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Introduction to Graphene
Published in Titash Mondal, Anil K. Bhowmick, Graphene-Rubber Nanocomposites, 2023
Ajith Pattammattel, Challa V. Kumar
A single graphene sheet represents a network of sp2-hybridized carbon atoms. The adjacent carbon atoms in the hexagon structure are bonded with a sigma bond comprising of two sp2 orbitals, while a π-bond is formed from the remaining p-orbital of each carbon atom (Figure 1.1). The extensive 2D-network of sigma bonds of high bond energies provides the material with high mechanical strength, flexibility, and unusual physical properties. The pz-orbitals of the carbons provide a continuous network of π-electrons which play a vital role in defining the unique optoelectronic and mechanical properties of graphene and graphene-family materials. Extensive delocalization of the network of π-bonds endows graphene with unusual optoelectronic properties, including high thermal and electrical conductivities (Sang et al. 2019).
Inverted Organic Solar Cells
Published in Madhu Bhaskaran, Sharath Sriram, Krzysztof Iniewski, Energy Harvesting with Functional Materials and Microsystems, 2017
Purna P. Maharjan, Qiquan Qiao
OPV devices are based on carbon-based organic semiconductors in which charge transport is mainly by the delocalization of electrons along conjugated polymer backbones [16]. Carbon atoms are sp2 hybridized and form three sigma bonds with neighboring atoms. The fourth orbital (Pz) is perpendicular to the plane of sp2 orbitals. Delocalization of the charges in organic semiconductors is due to the overlapping of the Pz electron wave functions [46]. Upon light illumination, photons are absorbed by an active layer and electron– hole pairs are generated as excitons. Unlike inorganic semiconductors, organic semiconductors have a relatively low dielectric constant (εr ≈ 2–4). This causes the coulombic interaction between electron and hole to be strong and form tightly bonded excitons. Typical binding energy of Frenkel exciton is around 0.3–1.0 eV [13,47].
Computational study of linear and nonlinear optical properties of substituted thiophene imino dyes using long-range corrected hybrid DFT methods
Published in Molecular Physics, 2020
Lamia Kara Zaitri, Sidi Mohamed Mekelleche
Experimentally [37–39], it has been found that the synthesised substituted thiophene imino dyes A-C (Scheme 2) are characterised by high molecular hyperpolarisabilities, namely, 3.0, 3.8, and 6.7 times than that of pNA (paranitroaniline) respectively. We note that pNA is one of the prototypical molecules used in the study of the NLO properties of D-π-A type molecular systems [40–44]. Our main objective in the present work is to perform a computational study for both the s-cis and s-trans (about the C-C sigma bond) configurations of the entitled compounds in order to rationalise the measured molecular hyperpolarisabilities and maximum absorption wavelengths for dyes A-C and to predict the NLO properties of a new designed derivative (dye D) by substituting the thiophene ring with a strong electron withdrawing group (R = TCV). The intramolecular charge transfer (ICT) was analysed by the calculation of the HOMO-LUMO gaps, ΔEH-L, chemical hardnesses, η, and second-order stabilisation energies E(2).
The elusive diiodosulphanes and diiodoselenanes
Published in Molecular Physics, 2022
On the contrary, the energy of certain molecular orbitals increases when the dihedral goes to 81.87°. The most significant one is the 40th molecular orbital which is a π partially antibonding between S and I atoms, but partially bonding between both S atoms. That orbital becomes less bonding between both S, thus its energy increases by +24 or +29 mH, with respect to the anti or the syn conformations respectively. The molecular orbital 39, which is clearly a sigma bond S–S with some sigma bond between both iodines in the 0° conformation, is less bonding and more energetic in the 81.87° conformation by +29 mH.