Print Page | Contact Us | Report Abuse | Sign In | Register
Graphene Updates
Blog Home All Blogs

Engineers mix and match materials to make new stretchy electronics

Posted By Graphene Council, Saturday, February 8, 2020
At the heart of any electronic device is a cold, hard computer chip, covered in a miniature city of transistors and other semiconducting elements. Because computer chips are rigid, the electronic devices that they power, such as our smartphones, laptops, watches, and televisions, are similarly inflexible.

Now a process developed by MIT engineers may be the key to manufacturing flexible electronics with multiple functionalities in a cost-effective way.

The process is called  “remote epitaxy” and involves growing thin films of semiconducting material on a large, thick wafer of the same material, which is covered in an intermediate layer of graphene. Once the researchers grow a semiconducting film, they can peel it away from the graphene-covered wafer and then reuse the wafer, which itself can be expensive depending on the type of material it’s made from. In this way, the team can copy and peel away any number of thin, flexible semiconducting films, using the same underlying wafer.

In a paper published today in the journal Nature, the researchers demonstrate that they can use remote epitaxy to produce freestanding films of any functional material. More importantly, they can stack films made from these different materials, to produce flexible, multifunctional electronic devices.

The researchers expect that the process could be used to produce stretchy electronic films for a wide variety of uses, including virtual reality-enabled contact lenses, solar-powered skins that mold to the contours of your car, electronic fabrics that respond to the weather, and other flexible electronics that seemed until now to be the stuff of Marvel movies.

“You can use this technique to mix and match any semiconducting material to have new device functionality, in one flexible chip,” says Jeehwan Kim, an associate professor of mechanical engineering at MIT. “You can make electronics in any shape.”

Buying time

Kim and his colleagues reported their first results using remote epitaxy in 2017. Then, they were able to produce thin, flexible films of semiconducting material by first placing a layer of graphene on a thick, expensive wafer made from a combination of exotic metals. They flowed atoms of each metal over the graphene-covered wafer and found the atoms formed a film on top of the graphene, in the same crystal pattern as the underlying wafer. The graphene provided a nonstick surface from which the researchers could peel away the new film, leaving the graphene-covered wafer, which they could reuse. 

In 2018, the team showed that they could use remote epitaxy to make semiconducting materials from metals in groups 3 and 5 of the periodic table, but not from group 4. The reason, they found, boiled down to polarity, or the respective charges between the atoms flowing over graphene and the atoms in the underlying wafer.

Since this realization, Kim and his colleagues have tried a number of increasingly exotic semiconducting combinations. As reported in this new paper, the team used remote epitaxy to make flexible semiconducting films from complex oxides — chemical compounds made from oxygen and at least two other elements. Complex oxides are known to have a wide range of electrical and magnetic properties, and some combinations can generate a current when physically stretched or exposed to a magnetic field.

Kim says the ability to manufacture flexible films of complex oxides could open the door to new energy-havesting devices, such as sheets or coverings that stretch in response to vibrations and produce electricity as a result. Until now, complex oxide materials have only been manufactured on rigid, millimeter-thick wafers, with limited flexibility and therefore limited energy-generating potential.

The researchers did have to tweak their process to make complex oxide films. They initially found that when they tried to make a complex oxide such as strontium titanate (a compound of strontium, titanium, and three oxygen atoms), the oxygen atoms that they flowed over the graphene tended to bind with the graphene’s carbon atoms, etching away bits of graphene instead of following the underlying wafer’s pattern and binding with strontium and titanium. As a surprisingly simple fix, the researchers added a second layer of graphene.

“We saw that by the time the first layer of graphene is etched off, oxide compounds have already formed, so elemental oxygen, once it forms these desired compounds, does not interact as heavily with graphene,” Kim explains. “So two layers of graphene buys some time for this compound to form.”

Peel and stack

The team used their newly tweaked process to make films from multiple complex oxide materials, peeling off each 100-nanometer-thin layer as it was made. They were also able to stack together layers of different complex oxide materials and effectively glue them together by heating them slightly, producing a flexible, multifunctional device.

“This is the first demonstration of stacking multiple nanometers-thin membranes like LEGO blocks, which has been impossible because all functional electronic materials exist in a thick wafer form,” Kim says.

In one experiment, the team stacked together films of two different complex oxides: cobalt ferrite, known to expand in the presence of a magnetic field, and PMN-PT, a material that generates voltage when stretched. When the researchers exposed the multilayer film to a magnetic field, the two layers worked together to both expand and produce a small electric current. 

The results demonstrate that remote epitaxy can be used to make flexible electronics from a combination of materials with different functionalities, which previously were difficult to combine into one device. In the case of cobalt ferrite and PMN-PT, each material has a different crystalline pattern. Kim says that traditional epitaxy techniques, which grow materials at high temperatures on one wafer, can only combine materials if their crystalline patterns match. He says that with remote epitaxy, researchers can make any number of different films, using different, reusable wafers, and then stack them together, regardless of their crystalline pattern.

