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Nanomaterials & Nanotechnology
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What does “Nano” mean?
• Nano (Greek): dwarf
• Nano(technology)
refers to particle
sizes at nano-scale:
1 nm = 10–9 m =
0.000,000,001 m
• Comparison of 1 m
with 1 nm equals
approximately the
size of the earth
compared with the
size of dice
vs.
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The Nanometer Size Scale
Nanotube
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A Few “Funny” Historical Citations
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The Most “Interesting” Citation…
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A Brief History
The history of nanotechnology reaches back to the late 19th century,
when colloidal science first took root.
The first mention of some of the distinguishing concepts in nanotechnology was in “There’s Plenty of Room at the Bottom”, a talk
given by physicist Richard Feynman at an APS meeting in 1959.
The term “nanotechnology” was defined by Prof. N. Taniguchi (“On
the Basic Concept of ‘Nano-Technology’,” Proc. Intl. Conf. Prod.
Eng. Tokyo, Part II, JSPE, 1974.) as follows: “‘Nano-technology’
mainly consists of the processing of, separation, consolidation, and
deformation of materials by one atom or one molecule.”
In the 1980s the basic idea of this definition was explored in much
more depth by Dr. K. E. Drexler, who promoted the technological
significance of nanoscale phenomena and devices through
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speeches and books.
Nanotechnology and nanoscience got started in the early 1980s
with two major developments: the birth of cluster science and the
invention of the scanning tunneling microscope (STM).
This development led to the discovery of fullerenes and carbon
nanotubes.
The synthesis and properties of semiconductor nanocrystals were
studied. This led to a fast increasing number of metal oxide
nanoparticles of quantum dots (QDs).
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A QD is a semiconductor nanostructure that confines the motion of
conduction band electrons, valence band holes, or excitons (pairs of
conduction band electrons and valence band holes) in all three
spatial directions.
The confinement can be due to (1) electrostatic potentials (generated
by external electrodes, doping, strain, impurities), (2) the presence of
an interface between different semiconductor materials (e.g., in the
case of self-assembled quantum dots), (3) the presence of the semiconductor surface (e.g., in the case of a semiconductor nanocrystal),
or (4) a combination of these.
A QD has a discrete quantized energy spectrum. The corresponding
wave functions are spatially localized within the QD but extend over
many periods of the crystal lattice.
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QDs can be contrasted to other semiconductor nanostructures:
1) quantum wires, which confine the motion of electrons or holes
in two spatial directions and allow free propagation in the third;
2) quantum wells, which confine the motion of electrons or holes
in one direction and allow free propagation in two directions.
In contrast to atoms, the energy spectrum of a QD can be
engineered by controlling the geometrical size, shape, and the
strength of the confinement potential.
In QDs that confine e– and h+, the interband absorption edge is
blue shifted due to the confinement compared to the bulk
material of the host semiconductor material. As a consequence,
QDs of the same material, but with different sizes, can emit light
of different colors.
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The larger the dot, the redder the fluorescence; the smaller the dot,
the bluer it is.
Quantitatively speaking, the band gap that determines the energy
(and hence color) of the fluoresced light is inversely proportional to
the square of the size of the quantum dot. Larger QDs have more
energy levels which are more closely spaced. This allows the QD
to absorb photons containing less energy, i.e. those closer to the
red end of the spectrum.
The ability to tune the size of QDs is advantageous, as the larger
and more red-shifted the QDs, the less the quantum properties are.
The small size of the QD allows people to take advantage of these
quantum properties.
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Fluorescence in various sized CdSe QDs