Quantum dots – novi materijal za sigurnosni tisak

Iva Minga, Damir Modrić, Damir Vusić

Sažetak

U ovom radu prikazane su potencijalne primjene tzv. kvantnih točkica (quantum dots - QD) kao zaštitnog elementa u tisku raznih vrijednosnica u borbi protiv krivotvorenja. Kvantne točke ili nanokristali su poluvodičke čestice veličine u nanometarskom području koje mogu imati optička svojstva koja proizlaze iz kvantnog ograničenja (zatočenja). QD je nanokompozitna mješavina koja ima jedinstvena optička svojstva. Određene kompozicije, strukture i / ili veličine kvantnih točkaka mogu se odabrati da bi se postigao željeni spektar valnih duljina svjetlosti koji emitiraju kvantne točke nakon stimulacije s određenim izvorom pobude. Budući da željeni spektralni odgovor ovisi o veličini QD, kombinacijom QD različitih veličina, može se dizajnirati jedinstveni spektar koji zatim karakterizira određeni proizvod. Tehnika ispisa koja je najviše koristi je inkjet ispis, međutim, ne postoje ozbiljne prepreke primjene QD i u drugim tehnikama tiska (offset, flexo, ...). Generiranjem preciznog sastava korištenjem QD različitih veličina se može postići takav spektralni odgovor koji bi predstavljao personaliziranu zaštitu od krivotvorenja raznih tiskovina.

Ključne riječi: kvantne točkice, inkjet tisak, personalizirani sigurnosni tisak

 

QUANTUM DOTS - a unique material for security systems

Iva Minga, Damir Modrić, Damir Vusić

Abstract

In this paper we will present the potential applications of so-called quantum dots (QD) as a protective element in the press of various securities in the fight against counterfeiting. Quantum dots or nanocrystals are nanometer sized semiconductor particles that can have optical properties arising from quantum confinement. QD is a nano composite mixture which has unique optical properties. The particular composition(s), structure, and/or size of a quantum dot can be selected to achieve the desired wavelength spectrum of light to be emitted from the quantum dot upon stimulation with a particular excitation source. Since the desired spectral response depends on the size of the QD, with the combination of QD of different sizes, we can design a unique spectrum that then characterizes particular product. Printing technique which is the most used is inkjet printing, however, there are no serious obstacles to is implementation of the QD in the other printing techniques (offset, flexo, ...). Generating precise composition of the QD by their sizes can be achieved such a spectral response that would represent personalized protection against counterfeiting of various printed matter.

Key words: quantum dots, ink jet printing, personalized security print

Introduction
Uvod

Counterfeiting is a worldwide problem that leads to economic losses of hundreds of millions of dollars each year. The influx of counterfeit items on the market negatively affected by both the consumers and the producers. Corporations that produce most commonly counterfeited items are losing millions of dollars of income. For consumers, the presence of these counterfeit items increases the risk of purchase, instead of legitimate, incorrect or poor quality products. Clothing, documents and money [ ] are usually forged. International Chamber of Commerce estimates that more than 7% of world trade is in counterfeit products [ ].
To prevent this problem, anti-counterfeiting technology is constantly evolving and improving. This technology aims to mark authentic products in a way that is very difficult to copy. However, given the significant amount of the total world trade relating to counterfeit products, in order to keep up with new methods of counterfeiting, manufacturers of products that are most often counterfeited are willing to spend significant funds. Namely, it is desirable that anti-counterfeiting technology is such that it is very difficult for forgers to copy, and at the same time that the users can easily identify the desired positive subject.

