FABRICATING POLYMER NANOSTRUCTURES WITH CONTROLLED MORPHOLOGY: DIFFERENT TOP-DOWN APPROACHES. Ignacio Martín-Fabiani

Transcription

1 FABRICATING POLYMER NANOSTRUCTURES WITH CONTROLLED MORPHOLOGY: DIFFERENT TOP-DOWN APPROACHES Ignacio Martín-Fabiani Instituto de Estructura de la Materia, IEM-CSIC, Madrid, Spain 1

2 Motivation POLYMERS Synthetic Natural High molecular weight (more than 1000 atoms and up to millions) Carbon-based Glass transition temperature (T g ) Unique internal structure: amorphous and crystalline phases coexist ~ m ~ 10-8 m ~ 10-9 m Functionality Low cost Flexibility Spherulites Crystalline lamellae Unit cell Durability Light weight 2

3 Motivation POLYMERS Light weight Functionality Low cost Flexibility Durability NANOSTRUCTURATION Laser Induced Periodic Surface Structures (LIPSS) Nanoimprint Lithography (NIL) Imprinting of Optical Near Fields (ONFs) Melt Wetting of Nanoporous Templates Feasibility of nanostructuration methods in polymers Appropiate characterization techniques Optimization of the process/nanostructures Understanding of the physics that lie underneath LET S GO! APPLICATIONS Photovoltaics Plasmonics Photonics Electronics

4 Preparation and characterization of polymer thin films Preparation and characterization of polymer thin films PC Spin coating of a PTT/TFA solution (20 g/l) on a Si substrate PTT PET T g = 44ºC ; T m = 232 ºC ω a ω a Atomic Force Microscopy (tapping mode) Absorbance 10 x 10 μm 2 5 x 5 μm 2 Rugosity 1,1 ± 0,5 nm Film thickness 147 ± 25 nm Good absorbance only in the near UV range 190 nm < λ < 400 nm

5 Laser Induced Periodic Surface Structures (LIPSS) Interference between incident and reflected light generates ripples with period L~λ It involves a feedback effect and thus pulse repetition Rugosity plays a relevant role Laser Induced Periodic Surface Structures (LIPSS) H n Laser light θ λ L = n senθ LIPSS Original surface Open questions L Can we assess the structural order of LIPSS on polymer films? Is it possible to optimize LIPSS formation with laser parameters? Irradiation of polymer thin films Nd:YAG laser (λ = 266 nm) Linearly polarized τ = 6ns ; f = 10 Hz Nnormal incidence (θ = 0⁰) L = λ n senθ = λ n Fluence dependence (Energy of a single pulse over a determined area integrated over time, mj/cm 2 ) Number of pulses dependence Characterization AFM in tapping mode Bolle, M.; Lazare, S., Appl. Surf. Sci. 1993, 69 (1-4), Csete, M.; Bor, Z., Appl. Surf. Sci. 1998, 133 (1-2),

6 Laser Induced Periodic Surface Structures (LIPSS) Martin-Fabiani, I.; Rebollar, E.; Perez, S.; Rueda, D. R.; Garcia-Gutierrez, M. C.; Szymczyk, A.; Roslaniec, Z.; Castillejo, M.; Ezquerra, T. A., Langmuir 2012, 28 (20), Dependence with number of pulses for a fixed fluence F = 7 mj/cm Dependence with fluence for a fixed number of pulses (600) 4 mj/cm 2 5 mj/cm 2 9 mj/cm2 13 mj/cm 2 Period close to the wavelength Height increases with number of pulses There is a strong dependence of morphology with laser parameters Is it possible to optimize LIPSS formation with laser parameters?

