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.”