# Nanofabrication Techniques in Large-Area Molecular Electronic Devices
## 2 Fabrication of Molecular Films, Deposition Techniques
### The Langmuir-Blodgett Technique  
In contrast to the SA method, in which molecules are first assembled onto the substrate, and subsequently, they are organized, in the Langmuir-Blodgett method, molecules (amphiphilic in nature) are first organized at the air-liquid interface (the liquid is usually water). The driving forces operating for the arrangement of the molecules at the air-liquid interface are mainly van der Waals interactions, hydrogen bonding, and/or electrostatic interactions. In addition, the pH of the liquid subphase, as well as the presence of ions can also govern the organization of the molecules [261]. Once the monolayer at the air-water interface, Langmuir film, is well-formed, it can be transferred onto a solid substrate to fabricate a Langmuir-Blodgett (LB) film. The transference can be done either by the emersion or the immersion of a vertical substrate (electrode) with respect to the air-water interface or by the horizontal lifting of a substrate located parallel to the subphase (Langmuir-Schaefer, LS, methodology). Depending on the nature of the solid substrate and the anchor groups present in the molecules, the molecules can be physisorbed or chemisorbed [126]. The main advantages associated with the Langmuir-Blodgett technique include:  
* Fabrication of high-quality monolayers with high internal order;
* Fabrication of homo and heterogeneous multi-layered systems [262], resulting in highly ordered 3D molecular architectures;
* Fabrication of directionally oriented monolayers when asymmetric molecules are used [37, 114]. * Large control of the orientation and the packing density of the molecules within the LB monolayers through optimization of different parameters that can be modified upon the manufacturing process, including the nature of the subphase, the spreading solvent (or even mixture of solvents), the temperature, the closing barriers speed, the dipping speed, the transference pressure, or transference direction of the electrode;_In situ_ characterization of the Langmuir film upon the compression process by a wide variety of techniques [263; 264; 265; 266; 267; 268; 269; 270] as surface pressure vs. area per molecule and surface potential vs. area per molecule isotherms, Brewster Angle Microscopy (BAM), ellipsometry, X-ray reflectometry, dilational rheology, Infrared reflection spectroscopy or UV-vis reflection spectroscopy, etc. These techniques provide complementary information for the understanding of the intermolecular interactions in the film;  
A really large number of different anchor groups can be used in the LB methodology in contrast with the SA method since these groups can be not only chemisorbed but also physisorbed onto the electrode. Furthermore, the transference of these films is possible to almost all types of substrates (e.g., metals, conducting polymers, silicon, carbon-and graphene-based electrodes, etc.). The historical background and comprehensive descriptions of the technique have been reported before [90; 91; 261; 271; 272]. The Langmuir-Blodgett technique, illustrated in Figure 3, involves a more complex monolayer formation process compared to the SA method. Briefly, a Langmuir though is used. The basic elements of a Langmuir though include a cuvette made of a hydrophobic material, one or two barriers, and a Wilhelmy plate to determine the surface pressure (defined as the difference between the surface tension of pure water minus the surface tension of the water with the monolayer). Additionally, the transference of the monolayer onto the substrate requires a dipper to introduce or withdraw the substrate. The starting point in the LB process is the spreading of a solution of the molecule in an organic solvent. The solvent must dissolve the material, be highly volatile, and exhibit a large spreading coefficient. A certain volume of this solution is carefully spread onto the subphase. After waiting for the solvent evaporation, the compression process starts with the aid of one or two mechanical barriers, and the surface pressure vs. area per molecule is recorded.