# Nanofabrication Techniques in Large-Area Molecular Electronic Devices
## 3 Fabrication of the Top Contact Electrode
Electrodeposition could be an attractive method for the metallization of monolayers because it does not require expensive vacuum equipment, and it is often easier to control. However, this methodology has so far been unsuccessful since the formation of clusters, or the penetration of metal ions from the solution or even metal wires through the defect sites in the monolayer are often observed--resulting in very low yield devices [84; 359; 360]. Although these problems--which are mainly associated with the existence of imperfections in the monolayer and the presence of free metal ions in the solution--have been able to be overcome or reduced [361; 362; 363; 364; 365; 22], still exits serious limitations in this approach. Only 1/3 of the monolayer is covered by the top metal layer; albeit this value can be increased until an almost completely covered SAM by repeating the adsorption-electrochemical reduction cycle in a metal ion free solution, but with the inconvenience of increasing the presence of short-circuits [364; 365]. An alternative to reduce or eliminate the inconvenient of metal penetration is electroless deposition. This approach allows the deposition of metals and other materials on a variety of substrates [84; 366]. It is very similar to electrodeposition in the sense that the metal deposition results from the reduction of metal ions from solution, but without applying an external potential, which additionally makes this method compatible with insulating or low conductivity materials. However, electroless deposition often requires the use of a catalyst to be adsorbed on the surface prior to the metal deposition, which can contaminate the organic layer affecting the final functionality of the junction [367; 368; 369]. To control the diffusion of the catalyst, the organic layer can be functionalized to selectively adsorb the catalyst,limiting the metallization to the functionalized areas [370; 371]. A modification of this method is the electroless nanowire deposition on micropatterned substrates, which is employed to direct the growth of metallic nanowires on the monolayer surface [368]. To solve the problems that arise from damage to the monolayer or formation of short-circuits, different non-destructive methods have been developed. Nanotranser printing (nTP) is one of these methods that fabricate soft top-contact electrodes by transferring the metal contact to the monolayer (by mechanical contact) from elastomeric poly(dimethylsiloxane) (PDMS) or perfluoropolyether (PFPE) stamps [372; 373]. The transference is a consequence of the chemical affinity between the metal contact and the anchoring groups present in the monolayer, for instance, a thiol-terminated monolayer and a gold metal contact [374]. By using the lift-off float-on (LOFO) method [374], a metal film can be transferred onto a monolayer via capillary interactions avoiding the physical damage in the monolayer. This approach includes basically four steps: (i) Evaporation of a metal film onto a solid support (denoted as leaf); (ii) detachment of the metal leaf from the solid support by floating the leaf at the liquid surface (lift-off); (iii) adsorption of a monolayer onto a solid support; and (iv) attachment of the metal film at the liquid surface to the monolayer supported in the solid substrate using a liquid-mediated process (float-on). Several different types of molecular junctions without any observable damage to the monolayer have been formed using this method [9]; however, the wrinkling of the leaf, the presence of air gaps between the monolayer and the leaf, as well as small monolayer-metal contact areas limit LOFO methodology for further applications and mass production. The transfer of a top metal layer with a hydrophobic polymer, polymer-assisted lift-off (PALO, which combines aspects of nTP with LOFO), solves the wrinkling problem and the small metal contact area in LOFO. Using PALO, metal electrodes with dimensions from 100 \(\upmu\)m\({}^{2}\) to 9 mm\({}^{2}\) can be produced with high yield (\(\geq\)90%) [375].