Scanning Tunneling Microscopy and Atomic Force Microscopy

1. EEW508 Scanning probe microscopy Scanning Tunneling Microscopy and Atomic Force Microscopy

2. Scanning Tunneling Microscopy (STM) - History - Principle of STM - Operation modes – constant current mode, constant height mode, conductance mapping, tunneling spectroscopy - Examples of STM studies – atomic structures, dynamics, STM manipulation Atomic Force Microscopy (AFM) - History - Principle of AFM - Operation modes – contact, non-contact, intermittent modes - Variation of AFM – friction force microscopy, conductive probe AFM, electrostatic force microscopy, etc. - Examples of AFM studies – atomic stick-slip, friction, adhesion properties of surfaces EEW508 Scanning probe microscopy

3. Invention of scanning tunneling microscopy (1982) Gerd Binnig & Heine Rohrer, IBM Zurich (nobel prize in 1986) EEW508 Scanning probe microscopy Beginning of Scanning Probe Microscopy First STM image of Si (7x7) Reconstruction on Si (111) surface Phys Rev Lett (1983)

4. EEW508 Scanning probe microscopy Principle of Scanning Tunneling Microscopy I STM tip I ~ e –2d (d: tip-sample separation, K is the constant) A V Sample surface Tunneling current (I) d The key process in STM is the quantum tunneling of electrons through a thin potential barrier separating two electrodes. By applying a voltage (V) between the tip and a metallic or semiconducting sample, a current can flow (I) between these electrodes when their distance is reduced to a few atomic diameters.

5. EEW508 Scanning probe microscopy Principle of Scanning Tunneling Microscopy Because the density of state of the sample contributes the tunneling current, STM is effective technique for the conductive surface (semiconductor or metallic surface).

6. EEW508 Scanning probe microscopy Schematic of Scanning Tunneling Microscopy The instrument basically consists of a very sharp tip which position is controlled by piezoelectric elements (converting voltage in mechanical deformation) • Figure: Michael Schmid, TU Wien

7. EEW508 A Scanning probe microscopy Imaging modes of Scanning Tunneling Microscopy STM tip Constant height mode Feedback off Constant current mode Feedback on STM topographical imaging (constant current mode) The tip is moved over the surface (x direction), while the current, and consequently the distance between the tip and the sample are kept constant. In order to do so, the vertical (z) position of the tip is adjusted by a feedback loop. Thus reading the z position of the tip, one obtains real-space imaging of the sample surface.

8. EEW508 Scanning probe microscopy Scanning Tunneling Spectroscopy Silicon (100) (2x1) dimer row reconstruction structure Tunneling spectroscopy reveals the bandgap of 0.7 eV due to the presence of surface states (M. Crommie group)

9. EEW508 Scanning probe microscopy STM instrumentation Beetle type walker Commercial system

10. EEW508 Scanning probe microscopy Examples of STM studies 1. Atomic manipulation (Don Eigler, IBM) A node in the electron standing wave Fe atoms Xe atoms on Ni (100) at 8K assembled by atomic manipulation Quantum corral (D. Eigler) Iron on Copper (111) assembled by atomic manipulation

11. EEW508 Scanning probe microscopy Examples of STM studies (Dynamics of molecules) Water dimers diffuse much faster than monomer and trimer Water molecules on Pd(111) surface T. Mitsui et al. Science (2002)

12. EEW508 Scanning probe microscopy (Somorjai group) High pressure STM reaction studies

13. EEW508 Scanning probe microscopy Examples of STM studies – Correlating the atomic structure with electronic properties STM image and spectroscopy of single walled carbon nanotube (C. M. Lieber group) (n,m) nanotube, if n − m is a multiple of 3, then the nanotube is metallic, otherwise the nanotube is a semiconductor.

14. EEW508 Scanning probe microscopy Examples of STM studies – revealing periodicity and aperiodicity STM image of two-fold surface of Al-Ni-Co decagonal quasicrystal surface Fibonacci sequence A progression of numbers which are sums of the previous two terms f(n+1) = f(n) + f (n-1), J. Y. Park et al. Science (2005)

15. EEW508 Scanning probe microscopy STM Fabrication and Characterization of Nanodots on Silicon Surfaces This involves field evaporation from either an Al- or Au-coated tungsten STM tip. This has the advantage of allowing imaging of the structures subsequent to fabrication, with the same tip. Application of a short voltage pulse to a tip held in close proximity to the surface produces nanodots with a probability and dot size which depend on the size and polarity of the pulse. It has been also demonstrated the modification of existing nanodots, via the application of additional, larger voltage pulses of both polarities. Left: STM image of Au dots (approx. 10 nm dia. x 1.2 nm ht.) deposited on oxidized Si(100) by application of -8V, 10 msec pulses to the tip. Right: Same Au dots after modification by application of +10 v pulse (left, dot erased) and –10 v pulse (right, dot enlarged). J. Y. Park, R. J. Phaneuf, and E. D. Williams, Surf. Sci. 470, L69 (2000).

