Terminating layer

Fig. 59. Molecular modification of semiconductor silicon surfaces. Removal of the oxide generates a hydrogen-terminated layer that reacts with a range of molecular functional groups including alkenes. 

Because of the poor oxide, passivation of germanium surfaces is required for practical use of this semiconductor in devices. Although an ideally passivated surface would resist oxidation and degradation perfectly, such complete resistance is not possible in practice. For this discussion, we consider passivated surfaces as those that strengthen resistance to oxidation in both ambient air and aqueous solution. Three different surface terminating layers are reviewed sulfide-, chloride-, and hydride-terminated germanium. To date, sulfide termination creates the most ideal passivating layer, whereas both chloride and hydride termination add limited stability sufficient to alter the surface reactivity in a way that allows for further reaction. 

Figure 5.9. Functionalization of GaN using the UV-photoinduced reaction with alkenes. Using a shadow mask for the irradiation allows for patterned functionalization. In this study, the alkyl terminated layer was later functionalized with DNA. Figure reproduced with permission from Ref. [152]. Copyright...

For epitaxial layers on sapphire, their polarity depends on the buffer structure [9], Most of such layers contain defects where the polarity is reversed [10], For SiC substrates, the nitride layers reproduce the substrate polarity the Si-face is overgrown with a Ga-terminated layer and the C-face is overgrown with an N-terminated layer. 

Fig. 23 HREM images of the NbSc2 nanotubes. The tube in (a) has a closed tip with a 90° bend in one corner, while the tube in (b) is near-conical in shape due to several terminated layers in the walls. The inset in (a) shows a typical ED pattern. (Reproduced with permission from ref. 70).

Fig. 43 The innermost terminated layer of the BN nanotube shows a spot assigned to the Pb atom (or cluster), which may prevent tube closure. (Reproduced with permission from ref. 44).

For the purpose of the present chapter, a comment on the interpretation of oxygen Is binding energies in oxides seems appropriate. The contribution of the surface terminating layer and of a possible adsorbate to the total O Is spectrum of an oxide catalyst is low (an estimated 15% for laboratory XPS) as in oxides the depth of information [33] is commonly larger than estimated from the universal relation between kinetic energy and escape depth, which is vahd for metals only. 

It is expected that addition of this terminal layer of oxygen will modify the surface structure. The Cr-terminated surface has been investigated by LEED I-V analysis and found to be strongly inwardly relaxed, as is the Fe-terminated a-Fe203(0001) surface. Rohr et al. [41, 42] measured inward relaxations of-38%, -21%, -25%, and +11% of the associated bulk values. The structure of the chromyl-terminated surface has not been determined. 

Fig. 9 Two epitaxial rafts of (Ixl)-Fe304(001) on MgO(OOl) displaced by (l/4)ao<110> relative to one another (a). Two rafts of (V2xV2)R45°- FcsOaCOOI) on MgO(OOl) displaced by (l/4)ao relative to one another which show only the tetrahedral Fe(IIl) terminal layer.

While SrTiOj is the most well-studied example, the influence of termination layer on film growth has been demonstrated to occur in other systems as well. For example, [0001] oriented sapphire can be terminated by three distinct planes. One is oxygen terminated and the other two are A1 terminated. Bench et al. [25] have observed that sapphire (0001) surfaces heated... 

The results cited above were selected to clearly illustrate the influence that the character of a surface has on its properties. To be more specific, the ability of an oxide surface to catalyze specific reactions or nucleate specific phases is known to depend on both the surface orientation and the termination layer. We now turn to the question of what determines the surface character and what characters are reasonably attainable. 

The first structure (shown in Fig. 4b) consisted of a presumably pure Pt Kagome net terminating layer. The close packing directions of the surface... 

Experimental data and calculations show that monolayers of compounds capable of formation of intermolecular H-bonds (such as amino- or hydroxy-terminated layers) have a network of such bonds on the surface of dry monolayers, but exposure to water vapours results in adsorption of water and disruption of the intermolecular bonds239,240. 

A recent study has further proven the applicability of gas-phase reactions to functional groups at the monolayer surface. Alcohol terminal groups were quantitatively converted to trifluoroacetates by exposure to the vapours of trifluoroacetic anhydride (equation 7). Reaction with perfluoropropionic and perfluorobutyric anhydrides proceeded similarly, but with ca 80% yield (XPS). Very high contact angles (>120°) of perfluoro-terminated layers proved the formation of esters333. 

Alternatively, hyperbranched polymers can be prepared in situ on the monolayer surface. Treatment of the acid-terminated layer with chloroformate and amine-terminated poly(f-butyl acrylate) produces ester groups on the surface. Hydrolysis of the ester functions with tosic acid gives carboxyl groups. The whole procedure is then repeated several times... 

An alternative approach to formation of gold-thiol multilayers utilizes inorganic chemistry. Copper(II) carboxylate groups can be generated on the monolayer surface by treatment of the acid-terminated layer with the solution of a copper salt. The copper(II) ions have strong affinity for sulphur(II) compounds, and thiols self-assemble on top of this Cu-terminated film (equation 17)360.361. 

Two examples of intercalation reactions which can be useful in photoelectrochemistry are depicted in Fig. 23. The first one (a) concerns the already discussed passivation and neutralization of step sites, which act as traps and recombination centers for charge carriers (A in Fig. 23 a). As already shown by Parkinson and others27 partially intercalated molecules can favourably alter the electronical properties of terminating layers, thus improving the efficiency of the electrode material. The second example (b) vis-... 

Z, a position of the penetration of a fivefold symmetry axis (in E ) into a single terminating layer. 

The height of the plateau of the graph in Figure 12-11 top)/ bottom) defines the densities of the terminations to be 0.134A (Table 12-4). Both solutions for the sequences of the fivefold terminations are presented in Figure 12-12. Whereas the volume density of the model (for i-AlPdMn) is 0.065 (averaged on the patch of size 100 x 100 x 100 A ), the volume density in the fivefold terminating layer, which is exactly 2.52 A broad, is 0.106 A. ... 

---