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Scanning Probe Microscopy – Principle of Operation, Instrumentation, and Probes

Scanning Probe Microscopy – Principle of Operation, Instrumentation, and Probes. Scanning Probe Microscopy-STM.

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Scanning Probe Microscopy – Principle of Operation, Instrumentation, and Probes

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  1. Scanning Probe Microscopy – Principle of Operation, Instrumentation, and Probes

  2. Scanning Probe Microscopy-STM • The principle of electron tunneling was proposed by Giaever. He envisioned that if a potential difference is applied to two metals separated by a thin insulating film, a current will flow because of the ability of electrons to penetrate a potential barrier. • R. Young developed field emission topograph profiler. • Binnig and Rohler introduced vacuum tunneling combined with lateral scanning. The vacuum provides the ideal barrier for tunneling. The lateral scanning allows one to image surfaces with exquisite resolution, lateral-less than 1 nm and vertical-less than 0.1 nm, sufficient to define the position of single atoms.

  3. Calibration standards Crystals as ‚rulers‘ h = d111 = 0,31 nm STM Image of HOPG (Highly Oriented Pyrolytic Graphite)Image size: 10nm x10nm AFM image of silicon (111) single atomic steps with native oxide

  4. Since the introduction of the STM in 1981 and AFM in 1985 by Binnig and Rohler , many variations of probe based microscopies, referred to as SPMs, have been developed.

  5. Scanning Probe Microscopy Family of SPM • STM scanning tunneling microscope • AFM Atomic force microscope • FFM (or LFM) (Lateral or Friction) force microscope • SEFM Scanning electrostatic force microscope • SFAM scanning force acoustic microscope • AFAM atomic force acoustic microscope • SMM scanning magnetic microscope • MFM magnetic force microscope • SNOM scanning near field optical microscope • SThM scanning thermal microscope • SEcM scanning electrochemical microscope • SKpM scanning Kelvin Probe microscope • SCPM scanning chemical potential microscope • SICM scanning ion conductance microscope • SCM scanning capacitance microscope

  6. Non-contact scanning probe microscope (SPM) • Scanning tunneling microscope • Atomic force microscope • Scanning near field optical microscope • Scanning magnetic force microscope

  7. Optical microscope and Transmission electron microscope Operating voltage 50~100kV; Wave length: 0.004~0.006 nm; Resolution: <1nm

  8. Laser autofocus system

  9. Field-ion microscope Radius of the tip: 10nm Magnification: 106; Operating voltage ~100kV; Resolution: <1nm

  10. Field emission profiler -- R.Young Operating voltage ~100V; Resolution: vertical~3nm; Lateral~400nm

  11. Principle of operation of the STM made by Binnig and Rohrer JT ∝ VT exp(−Aφ1/2d) JT― the tunnel current, a sensitive function of the gap width d ; VT ― the bias voltage; φ― the average barrier height (work function) ; A ―constant= 1.025 eV−1/2 Å−1. With a work function of a few eV, JTchanges by an order of magnitude forevery angstrom change of d.

  12. The tip is scanned over a surface while the tunneling current changes withit, so the surface is measured. • A sharp metal tip to the surface: 0.3–1 nm; • At a convenient operating voltage (10mV–1V); • The tunneling current varies from 0.2 to 10 nA (which is measurable).

  13. the head which houses the piezoelectric tube scanner for three dimensional motion of the tip and • the preamplifier circuit (FET input amplifier) mounted on top of the head for the tunneling current, • the base on which the sample is mounted, and the base support, which supports the base and head Nanoscope STM consists of three main parts:

  14. Scanning Probe Microscopy-STM Principle of operation of a commercial STM, a sharp tip attached to a piezoelectric tube scanner is scanned on a sample The motion of the tip due to external vibrations is proportional to the square of the ratio of vibration frequency to the resonant frequency of the tube. Therefore, to minimize the tip vibrations, the resonant frequencies of the tube are high at about 60 kHz in the vertical direction and about 40 kHz in the horizontal direction.

  15. Scanning Probe Microscopy-STM • STM cantilevers with sharp tips are typically fabricated from metal wires of tungsten (W), platinum iridium (Pt-Ir), or gold (Au) and sharpened by • grinding, cutting with a wire cutter or razor blade, • field emission/ evaporator, • ion milling, fracture, or • electrochemical polishing/etching. • The two most commonly used tips are made from either a Pt-Ir (80/20) alloy or tungsten wire.

  16. Schematic of a typical tungsten cantilever with a sharp tip produced by electrochemical etching. The resonant frequencies of the tube are high at about 60 kHz in the vertical direction and about 40 kHz in the horizontal direction. A lateral resolution of about 2 nm requires tip radii on the order of 10 nm.

  17. Scanning Probe Microscopy-STM Schematics of (a) CG(controlledgeometry) Pt-Ir probe, and (b) CG Pt-Ir FIB (focused ion beam) milled probe.

  18. Constant-currentmode: a feedback network changes the height of the tip z to keep the current constant. → topographic mapyielded bythe displacement of the tip. Constant height mode: a metal tip scannes across a surface at nearly constant height and constant voltage while the current is monitored → topographic map yielded by the change of thecurrent. STM can be operated in either the constant-current or the constant height mode. The images are of graphite in air.

