| Humans rely on a number of senses (sight, hearing, touch, smell,
and taste) to get information about their surroundings. For
quite some time humans have used many tools to extend their natural
senses.
To see the very small, traditional microscopes such as the Optical Microscopes
and Electron microscopes were used to extend our sense of sight at the
microscopic size scale. A Scanning Probe Microscope (SPM) unlike
optical and electron microscopes "feels" the surface of a sample instead
of "looking" at the sample with light waves or electrons. The power
of SPMs is in their versatility. By changing the nature of the mechanical
tip and the detection scheme used, surface topography as well as properties
can be investigated at the microscopic and nanoscopic scale.
Scanning Tunneling Microscope (STM)
Schematic showing configuration of various detection components
of STM (not to scale).
This powerful technique provide atomic-resolution images of a material's
surface. It relies on a quantum mechanical effect known as tunneling.
When the atoms on the tip and the atoms on the surface are placed in close
proximity, electrons can travel between them even if they do not have sufficient
energy to jump between them. Because this technique is dependent
on the detection of tunneling electrons, it is mostly applicable to electrically
conducting materials.
Atomic Force Microscope (AFM)
Schematic showing configuration of various detection components
of AFM (not to scale).
Most materials can be investigated by this technique because, regardless
of the material, a repulsive force results when the tip and the surface
is nudged closed enough together. This is because the outer part
of all atoms making up the substances around us consist of clouds of electrons.
Probes
All microscopes in this family use a sharp mechanical tip (probe) to “feel”
the surface of the sample by detecting the forces acting between the tip
and the sample surface or the amount of tunneling current flowing between
them.
STM Tips
Sharp metallic tips are required for the STM technique. Common
materials used for making these tips are tungsten and platinum/iridium
alloys. Tungsten wires are etched electrochemically in a solution
of potassium hydroxide (KOH). The platinum/iridium tips are also
electrochemically etched, but within a solution of potassium chloride (KCl).
A very sharp tip can be made because electrical fields are more intense
at sharp edges of objects. As a result, etching chemicals preferentially
migrate to these areas to further erode the wire into a tip.
AFM Tips and Cantilever
The tips are commonly fabricated from silicon, silicon nitride or diamond
(for hardness testing only) and are left bare or coated with a special
material (metal for EFM and rare-earth boride for MFM) to suit a particular
application. The tip is mounted on a thin cantilever whose deflections
provide a measure of the interaction. forces.
Currently commercially available AFM tips are microfabricated in three
geometries, namely: (1) conical, (2) tetrahedral, and (3) pyramidal.
Conical tips can be made sharp with a high aspect ratio (the ratio of tip
length to tip diameter) making them especially useful for imaging features
that are deep and narrow. Conical tips with diameters of 5 nm have
been made, but they are easily broken. The pyramidal and tetrahedral
tips have lower aspect ratios with tip diameter ranging 10nm to 50 nm.
These latter tip configurations are duller, but more durable.
The first step in the fabrication of an AFM tip is the etching of a
single-crystal silicon wafter with specific crystalline orientation.
This results in the forming of square pyramidal tips with characteristic
angles.
Steps used in fabricating pyramidal AFM tip and cantilever.
Cantilevers are usually fabricated as triangles or straight. During
their use in the instruments, small deflections are normally used, so the
forces measured obey Hooke's Law:
In the above relation, the force is F (Newtons), the spring constant
for the cantilever is k (Newtons/meter) and the deflection is z (meter).
The spring constant (k) of the cantilever is strongly dependent on its
physical dimensions (i.e., width - w, length - l, thickness - t) and the
elasticity of material (Modulus of Elasticity - E) that it is composed
of.
The spring constant for the triangular cantilever is approximately
expressed by:
For the straight cantilevers the spring constant can be estimated by:
Electron microscopy image of an AFM tip mounted on a cantilever.
Scanner
At present no mechanical motors can achieve precise, reproducible movement
at the atomic scale. The scanning probe microscopes utilize the phenomenon
of piezoelectricity to move the tips relative to the sample surface, at
the atomic sized scale. A scanner fabricated from ceramic, piezoelectric
component, typically made from sintered ceramic lead zirconium titanate
(PZT), is used to move the probe and the sample relative to each other
in all three (x, y, z) directions. An electrical voltage is applied
to the piezoelectric to make the scanner mechanically expand on contract.
The typical expansion coefficient for piezoelectric components range from
0.1 nm / Volt up to 300 nm / Volt.
As a result of these mechanical movements, the tip can be rastered over
the sample, as shown schematically below.
