# Nanofabrication Techniques in Large-Area Molecular Electronic Devices
## 2 Fabrication of Molecular Films, Deposition Techniques
### The Self-Assembly Technique  
The self-assembly technique is based on the autonomous and spontaneous reaction of a certain functional group on a surface (chemisorption) [150] and subsequent organization of the backbone of the molecule, due to non-covalent lateral intermolecular forces between neighbor molecules, resulting in long-range molecularly ordered domains within the monolayer. The cooperative effect of this strong-soft combination of forces often results in dynamic behavior of the molecules in the self-assembly monolayer (SAM), exhibiting lateral diffusion, conformational isomerism, and even reconstruction [151]. A general procedure for SAMs fabrication is illustrated in Figure 2. The substrate (e.g., Au, Ag, Pt, Cu, Pd, SiO\({}_{2}\), etc.) is introduced in an organic solution containing the molecule of interest (typically in the 10 uM to 10 mM range). A previous thorough cleaning process of the substrate is required, and often, a pre-treatment of the surface is also applied (annealing, plasma, etching, etc.). After a certain incubation time, the sample is withdrawn from the solution and exhaustively rinsed to remove any physisorbed material. In order to obtain high-quality monolayers with a large surface coverage, it is necessary to optimize a number of influential parameters, such as the incubation time, the concentration, the solvent, the temperature or the ambient relative humidity, due to the water adsorption on the surface of the substrate prior to the immersion into the solution containing the molecule of interest [152; 153]. The self-assembly method has been widely used [154; 155; 156] in the field of molecular electronics since it is probably the most useful approach for the fabrication of well-organized and tightly-packed monolayers, due to its simplicity and versatility. The main advantages of the self-assembly methodology include:  
* Fabrication of high-quality monolayers exhibiting 2D-crystalline long-range molecularly ordered regions. * Low-cost process: (i) No specific instrumentation is required; (ii) it takes place under ambient conditions (no annealing neither low pressures required); (iii) no significant contamination problems occur upon the monolayer fabrication; (iv) the process is quite straightforward for the operator. * The SA method is easily scalable. * The assembly process can be in situ followed by a quartz crystal microbalance, QCM, that provides information about the deposition rate and surface coverage. * Further functionalization of the monolayer either to form multi-layered films [157] or to deposit the top-contact electrode trough a strong molecule-electrode interaction is possible by an appropriate design of the material with the inclusion of a terminal group with the desired functionality. Figure 2: Scheme that illustrates the procedure for the preparation of a self-assembled monolayer. First, a clean substrate is incubated in a solution containing the molecules of interest for a certain time. Second, the substrate is withdrawn and thoroughly rinsed to remove the physisorbed material. Chemisorption is highly specific in nature, and therefore, the terminal functional group of the organic moiety needs to be carefully designed to find a compromise between the strength of the anchoring process on the surface and the final performance of the electrode | monolayer junctions [158]. In the self-assembly methodology, the main limitation comes from the number of functional groups showing the specific affinity and robust interactions with the electrode. As a consequence, a vast majority of the SAMs for molecular electronics incorporate molecules having a thiolate derivative as the head group interacting with the metal substrate [159; 160; 161; 162; 163; 164; 165; 166; 167; 168; 169; 170; 171; 172; 173; 174; 175]. In fact, most of the pioneering work in the study of electrical properties of molecules done in the late 1990s and early years of the 21 st century was based on thiol on gold contacts [176; 177; 178; 179].