The many ways of making graphene

Published: Friday, 21 March 2014

There are several ways of producing graphene, only some of which make use of natural graphite, a process featured in the November 2013 issue of IM. Marko Spasenovic* discusses some of the other methods for making the carbon wonder material and why some are considered to be better than others.

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