Synergistic Photon Management and Strain-Induced Band Gap Engineering of Two-Dimensional MoS2 Using Semimetal Composite Nanostructures.

2D MoS2 attracts increasing attention for its application in flexible electronics and photonic devices. For 2D material optoelectronic devices, the light absorption of the molecularly thin 2D absorber would be one of the key limiting factors in device efficiency, and conventional photon management techniques are not necessarily compatible with them. In this study, we show two semimetal composite nanostructures deposited on 2D MoS2 for synergistic photon management and strain-induced band gap engineering: (1) the pseudo-periodic Sn nanodots, (2) the conductive SnOx (x < 1) core-shell nanoneedle structures. Without sophisticated nanolithography, both nanostructures are self-assembled from physical vapor deposition. Optical absorption enhancement spans from the visible to the near-infrared regime. 2D MoS2 achieves >8× optical absorption enhancement at λ = 700-940 nm and 3-4× at λ = 500-660 nm under Sn nanodots, and 20-30× at λ = 700-900 nm under SnOx (x < 1) nanoneedles. The enhanced absorption in MoS2 results from strong near-field enhancement and reduced MoS2 band gap due to the tensile strain induced by the Sn nanostructures, as confirmed by Raman and photoluminescence spectroscopy. Especially, we demonstrate that up to 3.5% biaxial tensile strain is introduced to 2D MoS2 using conductive nanoneedle-structured SnOx (x < 1), which reduces the band gap by ∼0.35 eV to further enhance light absorption at longer wavelengths. To the best of our knowledge, this is the first demonstration of a synergistic triple-functional photon management, stressor, and conductive electrode layer on 2D MoS2. Such synergistic photon management and band gap engineering approach for extended spectral response can be further applied to other 2D materials for future 2D photonic devices.

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

Visualizing and manipulating the optical contrast of single-layer graphene (SLG) and other 2D materials has continuously been an interesting topic to understand fundamental light-matter interaction down to atomic thickness. Because the optical properties of SLG can be tuned by gating, demonstrating and manipulating the color contrast of SLG also has significant potential applications in ultrathin flexible color display. However, previous demonstrations of optical contrast of SLG are mostly limited to reflection intensity contrast under monochromatic illumination using the interference effect. The reported spectral contrast in SLG has mostly been narrow-band or at resonant wavelengths, and it required precise thickness control and/or nanolithography that are hardly scalable to large enough area for display applications. In this paper, we demonstrate novel color contrast optical visibility of SLG under white light using broadband photon management induced by nanoneedle-structured SnOx (x ≤ 1) transparent conductive oxides (TCOs), which is scalable to large-area color display. The low-temperature fabricated, self-assembled, nanoneedle-structured SnOx (x ≤ 1) thin films help to significantly increase the broadband optical absorption in SLG by enhancing the electromagnetic field and increasing the scattering efficiency at the SnOx/SLG interface. With nanoneedle-structured SnOx, the optical absorption in SLG on a fused quartz (SiO2) substrate is drastically increased from ∼1.4 to >10% at λ = 560-990 nm (from yellow to near infrared spectral regimes), leading to a clear color contrast to the surrounding region without SLG. The self-assembly approach, rather than sophisticated and costly nanolithography, allows scalable fabrication of large area 2D photonic devices with a broadband and highly efficient photon management effect. Therefore, this approach can be further extended to color-tunable TCO/dielectric/SLG 2D photonic devices by adjusting the free carrier concentrations/Fermi levels in the TCO and SLG layers via gating-a stepping stone toward ultrathin flexible color display technologies utilizing 2D materials and nanostructured thin films.