Silicon—the shiny, brittle metal commonly used to make semiconductors—is an essential ingredient of modern-day electronics. But as electronic devices have become smaller and smaller, creating tiny silicon components that fit inside them has become more challenging and more expensive.
Now, UCLA chemists have developed a new method to produce nanoribbons of graphene, next-generation structures that many scientists believe will one day power electronic devices.
This research is published online in the Journal of the American Chemical Society.
The nanoribbons are extremely narrow strips of graphene, the width of just a few carbon atoms. They’re useful because they possess a bandgap, which means that electrons must be “pushed” to flow through them to create electrical current, said Yves Rubin, a professor of chemistry in the UCLA College and the lead author of the research.
“A material that has no bandgap lets electrons flow through unhindered and cannot be used to build logic circuits,” he said.
Rubin and his research team constructed graphene nanoribbons molecule by molecule using a simple reaction based on ultraviolet light and exposure to 600-degree heat.
“Nobody else has been able to do that, but it will be important if one wants to build these molecules on an industrial scale,” said Rubin, who also is a member of the California NanoSystems Institute at UCLA.
The process improves upon other existing methods for creating graphene nanoribbons, one of which involves snipping open tubes of graphene known as carbon nanotubes. That particular approach is imprecise and produces ribbons of inconsistent sizes—a problem because the value of a nanoribbon’s bandgap depends on its width, Rubin said.
To create the nanoribbons, the scientists started by growing crystals of four different colorless molecules. The crystals locked the molecules into the perfect orientation to react, and the team then used light to stitch the molecules into polymers, which are large structures made of repeating units of carbon and hydrogen atoms.
The scientists then placed the shiny, deep blue polymers in an oven containing only argon gas and heated them to 600 degrees Celsius. The heat provided the necessary boost of energy for the polymers to form the final bonds that gave the nanoribbons their final shape: hexagonal rings composed of carbon atoms, and hydrogen atoms along the edges of the ribbons.
“We’re essentially charring the polymers, but we’re doing it in a controlled way,” Rubin said.
The process, which took about an hour, yielded graphene nanoribbons just eight carbon atoms wide but thousands of atoms long. The scientists verified the molecular structure of the nanoribbons, which were deep black in color and lustrous, by shining light of different wavelengths at them.
“We looked at what wavelengths of light were absorbed,” Rubin said. “This reveals signatures of the structure and composition of the ribbons.”
The researchers have filed a patent application for the process.
Rubin said the team now is studying how to better manipulate the nanoribbons—a challenge because they tend to stick together.
“Right now, they are bundles of fibers,” Rubin said. “The next step will be able to handle each nanoribbon one by one.”
More information: Robert S. Jordan et al. Synthesis of N = 8 Armchair Graphene Nanoribbons from Four Distinct Polydiacetylenes, Journal of the American Chemical Society (2017). DOI: 10.1021/jacs.7b08800
Researchers have designed a light-emitter and detector that can be integrated into silicon CMOS chips. This illustration shows a molybdenum ditelluride light source for silicon photonics. Image: Sampson Wilcox
Ultrathin films of a semiconductor that emits and detects light can be stacked on top of silicon wafers.
The huge increase in computing performance in recent decades has been achieved by squeezing ever more transistors into a tighter space on microchips.
However, this downsizing has also meant packing the wiring within microprocessors ever more tightly together, leading to effects such as signal leakage between components, which can slow down communication between different parts of the chip. This delay, known as the “interconnect bottleneck,” is becoming an increasing problem in high-speed computing systems.
One way to tackle the interconnect bottleneck is to use light rather than wires to communicate between different parts of a microchip. This is no easy task, however, as silicon, the material used to build chips, does not emit light easily, according to Pablo Jarillo-Herrero, an associate professor of physics at MIT.
Now, in a paper published today in the journal Nature Nanotechnology, researchers describe a light emitter and detector that can be integrated into silicon CMOS chips. The paper’s first author is MIT postdoc Ya-Qing Bie, who is joined by Jarillo-Herrero and an interdisciplinary team including Dirk Englund, an associate professor of electrical engineering and computer science at MIT.
