Written By Jesus de La Fuente
There are different ways in which graphene monolayers can be created or isolated, but by far the most popular way at this moment in time is by using a process called chemical vapour deposition. Chemical vapour deposition, or CVD, is a method which can produce relatively high quality graphene, potentially on a large scale. The CVD process is reasonably straightforward, although some specialist equipment is necessary, and in order to create good quality graphene it is important to strictly adhere to guidelines set concerning gas volumes, pressure, temperature, and time duration.
The CVD Process
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 reaction chamber (which is heated), a reaction occurs that create 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 a very small amount, at a very slow speed, often described in microns of thickness per hour. The process is similar to physical vapour deposition (PVD), the only difference being that the precursors are solid compounds, rather than gases, and therefore the process is slightly different. The solid compound or compounds is/are vaporized, and then deposited onto a substrate via condensation.
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 grained and increased hardness over other coating methods. It is a common solution for the deposit 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.
Although there are a number of different formats of CVD, most modern processes come under two headings separated by the chemical vapour deposition operating pressure: LPCVD, and UHVCVD. LPCVD (low pressure CVD) is the CVD procedure carried out under sub-atmospheric pressures. This low pressure helps to prevent unwanted reactions and produce more uniform thickness of coating on the substrate. UHVCVD (ultra-high vacuum CVD) is a process is which CVD is carried out under extremely low atmospheric pressures; usually in the region of 10-6 Pascals.
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. During the CVD process, the toxic by-products are removed from the reaction chamber by gas flow to be disposed of properly.
Fundamental Processes in the Creation of CVD Graphene
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. The first stage, the pyrolysis to disassociated carbon atoms, must be carried out on the surface of the substrate to prevent the precipitation of carbon clusters (soot) during the gas phase. The problem with this is that the pyrolytic decomposition of precursors requires extreme levels of heat, and therefore metal catalysts must be used to reduce the reaction temperature.
The second phase of creating the carbon structure out of the disassociated carbon atoms, also requires a very high level of heat (over 2500 degrees Celsius without a catalyst), so a catalyst is imperative at this stage to reduce the temperature needed for a reaction to occur to around 1000 degrees Celsius. The problem with using catalysts is that you are effectively introducing more compounds into the reaction chamber, which will have an effect on the reactions inside the chamber. One example of these effects is the way the carbon atoms dissolve into certain substrates such as Nickel during the cooling phase.
What all this means is that it is vitally important that the CVD process is very stringently co-ordinated, and 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 of the highest attainable.
Problems Associated with the Creation of CVD Graphene
In order to create monolayer or few layer graphene on a substrate, scientists must first overcome the biggest issues with the methods that have been observed so far.
The first major problem is that 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 been a bit of a stumbling block. The reason for this is primarily because the relationship between graphene and the substrate it is ‘grown’ on is not yet fully understood, so it is not easy to achieve separation without damaging the structure of the graphene or affecting the properties of the material. The techniques on how to achieve this separation differ depending on the type of substrate used. Often scientists can choose to dissolve the substrate in harmful 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 researched involves the creation of CVD graphene on a copper (Cu) substrate (in this example, Cu is used as a catalyst in the reaction). During CVD a reaction occurs between the copper substrate and the graphene that create a high level of hydrostatic compression, coupling the graphene to the substrate. It has been shown to be possible, however, to intercalate a layer of copper oxide (which is mechanically and chemically weak) between the graphene and the copper substrate to reduce this pressure and enable the graphene to be removed relatively easily (also, in this instance, the substrate can be reused).
Scientists have also been looking into using Poly(methyl methacrylate) (PMMA) as a support polymer to facilitate the transfer of graphene onto an alternate substrate. With this method, graphene is coated with PMMA, and the previous substrate is etched. Then, the coated graphene is strong enough to be transferred to another substrate without damaging the material. Other support polymers that have been tested include thermal release tape and PDMS (Polydimethylsiloxane). However, PMMA has been shown to be the most effective at transferring the graphene without excessive damage.
Another major hurdle is creating a completely uniform layer of graphene on a substrate. This is difficult to achieve as the kinetic transport dynamics of gas is affected by diffusion and convection, meaning that these values change within the space of a reaction chamber, in turn affecting the chemical reactions on the substrate. Also, due to fluid dynamics, there might be a depletion of reactants by the time gas reaches the further ends of the substrate, meaning that no reaction will occur. Some scientists have reported overcoming this issue by modifying the concentration of gases and also by incorporating spin coating methods.
Current and Potential Solutions
In terms of overcoming these issues, scientists have been developing more complex techniques and guidelines to follow in order to create the highest quality of graphene possible. One introductory technique to reducing the effects of these issues is by treating the substrate before the reaction takes place. A copper 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 graphene flakes that contain fewer imperfections.
This point of treating the substrate prior to deposition is something that will continue to be researched for a long time, as we slowly learn how to modify the structure of graphene to suit different applications. For example, in order to enable graphene to be effectively used in superconductors, doping must be carried out on the material in order to create a band-gap. This process could potentially be something that is carried out on a substrate before deposition occurs rather than treating the material after CVD.