16. EEW508 Scanning probe microscopy Atomic Force Microscopy

17. Invention of atomic force microscopy (1985) Binnig, Quate, Gerber at IBM and Stanford EEW508 Scanning probe microscopy History of Atomic Force Microscopy Binnig et al. PRL (1985)

18. EEW508 Position sensitive Photodiode array Laser beam Bending Torsion FRICTION LOAD Scanning probe microscopy Principle of Atomic Force Microscopy Forces: Van der Waals force electrostatic force Magnetic force Chemical force Pauli repulsive force When the tip is brought into proximity of a sample surface, forces between the tip and the sample lead to a deflection of the cantilever according to Hooke's law. This deflection is characterized by sensing the reflected laser light from the backside of cantilever with the position sensitive photodiode. Because force signal (including Van der Waals force, electrostatic force, Pauli repulsive force) is measured, various samples including insulator can be imaged in AFM.

19. EEW508 Scanning probe microscopy Constant height and force mode AFM laser detection cantilever Constant force mode (force feedback on) Constant height mode (force feedback off) AFM topographical imaging (constant force mode) The tip is moved over the surface (x direction), while the force, and consequently the distance between the tip and the sample are kept constant. In order to do so, the vertical (z) position of the tip is adjusted by a feedback loop. Thus reading the z position of the tip, one obtains real-space imaging of the sample surface.

20. EEW508 micromotor Beetle type walker photodiode cantilever Scanning probe microscopy AFM instrumentation

21. EEW508 Tip Coating (TiN) back Coating (Au) for laser reflection Scanning probe microscopy Cantilevers in atomic force microscopy Cantilevers can be seen as springs.the extension of springs can be described by Hooke's Law F = - k * s.This means: The forceF you need to extend the spring depends in linear manner on the range s by which you extend it. Derived from Hooke's law, you can allocate a spring constantk to any spring. Damping spring of wheel in the car : 10000 N/m, spring in the ball point pencil : 1000 N/m, spring constant of commercial cantilever :0.01 – 100 N/m

22. EEW508 Scanning probe microscopy Imaging mode in atomic force microscopy Feedback : lever deflection the feedback system adjusts the height of the cantilever base to keep this deflection constant as the tip moves over the surface (friction force microscopy, conductive probe AFM) Feedback : oscillation amplitude The cantilever oscillates and the tip makes repulsive contact with the surface of the sample at the lowest point of the oscillation (Tapping mode AFM) Feedback : oscillation amplitude the cantilever oscillates close to the sample surface, but without making contact with the surface. Electrostatic / magnetic force microscopy Feedback : lever deflection the tip does not leave the surface at all during the oscillation cycle. (interfacial force microscopy)

23. EEW508 Scanning probe microscopy Friction force microscopy AFM topographical and friction images of C16 silane self-assembled monolayer on silicon surface revealing lower friction of molecule layers

24. EEW508 Scanning probe microscopy Measurement of adhesion force between tip and sample with force-distance curve At the point A, the tensile load is the same with the adhesion force (FAB corresponds to the adhesion force)

25. Three dimensional mapping the adhesion force and Young’s modulus EEW508 28nN 35 30 25 18nN 20 15 10 5 0 5 10 15 20 25 30 Adhesion force (nN) Scanning probe microscopy Force Volume Mapping CdSe tetrapod Adhesion mapping topography

26. EEW508 Scanning probe microscopy AFM images of various materials Contact mode AFM topography (left), friction (right) images of graphite surface Contact mode topographical (up) and friction images (bottom) of polymer Contact mode friction image (left) and its line profile of mica surface which show atomic stick-slip process

27. EEW508 Friction at the single asperity Friction at the Macroscopic scale Scanning probe microscopy Nanoscale material properties is different from macroscopic properties – for example, friction Single asperity Real contact AFM

28. EEW508 Scanning probe microscopy Perspective of SPM • 1981 : First STM results in the lab • 1985 : Invention of AFM in Stanford (Quate group) • 1986 : Nobel Prize for Rusk , Binnig & Rohrer • : First commercial instruments (Park Scientific Instrumentation from Stanford, Digital Instrumentation from Paul Hansma) • : first year > 1000 STM papers published • 2005 : Over 2000 STM and 6500 AFM papers published • Scanning Probe Microscopy is one of major tools to characterize and control nanoscale objects

29. EEW508 Scanning probe microscopy Summary Scanning tunneling microscopy (STM): tunneling current between the sharp tip and conductive surface is detected and used to acquire STM images. Atomic force microscopy (AFM): Force between the cantilever and the surface is measured and used for AFM imaging Both insulating and conductive materials can be imaged in AFM