24 research outputs found
Evolution of Asymmetric Raman line-shape from nano-structures
A step-by-step evolution of an asymmetric Raman line-shape function from a
Lorentzian line-shape is presented here for low dimensional semiconductors. The
evolution reported here is based on the phonon confinement model which is
successfully used in literature to explain the asymmetric Raman line-shape from
semiconductor nano-structures. Physical significance of different terms in the
theoretical asymmetric Raman line-shape has been explained here. Better
understanding of theoretical reasoning behind each term allows one to use the
theoretical Raman line-shape without going into details of theory from first
principle. This will enable one to empirically derive a theoretical Raman
line-shape function for any material if information about its phonon
dispersion, size dependence etc is known
Effect of silicon resistivity on its porosification using metal induced chemical etching
A comparison of porous structures formed from silicon (Si) wafers with
different resistivities has been reported here based on the morphological
studies carried out using scanning electron microscope (SEM). The porous Si
samples have been prepared using metal induced etching (MIE) technique from two
different Si wafers having two different resistivities. It is observed that
porous Si containing well aligned Si nanowires are formed from high resistivity
(1-20 cm) Si wafer whereas interconnected pores or cheese like
structures are formed from low resistivity (0.02 cm ) Si wafers after
MIE. An explanation for the different porosification processes has also been
proposed based on the initial doping level where number of dopants seems to be
playing an important role on the etching process. Visible photoluminescence
have been observed from all the porous samples possibly due to quantum
confinement effect.Comment: 9 Pages, 5 Figure
Intrinsic Half-metallicity in Edge Fluorinated Armchair Boron Nitride Nanoribbons
We predict intrinsic half-metallicity in armchair boron nitride nanoribbons
(ABNNRs) via edge fluorination. The stability, electronic and magnetic
properties of bare and edge fluorinated ABNNRs have been systematically
analyzed by means of first-principles calculations within the local
spin-density approximation (LSDA). The ribbons whose only edge-B atoms
passivated with F atoms (i.e., edge-N atoms are un-passivated), regardless of
width, are found half-metallic with a half-metal gap of 0.3 eV. A 100 \% spin
polarized charge transport across the Fermi level is expected for such ribbons
as the spin polarized states are 0.4 eV more stable than the spin
un-polarized states and only single-spin conducting channels are present across
the Fermi level owing to the gigantic spin splitting. The existence of
half-metallicity is attributed to the localization of electronic charge at bare
edge-N atoms as revealed from the analysis of Bloch states and projected
density of states (PDOS).The sufficiently large half-metal gap (0.3 eV) with
huge difference in the energies ( 0.4 eV) of spin polarized and spin
compensated states projects these half-metallic ABNNRs as potential candidate
for spintronics applications.Comment: Error in Typing in older versio
In-situ spectroscopic studies of viologen based electrochromic device
Fabrication and operation of simple solid state electrochromic devices using
ethyl viologen diperchlorate in a polymer matrix is presented here. In-situ
Raman and transmission/absorption studies have been done to establish the
origin of bias induced color change, between a transparent and navy blue color,
in the electrochromic device. The origin of bias induced color change has been
attributed to the bias induced redox switching between its viologen dication
and free redicle forms. Fundamental reason behind colour changes of viologen
molecule has been established. In-situ UV-Vis spectra reveals that the navy
blue color of the device under biased condition is not due to increase in the
transparency corresponding to blue wavelength but due to suppression of the
transparency corresponding to the complementary colors. Absorption modulation
has been reported from the device with good ON/OFF contrast of the device.Comment: 14 Pages, 6 figure
Tent--Shaped Surface Morphologies of Silicon: Texturization by Metal Induced Etching
Nano--metal/semiconductor junction dependent porosification of silicon (Si)
has been studied here. The silicon (Si) nanostructures (NS) have been textured
on n-- and p-- type silicon wafers using Ag and Au metal nano particles induced
chemical etching. The combinations of n--Si/Ag and p--Si/Au form ohmic contact
and result in the same texturization on the Si surface on porosification where
tent--shaped morphology has been observed consistently with n-- and p--type Si.
Whereas, porosification result in different surface texturization for other two
combinations (p--Si/Ag and n--Si/Au) where Schottkey contacts are formed.
