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Thermal Inkjet Printing of Quantum Dot Inks for Overt and Covert Security Printing James Tom 1 Etheridge , Steve 2 Simske , Tim 1 Strecker , and Garry 1 Hinch Development Operations, Hewlett-Packard Company, Corvallis, Oregon; 2Print Production Automation Laboratory, Hewlett-Packard Company, Fort Collins, Colorado Relevant Quantum Dot Properties Motivation •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 Architecture of a typical-core shell (e.g CdSe/ZnS) quantum dot Semiconducting nanoparticles have unique optical and electronic properties determined by the quantum mechanics of reduced dimensional (confined) systems. Components of a water-based TIJ ink formulation (First step: modifying off-the-shelf ink jet inks for QD’s) Water: major formulation component – all other components must be water-stable Printed green QD's (on Teslin) 350000 Co-solvent/humectant: control boiling point/evaporation of ink solvent (vehicle); promotes nozzle health 300000 250000 Colorant: dyes or pigments (dissolved or suspended in ink vehicle) = Colloidal suspension of quantum dots Green QD's 1x Green QD's 2x 200000 Green QD's 3x Green QD's 4x 150000 Green QD's 5x C o re 5 – 10nm Shell e.g. CdSe e.g. ZnS Fixative/penetrant: modify interaction of ink with substrate (control migration of ink through substrate via wicking) Surfactant: modifies surface tension of ink, critical to surface wetting properties and proper jetting performance of ink Resins: used to improve image permanence – potential issues with nozzle plugging Biocide/fungicide: provide capability for long–term ink storage li g a nd ca p s e.g. tri-n-octylphosphine oxide Thermal Inkjet (TIJ) Drop Relevant Quantum Dot Properties Ejection C o re Shell Dot Diameter Eliminated from TIJ Buffer: provide stability for other ink components (primarily colorants) ink formulation Fluorescence C o re Why? -A very small ink film participates in the nucleation (<50 nm) event. Less than ~ 1% of the droplet is exposed to high temperatures. nozzle resistor Ink reservoir The TIJ Ink “Laundry List”: A (surprisingly) large number of inks can be engineered through surface tension, viscosity, DHvap, chamber geometry, etc. Recent work by Hewlett-Packard and other groups have shown that PANI many other materials areand usable: •1-part 2-part UV curable •PEDOT, (conductive epoxies •Small organic molecules in water • DMSO •Antibodies •Enzymes •Cells and other biological materials p But “ligands” can easily be displaced from surface by solvent, other formulation components li g a nd ca p s C o re Shell Which can lead to particle aggregation, surface reaction, and loss of sizedependent properties (e.g., fluorescence) • Ink stability is highly dependent on co-solvent used in ink vehicle • There is a limit on using solution viscosity to stabilize nanoparticle dispersion (high viscosity can lead to poor jetting) • Solvent initially chosen for jettability (HEP) provides limited solution stability for red-emitting QD’s • Other solvents show possibility for improved solution stability (2-P, 1,2-HD) Red QD stability in ink vehicles Ink vehicle solvents Fluorescence intensity (normalized) Track and Trace HEP: 1-(2hydroxyethyl)-2pyrrolindinone HEP/water 2-P/water 1,2-HD/water DGBE/water 2-P: 2-pyrrolidinone 1,2-HD: 1,2hexanediol 0 20 40 60 80 Time (h) •Emission and Adsorption wavelengths determined by size 555 575 595 615 635 emission wavelength (nm) • Fluorescence spectra obtained on Photon Technologies QM-4/2006 spectrofluorimeter • Emission intensity proportional to amount of material printed (negligible selfabsorption) • Amount of material controlled with number of print passes (1X-5X for these samples) • Experiment demonstrates basis for creating information within security mark based on emission amplitude (also demonstrated at other emission wavelengths) Barcode printed with QD-containing ink shown under UV (254 nm) illumination Interrogation Wavelength = 254 nm (UV) 450 500 550 600 650 700 750 100 120 140 160 DGBE: dipropylene glycol butyl ether Experimental Printing Test Beds and Printing Details • 2-D barcode printed with two QD “colors” • Relative peak areas depend on sample position (spot sampled is larger than barcode pixels) • Sharp, well-resolved peaks allow precise specification of emission wavelength and amplitude to generate covert “signature” Increasing information “Payload” of QD inks •Ink formulation contains two different sizes of CdSe/ZnS quantum dots •Relative peak intensity dependent on concentration of quantum dot sizes in ink •Line widths sufficiently narrow to allow data encoding •Composition of ink can be continuously varied to create dynamic information content Varying the “information content” of the ink by incorporating QD’s with different diameters Challenges and Path Forward QD Ink Development Challenges Electronic Materials Printer for fine “tuning”ink formulation •Sharp, well separated emission and adsorption peaks Original Package: Security marks include static content, include branding, regulatory compliance, recall sell, point of sale, track and trace •Resolved spectral features can provide increased information “payload” density •Mixtures of QD’s enable highly complex spectra •inorganic nanoparticles offer potential for increased stability vs. organic fluorophores QD stability in ink vehicles studied by measuring solution fluorescence Evidentiary/Forensics Why QD-inks enable new security printing methods: •Visible and “invisible” emission enabling overt, covert and forensic applications 535 li g a nd ca p s Control of quantum dot size provides tunable fluorescent emission Classification of security marks • Overt – Observable without device: naked eye, feel, smell – Limited personnel training required Brand identification • Covert – Often not perceptible to untrained or Investigation/Lead Generation with naked eye alone – Machine identifiable or readable • Forensic Product Authentication – Laboratory required for checking Brand identification Product Anticounterfeiting Document Anti-counterfeiting Track and Trace Product Authentication Evidentiary 515 emission wavelength (nm) Security Printing Overview • • • • • • 495 Emission spectrum from printed barcode ~650 nm Security and Forensics Printing Applications High Temp Region <0.05 µm Shell The absorption and emission peaks are precisely determined by the QD diameter. Peaks are typically sharp and well separated providing a unique “signature”. …however for many inks, there is minimal degradation resulting from the ejection event QD synthesis, stability provided by incorporation of “ligand” cap Photo by Xiaohu Gao The fluorescence spectra of quantum dots as a function of dot diameter at a fixed excitation wavelength Major caveat: For TIJ, all inks are required to boil… 475 Addition of quantum dots to ink formulation li g a nd ca p s Wavelength (nm) 0 Shell li g a nd ca p s ~400 nm 50000 Varying Emission Wavelength: Overt and Covert Marks Ink formulation and quantum dot stability …the art of adding dots to solvent C o re 100000 Fluor. intensity • 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” polymers) •Silver and gold nanoparticle suspensions •Quantum dots •Carbon nanotubes, nanowires,… •Ethanol, Methanol, IPA •OLED precursor solution •Toluene, gasoline •Acetonitrile, Chloroform, HEMA •Zinc Tin Oxide, ITO precursors Engineering Emission Intensity by Multi-Pass Printing counts 1Technology 1 Stasiak , Security Marks Added to Packaging: Unique ID, mass serialization, steganography, QA/inspection marks, 1 and 2 dimensional barcodes, microtext , and covert (invisible in optical spectrum)security ink marking based on emissive quantum dots? Experimental QD Ink printing using a standard desktop ink jet printer/printPaper head platen • Ink = Water + humectant + surfactant Cartridge 6540 TIJ resolution • Print System = HP 95 cartridge in DeskJet Encoder printer= 1 mm X,Y axis accuracy = +/- 5 mm • Quantum Dots = blue- and red-emitting CdSe:ZnS with TOPO X,Y axis repeatability = +/- 1 mm ligand • Media = Low-fluorescence office paper Quantum Dots 1. Elimination of heavy metals (HP’s commitment to the environment forbids introduction of any product containing Cd, Pb, or Hg) 2. Longer life 3. Broader color selection 4. More robust “ligand” sphere 5. Price 6. Improved optical properties 7. …. Water-based Inks 1. Improved ink stability 2. Greater solvent flexibility 3. Longer shelf live 4. … Functional Inkjet Inks – enabling the printing-ofthings MIT Cabot iTi & NIST Inorganic TFT Sirringhaus,et al. L = 5 mm Organic TFT (PIJ) OLED (TIJ) PZT actuators (TIJ) Metals (PIJ) HP HP Nanowires (TIJ) HP HP Quantum dots (TIJ) Clemson U. CNT’s on paper (TIJ) Printed neurons (TIJ)