Water adsorption on a sodium smectite clay surface: an ab initio study

Using calculations based on density-functional theory, we explore the configurations that water molecules (2,3,4;6) assume on the surface a sodium 2:1 smectite clay with isomorphic substitutions both on octahedral and tetrahedral sheets. The hydrophilicity of the surface is inhomogeneous, depending on the specific location of the counterions and of the cation replacements in the siloxane rings. The counterion does not complete the first hydration shell. Adsorption may occur in the form of coexisting monomers, dimers, and trimers, but at the level of six molecules, a water ring bound to two sodium ions becomes the most stable configuration. This structural transition observed for the adsorbate can be seen as marking the onset of the formation of water networks on the clay surface.

Adsorption and electronic properties of pentacene on thin dielectric decoupling layers

Alexander Kabakchiev, Klaus Kern, Sebastian Koslowski, Daniel Pablo Rosenblatt, Uta Schlickum

With the increasing use of thin dielectric decoupling layers to study the electronic properties of organic molecules on metal surfaces, comparative studies are needed in order to generalize findings and formulate practical rules. In this paper we study the adsorption and electronic properties of pentacene deposited onto h-BN/Rh(111) and compare them with those of pentacene deposited onto KCl on various metal surfaces. When deposited onto KCl, the HOMO and LUMO energies of the pentacene molecules scale with the work functions of the combined KCl/metal surface. The magnitude of the variation between the respective KCl/metal systems indicates the degree of interaction of the frontier orbitals with the underlying metal. The results confirm that the so-called IDIS model developed by Willenbockel et al. applies not only to molecular layers on bare metal surfaces, but also to individual molecules on thin electronically decoupling layers. Depositing pentacene onto h-BN/Rh(111) results in significantly different adsorption characteristics, due to the topographic corrugation of the surface as well as the lateral electric fields it presents. These properties are reflected in the divergence from the aforementioned trend for the orbital energies of pentacene deposited onto h-BN/Rh(111), as well as in the different adsorption geometry. Thus, the highly desirable capacity of h-BN to trap molecules comes at the price of enhanced metal-molecule interaction, which decreases the HOMO-LUMO gap of the molecules. In spite of the enhanced interaction, the molecular orbitals are evident in scanning tunnelling spectroscopy (STS) and their shapes can be resolved by spectroscopic mapping.

Beilstein Institut2017

First-principles molecular-dynamics study of metal-supported nanosystems

Massimiliano Stengel

This thesis is concerned with the theoretical study of the adsorption of molecules and thin films to single-crystal metal surfaces. First-principles electronic calculations are performed in the framework of density functional theory in the local density approximation (LDA-DFT), by using a plane-wave formalism, norm-conserving pseudopotentials and the supercell method. For the calculation of the electronic ground state and the subsequent structural optimizations we use the Car-Parrinello technique, modified for the optimal treatment of metallic systems. This method has been improved in this work with the introduction of a Lagrangian formalism and a mathematically conserved constant of motion in the presence of variable fractional occupancies. Complex mechanisms involving substantial reconstructions of the substrate are elucidated by the study of the C60/Al(111) system. We find that the interaction of C60 molecules with the Al(111) surface is predominantly covalent, and that the adsorbates bind optimally to the surface if an Al vacancy is created directly underneath. The removed Al atoms form a (6 x 6) array of ad-dimers in the interstices of the C60 overlayer, to which they strongly bind. Large-scale structural relaxations performed directly on a surface unit cell containing three C60 molecules lead to a reconstructed structure with two vacancies and an Al ad-dimer which is significantly more stable than the unreconstructed one. This spontaneous local process, rather than the compression state of the C60 overlayer, explains why one C60 molecule out of three protrudes from the surface upon reconstruction. The delicate interplay of many competing factors occurring at the interface between different media is investigated through the study of ultrathin MgO films deposited on the Ag(100) surface. We consider pseudomorphic MgO films of thickness ranging from one to three atomic layers, and we study the evolution of the surface electronic properties as a function of coverage. We find that already three MgO monolayers are sufficient to fully develop the electronic structure of the single-crystal MgO surface, in agreement with experimental findings. We discuss the reliability of the LDA-DFT results for the band structure by using simple 1D models for the dielectric properties of the system.

EPFL2004

First-principles molecular-dynamics study of metal-supported nanosystems

Massimiliano Stengel

EPFL2004