Translator: Kuan-Yi Li
Reviewer: Robert Tucker 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 600x600 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 magnet 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. Thank you. (Applause)
