Plasmon resonance. Optical properties of nanoparticles Localized scattering and absorption resonances in nanoparticles

, polariton , plasmon , nanophotonics Definition plasmon resonance (in the case of nanosized metal structures - localized plasmon resonance) is the excitation of a surface plasmon at its resonant frequency by an external electromagnetic wave.

Description

The surface plasmon is not directly related to electromagnetic radiation in the environment adjacent to the metal, since its speed is less than the speed of light. A technique that allows the use of surface plasmons in optics is based on the use of total internal reflection. In total internal reflection, an electromagnetic wave propagates along a light-reflecting surface, the speed of which is less than the speed of light and depends on the angle of incidence. If, at a certain angle of incidence, the speed of this wave coincides with the speed of the surface plasmon on the metal surface, then the conditions for total internal reflection will be violated, and the reflection will cease to be complete, and a surface plasmon resonance will arise.

In nanosized metal systems, modification of collective electronic excitations occurs. Collective electronic excitation of metal nanoparticles whose size is smaller than the wavelength of electromagnetic radiation in the environment - localized surface plasmon - oscillates at a frequency less than the frequency of the bulk plasmon by a factor of about 3, while the frequency of the surface plasmon is approximately 2 times less than the frequency of the bulk plasmon plasmon.

Currently, the phenomenon of surface plasmon resonance is widely used in the creation of chemical and biological sensors. When in contact with biological objects (DNA, viruses, antibodies), plasmonic nanostructures make it possible to increase the intensity of fluorescence signals by more than an order of magnitude, i.e. significantly expand the capabilities of detection, identification and diagnosis of biological objects.

Naimushina Daria Anatolyevna

Links

Perlin E.Yu., Vartanyan T.A., Fedorov A.V. Solid state physics. Optics of semiconductors, dielectrics, metals: Textbook. - St. Petersburg: St. Petersburg State University ITMO, 2008. - 216 p.
Pompa P.P., Martiradonna L. et al. Metal-enhanced fluorescence of colloidal nanocrystals with nanoscale control // Nature Nanotechnology - vol. 1, 2006 - P. 126 -130
Nashchekin A.V. and others. Biosensors based on surface plasmon resonance // Collection of abstracts of sectional reports, poster presentations and reports of participants in the competition of scientific works of young scientists - Second International Forum on Nanotechnology, 2008

Illustrations Tags Sections Methods for diagnostics and research of nanostructures and nanomaterials
The science

Encyclopedic Dictionary of Nanotechnologies. - Rusnano. 2010 .

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When electromagnetic radiation interacts with metal nanoparticles, the mobile conduction electrons of the particles are displaced relative to the positively charged metal ions of the lattice. This displacement is collective in nature, in which the movement of electrons is consistent in phase. If the particle size is much smaller than the wavelength of the incident light, then the movement of electrons leads to the appearance of a dipole. As a result, a force arises that tends to return the electrons to the equilibrium position. The magnitude of the restoring force is proportional to the magnitude of the displacement, as for a typical oscillator, so we can talk about the presence of a natural frequency of collective oscillations of electrons in the particle. If the frequency of oscillations of the incident light coincides with the natural frequency of oscillations of free electrons near the surface of a metal particle, a sharp increase in the amplitude of oscillations of the “electron plasma” is observed, the quantum analogue of which is a plasmon. This phenomenon is called surface plasmon resonance (SPR). A peak appears in the light absorption spectrum. For noble metal particles with a size of the order of 10-100 nm, SPR is observed in the visible region of the spectrum and in the near-infrared range. Its position and intensity depend on the size, shape of nanoparticles and the local dielectric environment. Spherical silver nanoparticles with a diameter of 10-25 nm have an absorption peak near 400-420 nm (Fig. 1a), spherical gold nanoparticles - 520 nm, copper (I) oxide nanoparticles - 450-700 nm.

Nanorods have anisotropic symmetry, and therefore two peaks are observed in the absorption spectrum, corresponding to the transverse and longitudinal plasmons. The transverse plasmon gives an absorption peak at 400 nm, and the longitudinal one can appear in the range from 500-1000 nm, i.e. V

near infrared region. Its position is determined by the dimensional factors of the nanorod, namely the ratio of length to width.

