Transcript Scanned-proximity Probe Microscopy (SPM) Background Emphasis on Atomic Force

Scanned-proximity Probe
Microscopy (SPM)
Background
Emphasis on Atomic Force
Microscopy (AFM)
•Reading
•SPM Features
•AFM Specifics
•AFM Operation (Conceptual)
•AFM and Nanotechnology
Reading
• “Window on a Small World,” McGuire, Today’s Chemist,
June 2002
Vol. 11, No. 5 pp 21–24
http://pubs.acs.org/subscribe/journals/tcaw/11/i06/html/06inst.html
• “AFM Technology Overview” From Veeco (Digital
Instruments) the manufacturer of the AFM that will be
used.
• http://nano.nd.edu/SC190/scanningprobe.pdf
“AFM Nanomanipulation” From Veeco on the use of the
AFM in direct nanotechnology applications.
http://www.veeco.com/pdfs.php/70
• Scanning Probe Microscopy (SPM) Instructions
http://frontpage.okstate.edu/nanotech/Lab/lab3/SPM_Ins
tructions_082003.pdf
Scanned Proximity Probe
Microscopy Common Features
• Piezoelectric (PZ) positioning
– PZ crystals expand/contract under applied voltage
(d11 ≈ 4x10-10 m/V)
• Example: A commercial linear actuator produces 15 mm
displacement for 100 V applied with sub nm resolution
– PZ crystals generate voltages under applied force
(g33 ≈ 1x10-2 V/Nm)
• Deflection Feedback Loop
– Height (z) dependent signal
• Example: Tunneling current for Scanning Tunneling Microscope
(STM)
• Example: Van der Waals forces for Atomic Force Microscope (AFM)
• Probe/Tip
– Photolithographic Fabrication Techniques
– Resolution determined by probe size (not diffraction)
AFM Capabilities
(Advantages)
• Able to achieve a resolution of 10 pm with
special tips (typical resolution ≈ 10 nm)
• Able to image samples in air and under
liquids
• Able to measure in 3 D (within limits)
• Able to measure non-conductive surface
(unlike SEM or STM)
AFM Capabilities
(Disadvantages)
• Incorrect tip choice can lead to measurement
artifacts or sample damage
• Depth of field limited by cantilever / z
positioning PZ
• Scan area limited by PZ scanners
• Slow scan rate compared to SEM
AFM Components
Position Sensitive
Photodetector
(PSPD)
Laser
Optical
Beam
Z Piezoelectric
Positioning/ Force
Sensor
Probe Tip
Sample
Substrate
Cantilever
X-Y Piezoelectric
Positioning/
Scanning
AFM Probe
• Tip
– Modifies measurements
– Typically Si or SiN for ease of fabrication
– Many variations depending on application
• Cantilever (Tip at the end)
– Low spring constant (Hooke’s Law  F = -kz)
1
– Low weight for high resonant frequency ( f 
2
– Coated for reflectivity
Tip on apex of
cantilever
Contact
Non-Contact
k (N/m)
0.2  0.006
140  2
f (kHz)
70  10
1100  70
Images from http://stm2.nrl.navy.mil/how-afm/howafm.html#General%20concept
k
)
m
AFM Modes
• Contact mode (Repulsive-Static)
– AFM tip rides on the sample in close contact with the sample
surface (low k)
– The force in the feedback loop is friction
– May interact with the sample surface
• Non-contact mode (Attractive-Dynamic)
– AFM tip hovers 5-15 nm away from the sample surface
– The force in the feedback loop is typically van der Waals (VDW)
forces
– Applied force (dependent on height z) changes cantilever
oscillation frequency.
• Tapping mode (Repulsive-Dynamic)
– AFM tip taps surface as it maps z.
– Eliminates lateral forces or hysteresis due to the tip sticking on
the sample.
– Less likely to damage the sample
AFM Measurement
Changes of the surface
properties along the scan line
Changes of interaction forces between
the probe tip and sample surface
Contact Mode
Non-Contact Mode
Deflection of the cantilever
Change of oscillation amplitude
and phase of the cantilever
Change in laser position on PSPD
Laser beam position oscillates on
the PSPD
Electronic signal to control and
recording electronics
Signal processing to form
image
AFM Visualization (Contact Mode)
PSPD measures change
optical beam position 
change in cantilever height
Sample is
raster scanned
Forces at Work
• Atomic Forces (approx.)
U
12 B 6 A
Fa  U  13  7
z
z
Repulsive
Attractive
B and A coefficients depend on the surfaces involved.
Detectable forces for an AFM 1 nN in the contact regime and 1
pN in the noncontact regime (Theoretical Limit 10-18 N with
heroic measures.) R. Wiesendanger, "Chapter 11. Future Nanosensors." In H. Meixner, R. Jones, eds.,
Volume 8: Micro- and Nanosensor Technology / Trends in Sensor Markets. In W. Gopel, J. Hesse, J.N. Zemel, eds., Sensors: A
Comprehensive Survey, VCH Verlagsgesellschaft mbH, Weinheim Germany, 1995; pp. 337-356.
Approach-Retract Phenomenon
Approach
Force
Impress
Retract
(proportional to cantilever
deflection)
Snap On
Snap Off
(NOT the tip Z position!)
1.
Approach—VDW forces pull tip toward surface
2.
Snap On—When close enough, VDW grow stronger than restoring spring force in the cantilever.
3.
Impress—Tip pressed into sample with positive force after reaching the set point deflection
4.
Retract—Tip adheres to surface giving rise to hysteresis on measured force/position curve
5.
Snap Off—Spring force dominates the tip-sample adhesion and tip leaves surface
Alternate Scanning Probe Systems
• Magnetic Force—Maps surface magnetic field (think
hard drives…)
• Electrostatic Force—Maps surface potential (100 nm
resolution)
• Lateral Force—Maps friction force experienced by
scanning probe on surface orthogonal to scanning
direction
• Magnetic Resonance—Possible 3D imaging of individual
molecules via resonant electron spin flipping with an
antenna on the cantilever
• Near Field Optical Microscopy
– Probe tip has a small aperture (radius << l) for optical
wavelength measurements
• And many, many more…
Nanotech Applications
•
•
•
•
Measurement of nanostructures
Nano indention of surfaces
Dip Pen Nanolithography
Manipulating nanoparticles (building
nanostructures)
• Precision electro chemistry—supply
electrons by applying voltage across AFM
tip and substrate