STM and AFM Tutorial Katie Mitchell January 20, 2010
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STM and AFM Tutorial
Katie Mitchell
January 20, 2010
Overview
• Scanning Probe Microscopes
• Scanning Tunneling Microscopy (STM)
• Atomic Force Microscopy (AFM)
– Contact AFM
– Non-contact AFM
• RHK UHV350 AFM/STM
Scanning Probe Microscopes
• Sharp probe tip is scanned across surface
• Piezoelectric materials provide atomic scale control of x,y,z motion
• Detect signal that reflects topography or other spatially varying property of interest
• Good vibration isolation required
Scanning tunneling microscope
Scanning Tunneling Microscopy
Image from Will's Nanotechnology Blog
V ~ 0.1 - 2 V
I ~ pA – nA
can operate in
air, liquid or
vacuum
sample and tip
must be
conducting
Scanning Tunneling Microscopy
zw n 2exp02
2
2
m
Tip
Sample
Tip
Sample
EF
EVacuum (0 eV)
a)
b)
e-
Tip
Sample
c)
e-
EF
EF
EF
EF
EF
eVeV
z
1D Potential Barrier
Current-Distance Dependence
0.0 0.5 1.0 1.5 2.0 2.5 3.0-7
-6
-5
-4
-3
-2
-1
0
1
Natu
ral
Lo
g o
f T
un
neli
ng
Cu
rren
t
Tip Displacement (Å)
= 1.0±0.1 Å-1
W and Pt-Ir tips on Si(111)-7x7
M. Rempel, MSc Thesis, Univ. of Saskatchewan, 2003
Scanning Tunneling Microscopy3D Picture
EEMw 22
dMEeVEe
I FF
eV2
0
4
dEeVEe
I FF
eV
0
4
Transition probability
Tunneling current
Assuming s-wave tip state
STM on Au(111)
60°
120°
100 x 100 nm, 25 mV, 1.15 nA
Au(111): High Resolution
5 x 5 nm, 90 mV, 1.09 nA50 x 50 nm, 25 mV, 1.15 nA
Highly Ordered Pyrolytic Graphite (HOPG)
HOPG lattice
5 nm x 5 nm
STM image
0.1V, 1.0 nA
Pt-Ir tip
Images,
Mark Rempel
Si(111) – 7x7
Model of Si(111)-7x7
surface structure; the side
length of the surface unit
cell is 26.6 Å
-1.5 V, 0.42 nA - filled states
+1.5 V, 0.50 nA - empty states
STM images, Dale Heggie
Si(111) – 7x7
+1.5 V, 0.12 nA +1.5 V, 0.78 nA +1.5 V, 0.80 nA
+2.0 V, 0.34 nA STM images, Dale Heggie
InP(100)
-1.81 V, 0.35 nA -2.0 V, 0.23 nA
STM images, Dale Heggie
InP(100)
-1.8 V, 0.58 nA
“ghost” tip images
noise
Scanning Tunneling Spectroscopy
• Measure I(V)
• Calculate sample density of states
eVE
V
I
dV
dI
F
Si(111)-7x7
• (dI/dV)/(I/V) as a function of
sample bias, for Si(111)-7x7
• (a) – (d) W tip
• (e) – (f) Pt-Ir tip
• features can be compared
to expected density of states
for Si(111)-7x7 surface
Figure, Mark Rempel
STM Tips
• Most common tips
– Electrochemically etched W wire
– Mechanically cut Pt-Ir wire
• Desirable properties
– Conducting
– Hard
– Inert (for imaging in air)
– Best shape varies with application
a)
b)
SEM images
100 m 1 m
Tungsten STM tip imaged at 150x and 20,000x magnification
19
DC Drop-off Method for Tungsten
• Anode (W wire) and cathode submerged in aqueous solution of NaOH or KOH
• W wire etches and breaks at the air/solution interface
• Remaining wire is used as the tip
C.J. Chen, Introduction to Scanning Tunneling Microscopy. OUP (1993)
20
Etching Mechanism
AnodeW(s) + 6OH- WO3 (s) + 6e- + 3H2O
WO3(s) + 2OH- WO42-(aq) + H2O
Cathode6H2O + 6e- 3H2(g) + 6OH-
OverallW(s) + 2OH- + 2H2O 3H2(g) +WO4
2-(aq)
