Material Properties and Characterization

ETH lecture: 151-0637-00 Material Properties and Characterization Manfred Heuberger Materials Science & Technology Advanced Fibers, Empa, 9014 St. Gallen Manfred Heuberger EMPA - ETH 1

8h about Surfaces Part 1 Introduction When the material property depends on the surface 1.5h Surface Properties What are the relevant quantities? Analysis and Characterization of Surfaces (XPS, SIMS) 1.5h Intermolecular Interactions intermolecular forces (Van der Waals, electrostatic forces, entropic forces) Integration: from intermolecular to surface forces Macroscopic forces (Kapillary Forces, Young-Laplace equation) 1h Exercises (magnitudes, ranges, XPS spectra) Part 2 1.5h Static Surface Forces Lifschitz Theory Hamaker constant Forces at solid-liquid interfaces (Hydrophilic, double-layer, depletion, structural forces) Forces at polymer interfaces (steric forces, brush, bridging, ) Forces on biological surfaces (membranes, specific interaction) 1.5h Dynamic Surface Forces Adhesion hysteresis (dynamics of surfaces, WLF) Macroscopic friction (with / without lubrication) Introduction to Nanotribologie Measurement of surface forces (AFM, SFA) 1h Exercises (integration, Hamaker, steric repulsion) Why study surfaces? Example 1: Miniaturization everywhere Memory stick Harddisk Integrated circuit 2

Why study surfaces? Example 2: MEMS MEMS ratchet MEMS hinge Why study surfaces? Example 3: Forces and Actuation MEMS Post Style Actuator Large Force Electrostatic MEMS Comb Drive 3

Why study surfaces? Example 4: Nano-Structures Nanoporous Bio-Assay E-spun nano-fiber Why study surfaces? Example 5: Nano-Particles Stabilized Nano-particle Quantum Dots V 2 O 5 Nano-wire 4

Introduction When the material property depends on the surface Surface to Volume Ratio 500x10 6 400 Ratio F/V [m [m -1-1 ] ] 300 200 100 R 0 10-10 10-5 10 0 10 5 10 10 Radius of Sphere [m] 5

Playing with Nano nano (greek) = dwarf Just a piece of surface Number of atoms? 7x7x7=343 atoms Number of surface atoms? 210 atoms (60%) 1 nm = 10-9 m = 0.000 000 001m 6

Size matters Mechanical properties Thermodynamic properties Electronic properties Magnetic properties Optical properties Chemical properties Biological properties Surface Properties What are the relevant quantities? 7

? Question! name 3 surface properties (3 ) How can they be measured? Surface properties Hydrophobic/hydrophilic ratio Surface energy Surface potential Surface chemistry (functional groups) Biologic activity (receptor sites) Swelling potential (hydrogel character) Molecular mobility, relaxations Topography (roughness, texture) Crystalline/amorphous character Orientation (tilt angle, conformation) Domain, phase separation... 8

Analysis and Characterization Devil s Work The physicist Wolfgang Pauli (1945 Nobel Prize in Physics) once remarked: "God made solids, but surfaces were the work of the devil" Problem 1: Surface Sensitivity Problem 2: Cleanliness 9

Surface Sensitivity Cube: 1cm x 1cm x 1cm Question: How many atoms are in this cube? Answer: ~10 24 Atoms 1cm Question: How many atoms are on one surface? Answer: ~10 16 Atoms Volume to Surface Ratio: 100 000 000x Interacting with a surface photons (light, X-ray, IR) electrons ions atoms forces (touch, adhesion, friction, confinement) Liquids interfacing (spreading, wetting) chemical reactions (catalysis) specific interactions (protein activity) non-specific interactions (adsorption)... 10

Surface Analysis 1. 2. 3. surface 1. Probe (excitation) 2. Interaction (surface process) 3. Signal (information) Surface Selectivity Requirement The Probe, Interaction or the Signal must have an exponentially (or steeper) decay within the solid. 11

