RU2386173C2 - Method of forming nanosized structure for protection from forgery and checking authenticity of valuable articles using giant raman scattering effect - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: method of forming nanosized structure for protection from forgery and checking authenticity of valuable articles using giant Raman scattering effect, where during use of the articles a high degree of reproducibility of the protective elements is provided, and the material of the protective element is nanosized structures which ensure additional amplification of the identification feature due to interference of the probing electromagnetic signal in the visible optical range in the said structures.

EFFECT: invention provides high degree of reproducibility of protective elements for valuable articles, as well as amplification of the identification feature and easier checking for the presence and authenticity of the protective element.

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Description

The invention relates to methods for protecting valuable products from counterfeiting and can be used to protect against museum counterfeit property, including paintings, jewelry, as well as expensive medicines, intellectual property, banknotes, credit and other securities, as well as to enable subsequent determining their authenticity using technical means.

Technical solutions of a similar nature are well known in the art.

So, from the prior art, individual means of protecting documents are known in the form of perforations, the pattern of which has recognizable irregularities. Perforation is carried out using a laser beam based on a conventional pattern, while the laser is controlled by a computer in such a way that each perforation has an individual irregularity depending on the initial value, see, for example, description of application DE No. 0368353, B44F 1/12, 1988 [ one].

The disadvantages of this method can be attributed to the fact that they can be quite easily reproduced with a high degree of compliance with the original using modern means, widely known and available to a wide range of specialists.

So, the prior art method of protection against counterfeiting and authenticity control of valuable products, disclosed in the description of the patent of the Russian Federation No. 2074420, G07D 7/00, G01N 24/08, 02/27/1997 [2]. The method consists in introducing into the material of the protected object or applying a mark on it, which is used as a stable isotope osmium-187 or its compound, and the determination of its presence is carried out by nuclear magnetic properties. The introduction into the material of the protected object or the deposition of the stable osmium-187 isotope on it can be carried out in a chemical compound that provides constant orientation of the magnetic moments of the electron shells of osmium-187 atoms. This method allows you to simplify and reduce the cost of protection against counterfeiting banknotes, securities and documents while ensuring a high degree of security.

However, the prior art method of protection against counterfeiting of valuable products, disclosed in the description of the patent of the Russian Federation No. 2144216, G07D 7/00, G07D 7/06, G06K 19/08, 10.01.2000 [3]. According to this method, an isotopic indicator based on a mixture of stable isotopes is used as a means of protection. A protective label is formed by the said isotope indicator in such a way that it is possible to control its presence on the protected product (during detection) by at least one of the spectral analysis methods (for example, X-ray fluorescence or luminescent methods). This security tag can be formed directly on the protected product or independently of it in any known form and using known technologies.

In addition, technologies of a similar purpose disclosed in the descriptions of foreign security documents, for example GB 1193511, JP 9119867, US 4533244, are known from the prior art.

Also known from the prior art is a method of protection against counterfeiting and authenticity control of valuable products, disclosed in the description of the patent of the Russian Federation No. 2276409, G07D 7/06, G06K 19/14, 05/10/2006 [4]. According to this method, a passive protective means of a given structure is formed on the product, which provides the ability to control the presence and authenticity of the said means by a physical method for analyzing resonance effects during external exposure to it by probing electromagnetic radiation of a given radio frequency and detecting parameters of certain informative features in the resonant response of the protective means to mentioned external impact followed by automatic matching registered OF DATA parameters of these informative features with reference values. As a passive protective agent, a metallized at least three-layer resonant filter structure is used. The microwave frequency is used as the probing radiation, and characteristic peak values of the frequency response of the direct transmission and back reflection coefficients are used as informative signs.

The disadvantages of all of the above analogues should include their lack of reliability. This is due, first of all, to the fact that the current level of development of computational, analytical and multiplying equipment allows reproducing with a high degree of identity almost any security paper in unlimited quantities at a relatively low material cost.

