SCANNING TUNNELING MICROSCOPE
MCG 5138 -Advanced Topics in Mechanical Engineering
Prepared by :
Jishang Tailor - 7462006
Roshan Teli - 7476661
Course Instructor - Prof. M. Yandouzi
Department of Mechanical Engineering,
University of Ottawa
• The principle of scanning tunneling microscopy is in quantum mechanics
which is different from classical mechanics.
• While classical mechanics deals up to macroscopic level, quantum
mechanics deals with microscopic level.
• Quantum mechanics explains the wave and particle like behavior of tiny
particles like photons and electrons.
• The quantum mechanics phenomenon which explains tunneling effect is
the working principal of scanning tunneling microscopy.
What is tunneling effect?
It is a phenomenon where a particle tunnels through a barrier that
it classically could not surmount.
In a metal, the energy levels of the
electrons are filled up to a particular
energy, known as the ‘Fermi energy’ 𝐸 𝐹. In
order for an electron to leave the metal, it
needs an additional amount of energy Φ,
the so-called ‘work function’.
The electrons need to overcome a barrier
Φ to travel from tip to specimen or vice
When a sharp metallic tip is brought very
close to the surface of a conductor, an
electric current can be detected due to the
tunneling of electrons through the air gap.
In this case, the air gap is considered as the
barrier and is only few 𝐴° thick.
When an electrical voltage V is applied
between sample and tip, this tunneling
phenomenon results in a net electrical
current, the ‘tunneling current’.
Components of STM
The main components of STM include scanning tip, Piezoelectric scanner,
Distance control and scanning unit, Vibration isolation system and Data
processing unit (Computer).
1. Scanning Tip :
STM tips are usually made from tungsten metal or a platinum-iridium alloy
where at the very end of the tip (called apex) there is one atom of the
material. Scanning tip is the most important aspect of the STM as tunneling
current is carried by that particular atom.
STM tip under 103
x and 105
x magnificationSample and Tip at atomic level
2. Piezoelectric Scanner :
The scanner tip is attached to a piezoelectric tube scanner. Piezoelectric
effect is a phenomenon under which the material changes its length
accordingly when put under an electrical voltage.
By adjusting the voltage on the piezoelectric element, the distance
between the tip and the surface can be regulated.
Piezoelectric crystals expand and contract very slightly depending on the
voltage applied to them and this principle is used to control the horizontal
position x, y, and the height z of the scanning tip.
Components of STM
3. Distance Control and Scanning unit :
Position control using piezoelectric means is extremely fine, so a coarse
control is needed to position the tip close enough to the sample before the
piezoelectric control can take over.
4. Data Processing Unit (Computer) :
The computer records the tunneling current and controls the voltage to the
piezoelectric tubes to produce a 3-dimensional map of the sample surface.
Components of STM
5. Vibration Isolation System :
STM deals with extremely fine position measurements so the isolation of
any vibrations is very important.
The tip and surface distance must be maintained in 𝐴°(0.1 𝑛𝑚)to get
desired atomic resolution.
Due to extremely high sensitivity of tunneling current between tip and
sample surface height, it is absolutely necessary to reduce inner vibrations
and to isolate the system from external vibration.
Damping can be achieved by : Pneumatic systems
Eddy current system
Components of STM
• First, a voltage bias is applied and the tip is brought close to the sample by
coarse sample-to-tip control, which is turned off when the tip and sample
are sufficiently close. At close range, fine control of the tip in all three
dimensions near the sample is typically piezoelectric, maintaining tip-
sample separation W typically in the 4-7 𝐴° (0.4-0.7 nm) range.
• In this situation, the voltage bias will cause electrons to tunnel between the
tip and sample, creating a current that can be measured. Once tunneling is
established, the tip's bias and position with respect to the sample can be
varied and data are obtained from the resulting changes in current.
• If the tip is moved across the sample in the x-y plane, the changes in
surface height and density of states cause changes in current. These
changes are mapped in images. This change in current with respect to
position can be measured itself, or the height, z, of the tip corresponding to
a constant current can be measured.
