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Exfoliation and Synthesis of 2D Materials
Emily Threatt, Christian Academy of Knoxville
Cameron Jeske, Hardin Valley Academy
Ali Mohsin, Dr. Gong Gu, Wan Deng
The exfoliation and synthesis of 2D materials focuses on exploring the atomic
properties of Graphene, boron nitride, and similar substances. With this
information, we can look forward to their future application to electronic devices.
The process involves preparing, testing, analyzing, and gathering information to
further the knowledge of 2D materials.
I. Introduction
The research of 2D materials is the foundation of future technology. Materials such as
graphene, boron nitride, and molybdenum disulfide, will make new devices more advanced than
ever. The characteristics that accompany 2D materials, like conductivity, transparency, and
strength are the key properties desired when designing new technology. Future applications
include transistors, mobile phones, and the possible replacement of silicon as the primary chip
base.
II. Literature Review
2D materials are characterized by their ability to exist as a single layer of atoms.
Graphene, boron nitride, and molybdenum disulfide all have this property. The research of 2D
materials is focused on graphene. It is able to exist as a single layer of atoms, because its
hexagonal structure does not leave any dangling electrons. The hexagonal structure also provides
it with flexibility and tremendous strength. Its strength is measured in relation to the monolayers
of other elements. In this respect, graphene is the strongest material in existence. Finally, it is
transparent, due to its thinness, and conductive. Transparency and conductivity make it a
promising material for companies like Apple and Samsung. Materials like boron nitride and
molybdenum disulfide have a very similar atomic structure to that of graphene. So, when
graphene was not available for a particular stage of the research we used them instead.
Illustrates the atomic structure of the materials used
III. Method
To start researching these 2D materials, we needed to prepare a substrate for the
exfoliation into one-atom thick layers. First, we cleaned a wafer of silicon dioxide/silicon, which
was going to serve as our substrate. The process of cleaning started with placing the wafer in a
glass beaker filled with acetone. Then it was placed in a sonicator, to clean the substrate with
vibrations. After this, we repeated the process, instead using isopropyl alcohol as a cleaning
solution. Next, we used a nitrogen gas gun, to blow off any extra residue that might’ve been on
the substrate. Then, we heated the substrate at 350 degrees celsius for 5 min, to finalize the
cleaning process.
After the cleaning process was completed, we began exfoliation. To do this, we placed a
piece of molybdenum disulfide onto a long piece of tape. This left a residue on the tape. We
further exfoliated the material by repeatedly folding the tape back onto itself. Eventually, the
long strip of tape was completely covered in thin flakes of molybdenum disulfide. Next, we
placed the tape onto a silicon dioxide/ silicon wafer substrate, transferring the material. With a
microscope we focused onto the darkest colored flakes, and taking pictures for later analyzation.
To analyze the pictures, we split them into RGB channels, which showed the contrast of the
silicon dioxide to the molybdenum disulfide much better. Finally, we compared the color
concentration of the red channel picture to a another picture which identified the number of
layers based on contrast.
Next, we learned the process of making graphene. There are two methods to growing
graphene: growing with a solid precursor, and growing with a gaseous precursor. To start the
process, we placed the copper substrates into the quartz tube, which is situated in the dual tube
furnace. Then, we vacuumed out all of the oxygen in the tube, and replaced it with argon and
hydrogen, preventing combustion. We then proceeded to heat the furnace to 1000 degrees
celsius. Finally, the methane precursor is introduced to the furnace. The solid precursor method
begins with the same three steps as the gaseous precursor, but instead, the solid precursor is
heated externally to 120 degrees celsius. Between the two methods, the gaseous precursor
worked more efficiently, producing more graphene than the solid precursor. These methods are
known as Chemical Vapor Deposition.
