Graphene is a single layer of carbon atoms arranged in a hexagonal lattice. Its extraordinary properties earned Geim and Novoselov the 2010 Nobel Prize in Physics:

• Electronic properties: Zero bandgap semiconductor with linear dispersion (Dirac fermions). Carrier mobility exceeds 200,000 cm2/V.s at room temperature.
• Mechanical strength: Young's modulus of ~1 TPa, intrinsic strength of 130 GPa. Strongest material ever measured.
• Thermal conductivity: 3,000-5,000 W/m.K, exceeding diamond and carbon nanotubes.
• Optical properties: 2.3% light absorption per layer, useful for transparent electrodes.

Synthesis methods

• Mechanical exfoliation: Scotch tape method, highest quality but low yield
• CVD growth: Cu/Ni substrates, scalable for electronics, 30" wafers demonstrated
• Liquid exfoliation: Sonication in solvents for bulk production, lower quality
• Epitaxial growth: SiC decomposition for wafer-scale integration

TMDs (MX2, where M = Mo, W, etc. and X = S, Se, Te) are semiconducting 2D materials with direct bandgaps in monolayer form:

MXenes are 2D transition metal carbides/nitrides (Mn+1XnTx) discovered in 2011:

• Composition: M = Ti, V, Nb, Mo, etc.; X = C or N; Tx = surface terminations (-OH, -O, -F)
• Synthesis: Selective etching of MAX phases (HF or LiF/HCl), milder routes developed
• Properties: Metallic conductivity (up to 20,000 S/cm), hydrophilic surfaces, tunable chemistry
• Applications: Supercapacitors (1,500 F/cm3), EMI shielding, batteries, catalysis

Ti3C2Tx is the most studied MXene, with record capacitance values and excellent EMI shielding effectiveness (>90 dB).

Stacking different 2D materials creates heterostructures with emergent properties:

• Graphene/hBN: Atomically flat substrate preserves graphene mobility, enables moiré superlattices
• Twisted bilayer graphene: "Magic angle" (1.1°) creates flat bands, unconventional superconductivity at 1.7 K
• TMD heterostructures: Type-II band alignment for efficient exciton separation, interlayer excitons with ms lifetimes
• Mixed-dimensional: 2D/0D, 2D/1D combinations for photodetectors and sensors