Thermal Inkjet Printing of Quantum Dot Inks for Overt and Covert Security Printing James Stasiak1, Tom Etheridge1, Steve Simske2, Tim Strecker1, and Garry Hinch1 1Technology Development Operations, Hewlett-Packard Company, Corvallis, Oregon; 2Print Production Automation Laboratory, Hewlett-Packard Company, Fort Collins, Colorado Motivation Relevant Quantum Dot Properties 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 Co-solvent/humectant: control boiling point/evaporation of ink solvent (vehicle); promotes nozzle health Colorant: dyes or pigments (dissolved or suspended in ink vehicle) = Colloidal suspension of quantum dots 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 Buffer: provide stability for other ink components (primarily colorants) Engineering Emission Intensity by Multi-Pass Printing Architecture of a typical-core shell (e.g CdSe/ZnS) quantum dot Ink formulations consisting emissive nanoparticles (quantum dots) can be developed and engineered to be optically active (emission and absorption) at precise wavelengths. Semiconducting nanoparticles have unique optical and electronic properties determined by the quantum mechanics of reduced dimensional (confined) systems. Water-based colloidal suspensions of quantum dot “inks” can provide new security printing applications using thermal ink jet printing methods 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” 5 – 10nm e.g. CdSe e.g. ZnS Fluorescence spectra obtained on Photon Technologies QM-4/2006 spectrofluorimeter Emission intensity proportional to amount of material printed (negligible self-absorption) 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) Eliminated from TIJ ink formulation e.g. tri-n-octylphosphine oxide Thermal Inkjet (TIJ) Drop Ejection Ink formulation and quantum dot stability …the art of adding dots to solvent Varying Emission Wavelength: Overt and Covert Marks Relevant Quantum Dot Properties Emission spectrum from printed barcode Dot Diameter Addition of quantum dots to ink formulation Fluorescence ~400 nm ~650 nm Wavelength (nm) But “ligands” can easily be displaced from surface by solvent, other formulation components Photo by Xiaohu Gao QD synthesis, stability provided by incorporation of “ligand” cap Barcode printed with QD-containing ink shown under UV (254 nm) illumination Interrogation Wavelength = 254 nm (UV) 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” The fluorescence spectra of quantum dots as a function of dot diameter at a fixed excitation wavelength Control of quantum dot size provides tunable fluorescent emission Which can lead to particle aggregation, surface reaction, and loss of size-dependent properties (e.g., fluorescence) Major caveat: For TIJ, all inks are required to boil… The absorption and emission peaks are precisely determined by the QD diameter. Peaks are typically sharp and well separated providing a unique “signature”. Increasing information “Payload” of QD inks …however for many inks, there is minimal degradation resulting from the ejection event QD stability in ink vehicles studied by measuring solution fluorescence Security Printing Overview Security and Forensics Printing Applications Brand identification Product Anticounterfeiting Document Anti-counterfeiting Track and Trace Product Authentication Evidentiary Investigation/Lead Generation Overt Observable without device: naked eye, feel, smell Limited personnel training required Covert Often not perceptible to untrained or with naked eye alone Machine identifiable or readable Forensic Laboratory required for checking Evidentiary/Forensics Classification of security marks 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 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) 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 High Temp Region <0.05 µm Ink vehicle solvents HEP: 1-(2-hydroxyethyl)-2-pyrrolindinone 2-P: 2-pyrrolidinone 1,2-HD: 1,2-hexanediol DGBE: dipropylene glycol butyl ether Varying the “information content” of the ink by incorporating QD’s with different diameters Challenges and Path Forward Experimental Printing Test Beds and Printing Details The TIJ Ink “Laundry List”: 1-part and 2-part UV curable epoxies Small organic molecules in water DMSO Antibodies Enzymes Cells and other biological materials PEDOT, PANI (conductive 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 Why QD-inks enable new security printing methods: 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 many other materials are usable: Emission and Adsorption wavelengths determined by size Sharp, well separated emission and adsorption peaks Visible and “invisible” emission enabling overt, covert and forensic applications 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 Ink Development Challenges Functional Inkjet Inks – enabling the printing-of-things Electronic Materials Printer for fine “tuning”ink formulation CNT’s on paper (TIJ) Nanowires (TIJ) Inorganic TFT Metals (PIJ) L = 5 mm Organic TFT (PIJ) PZT actuators (TIJ) Printed neurons (TIJ) OLED (TIJ) Quantum dots (TIJ) HP Cabot MIT iTi & NIST Sirringhaus,et al. Clemson U. Quantum Dots Elimination of heavy metals (HP’s commitment to the environment forbids introduction of any product containing Cd, Pb, or Hg) Longer life Broader color selection More robust “ligand” sphere Price Improved optical properties …. Water-based Inks Improved ink stability Greater solvent flexibility Longer shelf live … Original Package: Security marks include static content, include branding, regulatory compliance, recall sell, point of sale, track and trace Experimental QD Ink printing using a standard desktop ink jet printer/print head Paper platen Ink = Water + humectant + surfactant Print System = HP 95 cartridge in DeskJet 6540 TIJ printer Quantum Dots = blue- and red-emitting CdSe:ZnS with TOPO ligand Media = Low-fluorescence office paper 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? Cartridge Encoder resolution = 1 mm X,Y axis accuracy = +/- 5 mm X,Y axis repeatability = +/- 1 mm