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




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 "prolong the age" of the optical method by pairing it with another principle used in biophysical and biochemical research - plasmon resonance. It turned out that in the spectrum of elastic scattering on gold nanoparticles introduced into a cell, specific “dips” can appear, 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 dozens.

Optical spectrometry makes it possible to study proteins with optical density in the visible range of electromagnetic radiation (chromoproteins) by measuring the absorption of light at certain ( "characteristic" for specific molecules) wavelengths. However, such measurements require rather high protein concentrations, and the spatial resolution of this method is very low (usually, solutions of molecules in spectrometric cuvettes are studied, and there is simply no question of where exactly the studied molecules are located in the cell). Methods based on measuring are much more sensitive. fluorescence(together with confocal microscopy, they allow determining the location of molecules inside a living cell), but here it is necessary to modify the studied molecules with special marker molecules, which is not always desirable and possible. Another method often used in biology - nuclear magnetic resonance spectroscopy - also requires rather high protein concentrations and often - isotope labeling of an object that is difficult under the conditions of living systems.

The method proposed by scientists from Berkeley (the article was published in the journal Nature Methods) is based on the introduction of nanoscopic gold particles of a 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 exposure to light of a certain wavelength - a phenomenon known as plasmon resonance(see sidebar). The resonant frequencies of these nanoparticles are much easier to register than a 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 angle of reflection,
Watching the law
Setting landscapes in motion.

repeating words,
void of any meaning,
But without tension
Without tension.
B.G.

Literature

  1. 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;
  2. New nanoparticle technique captures chemical reactions in single living cell with amazing clarity . (2007). ScienceDaily.

Chapter 1. Review of the current state of research on the topic of the dissertation

1.1. Methods for calculating the absorption and scattering spectra of light by small particles.

1.1.1. Exact solution of the problem for a homogeneous sphere and its generalization to the case of a multilayer spherical particle

1.1.2. Solution of the problem for cylinders and spheroids.

1.1.3. Numerical methods for calculating the optical properties of nanostructures

1.2. Experimental research in the field of hybrid nanostructures and nanomaterials

1.2.1. Synthesis and structure of organometallic nanoparticles of spherical and elongated shape.

1.2.2. Investigation of absorption, luminescence and light scattering spectra by nanoparticles.

1.3. Hybrid organometallic nanosystems and metal nanoshells.

1.3.1. Properties of metallic nanoparticles. Localized surface plasmons

1.3.2. Metal nanoshells with a dielectric core

1.3.3. Molecular R-aggregates of cyanine dyes

1.3.4. Organometallic nanosystems of various shapes and sizes.

Chapter 2

2.1. Introductory remarks.

2.2. Basic formulas for calculating the absorption and scattering cross sections of light by two-layer particles.

2.2.1. Initial expressions for coefficients of multipole expansion. The contributions of TM and TE mod.

2.2.2. Quasi-static expressions for the cross section and dipole polarizability of a composite particle.

2.2.3. Dielectric Permittivities of the Core, L-Aggregate Shell, Organic Spacer, and Environment

2.3. Main regularities in the photoabsorption spectra of hybrid metal/L-aggregate nanoparticles.

2.4. Comparison of the contributions of absorption and scattering of light to the extinction cross section.

2.5. Analysis of the role of size effects.

2.6. Dependences of hybrid mode frequencies and absorption peak intensities on optical constants and geometric parameters of a nanoparticle.

2.6.1. Analytical Model for Calculating Hybrid Mode Frequencies

2.6.2. Dependences of the frequencies of hybrid modes and the intensities of the maxima of the photoabsorption peaks on the strength of the transition oscillator in the A-band of the dye.

2.6.3. Dependences of the positions of the maxima of the spectral peaks on the radius of the core and the thickness of the nanoparticle shell

2.7. Comparison of theory with experiment for metallic particles (Ag, Au, Cu, Al) with an A-aggregate shell

2.8. Assessment of the role of nonlocal phenomena

2.9. conclusions

Chapter 3

3.1. Preliminary remarks.

3.2. Initial formulas for the absorption and scattering cross sections of light by three-component particles.

3.2.1. Multipole expansion coefficients for the contributions of the TM and TE modes

3.2.2. Formulas for the quasi-static approximation

3.3. Absorption spectra and field structure for particles with a metal core, an outer J-aggregate shell, and an intermediate passive layer.

