AVS2009 Session SS-ThP: Surface Science Poster Session

Low temperature atomic layer deposition (ALD) is favored for more uniform thin film. The authors have investigated the surface reaction mechanism of first step of Hf ALD agent tetrakis(dimethylamido)hafnium (TDMAH) reacting on the H-Si(100) surface for low temperature range 60-250ºC by in situ ATR-FTIR. The formation H-Hf species was observed in the chemisorbed layer and a gas phase initiated adsorption mechanism was hypothesized by ab initio calculation. In situ transmission FTIR will be used to monitor the gas phase decomposition dynamics and products of TDMAH. Two most possible reactions: insertion and β-hydride elimination are expected to be observed. Deuterium water prepared background will be used to verify the Hf-H species on H-Si(100). Interfacial bonds of Hf-N and Hf-Si are to be of about 1:1.5 in concentration from thermodynamic calculation. Minor C-Si bond from silylation by product is expected to be observed during desorption. Experimental measurement of decomposition rate will be compared with the ab initio kinetic data.

Molecular beams, temperature programmed desorption, and infrared spectroscopy were used to study the diffusivity in nanoscale supercooled liquid films of alkanes and alcohols. Amorphous films were grown on top of a layer of rare-gas atoms on a Pt(111) substrate at 25 K in UHV. As the films are heated they transform from amorphous to supercooled liquid films around the glass transition temperature. The rare-gas atoms were found to diffuse through the films in the supercooled liquid state. In a series of experiments, the layer thickness and heating rate were varied to extract diffusivities over a range of different temperatures. Numerical simulations of the rare-gas atoms diffusing through the supercooled films were used to quantitatively model the experimental results. Mixing of initially layered isotopically labeled alcohol and alkane films were also performed. The extracted diffusivities for the rare-gas atoms were in good agreement with the self-diffusion coefficients extracted from the experiments with isotopically labeled species. The details of the experiment and the interpretation of the results will be discussed in detail.

The research described here was performed in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory.

This work was supported by the DFG within the GrK 611 and the SFB 668-A5 and by the European Union in the project “SPiDMe”.

[1] W. Auwärter et al., "Molecular nanoscience and engineering on surfaces", Int J. Nanotechnology Vol. 5, 1171 (2008).

[2] S. Kuck et al., "Naked" Iron-5,10,15-triphenylcorrole on Cu(111): Observation of Chirality on a Surface and Manipulation of Multiple Conformational States by STM, J. Am. Chem. Soc., 130, 14072 (2008).

This work was supported by the DFG within the GrK 611 and the SFB 668-A5 and by the European Union in the project “SPiDMe”.

The interaction of water molecules with single crystalline surfaces provides a model for a fundamental understanding of the wetting properties of metals. We investigate one and two dimensional water structures on a clean and on an oxygen precovered Cu(110) surface using scanning tunneling microscopy and non-contact atomic force microscopy. In previous work it has been proposed that at sub-monolayer coverage water forms pentagon based chain-like structure on the clean Cu(110) surface at low temperature. Water pentagons have a strong water-metal interaction as well as a minimal strain in hydrogen bond¹. On an oxygen precovered Cu(110) surface, hexagonal based honeycomb structures are preferred to form along the Cu-O stripes. However, it is not clear whether the hexamers are composed of intact or half-dissociated water molecules². We will discuss how the Cu-O stripes affect the formation of the honeycomb structures and chains of water pentamers.

¹J. Carrasco et.al Nature Mater. 8, 427 (2009)

²J. Ren et.al Phys. Rev. B 77, 054110 (2008)

Hydrophobic coatings have been used widely to apply for various engineering products. For assessing these properties, static wettability such as the contact angle or sliding angle is not always a useful criterion for materials design. Recently, the recognition of the importance of dynamic wettability such as droplet’s sliding velocity or acceleration on a tilted surface has been growing gradually.

Water droplets are known to slide down on the hydrophobic solid surface by a caterpillar-like rolling motion with or without slippage at the solid-liquid boundary; a direct observation method for internal fluidity of sliding water droplets was established recently using particle image velocimetry (PIV). The contribution ratio of rolling and slipping motions to overall sliding acceleration reportedly depends on nanoscale roughness and chemical heterogeneity of the hydrophobic coatings.

