21 Atomic level modeling of

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1 110 CSC Report on Scientific Computing Atomic level modeling of surface processes Tapio T. Rantala Department of Physical Sciences, University of Oulu 21.1 Theoretical surface science All of the interactions between the pieces of material take place at their surfaces. Therefore, surface properties of materials are actually behind many of the essential features of apparatus and devices from everyday tools to electronic and optical components. Theoretical surface science aims at explaining the observed surface phenomena by modeling the relevant structures and phenomena at the atomic level. Furthermore, with a detailed enough model and understanding, it is possible to even predict new phenomena and engineer materials with properties at will Atomic structure and molecular dynamics at surfaces Materials and their surfaces are composed of atoms, which are chemically bound to molecules and crystals. Thus, tracing the origin of surface properties is ultimately reduced to the description, how electrons form the chemical bonds. The set of quantum mechanical states of electrons is called the electronic structure of the material. At the ab initio level it is a solution of the Schrödinger equation, a complex entity that can be found only by numerical calculations [1]. The energetics of the electronic structure related to the atomic positions yields the sc. force field, the forces between the atoms. An approximation to the force field is needed to compute the motions of the atoms, that is the idea behind molecular dynamics.

2 21. Atomic level modeling of surface processes 111 K in K out O2 K 2 K 6 K 1 Ð O2 K 4 O Ð K 3 K 5 O depletion layer SnO 2 e Ð e Ð Figure 21.1: Oxygen reactions at the SnO 2 surface form the basis of sensor function Tin dioxide surface chemistry Tin dioxide is used in gas sensing applications due to its environment dependent surface chemistry. Surrounding gas atmosphere components are detected through the chemical reactions catalyzed by the metal oxide surface and measured through the response in the electrical properties of the grainy material. To explain these mechanisms we have carried out ab initio level studies of the SnO 2 surface. This semiconducting metal oxide is composed of Sn +n cations and O m anions, which at the surface are responsible for the chemical properties. As the surface is formed the atoms relax to new lowenergy positions, different from those of extrapolated bulk lattice, resulting in charge balancing and changes in the surface potential. Such detailed surface structure, atom by atom, is the basis of understanding and predicting the surface chemistry [2]. The basis of the sensor function is the surface oxygen chemistry. Oxygen balance controls the thickness of charge carrier depletion layer, and consequently, the measurable conductivity of the material [3]. Other gases in the surrounding atmosphere react with these oxygen species, distort the oxygen balance, effect on the conductivity and become detected. To understand this mechanism in details, we have simulated the oxygen exchange at the SnO 2

3 112 CSC Report on Scientific Computing surface using the kinetic Monte Carlo method. This is based on the model shown in Fig. 21.1, where physisorbed, ionosorbed and chemisorbed oxygen are in a temperature dependent balance with the lattice oxygen Xe atoms adsorbed in molecular sieves Microporous solids contain channels and cavities, that are capable of adsorbing atoms and molecules onto their surfaces. Thus, macroscopic absorption turns to adsorption, if studied at the molecular level. Such materials, as zeolites, are used in a variety of applications, molecular sieves, absorbents, catalysts and ion exchangers, which motivates the effort to search for molecular level understanding of the processes (Fig on page 199). We have carried out molecular dynamics simulations of Xe atoms in zeolite NaA [4] and aluminophosphate AlPO 4 [5]. This work is related to the NMR spectroscopy at the Department of Physical Sciences, where 129 Xe is used due to its experimental convenience. Xe atoms can be used as a probe in investigation of the pore and channel network, as the observed NMR parameters of Xe are sensitive to the environment and dynamics of the atoms. One of the important results we found is that the proper treatment of the dynamics of surrounding cells and channels is crucial. Though this sounds obvious, due to the lack of computing power this has been omitted in almost all of the previously published studies of this type Structures of biosurfaces Biological membranes form the surfaces of many organic structures. There they are responsible for several important functions, starting from holding the things in one piece and contents inside. With molecular dynamics we have simulated the structure and dynamics of a phospholipid (PLPC; 16:0/18:2 9,12 ) bilayer and found the significance of polyunsaturation of the constituent molecules [6]. This relates to the studies of small cholesterol transferring particles in human blood plasma [7] Tools of computational research In the research work described above the two most important expedients, counting out our expertise in the field, are fast computers and commercial software. In addition to those, we of course write computer programs ourselves, too. In ab initio work the problems always scale up with increasing computing power, in trying to be the first to solve certain problems with ever higher accuracy. Therefore, all computational resources that become available will al-

4 21. Atomic level modeling of surface processes 113 ways be used to solve ever larger and more complex problems. Development of scientific software usually starts in research laboratories. In a matured field some of these become commercialized, when the research moves from development and code writing to applications. Nevertheless, high capacity computers will be inevitable for the development of computational research in various fields of science. CSC has provided us with these basic resources, without which we could not have kept us at the international level. In future, too, to keep oneself at the frontier level in research we will need computing power and certain commercialized software. We also hope to be able to enjoy all of the other services, like support to solve more practical problems, databases and training of our students at various courses. Bibliography [1] Tapio T. Rantala, Ab initio studies of compound semiconductor surfaces, Computational Studies of New Materials, Eds. D.A. Jelski and T.F. George (World Scientific, Singapore, 1999), in print. [2] Tapio T. Rantala, Tuomo S. Rantala and Vilho Lantto, Surface relaxation of the (110) face of rutile SnO 2, Surface Science [3] Tuomo S. Rantala, Tapio T. Rantala and Vilho Lantto, Computational Studies for the Interpretation of Gas Response of SnO 2 (110) Surface, Sensors and Actuators 1999, in print. [4] J.-H. Kantola, J. Vaara, T.T. Rantala and J. Jokisaari, Molecular dynamics simulations for Xe absorbed in zeolites, Materials Theory, Simulations and Parallel Algorithms, Edited by E. Kaxiras, J. Joannopoulos, P. Vashishta and R.K. Kalia (Materials Research Society Symposium Proceedings, Volume 408, 1996), pp [5] Juha-Heikki Kantola, Juha Vaara, Tapio T. Rantala and Jukka Jokisaari, 129 Xe adsorbed in AlPO 4-11 molecular sieve: Molecular dynamics simulation of adsorbate dynamics and NMR chemical shift, Journal of Chemical Physics [6] Marja T. Hyvönen, Tapio T. Rantala and Mika Ala-Korpela, Structure and Dynamic Properties of Diunsaturated PLPC Lipid Bilayer from Molecular Dynamics Simulation, Biophysical Journal [7] Marja T. Hyvönen, Tapio T. Rantala and Mika Ala-Korpela, Biokalvojen rakenteen ja toiminnan simulointia tietokoneella, (Simulating the structure and function of biomembranes with a computer), Kemia Kemi 1999, in print.

5 38. Chemistry Chemistry Figure 38.1: Xe atom in an α cage of zeolite NaA (4A). One α cage can accommodate up to eight Xe atoms. See Chapter 21 on page 110.