In addition, a method of protection against falsification and authenticity control of a valuable product disclosed in the description of application No. 2007104844, G07D 7/06, 2007 is known. In this case, a metal with an electrochemically treated surface with the formation of a roughness of nanoscale level is used as a protective agent. , and as detected informational signs, the components of the “giant” Raman scattering formed by such a surface are used.

Despite the potential advantages of protective agents using the effect of "giant" Raman scattering (GCR), formed by a metal surface with a roughness of nanoscale level, the task of creating technological, highly reproducible in terms of characteristics of materials and structures for the formation of elements of protective structures requires solving.

The problem to which the invention is directed is to increase the level of reliability of protection against counterfeiting and copying valuable products.

When implementing this invention, several technical results are achieved, consisting in increasing the degree of complexity of a protective agent on a valuable product, ensuring technological reproducibility of the elements of the protective structures of the same valuable products, and further enhancing the intensity of the detected information feature.

This problem is solved by the fact that the method of forming the protective structure uses the technology of controlled structuring of the metal layer and signal amplification due to the phenomenon of interference in the SERS - the active nanostructure of the protective elements.

The phenomenon of traditional Raman scattering (Raman scattering) of light, also called Romanov scattering or the Roman effect, consists in the appearance of new spectral components in the spectrum of light scattered by the medium. In this case, the frequency differences of these spectral lines in the Raman spectrum, called the Stokes ω _{st, i} , and anti-Stokes ω _ast.i , and the exciting light frequencies ω _L coincide with the frequencies of intramolecular vibrations Ω _i , the set of which, in turn, is a unique characteristic of the molecule Wednesday. In other words,

ω _{st, i} = ω _L -Ω _i and ω _ast.i = ω _L + Ω _i . The mechanism of the appearance of new lines in the spectrum of scattered light has a clear radiophysical analogy and is associated with the appearance of new combination frequencies (side bands) in the spectrum when the amplitude of the electromagnetic oscillation of the fundamental frequency is modulated by a modulating oscillation of a different frequency.

The intensity of the scattered light is determined by the corresponding molecular susceptibilities and the number of molecules falling into the scattering volume (which, in turn, depends on the density of the substance, and therefore on its state of aggregation). A typical ratio of the Stokes component intensity to the incident radiation intensity in condensed matter (in liquid and solid bodies) is I (ω _{st, i} ) / I (ω _L ) = 10 ^-6 , which allows us to consider Raman scattering as a very weak effect.

In 1974, the English electrochemist M. Fleishman decided to increase the effective number of molecules involved in scattering from a monolayer, increasing the surface area while maintaining the area illuminated by pump radiation. To do this, he “roughened” the silver surface in an electrolyte (KCl aqueous solution) by anodic etching, and then in the same electrochemical cell he adsorbed a monolayer of C ₅ H ₅ N pyridine molecules onto an overgrown silver surface. M. Fleishman easily managed to observe the Raman spectrum of adsorbed molecules that he interpreted as a consequence of an increase in the effective area of the monolayer. However, in the late 70s, it became clear that the expressed interpretation was inconsistent with the fact that the recorded Raman intensity increased by 10 ⁶ -10 ⁷ times, while the effective area increased only by an order of magnitude. This indicated that a nontrivial effect, which was later called “giant” Raman scattering [1], may be behind the observed enhancement of Raman scattering.

Even more significantly, the local field is modified near the rough metal surface and near the surface of small (with a size much smaller than the incident field wavelength λ) metal particles.

However, for the technical implementation of effective protective elements, the most promising are technological developments for the formation of arrays of metal nano-objects combined in a system with controlled variation of parameters of both individual nano-objects (shape, size) and the system as a whole (distance between nano-objects, degree of ordering, symmetry) .

Another technological task that needs to be addressed when implementing protective structures is to optimize the design of the HRS protection elements with the use of additional effects to enhance the HRS signal.

The main directions of creating effective HCR structures are as follows:

- the creation of GCR-active structures based on island films of noble metals,

- creation of structures for the interference amplification of the SERS signal, which provide an additional increase in the electric field in the near optical zone of the nanoparticles due to the fulfillment of the conditions for interference amplification of the exciting radiation in the SERS active layer of the multilayer structure.