“The big picture of this work is, you can combine totally different materials in one place together,” Kim says. “Now you can imagine a thin, flexible device made from layers that include a sensor, computing system, a battery, a solar cell, so you could have a flexible, self-powering, internet-of-things stacked chip.”

The team is exploring various combinations of semiconducting films and is working on developing prototype devices, such as something Kim is calling an “electronic tattoo” — a flexible, transparent chip that can attach and conform to a person’s body to sense and wirelessly relay vital signs such as temperature and pulse. “We can now make thin, flexible, wearable electronics with the highest functionality,” Kim says. “Just peel off and stack up.”

Tags:  Electronics  Graphene  Jeehwan Kim  MIT  Semiconductor 

Share |
PermalinkComments (0)
 

Crystal-stacking process can produce new materials for high-tech devices

Posted By Graphene Council, Saturday, February 8, 2020
The magnetic, conductive and optical properties of complex oxides make them key to components of next-generation electronics used for data storage, sensing, energy technologies, biomedical devices and many other applications.

Stacking ultrathin complex oxide single-crystal layers -- those composed of geometrically arranged atoms -- allows researchers to create new structures with hybrid properties and multiple functions. Now, using a new platform developed by engineers at the University of Wisconsin-Madison and the Massachusetts Institute of Technology, researchers will be able to make these stacked-crystal materials in virtually unlimited combinations.

Epitaxy is the process for depositing one material on top of another in an orderly way. The researchers' new layering method overcomes a major challenge in conventional epitaxy -- that each new complex oxide layer must be closely compatible with the atomic structure of the underlying layer. It's sort of like stacking Lego blocks: The holes on the bottom of one block must align with the raised dots atop the other. If there's a mismatch, the blocks won't fit together properly.

"The advantage of the conventional method is that you can grow a perfect single crystal on top of a substrate, but you have a limitation," says Chang-Beom Eom, a UW-Madison professor of materials science and engineering and physics. "When you grow the next material, your structure has to be the same and your atomic spacing must be similar. That's a constraint, and beyond that constraint, it doesn't grow well."

A couple of years ago, a team of MIT researchers developed an alternate approach. Led by Jeehwan Kim, an associate professor in mechanical engineering and materials science and engineering at MIT, the group added an ultrathin intermediate layer of a unique carbon material called graphene, then used epitaxy to grow a thin semiconducting material layer atop that. Just one molecule thick, the graphene acts like a peel-away backing due to its weak bonding. The researchers could remove the semiconductor layer from the graphene. What remained was a freestanding ultrathin sheet of semiconducting material.

Eom, an expert in complex oxide materials, says they are intriguing because they have a wide range of tunable properties -- including multiple properties in one material -- that many other materials do not. So, it made sense to apply the peel-away technique to complex oxides, which are much more challenging to grow and integrate.

"If you have this kind of cut-and-paste growth and removal, combined with the different functionality of putting single-crystal oxide materials together, you have a tremendous possibility for making devices and doing science," says Eom, who connected with mechanical engineers at MIT during a sabbatical there in 2014.

The Eom and Kim research groups combined their expertise to create ultrathin complex oxide single-crystal layers, again using graphene as the peel-away intermediate. More importantly, however, they conquered a previously insurmountable obstacle -- the difference in crystal structure -- in integrating different complex oxide materials.

"Magnetic materials have one crystal structure, while piezoelectric materials have another," says Eom. "So you cannot grow them on top of each other. When you try to grow them, it just becomes messy. Now we can grow the layers separately, peel them off, and integrate them."

In its research, the team demonstrated the efficacy of the technique using materials such as perovskite, spinel and garnet, among several others. They also can stack single complex oxide materials and semiconductors.

"This opens up the possibility for the study of new science, which has never been possible in the past because we could not grow it," says Eom. "Stacking these was impossible, but now it is possible to imagine infinite combinations of materials. Now we can put them together."

The advance also opens doors to new materials with functionalities that drive future technologies. "This advance, which would have been impossible using conventional thin film growth techniques, clears the way for nearly limitless possibilities in materials design," says Evan Runnerstrom, program manager in materials design in the Army Research Office, which funded part of the research. "The ability to create perfect interfaces while coupling disparate classes of complex materials may enable entirely new behaviors and tunable properties, which could potentially be leveraged for new Army capabilities in communications, reconfigurable sensors, low power electronics, and quantum information science."

Tags:  Chang-Beom Eom  Evan Runnerstrom  Graphene  Jeehwan Kim  Massachusetts Institute of Technology  Semiconductor  U.S. Army Research Office  University of Wisconsin-Madison 

Share |
PermalinkComments (0)