After we grew the graphene, we looked at the substrate through a Scanning Electron
Microscope. A SEM works by focusing a beam of electrons onto an object. The primary
electrons get scattered, resulting in secondary electrons that are collected by the detector for
imaging. This then projects a much higher magnification image than that of a normal
microscope. In this images below, graphene is shown on the left as the darker-shaded hexagons,
while the lighter area is the copper substrate. On the left, boron nitride particles are shown by the
darker triangles.
Graphene Boron Nitride
The final portion of our project consisted of the testing of electronic devices. The devices
we tested, transistors, consisted of two pieces of gold-covered titanium, bridged together by a
piece of graphene.There were sixty-four transistors per a substrate. First, we looked the
transistors under a microscope to see if there was a visibly good connection between the
graphene and the metal contacts. We recorded the locations of these transistors. In preparation
for the next step, we used a diamond pen to scratch the substrate. Then, we transferred the
substrate to a humidity controlled environment that contained a hot plate and another
microscope. We placed the substrate on the hot plate to remove anything that could have fallen
on top. Then, we moved it to the microscope. The microscope has three probes. The first probe is
used to ground the substrate, ensuring that it will not move while you are working. The other two
probes are landed on the metal contacts of a transistor. To test, we ran an electrical current
through the probes, and recorded whether or not the current that was run through the device
remained unbroken.
IV. Results
Throughout the course of the project, we were able to gather numerous results from a
variety of experiments. The first of these results came from testing different methods of
transferring materials to a polymer. The first method consisted of placing the boron nitride and
copper onto a hot plate, thus making the copper oxidize. This allowed us to recognize the
contrast between the boron nitride and the copper. Next, we put a drop of polymer solution onto
the boron nitride, and put it into a spin-coating machine. After this, we put it into a copper-
etching solution, which left a layer of boron nitride on a film of polymer. Our second method of
transfer consisted of placing a different polymer (copper mesh coated in gold, with a layer of
carbon) onto the boron nitride. A few drops of isopropyl alcohol on top. As the IPA evaporated,
it lifted the molybdenum disulfide into the polymer. This method turned out the be the better of
the two. By the end of the project, we were able to complete the process of exfoliating,
analyzing, synthesizing, and the testing of graphene. We tested 64 different devices, with only 2
working properly.
This graph is from one of the working devices. The “V” shape of the line indicated that
the current never broke, and the voltage was successfully able to go from positive to negative.
This also indicates that the graphene successfully bridged from one piece of metal to the other.
But, the ideal result would be a graph that showed no fluctuation of results, thus the lines would
be directly on top of each other.
V. Conclusion
In the field of 2D materials, there is still much research to be done, especially before
commercial application. To make this possible, there must be further exploration of graphene’s
properties, better methods of mass production, and more efficient application to technologies.
Currently, graphene is not the most cost effective material for producing devices. The cost must
be less than its top competitor, silicon. Despite this, companies such as Samsung and Apple hold
patents on graphene technology, and have started making models that utilize the 2D material.
VI. Acknowledgements
We would like to thank our mentors, Ali Mohsin and Wan Deng, and our professor, Dr.
Gu. Additionally, we would like to thank Mr. Erin Wills, Dr. Chen, and Dr. Costinett for the time
and energy they have invested into the Young Scholars Program.
This work was supported in part by the Engineering Research Center, Program of the
National Science Foundation, and the Department of Energy under NSF Award Number EEC-
1041877 and the CURENT Industry Partnership Program.
VII. References
https://en.wikipedia.org/wiki/Graphene
https://en.wikipedia.org/wiki/Boron_nitride
https://en.wikipedia.org/wiki/Carbon
https://en.wikipedia.org/wiki/Scanning_electron_microscope
https://www.wearable-technologies.com/wp-content/uploads/2014/10/graphene.png
http://www.gizbeat.com/wp-content/uploads/2014/04/flexible-graphene-phone.jpg
http://image.guardian.co.uk/sys-images/Observer/Pix/pictures/2011/11/11/Graphene.jpg?guni=Article:in%20body%20link
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