1.
STM

2.
SCANNING TUNNELING MICROSCOPE
 The development of the family of scanning probe microscopes started with the original invention of
the STM in 1981.
 Gerd Binnig & Heinrich Rohrer developed the first working STM while working at IBM Zurich Research
Laboratories in Switzerland. This instrument would later win Binnig and Rohrer the Nobel prize in
physics in 1986.
 The scanning tunneling Microscope is an electron microscope that transmits three - dimensional
images of the electron cloud around the nucleus.
 The scanning tunneling Microscope (STM) works by scanning a very sharp metal wire tip over a
surface. By bringing the tip very close to the surface, and by applying an electrical voltage to the tip or
sample, we can image the surface at an extremely small scale - down to resolving individual atoms.
 The STM allows the inspection of the properties of a conductive solid surface at an atomic size.

3.
The main components of STM include Scanning tip, Piezoelectric
scanner, Distance control and scanning unit, Vibration isolation
system and Data processing unit (Computer).
 SCANNING TIP - STM tips are usually made from Tungsten metal
or 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.
 DISTANCE CONTROL & 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.
 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.

4.
 PIEZOELECTRIC SCANNER - Piezoelectric materials are ceramics that change dimensions in
response to an applied voltage and conversely, they develop an electrical potential in response to
mechanical pressure.
 Piezoelectric scanners can be designed to move in x, y, and z by expanding in some directions
and contracting in others. Usually fabricated from lead zirconium titanate, (PZT) by pressing
together a powder, then sintering the material. They are doped to create specific materials
properties.
 They are polycrystalline solids. Each of the crystals in a piezoelectric material has its own electric
dipole moment. These dipole moments are the basis of the scanner's ability to move in response
to an applied voltage.
 After sintering, the dipole moments within the scanner are randomly aligned. If the dipole
moments are not aligned, the scanner has almost no ability to move. A process called poling is
used to align the dipole moments.
 During poling the scanners are heated to about 200°C to free the dipoles, and a DC voltage is
applied to the scanner. Within hours most of the dipoles become aligned. At that point, the
scanner is cooled to freeze the dipoles into their aligned state.

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 by A⁰ (0.1 nm) to get desired atomic
resolution.
 Due to extremely high sensitivity of tunneling current between tip and the 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
- Spring system
- Eddy current system

6.
 The STM is based on several principles. One is the quantum mechanical effect of tunneling. It is this
effect that allows us to “see” the surface.
 Another principle is the piezoelectric effect. It is this effect that allows us to precisely scan the tip
with angstrom-level control.
 Lastly, a feedback loop is required, which monitors the tunneling current and coordinates the
current and the positioning of the tip.
 This is shown schematically below where the tunneling is from tip to surface with the tip rastering
with piezoelectric positioning, with the feedback loop maintaining a current setpoint to generate a
3D image of the electronic topography:

7.
 Tunneling is a quantum mechanical effect. A tunneling current occurs when electrons move
through a barrier that they classically shouldn’t be able to move through. In classical terms, if
you don’t have enough energy to move “over” a barrier, you won’t.
 However, in the quantum mechanical world, electrons have wavelike properties. These waves
don’t end abruptly at a wall or barrier, but taper off quickly. If the barrier is thin enough, the
probability function may extend into the next region, through the barrier.
 Because of the small probability of an electron being on the other side of the barrier, given
enough electrons, some will indeed move through and appear on the other side. When an
electron moves through the barrier in this fashion, it is called tunneling.
 Quantum mechanics tells us that electrons have both wave and particle-like properties.
tunneling is an effect of the wavelike nature.

8.
 The top image shows us that when an electron (the wave) hits a barrier, the wave doesn’t
abruptly end, but tapers off very quickly - exponentially. For a thick barrier, the wave doesn’t
get past.
 The bottom image shows the scenario if the barrier is quite thin (about a nanometer). Part of
the wave does get through and therefore some electrons may appear on the other side of the
barrier.
 Because of the sharp decay of the probability function through the barrier, the number of
electrons that will actually tunnel is very dependent upon the thickness of the barrier. The
current through the barrier drops off exponentially with the barrier thickness.
Schematic of electron wavefunction

9.
11
 The piezoelectric effect was discovered by Pierre Curie in
1880. The effect is created by squeezing the sides of certain
crystals, such as quartz or barium titanate.
 The result is the creation of opposite charges on the sides.
The effect can be reversed as well; by applying a voltage
across a piezoelectric crystal, it will elongate or compress.
 These materials are used to scan the tip in an scanning
tunneling microscopy (STM) and most other scanning probe
techniques. A typical piezoelectric material used in scanning
probe microscopy is PZT (lead zirconium titanate).

