1.3 billion people around the world live without access to “modern electricity.” Therefore, in these regions, the significance of small scale energy harvesting devices is increasing. This research used graphene, that has exceptional features of high transparency and flexibility, to create a bilayer graphene triboelectric nanogenerator (T.E.N.G). However, the issue is that currently, not only is there no confirmed optimal fabrication procedure to create graphene T.E.N.G, but creating a multilayer graphene is very difficult in the first place. Therefore, this research merged two methods of graphene transfer together- a stable dry transfer method for the first layer, and high quality wet transfer for the second, to fabricate a bilayer graphene T.E.N.G that is both stable and high quality. Raman Spectroscopy was used to prove that the bilayer was successfully synthesized, and the nanogenerator showed that transparency and flexibility of graphene was maintained all throughout the fabrication procedure used. Results showed that efficiency increased, with voltage from the bilayer being almost twice the voltage from monolayer graphene. This research serves as a fundamental stepping stone for further development of multilayer graphene sheets and to finding the optimal fabrication procedure for graphene T.E.N.Gs. There is also vast potential in which this nanogenerator could be used for application, as regions where there are extremely low access to energy are able to significantly benefit from such small-scale and labor intensive energy harvesting device like the bilayer graphene T.E.N.G.
Ever since graphene was first discovered in 2004, its vast potential to be applied in so many different fields of science was what made it a dream material for many scientists. However, although there are many advantages that graphene has, there is still no optimal way in which graphene can be synthesized, or the optimal fabrication procedure to which graphene can be stacked in multiple layers to enhance efficiency. This is because of graphene’s tendency to be easily crumpled or damaged.
Thus, the objective of this project is to research the most optimal fabrication procedure in which a multilayer graphene can be produced that is both high quality and stable, to enhance the efficiency of the overall nanogenerator. In this experiment, I merged two methods for transferring graphene- dry transfer for the first layer and wet transfer for the second.
This experiment was designed to clearly distinct how previous methods of creating a multilayer graphene triboelectric nanogenerator, differentiate from the fabrication process used in this research. Although there are not much cases in which bilayer graphene was used, among those several cases, it only used either methods- wet or dry. In this project, I merged the two methods together. The hypothesis was that if the two methods are combined, it will produce a bilayer graphene sheet that is of good quality as well as stable because the dry transfer method has high stability while the wet transfer is able to maintain the synthesized high-quality graphene.
Energy is fundamental to modern life; However, 1.3 billion people around the world live without access to “modern electricity.” Therefore, in regions with extremely low energy access, the significance of small scale energy harvesting devices and labor intensive appropriate technology is increasing. This research used graphene, that has various benefits, in order to create a graphene T.E.N.G(Triboelectric nanogenerator) that functions through a category of energy harvesting, triboelectricity.
1. Energy Harvesting
Energy Harvesting is the process of capturing small amounts of energy from external sources that would otherwise be lost as heat, light, sound, vibration or movement, most commonly during the process of energy generation. These small amount of energy captured can be put to use in urgent situations or when there is little energy required. Energy Harvesting devices depend on various energy sources such as solar, thermal, piezoelectric, triboelectric, or pyroelectric sources.
Triboelectric effect is when certain materials become electrically charged after friction contact is made with a different material. When two different materials come into contact, charges transfer from one material to the other to equalize their electrochemical potential, and when the two materials are separated, some of the bonded atoms keep extra electrons, while some give them away, creating triboelectricity. Triboelectricity have shown vast potential to facilitate various wearable and portable devices.
Graphene, a 2-D allotrope of carbon organized in a shape of a honeycomb lattice, has shown high potential for technological applications in the future with excellent conductivity, high transparency of 97%, flexibility, and impermeability. Moreover, graphene is also known to be the thinnest substance in the world, with a thickness 0.2 nm, which is the thickness of a single atom.
Past studies of fabricating a graphene T.E.N.G usually consisted of a monolayer graphene, or in several cases, bilayer graphene. Because of graphene’s tendency to be easily crumpled and damaged, this makes it so arduous to make a multilayer graphene, with no optimal fabrication procedure until now. Past studies have used methods wet transfer or dry transfer to produce a monolayer graphene sheet. In very scarce cases, bi-layer graphene was created using the wet transfer. However, this too has consequences with it being very unstable.
Approach of This Research
At first, referencing past studies, I used the same procedure of using the wet transfer method to produce a bilayer graphene. However, because of its extremely high instability, I could not come upon a successful outcome. Therefore, I took a new approach and tried combining the two methods to transfer the graphene sheet unto PET. I used dry method for the first layer because the method itself has high stability, and then wet method for the second layer because it is able to maintain the high-quality graphene.
