There are various methods to produce graphene materials, such as mechanical exfoliation of graphite, liquid-phase exfoliation and reduction of graphene oxide (GO), each with its own set of advantages and disadvantages. Chemical vapor deposition (CVD) is a method of producing graphene that has attracted much attention in the last decade and despite several shortcomings, is considered a leading approach to manufacture graphene, especially for applications like high-performance electronics and sensors, as it can yield high quality graphene sheets with a low defect count and good uniformity.
The CVD process
Generally speaking, the CVD method is based on gaseous reactants that are deposited on a substrate. The graphene is grown on a metallic surface like Cu, Pt or Ir, after which it can be separated from the metal and transferred to specifically-required substrates. When the gases contact the substrate inside the heated reaction chamber, a reaction occurs that creates a film of material on the substrate. The process can be simply explained as carbon-bearing gases that react at high temperatures (900–1100 °C) in the presence of a metal catalyst, which serves both as a catalyst for the decomposition of the carbon species and as a surface for the nucleation of the graphene lattice.
As was mentioned before, conditions like temperature, pressure, duration of time and many more can have a significant influence on the process and so must be carefully monitored.
Advantages and disadvantages
CVD can yield high quality graphene, with common characteristics that may include high homogeneity, imperviousness, high purity, fine grains, good control over layer number and more. However, CVD also comes with several disadvantages - namely a relatively high price of the equipment, toxic gaseous by-products and, as stated before, it is a rather sensitive process that is easily influenced by parameter changes. It is important to note that while CVD is a rather expensive method compared to other methods to produce lower-quality graphene, it is still probably the best way (to date) to acquire high quality graphene, since other ways to do that are even more expensive or complicated.
In addition, the separation (or exfoliation) of graphene from the substrate is known as challenging and it is tricky to accomplish without damaging the structure of the graphene or affecting the properties of the material. Another harrowing task is the creation of a uniform layer of graphene on a substrate, something that is continuously proving to not be easy at all.
Some approaches were and are still being developed to overcome this issue, like modifying the concentration of gases and incorporating spin coating methods, but this remains a challenge. However, despite these challenges, it is important to note that CVD is widely considered as an important and promising method to produce graphene, which is already in use, and will probably be even more so in the future, once further advancements are made. It is noteworthy that CVD is already a dominant manufacturing route for many other nanomaterials, and will assumably be in extremely common use once progress is made to resolve the issues that are currently hindering its acceptance.
While using the CVD method to produce graphene is definitely one the leading approaches in the world, it is still hindered by the challenges mentioned above. Thus, it is currently mostly limited to relatively small volumes and mainly restricted to R&D and academic uses.
However, graphene sheets produced via CVD methods are used in several applications like sensors, touch screens and heating elements. It is believed that once better answers are found to the questions of price and handling of CVD graphene, many more applications will start to appear.
Looking to buy CVD graphene sheets?
If you are interested in buying CVD graphene sheets, check out the Graphene Catalog that lists various CVD produced graphene sheets from several producers.
The latest CVD graphene news:
Researchers from Nanjing University in China have developed a method to make large graphene films free of any wrinkles. The ultra-smooth films could enable large-scale production of electronic devices that harness the unique physical and chemical properties of graphene and other 2D materials.
Chemical vapor deposition (CVD) is the best-known method for making high-quality graphene sheets. It typically involves growing graphene by pumping methane gas onto copper substrates heated to temperatures around 1,000 °C, and then transferring the graphene to another surface such as silicon. But some of the graphene sticks to the copper surface, and as the graphene and copper expand and contract at different rates, wrinkles form in the graphene sheets. Such wrinkles often present hurdles for charge carriers and lower the film’s conductivity. Other researchers have tried to reduce wrinkles using low growth temperatures or special copper substrates, but the wrinkles have proven difficult to eliminate entirely, according to Libo Gao, a physicist at Nanjing University.
Graphenea has announced the launch of a new product – highly flat monolayer graphene. The graphene is grown by CVD on copper thin film on a 2” sapphire substrate. With extremely low roughness that is less than 4 nm, this new product is targeted at applications in photonics, high-performance electronics, magnetic memory, and freestanding membranes.
The product aims to meet wafer-scale integration requirements to build uniform graphene devices in a fashion compatible with current industrial fabrication methods. The flat graphene product is ready to be transferred by electrochemical delamination or dry methods since the sapphire substrate is robust enough to withstand mechanical damage, preventing tearing and wrinkling of the thin Cu sheet. The total wafer thickness is 430 micrometers. Full product information can be found in Graphenea's online store.
In order for CVD graphene to be used in its intended application, it needs to be transferred from the growth substrate to a target substrate – a challenging but extremely important process step. Typically the transfer is done by spin-coating a supporting polymer layer and then chemically dissolving away the copper to release the graphene film from the substrate. The transferred graphene produced in this way is prone to contamination from the chemical agents used to remove the growth substrate as well as defective amorphous carbon generated during the high-temperature CVD growth. It also frequently leads to a substantial amount of polymer particle residue on the graphene generated during the transfer process. A third source of contamination could be airborne particles that are adsorbed onto the graphene surface.
Researchers from Peking University and Tsinghua University in China and University of Manchester in the UK have recently demonstrated that the amorphous carbon contaminants on CVD-produced graphene, which could greatly degrade its properties, can be removed by an activated carbon-coated lint roller, relying on the strong interactions between the amorphous carbon and activated carbon.
According to our information, LG Electronics is aiming to start supplying CVD graphene materials worldwide soon, with an aim to accelerate the adoption of CVD graphene in various applications. LG is collaborating with research groups to identify new applications for graphene sheets.
LG Electronics developed its own roll-to-roll production process in addition to a specific quality control system for its graphene. LG says that its inspection system can manage uniformity deviations in crystal size, defects and electrical properties in its graphene to within 10%.
Researchers from ICN2 and Imec managed to demonstrate the longest spin lifetime and relaxation length ever seen in a graphene sheet.
The researchers used CVD graphene, grown on a platinum foil. By optimizing several standard processes, the researchers managed to reduce the impurity level of the graphene.