By Marko
Spasenovic
Since the first graphene was made in a laboratory in
Manchester ten years ago, the volume of research on the
material has grown at an exponential pace.
The fact that graphene is extremely
stiff, nearly transparent across the spectrum, ultra-strong, a
good thermal conductor, all while being only a single atom
thick, has quickly propelled the material to global prominence,
both for scientific research and commercial applications.
Although academic research has
confirmed many of graphene’s predicted properties, earning
its discovers Andre Geim and Konstantin Novoselov the 2010
Nobel Prize for Physics, the material has so far not made it to
any widely available commercial products.
However, this looks likely to
change in the years to come. Sports equipment maker HEAD’s
graphene racket was one of the first products to hit the
market, with the likes of Novak Djokovic and Maria Sharapova
sporting the supercarbon-enhanced tool last year.
Graphene is promising to make an
impact on other sports equipment and also on nearly everything
from ultrafast flexible smartphones to lightweight aircraft and
smart buildings. All these products, however, make use of a
different property of graphene, and consequently will contain
graphene made by different methods.
Liquid phase and thermal exfoliation of natural
graphite
Graphene has the highest electron
mobility (a quantity related to the speed at which electrical
signals move through a material) on the planet.
It has a Young’s modulus
(stiffness) five times greater than steel of the same
thickness, very high thermal conductivity and almost negligible
absorption of light.
All these properties are available
in perfect single layer graphene, but most production methods
yield multilayer graphene, which, in addition, contains
defects. Luckily, many of these manufacturing processes can be
tweaked to make graphene that excels in one of the properties,
while trading off some others, allowing for a choice of
production method based on the intended use.
Liquid phase and thermal
exfoliation are processes in which natural graphite is exposed
to solvents or a thermal shock, respectively, which allows the
splitting of individual graphene flakes (see November 2013
issue of IM: A natural advantage? Making
graphene from mined graphite).
This results in
‘nanoplatelets’ of graphene, which are typically
small flakes, a few nanometers to micrometers across, of
few-layer graphite. The chemicals used in the process tend to
damage the graphene structure, compromising its electronic
mobility and structural integrity.
There are ways to reverse some of
the damage, for example by oxidation methods, leading to
graphene oxide nanoplatelets. Such nanoplatelets have become a
common item on the product list of graphene producers, and are
of sufficient quality for certain applications, such as
conducting inks.
Surface sublimation of SiC
Graphene can also be made by
sublimation on the surface of silicon carbide (SiC). The
process follows a recipe of subjecting a SiC substrate to
specific pressure and temperature conditions, such that some of
the carbon forms a smooth graphene layer on one of the surfaces
of the SiC.
SiC is widely used as an abrasive,
due to its hardness and ceramics made of SiC are used regularly
in car brakes, clutches and bulletproof vests. SiC is also used
in speciality electronics that require operation at high
temperatures and high voltages.
The number of graphene layers grown
on the face of SiC can be precisely controlled and the quality
of such graphene is very high over a wide area (crystallites of
some hundreds of micrometers). The drawbacks, however, are the
high cost of SiC and the high temperatures required to initiate
sublimation.
Chemical Vapour Deposition
(CVD)
The most popular method of
producing single layer, high-quality, large area graphene, is
chemical vapour deposition (CVD).
Simply put, CVD is a way of
depositing gaseous reactants onto a substrate. The way CVD
works is by combining gas molecules (often using carrier gases)
in a reaction chamber which is typically set at ambient
temperature.
When the combined gases come into
contact with the substrate within the heated chamber, a
reaction occurs that creates a material film on the substrate
surface. The waste gases are then pumped from the reaction
chamber.
The temperature of the substrate is
a primary condition that defines the type of reaction that will
occur, so it is vital that the temperature is correct. During
the CVD process, the substrate is usually coated with a very
small amount of the desired deposit, at a very slow speed,
often described in microns of thickness per hour.
The benefits of using CVD to
deposit materials onto a substrate are that the quality of the
resulting materials is usually very high. Other common
characteristics of CVD coatings include imperviousness, high
purity, fine grain and increased hardness compared to other
coating methods.
CVD is a common solution for
deposition of films in the semiconductor industry, as well as
in optoelectronics, due to the low costs involved compared to
the high purity of films created.
