Chemistry Reference
In-Depth Information
This method was fundamentally different from silver halide chemistry, in which the
latent image (small cluster of Ag
0
atoms) on photosensitive silver bromide (AgBr)
nanocrystals was photographically developed to form a multilayer diffraction grating
[
5
]. In TEM images, there was a distinction in the reduced size of Ag
0
NPs in
hydrogel matrices before and after photochemical patterning. Based on the angular-
resolved measurements, it was inferred that the photochemical patterning formed
Bragg diffraction gratings as well as a blazed grating (tooth saw) or a transmission
grating, as predicted by the simulations based on the superposition of waves [
1
].
While the optical microscopic images of the surface gratings supported the simulated
results, no direct evidence of the multilayer structure was found in SEM, ESEM and
TEM images. This could be attributed to a low density of Ag
0
NPs within the
hydrogel matrices, which might be due to the low (<1 %) diffraction ef
ciency of the
photonic structure. However, the period (
m) inferred from the transmission
gratings indirectly supported the existence of the multilayer structure [
6
,
7
].
An advantage of holographic sensors fabricated through in situ size reduction of
Ag
0
NPs is that this method does not require a gentle reduction step after laser
exposure. This allows the use of any reducing agent, not limiting the reduction
process to weak developers. For example, non-aqueous agents (e.g. hydroquinone
in THF) can be used, and tedious control over exposure parameters and Ag
0
NP
growth is not needed. The present technique shows that Bragg diffraction gratings
can be produced quickly with reduced complexity as compared to silver-halide
chemistry-based fabrication. This opens holographic sensors into a broader range of
applications and materials. For example, gold, copper or iron NPs can be used to
produce the multilayer gratings. A limitation of the present photochemical pat-
terning of Ag
0
NPs is that the photonic structure is not compatible with bleaching
with bromine (Br
2
), chlorine (Cl
2
), and iodine (I
2
). This may be attributed to the
uncontrolled growth of AgBr nanocrystals at the antinode as well as the node
regions. Another explanation for this phenomenon is that the Ag
0
NPs, which may
be burnt in the hydrogel matrix by the high-power of the laser light, become
detached from the matrix so that they loose their spatial integrity. Therefore, it may
not be possible to fabricate Ag
0
NP holograms with diffraction ef
3
µ
*
ciency >10 %.
This is also a limitation in sensing samples with divalent cations such as Cu
2+
,
which cause a decrease in the diffraction ef
ciency due to bleaching. The future
work may explore titanium(IV) oxide NPs, which have a refractive index of
2.8
at 632.8 nm [
8
,
9
]. These particles may be incorporated into the hydrogel matrices
to form diffraction gratings. Other attractive materials and structures might include
graphene [
10
,
11
], nanocones [
12
], graphite and carbon nanotubes [
13
]. The per-
fusion of Ag
+
ions into hydrogel matrices and subsequent reduction in situ allows
forming Ag
0
NPs in the upper half of the hydrogel matrix (
*
*
5
-
10
µ
m) down to
5
6 nm from the hydrogel-air surface [
14
]. There are two main contributors to this
issue: (i) The depletion of the developer strength as it perfuses into the hydrogel
matrix, and (ii) the use of Ag
+
ions dissolved in aqueous solutions, which may not
allow the Ag
+
ions to diffuse throughout the matrix. Hence, methods to obtain Ag
0
NPs throughout the hydrogel matrix should be developed to increase the number of
multilayer gratings, which will
-
increase the diffraction ef
ciency. A plausible
