RU2530238C2 - Method of creating hidden luminescent labels - Google Patents

Abstract

FIELD: physics, optics.

SUBSTANCE: invention relates to means of labelling articles. The technical result is more secure labelling. The method is based on embedding quantum nanorods on track pores of polymer membranes and generating photo-induced luminescence anisotropy in the layer of nanorods. The method includes, in linearly polarised light, selective action of light of a certain wavelength on a portion of the nanorods, spatial orientation in a sample of which matches the direction of the electric field vector acting on the sample.

EFFECT: simple method of manufacture, broader technological approach and lower requirements for accuracy of monitoring parameters of hidden labels with polarisation contrast during manufacture thereof.

6 dwg

Description

The present invention relates to printing, in particular to the manufacture of security tags that can be used for covert marking of various objects in order to prevent unauthorized production of these objects and simplify the process of verifying their authenticity.

There is a method of creating labels based on semiconductor quantum dots (CT) by introducing them into ink, paper, plastic and explosives "Method of protecting devices using quantum dots" (US Patent No. 6692031 B2, application 09/955808, publication date 02/17/2004 , priority date 02.21.2002) [1]. Due to the spectral-luminescent features of quantum dots: a narrow luminescence spectrum, the dependence of the position of the luminescence band on the size of the quantum dots (Efros, AL, DJLockwood, et al. Semiconductor Nanocrystals: From Basic Principles to Applications, Springer, 2003) [2] unique labels based on combinations of various types of nanoparticles, the use of which will accurately determine the manufacturer of a particular product. The common disadvantages of this method include the fact that to identify the tag and, accordingly, verify the authenticity of the marked object, it is necessary to analyze the spectral characteristics of the tag, which, in turn, significantly increases the cost and complexity of the verification process.

A known method of creating hidden fluorescent labels using two types of ink "Hidden fluorescent signs" (US Patent No. 7422158 B2, application 10/692569, publication date 09/09/2008, priority date 10/24/2003) [3]. To obtain such marks, two types of ink are used that have the same color in daylight. However, the second type of ink contains luminescent additives, which allows the formation of a hidden label. The disadvantages of this method include the low resistance of the obtained labels to photodegradation, since organic dyes are used as luminescent additives.

Closest to the claimed invention and adopted as a prototype "Environment for object recognition and method of its use" (US Patent No. 7391546 B2, application 10/557001, publication date 06/24/2008, priority date 12/07/2007) [4]. According to the description of the patent, in this case, the method of forming hidden labels consists in using bilayer structures of anisotropic cholesteric liquid crystal polymers. Such material, depending on the thickness of the layer, can selectively reflect light with left- or right-handed circular polarization in a certain spectral range. This allows you to create images that are indistinguishable in daylight, but easily visible when viewed with the help of special filters that transmit circularly polarized light. The use of additional color and polarization coding allows increasing the degree of protection. However, the described method has several significant disadvantages. Such disadvantages, in particular, include the complex manufacturing process of such a label: first, two anisotropic layers (reflecting light with right-handed and left-handed polarization) are polymerized on auxiliary isotropic substrates. Then, using a photolithographic technique, a unique image is formed on each of these layers. Then these layers are sequentially transferred by stamping to the marked object. An additional complication is the need for accurate selection of the thickness of each of the anisotropic layers, otherwise the material will not have the indicated selectivity with respect to different circular polarization and the wavelength of the incident light.

The technical problem to which the invention is directed is to simplify the method of creating hidden labels with polarization coding.

