# Nanofabrication Techniques in Large-Area Molecular Electronic Devices
## 2 Fabrication of Molecular Films, Deposition Techniques
### The Self-Assembly Technique
DEP is based on the movement of a polarizable nanoscale object (neutral or charged) caused by the polarization of such nano-object induced by a non-uniform electric field [244; 245; 246; 247; 248; 249; 250; 251; 252]. Coulomb interactions between the induced surface charges of the nano-object and the electric field occur. If the nano-object is in a uniform electric field, the net force acting on the nano-object is zero. In contrast, in a non-uniform electric field, there is a net force acting on the nano-object. This net force results in the motion of the nano-object since one end of the dipole is in a weaker field than the other, and the nano-object is pulled electrostatically along the electric field gradient. There are two types of DEP: positive and negative. If the nano-object experiences a force towards the high-field intensity region, the phenomenon is known as positive dielectrophoresis. In positive DEP, the nano-object has a larger polarizability than the surrounding medium, and it is pushed towards the region of a higher electric field. In contrast, if the nano-object experiences a force towards the low-field intensity region, the phenomenon is called negative dielectrophoresis. In negative DEP, the nano-object has smaller polarizability than the surrounding medium. Deposition through dielectrophoresis can be experimentally controlled by adjusting a series of parameters, including the dielectric constant of the nano-object and its surrounding medium, magnitude and frequency of applied electric field, and electric field gradient. As mentioned in the introduction, this vast body of research related to SAMs of organic, organometallic, and biomaterials may find direct use in the incorporation of molecules in large-area devices that may work as molecular wires, diodes, molecular switchers, rectifiers, single-molecule or protein-based transistors, etc. The control of the thermoelectric properties of SAMs (by using an appropriate anchor group, where the Seebeck coefficient can be boosted by more than an order of magnitude), represents a critical step towards functional ultra-thin-film devices for future molecular-scale electronics [253, 254]. Other applications of SAMs beyond the field of molecular electronics include, just to mention a few examples, the protection of electrodes from otherwise highly detrimental environments preventing electrochemical corrosion. This is a useful finding to increase, for instance, the lifetime of electrochromic devices [102]. Moreover, nanotemplating (which resembles the host-guest interaction in supramolecular chemistry) can be applied in molecular separation, chemical sensors and nanoreactors [255]. This technique has been also used as a tool to fabricate hybrid organic/inorganic nanostructures with application in sub-5 nm bottom-up patterning nanolithographic processes, which is an important point for the development of future electronic devices [256]. Other research fields where SA appears as a useful and versatile working technique include the development of drug delivery systems [257, 258], the merge of advanced nanomaterials and optical fibers, known as lab-on-fiber optrodes [259], as well as biopharmaceutical applications [260], among many others.