Broad-range modulation of light emission in two-dimensional semiconductors by molecular physisorption gating.

In the monolayer limit, transition metal dichalcogenides become direct-bandgap, light-emitting semiconductors. The quantum yield of light emission is low and extremely sensitive to the substrate used, while the underlying physics remains elusive. In this work, we report over 100 times modulation of light emission efficiency of these two-dimensional semiconductors by physical adsorption of O2 and/or H2O molecules, while inert gases do not cause such effect. The O2 and/or H2O pressure acts quantitatively as an instantaneously reversible "molecular gating" force, providing orders of magnitude broader control of carrier density and light emission than conventional electric field gating. Physi-sorbed O2 and/or H2O molecules electronically deplete n-type materials such as MoS2 and MoSe2, which weakens electrostatic screening that would otherwise destabilize excitons, leading to the drastic enhancement in photoluminescence. In p-type materials such as WSe2, the molecular physisorption results in the opposite effect. Unique and universal in two-dimensional semiconductors, the effect offers a new mechanism for modulating electronic interactions and implementing optical devices.

Thermally driven crossover from indirect toward direct bandgap in 2D semiconductors: MoSe2 versus MoS2.

Layered semiconductors based on transition-metal chalcogenides usually cross from indirect bandgap in the bulk limit over to direct bandgap in the quantum (2D) limit. Such a crossover can be achieved by peeling off a multilayer sample to a single layer. For exploration of physical behavior and device applications, it is much desired to reversibly modulate such crossover in a multilayer sample. Here we demonstrate that, in a few-layer sample where the indirect bandgap and direct bandgap are nearly degenerate, the temperature rise can effectively drive the system toward the 2D limit by thermally decoupling neighboring layers via interlayer thermal expansion. Such a situation is realized in few-layer MoSe(2), which shows stark contrast from the well-explored MoS(2) where the indirect and direct bandgaps are far from degenerate. Photoluminescence of few-layer MoSe(2) is much enhanced with the temperature rise, much like the way that the photoluminescence is enhanced due to the bandgap crossover going from the bulk to the quantum limit, offering potential applications involving external modulation of optical properties in 2D semiconductors. The direct bandgap of MoSe(2), identified at 1.55 eV, may also promise applications in energy conversion involving solar spectrum, as it is close to the optimal bandgap value of single-junction solar cells and photoelechemical devices.

Large reaction rate enhancement in formation of ultrathin AuSi eutectic layers.

Metal-semiconductor eutectic liquids play a key role in both the fundamental understanding of atomic interactions and nanoscale synthesis and catalysis. At reduced sizes they exhibit properties distinct from the bulk. In this work we show an unusual effect that the formation of AuSi eutectic liquid layers is much easier for smaller thicknesses. The alloying reaction rate is enhanced by over 20 times when the thickness is reduced from 300 to 20 nm. The strong enhancement is attributed to a strain-induced increase in the chemical potential of the solid layer prior to the alloying reaction.