NonMetal Nanoparticles

1
Non-Metal Nanoparticles
First, though, one final look at gold
nanoparticles
One-electron energy states
Plasmon energy states
2
Non-Metal Nanoparticles
First, though, one final look at gold
nanoparticles
One-electron energy states
Plasmon energy states
3
Non-Metal Nanoparticles
First, though, one final look at gold
nanoparticles
E
One-electron energy states
Plasmon energy states
Partially filled energy band (metal) of
one-electron energy states
4
Non-Metal Nanoparticles
First, though, one final look at gold
nanoparticles
E
One-electron energy states
Plasmon energy states
Partially filled energy band (metal) of
one-electron energy states
Conduction electrons
5
Non-Metal Nanoparticles
First, though, one final look at gold
nanoparticles
E
Surface Plasmon Band (narrow)
One-electron energy states
Plasmon energy states
Partially filled energy band (metal) of
one-electron energy states
Conduction electrons
6
Non-Metal Nanoparticles
First, though, one final look at gold
nanoparticles
E
Surface Plasmon Band (narrow)
h? energy jump
One-electron energy states
Plasmon energy states
Partially filled energy band (metal) of
one-electron energy states
Conduction electrons
7
What about insulating nanoparticles?
E
Unfilled orbitals
One atom
Filled orbitals
8
What about insulating nanoparticles?
E
Unfilled orbitals
One atom
Filled orbitals
E
Empty energy band
Nanoparticle (d gt 10 nm) or bulk sample.
Filled energy band
9
What about insulating nanoparticles?
E
Unfilled orbitals
One atom
Filled orbitals
E
Empty energy band
Nanoparticle (d gt 10 nm) or bulk sample.
Filled energy band
The band gap in energy between bands is too
great for electrons to jumps up with just thermal
energy.
10
What about insulating nanoparticles?
No UV-Vis absorption except possibly at high,
high UV and X-ray energies
11
What about semiconducting nanoparticles? (e.g.,
CdSe QDs)
E
One atom (like an insulator)
Unfilled orbital
Filled orbital
E
Smaller band gap than insulators small enough
for some electrons to thermally jump up
Valence band
12
What about semiconducting nanoparticles? (e.g.,
CdSe QDs)
E
One atom (like an insulator)
Unfilled orbital
Filled orbital
Nanoparticle (d gt 10 nm) or bulk sample. There
are relatively few conduction electrons in the
upper band, but some.
E
Smaller band gap than insulators small enough
for some electrons to thermally jump up
Valence band
13
Energy Excitations in Semiconducting
Nanoparticles
Surface Plasmons? Yes, weak ones since
conduction electrons are few in number.
14
Energy Excitations in Semiconducting
Nanoparticles
Surface Plasmons? Yes, weak ones since
conduction electrons are few in number. Other
excitations? Yes, excitons.
15
Energy Excitations in Semiconducting
Nanoparticles
Surface Plasmons? Yes, weak ones since
conduction electrons are few in number. Other
excitations? Yes, excitons.
E
conduction
Exciton a photon excites an electron from the
filled valence band to the conduction band,
leaving an electron vacancy in the valence band.
The conduction electron the valence band hole
are the exciton a two-particle excitation.
photon
valence
E
Ephoton gt Eband gap
16
The electron (e-) and the hole (e) behave
somewhat like a hydrogen atom or, more exactly,
like a positronium atom.
bound together but kept apart with kinetic
energy and angular momentum (like the Earth-Sun
system).
The exciton has discrete excited energy states,
like hydrogen and positronium.
17
Poole Owens Intro to Nanotech (2003), Fig. 4.20.
690 nm
345 nm
Optical absorption spectrum for two CdSe
nanoparticles (20 Å and 40 Å) at 10 K the peaks
would be broader at 300 K.
18
Intensity
Energy
Looking at the spectrum, notice its absorption
edgethe lowest energy of absorption. The
absorption edge measures the size of the band gap,
E
The photon must have at least this much energy in
order to be absorbed
conduction
valence
19
690 nm
345 nm
The absorption edge is shifted to higher energy
(shorter wavelength) as the particle becomes
smaller, i.e., the band gap increases in size as
the particle size decreases.
20
Using this data Poole Owen figure 4.20 allows
us to make a beginning at correlating particle
size and band gap size.