28 research outputs found
Blue luminescence of Au nanoclusters embedded in silica matrix
Photoluminescence study using the 325 nm He-Cd excitation is reported for the
Au nanoclusters embedded in SiO2 matrix. Au clusters are grown by ion beam
mixing with 100 KeV Ar+ irradiation on Au [40 nm]/SiO2 at various fluences and
subsequent annealing at high temperature. The blue bands above ~3 eV match
closely with reported values for colloidal Au nanoclusters and supported Au
nanoislands. Radiative recombination of sp electrons above Fermi level to
occupied d-band holes are assigned for observed luminescence peaks. Peaks at
3.1 eV and 3.4 eV are correlated to energy gaps at the X- and L-symmetry
points, respectively, with possible involvement of relaxation mechanism. The
blue shift of peak positions at 3.4 eV with decreasing cluster size is reported
to be due to the compressive strain in small clusters. A first principle
calculation based on density functional theory using the full potential linear
augmented plane wave plus local orbitals (FP-LAPW+LO) formalism with
generalized gradient approximation (GGA) for the exchange correlation energy is
used to estimate the band gaps at the X- and L-symmetry points by calculating
the band structures and joint density of states (JDOS) for different strain
values in order to explain the blueshift of ~0.1 eV with decreasing cluster
size around L-symmetry point.Comment: 13 pages, 7 Figures Only in PDF format; To be published in J. of
Chem. Phys. (Tentative issue of publication 8th December 2004
Multi-phonon Raman scattering in GaN nanowires
UV Raman scattering studies show longitudinal optical (LO) mode up to 4th
order in wurtzite GaN nanowire system. Frohlich interaction of electron with
the long range electrostatic field of ionic bonded GaN gives rise to
enhancement in LO phonon modes. Good crystalline quality, as indicated by the
crystallographic as well as luminescence studies, is thought to be responsible
for this significant observation. Calculated size dependence, incorporating
size corrected dielectric constants, of electron-phonon interaction energy
agrees well with measured values and also predict stronger interaction energy
than that of the bulk for diameter below ~3 nm.Comment: 13 pages, 5 figures, Journa
Nitrogen ion beam synthesis of InN in InP (100) at elevated temperature
InN phase is grown in crystalline InP(100) substrates by 50 keV N+
implantation at an elevated temperature of 400 deg C followed by annealing at
525 deg C in N2 ambient. Crystallographic structural and Raman scattering
studies are performed for the characterization of grown phases. Temperature-
and power-dependent photoluminescence studies show direct band-to-band
transition peak ~1.06 eV at temperatures <=150K. Implantations at an elevated
temperature with a low ion beam current and subsequent low temperature
annealing step are found responsible for the growth of high-quality InN phase.Comment: 11 pages, 4 figures, Journa
Optical characterization of GaN by N+ implantation into GaAs at elevated temperature
Both hexagonal wurtzite and cubic zinc blend GaN phases were synthesized in
GaAs by 50 keV N+ implantation at 400 deg C and subsequent annealing at 900 deg
C for 15 min in N2 ambient. Crystallographic structural and Raman scattering
studies revealed that GaN phases were grown for fluence above 2x1017 cm-2.
Temperature-dependent photoluminescence study showed sharp direct band-to-band
transition peak ~3.32 eV at temperature <= 200K. The intermediate bandgap
value, with respect to ~3.4 eV for hexagonal and ~3.27 eV for cubic phases of
GaN is an indicative for the formation of mixed hexagonal and cubic phases.Comment: 9 pages, 4 figuresn Journa
Upper critical field in borocarbides
Resistive measurement of superconducting transitions under externally applied magnetic fields in polycrystalline samples of YPd<SUB>5</SUB>B<SUB>3</SUB>C<SUB>0.3</SUB> and YNi<SUB>2</SUB>B<SUB>2</SUB>C has revealed a marked positive curvature in the temperature dependence of H<SUB>c2</SUB>, with the slope - dH<SUB>c2</SUB>/dT increasing monotonically from T<SUB>c</SUB> to 4.2 K. This observed positive curvature is in marked contrast with the predictions of the conventional theories of upper critical field. It is possible that a compensation mechanism may have to be invoked to account for the observed results