electronics that enables you to read this post) depends on doping of electronic materials to function.

In the decade-and-a-half since graphene was first isolated, researchers have been mesmerized by its extraordinary properties and by logical extension its enormous potential in electronics. However, graphene in its pure state is not a semiconductor, but rather a conductor. In order for it to be useful in electronic applications, especially digital electronics, it needs to behave as a semiconductor: possessing the capability of starting and stopping the flow of electrons through it thereby creating the on/off states for binary digital logic.

Of course, researchers have spent countless hours researching on how to best exploit graphene for electronics—attracted to its extraordinary electronic properties—and have often been funded handsomely to do so. This funding—which has come at the expense of other lines of research (making graphene research an easy target for envy)—has been so strong because of the hope that it would lead to some breakthrough that would stave off the end of Moore’s Law. 

Moore’s Law argued back in 1965 that the number of transistors placed in an integrated circuit (IC) or chip doubles approximately every two years.  It turns out graphene hasn’t saved Moore’s Law as of yet. And Moore’s Law may have seen its road come to a dead end two years ago when chip makers just threw up their hands at a 7-nm node and said, “No more.”

This means for the last decade there has been a pressing need and a fervent hope that graphene could come to the rescue of complimentary metal-oxide-semiconductor (CMOS) digital electronics. To meet this need and interest, graphene research had to devote much of its time to doping of the material.

It is a worthy argument to contest how effective this has all been in bringing graphene closer as a viable alternative in a post-CMOS world. However, it would be silly to argue that such research should never have been undertaken, or even taken on so aggressively and broadly. There was a need and the market pulled for such a use for the material. This was not an idle or wasted effort as the ACS Nano article insinuates.

The gist of the ACS Nano article in the form of a joke was to suggest that all doping of graphene research is nonsense because so much of it is performed and published. “Look how funny it would be to dope graphene with chicken droppings. That will really stick their noses in it.” While this may be funny to some, it’s highly detrimental to the spirit and inspiration for scientific inquiry.

And it certainly doesn’t do justice to the many innovative ways that graphene and a whole class of 2D materials are being applied to solve existing and new engineering challenges. 

Tags:  2D materials  American Chemical Society  Electronics  Graphene  Lu Wang  Martin Pumera  Semiconductors  Zdenek Sofer 

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Physicist obtain atomically thin molybdenum disulfide films on large-area substrates

Posted By Graphene Council, Thursday, January 23, 2020
Researchers from the Moscow Institute of Physics and Technology have managed to grow atomically thin films of molybdenum disulfide spanning up to several tens of square centimeters. It was demonstrated that the material's structure can be modified by varying the synthesis temperature. The films, which are of interest to electronics and optoelectronics, were obtained at 900-1,000 degrees Celsius. The findings were published in the journal ACS Applied Nano Materials.

Two-dimensional materials are attracting considerable interest due to their unique properties stemming from their structure and quantum mechanical restrictions. The family of 2D materials includes metals, semimetals, semiconductors, and insulators. Graphene, which is perhaps the most famous 2D material, is a monolayer of carbon atoms. It has the highest charge-carrier mobility recorded to date. However, graphene has no band gap under standard conditions, and that limits its applications.

Unlike graphene, the optimal width of the bandgap in molybdenum disulfide (MoS2) makes it suitable for use in electronic devices. Each MoS2 layer has a sandwich structure, with a layer of molybdenum squeezed between two layers of sulfur atoms. Two-dimensional van der Waals heterostructures, which combine different 2D materials, show great promise as well. In fact, they are already widely used in energy-related applications and catalysis. Wafer-scale (large-area) synthesis of 2D molybdenum disulfide shows the potential for breakthrough advances in the creation of transparent and flexible electronic devices, optical communication for next-generation computers, as well as in other fields of electronics and optoelectronics.

"The method we came up with to synthesize MoS2 involves two steps. First, a film of MoO3 is grown using the atomic layer deposition technique, which offers precise atomic layer thickness and allows conformal coating of all surfaces. And MoO3 can easily be obtained on wafers of up to 300 millimeters in diameter. Next, the film is heat-treated in sulfur vapor. As a result, the oxygen atoms in MoO3 are replaced by sulfur atoms, and MoS2 is formed. We have already learned to grow atomically thin MoS2 films on an area of up to several tens of square centimeters," explains Andrey Markeev, the head of MIPT's Atomic Layer Deposition Lab.

