We report the influence of uniaxial tensile mechanical strain in the range 0-2.2% on the phonon spectra and bandstructures of monolayer and bilayer molybdenum disulfide (MoS2) two-dimensional crystals. First, we employ Raman spectroscopy to observe phonon softening with increased strain, breaking the degeneracy in the E' Raman mode of MoS2, and extract a Grüneisen parameter of ~1.06. Second, using photoluminescence spectroscopy we measure a decrease in the optical band gap of MoS2 that is approximately linear with strain, ~45 meV/% strain for monolayer MoS2 and ~120 meV/% strain for bilayer MoS2. Third, we observe a pronounced strain-induced decrease in the photoluminescence intensity of monolayer MoS2 that is indicative of the direct-to-indirect transition of the character of the optical band gap of this material at applied strain of ~1%. These observations constitute a demonstration of strain engineering the band structure in the emergent class of two-dimensional crystals, transition-metal dichalcogenides.

We investigate electrical gating of photoluminescence and optical absorption in monolayer molybdenum disulfide (MoS2) configured in field effect transistor geometry. We observe an hundredfold increase in photoluminescence intensity and an increase in absorption at ∼ 660 nm in these devices when an external gate voltage is decreased from +50 V to -50 V, while the photoluminescence wavelength remains nearly constant. In contrast, in bilayer MoS2 devices we observe almost no changes in photoluminescence with gate voltage. We propose that the differing responses of the monolayer and bilayer devices are related to the interaction of the excitons in MoS2 with charge carriers. arXiv:1211.0341v1 [cond-mat.mes-hall]

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The Archimedean spiral geometry presents a platform for exploration of complex plasmonic mechanisms and applications. Here we show both through simulations and experiment that more complex plasmonic modes with unique near-field structure and larger mode volumes can be realized within a single, topologically robust structure. In the spiral, complex polarization response, resonant interactions and symmetry-breaking features are defined by the width and spacing of the spiral tracks and by the winding number of the spiral.