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Article
Visual Understanding of Light Absorption and Waveguiding in Standing Nanowires with 3D fluorescence confocal microscopy Rune Frederiksen, Gözde Tütüncüoglu, Federico Matteini, Karen L. Martinez, Anna Fontcuberta i Morral, and Esther Alarcon Llado ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00434 • Publication Date (Web): 21 Aug 2017 Downloaded from http://pubs.acs.org on August 23, 2017
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Visual Understanding of Light Absorption and Waveguiding in
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Standing Nanowires with 3D fluorescence confocal microscopy
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Rune Frederiksen,1 Gozde Tutuncuoglu,2 Federico Matteini,2 Karen L. Martinez,1 Anna Fontcuberta i Morral,2
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Abstract
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Semiconductor nanowires are promising building blocks for next generation photonics. Indirect proofs of
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large absorption cross-sections have been reported in nano-structures with sub-wavelength diameters, an
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effect that is even more prominent in vertically standing nanowires. In this work we provide a 3-
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dimensional map of the light around vertical GaAs nanowires standing on a substrate by using
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fluorescence confocal microscopy, where the strong long-range disruption of the light path along the
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nanowire is illustrated. We find that the actual long distance perturbation is much larger in size than
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calculated extinction cross-sections. While the size of the perturbation remains similar, the intensity of the
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interaction changes dramatically over the visible spectrum. Numerical simulations allow us to distinguish
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the effects of scattering and absorption in the nanowire leading to these phenomena. This work provides a
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visual understanding of light absorption in semiconductor nanowire structures, which is of high interest
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for solar energy conversion applications.
Esther Alarcon-Llado2,3* 1
Bio-Nanotechnology and Nanomedicine Laboratory, Department of Chemistry & Nano-Science Center, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark
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Laboratory of Semiconductor Materials, Institute of Materials, School of Engineering, EPFL, 1015 Lausanne,
Switzerland 3
Center for Nanophotonics, AMOLF, Science Park 104, 1098XG Amsterdam, Netherlands
*Corresponding Author:
[email protected] 24 25 26
Keywords: semiconductor nanowire, optical properties, solar energy, confocal microscopy
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Since the discovery of photonic properties of dielectric nanostructures,1–7 their great potential for solar
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applications at lower cost has been unravelled.8–18 Nanowire (NW) ensembles constitute a new class of
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metamaterial, where the optical properties cannot be directly deduced from individual parts.19–22 Optical
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properties of NW arrays are tuned by the individual NW type, geometry and collective arrangement,
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which provide a new playground in solar energy conversion and the solid-state lighting arenas.23–32 The
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effective absorption cross-section in vertically standing nanowires can be much larger than the
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geometrical one.6,8,33 This large absorption cannot be explained by standard Mie-like resonance
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formulism, as in the case of horizontally lying structures.34,35 Assessing the absorption cross-section in
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vertical structures is not straightforward and it is important to understand how light is interacting with
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nanostructures and essential for the accurate quantum efficiency determination in photodevices. Many
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approaches have been shown to measure the optical local density of states in and around photonic and
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plasmonic structures, which include cathodoluminescence tomography,36 near-field optical probe
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techniques, single molecules attached to an AFM tip37,38 or fluorescent solutions probed with super-
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resolution microscopes39–41, among others. Although complex computational work has shown that super-
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resolution microscopy can be used to scan in all three dimensions,42 no studies have shown 3D imaging of
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local density of states at microscale surroundings of photonic nano-structures.
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In this work, we are interested in the 3D visualization of long-range light-matter interactions in GaAs
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standing nanowires, in particular within the region of large absorption. To this end, we use a simple
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confocal microscope set-up. The reconstructed fluorescence image captures the evolution of light
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extinction around the nanowire and along its length. By combining the experiments with electromagnetic
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simulations, we elucidate the mechanisms behind self-concentrating light effects and large absorption
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cross-sections found in semiconductor nanowires and their relation to the nanowire waveguiding
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properties. While the resolution is diffraction limited, we show that this method provides insightful 3D
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extinction maps around nanostructures, which can be an asset to characterize more complex
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morphologies.
