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How Does Atomic Force Microscopy Work and What Can It Do? Senior Staff Applications Scientist, Chunzeng Li
How Does Atomic Force Microscopy Work and What Can It Do?
Senior Staff Applications Scientist, Chunzeng Li
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Scanning Probe Microscopy
• STM-Scanning Tunneling Microscopy (invented 1981, by Binnig and Rohrer at IBM Zurich)
• AFM-Atomic Force Microscopy
While the primary use is imaging, the boundary has been pushed beyond.
STM ProbeAFM Probes
Si (111) 7x7 by STM
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Atomic Force Microscope ModesPrimary Imaging Modes
AFM Revolutions center around
force control.
•1986 Contact Mode, Binnig
(IBM), Quate, and Gerber
•1993 Tapping mode, Digital
Instruments
•2009 PeakForce Tapping Mode,
Veeco/Bruker
Laser Diode
Photodetector
Sample
Scanner
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Measure the Force-Beam Deflection Detection
Laser Diode
Photodetector
Sample
Scanner
Beam deflection detection in the MultiMode Head
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Contact Mode AFM
• Feedback loop maintains constant deflection (force) of AFM cantilever
• Constant contact between the tip and sample surface facilitates:
• High resolution imaging
• Fast scanning speeds / fast data acquisition times
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Tapping Mode AFM
• AFM cantilever excited at fundamental resonance frequency
• Feedback loop maintains constant amplitude of oscillation of AFM cantilever
Amplitude reduced
"Free" Amplitude
"Tapping"Fluid layer
10-100 nm
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PeakForce Tapping Mode
A force-distance curve is obtained in each tapping cycle with controlled peak force.
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Fluid Imaging at 50 pN Peak ForceGentle Touch and High Resolution
OmpG Membrane Pore (Data courtesy of C. Bippes and D. Muller, Dresden Univ.)
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Simultaneously obtain quantitative data:
Topography
DMT Modulus
~1MPa – 100GPa
Adhesion
Energy Dissipation
DeformationDeformation
Quantitative Nanomechanical Property Mapping
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QNM - Approach to Material Assignments
Multi-component polymer blend (7 µm scan )
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Adhesion Topography
Healthy Erythrocyte Infected Erythrocyte (IE)
Adapted from the Plasmodium Genome Resource.
Adapted from http://cmr.asm.org/cgi/content-nw/full/13/3/439/F1.
• Malaria-infected erythrocytes (IE’s, Red blood cell) are misshapen with knob-like structures on surface.
• IE’s exhibit cytoadherence.
• Prevents IE elimination by the spleen and causes vascular blockages.
PeakForce QNM Molecular Recognition Mapping
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Bruker Nano Surfaces Division
20nm
Low [Ni++] High [Ni++]
Consecutive images
Unanchored parts
(mobile)
48nm
Movie of DNA on mica (Ni ++)
PeakForce TappingResolving the DNA Double HelixFastScan Bio AFM (FastScan-Dx probe, 0.25N/m)
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ICON EC Setup
Scanner Head
Fluid Probe Holder
EC Cell
ICON EC Chuckw/Heater RT~65°
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Lithium Battery: SEI on Si-Anode during Charginga-Si coated with 200nm Cu with p-Si sputtered islands
Potential Ramp Profile:
0.2 --> 1.5 --> 0.05V --> 1.5V
Potential Ramp Profile:
1.5 --> 0.36
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Atom Manipulation with LT-UHV-STM
Donald Eigler and Erhard Schweizer of the IBM
Almaden Research Center in San Jose, California,
used a scanning tunneling microscope, which can
discern individual atoms, to position xenon atoms
to form the initials "IBM". They shot a beam of
xenon atoms at a chilled crystal of nickel. These
were manipulated into patterns using the scanning
tunneling microscope. In addition to the IBM
initials, they created chains of xenon atoms similar
in form to molecules.
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Nano-ManipulationSingle Wall Nanotubes
1. Image with TappingMode. 2. Manipulate tubes with AFM tip. 3. Image again.
• All in just a few minutes
• Manipulation Mode: constant height (AFM tip) point-and-click, indicated by
arrows
900nm
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Secondary Modes (Piggy Back Modes)
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Conductivity Measurement
Laser Diode
Photodetector
Sample
Current Amplifier
Scanner
AFM
Conductive Probe
TUNA Module
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Carbon Nanotubes with Peak Force TUNA
Current map clearly identifies electrical connectivity of individual carbon nanotubes .
