  Few inventions have shaped the world like the incandescent bulb. While Edison
used thermal radiation from ohmically heated conductors, some noble metals
exhibit "cold" electroluminescence (EL) in percolation films, tunnel diodes,
electromigrated nanoparticle aggregates, optical antennae, or
scanning-tunnelling microscopy (STM). The origin of this radiation, which is
spectrally broad and depends on applied bias, is controversial given the low
radiative yields of electronic transitions. Nanoparticle EL is particularly
intriguing since it involves localized surface-plasmon resonances with large
dipole moments. Such plasmons enable very efficient non-radiative fluorescence
resonance energy transfer (FRET) coupling to proximal resonant dipole
transitions. We demonstrate nanoscopic FRET-LEDs which exploit the opposite
process, energy transfer from silver nanoparticles to exfoliated monolayers of
transition-metal dichalcogenides (TMDCs). In diffraction-limited hotspots
showing pronounced photon bunching, broadband silver EL is focused into the
narrow excitonic resonance of the atomically thin overlayer. Such devices may
offer alternatives to conventional nano-LEDs in on-chip optical interconnects.
