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Theoretical Study on the Mechanism for Small Molecule Adsorption on the Si(111)-7×7 Surface

Author: PengXinYu
Tutor: WangXueYe
School: Xiangtan University
Course: Physical and chemical
Keywords: water pyrrole ammonia Si(111)-7×7 surface density functional theory (DFT)
CLC: O641.1
Type: Master's thesis
Year: 2009
Downloads: 46
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Abstract

Chemical modification of semiconductor surface is of great interest because of its potential applications. Modified semiconductors is expected to result in new device applications, including molecular electronics, sensors, nanotechnology, and so on. The paper applied the density functional cluster model methods and systematically investigated the mechanisms of surface reactions between small molecules and Si(111)-7×7 surface. The following 3 systems were studied: (1) H2O decomposition on the Si(111)-7×7 surface; (2) Pyrrole decomposition on the Si(111)-7×7 surface; (3) NH3 decomposition on the Si(111)-7×7 surface.Firstly, the mechanisms for the complete decomposition of water molecules on the Si(111)-7×7 surface were investigated theoretically. The reaction pathways for dissociation of four water molecules over the adatom and rest atom sites were calculated using the density functional theory (DFT) in conjunction with the B3LYP functional. The calculated results demonstrated that the initial O-H bond dissociation from the first H2O to form the adsorbed OH species is more preferential on the adatom site (Sia) than the rest atom site (Sir) of Si(111)-7×7. Four water molecules dissociate successively over the adatom site, backbonds of adatoms which are saturated by OH species can reasonably be the place of insertion of oxygen atoms, yielding a tetrahedral SiO4 structure with one on top and three inserted oxygen atoms.Sencondly, the chemisorption and decomposition of pyrrole (C4H4NH) on the reconstructed Si(111)7×7 surface have been investigated by means of hybrid density functional B3LYP method in combination with the cluster model approach. Three chemisorption mechanisms, N-H dissociative adsorption, CC[4+2] cycloaddition, CN[2+2] ycloaddition adsorption of pyrrole onto a rest atom-adatom pair site, have been considered comparatively. It is shown that the dissociaton of C4H4NH occurs readily on either the adatom site (Sia) or the rest atom site (Sir), giving rise to C4H4N and H adspecies, and forming NadsPa and NadsPr products respectively. The calculations showed that the Cαadsorption process is barrierless and favorable over the N adsorption one. Through compare and analysis the three chemisorption mechanisms, N-H dissociative adsorption on a rest atom-adatom pair of the Si(111)7×7 surface is the most favorable, whereas C-H and N-H dissociative adsorption on a rest atom-adatom pair is the most difficult.Finally, the mechanism for the complete dissociation of NH3 on the Si(111)-7×7 surface was investigated theoretically. The calculations demonstrated that the initial N-H bond dissociation from NH3 to form the adsorbed NH2 species is more favorable on the rest atom site (Sir) of Si(111)-7×7. Subsequently NH2 insertion into Sia(r)-Si backbond bond can be facilitated by the elevated temperature, followed by an H-transfer process. It was founded that the NH2 insertion into the Sia-Si backbond on the Si(111)-7×7 surface is easier than that into the Sir-Si backbond. Similar to NH2, the NH insertion and an H-transfer process can also be facilitated by the elevated temperature. the NH insertion is more feasible on the adatom site (Sia) of Si(111)-7×7. H2 liberations can be achieved at even high temperature to form a surface Si=N unit or a subsurface Si3N unit.The mechanisms of chemical modification of Si surface is explained through the study of the surface Si(111)-7×7 using quantum chemical methods, which provided the theory direction for experiment.

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