Nanometrology

The word nanometrology has been derived from the Greek word “nanos'' which means one
billionth and “metrologia” means a theory of ratios. In other words, nanometrology can be
defined as the subfield of metrology concerned with science of measurement at the nanoscale
dimension. Nanometrological measurements are associated with measuring at a nanoscale
dimension (shape, aspect ratio, and size distribution) and the assessment of nanoparticle
concentrations, chemical composition, and determination of electrical, optical, mass, force,
and various other properties of nanomaterials. Nanometrology deals with the measurement of
dimensions within nanomaterials and devices.
Various techniques such as Scanning Tunnelling Microscopy (STM), near field optical
microscopy, Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), and
Transmission Electron Microscopy (TEM) are used for visual analysis of surfaces of
materials at nanoscale level.
Importance of Nanometrology
The progress of nanotechnology is strongly associated with advancements in nanometrology.
Nanotechnology could not have received its due popularity without the existence of
nanometrology. For the development of highly accurate and reliable nanodevices, the
characterization of nanomaterials, i.e., evaluation of their size, shape, physical, optical, and
other properties are extremely crucial. The foremost important feature of nanomaterials is
their nanoscale dimension and unique properties. The understanding of nanostructures, i.e.,
the arrangement of atoms or particles, is also important for research. For exploring new
applications of nanomaterials, a thorough characterization and understanding of the
nanomaterials are vital. Nanometrology has innumerable applications in scientific research,
especially, biological, medical, and environmental science. This has been made possible by
advancements in techniques such as Scanning Probe Techniques (SPT), Electron Beam
Techniques (EBM), and others. Some of the techniques associated with nanometrology that
are used in scientific applications are –
Scanning Electron Microscope (SEM)
Scanning electron microscope (SEM) is a type of electron microscope that produces images
of a sample by scanning the surface with a focused beam of electrons. The electrons interact
with atoms in the sample, producing various signals that contain information about the
surface topography and composition of the sample.
Working
SEM produces a largely magnified image by using electrons instead of light to form an
image. A beam of electrons is produced at the top of the microscope by an electron gun.
Electron gun produces a beam of electron beam and is accelerated towards a specimen which
follows a vertical path and is held within a vacuum. The beam travels through a anode
(positive potential) which attracts the beam and is provided with a hole so that electrons can
pass through it. Magnetic lens is used which focuses the electron to a scanning coil. Scanning
coil deflects a beam to a particular direction. Electron beam is focused on a desired spot on
the sample using objective lens. Once the beam hits the sample, they eject secondary and
backscattered electrons (BSE), as well as x-rays from the sample. Detectors collect these
backscattered electrons, and secondary electrons as well as x-rays and convert them into a
signal that is sent to a screen similar to a television screen. This produces the final image.

Applications
1. Scanning electron microscope is widely used in energy-Dispersive X-ray Spectroscopy for
spot chemical analysis.
2. It is prominently employed in biology laboratories to study the internal structures of
microorganisms at the cellular level.
3. A scanning electron microscope has multiple applications in industries. It can be used to
study the surface of solid objects and analyse the distribution of atoms in various elements.
4. Scanning electron microscope is also used in qualitative chemical analysis of elements by
proving a clear and magnified image of the crystalline structures.
5. It can be used to distinguish different phases of a multiphase sample.
6. Some of the scanning electron microscopes are equipped with diffracted backscattered
electron detectors, which helps to examine and determine the micro fabric and
crystallographic orientation of substances.
Advantages of the Scanning Electron Microscope
A scanning electron microscope is quite advantageous over the other microscopes. Some of
these advantages are listed below:
1. Scanning electron microscopes are user friendly and easy to use.
2. They can produce and generate results in digital format.
3. Scanning electron microscopes are able to provide quick results, i.e., data can be obtained
within a few minutes.
4. A scanning electron microscope requires minimum sample preparation.
5. The resolution of scanning electron microscopes is significantly high.
Disadvantages of the Scanning Electron Microscope
1. Scanning electron microscopes are comparatively expensive.
2. The room must be free of vibrations and electromagnetic radiation.
3. Scanning electron microscopes have a bulky structure.
4. A consistent voltage level must be maintained for the proper operation of a scanning
electron microscope. This may require additional electronic circuitry or voltage regulators to
fix the voltage magnitude to a constant value..
5. The sample to be examined with the help of a scanning electron microscope needs to be
solid in nature. Wet samples are unsuitable and are required to be decrepitated first.
6. A scanning electron microscope cannot be used for light materials such as hydrogen,
helium, lithium, etc.
Transmission electron microscope (TEM)
A transmission electron microscope (TEM) is one of the most powerful microscopes with
respect to its magnification and resolution. TEM is the technique where an electron beam is
transmitted through thin specimen which interacts with the specimen as the electron passes
through it. The two important features of the TEM are its high lateral spatial resolution and
its capability to provide both the image and the diffraction of the sample. This technique is
used to obtain the full morphological, crystallographic, atomic structural, micro analytical
(chemical composition, bonding), electronic structure.
Components of TEM
 Vacuum system
 Specimen stage
 Electron gun
 Electron lens

