Graphene And Their Theoretical Research, The Nature Of The Electronic Structure Of Doped

Graphene, a single layer of carbons, is found to exist as a free-standing form and exhibits many unusual and intriguing physical, chemical and mechanical properties. In the past few years, graphene which has caused researchers' extensive attention is becoming the leading edge and hotspot in various relative fields. In this paper, electronic structures of intrinsic and doped graphene have been simulated and computed based on the first principles of density functional theory(DFT), which could provide theoretical guidance for their applications in nano-electronic devices, energy storage and novel gas sensors.It indicated that finite size graphene had a band gap which decreased on increasing the dimension of the sheet to near metallic situation through compairing with three dimension graphene sheets by DFT. In our investigation we also observed high electron density along edges especially at the zigzag edges. Furthermore, we deviced a serials of graphene dots. By using DFT we found that it had both luminescent and ferromagnetic properties when graphene quantum dot was consists of six equilateral triangle structures of the benzene rings. We also obtained the electronic absorption spectra of the quantum dot by time-dependent density functional theory(TD-DFT) and our results were well agreement with experimental observations. We proposed that strong Luminescence of graphene quantum dot from the zigzag edge effect. In addition, the energy of the triplet state was lower than the singlet one which could be explained as the spin containing units with unpaired electrons for synthesis and design of the molecular magnetic materials.The adsorptions of H2 molecules on the intrinsic and doped graphenes were also investigated. It is found that H2 molecules are only weakly adsorbed onto the intrinsic graphene with small binding energy value and large distance between the H2 molecules and graphene. The electronic structure and electrical conductivity of the intrinsic graphene have a limited change caused by the adsorption of H2 molecules. However, the H2 molecule has strong interaction with the Al or Li+ doped graphene, forming strong physisoption that introduces a large amount of shallow acceptor states into the system. In this case, the remarkable variation of the electrical conductivity is induced by the H2 adsorption, possessing an excellent characteristic of high sensitivity for H2 gas detection.The structures and electronic states of sodium ion (Li+) trapped on graphene have been investigated by means of density functional theory (DFT) calculation to elucidate the nature of interaction between Li+ and graphenes. The VASP calculation showed that the Li+ ion is stabilized in hexagonal site and the height of Li+ ion at the energy minimum is 1.8A., Li+ ion is preferentially bound to a hexagonal site of the graphene, In addition, the DOS indicated that there were some charges transferred from graphene surface to Li ion, and we also found that there was a high potential barrier about 1.726eV for Li ion cross the carbon ring, and the potential barrie increased sharply as the increase of grapheme layers. All results showed that graphene as a new type of lithium-ion battery anode material, had great advantages and applicability.Pentacene(C22H12) can also be known as a graphene dot.We introduce polar substituents to pentacene such as F, Cl, Br, to enhance their dissolubility in common organic solvents while keeping the high charge-carrier mobilities of pentacene. Geometric structures, dipole moments, frontier molecule orbits, ionization potentials and electron affinities, as well as reorganization energies of those molecules, and of pentacene as a comparision, have been successively calculated by the density functional theory. The results indicate that halopentacenes have rather small reorganization energies (<0.2eV), and when the substituents in the 2 position or 2,9 positions they are polarity molecules, so we conjecture they can easily dissolve in common organic solvents, and are promising candidates for organic semiconductors.