Graphene is a two dimensional carbon material consisting of a single layer of carbon atoms arranged in a hexagonal lattice. Each carbon atom is bonded to three neighbouring carbon atoms through strong covalent interactions, while the remaining electronic orbital contributes to a delocalised electronic system extending across the sheet. This structure gives Graphene exceptional electrical, thermal and mechanical properties under ideal conditions.
It is often described as an atomically thin crystalline sheet, although real samples can contain defects, wrinkles, grain boundaries, contamination and multiple layers. The distinction between monolayer Graphene and few layer or multilayer graphitic materials is important because electronic and other properties change with thickness. Graphene became a major research subject after reliable isolation of individual layers was demonstrated in 2004, although theoretical work on the electronic structure of graphite and related systems had existed much earlier.
The electronic structure of Graphene is responsible for many of its unusual properties. The arrangement of carbon atoms produces electronic bands that meet at specific points in momentum space, leading to charge carriers that can be described by a relativistic like mathematical equation in the low energy approximation. These points are commonly called Dirac points, and the corresponding charge carriers are often described as Dirac fermion like quasiparticles. Graphene can therefore exhibit very high carrier mobility under appropriate conditions.
The electronic behaviour can be modified by chemical functionalisation, defects, substrate interactions, electrostatic gating and strain. Graphene does not possess the conventional band gap required for many ordinary digital transistor applications, which is both an advantage for some electronic uses and a limitation for others. Researchers have investigated methods of opening or engineering a band gap through nanostructuring, bilayer configurations and chemical modification.
Graphene has exceptional mechanical properties relative to its thickness. Strong carbon carbon bonding within the lattice produces a high in plane stiffness and tensile strength, while the low areal density gives a very high strength to mass ratio. These properties have encouraged research into reinforcement of polymers and other materials. However, transferring the intrinsic properties of an individual Graphene sheet into a bulk composite is not straightforward.
Sheets can fold, restack and aggregate, reducing accessible surface area and limiting effective reinforcement. Interfacial bonding between Graphene and the surrounding matrix determines how effectively mechanical stress is transferred. Defects and grain boundaries can also reduce strength and alter electrical transport. Consequently, material performance depends on sheet size, defect density, layer number, surface chemistry, dispersion and orientation as well as on the intrinsic properties of ideal Graphene.
Graphene conducts heat efficiently in the plane of the sheet because phonons can transport energy through its highly ordered carbon lattice. Its electrical conductivity can also be very high, although measured values depend on carrier density, disorder, substrate interactions, temperature and contacts. Graphene's large surface area and tunable electronic properties make it attractive for sensors and electrochemical devices. Its surface is chemically active enough to be functionalised while remaining relatively inert in its pristine state under many conditions.
Graphene oxide introduces oxygen containing groups and therefore has substantially different chemical, electrical and structural properties from pristine Graphene. Reduction of Graphene oxide can partially restore conductivity but generally does not reproduce perfect Graphene. The distinction among pristine Graphene, oxidised forms and chemically modified derivatives is therefore essential because the names are sometimes used loosely despite significant differences in properties.
Graphene can be incorporated into composites to improve electrical conductivity, mechanical reinforcement, thermal transport, barrier performance or surface properties. Polymer Graphene composites can create conductive pathways at relatively low loading levels when the sheets are sufficiently dispersed and connected. Barrier applications exploit the long diffusion paths that gases or liquids may encounter when moving through well dispersed layered structures. Coatings can use Graphene based materials for electrical, mechanical or chemical functionality.
Energy research investigates Graphene and related structures as electrode additives, conductive networks and components of electrochemical devices. Its high surface area is useful for electrode architectures, although practical performance depends on accessible surface, porosity, conductivity, electrolyte compatibility and processing. The presence of Graphene therefore does not guarantee improved performance. Its value depends on how effectively the nanoscale sheet structure is integrated into the complete device or material.
Graphene has also attracted attention in membranes, sensors and separation technologies. Atomically thin structures can potentially provide highly selective transport when pores or defects are engineered with suitable dimensions and chemical properties. Graphene based membranes can be modified to control interactions with water, gases or dissolved substances. Chemical functionalisation can introduce selectivity that pristine Graphene lacks. Sensor systems can exploit changes in electrical properties caused by adsorption or chemical reactions at the surface.
These applications depend strongly on defect control because a single nanoscale defect can alter transport through an otherwise impermeable sheet. Manufacturing large area Graphene with consistent properties remains a major challenge for many applications. Chemical vapour deposition can produce large sheets on suitable substrates, while mechanical exfoliation can provide high quality small flakes. The best production method therefore depends on whether the priority is structural quality, area, cost, thickness control or scalability.
The modern history of Graphene is associated with the experimental isolation of individual layers by Andre Geim, Konstantin Novoselov and colleagues using mechanical exfoliation of graphite. Their work demonstrated that stable single atomic layers could be isolated and studied under ordinary laboratory conditions, challenging assumptions that such structures might be difficult to maintain. The 2010 Nobel Prize in Physics recognised their experiments on Graphene. Research subsequently expanded rapidly into electronics, photonics, composites, membranes, sensors and energy technologies. The field also stimulated broader research into two dimensional materials with other compositions and structures. Despite the enormous scientific interest, commercial deployment has been more selective than early predictions suggested because high quality production, integration, defect control and cost remain significant engineering constraints.
Graphene is frequently described using claims that are broader than the underlying evidence supports. It is not the strongest, most conductive or most useful material for every possible application. Its extraordinary intrinsic properties are measured under particular conditions and do not automatically determine the behaviour of a commercial Graphene containing product. Graphene also does not mean any carbon material that contains a thin layer or graphite particles.
Graphite consists of many stacked carbon layers with interlayer interactions, while monolayer Graphene consists of one atomic layer. Carbon Nanotubes can be regarded structurally as related to rolled Graphene sheets but have different geometries and electronic behaviour. Accurate discussion therefore requires specifying layer number, lateral dimensions, defect density, synthesis method, surface chemistry and application context.
