136
Y. Min et al.
by alkanedithiols, of which 60% of the alkanedithiol are bound to NP by both groups,
while 40% are bound with only one thiol group. All films showed ohmic I–V characteristics and Arrhenius-type activation of charge transport. Interlinked NP assemblies help to enhance the conductivity of the films compared to free NP. Indeed,
the electron-tunneling decay constant β N was much lower for non-covalently linked
NP film. A fast and fully reversible increase of film resistance was measured upon
exposure to vapors of toluene or tetrachloroethylene. The resistance increases exponentially with increasing alkanedithiol chain length at a given concentration. This
type of film was forecasted as a promising material for sensing application, which
was studied later extending the ligand to alkanedithiol-C 12 , 4,4
-terphenyldithiol and
[4] -staffane-3,3
-dithiol to make chemiresistors. This study has revealed that the
flexibility and resistivity of the interlinkage has a profound impact on the response
characteristics of the sensors [49]. Thus, the use of the flexible 1,12-dodecanedithiol
as linker induces the interlinked film to respond with an increase in resistance. Oppositely, with the rigid staffane linker the interlinked film responded with a decrease in
resistance.
In a similar attempt of investigating the influence of dithiol ligands, Wessels et al.
synthesized Au NP assembly films from six different dithiols, which were classified
into three groups according to the nature of the group in the middle and at the
end (Table 5.2) [25]. The optical and electrical properties of the films were highly
influenced by the nature of the ligand. Especially, the conductivity increased by one
order of magnitude for linkers that contain a cyclohexane ring instead of a benzene
ring. As the molecule consists in non-conjugated and conjugated parts, according
to the electron-tunneling decay constant (β N-CON ), the conductivity can be tuned
from the insulating to the metallic limit, regardless of the inter-particle spacing (4 ±
0.8 nm, d NP ~ 1 nm).
Daskal et al. followed the Au NP assembly deposition using a quartz crystal
microbalance. This is a suitable method as it can sense material deposition in the
nanogram range [50]. The assembly efficiency is higher for shorter alkyldithiols
and ligands with more strongly interacting functional groups such as alkylbisdithiocarbamates, which outperformed alkyldiamines. Comparing plasmon resonance, a
typical and well-studied feature of Au NP, the spectrum of the bisdithiocarbamate
composite exhibits a distinct blue shift, which was attributed to more delocalized
electron charge at the NP, because the linker possesses more bulky groups allowing
a longer structure.
The number of the deposition cycles performed to produce multilayer films on
substrate counts importantly for properties and applications. Thus, enhanced localized surface plasmon resonance sensing was obtained with multilayer structures
fabricated with 1,10-decanedithiol as linker from four NP deposition cycles [51].
This was reflected by a ~fourfold improvement of the sensitivity to the changes of the
environmental refractive index compared to the submonolayer structure. Dhar et al.
produced Au and Ag NP composite thin films with a carboxyl-functionalized chitosan
polymer bearing COO
− and –NH 2 groups (Scheme 5.1), which were deposited on flat
quartz substrates [52]. The process of assemblies allows producing up to more than
40 layers, the growth of which was monitored by UV-visible spectroscopy, atomic
Y. Min et al.
by alkanedithiols, of which 60% of the alkanedithiol are bound to NP by both groups,
while 40% are bound with only one thiol group. All films showed ohmic I–V characteristics and Arrhenius-type activation of charge transport. Interlinked NP assemblies help to enhance the conductivity of the films compared to free NP. Indeed,
the electron-tunneling decay constant β N was much lower for non-covalently linked
NP film. A fast and fully reversible increase of film resistance was measured upon
exposure to vapors of toluene or tetrachloroethylene. The resistance increases exponentially with increasing alkanedithiol chain length at a given concentration. This
type of film was forecasted as a promising material for sensing application, which
was studied later extending the ligand to alkanedithiol-C 12 , 4,4
-terphenyldithiol and
[4] -staffane-3,3
-dithiol to make chemiresistors. This study has revealed that the
flexibility and resistivity of the interlinkage has a profound impact on the response
characteristics of the sensors [49]. Thus, the use of the flexible 1,12-dodecanedithiol
as linker induces the interlinked film to respond with an increase in resistance. Oppositely, with the rigid staffane linker the interlinked film responded with a decrease in
resistance.
In a similar attempt of investigating the influence of dithiol ligands, Wessels et al.
synthesized Au NP assembly films from six different dithiols, which were classified
into three groups according to the nature of the group in the middle and at the
end (Table 5.2) [25]. The optical and electrical properties of the films were highly
influenced by the nature of the ligand. Especially, the conductivity increased by one
order of magnitude for linkers that contain a cyclohexane ring instead of a benzene
ring. As the molecule consists in non-conjugated and conjugated parts, according
to the electron-tunneling decay constant (β N-CON ), the conductivity can be tuned
from the insulating to the metallic limit, regardless of the inter-particle spacing (4 ±
0.8 nm, d NP ~ 1 nm).
Daskal et al. followed the Au NP assembly deposition using a quartz crystal
microbalance. This is a suitable method as it can sense material deposition in the
nanogram range [50]. The assembly efficiency is higher for shorter alkyldithiols
and ligands with more strongly interacting functional groups such as alkylbisdithiocarbamates, which outperformed alkyldiamines. Comparing plasmon resonance, a
typical and well-studied feature of Au NP, the spectrum of the bisdithiocarbamate
composite exhibits a distinct blue shift, which was attributed to more delocalized
electron charge at the NP, because the linker possesses more bulky groups allowing
a longer structure.
The number of the deposition cycles performed to produce multilayer films on
substrate counts importantly for properties and applications. Thus, enhanced localized surface plasmon resonance sensing was obtained with multilayer structures
fabricated with 1,10-decanedithiol as linker from four NP deposition cycles [51].
This was reflected by a ~fourfold improvement of the sensitivity to the changes of the
environmental refractive index compared to the submonolayer structure. Dhar et al.
produced Au and Ag NP composite thin films with a carboxyl-functionalized chitosan
polymer bearing COO
− and –NH 2 groups (Scheme 5.1), which were deposited on flat
quartz substrates [52]. The process of assemblies allows producing up to more than
40 layers, the growth of which was monitored by UV-visible spectroscopy, atomic