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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 An Introduction to Atomic Force Microscopy Peter Grutter Physics Department www.physics.mcgill.ca/~peter/
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 Outline 1. Introduction 2. Magnitude of forces How to measure forces Ultimate limits 3. Components of an AFM Cantilever Deflection sensing Feedback Piezo scanners Image processing Approach mechanisms 4. What forces? Repulsive forces van der Waals forces Electrostatic forces Magnetic forces Capillary forces 5. Operation modes Normal and lateral forces Force spectroscopy Modulation techniques AC techniques Dissipation 6. Imaging artifacts 7. Summary
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 Scanning Tunneling Microscope (STM) Based on quantum mechanical tunneling current Works for electrically conductive samples Imaging, spectroscopy and manipulation possible D. Eigler, IBM Almaden
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 Forces between atoms Bonding energies: Quantum mechanical (covalent, metallic bonds): 1-3 nN Coulomb (dipole, ionic): 0.1-5 nN Polarization (induced dipoles): 0.02-0.1 nN J. Israelachvili ‘Intermolecular and Surface Forces’ Academic Press ‘Back of the envelope’: Atomic energy scale: E bond ~ 1-4 eV ~ 2-6 10 -19 J Typical bonding length: a ~ 0.2 nm Typical forces: F = E/a ~ 1-3 nN
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 Measuring forces Force: F = k z Force gradient F’ : F’= 2k f f approximation good if d 2 V / dz 2 = constant otherwise: Giessibl, APL 78, 123 (2001) zz spring constant k Harmonic oscillator: f 2 = k /m F’ acts like a spring in series: f 2 = (k+F’)/m
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 Ultimate limits of force sensitivity 1. Brownian motion of cantilever! thermal limits Martin, Williams, Wickramasinghe JAP 61, 4723 (1987) Albrecht, Grutter, Horne, and Rugar JAP 69, 668 (1991) D. Sarid ‘Scanning Force Microscopy’ 2. Other limits: - sensor shot noise - sensor back action - Heisenberg D.P.E. Smith RSI 66, 3191 (1995) Roseman & Grutter, RSI 71, 3782 (2000) A 2 = k B T/k A…rms amplitude Bottom line: Under ambient conditions energy resolution ~ 10 -24 J << 10 -21 J/molecule T=4.5K
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 Atomic Force Microscope deflection sensor approach Data acquisition scanner tip feedback force sensor vibration damping sample
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 The force sensor Microfabrication of inte- grated cantilevers with tips
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 Spring constants k and resonant frequency f of cantilevers Spring constant k : typical values: 0.01 - 100 N/m Young’s modulus E Y ~ 10 12 N/m 2 Resonant frequency f o : typical values: 7 - 500 kHz W L t
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 Calibration of cantilever spring constant k Methods: Thermal Hutter and Bechoefer, RSI 64, 1068 (1993) Sader method (measure geometry) Sader RSI 66, 9 (1995) Reference spring method M. Tortonese, Park Scientific Added mass Walters, RSI 67, 3583 (1996) Excellent discussion and references: www.asylumresearch.com/springconstant.asp
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 Deflection sensors A) Beam deflection B) Interferometry C) Piezoresisitive D) Piezoelectric Giessibl, APL 73, 3956 (1998) D A B Meyer and Amer, APL53, 1045 (1988) Rugar et al., APL 55, 2588 (1989)
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 Feedback modes z = constantF = constant
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 Piezoelectric scanners Properties: 1. Hysterisis (non-linear) 2. Creep (history dependent) 3. Aging (regular recalibration) +y -x +x -y Piezo tube (1) (2)
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 Creating an image from the feedback signal line scan gray scale image processed image
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 Image processing Raw data shows ‘jumps’ in slow scan direction. (Due to pointing instabilities of laser). Processing (here ‘flatten’) can remove them, but can create new artifacts. Beware of introducing image processing artifacts ! Understand and know what you are doing
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 Tip-sample approach Dynamic range from mm to nm Coarse & fine approach! Many possibilities: 1. Lever arms 2. Piezo walkers Micrometer screw 1 Micrometer screw 2 Fixed point
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 And finally: thermal drift! Touching the microscope (e.g. sample, cantilever) will change its temperature T. Shining light on it too! Cantilever has a mass of ~ 1 ng, and thus a VERY small heat capacity. So what!?! L/L = const T const ~ 10 -5
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 The first AFM G. Binnig, Ch. Gerber and C.F. Quate, Phys. Rev. Lett. 56, 930 (1986)
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 Repulsive Contact Forces Diblock co-polymers used as self assembled etch mask Rubbed Nylon LCD alignment layer Meli, Badia, Grutter, Lennox, Nano Letters 2, 131 (2002) Ruetschi, Grutter, Fuenfschilling and Guentherodt, Science265, 512 (1994)
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 Van derWaals forces F vdW = AR/6z 2 A…Hamaker const. R…Tip radius z…Tip - sample separation A depends on type of materials (polarizability). For most materials and vacuum A~1eV Krupp, Advances Colloidal Interface Sci. 1, 113 (1967) R~100nm typical effective radius -> F vdW ~ 10 nN at z~0.5 nm
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 Electrostatic forces F electrostatic = RU 2 / z U…Potential difference R…Tip radius z…Tip - sample separation R~100nm typical effective radius U=1V -> F electrostatic ~ 5 nN at z~0.5 nm Tans & Dekker, Nature404, 834 (2000)
