Strain can significantly affect the electronic and optical properties of graphene, a two-dimensional 2D material composed of carbon atoms arranged in a honeycomb lattice. When graphene is subjected to strain, its lattice structure is deformed, which in turn alters its electronic band structure and excitonic properties. Here, we will discuss the changes in these properties when graphene is strained.1. Electronic properties:The electronic band structure of graphene is characterized by the presence of Dirac cones at the K and K' points in the Brillouin zone. These cones represent the linear dispersion relation between energy and momentum for electrons and holes in graphene, which gives rise to its unique electronic properties, such as high carrier mobility and massless Dirac fermions.When strain is applied to graphene, the symmetry of the lattice is broken, and the Dirac cones are shifted and distorted. This can lead to several changes in the electronic properties:a. Bandgap opening: Under certain strain conditions, a bandgap can be induced in graphene, which is otherwise a zero-gap semiconductor. This can be useful for tuning its electronic properties for specific applications, such as transistors and other electronic devices.b. Charge carrier mobility: Strain can also affect the carrier mobility in graphene. Depending on the type and magnitude of the strain, the mobility can either increase or decrease. This can be attributed to changes in the scattering mechanisms and the effective mass of the charge carriers.c. Pseudo-magnetic fields: Inhomogeneous strain can lead to the generation of pseudo-magnetic fields in graphene, which can affect the electronic properties and lead to the formation of Landau levels.2. Optical properties:The optical properties of graphene are closely related to its electronic structure and excitonic properties. Strain can affect these properties in the following ways:a. Exciton binding energy: Strain can modify the exciton binding energy in graphene, which is the energy required to separate an electron-hole pair exciton into free carriers. This can lead to changes in the optical absorption and emission spectra of graphene.b. Optical selection rules: The optical selection rules, which determine the allowed optical transitions between electronic states, can also be affected by strain. This can result in the appearance of new peaks or the suppression of existing peaks in the optical spectra of graphene.c. Optical anisotropy: Strain can induce optical anisotropy in graphene, which means that its optical properties become direction-dependent. This can lead to polarization-dependent optical absorption and emission, which can be useful for applications in optoelectronics and photonics.In summary, strain can significantly affect the electronic and optical properties of graphene by altering its energy band structure and excitonic properties. These changes can be exploited to tune the properties of graphene for various applications, such as electronics, optoelectronics, and photonics. However, it is essential to carefully control the strain to achieve the desired effects and avoid any detrimental impact on the material's performance.