The device is built from a semiconductor material called molybdenum ditelluride. This ultrathin semiconductor belongs to an emerging group of materials known as two-dimensional transition-metal dichalcogenides.
Unlike conventional semiconductors, the material can be stacked on top of silicon wafers, Jarillo-Herrero says.
“Researchers have been trying to find materials that are compatible with silicon, in order to bring optoelectronics and optical communication on-chip, but so far this has proven very difficult,” Jarillo-Herrero says. “For example, gallium arsenide is very good for optics, but it cannot be grown on silicon very easily because the two semiconductors are incompatible.”
In contrast, the 2-D molybdenum ditelluride can be mechanically attached to any material, Jarillo-Herrero says.
Another difficulty with integrating other semiconductors with silicon is that the materials typically emit light in the visible range, but light at these wavelengths is simply absorbed by silicon.
Molybdenum ditelluride emits light in the infrared range, which is not absorbed by silicon, meaning it can be used for on-chip communication.
To use the material as a light emitter, the researchers first had to convert it into a P-N junction diode, a device in which one side, the P side, is positively charged, while the other, N side, is negatively charged.
In conventional semiconductors, this is typically done by introducing chemical impurities into the material. With the new class of 2-D materials, however, it can be done by simply applying a voltage across metallic gate electrodes placed side-by-side on top of the material.
“That is a significant breakthrough, because it means we do not need to introduce chemical impurities into the material [to create the diode]. We can do it electrically,” Jarillo-Herrero says.
Once the diode is produced, the researchers run a current through the device, causing it to emit light.
“So by using diodes made of molybdenum ditelluride, we are able to fabricate light-emitting diodes (LEDs) compatible with silicon chips,” Jarillo-Herrero says.
The device can also be switched to operate as a photodetector, by reversing the polarity of the voltage applied to the device. This causes it to stop conducting electricity until a light shines on it, when the current restarts.
In this way, the devices are able to both transmit and receive optical signals.
The device is a proof of concept, and a great deal of work still needs to be done before the technology can be developed into a commercial product, Jarillo-Herrero says.
This paper fills an important gap in integrated photonics, by realizing a high-performance silicon-CMOS-compatible light source, says Frank Koppens, a professor of quantum nano-optoelectronics at the Institute of Photonic Sciences in Barcelona, Spain, who was not involved in the research.
“This work shows that 2-D materials and Si-CMOS and silicon photonics are a natural match, and we will surely see many more applications coming out of this [area] in the years to come,” Koppens says.
The researchers are now investigating other materials that could be used for on-chip optical communication.
Most telecommunication systems, for example, operate using light with a wavelength of 1.3 or 1.5 micrometers, Jarillo-Herrero says.
However, molybdenum ditelluride emits light at 1.1 micrometers. This makes it suitable for use in the silicon chips found in computers, but unsuitable for telecommunications systems.
“It would be highly desirable if we could develop a similar material, which could emit and detect light at 1.3 or 1.5 micrometers in wavelength, where telecommunication through optical fiber operates,” he says.
To this end, the researchers are exploring another ultrathin material called black phosphorus, which can be tuned to emit light at different wavelengths by altering the number of layers used. They hope to develop devices with the necessary number of layers to allow them to emit light at the two wavelengths while remaining compatible with silicon.
“The hope is that if we are able to communicate on-chip via optical signals instead of electronic signals, we will be able to do so more quickly, and while consuming less power,” Jarillo-Herrero says.
The research was supported by Center for Excitonics, an EFRC funded by the U.S. Department of Energy.
Graphene is considered as one of the most promising new materials. However, the systematic insertion of chemically bound atoms and molecules to control its properties is still a major challenge. Now, for the first time, scientists of the Friedrich-Alexander-Universität Erlangen-Nürnberg, the University of Vienna, the Freie Universität Berlin and the University Yachay Tech in Ecuador succeeded in precisely verifying the spectral fingerprint of such compounds in both theory and experiment. Their results are published in the scientific journal Nature Communications.
