Atom Under The Microscope
Electron & Atomic Force Microscopy
What is an Atom?
Essentially, an atom is the smallest unit of an
element that retains the properties of the same element (iron, copper, carbon
etc). This means that divided further, its components (electrons, protons, and neutrons) do not retain the properties of the element.
* The word atom is derived from the Greek word
"atomos" which means uncuttable/indivisible.
Atoms are extremely small measuring about 1 x
10-10 meters in diameter. Because of their small size, it's impossible to view
them using a light microscope. While it may not be possible to view an atom
using a light microscope, a number of techniques have been developed to observe
and study the structure of atoms.
Some of these techniques include:
Recently, researchers have been working on
improving electron microscopes to be able to penetrate down to the subatomic
level in order to observe electrons.
According to one of the studies in Vienna
University of Technology, researchers working on energy-filtered transmission electron
microscopy (EFTEM) found out that under given conditions, it is actually
possible to view images of individual electrons in their orbit.
As well, a
new electron microscope (Nion Hermes Scanning Transmission Electron Microscope)
was unveiled in the UK and is capable of resolutions down to the atomic level
and thus capable of capturing images of individual atoms. According to the
researchers, the microscope is capable of imaging objects that are a million
times smaller than human hair.
Some of the techniques that have found
success so far include STEM techniques.
This technique has been used to observe
interfacial atoms that are located between metal nanoparticles and supports. In
2015, a group of researchers used STEM Depth sectioning to directly observe
gold atoms on titanium dioxide. This procedure was chosen due to the fact that
on Titanium dioxide, gold tends to shown high catalytic activity.
involved the following steps:
- Precipitation of gold on Titania
support in order to prepare the gold catalyst.
- Calcination of the
preparation in air (or reduction) under hydrogen (H2) at high
Once the preparation was ready, the sample was
observed using the aberration corrected STEM microscope (with installed JEOL
This technique was capable of observing the atoms, which were brighter
than the Titanium atoms. By recording the focal series of Z-contrast images from
the fold nanocrystal at the interfacial regions, the researchers were able to
locate the atoms in 3D.
ABF_STEM (Annular Bright-Field Scanning Transmission Electron Microscopy)
While researchers had experienced some
challenges, it became evident that using this technique, it was possible to
observe lithium atoms (lithium with atomic number 3). This was achieved by
observing such material as Lithium manganese oxide (LiMn2O4) in order to view
By viewing this compound/crystal under the microscope, the
researchers could identify the different atoms (Li, Mn, and O) and thus
identify the lithium atoms on their own. However, this was only possible when
using the STEM microscope with a resolution of 0.1 nm or lower with corrected
* Recently, a student from
the University of Oxford captured the image of a single, floating atom
(strontium atom) using typical camera.
Main parts of the scanning tunnelling microscope:
- Sharp metallic tip - This is the part that is
brought close to the sample (conductor)
- Scanning control, distance
Controls the distance between the tip and sample surface and regulates scanning
- Computer for data
processing and display - Output through which information is relayed. The computer also
controls the piezoelectric tube.
- A piezoelectric controlled
probe - During
operation, the Piezoelectric will contract and expand with varying voltage,
which in turn controls both the horizontal and height positions of the scanning
The scanning tunneling is one of the techniques
that was developed in the earl 1980s in Switzerland by Gerd Binnig and Heinrich
Essentially, this technique works by passing an electronic wave over the
surface of the sample (element). By passing the wave of electrons on the
surface of the sample, it becomes possible to position and thus detect the
The scanning tunneling microscope has a
small sharp/pointed metal tip that is brought very close to the surface of the
sample. Here, the distance between the pointed metal tip and the sample is very
close that they almost come in contact (about 1 nm).
With the tip very close
the surface of the sample, the two are put under a small voltage, which allows
a tunneling current to flow. With current flowing between the two, the surface
is scanned to reveal a three dimensional picture of the surface, and thus the
general appearance of atoms on the surface of the sample.
With this technique, electrons may also flow
only from the tip of the pointed metal to the sample or from the sample to the
tip. As the current originates from the metal tip, the scanner moves it (the
tip) rapidly across the surface of the sample.
Once the metal tip locates an
atom at the surface of the sample, electrons flow between the two changes while
the computer registers the change. This change is recorded in the x-y position,
as the tip continues moving and identifying more points of atom locations that
are then registered.
These points on the surface represent the presence of the
atoms that can then be scanned and viewed. This in turn makes it possible to
identify their structure.
What is Electron Tunneling?
From quantum physics, electrons should not be
able to pass through given barriers (such as air). However, when they are able
to pass through such barriers, the electrons produce what is refered to as a
tunneling current. This makes it possible to observe various materials at the
By bringing the metallic tip of the scanning tunneling
microscope very close to the surface of the sample material (conductor), a
small gap that contains is left between the tip and the material surface.
However, electrons are able to tunnel through the gap producing an electric
current that can be detected and measured.
As the metallic tip is passed across
the surface of the sample material, the current produced will vary depending on
the peaks and valleys of the surface (surface profile) which allows for
individual atoms to be located.
* Unlike the light microscope, the scanning
tunneling microscope relies on electrons to locate and position atoms
* Rather than behaving as particles, electrons in
this technique behave like a wave, which allows them to pass through the
With the scanning tunneling microscope, there
are two main modes of operation used when studying the surface of the sample
material. This includes the constant current mode and the constant height mode.
