Nanostructures
Lecture 13
MTX9100
Nanomaterials
OUTLINE
-What is quantum confinement?
- How can zero-dimensional materials be used?
-What are one –dimensional structures?
-Why does graphene attract so much attention?
1
General characteristics of nanomaterial
classes and their dimensionality
2
M. Ashby, P. Ferreira, D. Schodek; Nanomaterials, Nanotechnologies and Design; Copyright 2009 Elsevier Ltd.
Zero-dimensional materials
Nanocrystals absorb light then reemit the light in a different color
–the size of the nanocrystal (at
the Angstrom scale) determines
the color.
Six different quantum dot
solutions are shown excited with a
long wave UV lamp.
3
A quantum dot is a semiconductor whose excitons are
confined in all three spatial dimensions. As a result, they
have properties that are between those of bulk
semiconductors and those of discrete molecules.
The term "Quantum Dot" was coined by Mark Reed at Texas
Instruments.
Quantum confinement in
semiconductors
In an unconfined (bulk) semiconductor, an electron-hole
pair is typically bound within a characteristic length
called the Bohr exciton radius.
If the electron and hole are constrained further, then
the semiconductor's properties change.
This effect is a form of quantum confinement, and it is
a key feature in many emerging electronic structures.
Specifically, the effect describes the phenomenon results from
electrons and electron holes being squeezed into a dimension that
approaches a critical quantum measurement.
An exciton is a bound state of an electron and an imaginary
particle called an electron hole in an insulator or semiconductor,
and such is a Coulomb-correlated electron-hole pair.
4
Quantum confinement
For 0-D nanomaterials,
where all the
dimensions are at the
nanoscale,
an electron is confined
in 3-D space.
5
Quantum
confinement is
responsible for the
increase of energy
difference between
energy states and
band gap.
A phenomenon
tightly related with
the optical and
electronic properties
of the materials.
No electron delocalization (freedom to move) occurs.
Fundamentals of light
Light is a form of electromagnetic (interacting
electric and magnetic fields) radiant energy
that propagates through space in a way that
can be characterized as waves.
Electromagnetic radiation can be defined in
terms of its wavelength or frequency
(reciprocals of one another).
The electromagnetic spectrum is very large
ranging, from wavelengths at the kilometer
scale to those near the atomic scale.
6
Solid-state lighting
The direct conversion of electricity
to light using semiconductor
materials (normally in the form of
light emitted diodes - LEDs)
LEDs fundamentally produce light via
a special form
of electroluminescence.
One of the fundamental problems with LEDs—that of narrow sets of emission
frequencies that limit their uses — is solvable via quantum dot technologies.
A new generation of solid-state lighting—quantum light-emitting diodes
(QLEDs) made of quantum dot networks—are coming into use that would
work similarly to traditional LEDs but have greatly improved functionalities
and new uses.
7
Quantum dots
Quantum dots offer great potential in the form of
QLEDs which are made out of networks of quantum
dots and can also build on, yet dramatically improve,
existing LED technologies.
Quantum dots are essentially nanometer-size crystals
of semiconductor materials (e.g., silicon or germanium)
for which the electronic properties are strongly
dependent on their size.
Wavelengths in quantum dots can be controlled in
nanocrystalline materials. The energy separation
between valence and conduction bands can be altered
in nanocrystalline quantum dots by changing the size of
the nanoparticles. Resulting energy levels can thus be
varied.
8
Making quantum dots
quantum dot of
gallium arsenide
There are several ways to confine
excitons in semiconductors,
resulting in different methods to
produce quantum dots.
The total electron charge density
(shown in green) of a quantum dot of
gallium arsenide, containing just 465
atoms. (Image: Lin-Wang Wang)
9
www.lbl.gov
In general, quantum wires, wells
and dots are grown by advanced
epitaxial techniques in
nanocrystals produced by chemical
methods or by ion implantation,
or in nanodevices made by stateof-the-art lithographic techniques
Applications
Quantum dots are particularly
significant for optical
applications due to their
theoretically high quantum
yield.
a measure of the efficiency with which absorbed light
produces some effect.
the probability that a given quantum state is formed
from the system initially prepared in some other
quantum state
In electronic applications they
have been proven to operate like the increased resistance at small bias voltages of an
electronic device comprising at least one lowa single-electron transistor and capacitance tunnel junction.
show the Coulomb blockade
effect.
Quantum dots have also been
suggested as implementations of
qubits for quantum information
processing.
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concerns information science that depends on
quantum effects in physics
Quantum Dot infrared photodetectors
AFM micrographs of: a 1 μm x 1 μm
surface imaging of InAs quantum dots
on GaAs/InP, (inset) a single InAs
quantum dot.
Researchers have
demonstrated
the self-assembled growth
of In(Ga)As quantum dots in
both InP-based and GaAs
based systems using
metalorganic chemical vapor
deposition.
11
Such QDIPs have many advantages compared to the
conventional quantum well infrared photodetector
(QWIP), including: higher responsively, higher
temperature operation, higher light coupling to normal
incidence light, and capability of narrow band te nability.
The smallest nanostructures
Nanotubes (1 - D):
– Long, cylindrical tubes of
carbon formed by a catalytic
growth process.
– Nanometer-scale drop of
molten iron is typical
catalyst.
– Can behave like a
conductive metal wire or like
a semiconductor
12
Quantum dots (0 – D):
– Crystals containing only a few hundred
atoms
– Electrons are confined to widely
separated energy levels -> dot emits one
wavelength of light when excited
– Size of the dot determines electronic,
magnetic, and optical properties
– Used as biological markers (illuminating
sample with ultraviolet light crystals will
fluoresce at a specific wavelength)
Carbon structures - publishing
13
One-dimensional nanomaterials
TEM images of ZnO
nanorods synthesized
at different
temperatures:
180C (a, b);
room temperature (c);
insets indicate crystal
orientations.
