# Nanofabrication Techniques in Large-Area Molecular Electronic Devices
## 2 Fabrication of Molecular Films, Deposition Techniques
### The Langmuir-Blodgett Technique
Therefore, as the available area per molecule is being reduced, the molecules get closer resulting in intermolecular interactions. This gradual change in a molecular arrangement is reflected in surface pressure variations registered with the Wilhelmy balance. Thus, upon the compression process, the monolayer undergoes several bi-dimensional phases (gas, liquid expanded, liquid condensed, and solid) and phase transitions. Once the monolayer reaches the target surface pressure of transference, the Langmuir film can be deposited onto a solid substrate, forming an LB monolayer (LBM). Possibly the main drawback of the LB method is that it is a time-consuming technique. Concerning the fabrication process, a slow and critical step is the manual spreading process. Here, the electrospray (ES) spreading methodology could be an alternative to make this technique more functional and likewise to improve reproducibility. For instance, this methodology has been recently  
Figure 3: Scheme showing the main steps involved in the Langmuir–Blodgett monolayer (LBM) fabrication. First, an organic solution containing the molecule of interest is spread on the water surface. Second, the molecules are organized upon an isothermal compression resulting in a Langmuir film. Finally, the monolayer is transferred onto a solid support—forming the LBM. employed by [PERSON] et al. [273] to obtain LBMs incorporating clay mineral nanosheets hybridized with the ODAH\({}^{+}\), where a comprehensive analysis of the influence of the infusion rates on the film properties is carried out. In general terms, the materials typically used for the fabrication of LBMs are water-insoluble molecules with a head (hydrophilic)-tail (hydrophobic) amphiphilic structure [90; 262; 271; 274]. Molecular assemblies using the LB technique for fabricating large-area molecular electronic devices of highly conjugated materials with monodetated anchor groups, such as thiols [114; 275], nitriles [79], amines [21; 276], trimethylsilylethynyl group [113; 277], acetylenes [120], pyridines [125; 278], or a viologogen moiety [83], have been constructed. Additionally, the use of multidentate or multipodal anchor structures to improve the robustness and stability of the molecular junction has also been explored. For instance, using acids [116; 118; 279] or diacids [37], methyl esters [121], tetrathiofulvalenes [280], a 2-aminopyridine group [127], a pyrazole moiety [81], or the tripodal head group 2,6-bis((methylthio)methyl)pyridine [126]. Conductance in large-area molecular electronic devices incorporating carboxylic groups with lateral intermolecular H-bonding can be enhanced by deprotonation in a basic media with the subsequent rupture of those H-bonds. Therefore, this demonstration exploits the switching behavior in the conductance under protonation\(-\)der protonation conditions for the construction of pH sensors based on molecular junctions [37]. Moreover, an LB film of a dyad consisting of an electron-rich \"\(\pi\)-donor\" (D) and an electron-poor \"\(\pi\)-acceptor\" (A) separated by a rigid, insulating \"spacer\" (featuring three main components of the original Aviram-Ratner rectifier design) has shown rectification behavior validating the rectification proposal by Aviram, and thus, allowing the possibility of producing organic molecular rectifiers [280]. In addition to the typical amphiphilic molecules described above, the LB technique is largely enlarged towards the assembly of more complex structures with interest in molecular electronics (and organic electronics). Polymers [281; 282; 283], metalloporphyrins [284], DNA [285], perylenes [286], perylene\(-\)NH\({}_{2}\)[287], ruthenium complexes [288], organo-modified inorganic nanoparticles in combination with polymer nanospheres (nano-mille-feuille system) [289], pillar[5]arene derivatives [290], polymer-coated CsPbBr\({}_{3}\) nanowires [291], aligned SWCNTs [145], semi-conductive 2D materials based on 2,3,6,7,10,11-hexaiminotriphenylene (HATP) [292], or rGO [146] have been used.