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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 PowerPoint Presentation
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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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Presentation Transcript
slide1

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.

slide2

Atoms were seen for the first time at the end of very sharp needle of tungsten ( 20 atom radius) by applying very high voltage

slide5

It took us a few seconds to write

this over an area of 400x400m2 by using Pb atoms on the tip

and dropping them off with high voltage pulse

slide7

Histogram of height distribution

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

  • The discrete energy levels of the confined electrons dictate the film morphology.
slide8

The width corresponds to the island height and the wave

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

diffraction from thin films

Whenever

intensity is a maximum.

Whenever

intensity is a minimum.

Diffraction from Thin Films

We use a diffraction technique similar to optical diffraction from “air wedges” to determine the island thickness.

h

intensity oscillations with electron energy similar to optical fringes
Intensity Oscillations with electron energy similar to optical “fringes”

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)

slide11

Spin polarized spectroscopy with magnetic tips

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)

slide16

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.

slide17

Mg deposition at 102K, annealed to RT with

screw dislocation network

100x100nm2

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

slide19

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.

slide20

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)

slide21

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.

slide22

The force between tip and sample is not measured directly,

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.

slide24

Contact Mode

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

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.

slide25

Force Modulation

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.

Phase Imaging

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,

and viscoelasticity.

slide26

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.

slide27

Left: Potential diagram showing the motion of the probe in vibrating

mode. Right: The probe vibrates as it scans across a surface.

slide37

Most AFMs use a laser beam deflection system.

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.