9 Aperiodic Order in Nanoplasmonics
357
where E loc (λ p ) is the local electromagnetic field at the pumping frequency and
E loc (λ s ) is the local electromagnetic field at the scatted signal frequency. Because
of this distinctive scaling, in SERS applications it is very important to engineer strong
local fields at these two frequencies in order to obtain very high Raman enhancement
factors, enabling optical sensing of few or even single molecules.
Currently, the best approaches to generate efficient SERS substrates rely on random roughening of metal surfaces by etching or by colloidal synthesis of nanoparticles resulting in aggregates statistically described by fractal morphologies [1, 3, 114,
115]. The giant fluctuations of the local fields characteristic of self-similar (fractal)
structures leads to an efficient transfer of excitations towards progressively smaller
length scales of the aggregates where the electromagnetic enhancement reaches the
10 12 level needed to observe single molecule SERS [1, 116, 117]. However, while
fractal aggregates and rough metal surfaces led to successful applications in single molecule spectroscopy [101, 105, 106], reproducibility of results can vary from
sample to sample and engineering design rules are not easy to formulate. On the
other hand, these approaches clearly demonstrated the importance of the morphology dependent “structural enhancement”, which is characteristic of multi-scale complex systems where both electrodynamical interactions and electrostatic coupling
(lightning rod effect, quasi-static plasmon coupling) contribute to the overall SERS
enhancement.
In contrast to randomly roughened surfaces, it is possible to fabricate reproducible
metal nanostructures using Electron beam lithography (EBL) to be used as efficient
SERS substrates. Following the pioneering work of Wokaun et al. [118], it became
possible to obtain SERS signals from lithographically defined metal nanostructures
[119–123]. By using EBL, it is possible to fabricate uniform SERS substrates by
controlling not only the shapes but also the positions of each particle at the nanoscale.
The SERS enhancement achievable using periodic arrays of metal nanoparticles can be further increased by reducing their inter-particle separations [122, 123].
However, the width of the inter-particle gaps achievable by EBL is limited to approximately 20 nm, which is much larger than the inter-particle separations typically
obtained using random colloidal aggregates (1–5 nm). Therefore, in order to take
full advantage from structural enhancement effects in highly reproducible EBL-based
substrates, our group recently proposed to engineer the subtle interplay between diffractive long-range and near-field coupling interactions in nanoparticles arrays with
complex, yet deterministic, morphologies [124, 125].
In particular, by developing DANS arrays of Au nanoparticles as a novel approach
for the design and implementation of “engineered roughness” in SERS substrates, we
demonstrated large values (≥10 7 ) of spatially averaged enhancement factors (i.e.,
defined over the entire excitation area) localized within 25 nm gaps using molecular
pMA (p-mercaptoaniline) monolayers. In our studies, we used pMA as the Raman
marker because of its ability to form reproducible saturation coverage on gold surfaces, which reduces the ambiguity associated with the experimental quantification
of the SERS enhancement. Moreover, we introduced novel multi-scale aperiodic
nanostructures, called “plasmonic nanogalaxies”, which yield reproducible SERS
enhancement (spatially averaged) values up to ≥10 8 [124]. These deterministic ape-
357
where E loc (λ p ) is the local electromagnetic field at the pumping frequency and
E loc (λ s ) is the local electromagnetic field at the scatted signal frequency. Because
of this distinctive scaling, in SERS applications it is very important to engineer strong
local fields at these two frequencies in order to obtain very high Raman enhancement
factors, enabling optical sensing of few or even single molecules.
Currently, the best approaches to generate efficient SERS substrates rely on random roughening of metal surfaces by etching or by colloidal synthesis of nanoparticles resulting in aggregates statistically described by fractal morphologies [1, 3, 114,
115]. The giant fluctuations of the local fields characteristic of self-similar (fractal)
structures leads to an efficient transfer of excitations towards progressively smaller
length scales of the aggregates where the electromagnetic enhancement reaches the
10 12 level needed to observe single molecule SERS [1, 116, 117]. However, while
fractal aggregates and rough metal surfaces led to successful applications in single molecule spectroscopy [101, 105, 106], reproducibility of results can vary from
sample to sample and engineering design rules are not easy to formulate. On the
other hand, these approaches clearly demonstrated the importance of the morphology dependent “structural enhancement”, which is characteristic of multi-scale complex systems where both electrodynamical interactions and electrostatic coupling
(lightning rod effect, quasi-static plasmon coupling) contribute to the overall SERS
enhancement.
In contrast to randomly roughened surfaces, it is possible to fabricate reproducible
metal nanostructures using Electron beam lithography (EBL) to be used as efficient
SERS substrates. Following the pioneering work of Wokaun et al. [118], it became
possible to obtain SERS signals from lithographically defined metal nanostructures
[119–123]. By using EBL, it is possible to fabricate uniform SERS substrates by
controlling not only the shapes but also the positions of each particle at the nanoscale.
The SERS enhancement achievable using periodic arrays of metal nanoparticles can be further increased by reducing their inter-particle separations [122, 123].
However, the width of the inter-particle gaps achievable by EBL is limited to approximately 20 nm, which is much larger than the inter-particle separations typically
obtained using random colloidal aggregates (1–5 nm). Therefore, in order to take
full advantage from structural enhancement effects in highly reproducible EBL-based
substrates, our group recently proposed to engineer the subtle interplay between diffractive long-range and near-field coupling interactions in nanoparticles arrays with
complex, yet deterministic, morphologies [124, 125].
In particular, by developing DANS arrays of Au nanoparticles as a novel approach
for the design and implementation of “engineered roughness” in SERS substrates, we
demonstrated large values (≥10 7 ) of spatially averaged enhancement factors (i.e.,
defined over the entire excitation area) localized within 25 nm gaps using molecular
pMA (p-mercaptoaniline) monolayers. In our studies, we used pMA as the Raman
marker because of its ability to form reproducible saturation coverage on gold surfaces, which reduces the ambiguity associated with the experimental quantification
of the SERS enhancement. Moreover, we introduced novel multi-scale aperiodic
nanostructures, called “plasmonic nanogalaxies”, which yield reproducible SERS
enhancement (spatially averaged) values up to ≥10 8 [124]. These deterministic ape-