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abstract

The WRC includes:

The explanatory note contains 63 pages, 18 figures, 7 tables, 53 sources;

Presentation 25 slides.

HYDROCHEMICAL SYNTHESIS METHOD, QUANTUM DOTS, LEAD SULFIDE, CADMIUM SULFIDE, SOLID SOLUTION, PHOTON CORRELATION SPECTROSCOPY.

The object of study in this work was the quantum dots of CdS, PbS and CdS-PbS solid solution obtained by hydrochemical precipitation.

The purpose of this final qualification work is to obtain colloidal quantum dots CdS, PbS and in the CdS-PbS system by hydrochemical synthesis from aqueous media, as well as to study their particle sizes and study the dependence of luminescence on sizes.

Achieving this goal requires optimizing the reaction mixture, studying the composition, structure, particle size, and properties of the synthesized colloidal solutions.

For a comprehensive study of quantum dots, the method of photon correlation spectroscopy was used. The experimental data were processed using computer technology and analyzed.

Abstract 3

1.LITERARY REVIEW 7

1.1. The concept of "quantum dot" 7

1.2 Application of quantum dots 9

1.2.1.Materials for lasers 10

1.2.2. Materials for LEDs 11

1.2.3 Materials for solar cells 11

1.2.4 Materials for field-effect transistors 13

1.2.5 Use as biotags 14

1.3. Methods for learning quantum dots 15

1.4 Properties of quantum dots 18

1.5 Methods for determining particle sizes 21

1.5.1 Spectrophotometer Photocor Compact 21

2. Experimental procedure 25

2.1 Hydrochemical synthesis method 25

2.2 Chemical reagents 27

2.3 Disposal of waste solutions 27

2.4. Measurement technique on the particle analyzer Photocor Compact 28

2.4.1 Fundamentals of the method of dynamic light scattering (photon correlation spectroscopy) 28

3.Experimental part 30

3.1 Synthesis of quantum dots based on cadmium sulfide 30

3.1.1 Effect of cadmium salt concentration on particle size of CdS 32 QDs

3.2 Synthesis of quantum dots based on lead sulfide 33

3.2.1 Effect of lead salt concentration on particle size of PbS 34 QDs

3.3 Synthesis of quantum dots based on CdS-PbS 35 solid solution

4. Life safety 39

4.1.Introduction to the section on life safety 39

4.2. Harmful and dangerous production factors in the laboratory 40

4.2.1 Harmful substances 40

4.2.2. Microclimate parameters 42

4.2.3 Ventilation 43

4.2.5.Illumination 45

4.2.6 Electrical safety 46

4.2.7 Fire safety 47

4.2.8 Emergencies 48

Conclusions on section BDZ 49

5.2.4. Calculation of costs for third-party services 55

General conclusions 59

References 60

Introduction

A quantum dot is a fragment of a conductor or semiconductor whose charge carriers (electrons or holes) are limited in space in all three dimensions. The size of a quantum dot must be so small that quantum effects are significant. This is achieved if the kinetic energy of the electron is noticeably greater than all other energy scales: first of all, it is greater than the temperature expressed in energy units.

Quantum dots, depending on their size and chemical composition, exhibit photoluminescence in the visible and near infrared ranges. Due to the high size uniformity (more than 95%), the proposed nanocrystals have narrow emission spectra (fluorescence peak half-width 20-30 nm), which ensures phenomenal color purity.

Of particular interest are photoluminescent quantum dots, in which the absorption of a photon gives rise to electron-hole pairs, and the recombination of electrons and holes causes fluorescence. Such quantum dots have a narrow and symmetrical fluorescence peak, the position of which is determined by their size. Thus, depending on the size and composition, QDs can fluoresce in the UV, visible, or IR spectral region.

LITERATURE REVIEW

1. The concept of "quantum dot"

Colloidal quantum dots are semiconductor nanocrystals with a size in the range of 2-10 nanometers, consisting of 10 3 - 10 5 atoms, created on the basis of inorganic semiconductor materials, coated with a stabilizer monolayer ("coat" of organic molecules, Fig. 1). Quantum dots are larger in size than molecular clusters traditional for chemistry (~ 1 nm with a content of no more than 100 atoms). Colloidal quantum dots combine the physical and chemical properties of molecules with the optoelectronic properties of semiconductors.

Fig.1.1 (a) Quantum dot covered with a “coat” of a stabilizer, (b) transformation of the semiconductor band structure with decreasing size.

Quantum size effects play a key role in the optoelectronic properties of quantum dots. The energy spectrum of a quantum dot is fundamentally different from that of a bulk semiconductor. An electron in a nanocrystal behaves like in a three-dimensional potential “well”. There are several stationary energy levels for an electron and a hole with a characteristic distance between them , where d is the size of the nanocrystal (quantum dot) (Fig. 1b). Thus, the energy spectrum of a quantum dot depends on its size. Similar to the transition between energy levels in an atom, when charge carriers pass between energy levels in a quantum dot, a photon can be emitted or absorbed. Transition frequencies, i.e. wavelength of absorption or luminescence, it is easy to control by changing the size of the quantum dot (Fig. 2). Therefore, quantum dots are sometimes referred to as "artificial atoms". In terms of semiconductor materials, this can be called the ability to control the effective band gap.

There is another fundamental property that distinguishes colloidal quantum dots from traditional semiconductor materials - the possibility of existence in the form of solutions, or more precisely, in the form of sols. This property provides a wide range of possibilities for manipulating such objects and makes them attractive to technology.

The dependence of the energy spectrum on size provides a huge potential for the practical application of quantum dots. Quantum dots can find applications in optoelectric systems such as light emitting diodes and flat light emitting panels, lasers, solar cells and photoelectric converters, as biological markers, i.e. wherever variable, wavelength-tunable optical properties are required. On fig. Figure 2 shows an example of the luminescence of CdS quantum dot samples:

Fig.1.2 Luminescence of samples of CdS quantum dots with a size in the range of 2.0-5.5 nm, prepared in the form of sols. Above - without illumination, below - illumination with ultraviolet radiation.

Applications of quantum dots

Quantum dots have great potential for practical applications. First of all, this is due to the ability to control the effective band gap when changing the size. In this case, the optical properties of the system will change: the luminescence wavelength, the absorption region. Another practically important feature of quantum dots is the ability to exist in the form of sols (solutions). This makes it easy to obtain coatings from quantum dot films by cheap methods, such as spin-coating, or to apply quantum dots using inkjet printing on any surface. All these technologies make it possible to avoid expensive vacuum technologies traditional for microelectronics when creating devices based on quantum dots. Also, due to solution technologies, it is possible to introduce quantum dots into suitable matrices and create composite materials. An analogy may be the situation with organic luminescent materials that are used to create light emitting devices, which led to a boom in LED technology and the emergence of the so-called OLED.

Materials for lasers

The possibility of varying the luminescence wavelength is a fundamental advantage for creating new laser media. In existing lasers, the luminescence wavelength is a fundamental characteristic of the medium, and the possibility of varying it is limited (lasers with tunable wavelengths use the properties

resonators and more complex effects). Another advantage of quantum dots is their high photoresistance compared to organic dyes. Quantum dots demonstrate the behavior of inorganic systems. The possibility of creating laser media based on CdSe quantum dots was demonstrated by a research team led by Viktor Klimov at the Los Alamos National Laboratory, USA. Further, the possibility of stimulated emission for quantum dots based on other semiconductor materials, such as PbSe, is shown. The main difficulty is the short lifetime of the excited state in quantum dots and the side process of recombination, which requires a high pump intensity. At the moment, both the process of stimulated generation has been observed, and a prototype of a thin-film laser has been created using a substrate with a diffraction grating.

Fig.1.3. Use of quantum dots in lasers.

Materials for LEDs

The possibility of varying the luminescence wavelength and the ease of creating thin layers based on quantum dots present great opportunities for creating light-emitting devices with electrical excitation - light-emitting diodes. Moreover, the creation of flat screen panels is of particular interest, which is very important for modern electronics. The use of inkjet printing would lead to a breakthrough in

OLED technology.

To create a light emitting diode, a monolayer of quantum dots is placed between layers having p- and n-type conductivity. These can be conductive polymeric materials, which are relatively well developed in connection with OLED technology, and can be easily coupled with quantum dots. The development of technology for creating light-emitting devices is carried out by a scientific group led by M. Bulovic (MIT).

Speaking of LEDs, one cannot fail to mention the “white” LEDs, which can become an alternative to standard incandescent lamps. Quantum dots can be used to light-correct semiconductor LEDs. Such systems use optical pumping of a layer containing quantum dots using a semiconductor blue LED. The advantage of quantum dots in this case is a high quantum yield, high photostability, and the ability to compose a multicomponent set of quantum dots with different emission lengths in order to obtain a radiation spectrum closer to “white”.

Materials for solar panels

The creation of solar batteries is one of the promising areas of application of colloidal quantum dots. Currently, traditional silicon batteries have the highest conversion rate (up to 25%). However, they are quite expensive and existing technologies do not allow creating batteries of a large area (or this is too expensive to manufacture). In 1992, M. Gratzel proposed an approach to the creation of solar cells based on the use of 30 materials with a large specific surface area (for example, nanocrystalline TiO2). Activation to the visible range of the spectrum is achieved by adding a photosensitizer (some organic dyes). Quantum dots can perfectly act as a photosensitizer, since they allow you to control the position of the absorption band. Other important advantages are the high extinction coefficient (the ability to absorb a significant fraction of photons in a thin layer) and the high photostability inherent in the inorganic core.