Two-dimensional graphene consists of single layers of carbon atoms and exhibits intriguing properties. The transparent material conducts electricity and heat extremely well. It is at the same time flexible and solid. Additionally, the electrical conductivity can be continuously varied between a metal and a semiconductor by, e.g., inserting chemically bound atoms and molecules into the graphene structure – the so-called functional groups. These unique properties offer a wide range of future applications as e.g. for new developments in optoelectronics or ultrafast components in the semiconductor industry. However, a successful use of graphene in the semiconductor industry can only be achieved if properties such as the conductivity, the size and the defects of the graphene structure induced by the functional groups can already be modulated during the synthesis of graphene.
In an international collaboration scientists led by Andreas Hirsch from the Friedrich-Alexander-Universität Erlangen-Nürnberg in close cooperation with Thomas Pichler from the University of Vienna accomplished a crucial breakthrough: using the latter’s newly developed experimental set-up they were able to identify, for the first time, vibrational spectra as the specific fingerprints of step-by-step chemically modified graphene by means of light scattering. This spectral signature, which was also theoretically attested, allows to determine the type and the number of functional groups in a fast and precise way. Among the reactions they examined, was the chemical binding of hydrogen to graphene. This was implemented by a controlled chemical reaction between water and particular compounds in which ions are inserted in graphite, a crystalline form of carbon.
“This method of the in-situ Raman spectroscopy is a highly effective technique which allows controlling the function of graphene in a fast, contact-free and extensive way already during the production of the material,” says J. Chacon from Yachay Tech, one of the two lead authors of the study. This enables the production of tailored graphene-based materials with controlled electronic transport properties and their utilisation in semiconductor industry.
No more error-prone evaporation deposition, drop casting or printing: Scientists at LMU Munich and FSU Jena have developed organic semiconductor nanosheets, which can easily be removed from a growth substrate and placed on other substrates.
Today’s computer processors are composed of billions of transistors. These electronic components normally consist of semiconductor material, insulator, substrate, and electrode. A dream of many scientists is to have each of these elements available as transferable sheets, which would allow them to design new electronic devices simply by stacking.
This has now become a reality for the organic semiconductor material pentacene: Dr. Bert Nickel, a physicist at LMU Munich, and Professor Andrey Turchanin (Friedrich Schiller University Jena), together with their teams, have, for the first time, managed to create mechanically stable pentacene nanosheets.
The researchers describe their method in the journal Advanced Materials. They first cover a small silicon wafer with a thin layer of a water-soluble organic film and deposit pentacene molecules upon it until a layer roughly 50 nanometers thick has formed. The next step is crucial: by irradiation with low-energy electrons, the topmost three to four levels of pentacene molecular layers are crosslinked, forming a “skin” that is only about five nanometers thick. This crosslinked layer stabilizes the entire pentacene film so well that it can be removed as a sheet from a silicon wafer in water and transferred to another surface using ordinary tweezers.
Apart from the ability to transfer them, the new semiconductor nanosheets have other advantages. The new method does not require any potentially interfering solvents, for example. In addition, after deposition, the nanosheet sticks firmly to the electrical contacts by van der Waals forces, resulting in a low contact resistance of the final electronic devices. Last but not least, organic semiconductor nanosheets can now be deposited onto significantly more technologically relevant substrates than hitherto.
Of particular interest is the extremely high mechanical stability of the newly developed pentacene nanosheets, which enables them to be applied as free-standing nanomembranes to perforated substrates with dimensions of tens of micrometers. That is equivalent to spanning a 25-meter pool with plastic wrap. “These virtually freely suspended semiconductors have great potential,” explains Nickel. “They can be accessed from two sides and could be connected through an electrolyte, which would make them ideal as biosensors, for example”. “Another promising application is their implementation in flexible electronics for manufacturing of devices for vital data acquisition or production of displays and solar cells,” Turchanin says.
In 2016, annual global semiconductor sales reached their highest-ever point, at $339 billion worldwide. In that same year, the semiconductor industry spent about $7.2 billion worldwide on wafers that serve as the substrates for microelectronics components, which can be turned into transistors, light-emitting diodes, and other electronic and photonic devices.