Detection Limits Surface Characterization Techniques Electron spectroscopies ESCA/XPS, AES Ion spectroscopies SIMS, ISS Vibrational spectroscopies ATR/FT-IR, HREELS Direct force methods STM, AFM/LFM, SFA Contact angle methods Contact angles, Wilhemly balance Diffraction methods TEM, grazing-xrd Evanescent field methods SPR, OWLS Interference methods QCM Optical techniques SEM (e - ), CLSM, Ellipsometry (laser) 12

Surface Cleanliness The monolayer time, τ, is the time it takes to cover a surface with a single layer of molecular adsorbates. It can be estimated from: τ = 3.2 x 10-6 P where t is in seconds, and P is in millibar. This assumes unity "sticking coefficient". Vacuum technology Idealized initial pumpdown of a 100 l system, size 50 x 50 x 40 cm, with a 6 CFM roughing pump and 200 l/s UHV pump. Typical pumpdown cycle 13

Clean surfaces are high energy surfaces They cover themselves with adsorbents to lower energy. Sources of Contaminants: airborne, pump oil, plasticisers, degasing, fingerprint A single fingerprint can be too much Surface specific techniques are therefore extremely sensitive to surface contamination (the more surface specific, the worse...) and this can be catastrophic for surface identification XPS and SIMS XPS SIMS 14

Overview: XPS and SIMS XPS SIMS Primary beam hν Ions (Ga+) Secondary particles to detector e- photo Ions & molecular ions Depth analysed Sample under UHV - 10-9 mbar vacuum technique Information: Spectra Depth profiles XPS SIMS XPS, SIMS Imaging XPS, SIMS X-Ray Photoelectron Spectroscopy: XPS Electron Spectroscopy for Chemical Analysis (ESCA) 15

1921 Nobel Prize In 1921 Einstein won the Physics Nobel Prize, not for relativity, but for the photoelectric effect. Electrons can absorb energy from photons. They follow an "all or nothing" principle. All of the energy from one photon must be absorbed and used to liberate one electron from atomic binding, or the energy is re-emitted. If the photon is absorbed, some of the energy is used to liberate it from the atom, and the rest contributes to the electron's kinetic (moving) energy as a free particle. Einsteins conclusion The light is quantized launched quantum theory. Photoemission Experiment primary: interaction: secondary: X-Ray (~1.5KeV) photo-electric effect electrons E kin photo emission hν 16

X-Ray Sources Cu Mg Al Crystal-Monochromator Synchrotron Al K α Using Photoemission Idea! Determination of electronic binding energies (E b ) of electrons emitted from the core levels of an atom resulting from X-ray irradiation of the sample (photoelectric effect). The emitted photoelectrons are collected by an electrostatic energy analyzer as a function of their kinetic energy (E kin ), from which the binding energies (E b ) can be obtained using the Einstein relation E b = hν -E kin - φ 2p3/2 (L3) 2p3/2 (L3) 2p1/2 (L2) 2p1/2 (L2) 2s 2s (L1) (L1) hν hν e e - - photo photo 1s 1s (K) (K) ESCA/XPS ESCA/XPS Photoelectron Photoelectron emission emission 17

Where the energy can go 1) Photoemission O 1s Photon b) 1s 2s 2p Auger electron a) 1s 2s 2p photoelectron 2) Relaxation a) Auger e - emission b) X-Ray fluorescence Measuring kinetic energy Main X-ray sources E = hν ΔE Al K α1,2 1486.6 ev 0.85 ev Mg K α1,2 1253.6 ev 0.7 ev Energy analyser CHA Mg K α1,2 source h ν e - E lectron optics Electron detector Sample 18

Typical XPS spectrum PMMA Intensity [a.u.] [a.u.] oxygen KLL Auger 1000 1000 800 800 core levels 600 600 400 400 Binding Energy [ev] 200 200 Fermi level 0 valence band (UPS) C 1s XPS Reference Data O1s Handbook of X-ray Photoelectron Spectroscopy; J. Chastain, R.C. King ed., 1995, Physical electronics Inc., Minnesota, US 19