Fig.1.4. The use of quantum dots in solar cells.

A photon absorbed by a quantum dot leads to the formation of a photoexcited electron and hole, which can pass into electron and hole transport layers, as shown schematically in the figure. Conducting polymers of n- and p-types of conductivity can act as such transport layers; in the case of an electron transport layer, by analogy with the Gratzel element, it is possible to use porous layers of metal oxides. Such solar cells have an important advantage, such as the possibility of creating flexible elements by applying layers to polymer substrates, as well as relative cheapness and ease of manufacture. Publications on possible applications of quantum dots for solar cells can be found in the work of P. Alivisatos and A. Nozic.

Materials for FETs

The use of arrays of quantum dots as conductive layers in microelectronics is very promising, since it is possible to use simple and cheap “solution” deposition technologies. However, the applicability is currently limited by the extremely high (~1012 Ohm*cm) resistance of quantum dot layers. One of the reasons is the large (by microscopic standards, of course) distance between individual quantum dots, which, when using standard stabilizers such as trioctylphosphine oxide or oleic acid, is from 1 to 2 nm, which is too large for efficient tunneling of charge carriers. However, when shorter chain molecules are used as stabilizers, it is possible to reduce the interparticle distances to a level acceptable for charge carrier tunneling (~0.2 nm when using pyridine or hydrazine.

Fig.1.5. The use of quantum dots in field-effect transistors.

In 2005, K.Murray and D.Talapin reported on the creation of a thin-film field-effect transistor based on PbSe quantum dots using hydrazine molecules for surface passivation. As shown, lead chalcogenides are promising for creating conducting layers due to their high dielectric constant and high density of states in the conduction band.

Use as biotags

The creation of fluorescent labels based on quantum dots is very promising. The following advantages of quantum dots over organic dyes can be distinguished: the ability to control the luminescence wavelength, high extinction coefficient, solubility in a wide range of solvents, stability of luminescence to the environment, high photostability. We can also note the possibility of chemical (or, moreover, biological) modification of the surface of quantum dots, which makes it possible to selectively bind to biological objects. The right figure shows the staining of cell elements using water-soluble quantum dots that luminesce in the visible range. Figure 1.6 shows an example of using a non-destructive method of optical tomography. The photo was taken in the near IR range using quantum dots with luminescence in the range of 800–900 nm (the transparency window of warm-blooded blood) introduced into a mouse.

Fig.1.6 The use of quantum dots as biotags.

Methods for learning quantum dots

Currently, methods have been developed for obtaining nanomaterials both in the form of nanopowders and in the form of inclusions in porous or monolithic matrices. In this case, ferro- and ferrimagnets, metals, semiconductors, dielectrics, etc. can act as a nanophase. All methods for obtaining nanomaterials can be divided into two large groups according to the type of formation of nanostructures: “Bottom-up” methods are characterized by the growth of nanoparticles or the assembly of nanoparticles from individual atoms; and “Top-down” methods are based on the “crushing” of particles to nanosize (Fig. 1.7).

Fig.1.7. Methods for obtaining nanomaterials.

Another classification involves the division of synthesis methods according to the method of obtaining and stabilizing nanoparticles. The first group includes the so-called.

high-energy methods based on the rapid condensation of vapors in

conditions precluding aggregation and growth of formed particles. Main

the differences between the methods of this group are in the way of evaporation and stabilization of nanoparticles. Evaporation can be carried out by plasma excitation (plasma-ark), using laser radiation (laser ablation), in

volt arc (carbon arc) or thermal impact. Condensation is carried out in the presence of a surfactant, whose adsorption on the particle surface slows down growth (vapor trapping), or on a cold substrate, when growth

particles is limited by the diffusion rate. In some cases, condensation

are carried out in the presence of an inert component, which makes it possible to obtain nanocomposite materials with different microstructures in a targeted manner. If

the components are mutually insoluble, the particle size of the resulting composites can be varied by heat treatment.

The second group includes mechanochemical methods (ball-milling), which make it possible to obtain nanosystems by grinding mutually insoluble components in planetary mills or by the decomposition of solid solutions with

the formation of new phases under the action of mechanical stresses. The third group of methods is based on the use of spatially limited systems - nanoreactors (micelles, drops, films, etc.). These methods include synthesis in reverse micelles, Langmuir-Blodgett films, adsorption layers, or solid-phase nanoreactors. Obviously, the size of the particles formed in this case cannot exceed

the size of the corresponding nanoreactor, and therefore these methods make it possible to obtain monodisperse systems. In addition, the use

colloidal nanoreactors makes it possible to obtain nanoparticles of various shapes and anisotropy (including small ones), as well as particles with coatings.

This method is used to obtain almost all classes of nanostructures, from single-component metal to multi-component oxide. This also includes methods based on the formation of ultramicrodispersed and colloidal particles in solutions during polycondensation in the presence of surfactants that prevent aggregation. It is important that this particular method, based on the complementarity of the formed structure to the original template, is used by wildlife for the reproduction and functioning of living systems (for example, protein synthesis, DNA, RNA replication, etc.) The fourth group includes chemical methods for obtaining highly porous and finely dispersed structures (Rieke metals, Raney nickel), based on the removal of one of the components of the microheterogeneous system as a result of a chemical reaction or anodic dissolution. These methods also include the traditional method of obtaining nanocomposites by quenching a glass or salt matrix with a dissolved substance, resulting in the release of nanoinclusions of this substance in the matrix (glass crystallization method). In this case, the introduction of the active component into the matrix can be carried out in two ways: by adding it to the melt, followed by quenching, and by direct introduction into the solid matrix using ion implantation.

Properties of quantum dots

The unique optical properties of quantum dots (QDs) make them a promising material for applications in various fields. In particular, developments are underway on the use of QDs in light emitting diodes, displays, lasers, and solar cells. In addition, they can be conjugated to biomolecules through covalent bonding between groups of ligands covering QDs and functional groups of biomolecules. As such, they are used as fluorescent labels in a wide range of bioassay applications, from immunoassays to tissue imaging and drug tracking in the body. The use of QDs in bioanalysis is currently one of the promising fields of application of luminescent nanocrystals. Such unique characteristics of QDs as dependence of emission color on size, high photostability, and wide absorption spectra make them ideal fluorophores for ultrasensitive, multicolor detection of biological objects and medical diagnostics, which require registration of several parameters simultaneously.

Semiconductor QDs are nanocrystals whose dimensions in all three directions are less than the Bohr exciton radius for a given material. In such objects, a size effect is observed: optical properties, in particular, the band gap (and, accordingly, the emission wavelength) and the extinction coefficient, depend on the size of nanoparticles and their shape. Due to such a significant spatial limitation, QDs have unique optical and chemical characteristics:

High photostability, which makes it possible to multiply the power of the excited radiation and to observe the behavior of the fluorescent label in real time for a long time.

Wide absorption spectrum - due to which QDs with different diameters can be simultaneously excited by a light source with a wavelength of 400 nm (or other), while the emission wavelength of these samples varies in the range of 490 - 590 nm (fluorescence color from blue to orange-red) .

The symmetrical and narrow (peak width at half maximum does not exceed 30 nm) QD fluorescence peak simplifies the process of obtaining multi-colored labels.

The luminosity of QDs is so high that they can be detected as single objects using a fluorescent microscope.

To use QDs in bioanalysis, they are subject to requirements related to water solubility and biocompatibility (since the inorganic core is insoluble in water), as well as a clear particle size distribution and storage stability. To impart water-soluble properties to QDs, there are several approaches to the synthesis: either QDs are synthesized directly in the aqueous phase; or QDs obtained in organic solvents are then transferred to aqueous solutions by modifying the ligand layer covering QDs.

Synthesis in aqueous solutions makes it possible to obtain hydrophilic QDs; however, in a number of characteristics, such as the fluorescence quantum yield, particle size distribution, and stability over time, they are significantly inferior to semiconductor QDs obtained in organic phases. Thus, for use as biolabels, QDs are most often synthesized at high temperatures in organic solvents according to the method first applied in 1993 by the scientific group of Murray et al. The main principle of the synthesis is the injection of solutions of Cd metal precursors and Se chalcogen into a coordination solvent heated to high temperatures. With an increase in the process time, the absorption spectrum shifts to the long wavelength region, which indicates the growth of CdSe crystals.

CdSe nuclei have a low fluorescence brightness—their quantum yield (QE), as a rule, does not exceed 5%. To increase the CV and photostability, fluorescent CdSe cores are coated with a layer of a wider-gap semiconductor with a similar structure and composition, which passivates the surface of the core, thereby significantly increasing the fluorescence CV. A similar crystal structure of the shell and core is a necessary condition, otherwise there will be no uniform growth, and the difference in structures can lead to defects at the phase boundary. To coat cadmium selenide cores, wider-gap semiconductors such as zinc sulfide, cadmium sulfide, and zinc selenide are used. However, zinc sulfide, as a rule, grows only on small cadmium selenide nuclei (at d(CdSe)< 3 нм). Согласно , наращивание оболочки ZnS на ядрах CdSe большего диаметра затруднительно из-за большой разницы в параметрах кристаллических решёток CdSe и ZnS. Поэтому при наращивании ZnS непосредственно на ядрах CdSe диаметром более ~3 нм между ядром и сульфидом цинка помещают промежуточный слой – наращивают оболочку селенида цинка или сульфида кадмия, которые имеют промежуточные между CdSe и ZnS параметры кристаллической решётки и величину запрещённой зоны .