A new techniquedeveloped by MIT engineers may vastly reduce the overall cost of wafer technology and enable devices made from more exotic, higher-performingsemiconductormaterials than conventional silicon.
The new method, reported today in Nature, uses graphene—single-atom-thin sheets of graphite—as a sort of “copy machine” to transfer intricate crystalline patterns from an underlying semiconductor wafer to a top layer of identical material.
The engineers worked out carefully controlled procedures to place single sheets of graphene onto an expensive wafer. They then grew semiconducting material over the graphene layer. They found that graphene is thin enough to appear electrically invisible, allowing the top layer to see through the graphene to the underlying crystalline wafer, imprinting its patterns without being influenced by the graphene.
Graphene is also rather “slippery” and does not tend to stick to other materials easily, enabling the engineers to simply peel the top semiconducting layer from the wafer after its structures have been imprinted.
Jeehwan Kim, the Class of 1947 Career Development Assistant Professor in the departments of Mechanical Engineering and Materials Science and Engineering, says that in conventional semiconductor manufacturing, the wafer, once its crystalline pattern is transferred, is so strongly bonded to the semiconductor that it is almost impossible to separate without damaging both layers.
“You end up having to sacrifice the wafer—it becomes part of the device,” Kim says.
With the group’s new technique, Kim says manufacturers can now use graphene as an intermediate layer, allowing them to copy and paste the wafer, separate a copied film from the wafer, and reuse the wafer many times over. In addition to saving on the cost of wafers, Kim says this opens opportunities for exploring more exotic semiconductor materials.
“The industry has been stuck on silicon, and even though we’ve known about better performing semiconductors, we haven’t been able to use them, because of their cost,” Kim says. “This gives the industry freedom in choosing semiconductor materials by performance and not cost.”
Kim’s research team discovered this new technique at MIT’s Research Laboratory of Electronics. Kim’s MIT co-authors are first author and graduate student Yunjo Kim; graduate students Samuel Cruz, Babatunde Alawonde, Chris Heidelberger, Yi Song, and Kuan Qiao; postdocs Kyusang Lee, Shinhyun Choi, and Wei Kong; visiting research scholar Chanyeol Choi; Merton C. Flemings-SMA Professor of Materials Science and Engineering Eugene Fitzgerald; professor of electrical engineering and computer science Jing Kong; and assistant professor of mechanical engineering Alexie Kolpak; along with Jared Johnson and Jinwoo Hwang from Ohio State University, and Ibraheem Almansouri of Masdar Institute of Science and Technology.
Since graphene’s discovery in 2004, researchers have been investigating its exceptional electrical properties in hopes of improving the performance and cost of electronic devices. Graphene is an extremely good conductor of electricity, as electrons flow through graphene with virtually no friction. Researchers, therefore, have been intent on finding ways to adapt graphene as a cheap, high-performance semiconducting material.
“People were so hopeful that we might make really fast electronic devices from graphene,” Kim says. “But it turns out it’s really hard to make a good graphene transistor.”
In order for a transistor to work, it must be able to turn a flow of electrons on and off, to generate a pattern of ones and zeros, instructing a device on how to carry out a set of computations. As it happens, it is very hard to stop the flow of electrons through graphene, making it an excellent conductor but a poor semiconductor.
Kim’s group took an entirely new approach to using graphene in semiconductors. Instead of focusing on graphene’s electrical properties, the researchers looked at the material’s mechanical features.
“We’ve had a strong belief in graphene, because it is a very robust, ultrathin, material and forms very strong covalent bonding between its atoms in the horizontal direction,” Kim says. “Interestingly, it has very weak Van der Waals forces, meaning it doesn’t react with anything vertically, which makes graphene’s surface very slippery.”
Copy and peel
The team now reports that graphene, with its ultrathin, Teflon-like properties, can be sandwiched between a wafer and its semiconducting layer, providing a barely perceptible, nonstick surface through which the semiconducting material’s atoms can still rearrange in the pattern of the wafer’s crystals. The material, once imprinted, can simply be peeled off from the graphene surface, allowing manufacturers to reuse the original wafer.
