Rainer Hoft - Academia.edu (original) (raw)

Papers by Rainer Hoft

Research paper thumbnail of Electron tunneling through alkanedithiol molecules

Biomems and Nanotechnology Ii, 2006

ABSTRACT We report on first principles calculations of the tunneling current across n-alkanedithi... more ABSTRACT We report on first principles calculations of the tunneling current across n-alkanedithiol molecules (n = 4,6,8,10,12) sandwiched between two Au {111} electrodes. The conductance drops exponentially with increased chain length with decay parameter betan = 0.9. The results are compared with scanning tunneling microscopy measurements on decanedithiol and with other n-alkanedithiol (n = 6,8,10) results in the literature. The theoretical results are found to be an order of magnitude larger than experimental values but follow the same trend. However, two additional, more realistic, geometries are modeled by changing the bond type and by combining the first-principles results with a Wentzel-Kramer-Brillouin (WKB) expression for tunneling across the air gap that is invariably present during scanning tunneling microscopy (STM) measurements. These results are more compatible with the experimental data.

Research paper thumbnail of Adsorption of Amine Compounds on the Au(111) Surface: A Density Functional Study

The Journal of Physical Chemistry C, 2007

ABSTRACT A Density Functional Theory study of the adsorption energetics of various amine compound... more ABSTRACT A Density Functional Theory study of the adsorption energetics of various amine compounds on the gold(111) surface revealed that preferential binding occurs in under-coordinated sites. The largest binding energy is obtained when a gold adatom is placed in the fcc position and the amine positioned with the nitrogen above the adatom. The results are compared with previous calculations for thiols, phosphines, and ethynylbenzene molecules to provide a meaningful comparison within a consistent computational framework. The systematic increase in binding energy with methyl group substitution previously observed for phosphine compounds is not observed for the amine analogues. The binding energy of the amines is considerably lower than that for thiols and binding is indicated for only the adatom geometrya result consistent with experimental data.

Research paper thumbnail of Exploring the Performance of Molecular Rectifiers:  Limitations and Factors Affecting Molecular Rectification

Nano Letters, 2007

There has been significant work investigating the use of molecules as nanoscale rectifiers in so-... more There has been significant work investigating the use of molecules as nanoscale rectifiers in so-called "molecular electronics". However, less attention has been paid to optimizing the design parameters of molecular rectifiers or to their inherent limitations. Here we use a barrier tunneling model to examine the degree of rectification that can be achieved and to provide insight for the design and development of molecules with optimum rectification responses.

Research paper thumbnail of Adsorption of Benzene on Copper, Silver, and Gold Surfaces

Journal of Chemical Theory and Computation, 2006

The adsorption of benzene on the Cu(111), Ag(111), Au(111), and Cu at low coverage is modeled usi... more The adsorption of benzene on the Cu(111), Ag(111), Au(111), and Cu at low coverage is modeled using density-functional theory (DFT) using periodic-slab models of the surfaces as well as using both DFT and complete-active-space self-consistent field theory with second-order Møller-Plesset perturbation corrections (CASPT2) for the interaction of benzene with a Cu 13 cluster model for the Cu(110) surface. For the binding to the (111) surfaces, key qualitative features of the results such as weak physisorption, the relative orientation of the adsorbate on the surface, and surface potential changes are in good agreement with experimental findings. Also, the binding to Cu(110) is predicted to be much stronger than that to Cu(111) and much weaker than that seen in previous calculations for Ni(110), as observed. However, a range of physisorptive-like and chemisorptive-like structures are found for benzene on Cu(110) that are roughly consistent with observed spectroscopic data, with these structures differing dramatically in geometry but trivially in energy. For all systems, the bonding is found to be purely dispersive in nature with minimal covalent character. As dispersive energies are reproduced very poorly by DFT, the calculated binding energies are found to dramatically underestimate the observed ones, while CASPT2 calculations indicate that there is no binding at the Hartree-Fock level and demonstrate that the expected intermolecular correlation (dispersive) energy is of the correct order to explain the experimental binding-energy data. DFT calculations performed for benzene on Cu(110) and for benzene on the model cluster indicate that this cluster is actually too reactive and provides a poor chemical model for the system.

