Theoretical Study On Radical Chain-reaction Mechanism And Properties Of Organic Molecular Wire On Silicon Surface

Due to the unique properties and potential usage in several practical applicationssuch as nonlinear optical materials, microelectronic devices, sensors and biologicaletc., the hybrid organic molecule/inorganic semiconductor silicon materials haveattracted great interest during the past decades.In this thesis, we have launched systematically theoretical studies on thestructure, properties and formation mechanism of organic molecular wires on thewater-saturated Si(100)-(2×1) and H-Si(100)-(2×1) surface using the densityfunctional theory (DFT) calculations. The thesis includes two major contents: the firstpart focuses on the radial initiated chain reaction mechanism of the unsaturatedorganic molecules such as styrene, propylene, aldehyde and benzaldehyde on watersaturated Si(100)-(2×1), In the second part of the thesis, we investigated the structural,electronic and magnetic properties of one-dimensional organometallic sandwichmolecular wires [TM(styrene)]∞(TM=Sc, Ti, V, Cr and Mn) on H-Si(100)-(2×1)surface.In the first part of thesis, the radical initiated hydrosilylation of organicmolecules including propylene, styrene, benzaldehyde, and aldehyde on watersaturated Si(100)-(2×1) is systematically studied using density functional theory (DFT)calculations. In the study, three kinds of H-atom or OH-group abstraction pathwaysalong different directions are studied. We also predict two possible radicalchain-reaction mechanisms which include the H-abstraction from surface Si-OH orSi-H group and the direct abstraction of a-OH group. It is found the surface-OHgroup acts as a medium for the radical chain-reaction. The chain-reaction can proceedthrough the H-abstraction from surface-OH group, which has lower energy barrierthan the direct abstraction of a-OH group or conventional H-abstraction from thesurface Si-H group. The O-adatom caused by the H-abstraction from the surface-OHgroup enhances the reactivity of newly generated surface dangling bond. However, itshows double-side effects on the following H-abstraction along the dimer rowdirection, it can either facilitate the inter-dimer H-abstraction of the sequent attachedmolecule or significantly increase the H-abstraction barrier depends on its insertionmode into the surface Si-Si skeleton. Based on the computed reaction rates ofH-abstractions in different pathways, we predict the radical chain-reaction ofpropylene and aldehyde on water-saturated Si(100)-(2×1) will lead to disordered structures. The styrene and benzaldehyde will grow quasi-one-dimensional molecularwire within single surface dimer row.In the second part of thesis, the structural, electronic and magnetic properties ofone-dimensional organometallic sandwich molecular wires [TM(styrene)]∞and double[TM(styrene)]∞/[TM(styrene)]∞with TM=Sc, Ti, V, Cr and Mn supported onH-Si(100)-(2×1) surface are systematically investigated by means of spin-polarizeddensity functional theory as implemented in the Vienna Ab initio SimulationPackage(VASP). Through the calculated result of the energy and magnetic moment onferromagnetic (FM) and antiferromagnetic (AFM) state, the ground state of molecularwires is determined. We demonstrate that the [TM(styrene)]∞sandwich molecularwires (SMWs) on H-Si(100) surface are AFM for TM=Ti and V and FM for TM=Mn and Cr, but NM for TM=Sc. Despite of the magnetic properties of[TM(styrene)]∞SMWs are much different from their gas-phase counterparts, we findthat the stability trend of transition metal-styrene sandwich molecular wire in solidphase is consistent with the gas-phase transition metal-benzene sandwich molecularwire. The [Sc(styrene)]∞,[Ti(styrene)]∞, and [V(styrene)]∞SMWs have larger bindingenergies than those of [Cr(styrene)]∞, and [Mn(styrene)]∞SMWs. At the same time,we also compute the spin-polarized projected density of states (PDOS) and bandstructure. The analysis of the spin polarization and magnetic properties of different[TM(styrene)]∞SMWs on H-Si(100)-(2×1) show that [Cr(styrene)]∞and[TM(styrene)]∞wires are spin polarized. In particular, the [Cr(styrene)]∞wire onH-Si(100) surface displays a half-metallic ferromagnetic ground state. A quasihalf-metallic ferromagnetism is predicted for [Mn(styrene)]∞. Furthermore, we studythe charge transfer and magnetic contribution of transition metal-styrene sandwichwires by combining the spin density analysis, charge density difference analysis andBader charge analysis method. We found that the magnetic moment of [TM(styrene)]∞on the hydrogen-terminated silicon surface comes mainly from the3d orbitals of TMatoms and the electron transfer from TM atoms to C atoms in benzene rings is foundas well. We also show that the magnetism of double [TM(styrene)]∞SMWs can besimply added or subtracted without any coupling of two neighboring molecular wireson the same Si-Si dimer row of H-Si(100)-(2×1) surface.