Constant Current Mode
already mentioned, the amount of current between the metallic sharp tip and the
sample surface varies depending on the surface profile (peaks and depths) if
the distance between the tip and the surface is more, then there is little
current. However, a short distance between the two will result in more/high
In the constant current mode, the current level is kept at a constant
by moving the tip up and down as it moves across the surface of the sample to
retain the same height. Given that the contours across the sample surface
change, adjusting the tip by moving up and down allows for the current to
With this technique, atoms can be located and positioned by
recording the adjustment of the metallic tip (as it vertically moves up and
Constant Height Mode
this mode of operation, the height of the tip does not change as it moves
across the surface of the sample. As a result, only the current changes
depending on the contours of the sample surface.
For this technique, atoms are
located and positioned by the recording of the changing current.
Atomic force microscopy is also a type of
scanning probe microscope that works by recording such properties as height,
magnetism and friction.
By measuring these properties using a probe, it becomes
possible to get the image of a given surface area. This technique was developed
with the aim of improving the limitations of scanning tunneling microscope
given that atomic force microscope is capable of studying such non-conductive
materials as proteins (the scanning tunneling microscope is only used to
investigate conductive material).
Main parts of the AFM
- Sharp tip (Probe) - The
sharp tip or AFM (atomic force microscope) probe moves over the surface of the
sample for scanning
- Optical lever - The optical
lever allows for measurements to be made by measuring deflections of the
- The piezoelectric scanner -
This part serves to move the sharp tip across the surface of the sample
- Cantilever - This is the
soft girder on which the tip is attached
The atomic force microscope probe (made through
micro-fabrication) is very sensitive and is the part that comes in contact with
As the tip moves across the surface of the sample, it senses its contours. As such, it does not rely on electrons or light to view
the sample surface. This has been shown to be one of the biggest strengths of
this technique, allowing for higher resolution and efficiency.
When the AFM tip approaches the sample surface,
attractive force between the surface of the sample and the tip results in the
cantilever bending towards the surface of the sample. However, as the tip comes
closer to the proximity of the sample, deflection results from repulsive forces
causing the cantilever to bend away from the sample surface (this is why the
cantilever has to be very soft and flexible).
Whereas the z-scanner moves the
cantilever up and down, the x-y scanner moves the sample back and forth. These
movements make it possible to scan the entire surface area of the sample. And so, a position detector/sensor (optical lever) in place records the
bending of the cantilever.
The position sensor records the beam changes that
are reflected off the top of the cantilever. As the cantilever moves, there are
also changes in the beams, which are all recorded. With all these changes, the
topography of the sample surface is recorded to give an accurate
* A laser diode produces a laser beam, which is
reflected off the flat back of the cantilever and on to the photo detector
(position detector) As the sharp tip moves across the surface, it causes the
cantilever to move, which in turn causes changes in the deflected beam. This is
then detected as varying light intensity.
The movement of the AFM tip is typically
controlled by a scanner that is made up of piezoelectric material and thus the piezoelectric
scanner - This type of material (piezoelectric material) is largely preferred
for both AFM and STM due to the fact that they move the tip in a very precise
manner along the x, y, z axes.
For such small displacements as the tip moves
across the sample surface, this material allows for very good reproducibility.
Modes of Operation
In contact mode or contact AFM, the probe/tip
comes in contact with the sample surface and slightly dragged across the
contours of the surface. As the probe moves across the surface while in
contact, it causes deflections of the cantilever, which in turn allows for the
surface to be scanned using laser beams.
While this method has been shown to
have the advantage of being easy to use due to its simple set up, it has
several disadvantages including damaging the surface of the sample as well as
the probe itself. In particular, "dragging" the tip across the
surface causes it to be gouged, which in turn may affect the quality of the
Here, it is worth noting that in some cases, the
sample surface is scratched intentionally. For instance, some researchers will
scratch the sample surface using the contact mode in order to deposit other
samples in the scratched region. This is particularly the case with some form
of electroplating. The technique is also used for measuring friction at the nanoscale. This largely involves scratching the surface by
dragging the cantilever tip on the sample surface.
known as dynamic force microscopy (DFM) non-contact atomic force microscopy
involves passing the probe very close to the sample surface without really
dragging it on the sample surface.
Here, the cantilever oscillates just above
the surface as scanning takes place. A precise high speed loop ensures that the
cantilever, and thus the tip does not crash onto the surface of the sample.
With the tip being close to the surface, van der Waal forces that result decreases the cantilever's resonance frequency, which together with the
feedback loop ensures that a constant oscillation is maintained.
As the tip
oscillates and moves across the surface of the sample, scanning allows for a
3-D image of the surface to be constructed.
This technique has a big advantage in that the
sharpness of the tip is maintained while the sample remains undamaged. Given
that the tip is protected from damage, it can be used over and over again while
providing quality images of the sample surface.
tapping mode involves having the tip of the cantilever touch the sample surface
only for a short period to time. This technique helps prevent problems
associated with lateral force and dragging that occurs across the surface of
The cantilever tip oscillates at a higher amplitude (20-100 nm),
which in turn makes the deflection signal large enough for the control circuit.
This technique is largely used to scan the surface of damaged samples where it
reduces high resolution.
Atoms are composed of a nucleus (containing
protons and neutrons) and electrons that surround the nucleus. Whereas protons
have a +1 charge, electrons have a -1 charge. For all atoms, the atomic number
is the number of protons while the arrangement of electrons gives the
electronic structure of the atom.
* Unlike other elements, hydrogen does not have
Return from Atom under the Microscope to MicroscopeMaster Home
Wenpei Gao, Shankar Sivaramakrishnan, Jianguo
Wen, Jian-Min Zuo (2015) Direct Observation of Interfacial Au atoms Using STEM
IFM – The Department of Physics, Chemistry and
Biology. Scanning Tunneling Microscopy.