Structure and transport properties of atomic chains and molecules

Publication: ResearchPh.D. thesis – Annual report year: 2008

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The work presented in this thesis is based on density functional theory (DFT)applied mainly to calculate conductance properties of nano-scale systems. A full charecterization of Ag-oxygen chains between Ag contacts has been performed. Using spin DFT the electronic and magnetic properties of atomically thin, suspended chains containing silver and oxygen atoms in an alternating sequence has been studied. The conductances of the chains exhibit weak even-odd oscillations around an anomalously low value of 0.1G0 (G0 = 2e2/h) in agreement with experiments [1] in the long chain limit. These unusual conductance properties are explained in terms of a resonating-chain model, which takes the reflection probability and phase-shift of a single bulk-chain interface as the only input. The stability of silver-oxygen chains was studied with a thermodynamic model. This model has been developed in this work to describe tip-suspended atomically thin chains between macroscopic size electrodes. It has been tested with the use of DFT calculations on metal chains for which good agreement with experiments was obtained. To ensure the correctness of the DFT based transport calculations presented here, and in more general in the literature, a set of benchmark calculations for the Kohn-Sham elastic transmission function of representative single-molecule junctions has been performed. The transmission functions are calculated using two different density functional theory methods, namely an ultrasoft pseudopotential plane-wave code Dacapo [2] in combination with maximally localized Wannier functions and the norm-conserving pseudopotential code Siesta [3]which applies an atomic orbital basis set. For the systems studied we find that the Siesta transmission functions converge toward the plane-wave result as the Siesta basis is enlarged. Overall, we find that a double zeta polarized atomic basis is generally sufficient, and in some cases necessary, to ensure quantitative agreement with the plane-wave calculation. In a detailed DFT study of the carbon monoxide molecule between Pt electrodes, a particularly stable tilted bridge configuration is found, with a conductance of 0.5G0 over a wide range of electrode displacements. This is in agreement with the observed peak at 0.5G0 in the experimentally obtained conductance histogram for Pt/CO [4]. Also, for homogenous Pt point contacts and short chains good agreement with experiments is obtained. A study of CO in Au, Cu and Ni, reveals that the conductance for CO in the tilted bridge configuration for Ni is 0.5G0, in agreement with experiments [5]. For Au/CO and Cu/CO we find the effect of CO compared to the homogenous metal contacts is much smaller, in qualitative agreement with the experimental findings [5]. The observed conductance properties of Metal/CO are shown to be determined by the local d-band at the Metal apex atoms. For carbon nanotubes it is shown that the conductance may be controlled by site selective adsorption of molecules. A model to explain this behavior is verified by direct visualization of Kohn-Sham eigenchannel states. The possibility of non-carbon based nanotubes is also discussed. Both calculations of the strain energy of infinite PtO2 nanotubes that this material could be a candidate for non-carbon based nanotubes, as was recently suggested [6].
Original languageEnglish
Publication dateSep 2008
StatePublished
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