1.1 Materials Self-Assembly

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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 109 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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Why Nano?
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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 nano-technology was in
Theres 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 speeches and books.
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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
semi-conductor 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