Copyright © 2005 SRI International

Unique Properties at the NanoscaleThe Science Behind Nanotechnology

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What’s So Special About Nano?

• Using new scientific tools, we have found that nanosized particles of a given substance exhibit different properties than larger particles of the same substance

• As we study these materials at the nanoscale, we are– Learning more about the nature of matter– Developing new theories– Learning how to manipulate their properties to

develop new products and technologies

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Size-Dependent Properties

• Properties of a material– Describe how the material acts under

certain conditions– Are often measured by looking at large

(~ 1023) aggregation of atoms or molecules

• Types of properties that often change– Optical (e.g. color, transparency)– Electrical (e.g. conductivity)– Physical (e.g. hardness, boiling point)– Chemical (e.g. reactivity, reaction rates)

Sources: http://www.bc.pitt.edu/prism/prism-logo.gif http://www.physics.umd.edu/lecdem/outreach/QOTW/pics/k3-06.gif

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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.jpg http://www.foresight.org/Conferences/MNT7/Abstracts/Levi/

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

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“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.jpg http://www.abc.net.au/science/news/stories/s1165709.htm http://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

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Meaningless Properties

• At the nanoscale, some properties don’t make sense

• One example is boiling temperature– Vapor pressure becomes less and less

meaningful when you have smaller and smaller numbers of particles

– When you have 50 molecules there are no bubbles!

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Why Do Properties Change?

• Four important ways in which nanoscale materials may differ from macroscale materials– Gravitational forces become negligible and

electromagnetic forces begin to dominate– Quantum mechanics is used to describe

motion and energy instead of classical mechanics

– Greater surface to volume ratios– Random molecular motion becomes more

important

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Dominance of Electromagnetic Forces

• Because the mass of nanoscale objects is so small, gravitational force becomes negligible– Gravitational force is a function of mass and is

weak between nanosized particles– Electromagnetic forces are not affected by

mass, so they can be very strong even when we have nanosized particles

– Electromagnetic forces between two protons are 1036 times stronger than gravitational forces

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Quantum Mechanical Model Needed

• Classical mechanical models explain phenomena well at the macroscale level, but often break down at the nanoscale level

• Four phenomena that quantum mechanical models can explain (but classical mechanical models cannot)– Discreteness of energy– The wave-particle duality of light and matter– Quantum tunneling– Uncertainty of measurement

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Surface to Volume Ratio Increases

• As surface to volume ratio increases – A greater amount of a

substance comes in contact with surrounding material

– This results in better catalysts, since a greater proportion ofthe material is exposed for potential reaction

Source: http://www.uwgb.edu/dutchs/GRAPHIC0/GEOMORPH/SurfaceVol0.gif

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Source: http://galileo.phys.virginia.edu/classes/109N/ more_stuff/Applets/brownian/brownian.html

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are needed to see this picture.

Random Molecular Motion is Significant

• Random motion at the macroscale– Small compared the size of the object– We can barely detect motion of dust particles

on the surface of water• Random motion at the the nanoscale

– Large when compared to the size of the object– The molecules that make up the dust particle

are moving wildly compared with the size of the particle

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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 to volume ratio– Random (Brownian) motion

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