# Nanofabrication Techniques in Large-Area Molecular Electronic Devices
## 3 Fabrication of the Top Contact Electrode
In this methodology, the alignment of the top and bottom contacts without using cross-bar geometries is a significant difficulty. Other relevant methods for fabricating molecular junctions are via the construction of a hole, such as a nanopore or a nanowell [375; 376; 377]. To fabricate a nanopore device, a thin Si\({}_{3}\)N\({}_{4}\) layer is deposited in a double-sided polished silicon wafer. A window, suspended over the silicon wafer, is obtained by optical lithography, reactive ion etching (RIE), and wet etching techniques. After that, a gold layer is evaporated onto the top side of the membrane, filling the pore (the top electrode). Then, the sample is immediately incubated in a solution to assemble a molecular layer on the top electrode surface. Finally, the sample is placed in a vacuum chamber to thermally deposit the bottom metal (Au) electrode. Recently, large-area junctions based on SAMs deposited in AlO\({}_{x}\) micropores fabricated on ultraflat template stripped bottom electrodes of gold (Au\({}^{\text{TS}}\)) have been fabricated with high mechanical stability [378]. The construction of a nanowell is more simplified, and occurs in a planar device in contrast with the nanopore. In this case, a nanowell device is fabricated on silicon wafers incorporating pre-patterned gold electrodes covered with silicon dioxide prepared with the focus ion beam (FIB) technique. After the hole is created, molecules are self-assembled on this bottom electrode, and the hole is filled with gold to close the circuit. The disadvantages of these approaches are: The requirement of delicate processes during the sample fabrication, the non-reproducible electrical properties from sample to sample, and a higher ratio of short-circuits by penetration of the metal, overall in nanopore devices. However, to overcome these disadvantages, a conducting buffer interlayer can be inserted between the metal layer and the monolayer to avoid damaging the organic layer, which allows the creation of large-area molecular junctions [379; 380; 378; 358; 359; 381; 358; 359; 358; 359; 360; 358; 359; 370; 359; 382; 358; 359; 390; 391; 392; 393; 394;  
Other radical different alternatives to fabricate \"soft\" top-contact electrodes have been developed recently. Examples include the so-called Thermal Induced Decomposition of an Organometallic Compound (TIDOC) method in which an organometallic film is thermally decomposed--resulting in the formation in situ of gold nanoislands on top of the film without producing short-circuits by penetration of the metal top-contact electrode through the layer [137; 138]. Moreover, the fabrication of nanotrenches of controlled width by means of the shadow edge evaporation method resulting in molecular junctions with very good stability and reproducibility, which offer the advantages of large scale integration, reduced leakage currents, and easier access to the molecular layer with external tools [335]. Additionally, the DEP method (described before in this review) has also been used for the controlled deposition of a top contact electrode. [PERSON] et al. used Au nanowires electrostatically trapped between two microelectrodes. The Au nanowire act as the top-contact electrode in a flat gold | protein monolayer | Au nanowire junction [148]. While the ordinary metal electrode molecular junctions described above remain operational, experimental platforms based on non-metal materials are being constructed, leading to new possibilities for molecular-scale electronics. Carbon electrode--molecule junctions are one of these new testing systems, where several carbon materials, such as reduced graphene oxide (rGO), graphene, or single-walled carbon nanotubes (SWNTs) are used as top-contact electrodes because of their unique advantages [9]. Graphene or rGO films fabricated by chemical vapor deposition have been widely used as the conductive interlayers in molecular junctions [379; 384].