λ, nm

Fig.1a Optical absorption spectrum of silver nanoparticles

Fig.1b Optical absorption spectrum of rod-shaped silver nanoparticles

Experimental part Processing and presentation of laboratory results

The report must provide:

Scheme and equation of the reaction for the synthesis of nanoparticles

Records of solution color changes during synthesis

Records of the influence (or lack of influence) of the concentration of a reducing agent and/or stabilizer on the size and stability of the resulting nanoparticles

Absorption spectrum of a solution of nanoparticles

Conclusions about the shape and size of nanoparticles in the synthesized solution

Laboratory work No. 1 Obtaining Ag nanoparticles using the citrate method

This method allows one to obtain relatively large silver particles with a diameter of 60-80 nm. Absorption maximum 420 nm.

Reagents and equipment

Reagents: 0.005M solution of silver nitrate AgNO 3, sodium citrate Na 3 C 6 H 5 O 7 ∙6H 2 O (1% solution), distilled water.

Equipment: scales, spectrophotometer, quartz cuvettes with an optical path length of 1 cm, 200 ml flasks, 50 ml beakers, heated stirrer, graduated cylinder.

Work order

Prepare a 0.005M (0.085%) solution of AgNO 3 in water. To do this, dissolve 0.0425 g of the substance in 50 ml of distilled water.

Transfer 25 ml of the prepared solution into a flask and add 100 ml of water.

Prepare a 1% sodium citrate solution by dissolving 0.5 g of it in 50 ml of water.

Heat 125 ml of the resulting silver nitrate solution to a boil on a hotplate with a stirrer.

As soon as the solution begins to boil, add 5 ml of 1% sodium citrate solution into it.

Heat the solution until the color turns pale yellow.

Allow the solution to cool to room temperature with the stirrer running.

Bring the volume of the solution, which has decreased due to boiling, to 125 ml with water.

Record the absorption spectrum of the resulting colloidal solution in the range of 200 – 800 nm. Use water as a reference solution.

Take the absorption spectrum after a day or a week.

Add 5 ml of diluted HCl dropwise to 5 ml of a solution of the obtained silver nanoparticles.

Repeat the experiment with acetic acid CH 3 COOH. Observe the gradual dissolution of silver nanoparticles and the formation of a white precipitate when adding hydrochloric acid and discoloration of the solution when adding acetic acid. Write down conclusions, observations and reaction equations in your notebook. Optical absorption spectroscopy is one of the oldest methods for the physicochemical analysis of biomolecules. However, its low sensitivity and spatial resolution do not allow studying processes involving low protein concentrations. Scientists from Berkeley managed to “extend the life” of the optical method by combining it with another principle used in biophysical and biochemical research - plasmon resonance. It turned out that specific “dips” may appear in the elastic scattering spectrum of gold nanoparticles introduced into a cell, corresponding to the frequencies at which some biological molecules (for example, metalloproteins) absorb. Researchers call this effect migration of plasmon resonance energy.

and explain it by the direct interaction of gold particles with protein molecules adsorbed on them. The proposed method has unprecedented sensitivity: it can be used to determine, if not single protein molecules, then at least their tens Optical spectrometry allows you to study proteins that have optical density in the visible range of electromagnetic radiation (chromoproteins) by measuring light absorption at certain ( "characteristic" for specific molecules) wavelengths. However, such measurements require rather high concentrations of protein, and the spatial resolution of this method is very low (usually solutions of molecules located in spectrometric cuvettes are studied, and there is simply no question of where exactly in the cell the molecules being studied are located). Measurement-based methods are much more sensitive

The methodology proposed by Berkeley scientists (article published in the journal Nature Methods) is based on the introduction of nanoscopic gold particles of controlled size (20–30 nm) into living cells. Electrons on the surface of particles made of metals such as gold or silver collectively oscillate in response to irradiation with light of a specific wavelength - a phenomenon known as plasmon resonance(see sidebar). The resonant frequencies of these nanoparticles are much easier to register than the weak (due to very low concentrations) optical signal from biological molecules, which makes it possible to carry out measurements.

