Transcript Scanning Probe Microscopy STM

Scanning Probe Microscopy –
Principle of Operation,
Instrumentation, and Probes
Scanning Probe Microscopy-STM
• The principle of electron tunneling was proposed by
Giaever. He envisioned that if a potential difference is
applied to two metals separated by a thin insulating film,
a current will flow because of the ability of electrons to
penetrate a potential barrier.
• R. Young developed field emission topograph profiler.
• Binnig and Rohler introduced vacuum tunneling
combined with lateral scanning. The vacuum provides
the ideal barrier for tunneling. The lateral scanning
allows one to image surfaces with exquisite resolution,
lateral-less than 1 nm and vertical-less than 0.1 nm,
sufficient to define the position of single atoms.
Calibration standards
Crystals as ‚rulers‘
h = d111 = 0,31 nm
STM Image of HOPG
(Highly Oriented Pyrolytic Graphite)
Image size: 10nm x10nm
AFM image of silicon (111) single atomic
steps with native oxide
Since the introduction of the STM in 1981 and
AFM in 1985 by Binnig and Rohler , many variations
of probe based microscopies, referred to as SPMs,
have been developed.
Scanning Probe Microscopy
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Family of SPM
STM scanning tunneling microscope
AFM Atomic force microscope
FFM (or LFM) (Lateral or Friction) force microscope
SEFM Scanning electrostatic force microscope
SFAM scanning force acoustic microscope
AFAM atomic force acoustic microscope
SMM scanning magnetic microscope
MFM magnetic force microscope
SNOM scanning near field optical microscope
SThM scanning thermal microscope
SEcM scanning electrochemical microscope
SKpM scanning Kelvin Probe microscope
SCPM scanning chemical potential microscope
SICM scanning ion conductance microscope
SCM scanning capacitance microscope
Non-contact scanning probe
microscope (SPM)
• Scanning tunneling microscope
• Atomic force microscope
• Scanning near field optical microscope
• Scanning magnetic force microscope
Optical microscope and Transmission electron
microscope
Operating voltage
50~100kV;
Wave length:
0.004~0.006 nm;
Resolution: <1nm
Laser autofocus system
Field-ion microscope
Radius of the tip:
10nm
Magnification:
106;
Operating
voltage ~100kV;
Resolution:
<1nm
Field emission profiler -- R.Young
Operating
voltage ~100V;
Resolution:
vertical~3nm;
Lateral~400nm
Principle of
operation of the
STM made by
Binnig and Rohrer
JT ∝ VT exp(−Aφ1/2d)
JT ― the tunnel current, a sensitive function of the gap width d ;
VT ― the bias voltage;
φ― the average barrier height (work function) ;
A ― constant = 1.025 eV−1/2 Å−1.
With a work function of a few eV, JT changes by an order of
magnitude for every angstrom change of d.
The tip is scanned over a
surface while the
tunneling current
changes with it, so the
surface is measured.
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A sharp metal tip to the surface: 0.3–1 nm;
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At a convenient operating voltage (10mV–1V);
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The tunneling current varies from 0.2 to 10 nA (which is
measurable).
Nanoscope STM consists of
three main parts:
• the head which houses the piezoelectric tube
scanner for three dimensional motion of the tip and
• the preamplifier circuit (FET input amplifier)
mounted on top of the head for the tunneling
current,
• the base on which the sample is mounted, and the
base support, which supports the base and head
Scanning Probe Microscopy-STM
Principle of operation
of a commercial STM,
a sharp tip attached
to a piezoelectric tube
scanner is scanned
on a sample
The motion of the tip due to external vibrations is proportional
to the square of the ratio of vibration frequency to the resonant
frequency of the tube. Therefore, to minimize the tip vibrations,
the resonant frequencies of the tube are high at about 60 kHz
in the vertical direction and about 40 kHz in the horizontal
direction.
Scanning Probe Microscopy-STM
• STM cantilevers with sharp tips are typically fabricated
from metal wires of tungsten (W), platinum iridium (Pt-Ir),
or gold (Au) and sharpened by
• grinding, cutting with a wire cutter or razor blade,
• field emission/ evaporator,
• ion milling, fracture, or
• electrochemical polishing/etching.
• The two most commonly
used tips are made from
either a Pt-Ir (80/20) alloy
or tungsten wire.
Schematic of a typical
tungsten cantilever with a
sharp tip produced by
electrochemical etching.
The resonant frequencies of the tube are high at
about 60 kHz in the vertical direction and about 40 kHz
in the horizontal direction.
A lateral resolution of about 2 nm requires tip radii
on the order of 10 nm.
Scanning Probe Microscopy-STM
Schematics of (a) CG(controlled geometry) Pt-Ir probe, and (b)
CG Pt-Ir FIB (focused ion beam) milled probe.
