Scanning Tunneling Microscopy (STM)

Scanning Probe Microscopy
A plethora of possibilities
Topics
• The basic idea
• The scanning part
• Scanning Tunneling Microscopy (STM)
– Theoretical backround
– Applications
• Various other modes of interest
–
–
–
–
Atomic Force Microscopy (AFM)
Lateral (sometimes called Friction) Force Microscopy (LFM)
Magnetic Force Microscopy (MFM)
Others …
• Scanning Nanoindentation – Quantitative mechanical/tribological
properties on the nanoscale
Brief historic overview
•
Topografiner: 1965-1971 by
Russel D. Young (piezo-translators)
at NBS
•
STM: 1982 by
Heinrich Rohrer (1933-) and
Gerd Karl Binning (1947-) at IBM Zürich
Nobel prize 1986
•
AFM: 1986 by
Heinrich Rohrer (l) and Gerd Karl Binning (r)
Gerd Karl Binning,
Christoph Gerber and
Calvin Quate at IBM Zürich & Stanford University
Basic idea
The SPM family
The scanning part of SPMs
•
Based on the piezoelectric
effect:
– Piezo Tri-Pods
– Piezo-Tube-Scanners
•
Problems of these
scanners are:
– Hysteresis, creep
– Aging
– Cross-correlations
between the individual
axis
•
These are addressed by
extensive calibrationfunctions or closed-loopsystems utilizing laserinterferrometry
Piezo-tube scanner and
sketch of a piezo tripod
Scanning Tunneling Microscopy (STM)
•
•
First success by Binning
and Rohrer (IBM) in 1982
Atomic resolution is
achieved due to the high
sensitivity of the tunneling
current with respect to the
tip-sample distance s
I∝ ∑ ψ
1,2
°
1
2
ψ °2 2e −2k s
Operation modes of STM
•
Constant current
– A feeback-loop tries to keep the current between tip and sample constant while
scanning → z-piezo movement = constant density of states (≈ topography)
•
Constant height
– Is an alternative approach: here the height is kept constant while the current is
measured during scanning
•
Scanning tunneling spectroscopy STS
– Probes the density of states at a given surface position by ramping the voltage
while measuring the tunneling current
Applications of STM
•
Examples
Binning and Rohrer original scan of the
7×7 reconstruction of the Si(111)
surface
20 years later:
IBM work shows the density of states (DOS)
of 48 iron atoms on a Cu (111) surface
Applications of STS
•
Principles:
– By ramping of VT the density
of states is probed
– Detailed information about the
electronic configuration of the
sample is available
PbS quantum boxes – electric states: http://www.evsf2.sci.kun.nl/Eric/...
AFM - interaction
• Lennard-Jones
potential is often cited
• Consisting of a vander-Waals and a
Pauli-part
• Distance-dependence
of interaction is
changed in case of
nanoscale objects
• Basic behavior,
however, is
comparable
Various AFM modi
Contact mode
pro
• High resolution is possible even though rather the exception
• Often one deals with a kind of Moirée-effect and is not able to
identify individual defects (better resolution is reached by noncontact AFM under UHV conditions)
• Easy & artifacts are usually easy to handle
cons
• Capillary forces can cause large forces between tip and surface
• Moving of objects on the surface
• Indentation effects / scratching of soft surfaces
• Friction based artifacts depending on scan direction
• Adhesive samples cause troubles
Non-contact mode
• Idea here is to
sense the sample
without touching it
→ essential in the
context of most
polymer and
biological
samples
• Cantilever is
operated close to
its resonance
frequency via a
piezo actuator
Non-contact AFM
• Equation of motion – free cantilever
Non-contact AFM
• What happens if we add a sample?
Lateral/Friction Force Microscopy (LFM)/(FFM)
•
Set up:
– Cantilever based
– quadrant photo detector
•
Comments:
– Challenging to calibrate
– Friction-loop is
necessary to „remove“
topography-effect
– Geometry of the tip
– Single or multiple
asperity contact
– Stability of the tip
F-d measurements
•
•
While moving the stage,
the deflection of the
cantilever is monitored
Factors that have an
influence on the
experimental outcome:
–
–
–
–
Adhesion
Elasticity
Visco-elastic behavior
Plastic yield
Other techniques
Beschreibung der Topographie
Die Oberfläche eines ebenen Halbraumes sei durch z (x, y) gegeben. Statistisch lässt
sich nun die Oberfläche durch die Funktion φ (z), die die Wahrscheinlichkeit angibt, dass
ein Punkt der Oberfläche eine Höhe zwischen z und z + dz besitzt, beschreiben.
Folgende Kennwerte findet man bei der Beschreibung der Rauheit einer Oberfläche:
kumulative Höhenverteilung, Φ (z):
∞
Φ (z ) ≡ ∫ φ (z ′) dz ′
z
L
Mittelwert der Oberfläche, <z>:
1 Lx y
z≡
∫ ∫ z dx dy
Lx Ly 0 0
RMS (root-mean-squared), s(Rq):
1 Lx y
2
s≡
∫ ∫ [z − z ] dx dy
Lx Ly 0 0
L
Beschreibung der Topographie
Weitere Parameter: Die mittlere lokale Steigung s' sowie die mittlere lokale Krümmung s''
der Oberfläche. Die folgenden zwei Gleichungen geben die Definition dieser Größen für
den eindimensionalen Fall wieder:
s ′x ≡
1
Lx
Lx
 ∂z 
2
∫   dx
0  ∂x 
1
Lx
s ′x′ ≡
2
 ∂2z 
∫  2  dx
0 
 ∂x 
Lx
Viele reale Oberflächen weisen eine Normalverteilung (Gaussverteilung) der Höhen auf.
Für abweichende Fälle werden in der Literatur zwei weitere Parameter eingeführt. Hierbei
wird mittels der Größen „Schräge der Verteilung“ (skewness) Sk sowie „Schärfe der
Verteilung“ (kurtosis) K die Form der Wahrscheinlichkeitsverteilung φ (z) charakterisiert.
Sk ≡
1
s3
∞
∫ z φ (z ) dz
−∞
3
K≡
1
s4
∞
4
∫ z φ (z ) dz
−∞
Bei einer Oberfläche, deren Wahrscheinlichkeitsverteilung φ (z) der Gaussverteilung
entspricht, erhält man eine Schräge Sk = 0 und eine Schärfe K = 3
Challenges in the context of SPMs
•
Scanners:
–
–
–
–
•
Hysteresis
Creep
Non-Linearity's
Aging
Tip artifacts
– Convolution
– Change over time
•
Porous Aluminum
Tip radis 1nm
Uncertainties about the
mechanism of interaction
Tip radis 10nm
http://www.spmtips.com/products/pa/
Tip convolution
Sampling rate can also have an effect on the results