There are two main approaches for converting hydrophobic QDs into aqueous solutions: the ligand replacement method and coating with amphiphilic molecules. In addition, the coating of QDs with a silicon oxide shell is often distinguished as a separate category.

Particle Sizing Methods

The above properties of colloidal quantum dots are manifested in the presence of a size effect, therefore, it is necessary to measure the particle size.

In this WRC, the measurements were carried out on a Photocor Compact device installed at the Department of Physical and Colloidal Chemistry of the Ural Federal University, as well as on a Zetasizer Nano Z device at the Institute of Solid State Chemistry, Ural Branch of the Russian Academy of Sciences.

SpectrophotometerPhotocor Compact

The layout of the Photocor Compact laboratory spectrometer is shown in Fig. 1.8:

Fig.1.8. Scheme of the Photocor Compact spectrometer.

The instrument uses a thermally stabilized diode laser with a wavelength of λ = 653.6 nm. The laser beam passes through a focusing lens L1, with a focal length of 90 mm, is collected on the sample under study, where it is scattered by microscopic fluctuations of nanoparticles. Scattered light is measured at a right angle, passes through a diaphragm d = 0.7 mm, is focused by the lens L2 on the second diaphragm 100 μm, then is divided in half by a translucent mirror and falls on two PMTs. To maintain collection coherence, the pinhole in front of the PMT should have a size on the order of the first Fresnel zone. At smaller sizes, the signal-to-noise ratio decreases; as the size increases, the coherence decreases and the amplitude of the correlation function decreases. The Photocor-Compact spectrometer uses two PMTs, the cross-correlation function of their signals is measured, this allows you to remove PMT noise, since they are not correlated, and the cross-correlation function of signals from PMTs will be equivalent to the correlation function of scattered light. A multichannel (288 channels) correlator is used, the signals from which are read by a computer. It controls the device, the measurement process and the processing of measurement results.

The resulting solutions were measured on a correlation spectrometer. Using the Photocor Software, you can monitor the progress of measurements and control the correlator. During measurements, a division into parts of the total measurement time is used, the resulting correlation functions and scattering intensities are analyzed, and if the average intensity in some time interval is greater than in the others, the measurements for this interval are ignored, the rest are averaged. This makes it possible to remove distortions of the correlation function by rare dust particles (a few microns in size).

Figure 1.9 shows the software of the Photocor Software correlation spectrometer:

Fig.1.9 Photocor Software correlation spectrometer software.

Graphs 1,2,4 - measured correlation functions on a logarithmic scale: 1 - c.f., measured at a given time, 2 - measured functions, 4 - total correlation function is displayed; 3 graph - sample temperature; 5 graph - scattering intensity.

The program allows you to change the laser intensity, temperature (3), the time of one measurement and the number of measurements. The measurement accuracy depends on the set of these parameters, among other things.

The accumulated correlation function was processed by the DynaLS program, its software is shown in Fig. 1.10:

Rice. 1.10. Correlation Function Processing Software, DynaLC.

1 – measured correlation function, approximated by the theoretical one; 2 – difference between the obtained theoretical and measured exponential functions; 3 - the resulting size distribution, found by approximating the theoretical function to the experimental one; 4 - table of results. In the table: the first column is the number of solutions found; the second is the “area” of these solutions; the third is the average value; the fourth is the maximum value; the latter is the scatter of the solution (error). A criterion is also given that shows how well the theoretical curve coincides with the experimental one.

Experimental technique

Hydrochemical synthesis method

Chemical precipitation from aqueous solutions is of particular attractiveness and wide prospects, in terms of final results. The hydrochemical deposition method is distinguished by high productivity and economy, simplicity of technological design, the possibility of depositing particles on a surface of complex shape and different nature, as well as doping the layer with organic ions or molecules that do not allow high-temperature heating, and the possibility of "soft chemical" synthesis. The latter allows us to consider this method as the most promising for obtaining compounds of metal chalcogenides of complex structure that are metastable in nature. Hydrochemical synthesis is a promising method for fabricating metal sulfide quantum dots, potentially capable of providing a wide variety of their characteristics. The synthesis is carried out in a reaction bath containing a metal salt, an alkali, a chalcogenizer and a complexing agent.

In addition to the main reagents that form the solid phase, ligands are introduced into the solution that are capable of binding metal ions into stable complexes. An alkaline environment is necessary for the decomposition of the chalcogenizer. The role of complexing agents in hydrochemical synthesis is very important, since its introduction significantly reduces the concentration of free metal ions in the solution and, consequently, slows down the synthesis process, prevents the rapid precipitation of the solid phase, ensuring the formation and growth of quantum dots. The strength of the formation of complex metal ions, as well as the physicochemical nature of the ligand, has a decisive influence on the process of hydrochemical synthesis.

KOH, NaOH, NH are used as alkali 4 OH or ethylenediamine. Various types of chalcogenizers also have a certain effect on hydrochemical precipitation and on the presence of synthesis by-products. Depending on the type of chalcogenizer, the synthesis is based on two chemical reactions:

(2.1)

, (2.2)

Where is the complex metal ion.

The criterion for the formation of an insoluble metal chalcogenide phase is supersaturation, which is defined as the ratio of the ionic product of the ions that form quantum dots to the solubility product of the solid phase. At the initial stages of the process, the formation of nuclei in the solution and the particle size increase quite rapidly, which is associated with high concentrations of ions in the reaction mixture. As the solution becomes depleted of these ions, the rate of solid formation decreases until equilibrium is reached in the system.

The procedure for draining the reagents for preparing the working solution is strictly fixed. The need for this is due to the fact that the process of precipitation of chalcogenides is heterogeneous, and its rate depends on the initial conditions for the formation of a new phase.

The working solution is prepared by mixing the calculated volumes of the starting materials. Synthesis of quantum dots is carried out in a glass reactor with a volume of 50 ml. First, the calculated volume of cadmium salt is introduced into the reactor, then sodium citrate is introduced and distilled water is added. After the solution is alkalized, and thiourea is added to it. To stabilize the synthesis, the calculated volume of Trilon B is introduced into the reaction mixture. The obtained quantum dots are activated in ultraviolet light.

This method was developed at the Department of Physical and Colloidal Chemistry of the Ural Federal University and was mainly used to obtain thin films of metal chalcogenides and solid solutions based on them. However, the studies carried out in this work have shown its applicability for the synthesis of quantum dots based on metal sulfides and solid solutions based on them.

Chemical reagents

For hydrochemical synthesis of quantum dots CdS, PbS, Cd x Pb 1- x S,

The following chemicals were used:

cadmium chloride CdCl 2 , h, 1 M;

lead acetate Pb(CH 3 COO) 2 ZH 2 0, h, 1 M;

thiourea (NH 2) 2 CS, h, 1.5 M;

sodium citrate Na 3 C 6 H 5 O 7 , 1 M;

sodium hydroxide NaOH, analytical grade, 5 M;

Surfactant Praestol 655 BC;

Surfactant ATM 10-16 (Alkyl C10-16 trimethylammonium chloride Cl, R=C 10 -C 16);

Ethylenediaminetetraacetic acid disodium salt

C 10 H 14 O 8 N 2 Na 2 2H 2 0.1 M.

The determination of the CMC of stabilizers was carried out using an ANION conductometer.

Disposal of waste solutions

The filtered solution after hydrochemical precipitation, containing soluble salts of cadmium, lead, complexing agents and thiourea, was heated to 353 K, copper sulfate was added to it (105 g per 1 liter of the reaction mixture, 1 g was added until a violet color appeared), heated to boiling and withstood in within 10 minutes. After that, the mixture was left at room temperature for 30-40 minutes and the precipitate that formed was filtered off, which was then combined with the precipitate filtered in the previous stage. The filtrate containing complex compounds with a concentration below the maximum allowable concentration was diluted with tap water and poured into the city sewer.

Particle Analyzer Measurement ProcedurePhotocorCompact

The Photocor Compact particle size analyzer is designed to measure particle size, diffusion coefficient and molecular weight of polymers. The device is intended for traditional physical and chemical research, as well as for new applications in nanotechnology, biochemistry and biophysics.

The principle of operation of the particle size analyzer is based on the phenomenon of dynamic light scattering (photon correlation spectroscopy method). Measuring the correlation function of scattered light intensity fluctuations and the integrated scattering intensity makes it possible to find the size of dispersed particles in a liquid and the molecular weight of polymer molecules. The range of measured sizes is in the range from fractions of nm to 6 µm.