Secondary Peaks X-ray source satelites surface plasmons electron system shake-ups equal distance π > π Auger electrons (shift with X-ray energy) binding energy Energy of Auger electrons Ekin (Auger) = 509eV Al Kα = 1486.6 ev Apparent BE = 977.6 ev 20

Typical XPS spectrum PMMA Intensity [a.u.] [a.u.] oxygen KLL Auger 1000 1000 800 800 600 600 400 400 Binding Energy [ev] 200 200 0 Electron mean free path Estimation of inelastic mean free path (Seah & Dench relation): A λ = + E 0.5 B( ae ) 2 kin kin As first approximation: λ = BE kin 0.5 With : B = 0.087 for organics B = 0.096 for inorganics usually: 0.4 nm < λ < 4 nm Example: (MgK α ) λ(c1s) 34 Å λ(o1s) 29 Å λ(n1s) 32 Å Other estimations by Tanuma et al. give similar results in the 700-1100 ev E kin range. M. P. Seah and W. A. Dench, Surface and Interface Analysis, 1, 2-11 (1979) S. Tanuma, C. J. Powell and D. R. Penn, Surface and Interface Analysis, 11, 577-589 (1988) and 21, 165-176 (1993). 21

Typical XPS spectrum PMMA Intensity [a.u.] [a.u.] 1000 1000 800 800 600 600 400 400 Binding Energy [ev] 200 200 0 Surface Sensitivity http://www-group.slac.stanford.edu/sms/images/xps.jpg 22

Quantitative XPS Theory The peak intensity detected for element A (I A ) is a function of the corresponding element concentration distribution throughout the sample: I A = I RX σ A f(e kin ) 0 C A (z)exp( z λ A sinα )dz z hν e - photo α Sample I A : intensity of the measured photoelectron signal I RX : intensity of the primary X-ray source σ A : cross section for emission of a photoelectron from an inner core shell of A f(ekin): detection efficiency of the spectrometer for each electron emitted C A : concentration of A as a function of depth (z) λ A : inelastic mean free path of a photoelectron emitted from A α: emission angle of the photoelectron with respect to the surface of the sample. Quantitative XPS Measurement The concentration of element A in a homogeneous matrix of n elements can be determined from a quantitative XPS measurement by the relation: C A = I A s A Σ n i=1 I i s i s i : atomic sensitivity factor: accounts for instrumental factors and for the cross-section of the photon-electron collision. 23

AR-XPS Primary Xray source Detector Primary Xray source Detector θ Substrate Information depth Substrate Normal emission angle θ Grazing emission angle θ Information or sampling depth (95% of total signal intensity): Z = 3λ sinθ AR-XPS DDP / Nb 2 O 5 120x10 3 100 XPS Survey spectrum 45 O1s Nb3d counts [a.u.] 80 O KLL 60 40 20 0 75 45 15 Nb3p C1s P2s P2p Nb4p 1000 800 600 400 200 0 binding energy [ev] 24

75 45 15 AR-XPS DDP / Nb 2 O 5 35x10 3 30 'Nb3d@15 ' 'Nb3d@75 ' Nb3d 25 35x10 3 30 25 C1s 'C1s@15 ' 'C1s@75 ' counts [a.u.] 20 15 10 5 0 40x10 3 O1s 'O1s@15 ' 'O1s@75 ' counts [a.u.] 20 15 10 200 205 210 215 binding energy [ev] counts [a.u.] 30 20 10 5 0 0 280 285 290 295 300 525 530 535 binding energy [ev] 540 545 binding energy [ev] Quantitative XPS Overlayers e - e - e - e - T(E kin ) is the transmission function of the XPS analyzer. β [%]: over-layer surface coverage 25