We stood on the plane
With variable reflection angle,
Watching the law
Setting landscapes in motion.

Repeating the words
Devoid of all meaning
But without tension,
No tension.
B.G.

Literature

Gang Logan Liu, Yi-Tao Long, Yeonho Choi, Taewook Kang, Luke P Lee. (2007). Quantized plasmon quenching dips nanospectroscopy via plasmon resonance energy transfer. Nat Methods. 4 , 1015-1017;
New nanoparticle technique captures chemical reactions in single living cell with amazing clarity. (2007). ScienceDaily.

Metal nanoparticles exhibiting surface plasmon resonance at the metal-dielectric interface have great potential for use as highly sensitive sensors for biological and medical research. Surface plasmon resonance occurs at the interface between metal and dielectric. The frequency of surface plasmon resonance depends on both the dielectric constants of the adjacent surfaces and the shape of the surface. The dependence of the position of the maximum of surface plasmon resonance on the geometric dimensions of nanoparticles makes it possible to produce nanoparticles for biological research, the resonant frequency of which coincides with the natural frequency of vibrations of various biological reagents. In this work, we consider spherical nanoparticles consisting of a semiconductor core surrounded by a metal shell. The complex dependence of the dielectric constant of a semiconductor on frequency can lead to the appearance of additional surface plasmon resonance at various frequencies. The work considers the dielectric constant tensor of a semiconductor and takes into account the frequency dependence of the tensor components. The dielectric constant of a metal is calculated using the Drude formalism. The work calculates the absorption cross section of nanoparticles with a gold shell and shows that the dependence of the components of the semiconductor dielectric constant tensor on the external magnetic field makes it possible to change the position of the surface plasmon resonance maximum by changing the magnetic field. Thus, it has been shown that nanoparticles with a semiconductor core and a metal shell can serve as sensors for different biomolecules depending on the magnetic field strength

nanoparticles

surface plasmon resonance

semiconductor

Drude model

1. Bass, F.G. High-frequency properties of semiconductors with superlattices / F.G. Bass, A.A. Bulgakov, A.P. Tetervov – M: Nauka, 1989. – 288 p.

2. Boren, K. Absorption and scattering of light by small particles / K. Boren, D. Huffman. – M.: Mir, 1986. – 340 p.

3. Golovkina M.V. Reflection of an electromagnetic wave from a superconductor-semiconductor system / M.V. Golovkina // Modern high technology. 2009. No. 8. P. 8-10.

4. Dykman L.A. Biomedical application of multifunctional gold nanocomposites / L.A. Dykman, N.G. Khlebtsov // Advances in biological chemistry. – 2016. – T. 56. – P. 411-450.

5. Klimov, V.V. Nanoplasmonics / V.V. Klimov. – M.: Fizmatlit, 2009. – 480 p.

6. Synthesis of magnetite-gold nanoparticles having a core-shell structure / P.G. Rudakovskaya [and others] // Bulletin of Moscow University. Series 2. Chemistry. – 2015. –T. 56. – No. 3 – P. 181-189.

7. Golovkina M.V. Periodic semiconductor structures with metamaterials. Proceedings International Siberian Conference on Control and Communications, SIBCON-2009. Tomsk, 2009. – pp. 133-137.

Nanoparticles and nanocomposite structures based on them have recently increasingly come under the close attention of scientists and engineers. The advances that nanoparticle manufacturing technology has currently achieved make it possible to produce nanoparticles with a radius of several nanometers, spherical and ellipsoidal in shape, as well as nanoparticles with a complex structure with a shell. Nanoparticles and nanocomposite materials based on them are used for the manufacture of solid-state photonics and optoelectronics devices, such as filters, amplifiers, and can also be used as highly efficient, highly sensitive sensors. Metal nanoparticles or nanoparticles with a metal shell that have surface plasmon resonance are used in various fields of science and technology, such as chemistry, physics, biology, medicine, nanotechnology, biotechnology, etc. Magnetic nanoparticles also have great potential for use in biology and medicine. Due to their high specific magnetization and the ability to bind to the surface of biological molecules, iron oxide-based magnetic nanoparticles are a promising material for the development of selective contrast agents for magnetic resonance imaging. To solve biochemical problems of varying degrees of complexity, a combination of magnetic properties and special surface properties that are observed in nanoparticles is required. Magnetite nanoparticles functionalized with biomolecules (antibodies, enzymes, nucleotides, etc.) to target or recognize biological systems can be used as materials for targeted drug delivery.