Constant-current mode:
a feedback network
changes the height of the
tip z to keep the current
constant. → topographic
map yielded by the
displacement of the tip.
Constant height mode: a
metal tip scannes across a
surface at nearly constant
height and constant voltage
while the current is monitored
→ topographic map yielded
by the change of the current.
STM can be operated in either the constant-current or the constant
height mode. The images are of graphite in air.
Scanning Probe Microscopy-STM
STM images of
evaporated C60 film
on a gold-coated
freshly-cleaved mica
using a mechanically
sheared Pt-Ir (80-20)
tip in constant height
mode.
Atomic Force Microscope
Contact
Non-contact mode/Tapping
Tip radius ~ 2 ... 50 nm
Force ~ 0.01 nN ... 1 nN
sample: nearly any sample
Atomic force microscope
Principle of operation of the AFM. Sample mounted on a
piezoelectric tube scanner is scanned against a short tip and
the cantilever deflection is measured, mostly, using a laser
deflection technique. Force (contact mode) or force gradient
(noncontact mode) is measured during scanning.
Atomic force microscope
• The AFM combines the principles of the STM and the stylus
profiler .
• During initial contact, the atoms at the end of the tip
experience a very weak repulsive force due to electronic
orbital overlap with the atoms in the sample surface. The
force acting on the tip causes a cantilever deflection which
is measured by tunneling, capacitive, or optical detectors.
• In an AFM, the force between the sample and tip is
detected, rather than the tunneling current.
• The deflection can be measured to within 0.02 nm, so for
typical cantilever spring constant of 10N/m a force as low as
0.2 nN can be detected.
• The AFM can be used either in a static or dynamic mode.
Schematics of the four more commonly used detection
systems for measurement of cantilever deflection.
In each set up, the sample mounted on piezoelectric body
is shown on the right, the cantilever in the middle, and the
corresponding deflection sensor on the left
An illustration of the optical beam deflection system
that detects cantilever motion in the AFM. The voltage
signal VA−VB is proportional to the deflection
long-range (up to 100 nm) ：
Van der Waals ，Electrostatic ，Magnetic forces
short-range (fractions of a nm) :
Chemical forces (bonding energy， equilibrium distance)
AFM cantilever
• The cantilever is characterized by 3 important
coefficients :
• Spring constant k ;
• Eigenfrequency f0 ;
• Quality factor Q - is typically a few hundred but
can reach hundreds of thousands in vacuum.
• A variety of silicon and silicon nitride
cantilevers are commercially available with
• －micron-scale dimensions,
• －spring constants ranging from 0.01 to
100N/m, and
• －resonant frequencies ranging from 5
kHz to over 300 kHz.
Spring constant of cantilever
To obtain atomic resolution with the AFM, the spring
constant of the cantilever should be weaker than the
equivalent spring between atoms.
• The vibration frequencies ω of atoms bound in a
molecule or in a crystalline solid are typically 1013 Hz or
higher;
• The mass of the atoms m on the order of 10−25;
• Interatomic spring constants k, given by ω2m, on the
order of 10N/m.
• Therefore, a cantilever beam with a spring constant of
about 1N/m or lower is desirable.
AFM，a powerful surface tool
on atomic/molecular scales
1N/m， 1ng
0.01nN
0.1nm
100 kHz
0.01–5N/m
Interatomic forces with one or several atoms in contact are
20–40 or 50–100 pN, respectively.
Thus, atomic resolution with an AFM is only possible with a
sharp tip on a flexible cantilever at a net repulsive force of
100 pN or lower.
Contact (static) Mode
In the contact (static) mode, the
interaction force between tip and
sample is measured by measuring
the cantilever deflection.
Tip approach sample: A-B-C
A-B ― <10-10N (attractive force);
B-C ― >10-10N (repulsive force);
Tip leaves sample: C-B-D-A
C-B ― > 10-10N (repulsive force);
B-D ― <10-10N (attractive force);
D-A ― <10-10N (repulsive force);
NonContact (dynamic) Mode
q´(t) ― the deflection of the tip of the
cantilever, It oscillates with an
amplitude A at a distance q(t) to a
sample.
kts varies in orders of magnitude during
one oscillation cycle,
NonContact (static) Mode
Tapping mode
Schematic of tapping mode used for surface
roughness measurements
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Tapping mode，a powerful surface tool on
atomic/molecular scales, because of :
(1) it has true atomic resolution,
(2) it can measure atomic force (so-called atomic
force spectroscopy),
(3) it can observe even insulators, and
(4) it can measure mechanical responses such as
elastic deformation.
Scanning Probe Microscopy-AFM
A commercial small sample AFM/FFM,
A large sample
AFM/FFM
Schematics of a commercial
AFM/FFM made by Digital
Instruments Inc.
