All about Graphene

a.      Fabrication and Characteristics

Graphene is two-dimensional crystalline form of carbon: a single layer of carbon atoms arranged in hexagons, like a honeycomb, with sp² bonding, unlike diamond and amorphous carbon materials as having sp³ bonding. Chemical functionalization of the main graphene sheet (not the edges) is achieved by either covalent or non-covalent methods. Covalent functionalization requires the breaking of sp² bonds and can be achieved using a wide range of reactions. Non-covalent functionalization relies on van der Waals forces often due to  pi-pi stacking between aromatic molecules and the graphene lattice. Regarding its electronic properties, a good approximation to the band structure of mono-layer graphene can be obtained from a simple nearest neighbor tight-binding calculation. Inspection of this band structure immediately reveals three electronic properties of mono-layer graphene which have excited such interest in this material: the vanishing carrier density at the Dirac points, the existence of pseudo-spin and the relativistic nature of carriers [1].  In case of magnetic properties of graphene, weak paramagnetism at low temperatures is reported for relatively defect free graphene, and the use of ion irradiation to add vacancies can increase the paramagnetism [2].

Semiconductor Physics and Devices: Solar Cell

Solar cells are devices that convert photon energy into electricity. There are several types of solar cells regarding the materials which are used. The common structure of solar cell is configured by p-n junction where electron-hole pair generation takes place, and electrodes to transfer the electrons. There are other components installed in some solar cells in order to improve efficiency such as inserting intrinsic semiconductor in enlarge the area of photocurrent generation or reflector to enhance photon energy. The example of organic and inorganic solar cells is presented in Fig. 1. The mechanism of power generation of solar cells is due to photovoltaic effect  which explains that light of certain wavelengths is able to ionize the atoms in semiconductor materials and the internal electric field produced by the p-n junction separates some of the positive charges ("holes") from the negative charges (electrons) which then are transferred to the electrodes. Here, the band gap between p-type and n-type semiconductors determines the specific absorbance of light.

QA on Delectric Materials

Q1. Describe the definition of dielectric materials

Dielectric materials are electrical insulators which can be easily polarized by external electric field.  The presence of electric field results in charges re-arrangement in which the positive charges are displaced toward the field whereas negative charges are in opposite direction so called dielectric polarization. They are categorized as electrical insulator since there is no field induced-charges flow as it happens on conductor. This is due to large energy band gap and no free electron exists at conduction band. Closely seeing to the atomic model, the external electric field will distort electron cloud surrounding nucleus and give rise to electric dipole.