Humans have
always yearned to know more about the world around them.
This desire is the basis for all scientific investigation
and drives our steady pace of discovery. Arguably one
of the greatest leaps forward in the history of science
was the invention of the microscope, and since its somewhat
cloudy invention in the Netherlands in the late 1500's,
the microscope has steadily become an ever stronger
and more important instrument in the search for knowledge
today.
The ability of
any microscope to magnify a sample is dependent upon
the wavelength of the investigating medium. Visible
light's wavelength is much greater than the size of
an atom (around 4,000 angstroms (10 billionths of a
meter or around the diameter of an atom)) and thus,
it is impossible to image on the atomic scale with visible
light. However, several methods have arisen within the
last decade that make atomic resolution possible. The
most popular of these are scanning tunneling microscopy
(STM), atomic force microscopy (AFM), transmission electron
microscopy (TEM), and scanning electron microscopy (SEM).
Both scanning tunneling
microscopy (STM) and atomic force microscopy (AFM) are
scanning probe techniques which in general involve moving
a probe over a sample and recording the interaction
force. In particular, STM makes use of a quantum behavior
known as tunneling. The probe is brought within angstroms
(10 billionths of a meter) of a conducting sample and
a small voltage applied between the two. Due to their
incredible proximity, electrons leak or tunnel between
the probe and the sample. The amount of electrons tunneling
between the two creates a current whose size is proportional
to the distance between them. Attempting to keep the
current at a constant value by changing the location
of the probe as it scans over the sample surface, the
STM can create a three dimensional image of the sample.
Atomic force microscopy
was invented in 1986 by Binnig, Quate, and Gerber, and
like STM is a scanning probe technique. The AFM uses
a probe attached to a cantilever which bends in response
to the repulsive forces between the probe tip and the
atoms on the sample surface. The bending of this cantilever
is detected using an optical system composed of a laser
and photo detectors. As the cantilever moves, the intensity
of the laser reflected into the bank of photo detectors
changes. By looking at the change in intensity across
the photo detector bank, the motion of the cantilever
in response to the surface can be extrapolated, and
thereby the surface morphology. The cantilever arm itself
is mounted on extremely precise piezoelectric ceramic
(piezoelectric materials are simply materials which
change shape when different voltages are applied across
them) devices to allow for sub-angstrom movements of
the tip.
Another modality
to generating an atomic resolution image is electron
microscopes. Since electrons possess wavelengths (half
an angstrom) much smaller than light (It's worth noting
that quantum mechanics is built in part upon this principle
of particles existing as both particles and waves) they
are well suited to illuminate samples. In TEM, electrons
are "shot" at a thinly cut sample (usually
around a thousand angstroms). These electrons are scattered,
absorbed, and transmitted in certain percentages depending
upon the surface characteristics like any incident radiation
would be upon contact with a surface. However, like
in a medical X-ray examination the transmitted electrons
reach a photographic plate below the sample and create
a magnified image. With this method, samples can be
magnified up to a million times.
In the scanning
electron microscope, a highly focused electron beam
is scanned over the entire sample one voxel at a time
(voxel is a three dimensional box whose size is analogous
to resolution - smaller voxel, better resolution). As
this beam moves, its electrons are scattered or cause
secondary scatterings from the atoms in the sample.
These scattered electrons are counted by detectors and
a contrast image formed according to these values. SEMs
can magnify objects 100,000 times.
The Dutch inventor
approximately 400 years ago would be amazed at the quality
and resolution of images available now. What once was
unimaginable became theory, and what was once only theory
can at present be seen. The microscope was truly a huge
step forward in the progress of knowledge, an incredible
tool in the pursuit of scientific discovery. |
