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