Core-shell nanoparticles, which have the ability to combine the properties of several materials in a single particle, deserve special attention. Thus, the use of magnetite as a core for core-shell materials avoids all the disadvantages that colloidal solutions of iron oxide nanoparticles exhibit. They are toxic, exhibit a tendency to rapid aggregation in various biological solutions, and are difficult to functionalize the surface. To overcome these disadvantages, shelled nanoparticles can be used. Inorganic materials are used as shells to ensure stability, ease of surface functionalization, and biocompatibility. A suitable and optimal material for the formation of nanoparticle shells is gold, which is biocompatible and highly stable.

Colloidal solutions of small gold particles have been used for medicinal purposes since ancient times. However, only in recent decades, thanks to the emergence of new data on the unique optical and physicochemical properties of gold nanoparticles, their active use for various biological and medical purposes began, both in experimental biology and medicine, and in practice.

Interest in gold and other noble metal particles (such as silver nanoparticles) stems from their unique optical properties associated with the excitation of localized plasmon resonances in metal nanoparticles interacting with light. These excitations of surface plasmons lead to a whole class of plasmon-enhanced linear properties, such as resonant absorption, scattering, generation of strong local fields, giant Raman scattering.

Nanotechnology, which is used in biomedical research, uses gold and silver nanoparticles that adsorb various biological reagents well. Thus, biological macromolecules, molecules that work as a probe, for example, antibodies, can be attached to metal nanoparticles. Nanostructures containing a nanoparticle as a core with biological molecules attached to them are called bioconjugates or conjugates. In this case, the attachment of biomacromolecules to nanoparticles is called functionalization. In such a case, the biomacromolecule of the conjugate is used to attach to the biological entity that is the target. Gold nanoparticles are also widely used in biomedicine due to their good biocompatibility, low chemical reactivity, and good functionalization. However, some metals, such as silver, can exhibit high chemical reactivity, so such metal nanoparticles, which operate on the basis of surface plasmon resonance, must be surrounded by a dielectric shell that will protect them from contact with the biological analyte. Thus, shelled nanoparticles are suitable for practical applications: dielectric nanoparticles with a gold shell, which provide good functionalization, or metal nanoparticles with a dielectric shell, which prevents chemical contact with the analyte. However, it must be remembered that an increase in the thickness of the dielectric shell of nanoparticles leads to a decrease in sensitivity, that is, to a decrease in the shift of the plasmon resonance maximum when the refractive index of the analyzed solution changes. Therefore, an important task is to optimize the parameters of nanoparticles and select their geometric dimensions and shell thickness, which will increase the sensitivity of the spectral shift of the surface plasmon resonance maximum.

For biomedical purposes, semiconductor nanoparticles are used, which can be used as luminescent probes or labels, which are used, for example, for fluorescence tomography. Moreover, the use of semiconductor nanoparticles with a shell made of a wider-gap semiconductor leads to increased luminescence.

This work considers spherical nanoparticles with a semiconductor core surrounded by a metal shell. The dielectric constant of a metal is considered within the framework of the Drude model and is expressed by the following formula:

where ep is the lattice part of the dielectric constant,

wр - plasma frequency for metal,

g is the collision frequency for the metal. The collision frequency determines the presence of attenuation in the medium.

The dielectric constant tensor of a semiconductor is written in the known form:

, (2)

where the components of the dielectric constant tensor have the following form:

Here wpp is the plasma frequency for the semiconductor,

wc - cyclotron frequency,

n is the collision frequency for the semiconductor,

e0р is the lattice part of the dielectric constant for a semiconductor.

The results of calculating the frequency dependence of the dielectric constant tensor components of the model semiconductor are presented in Figures 1 and 2.

