Boosting the Photoluminescence of Monolayer MoS2 on High‐Density Nanodimer Arrays with Sub‐10 nm Gap

DOI: 10.1002/adom.201700984 and nonlinear optics[5] to photocatalysis,[6] chemical synthesis,[7] and biosensing.[8] However, monolayer MoS2 exhibits a relatively low quantum yield due to defectmediated nonradiative recombination and biexcitonic recombination at higher excitation powers.[9] Moreover, the inherent atomic thickness of 2D MoS2 results in poor light absorption and emission which pose significant challenges for integrating MoS2 in optoelectronic devices. To date, various strategies including chemical doping and treatment,[10] defect engineering,[11] and coupling with photonic crystals[12] have been reported to enhance the photoluminescence (PL) response of MoS2. Particularly, integration of 2D TMDs materials with plasmonic nanostructures has emerged as an efficient scheme to boost and modulate their PL emission.[13] Plasmonic nanostructures made of noble metals can control light within nanoscale confined regions and provide strongly enhanced electromagnetic (EM) fields in their vicinity.[14] The condensed EM fields, termed ‘hot spots,’ are of high interest in a wide range of applications such as surface-enhanced spectroscopy,[15] biochemical sensing,[16] photovoltaics,[17] and optoelectronics.[18] Through integration with plasmonic nanoparticles, improved PL emission of MoS2 with an ensemble enhancement factor (EF) of 40-folds has been achieved.[19] Nevertheless, larger PL enhancement can be expected by optimizing the plasmonic nanostructures. The characteristics of plasmonic systems depend strongly on the morphology, materials composition, and arrangement of the nanostructures.[20] Plasmonic dimers with sub-10 nm gaps are promising in providing superior PL enhancement due to their ability to strongly confine EM fields in their gaps.[21] However, efficient fabrication of large-area dimer patterns with uniform and well-defined gaps remains an experimental challenge. For example, electron-beam lithography has been explored to fabricate uniform nanodimer arrays with a sub-10 nm precision.[22] Yet, this technique is expensive and time-consuming, not suitable for wafer-scale production. Chemical synthetic methods are efficient in high-throughput fabrication of plasmonic dimers,[23] but suffer from poor control over the assembly patterns due to aggregation of the colloidal particles. In this work, we fabricate patterned plasmonic dimers by a facile approach utilizing porous anodic aluminium oxide (AAO) templates during angle-resolved shadow deposition. The AAO nanotemplate enables centimetre-scale fabrication of highly uniform nanoarrays[24] thereby the plasmonic modes Patterned plasmonic nanodimers are fabricated exploiting an ultrathin porous anodic aluminum oxide membrane as a mask during angle-resolved shadow deposition. The fabricated nanodimer arrays exhibit consistent sub-10 nm gaps and a high particle density up to 1.0 × 1010 cm−2 over a large area. The ultrasmall dimer gaps provide highly confined electromagnetic fields, which strongly enhance the photoluminescence (PL) emission and Raman scattering from the surrounding monolayer molybdenum disulphide (MoS2). The ensemble PL intensity from MoS2/dimers is enhanced by up to a factor of ≈160 by resonant excitation of the dimer modes. Anisotropic polarizationdependent characteristics of PL and Raman from the MoS2/dimers confirm that the dominant enhancement originates from the dimer configuration. These experiments demonstrate a facile approach for the fabrication of lowcost high-performance 2D material-based optoelectronic devices.