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 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(-2keffz)

– 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 - |eV|/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.