Scanning Probe Microscopy

STM: scanning tunneling microscope

AFM: atomic force microscopemeasuring of the force on the probe

Resolution limit ~STM, but more difficult to achieve.Any sample type.

MFM:magnetic force microscope, Measure magnetic domains –

combine with topography

yx

tunneling of electrons between probe and surface, Resolve individual atoms, measure electrical, properties, induce photo luminesence.Only conducting samples.

AFM with magnetical probe

•In some cases can image individual Molecules - Å level resolution•Many artifacts are possible!•Some limitations on the types of samples that may be imaged.•Relatively Inexpensive ($50K-$200K) & easy to learn… difficult to master

SPM: Scanning Probe MicroscopySPM: Scanning Probe Microscopy

0 V - V+ V

No applied voltage ExtendedContracted

� In some versions, the piezo tube moves the sample relative to the tip. In other models, the sample is stationary while the scanner moves the tip.

� AC signals applied to conductive areas of the tube create piezo movement along the three major axes.

Piezoelectric ScannersPiezoelectric Scanners

SPM scanners are made from a piezoelectric material that expands and contracts proportionally to an applied voltage.

Whether they expand or contract depends upon the polarity of theapplied voltage. Digital Instruments scanners have AC voltage ranges of +220 to -220V.

Nanomanipulation and electron density

waves by STM: Quantum Corrals (Iron on Copper (111)

(Don Eigler IBM)

Various stages during the construction of the circular corral.

http://www.almaden.ibm.com/vis/stm/corral.html

Using STM to study nanostructures on atomic scale

Scanning Tunneling Microscope

LEED vs. STMPt (110)-(1x2)

Some general concepts can also be derived:• Metals:High density of states at atoms => atoms appear as bright protrusions

• Insulators:No conduction possible => we crash• Semiconductors and thin oxides:complex electronic structure at fermi level => be carefull!

Measuring empty or filled statesby bias switching

GaAs (110)- an example of different filled/empty state imaging

AFM probe scans over the surface (in contact)

laser photodiode

piezo-element

probe

AFM: Atomic Force MicroscopyAFM: Atomic Force Microscopy

The atomic force microscope measures topography with a force probe

Cantilever touching a sample

Optical lever

Tube scanner measures 24 mm in diameter, while the cantilever is 100 µm long.

AFM: Atomic Force MicroscopyAFM: Atomic Force Microscopy

cantilever tip

laser

cantileverpiezo

y

z

x

photodiode

AFM: Atomic Force MicroscopyAFM: Atomic Force Microscopy

AFMs use feedback to regulate the force on the sample

The AFM feedback loop. A compensation network (which in AFM is a computer program) monitors the cantilever deflection and keeps it constant by adjusting the height of the sample (or cantilever).

AFM: Atomic Force MicroscopyAFM: Atomic Force Microscopy

The presence of a feedback loop is one of the subtler differences between AFMs and older stylus-based instruments such as record players and stylus profilometers. The AFM not only measures the force on the sample but also regulates it, allowing acquisition of images at very low forces.

The feedback loop (figure 6) consists of the tube scanner that controls the height of the entire sample; the cantilever and optical lever, which measures the local height of the sample; and a feedback circuit that attempts to keep the cantilever deflection constant by adjusting the voltage applied to the scanner.

One point of interest: the faster the feedback loop can correct deviations of the cantilever deflection, the faster the AFM can acquire images; therefore, a well-constructed feedback loop is essential to microscope performance. AFM feedback loops tend to have a bandwidth of about 10 kHz, resulting in image acquisition times of about one minute.

AFM: Atomic Force MicroscopyAFM: Atomic Force Microscopy

AFM tips

tipholder

motor control

SPM tip

sample

piezo translator

laser beam

mirror

photodiode

samplex,y,z piezo translator

fluid in fluid out

fluid cell

O-ring

in air and in buffer solutions

Atomic Force MicroscopeAtomic Force Microscope

path of AFM tip

AFM tip

superhelical DNA plasmid

DNA double helix

Mg2+

negatively charged mica surface

Mg2+Mg2+Mg2+Mg2+Mg2+

Movement of the AFM Tip along the SampleMovement of the AFM Tip along the Sample

AFM Image of a 6.8 kb AFM Image of a 6.8 kb SuperhelicalSuperhelical PlasmidPlasmid

AFM tip

AFM: Image of DNA Strands AFM: Image of DNA Strands

AFM: Image of Carbon AFM: Image of Carbon NanotubesNanotubes

In its repulsive "contact" mode, the instrument lightly touches a tip at the end of a leaf spring or "cantilever" to the sample. As araster-scan drags the tip over the sample, some sort of detection apparatus measures the vertical deflection of the cantilever, which indicates the local sample height. Thus, in contact mode the AFMmeasures hard-sphere repulsion forces between the tip and sample.

In noncontact mode, the AFM derives topographic images from measurements of attractive forces; the tip does not touch the sample.

AFMs can achieve a resolution of 10 pm, and unlike electron microscopes, can image samples in air and under liquids.

AFM: Atomic Force MicroscopyAFM: Atomic Force Microscopy

A tip is scanned across the sample while a feedback loop maintains a constant cantilever deflection (and force)

The tip contacts the surface through the adsorbed fluid layer.

Forces range from nano to micro N in ambient conditions and even lower (0.1 nN or less) in liquids.

Contact Mode AFMContact Mode AFM

A cantilever with attached tip is oscillated at its resonant frequency and scanned across the sample surface.

A constant oscillation amplitude (and thus a constant tip-sample interaction) are maintained during scanning. Typical amplitudes are 20-100nm.

Forces can be 200 pN or less

The amplitude of the oscillations changes when the tip scans over bumps or depressions on a surface.

Tapping ModeTapping Mode™™ AFMAFM

The cantilever is oscillated slightly above its resonant frequency.

Oscillations <10nm

The tip does not touch the sample. Instead, it oscillates above the adsorbed fluid layer.

A constant oscillation amplitude is maintained.

The resonant frequency of the cantilever is decreased by the vander Waals forces which extend from 1-10nm above the adsorbed fluid layer. This in turn changes the amplitude of oscillation.

NonNon--contact Mode AFMcontact Mode AFM

Contact Mode

Advantages:

• High scan speeds

• The only mode that can obtain “atomic resolution” images

• Rough samples with extreme changes in topography can sometimes be scanned more easily

Disadvantages:

• Lateral (shear) forces can distort features in the image

• The forces normal to the tip-sample interaction can be high in air due to capillary forces from the adsorbed fluid layer on the sample surface.

• The combination of lateral forces and high normal forces can result in reduced spatial resolution and may damage soft samples (i.e. biological samples, polymers, silicon) due to scraping

Advantages and Disadvantages Advantages and Disadvantages of the 3 main Types of AFMof the 3 main Types of AFM

Tapping Mode AFM

Advantages:

• Higher lateral resolution on most samples (1 to 5nm)

• Lower forces and less damage to soft samples imaged in air

• Lateral forces are virtually eliminated so there is no scraping

Disadvantages:

• Slightly lower scan speed than contact mode AFM

Advantages and Disadvantages Advantages and Disadvantages of the 3 main Types of AFMof the 3 main Types of AFM

In principle, AFM resembles the record player as well as the stylus profilometer. However, AFM incorporates a number of refinements that enable it to achieve atomic-scale resolution:

• Sensitive detection

• Flexible cantilevers

• Sharp tips

• High-resolution tip-sample positioning

• Force feedback

AFM: Atomic Force MicroscopyAFM: Atomic Force Microscopy

Tip Artifacts