# Nanofabrication Techniques in Large-Area Molecular Electronic Devices
## 3 Fabrication of the Top Contact Electrode
The incubation of a functionalized monolayer into a metal nanoparticle dispersion results in the chemical deposition of the metal nanoparticles on top of the organic layer. In this context, gold nanoparticles and thiol functional groups have been widely used [350]. However, this methodology results in an incomplete metallization of the organic layer [350]. Therefore, alternatives to increase the surface coverage of the metal deposits have been developed. For instance, the photoreduction by UV-vis light of a metal precursor provides surface coverage of the monolayer surface as large as 76% [115; 277]. In this methodology, the metal precursor is incorporated from an aqueous subphase to an LBM upon the transference process, generating in situ metal nanoislands on top of the monolayer by photoreduction of the precursor. This method has been proved not to damage the underlying organic film, nor penetration of the nanoparticles through the film Additionally, anisotropic palladiumnanostructures, previously produced by a CO-confined growth method [351; 352], were deposited onto the monolayer to generate large palladium nanodeposits (PdND) across the surface without any damage to the monolayer, and importantly, a surface coverage of 85% [111]. Moreover, the incorporation of in situ generated uncapped gold nanoparticles (NPs) has been explored. Here, the NPs were attached to a terminal alkyne functionalized monolayer through the formation of Au-C \(\sigma\)-bonds, via a heterolytic cleavage of the alkyne C-H bond [107]. An interesting methodology for the soft deposition of metallic contacts such as Au, Pt and Cu with yield \(>\)90% is the surface-diffusion-mediated deposition (SDMD) [353; 354]. In this approach, firstly, a SiO\({}_{2}\) etch mask patterned on a pyrolyzed photoresist film (PPF) layer, is fabricated by optical lithography on a silicon substrate with a thermal SiO\({}_{2}\) insulating layer. After that, the molecular layer is formed on the conducting PPF substrate through a C-C bond by electrografting. In order to protect the monolayer from the source radiation or from the direct impingement of metallic vapor generated by using electron-beam evaporation, an adjacent silicon dioxide layer is used. Surface diffusion of the metal atoms from the silicon dioxide to the molecular layer leads to the formation of a \"soft\" top contact, albeit this approach has a significant addressability problem. Atomic layer deposition (ALD) is a chemical process related to CVD but differs from it in several key aspects [355; 356; 357]. ALD involves a well-defined chemical reaction route to form the top-contact. In this methodology, the related chemical reactants (or precursors) are sequentially introduced into the reaction chamber via short pulses (each of them followed with a purge of inert gas to remove excess reactants). A layer-by-layer deposition onto the organic film takes place. Here, the free-ends of the molecule have been specially designed to exhibit affinity by these chemical reactants (or precursors) favoring a selective reaction. Since the film growth is self-limiting, atomic-scale control of the film thickness and minimization of the defects (pinhole-free films) is achieved. Subsequently, this protective molecular layer can be used to deposit a thicker metal film using traditional methods. ALD is very advantageous in fabricating metal-insulator-metal tunnel junctions (MIMTJ) and provides a high control of the thickness of the ultrathin insulating layer inserted in the junction [356]. For these reasons, ALD is considered an enabling technology for future electronics. In addition, based on this methodology, the conductor polymer PEDOT:PSS (where PSS is poly(4-styrenesulphonic acid)) has been used as an interlayer in large-area molecular junctions resulting in very stable devices with a shelf life of more than several months. The main drawback of this methodology may be associated with the presence of hydrophobic end groups in the monolayer that can difficult the subsequent deposition of the hydrophilic PEDOT:PSS [358].