Nanoparticle-host interactions in natural systems

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Metal nanoparticles in two dimensions

A classical example of two-dimensional nanoparticles or two-dimensional nanoparticle-host interactions is the adsorption of metallic clusters, which are often not more than one atomic layer high, on atomically flat mineral surfaces. An obvious choice of such a mineral surface is molybdenite (MoS2), which has perfect cleavage along the (001) plane. Thus, in order to understand the adsorption mechanism of metal atoms and clusters especially to semiconducting surfaces, the vapour phase adsorption and subsequent surface diffusion of Ag, Au, and Cu on the (001) molybdenite surface were studied as a model system (Becker et al., 2003). The size, thickness, and surface diffusion of the metal clusters can be observed, at the atomic scale, using scanning tunnelling microscopy (STM). The adsorption energy, partial electron transfer as a result of polarization, the stability of the adsorbates as a function of size, and spin ordering are evaluated using a combination of quantum-mechanical and empirical force-field calculations. Finally, the nature of the bond between the metal and sulphide surface is further investigated using ultraviolet electron spectroscopy (UPS) and scanning tunnelling spectroscopy (STS).

Geologic, experimental and computational background

Sulphide minerals are one of the most important mineral groups for the adsorption of heavy and noble metal atoms and ions (Gervilla et al., 1998; Wareham et al., 1998; Watanabe et al., 1998). Within this context, it is important to understand whether the association of certain adsorbates with specific mineral surfaces is due to a favourable adsorption mechanism (e.g., strong binding energy) or is mainly influenced by the co-existence of certain metal-sulphide combinations in certain geochemical conditions.

Even though MoS2 is apparently only of minor importance for noble metal adsorption in nature (Grenne et al., 1999; Lexa et al., 1999; Maloof et al., 2001; Sotnikov et al., 2001), MoS2 (001) is an excellent model surface for several reasons. Pristine, atomically flat surfaces that are relatively inert can be easily prepared which circumvents problems that may arise from the regular influx of contamination or from complicated surface nanotopography. These properties, when combined with its high electrical conductivity, allow for excellent imaging conditions of both the surface and of adsorbates at the atomic scale using STM (Manivannan et al., 1994; Miyake et al., 1994). Finally, the surface structure serves as an excellent model substrate to understand the packing structure, wetting behaviour, effects of strain arising from lattice mismatch, and diffusion of deposited metals on a hexagonal lattice during the initial stages of heteroepitaxial overlayer growth. In addition, it can be studied how a metal with a cubic bulk structure changes its electronic structure when forming an approximately epitaxial interface with a hexagonal surface.

Besides STM, ultraviolet photoelectron spectra (UPS) of the substrate, the bulk metal, and the combination of substrate and adsorbate can be used to help understand changes in the electron density of states during the adsorption process. Computer simulations elucidate which states are involved in the metal-mineral bonding and the charge transfer processes. Experiments performed in ultra-high vacuum (UHV) significantly reduce the effects of unknown contaminants and isolate fundamental metal mineral interactions. Quantum-mechanical calculations can also be applied to explain the UPS observations. In addition, they improve the derivation of specific and more accurate empirical interatomic force-field potentials. The empirical methods reduce the computational burden of calculations on larger systems. For instance for the calculation of Ag islands, the combined unit cell of molybdenite and silver has to be large enough to host the Ag cluster with a significant space around it such that different Ag clusters do not interact with each other. Then the adsorption energetics can be calculated and the structural relationship between substrate and adsorbate.

By using such a model setup, it is possible to develop the tools to study the structure, electronics, and thermodynamics that can be used for other systems involving different noble and heavy metals and other, especially semiconducting, surfaces. This further allows improved analysis of what metal-sulphur associations are due to the surface properties, chemical, or other environmental parameters.
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