new applications of graphene

Ruchica Kumar

Novocus Legal LLP

11/8/2016

New Applications of Graphene

RUCHICA KUMAR

11/8/16

NEW APPLICATIONS OF GRAPHENE

INTRODUCTION

Purpose of this document is to provide readers with a glimpse of new applications of well-

known nanomaterial – Graphene. These applications have been reported within date range 3

November 2016 and 8 November 2016. We have compiled this document from reported facts

and our sources are also given herein.

USE OF GRAPHENE TEMPLATES TO MAKE NEW METAL-OXIDE NANOSTRUCTURES

METAL-OXIDE FILMS WITH WRINKLES AND CRUMPLES TRANSFERRED FROM GRAPHENE

TEMPLATES HAVE IMPROVED PROPERTIES AS CATALYSTS AND ELECTRODES

(Researchers from Brown University have developed a method of using graphene templates to make

metal-oxide films with intricate surface textures. A study shows that those textures can enhance the

performance of the films as battery electrodes and as photo catalysts. Credit: Hurt lab / Wong lab /

Brown University)1

Researchers from Brown University have found a new method for making ultrathin metal-

oxide sheets containing intricate wrinkle and crumple patterns2. In a study published in the

journal ACS Nano, the researchers show that the textured metal-oxide films have better

performance when used as photo catalysts and as battery electrodes.

1 http://phys.org/news/2016-11-graphene-templates-metal-oxide-nanostructures.html 2 https://news.brown.edu/articles/2016/11/templates

http://phys.org/news/2016-11-graphene-templates-metal-oxide-nanostructures.html

https://news.brown.edu/articles/2016/11/templates

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The new findings build on previous work done by the same research group in which they

developed a method for introducing finely tuned wrinkle and crumple textures into sheets of

the nanomaterial graphene oxide. The study showed that the process enhanced some of

graphene’s properties. The textures made the graphene better able to repel water, which

would be useful in making water-resistant coatings, and enhanced graphene’s ability to

conduct electricity.

The researchers thought that similar structures might enhance the properties of other

materials — specifically metal oxides — but there’s a problem. To introduce wrinkle and

crumple structures in graphene, the team compressed the sheets multiple times in multiple

orientations. That process won’t work for metal oxides.

“Metal oxides are too stiff,” said Po-Yen Chen, a Hibbitt Postdoctoral Researcher in Brown’s

School of Engineering who led the work. “If you try to compress them, they crack.”

So, Chen, working with the labs of Robert Hurt and Ian Y. Wong, both engineering professors

at Brown, developed a method of using the crumpled graphene sheets as templates for

making crumpled metal-oxide films.

“We showed that we can transfer those surface features from the graphene onto the metal

oxides,” Chen said.

The team started by making stacks of crumpled graphene sheets using the method they had

developed previously. They deposited the graphene on a polymer substrate that shrinks when

heated. As the substrate shrinks, it compresses the graphene sitting on top, creating wrinkle

or crumple structures. The substrate is then removed, leaving free-standing sheets of

crumpled graphene behind. The compression process can be done multiple times, creating

ever more complex structures. The process also allows control of what types of textures are

formed. Clamping shrink film on opposite sides and shrinking it in only one direction creates

periodic wrinkles. Shrinking in all directions creates crumples. These shrinks can be performed

multiple times in multiple configurations to create a wide variety of textures.

To transfer those patterns onto metal oxides, Chen placed the stacks of wrinkled graphene

sheets in a water-based solution containing positively charged metal ions. The negatively

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charged graphene pulled those ions into the spaces between the sheets. The particles bonded

together within the interlayer space, creating thin sheets of metal that followed the wrinkle

patterns of the graphene. The graphene was then oxidized away, leaving the wrinkled metal-

oxide sheets. Chen showed that the process works with a variety of metal oxides — zinc,

aluminium, manganese and copper oxides.

Once they had made the materials, the researchers then tested them to see if, as was the

case with graphene, the textured surfaces enhanced the metal oxides’ properties.

They showed that wrinkled manganese oxide, when used as a battery electrode, had charge-

carrying capacity that was four times higher than a planar sheet. That’s probably because the

wrinkle ridges give electrons a defined path to follow, enabling the material to carry more of

them at a time, the researchers say.

The team also tested the ability of crumpled zinc oxide to perform a photocatalytic reaction

— reducing a dye dissolved in water under ultraviolet light. The experiment showed the

crumpled zinc oxide film to be four times more reactive than a planar film. That’s probably

because the crumpled films have higher surface area, which give the material more reactive

sites, Chen said.

In addition to improving the properties of the metals, Chen points out that the process also

represents a way of making thin films out of materials that don’t normally lend themselves to

ultrathin configurations.

“Using graphene confinement, we can guide the assembly and synthesis of materials in two

dimensions,” he said. “Based on what we learned from making the metal-oxide films, we can

start to think about using this method to make new 2D materials that are otherwise unstable

in bulk solution. But with our confinement method, we think it’s possible.”

