Why Size Matters

Adapted from Nanosense

http://nanosense.org/activities/sizematters/properties/SM_PropSlides.ppt

STEM ED/CHM Nanotechnology 2012

Relative sizes (review)

• Atomic nuclei ~ 10-15 meters = 10-6 nanometers

• Atoms ~ 10-10 meters = 0.1 nanometers

• Nanoscale ~ 1 to 100 nanometers ~ 10 to 1000 atoms

• Everyday world ~ 1 meter = 109 nanometers

The Basic Physics

• At the everyday scale, Newton’s laws (F=ma, etc.) work fine

• At the atomic and molecular level, quantum mechanics is needed to describe phenomena and properties– Discrete energy levels, tunneling

• 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, 10-6 nm

• Weak Nuclear Force – small, responsible for nuclear beta decay, act only at nuclear distances, 10-6 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• 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

Sources: http://www.sharps-jewellers.co.uk/rings/images/bien-hccncsq5.jpghttp://www.foresight.org/Conferences/MNT7/Abstracts/Levi/

12 nanometer gold particles look red“Bulk” gold looks yellow

“Traditional” ZnO sunscreen is white

Zinc oxide nanoparticles

Nanoscale ZnO sunscreen is clear

Sources: http://www.apt powders.com/images/zno/im_zinc_oxide_particles.jpghttp://www.abc.net.au/science/news/stories/s1165709.htmhttp://www.4girls.gov/body/sunscreen.jpg

Optical Properties Example: Zinc Oxide (ZnO)

• Large ZnO particles – Block UV light– Scatter visible light – Appear white

• Nanosized ZnO particles– Block UV light– So small compared to the

wavelength of visible light that they don’t scatter it

– Appear clear

• Application to sunscreen

Source: http://www.weizmann.ac.il/chemphys/kral/nano2.jpg

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

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 or 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

Sources: http://puffernet.tripod.com/thermometer.jpg and image adapted from http://serc.carleton.edu/usingdata/nasaimages/index4.html

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

In contact with 3 atoms

In contact with 7 atoms

– Surface atoms require less energy to move because they are in contact with fewer atoms of the substance

A flower or a person at the edge of a crowd has fewer neighbors than one in the middle

People at the edge can move more easily

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 (even with feathers, fur)

• Smallest warm blooded organisms are hummingbirds and the shrew, a small mouse-like mammal

What Does This All Mean?• Key factors 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

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 (cards, blocks)

• Only atoms on the surface can interact with another material and take part in a chemical reaction (Alka Seltzer)

Activities

• Groups of 3 people• Write ups, cards, Alka Seltzer materials• Explore the effects of increasing size with the

cards • Do the Alka- Seltzer experiment to see the

effect of particle size on chemical processes