The team found that its technique, which they term “remote epitaxy,” was successful in copying and peeling off layers of semiconductors from the same semiconductor wafers. The researchers had success in applying their technique to exotic wafer and semiconducting materials, including indium phosphide, gallium arsenenide, and gallium phosphide—materials that are 50 to 100 times more expensive than silicon.
Kim says that this new technique makes it possible for manufacturers to reuse wafers—of silicon and higher-performing materials—”conceptually, ad infinitum.”
An exotic future
The group’s graphene-based peel-off technique may also advance the field of flexible electronics. In general, wafers are very rigid, making the devices they are fused to similarly inflexible. Kim says now, semiconductor devices such as LEDs and solar cells can be made to bend and twist. In fact, the group demonstrated this possibility by fabricating a flexible LED display, patterned in the MIT logo, using their technique.
“Let’s say you want to install solar cells on your car, which is not completely flat—the body has curves,” Kim says. “Can you coat your semiconductor on top of it? It’s impossible now, because it sticks to the thick wafer. Now, we can peel off, bend, and you can do conformal coating on cars, and even clothing.”
Going forward, the researchers plan to design a reusable “mother wafer” with regions made from different exotic materials. Using graphene as an intermediary, they hope to create multifunctional, high-performance devices. They are also investigating mixing and matching various semiconductors and stacking them up as a multimaterial structure.
“Now, exotic materials can be popular to use,” Kim says. “You don’t have to worry about the cost of the wafer. Let us give you the copy machine. You can grow your semiconductor device, peel it off, and reuse the wafer.”
University of Utah engineers have discovered a new kind of 2D semiconducting material for electronics that opens the door for much speedier computers and smartphones that also consume a lot less power.
The semiconductor, made of the elements tin and oxygen, or tin monoxide (SnO), is a layer of 2D material only one atom thick, allowing electrical charges to move through it much faster than conventional 3D materials such as silicon. This material could be used in transistors, the lifeblood of all electronic devices such as computer processors and graphics processors in desktop computers and mobile devices. The material was discovered by a team led by University of Utah materials science and engineering associate professor Ashutosh Tiwari.
A paper describing the research was published online Monday, Feb. 15, 2016 in the journal, Advanced Electronic Materials. The paper, which also will be the cover story on the printed version of the journal, was co-authored by University of Utah materials science and engineering doctoral students K. J. Saji and Kun Tian, and Michael Snure of the Wright-Patterson Air Force Research Lab near Dayton, Ohio.
Transistors and other components used in electronic devices are currently made of 3D materials such as silicon and consist of multiple layers on a glass substrate. But the downside to 3D materials is that electrons bounce around inside the layers in all directions.
The benefit of 2D materials, which is an exciting new research field that has opened up only about five years ago, is that the material is made of one layer the thickness of just one or two atoms. Consequently, the electrons “can only move in one layer so it’s much faster,” says Tiwari.
While researchers in this field have recently discovered new types of 2D material such as graphene, molybdenun disulfide and borophene, they have been materials that only allow the movement of N-type, or negative, electrons. In order to create an electronic device, however, you need semiconductor material that allows the movement of both negative electrons and positive charges known as “holes.” The tin monoxide material discovered by Tiwari and his team is the first stable P-type 2D semiconductor material ever in existence.
“Now we have everything—we have P-type 2D semiconductors and N-type 2D semiconductors,” he says. “Now things will move forward much more quickly.”
Now that Tiwari and his team have discovered this new 2D material, it can lead to the manufacturing of transistors that are even smaller and faster than those in use today. A computer processor is comprised of billions of transistors, and the more transistors packed into a single chip, the more powerful the processor can become.
Transistors made with Tiwari’s semiconducting material could lead to computers and smartphones that are more than 100 times faster than regular devices. And because the electrons move through one layer instead of bouncing around in a 3D material, there will be less friction, meaning the processors will not get as hot as normal computer chips. They also will require much less power to run, a boon for mobile electronics that have to run on battery power. Tiwari says this could be especially important for medical devices such as electronic implants that will run longer on a single battery charge.
“The field is very hot right now, and people are very interested in it,” Tiwari says. “So in two or three years we should see at least some prototype device.”