Two-dimensional, close-packed monolayer arrays of nanoparticles underpin a variety of technologies and scientific studies. For example, nanoparticle monolayers can be deployed as functional layers on sensors or they can be used to produce colloidal masks for nanosphere lithography. But how does one efficiently and reliably takes a solution of nanoparticles (with three-dimensional freedom) and confine these particles to a (two-dimensional) monolayer across a large substrate?
Traditional techniques for nanoparticle deposition
A large variate of techniques are available for nanoparticle deposition. Some relatively simple and fast methods include solvent evaporation, dip coating and spin coating. However these techniques can waste large quantities of nanoparticles and offer very limited control of the density and coordination of nanoparticles.
Solvent evaporation is prone to a so-called coffee ring effect which is caused by the Marangoni flow. This will lead to non-uniform deposition; a sparse array of nanoparticles in the center and formation of the multilayer at the edges.
Dip coating on the other hand would offer an excellent technique if the purpose is just to cover the substrate with nanoparticles. However, the deposition of a nanoparticle monolayer is challenging with this technique. Also, dip coating requires a large amount of nanoparticles which is a problem when working with more expensive nanoparticles.
Spin coating is an attractive method as it is easy to scale up and the technique is well known from the semiconductor industry. The quality of the film is however dependent on several process parameters such as spin acceleration, speed, size of the nanoparticles, the wettability of the substrate and the solvent used. This makes the precise control of the layer properties difficult. Also, the volume of nanoparticle solution needed is generally quite high.
Monolayer formation at air-liquid interface
Forming a nanoparticle monolayer at air-liquid interface offer a controlled mean to deposit it on practically any substrate. Nanoparticles are trapped at the air-liquid interface, then the interfacial area is gradually decreased to pack the particles closer together. Controlling the packing density is possible as the amount of nanoparticles in a given area is easily calculated. The amount of nanoparticles needed can also be kept very low.
After the monolayer has been formed it can be transferred to the substrate by simply pulling or pushing the substrate through the interface.
If you are interested in how to make nanoparticle monolayers, please watch the webinar titled “Practical guide to the deposition of nanoparticle monolayers” given by Alaric Taylor (Ph.D) from University College London (UCL).
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Alaric Taylor is an EPSRC Research Fellow at University College London and has a special interest in the fabrication of nanophotonic materials, principally through the exploitation of colloidal monolayer self-assembly at the air-water interface.