7 q z Grazing Incidence Small Angle X-ray Scattering (GISAXS) q y Detector q z = 2π λ m ω=0 Correlations perpendicular to the sample plane senα i + senα Correlations parallel to the sample plane q y = 2π λ senωcosα Grazing Incidence Small Angle X-Ray Scattering Allows characterizing submicrometric films Reflection geometry is extremely sensitive to surface features By changing the incidence angles different depths within the sample can be probed Powerful tool to characterize nanostructures DESY, Hamburg (Germany) h α f ω m α i h α=0 Sample α i Incident beam BW4 beamline (Doris ring) λ = 0.14 nm MARR CCD pixel size 79.1 x 79,1 µm 2 α i 0.4⁰ Muller-Buschbaum, P., Analytical and Bioanalytical Chemistry 2003, 376 (1),

8 GISAXS modeling of nanostructures Local monodisperse aproximation (LMA) (monodisperse domains that interfere incoherently between them) Distorted Wave Born Approximation (DWBA) (4 contributions to the form factor) dσ dω q =< F 2 > S q α Iscattered Central geometrical parameters determined by AFM, assuming a variation σ R R ~ σ H H ~0.1 The probability of finding the next box at a distance L is determined p x = 1 (x L)2 exp σ 2π 2σ 2 by a Gaussian function Paracrystalline lattice Paracrystalline distortion parameter L L+ΔL 1 L L-ΔL 2 IsGISAXS software Freeware g = σ/l g = 0 Crystalline lattice g Disordered system Hosemann, R., Zeitschrift Fur Physik 1950, 128, Lazzari, R., Journal of Applied Crystallography 2002, 35,

9 100 AFM Dependence with number of pulses for a fixed fluence F = 7 mj/cm 2 GISAXS 100 Laser Induced Periodic Surface Structures (LIPSS) Cut at α = 0.2 ⁰ Modelling

10 Laser Induced Periodic Surface Structures (LIPSS) Paracrystalline distortion parameter g = σ/l g = 0 Crystalline lattice g Disordered system Optimum LIPSS formation LIPSS generated by ns laser pulses can be described as paracrystalline 1D lattices Optimum value of the number of pulses for a fixed frequency! GISAXS and AFM information are in agreement and complement each other 10

11 AFM 5 mj/cm 2 GISAXS 5 mj/cm 2 Laser Induced Periodic Surface Structures (LIPSS) Dependence with fluence for a fixed number of pulses (600) Cut at α = 0.2 ⁰ Modelling 5 mj/cm 2 9 mj/cm 2 9 mj/cm 2 9 mj/cm 2 13 mj/cm 2 13 mj/cm 2 13 mj/cm 2

12 Laser Induced Periodic Surface Structures (LIPSS) Structural order of LIPSS improves with increasing fluency! GISAXS and AFM information are in agreement and complement each other 12

13 Laser Induced Periodic Surface Structures (LIPSS) Formation mechanism Solving numerically the heat equation in one dimension: 2 T(x, t) x 2 T x, t 2 a x = α κ P t exp αx F 0 The maximum temperatures reached on the surface of the polymer film are around ⁰C. These temperatures are well above T g and below T m, suggesting that a reorganization of material takes place in the formation of LIPSS (no ablation). The Raman spectra of irradiation samples remain unaffected except for a slight oxidation in the case of high number of pulses Rebollar, E.; Perez, S.; Hernandez, J. J.; Martin-Fabiani, I. et al. Langmuir 2011, 27,

14 Laser Induced Periodic Surface Structures (LIPSS) Au-coated polymer LIPSS as Surface-Enhanced Raman Scattering (SERS) substrates The presence of metallic nanoparticles can enhance dramatically the Raman intensity PTT LIPSS Au-coated LIPSS A SERS enhancement of an order of magnitude is obtained Pulsed Laser Deposition Nd:YAG laser λ = 213 nm; τ = 15 ns Ν = 10 Hz Thinner Au coatings yield higher enhancements Linear relationship between analyte concentration and Raman intensity These hybrid substrates require a small amount of gold They benefit from the light weight, flexibility and durability of polymeric materials The linear relationship of the Raman intensity with analyte concentration is advantageous for potential application in sensors Rebollar, E. et al. Phys. Chem. Chem. Phys. 2012, 14,

