Understanding Nanoscale Properties: Why Size Matters in Nanotechnology

1. STEM ED/CHM Nanotechnology 2012 Why Size Matters Adapted from Nanosense http://nanosense.org/activities/sizematters/properties/SM_PropSlides.ppt

2. 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

3. 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

4. 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

5. 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

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

7. 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 Nanoscale ZnO sunscreen is clear “Traditional” ZnO sunscreen is white Zinc oxide nanoparticles 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

8. 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

9. 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

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

11. 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

12. 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

13. 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

14. 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)

15. 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