Page 102 - Organic Electronics in Sensors and Biotechnology

Or ganic Thin-Film Transistors for Inor ganic Substance Monitoring 79

case of gold, however, we found that this results in just a slight size
modulation of the Au-NPs cores.
A set of electrosyntheses were carried out by applying different
−2
current density values J to the cell, ranging from −0.5 mA/cm to
app
−2
−15.0 mA/cm . At the end of the process, the J values were cor-
app
rected by taking into account the effective electroconversion yield,
and the resulting effective current density J was used for further
eff
quantitative considerations. Both UV-vis spectroscopy and Trans-
200
mission Electron Microscopy (TEM) were used to study the electro-
synthesized NPs, and the correlation between the results and the J
eff
values is shown in Fig. 2.10. 201
In particular, UV-vis spectroscopy was used to study the surface
plasmon resonance (SPR) peak position of Au-NPs. This is a well-
known size-dependent spectral feature and can be used to achieve
information on the NP size modulation. 202–203 In Fig. 2.10c, the change
in the SPR position vs. the current density is reported. An even more
evident size-dependent correlation can be achieved by comparing the
Full Width at Half Maximum (FWHM) of NP samples synthesized at
different current densities (Fig. 2.10b). When the current density
decreased, a corresponding decrease in the FWHM was observed.
According to Link and coworkers, this evidence is correlated to NP
size changes, and, in particular, a FHWM decrease corresponds to an
increase of the nanophase mean diameter. The differences in the
204
NP-core size resulted in slight changes of the SPR peak position and
much more evident changes of its FWHM value. These results pro-
vide useful diagnostic tools for spectroscopic investigation of Au-NPs.
Furthermore, they are in excellent agreement with the theoretical pre-
dictions and with the TEM investigations reported in the Fig. 2.10a.
The latter panel shows that milder electrosynthesis conditions led
to greater core diameter (7.9 nm) with a narrower size dispersion
(±1.0 nm), while higher current densities resulted in smaller core
diameters (5.5 nm) and broader diameter distribution (±2.2 nm).
Before use as a gate material in field-effect sensors, Au-NPs were
subjected to a mild thermal treatment at 200°C for 1 h (higher tem-
peratures or longer annealing times resulted in excessive change of
the Au-NP morphology). The effect of the annealing treatment on the
Au-NP morphology was studied using Scanning Electron Micros-
copy (SEM) and TEM (data not shown). Both techniques clearly show
that the material is still nanostructured, and there is a homogeneous
in-plane distribution of Au-NPs. Gold clusters show a spherical mor-
phology, but an appreciable increase in their size (up to 50 nm) can be
observed with respect to pristine particles. 175
X-ray photoelectron spectroscopy (XPS) was used to study the
surface chemical composition of both pristine and thermally annealed
nanomaterials. Independent of the electrolysis conditions, carbon,
nitrogen, and chlorine are the most abundant elements on the surface