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