Scanning Force Microscopy - Contact Mode
- Snap to Contact
Long-range contributions have force gradients like dF/dz » 1-10 N/m.
Spring constants of k ≈ 0.01-1 N/m provokes “jump-into-contact” when k < dF/dz.
Adhesion causes hysteresis in force vs. distance curves.
- Force Distance Curves to obtain Chemical Specifity
- Snap to Contact
Contact Radius:
• in atmosphere: 2-10 nm (1nN < F < 100nN)
• in UHV: 1-4 nm (0.1nN < F < 10nN)
- Pseudo Atomic Resolution
• contact AFM on NaF(001)
• observation of atomic lattice Moirée-effect)
• step width approximately 1nm indication of tip radius
• single atoms or atomic defects not visible
- The Millipede
A 2-dimensional SFM cantilever array, called "Millipede", of the IBM Zurich Research laboratory [1] was
designed for high density data storage. A 32x32 um2 cantilever array is positioned above a polymer surface. The
entire cantilever array chip is scanned in x-y-direction. The approach in z-direction is controlled by 3
piezoresistive cantilevers and 3 corresponding magnetic actuators, which allow to control not only the z-distance
but also the z-leveling.
During the storage operation, the chip is scanned by a magnetic x-y-actuator. Each SFM writes and reads the
data in its own storage field (100x100 um2). Initial storage densities of 100-200 Gb/in2 were achieved, where dots
of 50nm in diameter were written into PMMA. Assuming this storage density the entire 32x32-array has a capacity
of 10Gb on 3x3mm2. The Millipede is based on a thermomechanical write/read process in nanometer-thick
polymer films. The tip is heated with an integrated heater and a thermal sensor is used for reading.
Scanning Force Microscopy - Applications in Biology
- AFM in Buffer Solution
Zur Anzeige wird der QuickTime™
Dekompressor “Cinepak”
benötigt.
- For survival pores close at low pH
- Conformational change of bacterial pores in cell membranes
- Unzipping Bacteriorhodopsin
Friction Force Microscopy (FFM)
- Scanning
- Friction Contrast
Material-specific contrast in FFM. Topography (a) and
lateral force (b) recorded on a mixed Langmuir-Blodgett
film(C21H43COO -/C9F19C2H4OCC2H4COO -)
Circular
areas
of
hydrocarbon molecules
are surrounded by
fluorocarbon molecules.
Within the hydrocarbon
islands, material has
been removed by the
action of the tip (low
scanning velocity, high
load).
- Spring Model of Experiment
• the effective stiffness dominated by the contact stiffness
Atomic stick-slip as a function of normal force: Observation of “Superlubricity”
AFM non-contact (ncAFM) or Dynamic Force Microscopy (DFM)
- Snap to Contact
Contact Radius:
• in atmosphere: 2-10 nm (1nN < F < 100nN)
• in UHV: 1-4 nm (0.1nN < F < 10nN)
- How to achieve True Atomic Resolution?
- Principle of Dynamic Force Microscopy with Atomic Resolution
- Dynamic SFM
Measurement of the dynamical properties of the cantilever:
resonance frequency shift & energy loss per oscillation period
Imaging with True Atomic Resolution
Atomic Resolution on Si(111)-7x7
• height difference between inequivalent adatoms due to small differences in reactivity
- Site Specific Forces
- Comparison to DF-Theory
• good agreement of maximum force but strong relaxation effects
• single covalent bond between tip apex and adatom
• well characterized tip are needed !
Atomic Resolution, Exchange Forces Site Specific Interaction Forces
• energy loss mechanism is non-trivial !
• energy dissipation 4 orders lager than predicted by stochastic force fluctuation theory
• energy loss caused by atomic instabilities
- Dissipation by Stochastic Friction Forces
“The energy dissipation is attributed to stochastic, non-equilibrium processes which cause
the appearance of a friction force due to surface atoms opposing the tip movement”
Dissipation calculated for various Lennard-Jones Interaction Potentials.
But: at the moment theory predicts dissipation to be 100.000x smaller than observed!
- Dissipation by Atomistic Instabilities
Modeling of Relaxation Effects
Approach tip to surface and calculate apexatom–sample distance versus tip–sample distance