The problem is solved by creating labels with polarized luminescence of semiconductor quantum nanorods (NS). Semiconductor quantum nanorods, along with the optical properties that are inherent in spherical nanocrystals, quantum dots (the dependence of the color of the luminescence on the diameter of the NS, high absorption capacity, high quantum yield of luminescence), have linearly polarized luminescence, the direction of the electric vector of which coincides with the long axis of the nanorod ( Hu, J., L.-s. Li, et al. (2001). "Linearly Polarized Emission from Colloidal Semiconductor Quantum Rods" Science 292: 2060-2064; Chen, X., A. Nazzal, et al. (2001 ). "Polarization spectroscopy of single CdSe quantum rods." Physical Review B 64 (24): 245304) [5, 6]. Obviously, with a random arrangement of nanorods in the sample, its luminescence will be completely depolarized. In this regard, in order to create a label with polarized luminescence of nanorods, it is necessary in one way or another to order the nanocrystals in the sample. One possible approach in this case is photoinduced ordering of an array of nanorods, which is as follows. An array of semiconductor nanorods randomly oriented relative to each other is formed in the polymer matrix. The luminescent response of such a sample will be completely depolarized. At the next stage, a polymer matrix with embedded NSs is illuminated by linearly polarized light, the radiation wavelength of which corresponds to the energy of the interband transition of the NSs. Irradiation with linearly polarized light under certain conditions will lead to photochemical processes on the surface of the subassemble of nanorods oriented in a certain direction in the sample, and, as a result, to the appearance of polarization of the luminescence of the sample. The mechanism of the appearance of anisotropy of the luminescent response of a sample with embedded NSs is as follows. Linearly polarized radiation incident on a sample with randomly arranged nanorods will be predominantly absorbed by those NSs whose dipole moments of interband transitions coincide with the direction of the electric field vector of the radiation incident on the sample. Thus, only the NS sub ensemble is affected in the matrix, the dipole moments of interband transitions of which, and therefore the long axes of the nanocrystals, are oriented in a certain direction. It is known that the quantum yield of luminescence of semiconductor nanocrystals strongly depends on the quality of their surface and the presence of a shell of semiconductor material with a larger band gap than the nanocrystal core (Yu, WW, L.Qu, et al. (2003). "Experimental Determination of the Extinction Coefficient of CdTe, CdSe, and CdS Nanocrystals. "Chemistry of materials 15 (14): 2854-2860) [7]. This is due to the presence of surface defects in nanocrystals, which are centers of nonradiative deactivation of their excited state. There are a number of works in which it was demonstrated that illumination of nanocrystal samples under certain conditions can cause photodegradation of nanocrystals. It was shown that at the initial stage of photodestruction, the number of surface defects of nanocrystals decreases, which leads to a decrease in the number of centers of nonradiative recombination of NSs and, accordingly, to a noticeable increase in the quantum yield of luminescence of nanocrystals (Zhang, Y., J. He, et al. (2006) . "Time-Dependent Photoluminescence Blue Shift of the Quantum Dots in Living Cells: D Effect of Oxidation by Singlet Oxygen." Journal of the American Chemical Society 128 (41): 13396-13401) [8].

Thus, exposure to a sample with nanorods of linearly polarized radiation of a certain energy will lead to a noticeable increase in the quantum yield of luminescence of nanorods macroscopically oriented in the matrix in a certain chosen direction. This, in turn, will lead to the appearance of luminescence polarization of the entire sample. It should be noted that, in contrast to the spatial ordering of the entire array of nanorods embedded in the matrix, in this case only the anisotropy of the luminescent response will appear in the sample, and dichroism of absorption will not be observed. This property allows to obtain a luminescent label with a high degree of protection, since it allows you to create a sample in which the degree of anisotropy of the optical properties (absorption and radiation) is different. It should be noted that for the process of photodegradation of semiconductor nanorods, molecular oxygen must be present in their immediate environment (Sark, WGJHM v., PLT. M. Frederix, et al. (2002). "Blueing, Bleaching, and Blinking of Single CdSe / ZnS Quantum Dots. "ChemPhysChem 3 (10): 871-879. [9]. Therefore, to preserve the polarized radiation of the label, it is sufficient to stop the access of oxygen to the nanorods embedded in the matrix. One possible solution is to laminate a sample with polarized luminescence of nanorods, polyethylene terephthalate lenka, which refers to low permeability polymers (Fakirov, S. (2002). Handbook of thermoplastic polyesters: homopolymers, copolymers, blends, and composites, Wiley-VCH) [10].

