Valence band energetic positions

Scheme I Interface energetics for an n-type Si photoanode at the flat-band condition showing the formal potential for a surface-confined ferricenium/ferrocene reagent relative to the position of the top of the valence band, E ,and the bottom of the conduction band,E , at the interface between the Si substrate and the redox/electrolyte system. Interface energetics apply to an EtOH/0.1 M [n-Bu N]C10 electrolyte system.

The oxide layer of a metal such as copper may be seen as a semiconductor with a band gap, which may be measured by absorption spectroscopy or photocurrent spectroscopy and photopotential measurements. Valuable additional data are obtained by Schottky Mott plots, i.e. the C 2 E evaluation of the potential dependence of the differential capacity C. For thin anodic oxide layers usually electronic equilibrium is assumed with the same position of the Fermi level within the metal and the oxide layer. The energetic position of the Fermi level relative to the valence band (VB) or conduction band (CB) depends on the p- or n-type doping. Anodic CU2O is a p-type semiconductor with cathodic photocurrents, whereas most passive layers have n-character. 

The photoelectrochemical and UPS results yield the energy diagram of Fig. 44 for a Cu electrode covered with the anodic oxides. For these diagrams an electronic equilibrium is assumed that leads to the same energy position of the Fermi level for Cu and its two anodic oxide layers. This situation defines an energetic difference of the upper valence band edge of CU2O and the Fermi level of 0.8 eV. 

By decreasing the particle size, it is possible to shift the conduction band to more negative potentials and the valence band to more positive potentials as a result of quantization effects [106]. Because of the increase in the effective bandgap, the band edges of the quantized semiconductor particle attain new positions relative to the band edges of the bulk material. Hence, redox processes that cannot occur in bulk materials can be energetically favored in quantized small particles as the conduction and the valence bands become stronger reductant and oxidants, respectively. 

Fig. 4.33. Energetic positions of valence band maxima in the Cu(In,Ga)Se2/ CdS/ZnO sequence showing the transitivity of band alignment. The valence band offsets for CdS/ZnO and Cu(In,Ga)Se2/ZnO are discussed in this chapter. The valence band offset for the Cu(In,Ga)Se2/CdS interface is taken from literature [36,106,123,125]...

The analogous situation is met if we excite an electron from the ground state (electron in the valence band, VB) to an energetically higher position (in the conduction band, CB), e.g. in silicon, according to... 

Absorption of UV/VIS radiation in the solid state is different from UV/VIS absorption in the liquid or gaseous phase with respect to photophysical processes taking place in the crystal lattice and to the metallic, semiconductor (SC) or insulator properties of the absorbing solid (Bottcher, 1991). In crystals, multiple atomic or molecular orbitals are combined to form broad energy bands, i.e. a valence band (vb) fully occupied by electrons and a conduction band (cb) unoccupied or only partly occupied by electrons. Conduction bands and valence bands have different energetic positions relative to one another depending on the specific substrate. In a SC cluster, electronic transitions between the valence band and the conduction... 

Fig. 26. Total and atom projected (P)DOS curves of the valence band region fo the M015O56H22 based vacancy cluster with an 0(3) surface vacancy, (a) Total DOS (thick solid) with decomposition into Mo (thin solid) and O (dashed) contributions, (b) PDOSs of the differently coordinated oxygen centers 0(1) (solid), 0(2) (dotted), and 0(3) (dashed). A gaussian level broadening of 0.4 eV is applied and the energetic position of the highest occupied cluster orbital Ehomo at -5.3 eV is marked by a thin vertical line.

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