Thermal origin of light emission in nonresonant and resonant nanojunctions

Electron tunneling is associated with light emission. In order to elucidate its generating mechanism, we provide an experimental ansatz that employs fixed-distance epitaxial graphene as metallic electrodes. In contrast to previous experiments, this open geometry permits an unobscured light spread from the tunnel junction, enabling both a reliable calibration of the visible to infrared emission spectrum and a detailed analysis of the dependence of the parameters involved. In a nonresonant geometry, the emitted light is perfectly characterized by a Planck spectrum. In an electromagnetically resonant environment, resonant radiation is added to the thermal spectrum, both being strictly proportional in intensity. In full agreement with a simple heat conduction model, we provide evidence that in both cases the light emission stems from a hot electronic subsystem in interaction with its linear electromagnetic environment. In a long-running discussion whether the light is of thermal or electromagnetic origin, these results on graphene nanojunctions clearly favor the thermal picture.

Figure 1

(a), (b) Sketch of a graphene tunnel contact (GTC) and a graphene point contact (GPC), in which presumably a single carbon-carbon bond forms the contact. (c) For the GTC, the light emission is plotted as a function of the detection wavelength for various bias voltages. The spectra resemble Planck spectra, however, with an added spectral weight around 720 nm that is further analyzed in Fig. 2. Dashed lines indicate fits of Planck's law to the IR data; the obtained temperatures are given in the legend. (d) For GPC, the spectra are in good agreement with Planck's law displaying high electronic temperatures well beyond the damage threshold of the material.

Figure 2

(a) Planck temperature as derived from the Planck spectra as a function of $\sqrt{P_{\mathrm{el}}}$ of a GPC sample. Two strictly linear regions can be identified in full accordance with Eq. (1), and the kink may indicate a crossover in cooling mechanisms. (b) Intensity dependencies on $P_{\mathrm{el}}^{-1/2}$ (GPC and GTC samples), which on the logarithmic scale appear as a straight line over six orders of magnitude (the constant dark count rate has been subtracted). (c) Analysis of the shoulderlike feature in Fig. 1, which appears as a self-similar resonance. (d) The amplitude of the latter scales strictly linear with the amplitude of the underlying blackbody radiation. The simplicity of all parameter dependencies (dashed straight lines are guides to the eye) is remarkable and underscores the validity of the model.