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 Chemical forces F Morse = E bond /z (2e - (z- ) - e -2 (z- ) ) E bond …Bond energy …decay length radius …equilibrium distance Other popular choice: 12-6 Lennard Jones potential Si(111) 7x7 Lantz et al, Science 291, 2580 (2001)
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 Magnetic Forces F magntic = m tip H sample Comprehensive review: Grutter, Mamin and Rugar, in ‘Scanning Tunneling Microscopy II’ Springer, 1991 Melting of flux lattice in Nb Images stray field and thus very useful in the magnetic recording industry, but also in science. Roseman & Grutter, unpublished
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 Magnetic Force Microscopy hard disk floppy disk image size 10 and 30 micrometers. M. Roseman (McGill) Magnetic reversal studies by MFM particles size 90 x 240 x 10 nm X. Zhu (McGill) Tracks on
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 Capillary forces (water layer) Total force on cantilever = sum of ALL forces There is always a water layer on a surface in air! F capillary = 4 R cos …surface tension, ~10-50 mJ/m 2 …contact angle Surface Water Tip Can be LARGE (several 1-10 nN)
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 Different operation modes Imaging (DC) Lateral or frictional forces Force spectroscopy (F(z), snap-in, interaction potentials, molecular pulling and energy landscapes) Modulation techniques (elasticity, electrical potentials, …) AC techniques (amplitude, phase, FM detection, tapping) Dissipation
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 DC Imaging, lateral forces Meli, Badia, Grutter, Lennox, Nano Letters 2, 131 (2002) Diblock co-polymer: Normal forcesFriction
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 Force Spectroscopy Snap in condition: k < F’ For meaningful quantitative analysis, k > stiffness of molecule a a water force distance
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 W(111) tip on Au(111) Cross et al. PRL 80, 4685 (1998) Schirmeisen et al, NJP 2, 29.1 (2000 ) Field ion microscope manipulation of atomic structure of AFM tip
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 Site specific chemical interaction potential: Si(111) 7x7 Lantz, Hug, Hoffmann, van Schendel, Kappenberg, Martin, Baratoff, and Guentherodt, Science 291, 2580 (2001)
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 AFM Elasticity Maps of Smooth Muscle Cells HANKS buffer no serotonin HANKS buffer 1 M serotonin topography elasticity contrast Induced contraction Cells stiffness increased B. Smith, N. Durisic, P. Grutter, unpublished
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 AFM probe Au surface DNA “Unwinding” Nature - DNA replication, polymerization Experiment - AFM force spectroscopy Anselmetti, Smith et. al. Single Mol. 1 (2000) 1, 53-58
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 DNA Structural Transitions AFM Force Spectroscopy in TRIS Buffer 300 450600 750 800 400 0 Duplex poly(dG-dC) B-S Transition ~ 70 pN Melting Transition ~ 300 pN 50 75 100 125 800 400 0 Force [pN] Duplex poly(dA-dT) B-S Transition ~ 40 pN Simulation data from Lavery and Lebrun 1997. B S ssDNA Elasticity Model Molecular Extension [nm]
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 Typical forces and length scales Gaub Research Group, Munchen
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 F(z) as a function of pulling speed Clausen-Schaumann et al., Current Opinions in Chem. Biol. 4, 524 (2000) Merkel et al., Nature 397, (1999) Allows the determination of energy barriers and thus is a direct measure of the energy landscape in conformational space. Evans, Annu. Rev. Biophys. Biomol. Struct., 30, 105 (2001)
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 Modulation techniques Concept: modulate at frequency f mod and use e.g. lock-in detection. Elasticity Viscoelasticity Kelvin probe Electrical potential Piezoresponse …. Carbon fibers in epoxy matrix, 40 micrometer scan Digital Instruments
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 AC techniques Change in resonance curve can be detected by: Lock-in (A or ) * FM detection ( f and A drive ) Albrecht, Grutter, Horne and Rugar J. Appl. Phys. 69, 668 (1991) (*) used in Tapping™ mode ff AA f 1 f 2 f 3
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 Some words on Tapping™ Amount of energy dissipated into sample and tip strongly depends on operation conditions. Challenging to determine magnitude or sign of force. NOT necessarily less power dissipation than repulsive contact AFM. Anczykowski et al., Appl. Phys. A 66, S885 (1998 )
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 Dissipation The cantilever is a damped, driven, harmonic oscillator Magnetic dissipation due to domain wall oscillations. Sensitivity better than 0.019 eV per oscillation cycle Y. Liu and Grutter, J. Appl. Phys. 83, 7333 (1998) Dissipation due to non-conservative tip- sample interactions such as: Inelastic tip-sample interactions Adhesion hysterisis Joule losses Magnetic dissipation
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 Imaging Artifacts ‘High’ resolution and double tip: Blunt tip :
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P. Grutter, McGill University BIMR Wokshop, May 15th, 2002 Outlook AFM provides imaging, spectroscopy and manipulation capabilities in almost any environment: ambient, UHV, liquid at temperatures ranging from mK - 900K with atomic resolution and sensitivity (at least in some cases)
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