576
M. Saini et al.
remains almost constant for the parallel polarization. The difference in the magnitude shows dependence on the layer thickness of Ag. For all three cases, there is
>80% IR transmission, however, transmission is weak in visible region due to LSPR.
This study paves a way towards transparent conductive electrode made of ultrathin
metal nanowires with >80% transmission, dielectric response similar to the bulk
metal (superior electrical conductivity), and as an optical scatterer for more efficient
absorption of light.
3.3.5 Anisotropic Specimen: Ag-Nanoparticles on Self-Organized
Nanodots
The nanodot patterns are fabricated on GaSb (100) substrates in a self-organized
manner using Ar ions at normal incidence and the ion-energy in the range of 600–
1200 eV. Further, these GaSb nanodots were used as templates for growth of Agnanoparticles at a growth angle of 70°. This section describes the plasmonic property
of self-organized Ag-nanoparticle decorated on GaSb nanodots. The optical constants
of the nanostructure are extracted using spectroscopic ellipsometry (SE) which are
further used in the analysis of the observed reflection and SERS spectra of silver
nanodots. These experiments were carried out to measure (, Δ) in a wide wavelength range (210–1688 nm) at a fixed incidence angle (i.e. 75°) for all the specimen
[45]. It is worth noting that there is no change in the measured reflection spectra
by providing an azimuthal sample rotation. Hence, nanoscale porous structures can
be modelled using the Maxwell–Garnett or Bruggeman EMT which are discussed
in Sect. 2.3.2. The Bruggeman formalism is chosen for the modeling as it can deal
with anisotropic systems and remains valid for high volumetric contents also [46]
and hence, it is appropriate for modeling of silver nanodots.
In order to account the increasing diameter of nanodots and to develop a unique
optical model, various approaches were considered from the reported literature [47].
However, Lorentz oscillator model is employed to account the optical response of
Ag-nanoparticles [48]. Moreover, two layer EMA method is used to model GaSb
nanodots in such a way that a fraction of silver along with voids is present in top
layer, however, the bottom layer have nanodots intermixed with voids. Anisotropic
EMA approach is adopted to model silver nanodots by considering the depolarization
factor, q z and plasmonic anisotropy in z-direction.
This procedure is called Bruggeman Biaxial as it keeps account of screening of
the polarization in x-, y- and z-directions and therefore, results in a distinct optical
response essential for in-plane and out-of-plane anisotropies. Figure 26(a) shows a
schematic diagram for the screening factor, q z where charge is not screened by the
columnar structures along the z-axis.
The advantage of this approach (compared to the conventional EMA) is that the
screening factor splits up in the x-, y-, and z-axes which allows the geometry of the
structure to be a part of the optical model. There is no significant reduction in the
calculated values of MSE with the introduction of in-plane screening parameters,
M. Saini et al.
remains almost constant for the parallel polarization. The difference in the magnitude shows dependence on the layer thickness of Ag. For all three cases, there is
>80% IR transmission, however, transmission is weak in visible region due to LSPR.
This study paves a way towards transparent conductive electrode made of ultrathin
metal nanowires with >80% transmission, dielectric response similar to the bulk
metal (superior electrical conductivity), and as an optical scatterer for more efficient
absorption of light.
3.3.5 Anisotropic Specimen: Ag-Nanoparticles on Self-Organized
Nanodots
The nanodot patterns are fabricated on GaSb (100) substrates in a self-organized
manner using Ar ions at normal incidence and the ion-energy in the range of 600–
1200 eV. Further, these GaSb nanodots were used as templates for growth of Agnanoparticles at a growth angle of 70°. This section describes the plasmonic property
of self-organized Ag-nanoparticle decorated on GaSb nanodots. The optical constants
of the nanostructure are extracted using spectroscopic ellipsometry (SE) which are
further used in the analysis of the observed reflection and SERS spectra of silver
nanodots. These experiments were carried out to measure (, Δ) in a wide wavelength range (210–1688 nm) at a fixed incidence angle (i.e. 75°) for all the specimen
[45]. It is worth noting that there is no change in the measured reflection spectra
by providing an azimuthal sample rotation. Hence, nanoscale porous structures can
be modelled using the Maxwell–Garnett or Bruggeman EMT which are discussed
in Sect. 2.3.2. The Bruggeman formalism is chosen for the modeling as it can deal
with anisotropic systems and remains valid for high volumetric contents also [46]
and hence, it is appropriate for modeling of silver nanodots.
In order to account the increasing diameter of nanodots and to develop a unique
optical model, various approaches were considered from the reported literature [47].
However, Lorentz oscillator model is employed to account the optical response of
Ag-nanoparticles [48]. Moreover, two layer EMA method is used to model GaSb
nanodots in such a way that a fraction of silver along with voids is present in top
layer, however, the bottom layer have nanodots intermixed with voids. Anisotropic
EMA approach is adopted to model silver nanodots by considering the depolarization
factor, q z and plasmonic anisotropy in z-direction.
This procedure is called Bruggeman Biaxial as it keeps account of screening of
the polarization in x-, y- and z-directions and therefore, results in a distinct optical
response essential for in-plane and out-of-plane anisotropies. Figure 26(a) shows a
schematic diagram for the screening factor, q z where charge is not screened by the
columnar structures along the z-axis.
The advantage of this approach (compared to the conventional EMA) is that the
screening factor splits up in the x-, y-, and z-axes which allows the geometry of the
structure to be a part of the optical model. There is no significant reduction in the
calculated values of MSE with the introduction of in-plane screening parameters,