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The nanoscale• ‘Nano’ is the unit prefix representing 10–9.

Some common unit prefixes

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• Carbon can exist in two forms in daily life:– Diamond (tetrahedral)

– Graphite (hexagonal)

Diamond Graphite

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• Carbon atoms can also be arranged in many other ways to produce other forms, e.g.– Carbon nanotube (roll up layer of carbon atoms from graphite)

– Fullerene (C-60: 20 hexagons + 12 pentagons)

Carbon nanotubeR ~ few nm

C-60 (a special kind of fullerene)Others: C-70, C-76, C-84, etcR ~ 1 nm

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• exhibits different behaviour than the bulk form.

Physical properties of nanomaterials

• much stronger and stiffer than their bulk forms.

• both strong and ductile.

Badminton racquets made of fibres mixed with carbon nanotubes

Mechanical properties

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Strange optical properties appear in nanomaterials.e,g 1 gold

Gold coin (bulk form) Aqueous colloidal goldParticle:10 – 100 nmBrilliant red in color

Optical properties

e.g.2

Zinc oxide & titanium oxide in nanoscale can block UV that they can be user in sunscreen and cosmetics

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• Most nanomaterials have a lower melting point than their bulk forms.

Variation of the melting point of gold nanoparticles with size

Thermal properties

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A CPU heat sink

• The thermal conductivity of a material depends on its form (e.g. diamond is a good thermal conductor but nanotubes are 2 times better than diamond).

A heat sink with carbon nanotubes

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• Some insulators become conductors in their nano forms.

e.g. SiO2

• The resistivity of metals (normally small) greatly increases when the metal sample is reduced to nanosize.

Electrical properties

Magnetic properties

Hard disk drives record data using ferromagnetic materials.

Most ferromagnetic

materials (can keep magnetization) become paramagnetic (cannot keep magnetization) when they are reduced to nano size.

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Observing at the nanoscale

• Factors affecting the resolving power of a microscope– Optical aberrations (= image error) of the lenses

Limitations of optical microscopes

Spherical aberration :due to curve surface

Chromatic aberration: due to frequency

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• Factors affecting the resolving power of a microscope– Diffraction of light

Light diffracts when it passes through a circular aperture.

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1.circular lens ~ circular aperture2.light diffracts when it passes through a lens of an optical device.

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The diffraction patterns of the two point sources overlap:

difficult to resolve them.

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The Rayleigh criterion

Two objects are just resolvable when the first minimum of

one diffraction patterns falls on the central maximum of the

other.

• For circular aperture, the minimum angular separation of two resolvable objects is

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The resolving power of a microscope is the minimum linear separation (s ) between two resolvable objects. S ~ λ .

Assume u ~ f, ~ (s/f) and f > (D/2)

S ~ 0.6 λ ~ λ

The resolving power of a microscope is limited by λ

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• The transmission electron microscope (TEM) uses electron beams of short wavelengths instead of visible light.

Transmission electron microscopes (TEM)

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E. Coli (大腸桿菌 ) Salmonella bacteria (沙門氏菌 )

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Comparison between a traditional optical microscope and a TEM

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• The scanning tunnelling microscope (STM) scans a surface by detecting the changes in the tunnelling current between the probe and the surface.

Scanning tunnelling microscopes (STM)

An STM Probe of an STM

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• The current is produced by a phenomenon called quantum tunnelling, and is very sensitive to the distance between the tip of the probe and the surface.

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An iron surface with chromium impurities (little bumps) on it

A chain of caesium atoms (orange)on a gallium arsenide surface

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Iron atoms arranging into a circle on a copper surface

Iron atoms arranging to form characters

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Comparison between the TEM and STM

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Recent development in nanotechnology

The technology used in CPUs nowadays reaches the nanoscale.

Electronics and computer hardware

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A CRT display

An FED display(Field emission display)

(Efficient electron emitter)

FED :

1.Reduce thickness

2.Use less power

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Material science

Badminton racquets made of fibres mixed with carbon nanotubes

New bullet-proof vest produced with carbon nanotubes

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Nanomedicine

A nanorobot performing a surgery on a cell

1.Some nanomaterials have specific interactions with special cells

2.target on defective cells/ deliver appropriate treatment

3.nanodoctors: repair cells & defective DNA

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Energy

A solar cell made of nanocrystals of titanium dioxide:

To facilitate conversion of solar energy to electrical energy

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little knowledge about the potential hazards of nanotechnology.Nanoparticles are chemically very reactive : 1.toxic, 2.highly penetrating (even reach DNA and cause DNA mutation), 3.easily absorbed by plants/animals/human body/water sources/food

Potential hazards of nanotechnology