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MM-407: NANOSTRUCTURED MATERIALS
NANOTECHNOLOGY
Nanotechnology deals with small structures or smallsized materials
Prefix of nanotechnology i.e. ‘nanos’ comes from the
Greek word for ‘dwarf’
Nanometer (nm) is one billionth of a meter, or l0-9 m
One nanometer is approximately the length
equivalent to 10 hydrogen or 5 silicon atoms aligned
in a line
Small features permit more functionality
Physical Properties of Nanomaterials
Copper which is an opaque substance become
transparent.
Platinum which is an inert material become catalyst.
Aluminum which
combustible.
is
a
stable
material
turns
Silicon insulators become conductors.
Gold which is solid, inert and yellow on room
temperature at micro scale becomes liquid and red in
color at nano scale on room temperature. It also gets
unusual catalytic properties not seen at macro scale.
Nanotechnology Definitions
The development and use of devices that have a size
of only a few nanometres
www.physics.about.com
“Research and technology development at the atomic,
molecular or macromolecular level in the length scale
of approximately 1 - 100 nm range, to provide a
fundamental understanding of phenomena and
materials at the nanoscale and to create and use
structures, devices and systems that have novel
properties and functions because of their small and/or
intermediate size.”
www.nano.gov
“Branch of engineering that deals with things smaller
than 100 nm (especially with the manipulation of
individual molecules).”
www.hyperdictionary.com
“The art of manipulating materials on an atomic or
molecular scale especially to build microscopic
devices.”
Miriam Webster Dictionary
“Nanotechnology, or, as it is sometimes called,
molecular manufacturing, is a branch of engineering
that deals with the design and manufacture of
extremely small electronic circuits and mechanical
devices built at the molecular level of matter.”
www.whatis.com
OLD NANOTECHNOLOGY
Stained-glass windows –
Silver-Halide Photography
AR-coated lenses
(anti-reflecting)
Viruses are nanomachines
NEW NANOTECHNOLOGY
Vastly improved catalysts enhance surface area
to volume ratios
Designer drugs
Cheap, sensitive medical diagnostics
Transparent Sunblock
Nanotube-strengthened cables
Difference:
Designing and manipulating at the molecular level whereas
before it was either evolution that did it for us or results
happened which we never really understood and so
couldn’t optimize
HISTORY OF SCIENTIFIC REVOLUTIONS
Discovery type
Name
Bronze
Start date
2,200,000
BC
3500BC
Steam power
Industrial
1764
Automation
Mass production
Consumer
1906
Automation
Computing
Genetic
Engineering
Nanotechnology
Information
1946
Genetic
1953
Nano age
1991
Molecular
assemblers
Assembler
age?
2020?
Life age
2050?
Industrial
Tools
Industrial
Metallurgy
Industrial
Health
Industrial
Automation
Health, industrial,
automation
Life assemblers
Age
Stone
Wilson et al. 2002. Nanotechnology: basic science and emerging
technologies. Chapman & Hall/CRC. New York.
PERSPECTIVE OF SIZE
Water molecules – 3 atoms
DNA molecules – millions of atoms
Carbon nanotubes – millions of atoms
Molecule of DNA
Carbon nanotubes
Examples of zero-dimensional nanostructures or nanomaterials
with their typical ranges of dimension
SURFACE VS. VOLUME
a
Diamond unit cell
Si has a diamond structure with a = 5.43 Å
A Si nanocube 10 nm on a side is composed of:
~6250 unit cells
~50,000 atoms
Each nanocube face is composed of:
~340 unit cells per face
~680 surface atoms per face
Total surface area is:
~4080 atoms (~10% surface atoms)
A bulk Si film 1 µm thick on a 10 cm square:
~6.3 X 1019 unit cells
~5 X 1020 atoms
~1.4 X 1017 surface atoms (~0.03% surface atoms)
MORE THAN SIZE….
Interesting phenomena:
Chemical –
Take advantage of large surface to volume ratio,
interfacial and surface chemistry important, systems
too small for statistical analysis
Electronic –
Quantum confinement, bandgap engineering, change
in density of states, electron tunneling
Magnetic –
Giant magneto-resistance by nanoscale multilayers,
change in magnetic susceptibility
Mechanical –
Improved strength hardness in light-weight
nanocomposites and nanomaterials, altered bending,
compression properties, nanomechanics of molecular
structures
Optical –
Absorption and fluorescence of nanocrystals, single
photon phenomena, photonic band gap engineering
Fluidic –
Enhanced flow properties with
nanoscale adsorbed films important
nanoparticles,
Thermal –
Increased thermoelectric performance of nanoscale
materials, interfacial thermal resistance important
WHAT ARE NANOSTRUCTURES?
