Nanochemistry
NAN 601
Instructor:
Dr. Marinella Sandros
Quantum Dots
1
2
3


•
•
Polymers, either in solution for coating thin
films or as particles for construction
Metal nanoparticles and nanowires
Semiconductor nanospheres, rods, wires,
tetrapods
Carbon nanotubes
4


Quantum dots are semiconductors whose
excitons are confined in all three dimensions of
space.
Quantum dots have properties combined
between
◦ Those of bulk semiconductors
◦ Those of atoms
5


A material with electrical conductivity due to
electron flow.
A substance whose resistivity lies between that of a
conductor and an insulator. The resistance of a
semiconductor decreases as the temperature
increases.
6
7


As the temp. of the semiconductor is
increased, these electrons gain more energy.
Some gain enough energy to break free of
their bonds, and wander through the piece of
material.
Once an electron moves out of a bond, it
leaves behind a ‘hole’ in that bond
8


This hole is positive, and so can attract nearby
electrons which then move out of their bond
etc.
Thus, as electrons move in one direction,
holes effectively move in the other direction
Electron moves to fill hole
As electron moves in one direction hole
effectively moves in other
9

There is a difference between conduction in
metals and semiconductors, in metals
conduction is due solely to movement of
electrons, in semiconductors it is due to
movement of negative electrons and positive
holes.
10



States in a bulk semiconductor are so closely
spaced that the conduction and valence bands
appear to be continua.
The number of these states per unit volume in
an energy band is described by the density of
states.
Because no available states exist within the
bandgap, the density of states is measured
from the bottom of the conduction band and
the top of the valence band from which it is,
for a bulk semiconductor, a continuously
increasing function with carrier energy .
11


Electrons in conduction band (and holes in the valence band) are free to
move in all three dimensions of space.
So their energy spectrum is almost continuous, and the density of
allowed electron states-per-unit energy increases as the square root of
the energy
a smooth square root dependence
12
B.E.A. Saleh,
M.C. Teich.
Fundamentals
of Photonics.
fig. 16.1-10
and 16.1-29.


Electrons in conduction band (and holes in the
valence band) are free to move in two dimensions.
Confined in one dimension by a potential well.
◦ Potential well created due to a larger bandgap of the
semiconductors on either side of the thin film.
◦ Thinner films lead to higher energy levels.
• Quantum wells are formed in semiconductors by having a
material, like gallium arsenide sandwiched between two layers of a
material with a wider bandgap, like aluminium arsenide.
B.E.A. Saleh, M.C. Teich.
Fundamentals of Photonics.
fig. 13.1-11 and 16.1-29.
Distinct Steps: with steps occurring at the energy of
each quantized level

Thin semiconductor wire surrounded by a material
with a larger bandgap.
◦ Surrounding material confines electrons and holes in two
dimensions (carriers can only move in one dimension) due to
its larger bandgap.
B.E.A. Saleh,
M.C. Teich.
Fundamentals
of Photonics.
fig. 16.1-29.
14



Electrons and holes are confined in all three dimensions
Discrete energy levels (artificial atom)
Like bulk semiconductor, electrons tend to make
transitions near the edges of the bandgap in quantum
dots.
3D
2D
1D
0D
B.E.A. Saleh,
M.C. Teich.
Fundamentals
of Photonics.
fig. 16.1-29.
15
16
Nontraditional Semiconductors
Quantum Dots, also known as nanocrystals, are a non-traditional type of
semiconductor with limitless applications as an enabling material across
many industries. Quantum dots have a specified, unique composition and
size that give them novel quantum properties.
Shortcomings of Traditional Semiconductors
Traditional semiconductors have a shortcoming- they lack versatility. Their
optical and electronic qualities are costly to adjust, because their bandgap
cannot be easily changed. Their emission frequencies cannot be easily
manipulated by engineering.
17
•
•
There exists a forbidden
range of energy levels in
any material called the
band gap.
The electrons in bulk
(much bigger than 10 nm)
semiconductor material
have a range of energies.
18
•One electron with a different
energy than a second electron is
described as being in a different
energy level, and it is established
that only two electrons can fit in any
given level.
•In bulk, energy levels are very close
together, so close that they are
described as continuous, meaning
there is almost no energy difference
between them.
19
•
•
•
It is also established that some
energy levels are simply off limits
to electrons; this region of
forbidden electron energies is
called the bandgap, and it is
different for each bulk material.
Electrons occupying energy levels
below the bandgap are described
as being in the valence band.
Electrons occupying energy levels
above the bandgap are described
as being in the conduction band
20
1. In natural bulk semiconductor material, an
extremely small percentage of electrons
occupy the conduction band the
overwhelming majority of electrons occupy
the valence band, filling it almost completely.
2. The only way for an electron in the valence
band to jump to the conduction band is to
acquire enough energy to cross the bandgap,
and most electrons in bulk simply do not
have enough energy to do so.
3. Applying a stimulus such as heat, voltage, or
photon flux can induce some electrons to
jump the forbidden gap to the conduction
band.
4. The valence location they vacate is referred
to as a hole since it leaves a temporary
"hole" in the valence band electron structure.
21
1. Electrons in natural semiconductor bulk that
have been raised into the conduction band will
stay there only momentarily before falling back
across the bandgap to their natural, valence
energy levels.
2. As the electron falls back down across the
bandgap, electromagnetic radiation with a
wavelength corresponding to the energy it loses
in the transition is emitted.
3. The great majority of electrons, when falling from
the conduction band back to the valence band,
tend to jump from near the bottom of the
conduction band to the top of the valence bandin other words, they travel from one edge of the
bandgap to the other.
4. Because the bandgap of the bulk is fixed, this
transition results in fixed emission frequencies.
5. Quantum dots offer the unnatural ability to tune
the bandgap and hence the emission
wavelength.
22
•
The average distance
between an electron and a
hole in a exciton is called
the Excited Bohr Radius.
•
When the size of the
semiconductor falls below
the Bohr Radius, the
semiconductor is called a
quantum dot.
23