The TEM works with a high voltage electron beam to create the image, where the electron
gun is placed at the top which produces the electron beam that travels through the anode in a
vacuum tube. The condenser lens is used to focus the fine beam of electrons that passes
through the thin specimen. The specimen scatters the electrons focusing them on the
objective and intermediate forming a large and clear image. The image is magnified with the
help of a projector lens and image is seen on the fluorescent screen which is present at the
bottom of the microscope. The obtained image can be directly studied or recorded using
digital cameras.
Applications of Transmission Electron Microscope (TEM)
TEM is used in a wide variety of fields From Biology, Microbiology, Nanotechnology,
forensic studies, etc. Some of these applications include:
1. To visualize and study cell structures of bacteria, viruses, and fungi.
2. To view the shapes and sizes of microbial cell organelles.
3. To study and differentiate between plant and animal cells.
4. It is also used in nanotechnology to study nanoparticles such as ZnO nanoparticles.
5. It is used to detect and identify fractures, damaged microparticles which further
enable repair mechanisms of the particles.
Advantages of Transmission Electron Microsc.ope (TEM)
1. It has a very powerful magnification of about 2 million times that of the Light
microscope.
2. It can be used for a variety of applications ranging from basic Biology to
Nanotechnology, to education and industrial uses.
3. It can be used to acquire vast information on compounds and their structures.
4. It produces very efficient, high-quality images with high clarity.
5. It can produce permanent images.

Disadvantages of Transmission Electron Microscope (TEM)

1. Generally, the TEMs are very expensive to purchase.
2. They are very big to handle.
3. The preparation of specimens to be viewed under the TEM is very tedious.
4. They are extremely sensitive to vibrations and electro-magnetic movements hence they
are used in isolated areas, where they are not exposed.
5. It produces monochromatic images, unless they use a fluorescent screen at the end of
visualization.

Atomic Force Microscope (AFM)

Atomic-force microscopy (AFM) is a surface scanning technique that has sub-nano meter
scale resolution. AFM describes a group of techniques used for non-destructive surface
studies at the nano scale. They have a resolution on the order of 103 times better than optical
microscopy’s resolution limit. AFM is used widely to collect data on various mechanical,
functional and electrical properties at the nano scale as well as for topography (surface)
studies. It is high resolution scanning type probe microscope.

Working
An AFM consists of four key components, the first being a cantilever with a sharp tip. The
back of the cantilever is coated with a reflective material so that it reflects light like a mirror.
A laser is directed to the back of the cantilever and the reflected light is then collected by a
photo-detector, which is very sensitive to the positions of the laser beam. The cantilever,
laser, and detector system are precisely scanned by an electric controller. The sample, can
also be moved forward. The cantilever is used as a force sensor. When the tip of the
cantilever is scanned across a sample, the tip acts like an elastic. Depending on the amount of
force between the tip and the sample, the cantilever compresses and stretches. The photo-
detector records the changes to the reflected laser beam position proportional to the
movement of the cantilever. A detailed topographical image of the sample is captured by
scanning detectors across the surface of the material.