The main and serious drawback of sensitive elements based on island films made by vacuum deposition is the relative irreproducibility and heterogeneity at the micro and nanoscale levels: the characteristic sizes of nanoparticles range from a few to a hundred nanometers.

As such, island films appear under certain conditions of vacuum condensation of metal vapor on a cold substrate. According to electron microscopic observations, the process of vapor condensation on a substrate begins with the sudden appearance of particles of approximately the same size. Then the volume of particles increases, and their sizes increase more rapidly in directions parallel to the substrate than in the direction normal to it. Finally, the particles come in contact with each other - they coagulate (combine) or coalesce (merge). The granular (granular) structure of the film, as a rule, is preserved during its subsequent growth to a continuous coating of the substrate [3].

The main parameters that determine the structure, optical properties, and SERS activity of thin metal films are: a) the substrate material; b) application conditions (deposition rate, vacuum level, substrate temperature); c) the thickness of the deposited metal layer; d) the conditions for its subsequent heat treatment.

The research results allow us to conclude that the efficiency control of SERS-active single-layer structures is possible only in a limited range due to variations in the technological parameters of the formation of island films.

For the purposes under consideration, the use of a multilayer structure is the most promising.

Below is a description of the graphic materials, in no way limiting all possible embodiments of the claimed invention.

Figure 1 presents the optical diagram of the device used to excite the SERS and detect its components formed by the protective structure.

Figure 2 and figure 3 shows a schematic representation of the options for SERS-active structures. Below are the numbers of the main blocks of the SERS excitation device and the detection of its components, as well as the SERS layers - active nanoscale structures, their names and abbreviations used below.

1 - laser (L),

2 - laser beam (LL),

3 - laser spot (LP),

4 - protective structure (AP),

5, 6 - components of the GKR (KGKR),

7, 8 - narrow-band optical filters KGKR (UOF),

9, 10 - photo-recording devices (FRU),

11 - electronic computing device (EVU),

12 - upper GCR-active layer (AC),

13 - resonator (P),

14 - mirror (S),

15 - porous layer (PS),

16 - the substrate of the SERS active structure (P).

The following is an example embodiment of the invention, in no way limiting all possible options for its implementation.

Excitation of SERS is carried out using L (1), LL (2) of which is directed to a specific section of the GL (4), forming the SAG (5, 6).

The detection of the identification sign formed on the surface of the CS (4) in the region of the PL (3) —KGKR (5, 6) is carried out using a switchgear (9, 10) with spectral selectivity at the frequencies ω _{st, i} = ω _L -Ω and ω _{st , i} = ω _L + Ω due to the use of UOF at their inputs (7, 8). The processing of an identification feature is carried out using an EVU (11).

Below are the options for the formation of SERS - active structures. Option 1.

The use of a three-layer structure of the elements of the protective structure.

In this case, AS (12) can be made, for example, from an island - 5 nm or quasi-continuous - 15 nm film of gold (Au). The resonator (13) is made, for example, of silicon dioxide (SiO ₂ ) transparent to the exciting radiation and SERS. The lower layer Z (14) is made of a material with a high reflection coefficient, for example, aluminum (Al), from which P (16) GCR - an active structure can be made.

The formation of AS (12) and P (13) elements of the protective mark can be carried out, for example, at a Plassys vacuum deposition unit equipped with thermal resistive (for Ag, Al) and electron-beam (for Au, Cr, SiO ₂ ) evaporators. The characteristic operating pressure should be 5 · 10 ^-6 mbar when spraying metal films and 5 · 10 ^-4 mbar when spraying SiO ₂ layers.

When the condition is reached when the amplitude of the radiation reflected from the three-layer structure is minimal, the main part of the incident light is absorbed in a thin gold film. Figure 4 shows the characteristic dependences of the absorption of incident light (a) and the intensity of the SERS signal (b) on the thickness of the silicon dioxide layer (mass thickness of gold 5 nm) [2].

