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Comptes Rendus Physique
Volume 17, n° 3-4
pages 471-480 (mars 2016)
Doi : 10.1016/j.crhy.2015.12.007
Insulating oxide surfaces and nanostructures
Surfaces d'oxyde isolantes et nanostructures
 

Fig. 1




Fig. 1 : 

(a): Shifts of the cation and oxygen effective atomic orbital energies ϵ C and ϵ O between a bulk and a low-coordinated environment, due to the reduction of the Madelung potential. (b): Electron transfer Δ per Ti-O bond as a function of the difference δV Mad of the Madelung potentials acting on the Ti and O atoms (in Hartree per electron), and as a function of the Ti-O bond length d (Ti-O) (in atomic units). Results for neutral stoichiometric clusters, charged clusters, bulk rutile TiO2 and TiO2 (110) surface are represented by filled circles, stars, plus signs and diamonds, respectively (from Reference [24]).


Fig. 2




Fig. 2 : 

Left panels: Primitive cells of six ZnO polymorphs. Small red balls are O atoms, big gray ones are Zn. Right panel: Variations of the gap widths δG versus the variation of electrostatic potential difference δV ), each with respect to its value in the wurtzite structure. The negative slope points towards band width effects rather than electrostatic modifications of ϵ C ϵ O . Black, red and blue symbols refer to different levels of approximation in the simulation (from Reference [28]).


Fig. 3




Fig. 3 : 

From left to right: structure of an MgO(100) island deposited on a metal substrate, with gray, red and green dots representing substrate, oxygen and magnesium atoms, respectively. Color representation of the mean Mg-O distance <d Mg-O > around each atom (larger to smaller from red to green). Local electronic property profiles across the island diagonal: Top panel: absolute values of Bader charges Q ; Middle panel: electrostatic potential V on oxygen atoms; Lower panel: ϵ C ϵ O values on neighboring atoms.


Fig. 4




Fig. 4 : 

Classification of compound surfaces (profile views) according to Tasker [30]. At type-1 surfaces, the layers are neutral and the repeat unit bears no dipole moment. In more complex crystallographic structures, the layers are non-neutral, but when the repeat unit bears no dipole moment, as schematized in type-2 surfaces, the surfaces are stable. At type-3 surfaces, both the layer charge and the dipole moment borne by the repeat unit are non-zero. These “polar” surfaces are unstable.


Fig. 5




Fig. 5 : 

Schematic representation of bulk (dashed lines) and surface (plain lines) DOS in insulating oxides. The surface VB and CB bands are more narrow than bulk bands and closer to each other, thus diminishing the gap width G s with respect to the bulk value G b . The gray shading thus denotes surface states.


Fig. 6




Fig. 6 : 

Complex phase diagram of MnOx /Pd(100) films as a function of the oxygen chemical potential μ O or partial pressure p (O2 ) (from reference [43]).


Fig. 7




Fig. 7 : 

Schematic representation of the charge transfer (CT) and rumpling (R) dipole moments (shown by arrows), for the two cases of negative (a) and positive (b) metal charging. In the first case, oxygen atoms of the oxide film are repelled by the negative charge of the metal and pushed outwards. In the second case, they are attracted by the substrate. Cations, oxygens and metal atoms are represented by blue, red and brown circles, respectively.


Fig. 8




Fig. 8 : 

(a): Capacitor model of a polar surface with alternating layers of charge density ±σ and spatial variation of the electrostatic potential V . (b): same but compensating charges Δσ =σR 1 /(R 1 +R 2 ) are added on the outer layers, suppressing the monotonic variation of the electrostatic potential.


Fig. 9




Fig. 9 : 

Left panels: STM image of the Zn-terminated ZnO(0001) surface and atomistic model of magic triangles (from reference [50]). Right panels: STM images [52] and model [51] of the reconstructed SrTiO3 (110) (3×1) surface. Surface TiO4 tetrahedra are shown in blue, bulk TiO6 octahedra in yellow, oxygen anions in red and strontium cations in orange.


Fig. 10




Fig. 10 : 

Left panels: transformation of a ZnO(0001) bilayer from wurtzite to h-BN structure and top view of a h-BN(0001) layer [55]. Right panels: phase transition of a MoS2 polar zig-zag nano-ribbon as a function of width and top view of 1H and 1T ribbon structures [58]. Mo and S atoms are represented by large gray and small yellow balls, respectively.

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