3.3.1. Analysis of the behavior of photoabsorption spectra with a change in the geometric parameters of the particle.

3.3.2. Comparison of the results of theory with experiment

3.4. Spectra and field structure in metal nanoshells with a dielectric core coated with an outer layer of molecular J-aggregates.

3.5. Metallic nanoshells with a semiconductor core and semiconductor/metal/J-aggregate particles.

3.5.1. Features of the photoabsorption spectra of hybrid nanoparticles with a high refractive index of the nucleus

3.5.2. Mie resonances in nanoballs with a silicon core and a metal shell

3.6. conclusions

Chapter 4. Studying the influence of the shape of hybrid nanoparticles on their spectral characteristics

4.1. Calculation and analysis of the extinction spectra of spheroidal nanoparticles in the quasi-static approximation.

4.1.1. Behavior of the photoabsorption spectra of spheroidal silver nanoparticles with a change in the ratio of the lengths of the major and minor semiaxes.

4.1.2. Effect of plasmon-exciton interaction in metal/L-aggregate nanoparticles on the extinction spectra of spheroidal nanoparticles.

4.2. Calculation of the absorption and scattering cross sections of light by hybrid spheroidal, cylindrical, and dumbbell-shaped nanoparticles using the FDTD method.

4.2.1. Adaptation of the FDTD method for calculating the optical properties of composite nanostructures

4.2.2. Results of calculating the spectral characteristics of spheroidal nanoparticles

4.2.3. Results of calculating the spectral characteristics of nanoparticles of complex shape.

4.3. Spectral features of electromagnetic field propagation along a chain of nanoparticles.

4.3.1. Model of a chain of nanoparticles.

4.3.2. Calculation results.

4.4. conclusions

Recommended list of dissertations

  • Optical properties of gold and silver nanoparticles in connection with the tasks of biodiagnostics 2007, Candidate of Physical and Mathematical Sciences Trachuk, Lyubov Anatolyevna

  • Plasmonic resonance nanoparticles for biomedical applications 2010, Doctor of Physical and Mathematical Sciences Khlebtsov, Boris Nikolaevich

  • Photoinduced Rearrangement of Molecules and Molecular Aggregates in the Near Field of Metallic Nanostructures 2013, Candidate of Physical and Mathematical Sciences Toropov, Nikita Aleksandrovich

  • Interaction of low-energy laser radiation with heterogeneous media based on solutions of organic dyes and metal nanostructures 2012, Candidate of Physical and Mathematical Sciences Panamarev, Nikolai Semyonovich

  • Study of the Optical Properties of Semiconductor Quantum Dots in the Near Field of Plasmonic Nanoparticles 2016, Candidate of Physical and Mathematical Sciences Bakanov, Alexey Georgievich

Introduction to the thesis (part of the abstract) on the topic "Theoretical study of the absorption and scattering spectra of light by hybrid organometallic nanoparticles"

The relevance and novelty of the problems solved in the dissertation is determined here by the fact that, until recently, the effects of the interaction of Frsnkel excitons with surface plasmons were studied mainly in hybrid organometallic nanostructures with planar geometry (for example, in thin metal films coated with molecular J-aggregates. In this In this case, the electromagnetic coupling of Frenkel excitons is carried out with plasmons propagating along a flat metal/dielectric or metal/semiconductor interface.However, in a number of articles, J-aggregation of cyanine dye on the spherical surface of noble metal nanoparticles in an aqueous solution was demonstrated, which opened up a fundamentally new opportunity to study Effects of Coherent Coupling of Molecular Excitons with Localized Plasmons An urgent task is also the development of methods for computer simulation of this kind of hybrid nanoparticles and composite nanomaterials, as well as the creation of numerical algorithms. algorithms and specific programs for calculating their various physical parameters This applies, first of all, to the adaptation of already developed numerical methods for solving problems of electrodynamics (widely used in radiophysics) for the problems posed in the dissertation in the field of nanooptics and nanophotonics. We are talking here, first of all, about the method of finite differences in the time domain (Finite Difference Time Domain Method - FDTD).