Advantages
1) AFM provides a three-dimensional surface profile.
2) AFM do not require any special treatments (such as metal/carbon coatings) that would
irreversibly change or damage the sample,
3) AFM can provide higher resolution than SEM.
4) AFM can also be combined with a variety of optical microscopy and spectroscopy
techniques expanding its applicability.
Disadvantages
1) The relatively slow rate of scanning during AFM imaging often leads to thermal drift
in the image.
2) AFM can only image a maximum scanning area of about 150×150 micrometers and a
maximum height on the order of 10–20 micrometers.
3) Tip or sample can be damaged.
Applications-
 Semiconductor science and technology
 Thin film and coatings
 Tribology (surface and friction interactions)
 Surface chemistry
 Polymer chemistry and physics
 Cell biology
 Molecular biology
 Energy storage (battery) and energy generation (photovoltaic) materials
 Piezoelectric and ferroelectric material

X-Ray Diffraction (XRD)

X-ray diffraction is a technique that is used for the determination of a crystallographic
structure, chemical composition, and can provide information on unit cell dimensions of a
material. It is based on the constructive interference of monochromatic X-rays and a
crystalline sample. If the crystal size is too small, it can determine the atomic and molecular
structure of a crystal. A crystal may be defined as a solid composed of atoms, ions or
molecules arranged in a pattern periodic in three dimensions.

The atomic planes of a crystal cause an incident beam of X-rays to interfere with one another
as they leave the crystal. The phenomenon is called X-ray diffraction. X-rays are used to
produce the diffraction pattern because their wavelength, λ, is often the same order of
magnitude as the spacing, d, between the crystal planes.
This technique sends x-ray beams through it. The x-rays then pass through the sample,
“bouncing” off of the atoms in the structure, and changing the direction of the beam at some
different angle, theta, from the original beam. This phenomenon is known as elastic
scattering; the electron is known as the scatterer. The interaction scatters the incident X-ray
beam in many directions. In the majority of directions, these waves cancel each other out
through destructive interference, however, they add constructively in a few specific directions
which principally satisfies Bragg's Law. Mathematically the Bragg’s law is 2d sin θ= nλ
 where lambda is the wavelength added, theta is the angle of diffraction, and d is the distance
between atomic planes and n is an integer. The distance between atomic plates can then be
used to determine composition or crystalline structure.

Bragg’s law states that when the X-ray is incident onto a crystal surface, its angle of
incidence, θ, will reflect with the same angle of scattering, θ and, when the path difference, d
is equal to a whole number, n, of wavelength, λ, constructive interference will occur.

Bragg’s law diagram

When the geometry of the incident X-rays impinging the sample satisfies Bragg’s law,
constructive interference occurs and a peak in intensity appears. A detector records and
processes this X-ray signal and converts the signal to a count rate, which is then output to a
device such as a printer or a computer monitor.

Representation of XRD working principle

Applications
1. Measurement of sample purity.
2. Determination of unit cell dimensions.
3. Characterization of crystalline materials and determine structural properties including: Lattice
parameters- Strain- Grain size- Epitaxy- Phase composition- Preferred orientation.
4. Characterize thin films samples and measure the thickness of thin films and multi-layers.
5. Determine atomic arrangement.
6. Identification of fine-grained minerals such as clays and mixed layer clays that are difficult to
determine optically.

Advantages
1.X rays are the least expensive, most convenient, and the most widely used method to
determine crystallographic structure.
2.It only requires preparation of a minimal sample for analysis.
3.X rays are not absorbed very much by air, so the sample need not be in an evacuated
chamber.
4.XRD measurement instruments are widely available.
5.It is a non destructive technique.
6.It is best method for phase analysis.
Limitations-
1) X rays do not interact very strongly with light elements.
2) For unit cell determination, indexing of patterns for non-isometric crystal systems is
complicated.
3) It cannot identify the amorphous materials and does not gives information on profile
depth.