## 4 A Test Case: Alkane Chains  
### Role of the Anchoring Groups: Symmetric Junctions  
Alkane polymers probably constitute the most common and the simplest molecular chains that have been studied in molecular electronics. Indeed, from their simple chemical nature, their easy availability and ability to connect to different chemical groups or metals and despite their very large gap, which would in principle reduce the conductance, it represents a toy model for electrical conduction in molecular systems. Hence, it is no surprise that this system has been measured in numerous works using different metallic electrodes (mainly noble metals Au, Ag, Cu) and different anchoring groups. Here, we briefly present the well-documented results in the literature on symmetric alkane-based molecular junctions, before exploring further the underlying physical mechanisms through the study of hybrid junctions. For example, anchoring groups like thiol (-SH) [33], amine (-NH\({}_{2}\)) [34], carboxylic acid (-COOH) [35], isocyanide (-NC) [36], methyl sulfide (-SMe) [37], etc., have been studied extensively. What is particularly interesting is the comparison of the measured attenuation factors for several junctions with different anchoring groups. For example, [PERSON] et al. [38] performed such a comparison between thiol, amine and carboxylic acid anchoring groups. The first important difference in the respective electronic properties of these junctions lies in the contact resistance, which is inversely related to the prefactor A in Equation (1). Indeed, the contact resistance is smaller for thiol, bigger for amine and even bigger for carboxylic acid, which yields overall conductance higher for thiol than for amine and then higher for amine than for carboxylic acid. This is due obviously to the different kinds of electronic coupling at the interface, namely of covalent nature for the thiol, much weaker for the amine and related to a deprotonation process of the carboxylic acid to contact the electrode. In this respect, geometries at the interfaces are also affected and the involved symmetries are different, as we will see in more detail later. In addition, the electronic properties seem to be rather different, since the HOMO level is located at around 2.0 eV from the Fermi level in the thiol case, 5.5 eV for the amine case and 1.1 eV in the carboxylic acid case. These differences in HOMO level positions are attributed to different symmetry couplings at the interfaces, as will be discussed in the next section. However, despite these important differences, the attenuation factors seem to remain rather similar for the three junctions, around 0.8-0.9 per C atom (or -CH\({}_{2}\) unit). Considering such similarities in the attenuation factors for different anchoring groups, one can wonder what the real impact of the anchoring groups on the attenuation factor is. From these first results, the only incidence that we can deduce is a variation in the contact resistance and consequently a variation in the overall conductance. In the next section, we will consider the role of the electrode, using different anchoring groups, in the electronic transport in molecular junctions. The main idea is to compare the influence of the anchoring groups with the same or different electrodes.