A stable inkjet ink containing ZnS:Mn nanoparticles as pigment Peter D. Angelo & Ramin R. Farnood University of Toronto Department of Chemical Engineering & Applied Chemistry Thursday, June 25 th, 2009, 11:15 AM
Piezoelectric inkjet technology piezoelectric printer computer/software t = 0, V = 0 peak V t = t pulse, V = 0 ink reservoir feed V t voltage drive waveform nozzle (pre-jetting) ink reservoir changes shape drop detachment t pulse droplet formation/ejection Tuesday, July 14, 2009 2
Applications Standard printed matter Conductive films Ag, Cu, Al, C, PEDOT:PSS Photovoltaics Transistors (TFTs) Displays PLEDs, OLEDs Broad emission spectra Poor jettability (polymers) Inorganic EL? Images courtesy of Cabot Corporation, General Electric, Orgacon Tuesday, July 14, 2009 3
Previous work: printed/coated EL 1) Inkjetted cathode (PEDOT:PSS) 2) Coated insulator/resin 3) Coated phosphor/resin 4) Inkjetted translucent anode (PEDOT:PSS) Device off Ambient light, device on (V th ) No light, device on (V th ) Tuesday, July 14, 2009 4
Completely printed EL: motivation Printed/coated device on Printed/coated device off Easier processing No realignment of screens or coating templates Single unit operation Material conservation Drop-on-demand Nanomaterials required Nanophosphors: potential for lower V th, higher η Colour tunability (quantization) Tuesday, July 14, 2009 5
Project objectives i) Synthesis of crystalline, nanoparticulate, inorganic EL emitter (doped ZnS) ii) Incorporation of nanoparticles into ink as pigment for jetting in an inkjet printer iii) Establishment of rheological properties and stability of inkjet ink, incorporating a binder polymer to form composite films upon jetting iv) Characterization of emissive properties of cured jetted films Tuesday, July 14, 2009 6
Typical inkjet ink formulation particulate pigment (1-5 w/w%) AND / OR organic dye (1-5 w/w%) (e.g. merocyanine) solvent (low T bp, 50-90 w/w%) + co-solvent (high T bp, 0-50 w/w%) dispersant (0-5 w/w%) dispersion + surfactant (0-5 w/w%) viscosity modifier buffer humectant (0-20 w/w%) inkjet ink Tuesday, July 14, 2009 7
Ink formulation: restraints Ink property Desired range Controlled by Viscosity Surface tension Particle size Droplet stability Drying time 2 10 cp 20 40 mn/m < 200 nm Spherical drops; no satellite droplets Variable; depends on application Solvent content Viscosity modifiers Surfactant(s) Choice of solvent Dispersant Pigment preparation Jetting waveform Fluid properties Humectant content Solvent type/amount Tuesday, July 14, 2009 8
Pigment synthesis ZnS:Mn (common PL/EL phosphor) readily synthesized as nanoparticles using wet methods acrylic acid: dispersant, PL enhancer, polymerizable Zn(CH 3 COO) 2 2H 2 O Mn(CH 3 COO) 2 4H 2 O Sodium citrate + Na 2 S 9H 2 O ZnS:Mn (NPs in H 2 O) + H 2 O + Na 2 S (aq) ZnS:Mn (nanoparticles) centrifugation Zn 2+ -citrate Mn 2+ Na + Cit 3- Ac - redispersion in acrylic acid ZnS:Mn-AA Tuesday, July 14, 2009 9
Ink formulation: ZnS:Mn + acrylic acid Solvent selection Co-solvent selection Check compatibility with acrylic acid dispersant Solvent content Surfactant content Solids content Rheological characterization i) Dynamic viscosity ii) Surface tension iii) Particle size Additives: i) defoamer ii) x-linker Images courtesy of Fujifilm-Dimatix Tuesday, July 14, 2009 10 Establish jettability using Dimatix DMP2800 printer
Semiconductive properties XRD: crystallinity, particle size, bandgap Particle size estimated from spectrum (1) Bandgap estimated from quantization relationship (2) Photoluminescence: inks (jetted and in solution) (1) Scherrer s equation (2) Energy of quantization β hkl = peak width at half-max. L hkl = particle diameter θ hkl = peak angle λ = x-ray wavelength (1.54 Å) K = 0.89 to 1 Tuesday, July 14, 2009 11 E G (R)= quantized bandgap E G ( ) =bulk bandgap R = particle radius m* = effective mass of carrier h = Planck s constant
Sample preparation centrifugation wet synthesis of ZnS:Mn nanoparticles drying dried nanopowder redispersion in acrylic acid ZnS:Mn dispersion ink formulation rheological testing ZnS:Mn ink XRD analysis inkjetting AA polymerization inkjetting PL analysis Tuesday, July 14, 2009 12 PL analysis
Results: X-ray diffraction Peak 1: 2θ = 28.0 Peak 2: 2θ = 46.9 Peak 3: 2θ = 56.1 Bragg angles for ZnS: (28.5, 47.5, 56.6 ) Lattice planes: (111) (220) (311): cubic Particle size estimated at 4.1 ± 1.1 nm; no bandgap modification; E g (R) = 3.54 ev Tuesday, July 14, 2009 13
Results: ink formulation Component Material Proportioning Pigment ZnS:Mn NPs 3-8 w/w% Dispersant/ monomer Acrylic acid 27-72 w/w% X-linker Diacrylate 2.7-7 w/w% Solvent N,N-DMF 0-54 w/w% Co-solvent Sulfolane 10 w/w% Surfactant Fluorosurfactant 2 w/w% Tuesday, July 14, 2009 14
Results: rheology & jettability 100 µm 00:00.00 00:00.10 00:00.20 00:00.30 00:00.40 00:00.50 Data for 6 w/w% ZnS:Mn ink. Solids/solvents loadings had little effect on γ or µ. Tuesday, July 14, 2009 15
Results: ink solids loading vs. PL Jetted films 3 jetted layers Cured 15 min at 90 C PL intensity at λ = 660 nm In solution 1 ml in glass cuvette PL intensity at λ = 655 nm Tuesday, July 14, 2009 16
Results: PL in inks vs. bulk ZnS:Mn Excitation λ = 325 nm Red-shifted for NPs (due to AA complexation) Shoulder at ~575 nm (polymer binders) Tuesday, July 14, 2009 17
PL spectra: varying excitation λ 330 340 360 370 350 380 λ ex = 360 nm: highest emission intensity in visible range λ ex = kλ em excitation wavelengths 390 260 250 270 280 290 320 400 410 420 300 310 Tuesday, July 14, 2009 18
Jetted pattern: photoluminescence Jetted 6 w/w% ZnS:Mn ink Cellulose acetate (CA) substrate Cured 15 min at 90 C Ambient light Jetted 6 w/w% ZnS:Mn ink Cellulose acetate (CA) substrate Cured 15 min at 90 C Short-λ UV (238 nm) excitation Tuesday, July 14, 2009 19
Conclusions Inkjet deposition of photoluminescent nanophosphor/polymer matrices feasible Ink is stable indefinitely; no precipitation observed Transparent after curing Simple wet methods suitable for preparation Applicable to several substrates (CA, photo paper, inkjet paper) and flexible after deposition Electroluminescence achievable Intensity/spectrum remain to be quantified Tuesday, July 14, 2009 20
Acknowledgements Prof. Ramin Farnood, Prof. Edgar Acosta & Prof. Timothy Bender (Univ. of Toronto) Gregory Cole (Univ. Of Waterloo) _ Tuesday, July 14, 2009 21
Thank you for your attention! Questions?