Chapter 2 - Size and Quantum effects of nanoparticles Flashcards
How does NP structure change with size?
- large NPs have bulk-like crystalline structure, and low surface to volume ratios
- smaller NPs can have different geometrical structures duet o competition between bulk and surface energies
- small cluster - structures result from complex interplay between geometric and electronic effects
What properties are intimately connected to their nanoscale size and atomic-scale structure?
- chemical properties: reactivity, catalysis
- thermal properties: melting point
- mechanical properties: adhesion, capillary forces
- optical properties: absorption and light scattering
- electrical properties: tunneling current
- magnetic properties: super paramagnetic effect
What are the 2 factors that cause nanomaterials to differ significantly from bulk?
- increased surface area
- quantum effects
What is the surface area of a cylinder?
2pi()r^2 + 2pi()r*h
What is the volume of a cylinder?
pi()r^2h
What is dispersion?
The fraction of atoms at the surface
How does dispersion scale?
it scales with SA/V, or inverse of the radius or diameter (thus also with N^-1/3 or r^-1 or d^-1)
*N = total number of atoms in a sphere
Classification of nanomaterials?
how does material deal with electrons?
- metals (conductors): materials with delocalized electrons
- insulators: materials with localized electrons
*semi-conductors exist between the above 2 classifications - material with new structures and properties due to their nanostructure (ex., carbon nanotubes)
How do metals work?
Their valence band and conductive band overlap (no band gap), allowing electrons to flow freely
How do insulators work?
There is a wide band gap between the conduction and valence bands, and it requires significant energy to jump between them
How do semiconductors work?
They have a small band gap, so adding energy allows the electrons to jump into the delocalized state
What is Fermi energy?
The energy of the highest occupied electron state at 0K
What happens to energy levels as they change from bulk to nanoscale?
- the energy states become discretized and the levels get similar to a molecular orbital structure rather than a conductive and valence band (as in the bulk).
The indefinite situation in a bulk material is the highest occupied molecular orbital becomes the Fermi energy of the free electron
What is the Quantum effect?
they begin to dominate the properties of matter as size reduces to the nanoscale
They can affect the optical, electrical, catalytic, and magnetic behaviour of materials
What is Quantum physics
describes the science of things smaller than a billionth of a meter
- scale of atoms, subatomic particles and waves of light
What is Quantum?
Quanta - is the minimum amount of any physical entity involved in an interaction
Quanta of matter or electricity = electrons
Follows the E = hf (h is plank’s constant and f is frequency of wavelength)
What are Quantum effects?
Effects that are not properly predicted by classic physics but is predicted by quantum theory
What is quantum mechanics?
solid particles behave like waves
What is the quantum wavefunction
matter and energy sometimes act like particles and sometimes act like waves
What is the wave-particle duality?
electrons spread as waves of various wavelengths called DeBroglie wavelength:
lambda = h/mv
- delocalized electrons are possible as long as the dimension of the metal particle is a multiple of the DeBroglie wavelength
What is Quantum confinement?
the restriction of quasi-freely mobile electrons in a piece of bulk metal
- accomplished by reduction of the volume of the bulk material and by reducing the dimensionality
What are the 3 different types of quantum confinement?
- quantum well - restricted electrons to move in 2 dimensions
- quantum wire - restricted electrons to move in 1 dimension
- quantum dot - restricted electron movement in all three directions
What are defects?
How do they apply to NP properties?
- apply only to localized electrons
- defects are most significant contributor to their properties at nanoscale
What different types of defects?
in lattice structure (more point than anything)
- n-fold coordination (has less than 6 bonded atoms)
- kink - atom is missing
- adatom - atom is added
- valley - atom is missing and surrounded by all sides
- terrace - a flat point (series of 3-fold coordination)
How to calculate coordination
-count number of atoms around them
ex. surface atoms are 5-coordination
- edge atoms are 4-coordination
- corner atoms are 3-coordination
How do defects impact NPs
- more abundant at surface than in bulk, so more predominant at nanoscale (since NPs possess a large surface area relative to their volume)
- surface atoms are responsible for many of the unique properties of NPs
Types of defects?
- common point defects -
- interstital - a small solute atom sits between gaps of solvent atom
- substitutional - 2 atoms are swap places - dislocation - crystal formation in different planes
- schottky defects - missing atoms
- Frenkel defects - missing atoms located in interstitial locations (for each missing, have an interstitial)
- twin boundary - multiple dislocations
- grain boundary - many out of phase crystal planes that line up against each other
