109 research outputs found
Controlled Irradiative Formation of Penitentes
Spike-shaped structures are produced by light-driven ablation in very
different contexts. Penitentes 1-4 m high are common on Andean glaciers, where
their formation changes glacier dynamics and hydrology. Laser ablation can
produce cones 10-100 microns high with a variety of proposed applications in
materials science. We report the first laboratory generation of
centimeter-scale snow and ice penitentes. Systematically varying conditions
allows identification of the essential parameters controlling the formation of
ablation structures. We demonstrate that penitente initiation and coarsening
requires cold temperatures, so that ablation leads to sublimation rather than
melting. Once penitentes have formed, further growth of height can occur by
melting. The penitentes intially appear as small structures (3 mm high) and
grow by coarsening to 1-5 cm high. Our results are an important step towards
understanding and controlling ablation morphologies.Comment: Accepted for publication in Physical Review Letter
Fabrication, Study of Optical Properties and Structure of Most Stable (CdP2)n Nanoclusters
CdP2 nanoclusters were fabricated by incorporation into pores of zeolite Na-X
and by laser ablation. Absorption and photoluminescence (PL) spectra of CdP2
nanoclusters in zeolite were measured at the temperatures of 4.2, 77 and 293 K.
Both absorption and PL spectra consist of two bands blue shifted with respect
to bulk crystal. We performed the calculations aimed to find the most stable
clusters in the size region up to size of the zeolite Na-X supercage. The most
stable clusters are (CdP2)6 and (CdP2)8 with binding energies of 9.30 eV and
10.10 eV per (CdP2)1 formula unit respectively. Therefore, we attributed two
bands observed in absorption and PL spectra to these stable clusters. The Raman
spectrum of CdP2 clusters in zeolite was explained to be originated from
(CdP2)6 and (CdP2)8 clusters as well. The PL spectrum of CdP2 clusters produced
by laser ablation consists of the asymmetric band with low-energy tail that has
been attributed to emission of both (CdP2)8 cluster and CdP2 microcrystals.Comment: Accepted for publication in Physica E: Low-dimensional Systems and
Nanostructure
Cr2O3 thin films grown at room temperature by low pressure laser chemical vapour deposition
Chromia (Cr2O3) has been extensively explored for the purpose of developing
widespread industrial applications, owing to the convergence of a variety of
mechanical, physical and chemical properties in one single oxide material.
Various methods have been used for large area synthesis of Cr2O3 films.
However, for selective area growth and growth on thermally sensitive materials,
laser-assisted chemical vapour deposition (LCVD) can be applied advantageously.
Here we report on the growth of single layers of pure Cr2O3 onto sapphire
substrates at room temperature by low pressure photolytic LCVD, using UV laser
radiation and Cr(CO)6 as chromium precursor. The feasibility of the LCVD
technique to access selective area deposition of chromia thin films is
demonstrated. Best results were obtained for a laser fluence of 120 mJ cm-2 and
a partial pressure ratio of O2 to Cr(CO)6 of 1.0. Samples grown with these
experimental parameters are polycrystalline and their microstructure is
characterised by a high density of particles whose size follows a lognormal
distribution. Deposition rates of 0.1 nm s-1 and mean particle sizes of 1.85
{\mu}m were measured for these films.Comment: 16 pages, 6 figures, accepted for publication in Thin Solid Film
Optical Properties and Structure of Most Stable Subnanometer (ZnAs2)n Clusters
ZnAs2 nanoclusters were fabricated by incorporation into pores of zeolite
Na-X and by laser ablation. Absorption and photoluminescence spectra of ZnAs2
nanoclusters in zeolite were measured at the temperatures of 4.2, 77 and 293 K.
Both absorption and PL spectra consist of two bands which demonstrate the blue
shift from the line of free exciton in bulk crystal. We performed the
calculations aimed to find the most stable clusters in the size region up to
size of the zeolite Na-X supercage. The most stable clusters are (ZnAs2)6 and
(ZnAs2)8 with binding energies of 7.181 eV and 8.012 eV per (ZnAs2)1 formula
unit respectively. Therefore, we attributed two bands observed in absorption
and PL spectra to these stable clusters. The measured Raman spectrum of ZnAs2
clusters in zeolite was explained to be originated from (ZnAs2)6 and (ZnAs2)8
clusters as well. The PL spectrum of ZnAs2 clusters produced by laser ablation
consists of a single band which has been attributed to emission of (ZnAs2)8
cluster.Comment: Article accepted for publication in Physica B: Physics of Condensed
Matte
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