Nanoparticle deposition refers to the process of attaching nanoparticles to solid surfaces called substrates to create coatings of nanoparticles. The coatings can have a monolayer 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.
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, which originates from their high surface-to-bulk ratio. Bare nanoparticles tend to stabilize themselves either by sorption of molecules from the surroundings or by lowering the surface area through coagulation and agglomeration. [1] 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.
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.
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. [2]
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, [3] 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. As a disadvantage, a successful Langmuir-Blodgett deposition requires optimization of multiple measurement parameters such as dipping speed, temperature and dipping packing density.
The spin and dip coating methods are simple methods for nanoparticle deposition. [4] 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 possible deposition methods include methods utilizing particle self-assembly by solvent evaporation, doctor blade, chemical vapor deposition 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. [5]
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
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.
A thin film is a layer of material ranging from fractions of a nanometer (monolayer) to several micrometers in thickness. The controlled synthesis of materials as thin films is a fundamental step in many applications. A familiar example is the household mirror, which typically has a thin metal coating on the back of a sheet of glass to form a reflective interface. The process of silvering was once commonly used to produce mirrors, while more recently the metal layer is deposited using techniques such as sputtering. Advances in thin film deposition techniques during the 20th century have enabled a wide range of technological breakthroughs in areas such as magnetic recording media, electronic semiconductor devices, integrated passive devices, LEDs, optical coatings, hard coatings on cutting tools, and for both energy generation and storage. It is also being applied to pharmaceuticals, via thin-film drug delivery. A stack of thin films is called a multilayer.
Self-assembled monolayers (SAM) of organic molecules are molecular assemblies formed spontaneously on surfaces by adsorption and are organized into more or less large ordered domains. In some cases molecules that form the monolayer do not interact strongly with the substrate. This is the case for instance of the two-dimensional supramolecular networks of e.g. perylenetetracarboxylic dianhydride (PTCDA) on gold or of e.g. porphyrins on highly oriented pyrolitic graphite (HOPG). In other cases the molecules possess a head group that has a strong affinity to the substrate and anchors the molecule to it. Such a SAM consisting of a head group, tail and functional end group is depicted in Figure 1. Common head groups include thiols, silanes, phosphonates, etc.
A Langmuir–Blodgett trough is a laboratory apparatus that is used to compress monolayers of molecules on the surface of a given subphase and measures surface phenomena due to this compression. It can also be used to deposit single or multiple monolayers on a solid substrate.
A Langmuir–Blodgett (LB) film is a nanostructured system formed when Langmuir films—or Langmuir monolayers (LM)—are transferred from the liquid-gas interface to solid supports during the vertical passage of the support through the monolayers. LB films can contain one or more monolayers of an organic material, deposited from the surface of a liquid onto a solid by immersing the solid substrate into the liquid. A monolayer is adsorbed homogeneously with each immersion or emersion step, thus films with very accurate thickness can be formed. This thickness is accurate because the thickness of each monolayer is known and can therefore be added to find the total thickness of a Langmuir–Blodgett film.
In physics, a "coffee ring" is a pattern left by a puddle of particle-laden liquid after it evaporates. The phenomenon is named for the characteristic ring-like deposit along the perimeter of a spill of coffee. It is also commonly seen after spilling red wine. The mechanism behind the formation of these and similar rings is known as the coffee ring effect or in some instances, the coffee stain effect, or simply ring stain.
Dip coating is an industrial coating process which is used, for example, to manufacture bulk products such as coated fabrics and condoms and specialised coatings for example in the biomedical field. Dip coating is also commonly used in academic research, where many chemical and nano material engineering research projects use the dip coating technique to create thin-film coatings.
Layer-by-layer (LbL) deposition is a thin film fabrication technique. The films are formed by depositing alternating layers of oppositely charged materials with wash steps in between. This can be accomplished by using various techniques such as immersion, spin, spray, electromagnetism, or fluidics.
Graphite oxide (GO), formerly called graphitic oxide or graphitic acid, is a compound of carbon, oxygen, and hydrogen in variable ratios, obtained by treating graphite with strong oxidizers and acids for resolving of extra metals. The maximally oxidized bulk product is a yellow solid with C:O ratio between 2.1 and 2.9, that retains the layer structure of graphite but with a much larger and irregular spacing.
