The Growth Mechanisms, Electronic And Magnetic Properties Of Conductive Molecular Wires On Semiconducting Surfaces

With increasing demand for computing power, the size of electronic devices is becoming smaller and smaller and approaching the molecular scale. Molecular wires become an important building block in electronic devices because of their fascinating mechanical,electronic and magnetic properties. Therefore, it is crucial that integrating these molecular wires with semiconducting surfaces meanwhile keeping their own merits. In recent decades,progress has been made in bonding mechanism between molecular wires and semiconducting surface as well as the controlling of their electronic properties both theoretically and experimentally. In this dissertation, by employing density functional theory (DFT)calculations, we investigate the electronic and magnetic properties of one-dimensional molecular wires and wires on semiconducting Si/Ge surfaces. Possible ways to fabricate molecular wires on semiconducting substrates with the selective adsorption and self-assembled growth of organic molecules are proposed. Moreover, the electronic properties of these supported molecular wires are tunable and can be even half-metallic by applying suitable external electric field and electron doping. The main conclusions are summarized as follows:1) Fabricating a conductive molecular wire on silicon surface via an in situ surface polymerization reaction. Integrating molecular wires on traditional semiconducting surface limits their application in electronic device. We propose a two-step surface reaction to fabricate a conductive polymer on H-Si(001)-2×1 surface. Firstly, HPyMB molecules selectively adsorb on H-Si(001)-2×1 surface with a line of Si dangling bonds; and then, an in situ surface polymerization reaction occurs for surface pre-adsorbed HPyMB molecules via a dehydration reaction. Our study reveals that this polymerized molecular wire is chemically bonded onto the surface by Si-N and mechanically stable. The formed polymer/Si(001) is metallic, however, the system conductivity is not good. By removing 1.1 electrons, good conductivity can be obtained and is attributed to the polymer chain. Furthermore, its conductivity remains robust even under a strong gate voltage.2) Self-assembly of 1,3,5-triethynylbenzene on Si(100)-2×1 and in situ polymerization via reaction with CO to fabricate a single surface-grafted polymer. We propose a two-step surface reaction for fabricating a conductive molecular wire on hydrogen-terminated H-Si(100)-2×1 surfaces taking advantage of alkyne molecules can self-assemble on Si(100)-2×1 surface. The first step is the self-assembled growth of 1,3,5-triethynylbenzene (TEB) molecules and formation of aligned molecular arrays on H-Si(100)-2×1 surface; and the second step is the in situ polymerization of the adsorbed molecules with CO (carbon monoxide) via formal [2+2+1] cycloaddition reactions to produce a surface-grafted molecular wire. Like some other molecular wire/substrate structures, the newly formed polymer/Si(100)-2×1 structure is semiconducting and can be tuned to be conductive by electron doping; in this structure the Si substrate retains its semiconducting character while the bands crossing Fermi level are dominated by the molecular wires.3) Fabrication of surface-grafted polythiophene on H-Si(100)-2×1 Surface via self-assembling and in situ surface polymerization. Polythiophene has widely potential applications in electroluminescent devices, chemical sensors, and so on, but integrating it with semiconducting substrate is still a challenge. We study the self-assembled growth of a series of thiophene substituted alkenes, [H2C=CH-(CH2)n-thiophene] (n=0-3), on H-Si(100)-2×1 and H-Ge(100)-2×1 surfaces into aligned one-dimensional molecular arrays,which are chemically bonded to the surfaces via the alkane chain. The thiophene rings at the top of the molecular arrays are situated side by side and can undergo an polymerization reaction into polythiophene once radicals are introduced into the thiophene rings, forming polyalky lthiophenes-Si/ Ge( 100)-2×1 surface-grafted polymers. Like other conductive polymers, these surface polymer chains show semiconducting characters and can be conductive either by by application of an external electric field or by p-doping. More importantly, both surface-grafted polymers and semiconducting substrates maintain their electrical properties, and the polythiophene chains are the sole conductive channels in polyalkylthiophenes-Si/Ge(100)-2×1 which helps prevent electric leakage.4) [EuCOTB]? sandwich molecular wires and the influence of H-Ge(001)-2×1 surface on the electronic and magnetic properties of molecular wires. Lanthanide organometallic sandwich molecular wires are widely studied both experimentally and theoretically, however, their properties on semiconducting substrates are rarely reported. By using spin-polarized DFT calculations, We investigate the structure, electronic and magnetic properties of two kinds of boron doped europium cyclooctatetraene sandwich molecular wires(SMWs) and the influence of H-Ge(001)-2×1 surface on electronic and magnetic properties of SMWs systematically. It is found that the B doping in COT ligand can enhance the structure stability and spin stability as well as change the electronic properties of SMWs evidently. [EuCOTB]? and [Eu-COTB-Eu-COT]? are metallic and half-metallic ferromagnets.Furthermore, it is found that semiconducting Ge surface has little influence on the magnetic properties of [EuCOTB]? and [Eu-COTB-Eu-COT]?. Both [EuCOTB]? and[Eu-COTB-Eu-COT]? SMWs anchoring on semiconductor germanium surface maintain ferromagnetic and show conductivity. In particular, [Eu-COTB-Eu-COT]/Ge(001) is quasi-half-metallic, which can be tuned to be full half-metallicity under a mild external electric field.