1. Tutorial 4 Derek Wright Wednesday, February 9 th, 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  N tip  N sample –N tip, N sample = density of states I  exp(-2  k eff  z) –z is the distance between the tip and sample –I drops off exponentially with the distance –I drops off exponentially with k eff

8. STM Equations k eff = (2m e B/h 2 ) + |k || | 2 –k eff = inverse effective decay length –m e = 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

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.