A comparative analysis of the dependence of the reflection coefficient and the intensity of the SERS signal on the thickness of the silicon oxide layer allows us to identify the optimal resonator parameters. The maximum value of the SERS signal at a mass thickness of gold of 5 nm is achieved with a dielectric thickness corresponding to 120-125 nm, and at 15 nm - 150-160 nm. This difference is explained by the difference in the optical properties of gold films of different mass thickness. It was found that the SERS signal can be amplified by more than an order of magnitude (i.e., 11-44 times depending on the morphology of the gold layer) as compared with the signal from a similar SSC active layer formed on the surface of SiO ₂ in the absence of the « resonator-mirror ”[2].

Option 2. The use of quasi-standard technology for the fabrication of SERS active structures based on the use of the surface of a porous material for structuring the SSC active layer - a gold film. The proposed technology allows you to create efficient and highly reproducible characteristics of the structure.

The porous layer of PS (15) alumina is formed by the electrochemical method in aqueous solutions of organic and inorganic acids of various concentrations. The effect of pore formation is based on the equilibrium between simultaneously occurring electrochemical processes of local growth and dissolution of aluminum oxide. The main technological parameters of the process of formation of the porous layer are the anodization voltage, composition, concentration and temperature of the electrolytic solution. The smallest pores in diameter (4 nm) can be obtained by anodizing in a 10% aqueous solution of sulfuric acid at a voltage of 5 V, the largest pores of 200 nm in a 5% solution of phosphoric acid. The pore diameter increases linearly with increasing voltage, an additional increase in pore size is achieved by chemical etching in a 3% aqueous solution of phosphoric acid. Anodizing can be carried out in a potentiostatic mode using 3 types of aqueous solutions of acids (sulfuric, oxalic and phosphoric), and the voltage can vary in the range of 10-20 V for H ₂ SO ₄ , 20-60 V for (COOH) ₂ and 40- 60 V for H ₃ PO ₄ . The primary anodization time is 60 minutes, and the secondary duration is determined by the required thickness of the porous layer.

According to the results of the experiments, it was found that a distinctive feature of the porous layers formed in the H ₂ SO ₄ solution is a high degree of ordering and the correct pore shape [2].

Thus, the application of the proposed methods based on the use of modern technologies for the formation of structures of the nanoscale level allows the creation of highly reproducible protective structures with predetermined characteristics, which further enhance the effect of giant Raman scattering, which is an identification sign of the authenticity of protected valuable products.

Literary sources

1. Akcipetrov O.A. Giant nonlinear optical phenomena on the surface of metals. Soros Educational Journal, Volume 7, No. 7, 2001.

2. Koscheev S.V. Optimization of active sensor elements using the giant Raman scattering effect. Abstract for the degree of candidate of technical sciences, St. Petersburg State Electrotechnical University "LETI" named after V.I. Ulyanova (Lenina), 2006.

3. Petrov A.Yu. Clusters and small particles. M .: Nauka, 1986.

Claims (3)

1. The method of forming a nanoscale structure for protection against counterfeiting and authenticity of valuable products using the giant Raman scattering effect, characterized in that when it is used, a high degree of reproducibility of the protective elements for the same valuable products is provided, and nanosized structures are used as the protective material, providing additional amplification of the identification feature due to the interference of the sounding electromagnetic signal in them optical range. 2. The method according to claim 1, characterized in that as a nanoscale structure enhancing the effect of giant Raman scattering, use a three-layer structure, sequentially containing an insular or quasi-solid metal film made, for example, of gold, optically transparent for probe radiation and giant Raman scattering dielectric, for example silicon dioxide, and a metal with good reflectivity, for example aluminum. 3. The method according to claim 1, characterized in that as a nanoscale structure enhancing the effect of giant Raman scattering, use a metal film, for example, of gold, structured by a porous surface, for example, aluminum dioxide formed on the surface of aluminum by electrochemical etching in an aqueous solution, for example, sulfuric acid.

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