Fundamentals of the method of dynamic light scattering (photon correlation spectroscopy)

Correlator Photocor-FC is a universal instrument for measuring time correlation functions. The cross-correlation function G 12 of two signals l 1 (t) and l 2 (t) (for example, light scattering intensity) describes the relationship (similarity) of two signals in the time domain and is defined as follows:

where is the delay time. Angle brackets denote time averaging t. The autocorrelation function describes the correlation between the signal I 1 (t) and the delayed version of the same signal 1 2 (t+):

In accordance with the definition of the correlation function, the correlator operation algorithm includes the following operations:

The Photocor-FC correlator is designed specifically for the analysis of photon correlation spectroscopy (PCS) signals. The essence of the PCS method is as follows: when a laser beam passes through the test liquid containing suspended dispersed particles, part of the light is scattered by fluctuations in the concentration of the number of particles. These particles perform Brownian motion, which can be described by the diffusion equation. From the solution of this equation, an expression is obtained that relates the half-width of the scattered light spectrum Г (or the characteristic relaxation time of fluctuations Тс) with the diffusion coefficient D:

Where q is the modulus of the wave vector of the fluctuations on which the light is scattered. The diffusion coefficient D is related to the hydrodynamic particle radius R by the Einstein-Stokes equation:

where k is the Boltzmann constant, T is the absolute temperature, - shear viscosity of the solvent.

Experimental part

1. Synthesis of quantum dots based on cadmium sulfide

The study of CdS quantum dots, along with PbS QDs, is the main direction of this WRC. This is primarily due to the fact that the properties of this material in hydrochemical synthesis are well studied and, at the same time, it is little used for the synthesis of QDs. A series of experiments was carried out to obtain quantum dots in the reaction mixture of the following composition, mol/l: =0.01; = 0.2; = 0.12; [TM] = 0.3. In this case, the sequence of pouring the reagents is strictly defined: a solution of sodium citrate is added to the cadmium chloride solution, the mixture is thoroughly mixed until the precipitate formed is dissolved and diluted with distilled water. Next, the solution is alkalized with sodium hydroxide and thiourea is added to it, from this moment the reaction time begins. Last, as a stabilizing additive, the most suitable stabilizer is added, in this case Trilon B (0.1M). The required volume was determined experimentally. The experiments were carried out at a temperature of 298 K, activation was carried out in UV light.

The volumes of the added reagents were calculated according to the law of equivalents using the values ​​of the initial concentrations of the initial substances. The reaction vessel was chosen with a volume of 50 ml.

The reaction mechanism is similar to the mechanism of thin film formation, but in contrast to it, a more alkaline medium (pH=13.0) and a Trilon B stabilizer are used for the synthesis of QDs, which slows down the reaction due to the envelopment of CdS particles, and makes it possible to obtain particles of small size ( from 3 nm).

At the initial moment of time, the solution is transparent, after a minute it begins to glow yellow. When activated in ultraviolet light, the solution is bright green. When choosing the optimal concentrations, as well as stabilizers (in this case, Trilon B), the solution retains its dimensions for up to 1 hour, after which agglomerates are formed and a precipitate begins to precipitate.

The measurements were carried out on a Photocor Compact particle size analyzer; the results were processed using the DynaLS program, which analyzes the correlation function and recalculates to the average particle radius in solution. On fig. Figures 3.1 and 3.2 show the interface of the DynaLS program, as well as the results of processing the correlation function for measuring the particle size of CdS QDs:

Fig.3.1. Interface of the DynaLS program when removing the correlation function of the CdS QD solution.

Fig.3.2. Results of processing the correlation function of the CdS QD solution.

According to fig. 3.2 shows that the solution contains particles with a radius of 2 nm (peak No. 2), as well as large agglomerates. Peaks 4 to 6 are displayed with an error, since not only the Brownian motion of particles is present in the solution.

Effect of Cadmium Salt Concentration on the Size of QD ParticlesCDS

To achieve the size effect of quantum dots, it is necessary to choose the optimal concentrations of the initial reagents. In this case, the concentration of cadmium salt plays an important role; therefore, it is necessary to consider the change in the size of CdS particles with varying the concentration of CdCl 2 .

As a result of changing the concentration of cadmium salt, the following dependencies were obtained:

Fig.3.3. Effect of cadmium salt concentration on the particle size of CdS QDs at =0.005M (1), =0.01M (2), =0.02M.

Figure 11 shows that with a change in the concentration of CdCl 2, there is an insignificant change in the size of CdS particles. But as a result of the experiment, it was proved that it is necessary to stay in the optimal concentration range, where particles are formed that can create a size effect.

Synthesis of quantum dots based on lead sulfide

Another interesting direction of this WRC was the study of quantum dots based on lead sulfide. The properties of this material in hydrochemical synthesis, as well as CdS, are well studied, in addition, lead sulfide is less toxic, which expands its scope in medicine. For the synthesis of PbS QDs, the following reagents were used, mol/L: [PbAc 2 ] = 0.05; = 0.2; = 0.12; [TM] = 0.3. The pouring order is the same as for the CdS formulation: sodium citrate solution is added to the acetate solution, the mixture is thoroughly mixed until the precipitate formed is dissolved and diluted with distilled water. Next, the solution is alkalized with sodium hydroxide and thiourea is added to it, from this moment the reaction time begins. The last, as a stabilizing additive, is the surfactant praestol. The experiments were carried out at a temperature of 298 K, activation was carried out in UV light.

At the initial moment of time, the reaction mixture is transparent, but after 30 minutes it begins to slowly become cloudy, the solution becomes light beige. After adding praestol and stirring, the solution does not change color. At 3 minutes, the solution acquires a bright yellow-green glow in UV light, passing, as in the case of CdS, the green part of the spectrum.

The measurements were carried out on a Photocor Compact size analyzer. The correlation function and measurement results are shown in Figs. 3.4 and 3.5 respectively:

Fig.3.4. Interface of the DynaLS program when removing the correlation function of the PbS QD solution.

Rice. 3.5. Results of processing the correlation function of the PbS QD solution.

According to fig. 13 shows that the solution contains particles with a radius of 7.5 nm, as well as agglomerates with a radius of 133.2 nm. Peaks numbered 2 and 3 are displayed with an error, due to the presence of not only Brownian motion in the solution, but also the course of the reaction.

Effect of Lead Salt Concentration on the Size of QD ParticlesPbS

As in the case of the synthesis of CdS colloidal solutions, so in the synthesis of PbS solutions, the concentrations of the initial reagents should be selected to achieve the size effect. Let us consider the effect of lead salt concentration on the dimensions of PbS QDs.

As a result of changing the concentration of lead salt, the following dependencies were obtained:

Rice. 3.6. Effect of lead salt concentration on the particle size of PbS QDs at [PbAc 2 ]=0.05M (1), [PbAc 2 ]=0.01M (2), [PbAc 2 ]=0.02M.

According to fig. It can be seen from Fig. 14 that, at the optimum lead salt concentration (0.05 M), the particle sizes do not tend to grow steadily, while at the lead salt concentrations of 0.01 and 0.02 M, the particles grow almost linearly. Consequently, a change in the initial lead salt concentration significantly affects the size effect of PbS QD solutions.

Synthesis of quantum dots based on a solid solutionCDS- PbS

The synthesis of quantum dots based on substitutional solid solutions is extremely promising, since it allows one to vary their composition and functional properties over a wide range. Quantum dots based on substitutional solid solutions of metal chalcogenides can significantly expand the scope of their application. This is especially true for supersaturated solid solutions that are relatively stable due to kinetic hindrances. We have not found a description of experiments on the synthesis of quantum dots based on solid solutions of metal chalcogenides in the literature.

In the present work, an attempt was made for the first time to synthesize and study quantum dots based on supersaturated solid solutions of CdS–PbS substitution by lead sulfide. In order to determine the properties of the material, a series of experiments was carried out to obtain quantum dots in the reaction mixture of the following composition, mol/l: = 0.01; [PbAc 2 ] = 0.05; = 0.2; = 4; [TM] = 0.3. This formulation makes it possible to obtain supersaturated substitutional solid solutions with a cadmium sulfide content in their composition from 6 to 8 mole %.

At the same time, the sequence of pouring the reagents is strictly defined: sodium citrate is added to the lead acetate solution in the first vessel, and a white precipitate is formed, which dissolves easily, the mixture is thoroughly mixed and diluted with distilled water. In the second vessel, an aqueous ammonia solution is added to the cadmium chloride solution. Next, the solutions are mixed and thiourea is added to them, from this moment the reaction time begins. The last, as a stabilizing additive, is the surfactant praestol. The experiments were carried out at a temperature of 298 K, activation was carried out in UV light.

After adding Praestol, the solution no longer changes color, in the visible area it glows brown. In this case, the solution remains transparent. When activated with UV light, the solution begins to luminesce bright yellow, and after 5 minutes - bright green.

After a few hours, a precipitate begins to form and a gray film forms on the walls of the reactor.

Particle size studies were carried out on a Photocor Compact instrument. The interface of the DynaLS program with the correlation function and the results of its processing are shown in fig. 3.7 and 3.8 respectively:

Fig.3.7. Interface of the DynaLS program when removing the correlation function of a QD solution based on CdS-PbS HRT.

Rice. 3.8. Rice. 3.5. Results of processing the correlation function of the QD solution based on CdS-PbS TRZ.

According to fig. 3.8. It can be seen that the solution contains particles with a radius of 1.8 nm (peak no. 2), as well as agglomerates with a radius of 21.18 nm. Peak no. 1 corresponds to the nucleation of a new phase in the solution. This means that the reaction continues to go. As a result, peaks No. 4 and 5 are displayed with an error, since there are other types of particle motion besides Brownian.