Quantitative analysis have been done using ImageJ to process the SEM images of
SiNS, which confirms that the tent like SiNS are formed when etching of silicon
wafer is done by AgNPs and AuNPs on n and p type Si wafer respectively. These
easily prepared sharp tent--shaped Si NSs can be used for enhanced field
emission applications.Comment: 10 Pages, 5 Figure
Generalization of Phonon Confinement Model for Interpretation of Raman Line-Shape from Nano-Silicon
A comparative analysis of two Raman line-shape functions has been carried out
to validate the true representation of experimentally observed Raman scattering
data for semiconducting nanomaterials. A modified form of already existing
phonon confinement model incorporates two basic considerations, phonon momentum
conservation and shift in zone centre phonon frequency. After incorporation of
the above mentioned two factors, a rather symmetric Raman line-shape is
generated which is in contrary to the usual asymmetric Raman line-shapes
obtained from nanostructured semiconductor. By fitting an experimentally
observed Raman scattering data from silicon nanostructures, prepared by metal
induced etching, it can be established that the Raman line-shape obtained
within the framework of phonon confinement model is a true representative Raman
line-shape of sufficiently low dimensions semiconductors.Comment: 8 pages, 3 figure
Observation, Evidence and Origin of Room Temperature Magnetodielectric Effect in Mn doped LaGaO3
We report an observation of room temperature magnetodielectric (RTMD) effect
in Mn doped LaGaO3. Results of frequency dependent magnetoresistance (FDMR)
measurements discards the possibility of any magnetoresistive contribution in
the observed MD effect. The intrinsic nature of MD coupling has been
validated/evidenced by means of magnetic field dependent Raman spectroscopy and
explained in terms of modified volume strain governed by magnetic field induced
rerotation of spin coupled Mn-orbitals. Ultimately, present RTMD effect is
attributed to magneto-compression/magnetostriction associated with spin-phonon
coupling as evidenced in the form of magnetic field induced hardening of
symmetric stretching (SS) MnO6 octahedral Raman modes. Presently studied Mn
doped LaGaO3 can be a candidate for magnetodielectric applications
Role of Metal Nanoparticles on porosification of silicon by metal induced etching (MIE)
Porosification of silicon (Si) by metal induced etching (MIE) process have
been studies here to understand the etching mechanism. The etching mechanism
has been discussed on the basis of electron transfer from Si to metal ion
(Ag) and metal to HO. Role of silver nanoparticles (AgNPs) in the
etching process has been investigated by studying the effect of AgNPs coverage
on surface porosity. A quantitative analysis of SEM images, done using Image J,
shows a direct correlation between AgNPs coverage and surface porosity after
the porosification. Density of Si nanowires (NWs) also varies as a function of
AgNPs fractional coverage which reasserts the fact that AgNPs governs the
porosification process during MIE.Comment: 8 pages, 5 figure
Interplay between Phonon Confinement and Fano Effect on Raman line shape for semiconductor nanostructures: Analytical study
Theoretical Raman line shape functions have been studied to take care of
quantum confinement effect and Fano effect individually and jointly. The
characteristics of various Raman line shapes have been studied in terms of the
broadening and asymmetry of Raman line shapes. It is shown that the asymmetry
in the Raman line-shape function caused by these two effects individually does
not add linearly to give asymmetry of line-shape generated by considering the
combined effect. This indicates existence of interplay between the two effects.
The origin of interplay lies in the fact that Fano effect itself depends on
quantum confinement effect and in turn provides an asymmetry. This can not be
explained by considering the two effects contribution independent of each
other.Comment: 10 Page
Comparison of porous silicon prepared using metal-induced etching (MIE) and laser-induced etching (LIE)
Porous silicon (p-Si), prepared by two routes (metal induced etching (MIE)
and laser induced etching (LIE)) have been studied by comparing the observed
surface morphologies using SEM. A uniformly distributed smaller (submicron
sized) pores are formed when MIE technique is used because the pore formation
is driven by uniformly distributed metal (silver in present case)
nanoparticles, deposited prior to the porosification step. Whereas in p-Si,
prepared by LIE technique, wider pores with some variation in pore size as
compared to MIE technique is observed because a laser having gaussian profile
of intensity is used for porosification. Uniformly distribute well-aligned Si
nanowires are observed in samples prepared by MIE method as seen using
cross-sectional SEM imaging. A single photoluminescence (PL) peak at 1.96 eV
corresponding to red emission at room temperature is observed which reveals
that the Si nanowires, present in p-Si prepared by MIE, show quantum
confinement effect. The single PL peak confirms the presence of uniform sized
nanowires in MIE samples. These vertically aligned Si nanowires can be used for
field emission application