To solve this problem, NSs are introduced into a polyethylene terephthalate track membrane, which in this case is used as a polymer matrix. This method of introducing semiconductor nanocrystals into polymer track membranes is described in detail in (A.O. Orlova, YAGromova, et al. (2011). "Track membranes with embedded semiconductor nanocrystals: structural and optical examinations." Nanotechnology 22 (45): 455201-455208) [11]. This allows one to obtain a disordered layer of quasi-isolated NS in the parietal layer of the membrane track pores. After that, the membrane is illuminated by linearly polarized light, the energy of which corresponds to the energy of the interband transition of the NS. The irradiation of the sample with linearly polarized light leads to flare-up of the luminescence of the NS sub-ensemble, whose long axes coincide with the direction of the electric vector of the incident radiation. If a part of a sample is irradiated, for example, with vertically polarized light, and another part is irradiated, for example, with horizontally polarized light, then, using the same exposure, the luminescence intensity will be the same for both parts, while the polarization of luminescence will be different, in other words, the directions the polarizations of these two parts of the sample will be mutually perpendicular.

The essence of the invention is as follows: to obtain hidden luminescent labels based on semiconductor nanorods, the image on which is formed using polarizing contrast, selective light exposure is used. In unpolarized light, such a mark looks like a uniformly luminescent region, while when viewed in linearly polarized light, contrast is noticeable.

The proposed method has the following advantages:

1. Simplification of manufacturing technology. This advantage is ensured by the fact that to obtain a hidden luminescent label based on semiconductor nanorods with polarizing contrast, a single-layer polymer matrix with embedded nanocrystals that are randomly oriented relative to each other and characterized by depolarized luminescence is sufficient.

2. Expansion of the technological approach. This advantage is provided by the fact that the induced anisotropy of the properties of the label is carried out by photochemical methods.

3. Simplification of control over the parameters of the label in the process of its manufacture. This advantage is ensured by the fact that the requirements for the thickness accuracy of a polymer matrix with embedded quantum nanorods are significantly weakened, while the exposure time is the only parameter that needs to be controlled.

The essence of the invention is illustrated in figures 1-6, which show:

Figure 1. Schematic representation of a polymer membrane with semiconductor nanorods embedded in the surface layers of track pores.

Figure 2. Schematic representation of the process of creating photoinduced anisotropy in an initially disordered layer of nanorods: irradiation of the membrane with linearly polarized light: 1 - PET track membrane with embedded NS, 2 - polarizer, 3 - LED.

Figure 3. The registration scheme of the luminescent response from a sample of a polymer track membrane with embedded semiconductor quantum nanorods using a spectrofluorimeter: 4 - radiation source; 5 - monochromator of the excitation channel; 6 - light filter; 7 - waveguide; 8 - track membrane with embedded NS; 9 - analyzer; 10 - waveguide; 11 - light filter; 12 - monochromator of the registration channel; 13 - PMT.

Figure 4. Scheme for observing the luminescent response from the mark in natural light: 14 — membrane with a mark; 15 - analyzer (orientation is chosen arbitrarily); 16 - human eye; 17 is a radiation source.

Figure 5. Luminescence spectra of a track membrane sample with embedded CdSe / ZnS semiconductor quantum rods with a diameter of 3.5 nm before and after irradiation with linearly polarized light with a wavelength of 595 nm: dashed line — before irradiation of the sample with linearly polarized light; solid line - the axis of the analyzer is oriented parallel to the membrane plane; dash-dotted line - the axis of the analyzer is oriented perpendicular to the plane of the membrane.

6. Luminescent images of a polymer membrane with embedded quantum rods, with two regions irradiated with linearly polarized light with a wavelength of 595 nm with different directions of the electric vector: a - registration without an analyzer; b - the axis of the analyzer coincides with the axis of the polarizer for region A; c - the axis of the analyzer coincides with the axis of the polarizer for region B.

