03 October 2012

Scanning tunneling microscope



The invention:

A major advance on the field ion microscope, the
scanning tunneling microscope has pointed toward new directions
in the visualization and control of matter at the atomic
level.


The people behind the invention:

Gerd Binnig (1947- ), a West German physicist who was a
cowinner of the 1986 Nobel Prize in Physics
Heinrich Rohrer (1933- ), a Swiss physicist who was a
cowinner of the 1986 Nobel Prize in Physics
Ernst Ruska (1906-1988), a West German engineer who was a
cowinner of the 1986 Nobel Prize in Physics
Antoni van Leeuwenhoek (1632-1723), a Dutch naturalist




The Limit of Light

The field of microscopy began at the end of the seventeenth century,
when Antoni van Leeuwenhoek developed the first optical microscope.
In this type of microscope, a magnified image of a sample
is obtained by directing light onto it and then taking the light
through a lens system. Van Leeuwenhoek’s microscope allowed
him to observe the existence of life on a scale that is invisible to the
naked eye. Since then, developments in the optical microscope have
revealed the existence of single cells, pathogenic agents, and bacteria.
There is a limit, however, to the resolving power of optical microscopes.
Known as “Abbe’s barrier,” after the German physicist and
lens maker Ernst Abbe, this limit means that objects smaller than
about 400 nanometers (about a millionth of a millimeter) cannot be
viewed by conventional microscopes.
In 1925, the physicist Louis de Broglie predicted that electrons
would exhibit wave behavior as well as particle behavior. This prediction
was confirmed by Clinton J. Davisson and Lester H. Germer
of Bell Telephone Laboratories in 1927. It was found that highenergy
electrons have shorter wavelengths than low-energy electrons
and that electrons with sufficient energies exhibit wave-
lengths comparable to the diameter of the atom. In 1927, Hans
Busch showed in a mathematical analysis that current-carrying
coils behave like electron lenses and that they obey the same lens
equation that governs optical lenses. Using these findings, Ernst
Ruska developed the electron microscope in the early 1930’s.
By 1944, the German corporation of Siemens and Halske had
manufactured electron microscopes with a resolution of 7 nanometers;
modern instruments are capable of resolving objects as
small as 0.5 nanometer. This development made it possible to view
structures as small as a few atoms across as well as large atoms and
large molecules.
The electron beam used in this type of microscope limits the usefulness
of the device. First, to avoid the scattering of the electrons,
the samples must be put in a vacuum, which limits the applicability
of the microscope to samples that can sustain such an environment.
Most important, some fragile samples, such as organic molecules,
are inevitably destroyed by the high-energy beams required for
high resolutions.


Viewing Atoms

From 1936 to 1955, ErwinWilhelm Müller developed the field ion
microscope (FIM), which used an extremely sharp needle to hold the
sample. This was the first microscope to make possible the direct
viewing of atomic structures, but it was limited to samples capable of
sustaining the high electric fields necessary for its operation.
In the early 1970’s, Russel D. Young and Clayton Teague of the
National Bureau of Standards (NBS) developed the “topografiner,”
a new kind of FIM. In this microscope, the sample is placed at a large
distance from the tip of the needle. The tip is scanned across the surface
of the sample with a precision of about a nanometer. The precision
in the three-dimensional motion of the tip was obtained by using
three legs made of piezoelectric crystals. These materials change
shape in a reproducible manner when subjected to a voltage. The
extent of expansion or contraction of the crystal depends on the
amount of voltage that is applied. Thus, the operator can control the
motion of the probe by varying the voltage acting on the three legs.
The resolution of the topografiner is limited by the size of the probe.
The idea for the scanning tunneling microscope (STM) arose
when Heinrich Rohrer of the International Business Machines (IBM)
Corporation’s Zurich research laboratory met Gerd Binnig in Frankfurt
in 1978. The STM is very similar to the topografiner. In the STM,
however, the tip is kept at a height of less than a nanometer away
from the surface, and the voltage that is applied between the specimen
and the probe is low. Under these conditions, the electron
cloud of atoms at the end of the tip overlaps with the electron cloud
of atoms at the surface of the specimen. This overlapping results in a
measurable electrical current flowing through the vacuum or insulating
material existing between the tip and the sample. When the
probe is moved across the surface and the voltage between the
probe and sample is kept constant, the change in the distance between
the probe and the surface (caused by surface irregularities)
results in a change of the tunneling current.
Two methods are used to translate these changes into an image of
the surface. The first method involves changing the height of the
probe to keep the tunneling current constant; the voltage used to
change the height is translated by a computer into an image of the
surface. The second method scans the probe at a constant height
away from the sample; the voltage across the probe and sample is
changed to keep the tunneling current constant. These changes in
voltage are translated into the image of the surface. The main limitation
of the technique is that it is applicable only to conducting samples
or to samples with some surface treatment.


Consequences

In October, 1989, the STM was successfully used in the manipulation
of matter at the atomic level. By letting the probe sink into the
surface of a metal-oxide crystal, researchers at Rutgers University
were able to dig a square hole about 250 atoms across and 10 atoms
deep.Amore impressive feat was reported in the April 5, 1990, issue
of Nature; M. Eigler and Erhard K. Schweiser of IBM’s Almaden Research
Center spelled out their employer’s three-letter acronym using
thirty-five atoms of xenon. This ability to move and place individual
atoms precisely raises several possibilities, which include the
creation of custom-made molecules, atomic-scale data storage, and
ultrasmall electrical logic circuits.
The success of the STM has led to the development of several
new microscopes that are designed to study other features of sample
surfaces. Although they all use the scanning probe technique to
make measurements, they use different techniques for the actual detection.
The most popular of these new devices is the atomic force
microscope (AFM). This device measures the tiny electric forces that
exist between the electrons of the probe and the electrons of the
sample without the need for electron flow, which makes the tech-
nique particularly useful in imaging nonconducting surfaces. Other
scanned probe microscopes use physical properties such as temperature
and magnetism to probe the surfaces.



                                                   Gerd Binnig and Heinrich Rohrer



Both Gerd Binnig and Heinrich Rohrer believe an early and
pleasurable introduction to teamwork led to their later success
in inventing the scanning tunneling microscope, for which they
shared the 1986 Nobel Prize in Physics with Ernst Ruska.
Binnig was born in Frankfurt, Germany, in 1947. He acquired
an early interest in physics but was always deeply influenced
by classical music, introduced to him by his mother, and
the rock music that his younger brother played for him. Binnig
played in rock bands as a teenager and learned to enjoy the creative
interplay of teamwork. At J. W. Goethe University in
Frankfurt he earned a bachelor’s degree (1973) and doctorate
(1978) in physics and then took a position at International Business
Machine’s Zurich Research Laboratory. There he recaptured
the pleasures of working with a talented team after joining
Rohrer in research.
Rohrer had been at the Zurich facility since just after it
opened in 1963. He was born in Buch, Switzerland, in 1933, and
educated at the Swiss Federal Institute of Technology in Zurich,
where he completed his doctorate in 1960. After post-doctoral
work at Rutgers University, he joined the IBM research team, a
time that he describes as among the most enjoyable passages of
his career.
In addition to the Nobel Prize, the pair also received the German
Physics Prize, Otto Klung Prize, Hewlett Packard Prize,
and King Faisal Prize. Rohrer became an IBM Fellow in 1986
and was selected to manage the physical sciences department at
the Zurich Research Laboratory. He retired from IBM in July
1997. Binnig became an IBM Fellow in 1987


See also :  Electron microscope  ; Mass spectrograph ; Neutrino detector ; Wikipedia

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