At least one dimension is between 1 - 100 nm
2-D structures (1-D confinement):
• Thin films
• Planar quantum wells
• Superlattices
1-D structures (2-D confinement):
• Nanowires
• Quantum wires
• Nanorods
• Nanotubes
0-D structures (3-D confinement):
• Nanoparticles
• Quantum dots
Dimensionality, confinement depends on structure:
• Bulk nanocrystalline films
• Nanocomposites
THIN FILMS
Nanoscale Thin Film
• Single “two dimensional” film, thickness < ~100
nm
• Electrons can be confined in one dimension;
affects wavefunction, density of states
• Phonons can confined in one dimension; affects
thermal transport
• Boundaries, interfaces affect transport
FABRICATION OF NANOSTRUCTURES AND
NANOMATERIALS
Group according to the growth media:
(1) Vapor phase growth, including laser reaction
pyrolysis for nanoparticle synthesis and atomic
layer deposition (ALD) for thin film deposition.
(2) Liquid phase growth, including colloidal
processing for the formation of nanoparticles and
self assembly of monolayers.
(3) Solid phase formation, including phase
segregation to make metallic particles in glass
matrix and two-photon induced polymerization
for the fabrication of three-dimensional photonic
crystals
.
4) Hybrid growth, including vapor-liquid-solid
(VLS) growth of nanowires.
Group the techniques according to the form of
products:
(1) Nanoparticles by means of colloidal processing,
flame combustion and phase segregation.
(2) Nanorods or nanowires by template-based
electroplating, solution liquid solid growth(SLS),
and spontaneous anisotropic growth.
(3) Thin films by molecular beam epitaxy (MBE)
and atomic layer deposition (ALD).
(4) Nanostructured bulk materials, for example,
photonic bandgap crystals
CHALLENGES IN NANOTECHNOLOGY
Integration of nanostructures/nanomaterials into
or with macroscopic systems that can interface
with people
Building & demonstration of novel tools to
study at the nanometer level what is being
manifested at the macro level
The small size and complexity of nanoscale
structures make the development of new
measurement technologies
New measurement techniques need to be
developed at the nanometer scale and may
require new innovations in metrological
technology.
Measurements
of
physical
properties
of
nanomaterials
require
extremely
sensitive
instrumentation, while the noise level must be kept
very low.
FABRICATION AND PROCESSING OF
NANOMATERIALS THE FOLLOWING
CHALLENGES MUST BE MET:
(1) Overcome the huge surface energy, a result
of enormous surface area or large surface to
volume ratio.
(2) Nanomaterials with desired size, uniform
size distribution, morphology, crystallinity,
chemical composition, and microstructure
(3) Prevent nanomaterials and nanostructures
from coarsening through either Ostwald ripening
or agglomeration as time evolutes.
For the fabrication of nanoparticles, a small size
is not the only requirement.
PRACTICAL APPLICATION
&
PROCESSING CONDITIONS FOR
NANOMATERIALS
(i)
Identical size of all particles (also called
monosized or with uniform size
distribution)
(ii)
Identical shape or morphology,
(iii) Identical chemical composition and
crystal structure that are desired among
different particles and within individual
particles, such as core and surface
composition must be the same
(iv)
Individually dispersed or monodispersed, i.e. no agglomeration.
If agglomeration does occur,
nanoparticles should be readily
redispersible.
EXCITING APPLICATIONS OF
NANOTECHNOLOGY INCLUDE:
� Nanopowders —
the unusual properties of particles less than 100 nm
allow a range of new and improved materials with a
breadth of applications, such as plastics that behave
like ceramics or metals; new catalysts for
environmental remediation; improved food shelf-life
and packaging; and novel drug delivery devices.
� Carbon nanotubes — graphite can be rolled into a
cylinder with a diameter of about 1 nm. These strong
but light ‘carbon nanotubes’ are being developed for
a raft of uses, such as sensors, fuel cells, computers
nd televisions.
� Nanomembrane filtration systems — these have
the potential to address one of the most pressing
issues of the 21st Century — safe, clean, affordable
water.
� Molecular electronic ‘cross bar latches’ —
Hewlett-Packard believes that silicon computer chips
will probably reach a technical dead end in about a
decade, to be replaced by tiny nanodevices described
as ‘cross bar latches’.
