DOI: 10.1002/ijch.201100129 Gas–Surface Interactions on Quasicrystals Stefano Curtarolo, [a] Wahyu Setyawan, [a] and Renee D. Diehl* [b] 1. Introduction Gas–surface interactions have been a topic of much study for the past 50 years, [1–4] both to increase our knowledge of fundamental interactions in matter and to enhance the many useful processes that occur at surfaces. [5] In studies of the fundamental behavior of rare gases at quasicrystal surfaces, it was demonstrated that under certain condi- tions the gas films adopt the quasicrystalline structure, and under other conditions, they adopt their natural peri- odic close-packed structure. [6–13] In light of the interesting frictional properties of quasicrystal surfaces, [14–18] this raised the question of whether the use of lubricants with quasicrystal surfaces might actually raise their coefficient of friction rather than reduce it. In this review, we describe first the studies of rare gases that resulted in a phenomenological rule that describes whether film growth on a quasicrystalline surface results in a periodic or an aperiodic structure. We then describe the studies of hydrocarbon adsorption on the same sur- face, which is found to follow the same rule, implying that large hydrocarbons, such as those typically used in lubricants, may not improve the lubricity of quasicrystal surfaces. 2. Growth of Rare Gas Films on Quasicrystalline Al-Co-Ni A study using the combination of low-energy electron dif- fraction (LEED) and grand canonical Monte Carlo (GCMC) was performed for Xe adsorption on the nomi- nally 10-fold surface of the decagonal Al-Co-Ni quasicrys- tal. This quasicrystalline phase is the Co-rich modification and consists of layers having aperiodic fivefold symmetric features stacked in a periodic arrangement that leads to overall 10-fold symmetry of the bulk quasicrystal. [19–23] The surface structure of this quasicrystal was determined by LEED to be consistent with a slightly-relaxed trunca- tion of the bulk crystal. [24–26] Using LEED, both the structure and the thermodynam- ics of gas adsorption can be measured at the same time. This is accomplished by performing equilibrium adsorp- tion isobars, where the pressure of gas above the crystal is held constant while the temperature is raised or low- ered. During this isobaric process, the gas adsorbs onto or desorbs from the surface, and the diffraction pattern can be acquired at many pressure–temperature (P-T) points during the process, providing both structural (from the diffraction) and thermodynamic (from P-T) information about the adsorption process. Using GCMC, a similar procedure can be followed, also resulting in both structural and thermodynamic infor- mation. Starting with a calculated adsorption potential, si- mulated density profiles are obtained for a sequence of (P-T) points, thereby producing both structural and ther- modynamic information. By comparing the information obtained in both experiments and simulations, it is possi- ble to assess the validity of the model and potentials, and then to extend the calculations to systems that are not easily accessed in experiments. Table 1 shows a comparison of the LEED [10] and GCMC [6, 12] results for Xe adsorption on Al-Co-Ni. Good correspondence was found between LEED and GCMC results for (1) the growth mode (layer-by-layer), (2) the observation of quasicrystalline order at low coverage and Abstract : To commemorate the awarding of the Nobel Prize for Chemistry to Daniel Shechtman for his discovery of qua- sicrystals, this paper reviews our recent studies of the inter- action of rare gases and hydrocarbon gases with the tenfold surface of quasicrystalline decagonal Al-Co-Ni. Keywords: ab initio calculations · electron diffraction · materials science · quasicrystals · surface analysis [a] S. Curtarolo, W. Setyawan Department of Mech. Eng. and Materials Science and Department of Physics Duke University Durham, NC 27708, USA [b] R. D. Diehl Department of Physics Penn State University University Park, PA 16802, USA phone: + 814 865-9251 fax: + 814 865-3604 e-mail: rdiehl@psu.edu 1304  2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Isr. J. Chem. 2011, 51, 1304 – 1313 Review R. D. Diehl et al.