From Figures 1 and 2 it is clear that the dependence of the dielectric constant tensor components e^ and ea on frequency is complex. At a certain frequency, depending on the cyclotron frequency wс, a change in sign of the tensor components is observed. If the core of the nanoparticle is made of a semiconductor, then such a change in the sign of the components e^ and ea leads to a change in the frequency of surface plasmon resonance at the interface with the metal shell and even to the emergence of new frequencies of surface plasmon resonance.

Fig.1. Graph of the dependence of the tensor component e^ of the dielectric constant of a semiconductor on frequency. Solid curve: wс=1.5×1014 rad/s, dotted line: wс=2×1014 rad/s, long dotted line: wс=4×1014 rad/s

Fig.2. Graph of the dependence of the tensor component ea of the dielectric constant of a semiconductor on frequency. Solid curve: wс=1.5×1014 rad/s, dotted line: wс=2×1014 rad/s, long dotted line: wс=4×1014 rad/s

The cyclotron frequency for a semiconductor is calculated as follows (in the CGS system):

where H is the strength of the external magnetic field.

Since the cyclotron frequency of a semiconductor depends on the magnitude of the external magnetic field, by changing the magnetic field, it is possible to change the frequency of surface plasmon resonance in nanoparticles with a metal shell. Thus, nanoparticles with a shell and a semiconductor core have the following important property: the electrodynamic parameters of such nanoparticles can be controlled by changing the magnitude of the external magnetic field.

Finding the position of the maximum of surface plasmon resonance, which is observed at the spherical semiconductor-metal interface, is a complex problem that does not have an analytical solution. To numerically determine the frequency of surface plasmon resonance, we will calculate the absorption cross section of a nanoparticle with a metal shell in accordance with the method described in the works. The results of calculating the absorption cross section are presented in Figure 3.

Fig.3. Calculation of the absorption cross section of a semiconductor nanoparticle with a gold shell. The core radius is 27 nm, the shell thickness is 17 nm. Solid curve: wс=1.5×1014 rad/s, dotted line: wс=2×1014 rad/s

From Figure 3 it is clearly seen that an increase in the cyclotron frequency of the semiconductor from 1.5 × 1014 rad/s to 2 × 1014 rad/s, observed with an increase in the external magnetic field, leads to a shift in the maximum of the surface plasmon resonance from 1.35 μm to 1. 58 microns.

The work examines shelled nanoparticles that can be used as sensors for biological and medical research. Nanoparticles consisting of a semiconductor core and a metal shell are considered. Taking into account the dielectric constant of the metal, calculated within the framework of the Drude model, and the frequency dependence of the components of the dielectric constant tensor of the semiconductor, calculations of the absorption cross section of the considered nanoparticles with a gold shell were carried out. It has been shown that a change in the magnetic field affects the position of the surface plasmon resonance in nanoparticles. The results obtained in this work can be used to create contrast agents for magnetic resonance imaging or as biological markers.

Bibliographic link

Orkina V.E., Golovkina M.V. CALCULATION OF PARAMETERS OF NANOPARTICLES WITH A SHELL FOR BIOLOGICAL RESEARCH // International Student Scientific Bulletin. – 2018. – No. 2.;
URL: http://eduherald.ru/ru/article/view?id=18408 (access date: 12/17/2019). We bring to your attention magazines published by the publishing house "Academy of Natural Sciences"

What are nanoparticles?
Features of optical processes occurring on nanometer scales
Spectral properties of semiconductor particles
Spectral properties of metal particles
Hybrid nanoparticles and their spectral properties

Terms used

Differential scattering cross section – physical quantity equal to the ratio of the number of particles scattered per unit time per unit solid angledΩ , to the flux density of incident particles
Total scattering cross section – is the differential scattering cross section integrated over the full solid angle
Absorption rate – the reciprocal of the distance at which the flux of monochromatic radiation forming a parallel beam decreases as a result of absorption in the medium ine once

What's happened nanoparticles?