15 Nanoimprint Lithography (NIL) Nanoimprint Lithography (NIL) Lithographic process which involves the transfer of nanometric motifs from a hard mold to a soft substrate Requires the use of white rooms The transfer is optimized when motifs in the mold are protruding from the substrate. Press 15

16 Fabricación del molde I: mesa Fabrication of a mesa-type structure Nanoimprint Lithography (NIL) Wet Optic etching Lithography UV light HF Mask Resist Si wafer H 2 O HNO 3 16

17 Fabricación del molde I: mesa Fabrication of a mesa-type structure Nanoimprint Lithography (NIL) AFM Height 650 nm Resist Wafer 17

18 Nanoimprint Lithography (NIL) Fabrication of motifs on the mesa structure e - e - PMMA Si (1) spin- coating 300 nm (2) e-beam lithography (3) Development + Metalization SF 6 C4 F 8 O nm (4) Removal of redundant resist and metal (5) Reactive Ion Etching (6) Mold

19 Si mold SEM AFM Nanoimprint Lithography (NIL) 60nm 200 x 200 µm 2 Mesa 300 nm 60 nm 100 nm Silanization process: Avoids sticking of the polymer to the Si mold TFOCS GISAXS arc-like pattern Radius depends on α i Spacing 300 nm LET S IMPRINT! Qin, D.; Xia, Y. N.; Whitesides, G. M. Nat. Protoc. 2010, 5,

20 Si mold Nanoimprint Lithography (NIL) Polymer replica 150 ºC 40 bar 5 minc 300 nm 60 nm 100 nm 60 nm 300 nm 52 nm AFM images shows regions where the effective length of the motifs is reduced 150 ºC 40 bar 5 minc Scattered intensity in the vertical direction 20

21 Nanoimprint Lithography (NIL) (⁰) (⁰) (⁰) (⁰) AFM Si mold GISAXS Modeling (⁰) (⁰) (⁰) Polymer replica (⁰) 200 μm 150 μm LIPSS (⁰) (⁰) 1 μm (⁰) (⁰) Effective length of the motifs decreases

22 (⁰) (⁰) (⁰) (⁰) (⁰) (⁰) Nanoimprint Lithography (NIL) GISAXS W/R = 1000 Form Factor Interference function Cut at ω = 0 ⁰ W/R = 100 (⁰) (⁰) (⁰) W/R = 10 (⁰) (⁰) (⁰) (⁰) The transition from an arc-like pattern, characteristic of polymer grating replicas fabricated by NIL, to a fringe pattern, characteristic of LIPSS, is determined by a reduction of the effective length of the motifs (⁰) (⁰) Rueda, D. R.; Martin-Fabiani, I.; Soccio, M. et al. J. Appl. Crystallogr. 2012, 45,

23 Optical Near Fields allow to overcome the diffraction limit Nanostructuring with Optical Near Fields (ONFs) Imprinting the Optical Field dispersed by a dielectric microsphere aser light ( λ/2) Vicinity of the sphere θ Ablation hole r Shadow region It is possible to visualize or imprint structures with resolution of λ/2 or smaller Period in the backward direction p bw 1 sin Laser light CD and DVD optical systems (phase change memories) Microscopy (SNOM) Sensors Materials processing Far field approximation θ p bw a Applications p fw b Period in the forward direction p fw 1 sin The sphere can be considered a point scatterer Carried out in several inorganic materials: silicon, amorphous silica, aluminium oxide, Ge 2 Sb 2 Te 5 Never in polymers! Kuehler, P. et al. Small 2009, 5,

24 Nanostructuring with Optical Near Fields (ONFs) 1. Sample preparation: spin-coating of SiO 2 microspheres (ϕ = 4.6 µm) on PTT polymeric thin films 2. Irradiation: single pulse from an excimer laser ArF at normal incidence (θ = 0⁰) or θ = 54⁰. Fluences around mj/cm 3. Characterization of the irradiated zone OM θ = 0⁰, F=300mJ/cm 2 AFM 150x150 µm2 10x10 µm2 Dendritic morphology! 100 µm Crystallinity hints? GIWAXS Some incipient shoulders corresponding to Bragg peaks appear Raman Spectroscopy Increased fluorescence Might be related to parallel arrangement of phenyl rings in PTT Luo, W.-a. et al. Macromolecules 2008, 41,

