132
Y. Min et al.
The review of the metal nanostructures will be classified and presented according
to the synthesis method. Table 5.1 shows representative examples of metal NP
networks produced by direct cross-linking methods.
5.2.1.1 Layer-by-Layer Method
Along the development of metal NP assemblies, LBL self-assembly of building
blocks has played a significant role, as it allows controllable growth over a substrate
turning into a nanocomposite demonstrating specific optical and electrical properties
[42, 43]. The LBL technique is a versatile approach to create ultrathin surface coatings
on a wide range of surfaces. It involves a ligand/linker exchange process during the
assembly cycle, by immersion of substrates into solution of NP and linker molecule.
In 1996, Andres et al. [24] reported an approach to prepare 2-D self-assembly of
Au NP connected by ditopic ligands, in which a solid substrate acted as a template
to obtain the 2-D growth. In this process, isolated metal NP were first produced in
the gas phase and protected by alkylthiols in solution. After that, being coated on a
substrate placed into aryl dithiol or di-isonitrile ligand solution, the NP are linked
into an assembly thanks to the exchange of ligands (Fig. 5.2). The authors have
demonstrated that by modifying the NP size or composition, the length and chemical
structure of the linker, and the nature of the substrate, a wide range of electronic
behavior can be achieved. The electrical conductance of NP assemblies (78 nS) was
lower than the one of unlinked NP (133 nS).
Zhong et al. have described a novel strategy toward the assembly of bimetallic
Au-Ag NP via carboxylate-Ag
+ binding at selective sites on the NP surface, giving
an assembly of composition Au 23 Ag 77 (Fig. 5.3) [44]. The possibility to modify the
composition of the Au-Ag NP as well as the chain length of the dicarboxylic acids
allows to modulate the optical and electronic properties. The Au-Ag but also Au
NP assemblies [45] with different inter-particle distances, adjusted by varying the
X-(CH 2 ) n -X length, were applied for vapor sensing, revealing a correlation between
sensitivity (electric conductivity) and inter-particle spacing [46].
Another alloyed Au-Ag NP assembly produced from the 4-aminothiophenol
(PATP) linker on substrate was investigated by Raman scattering [47]. The
LBL assembly was produced first by using the protonated pyridine groups of a
polyvinylpyridine-functionalized glass as anchoring sites for negatively charged Au
NP. PATP ligands were then adsorbed on Au NP through the formation of Au–S
bonds. The PATP molecule, which possesses two resonance structures (benzenoid
and quinonoid), can interact with Ag NP through the quinoid form. It was concluded
from the Raman study that the b2 vibrational mode of PATP, which is characteristic
of charge transfer between the metal NP and PATP ligands, is enhanced by charge
transfer from the Ag to Au NP by tunneling through PATP.
Alkanedithiols with different carbon chain length (C 6 , C 9 , C 12 , C 16 ), which are
typical linkers for Au NP assemblies, were investigated for sensors applications
[48]. X-ray photoelectron spectroscopy (XPS) analyses of Au NP assembly 2-D
films confirmed that dodecylamine ligands on Au NP were quantitatively exchanged
Y. Min et al.
The review of the metal nanostructures will be classified and presented according
to the synthesis method. Table 5.1 shows representative examples of metal NP
networks produced by direct cross-linking methods.
5.2.1.1 Layer-by-Layer Method
Along the development of metal NP assemblies, LBL self-assembly of building
blocks has played a significant role, as it allows controllable growth over a substrate
turning into a nanocomposite demonstrating specific optical and electrical properties
[42, 43]. The LBL technique is a versatile approach to create ultrathin surface coatings
on a wide range of surfaces. It involves a ligand/linker exchange process during the
assembly cycle, by immersion of substrates into solution of NP and linker molecule.
In 1996, Andres et al. [24] reported an approach to prepare 2-D self-assembly of
Au NP connected by ditopic ligands, in which a solid substrate acted as a template
to obtain the 2-D growth. In this process, isolated metal NP were first produced in
the gas phase and protected by alkylthiols in solution. After that, being coated on a
substrate placed into aryl dithiol or di-isonitrile ligand solution, the NP are linked
into an assembly thanks to the exchange of ligands (Fig. 5.2). The authors have
demonstrated that by modifying the NP size or composition, the length and chemical
structure of the linker, and the nature of the substrate, a wide range of electronic
behavior can be achieved. The electrical conductance of NP assemblies (78 nS) was
lower than the one of unlinked NP (133 nS).
Zhong et al. have described a novel strategy toward the assembly of bimetallic
Au-Ag NP via carboxylate-Ag
+ binding at selective sites on the NP surface, giving
an assembly of composition Au 23 Ag 77 (Fig. 5.3) [44]. The possibility to modify the
composition of the Au-Ag NP as well as the chain length of the dicarboxylic acids
allows to modulate the optical and electronic properties. The Au-Ag but also Au
NP assemblies [45] with different inter-particle distances, adjusted by varying the
X-(CH 2 ) n -X length, were applied for vapor sensing, revealing a correlation between
sensitivity (electric conductivity) and inter-particle spacing [46].
Another alloyed Au-Ag NP assembly produced from the 4-aminothiophenol
(PATP) linker on substrate was investigated by Raman scattering [47]. The
LBL assembly was produced first by using the protonated pyridine groups of a
polyvinylpyridine-functionalized glass as anchoring sites for negatively charged Au
NP. PATP ligands were then adsorbed on Au NP through the formation of Au–S
bonds. The PATP molecule, which possesses two resonance structures (benzenoid
and quinonoid), can interact with Ag NP through the quinoid form. It was concluded
from the Raman study that the b2 vibrational mode of PATP, which is characteristic
of charge transfer between the metal NP and PATP ligands, is enhanced by charge
transfer from the Ag to Au NP by tunneling through PATP.
Alkanedithiols with different carbon chain length (C 6 , C 9 , C 12 , C 16 ), which are
typical linkers for Au NP assemblies, were investigated for sensors applications
[48]. X-ray photoelectron spectroscopy (XPS) analyses of Au NP assembly 2-D
films confirmed that dodecylamine ligands on Au NP were quantitatively exchanged