The researchers determined that the structure of the film depends on the sulfurization temperature. The films sulfurized at 500 ? contain crystalline grains, a few nanometers each, embedded in an amorphous matrix. At 700 ?, these crystallites are about 10-20 nm across and the S-Mo-S layers are oriented perpendicular to the surface. As a result, the surface has numerous dangling bonds. Such structure demonstrates high catalytic activity in many reactions, including the hydrogen evolution reaction. For MoS2 to be used in electronics, the S-Mo-S layers have to be parallel to the surface, which is achieved at sulfurization temperatures of 900-1,000 ?. The resulting films are as thin as 1.3 nm, or two molecular layers, and have a commercially significant (i.e., large enough) area.

The MoS2 films synthesized under optimal conditions were introduced into metal-dielectric-semiconductor prototype structures, which are based on ferroelectric hafnium oxide and model a field-effect transistor. The MoS2 film in these structures served as a semiconductor channel. Its conductivity was controlled by switching the polarization direction of the ferroelectric layer. When in contact with MoS2, the La:(HfO2-ZrO2) material, which was earlier developed in the MIPT lab, was found to have a residual polarization of approximately 18 microcoulombs per square centimeter. With a switching endurance of 5 million cycles, it topped the previous world record of 100,000 cycles for silicon channels.

Tags:  2D materials  ACS Applied Nano Materials  Andrey Markeev  Graphene  Moscow Institute of Physics and Technology  Optoelectronics  Semiconductors 

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Graphene nanoribbons lay the groundwork for ultrapowerful computers

Posted By Graphene Council, Friday, September 13, 2019
Smaller, better semiconductors have consistently allowed computers to become faster and more energy-efficient than ever before.

But the 18-month cycle of exponential increases in computing power that has held since the mid 1960s now has leveled off. That’s because there are fundamental limits to integrated circuits made strictly from silicon—the material that forms the backbone of our modern computer infrastructure.

As they look to the future, however, engineers at the University of Wisconsin-Madison are turning to new materials to lay down the foundations for more powerful computers.

They have devised a method to grow tiny ribbons of graphene—the single-atom-thick carbon compound—directly on top of silicon wafers.

Graphene ribbons have a special advantage over the material when it’s in its more common form of a broad, flat sheet; namely, thin strips of graphene become excellent semiconductors.

“Compared to current technology, this could enable faster, low power devices,” says Vivek Saraswat, a PhD student in materials science and engineering at UW-Madison. “It could help you pack in more transistors onto chips and continue Moore’s law into the future.”

Saraswat and his colleagues published details of their work July 9, 2019, in the Journal of Physical Chemistry.

The advance could enable graphene-based integrated circuits, with much improved performance over today’s silicon chips.

“The main advantage of graphene nanoribbons is that electrons can travel faster through them, compared to silicon so you can make faster chips that use less energy,” says Mike Arnold, a professor of materials science and engineering at UW-Madison and a world expert in graphene growth.

Arnold is pioneer of a strategy to lay down long, thin strips of graphene—structures known as nanoribbons—on top a material called germanium.

That’s useful in many ways. However, since germanium isn’t a widely used semiconductor, it can’t form the basis for computer chips.

Meanwhile, other researchers have not been able to overcome a major barrier in layering graphene nanoribbons onto silicon. Graphene reacts with silicon to form an inert and less useful compound called silicon carbide.

Arnold’s group has developed an ingenious method to avoid that obstacle.

By laying down a thin protective layer of germanium before applying graphene, the researchers could successfully grow graphene nanoribbons on top of silicon wafers. The thin germanium layers protected graphene from reacting with silicon, yet didn’t interfere with the nanoribbons’ semiconducting capabilities.

It’s an important first step toward creating graphene-based integrated circuits. And because the base layer is composed of silicon, the graphene nanoribbon technology can be easily integrated into existing electronic/computing components.

“Our vision is to integrate graphene with existing devices,” says Arnold.

The scientists have patented their technology through the Wisconsin Alumni Research Foundation. One advantage of their synthesis approach is that it takes advantage of a scalable, industry-compatible chemical vapor deposition technique. Now, they’re working to improve the precision with which they lay down their nanoribbons so that they can achieve the complex patterns found in modern computer chips.

“We are using a few strategies to control the thickness and the orientation for the nanoribbons,” says Arnold. “We have a few really cool ideas.”

Tags:  Graphene  Graphene Nanoribbons  Mike Arnold  Semiconductors  University of Wisconsin-Madison  Vivek Saraswat 

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