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Scanning
probe microscope (SPM):

An SPM is an instrument which is used for studying surfaces at a
nanoscale level. They create images of surfaces with the use of a physical
probe that touches the surface of a sample in order to scan the surface and
collect data, they’re typically obtained as a two dimensional grid of data
points and are displayed as a computer image.

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The tracing of the surface of a specimen is carried out through
the use of a sharp, electrically charged probe. The SPM probe does not touch
the surface but instead traces the specimen nanometres above the surface level.
The probe can be used to interact with a specimen enabling researchers to
examine how a substance attracts or detracts and also how it responds to
electrical currents. Seeing as SPM technology can operate in a various amount
of environments, even that of non-conductive specimens can be observed and
manipulated.

An SPM includes a probe tip which is mounted on the end of a
cantilever. The tip is sharp and can be as sharp as a single atom. It can be
moved precisely and accurately back and forth across the surface, even atom by
atom. When the tip is near the sample surface, the cantilever is deflected by a
force. SPMs can measure deflections caused by many kinds of forces, including
mechanical contact, electrostatic forces, magnetic forces, chemical bonding,
van der Waals forces, and capillary forces. The distance of the deflection is
measured by a laser that is reflected off the top of the cantilever and into an
array of photodiodes. SPMs can detect differences in height that are a fraction
of a nanometer, about the diameter of a single atom. The tip is moved across
the sample multiple times. This is why they are known as ‘scanning’
microscopes. A computer combines the data to create an image. The images are mostly
colourless because they measure properties other than the reflection of light.
However, the images are often colourised, with different colours which
represent various properties for example height along the surface. Scientists
use SPMs in a number of different ways, depending on the information they’re
trying to gather from a sample. The two primary modes are contact mode and
tapping mode. In contact mode, the force between the tip and the surface is
kept constant. This allows a scientist to rapidly image a surface. In tapping
mode, the cantilever oscillates, intermittently touching the surface. The
tapping mode is especially useful when imaging a soft surface.

There are multiple types of SPMs. There is the Atomic force
microscope (AFM) which measures the electrostatic forces between the cantilever
tip and the sample. The Magnetic force microscope (MFM) which measures magnetic
forces. The Scanning tunnelling microscope (STM) which measures the electrical
current flowing between the cantilever’s tip and the sample.

 

 

 

 

What are
the advantages of SPM technology?

SPM technology provides researchers with a larger variety of
specimen observation environments using the same microscope and the same
specimen, in turn reducing the time required to prepare and study specimens.
Specialised probes, improvements and modifications to scanning probe equipment
continues to provide faster, more efficient, revealing specimen images with
little effort and modification.

What are
the disadvantages of SPM technology?

One of the disadvantages of a scanning probe microscope is that
the images in which they form in black, white or greyscale can in some
instances exaggerate a specimen’s shape or size. Computers are used to
compensate for these exaggerations and produce real time colour images which
facilitate researchers with information in real time including interactions
within cellular structures, harmonic responses and magnetic energy.

As researchers continue with improvement of techniques and
expanding the abilities of scanning probe microscopes, the technological
evolution will include better observation instruments, better quality data
analysis, and better processing equipment. Also, micro-manipulation of
molecules, DNA, biological and organic specimens incorporating this precision
equipment will produce a better understanding of and new methods for:

· Treating disease

· Manufacturing

· Astronomy

· Physics

· Energy

The evolution of SPMs has resulted in both scientists and
engineers being able to observe structure and detail with unprecedented
resolution, without the need for rigorous sample preparations.

Technical advances along with the development of sophisticated
techniques, have undoubtedly extended the capabilities of SPMs, in particular
AFMS, across a broad range of research into both materials and life sciences.

The Atomic
Force Microscope:

The Atomic Force Microscope (AFM) is a form of scanning probe
microscopy (SPM) involving the detection of interatomic forces that happen
between a probe tip and a sample. 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 and is used to scan across a
sample in an effort to obtain information about its surface (topography). The
tip is integrated into a cantilever which moves up and down tracing the
interaction of the surface of a sample. 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. The probe is scanned in a similar
pattern to the SEM where it moves in a raster pattern across the sample in
order to generate an image in an x, y and z pattern. The AFM can either use the
probe in a contact or non-contact mode. A great advantage of the AFM is that
the specimen being examined can be non-conducting, unlike for the SEM. In this
manner, the AFM can be used to study almost any sample.

The AFM generates resolution by calculating the vertical and
lateral deflections of the cantilever. This is completed by reflecting a laser
beam off the cantilever which is reflected to a position sensitive photodiode
(PSPD) that consists of two sections. Consequently, any small change in
deflection of the cantilever produces a magnified reflection of displacement on
the photodiode relative to the cantilever (American society for Microbiology,
2002).

AFM has considerable advantages over the SEM. The ATM provides
more information with regard to the 3D surface of the specimen. Another major
advantage of the AFM is that it does not require any pre-treatment that may cause
damage or disfigurement to the sample. AFM also does not require a vacuum and
can be performed in air and liquid environments. Disadvantages of the AFM
include the single scan image size produced. The AFM has a lower depth of field
compared to the SEM because it only scans an area of micrometres, not
millimetres like the SEM. During analysis, the AFM has a slower scanning speed
to the SEM. Due to the nature of AFM probes, they cannot normally measure steep
walls or overhangs. Specially made cantilevers and AFMs can be used to modulate
the probe sideways as well as up and down to measure sidewalls. These
cantilevers however are considerably more expensive and have a lower lateral
resolution.

What is
AFM used for?

· Inorganics, polymers, coatings and bio-samples

· Personal care products, the measuring of the change in nanoscale
mechanical properties (modulus and friction) of hair, teeth and skin.

· Investigation of the force in which is required to remove
nanoparticles from a surface.

· The topography and nano mechanical properties of coatings.

The
challenges faced with measurement

· Calibration, quantification and understanding of AFM modes
(including that of force spectroscopy,

multi-frequency modes, frequency modulation mode, lateral force
and amplitude modulation mode.

· Obtaining important and additional information from AFM
(mechanical, chemical, electrical).

· Imaging soft samples at a high resolution whilst working on
minimising damage.

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