Quantification of the short-range order in amorphous silicon has been formulized using Raman scattering by taking into account established frameworks for studying the spectral line-shape and size dependent Raman peak shift. A theoretical line-shape function has been proposed for representing the observed Raman scattering spectrum from amorphous-Si-based on modified phonon confinement model framework. While analyzing modified phonon confinement model, the term "confinement size" used in the context of nanocrystalline Si was found analogous to the short-range order distance in a-Si thus enabling one to quantify the same using Raman scattering. Additionally, an empirical formula has been proposed using bond polarizability model for estimating the short-range order making one capable to quantify the distance of short-range order by looking at the Raman peak position alone. Both the proposals have been validated using three different data sets reported by three different research groups from a-Si samples prepared by three different methods making the analysis universal.
Size-dependent asymmetric low-frequency Raman line shapes have been observed from silicon (Si) nanostructures (NSs) due to a quantum confinement effect. The acoustic phonons in Si NSs interact with an intraband quasi-continuum to give rise to Fano interaction in the low-frequency range. The experimental asymmetric Raman line shape has been explained by developing a theoretical model that incorporates the quantum-confined phonons interacting with an intraband quasi-continuum available in Si NSs as a result of discretization of energy levels with unequal separation. We discover that a phenomenon similar to Brillouin scattering is possible at the nanoscale in the low-frequency regime and thus may be called "Fano scattering" in general. A method has been proposed to extract information about nonradiative transitions from the Fano scattering data where these nonradiative transitions are involved as an intraband quasi-continuum in modulation with discrete acoustic phonons.
A suitably designed heterostructured
TiO2–Co3O4 core–shell
nanorod array has been found to exhibit improved supercapacitive as
well as electrochromic properties as compared to the nanowires of
either of the oxides when used individually. The core–shell
nanostructures have been grown on an FTO coated glass substrate by
preparing TiO2 nanorods through hydrothermal reaction followed
by growing a Co3O4 shell layer by electrodeposition.
The core–shell electrode shows high specific and areal capacitance
of ∼342 F/g and ∼140 mF/cm2 (at scan rate
of 100 mV/s), respectively. Such an improvement in supercapacitive
behavior, as compared to the behavior of the existing ones, is likely
due to increased surface area and modified charge dynamics within
the core–shell heterojunction. Additionally, these core–shells
also exhibit stable and power efficient bias induced color change
between transparent (sky blue) and opaque (dark brown) states with
coloration efficiency of ∼91 cm2/C. Porous morphology
and strong adhesion to the surface of transparent conducting glass
electrode give rise to superior cyclic stability in both energy storage
and electrochromic applications, which make these core–shell
structures suitable candidates for future electronic devices.
A fast and flexible
all-organic electrochromic device, fabricated
using polythiophene and PCBM as active materials and plastic substrate,
which shows very good power efficiency as well, has been reported
here. The device shows quantifiable improvement in electrochromic
performance using parameters like switching speed, coloration efficiency,
color contrast, and cycle life. Spectroscopic investigations have
been carried out using Raman and UV–vis to establish a bias
induced redox switching based mechanism for the reported improvement
in the performance. The device shows switching between magenta (OFF)
and transparent states (ON) with a very small bias of ±1 V, an
optical modulation of 50% and an absorbance switching contrast of
91%. An enhanced stability for a duration of longer than 2500 s and
250 cycles has been reported with an ultrafast response of few hundred
milliseconds. A very high coloration efficiency of 321 cm2/C is achieved, making the proposed device one of the best reported
P3HT-based electrochromic devices.
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