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To demonstrate the power of the technique we probe gallium arsenide (GaAs) nanowires vertically
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standing on a silicon (Si) substrate. GaAs is a compound semiconductor with a direct bandgap of 1.42 eV,
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which makes it an excellent candidate for solar energy applications.
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Figure 1a illustrates the experimental set-up that provides a 3-dimensional mapping of light distribution
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around nanostructures. The sample is embedded in a liquid solution with suspended fluorescent dye
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molecules (see supplementary information S1 for more details). The dye fluorescence intensities are
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mapped around the nanowires with a laser scanning confocal microscope. Dye molecules are excited with
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a laser beam focused with a confocal microscope objective and the emitted light is collected through the
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same objective and detected with a photomultiplier (PMT). The physical principle of this characterization
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method resides in the fluorescence intensity of the molecules being proportional to the local excitation
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light intensity. Thus, by scanning the laser beam over an x-y area for a number of z-stacks, a diffraction-
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limited resolution 3D map of the light intensity is obtained. The x-y and z optical resolution in our
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experiments are specified in the Supplementary Information.
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An example of a reconstructed 3D image of light around GaAs nanowires when illuminated with blue
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laser light is shown in Figure 1b. In this case, two vertical GaAs nanowires (10 µm long, 126 nm in
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diameter and 2.5 µm apart) stand vertically on a Si substrate. The sample is excited with blue light
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(λ=488 nm), for which theory predicts that it should not have the strongest interaction with these
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nanostructures (see Supplementary Information on the absorption spectrum of single nanowire). The
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represented grey transparency in the image is linked to the amount of detected light, so that the darkest
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areas in the image indicate positions where no light is detected. As a consequence, the substrate appears
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dark. Under these conditions, also two dark columns stand at the position of the nanowires and a dark
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layer where the silicon substrate starts, in accordance to the lack of fluorophores in that volume.
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Interestingly, a semi-transparent conical region surrounds the nanowires, and widens up to several
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microns when approaching the nanowires’ bottom. The y-z cross-section intensity contour plot of the
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same image (Figure 1c) provides a more quantitative representation of the conical “shadow”. These
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pictures represent a clear visual image of how the nanowire pair is capable of redistributing the
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electromagnetic field around them at a long-range distance. It is clear from this picture that there is less
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light arriving at the silicon substrate surface due to the presence of the nanowires. It is interesting to note
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both the dimension and the magnitude of the shaded silicon surface. A better understanding of how light
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is redistributed around the nanowires is discussed later on. First, to confirm and proof that our
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experimental set-up is truly representing the light intensity distribution around the nanowires of the
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incoming laser, FDTD simulations were performed. The steady-state field energy distribution was
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simulated by considering an incoming plane wave propagating in the -z direction and same wavelength as
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in the experiment (λ=488 nm). Due to the asymmetry of the nanowire pair, two sets of simulations were
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performed for light polarized in the x and y axes, respectively. The resulting intensity maps were averaged
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to obtain the final distribution map. Figure 1d displays the line profiles of the calculated electric field
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intensity squared superimposed to the fluorescence light intensity at 1.2 and 7.5um below the nanowires
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tip. The calculated field energy distributions are in remarkable agreement with the detected light
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distribution from the confocal images. This result confirms that the observed confocal images are indeed
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an excellent representation of the interaction between a vertically-impinging plane wave and the 4 ACS Paragon Plus Environment
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nanostructures, which can span up to several micrometers in distance. Such good agreement also rules out
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that the confocal image is strongly perturbed by the possible interactions between fluorophore emission
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and the nanostructures. In fact, light emitted by the fluorophores is only perturbed when very close to the
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nanowire surface (