(a) front view,
(b) optical head,
(c) base, and
(d) cantilever substrate
mounted on cantilever
mount (not to scale)
(b) optical head
(c) base
(d) cantilever substrate mounted on
cantilever mount (not to scale)
Block schematic of the feedback control loop of an AFM
The NPL Metrological Atomic Force Microscope (MAFM)
The NPL Metrological Atomic Force Microscope (MAFM)
AFM
(a) A schematic depiction
of an atomic force
microscope cantilever and
tip interacting with
materials on a surface.
Tips typically have points
of 50 nm or less in
diameter.
(b) Schematic of
multiplexed AFM tips
performing multiple
operations in parallel.
Probes in Scanning Microscopies
― SPM images are generated through
measurements of a tip-sample interaction.
― A well-characterized tip is the key element to
data interpretation and is typically the limiting factor.
Probes in Scanning Microscopies
Scanning Probe Microscope - cantilevers
and tips
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Si; tetraeder tip (~ 35 )
Si; beam cantilever
ElectronBeamD
eposited
Si; super sharp (~20 )
Si; Focussed Ion Beam
sharpened (~ 10 )
Si N ; triangular cantilever
... with pyramidal tip (~100 )
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4
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Oxide sharpening of silicon tips. (Si-SiO2 stress formation
reduces the oxidation rate at regions of high curvature. )
The left image shows a sharpened core of silicon in an
outer layer of SiO2. The right image is a higher
magnification view of such a tip after the SiO2 is removed.
Electron beam deposition (EBD).
Carbon Nanotube Tips
SEM micrograph of a multi-walled carbon nanotube
(MWNT) tip physically attached on the single-crystal
silicon, square-pyramidal tip
Carbon Nanotube Tips
A MWCNT attached to a tungsten spike
Assembled Cantilever Probe (ACP)
ACP Structure comprising two cantilevers glued together
A mechanically cut STM tip (left) and an
electrochemically etched STM tip (right）.
Probe Tip Performance
A tip model to explain the high resolution
obtained on ordered samples in contact
mode in AFM.
Electron beam deposition (EBD) Tips
Measurement of a step height standard using a normal
silicon tip (left) and a silicon tip with an EBD deposited tip on
the end
• The tunneling current is a monotonic function of
the tip-sample distance and has a very sharp
distance dependence.
• In contrast, in AFM, the tip-sample force has
long and short-range components and is not
monotonic.
一Jump-to-Contact and Other Instabilities;
一Contribution of Long-Range Forces;
一Noise in the Imaging Signal (1/f );
一Non-monotonic Imaging Signal (FM modulation)
一Jump-to-Contact and Other Instabilities
Plot of tunneling current It and force Fts (typical values)
as a function of distance z between front atom and
surface atom layer
Schematic view of 1/ f noise apparent in force detectors.
Static AFMs - from 0.01 Hz to a few hundred Hz.
Dynamic AFMs - around 10 kHz to a few hundred kHz.
Noncontact mode AFM images of Si(111) 7×7
reconstructed surface obtained using the Si tips
(a) Without and (b) with dangling bond. The scan area is
99Å×99 Å.
Low Temperature Scanning
Probe Microscopy
Probably the most important advantage of
the low-temperature operation of scanning probe
techniques is that they lead to a significantly
better signal-to-noise ratio than measuring at
room temperature.
One chamber UHV system
with variable temperature
STM based on a flow
cryostat design.
Three-chamber UHV
and bath cryostat system
for scanning force
microscopy, front view.
One for cantilever and
sample preparation,
which also serves as a
transfer chamber,
One for analysis
purposes, and
A main chamber that
houses the microscope.
The first demonstration of manipulating atoms was performed by
Eigler and Schweizer (1990), who used Xe atoms on a Ni(110)
surface to write the three letters “IBM” (their employer) on the
atomic scale
Final artwork greeting the new millennium on the atomic scale
Synthesis of biphenyl联苯 from two iodobenzene碘代苯 molecules on
Cu(111): First, iodine is abstracted from both molecules (i),(j), then the
iodine between the two phenyl groups is removed from the step (k), and
finally one of the phenyls is slid along the Cu-step (l) until it reacts with the
other phenyl (m); the line drawings symbolize the actual status of the
molecules
AFM
(c) AFM image of a quantum corral, a structure built
using AFM manipulation of individual atoms (from
the IBM Image Gallery)
AFM
(a) A schematic depiction of an
atomic force microscope
cantilever and tip interacting
with materials on a surface.
Tips typically have points of 50
nm or less in diameter.
(b) Schematic of multiplexed
AFM tips performing multiple
operations in parallel.
(c) AFM image of a quantum
corral, a structure built using
AFM manipulation of individual
atoms (from the IBM Image
Gallery)