25 Nanostructuring with Optical Near Fields (ONFs) θ = 54⁰, F = 350 mj/cm 2 25 x 25 μm 2 25 x 25 μm 2 Zoom in the backscattering region 100 µm L 2 L 1 Near-field induced topography modulation extends over the entire film thickness. Regions of local fluence enhancement correspond to topographic regions below the initial film surface Values of the period tend to reach the asymptotical value predicted in the far field 107 nm Asymptotical value in the far field p bw 1 sin

26 10 nm 0 nm Nanostructuring with Optical Near Fields (ONFs) Formation mechanism: patterning of GST single pulse from an excimer laser ArF Fluences around 75 mj/cm 2 θ = 54⁰, F = 75 mj/cm 2 Topographic contrast originates from the different density of both phases, the amorphous one being lower yielding to an elevation of the amorphous regions over the crystalline film (no ablation) Raman spectroscopy 10 nm 0 nm Siegel, J. et al. Appl. Phys. 2008, Under these irradiation conditions PTT presents chemical stability

27 Bulk Restricting dimensions 2D confinement Melt wetting of nanoporous AAO membranes Electrospinning 1D confinement Self-assembly Nanoporous templates Competition Length scales imposed by external geometry Pore diameter Pore depth Pore density Versus Length scales imposed by internal cooperativity Crystalline unit cell Chain folding Size of crystalline lamellae Under these conditions interesting effects may arise! Garcia-Gutierrez, M.-C.; Linares, A.; Hernandez, J. J.; Rueda, D. R.; Ezquerra, T. A.; Poza, P.; Davies, R. J. Nano Lett. 2010, 10, Zhang, M. F.; Dobriyal, P.; Chen, J. T.; Russell, T. P.; Olmo, J.; Merry, A. Nano Lett. 2006, 6, Shin, K.; Xiang, H. Q.; Moon, S. I.; Kim, T.; McCarthy, T. J.; Russell, T. P. Science 2004, 306,

28 Poly (trimethylene terephthalate) (PTT) T g = 44ºC ; T m = 232 ºC Disordered AAO membranes TOP Pore size ~ 20 nm Melt wetting of nanoporous AAO membranes Melt wetting PTT/SWCNTs nanocomposite (0.5 % weight concentration) Prepared by in-situ polimerization SWCNTs: diameter nm 3ºC/min Semicrystalline nanorods 1h at 253 ºC Solving AAO in NaOH 500 nm BOTTOM Pore size ~ 200 nm PTT /nanocomposite film (200 µm thick) 0 AAO membrane N nm Are you sure they are crystalline? Are they oriented? Let s check it! 28

29 ESRF (Grenoble) ID13 Microfocus beamline λ = 0,09951 nm 1 µm beam diameter FReLoN detector Transmission geometry X-Ray Microdiffraction setup Step scan (1 µm resolution) We record a 2D diffraction pattern each single micrometer containing WAXS and SAXS information WAXS Residual film Interface Infiltrated AAO template Crystallographic planes Orientation Scan direction SAXS X Beam orthogonal to the slide plane Orientation of crystalline lamellae Long spacing 29

30 Bulk PTT Inside AAO 102 Wide Angle X-Ray Scattering (WAXS) 002 c Bulk PTT/SWCNTs Inside AAO Kinetic selection a b Triclinic unit cell a=0.46nm ; b=0.61 nm ; c =1.86 nm ; α=97.5º ; β=92.1º ; γ =110º Wang, B. J et al. Polymer 2001, 42, Main nanopore axis 0kl reflections concentrated in the meridian in both infiltrated PTT and nanocomposite!! a-axis lies in the equator, b and c can rotate Steinhart, M. et al.. Phys. Rev. Lett. 2006, 97. b c a-axis is the radial growth direction for PTT spherulites! a Ho, R. M. et al. Macromolecules 2000, 33,