Summarizing the above, we can conclude that the topic of the dissertation corresponds to current scientific and technical problems and challenges facing modern nanophotonics and the industry of nanosystems, nanomaterials and nanodevices.

purpose of work

The purpose of this work is to perform numerical calculations and analyze the absorption and scattering spectra of organometallic nanoparticles of various compositions, shapes, and sizes, and to study, on this basis, the effects of the interaction of Frenkel excitons with dipole and multipole localized plasmons. As specific objects of research, the dissertation will study, among other things, two-component nanoparticles consisting of a metal core coated with a layer of molecular L-aggregates of cyanine dyes, three-component nanoparticles consisting of a metal core, a passive dielectric gasket and an outer L-aggregate layer, metal nanoshells with dielectric or semiconductor core and outer layer of molecular L-aggregates, as well as multilayer metal-organic nanosystems on these processes, as well as the development of effective methods for controlling the spectral characteristics and effects of plasmon-exciton interaction in such hybrid organometallic nanostructures

Tasks to be solved

In accordance with the goals set, the specific tasks to be solved in the dissertation can be briefly formulated as follows

1 Development of numerical algorithms for calculating the spatial structure of electromagnetic fields, polarizability tensors and spectral characteristics of two-component, three-component and multilayer nanoparticles of spherical, spheroidal and complex shape

2 Numerical calculations of the structure of fields and cross sections of absorption and scattering of light by the studied hybrid metal-organic nanoparticles and metal nanoshells with a dielectric or semiconductor core coated with an outer layer of molecular L-aggregates of cyanine dyes

3 Development of an analytical model for calculating natural frequencies of hybrid modes of a composite nanosystem and comparing its results with numerical calculations of the positions of spectral peaks of photoabsorption and light scattering Calculation and analysis of the spectral distribution of light absorption and scattering intensities

4 Elucidation of the main regularities in the effects of the interaction of Frenkel excitons with dipole and multipole localized surface plasmons in weak and strong coupling regimes. Analysis of the character of the plasmon-exciton coupling depending on the value of the transition oscillator strength in the J-band of the dye, the shape and geometric parameters of the system.

5. Investigation of the role of dimensional phenomena and elucidation of their influence on the optical properties of the hybrid nanostructures under study. Evaluation of the Influence of Nonlocal Effects in the Dielectric Function of the Metallic Core of a Nanoparticle on the Light Absorption Spectrum

6. Comparison of the results of calculations with the available experimental data and explanation of the results of experiments on the extinction spectra of hybrid metal-organic nanoparticles

7. Development of effective methods for controlling the optical properties, nature and magnitude of the plasmon-exciton interaction constant in the hybrid nanostructures under study.

Scientific novelty of the work

In general, the scientific novelty of the work consists in solving a number of new problems in the optics of composite organometallic nanostructures and in obtaining original results on their spectral characteristics, as well as in studying the nature of the plasmon-exciton coupling in the interaction of light fields with hybrid nanoparticles of various compositions, shapes and sizes.

In particular, in a wide range of wavelengths and geometric parameters of the system, new theoretical data were obtained on the spectral characteristics of hybrid two-component and three-component organometallic nanoparticles of various shapes and sizes, as well as nanoshells coated with J-aggregates of dyes with dielectric (semiconductor) cores. In this case, Ag, Au, Cu, and Al were used as the metal component of the nanoparticle in the calculations, and a set of various cyanine dyes (TC, OS, PIC, NK2567; the corresponding structural formulas are given below in Chapter 2) as the organic component, molecular J-aggregates which have absorption peaks in different spectral ranges of the visible spectrum and significantly different transition oscillator strengths in the J absorption band.

This made it possible for the first time to obtain a self-consistent physical picture of the processes of absorption and scattering of light by hybrid nanoparticles of this kind and to establish the influence on their optical properties of the effects of the interaction of Frenkel excitons both with electric dipole plasmons and with localized plasmons of a higher order of multipoleness Qualitatively new regularities in plasmon-exciton bonds were also found in the case when the frequency of the plasmon resonance of a metal nanoshell with a dielectric or semiconductor core coincides with the central frequency of light absorption in its outer L-aggregate shell. This leads to a radical change in the nature of the absorption spectrum of the entire hybrid nanosystem

The original result of the work is the development of a simple analytical model for calculating the hybrid modes of two-component spherical nanoparticles with a metal core coated with an outer layer of molecular A-aggregates of dyes, as well as a detailed analysis of the distribution of intensities and positions of the maxima of the spectral peaks of the nanosystems under study, based on accurate numerical calculations for spherical and spheroidal particles