Nanoparticles refer to objects ranging in size from a few nanometers to several hundred nanometers. As a rule, these are either nanometer-scale crystals ( nanocrystals), or large molecules

1 – fullerene C 60; 2 – single-layer semiconductor quantum dot; 3 – quantum dot of the “core-shell” type; 4 – TEM image of gold nanoparticles; 5 – TEM image of silver nanoparticles.

Quantum dots

We will mainly consider the special case of nanoparticles - quantum dots. Quantum dot is a crystal in which the movement of charge carriers (electrons or holes) is limited in all three dimensions. A quantum dot is made up of hundreds of atoms!

Currently, chemists are able to synthesize quantum dots of a wide variety of compositions. The most common quantum dots are cadmium-based (e.g. CdSe).

Nanooptics studies the physical properties, structure and methods of creating light fields localized on nanometer scales.
Traditional optics and laser physics deal with light fields in the far (wave) zone R" λ.
Specifics of the optical range– dipole approximation emitter size a« λ → a ~0.1 – 1 nm; λ ~0.2 – 1 µm (UV – IR).
Near field optics (subwavelength optics) deals with fields at distances from the source (object)R« λ (down to several nm).
In such conditions, in addition to ordinary (propagating) waves, localized (evanescent) waves must be taken into account! This is especially important when considering particle ensembles !

Taking into account near-field interaction leads to a qualitative change in the behavior of fields

Taking into account the influence of localized fields leads to the possibility of propagation of light whose polarization is directed along the direction of propagation. Such waves (called longitudinal) are not taken into account in conventional optics. However, when working with nanometer-sized objects, the intensities of such waves can exceed the intensities of conventional (transverse) electromagnetic waves.

The simplest nanophotonic splitter

Left: Polarization in direction X, along wave propagation

On right: Polarization in direction Y, across wave propagation

Features of optical processes occurring on nanometer scales

The influence of localized fields must be taken into account
Electromagnetic fields near nanostructures differ significantly from fields in free space and in bulk materials
These circumstances are especially important when considering effects occurring near the boundaries of nanostructures, as well as during the interaction of closely located nanoparticles
Localized fields exist in limited parts of space, but the intensities of such fields can be significant, which can lead to the occurrence of nonlinear optical phenomena
If the nanoobjects under study have sizes less than 10 nm, quantum effects may begin to play a role, leading to the inapplicability of the concept of dielectric constant

Spectral properties of semiconductor nanoparticles

In a bulk material, an electron can occupy any unoccupied position in the conduction band. The spectrum of photons emitted when an electron returns to the valence band is continuous.
In a quantum dot, there is a spatially limited decrease in the bottom of the conduction band and an increase in the top of the valence band. Due to the laws of quantum mechanics, the permissible energy levels of the electron form a discrete spectrum.

Energy levels in a quantum dot

The energy levels of the electron and hole are inversely proportional to the square of the width of the quantum dot! By choosing different sizes and shapes of quantum dots, you can make them emit or absorb light given wavelength. This allows using the same material, but different sizes and shapes, create light sources emitting in a given spectral range!

Emission spectra of quantum dots

Dependence of the fluorescence of CdSe/ZnS core-shell quantum dots irradiated with light with l = 470 nm on the core radius.

Normalized emission spectra of In(Ga)As quantum dots placed in a GaAs matrix.

As in the case of semiconductor nanoparticles, the spectral properties of metal particles depend significantly on their size and shape. However, unlike semiconductors, in the case of metals this phenomenon is mainly associated with excitation plasmons . When light interacts with electrons, which can move freely throughout the metal, the position of the electrons relative to the position of the ions in the crystal lattice begins to oscillate with a plasma frequency ωp. Quanta of plasma oscillations are called plasmons .

In the case of interaction of light with the surface of a metal, the electromagnetic wave penetrates into the metal only over very short distances (less than 50 nm for silver and gold), so the main contribution to the vibrations is made by electrons located near the surface. Their collective vibrations are called propagating surface plasmons . If free electrons are limited to a certain finite volume of the metal (which is the case with metal nanoparticles), the vibrations are localized, and their quanta are called localized surface plasmons .