Quantum dots
Kvantne točke

Quantum dots (QD) are used as an element of the encryption to create a large amount of different combinations (coding labels), and their application on various materials (paper, metal, ceramics, wood, textiles, ...) for further contact or remote sensing. This marking technology, due to the unique characteristics of the QD electromagnetic spectrum, is to protect the securities (stocks, commercial, corporate,...) and objects with a high degree of certainty of replacement or counterfeiting.
The fluorescence of the complex composition of the QD is perceived when it is exposed to UV, violet, blue or green light. Distinct feature is a combination of the original label color and a color that is a fluorescent response to irradiation of label [ ].
Quantum dots or nanocrystals are nanometer sized semiconductor particles that can have optical properties arising from quantum confinement. They exhibit energy band gap that determines required wavelength of radiation absorption and emission spectra. Quantum dots can have various shapes, including, but not limited to, a sphere, a rod, a disk, other shapes, and mixtures of various shaped particles. Quantum dots are nanocrystals of a semiconducting material with diameters in the range of 2-10 nanometers (10-50 atoms). They were first discovered in 1980 [ ]. Quantum dots display unique electronic properties, intermediate between those of bulk semiconductors and discrete molecules, that are partly the result of the unusually high surface-to-volume ratios for these particles [ , , ]. The most apparent result of this is fluorescence, wherein the nanocrystals can produce distinctive colors determined by the size of the particles. In essence, quantum dots may be tuned to emit light across the visible spectrum by changing their size [ ]. The narrow FWHM (full width at half maximum) of quantum dots can result in saturated color emission. The broadly tunable, saturated color emission over the entire visible spectrum of a single material system is unmatched by any class of organic chromophores [ ]. A monodisperse population of quantum dots will emit light spanning a narrow range of wavelengths. In particular, QD fluorescence depends on their size - so that small (~ 2 nm) CdSe nanocrystals generates luminescence in the blue region of the spectrum, while the size of about 7 nm in the red. This feature produces quantum dots with virtually any fluorescence wavelength from ultraviolet to near-infrared range by changing the particle size and the nature of the semiconductor forming the nanocrystal. It is equally important that the QD has a very wide (any wavelength less than the exciton absorption peak) absorption spectrum, and therefore, QD of different sizes can be excited by a single light source. This effect is used to multiplex the analysis of biological macromolecules (e.g., immunoanalysis). QD photoluminescence peaks are quite narrow (FWHM less than 30 nm in width) and symmetrical, it is also very important for the simultaneous identification of a plurality of fluorescent signals.
The quantum dots can be water soluble quantum dots or oil soluble quantum dots, preferably oil soluble quantum dots. The quantum dots are selected from one or more of doped or undoped quantum dots of zinc sulfide, zinc oxide, gallium nitride, zinc selenide, cadmium sulfide, gallium selenide, cadmium selenide, zinc telluride, cadmium telluride, gallium arsenide, indium phosphide and lead telluride. The quantum dots have a relatively small size in nano-level. They can be dispersed in a solvent easily and thus have a good dispersivity. By using the good dispersivity of the quantum dots, they can be used as the isolation layer among the pigment particles to prevent the aggregation of the pigment particles and thus to improve the stability of the pigment dispersion liquid. Additionally, due to the size effect of the quantum dots, it is possible to modify the quantum dots in terms of the adsorption of the light wave by controlling the size of the quantum dots, by which the color properties of the pigment can be improved and controlled.

a
Size dependent Photoluminescence (PL) spectra of CdSe QDs (from Benoit Dubertret and Hideki Ooba)
Ovisnost fotoluminescencijskih (PL) spektara o veličini CdSe QD (Benoit Dubertret i Hideki Ooba)

Apart from visual inspection of fluorescence can be read with a portable device. Variants of combination of simple code, along with inexpensive and compact device for determining the code, using the spectrophotometric readings, as well as marking complex composition allows the decoding information about the protected printed material.
The usefulness and application of QD technology continues to expand and research is striving to bring their benefits to more and more technologically applied fields.

Architecture of a typical-core shell (e.g CdSe/ZnS) quantum dot
Arhitektura tipične CdSe/ZnS kvantne točke

Semiconducting nanoparticles have unique optical and electronic properties determined by the quantum mechanics of reduced dimensional (confined) systems.

a

Application of QD
Primjena kvantnih točaka

The photoluminescence (PL) and electroluminescence emission from colloidal cadmium selenide (CdSe) quantum dots (QDs) can be tuned within the visible spectrum from wavelengths of 450 nm to 650 nm by controlling nanocrystal size. This versatility opens up a variety of potential applications for CdSe quantum dots in photonic devices, such as the following:

  • emitters for color displays
  • color modifiers for light emitting diodes (LEDs)
  • optical fiber amplifiers
  • low threshold lasers
  • self-assembled photonic sphere arrays
  • polymer-based photovoltaic cells
  • optical temperature probes
  • chemical sensors
  • high-speed signal-processing filters.
  • Anti-counterfeiting capabilities:  inject dots into liquid mixtures, fabrics, polymer matrices, etc.  Ability to specifically control absorption and emission spectra to produce unique validation signatures.  Almost impossible to mimic with traditional semi-conductors.
  • Counter-espionage / Defense applications:  Integrate quantum dots into dust that tracks enemies.  Protection against friendly-fire events.

