Fig. 2.

Light emission from vdWQT devices. a Optical microscope image of a set of devices. The three areas on the left, where gold electrodes (50 nm), h-BN crystal (N = 7) and graphene overlap, form the tunneling devices. Electrodes on the right serve as electrical contacts to the graphene sheet. Scale bar: 10 μm. b Spatial distribution of photons emitted into the air/upper half-space above the graphene sheet from the devices at an applied voltage of 2.0 V. Light is emitted from the entire area of the devices. Scale bar: 5 μm. c Corresponding distribution of photons emitted into to substrate/lower half-space. Light is observed primarily at the edges of the devices. Scale bar: 5 μm. Straight lines in b, c mark the outlines of gold electrodes (gold) and graphene (gray). Intensity units are as measured and not corrected for system efficiency. d Sketch of the vertical cross-section through the device, illustrating the mechanism of light generation. The inelastic component of the tunneling current j couples to the optical modes of the vdWQT device, i.e., the free-space continuum of optical states as well as surface-bound SPP modes (not to scale). Photons emitted into the air half-space give rise to the image in b, SPP scattering at the edges of the gold electrodes generate the image in c