Transcript Why Size Matters

STEM ED/CHM Nanotechnology 2007
Why Size Matters
Adapted from Nanosense
http://nanosense.org/activities/sizematters/properties/SM_PropSlides.ppt
Relative sizes
Atomic nuclei ~ 10-15 meters
= 10-5 nanometers
Atoms ~ 10-10 meters = 0.1 nanometers
Nanoscale ~ 1 to 100 nanometers
Everyday world ~ 1 meter
= 1010 nanometers
The Basic Physics
At the atomic and molecular level,
quantum mechanics is needed to describe
phenomena and properties

Discrete energy levels, tunneling
At the everyday scale, Newton’s laws
(classical physics) work fine
Nanomaterials are in a borderline region
where either or both approaches may be
appropriate
The Basic Forces
Strong Nuclear Force – huge, hold nuclei
together; act only at nuclear distances, 10-5 nm
Weak Nuclear Force – small, responsible for
nuclear beta decay, act only at nuclear
distances, 10-5 nm
Electric and Magnetic – dominant at atomic and
nanotech scales; 1039 × gravitational forces; long
ranged, 1/r2
Gravitational – long ranged, 1/r2; dominant at
everyday world scale, since most objects lack a
substantial net electrical charge
Properties of a Material
A property describes how a material acts
under certain conditions
Types of properties
 Optical (e.g. color, transparency)
 Electrical (e.g. conductivity)
 Physical (e.g. hardness, melting point,
diffusion rate)
 Chemical (e.g. reactivity, reaction rates)
Properties are usually measured by
looking at large (~1023) aggregations of
atoms or molecules
Optical Properties Example:
Gold
Bulk gold appears yellow in color
Nanosized gold appears red in color


The particles are so small that electrons
are not free to move about as in bulk gold
Because this movement is restricted, the
particles react differently with light
“Bulk” gold looks yellow
12 nanometer gold particles look red
Sources: http://www.sharps-jewellers.co.uk/rings/images/bien-hccncsq5.jpg
http://www.foresight.org/Conferences/MNT7/Abstracts/Levi/
Optical Properties Example:
Zinc Oxide (ZnO)
Large ZnO particles

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Block UV light
Scatter visible light
Appear white
Nanosized ZnO particles


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Block UV light
So small compared to the
wavelength of visible light that
they don’t scatter it
Appear clear
“Traditional” ZnO
sunscreen is white
Application to sunscreen
Sources: http://www.apt powders.com/images/zno/im_zinc_oxide_particles.jpg
http://www.abc.net.au/science/news/stories/s1165709.htm
http://www.4girls.gov/body/sunscreen.jpg
Nanoscale ZnO
sunscreen is clear
Zinc oxide nanoparticles
Physical Properties: Diffusion
Small particles (molecules in suspensions, dust
particles in air) move randomly
in zigzag paths (Brownian motion)
due to collisions
Particles spread out (diffuse) when introduced into a
medium at one point

Perfume in a room
Average kinetic energy ½ mv2 ~ temperature
Average particle speeds decrease as mass
increases, so more massive particles diffuse more
slowly
We explore this phenomenon with the gelatin
diffusion experiment
Electrical Properties Example:
Conductivity of Nanotubes
Nanotubes are long, thin cylinders of carbon

They are 100 times stronger than steel, very flexible,
and have unique electrical properties
Their electrical properties change with diameter,
“twist”, and number of walls

They can be either conducting or semi-conducting in
their electrical behavior
Electric current
varies by tube
structure
Multi-walled
Source: http://www.weizmann.ac.il/chemphys/kral/nano2.jpg
Physical Properties Change:
Melting Point of a Substance
Melting Point (Microscopic Definition)

Temperature at which the atoms, ions, or
molecules in a substance have enough energy
to overcome the intermolecular forces that hold
the them in a “fixed” position in a solid

Surface atoms require less
energy to move because they are
in contact with fewer atoms of the
substance
In contact with 3 atoms
In contact with 7 atoms
Sources: http://puffernet.tripod.com/thermometer.jpg and
image adapted from http://serc.carleton.edu/usingdata/nasaimages/index4.html
Physical Properties Example:
Melting Point of a Substance
At the
macroscale
The majority of …almost all on the
inside of the object
the atoms
are…
At the nanoscale
…split between the inside and
the surface of the object
Changing an
object’s size…
…has a very small
effect on the
percentage of atoms
on the surface
…has a big effect on the
percentage of atoms on the
surface
The melting
point…
…doesn’t depend on … is lower for smaller particles
size
Surface to Volume Ratio
Experiments
As a sample is made larger, a smaller fraction of
the atoms (or molecules) are on the surface
Atoms on the surface have fewer neighbors than
those on the interior

Students at the edge of the classroom have fewer
neighbors than those in the center
Explore this with two activities – cards, blocks
Only atoms on the surface can interact with
another material and take part in a chemical
reaction
Explore this with Alka Seltzer tablets and powder
Size Matters in Biology
Metabolism (heat generation) is limited by the
number of cells, or volume, L3
Heat loss to the environment is proportional to
the surface area, L2
As we look at smaller and smaller organisms,
the surface to volume ratio L2/ L3 = 1/L gets
larger and larger, making it harder to maintain
body temperature
Smallest warm blooded organisms are
hummingbirds and a small mouse-like mammal
What Does This All Mean?
The following factors are key for
understanding nanoscale-related
properties
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Dominance of electromagnetic forces
Importance of quantum mechanical models
Higher surface area to volume ratio
Random (Brownian) motion
It is important to understand these four
factors when researching new materials
and properties