Quantitative XPS Overlayers OIR does NOT depend on β! distinction of different over-layer morphologies? XPS Quiz! 1. What features in the XPS spectrum are due to elastic electrons? main Peaks 2. Why has the XPS spectrum stair-steps at each peak? inelastic scattering of electrons 3. What are the advantages / disadvantages of a monochromator? resolution / less intensity 4. Why is XPS a vacuum technique? 5. Is shake-up an alcoholic drink? electron scattering, Cleanliness no, an excitation of the π electrons 26

Secondary Ions Mass Spectroscopy: SIMS Time-of-flight Secondary Mass Spectroscopy Tof-SIMS The three modes of SIMS Static SIMS (S-SIMS) Minimal sample degradation, topmost surface atoms Static limit: total primary ion dose < 10 13 ions/cm 2 Sampling depth < 1 nm Dynamic SIMS (D-SIMS) Depth profiling applications (sputtering rates 100 Å/min) Sampling depth 1-5 nm Imaging SIMS (I-SIMS) Chemical imaging Sampling depth 3-10 nm 27

ABC + ABCAB + Secondary Ions from Ion Bombardement Primary ions 10 12 ions cm -2 max. ABCABCAB + ABCABCAB + A + C + BC + 1 monolayer 10 15 atoms cm -2 Cs+ + Mass Spectroscopy Cs + ion gun Animation: (Forschungszentrum Jülich) Tofsim.swf Electrodynamic Buncher IL ESA 3 Deflector Plates IL High Speed Deflector ESA 2 Energy slit ESA 1 sample Sec. El. Detector Electron Neutraliser TL 1 TL 2 Contrast Diaphragm Laser Liquid Metal Ion Gun Detector 28

Typical Spectrum I-SIMS Si 29

Comparing XPS and SIMS ESCA/XPS Elemental analysis (Z>2, 0.1-1% monolayer) Determination of oxidation state of elements Functional group identification (± 0.3 ev) Surface quality control Quantification (± 5-10%) Composition vs depth 2-10 nm (AR-XPS) > 10 nm (depth profiling) Determination of oxide or SAM thicknesses Chemical imaging (± 5-1000 µm) Sample charging Study of surface reorganisation (with or without cold-stage) ToF-SIMS Elemental analysis (10-3 monolayer, mass range easlily up to 10000 amu) Isotopes Chemical structure (m/δm > 3000) Surface quality control Limited quantification (matrix effects) Composition vs depth Sampling depth < 10 Å (static mode) > 1 nm (dynamic SIMS) Chemical imaging (> 100 nm) Sample charging very problematic Study of surface reorganisation (with cold-stage) Sample charging effects Problem related to insulating samples (bio-polymer, ceramics...) Cherging mechanism XPS: charging via emission of photoelectrons a positive charge builds up at the surface affects emission efficiency and the binding energy (drift to higher Eb). SIMS: charging due to positive primary ions bombardment Special problem for negative ions mass spectrum loss of intensity and high-mass fragments detection Charging generally not stable with time and not homogeneous throughout the sample (depends on chemical, morphological,..., heterogeneity). Neutralization with low energy electron flood gun: help but not ideal; can also induce significant sample damage. 30

? XPS Quiz! 1. Why is XPS surface-selective? Electron free path very short 2. Why is SIMS not as quantitative? Not all secondary fragments are charged 3. Why does one work in vacuum? Cleanliness, Scattering of electrons and ions 4. Why does Tof-SIMS use pulsed primary ions? To measure time-of-flight 5. How can one realize the imaging mode (i-xps, i-sims)? i-xps: electron optics; i-sims primary ion beam Intermolecular Interactions What are the forces between two molecules? 31

Forces in Nature Strong nuclear interaction (gluons) Electro-weak interaction (kaons) Grand Unifying Theory (GUT) Electrostatic Interaction (photons) -> (vacuum) surface forces Gravitational interaction (tachions) The nature of Surface Forces Q: Are Surface Forces fundamental forces? A: No, except charged surface interaction in vacuum. (Surface Forces are structure and thermodynamics) 32

