Scanning Probe Techniques

MSN506 notes

Imaging

• There are two approaches to microscopy– Parallel

• All pixels of an image is recorded simultaneously, or directly viewed by eye

– Sequential• Information from each pixel is collected

sequentially and recorded electronically, then the image is reconstructed

Scanning probes

Scanning probes

• Sequential data collection• A sharp tip interacting with the sample

surface• Relative tip location on the sample is

scanned in XY and Z• Interaction is recorded as a function of

location and other parameters• Image is reconstructed directly from

measured signals or after data processing

History of scanning probes• Profilometry (one dimensional topography)• Scanning Tunneling Microscope• Atomic force microscope

– A large number of spin-off variants• Scanning electrical probes

– Scanning Hall probe microscopy– Scanning Single electron transistor microscopy– Etc…

STMScanning Tunneling Microscope

Tunneling current: High resolution from a blunt tip ?

STS (Scanning Tunneling Spectroscopy)

• Feedback is momentarily turned off at a specific location

• Rapidly, the voltage is swept and current is recorded (IV curve)

• The quantity (dI/dV)/(I/V) is proportional to the density of states on the sample (under certain assumptions about the tip)

• Feedback is resumed to avoid a tip-crash due to drift

STS (Scanning Tunneling Spectroscopy)

Appl. Phys. Lett., Vol. 73, No. 1, 6 July 1998

STS (Scanning Tunneling Spectroscopy)

STS (Scanning Tunneling Spectroscopy)

Electronic structure of

atomically resolved

carbon nanotubes

Jeroen W. G. Wildoer*, Liesbeth C. Venema*,

Andrew G. Rinzler†, Richard E. Smalley† & CeesDekker

With STM, Imaging is possible …

• In air,• In fluids (elechtrochemistry)• Under vacuum• Under a large temperature range (mK to

1000 K)• … only on conductors

AFM

Profilometer• A stylus is dragged over a surface and

interaction is measured

Record player

AFM Cantilever Springs

Spring Constant N/m

Resonance Frequencies

Typical values 0.01 to 50 N/m

Typical 10 Khz to 500 Khz

Deflection Readout in AFM

• Beam deflection• Fiber optic interferometer• Piezoresistive

Deflection Readout in AFM

• Beam deflection

Quadrant photodiode(4 photodiodes closely placed)

Focused or Partly collimated laser

Signal proportional to deflectionLarge dynamic range

10-4 Angstroms/sqrt(Hz)sensitivity achievable

Fiber optic Fabry-Perot interferometer in AFM

Periodic output as a function of distance

10-6 Angstroms/sqrt(Hz) sensitivity achievable

Piezoresistive readout• Piezoresistance: resistance changes with

applied stress

Piezocoefficient

stress

For a rectangular cantilever beam of dimensions L, w, tStress is given by

Simple but not as sensitiveRequires extra fabrication stepsDuring fabrication of microcantilevers

Feedback

• Tip-sample separation must be kept constant during imaging

Tip-Sample Mechanical Interaction

• Contact• Interminent contact (tapping)• True noncontact• Long range interactions

Contact between a sharp tip and a surface

• Herzian Contact theory is frequently used

Using elastic properties of the tip and surface materials, a relation between the force felt by the tip and the sample as a function of tip sample separation can be calculated

Simplest tip-sample interaction model

Tip sample separation

surface

Force

Infinitely hard surface

Simplest tip-sample interaction model

Tip sample separation

surface

Force Finite hardness surface(tip can indent the surface)

Van der Waals forces

These are attractive forcesDue to polarization of the Tip and the sample surface atoms

Electrostatic Forces• Long range attractive forces due to

charges on the tip and the sampleV

Energy = ½ C(z) V2

Force = - d Energy / dz Tip-sample capacitance can be accurately modelled

A large number of force models

Force depend on many factors including tip-geometry, tip-sample separation, bias voltage, ambient atmosphere, presence of surface water film etc.

Typical Force-Distance curve

Directional Forces• Vertical and lateral forces can be

simultaneously imaged

Lateral forces can be interpreted as frictional forces

Interminent contact / Tapping AFM

• Cantilever oscillation amplitude and phase is measured

• Feedback is maintained to keep the oscillation amplitude fixed

• Amplitude, Phase and topography can be imaged simultaneously

Cantilever Mechanical Resonance(s)

Upon interaction with the sample, resonance properties change

Cantilever Mechanical Resonance(s)

Q factor (dissipation) is an importantFeature of the cantilever oscillation

Determines how much the oscillatorWill respond to an on-resonance excitation

Active Q-Control only affectsspeed response, does not change ultimate force sensitivity

Cantilever Mechanical Resonance(s)

Tapping mode AFM

Robust to DC drifts in the deflection detection system and electronicsVery small interaction forces compared to contact modeTip generally retains sharpness longer

Phase in tapping mode AFM

Freq. Shift

Phase in tapping mode AFM

Phase can be related to the stiffness of the sample

Phase in tapping mode AFM

Noncontact AFM

Cantilever is made into a self-oscillator (phase is locked by an external circuit)Can be extremely sensitive, and resolution can be compared to STMIn UHV

Noncontact AFMTrue atomic resolution generally only possible with true noncontactmodeIn UHV

Noncontact AFM

Effect of Ambient Conditions• Vacuum

– High Q (10000), no water film, true non-contact imaging, high resolution

• Air– Moderate Q (500), good tapping mode

performance, nanometer range XY resolution• Fluid

– Low Q (10), Q control needed for tapping mode, excitation problematic, contact mode imaging, good for biological applications

Spatial Resolution• XY resolution generally limited by tip radius

(typically 5 to 15 nm)• Z resolution limited by scanner electronics

– Noise performance, mechanical design, DAC resolution

• In true noncontact mode, on a flat surface, XY resolution is sub-Angstrom– A single adatom on the tip dominantly

interacts with the sample

Resolution• Highest resolution generally possible on a

flat substrate

Mica and graphite are generally used asflat substrates

Due to their layered structure, they cleavealong single planes

Mica

Example Applications of AFM

Other Scanning probe microscopes

• Electrostatic forces– Surface potentials and surface charge– Workfunction imaging (kelvin probe)

• Magnetic forces• Scanning Hall Probes• Scanning single electron transistors• Thermal imaging• Near field optical imaging• Tip enhanced Raman

Electrostatic/magnetic force imaging

Double pass scheme (lift mode etc.)

Electrostatic Imaging• Sub-electronic charge sensitivity

Surface potential imaging discriminates Au and Ag nanoparticles

MRFM• Tries to measure the NMR or ESR signal

for imaging in 3D

MRFM• Tries to measure the NMR or ESR signal

for imaging in 3D

Scanning Near Field Optical

Left : ‘aperture’ SNOM with optical fibres. Right : ‘apertureless’ (‘scattering’) SNOM.

Tip enhanced Raman

Tip enhanced Raman

Plasmonic enhancement

topo

raman

ForceDistance Spectroscopy

ForceDistance Spectroscopy