The calculations carried out in the dissertation made it possible for the first time to establish the influence of various dimensional phenomena on the peak widths and intensity distribution in the light absorption spectra of hybrid metal-organic nanoparticles. In this case, the role of both dimensional effects caused by the dependences of the polarizability of a hybrid nanoparticle on the radius of its core and the thicknesses of the outer layers was clarified separately , and effects associated with an increase in the damping rate constant of free electrons in the core of a metal-organic nanoparticle as a result of their scattering at the spherical interface of the mstall/L-aggregate in the case when the particle radius becomes significantly less than the mean free path of an electron in a bulk metal sample

New results of the work were also obtained in studying the influence of the shape of a nanoparticle on the nature of the absorption and scattering spectra of light and on the related effects of plasmon-exciton coupling in hybrid steel-organic nanostructures and metal nanoshells with an outer layer of molecular L-aggregates.

To calculate the spectral characteristics of hybrid nanostructures and study the effects listed above, the dissertation developed special numerical algorithms for calculating the spatial structure of fields, polarities, as well as the absorption and scattering cross sections of light. They are based on the Mie theory generalized to the case of multilayer spherical nanoparticles and modified to take into account size effects in the dielectric function, on the use of a number of fairly cumbersome solutions for a two-layer spheroid, and on the adaptation of the finite difference method in the time domain (FDT) to solve the set in work tasks.

Scientific and practical value

The scientific and practical value of the work is largely determined by the relevance of the topic and the novelty of the problems being solved. In general, the significance of the work for nanophotonics is associated with obtaining new results in the field of studying the optical properties of hybrid nanostructures synthesized on the basis of metals and molecular A-aggregates of dyes, with the aim of their potential use for the creation of new composite nano-materials and for the development of efficient photonic, optoelectronic and light-emitting devices.

The results obtained significantly expand the understanding of the nature and mechanisms of the interaction of light with composite organometallic nanostructures. They made it possible to give an adequate explanation of a number of experimentally observed phenomena in the absorption and scattering spectra of light due to plasmon-exciton interaction in two-layer and three-layer nanostructures of various compositions, shapes and sizes, created on the basis of metals, ordered molecular L-aggregates, and also a number of passive organic dielectrics. and semiconductor materials with a high refractive index. A significant part of the obtained theoretical results is directly oriented towards setting up new experiments to study the optical properties of hybrid metal-organic nanostructures.

For the intensively developed area - nanoplasmonics, of greatest interest are the new results obtained in the dissertation on the effects of electromagnetic coupling of Frenkel excitons in hybrid organometallic nanoparticles, spherical, spheroidal and more complex in shape with surface plasmons localized in the particle core (or in its intermediate metallic nanoshell) of various multipole order in weak and strong coupling regimes. These results significantly complement the extensive studies previously carried out in the literature on the effects of the interaction of Frenkel or Wannier-Mott excitons with traveling surface plasmons in composite nanosystems with planar geometry (for example, in metal films coated with L-aggregates of dyes), or the effects observed in the literature during the interaction of excitons. in semiconductor quantum dots with metal nanoparticles or nanowires isolated from them. In this regard, the results obtained in the dissertation concerning the elucidation of the role of dimensional phenomena in processes involving hybrid metal-organic nanoparticles are also important.

To create hybrid nanostructures and nanomaterials with desired optical properties, the methods proposed in this work for controlling the spectral characteristics of organometallic nanoparticles and metal nanoshells coated with L-aggregates of dyes are practically significant. Practically important for solving a number of similar problems in the optics of composite nanostructures are the developed methods and numerical algorithms for calculating the fields and spectral characteristics of nanoparticles of various sizes and shapes.

Basic provisions for defense

1. Variation of the geometric parameters of 2- and 3-layer metal-organic nanoparticles leads to a radical redistribution of the intensities of the spectral peaks of light absorption and scattering, to a strong shift in the positions of their maxima, and to a change in the total number of peaks. This makes it possible to control the spectral characteristics of such nanoparticles and indicates ways to create hybrid nanostructures based on them with desired optical properties.

