Tutorial 4 (PowerPoint)

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Transcript Tutorial 4 (PowerPoint)

Tutorial 4
Derek Wright
Wednesday, February 9th, 2005
Scanning Probe Techniques
Scanning Tunneling Microscope
Scanning Force Microscope
Imaging of Soft Materials
Manipulation of Atoms and Molecules
Chemical Reactions with the STM
Scanning Probes
• Atomic-sized probe is dragged across the
• Types of measurements taken:
– Current
– Magnetic
– Force
Scanning Tunneling Microscope
• Scanning:
– The tip is scanned across the
sample in a grid pattern
• Tunneling:
– There is a tunneling current
between the sample and the tip
which is measured
• Microscope:
– We can see atomic sized things
with it
Scanning Tunneling Microscope
• Tunneling current is a quantum effect
• e- aren’t points in space, they have a
probability of location
• This waves exist with a probability density
centered around the e– The e- is “smudged” in space
• If a thin barrier intersects this probability
density, the e- might have a chance of
“appearing” on the other side of the barrier
Scanning Tunneling Microscope
STM Equations
• I  V  Ntip  Nsample
– Ntip, Nsample = density of states
• I  exp(-2keffz)
– z is the distance between the tip and sample
– I drops off exponentially with the distance
– I drops off exponentially with keff
STM Equations
• keff = (2meB/h2) + |k|||2
– keff = inverse effective decay length
– me = mass of electron
– B = barrier height (has to do with the work functions of
the tip and sample and the applied voltage)
– k|| = parallel wave vector of the tunneling electrons
• B = (tip + sample)/2 - |eV|/2
– (tip + sample) are the work functions of the tip and
– V is the applied voltage
STM Modes
• There are two modes of operation
• Constant Distance (z-position const.)
– The tunneling current is plotted
• Constant Current
– The vertical movement of the tip is plotted
– This is the usual method
– Good because of the exponential nature of
the tunneling current + feedback
STM Constraints
• The STM tip must have excellent
mechanical stability
– Achieved through piezoelectric actuators
– Rests on heavy table with many dampers
• The tip must come to a very small point
– Can be achieved through electrochemical
– Carbon nanotube can be placed on the end to
improve accuracy
Scanning Force Microscope
• Sometimes called Atomic Force
Microscope (AFM)
• Setup very similar to STM except tip
deflection is measured instead of tip
• Can be used where current won’t flow
• Two modes of operation:
– Contact
– Non Contact
Scanning Force Microscope
• Contact Mode (z < 1 nm):
– The tip is dragged across the surface and the
deflection is measured optically
– Deflection is due to repulsion of tip particles with
surface particles
– Can scratch the surface – not recommended for soft
• Non-contact Mode (z > 1 nm):
– With the tip not actually touching the surface,
dominant forces are van der Waals, electrostatic, and
Scanning Force Microscope
• As the tip is brought from a distance closer
to the sample:
– First van der Waals forces pull the tip closer
– Then ionic repulsion pushes it away
• The tip’s deflection can be measures using
laser interferometery
Scanning Force Microscope
• Tip can be operated in “dynamic mode”
• The tip and cantilever (beam with the tip on it)
have a mechanical natural resonance
• The resonance will change as external forces
from the sample are exerted on it
• The tip’s vibration amplitude must be much less
than the distance between it and the sample to
ensure linear operation
– Like how a transistor amplifier is linear when the
signal is much less than the supply voltage
Scanning Force Microscope
Magnetic SFM
• Used to measure magnetic media
• The tip is a piece of magnetic material and
is of a single domain
– All dipoles are aligned in the tip
• The interaction of the tip’s magnetic field
and the sample create a force
• The force shows the sample’s domains
and boundaries between them
Electrostatic SFM
• A method that plots the sample’s static surface
• Tip is electrically isolated (cantilever is an
• Two pass method:
– First pass is a contact pass
– Second pass occurs at a constant distance from the
sample and measures the force due to the charge on
the sample and the charge induced in the tip
Piezoresponse Force Microscopy
• The tip and cantilever can bend in two
axes to give an idea of the 3D domain
structure of a sample
• An oscillating voltage is applied to the tip
• An oscillating current occurs (due to the
capacitance of the tip) which interacts with
the B-field of the sample
• This creates a measurable force and
bends the cantilever
Imaging of Soft Materials
• Contact with soft samples is bad
– The tip will damage the delicate sample
– Contact gives better resolution, but is too harsh
• Non-contact methods have been tailored for soft
– Special feedback circuits
– Special modulation frequencies
– High gap impedances (large gap between tip and
Manipulating Atoms and Molecules
• Tip is brought above a loose atom or
• Attractive forces between the two allow tip
to pick up the atom
• Tip drags the atom
• Tip raises to let go of the atom
Manipulating Atoms and Molecules
Quantum Corrals
• A ring of atoms can create a “quantum
– The ring forces electrons within into circular
wave patterns
• Doesn’t need to be a ring – any closed
structure will create resonance patterns
Quantum Corrals
Quantum Corrals
Quantum Corrals
Chemical Reactions with the STM
• Since the tip can:
– Manipulate atoms and molecules
– Provide energy in the form of a tunneling
• It is possible to make chemical reactions
occur by dragging the molecules together
and form or break bonds with the tunneling
Chemical Reactions with the STM
Thank You!
• This presentation will be available on the