Research ArticleNANOTECHNOLOGY

Ultra-broadband light trapping using nanotextured decoupled graphene multilayers

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Science Advances  26 Feb 2016:
Vol. 2, no. 2, e1501238
DOI: 10.1126/sciadv.1501238

Figures

  • Fig. 1 TPS pixel with nanostructured ultrathin emissivity coating on top surface.

    (A) TPS design with raised surface. Inset: Model of pixel in operation [hot scale, 20°C (blue); 800°C (red)]. (B) Top surface of TPS showing nanostructured ultrathin absorber. Area shown is denoted in red in (A). Inset: Comparison to a real “moth-eye” from the nymphalid Bicyclus anynana (50). Scale bar, 500 nm. (C) High-resolution scanning transmission electron microscopy (HR-STEM) from top of nanostructures, showing the graphene planes of the optical absorber layer. (D) Reflectance for the decoupled multilayer (DM) graphene surrounding the metal nanostructures (black curve), uncoated nanostructures (pink), nanostructures coated with amorphous carbon (a-C) (red), and a-C on a plain silicon substrate. The DM graphene absorber (black curve) shows broadband antireflection. For all cases, the transmittance is zero; therefore, the absorbance is 1 − reflectance. Inset: Transitions in the linear band structure of the DM graphene absorber involved in the broadband absorptivity and emissivity.

  • Fig. 2 Interaction of 4-μm wavelength radiation with metal nanostructures (without graphene absorber layer).

    (A) Interaction of electric field (in the y plane) (Ey) with nanostructures. Field strength of incident wave is 1 Vm−1. (B) Location of displacement currents (ID) generated by wave, indicated as regions of resistive heating (RH). These regions are the locations where the wave interacts with the nanostructures. Inset: Motion of displacement current around nanostructures. (C) Magnetic field (z direction) (Bz) generated by displacement currents in (B). (D) Total energy density of wave (time-averaged) (U) showing confinement of the wave’s energy to the spaces around the nanostructures.

  • Fig. 3 Evolution of absorber with annealing.

    (A and B) Raman spectroscopy of carbon films on Fe-coated (A) and bare Ti (B) nanostructures as a function of annealing, using a 514-nm laser. (C and D) Evolution of the D/G and the 2D/G peak height ratio (C) and FWHM of the D and G peaks, showing the formation of graphene structures for the Fe-coated nanostructures (D). (E and F) Optical absorption with annealing for films from (A) and (B), respectively. The increase in the broadband absorption is evident in the Fe-coated nanostructures in (E) and corroborated by emissivity measurements.

  • Fig. 4 Emissivity measurements in the 3- to 5-μm spectral range obtained by IR imaging.

    (A) Improvement of the emissivity with the formation of the multilayer graphene absorber around the Fe-coated nanostructures. Inset: Single Lorenzian peak in Raman spectrum indicating decoupled graphene layers. (B) Emissivity improvement from reference sample using a-C as the absorber (deposited directly on nanostructures without Fe catalyst). Inset: Increase in peak absorption intensity with reduction in the band gap of a-C films.

  • Fig. 5 Energy density in uncoated and carbon-coated nanostructure, obtained from the model.

    The wave travels from free space through the absorber region (in blue) to the bulk Ti surface (0 μm on the x axis) and is weakly absorbed by the uncoated nanostructures (black curve, left inset) but strongly absorbed by the carbon-coated nanostructures (red curve, right inset). The dashed green lines in the inset depict the scan lines where the curves were obtained. The inset shows (orange) the strong wave attenuation before reaching the surface for the carbon-coated case (right) compared to uncoated case (left). Wave confinement between the nanostructures results in energy densities rising above 100% at these locations.

Tables

  • Table 1 Evolution of the thermal emissivity (3-to 5-μm wavelengths) for samples with different layer structures, before and after annealing at 800°C for 10 min.

    The table shows strong enhancement and necessity to have both the nanostructured titanium and carbon active absorber combined to enhance the emissivity.

    Sample structureEmissivity before annealing (%)Emissivity after annealing (%)
    Si1010
    a-C/Si10–2010
    Ti-nano/Si1525
    a-C/Ti-nano/Si2095
    a-C/Fe catalyst/Ti-nano/Si2099

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