2. The influence of size effects on the nature of the absorption and scattering spectra of organometallic nanoparticles is reduced to three main factors: 1) the dependence of dipole polarizabilities on the total volume of the particle, as well as on the ratio ce of the inner and outer radii; 2) an increase in the role of the effects of the interaction of multipole plasmons with Frenkel excitons with increasing particle size; 3) the influence of the size of the particle core or the thickness of the metal nanoshell on its dielectric function due to an increase in the attenuation coefficient of free electrons when they are scattered at the boundary between the core and the shell, when the particle size becomes smaller than the mean free path of an electron in a bulk metal sample

3. Changing the shape of a hybrid nanoparticle from spherical to spheroidal or dumbbell-shaped leads to the appearance of new patterns in the absorption and scattering spectra of light. The resulting features in the behavior of the spectra are associated, in particular, with the splitting of localized plasmon resonance peaks in the metal core of a particle (or in a metal nanoshell) into longitudinal and transverse ones and with their interaction with Frenkel excitons in the outer A-aggregate layer. The intensity distribution at the maxima and the direction of the shift of new peaks strongly depend on the polarization of the incident light

4 The nature of the absorption and scattering spectra of light by metal-organic nanoparticles and the occurrence of one or another regime of plasmon-exciton coupling depends significantly on the strength of the oscillator of the transition in the L-bands of the dye and on the distance AL between the center of the absorption band of the molecular L-aggregate and the maximum of the plasmon resonance peak in the metal core (or intermediate metal layer) of the particle The regime of strong plasmon-exciton coupling is realized when the corresponding wavelengths of the indicated peaks are close. This leads to the appearance of a spectral minimum (in the vicinity of the maximum of the L-absorption band of the dye) and two practically equal in intensity photoabsorption peaks of the hybrid particle.

5. The developed approach and the performed numerical calculations make it possible to give an adequate explanation of the available experimental data on the photoabsorption spectra of hybrid nanoparticles containing a metal component and ordered molecular D-aggregates of dyes, and create a theoretical basis for setting up new experiments aimed at studying the effects of the interaction of localized plasmons with excitons Frenkel in organometallic nanostructures of various shapes, sizes and compositions.

Reliability of work results

The calculations carried out in the thesis and the analysis of the results are based on self-consistent analytical and numerical methods and approaches in the theory of light absorption and scattering. The calculations used modern and reliable data on the optical constants of the materials that make up the hybrid nanoparticles. The reliability of the results of the work is confirmed by comparing the results of theoretical calculations with the available experimental data, and in some cases with the results of calculations by other authors. The spectral characteristics of a number of hybrid nanoparticles under study were calculated in the thesis by various methods, which demonstrated identical results.

Approbation of work

12 reports were made at Russian and international conferences, symposiums and workshops, including the International Conference "Raman scattering - 80 years of research" (KR-80, Moscow, 2008); at the XX Conference on Fundamental Atomic Spectroscopy (FAS-XX, Voronezh, 2013); at the III Symposium on coherent radiation of semiconductor compounds and structures (Moscow-Zvenigorod, 2011); at the 53rd MIPT Scientific Conference "Modern Problems of Fundamental and Applied Sciences" (Moscow-Dolgoprudny-Zhukovsky, 2010); at the 55th MIPT Scientific Conference "Modern Problems of Fundamental and Applied, Natural and Technical Sciences in the Modern Information Society"11 (Moscow-Dolgoprudny-Zhukovsky, 2012); at the III All-Russian Youth School-Seminar with International Participation "Innovative Aspects of Fundamental Research on Actual Problems of Physics"11 ( FIAN, Moscow - FIAN Technopark, Troitsk, Moscow region, 2009); at the XIII School of Young Scientists 11Actual Problems of Physics" and the IV School-seminar "Innovative Aspects of Fundamental Research" (Moscow-Zvenigorod,

2010); at the XIV School of Young Scientists "Actual Problems of Physics" (Zvenigorod, 2012).

The results of the work were also reported at the Department of Quantum Radiophysics of the Moscow Institute of Physics and Technology, as well as at scientific seminars in the Optical Department. G.S. Landsberg and in the Department of Luminescence. S.I. Vavilov FIAN.

The dissertation is the result of the author's independent scientific work, carried out under the guidance of his supervisor. The personal contribution of the author of the dissertation consists, in particular, in independent carrying out of all numerical calculations, in active participation in the discussion of the formulation of specific problems to be solved, in joint analysis with co-authors of all the results of the work and in joint writing of articles. In general, the contribution of the author is decisive in the results presented in the dissertation.

The structure and scope of the dissertation

The dissertation consists of an introduction, four chapters of the main text and a conclusion. The total volume of the dissertation is 141 pages, including 60 figures and 2 tables. The list of references contains 118 titles.

