Tutorial 4

1. Tutorial 4 Derek Wright Wednesday, February 9th, 2005

2. Scanning Probe Techniques • Scanning Tunneling Microscope • Scanning Force Microscope • Imaging of Soft Materials • Manipulation of Atoms and Molecules • Chemical Reactions with the STM

3. Scanning Probes • Atomic-sized probe is dragged across the surface • Types of measurements taken: • Current • Magnetic • Force

4. Scanning Tunneling Microscope • Scanning: • The tip is scanned across the sample in a grid pattern • Tunneling: • There is a tunneling current between the sample and the tip which is measured • Microscope: • We can see atomic sized things with it

5. Scanning Tunneling Microscope • Tunneling current is a quantum effect • e- aren’t points in space, they have a probability of location • This waves exist with a probability density centered around the e- • The e- is “smudged” in space • If a thin barrier intersects this probability density, the e- might have a chance of “appearing” on the other side of the barrier

6. Scanning Tunneling Microscope

7. STM Equations • I  V  Ntip Nsample • Ntip, Nsample = density of states • I  exp(-2keffz) • z is the distance between the tip and sample • I drops off exponentially with the distance • I drops off exponentially with keff

8. STM Equations • keff = (2meB/h2) + |k|||2 • keff = inverse effective decay length • me = mass of electron • B = barrier height (has to do with the work functions of the tip and sample and the applied voltage) • k|| = parallel wave vector of the tunneling electrons • B = (tip + sample)/2 - |eV|/2 • (tip + sample) are the work functions of the tip and sample • V is the applied voltage

9. STM Modes • There are two modes of operation • Constant Distance (z-position const.) • The tunneling current is plotted • Constant Current • The vertical movement of the tip is plotted • This is the usual method • Good because of the exponential nature of the tunneling current + feedback

10. STM Constraints • The STM tip must have excellent mechanical stability • Achieved through piezoelectric actuators • Rests on heavy table with many dampers • The tip must come to a very small point • Can be achieved through electrochemical etching • Carbon nanotube can be placed on the end to improve accuracy

11. Scanning Force Microscope • Sometimes called Atomic Force Microscope (AFM) • Setup very similar to STM except tip deflection is measured instead of tip current • Can be used where current won’t flow • Two modes of operation: • Contact • Non Contact

12. Scanning Force Microscope • Contact Mode (z < 1 nm): • The tip is dragged across the surface and the deflection is measured optically • Deflection is due to repulsion of tip particles with surface particles • Can scratch the surface – not recommended for soft substrates • Non-contact Mode (z > 1 nm): • With the tip not actually touching the surface, dominant forces are van der Waals, electrostatic, and magnetic

13. Scanning Force Microscope • As the tip is brought from a distance closer to the sample: • First van der Waals forces pull the tip closer • Then ionic repulsion pushes it away • The tip’s deflection can be measures using laser interferometery

14. Scanning Force Microscope • Tip can be operated in “dynamic mode” • The tip and cantilever (beam with the tip on it) have a mechanical natural resonance • The resonance will change as external forces from the sample are exerted on it • The tip’s vibration amplitude must be much less than the distance between it and the sample to ensure linear operation • Like how a transistor amplifier is linear when the signal is much less than the supply voltage

15. Scanning Force Microscope

16. Magnetic SFM • Used to measure magnetic media • The tip is a piece of magnetic material and is of a single domain • All dipoles are aligned in the tip • The interaction of the tip’s magnetic field and the sample create a force • The force shows the sample’s domains and boundaries between them

17. Electrostatic SFM • A method that plots the sample’s static surface charge • Tip is electrically isolated (cantilever is an insulator) • Two pass method: • First pass is a contact pass • Second pass occurs at a constant distance from the sample and measures the force due to the charge on the sample and the charge induced in the tip

18. Piezoresponse Force Microscopy • The tip and cantilever can bend in two axes to give an idea of the 3D domain structure of a sample • An oscillating voltage is applied to the tip • An oscillating current occurs (due to the capacitance of the tip) which interacts with the B-field of the sample • This creates a measurable force and bends the cantilever

19. Imaging of Soft Materials • Contact with soft samples is bad • The tip will damage the delicate sample • Contact gives better resolution, but is too harsh • Non-contact methods have been tailored for soft samples • Special feedback circuits • Special modulation frequencies • High gap impedances (large gap between tip and sample)

20. Manipulating Atoms and Molecules • Tip is brought above a loose atom or molecule • Attractive forces between the two allow tip to pick up the atom • Tip drags the atom • Tip raises to let go of the atom

21. Manipulating Atoms and Molecules

22. Quantum Corrals • A ring of atoms can create a “quantum corral” • The ring forces electrons within into circular wave patterns • Doesn’t need to be a ring – any closed structure will create resonance patterns within

23. Quantum Corrals

24. Quantum Corrals

25. Quantum Corrals

26. Chemical Reactions with the STM • Since the tip can: • Manipulate atoms and molecules • Provide energy in the form of a tunneling current • It is possible to make chemical reactions occur by dragging the molecules together and form or break bonds with the tunneling current

27. Chemical Reactions with the STM

28. Thank You! • This presentation will be available on the web.