Li Intercalation in MoS₂: In Situ Observation of Its Dynamics and Tuning Optical and Electrical Properties

Two-dimensional layered materials like MoS₂ have shown promise for nanoelectronics and energy storage, both as monolayers and as bulk van der Waals crystals with tunable properties. Here we present a platform to tune the physical and chemical properties of nanoscale MoS₂ by electrochemically inserting a foreign species (Li⁺ ions) into their interlayer spacing. We discover substantial enhancement of light transmission (up to 90% in 4 nm thick lithiated MoS₂) and electrical conductivity (more than 200×) in ultrathin (∼2–50 nm) MoS₂ nanosheets after Li intercalation due to changes in band structure that reduce absorption upon intercalation and the injection of large amounts of free carriers. We also capture the first in situ optical observations of Li intercalation in MoS₂ nanosheets, shedding light on the dynamics of the intercalation process and the associated spatial inhomogeneity and cycling-induced structural defects

Second-Harmonic Generation in GaAs Photonic Crystal Cavities in (111)B and (001) Crystal Orientations

We demonstrate second-harmonic generation in photonic crystal cavities in (001)- and (111)­B-oriented GaAs. The fundamental resonance is at 1800 nm, leading to generated second harmonic below the GaAs band gap. Below-band-gap operation minimizes absorption of the second-harmonic and two-photon absorption of the pump. Photonic crystal cavities were fabricated in both orientations at various in-plane rotations of the GaAs substrate. The rotation dependence and far-field patterns of the second harmonic match simulation. We observe similar maximum efficiencies of 1.2%/W in (001)- and (111)­B-oriented GaAs