  19. Scanning Probe Microscopy-STM STM images of evaporated C60 film on a gold-coated freshly-cleaved mica using a mechanically sheared Pt-Ir (80-20) tip in constant height mode.

  20. Atomic Force Microscope Contact Non-contact mode/Tapping Tip radius ~ 2 ... 50 nm Force ~ 0.01 nN ... 1 nN sample: nearly any sample

  21. Atomic force microscope Principle of operation of the AFM. Sample mounted on a piezoelectric tube scanner is scanned against a short tip and the cantilever deflection is measured, mostly, using a laser deflection technique. Force (contact mode) or force gradient (noncontact mode) is measured during scanning.

  22. Atomic force microscope • The AFM combines the principles of the STM and the stylus profiler . • During initial contact, the atoms at the end of the tip experience a very weak repulsive force due to electronic orbital overlap with the atoms in the sample surface. The force acting on the tip causes a cantilever deflection which is measured by tunneling, capacitive, or optical detectors. • In an AFM, the force between the sample and tip is detected, rather than the tunneling current. • The deflection can be measured to within 0.02 nm, so for typical cantilever spring constant of 10N/m a force as low as 0.2 nN can be detected. • The AFM can be used either in a static or dynamic mode.

  23. Schematics of the four more commonly used detection systems for measurement of cantilever deflection. In each set up, the sample mounted on piezoelectric body is shown on the right, the cantilever in the middle, and the corresponding deflection sensor on the left

  24. An illustration of the optical beam deflection system that detects cantilever motion in the AFM. The voltage signalVA−VB is proportional to the deflection

  25. long-range (up to 100 nm)： Van der Waals ，Electrostatic ，Magnetic forces short-range (fractions of a nm) : Chemical forces (bonding energy，equilibrium distance)

  26. AFM cantilever

  27. The cantilever is characterized by 3 important coefficients : • Spring constant k; • Eigenfrequency f0 ; • Quality factor Q - is typically a few hundred but can reach hundreds of thousands in vacuum.

  28. A variety of silicon and silicon nitride cantilevers are commercially available with • －micron-scale dimensions, • －spring constants ranging from 0.01 to 100N/m, and • －resonant frequencies ranging from 5 kHz to over 300 kHz.

  29. Spring constant of cantilever To obtain atomic resolution with the AFM, the spring constant of the cantilever should be weaker than the equivalent spring between atoms. • The vibration frequencies ωofatoms bound in a molecule or in a crystalline solid are typically 1013 Hz or higher; • The mass of the atoms m on the order of 10−25; • Interatomic spring constants k, given by ω2m, on the order of 10N/m. • Therefore, a cantilever beam with a spring constant of about 1N/m or lower is desirable.

  30. AFM，a powerful surface tool on atomic/molecular scales 1N/m， 1ng 0.01nN 0.1nm 100 kHz 0.01–5N/m Interatomic forces with one or several atoms in contact are 20–40 or 50–100 pN, respectively. Thus, atomic resolution with an AFM is only possible with a sharp tip on a flexible cantilever at a net repulsive force of 100 pN or lower.

  31. Contact (static) Mode In the contact (static) mode, the interaction force between tip and sample is measured by measuring the cantilever deflection. Tip approach sample: A-B-C A-B ― <10-10N(attractive force); B-C ― >10-10N (repulsive force); Tip leaves sample: C-B-D-A C-B ―> 10-10N(repulsive force); B-D ― <10-10N (attractive force); D-A ― <10-10N (repulsive force);

  32. NonContact (dynamic) Mode q´(t) ―the deflection of the tip of the cantilever, It oscillates with an amplitude A at a distance q(t) to a sample. kts varies in orders of magnitude during one oscillation cycle,

  33. NonContact (static) Mode

  34. Tapping mode Schematic of tapping mode used for surface roughness measurements

  35. Tapping mode，a powerful surface tool on atomic/molecular scales, because of : • (1) it has true atomic resolution, • (2) it can measure atomic force (so-called atomic force spectroscopy), • (3) it can observe even insulators, and • (4) it can measuremechanical responses such as elastic deformation.

  36. Scanning Probe Microscopy-AFM A commercial small sample AFM/FFM,

  37. A large sample AFM/FFM

  38. Schematics of a commercial AFM/FFM made by Digital Instruments Inc. (a) front view, (b) optical head, (c) base, and (d) cantilever substrate mounted on cantilever mount (not to scale)

  39. (b) optical head

  40. (c) base

  41. (d) cantilever substrate mounted on cantilever mount (not to scale)

  42. Block schematic of the feedback control loop of an AFM

  43. The NPL Metrological Atomic Force Microscope (MAFM)

  44. The NPL Metrological Atomic Force Microscope (MAFM)

  45. AFM (a) A schematic depiction of an atomic force microscope cantilever and tip interacting with materials on a surface. Tips typically have points of 50 nm or less in diameter. (b) Schematic of multiplexed AFM tips performing multiple operations in parallel.

  46. Probes in Scanning Microscopies ― SPM images are generated through measurements of a tip-sample interaction. ― A well-characterized tip is the key element to data interpretation and is typically the limiting factor.

  47. Probes in Scanning Microscopies

  48. Scanning Probe Microscope - cantilevers and tips

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