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Optical Control of Hierarchical Graphene Systems in Ultraviolet to Mid-infrared Region through Varying Surface Profiles and Nanoparticle Coatings


Fundamental understanding of crumpled graphene and its optical properties are currently lacking. This paper addresses the gap in current literature by characterizing two hierarchical graphene systems: biaxially-strained randomly crumpled graphene (BRCG) and gold particle coated periodically crumpled graphene (GP-coated PCG), through rigorous coupled-wave analysis (RCWA) and finite-difference time-domain (FDTD) computations. In BRCG, the structure is primarily characterized by its correlation length (ξ) and root-mean-square amplitude (α), which are taken as the average value over the entire simulation space, as well as the film thickness (δ). In GP-coated PCG, the periodic crumple structure is characterized by its amplitude (A) and pitch (P) as well as the radius (r) and separation (λ) of gold particles. The results show by decreasing the ξ of BRCG from 1.5 to 0.25 μm, the spectral average absorbance in the ultraviolet to near-infrared (0.2–2 μm) region increased from 0.44 to 0.75 while reflectivity was lowered from 0.52 to 0.19. Increasing α from 0.25 and 1.5 μm, the spectral average absorbance increased from 0.38 and 0.75 while reflectivity was lowered from 0.59 to 0.19. The rise in absorbance observed with decreasing ξ and increasing α is attributed to the resulting geometrically induced enhancement in internal ix scattering between and within adjacent crumples. The simultaneous reduction in reflectivity is due to the amplification of interference effects and the lowering of specular reflection as surface roughness is increased. By decreasing δ from 3.5 to 0.35 nm, average absorbance was lowered from 0.45 to 0.31 due to a shorter optical length which diminishes internal scattering. Average reflectivity also increased from 0.51 to 0.66 and is driven by the reduction in absorbance. In GP coated PCG, decreasing the P from 1 to 0.14 μm led to an increase in average solar absorbance (Asolar) in the UV-NIR (0.05–1 μm) region from near-zero to 0.14 due to the associated enhancement in electric field confinement and plasmonic resonance. Concurrently, the average solar transmittance (Tsolar) in the same region decreased from 0.74 to 0.55 due to the reduced exposure of underlying transmissive graphene while the average solar reflectivity (Rsolar) is marginally improved from near-zero to 0.05 due to the diminished separation between reflective gold particles. The same mechanisms are applied to explain the changes in optical properties with varying A. Decreasing A from 0.2 to 0.025 μm. Asolar increased from 0.07 to 0.14, Tsolar decreased from 0.67 to 0.55, and Rsolar was again marginally improved from near-zero to 0.05. In the mid infrared (MIR, 1–20 μm), all optical signals were shifted along the wavelength axis due to Bragg diffraction without notable changes in their magnitude. Enlarging r from roughly 4.2 to 67 nm increased Asolar from near-zero to 0.15 due to the enhanced presence of gold particles which contribute to electric field (E-field) confinement and plasmonic resonance effects. The enlarged extinction cross-section of gold particles with increasing r also contributes to the rise in absorbance. Rsolar also increased from near-zero to 0.09 and Tsolar dropped from 0.74 to 0.49 due to the increased cross-section of gold particles and decreased exposure of underlying graphene, respectively. Changing λ from 0.2 to 0.05 μm, Asolar increased from 0.01 to 0.06 due to the enhancement in E-field confinement enabled by the closer proximity of gold particles. This was x accompanied by a reduction in Tsolar from 0.73 to 0.67 and a marginal improvement in Rsolar from near-zero to 0.01, attributed to the same mechanism discussed above. Marginal changes with varying particle radius and separation were observed in the MIR. Thermal analysis further demonstrates enhanced radiative heating and cooling through precisely controlling the structures’ solar absorptivity and infrared emissivity. This paper presents two approaches to creating hierarchical graphene systems through varying either the surface geometry of BRCG or nanoparticle dimensions of GP-coated PCG, and subsequently tune their optical properties from the UV-NIR to MIR region to enable more effective thermal radiation control.

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