PowerPointpresentatie

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Nanomaterials 2. Physical and Chemical Properties
Petra de Jongh September 2006 P.E.deJongh_at_chem.uu.
nl
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Lectures overview
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Overview of this lecture (2/6)

• Why different properties when things get small?
• Surface to volume ratio
• Electron confinement
• Mechanical properties
• Magnetic properties

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Why different properties when things get small?

• Energetically
• a large fraction of atoms is located at the
  surface -gt
• "surface energy" significant part of total energy
• Geometrically
• so few atoms that they often rearrange compared
  to bulk geometry
• Optically/electrically
• electronic properties are different due to
  electron confinement
• Mechanically
• different concentration and movement of
  dislocations
• Magnetically
• domains so small that they are easily switchable

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Overview of this lecture (2/6)

• Why different properties when things get small?
• Surface to volume ratio (-gt stability)
• melting temperature
• reactivity
• magic numbers I
• Electrical and optical properties
• Mechanical properties
• Magnetic properties

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Surface/volume ratio
For small particles, many of the atoms are
located at the surface
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Surface/volume ratio
Small particles are less stable than bulk
materials, due to the large contribution of the
surface energy
or to put it differently less energy is gained
due to bonding between atoms as the average atom
coordination number is lower

• This has important consequences for
• melting temperature
• evaporation temperature
• reactivity
• mobility

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Surface/volume ratio - melting temperature
example gold nanoparticles
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Surface/volume ratio - reactivity
High surface area is very important if the
functionality of the materials is related to its
surface/interface
e.g. catalysis always uses relatively small
particles (less expensive)
Nanoparticles have higher mobility -gt sintering
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Surface/volume ratio - reactivity
but, as bulk gold is so inert, how can small
particles be chemically reactive (e.g. for the
oxidation of CO)???
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effects in changed chemical (re)activity of
nanoparticles often complex
Surface/volume ratio - reactivity
Interaction with the support is important
Different electronic structure and stability due
to nanosizing Aun might play a role
Haruta et al., J. Catal. 187 (1999) 50
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Surface/volume ratio - reactivity
chemical reactivity is difficult to access
experimentally theoretical calculations can
provide insight

• all materials become less stable if they get
  smaller
• for some materials the energy shift is larger
  than for others
• -gt nanosizing changes reaction conditions

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Surface/volume ratio - magic numbers I
for very small particles (a limited number of
atoms) the surface to volume ratio does not vary
continuously as a function of the number of atoms
geometric "magic numbers" clusters of a certain
number of atoms are more stable than others
(especially for metallic nanoparticles)
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Surface/volume ratio - magic numbers I
so extra stable are (N) 13, 55, 147, 309, ...
This is confirmed by experiments
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Overview of this lecture (2/6)

• Why different properties when things get small?
• Surface to volume ratio
• Electrical and optical properties
• metal versus semiconductor revisited
• electron confinement
• magic numbers II
• Coulomb blockade and single-electron tunneling
• surface plasmon resonance
• Mechanical properties
• Magnetic properties

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Metal versus semiconductor revisited
The position and filling of the band determine
the bulk electrical material properties is it a
metal or a semiconductor? (see "Chemie 3")
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Metal versus semiconductor revisited
The position and filling of the band determine
the bulk electrical material properties is it a
metal or a semiconductor (see "Chemie 3")?

• valence band filled with valence electrons
• conduction band empty -gt not many mobile
  electrons
• extra mobile electrons can be introduced by
  heat, light, doping

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Electron confinement
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Electron confinement - semiconductors
nanoparticles are inbetween atom and bulk material
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Electron confinement - semiconductors
Light absorption can probe the bandgap
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Electron confinement - semiconductors
Confinement depends on dimensionality (and shape)
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Electron confinement - metals
electronic structure of nanoparticles inbetween
atom and bulk material

• if the particle is very
• small, it is no longer a metal!

EF
more details -gt lectures by Celso de Mello-Donega
atom
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Magic numbers II
electron states in nanoparticles resemble atom
orbitals (s, p, d-like) filling (energies) are
different, but some numbers are more favourable
due to closed shells (2, 8, (18), 20, 34, 40, 58,
...)
electronic "magic numbers" clusters with a
certain number of valence (free) electrons are
more stable than others due to closed shells
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Surface plasmon resonance
The colour of gold and silver nanoparticles
varies with size and shape Why?
This is a different phenomenon, not due to
electron confinement

• only (visible) for Au, Ag, and Cu...
• not for radiuslt 5 nm (no free electrons)
• not for 50 nmltradius (band broadened to give
  gold (yellow) colour)

K. L. Kelly et al., J.Phys.Chem.B 107 (2003) 669
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Overview of this lecture (2/6)

• Why different properties when things get small?
• Surface to volume ratio
• Electron confinement
• Magnetic properties
• Mechanical properties

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Magnetic properties magnetism of bulk solids
(Atkins)
Magnetism of solids revisited (Atkins)
permanent magnetic moment
induced magnetic moment counteracting the
field diamagnetic (no unpaired
electrons) aligned with the field paramagnetic
(unpaired electrons/free orbitals)
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Magnetic properties
For nanomaterials an extra magnetic state is
possible
Superparamagnetic (inbetween ferromagnetic and
paramagnetic) below TNeel the nanodomains are
oriented (ferromagnetic), but the domains are
small and can be differently oriented -gt no nett
magnetisation
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Magnetic properties
The fact that it costs little energy to change
the magnetisation in nanoparticles or thin layers
can be used to switchmagnetisation
-gt see applications
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Overview of this lecture (2/6)

• Why different properties when things get small?
• Surface to volume ratio
• Electron confinement
• Magnetic properties
• Mechanical properties

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Mechanical properties
Nanocomposites can show an increase in mechanical
strength
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What you should know
1. consequences of the high surface/volume ratio
of nanoparticles
2. electron confinement, and its consequences
for metals and semiconductors
3. Magic numbers
4. Why are small gold and silver particles
coloured
5. What is superparamagnetism
6. Why nanocomposites have different mechanical
properties than bulk materials
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Coulomb blockade and single-electron tunneling
Nanoparticles have a very small capacitance
(C),hence the charging energy Ec can become
significantly larger than kT
deviation from Ohms law VIR
Coulomb blockade if the applied voltage is not
high enough to supply the charging energy (DVltEc)
no current can flow
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Coulomb blockade and single-electron tunneling
Nanoparticles have a very small capacitance
(C),hence the charging energy Ec can become
significantly larger than kT
deviation from Ohms law VIR
Single electron tunneling at higher voltages one
(EcltDVlt4e2/2C) or more (4e2/2CltDV) electrons can
occupy the nanoparticle simultaneously