Nanoparticle deposition refers to the process of attaching

nanoparticle A nanoparticle or ultrafine particle is usually defined as a particle of matter that is between 1 and 100 nanometres (nm) in diameter. The term is sometimes used for larger particles, up to 500 nm, or fibers and tubes that are less than 1 ...

s to solid surfaces called substrates to create

coating A coating is a covering that is applied to the surface of an object, usually referred to as the Substrate (materials science), substrate. The purpose of applying the coating may be decorative, functional, or both. Coatings may be applied as liquid ...

s of nanoparticles. The coatings can have a

monolayer A monolayer is a single, closely packed layer of atoms, molecules, or cells. In some cases it is referred to as a self-assembled monolayer. Monolayers of layered crystals like graphene and molybdenum disulfide are generally called 2D materials. ...

or a multilayer and organized or unorganized structure based on the coating method used. Nanoparticles are typically difficult to deposit due to their physical properties.

Challenges

Nanoparticles can be made from different materials such as metals, ceramics and polymers. The stability of the nanoparticles can be an issue as nanoparticles have a tendency to lower their very high

surface energy In surface science, surface free energy (also interfacial free energy or surface energy) quantifies the disruption of intermolecular bonds that occurs when a surface is created. In solid-state physics, surfaces must be intrinsically less energe ...

, which originates from their high surface-to-bulk ratio. Bare nanoparticles tend to stabilize themselves either by

sorption Sorption is a physical and chemical process by which one substance becomes attached to another. Specific cases of sorption are treated in the following articles: ; Absorption: "the incorporation of a substance in one state into another of a dif ...

of molecules from the surroundings or by lowering the surface area through coagulation and agglomeration. Usually the formation of these aggregates is unwanted. The tendency of a nanoparticle to coagulate can be controlled by modifying the surface layer. In a liquid medium, suitable ligand molecules are commonly attached to the nanoparticle surface, as they provide solubility in suitable solvents and prevent coagulation.

Deposition methods

There are multiple different coating methods available to deposit nanoparticles. The methods differ by their ability to control particle packing density and layer thickness, ability to use different particles and the complexity of the method and the instrumentation needed.

Langmuir-Blodgett

In the Langmuir-Blodgett method, the nanoparticles are injected at air-water interphase in a special Langmuir-Blodgett Trough. The floating particles are compressed closer to each other with motorized barriers which allow to control the packing density of the particles. After compressing the particles to the desired packing density, they are transferred on a solid substrate using vertical (Langmuir-Blodgett) or horizontal (Langmuir-Schaefer) dipping to create a monolayer coating. Controlled multilayer coatings can be made repeating the dipping procedure multiple times. The benefits of the Langmuir-Blodgett method include a firm control over the packing density and the layer thickness achieved that have been shown to be better than with other methods, the ability to use different shapes and materials of substrates and particles and the possibility to characterize the particle layer during deposition for example a

Brewster Angle Microscope A Brewster angle microscope (BAM) is a microscope for studying thin films on liquid surfaces, most typically Langmuir films. In a Brewster angle microscope, both the microscope and a polarized light source are aimed towards a liquid surface at ...

. As a disadvantage, a successful Langmuir-Blodgett deposition requires optimization of multiple measurement parameters such as dipping speed, temperature and dipping packing density.

Dip coating and spin coating

The

spin Spin or spinning most often refers to: * Spinning (textiles), the creation of yarn or thread by twisting fibers together, traditionally by hand spinning * Spin, the rotation of an object around a central axis * Spin (propaganda), an intentionally b ...

and

dip coating image:Dip coating.svg, A schematic of the continuous dip coating process. Roll of coarse cloth Cloth Bath Liquid material Rollers Oven Scrapers Excess liquid falls back A coating remains on the fabric cloth. Dip coating is an industrial ...

methods are simple methods for nanoparticle deposition. They are useful tools especially in creating self-assembled layers and films where the packing density isn't critical. Accurate and vibration-free sample withdrawal speeds can be used to have control over the film thickness. Creating high density monolayers is typically very difficult since the methods are lacking the packing density control. Also, the volume of nanoparticle suspension required for both spin coating and dip coating is rather big which may be an issue when using expensive nanoparticle materials.

Other methods

Other possible deposition methods include methods utilizing particle self-assembly by solvent evaporation, doctor blade,

chemical vapor deposition Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high quality, and high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films. In typical CVD, the wafer (substra ...

and transfer printing. Some of these methods like solvent evaporation are extremely simple but produce low-quality films. Other methods such as the chemical vapor deposition are effective for certain types of particles and substrates but are limited in particle types that can be used and require heavier instrumentation investments. Also hybrid methods such as combining self-assembly to Langmuir-Blodgett have been used.

Nanoparticle coating applications

Coatings and thin films made from nanoparticles are being used in various applications including displays, sensors, medical devices, energy storages and energy harvesting. Examples include * Using graphene oxide for applications in electronics * Using nanoparticles of metal oxides, carbon nanotubes and

quantum dot Quantum dots (QDs) are semiconductor particles a few nanometres in size, having light, optical and electronics, electronic properties that differ from those of larger particles as a result of quantum mechanics. They are a central topic in nanote ...

s in photovoltaics, displays and sensors * Using polymers and nanocomposites in nanolithographic patterning

External links

Pdf: Fabricating Highly Organized Nanoparticle Thin Films

References

{{Reflist Nanoparticles Coatings