The features of the technology [ ]

Značajke tehnologije [10]

1. Photostability
Fotostabilnost

Non-organic (semiconductor) nature of the nanoparticles provides the phenomenal stability to the photobleaching for 10-15 years

a
The diagram of the fluorescence intensity variation (signal registration at 620 nm) of the silicone plate with QDs of the type CdSe/CdS/ZnS when exposed with blue LED (450 nm, 12 W) for 6000 hours at the temperatures from 30÷50° С.
Dijagram varijacija intenziteta fluorescencije (registracijski signal na 620 nm) silikonske pločice s QD tipa CdSe / CdS / ZnS kada je izložena plavom LED rasvjetom (450 nm, 12 W) tijekom 6000 sati na temperaturi od 30 ÷ 50 ° С.

2.Machine-readability
Strojna čitljivost

Marking on the basis of the quantum dots has fluorescent machine-readable properties different from all the known classes of the phosphors. In particular, the ability to identify marks when activated by the irradiation sources with the different emission wavelengths including those in the visible light range.

3. Encoding
Kodiranje

This approach provides the creation of over 1 million combinations (coding marks) to mark various objects (paper, polymers, metals).

 

a
Fluorescence spectrum of the luminescent mark on the basis of the mixture of 4 QDs types (1 – on the basis of ZnSe, 2 – on the basis of CdS/ZnS, 3 – on the basis of CdSe/ZnS, 4 – on the basis of CdSe/CdS/ZnS).
Fluorescencijski spektar luminiscentne oznake na osnovi mješavine 4 vrste QD (1 - na osnovi ZnSe, 2 - na osnovi CdS / ZnS, 3 - na osnovi CdSe / ZnS, 4 - na osnovi CdSe / CdS / ZnS).

4. Mark masking
Sakrivanje oznaka

The possibility to create a masked mark, which is identified just after the special development procedure, was realized.

5. Any given colour of the fluorescent mark
Bilo koja boja fluorescentne oznake

Fluorescent marks with the emission wavelength in UV, visible and near infrared spectrum ranges with the step of 5 nm are offered.

a
Absorption (left) and fluorescence (right) spectra of the fluorescent marks
Apsorpcijski (lijevo) i fluorescencijski (desno) spektri fluorcscentnih oznaka
.
Recently the high quantum yield (QY) of the nanocrystals fluorescence of 90% was reached that enables the detection of the small quantities of the applied marking mixtures (not discernible with the eye-sight). The quantum yield (QY) can be defined as follows:

a

Fluorescent nanoparticles also excel all the known colorants in photostability that guarantees the useful properties preservation at any long-term operating conditions.

Technical parameters
Tehnički parametri

Period of storage and identification of the set code at the marked object

Up to 20 years

Resistance to direct long-term sun-light exposure and to other environmental factors

yes

The range of temperature-humidity conditions, at which the set code recognizability is preserved

From -60о С to +60о С (long-term exposure)
Up to +250о С (short-term exposure)

Spectrum operating range

visible, near infrared (450-800 nm)

Inkjet printing has become an important technology for many applications, such as organic electronics, nanotechnology, and tissue engineering, on account of its ability to precisely deposit picolitre volumes of solutions or suspensions in well-defined patterns. This ability, sometimes termed ‘direct-write,’ is achieved by using computer-controlled translation stages and ink-dispensers, which readily facilitates the production of complex patterns.

Ink formulations consisting emissive nanoparticles (quantum dots) can be developed and engineered to be optically active (emission and absorption) at precise wavelengths. Water-based colloidal suspensions of quantum dot “inks” can provide new security printing applications using thermal ink jet printing methods.

Components of a water-based Thermal Ink Jet (TIJ) ink formulation
Komponente formulacije Thermal Ink Jet (TIJ) tinte na bazi vode

One of the possibilities formulation TIJ ink is modification of the off-the-shelf ink jet inks for QD’s.
Water is taken as major formulation component, which means that all other components must be water-stable. Next component is co-solvent/humectant whose function is to control boiling point/evaporation of ink solvent (vehicle) and thereby promotes nozzle health. Dyes or pigments (dissolved or suspended in ink vehicle) are commonly used as a colorant, but in our case it will be colloidal suspension of quantum dots. The next component is the fixative / penetrant which modifies the interaction of ink with substrates, ie controls the migration of ink through substrate via wicking.
Furthermore, we need a surfactant that modifies surface tension of ink, critical to surface wetting properties and proper jetting performance of ink. The next thing we need are resins that are used to improve image permanence, and it is closely related to potential issues with nozzle plugging, biocide / fungicide that will provide the capability for long-term storage of ink and a buffer that will provide stability for other ink components eg., primarily colorants if our QD mixed with some color.