In addition to Chen, Hurt and Wong, other authors on the paper were Muchun Liu, Thomas

Valentin, Zhongying Wang, Ruben Spitz Steinberg and Jaskiranjeet Sodhi. The work was

supported by the Hibbitt Engineering Postdoctoral Fellowship and seed funding from Brown

University.

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GRAPHENE BALLOONS SHOW THEIR COLORS

RESEARCHERS FROM THE GRAPHENE FLAGSHIP HAVE FOUND A NEW POTENTIAL

APPLICATION FOR GRAPHENE: MECHANICAL PIXELS.

By applying a pressure difference across graphene membranes, the perceived color of the

graphene can be shifted continuously from red to blue. This effect could be exploited for use

as colored pixels in e-readers and other low-powered screens. The research was a

collaborative effort from researchers at TU Delft, Netherlands, and Graphenea, Spain, and the

study has recently been published in the journal Nano Letters3.

(Artist's impression of graphene balloons showing colors. Under large deformations, Newton rings

appear. Credit: Delft University of Technology)

In graphene balloon devices, a double layer of graphene two atoms thick is deposited on top

of circular indents cut into silicon. The graphene membranes enclose air inside the cavities,

and the position of the membranes can be changed by applying a pressure difference

3 http://phys.org/news/2016-11-graphene-balloons.html

http://phys.org/news/2016-11-graphene-balloons.html

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between the inside and the outside. When the membranes are closer to the silicon they

appear blue; when the membranes are pushed away they appear red.

The color change effect arises from interference between light waves reflected from the

bottom of the cavity and the membrane on top. These reflected waves interfere

constructively or destructively depending on the position of the membrane – either adding

up or cancelling out different parts of the spectrum of white light. This interference enhances

or reduces certain colors in the reflected light.

Dr. Samer Houri, a researcher at TU Delft, led the exiting work. "At the beginning, we did not

pay attention to the colors of the membranes because graphene is 'colorless' when isolated.

However, we observed Newton rings and noticed their color changing over time," he said.

When the membranes are extremely deformed, their color is no longer homogeneous, but

instead circular rings appear. These rings are called Newton rings in honor of Sir Isaac Newton,

who studied them in 1717.

Santiago Cartamil-Bueno is a Ph.D. student at TU Delft, who carried out the experimental

work and was first to observe the change in color. "Not only does this provide the colorimetry

technique for characterizing suspended graphene, which is useful for companies developing

graphene mechanical sensors, but it also provides a means to implement display technology

based on interferometry modulation," says Cartamil-Bueno. Interferometry modulation, or

IMOD, is employed in displays that have low-power consumption requirements, such as smart

watches and e-books, and is increasing in importance for Internet of Things devices. Currently,

such displays are composed of mechanical pixels made of silicon materials. "By instead using

graphene, with its extraordinary mechanical properties," Cartamil-Bueno says, "a GIMOD

(Graphene IMOD) could drastically improve the device performance –power consumption,

pixel response time, failure rates, etc.– while enabling electrical integration and even flexible

devices." The researchers are now working to control the color of the membranes with

electricity, and hope to have a screen prototype for the Mobile World Conference 2017 in

Barcelona.

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ON-CHIP OBSERVATION OF THZ GRAPHENE PLASMONS

Researchers developed a technique for imaging THz photocurrents with nanoscale resolution,

and applied it to visualize strongly compressed THz waves (plasmons) in a graphene

photodetector. The extremely short wavelengths and highly concentrated fields of these

plasmons open new venues for the development of miniaturized optoelectronic THz devices4.

(THz plasmons of extremely short wavelength propagate along the graphene sheet of a THz detector, as visualized with

photocurrent images obtained by scanning probe microscopy.)

Radiation in the terahertz (THz) frequency range is attracting large interest because of its

manifold application potential for non-destructive imaging, next-generation wireless

communication or sensing. But still, the generating, detecting and controlling of THz radiation

faces numerous technological challenges. Particularly, the relatively long wavelengths (from

30 to 300 μm) of THz radiation require solutions for nanoscale integration of THz devices or

for nanoscale sensing and imaging applications.

In recent years, graphene plasmonics has become a highly promising platform for shrinking

THz waves. It is based on the interaction of light with collective electron oscillations in

graphene, giving rise to electromagnetic waves that are called plasmons. The graphene

4 http://www.nanogune.eu/newsroom/chip-observation-thz-plasmons

http://www.nanogune.eu/newsroom/chip-observation-thz-plasmons

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plasmons propagate with strongly reduced wavelength and can concentrate THz fields to

subwavelength-scale dimensions, while the plasmons themselves can be controlled

electrically.

Now, researchers at CIC nanoGUNE (San Sebastian, Spain) in collaboration with ICFO

(Barcelona, Spain), IIT (Genova, Italy) - members of the EU Graphene Flagship - Columbia

University (New York, USA), Radboud University (Nijmegen, Netherlands), NIM (Tsukuba,

Japan) and Neaspec (Martinsried, Germany) could visualize strongly compressed and

confined THz plasmons in a room-temperature THz detector based on graphene. To see the

plasmons, they recorded a nanoscale map of the photocurrent that the detector produced

while a sharp metal tip was scanned across it. The tip had the function to focus the THz

illumination to a spot size of about 50 nm, which is about 2000 times smaller than the

illumination wavelength. This new imaging technique, named THz photocurrent nanoscopy,

provides unprecedented possibilities for characterizing optoelectronic properties at THz

frequencies.