Applications to Real World
Through a more efficient and stable graphene T.E.N.G produced with this fabrication procedure, it will be significant help for urgent situations where immediate electricity is needed, or in regions that have extremely low access to energy.
Thermal C.V.D system was used to synthesise graphene. Argon gas is put inside the vacuum chamber to match air pressure, hydrogen treatment takes place, then the temperature rises to 1000℃, methane (CH4) is put inside the vacuum chamber to decompose. As a result, a monolayer graphene sheet is formed on the surface of the substrate copper, and flakes of graphene on the opposite side. The first layer of graphene on PET sheet is made through dry transfer method. The back side of the copper, which there are flakes of graphene, are taped on a OHP film tightly, then goes through R.I.E (Reactive Ion Etching). This will clean the sample’s surface without any organic material or flakes of graphene. After R.I.E, graphene copper sheet is placed on TRT (Thermal Release Tape), with the graphene side in contact with the tape and is pressed uniformly using PDMS. Next, the substrate copper is dissolved using copper etchant leaving TRT+monolayer graphene remaining. It is put on top of R.I.E processed PET, placed on top of an SiO2 wafer, and hot pressed. Lastly, TRT is removed carefully, which leaves monolayer graphene+PET left- this method is only used for the first layer of graphene.
The second layer, using the wet transfer method, starts with the synthesized graphene from C.V.D and has a equivalent first step which is taping the back side of the copper on OHP film. Next, instead of R.I.E, PMMA solution is spin coated on the surface of the graphene/cu sheet with 3000 rpm for 30 seconds. The sample is baked for 10 minutes on 100 degrees celsius heated hot plate to harden the PMMA. Next, R.I.E takes place to get rid of any organic material, or PMMA solution that might have gone under the sample. After the edges are cut away, copper is dissolved leaving only pmma/graphene sheet remaining at the end.
To ultimately form a bilayer graphene sheet, the first layer formed by the dry method is clipped with an sio2 wafer while the second layer, formed by the wet method is left floating on water. The first layer lifts up the second layer like a spoon, overlapping the two layers. The PMMA on the top is then removed using acetone, and the sample is baked for 10 minutes on 120 degrees heated hot plate.
Gold sheet was used for a more effective transfer of electricity, copper wire was used so that the electric wire had no direct contact with the nano generator, and was stuck using silver paste. A thin layer of PDMS was also used on the inside of the arch of the T.E.N.G to prevent damage of the graphene while making contact, and the two sheets of bilayer graphene pet sheet was taped with one side forming an arch shape.
(For monolayer graphene, only wet transfer was used )
Data Analysis Tool:
Raman Spectroscopy was used as a characterization data to validate that a bilayer graphene was successfully made through this fabrication procedure.
In order to verify that a bilayer graphene has been created, Raman Spectrum graph was used. Raman spectroscopy is considered a great tool for the characterization of graphene, especially in layer thickness as it is capable of differentiating multilayer graphene up to four layers. When a monolayer graphene is formed, the 2D band height is more than two times the height of the G band. As layers begin to build up, the G-band begins to be taller and with bilayer graphene, the height is similar or sometimes even higher than the 2-D band. As the results show, because the G-band is of similar height with the 2-D band, it is evident that a bilayer graphene has been formed through this fabrication procedure. As more layers build up, the G-band will grow taller and taller.
Graphene T.E.N.G. Test
Graphene T.E.N.G. showed remarkable transparency, unlike most other T.E.N.G. created from different substances such as gold. The letters “SIS” behind was able to be clearly shown with the graphene T.E.N.G. Graphene is 97% transparent, and approximately 2.3% of transparency decreases each time a layer is added. That means, even after stacking 10 layers of graphene, the thickness would only be about 2 nm with a transparency of about 80%. 80% would still have enough transparency to be able to read the words behind without trouble.
The fabricated graphene T.E.N.G. also directly maintained graphene’s flexibility, as it was able to be bent like an arch without breaking. Flexibility takes on a big role in effective T.E.N.G. For example, TiO2 (Titanium Dioxide) is a substance that is commonly used to fabricate T.E.N.G. However, they have the drawback of brittleness. Graphene can be able to solve the problem of fragility and develop flexible T.E.N.G.s in the future.
A digital multimeter was used to measure the voltage of the graphene T.E.N.G. Multiple tapping motions of 10~15 times were used in order to create triboelectricity, and there were intervals of 3~5 seconds before tapping again. The results showed that a monolayer graphene T.E.N.G. produced approximately 1~2 voltage from the tapping motions. The bilayer graphene T.E.N.G’s voltage was also measured using the same method of multiple tapping motions with intervals. As a result, the voltage came out to be higher with a wider range of voltage produced between 2~4 volts, which is twice the voltage of the monolayer graphene T.E.N.G.
