# Nanofabrication Techniques in Large-Area Molecular Electronic Devices
## 3 Fabrication of the Top Contact Electrode  
Once the molecules have been assembled using some of the methodologies above-described onto a bottom-electrode, the subsequent deposition of a top-contact electrode to close the circuit is one of the most crucial and delicate steps in molecular electronics. Even when the deposition of the top-contact electrode has been largely investigated, having a reliable control in the fabrication of this electrode is still a challenge to reach viable incorporation of molecular electronics in the market. Different techniques have been employed to deposit or make a top-contact electrode, including physical or chemical vapor deposition, atomic layer deposition, liquid metal droplets, break junctions, scanning probe tips, electrodeposition, electroless deposition, etc. Moreover, these techniques can be divided into two categories: (i) Those that can be used to integrate molecular constructs into devices; and (ii) those that are employed for characterizing molecular electronic properties. In this review, we will focus mainly on the first ones. Moreover, the reader can find elsewhere excellent and comprehensive reviews that describe in detail the methods used to characterize, in the laboratory, molecular electronic properties through the formation of temporal metal-molecule contacts, but that are unsuitable for large-scale integration such as: In situ break junction (STM-BJ), mechanically controlled break junction (MCBJ), \(I\)(s) (\(I\) = current, \(s\) = distance) and \(I\)(\(t\)) (\(I\) = current and \(t\) = time) methods based on an STM, electromigration breakdown junction, liquid metal droplets, etc. [9; 84; 169; 341]. In the MCBJ, firstly, a fine metal bridge is formed, and subsequently, this is cleaved upon bending the whole assembly allowing forming the molecular bridge. Similarly, in the STM-BJ method, break junctions are mechanically formed using an STM tip to create a metallic contact to the substrate, which is then cleaved, forming the molecular bridge, while monitoring the current. On the contrary, in the \(I\)(s) method, although it also uses an STM tip to form molecular junctions, there is no contact between the metal electrodes since the STM tip is brought close to the surface and then withdrawn, while the tunneling current is measured. Meanwhile, in the \(I\)(\(t\)) method, the STM tip is placed at a constant distance from the substrate, and the formation of molecular wires is monitored in the time domain. Top-contact electrodes on molecular assemblies have been fabricated by direct evaporation of metals using either physical vapor deposition (PVD) [342] or chemical vapor deposition (CVD) [343]. In PVD, the top-electrode is formed by evaporating a metal, at a sufficiently high temperature and low pressure, for the subsequent condensation of metal atoms in the molecular layer. Nevertheless, in this method, both the film thickness and the spatially selective growth of the metallic film are difficult to control. CVD is a more selective technique and permits higher control in the thickness of the metal deposition than PVD. However, CVD is characterized by rather slow growth rates unless high thermal activation temperatures (\(\geq\)200 \({}^{\circ}\)C) are employed, with these high temperatures being incompatible with most organic thin-films. Regardless if PVD or CVD are used, both methods often result in damage to the monolayer and also in penetration of metal atoms through the monolayer, and therefore, metallic contact between the top and bottom-electrodes [84; 344; 345]. Alternatives (such as metal evaporation onto a cooled substrate [346; 347], blocking the direct path between the crucible and the sample with baffles [348], or the use of an indirect evaporation method to reduce the exposure of the molecular layer to energetic metallic atoms and temperatures [349]) have been proposed to minimize damage to the monolayer, as well as the presence of short-circuits.