Example 1

To demonstrate the operability of the proposed method for creating hidden tags based on semiconductor nanorods embedded in polymer track membranes, a sample of a poly (ethylene trphthalate) track membrane with pores of 500 nm and a thickness of 12 μm was prepared; quantum nanorods were introduced from the solution in toluene into the surface pore layer CdSe / ZnS with a diameter of 3.5 nm and an aspect ratio of 1: 7, synthesized according to the procedure described in (Yu, WW, L.Qu, et al. (2003). "Experimental Determination of the Extinction Coefficient of CdTe, CdSe, and CdS Nanocrystals. "Chemistry of mater ials 15 (14): 2854-2860) [7]. For this, the track membrane was kept in a toluene solution with a concentration of HC With _HC = 10 ^-7 mol / L for 14 days; the procedure was described in more detail in (Orlova, A.O., YAGromova, et al. (2011). "Track membranes with embedded semiconductor nanocrystals: structural and optical examinations. "Nanotechnology 22 (45): 455201-455208) [11]. Figure 1 shows a schematic representation of the NS embedded in the pores of the track membrane.

Next, the sample was irradiated with an LED through the polarizer; the axis of the polarizer was oriented perpendicular to the membrane plane. An LED with a maximum radiation wavelength of 595 nm and a radiation flux of 27 lm was used as a radiation source. The process of photo-irradiation of the membrane to create photo-induced anisotropy in the initially disordered ensemble of NS is schematically depicted in Figure 2.

Figure 3 shows the registration scheme of the luminescent response from the polymer track membrane with embedded quantum rods using a spectrofluorimeter. In this case, the light from the source 4 through the monochromator 5 and the light filter 6 enters the waveguide 7, from which it is supplied to the sample 8. The use of the waveguide provides complete depolarization of the exciting radiation. The luminescent signal from the sample 8 passes through the analyzer 9 and is fed into the waveguide 10. After that, the light enters the filter 11, which emits the luminescent signal from the sample 8, and then sequentially enters the monochromator of the recording channel 12 and the photoelectronic multiplier 13.

Figure 4 shows a diagram of the visual registration of the luminescent response from the sample, in which the contrast image of the sample 14 can be observed visually by rotating the analyzer 15 and illuminating the sample with a portable radiation source 17, for example, an LED with a suitable radiation spectrum can be used.

The degree of photoinduced anisotropy of the luminescence of quantum nanorods in the samples was estimated by the formula:

$$R=\frac{I_H-I_V}{I_H+I_V},(\mathrm{one})$$

where I _{H, V} is the luminescence intensity of the sample upon excitation by linearly polarized light, the electric vector of which is located parallel to (H) and perpendicular to (V) the plane of the sample, respectively.

5. The luminescence spectra of a membrane sample with embedded CdSe / ZnS semiconductor nanorods are presented for different times of irradiation with linearly polarized light with a wavelength of 595 nm.

From the data shown in Fig. 5, it is seen that photoinduced anisotropy of the luminescence of the sample is observed as a result of irradiation of a membrane sample with NS linearly polarized light, the wavelength of which corresponds to the energy of the exciton transition. This is evidenced by the appearance of a difference in the luminescence intensity of the sample recorded at a mutually perpendicular position of the analyzer. It should be noted that an increase in the irradiation time of the membrane sample leads to a further increase in the quantum yield of luminescence and an increase in photoinduced anisotropy of the luminescence of NS. Thus, upon irradiation of the sample for 5 hours, the degree of polarization of the luminescence of the NS calculated by formula (1) turned out to be 0.08 and reached 0.12 with an increase in the irradiation time to 10 hours. The obtained data clearly demonstrate the effectiveness of our approach for obtaining a sample with polarized luminescence of semiconductor quantum rods.