Analyzing the data obtained, it can be said with confidence that the hydrochemical method for the synthesis of quantum dots is promising for their production. The main difficulty lies in the selection of a stabilizer for different initial reagents. In this case, the surfactant Praestol is best suited for colloidal solutions of TRZ based on CdS-PbS and CT based on lead sulfide, while Trilon B is best suited for CT based on cadmium sulfide.

Life safety

1. Introduction to Life Safety

Life safety (BZD) is a field of scientific and technical knowledge that studies the danger and undesirable consequences of their impact on humans and objects of the environment, the patterns of their manifestation and ways to protect against them.

The purpose of the BZD is to reduce the risk of occurrence, as well as protect against any types of hazards (natural, man-made, environmental, anthropogenic) that threaten people in everyday life, at work, in transport, in emergency situations.

The fundamental formula of the BJD is the prevention and anticipation of the potential danger that exists when a person interacts with the environment.

Thus, the BZD solves the following main tasks:

identification (recognition and quantitative assessment) of the type of negative environmental impacts;

protection from hazards or prevention of the impact of certain negative factors on humans and the environment, based on a comparison of costs and benefits;

elimination of negative consequences of exposure to dangerous and harmful factors;

creation of a normal, that is, a comfortable state of the human environment.

In the life of a modern person, an increasing place is occupied by problems related to life safety. Numerous negative factors of anthropogenic origin (noise, vibration, electromagnetic radiation, etc.) have been added to the dangerous and harmful factors of natural origin. The emergence of this science is an objective need of modern society.

Harmful and dangerous production factors in the laboratory

According to GOST 12.0.002-80 SSBT, a harmful production factor is a factor whose impact on a worker under certain conditions can lead to illness, reduced performance and (or) a negative impact on the health of offspring. Under certain conditions, a harmful factor can become dangerous.

A hazardous production factor is a factor whose impact on a worker under certain conditions leads to injury, acute poisoning or other sudden, sharp deterioration in health, or death.

According to GOST 12.0.003-74, all hazardous and harmful production factors are divided into the following groups according to the nature of their action: physical; chemical; biological; psychophysiological. In the laboratory where the studies were carried out, there are physical and chemical SanPiN 2.2.4.548-96.

Harmful substances

A harmful substance is a substance that, when in contact with the human body, can cause injuries, diseases or deviations in the state of health, detected by modern methods both in the process of contact with it and in the long-term life of this and subsequent generations. According to GOST 12.1.007-76 SSBT, harmful substances are divided into four hazard classes according to the degree of impact on the body:

I - substances are extremely dangerous;

II - highly hazardous substances;

III – moderately dangerous substances;

IV - substances of low hazard.

The maximum allowable concentration (MAC) is understood as such a concentration of chemical elements and their compounds in the environment, which, under daily influence for a long time on the human body, does not cause pathological changes or diseases established by modern research methods at any life time of the present and subsequent generations.

When carrying out work in the laboratory of oxide systems, harmful substances are used, indicated in Table. 4.1, to reduce the concentration of their vapors in the air, exhaust ventilation is turned on, which reduces the content of harmful substances to a safe level in accordance with GOST 12.1.005-88 SSBT.

Table 4.1 - MPC of harmful substances in the air of the working area

where: + - compounds that require special protection of the skin and eyes;

Cadmium, regardless of the type of compound, accumulates in the liver and kidneys, causing their damage. Reduces the activity of digestive enzymes.

Lead, when accumulated in the body, has adverse neurological, hematological, endocrine, and carcinogenic effects. Disrupts the functioning of the kidneys.

Thiocarbamide causes skin irritation, is toxic to the cardiovascular immune systems, as well as reproductive organs.

Trilon B may cause irritation of the skin, mucous membranes of the eyes and respiratory tract.

Sodium hydroxide is corrosive to the eyes, skin and respiratory tract. Corrosive action if swallowed. Inhalation of the aerosol causes pulmonary edema.

Oleic acid is poisonous. It has a weak narcotic effect. Possible acute and chronic poisoning with changes in the blood and hematopoietic organs, organs of the digestive system, pulmonary edema.

Synthesis of powders is carried out in ventilation cabinets, as a result of which the concentration of any particles in the air of the working space (of any size and nature) that are not part of the air tends to zero. In addition, personal protective equipment is used: special clothing; for respiratory protection - respirators and cotton-gauze bandages; to protect the organs of vision - goggles; to protect the skin of the hands - latex gloves.

Microclimate parameters

The microclimate is a complex of physical factors of the internal environment of the premises, which affects the heat exchange of the body and human health. Microclimatic indicators include temperature, humidity and air velocity, the temperature of the surfaces of enclosing structures, objects, equipment, as well as some of their derivatives: the air temperature gradient along the vertical and horizontal of the room, the intensity of thermal radiation from internal surfaces.

SanPiN 2.2.4.548-96 establishes the optimal and permissible values ​​of temperature, relative humidity and air velocity for the working area of ​​industrial premises, depending on the severity of the work performed, the seasons of the year, taking into account excess heat. According to the degree of influence on the well-being of a person and his performance, microclimatic conditions are divided into optimal, permissible, harmful and dangerous.

According to SanPiN 2.2.4.548-96, the conditions in the laboratory belong to the category of work Ib (work with an energy intensity of 140-174 W), performed while sitting, standing or walking and accompanied by some physical stress.

Area per worker, fact / norms, m 2 - 5 / 4.5

Volume per worker, fact / norms, m 2 - 24/15

The values ​​of microclimate indicators are given in Table 4.2.

In the working laboratory, there is no deviation from the optimal microclimate indicators. Maintenance of microclimate parameters is provided by heating and ventilation systems.

Ventilation

Ventilation - air exchange in rooms to remove excess heat, moisture, harmful and other substances in order to ensure acceptable meteorological conditions and air purity in the serviced or working area, according to GOST 12.4.021-75 SSBT.

In the laboratory of the Department of Physical and Colloidal Chemistry, ventilation is carried out by natural (through windows and doors) and mechanical ways (hoods, subject to the rules of sanitary, environmental and fire safety).

Since all work with harmful substances takes place in a fume hood, we calculate its ventilation. For approximate calculations, the amount of air required is taken according to the air exchange rate (K p) according to formula 2.1:

where V is the volume of the room, m 3;

L - total productivity, m 3 / h.

The air exchange rate shows how many times per hour the air in the room changes. The value of K p is usually 1-10. But for the ventilation of the fume hood, this figure is much higher. The area occupied by the cabinet is 1.12 m 2 (length 1.6 m, width 0.7 m, height (H) 2.0 m). Then the volume of one cabinet, taking into account the air duct (1.5), is equal to:

V \u003d 1.12 ∙ 2+ 1.5 \u003d 3.74 m 3

Since the laboratory is equipped with 4 fume hoods, the total volume will be 15m3.

From the passport data we find that an OSTBERG RFE 140 SKU fan with a capacity of 320 m 3 / h, a voltage of 230V is used for the hood. Knowing its performance, it is easy to determine the air exchange rate using formula 4.1:

h -1

The air exchange ratio of 1 fume hood is 85.56.

Noise is random fluctuations of various physical nature, characterized by the complexity of the temporal and spectral structure, one of the forms of physical pollution of the environment, adaptation to which is physically impossible. Noise above a certain level increases the release of hormones.

The permissible noise level is the level that does not cause significant anxiety and significant changes in the indicators of the functional state of systems and analyzers that are sensitive to noise.

Permissible sound pressure levels depending on the sound frequency are taken in accordance with GOST 12.1.003-83 SSBT, are presented in table 4.3.

Table 4.3 - Permissible sound pressure levels in octave frequency bands and equivalent noise levels at workplaces

Noise protection, according to SNiP 23-03-2003, should be provided by the development of noise-proof equipment, the use of means and methods of collective protection, the use of means and methods of collective protection, the use of personal protective equipment, which are classified in detail in GOST 12.1.003-83 SSBT.

The source of constant noise in the laboratory is operating fume hoods. The noise level is estimated at about 45 dB, i.e. does not exceed the established standards.

illumination

Illumination is a luminous quantity equal to the ratio of the luminous flux falling on a small surface area to its area. Lighting is regulated in accordance with SP 52.13330.2011.

Industrial lighting is:

natural(due to direct sunlight and scattered light of the sky, varies depending on the geographical latitude, time of day, degree of cloudiness, atmospheric transparency, season, precipitation, etc.);

artificial(created by artificial light sources). It is used in the absence or lack of natural light. Rational artificial lighting should provide normal conditions for work with an acceptable consumption of funds, materials and electricity;

use when there is insufficient natural light combined (combined) lighting. The latter is lighting in which natural and artificial light is used simultaneously during daylight hours.

In the chemical laboratory, natural lighting is provided by one side window. Natural light is not enough, so artificial lighting is used. It is provided by 8 OSRAM L 30 lamps. Optimal laboratory illumination is achieved with mixed lighting.

electrical safety

According to GOST 12.1.009-76 SSBT, electrical safety is a system of organizational and technical measures and means that protect people from the harmful and dangerous effects of electric current, electric arc, electromagnetic field and static electricity.

In a chemical laboratory, the source of electric shock is electrical equipment - a distiller, a thermostat, electric stoves, electronic scales, electrical outlets. General safety requirements for electrical equipment, including embedded computing devices, are established by GOST R 52319-2005.

