A localized surface plasmon (LSP) is the result of the confinement of a

surface plasmon Surface plasmons (SPs) are coherent delocalized electron oscillations that exist at the interface between any two materials where the real part of the dielectric function changes sign across the interface (e.g. a metal-dielectric interface, such ...

in a

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 10 ...

of size comparable to or smaller than the wavelength of

light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...

used to excite the

plasmon In physics, a plasmon is a quantum of plasma oscillation. Just as light (an optical oscillation) consists of photons, the plasma oscillation consists of plasmons. The plasmon can be considered as a quasiparticle since it arises from the quantiz ...

. When a small spherical metallic nanoparticle is irradiated by light, the oscillating electric field causes the conduction electrons to oscillate coherently. When the electron cloud is displaced relative to its original position, a restoring force arises from Coulombic attraction between electrons and nuclei. This force causes the electron cloud to oscillate. The oscillation frequency is determined by the density of electrons, the effective electron mass, and the size and shape of the charge distribution. The LSP has two important effects: electric fields near the particle's surface are greatly enhanced and the particle's optical absorption has a maximum at the

resonant frequency.

Surface plasmon resonance Surface plasmon resonance (SPR) is the resonant oscillation of conduction electrons at the interface between negative and positive permittivity material in a particle stimulated by incident light. SPR is the basis of many standard tools for measu ...

can also be tuned based on the shape of the nanoparticle. The plasmon frequency can be related to the metal dielectric constant. The enhancement falls off quickly with distance from the surface and, for

noble metal A noble metal is ordinarily regarded as a metallic chemical element that is generally resistant to corrosion and is usually found in nature in its raw form. Gold, platinum, and the other platinum group metals ( ruthenium, rhodium, palladium, o ...

nanoparticles, the resonance occurs at visible wavelengths. Localized surface plasmon resonance creates brilliant colors in metal colloidal solutions. For metals like silver and gold, the oscillation frequency is also affected by the electrons in d-orbitals. Silver is a popular choice in plasmonics, which studies the effect of coupling light to charges, because it can support a surface plasmon over a wide range of wavelengths (300-1200 nm), and its peak absorption wavelength is easily changed. For instance, the peak absorption wavelength of triangular silver nanoparticles was altered by changing the corner sharpness of the triangles. It underwent a blue-shift as corner sharpness of the triangles decreased. Additionally, peak absorption wavelength underwent a red-shift as a larger amount of reducing agent (HAuCl₄) was added and porosity of the particles increased. For semiconductor nanoparticles, the maximum optical absorption is often in the near-infrared and mid-infrared region.

Propagating surface plasmons

Localized surface plasmons are distinct from propagating surface plasmons. In localized surface plasmons, the electron cloud oscillates collectively. In propagating surface plasmons, the surface plasmon propagates back and forth between the ends of the structure. Propagating surface plasmons also need to have at least one dimension that is close to or longer than the wavelength of incident light. The waves created in propagating surface plasmons can also be tuned by controlling the geometry of the metal nanostructure.

Characterization and study of localized surface plasmons

A goal of plasmonics is to understand and manipulate surface plasmons at the nano-scale, so characterization of surface plasmons is important. Some techniques frequently used to characterize surface plasmons are dark-field microscopy, UV-vis-NIR spectroscopy, and surface-enhanced Raman scattering (SERS). With dark-field microscopy, it is possible to monitor the spectrum of an individual metal nanostructure as the incident light polarization, wavelength, or variations in the dielectric environment is changed.

Applications

Nanoparticles; scanning electron micrograph (5887833046)

The

resonant frequency is highly sensitive to the

refractive index In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, or ...

of the environment; a change in

results in a shift in the resonant frequency. As the resonant frequency is easy to measure, this allows LSP nanoparticles to be used for

nanoscale The nanoscopic scale (or nanoscale) usually refers to structures with a length scale applicable to nanotechnology, usually cited as 1–100 nanometers (nm). A nanometer is a billionth of a meter. The nanoscopic scale is (roughly speaking) a lo ...

sensing applications. Also, nanoparticles exhibiting strong LSP properties, such as gold

nanorod In nanotechnology, nanorods are one morphology of nanoscale objects. Each of their dimensions range from 1–100 nm. They may be synthesized from metals or semiconducting materials. Standard aspect ratios (length divided by width) are 3-5. Na ...

s, could enhance the signal in surface plasmon resonance sensing. Nanostructures exhibiting LSP resonances are used to enhance signals in modern

analytical technique Analytical technique is a method used to determine a chemical or physical property of a chemical substance, chemical element, or mixture. There is a wide variety of techniques used for analysis, from simple weighing to advanced techniques using high ...

s based on spectroscopy. Other applications that rely on efficient light to heat generation in the nanoscale are

heat-assisted magnetic recording Heat-assisted magnetic recording (HAMR) (pronounced "''hammer")'' is a magnetic storage technology for greatly increasing the amount of data that can be stored on a magnetic device such as a hard disk drive by temporarily heating the disk materia ...

(HAMR), photothermal cancer therapy, and thermophotovoltaics. So far, high efficiency applications using plasmonics have not been realized due to the high ohmic losses inside metals especially in the optical spectral range (visible and NIR)., Additionally surface plasmons have been used to create super lenses, invisibility cloaks, and to improve quantum computing. Another interesting area of research in plasmonics is the ability to turn plasmons "on" and "off" via modification of another molecule. The ability to turn plasmons on and off has important consequences for increasing sensitivity in detection methods. Recently, a supramolecular chromophore was coupled with a metal nanostructure. This interaction changed the localized surface plasmon resonance properties of the silver nanostructure by increasing the absorption intensity.

References

{{Reflist Nanoparticles Plasmonics Scattering, absorption and radiative transfer (optics)

Propagating surface plasmons

Characterization and study of localized surface plasmons

Applications

See also

References