Senior Secondary Science Programme
Sequel to Probing into nano scale
Teachers' Notes
Subject
Physics
Level
S4-S6
Duration:
15 minutes
Key Points
1. The working principle of scanning tunneling microscope.
2. Recent developments and application of nanotechnology in various areas related to daily life.
3. Potential hazards and risks of employing nonotechnolgy.
The programme consists four segments:
1.
Scanning tunneling microscopes
According to quantum mechanics, electron has both particle and wave properties. In the
macroscopic world, the particle nature prevails and while in the microscopic or nanoscale,
wave nature prevails. For a material to conduct, electrons have to overcome the electric
potential barrier. According to classical physics, if electron does not have enough kinetic
energy, it can never pass through the barrier. But in the microscopic world, the electron does
have a small probability of going through the barrier even if it does not have enough kinetic
energy. This process is called “tunnelling effect”. A new kind of microscope, applying the
tunnelling effect, was invented in 1981. It is called scanning tunnelling microscope (STM).
Scanning tunnelling microscope is used for scanning conductive surfaces on the atomic scale.
STM contains a tiny hard needle whose tip is only two or three atoms wide. When a sample is
examined, the sharp tip needle will scan across the surface of the sample. The surface is kept at
a very short distance of just a few nano from it. The needle is at a small positive potential with
respect to the surface. Some electrons are able to jump across from the surface to the tip of the
needle by tunnelling effect. The tunnelling current is very sensitive to the distance between the
tip of the needle and the surface of the specimen. Thus by monitoring the changes in the
tunneling current when the needle moves across the surface or keeping the current constant by
varying the height of the needle, the computer can use the data to construct a magnified
three-dimensional image of the surface.
2.
Nanotechnology in Nature
Phenomenon raised from nanoscale already exists in nature. Nanoparticles can be discovered
in the environment and found among living thing and non-living things.
Lotuses usually grow in muddy pond, but their leaves can remain clean and dry. When we look
at the lotus leafs in detail, we’ll find that the water on the leaf does not spread. Instead water
will form drop on the surface of the leaf. These drops move easily across the surface of the leaf
and take away the dust and dirt to achieve automatic cleaning. This is called “lotus effect”. The
lotus effect is actually related to nano phenomenon. Under a high resolution electronic
microscope, there are small micron-size bumps on the surface of the lotus leaf which are
coated with fine wax nanocrystals. The waxy materials are hydrophobic. Water on the
hydrophobic surface will form water drops and not making the surface wet. Bumps on a flat
hydrophobic surface will turn it into a super-hydrophobic surface. The contact size of water
and surface is small because of the bumpy and small surface, the water drops will become
rounded and roll easily on the leaf. In addition, due to the bumpy surface of the leaf, the
particles of the dirt can only stay on the bumpy surface instead of penetrating it. When the
water drops roll on the leaf surface, they can wash away the dirty particles to achieve the
automatic cleaning effect. Scientists have already tried simulating the lotus effect to our daily
applications. For example, special nanocoatings or nanopaint can be added to tiles and paints
to keep the surface of the house or buildings clean and dry; added to leather, clothes and
raincoats to improve their water-proof and antifouling functions.
Some butterflies’ wing scales, like those of the Paris Peacock, has beautiful colors. The
interested thing is that the colors vary with viewing angles. There are photonic crystals on the
butterfly wings. Photonic crystals are periodic optical nanostructures. The structure can operate
like grating system to produce various colors. For example, there are thousands scales on the
wing of the Paris Peacock. Each scale is scattered with long sharp ridge-like structure. The
bottom of the structure is a multilayer branch-like cuticle, like a big piece of Christmas tree.
The distance between each branch is around 100 nano, and the multilayer Christmas tree liked
structure produced the same effect similar to photonic crystal. It is responsible for the different
colour in the wings of the beautiful butterfly.
Pigeons, bees, birds and turtles have the ability to identify direction so they can fly or dive for
long distance without getting lose the way. It is because there have nanostructures inside their
bodies. These nanostructures are magnetic in property. When they are under the influence of
Earth’s magnetic field, the magnetic nanoparticles will give navigating function to enable the
living creatures to identify directions.
Although Van der Waals’ force is a weak force, there are millions of fine hairs each with a
diameter of a few hundred nano on the feet of a lizard. When millions of nanoscale fine hairs
work together, it will produce a very strong attractive force to support the weight of a lizard.
3. Nanotechnology in daily lives
Nano substances already exist in nature for a long time, but humans have in fact unknowingly
used them in our daily life. Medieval artisans knew that they could produce stained glass of
different colors by adding different metal oxides to the molten glass. Stained glass is a kind of
western architectural decorations, usually found on the wall of the church. Stained glass can also
be seen in the Islamic mosques. It can cause dazzling effect when sunlight shines on the glass.
At night time, the colorful lights made by lighting created various colors. This effect is caused
by the nanoparticles produced in the mixing process. It was these nanoparticles that gave the
glass distinctive colors.
Nanotechnology can be applied to building materials like tiles, glasses and paints to make it
with self-cleaning and anti-fouling function. The common self-cleaning tiles and glass can be
divided into two types. One of these is applying the lotus effect. The coating on the surface of
glass or tiles is nanoparticles which are hydrophobic. Droplets of water are formed on the
surface when rain falls on it. The droplets roll easily carrying away the dirt to achieve the
purpose of self-cleaning. Another type of self-cleaning tiles and glasses is using photocatalyst
on the surface. As the advancement of nanotechnology, some nanomaterials can accelerate
chemical reaction in the presence of light without themselves being changed. They are called
photocatalysts. Two common photocatalysts are titanium dioxide and zinc oxide nanoparticles.
When titanium dioxide nanoparticles are used as coating on a glass surface with Sun shining on
it, water falls on the surface will become hydrophilic and will spread evenly on it forming a thin
film rather than droplets. Dirt attached in the film of water can easily be removed as the film of
water slides down during raining. In addition, when photocatalyst are exposed to sunlight, free
radicals are produced.
These radicals can break down the organic dirt to produce self-cleaning,
sterilize and deodorant effect.
4. Nanotechnology and the future
Nanotechnology has developed rapidly. The products have various applications, like carbon
nanotube, nano battery, nano building materials, nano filter membrane, smart windows and
ultra-thin display, etc. On the one hand, nanotechnology has raised safety and environmental
concerns. Because of the high ratio of surface area to volume of nano particles, they have a very
high chemical activity and may cause unexpected reaction to human bodies. On the other hand,
the functional cells of human body have a size much larger than nano. Therefore, nanoparticles
or fiber can enter the human body through respiratory system, blood and skin, or even penetrate
into the genes through cell membrane. This will cause genetic changes. In addition,
nanoparticles are very active. If they leak out accidentally, they will spread by winds and water
and lead to uncontrollable nano pollution
If nanotechnology continues advancing, the prevailing perception and moral concept of human
beings will be challenged. Try to imagine, when nano materials become popular, undoubtedly it
will replace most of the materials we use today because it’s lighter, thinner, more supreme, more
durable and more functional. A display thinner than a sheet of paper; a computer that can fold
and put in the pocket; implanted navigation system; implanted nano detector; nano spaceship
that flies everywhere. The development of Nanotechnology is unavoidable. The urgent issue is
how to live with it and to set up a safety standards.
Suggested Learning Activities
Preparation before viewing the programme
1.
Teacher may discuss with students how nanotechnology is involved in daily life
Activities after viewing the programme
1.
2.
Student may, under the guidance of teacher, review the physical concepts presented in the
programme.
The teacher may discuss with students the impact and hazard when nanotechnology becomes
parts of our life.