74
PROPERTIES OF INDIVIDUAL NANOPARTICLES
than these characteristic lengths, it is possible that new physics or chemistry may
occur.
Perhaps a working definition of a nanoparticle is an aggregate of atoms between 1
and 100 nm viewed as a subdivision of a bulk material, and of dimension less than
the characteristic length of some phenomena.
4.2. METAL NANOCLUSTERS
4.2.1. Magic Numbers
Figure 4.2 is an illustration of a device used to form clusters of metal atoms. A highintensity laser beam is incident on a metal rod, causing evaporation of atoms from
the surface of the metal. The atoms are then swept away by a burst of helium and
passed through an orifice into a vacuum where the expansion of the gas causes
cooling and formation of clusters of the metal atoms. These clusters are then
ionized by UV radiation and passed into a mass spectrometer that measures their
mass : charge ratio. Figure 4.3 shows the mass spectrum data of lead clusters formed
in such an experiment where the number of ions (counts) is plotted as a function of
the number of atoms in the cluster. (Usually mass spectra data are plotted as counts
versus mass over charge.) The data show that clusters of 7 and 10 atoms are more
likely than other clusters, which means that these clusters are more stable than
clusters of other sizes. Figure 4.4a is a plot of the ionization potential of atoms as a
function of their atomic number Z, which is the number of electrons in the atom. The
ionization potential is the energy necessary to remove the outer electron from the
I
I
I
I ;
I I
I + -
METAL DISK
I
I
I
/
I
I
I
I
I
MASS
I SPECTROMETER
GASES INTRODUCED
Figure 4.2. Apparatus to make metal nanoparticles by laser induced evaporation of atoms
from the surface of a metal. Various gases such as oxygen can be introduced to study the
chemical interaction of the nanoparticles and the gases. (With permission from F. J. Owens
and C. P. Poole, Jr., New Superconductors, Plenum Press, 1999.)
PROPERTIES OF INDIVIDUAL NANOPARTICLES
than these characteristic lengths, it is possible that new physics or chemistry may
occur.
Perhaps a working definition of a nanoparticle is an aggregate of atoms between 1
and 100 nm viewed as a subdivision of a bulk material, and of dimension less than
the characteristic length of some phenomena.
4.2. METAL NANOCLUSTERS
4.2.1. Magic Numbers
Figure 4.2 is an illustration of a device used to form clusters of metal atoms. A highintensity laser beam is incident on a metal rod, causing evaporation of atoms from
the surface of the metal. The atoms are then swept away by a burst of helium and
passed through an orifice into a vacuum where the expansion of the gas causes
cooling and formation of clusters of the metal atoms. These clusters are then
ionized by UV radiation and passed into a mass spectrometer that measures their
mass : charge ratio. Figure 4.3 shows the mass spectrum data of lead clusters formed
in such an experiment where the number of ions (counts) is plotted as a function of
the number of atoms in the cluster. (Usually mass spectra data are plotted as counts
versus mass over charge.) The data show that clusters of 7 and 10 atoms are more
likely than other clusters, which means that these clusters are more stable than
clusters of other sizes. Figure 4.4a is a plot of the ionization potential of atoms as a
function of their atomic number Z, which is the number of electrons in the atom. The
ionization potential is the energy necessary to remove the outer electron from the
I
I
I
I ;
I I
I + -
METAL DISK
I
I
I
/
I
I
I
I
I
MASS
I SPECTROMETER
GASES INTRODUCED
Figure 4.2. Apparatus to make metal nanoparticles by laser induced evaporation of atoms
from the surface of a metal. Various gases such as oxygen can be introduced to study the
chemical interaction of the nanoparticles and the gases. (With permission from F. J. Owens
and C. P. Poole, Jr., New Superconductors, Plenum Press, 1999.)