Why Size MattersWhy Size Matters

Adapted from NanosenseAdapted from Nanosense

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

STEM ED/CHM Nanotechnology 2007

Relative sizesRelative sizes

Atomic nuclei ~ 10Atomic nuclei ~ 10-15-15 meters meters = 10= 10-5 -5 nanometersnanometers

Atoms ~ 10Atoms ~ 10-10 -10 meters = 0.1 nanometers meters = 0.1 nanometers

Nanoscale ~ 1 to 100 nanometersNanoscale ~ 1 to 100 nanometers

Everyday world ~ 1 meter Everyday world ~ 1 meter = 10= 1010 10 nanometers nanometers

The Basic PhysicsThe Basic Physics

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

At the everyday scale, Newton’s laws At the everyday scale, Newton’s laws (classical physics) work fine(classical physics) work fineNanomaterials are in a borderline region Nanomaterials are in a borderline region where either or both approaches may be where either or both approaches may be appropriateappropriate

The Basic ForcesThe Basic Forces

Strong Nuclear Force – huge, hold nuclei Strong Nuclear Force – huge, hold nuclei together; act only at nuclear distances, 10together; act only at nuclear distances, 10 -5 -5 nmnmWeak Nuclear Force – small, responsible for Weak Nuclear Force – small, responsible for nuclear beta decay, act only at nuclear nuclear beta decay, act only at nuclear distances, 10distances, 10-5 -5 nmnmElectric and Magnetic – dominant at atomic and Electric and Magnetic – dominant at atomic and nanotech scales; 10nanotech scales; 103939 ×× gravitational forces; long gravitational forces; long ranged, 1/rranged, 1/r22

Gravitational – long ranged, 1/rGravitational – long ranged, 1/r22; dominant at ; dominant at everyday world scale, since most objects lack a everyday world scale, since most objects lack a substantial net electrical chargesubstantial net electrical charge

Properties of a MaterialProperties of a MaterialA property describes how a material acts A property describes how a material acts under certain conditionsunder certain conditions

Types of propertiesTypes of properties Optical (e.g. color, transparency)Optical (e.g. color, transparency) Electrical (e.g. conductivity)Electrical (e.g. conductivity) Physical (e.g. hardness, melting point, Physical (e.g. hardness, melting point,

diffusion rate)diffusion rate) Chemical (e.g. reactivity, reaction rates)Chemical (e.g. reactivity, reaction rates)

Properties are usually measured by looking at Properties are usually measured by looking at large (~10large (~102323) aggregations of atoms or ) aggregations of atoms or moleculesmolecules

Optical Properties Example: Optical Properties Example: GoldGold

Bulk gold appears yellow in colorBulk gold appears yellow in color

Nanosized gold appears red in colorNanosized gold appears red in color The particles are so small that electrons The particles are so small that electrons

are not free to move about as in bulk goldare not free to move about as in bulk gold Because this movement is restricted, the Because this movement is restricted, the

particles react differently with lightparticles 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:Optical Properties Example: Zinc Oxide (ZnO) Zinc Oxide (ZnO)

Large ZnO particles Large ZnO particles Block UV lightBlock UV light Scatter visible light Scatter visible light Appear whiteAppear white

Nanosized ZnO particlesNanosized ZnO particles Block UV lightBlock UV light So small compared to the So small compared to the

wavelength of visible light that wavelength of visible light that they don’t scatter it they don’t scatter it

Appear clearAppear clear

Application to sunscreenApplication to sunscreen

Physical Properties: DiffusionPhysical Properties: Diffusion

Small particles (molecules in suspensions, dust Small particles (molecules in suspensions, dust particles in air) move randomly particles in air) move randomly in zigzag paths (Brownian motion) in zigzag paths (Brownian motion) due to collisions due to collisions Particles spread out (diffuse) when introduced into a Particles spread out (diffuse) when introduced into a medium at one pointmedium at one point

Perfume in a room Perfume in a room

Average kinetic energy ½ mvAverage kinetic energy ½ mv2 2 ~ temperature ~ temperatureAverage particle speeds decrease as mass Average particle speeds decrease as mass increases, so more massive particles diffuse more increases, so more massive particles diffuse more slowlyslowlyWe explore this phenomenon with the gelatin We explore this phenomenon with the gelatin diffusion experimentdiffusion experiment

