226 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
Intersubband transitions in nonpolar GaN/Al(Ga)N heterostructures in the short and mid-wavelength infrared regions
This paper assesses nonpolar m- and a-plane GaN/Al(Ga)N multi-quantum-wells
grown on bulk GaN for intersubband optoelectronics in the short- and
mid-wavelength infrared ranges. The characterization results are compared to
those for reference samples grown on the polar c-plane, and are verified by
self-consistent Schr\"odinger-Poisson calculations. The best results in terms
of mosaicity, surface roughness, photoluminescence linewidth and intensity, as
well as intersubband absorption are obtained from m-plane structures, which
display room-temperature intersubband absorption in the range from 1.5 to 2.9
um. Based on these results, a series of m-plane GaN/AlGaN multi-quantum-wells
were designed to determine the accessible spectral range in the mid-infrared.
These samples exhibit tunable room-temperature intersubband absorption from 4.0
to 5.8 um, the long-wavelength limit being set by the absorption associated
with the second order of the Reststrahlen band in the GaN substrates
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
Boron-doped superlattices and Bragg mirrors in diamond
International audienceA periodic modulation of the boron doping level of single crystal diamond multilayers over more than three orders of magnitude during epitaxial growth by microwave plasma-enhanced chemical vapor deposition is shown to yield Bragg mirrors in the visible. The thicknesses and doping level of the individual layers were controlled by in situ spectroscopic ellipsometry, enabling to tune the reflec-tance peak to the wavelength range of diamond color centers, such as NV 0 or NV À . The crystalline quality, periodicity, and sharpness of the doping transitions in these doping superlattices over tens of periods were confirmed by high resolution X-ray diffraction
Ferromagnetic (Ga,Mn)N epilayers versus antiferromagnetic GaMnN clusters
Mn-doped wurtzite GaN epilayers have been grown by nitrogen plasma-assisted
molecular beam epitaxy. Correlated SIMS, structural and magnetic measurements
show that the incorporation of Mn strongly depends on the conditions of the
growth. Hysteresis loops which persist at high temperature do not appear to be
correlated to the presence of Mn. Samples with up to 2% Mn are purely
substitutional GaMnN epilayers, and exhibit paramagnetic
properties. At higher Mn contents, precipitates are formed which are identified
as GaMnN clusters by x-ray diffraction and absorption: this induces a
decrease of the paramagnetic magnetisation. Samples co-doped with enough Mg
exhibit a new feature: a ferromagnetic component is observed up to
K, which cannot be related to superparamagnetism of unresolved magnetic
precipitates.Comment: Revised versio
Structure and magnetism of self-organized Ge(1-x)Mn(x) nano-columns
We report on the structural and magnetic properties of thin Ge(1-x)Mn(x)films
grown by molecular beam epitaxy (MBE) on Ge(001) substrates at temperatures
(Tg) ranging from 80deg C to 200deg C, with average Mn contents between 1 % and
11 %. Their crystalline structure, morphology and composition have been
investigated by transmission electron microscopy (TEM), electron energy loss
spectroscopy and x-ray diffraction. In the whole range of growth temperatures
and Mn concentrations, we observed the formation of manganese rich
nanostructures embedded in a nearly pure germanium matrix. Growth temperature
mostly determines the structural properties of Mn-rich nanostructures. For low
growth temperatures (below 120deg C), we evidenced a two-dimensional spinodal
decomposition resulting in the formation of vertical one-dimensional
nanostructures (nanocolumns). Moreover we show in this paper the influence of
growth parameters (Tg and Mn content) on this decomposition i.e. on nanocolumns
size and density. For temperatures higher than 180deg C, we observed the
formation of Ge3Mn5 clusters. For intermediate growth temperatures nanocolumns
and nanoclusters coexist. Combining high resolution TEM and superconducting
quantum interference device magnetometry, we could evidence at least four
different magnetic phases in Ge(1-x)Mn(x) films: (i) paramagnetic diluted Mn
atoms in the germanium matrix, (ii) superparamagnetic and ferromagnetic low-Tc
nanocolumns (120 K 400 K) and
(iv) Ge3Mn5 clusters.Comment: 10 pages 2 colonnes revTex formatte
Insertion of CdSe quantumdots in ZnSe nanowires : MBE growth and microstructure analysis
ZnSe 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
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
High In-content InGaN layers synthesized by plasma-assisted molecular-beam epitaxy: growth conditions, strain relaxation and In incorporation kinetics
We report the interplay between In incorporation and strain relaxation
kinetics in high-In-content InxGa1-xN (x = 0.3) layers grown by plasma-assisted
molecular-beam epitaxy. For In mole fractions x = 0.13-0.48, best structural
and morphological quality is obtained under In excess conditions, at In
accumulation limit, and at a growth temperature where InGaN decomposition is
active. Under such conditions, in situ and ex situ analysis of the evolution of
the crystalline structure with the growth thickness points to an onset of
misfit relaxation after the growth of 40 nm, and a gradual relaxation during
more than 200 nm which results in an inhomogeneous strain distribution along
the growth axis. This process is associated with a compositional pulling
effect, i.e. indium incorporation is partially inhibited in presence of
compressive strain, resulting in a compositional gradient with increasing In
mole fraction towards the surface
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