Sarfus is an optical quantitative imaging technique based on the association of:
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 that liquid's Brewster angle, in such a way for the microscope to catch an image of any light reflected from the light source via the liquid surface. Because there is no p-polarized reflection from the pure liquid when both are angled towards it at the Brewster angle, light is only reflected when some other phenomenon such as a surface film affects the liquid surface. The technique was first introduced in 1991.
Nanoparticles are classified as having at least one of three dimensions be in the range of 1-100 nm. The small size of nanoparticles allows them to have unique characteristics which may not be possible on the macro-scale. Self-assembly is the spontaneous organization of smaller subunits to form larger, well-organized patterns. For nanoparticles, this spontaneous assembly is a consequence of interactions between the particles aimed at achieving a thermodynamic equilibrium and reducing the system’s free energy. The thermodynamics definition of self-assembly was introduced by Nicholas A. Kotov. He describes self-assembly as a process where components of the system acquire non-random spatial distribution with respect to each other and the boundaries of the system. This definition allows one to account for mass and energy fluxes taking place in the self-assembly processes.
A nanosheet is a two-dimensional nanostructure with thickness in a scale ranging from 1 to 100 nm.
In nanobiotechnology, a peptoid nanosheet is a synthetic protein structure made from peptoids. Peptoid nanosheets have a thickness of about three nanometers and a length of up to 100 micrometers, meaning that they have a two-dimensional, flat shape that resembles paper on the nanoscale.
Nanosphere lithography (NSL) is an economical technique for generating single-layer hexagonally close packed or similar patterns of nanoscale features. Generally, NSL applies planar ordered arrays of nanometer-sized latex or silica spheres as lithography masks to fabricate nanoparticle arrays. NSL uses self-assembled monolayers of spheres as evaporation masks. These spheres can be deposited using multiple methods including Langmuir-Blodgett, Dip Coating, Spin Coating, solvent evaporation, force-assembly, and air-water interface. This method has been used to fabricate arrays of various nanopatterns, including gold nanodots with precisely controlled spacings.
Transition-metal dichalcogenide (TMD or TMDC) monolayers are atomically thin semiconductors of the type MX2, with M a transition-metal atom (Mo, W, etc.) and X a chalcogen atom (S, Se, or Te). One layer of M atoms is sandwiched between two layers of X atoms. They are part of the large family of so-called 2D materials, named so to emphasize their extraordinary thinness. For example, a MoS2 monolayer is only 6.5 Å thick. The key feature of these materials is the interaction of large atoms in the 2D structure as compared with first-row transition-metal dichalcogenides, e.g., WTe2 exhibits anomalous giant magnetoresistance and superconductivity.
A rapidly increasing list of graphene production techniques have been developed to enable graphene's use in commercial applications.
Liquid marbles are non-stick droplets wrapped by micro- or nano-metrically scaled hydrophobic, colloidal particles ; representing a platform for a diversity of chemical and biological applications. Liquid marbles are also found naturally; aphids convert honeydew droplets into marbles. A variety of non-organic and organic liquids may be converted into liquid marbles. Liquid marbles demonstrate elastic properties and do not coalesce when bounced or pressed lightly. Liquid marbles demonstrate a potential as micro-reactors, micro-containers for growing micro-organisms and cells, micro-fluidics devices, and have even been used in unconventional computing. Liquid marbles remain stable on solid and liquid surfaces. Statics and dynamics of rolling and bouncing of liquid marbles were reported. Liquid marbles coated with poly-disperse and mono-disperse particles have been reported. Liquid marbles are not hermetically coated by solid particles but connected to the gaseous phase. Kinetics of the evaporation of liquid marbles has been investigated.
Two dimensional hexagonal boron nitride is a material of comparable structure to graphene with potential applications in e.g. photonics., fuel cells and as a substrate for two-dimensional heterostructures. 2D h-BN is isostructural to graphene, but where graphene is conductive, 2D h-BN is a wide-gap insulator.
Emilie Ringe is an American chemist who is an Assistant Professor at the University of Cambridge. She was selected by Chemical & Engineering News as one of its "Talented Twelve" young scientists in 2021.