Preparation of Graphene Powder Substrate

Overview: To utilize dispersed graphene powder in non-liquid phases, various preparation methods are employed. These methods involve different processes such as vacuum processing in silicon carbide crystals and chemical vapor deposition (CVD). Here, we will explore these methods and their respective outcomes.

Methods of Preparation

1. Vacuum Processing in Silicon Carbide (SiC) Crystals
   • During heating, carbon diffuses through a nickel layer and forms graphene powder or graphite layers on the surface, depending on the heating rate.
   • The resulting graphene powder is easier to separate from the surface compared to that produced by simple SiC crystal growth without nitrogen (N).
2. Direct Growth on Substrates
   • Graphene powder can be grown directly on surfaces, where the size of the resultant layers is not dependent on the initial graphite crystal.
   • This method can either utilize pre-existing carbon in the substrate or add it via CVD.
3. Liquid Phase Exfoliation
   • Graphene oxide powder, a hydrophilic molecule, can be exfoliated in water through sonication or stirring.
   • The resulting negatively charged layers repel each other, preventing aggregation. After centrifugation, the graphene oxide powder is filtered and processed into graphene paper cakes.
4. Deposition Techniques
   • Graphene can be deposited on surfaces via simple drop-casting methods, where solutions are dripped onto substrates.
   • For more uniform coatings, the solution can be dispersed using centrifugal force or sprayed onto samples.

Chemical Vapor Deposition (CVD)

• Process Overview:
  • CVD is a well-known process where substrates are exposed to gaseous compounds that decompose on the surface to form thin films.
  • Heating samples with filaments or plasma, exposing nickel films to gas mixtures of H2, CH4, and Ar at 1000°C results in the formation of graphene layers.
  • Carbon from decomposed methane diffuses into the nickel and, upon cooling, graphene layers form on the surface.
• Layer Control:
  • The average number of layers depends on the thickness of the nickel, which can be controlled. The shape of the graphene can also be managed.
  • These layers can be transferred using polymer carriers that attach to the graphene. After etching the nickel, graphene is transferred to the desired substrate, and the polymer is removed.
  • Stacking several layers of graphene reduces resistance, and each layer’s electronic properties remain largely unchanged due to minimal interaction.
• Using Copper Substrates:
  • Replacing nickel with copper as the growth substrate results in less than 5% single-layer graphene, due to copper’s low solubility for carbon.
  • A 30-inch graphene layer can be grown on copper foil using CVD and then transferred to PET films through a rolling process.
  • Doping the graphene with HNO3 reduces sheet resistance, and stacking doped graphene layers on PET results in functional touch screen panels with high transparency and low resistance.

Summary of Methods and Their Performance

• Exfoliation Methods:
  • Advantages: Produce high-quality and pure graphene, ideal for laboratory research due to low complexity.
  • Limitations: Values obtained are also suitable for industrial production.
• Surface Growth Methods:
  • Advantages: Allows for the production of large and controllable graphene layers, suitable for industrial production.
  • Limitations: The purity is not as high, making these methods less suitable for laboratory research.
• CVD Methods:
  • CVD is already an established industrial method, enabling the epitaxial growth of graphene.
  • Allows for high control over layer formation and the ability to produce large-scale graphene suitable for various applications.