Sample courtesy of Prof. Hague, Rice University
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Point&Shoot I-V Curves on V-CNT
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SSRM-Scanning Spreading Resistance Microscopy
Resistance Measurements
Si
log(I)
VDC
conductive probe (10 pA - 0.1 mA)
R =ρρρρ
4 x radius
contact resistance
spreading resistancen p
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SSRM on Si dopant staircase
depth (µm)
20
1014
1016
1018
10
carrierconcentration
(atoms/cm3)
403020100103
105
107
109
resistance
(ΩΩΩΩ)
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SSRM Resolution: 100nm PMOS & NMOS transistors
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Scanning Capacitance Microscopy (SCM)
SiO2
Si
= MIS capacitor
ca
pa
cit
an
ce
voltage
AC
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Scanning Capacitance Microscopy on LOCOS isolation between 2 transistors
topography SCM dC/dV
Courtesy: A. Erickson, DI
p pn
n ninsulator
metal
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KPFM-Kelvin Probe Force Microscopy
AM Amplitude-Modulation
FMFrequency-Modulation Better spatial resolution
Better accuracy
Physical Review B 2005, 71(12) 125424
KPFM measures the work function difference of tip/sample.
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PeakForce KPFM VS FM-AMFM detection advantage maintained
240 mV
PeakForce KPFM
97 mV
PeaForce KPFM-AM
FM sees larger and more localized contrast leading to better accuracy.
AM contrast smaller and more convoluted.
Sn60Pb40 Alloy
Work functions: Sn 4.42 eV; Pb 4.25 eV
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Some Trends
Smaller force (better force control)Easier to use (ScanAsyst)More informationFaster ScanHigher resolution (routinely)Chemical Identification
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Trend: More Orthogonal InformationSimultaneous Morphology, Mechanical and Electrical Mapping (L333-Glue) with PF-TUNA
Height DMT Modulus Adhesion Current
+
Li[Ni1/3Mn1/3Co1/3]O2
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Image Specifications:
Size: 2.2um
ScanRate: 100Hz
Pixels:256 x 256
TipV: 440um/s
Frame Rate: 2.5s
Real Time Video
Duration: ~4min
Sample Courtesy:
Dr. Jamie Hobbs
University of Sheffield
Trend:Fasterfor Dynamics and Productivity
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• Spherulite growth rates are consistent with earlier measurements (Hobbs2003)
• Front growth rate of ~constfor each temperature
• Secondary Nucleation model ignores variation in growth rate of individual lamella
• Rate of nucleation (T) affects spherulite structure
22C
50C
70C
Spherulite growth rate (nm/s)
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High Resolution AFM A challenging task for instrumentation and operation
Gross, Meyer, 2006Low Temp UHV
Fukuma et al., 2005Mica Tapping in liquid
Lattice resolution of Mica,
contact mode
Atomic resolution with any probe on a
flexible sample platform? fluid & ambient?Rode et al., 2009
Calcite Tapping in liquid
Hoogenboom et al, 2006
Mica Tapping in liquid
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Trend: Higher Resolution AFM RoutinelyOxygen Atoms of CalciteDissolution crystal plane details revealed
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lattice infringement
F 0.147
C 0.154
H 0.1
S 0.180
O 0.120
Atom WdW R, nm
A.Enkelmann, Adv.Pol. Sci., 1984, 63, 91
• Peak Force Tapping provides much higher resolution
• Standard probes
• Material property maps
Polydiacetylene (PDA) in AirMultimode 8 Peak Force Tapping
b
Height Stiffness Adhesion
0.5 nm
1.5 nm
0.5 nm1.4 nm
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Trend: Nanoscale Chemical ProbingNanoscale IR + Raman SpectroscopyLeveraging Vibrational Spectroscopies
XYZ
+++
- - -
1. Use IR/Raman spectroscopy: Label-free, non-destructive chemical ID
2. Use tip as optical antenna: Isolate signal from nanoscale region
RESULT IS NANOSCALE CHEMICAL ID
WITH ~ 10nm SPATIAL RESOLUTION
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TERS Performance BenchmarkingMalachite Green on Au
• Malachite Green on Au, 633nm excitation, 1s integration, 1mW
• Signal contrast >100 (EF > 107) when raising tip 60nm
• High probe to probe consistency
• Innova-IRIS with IRIS TERS probe
Tip in feedback / Tip 60nm above surface
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Correlated Information on GrapheneRaman, IR-AFM and PeakForce KPFM
• Raman: how many layers, defects
• QNM: layer heights, mechanical properties
• KPFM: work function/fermi energy
• IR AFM: plasmonics, # of layers, fermi energy
KPFM work function IR 1730cm-1
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Summary
• AFM revolution centers around force control: Contact mode,
Tapping mode, to Peak Force Tapping mode.
• AFM has opened the door to the nanoscale world, making
accessible a wide range of material properties, including but not
limited to mechanical, electrical, magnetic, chemical properties.
• AFM’s diverse uses stem from its capability to work in any
environments: ambient, liquid and vacuum.
• AFM has been widely adopted by the academic and industrial world
alike.
• AFM Trends: Easier to use, Faster, Higher resolution, More
information more quantitatively, and chemical identification (TERS,
NanoIR).
