Layer by layer
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Layer-by-Layer (LbL) deposition is a thin film fabrication technique. The films are formed by depositing alternating layers of oppositely charged materials with wash steps in between. The first implementation of this technique is attributed to R. K. Iler of DuPont, who carried it out using microparticles in 1966. The method was later revitalized by the discovery of its applicability to a wide range of polyelectrolytes by Prof. Gero Decher at Johannes Gutenberg-Universität Mainz. A simple representation can be made by defining two oppositely charged polyions as + and -, and defining the wash step as W. To make an LbL film with 5 bilayers one would deposit W+W-W+W-W+W-W+W-W+W-W, which would lead to a film with 5 bilayers, specifically + - + - + - + - + - .
It is important to note that the representation of the LbL technique as a multilayer build-up based solely on electrostatic attraction is a simplification. As was demonstrated by Prof. Nicholas A. Kotov at Oklahoma State University–Stillwater (now at the University of Michigan), other interactions are involved in this process, including hydrophobic attraction. In general terms, multilayer build-up is enabled by multiple attractive forces acting cooperatively, typical for high-molecular weight building blocks, while electrostatic repulsion provides self-limitation of the absorption of individual layers. This range of interactions makes it possible to extend the LbL technique to hydrogen-bonded films, nanoparticles, similarly charged polymers, hydrophobic solvents, and other unusual systems. The bilayers and wash steps can be performed in many different ways including dip coating, spin-coating, spray-coating and flow based techniques. Characterization of LbL film deposition is typically done by optical techniques such as dual polarisation interferometry or ellipsometry or mechanical techniques such as QCM or QCMD.
LbL offers several advantages over other thin film deposition methods. LbL is simple and can be inexpensive. There are a wide variety of materials that can be deposited by LbL including polyions, metals, ceramics, nanoparticles, and biological molecules. Another important quality of LbL is the high degree of control over thickness, which arises due to the linear growth of the films with the number of bilayers. By the fact that each bilayer can be as thin as 1 nm, this method offers easy control over the thickness with 1 nm resolution.
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