Page 104 - Organic Electronics in Sensors and Biotechnology

Or ganic Thin-Film Transistors for Inor ganic Substance Monitoring 81

C/Au
Element C Au N Cl O Si Ratio
Pristine
92.3% 0.1% 3.9% 2.4% 1.3% — 923
Au-NPs
Annealed
34.6% 1.3% 3.2% — 36.2% 24.7% 27
Au-NPs

TABLE 2.1 Surface Atomic Concentrations Recorded by XPS on Pristine
and Annealed Au-NPs. The error in the atomic percentages is ±0.1% for gold
and ±0.3% for the other elements.

of pristine Au-NP films. This is due to the high concentration of the
surfactant present as base electrolyte in the colloidal solution. Gold
and oxygen are detected as well, but they are present at lower con-
centration levels. For pristine Au-NP films, the carbon/gold surface
3
atomic ratio is close to 10 . Thermal annealing causes an appreciable
removal of the organic matter from the sample surface: carbon and
nitrogen surface concentrations are noticeably decreased, while chlo-
rine is completely removed. Gold surface concentration is increased
significantly; consequently, the C/Au ratio is lowered by more than
one order of magnitude. Finally, oxygen and silicon signals increase,
and this occurs since a part of the Si/SiO /SiO substrate becomes
2 x
exposed upon heating (see Table 2.1 for details).
Interestingly, the heating treatment allowed a larger catalytically
metal area to be exposed to the gas molecules, increased the electrical
conductivity of nanostructured gold, and greatly improved its ther-
mal stability.
High-resolution XP spectra of pristine and annealed Au-NP are
reported in Fig. 2.11, where they can be compared to a reference spec-
trum recorded on bulk metallic gold.
The Au4f region of pristine NPs (Fig. 2.11a) is composed by two
doublets, relevant to two chemical states: nanostructured elemental
Au (binding energy BE = 83.0 ± 0.1 eV) and Au(I) chlorides, most
Au4f7/2
probably as (NR )AuCl species (BE = 84.5 ± 0.1 eV). 175
4 2 Au4f7/2
The Au4f region of annealed NPs (Fig. 2.11b) is composed only by
one doublet, ascribed to nano-Au (BE = 83.7 ± 0.1 eV). The
(0)
Au4f7/2
higher BE value of the latter peak, as compared to what detected in
case of pristine NPs, is ascribed to size-dependent chemical shifts 205
and is in perfect agreement with the size increase observed by elec-
tron microscopies.
The thermal annealing procedure is critical since, on the one
hand, it is necessary to increase the chemical stability and the conduc-
tivity of the nanostructured film while, on the other hand, it caused
the increase of the particle dimensions and partial degradation of the
protective organic shell. A sketch outlining the changes induced on
Au-NPs by thermal annealing is shown in Fig. 2.12.