Mixtures of QD-based inks can be developed to provide rich and complex optical spectra enabling the printing of:

  • overt and covert anti-counterfeiting patterns
  • marks with increased information “payloads”

a  a

Ink jet paper photo quality color printed rectangle with QD viewed under ambient light (left) and UV light (right) [ ]
Pravokutnik s QD otisnut Ink jet printerom u boji na papiru foto kvalitete promatran pod ambijentalnim svjetlom (lijevo) i UV svjetlom (desno) [11]

a  a

Logo printed on ink jet paper. Two passages doped with Mn2+ QD (left) and one passage doped with Cu2+ QD (right) viewed under UV light [ ]
Logo tiskan na ink jet papiru. Dva prolaza s QD dopiranim s Mn2+ (lijevo) i jedan prolaz s QD dopiranim s Cu2+ (desno) gledano pod UV svjetlom [12]

a
Sample of the marking inks on the basis of QDs - left, Sample of the flexoprint using QDs – right [10]
Uzorak od tinte za markiranje na temelju QD – lijevo; Uzorak flexo tiska korištenjem QD – desno [10]

Ink formulation and quantum dot stability [ ]
Formulacija tinte i stabilnost kvantnih točaka [13]

 

Quantum Dot
Material
System

Emission
Range

Quantum Dot
Diameter
Range

Quantum
Dot Type

Standard
Solvents

Qauntum Dot Example
Applications

CdSe

465nm - 640nm

1.9nm - 6.7nm

Core

Toluene

Research, Solar Cells, LEDs

CdSe/Zns

490nm -          620nm

2.9nm - 6.1nm

Core-Shell

Toluene

Visible Fluorescence
Applications,
Electroluminescence, LEDs

CdTe/CdS

620nm -
680nm

3.7nm - 4.8nm

Core-Shell

Toluene

Deep Red Fluorescence Apps

PbS

850nm – 2100nm

2.3nm - 9.8nm

Core

Toluene

Near Infrared Applications, Security Inks, Solar Cells, IR LEDs

PbSe

1200nm – 2340nm

4.5nm - 9nm

Core

Toluene

Opto-electronics, Optical
Switching, Non-linear
Applications, Photonics,
Telecommunications

Conclusion
Zaključak

Research has shown that the luminescent QD successfully prepared and formulated in a stable ink jet formulation. Inkjet printing is an attractive patterning technology, which has become increasingly accepted for a variety of industrial and scientific applications. It has been successfully used for printing digital characters and images that are invisible to the ambient light, but can be seen in different colors when viewed under UV light. The color depends on the size of the QD that were used. This new technology opens up significant potential application in ink-jet printing of security documents and labels in one color or full color signs and images, advertising, but also in new flexible luminescent displays.

Literatura:

1. First Global Conference in Combating Counterfeiting. 25 May 2004.
<http://www.anticounterfeircongress.org/wco2004/website.asp?page=press>.
2. Hilton, B., Choi, CJ, Chen, S, "The ethics of counterfeiting in the fashion industry: Quality, credence and profit issues." Journal of Business Ethics, 2004. 55: p. 344-354.
3. Vij, D.R. Luminescence of Solids, Plenum Press: New York,1998.
4. Ekimov, A. I.; Onushchenko, A. A. JETP Lett. 1981, 34, 345–349.
5. Kastner, M. A. Physics Today, 1993, 46(1), 24.
6. Ashoori, R. C. Nature, 1996, 379(6564), 413.
7. Collier, C. P.; Vossmeyer, T.; Heath, J. R. Annual Review of Physical Chemistry, 1998, 49, 371.
8. C. B. Murray; C. R. Kagan; M. G. Bawendi, Annual Review ofMaZerial Sci, 2000, 30: 545-610
9. Dabbousi et al.;  J.Phys. Chem. 101, 9463 (1997)
10. http://en.rusnano.com/portfolio/companies/nanotech-dubna
11. Small, A.C. Novel Hybrid Materials and Their Applications. PhD Thesis, Victoria University of Wellington, 2008, 1-248.
12. James H. Johnston,Aaron C. Small, Noel Clark, „Colour Tuneable Photoluminescent Quantum Dots for Ink-Jet Printing of Security Documents and Labels“, Chemistry in New Zealand, 2010.: p. 70-71
13. Stasiak J., Etheridge T., Simske S., Strecker T., Hinch G., „Thermal Inkjet Printing of Quantum Dot Inks for Overt and Covert Security Printing“, www.nseresearch.org/2009/presentations/_Stasiak.ppt

 


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