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Atoms at the surface of a crystal are well ordered. By diffracting Electrons from the top of crystals we can tell how atoms are arranged but the blobs are not the atoms.
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are well ordered. By diffracting
Electrons from the top of crystals
we can tell how atoms are arranged
but the blobs are not the atoms.
Atoms were seen for the first time at the end of very sharp needle of tungsten ( 20 atom radius) by applying very high voltage
this over an area of 400x400m2 by using Pb atoms on the tip
and dropping them off with high voltage pulse
2000 x 2000 Å growth on Si(111)-(7x7), =3 ML, T=185 K.
These STM results confirm earlier diffraction results showing that Pb islands of the same height are grown at low temperature
to the electrons inside the island
Electrons inside the islands are like notes on
violin “string”. When the notes are “tuned” we have
the selected height
Diffracted intensity oscillations have shown unusual 7-fold periodicity. This corresponds to 7-step islands at 180K
For single steps one oscillation would be observed
K. Budde, E. Abram, V. Yeh, M.C. Tringides, Phys. Rev. B Rapid Comm. B61 10602 (2000)
The tunneling current depends on the overlap of the
sample tip density of states
Both ferromagnetic and anti-ferromagnetic tips
can be used because only the last atom determines s
the polarized current
Magnetic samples have non-zero polarization
Only the same sample polarization as the tip is selected by
the tunneling process
At sufficiently low bias and for polarized tips
one is sensitive to spin states in the sample of the
same polarity so the spin dependent density
of states can be measured
M.Bode Rep. Prog. Phys. 66 523 2003)
Magnetic vortex for large enough island ( Larger than 5nm) then not a single domain structure.
This is easily seen in dI/dU maps because a different spin polarization is seen when the center is
Crossed. Magnetization “curles” around the center.
screw dislocation network
Tailored bimetelalic islands Co surrounding Pt cores
have the same magnetic properties as pure Co islands
Rusponi et al. Nat. Mat. 2 (2003) 546
Very preliminary experiments of Fe-Mg coadsorption
show the same decoration of the Mg edges by Fe
After Fe 0.3ML deposition at RT Fe adsorbs preferably
at the terrace edges
An AFM SEM image of the cantilever/probe used in an AFM force sensor (right).
SiN is used for creating probes that have very low force constants. The thin
films used for creating SiN probesmust have very low stress so the cantilevers
don’t bend naturally from the stress. Practically, most SiN films have some
residual stress and in fact, cantilevers made with SiN tend to have curvature along
their primary axis.
SiN cantilevers are typically triangular with two arms meeting at an apex.
The probe on SiN probes
are typically pyramidal and appear hollow at the top.
(Top) Si cantilevers are typically rectangular and the
probes tend to have a triangular shape to them.
Si probes are crystalline and are probe to chipping and breaking
if they crash into a surface. (Bottom)
Left: A piezoelectric cylinder has electrodes on the inside and outside so that the potential goes
through the wall of the ceramic. When activated the cylinder elongates. Right: A piezoelectric
9 Top: PZT materials have hysteresis. When a voltage ramp is placed on the ceramic, the
motion is nonlinear. Bottom: Creep occurs when a voltage pulse on a
PZT causes initial motion followed by drift.bimorph has two sheet of piezoelectric material
bonded together. The end of the bimorph expands in a parabolic motion when activated.
but calculated by measuring the deflection of the lever, and knowing the
stiffness of the cantilever.Hook’s law gives F = -kz, where F is the force, k is
the stiffness of the lever, and z is the distance the lever is bent.
As the tip is raster-scanned across the surface, it is
s deflected as it moves over the surface corrugation. In constant force mode, the tip is constantly adjusted to
maintain a constant deflection, and therefore constant height above the surface.
Because the tip is in hard contact with the surface, the stiffness of the lever needs to be less that the effective spring
constant holding atoms together, which is on the order of 1 - 10 nN/nm. Most contact mode levers have a spring constant of < 1N/m.
Lateral Force Microscopy
LFM measures frictional forces on a surface. By measuring the “twist” of the cantilever, rather than merely its deflection,
one can qualitatively determine areas of higher and lower friction.
Noncontact mode belongs to a family of AC modes, which refers to the use of an oscillating cantilever.
A stiff cantilever is oscillated in the attractive regime, meaning that the tip is quite close to the sample, but not
Touching it (hence, “noncontact”). The forces between the tip and sample are quite low, on the order of
pN (10 -12 N).
Dynamic Force / Intermittant-contact / “tapping mode” AFM
Commonly referred to as “tapping mode” it is also referred to as intermittent-contact or the more general term
Dynamic Force Mode (DFM).A stiff cantilever is oscillated closer to the sample than in noncontact mode.
Part of the oscillation extends into the repulsive regime, so the tip
intermittently touches or “taps” the surface. Very stiff cantilevers are typically used, as
tips can get “stuck” in the water contamination layer.
Force modulation refers to a method used to probe properties of materials through sample/tip interactions.
The tip (or sample) is oscillated at a high frequency and pushed into the repulsive regime.
The slope of the force-distance curve is measured which is correlated to the sample's elasticity.
In Phase mode imaging, the phase shift of the oscillating cantilever relative to the driving signal is measured.
This phase shift can be correlated with specific material properties that effect the tip/sample interaction.
The phase shift can be used to differentiate areas on a sample with such differing properties as friction, adhesion,
The forces applied to the surface by the probe in contact mode are given by Hook's law:
Left: Potential diagram showing the region of the probe while scanning in contact mode.
Right: In contact mode the probe glides over the surface.
mode. Right: The probe vibrates as it scans across a surface.
separated by 40nm
A laser is reflected from the
back of the reflective AFM lever
and onto a position-sensitive detector.
AFM tips and cantilevers are
microfabricated from Si or Si3N4.
Typical tip radius is from a few to 10s of nm.