Reduced-dimensionality materials for photonic and optoelectronic applications including energy conversion, solid-state lighting, biological sensing, and information technology are undergoing rapid development. This development is driven by new physics that arises from the reduction in dimensionality, spurring the continuous search for novel low-D materials and material structures. Importantly, understanding and optimizing the properties of these materials requires characterization at the relevant length scale, which is often below the diffraction limit.
Two-dimensional (2D) materials display diverse optical and electronic properties, ranging from zero-bandgap graphene to insulating boron nitride and semiconducting transition metal dichalcogenides (TMDCs) such as molybdenum disulfide (MoS2).
The emergence of 2D monolayer transition metal dichalcogenides (ML-TMDCs) as direct bandgap semiconductors has rapidly accelerated the advancement of room-temperature, 2D optoelectronic devices. ML-TMDCs are exquisite optoelectronic materials that synergize the effects of strong confinement, intense many-body interactions, and spin-coupled valley degrees of freedom in a robust, atomically thin semiconductor. This provides an unprecedented opportunity for exploring the internal quantum degrees of freedom of electrons and their potential for new electronic technologies and sensing devices.
The optical excitations in these materials manifest from a hierarchy of electrically and mechanically tunable, Coulombic free-carrier and excitonic many-body phenomena. Investigating the fundamental interactions underpinning these phenomena presents challenges, however, due to a complex balance of competing optoelectronic effects and interdependent properties.
In our group, we are able to exploit the optical detection of bound- and free-carrier photoexcitations (with high spatial and temporal resolution), allowing us to directly quantify and correlate carrier-induced changes of the quasiparticle band gap, exciton binding energies, and exciton lifetimes in 2D materials. Excitingly, this in turn permits us to explicitly disentangle the competing optoelectronic effects that ultimately result in material functionality.
In a related effort in collaboration with the Hone group, we have been studying monolayer WSe2 (ML-WSe2), an atomically thin semiconductor that hosts defect-bound exciton states, which behave as nanoscale single-photon quantum emitters at cryogenic temperatures. Recent work has shown that these defect-bound exciton states are localized to nanoscale strained regions referred to as “nanobubbles” and demonstrate a compelling route to systematically pattern single-photon emitters in monolayer semiconductors for quantum photonic technologies. However, the single-photon emitting states are largely suppressed with increasing temperature, posing a significant challenge to any practical realization of this phenomena at room temperature.
Using a model plasmonic-2D semiconductor architecture, we demonstrate that a nano-optical antenna activates optical excitation and luminescence from bound exciton states in nanobubbles of ML-WSe2 at room temperature and under ambient conditions. Enabling the room-temperature emission from such tightly localized bound exciton states is a first critical step towards realizing room-temperature quantum emitters in ML-WSe2 and sets the stage for more sophisticated photon anti-bunching measurements on these unique, nano-optical enabled states.