10.
• Electronics are needed to measure the current, scan the tip,
and translate this information into a form that we can use for
STM imaging. A feedback loop constantly monitors the
tunneling current and makes adjustments to the tip to
maintain a constant tunneling current.
• These adjustments are recorded by the computer and
presented as an image in the STM software. Such a setup is
called a constant current image.
• In addition, for very flat surfaces, the feedback loop can be
turned off and only the current is displayed. This is
a constant height image.
Feedback loop & electron tunneling
for STM

11.
• A voltage bias is applied and the tip is brought close to the sample by coarse sample - tip control,
which is turned off when the tip and the sample are sufficiently close. At close range, fine control
of the tip in all 3 dimensions near the sample is typically piezoelectric, maintaining tip - sample
separation W typically in the 4-7 A⁰ (0.4 - 0.7 nm) range.
• The voltage bias will cause electrons to tunnel between the tip and sample, creating a current
that can be measured. Once tunnelling is established, tip’s bias and position with respect to the
sample can be varied and data are obtained from the resulting changes in the 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.

12.
There are two main kinds of operation modes of the STM,
I) constant current mode, II) constant height mode.
CONSTANT CURRENT MODE - In this mode, feedback loop system forces the tip (using the z-driver) to
stay at a distance from the surface that would keep a constant current flowing between the tip and
the sample surface.
 By recording the voltage, which has to be applied to the z-driver in order to maintain the current
constant while scanning, a topographical image of the surface is produced.
 Mode can be applied to surfaces that are not necessarily flat on the atomic scale i.e. stepped
surfaces. The main disadvantage of this mode is the finite time of the feedback loop, which result
in slowing down the scanning process.

13.
CONSTANT HEIGHT MODE - In this mood, the tip is rapidly scanned over the surface at a
constant height.
 By recording the change in the value of the tunneling current as a function of position, a
topographical image of the surface is produced.
 This mode has the advantage of being faster than the constant current mode. On the other
hand, there are disadvantages to this mode, such as, extracting the topographical
information is harder and depending on the sample kind.
 The other disadvantage is the tip might get damaged due to a crash with a “step” on the
surface while scanning at high speeds. That is why this technique is used for flat surfaces
(atomic scale).

14.
16
In recent years, the microscope has been utilized to investigate biological molecules deposited on
suitable conducting surfaces, providing atomic resolution images of single molecules, with no
conformational averaging as occurs for spectroscopic techniques associated with the study of bulk
molecules.
 These studies show that the technique is a potentially valuable biophysical tool complementary
to the other well established methods that are extensively reviewed in this volume.
 To date, high resolution images of biological systems, such as DNA globular macromolecules,
such as vicilin phospholipid membranes have been obtained, with the body of scientific
literature increasing rapidly with time.
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 its surface as a function of lateral
position on the sample surface & energy.

15.
STMs are helpful because they can give researchers a three dimensional profile of a surface, which
allows researchers to examine a multitude of characteristics, including roughness, surface defects
and determining things about the molecules such as size and conformation
Other advantages of the scanning tunneling microscope include:
 Capable of capturing much more detail than lesser microscopes. This helps researchers better
understand the subject of their research on a molecular level.
 STMs are also versatile. They can be used in ultra high vacuum, air, water and other liquids and
gasses.
 They will operate in temperatures as low as zero Kelvin up to a few hundred degrees Celsius.

16.
There are very few disadvantages to using a scanning tunneling microscope.
The two major downsides to using STMs are:
 STMs can be difficult to use effectively. There is a very specific technique that
requires a lot of skill and precision. STMs require very stable and clean surfaces,
excellent vibration control and sharp tips.
 STMs use highly specialized equipment that is fragile and expensive.
 https://www.researchgate.net/publication/325334823_Scanning_Tunneling_Microscopy_
The_new_eyes_and_hands_of_the_scientists
 https://my.eng.utah.edu/~lzang/images/Lecture_6_STM.pdf