Thermodynamic Forces Thermodynamics Steric forces Osmotic forces Entropic forces important in colloid science What exactly are Surface Forces? Range of Surface Forces? Surface: Interface ±10nm Surface Forces <-> Intermolecular Forces Strength of Surface Forces? Where are Surface Forces important? What is a Dynamic Surface Force? How can one measure a Surface Force? 33

Who is studying Surface Forces? Prof. B. Derjaguin, Moscow (50 s) Prof D. Tabor, Cambridge, UK (70 s) Prof. Jacob Israelachvili, UCSB, USA Other labs Historic opinions McBain in Colloid Science, (1950) One of the fundamental problems of science is to establish the effective ranges of molecular attractions: 1) The classical physicists of the 19 th century considered that there were direct forces of attraction over distances... up to many microns. 2) During the first decades of the 20 th century, however, the recognition of the electrical structure of matter influenced scientific opinion to assume, without new evidence, that the direct range of molecular attraction amounted to only a few Ångstrom units; and an extreme view, under the influence of Langmuir, held that not adjacent molecules, but only their adjacent atoms had any important influence upon each other. 3) A third point of view recognizes the short range of direct attraction but considers that it must be relayed from molecule to neighboring molecule through impressive distances. The conflicting consequences of these beliefs are of the greatest importance. 34

First direct measurement Range 1µm Roughness Surface charging problematic Derjaguin, 1951 Intermolecular forces early concepts Van der Waals forces in gas equation, 1873 (P + a ) (V b) = R T 2 V Empiric potential by Mie, 1903 w(r) = C 1 r + C 2 n r m Intermolecular forces Surface Forces 35

Power laws and decay lengths 10 1 10 0 y=10-10n /x n 10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 pair potential [a.u.] n=4 n=5 n=3 n=2 n=1 10-9 n=6 10-10 10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 10 0 distance [m] Surface forces Starting point: w(r) = C r n assumption: pair additive r = (z2+x 2)1/2 z=0 z=d D x dz dv dx x z dv = 2π x dx dz W ms (D) = 2π C ρ n=6 W (n 2)(n 3)D n 3 ms (D) = π C ρ 6D 3 36

Electrostatic Surface Forces 8.2.1 Electrostatic Interactions Coulomb Potential w Cb (r) = Q 1 Q 2 4π ε 0 ε r = z 1 z 2 e 2 4π ε 0 ε r Interaction charge-surface Integration: additivity surface charge density, σ da=2πrdr 2 D2+r2 point charge, Q 1 2 distance, D D2+r2 r R F cs (r) = d dd w cs(r) = σ e 2π ε 0 ε D D 2 + R 2 1 R σ e 2π ε 0 ε strong interaction Nearly independent of D 37

Experimental Consequences Problems Difficult to measure other (weaker) interactions High electric fields in contact area Solutions Screening of charges by ions Removal of charges by conductance Electrostatic Surface Forces 8.2.2 Van der Waals forces and potentials Electrostatic Nature of matter e - polarizability, α permittivity, ε dispersion, ε(λ) permanent charges, σ permanent dipoles, µ p- 38

Van der Waals force Gecko and ist feet Johannes Diderik van der Waals (1837-1923 ) Van der Waals - many faces strong ubiquitous (always) attractive 39

The three most important contributions to Van der Waals interactions Dispersion interaction (London) Dipole-induced dipole interactions (Debye) Rotating dipole-dipole interactions (Keesom) The London interaction Description polarizability, α α r α London interaction energy (quantum mechanics) 3hυα 2 w(r) = 4(4πε 0 ε) 2 r 6 40

