Transcript Tutorial 4 (PowerPoint)

Tutorial 4
Derek Wright
Wednesday, February 9th, 2005
Scanning Probe Techniques
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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
surface
• 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
sample
– 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
etching
– 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
current
• 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
substrates
• Non-contact Mode (z > 1 nm):
– With the tip not actually touching the surface,
dominant forces are van der Waals, electrostatic, and
magnetic
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
charge
• Tip is electrically isolated (cantilever is an
insulator)
• 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
samples
– Special feedback circuits
– Special modulation frequencies
– High gap impedances (large gap between tip and
sample)
Manipulating Atoms and Molecules
• Tip is brought above a loose atom or
molecule
• 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
corral”
– The ring forces electrons within into circular
wave patterns
• Doesn’t need to be a ring – any closed
structure will create resonance patterns
within
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
current
• It is possible to make chemical reactions
occur by dragging the molecules together
and form or break bonds with the tunneling
current
Chemical Reactions with the STM
Thank You!
• This presentation will be available on the
web.