Band alignment

There are many published examples in which the coupling of two different materials leads to an increase in the photocatalytic activity. Many of them concern coupling and junctions between different nanopartides, considering also different topologies, like coupled and capped systems [72]. Tentative explanations based on possible heterojunction band profiles are given. However, in-depth analysis of the hetero junction band alignment, the physical structure of the junction, the role of (possible) interfadal traps and of spedfic catalytic properties of the material is still lacking. Some recently published models and concepts based on (nano)junction between different materials are briefly reviewed here. 

The electronic properties of CNTs are of particular importance for hybrid materials and strongly depend on the structure of the CNTs. Theoretical [45-48] and experimental results [49] reveal that SWCNTs are either metallic or semiconducting depending on diameter and chirality, while MWCNTs are generally metallic (due to band alignment upon interlayer interactions). 

A general requirement for the synthesis of CS NCs with satisfactory optical properties is epitaxial type shell growth. Therefore an appropriate band alignment is not the sole criterion for choice of materials but, in addition, the core and shell materials should crystallize in the same structure and exhibit a small lattice mismatch. In the opposite case, the growth of the shell results in strain and the formation of defect states at the core-shell interface or within the shell. These can act as trap states for photogenerated charge carriers and diminish the fluorescence QY.95 The structural parameters of selected semiconductor materials are summarized in Table 5.1. 

Fig. 1.10. Band alignment between II-VI compounds according to density functional theory calculations by Wei and Zunger [95]. The energy of the valence band maximum of ZnS is arbitrarily set to 0 eV. A comparison to experimental results is presented in Fig. 4.18 in Sect. 4.3.1 (page 150)...

Figure 1.11 shows an experimental determination of the band alignment at the ZnO/(Zn,Mg)0 interface using optical spectroscopy of quantum well structures [100]. The data indicate that the larger band gap of (Zn,Mg)0 is... 

Fig. 4.3. Experimental procedure for the determination of the band alignment using photoelectron spectroscopy...

Interfaces of sputter-deposited ZnO and ZnO Al films with different substrate materials (CdS, hi2S3, and Cu(In,Ga)Se2) in dependence on deposition parameters. The band alignments and chemical interactions at the interfaces are discussed. 

CdS [92]. The ionization potentials of CdTe(lll), (111), and (110) amount to 5.3, 5.6, and 5.65 eV, those of epitaxial CdS films deposited onto these surfaces are given by 6.25, 6.85, and 6.75eV, respectively. For comparison, the ionization potentials of polycrystalline CdTe and CdS films amount to 5.8 and 6.9 eV [92]. The lower ionization potential of the cation terminated (111) or (0001) surfaces is related to a smaller surface dipole. The different ionization potentials can also affect the band alignment at weakly interacting interfaces [93]. 

The CdS/ZnO interface is of particular importance in Cu(In,Ga)Se2 thin film solar cells because it is used in the standard cell configuration (Fig. 4.2). A first experimental determination of the band alignment at the ZnO/CdS interface has been performed by Ruckh et al. [102]. The authors have used ex-situ sputter-deposited ZnO films as substrates. The interface formation was investigated by stepwise evaporation of the CdS compound from an effusion cell. Photoelectron spectroscopy revealed a valence band offset of A Vb = 1.2eV. An identical value of 1.18eV has been derived using first-principles calculations [103]. With the bulk band gaps of CdS and ZnO of 2.4 and... 

The agreement between the calculations and the various experimental results is excellent. This indicates the wide applicability of the calculated band alignments. The general behavior also confirms the original value given... 

A determination of the band alignment at the CdS/ZnO interface where ZnO has been stepwise deposited by magnetron sputtering has been published by Venkata Rao et al. [71]. A more extended series of spectra recorded during ZnO deposition by dc magnetron sputtering onto CdS are presented in Fig. 4.19. During ZnO deposition the sample was held at room temperature. 

The interfaces prepared by sputter deposition of ZnO (filled square) or (Zn,Mg)0 (filled triangles) exhibit a valence band offset of AEyb = 1.2 eV. The ZnO and (Zn,Mg)0 films were prepared at room temperature in pure Ar and therefore exhibit a large disorder and a large BEve(Zn 2p3/2)- Compared with the interface with reverse deposition sequence, the offset is 0.35 eV larger. This indicates a rather strong influence of the deposition sequence on the band alignment at the CdS/ZnO interface, which is most likely related to the amorphous nucleation layer when ZnO is deposited onto CdS. 

The valence band offsets determined for the ZnO Al/CdS interfaces (1.4 0.1 eV) are 0.2-0.4eV larger than the values obtained for interfaces where undoped ZnO or (Zn,Mg)0 films have been used as substrate. This points toward an influence of the A1 content in the ZnO film on the band alignment. An explanation for this cannot be given yet. 

The results presented in this section further illustrate that there is a considerable dependence of the band alignment at the CdS/ZnO interface on the details of its preparation. An important factor is the local structure of the ZnO film. There is considerable local disorder when the films are deposited at room temperature in pure Ar, deposition conditions that are often used in thin film solar cells. It is recalled that the disorder is only on a local scale and does not affect the long range order of the films, as obvious from clear X-ray diffraction patterns recorded from such films (see discussion in Sect. 4.2.3.3). Growth of sputter deposited ZnO on CdS always results in an amorphous nucleation layer at the interface. The amorphous nucleation layer affects the valence band offset. 

To give an individual value for the band alignment is not possible. Structurally well-ordered interfaces, which are obtained e.g., by deposition of CdS onto ZnO layers deposited at higher temperatures and/or with the addition of oxygen to the sputter gas, show a valence band offset of A TV is = 1.2 eV in good agreement with theoretical calculations [103]. Sputter deposition of undoped ZnO at room temperature in pure Ar onto CdS also leads to a valence band offset of 1.2 eV. In view of the observed dependencies of the band offsets this agreement is fortuitous, as the influence of the local disorder and of the amorphous nucleation layer most likely cancel each other. 

Fig. 4.25. Influence of the amorphous nucleation layer of the ZnO film on the band alignment at a hypothetical CdS/ZnO interface (a) CdS and ZnO before contact (b) in contact with charge equilibrium established by space charge layers (c) in contact with equilibrium established by charges localized in an amorphous ZnO nucleation layer...

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