• There two modes : Constant height mode
Constant current mode
Working of STM
Constant Height Mode :
The voltage and height are both held constant while the current
changes to keep the voltage from changing; this leads to an image made
of current changes over the surface, which can be related to charge
Working of STM
The benefit to using a constant height
mode is that it is faster, as the piezoelectric
movements require more time to register
the height change in constant current
However, it is applicable only when sample
surface is real flat, the corrugation more
than 6 to 7 𝐴°
will lead tip to crash.
Generally less preferred due to the risk of
damaging the tip.
Constant Current Mode :
Feedback electronics adjust the height by a voltage to the piezoelectric
height control mechanism.
This leads to a height variation and thus the image comes from the tip
topography across the sample and gives a constant charge density
surface; this means contrast on the image is due to variations in charge
Working of STM
It is a time consuming method compared to
the constant height mode as the feed back
control has to adjust the current constant
according height as the tip moves along the
What STM measures?
STM images are not direct surface images of the sample as in the case of
optical microscopy rather it is measure of the local density of states of a
material at it surface as a function of lateral (x-y) position on the sample
surface and energy.
Within sample each electron has specific energy level and only certain
number of electrons can occupy that level at a time .
The distribution that gives number of electrons allowed per energy level
as a function of certain energy level is called the density of states.
So, the grey scale image generated is direct measurement atomic
corrugation of the surface.
Use of STM
Generation of images
The amount of adjustment done
by the feedback loop is recorded
and defines the grid value which
can be displayed as the grey scale
Once the grid values are assigned
one can use it to deform it
perpendicular to the surface and
get 3D image.
Now, different colors patterns are
used to color the image and
STMs are versatile. They can be used in ultra high vacuum, air, water and
other liquids and gasses.
STMs give three dimensional profile of a surface, which allows researchers
to examine a multitude of characteristics, including roughness, surface
defects and molecule size.
Lateral Resolution of 0.1 nm and 0.01 nm of resolution in depth can be
It is very expensive.
It need specific training to operate effectively.
STM need very clean surface, excellent vibration control while operation,
single atom tip.
Merits and Demerits
If the tip has is not prepared as per standard which contains multipoint
and if the tip get contaminated during the operation, it creates multi
signal which are resulted as the artifacts in the image.
Sometimes the frequency gained by amplifier is too high which makes the
tube to oscillate which gives noisy image.
Sometimes rapid change in voltage are noted due to scanner encounters a
large step during scanning. The tube will continue to move even if the
voltage remains fixed which results in hazy image.
Artifacts in Image
Paper Review :
The selected paper is research on Fabrication of nanoscale alumina on
NiAl(1 0 0) surface using scanning tunneling microscope under ultrathin
In the research, two modes of oxidation were observed under different
conditions in formation of the alumina on the surface of NiAl(1 0 0).
The first mode of oxidation observed under the tunneling current greater
than 0.4 nA and the power greater than 0.24 nW. Here, Al and O atoms
formed alumina on the surface in the proximity of the tip. The width and
thickness of the alumina strips growth are controlled by the current and
In the second mode oxidation smaller
power and a smaller bias (1.0V) were
used which resulted in growth of
crystalline alumina along direction [0 0
1] or [0 1 0] of NiAl(1 0 0) surface in
the tip scanned area of oxidized
surfaces, irrespective of the scanning
direction of tip.
Direction of crystalline alumina
Direction of scanning
The combination of above two oxidation modes were used to fabricate
crystalline alumina on NaAl(1 0 0) surface in which bias was kept constant
at 0.7 V and current was raised from 0.3 nA to 1.6 nA which created
abrupt alumina on the surface.
With same condition of oxidation on the surface keeping bias at 0.7 V and
lowering current to 0.3 nA just altered the topography of the surface and
it became more structured, indicating the gradual formation of the oxide
strips along direction [0 1 0] and [0 0 1] of NiAl (1 0 0)
The STM tip induced oxidation has been widely studied and this is very
important break through in research as industrialization of such
technology at economical rate can use to fabricate nanosensors, bio
It helps to change the property of the structure at nano-scale which helps
to create strong structure.