The team recorded photocurrent images of the graphene detector, while it was illuminated

with THz radiation of around 100 μm wavelength. The images showed photocurrent

oscillations revealing that THz plasmons with a more than 50 times reduced wavelength were

propagating in the device while producing a photocurrent.

“In the beginning we were quite surprised about the extremely short plasmon wavelength, as

THz graphene plasmons are typically much less compressed”, says former nanoGUNE

researcher Pablo Alonso, now at the University of Oviedo, and first author of the work. “We

managed to solve the puzzle by theoretical studies, which showed that the plasmons couple

with the metal gate below the graphene”, he continues. “This coupling leads to an additional

compression of the plasmons and an extreme field confinement, which could open the door

towards various detector and sensor applications”, adds Rainer Hillenbrand, Ikerbasque

Research Professor and Nanooptics Group Leader at nanoGUNE who led the research. The

plasmons also show a linear dispersion – that means that their energy is proportional to their

momentum - which could be beneficial for information and communication technologies. The

team also analysed the lifetime of the THz plasmons, which showed that the damping of THz

plasmons is determined by the impurities in the graphene.

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THz photocurrent nanoscopy relies on the strong photothermoelectric effect in graphene,

which transforms heat generated by THz fields, including that of THz plasmons, into a current.

In the future, the strong thermoelectric effect could be also applied for on-chip THz plasmon

detection in graphene plasmonic circuits. The technique for THz photocurrent nanoimaging

could find further application potential beyond plasmon imaging, for example, for studying

the local THz optoelectronic properties of other 2D materials, classical 2D electron gases or

semiconductor nanostructures.

ADDING HYDROGEN TO GRAPHENE

Adding hydrogen to graphene could improve its future applicability in the semiconductor

industry, when silicon leaves off. Researchers have recently gained further insight into this

chemical reaction. These findings extend the knowledge of the fundamental chemistry of

graphene and bring scientists perhaps closer to realizing new graphene-based materials5.

(Hydrogenation (in red) of bilayer graphene via Birch-type reaction begins from the edges. The

images show a graphene flake before (a), two minutes (b), and eight minutes (c), after exposure

to a solution of lithium and liquid ammonia (Birch-type reaction). Graphene gets gradually

hydrogenated starting from the edges. Credit: Zhang X et al, JACS, Copyright 2016 American

Chemical Society6)

5 https://www.sciencedaily.com/releases/2016/11/161103090801.htm

6 http://phys.org/news/2016-11-adding-hydrogen-graphene.html

https://www.sciencedaily.com/releases/2016/11/161103090801.htm

http://phys.org/news/2016-11-adding-hydrogen-graphene.html

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Adding hydrogen to graphene could improve its future applicability in the semiconductor

industry, when silicon leaves off. Researchers at the Center for Multidimensional Carbon

Materials (CMCM), within the Institute for Basic Science (IBS) have recently gained further

insight into this chemical reaction. Published in Journal of the American Chemical Society,

these findings extend the knowledge of the fundamental chemistry of graphene and bring

scientists perhaps closer to realizing new graphene-based materials.

Understanding how graphene can chemically react with a variety of chemicals will increase

its utility. Indeed, graphene has superior conductivity properties, but it cannot be directly

used as an alternative to silicon in semiconductor electronics because it does not have a

bandgap, that is, its electrons can move without climbing any energy barrier. Hydrogenation

of graphene opens a bandgap in graphene, so that it might serve as a semiconductor

component in new devices.

While other reports describe the hydrogenation of bulk materials, this study focuses on

hydrogenation of single and few-layers thick graphene. IBS scientists used a reaction based

on lithium dissolved in ammonia, called the "Birch-type reaction," to introduce hydrogen onto

graphene through the formation of C-H bonds.

The research team discovered that hydrogenation proceeds rapidly over the entire surface of

single-layer graphene, while it proceeds slowly and from the edges in few-layer graphene.

They also showed that defects or edges are actually necessary for the reaction to occur under

the conditions used, because pristine graphene with the edges covered in gold does not

undergo hydrogenation.

Using bilayer and trilayer graphene, IBS scientists also discovered that the reagents can pass

between the layers, and hydrogenate each layer equally well. Finally, the scientists found that

the hydrogenation significantly changed the optical and electric properties of the graphene.

"A primary goal of our Center is to undertake fundamental studies about reactions involving

carbon materials. By building a deep understanding of the chemistry of single-layer graphene

and a few layer graphene, I am confident that many new applications of chemically

functionalized graphenes could be possible, in electronics, photonics, optoelectronics,

sensors, composites, and other areas," notes Rodney Ruoff, corresponding author of this

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paper, CMCM director, and UNIST Distinguished Professor at the Ulsan National Institute of

Science and Technology (UNIST).