ICMCTF2006 Session H5-1: The Atomistics of Thin Film Growth: Computational and Experimental Studies

Transition metal oxides in ultrathin nanostructured layers on well-defined metal surfaces may form novel oxide phases that do not occur in nature. These "artificial oxide phases" display new physical and chemical properties, which make them potentially interesting materials for nanotechnology applications. They derive their formation, on the one hand, from the interactions at the interface between the oxide overlayer and the metal substrate and, on the other hand, from kinetic constraints during the growth process. The growth of a variety of novel low-dimensional vanadium, nickel, and manganese oxide structures on Rh and Pd single crystal surfaces has been followed and the surface phase diagrams and the atomic structures of oxide nanolayer phases have been characterised by the interplay of various experimental and theoretical methods. The influence of energetic and strain effects at the interface is important and determines the particular structures, which are observed on different substrates. The oxide structures to be discussed comprise highly oxidised, mixed valent, and more reduced vanadium oxide surface phases on Rh(111) and Pd(111) substrates, a c(4x2) wetting layer of an interfacial nickel oxide on Pd(100), which acts as an interlayer to cubic NiO growth, and various manganese oxide phases on Pd(100). It is shown that, in addition to the thickness confinement in the nanolayers, the lateral confinement as imposed by the regular step array on a vicinal substrate surface can also promote the growth of novel oxide nanostructures.

Supported by the Austrian Science Funds, the EU STREP Programme GSOMEN, and the Austrian Nanoinitiative.

The TiN_x-SiN_x system is attracting interest for the fabrication of superhard thin film materials of nanocomposites¹. In order to fully understand the relationship between film hardness and nanostructure in TiN_x-SiN_x nanocomposites, it is important to consider not only equilibrium phases, but the possible existence of metastable solid solutions² and epitaxially-stabilized SiN_x phasesfootnote3. Our recent in-situ STM and LEED investigations show that SiN_x overlayers form a variety of reconstructions on TiN(001) and TiN(111) surfaces as a function of coverage. To better interpret these results, ab initio DFT calculations and MD-simulations were carried out to probe atomistic interactions leading to the formation of reconstructed crystalline SiN_x surface phases on both TiN(001) and N-terminated TiN(111). Calculational supercells were constructed with lattice constants which matched the experimental values. Atomic rows as well as Si clusters were relaxed on (001) and (111) TiN surfaces resulting in the formation of ordered structural units. Our results suggest that SiN_x rows along <110> directions are stabilized at surface coverages near 33% through interactions with surface N atoms.

¹S. Veprek, M. G. J. Veprek-Heijman, P. Karvankova, J. Prochazka, Thin Solid Films 476 (2005) 1-29.
²A. Flink, T. Larsson, J. Sjölen, L. Karlsson, L. Hultman, "Influence of Si on the Microstructure of Arc Evaporated (Ti,Si)N Thin Films; Evidence for Cubic Solid Solutions and their Thermal Stability", in press in Surf. And Coat. Technol.
³H. Söderberg, M. Odén, J. M. Molina-Aldareguia, L. Hultman, J. Appl. Phys. 97 (2005) 114327.

The production of nano-sized cluster-assembled materials is usually based upon the creation of separated small clusters which are then combined into cluster molecules or bulk amounts. In this respect, materials based on silicon clusters, which may differ from normal silicon in several properties, could be applied to create new devices. Mixed silicon clusters containing atoms of transition metal elements, recently synthesized in the laboratory¹, when considered as elementary building blocks of condensed phases, may exhibit properties, different from those of metal silicides and silicon-transition metal alloys which already have multiple applications in the high-temperature electronics and as engineering materials.

We utilize first-principles simulations to study the dependence on size (n) and species (M) of structural and electronic properties of clusters with stoichiometry MSi_n. A total of 126 clusters (containing 1 to 14 Si atoms together with one transition metal atom among 9 different elements: Ti, Zr, Hf, V, Nb, Ta, Ni, Pd, Pt) were investigated. It was found that most of these metals exhibit a size-dependence for the cohesive energy, in which clusters with n= 7,10,12 appear as local maxima, independently of the metal involved, a behavior similar to that of previously studied by us MSi_n² (corresponding to 12 metals other than above). In most cases MSi₁₂ are highly symmetric, whereas the MSi₁₀ are the smallest endohedral species. Electronic properties of structurally equivalent clusters depend sensitively on the transition metal involved, providing the means to tailor specific properties when designing cluster-assembled materials. Simulations of extended phases based on MSi_n will be reported.

¹H. Hiura et al., Phys. Rev. Lett., 86 (2001) 1733.

² G. K. Gueorguiev et al., J. Chem. Phys., 119 (2003) 10313.