06525 lecture 3. Nanocrystals (Quantum Dots)

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06525 lecture 3. Nanocrystals (Quantum Dots) 1.
Historical Background Very many different kinds
of nanoparticles are present in nature, although
most nanoparticles have only been recognised as
such in the last two decades. New synthetic and
analytical tools and techniques had to be
developed and existing techniques modified to
prepare, purify and then identify the chemical
composition, purity, size, shape and structure of
nanoparticles, such as nancrystals, nanorods,
metal clusters, etc., as they are smaller than
the wavelength of light used in normal
spectroscopic methods of analysis. Analytical
techniques include FTIR UV-vis NMR XRD, SAXS,
WAXS, EXAFS TEM SEM STM AFM
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Colloidal Suspensions of Gold Nanoparticles ? d gt
10 nm ? appear almost any colour from red or
blue ? colour depends on the size of the
nanocrystals ? colour due to quantum
confinement ? first observed by Michael Faraday
in 1858 ? used since the end of the middle ages
to colour ruby glass Synthesis Nanoparticles of
gold (gt 10 nm) are formed as a powder by reaction
with water-soluble phosphine ligands, such as
P(m-C6H4SO3Na)3, which form an organic coating on
the nanoparticle surface. These powders can be
dissolved in water - forming blood-red solutions
- and then applied to a surface. Evaporation of
the solvent on a smooth surface creates a bright
film of metallic gold. Deposition of the same
solution on a porous surface leads to a red
colour.
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Stained Glass Windows ? nano-sized CdS dispersed
in the transparent amorphous glass matrix ?
blue-shift in the colour of some soda-lime
glasses due to nanoparticles ? attributed in the
twentieth century to very small polarised CdS
particles ? the blue-shift actually arises from
the quantum confinement Nebula Dust ? Red colour
of dust clouds (nebula) in our galaxy first
observed in 1980 ? red colour attributed to very
small quantum-confined silicon nanocrystals
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2. Quantum Confinement Reducing the size of bulk
solids changes the magnitude of the physical
properties of the nano-sized solid to
intermediate values between those of the original
bulk solid and those of individual atoms or
molecules, e.g., whereas metals conduct electric
charge, individual metal atoms do not. Therefore,
there is a critical size of some metal clusters
below which no conductivity can be observed,
i.e., these metal clusters contain the last "free
electrons" for electrical conductivity. Classical
mechanics are no longer applicable for such
small nanometer-sized particles and quantum
mechanics take over. The quasi-continuous
density of electronic states of a bulk material
is gradually reduced to a limited number of
discrete energy levels as the size of a particle
decreases and "particle in a box" quantised
energy levels are then observed at a critical
nanocrystal or nanocluster diameter.
Quantum-confinement in very small particles means
that the colour of a nanocrystal - often referred
to as a quantum dot - depends as much on the size
of the particle as on the nature of the material
itself, e.g., the colour of small CdSe (2.3 nm)
nanocrystals is turquoise, whereas that of larger
CdSe (5.5 nm) nanocrystals is orange. Quantum
effects in the absorption and emission of light
from nanoparticles were first observed in 1967.
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Chalcogenide Nanocrystals - Synthesis a)
Colloidal Dispersions Early 1980s colloidal
dispersions or suspensions of semiconductors,
such as CdS, were first synthesised by mixing
reagents in solvents in the as potential
catalysts in photochemical reactions. There was
no real attempt to make colloidal dispersions
containing nanoparticles of a controlled size and
shape as these are not of critical importance in
catalytic applications. Disadvantages The
colloidal dispersions of semiconductor
nanocrystals were often unstable the
nanocrystals could not be isolated and
characterised.
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b) Physical Confinement Solid nanocrystalline
particles of good optical quality were
synthesised within the nano-sized pores of solid
inorganic porous media, such as zeolites, clays
and glasses. Advantages The nanocrystals are
size-limited the nanocrystals grow in the pores
within the inorganic host up to, but not beyond,
the maximum size of the nanometer-sized pores of
the host material. The shape and size is
determined by the nanopore. Disadvantages The
nanocrystals cannot be removed and isolated from
the host nanoporous materials, as the
nanoparticles and the nanoporous host are both
inorganic and the nanoparticles are stuck within
the nanopores.
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c) Polymer Coatings Nanocrystals are covered in
an ionic polymer-coating by preparing them in an
aqueous solution containing a sodium
polyphosphate derivative e.g., sodium
hexaphosphate NaPO36. The cadmium ions form a
complex with the polyphosphonate (PP) chains in
aqueous solution Advantages The charged
polymer coating prevents agglomeration of the
nanocrystals due to electrostatic repulsion and
steric effects. The polymer controls the size
and shape of the nanocrystals formed by adding a
chalcogenide gas to the reaction, e.g., by
bubbling hydrogen suphide gas through it, which
provides the required sulphur S2- anions in this
case. The formation and growth of the
nanocrystals can be controlled by modifying the
pH of the reaction solution.