Research paper thumbnail of Electron tunneling through alkanedithiol molecules

Biomems and Nanotechnology Ii, 2006

ABSTRACT We report on first principles calculations of the tunneling current across n-alkanedithi... more ABSTRACT We report on first principles calculations of the tunneling current across n-alkanedithiol molecules (n = 4,6,8,10,12) sandwiched between two Au {111} electrodes. The conductance drops exponentially with increased chain length with decay parameter betan = 0.9. The results are compared with scanning tunneling microscopy measurements on decanedithiol and with other n-alkanedithiol (n = 6,8,10) results in the literature. The theoretical results are found to be an order of magnitude larger than experimental values but follow the same trend. However, two additional, more realistic, geometries are modeled by changing the bond type and by combining the first-principles results with a Wentzel-Kramer-Brillouin (WKB) expression for tunneling across the air gap that is invariably present during scanning tunneling microscopy (STM) measurements. These results are more compatible with the experimental data.

Research paper thumbnail of Adsorption of Amine Compounds on the Au(111) Surface: A Density Functional Study

The Journal of Physical Chemistry C, 2007

ABSTRACT A Density Functional Theory study of the adsorption energetics of various amine compound... more ABSTRACT A Density Functional Theory study of the adsorption energetics of various amine compounds on the gold(111) surface revealed that preferential binding occurs in under-coordinated sites. The largest binding energy is obtained when a gold adatom is placed in the fcc position and the amine positioned with the nitrogen above the adatom. The results are compared with previous calculations for thiols, phosphines, and ethynylbenzene molecules to provide a meaningful comparison within a consistent computational framework. The systematic increase in binding energy with methyl group substitution previously observed for phosphine compounds is not observed for the amine analogues. The binding energy of the amines is considerably lower than that for thiols and binding is indicated for only the adatom geometrya result consistent with experimental data.

Research paper thumbnail of Exploring the Performance of Molecular Rectifiers:  Limitations and Factors Affecting Molecular Rectification

Nano Letters, 2007

There has been significant work investigating the use of molecules as nanoscale rectifiers in so-... more There has been significant work investigating the use of molecules as nanoscale rectifiers in so-called "molecular electronics". However, less attention has been paid to optimizing the design parameters of molecular rectifiers or to their inherent limitations. Here we use a barrier tunneling model to examine the degree of rectification that can be achieved and to provide insight for the design and development of molecules with optimum rectification responses.

Research paper thumbnail of Adsorption of Benzene on Copper, Silver, and Gold Surfaces

Journal of Chemical Theory and Computation, 2006

The adsorption of benzene on the Cu(111), Ag(111), Au(111), and Cu at low coverage is modeled usi... more The adsorption of benzene on the Cu(111), Ag(111), Au(111), and Cu at low coverage is modeled using density-functional theory (DFT) using periodic-slab models of the surfaces as well as using both DFT and complete-active-space self-consistent field theory with second-order Møller-Plesset perturbation corrections (CASPT2) for the interaction of benzene with a Cu 13 cluster model for the Cu(110) surface. For the binding to the (111) surfaces, key qualitative features of the results such as weak physisorption, the relative orientation of the adsorbate on the surface, and surface potential changes are in good agreement with experimental findings. Also, the binding to Cu(110) is predicted to be much stronger than that to Cu(111) and much weaker than that seen in previous calculations for Ni(110), as observed. However, a range of physisorptive-like and chemisorptive-like structures are found for benzene on Cu(110) that are roughly consistent with observed spectroscopic data, with these structures differing dramatically in geometry but trivially in energy. For all systems, the bonding is found to be purely dispersive in nature with minimal covalent character. As dispersive energies are reproduced very poorly by DFT, the calculated binding energies are found to dramatically underestimate the observed ones, while CASPT2 calculations indicate that there is no binding at the Hartree-Fock level and demonstrate that the expected intermolecular correlation (dispersive) energy is of the correct order to explain the experimental binding-energy data. DFT calculations performed for benzene on Cu(110) and for benzene on the model cluster indicate that this cluster is actually too reactive and provides a poor chemical model for the system.