Here is the full transcript of Nai-Chang Yeh’s TEDx Talk: A New World Composed of Graphene-Based Technology at TEDxTaoyuan conference.
TRANSCRIPT:
Carbon is an essential element of life, and is also one of the most abundant elements on earth. Carbon occupies a place in the second row of the periodic table, right above silicon.
A single carbon atom consists of 6 protons, 6 or 7 neutrons, and 6 electrons. A well-known example of carbon is the tip of pencils. The tip of pencils consists of graphite, which refers to a collection of carbon atoms, layers of carbon atoms. Each layer of the carbon atoms forms a honeycomb structure which is called graphene. Although scientists knew that graphene layers are the constituents for graphite, for years they were not sure whether a monolayer of graphene could be isolated in nature.
Until 2004, when two British physicists finally demonstrated that a single atomic layer of graphene can be isolated and stabilized in nature. These two physicists, Dr Novoselov and Dr Geim of Manchester University, not only demonstrated that graphene can be isolated and stabilized in nature, they also carried out detailed studies of the properties of graphene. They found that graphene exhibited very interesting and unique properties that could be promising for a wide range of applications. Therefore, their research ignited great excitement throughout the world.
So, what are the unique properties of graphene that makes it so special? Graphene, actually, is both electrically and thermally highly conductive, so electrons on graphene can move like massless particles ballistically across the surface of graphene without being scattered. Graphene is very thin and optically transparent, so light can penetrate through graphene without being reflected.
The edges of graphene also exhibit very interesting properties that can be functionalized for chemical applications.
The tiny honeycomb structure of graphene can only allow electrons and protons to penetrate through, so, for this reason, graphene can be used for filtering of chemical elements.
Moreover, graphene is mechanically flexible and 200 times stronger than steel. You can imagine, with these unique properties, many applications become possible. For instance, the excellent electrical and thermal conductivity, combining with the optical transparency, make it possible for graphene to be applied to nanoelectronics in integrated circuits, to optoelectronic components like solar cells, light-emitting diodes, lasers and display panels.
The flexible and strong mechanical properties of graphene can be used for lightweight, high-strength materials applicable to things such as transportation vehicles, airplanes. The excellent chemical filtering capabilities of graphene makes it possible for graphene to be used in desalination, detoxification, chemical sensing, DNA sequencing and even delivery of medicine.
The edges of graphene are very interesting also. In fact, there are two distinctly different types of edges for graphene. One is called the armchair edges, the other is called the zigzag edges. The zigzag edges usually are chemically more reactive and so can be functionalized for all kinds of chemical applications. If you take a piece of graphene and cut it into small stripes, then you have graphene nanoribbons.
You can have either armchair or zigzag graphene nanoribbons. All of these nanoribbons have very large edge-to-area ratios, and so they can be very good, very effective, for charging and discharging. For this reason, graphene nanoribbons can be used in supercapacitors, batteries for energy storage. Despite all these wonderful properties of graphene, there are major challenges before we can fully realize the potential of graphene. In particular, we need to develop reliable large-scale production of graphene with high quality and low cost.
Currently, there are three typical ways of making graphene. The first one is called mechanical exfoliation from graphite. What it is, is actually involving the use of adhesive tapes, or a Scotch tape that you’re familiar with. You take a piece of Scotch tape, press it against graphite, and you peel it off. Then you get tiny flakes of graphite.
Then you keep repeating the process until you hopefully get little, little flakes of graphene that can be monolayer or bilayer or multilayers. As you can imagine, this is a method that’s labor-intensive and very slow in production. It’s not scalable: you have no control of the quality and size of the graphene samples. But this was the initial method used by the two Nobel laureates.
A second method is based on chemical reduction, which utilizes very toxic chemicals to oxidize graphite into graphite oxide, and then chemically reduces graphite oxide into tiny flakes of graphene. This method, again, is environmentally unfriendly, and also the produced graphene flakes are uncontrollable in size, number of layers and quality. As a matter of fact, most of these graphene flakes consist of lots and lots of impurities.
The third method is called chemical vapor deposition, which involves using multistep, long-term processes of growing graphene on metals such as copper or nickel. The growth process involves very, very high temperature — 1,000 degrees centigrade. Typically, this method can produce sufficiently large areas of graphene sheets, and the quality can be reasonable if you take a lot of time to go through many steps to produce the material.
Overall this process is very expensive. The other problem is that it’s incompatible with most device fabrications. And so, as you can see, all these three methods are not ideal for fully realizing the potential of graphene. To overcome these problems, we recently developed a new method, which is called plasma-assisted, chemical vapor deposition, or PECVD. This process takes place at room temperature, in a single step, and only takes a few minutes, and there we produce excellent quality of graphene.