G. Stephen Kelsey, J. Electrochem. Soc. 124(6), 814 (1977).
21
Etching Apparatus
Atomic Force Microscopy
Image from Will's Nanotechnology Blog
measure deflection of
a cantilever of known
spring constant, due to
tip-surface interaction
forces
F ~ nN
can operate in air,
liquid or vacuum
sample and tip may be
conducting or insulating
Atomic Force Microscopy
Wikipedia
• Contact Mode– Operate in region of repulsive tip-sample forces
– Scanning at constant cantilever deflection results in contour map of constant force, which represents topography
– Twisting of cantilever is a measure of lateral (friction) forces
– Contact mode can be very useful but can also be damaging to delicate surfaces
Atomic Force Microscopy
Wikipedia
• Non-Contact Mode (NC AFM)– Operates in region of attractive (van der Waals) forces
– Cantilever is forced to oscillate at its resonant frequency (100-300 kHz)
– Change in force experienced by tip changes the resonant frequency
– Change in resonant frequency is used as the feedback signal
– Very high Q cantilevers allow very small changes in resonant frequency to be detected, and thus small changes in force
– Ideal for delicate samples, and can give atomic resolution
Force-Distance Curves
Shahin, V. et al. J Cell Sci 2005;118:2881-2889
AFM Cantilevers and Tips
• Microfabricated cantilevers with integrated tips
– Silicon nitride, silicon oxide, silicon
– Spring constants: 0.1-1 N/m (contact), 10-100 N/m (non-contact)
– Resonance frequencies: 1-50 kHz (contact), 100-300 kHz (non-contact)
– Coatings depend on application: eg. conducting, magnetic, functionalized (specific molecules)
SEM image of cantilever
Cantilever Example
• Nanosensors PPP-NCL-10– Intended for non-contact mode
– Fabricated from n-type silicon
– no coating
– Specifications• t = 7.00 microns
• w = 38 microns
• l = 229 microns
• h = 10 – 15 microns
• c = 47 N/m
• f = 184 kHz
• Tip radius ~ 7 nm
RHK UHV 350 AFM/STM
• full range of AFM acquisition modes• AFM, STM or simultaneous AFM/STM
Sample holder
Cantilever
RHK Technology Inc.
Position sensitive detector
Design Features• Sample stationary, tip/cantilever scanned
• “Beetle” or “Walker” design– Piezo legs walk down ramp due to slip/stick
• Piezo legs used for approach and scanning
• Central piezo stack used for z motion
• Laser deflection and 4-quadrant PSD
AFM, Wikibooks
Design Features
• Head accepts STM tip or AFM cantilever holders
• Tip and cantilever holders can be transferred on and off in vacuum
• Sample may be heated or cooled while scanning
• Vibration isolation
– External airlegs
– Viton stack supporting sample stage
– Scan head designed to have high resonant frequencies
– Scan head decoupled from chamber
Sample Holders
Thermocouple connections
Sapphire washer to hold sample in place
Ramps for piezo legs
Control Hardware
• SPM 1000 controller
– Primary interface
– Front-panel controls
• PLL Pro
– Digital controller for AFM
• PMC 100 Piezo Motor Control
– Controls laser lens and position sensitive detector motors for AFM
• Small piezo control box (for X,Y,Z)
Control Software
• XPMPro
– STM/AFM data acquisition, processing and analysis
– May be used free for data analysis on any computer running Windows (XP, Vista)*
• PLLPro
– Controls the AFM mode, and all setup and parameters for AFM operation
*http://www.rhk-tech.com/downloads_files_V2.php
Specifications
From RHK
product brochure
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