4 research outputs found
Closing the ultrafine particle number concentration budget at road-to-ambient scale: Implications for particle dynamics
<p>Freshly emitted vehicle exhaust particles are diluted quickly as they mix into ambient air, but the contribution of evaporation, coagulation, and/or nucleation of new particles to the number concentration has been the subject of some debate. We analyzed one-second time resolution size distribution data from an early morning field campaign, data collected at a time at which dilution has a smaller (but still dominant; ∼70−80%) impact on particle concentrations. Because the plume is diluted over an hour, and a distance of 1500 m, we can constrain the processes with higher accuracy. We find that concentrations in the smaller size bins (5.6–23.7 nm) peak further downwind than the reference particles (42.1–562 nm), and decay significantly faster than larger particles particularly in the area 100−400 m downwind. Comparisons of the cumulative contributions of van der Waals enhanced coagulation, dry deposition, and dilution and the observed decay curves, imply that for up to the first 50–100 m there is nucleation and/or growth of particles smaller than 5.6 nm. In contrast, in the ∼100–400 m region, some of the smaller particles evaporate. In the further downwind areas (>400 m) the particles all appear to decay at rates consistent with the sum of dilution, coagulation, and deposition. We also find that a dry deposition parameterization at the low end of those available in the literature is most consistent with the observational data.</p> <p>© 2016 American Association for Aerosol Research</p
Site-Controlled VLS Growth of Planar Nanowires: Yield and Mechanism
The
recently emerged selective lateral epitaxy of semiconductor
planar nanowires (NWs) via the vapor–liquid–solid (VLS)
mechanism has redefined the long-standing symbolic image of VLS NW
growth. The in-plane geometry and self-aligned nature make these planar
NWs completely compatible with large scale manufacturing of NW-based
integrated nanoelectronics. Here, we report on the realization of
perfectly site-controlled growth of GaAs planar NW arrays with unity
yield using lithographically defined gold (Au) seed dots. The growth
rate of the planar NWs is found to decrease with the NW width at fixed
spacing, which is consistent with the conventional VLS model based
on the Gibbs-Thomson effect. It is found that in general, the planar
and out-of-plane NW growth modes are both present. The yield of planar
NWs decreases as their lateral dimension shrinks, and 100% yield of
planar NWs can be achieved at moderate V/III ratios. Based on a study
of the shape of seed particles, it is proposed that the adhesion between
the liquid-phase seed particle and the substrate surface is important
in determining the choice of growth mode. These studies represent
advances in the fundamental understanding of the VLS planar NW growth
mechanism and in the precise control of the planar NW site, density,
width, and length for practical applications. In addition, high quality
planar InAs NWs on GaAs (100) substrates is realized, verifying that
the planar VLS growth mode can be extended to heteroepitaxy
Direct Observation of Dopants Distribution and Diffusion in GaAs Planar Nanowires with Atom Probe Tomography
Intentional and unintentional doping
in semiconductor nanowires undoubtedly have significant impact on
the device performance. However, spatially resolved precise determination
of dopant concentration is challenging due to insufficient sensitivity
and resolution of conventional techniques. In this paper, quantitative
3D distribution of Si and Zn dopants in planar GaAs nanowires and
their interface with AlGaAs film underneath are obtained by using
a unique atom probe tomography technique, providing critical insights
for the growth and potential applications of these nanowires
Direct Electrical Probing of Periodic Modulation of Zinc-Dopant Distributions in Planar Gallium Arsenide Nanowires
Selective
lateral epitaxial (SLE) semiconductor nanowires (NWs),
with their perfect in-plane epitaxial alignment, ability to form lateral
complex p–n junctions <i>in situ</i>, and compatibility with planar processing, are a
distinctive platform for next-generation device development. However,
the incorporation and distribution of impurity dopants in these planar
NWs <i>via</i> the vapor−liquid−solid growth mechanism remain relatively unexplored. Here, we present
a detailed study of SLE planar GaAs NWs containing multiple alternating
axial segments doped with Si and Zn impurities by metalorganic chemical
vapor deposition. The dopant profile of the lateral multi-p–n
junction GaAs NWs was imaged simultaneously with nanowire topography
using scanning microwave impedance microscopy and correlated with
infrared scattering-type near-field optical microscopy. Our results
provide unambiguous evidence that Zn dopants in the periodically twinned
and topologically corrugated p-type segments are preferentially segregated
at twin plane boundaries, while Si impurity atoms are uniformly distributed
within the n-type segments of the NWs. These results are further supported
by microwave impedance modulation microscopy. The density functional
theory based modeling shows that the presence of Zn dopant atoms reduces
the formation energy of these twin planes, and the effect becomes
significantly stronger with a slight increase of Zn concentration.
This implies that the twin formation is expected to appear when a
threshold planar concentration of Zn is achieved, making the onset
and twin periodicity dependent on both Zn concentration and nanowire
diameter, in perfect agreement with our experimental observations