Afm

Atomic Force Microscopy, presentation

1. Principle, Instrumentation and Applications



Prepared by: -
Mulugeta Abera,




Summited to: -
Dr. Sathiesh Kumar




February 8, 2021
CHARACTERIZATION TECHNIQUES, MSE6105
Material Science and Engineering,

A S SIGNEMENT

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2. Atomic Force Microscopy (AFM). Principle, Instrumentation and Applications
By - Mulugeta Abera
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History
The atomic force microscope (AFM) was invented in the 1980s by Gerd Binnig and Heinrich
Rohrer. It was developed to overcome a basic drawback with STM that it can only image
conducting or semiconducting surfaces. The AFM has the advantage of imaging almost any type
of surface, including polymers, ceramics, composites, glass, and biological samples. AFMs are
capable of generating topological maps of material surfaces on an atomic scale. As such, it
promises breakthroughs in areas such as material science, nanoparticle characterization, bio-
nanotechnology, nano-indentation for high-density data storage systems and nanomachining.
In general, AFM is a powerful technique that enables the imaging of almost any type of surface,
including polymers, ceramics, composites, glass and biological samples. It is also used to measure
and localize many different forces, including adhesion strength, magnetic forces and mechanical
properties.

1. PRINCIPLE
1.1 General
In general, AFM works based on the principle of inter atomic forces (attractive or repulsive forces
at the atomic level). It is dramatically different from other available forms of microscopy, as it
doesn’t need a light source, electron beam, or lenses to generate an image. Furthermore, can
produce three-dimensional maps of material surfaces at extremely high resolutions.
The AFM consists of a cantilever with a sharp tip (probe) at its end that is used to scan the specimen
surface. The cantilever is typically silicon or silicon nitride with a tip radius of curvature on the
order of nanometers. When the tip is brought into proximity of a sample surface, forces between
the tip and the sample lead to a deflection of the cantilever according to Hooke's law. Figure (1)
shows the basic concept of AFM. Depending on the situation, forces that are measured in AFM
include mechanical contact force, van der Waals forces, capillary forces, chemical bonding,
electrostatic forces (EFM), magnetic forces (MFM), Casimir forces, solvation forces, etc. Along
with force, additional quantities may simultaneously be measured through the use of specialized
types of probes.

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Figure (1) Shows the basic concept of AFM
1.2 Imaging modes
AFM can be operated in a number of modes, depending on the application. As a principle, the
possible imaging modes are divided into static (also called contact) modes and a variety of dynamic
(non-contact or "tapping") modes where the cantilever is vibrated or oscillated at a given frequency

Figure (2) Operational modes

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When operated in contact mode, the microcantilever is brought in contact with the sample. The
distance between the tip and the sample is very less thus the cantilever is repelled due to the
repulsion of atoms. The sample is moved in a raster pattern by actuating the scanner. This causes
the cantilever to deflect due to the variations in the surface topology of the sample. This deflection
in turn causes a variation in the intensity of the reflected beam captured by the PSD. Based on the
changes recorded by the PSD, an image of the sample surface is generated by the AFM circuitry
and software. In this mode the resolution is high, due to contact both have high chance of getting
damaged. When operated in noncontact mode, the probe is brought into close proximity within a
few nanometers of the sample, thus the cantilever is attracted. The microcantilever is deliberately
vibrated at a particular frequency. Changes in the vibration amplitude or frequency are used to
detect the surface structure of the sample. In this mode the resolution is low and there is no damage
for the tip and the sample. In tapping mode, in ambient conditions most samples develop a liquid
meniscus layer, because of this tip stick to the surface and to prevent this dynamic contact mode
is used when operating in ambient condition or in liquids.The distance between the tip and the
sample is kept intermediate and thus the cantilever is kept oscillating. In this mode the resolution
is better when compare to non-contact mode.

Figure (3) Interaction force verses distance between AFM tip and Sample

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2. Instrumentation
The motion of the probe across the surface is controlled similar to the STM using feedback loop
and piezoelectronic scanners. The piezoelectric transducer moves the tip over the sample surface,
the force transducer senses the force between the tip and the surface and the feedback control feeds
the signal from the transducer back in to the piezoelectric to maintain a fixed force between the tip
and sample.

Figure (4) Block diagram showing the components in an AFM.
2.1 Piezoelectric Transducers (PT)
Piezoelectric materials are electromechanical transducers that convert electrical potential into
mechanical motion. When a potential is applied across two opposite sides of the piezoelectric, it
changes geometry. The magnitude of the dimensional changes depends on the material, the
geometry of the device, and the magnitude of the applied voltage.

Figure (5) Piezoelectric disk expand radially when a voltage potential is applied to the top and bottom
electrode

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2.2 Force Transducers (FT)
The force between a probe and a surface is measured with a force transducer. As illustrated in
Figure (6), when the probe comes into contact with the surface, the voltage output from the
transducer increases. It is important that the output of the transducer be monotonic and increase as
a greater force is applied between the probe and surface.

