The molecule-substrate interaction can be highly complex as, depending on the particular system in question, it can encompass vdW, coulombic and covalent contributions. With respect to surface engineering, it is particularly critical to consider the strength of the interaction and its impact on the functional properties and assembly of the adsorbed tectons. Where it is very weak, the molecule is only slightly perturbed by the underlying metal. It therefore retains much of its gas-phase physical and electronic structure [10,11] and is likely to be still capable of its originally desired functional properties [12]. This is typically labelled physisorption. Conversely, where it is strong, the organic species is considered to be chemisorbed, which often results in substantial conformational, chemical and electronic modifications to both the molecule and the surface. Furthermore, molecular diffusion can also be compromised and the assembly of chemisorbed molecules can therefore be severely hindered. The distinction between these two labels is not well defined and is based rather arbitrarily on the magnitude of the interaction strength. Consequently, it is perhaps of greater use to consider the possible implications to the tecton and the substrate when the two interact strongly. In the following section, the different repercussions are outlined, where they are loosely defined as either ‘structural’ – those that concern the conformation and composition of the molecule and surface – or ‘electronic’. The effects on adsorbate assembly can be highly complex and vary from case to case. This will be explored in more detail in Chapter 3.
29 1.2.1 Adsorbate and substrate structural modifications
Unless a molecule is extremely weakly adsorbed or is significantly rigid, it will undergo distortions upon adsorption that optimise its interaction with the substrate. In the mildest cases, only very small conformational changes are likely to occur [13,14], which ultimately have little to no effect on the molecular assembly or its functional properties. In other instances, the effects can be profound. For example, as a candidate for conducting wires or interconnections in molecular computing systems, poly-aromatic molecules are promising. For their successful implementation, the species has to be adsorbed upon, but lifted from, a metal substrate, which can be achieved by incorporating bulky chemical moieties at their peripheries (see Section 1.4.1 for more details). However, the strong interaction between π-electron systems and metals can be sufficient to bend the aromatic moiety away from the desired height to one much closer to the surface [15]. In doing so, the adsorbate’s electronic structure is significantly perturbed and is consequently no longer suitable for the role of a molecular wire.
The metal support is also not immune to adsorption-induced conformational changes, and these effects can often be quite radical. For example, where the molecule-surface interaction is sufficiently strong, metal atoms can be lifted out of the surface plane. Whilst this sounds relatively innocuous, the resulting stress can severely modify the assembly of the adsorbates [14,16]. In more extreme circumstances, the surface can even undergo radical reconstructions, resulting in a fundamentally different structure to that before molecular adsorption [17-19].
When sufficiently strong, the molecule-substrate interaction can also lead to chemical transformations in the adsorbate. In such cases, chemical bonds can be broken and sometimes additional bonds can be formed [20]. Moreover, chemical
30 modification of tectons are often not well defined, resulting in unpredictable products that can be difficult to identify even with exhaustive multidisciplinary studies [21-23]. While such reactions are the basis of the extensive success of reactive metal surfaces in heterogeneous catalysis [24,25], it should clearly be avoided when the goal is to transfer molecular functionality to a substrate.
On the other hand, controlled chemical reactions on surfaces can, in some situations, be a useful tool with which to fabricate the desired molecular species. For example, it has recently been demonstrated that the reactivity of metals and annealing treatments can be used to form covalent bonds between appropriate tectons, resulting in the formation of very large single-molecule species [5,20]. As adsorbates of such size cannot be easily prepared by other means, this is likely to become an important tool in surface engineering.
1.2.2 Electronic interactions between molecule and substrate An adsorbed molecule will experience a number of electronic perturbations because of its proximity to a metallic surface. Perhaps the most difficult to predict a priori is the hybridisation of molecular and surface electronic states. When an organic tecton is adsorbed on a metal, the molecular and metallic electronic states are in close spatial proximity. If they are also sufficiently proximal in energy, the electronic structures of the two can mix, or ‘hybridise’. Metal surfaces exhibit a large density of electronic states (DOS) over a wide energy range, and thus an adsorbate is always able to find electronic states with which to hybridise. The consequences for the molecule can be severe; the originally well-defined and localised molecular orbitals become spatially and energetically broadened [26-32], as depicted in Figure
31 1.2. In some cases this effect is relatively minor [11], but often the electronic structure can be transformed beyond recognition [33].
In addition to this hybridisation are two effects that can shift the energy of an adsorbate’s electronic states. The first occurs upon electron transfer to or from the molecule. When a charge is localised above the substrate (i.e. within the tecton), the electron density within the metal surface is redistributed. This results in an ‘image charge’ in the metal that, in terms of electron distribution and polarisation response, is equivalent to an equal but opposite charge with respect to that in the adsorbed species [34], as illustrated in Figure 1.3a. The coulombic attraction between the two charges stabilises that in the adsorbate, resulting in a reduced ionisation potential and an increased electron affinity. The net result is that the highest-occupied and lowest unoccupied molecular orbitals (HOMO and LUMO, respectively), are typically shifted in energy towards the Fermi level, resulting in a reduction of the ‘molecular
Energy DOS
Energy DOS
Gas phase Adsorbed on metal surface
Figure 1.2 Schematic representation of the electronic structure of a molecule (red dumbbell shape) in the gas-phase (left) and adsorbed on a metal surface (right). In the gas phase, the electronic structure is energetically well-defined. In contrast, when adsorbed on the metal, the hybridisation of the molecular and metallic electronic states results in their energetic and spatial distributions being broadened.
32 band gap’ in comparison to that observed in the gas phase, as illustrated in Figure