Electric current, passing through the human body, has the following types of effects on it: thermal, electrolytic, mechanical, biological. To ensure protection against electric shock in electrical installations, technical methods and means of protection must be used in accordance with GOST 12.1.030-81 SSBT.

In accordance with the rules for the installation of electrical installations of the PUE, all premises are divided into three categories in relation to the danger of electric shock to people: without increased danger; with increased risk; especially dangerous.

The laboratory room belongs to the category - without increased danger. To ensure protection against electric shock in electrical installations, technical methods and means of protection must be applied.

fire safety

According to GOST 12.1.004-91 SSBT, a fire is an uncontrolled combustion process, characterized by social and / or economic damage as a result of exposure to people and / or material assets of thermal decomposition and / or combustion factors, developing outside a special focus, as well as applied fire extinguishing agents.

The causes of a possible fire in the laboratory are a violation of safety regulations, a malfunction of electrical equipment, electrical wiring, etc.

In accordance with NPB 105-03, the premises are classified as "B1", i.e. fire hazardous, where there are flammable and slow-burning liquids, slow-burning substances and materials, plastic that can only burn. According to SNiP 21-01-97 the building has II degree of fire resistance.

In the event of a fire, evacuation routes are provided to ensure the safe evacuation of people. The height of the horizontal sections of the escape routes must be at least 2 m, the width of the horizontal sections of the escape routes must be at least 1.0 m. Escape routes are illuminated.

The laboratory complied with all fire safety rules in accordance with existing regulations.

Emergencies

According to GOST R 22.0.05-97, an emergency situation (ES) is an unexpected, sudden situation in a certain territory or economic facility as a result of an accident, a man-made disaster that can lead to human casualties, damage to human health or the environment, material losses and violation of the living conditions of people.

In a chemical laboratory, the following causes of emergencies are possible:

violation of safety regulations;

ignition of electrical appliances;

violation of the insulation of electrical equipment;

In connection with the possible causes of emergencies in the laboratory, Table 4.4 of possible emergencies has been compiled.

Ways to protect against possible emergencies are regular briefings on safety and behavior in emergencies; regular checking of electrical wiring; have an evacuation plan.

Table 4.4 - Possible emergency situations in the laboratory

Possible emergency	Cause	Measures to eliminate emergencies
Electric shock	Violation of safety regulations for working with electric current; Violation of the integrity of the insulation, as a result of the aging of insulating materials.	Turn off the electricity with a common switch; call an ambulance to the victim; provide first aid if necessary; report the incident to the employee responsible for the equipment, to determine the cause of the emergency.
Fire in the laboratory.	Violation of fire safety equipment; Short circuit;	De-energize the equipment operating in the laboratory; call the fire brigade, start extinguishing the fire with fire extinguishers; report the incident to the employee responsible for the equipment, to determine the cause of the emergency.

Conclusions on the BJD section

In the section on life safety, the following factors are considered:

microclimate parameters comply with regulatory documents and create comfortable conditions in the chemical laboratory;

the concentration of harmful substances in the air of the laboratory when receiving chalcogenide films meet hygienic standards. The laboratory has all the necessary individual and collective means of protection against the influence of harmful substances;

the calculation of the ventilation system of the fume hood, based on the OSTBERG RFE 140 SKU fan, with a capacity of -320 m 3 / h, a voltage of -230 V, ensures the possibility of minimizing the harmful effects of chemical reagents on humans and, according to the calculated data, provides a sufficient air exchange rate - 86;

noise at the workplace complies with standard norms;

sufficient illumination of the laboratory is realized mainly due to artificial lighting;

according to the danger of electric shock, the chemical laboratory belongs to the premises without increased danger, all current-carrying parts of the devices used are insulated and grounded.

The fire hazard of this laboratory room was also considered. In this case, it can be classified as category "B1", the degree of fire resistance is II.

To prevent emergencies, Ural Federal University regularly conducts briefings with those responsible for ensuring the safety of staff and students. As an example of an emergency, electric shock was considered in case of faulty electrical equipment.

Numerous spectroscopic methods that appeared in the second half of the 20th century - electron and atomic force microscopy, nuclear magnetic resonance spectroscopy, mass spectrometry - would seem to have sent traditional optical microscopy into retirement long ago. However, the skillful use of the phenomenon of fluorescence more than once extended the life of the "veteran". This article will talk about quantum dots(fluorescent semiconductor nanocrystals), which breathed new powers into optical microscopy and made it possible to look beyond the notorious diffraction limit. The unique physical properties of quantum dots make them ideal for ultra-sensitive multicolor registration of biological objects, as well as for medical diagnostics.

The paper gives ideas about the physical principles that determine the unique properties of quantum dots, the main ideas and prospects for the use of nanocrystals, and talks about the successes already achieved in their application in biology and medicine. The article is based on the results of research conducted in recent years at the Laboratory of Molecular Biophysics of the Institute of Bioorganic Chemistry. MM. Shemyakin and Yu.A. Ovchinnikov, together with the University of Reims and the Belarusian State University, aimed at developing a new generation of biomarker technology for various areas of clinical diagnostics, including cancer and autoimmune diseases, as well as at creating new types of nanosensors for the simultaneous registration of many biomedical parameters. The original version of the work was published in The Nature; To some extent, the article is based on the second seminar of the Council of Young Scientists of the IBCh RAS. - Ed.

Part I, theoretical

Figure 1. Discrete energy levels in nanocrystals."solid" semiconductor ( left) has a valence band and a conduction band separated by a band gap Eg. Semiconductor nanocrystal ( on right) is characterized by discrete energy levels similar to the energy levels of a single atom. In a nanocrystal Eg is a function of size: an increase in the size of a nanocrystal leads to a decrease Eg.

Reducing the particle size leads to the manifestation of very unusual properties of the material from which it is made. The reason for this is the quantum-mechanical effects that arise when the motion of charge carriers is spatially limited: the energy of the carriers in this case becomes discrete. And the number of energy levels, as quantum mechanics teaches, depends on the size of the "potential well", the height of the potential barrier and the mass of the charge carrier. Increasing the size of the "well" leads to an increase in the number of energy levels, which at the same time become closer to each other until they merge, and the energy spectrum becomes "continuous" (Fig. 1). The movement of charge carriers can be limited along one coordinate (forming quantum films), along two coordinates (quantum wires or filaments), or along all three directions - these will be quantum dots(CT).

Semiconductor nanocrystals are intermediate structures between molecular clusters and "solid" materials. The boundaries between molecular, nanocrystalline, and solid materials are not well defined; however, the range of 100 ÷ 10,000 atoms per particle can be roughly considered the "upper limit" of nanocrystals. The upper limit corresponds to the dimensions for which the interval between energy levels exceeds the energy of thermal vibrations kT (k is the Boltzmann constant, T- temperature), when charge carriers become mobile.

The natural length scale for electronically excited regions in "continuous" semiconductors is determined by the Bohr exciton radius a x, which depends on the strength of the Coulomb interaction between the electron ( e) and hole (h). In nanocrystals, the order of magnitude a x self size begins to influence the configuration of the pair e–h and hence the size of the exciton. It turns out that in this case the electronic energies are directly determined by the size of the nanocrystal - this phenomenon is known as the "quantum confinement effect". Using this effect, one can control the nanocrystal band gap ( Eg), simply by changing the particle size (Table 1).

Unique properties of quantum dots

As a physical object, quantum dots have been known for a long time, being one of the forms intensively developed today. heterostructures. A feature of quantum dots in the form of colloidal nanocrystals is that each dot is an isolated and mobile object in a solvent. Such nanocrystals can be used to build various associates, hybrids, ordered layers, etc., on the basis of which elements of electronic and optoelectronic devices, probes and sensors for analyzes in microvolumes of a substance, various fluorescent, chemiluminescent, and photoelectrochemical nanoscale sensors are constructed.

The reason for the rapid penetration of semiconductor nanocrystals into various fields of science and technology is their unique optical characteristics:

• narrow symmetrical fluorescence peak (in contrast to organic dyes, which are characterized by the presence of a long-wavelength “tail”; Fig. 2, left), whose position is controlled by the choice of the nanocrystal size and its composition (Fig. 3);
• a wide excitation band, which makes it possible to excite nanocrystals of different colors with one radiation source (Fig. 2, left). This advantage is fundamental when creating multi-color coding systems;
• high fluorescence brightness determined by high extinction value and high quantum yield (up to 70% for CdSe/ZnS nanocrystals);
• uniquely high photostability (Fig. 2, on right), which allows the use of high power excitation sources.

Figure 2. Spectral properties of cadmium-selenium (CdSe) quantum dots. Left: Nanocrystals of different colors can be excited by a single source (the arrow indicates excitation by an argon laser with a wavelength of 488 nm). The inset shows the fluorescence of CdSe/ZnS nanocrystals of different sizes (and, accordingly, colors) excited by a single light source (UV lamp). On right: Quantum dots are extremely photostable compared to other common dyes, which are rapidly destroyed under the beam of a mercury lamp in a fluorescent microscope.

Figure 3. Properties of quantum dots from different materials. Above: Fluorescence ranges of nanocrystals made from different materials. Bottom: CdSe quantum dots of various sizes cover the entire visible range of 460–660 nm. Bottom right: Scheme of a stabilized quantum dot, where the "core" is covered with a semiconductor shell and a protective polymer layer.

