M any chemical and physical processes and properties, such as crystal growth, crystal morphology, and adsorption, are controlled by the atomic structure of the surface and its energy. This is even more important for nanostructured materials, in which the surface energy gives a considerable contribution to the total energy. Knowledge of the surface structures and energies allows predicting and optimizing the synthesis and, eventually, processing of nanocrystals into genuine nanomaterials. In recent years, PbSe nanomaterials have been intensively studied because of their attractive physical properties and promises for near-infrared opto-electronic applications. 1Ϫ9 Through fusion of smaller nanocrystal units, larger nanostructures can be fabricated with a wide variety of single crystal morphologies, such as nanorods, spheres, cubes, stars, and rings. 1,10Ϫ15 The experiments suggest that the efficient oriented attachmentOthe mutual alignment of nanocrystals during or prior to fusionOcan be attributed to dipole interactions caused by the electrically charged, Pb-or Se-terminated {111} facets. 10,12Ϫ15 However, this is currently just a hypothesis. Although it is experimentally proven that the nanocrystals carry dipole moments, 16,17 the experiments are not sufficiently sophisticated to prove the presence of nonreconstructed, fully polar {111} facets, and theoretical investigations are absent.There are many experimental and theoretical studies on the properties of bulk PbSe, 18Ϫ22 while the literature on the surface structure and energy is scarce. Kimmura et al. 23 grew a PbSe {111} surface on the {111} surface of BaF 2 . From the results of the high-resolution Rutherford backscattering spectroscopy, they found that the PbSe {111} surface is dominated by Pb atoms.Moreels and co-workers prepared colloidal PbSe nanocrystals of a uniform size. 24 They also investigated the interplay between the Pb-coordinating ligands and the structure of the PbSe surfaces in solution by nuclear magnetic resonance spectroscopy (NMR). It was concluded that the ligands have a strong impact on the nanocrystal surfaces and that capped PbSe nanocrystals are nonstoichiometric with a surface that is composed mainly of oleic acid capped lead atoms. 24 Ma et al. investigated the geometric and electronic properties of PbX (X ϭ S, Se, and Te) {100} surfaces using the density functional method. 25 However, there is no information about the surface energies.With the fast increasing interest in colloidal semiconductor nanocrystals, it is of key importance to investigate the surface structures and related energies of colloidal PbSe

Detailed results of ab initio band structure calculations for tantalum (oxy)nitrides (TaON and Ta 3 N 5) are reported. The calculations are performed within the framework of density functional theory (DFT). We compare results obtained with the molecular dynamics pseudopotential (PP) approach of the Vienna Ab initio Simulation Program (VASP) and the Full Potential Linearized Augmented Plane Waves method (FP-LAPW) using the WIEN97 program. In agreement with neutron diffraction measurements, we show an ordering of the anions in TaON. The calculations also show that the valence band is composed mainly of the anion 2p orbitals hybridized with Ta 5d states. For TaON the top of the valence band is dominated by N 2p states. The bottom of the conduction band is mainly composed of Ta 5d states. Both TaON and Ta 3 N 5 are semiconductors with calculated indirect band gaps of respectively 1.8 and 1.1 eV (VASP calculations) and 2.0 and 1.2 eV (WIEN97 calculations). Optical diffuse-reflectance spectra show an energy gap of 2.08 eV for Ta 3 N 5 .

Angular momentum seems to not be conserved in chemical reactions where one of the reactants is magnetic; consequently, such reactions show a high activation barrier. An example is the production of hydrogen by electrolysis of water: practically all losses occur in the production of (magnetic) oxygen. Anodes with a low overvoltage (a measure of the losses) are based on the ruthenium dioxide (110) surface. First-principles electronic structure calculations show that this surface itself carries magnetic moments. This magnetic surface enables the production of oxygen in the ground state while conserving angular momentum.

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