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