What is Graphene? Here’s What You Need to Know

Overview: Graphene is a unique material with a structure and properties that make it stand out in the field of materials science. Let’s delve into its characteristics and understand what makes graphene so special.

Internal Structure

Graphene consists of carbon atoms arranged in a single layer, similar to a monolayer of graphite. Here are some key features:

• Each carbon atom has four valence electrons.
• Three of these electrons form sp2 bonds, while one electron is in the pz orbital.
• The pz orbitals of adjacent atoms form π bonds, which are perpendicular to the plane and contribute to graphene’s half-filled electronic states.
• Carbon atoms in graphene have a coordination number of 3.
• The bond length between adjacent carbon atoms is 1.42 Å, with a bond angle of 120°.
• This structure forms a honeycomb-like, hexagonal pattern, giving graphene its exceptional electrical and optical properties.

Mechanical Properties

Graphene is one of the strongest known materials while also being highly flexible and bendable:

• It has a theoretical Young’s modulus of 1.0 TPa and an intrinsic tensile strength of 130 GPa.
• Reduced graphene oxide modified by hydrogen plasma can have a modulus exceeding 0.25 TPa.
• Graphene paper, composed of multiple graphene sheets, tends to be brittle due to its porous nature.
• Functionalized graphene, derived from oxidized graphene, can be incredibly strong and resilient.

Electronic Properties

Graphene exhibits extraordinary electronic properties:

• At room temperature, graphene has a carrier mobility of about 15,000 cm²/(V·s), which is over ten times higher than that of silicon.
• Under certain conditions, such as low temperatures, the mobility can reach up to 250,000 cm²/(V·s).
• Graphene’s electron mobility is relatively unaffected by temperature changes, maintaining around 15,000 cm²/(V·s) across a wide temperature range (50K to 500K).
• Graphene demonstrates the half-integer quantum Hall effect, observable even at room temperature.
• It also exhibits a unique quantum tunneling effect, preventing backscattering of carriers and contributing to its local superconductivity and high carrier mobility.
• Electrons and photons in graphene have zero rest mass and move at a constant speed, independent of their energy.

Semiconductor Properties

Graphene is a semiconductor material with the following characteristics:

• Its conduction band and valence band intersect at Dirac points.
• These Dirac points occur at the edges of the Brillouin zone in momentum space, forming two sets of equivalent triplets.
• Unlike traditional semiconductors, which have their main points at zero momentum, graphene’s special properties arise from its unique band structure.

Thermal Properties

Graphene is an excellent thermal conductor:

• Defect-free graphene has a thermal conductivity of up to 5300 W/(m·K), the highest among carbon materials.
• It surpasses the thermal conductivity of single-wall carbon nanotubes (3500 W/(m·K)) and multi-wall carbon nanotubes (3000 W/(m·K)).
• When used as a substrate, its thermal conductivity can reach 600 W/(m·K).
• Graphene’s ballistic thermal conductivity also enhances the performance of carbon nanotubes.

Graphene’s remarkable properties make it a revolutionary material with numerous potential applications in electronics, materials science, and beyond. Its unique combination of strength, flexibility, electrical conductivity, and thermal properties positions it as a key material for future technological advancements.