Characterization of surfaces by AFM topographical, mechanical and chemical properties

Transcription

1 Characterization of surfaces by AFM topographical, mechanical and chemical properties Jouko Peltonen Department of physical chemistry Åbo Akademi University

2 Atomic Force Microscopy (AFM) Contact mode AFM Tapping mode AFM Fotodiode FN Laser FL Sample XYZ - piezounit Cantilever

3 Contact mode AFM cantilever deflection ( z) tip sample contact point sample moving down sample position Jump-into-contact force F attr sample moving up Jump-off-contact force F adh Cantilever deflection can be converted to force: F int = - zk - k = cantilever spring constant F attr is proportional to the Hamaker constant A: F attr = AR/(6d 2 ) - R = tip radius of curvature - d = tip-sample distance at contact (2-4 Å) F adh is proportional to surface energy γ: F adh = γ3 R

4 Contact mode AFM: OAT STARCH 1 week old sample 3 weeks old sample 5 weeks old sample morphology Friction (mechanical contrast) Carbohydrate Polymers 37 (1998) 7-12

5 Tapping mode AFM Free: W kin = 0.5 k λ 2 Imaging: W kin = 0.5 k λ 2 F = - k (λ/2) F = - k (λ/2) φ S(Q/k) A 1/2 E * (Q/k)

6 Fibre topography vs. chemical pulping 10 min cooked 120 min cooked Oxygen delignified phase image morphology Polymer 41 (2000)

7 Fibre topography vs. mechanical pulping Image size: 3 µm x 3 µm Colloids Surfaces A 225 (2003)

8 Chemical and mechanical surface properties of wood fibres adhesion contrast (low tapping) stiffness contrast (high tapping)

9 3D topography How to identify a 3D surface?

10 Ex: Protein adsorption onto an ultraflat substrate Y X Height scale: 5 nm Image size: 2 x 2 µm 2 Relative height höjd (nm) (nm) X (µm) ~4-7 nm ~2-3 nm Layer of fragmented antibodies (Fab ) Langmuir 18 (2002)

11 Tip-sample convolution in AFM imaging causes distortion of lateral dimensions can be eliminated by deconvolution AFM tip imaging image analysis corrected lateral dimensions by deconvolution

12 Protein surface coverage by AFM; image deconvolution Original AFM image Deconvoluted AFM image A z (nm) 4 z (nm) x (µm) x (µm) Sensors and Actuators B 102 (2004)

13 A set of roughness parameters for versatile topographic identification S q = RMS roughness S sk =skewness, height distribution asymmetry. Gaussian 0 S ku =kurtosis, height distribution sharpness (peakedness). Gaussian 3.0 S ds = density of summits, number of local maxima per unit area. S sc = mean summit curvature, principal curvature of local maxima. S dq = RMS slope, RMS value of the Z-slope within the sampling area. S dr = surface area ratio, ratio between the real interfacial area and projected area. S ci = core fluid retention index, measure of fluid volume in core zone. Gaussian 1.56 S vi = valley fluid retention index, measure of fluid volume in valley zone. Gaussian 0.11

14 Enhanced wetting of rough polar surfaces Θ Y = Young s contact angle model surface Θ Y Θ m = measured contact angle real surface cos Θ m = r cos Θ Y r = the ratio between the effective and projected surface area Θ m The Wenzel s roughness equation Wenzel, Ind. Eng. Chem. (1936)

15 3D imaging of paper board topview images of size 3 x 3 µm 2 sample 2 sample 1 Size distribution and 3D orientation of pigments Phase contrast images resolve structural boundaries Identification of the crystal form of pigments topograph phase image

16 Roughness analysis of paper board samples - influence of length scale S #2 S #1 S #4 S #5 S #3 Skewness (Ssk) um image um image Nanosized, but not microsized surface pores Density of summits (Sds) um image um image More fluctuations on nanoscale than on microscale

17 Wenzel s equation and porous surfaces; TiO 2 Image size: 3 x 3 µm S q : nm S sk : S dr : % Θ m : (measured) Θ Y : (calculated) Langmuir 20 (2004), in press

18 Precipitation of inorganic minerals onto TiO 2 films Experimental setup: 4 mm CaCO 3 solution, ph = 8.5, ~1 1 cm 2 TiO 2 -films Calcite crystals on TiO 2 films were observed after a couple of hours The number of crystals increased iniatially, continued by the growth in size of the crystals After 6 hours After 24 hours Scalebar: 100µm

19 Precipitation vs. substrate topography Calcium Phosphate Formation Index Sds Sds = density of summits, number of local maxima per unit area. The Sds-parameter obtained from AFMmeasurements gave the best correlation with calcium phosphate growth. Langmuir 20 (2004), in press