Scanning Tunneling Microscopy and Atomic Force Microscopy EEW508 Scanning probe microscopy

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Transcript of Scanning Tunneling Microscopy and Atomic Force Microscopy EEW508 Scanning probe microscopy

Dental Enamel StructuresScanning probe microscopy
Atomic Force Microscopy (AFM)
- 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
Scanning probe microscopy
Invention of scanning tunneling microscopy (1982)
Gerd Binnig & Heine Rohrer, IBM Zurich
(nobel prize in 1986)
Reconstruction on Si (111) surface
Phys Rev Lett (1983)
STM tip
A
V
I
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.
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).
Scanning probe microscopy
The instrument basically consists of a very sharp tip which position is controlled by piezoelectric elements (converting voltage in mechanical deformation)
Scanning probe microscopy
Imaging modes of Scanning Tunneling Microscopy
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.
Scanning probe microscopy
reconstruction structure
Tunneling spectroscopy reveals the bandgap of 0.7 eV due to the presence of surface states
Scanning probe microscopy
Quantum corral (D. Eigler)
assembled by atomic manipulation
Iron on Copper (111)
assembled by atomic manipulation
Fe
atoms
Water molecules on Pd(111) surface
Water dimers diffuse much faster
than monomer and trimer
Scanning probe microscopy
Scanning probe microscopy
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.
Scanning probe microscopy
J. Y. Park et al. Science (2005)
Fibonacci sequence
A progression of numbers which are sums of the previous two terms
f(n+1) = f(n) + f (n-1),
STM image of two-fold surface of
Al-Ni-Co decagonal quasicrystal surface
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.
J. Y. Park, R. J. Phaneuf, and E. D. Williams, Surf. Sci. 470, L69 (2000).
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
 
Invention of atomic force microscopy (1985)
Binnig, Quate, Gerber at IBM and Stanford
Binnig et al. PRL (1985)
Scanning probe microscopy
Principle of Atomic Force Microscopy
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.
Scanning probe microscopy
Constant height and force mode AFM
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.
Scanning probe microscopy
Cantilevers can be seen as springs.
the extension of springs can be described by Hooke's Law F = - k * s.
This means: The force F 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 constant k 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
Scanning probe 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)
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
Scanning probe microscopy
EEW508
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)
Scanning probe microscopy
Three dimensional mapping the adhesion force and Young’s modulus
CdSe tetrapod
AFM images of various materials
Contact mode AFM topography (left), friction (right) images of graphite surface
Contact mode friction image (left) and its line profile of mica surface which show atomic stick-slip process
Contact mode topographical (up) and friction images (bottom) of polymer
Scanning probe microscopy
Single
asperity
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
Perspective of SPM
Scanning probe microscopy
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
Scanning probe microscopy