The Layer-by-Layer (LbL) assembly

The Layer-by-Layer (LbL) assembly, also called Electrostatic Self Assembly Multilayer (ESAM), consists of the immersion of the substrate into alternate solutions with opposite electric charge [1]. This electrostatic attraction is the main force of adhesion to the surface of the substrate. It was discovered in 1966 although this technique went unnoticed until the nineties, when G. Decher and coworkers rediscovered the LbL technique [2]. Since then, the LbL assembly has become one of the most popular methods to obtain nanometric coatings.

The LbL assembly is a multistep process, in which the different electrostatic attraction forces of the polyelectrolyte solutions keep the coating together. The Layer-by-Layer process steps are:

  • Cleaning of the substrate and surface activation
  • The substrate is alternately submerged in cationic and anionic charge solutions. This creates the multilayer film. See figure.
  • If the charge of the substrate is positive, the initial monolayer will be a polyanion. If it is negative, the first deposited layer will be a polycation.
  • After each immersion, the substrate is rinsed with ultrapure water to remove the excess of material.
  • The process can be repeated as many times as needed

LBL

Fig. Layer-by-Layer process. Nadetech Innovations

Note that the polyanions and polycations overlap each other, producing a homogeneous optical material [2-4]. This pair of an anionic monolayer and one cationic monolayer will form a bilayer. The composition and thickness of a bilayer is determined by the precise control of the deposition parameters: concentrations, pH, temperature, immersion times, ionic strength… Nowadays, modern coating devices, like the ND-2&3D Layer-by-Layer coaters, are able to control and automatize all these parameters to obtain precise and controlled coatings. The ND-2D & ND-3D coaters are controlled by a software that allows to program different sequences and adjust the dipping parameters of each individual immersion step: immersion speed, withdrawal speed, vertical displacement, substrate initial position, immersion time, drying time,  as well as the sequence parameters such as the number and position of the immersions steps in a deposition cycle and number of cycles.

LbL technique offers some important advantages in comparison with other coating techniques. First, it is a simple technique, as no special equipment is needed. Second, this method can be used to coat complex geometries with homogeneous results and allows to control the thickness of the film. Many kinds of substances can be deposited by LbL technique, such as nanoparticles, polymers, fluorescent indicators, colometric compounds…

[1] Rivero Fuente, P.J.. “Contribution to the development of functional nanostructured coatings based on silver nanoparticles.” (2014). Universidad Pública de Navarra

[2] Choi, J.; Rubner, M. F. Influence of the Degree of Ionization on Weak Polyelectrolyte Multilayer Assembly. Macromolecules 2005, 38, 116-124.

[3] Hammond, P. T. Form and Function in Multilayer Assembly: New Applications at the Nanoscale. Adv Mater 2004, 16, 1271-1293.

[4] Choi, J.; Rubner, M. F. Influence of the Degree of Ionization on Weak Polyelectrolyte Multilayer Assembly. Macromolecules 2005, 38, 116-124.

© Nadetech Innovations 2017

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