The Royal
Swedish Academy of Sciences has decided to award the
1986 Nobel Prize in Physics by one half to
Professor Ernst Ruska, Fritz-Haber-Institut der
Max-Planck-Gesellschaft, Berlin, Federal Republic of
Germany, for his fundamental work in electron optics, and
for the design of the first electron microscope
and the other half, jointly to
Dr Gerd Binnig and Dr Heinrich Rohrer, IBM Research
Laboratory, Zurich, Switzerland, for their design of the
scanning tunnelling microscope.
Summary
One half of this year's Nobel Prize in Physics has
been awarded to Ernst Ruska for "his fundamental work in
electron optics and for the design of the first electron
microscope". The significance of the electron microscope
in different fields of science such as biology and
medicine is now fully established: it is one of the most
important inventions of this century.
Its development began with work carried out by Ruska as a
young student at the Berlin Technical University at the
and of the 1920's. He found that a magnetic coil could
act as a lens for electrons, and that such an electron
lens could be used to obtain an image of an object
irradiated with electrons. By coupling two electron
lensed he produced a primitive microscope. He very
quickly improved various details and in 1933 was able to
build the first electron microscope with a performance
clearly superior to that of the conventional light
microscope. Ruska subsequently contributed actively to
the development of commercial mass-produced electron
microscopes that rapidly found applications within many
areas of science.
Electron microscopy has since been developed through
technical improvements and through the advent of entirely
new designs, among them the scanning tunnelling electron
microscope. A number of researchers have taken part in
both this and the earlier development, but Ruska's
pioneering work is clearly the outstanding
achievement.
The other half of this year's prize has been awarded to
Gerd Binnig and Heinrich Rohrer for "their design of the
scanning tunneling microscope". This instrument is not a
true microscope ( i.e. an instrument that gives a direct
image of an object) since it is based on the principle
that the structure of a surface can be studied using a
stylus that scans the surface at a fixed distance from
it. Vertical adjustment of the stylus is controlled by
means of what is termed the tunnel effect - hence the
name of the instrument. An electrical potential between
the tip of the stylus and the surface causes an electric
current to flow between them despite the fact that they
are not in contact. The strength of the current is
strongly dependent on the distance, and this makes it
possible to maintain the distance constant at
approximately 10-7 cm (i.e. about two atom
diameters). The stylus is also extremely sharp, the tip
being formed of one single atom. This enables it to
follow even the smallest details of the surface it is
scanning. Recording the vertical movement of the stylus
makes it possible to study the structure of the surface
atom by atom.
The scanning tunneling microscope is completely new, and
we have so far seen only the beginning of its
development. It is, however, clear that entirely new
fields are opening up for the study of the structure of
matter. Binnig's and Rohrer's great achievement is that,
starting from earlier work and ideas. they have succeeded
in mastering the enormous experimental difficulties
involved in building an instrument of the precision and
stability required.
Background information
The invention of the conventional microscope represented
a great step forward for science, particularly in biology
and medicine. As better and better microscopes were
built, it was discovered that there exists a limit that
cannot be exceeded. This is connected with the wave
characteristics of light. Using light waves, it is
impossible to distinguish details smaller than the
wavelength of the light. The term "resolution" refers to
the distance between two details of an image that can
just be distinguished. For a conventional microscope
using visible light, the resolution is some 4 000 Å
(1 Å, ångstrom = l0-8cm).
The great breakthrough in microscopy came when it was
found possible to produce an image of an object using an
electron beam. The starting point was the discovery that
a magnetic coil can function like an optical lens. A
divergent bundle of electrons passing through the coil is
focused to a point. A suitable electric field can also
act as an electron-optical lens. Using a lens of this
type, an enlarged image can be obtained of an object
irradiated with electrons. the image is recorded on a
fluorescent screen or a photographic plate. It also
proved possible to combine two or more lenses to increase
the magnification. The work was carried out at the
Technical University of Berlin at the end of the
1920's.
The scientist who has made the greatest contribution to
this development is Ernst Ruska. As a young student
together with his supervisor Max Knoll, he began studying
simple magnetic coils, He found that the use of
suitably-designed iron encapsulation improved their
electron-optical properties. Above all, it now became
possible to build a lens of short focal length. This is
essential if high magnification is desired. Using two
coils in series, Ruska achieved a magnification of
fifteen times. Even though this was a modest result, it
nevertheless represents the first prototype of an
electron micrcscope. Ruska subsequently worked
purposefully to improve the details, and in 1933 he built
what can be described as the first electron microscope in
the modern sense - an instrument with considerably better
performance than a conventional light microscope 's. He
was then appointed by Siemens and took part in the
development of the first commercially-available,
mass-produced electron microscope, which entered the
market in 1939. This event may be considered the real
breakthrough for electron microscopy.
