6.2. NANOSTRUCTURED CRYSTALS
157
6.2.4. Crystals of Metal Nanoparticles
A two-phase water toluene reduction of AuC1,- by sodium borohydride in the
presence of an alkanethiol (C 2H25SH) solution produces gold nanoparticles Au,
having a surface coating of thiol, and embedded in an organic compound. The
overall reaction scheme is
AuCl,-(aq) + N(C8H17)4 + (C6H5Me) + N(C8HI7), + AuC14-(C6H5Me) (6.10)
mAuCl,-(C,H,Me) + n(C12H25SH)(C6H5Me) + 3me- +
4mCl-(aq) + (Au,)(C,,H,~SH),(C~H,M~) (6.1 I)
Essentially, the result of the synthesis is a chemical compound denoted as c-Au:SR,
where SR is (C12H25SH)n(C6H5Me), and Me denotes the methyl radical CH3. When
the material was examined by X-ray diffraction it showed, in addition to the diffuse
peaks from the planes of gold atoms in the nanoparticles Aum, a sequence of sharp
peaks at low scattering angles indicating that the gold nanoparticles had formed a
giant three-dimensional lattice in the SR matrix. The crystal structure was determined to be a body-centered cubic (BCC) arrangement. In effect, a highly ordered
symmetric arrangement of large gold nanoparticles had spontaneously selfassembled during the chemical processing.
Superlattices of silver nanoparticles have been produced by aerosol processing.
The lattices are electrically neutral, ordered arrangements of silver nanoparticles in a
dense mantel of alkylthiol surfactant, which is a chain molecule, n-CH3(CH2),,,SH.
The fabrication process involves evaporation of elemental silver above 1200°C into a
flowing preheated atmosphere of high-purity helium. The flow stream is cooled over
a short distance to about 400 K, resulting in condensation of the silver to nanocrystals. Growth can be abruptly terminated by expansion of the helium flow through a
conical funnel accompanied by exposure to cool helium. The flowing nanocrystals are condensed into a solution of alkylthiol molecules. The material produced
in this way has a superlattice structure with FCC arrangement of silver nanoparticles
having separations of t 3 n m . Metal nanoparticles are of interest because of the
enhancement of the optical and electrical conductance due to confinement and
quantization of conduction electrons by the small volume of the nanocrystal.
When the size of the crystal approaches the order of the de Broglie wavelength of
the conduction electrons, the metal clusters may exhibit novel electronic properties.
They display very large optical polarizabilities as discussed earlier, and nonlinear
electrical conductance having small thermal activation energies. Coulomb blockade
and Coulomb staircase current-voltage curves are observed. The Coulomb blockade
phenomenon is discussed in Chapter 9. Reduced-dimensional lattices such as
quantum dots and quantum wires have been fabricated by a subtractive approach
that removes bulk fractions of the material leaving nanosized wires and dots. These
nanostructures are discussed in Chapter 9.
157
6.2.4. Crystals of Metal Nanoparticles
A two-phase water toluene reduction of AuC1,- by sodium borohydride in the
presence of an alkanethiol (C 2H25SH) solution produces gold nanoparticles Au,
having a surface coating of thiol, and embedded in an organic compound. The
overall reaction scheme is
AuCl,-(aq) + N(C8H17)4 + (C6H5Me) + N(C8HI7), + AuC14-(C6H5Me) (6.10)
mAuCl,-(C,H,Me) + n(C12H25SH)(C6H5Me) + 3me- +
4mCl-(aq) + (Au,)(C,,H,~SH),(C~H,M~) (6.1 I)
Essentially, the result of the synthesis is a chemical compound denoted as c-Au:SR,
where SR is (C12H25SH)n(C6H5Me), and Me denotes the methyl radical CH3. When
the material was examined by X-ray diffraction it showed, in addition to the diffuse
peaks from the planes of gold atoms in the nanoparticles Aum, a sequence of sharp
peaks at low scattering angles indicating that the gold nanoparticles had formed a
giant three-dimensional lattice in the SR matrix. The crystal structure was determined to be a body-centered cubic (BCC) arrangement. In effect, a highly ordered
symmetric arrangement of large gold nanoparticles had spontaneously selfassembled during the chemical processing.
Superlattices of silver nanoparticles have been produced by aerosol processing.
The lattices are electrically neutral, ordered arrangements of silver nanoparticles in a
dense mantel of alkylthiol surfactant, which is a chain molecule, n-CH3(CH2),,,SH.
The fabrication process involves evaporation of elemental silver above 1200°C into a
flowing preheated atmosphere of high-purity helium. The flow stream is cooled over
a short distance to about 400 K, resulting in condensation of the silver to nanocrystals. Growth can be abruptly terminated by expansion of the helium flow through a
conical funnel accompanied by exposure to cool helium. The flowing nanocrystals are condensed into a solution of alkylthiol molecules. The material produced
in this way has a superlattice structure with FCC arrangement of silver nanoparticles
having separations of t 3 n m . Metal nanoparticles are of interest because of the
enhancement of the optical and electrical conductance due to confinement and
quantization of conduction electrons by the small volume of the nanocrystal.
When the size of the crystal approaches the order of the de Broglie wavelength of
the conduction electrons, the metal clusters may exhibit novel electronic properties.
They display very large optical polarizabilities as discussed earlier, and nonlinear
electrical conductance having small thermal activation energies. Coulomb blockade
and Coulomb staircase current-voltage curves are observed. The Coulomb blockade
phenomenon is discussed in Chapter 9. Reduced-dimensional lattices such as
quantum dots and quantum wires have been fabricated by a subtractive approach
that removes bulk fractions of the material leaving nanosized wires and dots. These
nanostructures are discussed in Chapter 9.