� Quantum dots — these are small devices that
contain a tiny droplet of free electrons — essentially
artificial atoms. The potential applications are
enormous, such as counterfeit-resistant inks, new biosensors, quantum electronics, photonics and the
possibility of tamper-proof data transmission.
� New technologies for clean and efficient energy
generation.
Nanostructures
Definition:
A nanostructure is an object of intermediate size
between molecular and microscopic (micrometersized) structures.
Number of dimensions on the nanoscale
Nanotextured surfaces one dimension on nanoscale
i.e., only the thickness of the surface of an object is
between 0.1 and 100 nm.
Nanotubes have two dimensions on the nanoscale,
i.e., the diameter of the tube is between 0.1 and 100
nm; its length could be much greater.
Spherical nanoparticles have three dimensions on
the nanoscale,
i.e., the particle is between 0.1 and 100 nm in each
spatial dimension.
The terms nanoparticles and ultrafine particles (UFP)
often are used synonymously although UFP can reach
into the micrometer range.
Two approaches to the synthesis
nanomaterials
and
the
fabrication
nanostructures:
of
of
1) Top-down
2) Bottom-up
Attrition or milling is a typical top-down method
in making nanoparticles.
Colloidal dispersion is a good example of bottom-up
approach in the synthesis of nanoparticles.
Lithography may be considered as a hybrid approach,
since the growth of thin films is bottom-up whereas
etching is top-down, while nanolithography and
nanomanipulation are commonly a bottom-up
approach.
BOTTOM-UP APPROACH
Build-up of a material from the bottom:
atom-by-atom
molecule-by-molecule
In organic chemistry and/or polymer science, we
know polymers are synthesized by connecting
individual monomers together.
In crystal growth, growth species, such as atoms, ions
and molecules, after impinging onto the growth
surface, assemble into crystal structure one after
another.
Advantages
Less defects,
Homogeneous chemical composition,
Better short and long range ordering
Reason
Driven mainly by the reduction of Gibbs
free energy,, therefore closer to a
thermodynamic equilibrium state
Examples:
Production of salt and nitrate in chemical industry,
Growth of single crystals and deposition of films in
electronic industry.
For most materials, there is no difference in physical
properties of materials regardless of the synthesis
routes, provided that chemical composition,
crystallinity, and microstructure of the material in
question are identical.
Top-down Approach
Disadvantages
Introduces internal stress,
Surface defects (i.e. imperfections)
Contaminations
SOLID SURFACES PHYSICAL CHEMISTRY
Nanomaterials possess a large fraction of surface
atoms per unit volume.
The ratio of surface atoms to interior atoms changes
on dividing macroscopic object into smaller parts.
The total surface energy increases with the overall
surface area.
Large surface energy therefore thermodynamically
unstable or metastable
The percentage of surface atoms changes with the palladium cluster
diameter.
SURFACE ENERGY REDUCTION
(i) Surface Relaxation
The surface atoms or ions shift inward
(ii) Surface Restructuring
Combining surface dangling bonds into new bonds
Original
Restructured
(iii) Surface adsorption
Chemical or physical adsorption of terminal
chemical species onto the surface by forming
chemical bonds or weak attraction forces such as
electrostatic or van der Waals forces
(iv) Composition segregation or impurity enrichment
Through solid-state diffusion.
AGGLOMERATION OF INDIVIDUAL
NANOSTRUCTURES
(1) Sintering
Individual structures merge together
Polycrystalline material
(2) Ostwald ripening
Large structures grow at the cost of
smaller ones
Appreciable solubility in a solvent
Single uniform structure
(a) Sintering (b) Ostwald ripening
Sintering
Solid-state diffusion
(i)
Surface diffusion
Requires the smallest activation energy
Start at relatively low temperature
(ii)
Volume diffusion
Require moderate temperatures,
Volume diffusion dominates
(iii)
Cross grain-boundary diffusion
Requires highest activation energy
Significant only at high temperatures
Evaporation-condensation
Nanomaterials have an appreciable vapor pressure at
the processing temperature.
Dissolution-precipitation
Solid is dispersed in a liquid in which the solid is
partially soluble
Vapor pressure of a number of liquids as a function of droplet
radius
Variation in solubility of silica with radius of curvature of surface
OSTWALD RIPENING
Can have either positive or negative influence on the
resulting materials, depends on process & application
Can either widen or narrow the size distribution,
depending on the control of the process conditions
Abnormal grain growth, leading to inhomogeneous
microstructure and inferior mechanical properties
Specifically, it has been used to narrow the size
distribution of nanoparticles by eliminating small one