AFM and STM (Scanning probe microscopy)

Scanning Probe microscopy (AFM and STM) head point
AFM: Configuration of AFM
Parts of AFM system and Principle of AFM
Three Modes of AFM
AFM Instrument
Advantage and disadvantage
STM
Schematic Diagram
AFM and STM

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Preeti Choudhary

1. Scanning Probe microscopy (AFM and STM) Preeti Choudhary [email protected] MSc (Applied Physics)

2. AFM Atomic-force microscopy (AFM) or scanning-force microscopy (SFM) is a very high-resolution type of scanning probe microscopy (SPM), with demonstrated resolution on the order of fractions of a nm, more than 1000 times better than the optical diffraction unit. Used to measure a roughness of a sample surface at a high resolution, to distinguish a sample based on its mechanical properties (for example, hardness and roughness) and, in addition, to perform a microfabrication of a sample (for example, an atomic manipulation)

3. Configurations of AFM An AFM typically consists of the following features: (1) Cantilever , (2) Support(Configured to support cantilever.),  (3) Piezoelectric element(Configured to oscillate cantilever at its eigen frequency.), (4) Tip (Fixed to open end of a cantilever, work as a probe of AFM, (5) Detector (Configured to detect the deflection and motion of the cantilever.), (6) Sample (Will be measure by AFM), (7) xyz-drive, (Moving a Sample (6) and Sample (8) Stage to be displaced in x, y, and z directions with respect to a tip apex(4)), and  (8) Stage.

4. Parts of AFM system 1. Laser – deflected off cantilever 2. Mirror –reflects laser beam to photodetector 3. Photodetector –dual element photodiode that measures differences in light intensity and converts to voltage 4. Amplifier 5. Register 6. Sample 7. Probe –tip that scans sample made of Si 8. Cantilever –moves as scanned over sample and deflects laser beam

5. Energy U and force F between tip and sample as a function of their distance z. The force is the derivative (= slope) of the energy. It is attractive at large distances (van der Waals force, non-contact mode), but it becomes highly repulsive when the electron clouds of tip and sample overlap (Pauli repulsion, contact mode). In AFM the force is kept constant, while in STM the current is kept constant. Principle of AFM F U repulsive attractive z

6. Three Modes of AFM Contact Mode Non-Contact Mode Tapping (Intermittent contact) Mode

7. 40  m AFM Cantilever and Tip To obtain an extra sharp AFM tip one can attach a carbon nanotube to a regular, micromachined silicon tip.

8. AFM: Instrument

9. Contact Mode Measures repulsion between tip and sample Force of tip against sample remains constant Feedback regulation keeps cantilever deflection constant Voltage required indicates height of sample Problems: excessive tracking forces applied by probe to sample

10. Non-Contact Mode Measures attractive forces between tip and sample Tip doesn ’ t touch sample Van der Waals forces between tip and sample detected Problems: Can ’ t use with samples in fluid Used to analyze semiconductors Doesn ’ t degrade or interfere with sample- better for soft samples

11. Tapping (Intermittent-Contact) Mode Tip vertically oscillates between contacting sample surface and lifting of at frequency of 50,000 to 500,000 cycles/sec. Oscillation amplitude reduced as probe contacts surface due to loss of energy caused by tip contacting surface Advantages: overcomes problems associated with friction, adhesion, electrostatic forces More effective for larger scan sizes

12. W hat are the limitations of AFM ? AFM imaging is not ideally sharp

13. Advantages and Disadvantages of AFM Easy sample preparation Accurate height information Works in vacuum, air, and liquids Living systems can be studie d Limited vertical range Limited magnification range Data not independent of tip Tip or sample can be damaged

14. The Future of Atomic Force Microscopy   Sharper tips by improved microfabrication processes: tip – sample interaction tends to distort or destroy soft biological molecules Atomic or angstrom resolution images of live cell surfaces: development of more flexible cantilever springs and less damaging and nonsticky probes needed

15. STM

16. STM A  scanning tunneling microscope  ( STM ) is an instrument for imaging surfaces at the atomic level. Its development in 1981 earned its inventors, Gerd Binnig and Heinrich Rohrer (at IBM Zurich), the Nobel Prize in Physics in 1986. For an STM, good resolution is considered to be 0.1 nm lateral resolution and 0.01 nm depth resolution. With this resolution, individual atoms within materials are routinely imaged and manipulated. The STM can be used not only in ultra-high vacuum but also in air, water, and various other liquid or gas ambients , and at temperatures ranging from near zero kelvin  to a few hundred degrees Celsius

17. Conti.. The STM is based on the concept of quantum tunneling. When a conducting tip is brought very near to the surface to be examined, a bias (voltage difference) applied between the two can allow electrons to tunnel through the vacuum between them. The resulting  tunneling current  is a function of tip position, applied voltage, and the local density of states (LDOS) of the sample. Information is acquired by monitoring the current as the tip's position scans across the surface, and is usually displayed in image form. STM can be a challenging technique, as it requires extremely clean and stable surfaces, sharp tips, excellent vibration control and sophisticated electronics

18. STM ….Conti.. The resolution of an image is limited by the radius of curvature of the scanning tip of the STM. Additionally, image artifacts can occur if the tip has two tips at the end rather than a single atom; this leads to “double-tip imaging,” a situation in which both tips contribute to the tunneling. The tip is often made of tungsten or platinum-iridium, though gold is also used.

19. Schematic Diagram

20. Scanning Tunneling Microscope (STM) feedback regulator high voltage amplifier z x y I probing tip sample xyz-Piezo-Scanner Negative feedback keeps the current constant (pA-nA) by moving the tip up and down. Contours of constant current are recorded which correspond to constant charge density.

21. Technology Required for a STM Sharp, clean tip (Etching, ion bombardment, field desorption by pulsing) Piezo -electric scanner (Tube scanner, xyz scanner) Coarse approach (Micrometer screws, stick-slip motors) Vibrational damping (Spring suspension with eddy current damping, viton stack) Feed-back electronics (Amplify the current difference, negative feedback to the z- piezo )

22. STM versus AFM STM is particularly useful for probing electrons at surfaces, for example the electron waves in quantum corrals or the energy levels of the electrons in dangling bonds and surface molecules. AFM is needed for insulating samples. Since most polymers and biomolecules are insulating, the probe of choice for soft matter is often AFM. This image shows DNA on mica, an insulator.

23. Thanking-You 