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Thesis presentation

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by

duho Kim

on 2 November 2012

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Transcript of Thesis presentation

Title Thesis presentation Purpose HF is a colorless gas with dreastic toxicity, and it is a commom industrial exhaust gas.
So sensors with high sensitivity to it are required for environmental safety. First-principles studies of HF molecule adsorption on intrinsic graphene and Al-doped graphene An excellent gas sensor should have high sensitivity, robustness
and a wide range of applications as well as low cost and potential
for miniaturization.
Graphene is newly reported to make highly sensitive sensors, by
detecting the change of conductance as CNTs(carbon nanotubes) do.





Another study revealed that the sensitivity of graphene to gas molecule
could be improved by doping metal atoms. Experimental procedure and results E a = adsorption energy E a = E HF+graphene - (E graphene + E HF ) the letter T = the top site of C atoms the letter B = bridge site of the C-C bond the letter H = the hollow site of the carbon hexagon The adsorption energies of the configurations are small, the initial adsorption configuration
H-H-H has the lowest adsorption energy up to -0.074eV.

The distacne from the HF molecule to the intrinsic graphene suface is 3.661A, there is no evidence
of formation of chemical bonds between the HF molecule and the graphene. - Table 2 presents the C-C bond lengths of intrinsic graphene with and without adsorption of HF molecule The C-C bonds are slightly shortened by 0.001A after adsorption.
Adsorption of HF molecule does no impact the structure of graphene distinctly. The net electron transfer is shown about 0.001e is transferred from the intrinsic graphene to the HF molecule
, indicating that the interaction between the HF molecule and the intrinsic graphene sheet is mainly electrostatic
in nature. - The band structure and the DOS of the HF-graphene system are shown in Fig.

And Comparing the DOS of HF-graphene with that of intrinsic graphene, we can see that there is no hybridization between the HF molecule and intrinsic graphene, and the graphene states are nearly unaltered by the adsorption of the HF molecule. - Fig. shows the electronic density differences images of HF-graphene.

No charge accumulation could be seen between the intrinsic graphene and the HF molecule All results indicate that the interaction between the HF molecule and the intrinsic graphene is of weak van der Waals interaction type, and the adsorption is by weak physisorption.

Therefore, intrinsic graphene cannot be a good sensor material to detect HF gas. The initial adsorption configuration H-T-H has the most stable relaxed structure.

The corresponding adsorption energy value of an HF molecule in the AL-doped system is -7.164eV,
which is over 100 times more than that in intrinsic graphene.

This indicates that Al-doped graphene is energetically favorable for the adsorption of HF molecule. - Table 2 presents the Al-C bond lengths of Al-doped graphene with and without adsorption of HF molecule The Al-C bond length in HF-Al-doped graphene is in the range from 1.797 to 1.817A. The net electron transfer from an HF molecule to the Al-doped graphene is 0.024e, which is 24 times the net electron transfer in the HF-intrinsic graphene system. Metal doping may improve the sensitivity of graphene to gas molecule


The structure of Al-doped graphene is shown in Fig., with one C atom in the center of the supercell substituted by an Al atom. - The band structure and the DOS of the Al-doped graphene system with adsorption are shown in Fig.

The DOS shows that the HF molecule hydridizes with Al atom and C atom at -11, -9 and 2.2 eV relative to the Fermi level. The highest occupied molecule orbital is -9 eV and the lowest unoccupied molecule orbital is 2.2 eV above the Fermi level. So, the overlap of orbitals would form a covalent bond between the HF molecule and the Al-doped graphene. - Fig. shows the electronic density differences images of Al-doped graphene and HF molecule.

There is charge accumulation between the HF molecule and the Al-doped graphene, and the Al-doped graphene gains electrons from the H atom of the HF molecule.

This demonstrates that the charge transfer would form an ionic bond between the HF molecule and the Al-doped grahphene and the HF molecule acts as the electron donor of the system. Discussion The ionic and covalent interaction between the HF molecule and the Al-doped graphene contribute to the bonding of the adsorption The adsorption of HF molecules on Al-doped graphene is by chemisorption, which would increase a increas of electrical conductivity in the Al-doped graphene.

Hence, Al-doped graphene can be a good sensor material to detect HF gas. Discussion
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