A photoluminescence
spectrum of the
room-temperature
synthesized ZnO
nanorods is displayed
in (d).
14
Nanostructures
Solution synthesis of
copper nanowires: (a)
an as-prepared Cu
nanowire suspension,
(b) FESEM image of
Cu nanowires,
(c) TEM image of a
Cu nanowire, and
(d) SAED pattern of
the nanowire in (c).
15
Nanowire nanosensors for biology
Central to detection is the signal transduction
associated with selective recognition of a biological or
chemical species of interest.
The diameters of the nanostructures are comparable to
the sizes of biological and chemical species being
sensed, and thus intuitively represent excellent primary
transducers for producing signals that ultimately
interface with macroscopic instruments.
The size-tunable colors of semiconductor
nanocrystals, together with their highly robust
emission properties, are opening up opportunities for
labelling and optical-based detection of biological
species that offer advantages compared with
conventional organic molecular dyes widely used today.
16
Nanowire field-effect sensors
The underlying mechanism for nanowire sensors
is a field effect that is transduced using fieldeffect transistors (FETs), the ubiquitous
switches of the microelectronics industry.
The electronic
characteristics of
nanowires are well
controlled during
growth in contrast
to carbon nanotubes
Si nanowires can be prepared as
single-crystal structures with
diameters as small as 2-3 nm
a single Si nanowire sensor device
17
Two-dimensional nanomaterials
18
M. Ashby, P. Ferreira, D. Schodek; Nanomaterials, Nanotechnologies and Design; Copyright 2009 Elsevier Ltd.
Nanocoatings and multilayers
19
M. Ashby, P. Ferreira, D. Schodek; Nanomaterials, Nanotechnologies and Design; Copyright 2009 Elsevier Ltd.
Graphene
The atomic structure
of isolated, single-layer
graphene:
hexagonal lattice; high
crystal quality
20
What is graphene?
Band structure of graphene.
The conductance band touches the
valence band at the K and K’ points.
charged massless particles
(Dirac fermions)
Charge carriers can travel
thousands of interatomic
distances without scattering
21
Intrinsic graphene is a
semi-metal or zero-gap
semiconductor.
Graphite - Graphene
Graphene
is one-atom-thick planar
sheet of sp2-bonded
carbon atoms that are
densely packed in a
honeycomb crystal lattice.
22
graphite itself
consists of many
graphene sheets
stacked together
The carbon-carbon bond length
in graphene is about 0.142 nm.
Graphene is the basic structural
element of some carbon
allotropes including graphite,
carbon nanotubes and fullerenes.
SCIENCE, June 2010
If there's a rock star in the world of materials, it's
graphene: single-atom–thick sheets of carbon prized for
its off-the-charts ability to conduct electrons and for
being all but transparent.
Those qualities make graphene a tantalizing alternative for
use as a transparent conductor, the sort now found in
everything from computer displays and flat panel TVs to
ATM touch screens and solar cells.
But the material has been tough to manufacture in
anything larger than flakes a few centimeters across.
Now researchers have managed to create rectangular
sheets of graphene 76 centimeters in the diagonal
direction and even use them to create a working touchscreen display
23
Unique nature of charge carriers
In condensed matter physics, the Schrodinger
equation rules the world, usually being quite
sufficient to describe electronic properties of
materials.
Graphene is an exception — its charge carriers
mimic relativistic particles and are more easily
and naturally described starting with the Dirac
equation rather than the Schrodinger equation.
24
Dirac fermions
Although there is nothing particularly relativistic about electrons
moving around carbon atoms,
their interaction with the periodic potential of graphene’s
honeycomb lattice gives rise to new quasiparticles that at low
energies E are accurately described by the Dirac equation with an
effective speed of light.
These quasi-particles, called massless Dirac fermions, can be seen
as electrons that have lost their rest mass m or as neutrinos that
acquired the electron charge e.
The relativistic like description of electron waves on honeycomb
lattices has been known theoretically for many years, never failing to
attract attention, and the experimental discovery of graphene now
provides a way to probe quantum electrodynamics (QED) phenomena
by measuring graphene’s electronic properties.
25
What kind of uses does
graphene have?
Graphene can be used to make excellent transistors.
It is so thin we can easily control whether or not it
conducts by applying an electric field.
We would like to be able to do this with metals, but we
cannot make metal films thin enough to affect their
conducting state in this way.
Electrons in graphene also travel ballistically over submicron distances. As a result, graphene-based
transistors can run at higher frequencies and more
efficiently than the silicon transistors we use now.
At the present moment we have no way to produce entire
integrated circuits from these transistors since we are
limited by the size of graphenes we can produce.
26
Gas Sensors
Gas molecules that land on graphene affect its
electronic properties in a measurable way - in
fact, we have measured the effect of a single
molecule associating with a graphene.
This means that we can create gas sensors
which are sensitive to a single atom or
molecule!
27
Example: how to make sensors…
bottom up
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Thermal conductivity of thin CN films
Thin films with oriented
nanotube directions can
have remarkably high
thermal conductivities
29
M. Ashby, P. Ferreira, D. Schodek; Nanomaterials, Nanotechnologies and Design; Copyright 2009 Elsevier Ltd.
3-D nanomaterials
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Opportunities for using nanomaterials
and nanotechnologies in automobiles
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