Depositing Monolayers and Thin Films of Nanoparticles
There are a number of techniques used to deposit nanoparticle as a thin film or monolayer onto a substrate, and the choice depends on the area that you’re trying to cover and the equipment available to you. We have provided a brief overview of several techniques below. In general, experimental parameters such as choice of solvent, particle concentration, and temperature will all affect the deposition process, and must be tuned to yield a film with the desired thickness and morphology. Many of the methods described in this section are discussed in the scientific literature and can serve as a guide for your specific material, solvent, and application.
For small substrates (~1 cm2), an easy and tunable deposition method is drop-casting – spreading a nanoparticle dispersion over a substrate and allowing it to dry under controlled conditions, i.e. pressure and temperature. In principle, film thickness depends on the volume of dispersion used and the particle concentration, both of which can be easily varied. There are also other variables that affect the film structure such as how well the solvent wets the substrate, evaporation rate, capillary forces associated with drying, etc.
Generally, it is desirable to use solvents that are volatile, wet the substrate, and are not susceptible to thin film instabilities (de-wetting). Water tends to be a poor solvent for drop-casting due to the low vapor pressure and large surface tension. In some cases alcohols can be in place of water, while organic solvents (such as hexane, toluene or halogenated solvents) are often very good choices for nanoparticles with hydrophobic capping ligands.
One drawback of drop-casting is that even under near ideal conditions, differences in evaporation rates across the substrate or concentration fluctuations can lead to variations in film thickness or internal structure. However, drop-casting does serve a quick and accessible method to generating thin films on relatively small substrates.
Spin-coating often provides more uniform film thicknesses across the substrate compared with drop-casting, and with the right equipment can accommodate much larger substrates. In this technique, a substrate is spun at high RPM and a volume of material with known particle concentration is introduced into the center. Centrifugal force leads to uniform spreading of the dispersion across the substrate, followed by evaporation of solvent to yield a thin particle film. Film thickness depends on the dispersion concentration, volume, and the rotational velocity. As with drop-casting, solvents other than water are favored.
Slowly withdrawing a substrate from a nanoparticle dispersion causes particles to be drawn into the meniscus and deposited as the thin liquid layer dries. This technique has been used to produce very uniform, close-packed particle films, but does have a number of inter-related variables to be tuned to produce good films. The substrate pull rate, particle concentration in the solution, and surface tension between substrate and solution are all important. Literature details specific methods for fabrication of monolayer and multilayer films, as well as more complex geometries such as uniformly spaced stripes of particles on a substrate.
Spray-coating utilizes nebulizers to generate a homogenous, aerosolized stream that applies evenly onto the target substrate. Typically, a syringe pump is used to supply a constant liquid flow to the nebulizer where the stream is combined with an inert gas. The resulting mixture forms aerosolized droplets that deposit onto the substrate in an even and homogenous manner. The nebulizer is usually attached to a movable platform that covers a wide range of area and can be controlled by the operator. Many of the spray-coating parameters are tunable which allows for tailoring of the resulting film deposition, i.e. liquid flow rate, nebulizer position speed, particle concentration. As with most of the thin film methods, volatile solvents are preferred to maximize liquid evaporation time and reduce any potential particle aggregation associated with capillary forces during drying. Spray-coating can apply a variety of particle mixtures to a wide range of substrates and allows the operator to engineer the thin film deposition by manipulating the tunable parameters.
Langmuir-Blodgett (LB) troughs offer a very high level of control over the particle deposition process since the formation of the nanoparticle film can be performed separately from the transfer of the film to the substrate. In using this technique, a dispersion of particles is evaporated onto an immiscible liquid substrate in the LB trough. The particle layer can then be compressed using a movable barrier to obtain uniform monolayer or sub-monolayer films over relatively large (~100 cm2) areas. A substrate can be dipped into the particle layer, or a pre-submerged substrate can be withdrawn, and the film will deposit at the liquid-solid interface. Computer control allows a constant pressure to be maintained on the particle film during deposition, leading to uniform film formation across the entire substrate. Recent literature results have shown that stripes of particles can be deposited onto substrates using this technique, due to complex drying dynamics during deposition.
Substrate/Particle Surface Functionalization
The final technique involves functionalizing your substrate and particles with complimentary coatings to allow chemical or electrostatic attachment of the particles. There are two general ways to accomplish this. The first method involves having a short chain linking molecule with two functional groups, one of which binds to the substrate and the second provides an attachment point for the particles. For example, having a thiol group exposed at the free surface and incubating your substrate in a solution containing citrate-coated Ag nanoparticles should lead to displacement of the citrate and binding of the Ag to the thiol groups. A related technique makes use of charge interactions between molecules bound to the surface, often cationic or anionic polymers, and oppositely charged nanoparticles. There are literature results showing the binding of negatively charged nanoparticles to cationic PDDA thin films on various substrates; similar techniques should work with any of our negatively charged PVP or citrate-coated Ag particles.
The pros of this technique is that coverage is essentially limited to depositing a monolayer at a time, and layers can be built up by alternating incubation of the film in solutions containing linker molecules or particles. The drawback of the technique is that it does require surface modification of your substrate, and the resulting particle layers are typically not at the highest packing density possible.