1. 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