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 >8x optical absorption enhancement at lambda = 700-940 nm and 3-4x at lambda = 500-660 nm under Sn nanodots, and 20-30x at lambda = 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 similar to 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.