Layer-by-layer deposition of nanoscale structures

Fabrication of ultrathin films of polymers or polymer nanocomposites with precise control of thickness and composition is of great interest due to a variety of possible applications for optics and electronics. Several techniques have been reported in the last few decades, amongst which are the Langmuir-Blodgett (LB) technique, self-assembled monolayer (SAM) technique, and the layer-by-layer adsorption (LbL) of materials. Both the LB and the SAM techniques are capable of assembling small organic molecules, polymers, proteins and nanoparticles for optical and electronic devices as well as surface modification. Both techniques provide ultrathin films of controlled thickness, composition and orientation. The LB technique requires expensive instrumentation and is a laborious process. One of the limitations of SAM assembly is the requirement of very specific substrates such as noble metals for thiols, or silica or metal oxides for silanes. A versatile technique for deposition of ultrathin films is the LbL assembly of materials, as demonstrated by Decher and co-workers [1]. In the last two decades, this technique has been widely adopted for the fabrication of nanostructures with tailored properties. The LbL technique is attractive due to its simplicity and versatility. In most cases, the deposition can be carried out from aqueous solution of molecules and polymers or suspensions of nanoparticles on the bench top. LbL technique can be extended to create conformal coatings on non-planar geometries including porous membranes, colloidal particles, fibers, living cells, etc [2-4]. In general we can classify LbL techniques by the kind of interactions which dominate the assembly process. Accordingly, the following distinct classes can be identified: (1) electrostatically assembled multilayers from highly charged polycations, polyanions or charged nanoparticles; (2) electrostatically assembled multilayers from partially charged polyelectrolytes; (3) LbL multilayers where interactions such as hydrogen bonding play an important role in adsorption; and (4) LbL multilayers produced via specific interactions, such as biotin-avidin affinity binding. With the appropriate choice of these interactions, the thickness and morphology of the films can be effectively controlled. In the electrostatic LbL process, spontaneous sequential adsorption of oppositely charged materials is carried out from dilute aqueous solutions on a charged substrate. During the deposition process, an oppositely charged substrate is dipped in a solution of either a cationic or an anionic polyelectrolyte (molecules or nanoparticles) for a certain period of time. If the concentration of the charged species (i.e., polyelectrolyte) is sufficient, adsorption occurs until there is a complete charge reversal at the surface. The process is then repeated with a solution of either an anionic or a cationic polyelectrolyte. Figure 1(a) shows a schematic diagram of the experimental procedure of adsorption in LbL films. After adsorption of each layer, a simple washing step is performed to remove weakly or nonspecifically bound polymers, molecules or particles adhering to the substrate. Moreover, this step also prevents the contamination of the solution in the next dipping Journal of Nanophotonics, Vol. 3, 030306 (10 September 2009)