Ultra-thin decoupling layers

An alternative approach is to use an ultra-thin film of insulating material – a ‘decoupling layer’ or ‘decoupling film’ – between the tecton film and the metal surface, as depicted in Figure 1.6a. The use of inorganic materials has proven highly successful in this respect. Such films can often be fabricated with the appropriate thicknesses and with sufficient band gaps [67], such that there is no electronic coupling of the molecular ‘overlayer’ with the decoupling film or with the surface [68]. Furthermore, the interaction between the overlayer and the decoupling film is often exceptionally weak, thus allowing the molecular assembly to proceed

39 unperturbed and as directed by their intermolecular interactions alone. The most commonly used materials are alkali halides and metal oxides, both of which are discussed in more detail below.

It is also possible, at least in principle, to achieve a similar decoupling effect using a multiple-layer thick film of the molecular tecton, as depicted in Figure 1.6b. The first layer(s) act as a ‘sacrificial’ decoupling film, whilst later layers are sufficiently separated from the surface to be electronically decoupled [33,57,59]. However, this approach offers less control over the final product; the properties of the sacrificial layer and the interaction between the different layers are not necessarily well defined.

1.4.2.1 Alkali halide decoupling films

Of the possible alkali halide options, NaCl has been by far the most popular material choice for a decoupling layer. This is perhaps due to its simple internal structure and the relative ease with which suitably thick films can be fabricated on a range of different materials [31,69-74]. Furthermore, the growth of NaCl has been optimised such that exclusively bilayer films can be formed on some surfaces [31,75], which is an ideal thickness for a complete electronic decoupling.

Figure 1.6 a) Schematic representation of the decoupling layer strategy. A thin layer of an insulating material is deposited onto the surface prior to adsorption of the molecular tecton. b) The ‘multilayer approach’ to decoupling, where a thick film of the organic species is deposited. The lower layers act as a decoupling layer.

40 The strong attraction between the ions within alkali halide films plays a critical role in their growth behaviour on surfaces. First, regardless of the substrate symmetry, alkali halides almost always form (100)-terminated films (shown in Figure 1.7a). Only one example of a non-(100) film has been reported, which was only obtained after a considerably complex synthesis [76]. Second, alkali halide films exhibit a ‘carpet-growth’ regime: As the film grows laterally on substrate terraces, it will encounter steps, which it spreads over smoothly to continue growth on the following terrace [69]. This is illustrated in Figure 1.7b. In doing so, the alkali-halide maintains its (100)-termination. Finally, one study reported that when NaCl is deposited onto a particularly highly stepped surface, the underlying metal is forced to reconstruct in a way that provides a better epitaxy for the NaCl (100) facets [71,77].

Figure 1.7 a) Side-view schematic of a NaCl bilayer decoupling film. The substrate is shown in grey, the Na ions are blue, the Cl in green, and the decoupled molecules are shown as red dumbbells. b) Depiction of the ‘carpet’ growth of NaCl over Ge(001), adapted from reference [69]. The substrate atoms are shown with partially shaded circles, the fully shaded circles show the Cl-_{ions within the NaCl structure.}

a)

41 It is perhaps unfortunate that other alkali halides have received very little attention in comparison to NaCl, as the electronic states of adsorbates on NaCl have been reported to be slightly broader than that expected [41]. This is not due to a residual molecule-substrate electronic coupling, but in fact arises from that between the molecular electronic states and phonons in the NaCl film [78]. In contrast, when RbI films are used, this broadening is not observed, allowing normally obscured electronic features to be revealed [42]. It is not clear if this effect extends to other alkali halide films due to their limited study.

1.4.2.2. Metal oxide decoupling films

Thin films of metal oxide on metals were originally studied as models for the substrates used to support catalytic metal dispersions [79]. As such films are both chemically inert and develop appropriately large band gaps within a few layers thickness [67], they are also suitable candidates for adsorbate decoupling. In fact, a number of studies have already demonstrated their effectiveness in this regard [54,58,80,81].

The most commonly used metal oxide decoupling film used is the surface oxide of NiAl(110). By annealing the (110) termination of a NiAl crystal in an O2

atmosphere, approximately half of the substrate becomes covered with a 0.5 nm thick oxide layer [82]. The oxide has a highly complex structure and composition, which has remained controversial until recently [82-84]. Other oxides have been explored, but their preparation is typically more complex; metal atoms need to be deposited onto the sample using thermal deposition procedures whilst a background pressure of O2 is maintained [74]. For films grown in this way, a lack of good

42 growth [79]. In comparison, the NiAl(110) surface oxide has a reportedly low defect density and exhibits epitaxial growth along one direction [82].

The imperfections in oxide films are possibly their greatest limitation towards their widespread use. This is because defects usually interact relatively strongly with overlayer adsorbates, in turn modifying their functional properties. Even on the NiAl(110) surface oxide, the effect can be quite pronounced. For example, STS investigations of adatoms [68,80], metal clusters [85,86] and molecules [54] deposited onto this film reveal that the electronic states of the adsorbates can vary significantly, as shown by the STS spectra in Figure 1.8, depending on their position with respect to the film. This is taken as evidence for adsorbates coupling to different defects in the film, although this is challenging to prove given the difficulty encountered when trying to atomically resolve the surface of the oxide film with STM.

It is important to note that the use of inorganic layers in ‘real world’ applications will ultimately require that their growth conditions are well optimised.

a) b) c)

Figure 1.8 a) and b) STM images of Pd atoms adsorbed on the NiAl(110) surface oxide film. The corresponding STS spectra of Pd atoms in each position are shown in c). Adapted from reference [68].

43 The low diffusion barriers of overlayer species, coupled with a preference for adsorbing on, or as near as possible to the metal surface [87-89] could result in differently decoupled adsorbates if non-uniform decoupling layers are used. This can be avoided by ensuring that the decoupling film covers the entire surface and is of uniform thickness.