Very small semiconductor particles with a size
comparable to the Bohr radius of the excitons
(separation of electron and hole).
◦ Typical dimensions: 1 – 10 nm
◦ Can be as large as several μm.
◦ Different shapes (cubes, spheres, pyramids, etc.)
24


The energy levels depend on the
size, and also the shape, of the
quantum dot.
Smaller quantum dot:
◦ Higher energy required to confine
excitons to a smaller volume.
 Energy levels increase in energy and
spread out more.
 Higher band gap energy.
Figures are from “Quantum Dots
Explained.” Evident Technologies. 2008.
25
Because quantum dots' electron
energy levels are discrete rather than
continuous, the addition or
subtraction of just a few atoms to the
quantum dot has the effect of altering
the boundaries of the bandgap.
Changing the geometry of the surface
of the quantum dot also changes the
bandgap energy, owing again to the
small size of the dot, and the effects
of quantum confinement.
26
3D Nanocomposites, cellular materials, porous materials,
nanocrystal arrays, block co-polymers
2D Quantum wells, superlattices, membranes
1D Nanotubes, nanowires, nanorods
0D Nano dots from the gas phase
Colloids and nanoparticles by other methods
27
28
29
30
31
Semiconductors derive their great importance from the fact that their
electrical conductivity can be greatly altered via an external stimulus (voltage, photon
flux, etc), making semiconductors critical parts of many different kinds of electrical
circuits and optical applications.
Quantum dots are a special class of materials known as
semiconductors, which are crystals composed of periodic groups of II-VI, III-V, or IV-VI
materials. Quantum dots are unique class of semiconductor because they are so
small, ranging from 2-10 nanometers (10-50 atoms) in diameter. At these small sizes
materials behave differently, giving quantum dots unprecedented tunability and
enabling never before seen applications to science and technology.
.
32
Quantum dots are ordered collections of hundreds to thousands of
semiconductor- type atoms.
The electrons associated with a dot are confined to this small set of
atoms.
On the nano-scale, when the electron is confined, the change in energy
levels becomes distinctly discrete – a condition known as “quantum
confinement”.
Therefore the band gap is a function of size.
When there are lots of atoms, such as in a traditional semiconductor,
the bandgap is not a function of size.
The significance of this nanoproperty is that different size quantum
dots will fluoresce in different colors.
33
34
The goal is to elaborate a method of synthesis, which
a) is reproducible
b) yields monodisperse nanoparticles
c) produces „perfect“ particles
d) may control the shape of the particles
e) is easy, cheap
The chemical methods are either based on the kinetic control of nucleation and growth of the particles,
on electrostatic stabilization in (aqueous) suspension, or on the introduction of spatial constraints. The
latter include particle formation within or at the interface of micelles, vesicles, or bilayer lipid
membranes, within the channels of zeolites, in interlayers of clay, in peptides, or in biological cells.
35
The stages of nucleation and growth for the preparation of monodisperse
NCs in the framework of the La Mer model. As NCs grow with time, a size
series of NCs may be isolated by periodically removing aliquots from the
reaction vessel.
36
37
An organometallic method is used for the fabrication of highly
monodisperse cadmium selenide nanocrystal quantum dots.
Nucleation and subsequent growth of QDs occurs after a quick
injection of metal and chalcogenide precursors into the hot, strongly
coordinating solvent such as a mixture of trioctylphosphine (TOP)
and trioctylphosphine oxide (TOPO) in the case shown. After a fixed
period, removing the heat source stops the reaction. As a result,
NQDs of a particular size form.
38
Size- and material-dependent emission spectra of several surfactant-coated
semiconductor nanocrystals in a variety of sizes. The blue series represents
different sizes of CdSe nanocrystals with diameters of 2.1, 2.4, 3.1, 3.6, and
4.6 nm (from right to left). The green series is of InP nanocrystals with diameters
of 3.0, 3.5, and 4.6 nm. The red series is of InAs nanocrystals with diameters of
2.8, 3.6, 4.6, and 6.0 nm.
39
40

http://www.youtube.com/watch?v=VjznErmcLnU
41

Name a difference between bulk and QDs
semiconductors in terms of their energy band
level?
42



In bulk, energy levels are very close
together, so close that they are described
as continuous, meaning there is almost no
energy difference between them.
When the size of the semiconductor falls below
the Bohr Radius, the semiconductor is called a
quantum dot.
Because quantum dots' electron energy
levels are discrete rather than continuous,
the addition or subtraction of just a few
atoms to the quantum dot has the effect of
altering the boundaries of the bandgap.
43