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  1. What are nanoparticles?
  2. Features of optical processes occurring on nanometer scales
  3. Spectral properties of semiconductor particles
  4. Spectral properties of metal particles
  5. Hybrid nanoparticles and their spectral properties

Terms used

  • Differential scattering cross section is a 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 total 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 nanoparticles?

Nanoparticles are objects ranging in size from a few nanometers to several hundreds of nanometers. As a rule, these are either nanoscale 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

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

At present, chemists are able to synthesize quantum dots of various compositions. The most common quantum dots are based on cadmium (for example, CdSe).


  • Nanooptics studies the physical properties, structure and methods of creating light fields localized on the nanometer scale.
  • 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 (up 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 ensembles of particles !

Accounting for the near-field interaction leads to a qualitative change in the behavior of the fields

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

  • It is necessary to take into account the influence of localized fields
  • Electromagnetic fields near nanostructures differ significantly from fields in free space and in bulk materials
  • These circumstances are especially important when considering the effects occurring near the boundary of nanostructures, as well as in the interaction of closely spaced nanoparticles.
  • Localized fields exist in limited parts of space, however, the intensities of such fields can be significant, which can lead to the appearance 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 using the concept of permittivity

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, a space-limited decrease in the bottom of the conduction band and an increase in the top of the valence band occur. By virtue of the laws of quantum mechanics, the admissible energy levels of an electron in this case form a discrete spectrum.

Energy levels in a quantum dot

The energy levels of an electron and a hole are inversely proportional to the square of the quantum dot width! By choosing different sizes and shapes of quantum dots, it is possible to achieve that they will emit or absorb light. given wavelength. This allows using the same material, but different sizes and shapes, create light sources that emit 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 embedded in a GaAs matrix.

As in the case of semiconductor nanoparticles, the spectral properties of metal particles significantly depend on their size and shape. However, in contrast to semiconductors, in the case of metals this phenomenon is mainly associated with the excitation plasmons . When light interacts with electrons that are free to move through the metal, the position of the electrons relative to the position of the ions in the crystal lattice begins to oscillate at the plasma frequency ωp. The quanta of plasma oscillations are called plasmons .

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

Plasmon resonance

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

Modes of plasmon oscillations excited by irradiating a nano-triangle with an electron beam 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

Dependences of the Spectra of Metallic Nanoparticles on Their Shape and Size

Maximums in 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 metallic 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, it is possible to ensure that the maximum of the extinction cross section falls within the spectral range we need
  • Using this property, it is possible to significantly increase the efficiency of solar cells by absorbing different parts of the solar spectrum by different nanoparticles.

Hybrid nanoparticles

hybrid nanoparticles are made up of various materials, such as metal and semiconductor. Since the properties of different materials change differently with a decrease in size, 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 organometallic core-shell nanoparticles consisting of a metal core and a dye shell in the so-called aggregate state.

Mutual arrangement of the unperturbed peaks of the plasmon resonance of the nucleus (Ag and Au) and the exciton peak of the J-aggregate shell of the dye (TC, OC, PIC)

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

The dependence of the nature of the photoabsorption spectra of hybrid Ag/J-aggregate 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 optical properties of hybrid nanoparticles on their shape

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

Dependence of the absorption spectrum of Ag/J-aggregate composite systems on geometrical 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 the nanoparticle components can lead to a shift in the positions of the peaks in the absorption cross sections, the appearance of new peaks, and 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 nonspherical particles, the positions of the absorption maxima depend on the polarization of the incident radiation
  • By choosing different geometrical parameters of a hybrid nanoparticle, it is possible to shift the absorption peaks to the required 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 those of the bulk material.
  • For almost all nanoparticles, the spectral characteristics change significantly with a change 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 passing to the consideration of 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 of which they are composed (the whole is not equal to the sum of the parts!)