Example 2

To demonstrate the possibility of visual observation of the contrast luminescent response from a membrane sample with photoinduced anisotropy of semiconductor quantum rods, a polymer membrane sample with a CdSe / ZnS NS with a diameter of 3.5 nm was irradiated with light with a wavelength of 595 nm in such a way that light was incident on region A of the sample through a polarizer, the axis which was oriented in parallel, and to region B through the polarizer, whose axis is oriented perpendicular to the plane of the sample. Figure 6 shows photographs of this sample of the membrane after irradiation, obtained without the use of the analyzer (a), using an analyzer whose axis is oriented parallel to (b) and perpendicular (c) to the plane of the sample.

According to the luminescent images of the sample shown in Fig.6, it can be seen that without the use of the analyzer, areas A and B glow with equal intensity. Photographing a sample using an analyzer leads to a contrast, which is due to the fact that, in regions A and B, mainly those quantum rods luminesce, whose long axes are oriented parallel and perpendicular to the plane of the sample, respectively.

The proposed method for creating hidden tags based on semiconductor nanorods embedded in polymer track membranes can be implemented without the use of additional devices. As shown in FIG. 2, to obtain a sample of a membrane with NS embedded in the pores and its subsequent irradiation with light with a certain wavelength, which leads to the appearance of photoinduced anisotropy of the luminescent response of the sample, no special devices and devices are required. This approach allows you to create a label with polarizing contrast. To protect such a label from oxygen access and from aggressive environmental influences, lamination or any similar method can be used.

Thus, the proposed method for creating a tag is simpler compared to the prototype, since a standard light source and a polarizer are required to obtain such a tag. In this case, the label is formed in a single-layer polymer matrix, which also simplifies the production process. An extension of the technological approach, which involves the use of photochemical methods to obtain polarization contrast in the luminescent response of a sample, allows the use of a sample with disordered NS space. In addition, to obtain the most contrasting label, it is necessary to select only one parameter — the optimal time of irradiation of the sample, which can be easily achieved by recording the spectral and luminescent characteristics of the sample.

Information sources

1. US patent No. 6692031 B2, application 09/955808, publication date 02/17/2004, priority date 02/21/2002.

2. Efros, A. L., D. J. Lockwood, et al. (2003). Semiconductor Nanocrystals: From Basic Principles to Applications, Springer.

3. US patent No. 7422158 B2, application 10/692569, publication date September 9, 2008, priority date 10/24/2003.

4. US patent No. 7391546 B2, application 10/557001, publication date 06/24/2008, priority date 07/12/2007.

5. Nu, J., L.-s. Li, et al. (2001). "Linearly Polarized Emission from Colloidal Semiconductor Quantum Rods" SCIENCE 292: 2060-2064.

6. Chen, X., A. Nazzal, et al. (2001). "Polarization spectroscopy of single CdSe quantum rods." Physical Review B 64 (24): 245304.

7. Yu, W.W., L.Qu, et al. (2003). "Experimental Determination of the Extinction Coefficient of CdTe, CdSe, and CdS Nanocrystals." Chemistry of materials 15 (14): 2854-2860.

8. Zhang, Y., J. He, et al. (2006). "Time-Dependent Photoluminescence Blue Shift of the Quantum Dots in Living Cells: Effect of Oxidation by Singlet Oxygen." Journal of the American Chemical Society 128 (41): 13396-13401.

9. Sark, W.G.J. H.M. v., P.L.T.M. Frederix, et al. (2002). "Blueing, Bleaching, and Blinking of Single CdSe / ZnS Quantum Dots." ChemPhysChem 3 (10): 871-879.

10. Fakirov, S. (2002). Handbook of thermoplastic polyesters: homopolymers, co-polymers, blends, and composites, Wiley-VCH.

11. Orlova, A.O., Y. A. Gromova, et al. (2011). "Track membranes with embedded semiconductor nanocrystals: structural and optical examinations." Nano-technology 22 (45): 455201-55208.

Claims (1)

A method for creating hidden labels in which anisotropic particles form a layer having a polarizing contrast, characterized in that the luminescent polarizing contrast of the label is formed by a photochemical reaction under the influence of polarized light in a single-layer polymer matrix into which luminescent quantum nanorods are embedded without prior spatial ordering.

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