Graphene and other 2D materials exhibit unique electronic and optical properties that differentiate them from traditional 3D materials. These properties arise due to their atomic-scale thickness and the arrangement of their atoms in a planar lattice. Some of the most studied 2D materials include graphene, transition metal dichalcogenides TMDCs , and hexagonal boron nitride h-BN .1. Electronic properties:Graphene: Graphene is a single layer of carbon atoms arranged in a hexagonal lattice. It is a zero-bandgap semiconductor, which means that its conduction and valence bands touch at the Dirac points. This results in exceptional electronic properties, such as high electron mobility up to 200,000 cm^2/Vs and high electrical conductivity. Graphene also exhibits ballistic transport, where electrons can travel long distances without scattering.TMDCs: These materials consist of a transition metal atom such as Mo or W sandwiched between two chalcogen atoms such as S, Se, or Te . TMDCs have a tunable bandgap that can range from 1 to 2 eV, depending on the specific material and its thickness. This makes them suitable for various electronic applications, such as transistors and photodetectors.h-BN: Hexagonal boron nitride is an insulating 2D material with a wide bandgap of ~6 eV. It is often used as a substrate or dielectric layer in 2D electronic devices.2. Optical properties:Graphene: Graphene is highly transparent, with an absorption of only ~2.3% of visible light. This property makes it suitable for transparent electrodes in optoelectronic devices. Additionally, graphene's linear dispersion relation leads to unique optical phenomena, such as the anomalous quantum Hall effect and ultrafast carrier dynamics.TMDCs: TMDCs exhibit strong light-matter interactions due to their direct bandgap in the monolayer limit. This results in high photoluminescence quantum yields and strong absorption coefficients. TMDCs also possess valley-selective optical properties, which can be exploited for valleytronics and spintronics applications.h-BN: Due to its wide bandgap, h-BN is transparent in the visible and ultraviolet regions of the spectrum. It also exhibits strong excitonic effects and hyperbolic dispersion, which can be utilized for nanophotonics and polaritonics.Comparison with 3D materials: Traditional 3D materials, such as silicon and gallium arsenide, typically have lower electron mobility and less tunable bandgaps compared to 2D materials. Additionally, 3D materials often exhibit indirect bandgaps, which result in weaker light-matter interactions and lower photoluminescence quantum yields.Quantum chemical calculations, such as density functional theory DFT and many-body perturbation theory MBPT , have been instrumental in predicting and understanding the electronic and optical properties of 2D materials. These calculations provide insights into the band structure, density of states, excitonic effects, and optical absorption spectra of 2D materials, enabling researchers to design and optimize materials for specific applications.