5 research outputs found
Measuring and Modeling the Growth Dynamics of Self-Catalyzed GaP Nanowire Arrays
The bottom-up fabrication of regular
nanowire (NW) arrays on a
masked substrate is technologically relevant, but the growth dynamic
is rather complex due to the superposition of severe shadowing effects
that vary with array pitch, NW diameter, NW height, and growth duration.
By inserting GaAsP marker layers at a regular time interval during
the growth of a self-catalyzed GaP NW array, we are able to retrieve
precisely the time evolution of the diameter and height of a single
NW. We then propose a simple numerical scheme which fully computes
shadowing effects at play in infinite arrays of NWs. By confronting
the simulated and experimental results, we infer that re-emission
of Ga from the mask is necessary to sustain the NW growth while Ga
migration on the mask must be negligible. When compared to random
cosine or random uniform re-emission from the mask, the simple case
of specular reflection on the mask gives the most accurate account
of the Ga balance during the growth
Dynamic Shaping of Femtoliter Dew Droplets
Herein,
we show that wetting properties such as giant wetting anisotropy
and dynamic shaping can be observed when femtoliter (submicron scale)
dew droplets are condensed on nanopatterned mildly hydrophilic surfaces.
Large-scale, optically transparent, nanopatterned TiO<sub>2</sub> surfaces
were fabricated by direct nanoimprinting lithography of sol–gel-derived
films. Square, infinitely elongated, or circular droplets were obtained
with square, line, or concentric patterns, respectively, and were
visualized <i>in situ</i> during formation and recession
using optical microscopy and environmental scanning electronic microscopy.
We first describe how extremely elongated droplets could form on mildly
hydrophilic surfaces, naturally contaminated in real environmental
conditions. In this configuration, the dew droplet shape can be dynamically
and reversibly varied by controlling the out-of-equilibrium conditions
associated with condensation/evaporation kinetics. As an example of
the application, we propose a “morphological” sensor
that exploits the shape of the dew droplets as a transduction mode
for detecting organic vapors in the outer atmosphere. Importantly,
this study is underlining that environmentally stable, purely hydrophilic
surfaces can be smartly engineered to induce wetting phenomena at
very small scale never observed so far for hydrophobic or heterogeneous
surfaces. Our versatile approach based on nanoimprinted, transparent
sol–gel films could open great perspectives for the implementation
of environmentally stable, mildly hydrophilic materials for “dew
engineering” applications such as open microfluidics, fuming
for fingerprints, vapor sensing, or water harvesting on glass windows,
for instance
Dynamic Shaping of Femtoliter Dew Droplets
Herein,
we show that wetting properties such as giant wetting anisotropy
and dynamic shaping can be observed when femtoliter (submicron scale)
dew droplets are condensed on nanopatterned mildly hydrophilic surfaces.
Large-scale, optically transparent, nanopatterned TiO<sub>2</sub> surfaces
were fabricated by direct nanoimprinting lithography of sol–gel-derived
films. Square, infinitely elongated, or circular droplets were obtained
with square, line, or concentric patterns, respectively, and were
visualized <i>in situ</i> during formation and recession
using optical microscopy and environmental scanning electronic microscopy.
We first describe how extremely elongated droplets could form on mildly
hydrophilic surfaces, naturally contaminated in real environmental
conditions. In this configuration, the dew droplet shape can be dynamically
and reversibly varied by controlling the out-of-equilibrium conditions
associated with condensation/evaporation kinetics. As an example of
the application, we propose a “morphological” sensor
that exploits the shape of the dew droplets as a transduction mode
for detecting organic vapors in the outer atmosphere. Importantly,
this study is underlining that environmentally stable, purely hydrophilic
surfaces can be smartly engineered to induce wetting phenomena at
very small scale never observed so far for hydrophobic or heterogeneous
surfaces. Our versatile approach based on nanoimprinted, transparent
sol–gel films could open great perspectives for the implementation
of environmentally stable, mildly hydrophilic materials for “dew
engineering” applications such as open microfluidics, fuming
for fingerprints, vapor sensing, or water harvesting on glass windows,
for instance
Dynamic Shaping of Femtoliter Dew Droplets
Herein,
we show that wetting properties such as giant wetting anisotropy
and dynamic shaping can be observed when femtoliter (submicron scale)
dew droplets are condensed on nanopatterned mildly hydrophilic surfaces.
Large-scale, optically transparent, nanopatterned TiO<sub>2</sub> surfaces
were fabricated by direct nanoimprinting lithography of sol–gel-derived
films. Square, infinitely elongated, or circular droplets were obtained
with square, line, or concentric patterns, respectively, and were
visualized <i>in situ</i> during formation and recession
using optical microscopy and environmental scanning electronic microscopy.
We first describe how extremely elongated droplets could form on mildly
hydrophilic surfaces, naturally contaminated in real environmental
conditions. In this configuration, the dew droplet shape can be dynamically
and reversibly varied by controlling the out-of-equilibrium conditions
associated with condensation/evaporation kinetics. As an example of
the application, we propose a “morphological” sensor
that exploits the shape of the dew droplets as a transduction mode
for detecting organic vapors in the outer atmosphere. Importantly,
this study is underlining that environmentally stable, purely hydrophilic
surfaces can be smartly engineered to induce wetting phenomena at
very small scale never observed so far for hydrophobic or heterogeneous
surfaces. Our versatile approach based on nanoimprinted, transparent
sol–gel films could open great perspectives for the implementation
of environmentally stable, mildly hydrophilic materials for “dew
engineering” applications such as open microfluidics, fuming
for fingerprints, vapor sensing, or water harvesting on glass windows,
for instance
Determination of n‑Type Doping Level in Single GaAs Nanowires by Cathodoluminescence
We
present an effective method of determining the doping level
in n-type III–V semiconductors at the nanoscale. Low-temperature
and room-temperature cathodoluminescence (CL) measurements are carried
out on single Si-doped GaAs nanowires. The spectral shift to higher
energy (Burstein–Moss shift) and the broadening of luminescence
spectra are signatures of increased electron densities. They are compared
to the CL spectra of calibrated Si-doped GaAs layers, whose doping
levels are determined by Hall measurements. We apply the generalized
Planck’s law to fit the whole spectra, taking into account
the electron occupation in the conduction band, the bandgap narrowing,
and band tails. The electron Fermi levels are used to determine the
free electron concentrations, and we infer nanowire doping of 6 ×
10<sup>17</sup> to 1 × 10<sup>18</sup> cm<sup>–3</sup>. These results show that cathodoluminescence provides a robust way
to probe carrier concentrations in semiconductors with the possibility
of mapping spatial inhomogeneities at the nanoscale