Self-Assembled, Ultrahigh Refractive Index Pseudo-Periodic Sn Nanostructures for Broad-Band Infrared Photon Management in Single Layer Graphene

Abstract

Graphene is a two-dimensional material with intriguing electrical and optical properties for infrared photonic devices. However, single layer graphene (SLG) suffers from a very low optical absorption of ∼1–2% depending on the substrate, which significantly limits its efficiency as photonic devices. In this Letter, we address this challenge by coating SLG with self-assembled, pseudoperiodic ultrahigh refractive index (n = 8–9 at λ = 1600–5000 nm) semimetal Sn nanostructures for highly effective, broad-band infrared photon management in SLG, offering a new approach for light trapping beyond plasmonics and high refractive index dielectric photonics. The infrared absorption in SLG on fused quartz (SiO2) is greatly increased from <1.5% to >15% in a very broad spectral range of λ = 900–2000 nm due to the near-field electromagnetic interactions between the ultrahigh refractive index Sn nanostructures and SLG, a significant advantage over relatively narrow-band plasmonic resonances for photon management in SLG. The optical absorption enhancement in SLG has also been confirmed by field-enhanced Raman peaks from SLG and supported by higher photoconductivities both at an infrared wavelength of λ = 1550 nm and at a visible wavelength of λ = 650 nm. This work also opens the door to the investigations of ultrahigh refractive index semimetal nanostructures for nanoscale photon management.

Color Contrast of Single-Layer Graphene under White Light Illumination Induced by Broadband Photon Management

“…This allows a much stronger near-field enhancement at a nanoscale (i.e., > 100× optical field intensity at the airgap between Sn nanostructures) and thereby a much higher absorption enhancement of SLG in the vicinity of Sn nanodots, as we have reported elsewhere. 4 A similar mechanism can be expected in the SnO x nanoneedle-based photon management strategy here considering that we have indeed observed Sn/ SnO core−shell nanoneedles from the BSE images in Figure 2d,f. The near-field enhancement contributed by Sn nanostructures is also consistent with the observation that the SLG absorption enhancement decreases dramatically with the GeO 2 thickness in Figure 4a, which defines the minimal spacing between Sn nanoneedles and SLG.…”

“…Such a highly effective and broadband light trapping effect leads to a strong color contrast between the regions with and without SLG under white light illumination, which is a notable improvement in visualizing SLG compared to the conventional interferenceinduced intensity contrast under optimal monochromatic illumination. Our previous work 4 reported self-assembled, pseudoperiodic ultrahigh refractive index Sn nanodots for SLG absorption enhancement in the NIR regime. However, it only demonstrated absorption intensity contrast instead of color contrast in the visible regime.…”

“…However, it only demonstrated absorption intensity contrast instead of color contrast in the visible regime. While reducing the nominal thickness of Sn nanodots could better match the SLG absorption enhancement with the visible spectrum, it also induces a much larger morphological difference between the Sn nanostructures deposited on SLG versus those deposited on quartz 4 that overwhelms the color contrast induced by photon management. By comparison, in this paper the optical color contrast is achieved in the visible spectrum regime, and it is predominantly contributed by the SLG absorption enhancement without any morphological differences between the SnO x nanostructures deposited on SLG versus those on quartz.…”

“…3 This number goes down to 1.4% if SLG is transferred on a fused quartz (SiO 2 ) substrate because of optical impedance mismatch. 4 Therefore, optically visualizing SLG has been a challenging topic to understand fundamental light−matter interactions down to atomic thickness.…”

“…Because of its atomic thickness and the unique conical band structures, the electronic and mechanical properties of SLG have been widely investigated for flexible devices and transparent electrodes in solar cell applications. , Optically, free standing SLG is perceived as nearly transparent because it only absorbs 2.3% of the incident light, independent of wavelength . This number goes down to 1.4% if SLG is transferred on a fused quartz (SiO 2 ) substrate because of optical impedance mismatch . Therefore, optically visualizing SLG has been a challenging topic to understand fundamental light–matter interactions down to atomic thickness.…”