• The principle of electron tunneling was proposed by
– I. Giaever: Energy gap in superconductors measured by
electron tunneling, Phys. Rev. Lett. 5 (1960) 147–148
• 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.
• To be able to measure a tunneling current, the two
metals must be spaced no more than 10 nm apart.
Principle of STM
The principle of the STM is straightforward. A
sharp metal tip (one electrode of the tunnel
junction) is brought close enough (0.3–1 nm)
to the surface to be investigated (the second
electrode) that, at a convenient operating
voltage (10 mV–1 V), the tunneling current
varies from 0.2 to 10 nA which is measurable.
The tip is scanned over a surface at a distance
of 0.3–1 nm, while the tunneling current
between it and the surface is measured.
Compact STM for use in controlled environments
2000, Toyohashi University of Technology, Japan
Source: BSc & MSc thesis of Mashiur Rahman, Toyohashi University of Technology
Scanning Tunneling Microscope (STM)
This method uses an
(tunneling current) that
begins to flow when a
very sharp tip moves
near to a conducting
surface and hovers at
about one nanometer
• The tip (about the size of a single atom) sits on
a piezoelectric tube. When you apply voltage
to electrodes attached to this tube, you can
make teensy adjustments to keep the
tunneling current constant — which also
keeps the tip at a constant distance from the
sample while an area is scanned. The
movement of the piezoelectric tube is
recorded and displayed as an image of the
Binnig et al.’s Design
VT = bias voltage
Ø = average barrier height
JT = tunnel current
A = constant 1.025 eV−1/2Å−1.
• STM for operation in ambient air, the sample is held in
position while a piezoelectric crystal in the form of a
cylindrical tube (referred to as PZT tube scanner) scans the
sharp metallic probe over the surface in a raster pattern
while sensing and outputting the tunneling current to the
• The digital signal processor (DSP) calculates the desired
separation of the tip from the sample by sensing the
tunneling current flowing between the sample and the tip.
• The bias voltage applied between the sample and the tip
encourages the tunneling current to flow. The DSP
completes the digital feedback loop by outputting the
desired voltage to the piezoelectric tube.
constant-current &constant-height mode
STM can be operated in
either the constant-current or
mode.The images are of
graphite in air.
Source: Springer Handbook of Nanotechnology by B. Bhushan
STM cantilever / tip
• Typically fabricated from metal
wires of tungsten (W), platinum-
iridium (Pt-Ir), or gold (Au).
• sharpened by grinding,cutting
with a wire cutter or razor blade,
ionmilling, fracture, or
Schematics of a) CG Pt-Ir
probe, and (b) CG Pt-Ir FIB
• The two most commonly used tips are
– Pt-Ir (80/20) Iridium: tips are generally
mechanically formed and are readily available.
provide better atomic resolution than tungsten
– Tungsten wire: are etched from tungsten wire with
an electrochemical process. Tungsten tips are
more uniformly shaped and may perform better
on samples with steeply sloped features.
Mechanically cut and electrochemically
etched STM tips
A mechanically cut STM tip (left) and an electrochemically etched STM tip (right),
p.383 Springer Handbook of Nanotechnology
Sample should be conductive
• Samples to be imaged with the STM must be
conductive enough to allow a few
nanoamperes of current to flow from the bias
voltage source to the area to be scanned.
• In many cases, nonconductive samples can be
coated with a thin layer of a conductive
material to facilitate imaging.
• The bias voltage and the tunneling current
depend on the sample.
• If you put electrodes on the opposite sides of
some crystals — quartz or topaz, for example —
and apply a voltage across the crystal, it will
expand or contract. Any movement of the crystal
in response to a voltage is called the piezoelectric
• The piezoelectric tube used in the scanning
tunneling microscope is simply a crystal that
expands or contracts depending upon the voltage
you apply to it.
Scanning electron microscope (SEM)
• An SEM shoots a beam of electrons at whatever
you’re examining, transferring energy to the spot
that it hits. The electrons in the beam (called primary
electrons) break off electrons in the specimen. These
dislodged electrons (called secondary electrons) are
then pulled onto a positively charged grid, where
they’re translated into a signal.
• Moving the beam around the sample generates a
whole bunch of signals, after which the SEM can
build an image of the surface of the sample for
display on a computer monitor.
SEMs can ferret out quite a bit of information about the
• Topography: surface features such as texture
• Morphology: shape, size, and arrangements of the
particles that compose the object’s surface
• Composition: elements that make up the sample
(This can be determined by measuring the X-rays
produced when the electron beam hits the sample.)
Transmission electron microscope (TEM)
• It’s a kind of nano-scale slide projector: Instead of shining a
light through a photographic image the TEM sends a beam of
electrons through a sample.
• The electrons that get through then strike a phosphor screen,
producing a projected image: Darker areas indicate that fewer
electrons got through; lighter areas are where more electrons
• A TEM can achieve a resolution of approximately 0.2
nanometers, roughly the size of many atoms.
• A TEM can produce images that show you just how the atoms
are arranged in a material.