Metal: Bulk to Nanoparticle Transition

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Metal Bulk to Nanoparticle Transition
Energy
Bulk (with a partially filled energy bandtypical
of metals)
A partially filled band
Filled states
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Metal Bulk to Nanoparticle Transition
Energy
Bulk (with a partially filled energy bandtypical
of metals)
A partially filled band
Filled states
Whats the connection between a bulk piece of
metal and a metal nanoparticle? (Closer than you
might think!)
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For metal clusters, bulk descriptions (using the
band energy picture) are accurate down to
nanoparticles with diameters of 10 nm about
30,000 atoms. NB a nanoparticle 20 nm across
would contain about 200,000even a 1 nm
nanoparticle would have 1000 within it.
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For metal clusters, bulk descriptions (using the
band energy picture) are accurate down to
nanoparticles with diameters of 10 nm about
30,000 atoms. NB a nanoparticle 20 nm across
would contain about 200,000even a 1 nm
nanoparticle would have 1000 within it.
What is happening in these bulk-like
nanoparticles?
Graph of all possible energy states (filled and
unfilled)
Number of States
Electron distribution at T 0 K.
Filled
Unfilled
Energy
EF Fermi energy
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For metal clusters, bulk descriptions (using the
band energy picture) are accurate down to
nanoparticles with diameters of 10 nm about
30,000 atoms. NB a nanoparticle 20 nm across
would contain about 200,000even a 1 nm
nanoparticle would have 1000 within it.
What is happening in these bulk-like
nanoparticles?
Number of States
Electron distribution at T 300 K.
Unfilled
Electrons excited above EF by thermal energy (kT)
Filled
Energy
EF Fermi energy
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For metal clusters, bulk descriptions (using the
band energy picture) are accurate down to
nanoparticles with diameters of 10 nm about
30,000 atoms. NB a nanoparticle 20 nm across
would contain about 200,000even a 1 nm
nanoparticle would have 1000 within it.
What is happening in these bulk-like
nanoparticles?
Number of States
Electron distribution at T 300 K.
Unfilled
Electrons excited above EF by thermal energy (kT)
Filled
Energy
EF Fermi energy
25 meV at 300 K
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Distribution of electrons by energy in a bulk
solid
Number of electrons
0 K
300 K
EF
Energy
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Distribution of electrons by energy in a bulk
solid
Number of electrons
0 K
300 K
Energy kT
EF
Energy
Raising the samples temperature from absolute 0
to room temperature provides energy for any
electron to increase in energy, BUT only those
near EF have open energy levels above them and
can take advantage.
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Distribution of electrons by energy in a bulk
solid
Number of electrons
Incoming light energy
0 K
300 K
EF
Energy
It is the uppermost (most energetic) electrons
that are also able to absorb light energyas
happens in generating a UV-Vis absorption
spectrum.
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Distribution of electrons by energy in a bulk
solid
Number of electrons
Incoming light energy
0 K
300 K
Energy h?
EF
Energy
It is the uppermost (most energetic) electrons
that also are able to absorb light energyas
happens in generating a UV-Vis absorption
spectrum.
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Result It is a relatively small number of
thermalized electrons that move if excited by
light. These thermally activated electrons are
free to oscillate if driven (electromagnetically)
by a light source surface plasmons. The
surface conduction electrons in larger
nanoparticles move in collective oscillations
plasmons.
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Result It is a relatively small number of
thermalized electrons that move if excited by
light. These thermally activated electrons are
free to oscillate if driven (electromagnetically)
by a light source surface plasmons. The
surface conduction electrons in larger
nanoparticles move in collective oscillations
plasmons.
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Time
(above a dipole plasmon)
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These collective oscillations are multipole
dipole, quadrupole, and higher symmetry patterns
will all occur at the same time if allowed.
These oscillations require a certain size of
nanoparticle if the particles diameter is less
than 20 nm, then only dipole plasmons contribute
significantly to UV-Vis absorption. (Dipole
plasmons require the lowest energy.)

• General facts for UV-Vis absorption due to
  surface plasmons
• Theres a redshift in absorption as the
  nanoparticle gets larger. (Larger particles
  absorb more red than smaller particles do.)
• The bandwidth of the absorption spectrum
  increases with increasing particle size

Absorp.
Absorp.
?
?
40 nm
100 nm
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Surface plasmon absorption of spherical
nanoparticles and its size dependence. From S.
Link M.A. El-Sayed, Shape Size Dependence of
Radiative, Non-Radiative and Photothermal
Properties of Gold Nanocrystals Int. Reviews of
Physical Chemistry (2000) vol. 19, no. 3, pages
409-453.
The redshift and increase in bandwidth with
increasing nanoparticle size