Atomic resolution images of the polar (0001) surface of lithium niobate (LiNbO 3 ) are achieved by frequency modulation atomic force microscopy operated at the solid-water interface. The measured data reveal a hexagonal surface unit cell. Its lattice constant corresponds to the bulk-truncated structure, suggesting that the hightemperature annealed surface does not reconstruct. Compared to the (0001) surface, high-resolution imaging on the oppositely charged (0001) surface is considerably more difficult to achieve. This finding is rationalized by density functional calculations that indicate a higher corrugation and softer bonds on the (0001) surface compared to the (0001) surface.

The effect of ferroelectric poling on the adsorption characteristics of water at lithium niobate surfaces is investigated by ab initio calculations. Thereby we model the adsorption of H 2 O monomers, small water clusters, and water thin films on the LiNbO 3 (0001) surface. The adsorption configuration and energy are determined as a function of the surface coverage on both the positive and negative (0001) surfaces. Confirming the results of temperature programed desorption measurements [Garra, Vohs, and Bonnell, Surf. Sci. 603, 1106 (2009)], polarization-dependent adsorption energies, geometries, and equilibrium coverage are found. Our calculations predict the adsorption to occur mainly nondissociatively, independently of the coverage. The water structures formed at the surface are coverage-dependent, though. The different affinity of water to the two surfaces is explained in terms of electrostatic interactions between the substrate and polar molecules. Water adsorption accentuates the surface relaxation, thus increasing the microscopic surface roughness.

The microscopic structure of the polar (0001) and (0001) surfaces of lithium niobate is investigated by atomic-resolution frequency modulation atomic force microscopy and first-principles calculations. It is found that the surface reconstructs at annealing temperatures sufficiently high to drive off external adsorbates. In particular a ( √ 7 × √ 7)R19.1• reconstruction is found for the (0001) 1 While these applications exploit the peculiar ferroelectric, piezoelectric, photorefractive, and electro-optical properties of the bulk, the strong electric fields and charges at the surfaces of ferroelectric materials have recently attracted the attention of scientists and engineers. For example, it is possible to grow group III nitrides with spatially varied, absolute polarity control on LN substrates.2 The dipole orientation can be switched at the nanoscale, bearing a great potential for domain-specific surface chemistry as a route towards the fabrication of nanoscale devices.3 LN surface charges were found to enable artificial photosynthesis 4 and to drive photocatalytic dye decolorization. 5 The integration of ferroelectric thin films within liquid environments is investigated in the context of laboratory-on-chip device designs, e.g., for localizing, sensing or activating charged biomolecules.6 High surface electric fields due to pyroelectricity were found to efficiently pole electro-optic polymers 7 and to lead to the reversible fragmentation and self-assembling of nematic liquid crystals. 8 Despite many exciting applications, our actual knowledge of the LN surface atomic structure and the associated surface electric field and surface charge is remarkably limited. LiNbO 3 can be thought of as a stapling of Nb-O 3 -Li trilayers along the [0001] direction (Fig. 1). This order gives rise to a spontaneous electric dipole moment and strongly polar (0001) and (0001) surfaces, commonly referred to as negative and positive Z cut, respectively. Using electron diffraction, Yun et al. 9 found no indication for reconstructions on LN(0001) surfaces at ambient conditions. From ion scattering spectroscopy it was concluded that both Z-cut surfaces are oxygen terminated. 9 On the other hand, coaxial impact-collision ion scattering suggested a Nb surface termination.10,11 The experimental investigations are hindered by the extremely challenging preparation and analysis conditions of strongly polar surfaces of insulating materials: Charging effects preclude the application of electron tunneling or diffraction techniques and unscreened surface charges hinder atomic force microscopy. Nonetheless, reconstructions in surfaces commonly referred to as weakly polar surfaces such as the SrTiO 3 (001) and BaTiO 3 (001), have been recently demonstrated by STM 12,13 and transmission electron microscopy 14 techniques. Moreover, the structural and physical properties of polar surfaces are strongly temperature dependent. 15 Only recently, atomic resolution images of the LN(0001) surface have been obtained by frequency modulation atomic force micro...

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The adsorption of single hydrogen atoms and hydroxyl radicals (OH) at the polar (0001) surface is simulated by means of first‐principles total energy calculations, in order to investigate the influence of ferroelectric poling on the surface physics and chemistry of LiNbO3. H and OH are found to adsorb at a similar site both at the positive and at the negative (0001) surface. Despite this, the adsorption energy is for both adsorbates strongly polarization dependent, with adsorption energy differences as high as 2 eV. This striking contrast is traced back to electrostatic effects, which lead to a different bonding scenario at the two LN(0001) sides. The polar radical OH lies relatively flat on the positive surface, with the adsorbate oxygen forming a covalent bond with the surface oxygen. At the negative face, OH adsorbs roughly perpendicular and the adsorbate oxygen forms a bond of covalent nature with the surface cations. The adsorbate dipole moment is directed against the spontaneous polarization of the substrate. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)