Nanoparticles and Plasmons are different entities but equally important in Physics as well as Chemistry.
What are Nanoparticles?
Nanoparticles are defined as particles of matter that are between 1 to 100 nanometers(10^-9m). Most nanoparticles are a combination of a few hundred atoms. These particles exhibit extremely different physical and chemical properties compared to their larger ones. For example, the bending of bulk copper occurs with the movement of copper atoms on about a 50 nm scale. In contrast, copper nanoparticles less than 50nm are considered as the super hard material that does not exhibit the same malleability and ductility as of the bulk copper metal.
Free Nanoparticles
Free nanoparticles are formed through the breaking down of larger particles or by the controlled assembly processes. The key features of nanoparticles are their large surface area to volume ratio, which is because as the size becomes smaller, the volume decreases more rapidly than the surface area.
Nanoparticles are more of an interest due to their chemical reactivity and optical behavior.
These particles can be found naturally in volcanic ash, ocean spray, fine sand, biological matters like viruses, etc. In addition they are also classified as hard (e.g. titania[titanium dioxide], silica[silica dioxide] and fullerenes) and soft (e.g.liposomes, vesicles and nanodroplets). The main uses of nanoparticles can be referred to in chemical reactions as catalysts (as more atoms of the particle are at the surface and able to Interact with the reactants), medicine to deliver drugs (successful transport and efficient release of drugs to the specific cell).
Types of Nanoparticles
Nanoparticles can be classified into carbon-based nanoparticles, ceramic nanoparticles, metal nanoparticles, semiconductor nanoparticles, polymeric nanoparticles, and lipid-based nanoparticles according to their size, morphology, physical and chemical properties.
Carbon-Based Nanoparticles
They mainly include carbon nanotubes and fullerenes. Carbon nanotubes are graphene sheets rolled into tubes. They are unique with the property that they are thermally conductive along the length and nonconductive across the tube. Fullerenes are allotropes of carbon having a hollow cage (or hollow football) structure with sixty or more carbon atoms. They have a commercial application with their electrical conductivity, structure, high strength, and electron affinity.
Ceramic Nanoparticles
They are inorganic solids made of oxides, carbides, carbonates, and phosphates. These particles have high heat resistance and chemical inertness. Their application in drug delivery is useful for bacterial infection, glaucoma, cancer, etc. They are also used as photocatalysis, photodegradation of dyes, and imaging.
Metal Nanoparticles
These are prepared from metals by chemical, electrochemical, or photochemical methods. These nanoparticles have applications in the research, detection, imaging of biomolecules, environmental, and bioanalytical applications.
Semiconductor Nanoparticles
They have properties like metals and non-metals. These particles have wide band gaps, which on tuning show different properties. These are used in photocatalysis, electronic devices, photo optics, and water splitting.
Polymeric Nanoparticles
They are organic-based nanoparticles. Depending on the preparation method, they have nanocapsules or nanosphere shape. Nanosphere particle has a matrix-like structure, whereas nanocapsule particles have core-shell morphology. The drug deliveries with polymeric nanoparticles are highly biodegradable and biocompatible.
Lipid-based Nanoparticles
It consists of a solid core made of lipid and a matrix containing a soluble lipophilic molecule. These particles are used in the biomedical field as drug carrier and RNA release in cancer therapy.
Plasmons
Plasmons are quantum of plasma oscillation, i.e., it is the minimum amount of any particle, with the physical property involved in the interaction of a rapid oscillation in an electron density.
Just as optical oscillation or light consists of photons, plasma oscillation consists of plasmons.
Drude Model
In detail, let us consider a metal with a large number of free electrons. If we apply the kinetic theory of gases (Drude model) to the metal, and take the electrons in metal as electron gas moving freely, to compensate for the cloud of negative charges, there should be positive charges. Drude considered the positive charges as immobile particles or positive ion core right. Now, to compensate this electric field, electrons will move towards left uncovering the positive ions which cancel the applied electric field.
But this displacement of the electric gas towards the left will create a force in opposite (right) direction due to the net interaction between electron gas and positive ions. Since the applied electric field is canceled, the electrons will move towards the right and this way making the electron gas to oscillate.
Plasma Oscillation
This oscillation of electron gas is called plasma oscillation, and quanta of this charge density are called plasmons.
Surface plasmons are originated from the coupling between photons and free electrons.
More surface plasmons can be seen in metals, with an abundant amount of free electrons like gold and silver. Later, heavily doped semiconductors and 2D materials are also demonstrated to exhibit abundant plasmonic responses.
The plasmonic frequency is given by
where N is the density of carriers (electrons or holes), m* is the effective mass of the carrier.
Plasmonic Nanoparticle
Plasmonic Nanoparticles are particles whose electron density can interact with the electromagnetic radiation or visible range of light of a wavelength that is larger than the size of the particle, to exhibit different chemical, mechanical and optical properties.
Let us take an example of gold. When a bulk amount of gold interacts with electromagnetic waves, there is not much difference in the characteristic features of gold; it has the same color.
Now consider a nanoparticle of gold interacting with an electromagnetic wave, the size of the particle is minute as compared to the wavelength of the wave. In the wave, we have oscillating electric and magnetic fields, and in the nanoparticle, we have free electrons.
This interaction leads to the delocalization of the electron clouds and gives rise to new types of material properties. As we take the nanoparticles with different sizes and shapes, we get different colors for the particles.
As illustrated in the figure, the gold nanoparticles of a spherical, short rod, medium rod, long rod are taken here; we can see that all these gold particles have different colors. The other importance of surface plasmons is a significant local enhancement of the electromagnetic field at some place of the illuminated nanoparticles. Gold and Silver nanoparticles for their plasmonic properties have aided the development of sensors for chemical and environmental analysis. It is broadly used in the field of science monic solar cells, spectroscopy, cancer treatment, etc.
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Featured Image: Nanoparticles and Plasmons
Nanoparticle Plasmon Resonance
M.Sc Physics, Mysore India
