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Atomic force microscopy (AFM)
Introduction
Atomic force microscopy (AFM) is a technique to obtain images and other information from a
wide variety of samples, at extremely high (nanometer) resolution
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10 nm) probe along the sample surface, carefully maintaining the force
between the probe and surface at a set, low level
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The laser is reflected by the cantilever onto a distant photo detector
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This set-up is known as an optical lever
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The
combination of the sharp tip, the very sensitive optical lever, and the highly precise movements
by the scanner, combined with the careful control of probe-sample forces allow the extremely
high resolution of AFM
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The Scanning Tunnelling Microscope was itself very
similar to an earlier instrument called the Topografiner, invented by Young
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However, both techniques were limited in what samples could be analyzed
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Commercial
instruments have been available since 1988, and since then there have been many developments
and new modes of operation devised for AFM
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A variety of probes have

been used but the most commonly used are micro-fabricated silicon (Si) or silicon nitride (Si3N4)
cantilevers with integrated tips
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Typically, probe radius varies from 5 to 20 nm
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This system magnifies the normal bending of the cantilever greatly, and is sensitive
to Angstrom-level movements
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The movement
of the probe over the surface is controlled by a scanner
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The signal from the photo detector passes through a
feedback circuit, and into the z-movement part of the scanner, in order to maintain the probesample distance at a set value (Figure 1)
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The amount by which the
scanner has to move in the z-axis to maintain the cantilever deflection is taken to be equivalent to
the sample topography
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A great advantage of AFM compared to for example TEM or SEM, is that it is simple to operate
in almost any environment, such as aqueous solutions, but also other solvents, in air, vacuum, or
other gases
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Z
(height) resolution is extremely high and can be sub -Angstrom, whereas lateral resolution could

be of the order of 1 nm
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Operating modes
The most commonly used modes of operation of an AFM are:


Contact mode AFM (C-AFM or CMAFM)
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

Noncontact mode (NC-AFM, close contact AFM or FM-AFM)
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Typically this is done by adding an
extra piezoelectric element that oscillates up and down at somewhere between 5-400 kHz to the
cantilever holder
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g
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Some authors prefer to refer to
the modes by their detection mechanisms, Tapping mode would then be called Amplitude
Modulation AFM (AM-AFM), and noncontact mode would be Frequency Modulation AFM
(FM-AFM)
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In NC-AFM, the cantilever stays close to the sample all the
times, and has much smaller oscillation amplitude
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Noncontact AFM, unlike the other AFM techniques can obtain true atomic resolution
images
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This is often the most stable mode to use in air, and so is currently more
commonly used than either noncontact or contact modes for most applications
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For example:


Magnetic Force Microscopy, MFM, measures the distribution of magnetic field in the
sample
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

Force Spectroscopy can measure individual molecular interactions
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

Thermal modes can measure thermal parameters, for example, thermal conductivity on
the nanoscale
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Dimension Icon AFM
system (Bruker) delivers uncompromised performance, robustness, and flexibility to perform
nearly every measurement at scales previously obtained by extensively customized systems
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With more features than any other AFM, the new NanoScope-based control system sets
the standard for power and flexibility
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The Dimension Icon
AFM system and overall instrument view are shown below in (Figure 2 a, b)
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AFM images
The thicknesses of gold, silver single and double layer films were measured by Atomic Force
Microscope (AFM)
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Figure 3 Thickness measured by AFM (a) single layer of Au 30, 45 & 50 nm

AFM characterization
The roughness of Ti and Mo thin films were measured by Atomic Force Microscope (AFM)
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Silicon surface
found to be smoother compared glass
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Figure 4 AFM morphology of Ti and Mo thin films on glass and silicon substrates
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Single layer
Ag: 30nm, 45nm, 50nm and 70nm
Au: 30nm, 45nm, 50nm and 70nm
Double layers
Ag+Au: 30+30nm, 45+5nm and 45+10nm
Au+Ag: 30+30nm, 45+5nm and 45+10nm
Figure 5 (a, b, c and d) illustrates different thickness combinations of metal thin films
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