The disadvantages to using CVD to
create material coatings are that the gaseous by-products of
the process are usually very toxic. This is because the
precursor gases used must be highly volatile in order to react
with the substrate, but not so volatile that it is difficult to
deliver them to the reaction chamber.
Challenges to making
pristine graphene using CVD
CVD graphene is created in two
steps: the precursor pyrolysis of a material to form carbon;
and the formation of the carbon structure of graphene using the
disassociated carbon atoms.
Both steps are relatively easy to
initiate, but mastering them to obtain a perfect monoatomic
layer is more challenging. It is vitally important that the CVD
process is stringently co-ordinated; that controls are put in
place at every stage of the process to ensure that the
reactions occur effectively; and that the quality of graphene
produced is pristine.
In order to create monolayer or few
layer graphene on a substrate, scientists must first overcome
the issues with the methods that have been observed so far.
While it is possible to create high
quality graphene on a substrate using CVD, the successful
separation, or exfoliation, of graphene from the substrate has
so far proved to be something of a stumbling block.
The primary reason for this is
because the graphene sticks strongly to the substrate, so it is
not easy to achieve separation without damaging the structure
of the graphene or affecting the properties of the
material.
The techniques for peeling off the
graphene differ depending on the type of substrate used. Often,
scientists can choose to dissolve the substrate in acids, but
this process commonly affects the quality of the graphene
produced, so other methods are currently being researched.
One alternative method that has
been investigated involves the creation of CVD graphene on a
copper (Cu) substrate.
During CVD, a reaction occurs
between the Cu substrate and the graphene, creating a high
level of hydrostatic compression, pressing the graphene into
the substrate. It has been shown to be possible to intercalate
a layer of copper oxide (Cu2O), which is
mechanically and chemically weak, between the graphene and the
Cu substrate to reduce this pressure and enable the graphene to
be removed relatively easily.
An additional benefit to this
method is that the substrate can be reused, because during
separation only the Cu2O is etched away.
Scientists have also been using
polymers as a support to facilitate the transfer of graphene
onto an alternate substrate. In this approach, graphene is
coated with a polymer and the substrate is etched.
This way the coated graphene is
strong enough to be transferred to another substrate without
damaging the material. The polymer transfer method has been
developed to such perfection that hovering membranes of
suspended graphene, akin to nano-scale drums, are now routinely
made. Such graphene drums are being explored for use as
pressure and force sensors.
Achieving
uniformity
Another major hurdle is creating a
completely uniform layer of graphene on a substrate.
This is difficult to achieve
because the gas distribution itself is not uniform within the
reaction chamber. Also, due to fluid dynamics, there might be a
depletion of reactants by the time the gas reaches the further
ends of the substrate, meaning that no reaction will occur away
from the centre of the substrate.
Some scientists have reported
overcoming this issue by modifying the concentration of gases
and also by incorporating spin coating methods.
One technique which is commonly
used to alleviate some of the negative effects described above
is treating the substrate before the reaction takes place.
A Cu substrate can be chemically
treated to enable reduced catalytic activity, increase the Cu
grain size and rearrange the surface morphology in order to
facilitate the growth of large graphene flakes that contain
fewer imperfections.
CVD - the production method
of choice?
Despite its technical challenges,
CVD has become the go-to technique for growing large-area,
high-quality graphene films, with applications primarily
directed towards flexible touchscreens and other transparent
conductor applications, such as smart windows.
Films up to 100 metres in length
have been demonstrated, with square metres of graphene now
routinely produced at several locations worldwide.
Yet it remains to be seen which
production method will ultimately emerge as the process of
choice for making large quantities of pristine graphene at a
commercially competitive cost.
Unsurprisingly, the most promising
methods are being quickly locked into patents and much of the
work performed to date has been kept out of the public
domain.
But with the European
Commission’s Û1bn ($1.38bn**) Graphene Flagship
scheme underway and billions of dollars being ploughed into
graphene research across the world, the industry is rapidly
progressing towards its goal of fully realising the
supercarbon’s potential.
*Marko Spasenovic writes
articles about graphene for Graphenea, in parallel to his
postdoctoral scientific research post at ICFO (Barcelona) and
administrative role at Graphene Tracker.
**Conversion made March 2014