The idea is the following: We take a piece of copper. The copper surface is very reactive and so usually is covered with oxide. We subject the copper piece to hydrogen plasma and other radicals like cyan. And then once we clean up the surface of copper, try to get rid of all of the copper oxide, we flow methane gas through this highly reactive copper. And so the copper surface will rip apart the chemical bonds between carbon and hydrogen, and then allow carbon to nucleate on the surface of the copper and turn into graphene.
And with this approach, we can actually grow very large areas of graphene in a very short time, which is of excellent quality, electrically, structurally and mechanically. And here I just show you an example from the images we took using our scanning tunneling microscope on our PECVD-grown graphene.
As you can see, at the small scale, you can visualize the honeycomb structure of carbon. We can also modify our process a little bit so that not only can we grow large sheets of graphene, we can also grow graphene nanoribbons. This little modification involves adding additional precursor molecules so that graphene can also grow vertically on the surface of the copper and turn into graphene nanoribbons.
So, overall, using our PECVD method, we effectively turn greenhouse gas – that’s methane – into very useful material: graphene and graphene nanoribbons.
So, what’s new in the future? As you can imagine, at this point, when we have these advances in the graphene production, many things become possible. So here I will give you two examples of our ongoing research to give you a flavor of what can be done.
The first example is a little exotic. It’s called nanoscale strain engineering of graphene. It’s purposed for novel optoelectronic applications. One of the most interesting properties of graphene is that if you distort the structure of graphene at the nanoscale, you can actually fundamentally change the electronic properties of graphene. And so now we have essentially flawless pieces of graphene, I can use nanofabrication technology to build nanostructures wherever I want them to be with whatever shapes I want them to be.
Then I can lay a layer of graphene over such nanostructures to produce the distortion that I want, and, therefore, I can get the electronic properties that I want. So, as an example, you see here, a 600×600 nanometer-sized layer of graphene can be put over a tetrahedral-like nanostructure. So, graphene is being distorted, and the consequence of it is that electrons on graphene would see this distortion as if you have very strong, spatially alternating magnetic fields. The magnetic fields are so strong, as you can see here, the picture shows – the blue color indicates negative magnetic fields, red color indicates positive magnetic fields, and they are as high as several hundred teslas, much higher than most fields you can produce in any laboratory on earth.
And so, if you design these nanostructures properly, and connect them in a special form, electrons will effectively see these alternating magnetic fields, so that when electrical currents on graphene try to pass through these distortions, they will be effectively accelerated as if they were under strong magnetic fields. This acceleration can cause so-called synchrotron radiation, so that photons can be radiated. So, this is similar to the principle of the free-electron laser, except that I don’t have to apply any real magnetic field, I just have to do strain engineering.
And also, instead of a gigantic lab that’s required to make a free-electron laser, I can actually make a tabletop free-electron laser using this concept. This is an ongoing research project.
Another very practical example of research that we are pursuing is a new generation of interconnects. As you know, one of the major challenges facing the continuing miniaturization of nanoelectronics in integrated circuits is that the interconnects are also shrinking to a very, very small scale. In current technology, the interconnects are based on copper.
When you shrink copper into tiny, tiny lines, you start facing very serious problems, because copper becomes granular and very resistive. And, therefore, if you pass electrical current through these interconnects, you’re going to generate lots of heat. Furthermore, copper under heat will diffuse into the underlying silicon, which will further degrade the interconnect characteristics and properties. So, these are major issues the semiconductor industries try to solve.
Now imagine, if I can put a barrier layer of graphene between the copper and the underlying substrate silicon, then I can prevent copper from diffusing into the silicon because, as I have told you, only electrons and protons can get through graphene.
Furthermore, graphene itself is an excellent electrical and thermal conductor. Therefore, with the presence of graphene, the overall interconnect properties are becoming better both electrically and for thermal dissipation, and so overall it will reduce energy consumption. So, these are just two examples of what we are working on with our new development.
In general, we can say that you can consider graphene as the next generation wonder material beyond silicon for science and technology. With the new development of advances in graphene production, and also the ingenuity of researchers all over the world, we can expect graphene to open up new frontiers for science and technology that can brighten our future.
I have mentioned these possibilities, including next-generation nanoelectronics, optoelectronics, very strong materials that are flexible and lightweight, renewable energy production and storage, and also for medicine and bioengineering. So, with all of these possibilities and reasons, I daresay that even the sky is not the limit.
