Core Shell

ligand caps

Core Shell

ligand caps

Thermal Inkjet Printing of Quantum Dot Inks for Overt and Covert Security PrintingThermal Inkjet Printing of Quantum Dot Inks for Overt and Covert Security PrintingJames StasiakJames Stasiak11, Tom Etheridge, Tom Etheridge11, Steve Simske, Steve Simske22, Tim Strecker, Tim Strecker11, and Garry Hinch, and Garry Hinch11

11Technology Development Operations, Hewlett-Packard Company, Corvallis, Oregon; Technology Development Operations, Hewlett-Packard Company, Corvallis, Oregon; 22Print Production Automation Laboratory, Print Production Automation Laboratory, Hewlett-Packard Company, Fort Collins, ColoradoHewlett-Packard Company, Fort Collins, Colorado

•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

• 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”

Motivation

Thermal Inkjet (TIJ) Drop

Ejection

nozzle

resistor

Ink reservoir

High Temp Region <0.05 µm

Why? -A very small ink film participates in the nucleation (<50 nm) event. Less than ~ 1% of the droplet is exposed to high temperatures.

…however for many inks, there is minimal

degradation resulting from the ejection event

•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

Original Package:

Security marks include static content, include branding, regulatory compliance, recall sell, point of sale, track and trace

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?

CartridgeEncoder resolution = 1 mX,Y axis accuracy = +/- 5 mX,Y axis repeatability = +/- 1 m

Paper platen

Experimental Printing Test Beds and Printing Details

Electronic Materials Printer for fine “tuning”ink formulation

• 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

Experimental QD Ink printing using a standard desktop ink jet printer/print head

Major caveat: For TIJ, all inks are required to boil…

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:

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

The absorption and emission peaks are precisely determined by the QD diameter. Peaks are typically sharp and well separated providing a unique “signature”.

Relevant Quantum Dot Properties

The fluorescence spectra of quantum dots as a function of dot diameter at a fixed excitation wavelength

Flu

ore

scen

ce

~400 nm ~650 nmWavelength (nm)

e.g. tri-n-octylphosphine oxide

Classification of security marksSecurity Printing Overview

Security and Forensics Printing Applications

• Brand identification• Product Anticounterfeiting• Document Anti-

counterfeiting• Track and Trace• Product Authentication• Evidentiary

Brand identification

Track and Trace

Product Authentication

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

• 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)

Ink vehicle solvents

HEP: 1-(2-hydroxyethyl)-2-pyrrolindinone

2-P: 2-pyrrolidinone

1,2-HD: 1,2-hexanediol

DGBE: dipropylene glycol butyl ether

Red QD stability in ink vehicles

0 20 40 60 80 100 120 140 160

Time (h)

Flu

ore

scen

ce i

nte

nsi

ty(n

orm

aliz

ed)

HEP/water

2-P/water

1,2-HD/water

DGBE/water

Relevant Quantum Dot Properties

5 – 10nme.g. ZnS

Core Shell

ligand caps

e.g. CdSe

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

Why QD-inks enable new security printing methods:

QD synthesis, stability provided by incorporation of “ligand” cap

Which can lead to particle aggregation, surface reaction, and loss of size-dependent properties (e.g., fluorescence)

Ink formulation and quantum dot stability…the art of adding dots to solvent

QD stability in ink vehicles studied by measuring solution fluorescence

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

Photo by Xiaohu Gao

Control of quantum dot size provides tunable fluorescent emission

• 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)

Engineering Emission Intensity by Multi-Pass Printing

Printed green QD's (on Teslin)

0

50000

100000

150000

200000

250000

300000

350000

475 495 515 535 555 575 595 615 635

emission wavelength (nm)

co

un

ts

Green QD's 1x

Green QD's 2x

Green QD's 3x

Green QD's 4x

Green QD's 5x

450 500 550 600 650 700 750

Flu

or.

inte

nsi

ty

emission wavelength (nm)

• 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”

Barcode printed with QD-containing ink shown under UV (254 nm) illumination

Emission spectrum from printed barcode

Varying Emission Wavelength: Overt and Covert Marks

Interrogation Wavelength = 254 nm (UV)

Challenges and Path Forward

•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

Increasing information “Payload” of QD inks

Varying the “information content” of the ink by incorporating QD’s with different diameters

CNT’s on paper (TIJ)

Nanowires (TIJ) Inorganic TFT

Metals (PIJ)

L = 5 m

Organic TFT (PIJ)

PZT actuators (TIJ)

Printed neurons (TIJ)

OLED (TIJ)

Quantum dots (TIJ)

HPHP

HPHP

HPHP

HPHP

CabotCabot

MITMIT

iTi & NISTiTi & NIST

Sirringhaus,et al.Sirringhaus,et al. Clemson U.

Clemson U.

Quantum Dots1. Elimination of heavy metals

(HP’s commitment to the environment forbids introduction of any product containing Cd, Pb, or Hg)

2. Longer life3. Broader color selection4. More robust “ligand” sphere5. Price6. Improved optical properties7. ….Water-based Inks1. Improved ink stability2. Greater solvent flexibility3. Longer shelf live4. …

QD Ink Development Challenges

Functional Inkjet Inks – enabling the printing-of-things

Core Shell

ligand caps

Dot Diameter

Addition of quantum dots to ink formulation

p Core Shell

ligand caps

But “ligands” can easily be

displaced from surface by

solvent, other formulation components