79 research outputs found
Swelling of phospholipid floating bilayers: the effect of chain length
The equilibrium distance between two lipid bilayers stable in bulk water and
in proximity of a substrate was investigated. Samples consisted of a
homogeneous lipid bilayer, floating near an identical bilayer deposited on the
hydrophilic surface of a silicon single crystal. Lipids were saturated di-acyl
phosphocholines, with the number of carbon atoms per chain, n, varying from 16
to 20. The average and r.m.s. positions of the floating bilayer were determined
by means of neutron specular reflectivity. Samples were prepared at room
temperature (i.e. with the lipids in the gel phase) and measurements performed
at various temperatures so that the whole region of transition from gel to
fluid phase was explored. Data have been interpreted in terms of competition
between the interbilayer potential and membrane fluctuations and used to
estimate the bending rigidity of the bilayer
Deterministic radiative coupling between plasmonic nanoantennas and semiconducting nanowire quantum dots
International audienceWe report on the deterministic coupling between single semiconducting nanowire quantum dots emitting in the visible and plasmonic Au nanoantennas. Both systems are separately carefully characterized through microphotoluminescence and cathodoluminescence. A two-step realignment process using cathodoluminescence allows for electron beam lithography of Au antennas near individual nanowire quantum dots with a precision of 50 nm. A complete set of optical properties are measured before and after antenna fabrication. They evidence both an increase of the NW absorption, and an improvement of the quantum dot emission rate up to a factor two in presence of the antenna
Unraveling the strain state of GaN down to single nanowires
GaN nanowires (NWs) grown by molecular beam epitaxy are usually assumed free of strain in spite of different individual luminescence signatures. To ascertain this usual assumption, the c/a of a GaNNW assembly has been characterized using both X-ray diffraction and Raman spectroscopy, with scaling the measurement down to the single NW. Free-standing single NWs have been observed free of strain defined as [c/a-(c/a)o]/(c/a)o within the experimental accuracy mounting to 1.25 × 10-4. However, in the general case, a significant portion of the NWs is coalesced, generating an average tensile strain that can be partly released by detaching the NWs from their substrates. It is concluded that at the scale of the single NW, the free surface and the residual doping do not generate a significant strain and only coalescence does
Optical properties of single ZnTe nanowires grown at low temperature
Optically active gold-catalyzed ZnTe nanowires have been grown by molecular
beam epitaxy, on a ZnTe(111) buffer layer, at low temperature 350\degree under
Te rich conditions, and at ultra-low density (from 1 to 5 nanowires per
micrometer^{2}. The crystalline structure is zinc blende as identified by
transmission electron microscopy. All nanowires are tapered and the majority of
them are oriented. Low temperature micro-photoluminescence and
cathodoluminescence experiments have been performed on single nanowires. We
observe a narrow emission line with a blue-shift of 2 or 3 meV with respect to
the exciton energy in bulk ZnTe. This shift is attributed to the strain induced
by a 5 nm-thick oxide layer covering the nanowires, and this assumption is
supported by a quantitative estimation of the strain in the nanowires
From diluted magnetic semiconductors to self-organized nanocolumns of GeMn in Germanium
While achieving high Curie temperatures (above room temperature) in diluted
magnetic semiconductors remains a challenge in the case of well controlled
homogeneous alloys, several systems characterized by a strongly inhomogeneous
incorporation of the magnetic component appear as promising. Incorporation of
manganese into germanium drastically alters the growth conditions, and in
certain conditions of low temperature Molecular Beam Epitaxy it leads to the
formation of well organized nanocolumns of a Mn-rich material, with a
crystalline structure in epitaxial relationship with the Mn-poor germanium
matrix. A strong interaction between the Mn atoms in these nanocolums is
demonstrated by x-ray absorption spectroscopy, giving rise to a ferromagnetic
character as observed through magnetometry and x-ray magnetic circular
dichroism. Most interesting, intense magneto-transport features are observed on
the whole structure, which strongly depend on the magnetic configuration of the
nanocolumns.Comment: SPIE Optics & Photonics Symposium, San Diego : \'Etats-Unis
d'Am\'erique (2008
Insertion of CdSe quantum dots in ZnSe nanowires: Correlation of structural and chemical characterization with photoluminescence
International audienceZnSe nanowires with CdSe quantum dot insertions were grown by molecular beam epitaxy using gold as a catalyst. Structural, chemical, and optical properties of the wires and quantum dots were characterized using electron microscopy and photoluminescence spectroscopy. We determined the crystalline structure, the chemical composition, and the size of the quantum dot and established a correlation between quantum dot size and luminescence. As expected, a blueshift of the luminescence was observed for decreasing quantum dot size. The comparison of calculated photoluminescence energy and experimental data seems to indicate that the quantum dots consist of a ZnxCd1-xSe ternary alloy rather than pure CdSe
Insertion of CdSe quantumdots in ZnSe nanowires : MBE growth and microstructure analysis
International audienceZnSe nanowire growth has been successfully achieved on ZnSe (100) and (111)B buffer layers deposited on GaAs substrates. Cubic [100] oriented ZnSe nanowires or [0001] oriented hexagonal NWs are obtained on (100) substrates while [111] oriented cubic mixed with [0001] oriented hexagonal regions are obtained on (111)B substrates. Most of the NWs are perpendicular to the surface in the last case. CdSe quantum dots were successfully incorporated in the ZnSe NWs as demonstrated by transmission electron microscopy, energy filtered TEM and high angle annular dark field scanning TEM measurements
Effect of extended defects on AlGaN QDs for electron-pumped UV-emitters
We study the origin of bimodal emission in AlGaN/AlN QD superlattices
displaying high internal quantum efficiency (around 50%) in the 230-300 nm
spectral range. The secondary emission at longer wavelengths is linked to the
presence of cone-like defects starting at the first AlN buffer/superlattice
interface and propagating vertically. These defects are associated with a
dislocation that produces strong shear strain, which favors the formation of
30{\deg} faceted pits. The cone-like structures present Ga enrichment at the
boundary facets and larger QDs within the defect. The bimodality is attributed
to the differing dot size/composition within the defects and at the defect
boundaries, which is confirmed by the correlation of microscopy results and
Schr\"odinger-Poisson calculations
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