1. Adapted from Nanosense
STEM ED/CHM Nanotechnology 2007
Why Size Matters
Adapted from Nanosense

2. Relative sizes Atomic nuclei ~ 10-15 meters = 10-5 nanometers
Atoms ~ meters = 0.1 nanometers
Nanoscale ~ 1 to 100 nanometers
Everyday world ~ 1 meter = nanometers

3. 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

4. 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

5. 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

6. 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
12 nanometer gold particles look red
“Bulk” gold looks yellow
Sources:

7. Optical Properties Example: Zinc Oxide (ZnO)
Large ZnO particles
Block UV light
Scatter visible light
Appear white
Nanosized ZnO particles
So small compared to the wavelength of visible light that they don’t scatter it
Appear clear
Application to sunscreen
Nanoscale ZnO sunscreen is clear
“Traditional” ZnO sunscreen is white
Zinc oxide nanoparticles
Sources: powders.com/images/zno/im_zinc_oxide_particles.jpg

8. 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

9. 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:

10. 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: and
image adapted from

11. Physical Properties Example: Melting Point of a Substance
At the macroscale
At the nanoscale
The majority of the atoms are…
…almost all on the inside of the object
…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 size
… is lower for smaller particles

12. 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

13. 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

14. What Does This All Mean?
The following factors are key for understanding nanoscale-related properties
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