Chemistry Reference
In-Depth Information
discoveries of these ''seeing'' technologies, which made it possible for us to
perform the intensive research work to realize ''single electron transistor'', ''solar
cells with an energy conversion efficiency of higher than 40 %'', and ''Giant
Magneto Resistive (GMR)'' and ''Tunneling Magneto Resistive (TMR)'' sensors
to produce Terabyte hard disks and a precise positioning encoder on an atomistic
scale [
1
]. When one starts investigating nature on an atomistic scale, such meth-
odologies lead to create new research fields beyond the old frame of science and
technology, such as physics, chemistry, biology, electronics, and mechanics.
The microscopes mentioned above allow to observe atoms on the surface and in
thin film with a typical thickness of several 10 nm [
2
]. But how can we investigate
the interior of a material on an atomistic scale without destroying the material?
Electric resistivity and magnetic susceptibility provide us information on the
characteristics of bulk materials. One would like to distinguish, however, the
electric and magnetic state of each atom of which the material consists, the crystal
structures, as well as their micro- or even nanostructures. X-ray, electron, and
neutron scattering methods are, for this purpose, further combined to obtain such
structures, i.e. the atomic arrangements in the material. These methods are based
on the interference of waves, such as electromagnetic radiation, electron and
neutron ''waves'' [
3
]. These methods, however, will have more difficulties to
observe ''the images of the structure'', when the micro- and nano-structures tend to
possess a unit of structure of the order of nanometers, because the number of atoms
inside the structure becomes too small to produce enough intensity of the inter-
ference pattern. Is there any other method to ''probe'' such small structure?
After the discovery of the Mössbauer effect in 1958 by Rudolf Mössbauer
[
4
-
6
], an atomic nucleus became such a scientific probe, which opens to study the
interior of materials through the study of the ''hyperfine interaction'', which is the
interaction between the nucleus and its surrounding electrons [
7
]. The energy
levels of the nucleus are determined by the hyperfine interaction, causing a shift or
a splitting of the nuclear energy levels. This level structure can be observed in a
Mössbauer spectrum, as has been explained in the former tutorials. The Mössbauer
spectrum can be detected via signals such as c-rays and/or electrons emitted from
the nucleus. These signals bring us information on the electric and magnetic states
of the probe atom from the interior of the materials, as is schematically shown in
Fig.
6.1
. The situation can be compared to a ''spy'', the nuclear probe, who sends a
code signal to deliver us secret information from inside the material.
The probe atoms can be one of the constituents of the material, or they can be
impurities introduced into the material from the outside by melting, by diffusion, or
by implantation. The former processes make use of the thermal motion of the
atoms, while the implantation process injects energetic probe atoms using an
accelerator. In this tutorial we will further discuss ''implantation Mössbauer
Spectroscopy'', i.e., the probe atoms will be first implanted into a material, and
subsequently Mössbauer spectra will be measured by detecting emitted c-rays and
electrons. The spectra will provide us with atomistic information on the probe
atoms through the hyperfine interactions. This situation may be well compared with
an analogy of a ''spy'' which is sent to a place to gather information, and he/she will
