Room-temperature high-speed electrical modulation of excitonic distribution in a monolayer semiconductor.

Excitons in monolayer semiconductors, benefitting from their large binding energies, hold great potential towards excitonic circuits bridging nano-electronics and photonics. However, achieving room-temperature ultrafast on-chip electrical modulation of excitonic distribution and flow in monolayer semiconductors is nontrivial. Here, utilizing lateral bias, we report high-speed electrical modulation of the excitonic distribution in a monolayer semiconductor junction at room temperature. The alternating charge trapping/detrapping at the two monolayer/electrode interfaces induces a non-uniform carrier distribution, leading to controlled in-plane spatial variations of excitonic populations, and mimicking a bias-driven excitonic flow. This modulation increases with the bias amplitude and eventually saturates, relating to the energetic distribution of trap density of states. The switching time of the modulation is down to 5 ns, enabling high-speed excitonic devices. Our findings reveal the trap-assisted exciton engineering in monolayer semiconductors and offer great opportunities for future two-dimensional excitonic devices and circuits.

Highly polarized light emission from hybrid of quantum dots and plasmons in the intermediate coupling regime.

Colloidal semiconductor quantum dots (QDs), with a size tunable bandgap and remarkably high quantum efficiency, have been recognized as ideal light sources in quantum information and light emitting devices. For light sources, besides the emission intensity and spectral profile, the degree of polarization (DoP) is an essential parameter. Here, by embedding a monolayer of QDs inside the nanogap between a bottom Au mirror and a top Ag nanowire, we have demonstrated highly polarized light emission from the QDs with an average DoP of 0.89. In addition to the anisotropic photoluminescence (PL) intensity, the PL spectra are distinct at different polarizations, with an asymmetric spectral shape or even two-peak features. Such an anisotropic emission behavior arises from the coupling between the QDs and the largely confined and polarization-dependent gap-plasmons in the Au/QD/Ag nanocavities in the intermediate coupling regime. Our results demonstrate the possibility of achieving highly polarized light sources by coupling spherical QDs to single anisotropic plasmonic nanocavities, to provide new opportunities in the future design of polarized QD-based display devices.

Room-Temperature Polarized Light-Emitting Diode-Based on a 2D Monolayer Semiconductor.

Transition metal dichalcogenide (TMD)-based 2D monolayer semiconductors, with the direct bandgap and the large exciton binding energy, are widely studied to develop miniaturized optoelectronic devices, e.g., nanoscale light-emitting diodes (LEDs). However, in terms of polarization control, it is still quite challenging to realize polarized electroluminescence (EL) from TMD monolayers, especially at room temperature. Here, by using Ag nanowire top electrode, polarized LEDs are demonstrated based on 2D monolayer semiconductors (WSe2 , MoSe2 , and WS2 ) at room temperature with a degree of polarization (DoP) ranging from 50% to 63%. The highly anisotropic EL emission comes from the 2D/Ag interface via the electron/hole injection and recombination process, where the EL emission is also enhanced by the polarization-dependent plasmonic resonance of the Ag nanowire. These findings introduce new insights into the design of polarized 2D LED devices at room temperature and may promote the development of miniaturized 2D optoelectronic devices.