Production technology

The synthesis of nanocrystals is carried out by rapid injection of precursor compounds into the reaction medium at a high temperature (300–350°C) and subsequent slow growth of nanocrystals at a relatively low temperature (250–300°C). In the “focusing” mode of synthesis, the growth rate of small particles is higher than the growth rate of large ones, as a result of which the spread in nanocrystal sizes decreases , .

Controlled synthesis technology makes it possible to control the shape of nanoparticles using the anisotropy of nanocrystals. The characteristic crystal structure of a particular material (for example, CdSe is characterized by hexagonal packing - wurtzite, Fig. 3) mediates the "selected" growth directions that determine the shape of nanocrystals. This is how nanorods or tetrapods are obtained - nanocrystals elongated in four directions (Fig. 4).

Figure 4. Different shapes of CdSe nanocrystals. Left: CdSe/ZnS spherical nanocrystals (quantum dots); in the center: rod-shaped (quantum rods). On right: in the form of tetrapods. (Transmission electron microscopy. Mark - 20 nm.)

Barriers to practical application

A number of limitations stand in the way of the practical application of nanocrystals from semiconductors of groups II–VI. First, the quantum yield of their luminescence depends significantly on the properties of the environment. Secondly, the stability of the "cores" of nanocrystals in aqueous solutions is also low. The problem lies in the surface "defects", which play the role of nonradiative recombination centers or "traps" for excited e–h steam.

To overcome these problems, quantum dots are enclosed in a shell consisting of several layers of wide-gap material. This allows you to isolate e-h pair in the nucleus, increase its lifetime, reduce nonradiative recombination, and hence increase the fluorescence quantum yield and photostability.

In this regard, to date, the most widely used fluorescent nanocrystals have a core/shell structure (Fig. 3). Advanced procedures for the synthesis of CdSe/ZnS nanocrystals make it possible to achieve a quantum yield of 90%, which is close to the best organic fluorescent dyes.

Part II: application of quantum dots in the form of colloidal nanocrystals

Fluorophores in medicine and biology

The unique properties of QDs make it possible to use them in almost all systems of labeling and visualization of biological objects (with the exception of only fluorescent intracellular labels expressed genetically - widely known fluorescent proteins).

To visualize biological objects or processes, QDs can be injected directly into the object or with “attached” recognition molecules (usually antibodies or oligonucleotides). Nanocrystals penetrate and are distributed throughout the object in accordance with their properties. For example, nanocrystals of different sizes penetrate biological membranes in different ways, and since the size determines the color of fluorescence, different areas of the object also turn out to be colored differently (Fig. 5) , . The presence of recognizing molecules on the surface of nanocrystals makes it possible to implement targeted binding: the desired object (for example, a tumor) is stained with a given color!

Figure 5. Coloring objects. Left: multicolor confocal fluorescent image of the distribution of quantum dots against the background of the microstructure of the cellular cytoskeleton and nucleus in the THP-1 cell line of human phagocytes. Nanocrystals remain photostable in cells for at least 24 hours and do not cause damage to the structure and function of cells. On right: accumulation of nanocrystals "cross-linked" with the RGD peptide in the tumor area (arrow). To the right - control, introduced nanocrystals without peptide (CdTe nanocrystals, 705 nm).

Spectral coding and "liquid microchips"

As already mentioned, the fluorescence peak of nanocrystals is narrow and symmetrical, which makes it possible to reliably isolate the fluorescence signal of nanocrystals of different colors (up to ten colors in the visible range). On the contrary, the absorption band of nanocrystals is wide, that is, nanocrystals of all colors can be excited by a single light source. These properties, as well as their high photostability, make quantum dots ideal fluorophores for multicolor spectral coding of objects - similar to a barcode, but using multicolor and "invisible" codes that fluoresce in the infrared region.

Currently, the term “liquid microchips” is increasingly used, which, like classical flat chips, where the detecting elements are located on a plane, can be used to analyze multiple parameters simultaneously, using sample microvolumes. The principle of spectral coding using liquid microchips is illustrated in Figure 6. Each element of the microchip contains a given number of QDs of certain colors, and the number of encoded variants can be very large!

Figure 6. The principle of spectral coding. Left:"regular" flat microchip. On right:"liquid microchip", each element of which contains a given number of CTs of certain colors. At n levels of fluorescence intensity and m colors, the theoretical number of encoded variants is n m-1. So, for 5–6 colors and 6 intensity levels, this will be 10,000–40,000 options.

Such coded trace elements can be used for direct labeling of any objects (for example, securities). Embedded in polymer matrices, they are extremely stable and durable. Another aspect of application is the identification of biological objects in the development of early diagnostic methods. The indication and identification method consists in the fact that a specific recognition molecule, , is attached to each spectrally encoded element of the microchip. The solution contains a second recognition molecule, to which the signal fluorophore is "sewn". Simultaneous appearance of microchip fluorescence and signal fluorophore indicates the presence of the studied object in the analyzed mixture.

Flow cytometry can be used to analyze encoded microparticles on the fly. A solution containing microparticles passes through a channel irradiated by a laser, where each particle is characterized spectrally. The software of the device allows you to identify and characterize events associated with the appearance of certain compounds in the sample - for example, markers of cancer or autoimmune diseases,.

In the future, based on semiconductor fluorescent nanocrystals, microanalyzers can be created for the simultaneous registration of a huge number of objects at once.

Molecular sensors

The use of QDs as probes makes it possible to measure the parameters of the medium in local areas, the size of which is comparable to the size of the probe (nanometer scale). The operation of such measuring instruments is based on the use of the Förster resonant energy transfer (FRET) effect. The essence of the FRET effect is that when two objects approach each other (donor and acceptor) and overlap fluorescence spectrum first since absorption spectrum second, the energy is transferred non-radiatively - and if the acceptor can fluoresce, it will glow with a vengeance.

We already wrote about the FRET effect in the article “ Tape measure for spectroscopist » .

Three parameters of quantum dots make them very attractive donors in FRET format systems.

1. The ability to select the emission wavelength with high accuracy to obtain the maximum overlap of the emission spectra of the donor and excitation of the acceptor.
2. Possibility of excitation of different QDs by one wavelength of one light source.
3. Possibility of excitation in the spectral region far from the emission wavelength (difference >100 nm).

There are two strategies for using the FRET effect:

• registration of the act of interaction of two molecules due to conformational changes in the donor-acceptor system and
• registration of changes in the optical properties of the donor or acceptor (for example, the absorption spectrum).

This approach made it possible to implement nanoscale sensors for measuring pH and the concentration of metal ions in a local area of ​​a sample. The sensitive element in such a sensor is a layer of indicator molecules that change their optical properties when bound to the registered ion. As a result of binding, the overlap of the fluorescence spectra of QDs and the absorption of the indicator changes, which also changes the efficiency of energy transfer.

An approach that uses conformational changes in the donor-acceptor system is implemented in a nanoscale temperature sensor. The action of the sensor is based on the temperature change in the shape of the polymer molecule that binds the quantum dot and the acceptor - fluorescence quencher. As the temperature changes, both the distance between the quencher and the fluorophore and the fluorescence intensity change, from which a conclusion about the temperature is already made.

Molecular Diagnostics

The rupture or formation of a bond between a donor and an acceptor can be registered in exactly the same way. Figure 7 demonstrates the "sandwich" principle of registration, in which the registered object acts as a link ("adapter") between the donor and the acceptor.

Figure 7. The principle of registration using the FRET format. The formation of a conjugate (“liquid microchip”)-(recorded object)-(signal fluorophore) brings the donor (nanocrystal) closer to the acceptor (AlexaFluor dye). By itself, laser radiation does not excite dye fluorescence; the fluorescent signal appears only due to the resonant energy transfer from the CdSe/ZnS nanocrystal. Left: energy transfer conjugate structure. On right: spectral scheme of dye excitation.

An example of the implementation of this method is the creation of a diagnosticum for an autoimmune disease systemic scleroderma(scleroderma). Here, quantum dots with a fluorescence wavelength of 590 nm served as a donor, and an organic dye, AlexaFluor 633, served as an acceptor. An antigen to an autoantibody, a marker of scleroderma, was "sewn" onto the surface of a microparticle containing quantum dots. Secondary antibodies labeled with a dye were introduced into the solution. In the absence of a target, the dye does not approach the surface of the microparticle, there is no energy transfer, and the dye does not fluoresce. But if autoantibodies appear in the sample, this leads to the formation of a microparticle-autoantibody-dye complex. As a result of the energy transfer, the dye is excited, and its fluorescence signal appears in the spectrum with a wavelength of 633 nm.

The importance of this work is also in the fact that autoantibodies can be used as diagnostic markers at the earliest stage of development of autoimmune diseases. "Liquid microchips" allow you to create test systems in which antigens are in much more natural conditions than on a plane (as in "ordinary" microchips). The results already obtained open the way to the creation of a new type of clinical diagnostic tests based on the use of quantum dots. And the implementation of approaches based on the use of spectrally encoded liquid microchips will make it possible to simultaneously determine the content of many markers at once, which is the basis for a significant increase in the reliability of diagnostic results and the development of early diagnostic methods.

Hybrid molecular devices

The possibility of flexible control of the spectral characteristics of quantum dots opens the way to nanoscale spectral devices. In particular, QDs based on cadmium-tellurium (CdTe) made it possible to expand the spectral sensitivity bacteriorhodopsin(bR), known for its ability to use light energy to "pump" protons across a membrane. (The resulting electrochemical gradient is used by bacteria to synthesize ATP.)