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c) Polymer Coatings Nanocrystals are covered in
an ionic polymer-coating by preparing them in an
aqueous solution containing a sodium
polyphosphate derivative e.g., sodium
hexaphosphate NaPO36. The cadmium ions form a
complex with the polyphosphonate (PP) chains in
aqueous solution Advantages The charged
polymer coating prevents agglomeration of the
nanocrystals due to electrostatic repulsion and
steric effects. The polymer controls the size
and shape of the nanocrystals formed by adding a
chalcogenide gas to the reaction, e.g., by
bubbling hydrogen suphide gas through it, which
provides the required sulphur S2- anions in this
case. The formation and growth of the
nanocrystals can be controlled by modifying the
pH of the reaction solution.
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d) Monolayer Coatings Soluble semiconductor
nanocrystals of a defined size and shape and with
an organic monolayer coating are prepared in the
aqueous droplets of reverse micelles.
Advantages The organic monolayer inhibits
flocculation and aggregation of the nanocrystals.
The addition of Cd2 and S2- reagents leads to
further growth of the nanoparticles. Addition of
phenyl(trimethylsilyl)selenium to a CdSe
nanocrystal microemulsion leads to the
replacement of the surfactant coating with a thin
layer of Se-Ph chemically bound to the
nanocrystal surface, which passivates the
reactive nanocrystal surface, which increases
stability and QE.
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e) Controlled Precipitation Controlled
precipitation of nanocrystals in colloidal
solutions containing stabilisers, e.g.,
trioctylphosphine and trioctylphosphine oxide,
allows the nanocrystals to remain in solution and
grow in a controlled fashion by suppressing
aggregation of individual nanocrystals.
Advantages nanocrystal powders soluble in
organic solvents organic coating Post-synthesis
processing improves size distribution and quantum
yield Good control over shape ands size of
nanocrystals by ? Temperature variation ? Rate
of addition of reagents Disdavantages High
temperatures Toxic and explosive reagents
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e) Controlled Precipitation continued The main
alternative variation of this method for the
synthesis of cadmium chalcogenides involves using
aliphatic thiol and aromatic thiophenol
stabilisers in colloidal, often aqueous,
solutions. Aliphatic stabilisers include
thioethanol, thioglycerine and thioglycolic acid
for the preparation of water soluble
nanocrystals. Aromatic thiols can be used and a
range of organically soluble cadmium chalcogenide
nanocrystals have been prepared. Advantages Low
er reaction temperatures Non-toxic reagents Water
soluble nanocrystals
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f) Core-Shell Nanocrystals The highly reactive
surface of nanocrystals with atoms that are not
completely co-ordinated allows a second layer of
a different nanocrystalline material to be
deposited by epitaxial growth to form core-shell
nanocrystals. For example pre-formed CdSe
nanocrystals in solution can be coated with CdS
in situ to form stable CdSe(CdS) core/shell
nanocrystals with a passivated inner
interface. Advantages Core shell nanocrystals
are more stable than simple nanocrystals Higher
quantum yield due to quantum confinement and no
dangling bonds Disadvantages Complex synthesis
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4. Applications of Semiconductor
Nanocrystals CdS nanocrystals are being
investigated as photoluminescent biological
tags ? protein coated nanocrystals bind
selectively to cancer cells, ? the size and
position of a cancer tumor are accurately mapped
PL, ? more sophisticated treatment with less
side effects is facilitated, ? nanocrystals
absorb laser light and destroy tagged cancer
cells selectively. Advantages Inorganic
nanocrystal labels are superior to organic dyes
? they are brighter, ? their emission is
narrower, ? their lifetime is longer, ? they are
also biocompatible, ? are non-toxic and cause no
adverse side-effects, ? the nanocrystals can also
transferred from one cancer cell to another.
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Light-Emitting Diodes Cadmium chalcogenide II-VI
semiconductor nanocrystals, such as CdS, CdSe and
CdTe, are being investigated as
electroluminescent components of hybrid
inorganic/organic light-emitting diodes as a new
kind of flat panel display device to compete with
with LCDs. Advantages ? wide viewing angles, ?
low power consumption, ? clean colours, e.g.,
green light not contaminated by yellow or blue
light, ? colour tuning by changing nanocrystals
size. Gas Sensors Metal oxide nanocrystals,
such as SnO2 and In2O3, are already being used in
commercial gas sensors. Advantages ? higher
sensitivity and selectivity
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Plastic Solar Cells Metal oxide quantum dots,
such as TiO2, are already being used in plastic
solar cells. Advantages ? high efficiency
(10) and comparable with solid-state silicon
solar cells, ? light, ? robust, ? cheap to
manufacture, ? produced in large-area formats.
Fuel Cells Fuel cells using TiO2 nanocrystals
are used to produce hydrogen photocatalytically
from water. Advantages ? water is cheap and
plentiful, ? hydrogen is a clean fuel water as a
by-product, ? very environmentally friendly.