Since then, development of the electron microscope has
been very extensive. Its resolving power could be
considered theoretically unlimited, since the electron is
a pointlike particle, However, according to quantum
mechanics, every particle has wave characteristics which
introduce an uncertainty into the determination of its
position. This sets a theoretical limit to resolution for
the acceleration potentials normally used of the order of
0.5 - 1 Å. In practice, resolutions down to about 1
Å have been achieved.
The type of elect on microscope developed by Ruska is
called the transmission microscope. The object to be
examined is in the form of a thin section. The electron
beam goes right through this in the same way that light
pierces the object in a light microscope. There are,
however, several other types of electron microscope, the
most important apart from the transmission microscope
being perhaps the scanning electron microscope. In this
extremely sharply focused electron beam strikes the
object The secondary electrons emitted are collected by a
detector and the current is recorded. Magnetic coils
cause the electron beam to scan the object in the same
way as the beam of a TV tube. The variations in the
emission of secendary electrons carn be used to build up
an image. The advantage is the large depth of focus which
gives a three-dimensional image as opposed to the
sectional image obtained with a transmission microscope.
However, the resolution is poorer. These two types of
microscope thus complement each other.
Electron microscopy has developed extremely over the last
few decades, with technical improvements and entirely new
designs. Its importance can scarcely be exaggerated and,
against this background, the importance of the earliest,
fundamental work becomes increasingly evident. While many
researchers were involved Ruska's contributions clearly
predominate. His electron-optical investigations and the
building of the first true electron microscope were
crucial for future development.
The latest contribution to the development of microscopy
is what is termed the scanning tunneling microscope. Its
principle differs completely from that of other
microscopes. A mechanical device is used to sense the
structure of a surface. To this extent, the principle is
the same as that of braille-reading. In braille, it is
the reader's fingers that detect the impressed characters
but a much more detailed picture of the topography of a
surface can be obtained if the surface is traversed by a
fine stylus, the vertical movement of which is recorded.
What determines the amount of detail in the image - the
resolution - is the sharpness of stylus and how well it
can follow the structure of the surface. Obviously if the
tip of the stylus is too sharp, it rapidly becomes
destroyed. At the same time, small structural details of
the surface can be damaged by mechanical contact, One
solution to this problem would be to maintain the stylus
at a small, constant distance from the surface The first
to succeed in doing this was the American physicist
Russel Young at the National Bureau of Standards in the
USA. He used the phenomenon known as field emission. If a
sufficiently high potential is applied between stylus and
surface, a current flows with a strength depending on the
stylus-surface distance. If regulated by a servo mecanism
controlled by the current, this distance can be kept
constant without mechanical contact. Young succeeded in
building an instrument that worked on this principle. The
distance between the stylus tip and the surface was
approximately 200 Å. Its resolution was thus
considerably poorer than that of an electron
microscope
However, Young realised, that it should be possible to
achieve better resolution by using the so-called tunnel
effect This is a quantummechanical effect that allows an
electron (and also other particles to cross an area
where, according to classical physics it cannot exist
since it lacks sufficiently high energy. It makes its way
so to speak, through a potential mountain by
quantum-mechanical tunneling; hence the name tunneling
microscope. This means here that if the tip of the stylus
is near enough to the surface (10 Å, i.e. 1-2 atom
diameters) a current flows even at low voltages. In the
same way as field emission, it should be possible to
control the stylus without mechanical contact. However,
Young was unable to convert this idea into practice owing
to the exceptionally large experimental difficulties
involved
The first researchers to succeed in building a scanning
tunneling microscope were Gerd Binnig and Heinrich Rohrer
at the IBM Research Laboratories in Zürich,
Switzerland. The reason for their success was the
exceptional precision of the mechanical design One
example of this is that disturbing vibrations from the
environment were eliminated by building the microscope
upon a heavy permanent magnet floating freely in a dish
of superconducting lead. Less bulky but equally effective
devices for stable, disturbance-free suspension of the
microscope have now been developed. Piezoelectrical
elements are used to control the horizontal movement of
the stylus in two perpend icular directions so that it
scans the surface a long parallel lines - hence the name
scanning microscope The vertical movement of the stylus
is controlled and measured using another piozoelement.
Using a special technique it has been possible to produce
styluses with tips consisting of a single atom.
Consequently, the precision of the image is particularly
great. Horizontal resolution is approximately 2 Å
and vertical resolution. approximately 0.1 Å. This
makes it possible to depict individual atoms, that is, to
study in the greatest possible detail the atomic
structure of the surface being examined.
It is evident that this technique is one of exceptional
promise, and that we have so far seen only the beginning
or its development. Many research groups in different
areas of science are now in using the scanning tunneling
microscope. The study of surfaces is an important part of
physics, with particular applications in semiconductor
physics and microelectronics In chemistry, also, surface
reactions play an important part, for example in
connection with catalysis. It is also possible to fixate
organic molecules on a surface and study their
structures. Among other applications, this technique has
been used in the study of DNA molecules.