In fact, a new hybrid material was obtained: the attachment of quantum dots to purple membrane- a lipid membrane containing densely packed bacteriorhodopsin molecules - extends the range of photosensitivity to the UV and blue regions of the spectrum, where "ordinary" bR does not absorb light (Fig. 8) . The mechanism of energy transfer to bacteriorhodopsin from a quantum dot that absorbs light in the UV and blue regions is still the same: this is FRET; In this case, the radiation acceptor is retinal- the same pigment that works in the photoreceptor rhodopsin.

Figure 8. "Upgrade" bacteriorhodopsin using quantum dots. Left: a proteoliposome containing bacteriorhodopsin (in the form of trimers) with CdTe-based quantum dots “sewn” to it (shown as orange spheres). On right: scheme for expanding the spectral sensitivity of bR due to QD: on the spectrum, the region takeovers CT is in the UV and blue parts of the spectrum; range emissions can be "customized" by selecting the size of the nanocrystal. However, in this system, energy emission by quantum dots does not occur: the energy nonradiatively migrates to bacteriorhodopsin, which does work (pumps H + ions into the liposome).

Proteoliposomes created on the basis of this material (lipid “vesicles” containing the bR-CT hybrid) pump protons into themselves under illumination, effectively lowering the pH (Fig. 8). This invention, insignificant at first glance, may form the basis of optoelectronic and photonic devices in the future and find application in the field of electric power and other types of photovoltaic conversions.

Summarizing, it should be emphasized that quantum dots in the form of colloidal nanocrystals are the most promising objects of nano-, bionano- and biocopper-nanotechnologies. After the first demonstration of the possibilities of quantum dots as fluorophores in 1998, there was a lull for several years associated with the formation of new original approaches to the use of nanocrystals and the realization of the potential that these unique objects possess. But in recent years, there has been a sharp rise: the accumulation of ideas and their implementation determined a breakthrough in the creation of new devices and tools based on the use of semiconductor nanocrystalline quantum dots in biology, medicine, electronic engineering, solar energy technology, and many others. Of course, there are still many unsolved problems along the way, but the growing interest, the growing number of teams that are working on these problems, the growing number of publications devoted to this area, allow us to hope that quantum dots will become the basis of the next generation of technology and technology.

Video recording of V.A. Oleinikov at the second seminar of the Council of Young Scientists of the IBCh RAS, held on May 17, 2012.

Literature

1. Oleinikov V.A. (2010). Quantum Dots in Biology and Medicine. Nature. 3 , 22;
2. Oleinikov V.A., Sukhanova A.V., Nabiev I.R. (2007). Fluorescent semiconductor nanocrystals in biology and medicine. Russian nanotechnologies. 2 , 160–173;
3. Alyona Sukhanova, Lydie Venteo, Jérôme Devy, Mikhail Artemyev, Vladimir Oleinikov, et. al. (2002). Highly Stable Fluorescent Nanocrystals as a Novel Class of Labels for Immunohistochemical Analysis of Paraffin-Embedded Tissue Sections . Lab Investment. 82 , 1259-1261;
4. C. B. Murray, D. J. Norris, M. G. Bawendi. (1993). Synthesis and characterization of nearly monodisperse CdE (E = sulfur, selenium, tellurium) semiconductor nanocrystallites . J. Am. Chem. soc.. 115 , 8706-8715;
5. Margaret A. Hines, Philippe Guyot-Sionnest. (1998). Bright UV-Blue Luminescent Colloidal ZnSe Nanocrystals . J Phys. Chem. B. 102 , 3655-3657;
6. Manna L., Scher E.C., Alivisatos P.A. (2002). Shape control of colloidal semiconductor nanocrystals . J. Clust. sci. 13 , 521–532;
7. Fluorescent Nobel Prize in Chemistry;
8. Igor Nabiev, Siobhan Mitchell, Anthony Davies, Yvonne Williams, Dermot Kelleher, et. al. (2007). Nonfunctionalized Nanocrystals Can Exploit a Cell's Active Transport Machinery Delivering Them to Specific Nuclear and Cytoplasmic Compartments . Nano Lett.. 7 , 3452-3461;
9. Yvonne Williams, Alyona Sukhanova, MaÅgorzata Nowostawska, Anthony M. Davies, Siobhan Mitchell, et. al. (2009). Probing Cell-Type-Specific Intracellular Nanoscale Barriers Using Size-Tuned Quantum Dots Nano pH Meter ;
10. Alyona Sukhanova, Andrei S. Susha, Alpan Bek, Sergiy Mayilo, Andrey L. Rogach, et. al. (2007). Nanocrystal-Encoded Fluorescent Microbeads for Proteomics: Antibody Profiling and Diagnostics of Autoimmune Diseases. Nano Lett.. 7 , 2322-2327;
11. Aliaksandra Rakovich, Alyona Sukhanova, Nicolas Bouchonville, Evgeniy Lukashev, Vladimir Oleinikov, et. al. (2010). Resonance Energy Transfer Improves the Biological Function of Bacteriorhodopsin within a Hybrid Material Built from Purple Membranes and Semiconductor Quantum Dots . Nano Lett.. 10 , 2640-2648;

Good time of the day, Khabrazhiteli! I think many have noticed that more and more advertisements for displays based on quantum dot technology, the so-called QD - LED (QLED) displays, began to appear, despite the fact that at the moment this is just marketing. Similar to LED TV and Retina, this is an LCD display technology that uses quantum dot LEDs as a backlight.

Your humble servant nevertheless decided to figure out what quantum dots are and what they are eaten with.

Instead of an introduction

quantum dot- a fragment of a conductor or semiconductor whose charge carriers (electrons or holes) are limited in space in all three dimensions. The size of a quantum dot must be so small that quantum effects are significant. This is achieved if the kinetic energy of the electron is noticeably greater than all other energy scales: first of all, it is greater than the temperature expressed in energy units. Quantum dots were first synthesized in the early 1980s by Alexei Ekimov in a glass matrix and Louis E. Brus in colloidal solutions. The term "quantum dot" was coined by Mark Reed.

The energy spectrum of a quantum dot is discrete, and the distance between the stationary energy levels of the charge carrier depends on the size of the quantum dot itself as - ħ/(2md^2), where:

1. ħ is the reduced Planck constant;
2. d is the characteristic point size;
3. m is the effective mass of an electron at a point

In simple terms, a quantum dot is a semiconductor whose electrical characteristics depend on its size and shape.

For example, when an electron moves to a lower energy level, a photon is emitted; since it is possible to control the size of the quantum dot, it is also possible to change the energy of the emitted photon, which means changing the color of the light emitted by the quantum dot.

Types of quantum dots

There are two types:

• epitaxial quantum dots;
• colloidal quantum dots.

In fact, they are named so according to the methods of their production. I will not talk about them in detail due to the large number of chemical terms (Google to help). I will only add that with the help of colloidal synthesis it is possible to obtain nanocrystals coated with a layer of adsorbed surface-active molecules. Thus, they are soluble in organic solvents, after modification also in polar solvents.

Construction of quantum dots

Usually a quantum dot is a semiconductor crystal in which quantum effects are realized. An electron in such a crystal feels like it is in a three-dimensional potential well and has many stationary energy levels. Accordingly, when moving from one level to another, a quantum dot can emit a photon. With all this, the transitions are easy to control by changing the size of the crystal. It is also possible to throw an electron to a high energy level and receive radiation from the transition between lower levels and, as a result, we get luminescence. Actually, it was the observation of this phenomenon that served as the first observation of quantum dots.

Now about displays

The history of full-fledged displays began in February 2011, when Samsung Electronics presented the development of a full-color display based on QLED quantum dots. It was a 4-inch display driven by an active matrix, i.e. each color quantum dot pixel can be turned on and off by a thin film transistor.

To create a prototype, a layer of quantum dot solution is applied to the silicon board and a solvent is sprayed on. After that, a rubber stamp with a comb surface is pressed into the layer of quantum dots, separated and stamped onto glass or flexible plastic. This is how the strips of quantum dots are deposited on the substrate. In color displays, each pixel contains a red, green, or blue subpixel. Accordingly, these colors are used with different intensities to obtain as many shades as possible.

The next step in development was the publication of an article by scientists from the Indian Institute of Science in Bangalore. Where quantum dots were described that luminesce not only in orange, but also in the range from dark green to red.

Why is LCD worse?

The main difference between a QLED display and an LCD is that the latter can only cover 20-30% of the color range. Also, in QLED TVs, there is no need to use a layer with light filters, since the crystals, when voltage is applied to them, always emit light with a well-defined wavelength and, as a result, with the same color value.

There was also news about the sale of a quantum dot computer display in China. Unfortunately, I have not had a chance to check it with my own eyes, unlike the TV.

P.S. It is worth noting that the scope of quantum dots is not limited to LED - monitors, among other things, they can be used in field-effect transistors, photocells, laser diodes, they are also being studied for the possibility of using them in medicine and quantum computing.

P.P.S. If we talk about my personal opinion, then I believe that they will not be popular for the next ten years, not because they are little known, but because the prices for these displays are exorbitant, but still I would like to hope that quantum points will find their application in medicine, and will be used not only to increase profits, but also for good purposes.

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