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SEM images of GaAs nanoneedles. (a) Two needles which enveloped each other during growth, showing the sidewall and tip deposition growth mode. (b) Nanoneedles grown on a GaAs substrate, viewed top down (left) and tilted by 30° (right) indicating the uniformity and alignment of the  nanoneedle growth axis to the substrate directions. (c) Nanoneedle viewed near to its side highlighting the facet smoothness, with the extremely sharp tip shown in the inset. (d) A linear array of nanoneedles made by mechanically roughening only a very thin line. (e) GaAs nanoneedle grown on 4° off-cut Si (111) substrate, with views of 30° tilted and top down. The nanoneedle tilt indicates good epitaxial alignment to substrate despite the 4% lattice mismatch.
GaAs nanoneedle growth mode and the fabrication of hollow nanoneedles. (a) The standard growth mode for the nanoneedles is via continual surface deposition, unlike VLS growth. This enables core-shell structures simply by adding additional gas precursors. (b) Sonication of the AlGaAs-shell, GaAs-core nanoneedles resulted in broken tips. A selective etch was then performed to remove part of the core GaAs material to form hollow nanoneedles, demonstrating the surface-deposition growth mode. (c) A planar-view SEM picture of the fabricated hollow nanoneedles. The AlGaAs cladding with thickness of is clearly seen after the removal of the GaAs core. (d) A 20°-tilt view SEM picture of another fabricated hollow nanoneedle with the sonication break closer to the tip. The same thickness AlGaAs cladding can still be clearly seen.
HRTEM images of GaAs nanoneedles. (a) zone axis HRTEM image of an as-grown GaAs nanoneedle, with a tip only wide. The insets show the zoomed-out view, and also the image FFT. (b) FFT from another nanoneedle on its zone axis with a distinct wurtzite pattern. The spacing is . (c) Top-down  TEM image of a nanoneedle. The image to the right shows a SAED pattern from the circled area, with distinct wurtzite spots matching the expected unique wurtzite spacing. The chevron spot shape is due to electron scattering from the two sidewalls contained in the circled area.
spectra of the pure GaAs nanoneedle and AlGaAs-coated GaAs nanoneedle. (a) Low-temperature spectrum. The pumping power is with a focused laser spot and a wavelength of . The peak wavelength of the GaAs nanoneedle is at . The AlGaAs-coated nanoneedle emits at a similar wavelength and is brighter than the pure GaAs nanoneedle by approximately a factor of 2. (b) Room-temperature spectrum. The pumping power is . The peak wavelength of the GaAs nanoneedle and AlGaAs-coated nanoneedle are both approximately . The AlGaAs-coated nanoneedle is brighter than the GaAs nanoneedle by a factor of 4. The PL linewidths of the pure GaAs and AlGaAs-coated nanoneedles are 50 and , respectively.
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