Bibliography

  • L. Novotny, B. Hecht, Basicsnanooptics, Moscow, Fizmatlit 2011
  • Y. Masumoto, T. Takagahara, Semiconductor Quantum Dots, Springer-Verlag Berlin Heidelberg New York, 2002
  • V.S. Lebedev et al, Colloids and Surfaces A 326, 204 (2008); Quantum Electronics 40, 246 (2010)
  • V.S. Lebedev, A.S. Medvedev, Quantum Electronics 42, 701 (2012); Quantum Electronics 43, No. 11(2013); J. Russ. Laser Res. 34,303(2013)
  • R.B. Vasiliev, D.N. Dirin, A.M. Gaskov, Uspekhi Khimiya, 80, 1190 (2011)
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  • H.-E. Schaefer, Nanoscience Springer Heidelberg Dordrecht London New York, 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 shift 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. The result is a force that tends to return the electrons to their 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 a particle. If the oscillation frequency of the incident light coincides with the natural oscillation frequency of free electrons near the surface of a metal particle, there is a sharp increase in the oscillation amplitude of the "electron plasma", 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 about 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 and shape of the 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 have an absorption peak of 520 nm, and copper (I) oxide nanoparticles have an absorption peak of 450–700 nm.

Nanorods have anisotropic symmetry, and therefore two peaks are observed in the absorption spectrum, corresponding to transverse and longitudinal plasmons. in

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

λ, nm

λ, nm

Fig.1a Optical absorption spectrum of silver nanoparticles

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

Experimental Processing and presentation of laboratory results

The report must include:

Scheme and equation of the reaction for the synthesis of nanoparticles

Records of solution color change during synthesis

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

Absorption spectrum of a solution of nanoparticles

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

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

This method makes it possible to obtain relatively large silver particles with a diameter of 60–80 nm. The absorption maximum is 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: balance, spectrophotometer, quartz cuvettes with an optical path length of 1 cm, 200 ml flasks, 50 ml beakers, heated stirrer, measuring cylinder.

Work order

    Prepare a 0.005 M (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% solution of sodium citrate 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 to it.

    Heat the solution until the color turns pale yellow.

    Leave the solution to cool to room temperature with the stirrer on.

    Reduced by boiling, bring the volume of the solution with water to 125 ml.

    Read the absorption spectrum of the obtained colloidal solution in the range of 200 - 800 nm. Take water as a reference solution.

    Remove the absorption spectrum in a day, a week. Compare the obtained spectra. What can be said about the stability of nanoparticles? What factors determine the stability of nanoparticles obtained by this method? What other ways are known to improve the stability of metal nanoparticles? Why is an aqueous solution of silver nitrate stored in a laboratory in a dark container?

    Add dropwise 5 ml of dilute HCl 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 hydrochloric acid is added and the solution becomes colorless when acetic acid is added. Write down the conclusions, observations and equations of reactions in a notebook.

, 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 medium 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. With total internal reflection, an electromagnetic wave propagates along a surface reflecting light, 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 velocity of this wave coincides with the velocity of a 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 nanoscale metallic systems, collective electronic excitations are modified. The collective electronic excitation of metal nanoparticles, the size of which is smaller than the wavelength of electromagnetic radiation in the environment - a localized surface plasmon - oscillates at a frequency that is ≈3 times lower than the frequency of the bulk plasmon, while the frequency of the surface plasmon is approximately ≈2 times lower than the frequency of the bulk plasmon. plasmon. Due to the small size of the system, the requirement that the velocity of propagation of the excitation and the electromagnetic wave in the external medium should coincide is eliminated, so that the localized surface plasmons are directly related to the radiation. When the frequency of the external field coincides with the frequency of the localized surface plasmon, a resonance arises, which leads to a sharp increase in the field on the surface of the particle and an increase in the absorption cross section.

The properties of localized plasmons critically depend on the shape of the nanoparticles, which makes it possible to tune the system of their resonances for effective interaction with light or elementary quantum systems.

At present, the phenomenon of surface plasmon resonance is widely used in the creation of chemical and biological sensors. Upon 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; significantly expand the possibilities of detection, identification and diagnostics of biological objects.

  • Naimushina Daria Anatolievna
Links
  1. Perlin E.Yu., Vartanyan T.A., Fedorov A.V. Solid state physics. Optics of semiconductors, dielectrics, metals: Textbook. - St. Petersburg: SPbGU ITMO, 2008. - 216 p.
  2. Pompa P.P., Martiradonna L. et al. Metal-enhanced fluorescence of colloidal nanocrystals with nanoscale control // Nature Nanotechnology - vol. 1, 2006 - P. 126-130
  3. Nashchekin A.V. 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 of diagnostics and research of nanostructures and nanomaterials
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Encyclopedic Dictionary of Nanotechnology. - Rusnano. 2010 .

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