As the tip is rastered over the sample surface, the lateral (x, y) location
and the height or property value at that position (z) is recorded.
Once the z-profile for each of the raster lines are put together, an image
of the surface can be constructed and visualized.
STM rastering to form an image of silicon atoms on a surface.
Rapid advances in the engineering of piezoelectric components permitted
the development of the SPM.
STM detection scheme
The detectable electrical currents resulting from the tunneling of electrons
between the tip and the surface are about several nanoamperes
(nA).
The atomic scale lateral resolution of the STM is made possible by the
rapid change of current between an individual atom on the tip and atoms
on the sample surface as a function of their vertical separation.
This behavior is illustrated by the graph below.
Exponential dependence of tunneling current as a function
of the separation between the tip and the sample surface.
The expression below quantitatively models this behavior:
where Itunneling is the tunneling current, Vbias
is the bias voltage applied between the tip and the sample,
f is the workfunction (the energy necessary
to extract an electron) of the sample surface, and d is the distance separating
the tip and the surface. The constants G and k
are dependent on the configurations of the intrument.
AFM detection scheme
Many schemes, such as interferometry, capacitance, piezoresistivity etc.
developed to detect the deflection of the tip by the sample surface, have
been described by Sarid (1994).
The beam deflection technique which uses a laser, mirror, and a detector
made from a set of photodiodes is currently the most widespread technique
and is used in most commercially available units to detect the minute movement
of the cantilever with sub-angstrom resolution.
A red laser, generated by a solid state photodiode, with wavelength
near 625 nm is typically used for detecting the vertical and lateral movements
of the AFM cantilever.
A position-sensitive photodetector (PSPD) used to measure the displacement
of the laser beam reflected by the cantilever as the probe interacts with
the sample surface is either a two-section or four-section detector.
In the former configuration (used by AFM, MFM, FM-AFM), only the vertical
displacement is measured, whereas the latter (used by LFM) can detect the
horizontal and vertical displacement of the laser. The commercially
available PSPDs can measure displacements of light as small as 1 nm.
However, the optical configuration of the AFM allows a mechanical amplification
of laser displacement resulting in sub-Angstrom (< .1 nm) detection
of the cantilever's vertical movement. This amplification is proportional
to the ratio of the laser's path length between the cantilever and the
PSPD and the length of the cantilever itself.
Typical signals detected as the laser, transverses two sections of the
PSPD vertically.
Signal sum used to optimize alignment of laser, cantilever,
mirror, and PSPD to obtain maximum laser intensity.
Signal difference between the two PSPD sections to align
optical components to maximize detectibility of vertical movements by cantilever.
Based on the signal detected by the PSPD and the spring constant of
the cantilever, a force-curve can be constructed to determine the distance
separating the tip from the sample surface and the magnitude and nature
(attractive or repulsive) of the forces interacting between them.
Force-curve above shows the distance separating the tip and the sample
surface and the nature and magnitude of the interacting forces. The
red line shows the forces as the tip approach the surface and the blue
line shows how the force varies as the tip is retracted. Note the
tip in the blue line due to temporary adhesion to the surface.
In a typical force microscope, cantilever deflections ranging from 0.1
Angstrom to a few micrometers are measured and the typical forces range
from 10-13 to 10-5 Newtons. There are a variety
of forces in Force Microscopy that one should pay attention to, for example:
a) Pauli repulsion and ionic repulsion which are very short range and negligible
beyond about 1nm, b) Van der Waals force that are forces between electric
dipoles and quite long ranged being significant up to 10 nm, c) Capillary
forces, and d) magnetic and electrostatic forces.
An actual force-curve from an AFM instrument. The
force between the tip and the surface changes with their separation distance.
The two lines trace the force felt as the tip approaches the surface and
as the tip is retracted from the surface. The dip on the retracting
curve is due to adhesion to the surface.
Feedback
STM
In the STM's constant height mode, which is often used for small scans
and fast scanning, the sample is scanned without feedback and the variation
of tunneling current is monitored.
When the STM is operated in the constant current mode, a feedback system
that controls the vertical position of the tip allows one to maintain a
constant tip current of a few nanoamperes while the tip is rastered over
the sample surface in the xy plane.
AFM
In the AFM's constant height mode, which is often used for small scans
and fast scanning, the sample is scanned without feedback and the variation
in the deflection of the cantilever is monitored.
When the AFM is operated in the constant force mode, a feedback system
that controls the vertical position of the tip allows one to maintain a
constant cantilever deflection as the tip interacts with the sample surface
while being scanned in the xy plane.
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