Figure (6) FT output an electronic signal when the probe interacts with the surface.
2.2.1 Tip-sample interaction forces
 Long-range electrostatic and magnetic forces (upto 100 nm)
 Capillary forces (few nm)
 Van der Waals forces (few nm) that are fundamentally quantum mechanical
(electrodynamic) in nature
 Casimir forces
 Short-range chemical forces (fraction of nm)
 Contact forces
 Electrostastic double-layer forces, Magnetic forces
 Solvation forces
 Nonconservative forces (Dürig (2003))
2.3 Feedback control
Feedback control is used in AFM for maintaining a fixed relationship, or force, between the probe
and the surface. The feedback control operates by measuring the force between the surface and
probe, then controlling the Z piezoelectric ceramic that establishes the relative position of the probe
and surface.
Atomic Force Microscopy has a feedback loop using the laser deflection to control the force and
tip position. As shown, a laser is reflected from the back of a cantilever that includes the AFM tip.

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As the tip interacts with the surface, the laser position on the photodetector is used in the feedback
loop to track the surface for imaging and measuring.
2.4 Beam deflection method
The deflection of the probe is typically measure by a “beam bounce” method. A semiconductor
diode laser is bounced off the back of the cantilever onto a position sensitive photodiode detector.
This detector measures the bending of cantilever during the tip is scanned over the sample. The
measured cantilever deflections are used to generate a map of the surface topography.

Figure (7) Beam deflection
3. Application
AFM has been applied to problems in a wide range of disciplines of the natural sciences, including
solid-state physics, semiconductor science and technology, molecular engineering, polymer
chemistry and physics, surface chemistry, molecular biology, cell biology and medicine.
It gives information about the toughness, roughness and smoothness values of the surface.
Application in the field of solid-state physics include
 Identification of atoms at a surface
 Elevation of interactions between a specific atom and its neighboring atoms
 Study of changes in physical properties arising from changes in an atomic arrangement
through atomic manipulation

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In molecular biology, study the structure and mechanical properties of protein complexes and
assemblies
 Image microtubules
 Measure their stiffness
In cellular biology, AFM attempt to distinguish cancer cells and normal cells based on a hardness
of cells and to evaluate interactions between a specific cell and its neighboring cells in a
competitive culture system.
Soft surface are analyzed by this technique without damaging it like liquid and covalent bond
strength is measured by this technique as well.
In general AFM used in material investigation such as thin and thick film, coatings, ceramics,
composites, glasses, synthetic and biological membranes, metals, polymers and semiconductors.
It uses to study phenomena of, abrasion, corrosion, etching (scratch), friction, lubricationg, plating
and polishing. And also AFM can image surface of material in atomic resolution and also measure
force at the nano-newton scale.

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Advantage of AFM
 Easy sample preparation
 Works in vacuum, air and liquid
 Accurate height information
 Living systems can be studied
 Surface roughness of the sample can be quantified
 3D image facilities
Disadvantage if AFM
 Limited vertical range
 Limited magnification range
 Data is dependent of the tip
 Damage of tip and sample is possible
 Limited scanning speed

References
1. G. Binnig, C.F. Quate, C.H. Gerber, “Atomic force microscope.” Phys. Rev. Lett. 56, 930–
933 (1986)
2. M. Brogly, H. Awada, O. Noel, “Contact atomic force microscopy: a powerful tool in
adhesion science, in Nanoscience and Technology.” Applied Scanning Probe Methods, vol.
XI (Springer, Berlin, 2009), p. 73
3. Paul E. West, (2014). “Introduction to Atomic Force Microscopy Theory Practice
Applications,”
4. Yangjie Wei, Zaili Dong, Chengdong Wu “Method for Simultaneously Obtaining Surface
Elasticity Image and Accurate Height Image Using Contact Mode AFM” IEEE
Transactions on Nanotechnology · November 2011
5. Qun (Allen) Gu, Ph.D., AFM Scientist, Pacific Nanotechnology “Today’s SPM in
Nanotechnology An introduction for Advanced Applications” IEEE Bay Area
Nanotechnology Council, August, 2007

10. Atomic Force Microscopy (AFM). Principle, Instrumentation and Applications
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6. Guanghong Zeng, Yusheng Duan, Flemming Besenbacher and Mingdong Dong
”Nanomechanics of Amyloid Materials Studied by Atomic Force Microscopy”
Interdisciplinary Nanoscience Center (iNANO), Aarhus University Denmark, may 2014.
7. Q. Jane Wang, Yip-Wah Chung “Atomic Force Microscopy (AFM)” Department of
Mechanical Engineering-Engineering Mechanics, Michigan Technological University,
Houghton, USA, 2013.
8. J. Adams, L. Hector, D. Siegel, H. Yuand, J. Zhong, “Adhesion, lubrication and wear on
the atomic scale.” Surf. Interface Anal. 31, 619–626 (2001)