Plasmon resonance

If plasmon oscillations excited in different parts of the crystal interfere constructively, the phenomenon occurs plasmonic resonance . In this case, the extinction cross section (absorption + scattering) increases significantly. The position of the peak in the spectrum, as well as its magnitude, significantly depend on the shape of the particle and its size.

Modes of plasmon oscillations excited by irradiation of a nano-triangle with a beam of electrons with different energies. Depending on the energy, the field maxima appear in the corners, near the centers of the faces and in the center of the triangle

Dependence of the spectra of metal nanoparticles on their shape and size

Maximums in the scattering spectra for various metal nanoparticles: a) silver nanoprisms; b) gold beads with a size of 100 nm; c) gold beads with a size of 50 nm; d) silver beads with a size of 100 nm; e) silver beads with a size of 80 nm; f) silver beads with a size of 40 nm.

Dependence of the extinction spectrum of silver nanoparticles on the particle shape.

Spectral properties of metal particles

The spectral properties of metal nanoparticles are associated with the phenomenon of resonance of localized surface plasmons
The position, magnitude and shape of the extinction spectra of metal nanoparticles depend on the shape and size of the nanoparticles
By varying the size and shape of a metal nanoparticle, we can ensure that the maximum extinction cross section falls into the desired spectral range
Using this property, it is possible to significantly increase the efficiency of solar cells due to the absorption of different parts of the solar spectrum by different nanoparticles

Hybrid nanoparticles

Hybrid nanoparticles consist of various materials, such as metal and semiconductor. Since the properties of different materials change differently as the size decreases, when describing the optical properties of hybrid nanoparticles it is necessary to take into account the interaction between the various components that make up the nanoobject.

Let us consider the optical properties of hybrid nanoparticles using the example of metal-organic nanoparticles of the “core-shell” type, consisting of a metal core and a dye shell in the so-called aggregate state.

Relative position of unperturbed plasmon resonance peaks of the core (Ag and Au) and the exciton peak of the dye J-aggregate shell (TC, OC, PIC)

Typical light absorption spectra of Ag/J-aggregate and Au/J hybrid nanoparticles-unit

Dependence of the nature of the photoabsorption spectra of Ag/J-aggregate hybrid nanoparticles ( peak positions and intensities) on the thickness of the outer shell of the dye at a fixed core radius

Shell thickness: ℓ=2 nm (1); ℓ= 4 nm (2); ℓ= 6 nm (3); ℓ= 8 nm (4); ℓ= 10 nm (5); ℓ=12 nm (6). The radius of the nanoparticle core does not change: r= 30 nm

Dependence of the optical properties of hybrid nanoparticles on their shape

Object of study: 2-layer spheroidal nanoparticles with a metal core (Ag, Au), coated with a J-aggregate of cyanine dye.

Dependence of the absorption spectrum of Ag/J-aggregate composite systems on geometric parameters

Spectral properties of hybrid nanoparticles

The spectral properties of hybrid particles differ significantly from the properties of the components that make up the nanoparticle
The interaction of nanoparticle components can lead to a shift in the position of peaks in the absorption cross sections, the appearance of new peaks, and also a change in the peak values of the absorption cross sections
The positions and number of peaks in the absorption cross sections depend on the shape of the nanoparticle
For non-spherical particles, the positions of absorption maxima depend on the polarization of the incident radiation
By choosing various geometric parameters of a hybrid nanoparticle, it is possible to achieve a shift of absorption peaks to the desired spectral region, which opens up the possibility of controlling the spectral properties of hybrid nanoparticles

conclusions

The optical properties of nanoparticles are radically different from the properties of bulk material
For almost all nanoparticles, the spectral characteristics change significantly with changes in the shape and size of the particles
By varying the geometric parameters of nanoparticles, it is possible to achieve the required optical properties
When moving to consider ensembles of nanoparticles, it is necessary to take into account the interaction between individual particles
The spectral properties of hybrid nanoparticles differ from the properties of the components from which they are composed (the whole is not equal to the sum of the parts!)

Bibliography

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Y. Masumoto, T. Takagahara, Semiconductor Quantum Dots, Springer-Verlag Berlin Heidelberg New York, 2002
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