A total of 8 monolayer and 8 bilayer graphene T.E.N.Gs were made with the fabrication procedure of this research to identify the patterns the results showed, which were that the beneficial characteristics of graphene such as high flexibility and transparency were directly maintained.
Recap of Project:
Recap of Results:
This experiment verified that the fabrication procedure of using both dry and wet method for transfer of graphene sheet produced a high-quality bilayer graphene T.E.N.G stably.
The method of dry transfer generally has high stability in producing graphene- meaning, there are no high possibilities of failure when synthesizing and stacking multiple layers of graphene using the dry method. However, one backlash that this method has is that it does not produce the most high-quality graphene. On the other hand, the method of wet transfer has the advantage of being able to produce a very high-quality graphene T.E.N.G. However, it is arduous to stably stack multiple layers, which is the benefit that the dry transfer has. Therefore, this research merged both methods together to create a bilayer graphene T.E.N.G stably with high quality.
This experiment also proved that the bilayer graphene T.E.N.G maintained graphene’s unique and exceptional properties of high transparency and flexibility. The graphene T.E.N.G. had an arch-type structure, which shows that graphene is able to be shaped and curved without breaking. Also, the printed word “SIS” was clearly seen without any blurriness even with one more layer added, which is a strong indicator that high transparency is evident up to several layers, with only 2~3% decreases each time a layer is stacked. Furthermore, the nanogenerator showed showed higher voltages with a range of 2~4 voltage measured.
The scientific implications of this research is profound, as it took a new approach in being one step closer to finding an optimal fabrication procedure for a multilayer graphene T.E.N.G. The greatest issue currently for graphene nanogenerators is that, even if there are so much potential and benefits of graphene, not only is there no confirmed optimal procedure in which a multilayer graphene T.E.N.G could be fabricated, but producing a multilayer graphene itself is very difficult to do in the first place.
In this experiment, the approach to combine two different methods, dry and wet, was taken in order to stably produce a high quality bilayer graphene, that also maintained the exceptional characteristics of graphene, which is high transparency and flexibility.
Although this graphene T.E.N.G is not the technology suitable for mass production of electricity, even such small energy produced through this nanogenerator can serve as great significance to 1.3 billion people in regions that are extremely low in electricity. The graphene T.E.N.G can even be a life changing application that makes it possible to satisfy basic human needs while minimizing our impact on the environment. For example, this electricity can be used to create alarm bracelets that sends warnings when natural disasters are coming, or portable flashlights that shines light through the triboelectricity created.
My name is Jihye Michelle Heo from Seoul, Korea, attending Seoul International School. When thinking about the activities that mean the most to me, it would definitely be ballet and science. Always devoted to pushing myself to the limit, I started professional ballet since 7th grade, which is pretty late considering most people have been practicing for years. However, I think this is also the reason why I am intrigued by the beauty of ballet everytime. Ballet is synonymous to science, as both of these stimulate me to persevere and transcend.
To me, my older brother is the most influential scientist. I have always enjoyed exploring with my brother, whether that was in our backyard, or even under our desk. I always loved how science, as definitive as it may feel, is so uncertain-the fact that there are so many possibilities for unique ideas to appear approached me in a positive way. My curiosity motivated me to dive deeper into the realm of science.
Science, to me, is like a treasure box-- I never know what to expect, yet I am excited. It also makes me eager to turn on my creative spirit which enables me to imagine the unprecedented. Winning this google science fair, rather than the immediate prize of winning itself, will provide me the stepping stone towards making my future endeavors widely recognized and therefore make a meaningful contribution to the world as a future engineer.
I, Jihye Michelle Heo, followed the safety rules and guidelines of school of engineering, Ajou University.
Professor Jaehyun Lee's Lab, Department of Materials Science & Engineering, Ajou University
Mentor: Professor Jaehyun Lee
Lab Advisor: Sanghwa Hyun
I greatly acknowledge Professor Jaehyun Lee in the Department of Materials Science & Engineering of Ajou University for providing me invaluable scientific advice and for allowing to perform the experiments using their facilities. I also greatly thank Sang-Hwa Hyun for providing experimental guidance, scientific advice as well as assisting the acquisition of Raman Spectra data as the lab advisor despite his busy schedules. I also would like to thank my parents for the great encouragement they gave me, holding frequent discussions on my topic; they meant a lot and was very helpful. Lastly, I would like to thank my older brother who is studying abroad, for giving me feedback to my ideas and further encouraging me.
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