On the other hand, evaporation is a fundamental phenomenon for liquids. Various studies have been conducted in relation to this phenomenon for liquid droplets on solid surfaces. Ultra-small, e.g. nanoliter-scale, droplets are evapolated and disappeared in a short time, which corresponds to a situation of large volume change ratio per unit period. It might possess similar dependence against nanoscale surface heterogeneity as well as dynamic wettability.

We prepared fluoroalkylsilane coatings with different nanoscale roughness. This study examined evaporation (including nanoliter-scale droplets) and sliding behaviors of water droplets on these coatings using an automatic microscopic contact angle meter and PIV method.

Evaporation and sliding behaviors of water droplets were investigated on smooth and rough coatings. Evaporation behaviors for these two coatings differed when nanoliter-scale droplets were used, although they were nearly identical for microliter-scale droplets. The droplets on the smooth coating exhibit greater sliding acceleration and a larger slipping velocity ratio than those on the rough coating. Both the evaporation behavior of nanoliter-scale droplets and sliding velocity of microliter-scale droplets were affected by nanoscale surface heterogeneity.

Moreover, considering power balance around the three phase contact area, line tension was measured for microliter-scale and sub-nanoliter-scale droplets of an ionic liquid on a highly smooth and homogeneous fluoroalkylsilane coating. Values for microliter-scale droplets were two orders larger than those for sub-nanoliter-scale droplets despite their identical combinations of solid surfaces and liquid. Scale factors related to this measurement and from the liquid play an important role in the discrepancy.

Passivation of clean Al(111) has been attempted against oxidation in O₂ or air at room temperature by various kinds of adlayers prepared on the surface. The goal of this attempt is to block the growth of oxide layer by introducing a covering monolayer or thin multilayer, whose thickness is smaller than 1 nm. Metallic aluminum surfaces stored in air are usually covered with more than a few nm of Al oxide layer, which prevent us from fabrication of nanostructures and nanoparticles of pure metallic Al. If an ultrathin passivation layer, composed of inorganic or organic materials in a controlled manner, can inhibit oxidation of Al substrate, it will help enhancing the accuracy and precision of nano-scale fabrication. This approach was successful in passivating Si wafer surfaces [1]. When a clean Al(111) surface was exposed to pure O₂ gas at room temperature, the surface was covered with an amorphous oxide layer observed by low energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS) [2]. The thickness of oxide layer was calculated from XPS signal intensities, with referring to spectra of a cleaned sapphire (γ-Al₂O₃) surface. The process of oxide accumulation was once saturated below ~10^-1 Torr of O₂ pressure, at which the thickness of oxide layer was <0.5 nm. Formation of such oxide layers was blocked with various kinds of thin adlayers, such as fluoride thin layers, alkanethiol monolayers etc., similarly to the formerly reported C₆₀ layer [3]. At O₂ pressures higher than 10^-1 Torr, another sort of rigorous oxidation process took place. The thickness of oxide layer was approximately proportional to the logarithm of O₂ exposure, reaching a few nanometers. Most of the monolayers from small precursor compounds could not block this process of oxidation. This process seems to involve penetration of O atoms deeply into the Al substrate. Practically, it is desirable to inhibit this rigorous process of oxidation. In this talk, we will present our attempts using monolayers including long linear molecules anchored on Al(111). The oxidation process was examined in the air up to the atmospheric pressure, involving O₂ and H₂O. Some kinds of adlayers on Al(111) were vulnerable in contact with H₂O. It is probably important to involve hydrophobic compounds to build a firm, oxidation-resistant monolayer.

[1] T. Yamada et al., J. Electroanal.Chem. 532 (2002) 247, T. Yamada et al., J. Chem. Phys. 121 (2004) 10660.

[2] H. Brune et al., J. Chem. Phys. 99 (1993) 2128, V. Zhukov et al., Surf. Sci. 441 (1999) 251.

[3] A. V. Hamza et al., Surf. Sci. 318 (1994) 368.

structure and transport. As such, they promise to provide a crucial first step towards understanding the complexities of the atomistic processes involved in hydrogen evolution and PEC corrosion.