The Debye interaction Description dipole moment µ rotational angle Θ 1 polarizability, α µ Θ r α Electric dipole field E(r,Θ) = μ 1+ 3cos2 Θ 4πε 0 ε r 3 The Debye interaction w(r,θ) = 1 2 α E2 (r) = μ 2 α (1+ 3cos 2 Θ) 2(4πε 0 ε) 2 r 6 After angular integration: w(r,θ) = μ 2 α (4πε 0 ε) 2 r 6 41

The Keesom interaction Description dipole moments µ 1 and µ 2 rotational angles Θ 1, Θ 2 and Θ 3 Θ 1 Θ 3 µ 1 r µ2 Θ2 w(r,θ 1,Θ 2,Θ 3 ) = μ 1 μ 2 4πε 0 ε r 3 (2cosΘ 1 cosθ 2 sinθ 1 sinθ 2 cosθ 3 ) Maximum interaction w(r,0,0,θ 3 ) = 2μ 1 μ 2 4πε 0 ε r 3 The Keesom interaction Angular-spatial Integration Integral Zero for free rotation Boltzmann distribution favors attractive angles Θ 1 Θ 3 µ 1 r µ2 Θ2 w(r) μ 2 2 1 μ 2 for kt > 3(4πε 0 ε) 2 kt r 6 μ 1 μ 2 4πε 0 ε r 3 42

Van der Waals Surface Forces Experimental evidence of retardation effects range <10nm retardation >5nm -> Hamaker constant What is surface energy? Broken symmetry at the surface Surface molecules have higher energy less interactions 43

Minimum system energy at equilibrium smallest surface to volume ratio sphere curvature 1/r r Lowering surface energy 44

Surfactants can lower surface energy non-polar polar polar Surface energy and surface tension Surface energy = [J/m 2 ] = [Nm/m 2 ] = [N/m] = surface tension 1m J/m2 1m 1N 1m 45

Surface tension of water 1 dyne = 10-5 Newton [dyne/cm] = [mn/m] = [mj/m 2 ] Young-Laplace equation r 2 r 1 dr area: A = 4 π R 2 da = 8 π R dr pressure, p i volume: V=4/3 π R 3 dv = 4 π R 2 dr outside pressure, p 0 work: ΔW = 0 = γ da (p i -p 0 ) dv = γ 8 π R dr Δp 4 π R 2 dr Δp = 2 γ / R = γ (1/r 1 + 1/r 2 ) 46

1805 independent derivation of equation Thomas Young (1773-1829) Pierre-Simon Laplace (1749-1827) Exercises 1) Plot the Lennard-Jones pair-potential and the resulting pair- force in a graph. At what separation, r, is the equilibrium distance (F=0)? Below which separation does the repulsive term start to dominate? 2) To directly measure Lennard-Jones-type forces between macroscopic bodies, does one need the entire bodies to be present? What is the critical exponent, n, in a potential of type w(r)=-a/r n, below which the total energy of interaction starts to depend on the entire body? 3) Compare the Keesom energy to kt. Up to what distance are dipolar typically molecules oriented? The dipole moment of water is µ=1.854d (Debye), where 1D=3.33564*10-30 Cm, the relative permittivity of water is ε=78.54 and the permittivity of vacuum is ε 0 =8.854*10 12 As/Vm, the Boltzmann constant is k=1.38*10-23 J/K. 4) The DLVO theory is known to fail at high salt concentrations and/or small distances. What could be the reason(s) for this, and, in which sense do you expect the real forces to deviate from the theory? 5) Using Archimedes law, show that two bodies of mass m 1 =V 1 *ρ 1 and m 2 =V 2 *ρ 2 are experiencing a repulsive gravitational force if immersed in a medium of density ρ 3 when either one of the conditions ρ 1 <ρ 3 <ρ 2 or ρ 2 <ρ 3 <ρ 1 applies. 47

? Question! What objects are shown in the background? What are their properties? Answer! Multi-walled carbon nano tubes (CNT) high tensile strength el. conductive 1nm-10nm diameter -> use in Advanced Fibers 48