31 Crystalline orientation Bulk Interface Inside AAO Wide Angle X-Ray Scattering (WAXS) Full Width at Half Maximum of the 102 reflection 102 Azimutal integration Orientation improves with pore depth! Template has been filled completely Increase of intensity related to: Augmented amount of material Increase in orientation 31

32 Expected : Shish Kebab? Obtained AAO vs SWCNTs SWCNT b c a Do the SWCNTs enter the pores? Main polymer chain paralell to the AAO wall (edge-on) Main polymer chain orthogonal to the AAO wall (flat-on) Yes, they do! Sticky walls Polymer melts Low surface energy AAO High surface energy Slippery walls The formation of flat-on lamellae is favored by the strong polymer-aao interaction This interaction, toguether with the kinteic selection, prevails over the templating features of the SWCNTs Ma, Y.; Hu, W. B.; Hobbs, J.; Reiter, G. Soft Matter 2008, 4,

33 Small Angle X-Ray Scattering (SAXS) Isotropic Edge-on Edge-on + Flat-on No long spacing PTT Scan direction Residual film 70 µm away 25 µm away 5 µm away Infiltrated AAO Main nanopore axis PTT/ SWCNTs 4-point patterns (shish-kebab) Slight increase in flat-on population (SWCNTS alignment with the pores) No long spacing 33

34 SAXS Model for PTT WAXS Isotropic WAXS Edge-on lamellae Edge-on/ flaton lamellae 002 b c a Bulk polymer Flow zone Interface Confinement zone Kinetic selection Martin-Fabiani, I. et al. ACS Applied Materials & Interfaces 2013, 5, AAO-polymer interaction 34

35 Conclusions Conclusions It has been proven that controlled nanostructuration of polymeric materials can be achieved by using laser light (LIPSS and imprinting of ONF), NIL and melt wetting of nanoporous AAO membranes. It was demonstrated that nanosecond laser pulses can be used in order to obtain LIPSS on polymer thin films. In addition, it has been proven that the characterization of LIPPS in real space by AFM can be complemented with the corresponding characterization in the reciprocal space by GISAXS. It was shown that numerical modeling of the GISAXS diagrams of LIPSS structures provides a description of them in terms of one-dimensional paracrystalline lattice of parallelepipeds. In addition to this, the study of the dependence of the paracrystalline distortion parameter has allowed determining the values of laser fluence and number of pulses that optimize LIPSS formation. The potential of hybrid gold-polymer LIPSS as SERS substrates has been proven. 35

36 Conclusions Conclusions Hard silicon mold nanogratings have been imprinted on polymeric thin films by NIL, fabricating successfully their replicas in the polymer. Numerical modeling of the GISAXS diagrams has allowed concluding that the transition from an arc-like pattern, characteristic of polymer grating replicas fabricated by NIL, to a fringe pattern, characteristic of LIPSS, is determined by a reduction of the effective length of the motifs. The nanostructuration by imprinting of ONF has been applied for the first time on a polymeric material. Structuration by ONF takes place by polymer ablation and the nature of the laser used determines the appearance of dendritic structures on the polymer film surface. It has been proven that the one-dimensional confinement of PTT in AAO nanopores induces the growth axis of the crystalline lamellae to align with the main axis of the nanopores. The template effect of the SWCNTs in the crystallization process of polymer nanocomposites is inhibited by the one-dimensional confinement in AAO membranes. 36

37 Acknowledgements Acknowledegements Soft and Polymeric Matter group T.A. Ezquerra A. Linares Collaborators E.Rebollar (IQFR-CSIC) J. Siegel (IO-CSIC) F. Pérez-Murano ( J. Boneberg 37

38 Thanks for your attention! Soft and Polymeric Matter group 38