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

Electrical Properties Example: Electrical Properties Example: Conductivity of NanotubesConductivity of Nanotubes

Nanotubes are long, thin cylinders of carbonNanotubes are long, thin cylinders of carbon They are 100 times stronger than steel, very flexible, They are 100 times stronger than steel, very flexible,

and have unique electrical propertiesand have unique electrical properties

Their electrical properties change with diameter, Their electrical properties change with diameter, “twist”, and number of walls“twist”, and number of walls They can be either conducting or semi-conducting in They can be either conducting or semi-conducting in

their electrical behaviortheir electrical behavior

Electric current varies by tube

structure

Multi-walled

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

Physical Properties Change:Physical Properties Change:Melting Point of a SubstanceMelting Point of a Substance

Melting Point (Microscopic Definition)Melting Point (Microscopic Definition) Temperature at which the atoms, ions, or Temperature at which the atoms, ions, or

molecules in a substance have enough energy molecules in a substance have enough energy to overcome the intermolecular forces that hold to overcome the intermolecular forces that hold the them in a “fixed” position in a solidthe them in a “fixed” position in a solid

In contact with 3 atoms

In contact with 7 atoms

Surface atoms require Surface atoms require lessless energy energy to move because they are in contact to move because they are in contact with with fewerfewer atoms of the substance atoms of the substance

Physical Properties Example:Physical Properties Example:Melting Point of a SubstanceMelting Point of a Substance

At the At the macroscalemacroscale

At the nanoscaleAt the nanoscale

The majority of The majority of the atoms the atoms are…are…

……almost all on the almost all on the inside of the objectinside of the object

……split between the inside and split between the inside and the surface of the objectthe surface of the object

Changing an Changing an object’s size…object’s size…

……has a very small has a very small effect on the effect on the percentage of atoms percentage of atoms on the surfaceon the surface

……has a big effect on the has a big effect on the percentage of atoms on the percentage of atoms on the surface surface

The melting The melting point…point…

……doesn’t depend on doesn’t depend on sizesize

… … is lower for smaller particlesis lower for smaller particles

Surface to Volume Ratio Surface to Volume Ratio ExperimentsExperiments

As a sample is made larger, a smaller fraction of As a sample is made larger, a smaller fraction of the atoms (or molecules) are on the surfacethe atoms (or molecules) are on the surfaceAtoms on the surface have fewer neighbors than Atoms on the surface have fewer neighbors than those on the interiorthose on the interior Students at the edge of the classroom have fewer Students at the edge of the classroom have fewer

neighbors than those in the centerneighbors than those in the center

Explore this with two activities – cards, blocksExplore this with two activities – cards, blocksOnly atoms on the surface can interact with Only atoms on the surface can interact with another material and take part in a chemical another material and take part in a chemical reactionreactionExplore this with Alka Seltzer tablets and powderExplore this with Alka Seltzer tablets and powder

Size Matters in BiologySize Matters in Biology

Metabolism (heat generation) is limited by the Metabolism (heat generation) is limited by the number of cells, or volume, Lnumber of cells, or volume, L3 3

Heat loss to the environment is proportional to Heat loss to the environment is proportional to the surface area, Lthe surface area, L22

As we look at smaller and smaller organisms, As we look at smaller and smaller organisms, the surface to volume ratio Lthe surface to volume ratio L22/ L/ L33 = 1/L gets = 1/L gets larger and larger, making it harder to maintain larger and larger, making it harder to maintain body temperaturebody temperatureSmallest warm blooded organisms are Smallest warm blooded organisms are hummingbirds and a small mouse-like mammalhummingbirds and a small mouse-like mammal

What Does This All Mean?What Does This All Mean?The following factors are key for The following factors are key for understanding nanoscale-related propertiesunderstanding nanoscale-related properties Dominance of electromagnetic forcesDominance of electromagnetic forces Importance of quantum mechanical modelsImportance of quantum mechanical models Higher surface area to volume ratioHigher surface area to volume ratio Random (Brownian) motionRandom (Brownian) motion

It is important to understand these four It is important to understand these four factors when researching new materials factors when researching new materials and propertiesand properties